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NBSIR 84-2852

RADC/NBS Workshop

Moisture Measurement and Control

for Semiconductor Devices, III

U S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Electronics and Electrical Engineering

Semiconductor Materials and Processes Division

Washington, DC 20234

May 1984

*0,>eau o*

U S. DEPARTMENT OF COMMERCE

U' VTION AL BUREAU OF STANDARDS

RATIONAL BUREA OF STANDARDS

. LIBRARY

( l / F

Oc

NBSIR 84-2852

RADC/NBS WORKSHOP

MOISTURE MEASUREMENT AND CONTROL

FOR SEMICONDUCTOR DEVICES, III

Proceedings of the RADC/NBS Workshop

held at the National Bureau of Standards

Gaithersburg, MD

November 2-4, 1 983

Benjamin A. Moore and Stanley Ruthberg, Editors

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Electronics and Electrical Engineering

Semiconductor Materials and Processes Division

Washington, DC 20234

This activity was sponsored by:

Rome Air Development Center

RBRE

Griffiss AFB, NY 1 3441

and

National Bureau of Standards

May 1984

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Emwt Ambler. Director

Table of Contents

Page

Preface vi

Abstract 1

1. Introduction 1

2. Session I Moisture Analysis ......... 3 Chairman, Robert W. Thomas, RADC

2.1 What's Happening at RADC

B. A . Moore (Rome Air Development Center) 3 2.2 Moisture Analysis at Oneida Research Services, Inc.

T. J. Rossiter and D. Feliciano-Welpe s Oneida

Research Services , Inc,) 8 2.3 Gas Analysis at Gollob Analytical Service

F. Gollob (Gollob Analytical Service) 19

2.4 Control Chart Analysis of Moisture in CERDIPs

R. B . Elo and A . M, Massoletti (American Microsystems

)

. 24

2.5 A Moisture Standard for IC Package Gas Analysis

R . F, Haack (Jet Propulsion Laboratory) 39

2.6 The Evolution of Method 1018

R. W. Thomas (Rome Air Development Center) 51

3. Session II Moisture Measurement Methods 59

Chairman, Robert K. Lowry, Harris Semiconductor

3.1 Selecting Moisture Measurement Methods for Semiconductor

Devices

R, K. Lowry (Harris Semiconductor) 59

3.2 The Influence of Filling Gas of Hermetically Sealed

Packages on Device Reliability: A Demand for a Complete

Residual Gas Analysis

L. J, Van Beek and H. A . Loos (Bell Telephone Mfg . Co,) . 62

3.3 Extended Computer Control and Data Processing for the Mass

Spectrometr ic Analysis of Package Atmospheres

L, J, Rigby (Standard Telecommunication Laboratories) , , 69

3.4 Gaseous Compositions of Hermetic Package Cavity Ambients II

R, K, Lowry (Harris Semiconductor) 86

3.5 Improvements in the Capacitance-Ratio Method of Using an

IC Die as a Moisture Sensor

R, P, Merrett (British Telecom Research Laboratories) . . 94

3.6 Comparison of Various Moisture Measurement Methods: Test

Procedures at RADC

N. K, Annamalai (Rome Air Development Center) 106

3.7 Laser Spectrometr ic Determination of Moisture

J, ‘A, Mucha (AT&T Bell Laboratories) 119

3.8 Total Water Measurement Equipment and Process Using Method

1018, Procedure 2 C, Sloneker and B, Church (Medtronic > Inc.) 128

iii
Page

4.

Session III Moisture Sensors 129

Chairman, Victor Fong, Panametrics

4.1 Fundamental Studies of the Aging Mechanism in Aluminum

Oxide Thin Films

C.-H. Lin and S. D. Senturia (Massachusetts Institute

of Technology ) 129

4.2 Recent Evaluations of AI2O3 In Situ Moisture Sensors for

Industrial Electronic Hermetic Components Application

M. Brizoux and J. Perdrigeat (Thomson-CSF) and D. Kane,

and R. Gauthier (I.N.S.A. Lyon) 140

4.3 Moisture Sources and Measurement in CERDIP Packages with

Alumina Sensors

H. Joss (Coors Porcelain Company) 153

4.4 Moisture Content Control Using Alumina Sensor

C. M . Roberts, Jr. (Analog Devices) 162

4.5 Monitoring of Moisture in CERDIP Devices Using a Moisture

Sensor Chip

J. Hunter (Advanced Micro Devices, Inc.) 171

5.

Session IV Hermeticity and Moisture 180

Chairman, Stanley Ruthberg, NBS

5.1 How to Raise the Permissible Leak Rate by Four Orders of

Magnitude

J. G. Davy (Westinghouse Manufacturing Systems and

Technology Center) 180

5.2 Hermeticity and Moisture Ingress

A. DerMarderosian (Raytheon Company) 196

5.3 Leak Detection, Gross and Fine, Using Helium as a Trace Gas

P. R. Forant (Varian/Lexington Vacuum Division) 217

5.4 Using Optical Correlation to Measure Leak Rates in Sealed

Packages

J. W. Wagner, L. C. Phillips, E. P. Mueller, and

R. E. Green, Jr. (Johns Hopkins University) 222

6.

Session V Moisture Physics 226

Chairman, Aaron DerMarderosian, Raytheon Company

6.1 The Physisorption of Water Onto Integrated Circuit Package

Components

J . M. Ammons , G. R. Hoff, M. Kovac, J. H. Linn, and

W. E. Swartz, Jr. (University of South Florida) 226

6.2 Simulating the Corrosion Threshold of LSI/VLSI Devices

Using Moisture Sensitive Test Patterns

J. B. Kiely (Intel Corporation) 236

6.3 The Effect of Humidity on Cermet Resistors

T. R. Homa and P. P. Vadala (IBM Corporation) 251

6.4 Moisture Effects on Passive Hybrid Components - 4 Year

Exposure Test

J. A. Ronning, Jr. and A. H. Jevne (Medtronic, Inc.) . . 263

IV
Page

7. Session VI Moisture and Organics 271

Chairman, Thomas J. Rossiter, Oneida Research Services

7.1 Mass Spectrometr ic Evaluation of Epoxy Adhesive Systems

W. Bardens (Beckman Instruments , Inc,) 271

7.2 Moisture Transport in Polyimide Films: Implications for

Moisture Measurement in IC Packages

D, D, Denton , D, R. Day, and S. D. Senturia (Massachusetts Institute of Technology

)

279

7.3 Moisture Content Variation in a High Polymer Content

Package

B, Church (Medtronic, Inc,) 288

7.4 Volatility Behavior of Organic Die Adhesive Materials

R, K. Lowry and K. L, Hanley (Harris Semiconductor) , . , 289

8. Session VII Moisture Control 296

Chairman, John Kiely, Intel Corporation

8.1 Moisture in Electronic Packaging: Measurement and the

Control of It

G, B. Cvijanovich (AMP Incorporated Research) 296

8.2 Evaluation of Moisture Sources in Hybrid Microcircuits

G, H, Ebel, H. Hammer, and S , Herman (Singer Company) . , 302

8.3 Implantable Pacemaker Moisture Control

J, A, Ronning, Jr, (Medtronic, Inc.) 307

8.4 Determination of Reliable Moisture Levels

B. Church (Medtronic, Inc.) 313

9. Workshop Participants 314

9.1 List of Speakers 314

9.2 List of Attendees 317

v
Preface

A third workshop on Moisture Measurement Technology for Semiconductor Devices

was held at the Gaithersburg, Maryland, facility of the National Bureau of

Standards on November 2 to 4, 1983, under the co-sponsorship of the Rome Air

Development Center (RADC) and the National Bureau of Standards (NBS) . It was another in a series of workshops that have been conducted as part of the

Semiconductor Technology Program of the NBS. These workshops have been dedicated to the furtherance of the measurement technology needed by the semiconductor device industry in its attempt to provide to its customers products

that are based on the most advanced technology, yet have high reliability and

the affordable costs which result from high yields. Representatives from

industrial, governmental, and academic organizations concerned with device

manufacture, analysis, and instrument design participated.

These workshops have provided a forum for reporting the continuous progress

that has been made in the measurement of moisture in hermetic semiconductor

devices, in the correlation between measurements by different procedures, in

determining the sources of moisture, in test environments, and in process

control. Thirty-four formal presentations were given.

The workshop was co-chaired by Benjamin A. Moore of RADC and Stanley Ruthberg

of NBS; Sara R. Torrence and Kathleen D. Kilmer of NBS coordinated workshop

arrangements; Pamela M. Baker of NBS was responsible for fiscal matters and

registration; E. Jane Walters of NBS provided editorial assistance and supervised processing of these Proceedings; and Brenda L* Kefauver, Marilyn L.

Stream, Jennifer L. Allen, and Josephine S. Halapatz assisted in operational

details.

Disclaimers

The views and conclusions expressed are those of the authors and do not

necessarily represent the official policies of the Department of Defense,

Department of Commerce, or the United States Government.

Certain commercial equipment, instruments, or materials are identified in

papers published in this report in order to adequately specify the

experimental procedure. In no case does such identification imply

recommendation or endorsement by the National Bureau of Standards, nor does

it imply that the material or equipment identified is necessarily the best

available for the purpose.

Papers in this volume have not been edited by the National Bureau of

Standards. Non-NBS authors are solely responsible for the content and

quality of their submissions.

vi
RADC/NBS Workshop

Moisture Measurement and Control for Semiconductor Devices, III

- A Workshop Report -

Benjamin A. Moore (RADC) and Stanley Ruthberg (NBS

)

The workshop, one of a series concerned with measurement problems

in integrated circuit processing and assembly, served as a forum to

examine the continuing progress that has been made in the measurement and control of moisture in hermetically packaged semiconductor

devices. Thirty-four presentations are included which contain

detailed information for securing hermetic packages with low moisture content. Agreement in measurement has been obtained with the mass spectrometer for cerdip and metal packages at the 5000 ppmv

level of moisture through the use of suitable moisture generators,

a 3-volume calibrator, calibrated dewpoint hygrometers, and appropriate operational procedures. An approach is given for a reproducible and reliable transfer package. However, the increased use

of organic materials in new and rapidly expanding technologies such

as VLSI/VHSIC and hybrid packaging presents new and more complex

challenges to accurate measurement of interior moisture.

Key words: analysis of moisture content; hermetically packaged

semiconductor devices; hybrid packages; mass spectrometer measurements; moisture; organic package materials; moisture generators;

moisture sensors; quality control; reliability of semiconductor

devices; semiconductor devices.

1. Introduction

A workshop on Moisture Measurement and Control for Semiconductor Devices was held at the National Bureau of Standards in Gaithersburg, Maryland, on November 2 to 4, 1983, co-sponsored by the Rome Air Development Center and the

National Bureau of Standards. It was another in a series concerned with

measurement problems related to integrated circuit processing, assembly, and

reliability. This was the third workshop at NBS concerned with the moisture

measurement and control problem. The first workshop was sponsored by

Defense Advanced Research Projects Agency/National Bureau of Standards

(DARPA/NBS) on March 22 to 23, 1978 (see NBS Special Publication 400-69,

Semiconductor Measurement Technology; ARPA/NBS Workshop V, Moisture

Measurement Technology for Hermetic Semiconductor Devices, May 1981). The

second workshop was sponsored by NBS/RADC on November 5 to 7, 1980 (see NT. Special Publication 400-72, Semiconductor Measurement Technology; NBS/RADC

Workshop, Moisture Measurement Technology for Hermetic Semiconductor Devic*

,

II) •

Progress has continued in the measurement and control of moisture in hermetically packaged semiconductor devices. At the time of the first workr.h

in 1978, inter laboratory measurement precision was poor, although it that individual laboratory precision of mass spectrometer measur omer.

1
be adequate. It also appeared that the use of moisture sensors was becoming

practical, but correlation to the mass spectrometer was low. Now, however,

as presented in the third workshop, agreement in measurement has been

obtained with the mass spectrometer for cerdip and metal packages at the 5000

ppmv level of moisture through the use of suitable moisture generators, a 3-

volume calibrator, calibrated dewpoint hygrometers, and specific operational

procedures. The procedures by which such agreement can be obtained,

including extended computer control and data processing, are presented.

A reproducible and reliable transfer standard, i.e., a simulated package of

known volume and moisture control, could be a very helpful device. Achievement of such a structure has been elusive. An approach for this was presented as based upon a demountable metal seal over a machined volume.

Nondestructive measurement techniques such as a capacitance-ratio method on the IC die and internal moisture sensors of both the oxide and surface con- ductivity types have become useful for production control and for diagnostics

of materials behavior. The thermal and the aging characteristics of the

oxide sensor, which affect the sensor's precision, are better known.

The sorption of water on package components and its effect on corrosion and

performance along with allowable levels within packages are topics that were

addressed. Of primary concern now is the increased use of organic materials

such as epoxies, polyimides, dessicants, etc. in new and rapidly expanding

technologies such as VLSI/VHSIC and hybrid packaging which present new and more complex challenges to the accurate measurement of moisture.

The workshop included 34 presentations in 7 sessions on topics such as Method

1018.2 of MIL-STD-883, measurement methods, sensors, hermeticity, moisture

physics, organic materials, and moisture control.

2
2. SESSION I METHOD 1018.2 MOISTURE ANALYSIS

2 . 1 WHAT '

S

HAPPENING AT RADC

BENJAMIN A. MOORE

ROME AIR DEVELOPMENT CENTER

GRIFFISS AFB NY 13441

(315)330-4055

ABSTRACT

The following information was initially prepared to be presented as informal

comments during a "rump” session or question/answer period during the Moisture

Workshop. Therefore, no formal text was prepared. However, due to a paper

withdrawal, the information was presented during the Method 1018.2 Moisture

Analysis Session (I). A brief discussion of the accompanying tables and figures

follows.

DISCUSSION

Table I outlines current efforts at RADC to improve the quality and scope of

moisture measurements while anticipating and meeting the challenges of new (VHS1C)

and burgeoning technologies (hybrid). Issues of concern with accompanying current

and planned activities to address them are presented.

The results of an in-house experiment to determine the feasibility of producing

an organic containing standards are presented in Table II. The potential is

demonstrated. However, work must be done to evaluate the moisture uptake in

epoxies and the best methods (i.e., pre-analysis bake) of assuring acceptable

analytical accuracy.

Figures 1-7 present some of our current efforts in advanced software

development. When completed, the software will provide the capabilities of

evaluating moisture desorption from package materials, temperature and sample

volume effects, and specie related phenomena (i.e. hydrogen pumpout

characteristics). Though not shown on the accompanying figures, date and

experimental information is now included.

3
As shown in Figure 1, any single mass scan from an analysis may be examined.

Figures 2 and 3 detail sample pressure measurement and pumpout curves. Unusual

pumpout phenomena can be detected using these features. Figure 4 shows the

pumpout curve for a typical "ideal" acting gas (^) in contrast to moisture as shown

in Figure 3. The option now exists to extrapolate the moisture curve until it reaches

background. The effects of moisture desorbing materials on a mass spectrometric

gas analysis can be seen in Figures 6 and 7. It should be noted that Figures 3-7

present both the raw data points and a "best-fit" polynominal to the data. This

information is used in comparing discrete (point-to-point) and integrated (Simpson's

Rule) gas analysis.

A recent in-house comparison of plating processes is detailed in Figure 8-11.

Examination of this data shows that gold over electrolytic plating will yield the

minimum package effect on microelectronic device ambients.

CONCLUSIONS

A very brief overview on current efforts at RADC has been presented. /While

much of the material is self-explanatory, questions or comments concerning these

areas are encouraged and should be directed to the author.

4
SAMPLE

PRESSURE

—

ION

GAUGE—

TORR-LOG

TABLE I

HUISTUHE MEASUREMENTS 1 EFFECTS

PROBLEM: ELIMINATION OF MOISTURE AIDED ( ABETTED FAILURES

ISSUE

I uUALlIt NEASUKfMf Nl ASSURANCE

• ACCEPTABLE nOI S! uk£ LEVELS

• LA8GKAIURT RECENT IF ICATIUN

KI1Y1I1ES

0 MEM GEJCJUIIQN STANDARDS

0 IMOEPEMDCMT STAHOARO-JPL

X RECERTIFICATION VIA OEUPOINT HYGROMETER

i HEM lECHNIOUES U COHWCIAL LASER SPECTROMETER SYSTEM

0 CAPACITANCE -RAT 10/MASS SPEC CORRELATION

X MEM PROCEDURES IN 1018

f MATERIALS EFFECTS VLSI/VHSIC

A HYBRIDS

• INCREASING SAMPLE VOLUMES

0 IN-HOUSE EVACUATION QF ANALITICAL

TECHNIQUES ANO CONDITIONS

0 IN-HOUSE DEVELOPMENT IMPROVED SOFTMARE

X CHANCES TO 1018 ( PRE-ANALYSIS SAXE. ETC.)

0 CURRENT ACTIVITIES X ‘PLANNED UEDEC COORDINATION REQUIRED)

TABLE II

IR HOUSE STAHOARQ PEVELQPimi

CONDITIONS

N

z

. I "L

N

t

. AIM

n r

AIM

l Hi tfOIT

SEAL HjO. PfMv

/tA) AM)

S/M)

S/M)

ANALT2ED HjO.

RAXED 2110

IS//U <580

59WO

MON- RAXED

2150 6/W

susu

S/SAO 1-2 u mams)

O

a

cn

MRS5

FIGURE 1

SECCNDS

FIGURE 2 5

FIGURE 3
a

<7* U3

aa

° 0.00 15.00 30. Q0 *45.00 60.00 75.00 9Q.0QE 0 SECONOS

ao

UJ\

SECONOS

FIGURE 4 FIGURE 5

CD

a

i

00

UJ

a

a ' 3

'.CO :S.OO 30.00 45. 00 50.00 75.00 90. COE 0 SECONDS

FIGURE 5

cnH—

o>

I >—

I—

cn

as

FIGURE 7 6
FIGURE 8 FIGURE 9

FIGURE 10

7 FIGURE 11
2.2 MOISTURE ANALYSIS AT ONEIDA RESEARCH SERVICES, INC.

Thomas J. Rossiter and Diane Feliciano-Welpe

Oneida Research Services, Inc.

3 Ellinwood Court

New Hartford, NY 13413

INTRODUCTION

For the past five years, Oneida Research Services, Inc., has been

offering as a standard service Residual Gas Analysis via the Mass

Spec trometr ic method outlined in Procedure 1 of MIL-Std 883, Method

1018. During that time, ORS has had a continuing program to

develop methods that accurately and reproducibly measure gases

sealed in electronic devices. Primary focus has been placed on

developing a method that measures moisture and other gases without

achieving steady state conditions in the mass spectrometer.

There are several reasons for developing a Dynamic Method. The

primary reason is that moisture is a bipolar molecule and easily

adsorbs on surfaces, such as a mass spectrometer's chamber walls.

The rates of adsorption are a function of many variables such as

temperature, surface conditions of the mass spectrometer, pressure,

and other gases present. Surface conditions, themselves, are

very difficult to control because it depends on the content of

prior samples, time between samples, pumping efficiency and

vacuum integrity. Rather than trying to develop elaborate

techniques to control the large number of variables, a method/

was developed that was less sensitive to the adsorption properties.

The objective of a Dynamic Method is to measure adsorbent gases

very quickly, before they have time to significantly adsorb on

the chamber surfaces. The rates of adsorption still have to be

controlled, but now to a much lesser degree. Because we are an

independent testing laboratory, we are faced with the task of

performing RGA (Residual Gas Analysis) on a wide variety of

device types ranging in size, construction, and gas content.

Therefore, it is essential that the proposed test method be

reproducible and easy to control. The Dynamic Method offers this

ability plus other interesting advantages.

THE DYNAMIC METHOD

The fundamentals behind the Dynamic Method of measurement require

an understanding of the dynamic behavior of residual gases. We

must start by looking at the physical design of the sample chamber

and inlet path to the mass spectrometer. A cross-section of a rapid cycle mass spectrometer system is shown in Figure 2. The

sample is held in place on the sample mount by suction from a small vacuum pump. The lid of the sample is sealed against a viton 0-ring. This mounting procedure places most of the surface

of the IC device outside of the realm of the mass spectrometer

8
analyzer, thus minimizing the effect of the exterior of the

package as one of the major adsorption variables. The mount is

sealed against the wall of the inlet block leaving a hermetic

cavity around the sample in which a vacuum may be maintained.

This vacuum around the exterior of the package is needed for

equipment protection in case the package should fracture, and is

unrelated to the vacuum in the inlet block itself. The puncture

pin mechanism, which is manually operated, guides the puncture

pin through the center of the small O-ring to pierce the lid of

the sample. The cavity gas is released into a transfer passage

or inlet and travels through a fixed orifice. This fixed orifice

provides for a slower and more controlled flow into the mass

analyzer. The pumping mechanism is maintained by a direct drive

turbomolecular pump which achieves background levels of lxlO“Storr.

In order to derive the dynamic behavior of gases in a mass

spectrometer, we can use a simple diagram shown below. The

system takes on three major components: the sample, the inlet

chamber, and the analyzer chamber. Prior to puncturing the

sample, the inlet and the analyzer are at lxl0“8torr while the

sample is generally at 1 atm. (less than 1 atm. for cerdips).

The conductance path (G^) between the inlet and the analyzer

chamber is set by the fixed orifice described earlier. When the

sample is punctured, a conductance path is created (cr^) between

the sample and the inlet chamber.

Sample Inlet Analyzer

Reg 1 Reg 2

1 .)

2 .)

3.)

Ideal Gas Law

NRT

P = V

Fluid Movement — = <n(Pi 5 p 2>

dt

- <7*2 ( P 2

)

Conservation of Matter

N = N + N 2 + n

3

P = pressure (torr)

N = number of molecules

T = temperature (°K)

V = volume (cm3)

R = Universal Gas Constant

cr = conductance

t = time (sec)

By using three fundamental laws of physical behavior, an equation

representing the flow of nonadsorbent molecules through the : i i orifice (o^) into the analyzer may be derived (Eq. 1). This

equation represents the value which is actually being measur'd ; the analyzer during pumpout.

9
= N [k 2 ( 1 - e”k l t )e"^2 t + k X (l - e'k

2 t )e“k l"t| (Eq. 1)

dt -1

where: kj - li *2 “ ^

cr2 R kj-100,000 x k 2

Since: V X «V2

<Ti» cr2

Because the sample puncture hole size is much larger than the

size of the fixed orifice and because the volume of the package

is smaller than the volume of the inlet chamber, k^ is several

orders of magnitude greater than kg. Application of this equation

shows that shortly after a sample is punctured, the term e“k l t

will essentially be equal to 0, leaving Eq. 2:

52l = Nk2 e"k 2

t (Eq. 2)

dt

Equation 2 is the mathematical representation of an exponential

pumpout rate. A linear plot of Equation 1, as shown in Figure 3,

illustrates the predicted behavior of residual gas (as measured

by the quadrupole analyzer) when the sample lid is punctured.

The gas rises instantaneously (50-100 microsec.) to a sharp and

stable maximum and begins to decline at an exponential rate.

Actual pumpout curves from the RGA systems have reproducibly

followed this theoretical exponential plot which when displayed

on a semilog scale will yield a straight line. A pumpout curve

for nitrogen (mass 29) is shown in Figure 4 on a semilog scale.

As seen theoretically, when burst into the system, the nitrogen

rises instantaneously to a sharp and stable maximum and begins

to decrease at a characteristic pumpout rate which is proportional

to the square root of the molecular weight. The larger the

molecular weight of a gas, the slower the pumpout rate.

These curves have shown such ‘a great degree of reproducibility

that, in effect, they may be used as quality control curves for

the given RGA system. In the case of the nitrogen pumpout curve

in Figure 4, the sharpness of the peak at the maximum indicates

that the ion source is properly tuned. The fact that the curve

can be fitted to a straight line (correlation coefficient = 0.9999)

demonstrates the quality of the fixed orifice, the linearity of

the analyzer over a dynamic range, and the signal to noise of the

detector

.

Given this dynamic behavior of ideal gases (nonadsorbent gases),

what is the best way to acquire the data and quantitate the

results? Contemporary methods acquire data over the full curve

or until 95% of the gas is pumped out of the inlet chamber. Data

is then summed or integrated for quantitation. Summation or

10
integration techniques usually result in higher peak-to-backgr ound

ratios, but this is not true when integrating under conditions of

a rapidly decreasing exponential data curve. If we evaluate the

peak-to-background levels of various sections of the dynamic curve,

we find that the peak-to-background is highest at the maximum of

the curve. Thus, by making several measurements at the peak of

the curve (before 10% of the gas is allowed to pump away) a 2 to

1 improvement in peak-to-background can be achieved as compared

to the integration method which sums data (and background noise)

over the entire curve. Clearly, the advantages to this form of

analysis is a more accurate method for measuring peak intensities.

Up to this point, we have only discussed ideal or nonadsorbent

gases, and find an improvement in signal processing by using a dynamic measurement. When we consider the behavior of adsorbent

gases, moisture in particular, we find that the Dynamic Method

provides even more advantages than the 2 to 1 peak-to-background

improvement described earlier. Figure 5 displays various moisture

pumpout curves on a semilog scale.

Matrix effects such as adsorption, desorption and outgassing of

moisture must be taken into account since the pumpout of moisture

changes radically with chamber surface conditions, the presence

of organic materials in the IC cavity, and the presence or

absence of gases such as hydrogen in the ambient. If one were to

measure moisture peak amplitude along any one of the curves

presented in Figure 5 to arrive at an integrated quantitative

analysis, several problems would result. In some cases, we would

be faced with integrating the moisture signal (and background)

to infinity since moisture does not always reach background

levels within the given measurement period. Since moisture background levels are non-trivial, this would result in a very poor

statistical averaging method even if it could be assumed that

the moisture background remained constant, which it does not.

The Dynamic Method overcomes a great number of the problems

encountered with integrated measurement by taking the required

measurements at the peak of the pumpout curve where there is

minimum adsorption effects, peak-to-background is substantially

large and there is confidence in background measurements for

accurate quantitation.

To make all these measurements within a short period of time

(•—'3 seconds) requires computer technology with high-speed

interfacing. A PDP11-23 system with 256K memory is used to

acquire data. An elaborate data acquisition routine and quantitation program has been written to maximize accuracy and

reproducibility.

PERFORMING THE ANALYSIS

ORS operates two mass spectrometer systems dedicated specifically

to RGA analysis. The sample is loaded into the mass spectron* t- r and allowed to reach thermal equilibrium at 100°C for a mini::'!-
of ten minutes. During this

the inlet and sample chamber

data acquisition program is

spectra, checks for minimum

into an Auto-Start routine,

peak (or any other selected

program when it senses a ris

Immediately following the re

puncture hole in the sample

an. initial set of repetitive

several full scale scans (0-

gases. One of the advantage

routine is that it controls

is a critical parameter.

time, the vacuum system pumps down

to lxl0”°torr. An automated computer

activated which collects a background

background requirements, then goes

This routine monitors the nitrogen

peak) and triggers the data acquisition

e in signal from the package puncture,

lease of residual gas through a lid, the Dynamic Method incorporates

moisture measurements followed by

100 AMU) to measure all other residual

s of the automated data acquisition

the time frame of the analysis which

Quantitation is calculated in % concentration as shown in the

following equation:

(£eak intens itjH sensi t ivity.. factor)

x 10Q% = % concentration

total gas

total (peak intensity^ (sensitivity factor)^

CALIBRATION

The calibration and determination of sensitivity factors are

equally as important as the actual test method. The key to

accuracy, in this case, is to perform the actual calibration under

the same conditions as the test method. To accomplish this,

calibration of each RGA system uses a Multi-Volume Calibrator

which is shown in Figure 6. This unit is mounted in close

proximity to the sample and simulates a sample burst into the

inlet and mass analyzer using various calibration gases. Three

volume sizes (.Qlcc, .lee, and .8cc) are available within the

unit with two additional extension volumes (1.8cc and 4.0cc) that

may be fitted for larger calibration bursts. By purging a given

calibration gas through the Multi-Volume Calibrator, an operator

may simulate sample bursts for several package sizes. A schematic

illustrating the Multi-Volume Calibrator with the calibration

hardware is shown in Figure 7. In the case of moisture, a General Purpose Humidifier is used to generate a known moisture

level which is confirmed by an NBS Traceable Dew Point Hygrometer.

For the calibration of other gases, room air and special gas mixtures are used.

SUMMARY

An advantage of the Mass Spec method of analysis is that all other

gases present in the ambient are measured as well as the moisture.

Clearly, this is more than just a pass-fail moisture measurement

test and can offer valuable information about the history of the

device. Data on other ambient gases can lead to the identification

12
of fine and gross leakers, leak test escapes, poor process control,

internal chemical reactions, and effects of organic materials and

dessicants

.

The Dynamic Method, also referred to as the instantaneous method,

provides improved sensitivity and reproducibility in measuring

moisture while minimizing adsorption effects as a measurement

variable. The method more closely measures the ambient gas in

the package because it precludes measuring outgassing products

from the package after the device is punctured. When the device

is punctured, the interior of the package is suddenly under

vacuum. This in itself is a stress condition which can result in

outgassing and mechanical stress on the device. It is our opinion

that an analytical method that measures gas content should not

also be a stress test on the device. Stress tests should be performed prior to analysis so as to generate realistic, worst case

conditions in the product. The mass spectrometer should then be

able to measure those conditions as they exist without introducing

additional stress. By collecting data within a very short period

of time, the Dynamic Method essentially accomplishes this.

The Dynamic Method has been utilized by ORS since 1978. In that

time, a number of the supporting factors in this method have been

revised (for example, upgrading the software and the computer

system) but the basic fundamentals presented in this paper have

remained unchanged. Throughout the years, this instantaneous

method of measurement has offered a data base with both accuracy

and reproducibility.

13
MINIMUM REQUIREMENTS OF MIL-STD 883, METHOD 1018, PROCEDURE 1

Accuracy il0% at 5000 ppm

Volume Range .Qlcc - lOcc

Temperatures Sample - 100°C for ten minutes

Inlet - 125°C

Puncturing Puncture a hole, do not break seal

Sample Ceramic Lids may be thinned

Prebake at 100°C for 12-24 hours if

unit contains dessicants or organics

Measure Chamber Pressure,

Moisture Content,

All Other Gases

.

TABLE 1
Fig. 1: Two adjoining mass spectrometer systems

VACUUM PUMP

Fig. 2: Cross-section of sample block and analyzer

15
Fig. 3: Signal Intensity vs. Time from Puncture

Fig. 4: Nitrogen(29) Pumpout Curve

16
Log

Mass

1

8

Peak

Fig. 5: Moisture Pumpout Curves

17
Fig. 6: Multi-Volume Calibrator

Fig. 7: Schematic of calibration system

18
2.3 GAS ANALYSIS AT GOLLOB ANALYTICAL SERVICE

Fred Gol 1 ob

Gollob Analytical Service

47 Industrial Road

Berkeley Heights, NJ 07922

(201) 464-3331

ABSTRACT

This discussion is concerned with

of hermetic packages, as per Method

introductory statement about the n background o-f its development, th

the performance of the procedure wi

include a schematic of the mass sp

sample system, including vacuum syst

Modifications to the mass spectrom

Method 1018.2 will then be disc

puncturing chambers, with provision

will be described. Two accessor

volume calibrator and the controlled

the equipment needed for the analysi

the dfl 3 1 ysi s of the g as cont en t

1018 nxi , Mi 1 St d 883B

.

After an

eed f or t he method and the

e eq ui p ment used at G . A. S. f or

1

1

be descr i

b

ed

.

This w i 1

1

ec tr ome t er show ing conventional

em and dat a acq u i s i t i on syst em

.

et er f or the per f or mance of

ussed

.

The des i gn of un i ver sal

f or cal i brat i on and punctur ing

i es to th is s yst em

,

the th r ee

humi d ity gen er at or comp 1 et e

After the description of equipment, the detailed analytical

procedure will be presented. Initially, conventional analytical

procedures will be described. This includes background and regular

scan analyses. Procedures specific for 1018.2 will then be

described, covering wet and dry gas "burst" calibrations, specific

measurement of the water peak (using the instantaneous reading

technique), and the sample volume measurement.

The final portion of the discussion will be concerned with the data

handling and calculation. It will include data storage and

interpolation of the background water correction.

INTRODUCTION

This discussion is concerned with the analysis of the gas content

of hermetic packages as per Method 1018.2 of MIL STD 883B. The

impetus for the development of this procedure came over ten years

ago from studies performed by Ben Moore and Bob Thomas at RADC.

Failure analysis showed a correlation between corrosive failure and

excessive moisture content. Initial analyses were performed at

RADC using a mass spectrometer with a "cookie cutter" sampling

inlet. This unique inlet was effective in determining water

content at a time when it was generally considered to be not

feasible to perform that deter mi nat i on by mass spec trometer . The

reason for difficulties with the det er mi nat i on was the strong

adsorption of water on surfaces within the analytical instrument,

and the reason for success at RADC was their unique sampling

procedure, - which provided dynamic sampling and analysis and

overcame this problem.

The basic problem is that relatively large amounts of moisture

adsorb on the surfaces of the gas sample handling equipment, and

establish a dynamic equilibrium with the gas phase. Inasmuch c<s

19
100 ppm of water in .01 cc of gas at .5 atmospheres pressure (the

long term goal for the lower limit for subject specification) is

only about 0.0004 micrograms, careful conditioning of the

analytically active surfaces must be performed to minimise

interference from this equilibrium.

If the walls of the system contain too much moisture, appreciable

amounts of moisture will desorb and produce a falsely high value

for the water content of the sample. In addition, slight

variations in temperature and technique can cause large variations

in this, false contribution. On the other hand, if the conditioning

cycle of heat and evacuation is too effective in removing moisture

from the walls, the walls can act as a getter, adsorb moisture from

the sample, and produce a falsely low result. Therefore, the

conditioning must be controlled so that desorption from the walls

contributes an amount of moisture that is small compared to the

amount that is actually present in the sample. As can be seen by a consi der at i on of the numbers, this desorption factor becomes much

less important as the sample size increases, and as the goal value

for moisture increases. Thus, a desorption of 0.01 micrograms of

moisture from the walls would destroy the accuracy of a 0.01 cc

sample at 100 ppm, would still be a major factor vis-a-vis 0.01 cc

at 5000 ppm , and would be insignificant when considering a 1.0 cc

sample at 5000 ppm.

For the fixed gases (ni trogen , oxygen , hydrogen , etc .), the sorption

problem is so small that it does not have a significant effect on

their determinati on . These inorganic gases are much less polar

than water. This means that 1) smaller amounts of these gases

would normally adsorb on surfaces, and 2) water will displace them.

Thus, omni-present water effectively covers all the surfaces and

helps eliminate a similar analytical problem for these other gases.

Although we cannot eliminate this adsorption problem, we can

minimize it and learn to control it. Materials of construction

should be smooth and not be moisture absorbant, and should be free

from moisture absorbing contaminants. Our systems are constructed

of stainless steel, heli-arc welded, with minimum amounts of

Teflon., where needed. As far as techniques are concerned, all

surfaces that contact the sample gas are thermostat i cal 1

y

maintained at 100 C.,or 125 C. , to minimize the amount of adsorbed

moisture and to minimize equilibrium excursions that are caused by

temperature fluctuations. In addition, we minimize the exposure of

these surfaces to atmospheric moisture, by keeping the system

sealed whenever feasible, and by using a dry nitrogen backflush

when the system is open.

CALIBRATION

Before I describe the specific equipment we use, and the

calculation procedures, I should like to discuss calibration. In

any analytical procedure, the only legitimate verification of the

accuracy of the results is to analyze a known standard using the

same conditions of analysis for the standard and the unknown. In

many situations, where all the variables that may affect the

analysis are known and understood, it is acceptable to calibrate

the equipment on an absolute basis. However, where it is known

20
that adsor p t i on-desor p t i on phenomena are significant, or where

system memory effects are large, standards must be analyzed. In

the present case, the ideal situation would be to analyze packages

(of the same type as the real samples) but which contain known

amounts of moisture in them. Many attempts have been made to

produce these standards, but these efforts have met with only

limited success.

In the absence of a large supply of package standards, we developed

a "burst" calibration procedure in which we tried to simulate the

puncturing of an ic package, and the subsequent release of its

gaseous contents. A "burst" valve was attached to the far end of

the sample puncturing chamber and connected to an external system

which supplied a flowing stream of nitrogen containing a controlled

amount of moisture. The moisture level of this flowing stream,

which could be accurately measured using a refrigerated mirror

dewpointer, could be continuously adjusted by varying the amounts

of a dry gas stream and a water saturated gas stream that were

blended. The "burst" valve could then be manually operated to

introduce a small amount of this gas stream. A skilled mass

spectrometr i st could, with a very high success ratio, introduce, in

a fraction of a second, an amount of gas, of known moisture

content, that was very similar to the amount of gas in the real

sample. Although this procedure required a lot of operator skill,

it did result in increased precision.

Subsequently, the well-known Three Volume Calibration Valve (TVCV)

and the NBS General Purpose Humidifier (GPH) were developed

elsewhere and have become available through Pernicka Corporation.

The TVCV and the GPH permit burst calibration to be performed with

excellent r eproduci bi 1 i ty and have resulted in the achievement of

acceptable correlation among the various analytical laboratories

and with the standards that have been circulated.

MASS SPECTROMETER EQUIPMENT

At Gollob Analytical Service, we perform this test procedure using

a modified DuPont 21-614 cycloidal focussing mass spect r omet er

.

This instrument is equipped with a relatively conventional static

gas sample system that we built ourselves. The system permits us

to introduce known amounts of the various stable gases into the

analyzer and to establish sensitivities for these gases, in terms

of divisions of peak height on the recorder per micron of each gas

that is present in the sample system. As mentioned earlier, the

adsorption problems associated with water render this calibration

procedure inaccurate for water, and the separate "burst" procedure

is used for that gas. Our conventional gas sample system is

operated at room temperature. However, as referred to earlier, it

is advisable to maintain those systems used for water det er mi nat i on

at a constant and elevated temperature. We therefore constructed a separate system for the Method 1018 work and connected it to the

conventional sample system as close as feasible to the analyzer.

Provision is made, using valves, to connect either the regular or

the 1018 system to the source. All surfaces exposed to the sample

gas in the 1018 system, from the breaking chamber through to the

gold leak entrance to the source are temperature controlled; the

breaker is maintained at 100 C and the sample transfer passages are

21
maintained at 125 C. The TVCV and the GF'H are connected to a port

at the -far end of the breaker, so that the "burst" calibration gas

must travel the full length of the gas transport system. This

ensures that the calibration gas will see all surfaces, and

experience all aberrations that might affect the sample. This is

part of the approach that the best calibration really is a package

of the same size and structure as the real sample, but with a known

gas content. In the absence of the standard, the analyst must use

his ingenuity to insure as complete simulation between the

calibration procedure and the sample analysis procedure as

i magi nabl e. UNIVERSAL BREAKING CHAMBERS

Over the years, we have designed and constructed a number of

breaking chambers, all of which are capable of providing analysis

as per Method 1018.2. These chambers can accomodate a wide variety

of samples, from tiny crystal carriers, through a variety of TO

cans, cerdips, flat-paks, hybrids, to other structures, such as

pacemakers, infra-red scanners, gyroscopes, implantable medical

devices, reed switches, resistors, capacitors, spark gaps, surge

arrestors, and the list goes on. Basically, the universal approach

used in building the chambers has permitted us to analyze the gas

content of devices , glass or metal, with internal gas content from

0.00001 to 1000 cc. What is universal about these chambers is that

they all have the same three connector fittings. One fitting is of

the size to connect to the mass spectrometer sample system. The

second fitting, at the other end of the chamber , accept s the TVCV.

The third fitting, which must be precisely located over the

^ sample

at the spot to be punctured, mates with the puncturing actuator.

For our universal actuator, we modify an all stainless steel

bellows valve and have a special fitting machined on the bonnet.

The mate of this fitting is then welded onto the breaker. There

are a variety of i nterchangab 1 e breaking, puncturing and smashing

tips than can be mounted in this actuator. After the appropriate

chamber and accessories are assembled and connected to the mass

spec trometer , the breaker is enclosed in a temperature controlled

environment. We thus wind up with a system in which we can

puncture a sample, collect all the gaseous contents (at 100 C>

without loss or contamination, transfer the gas (at 125 C) to the

analyzer, and "burst" calibrate. A puncture can usually be made

within 1/16 of an inch of the desired puncture point.

ANALYTICAL PROCEDURE AND CALCULATIONS

The analytical procedure used to perform this analysis is based

upon standard analytical procedures, as follows. On the day prior

to analysis, the samples are loaded into a suitable chamber. After

the chamber is attached to the mass spectromter, initial leak test

analyses are performed to insure that the system is free of leaks

and that no unusual gases are being evolved from the samples. The

next morning, usually sixteen hours later, final leak tests are

performed. The mass spectrometer is tuned to mass 18, and that

peak is monitored with the pump valve closed to be sure that rate

of increase of that peak is suitable. At this point, bursts of dry

gas are introduced, in order to establish how much moisture will be

desorbed into the system when the walls are hit with a burst of gas

22
similar in amount to the sample. Bursts can be introduced with

volumes of .01,. 10, 1.0 cc and larger, and are chosen to bracket the

sample size. Sample analysis then proceeds in a similar fashion.

The mass spectrometer is tuned to mass 18 and the sample is

punctured. There is an almost instantaneous rise in the water

peak. After the initial rise, the water value may then continue on

its previous slope, or may establish a new one. The value used for

the calculation of the water content is the instantaneous change

which occurred at the moment of puncture. This value correlates

with the true water value because it is measured before any of the

sorption effects (other than the burst effects) have had the

opportunity to influence it. After the water value has been read,

the mass spectrometer is scanned in the conventional manner and all

of the other gases present are measured. Suitable blank analyses

are performed per i odi cal 1 y . After all samples have been analyzed,

burst dry gas analyses, and burst wet gas analyses (the bursts

contain about 5000 ppm water as produced by the GF'H) are performed.

Finally, the mass spectrometer is calibrated in the normal fashion

for nitrogen, hydrogen, oxygen and other gases of interest. All of

the data is fed into a computer for the calculation. The computer

program corrects the water value of the sample for the dry burst

gas contribution and interpolates for differences in sample volume.

Then, using a sensitivity for moisture calculated from the wet

burst gas analyses, the partial pressure of water in the breaking

chamber is calculated. The partial pressure of all other gases is

calculated from their individual sensitivities, and the over-al

1

composition of the gas released from the sample is calculated.

Inasmuch as the sample gas is contained in a fixed calibrated

volume and is slowly bled into the analyzer through a fixed leak,

the analysis yields the actual partial pressure, and, therefore,

the total amount of each gaseous constituent. Thus the actual

total amount of gas that was present in the sample can be

determined accurately. This added bit of information is often of

great value in studying unusual samples.

In summar

r el eased

by measur

when the

produce

1 abor ator

y, at GAS, we use a mass spectrometer to analyze th

from various electronic packages. The water is det

ing the instantaneous change in the water peak that

sample is punctured. This procedure has been s good correlation with the other mass spect

ies and with package standards from RADC.

e gases

er mi ned

occurs

hown to

r ometer

23
2.4 CONTROL CHART ANALYSIS OF MOISTURE IN CERDIPS

by

Robert B. Elo, Ph.D. and Arnold M. Massoletti

American Microsystems, Inc.

3800 Homestead Road

Santa Clara, California 95051

INTRODUCTION

Beginning 3rd quarter of 1980, a development effort was undertaken to produce a state

of the art, very low moisture cavity, cerdip package. The 1st phase task was evaluation of the then current AMI package moisture level. The 2nd phase task was development of alternative processes which would produce lower cavity moisture. The 3rd

phase was selection and implementation of the optimum low moisture process. Cavity

moisture was continuously monitored during the implementation of this process as it

phased into manufacturing production.

Control chart methodology was selected as the format for the process monitor. The

data gathered by and the interpretation of these control charts is the subject of

this paper.

The piece parts of cerdip construction used at AMI are common to the industry (see

Table 1). The manufacturing assembly process flow is shown in Table 2.

For the purpose of evaluation and subsequent monitoring of cavity moisture, the following sampling methodology was used. A weekly sample of ten adjacent units were

taken from the belt after exiting from the furnace. A secondary monitor of ten units

with minimized processing was assembled using electrical reject die (dummy).

The dummy packages were put into seal on the belt beside normal product. Both product

and dummy samples were then processed through post seal assembly flow with the exception of the tin plate operation.

Low Moisture Evaluation and Development

Results of the development phase of 1981 are presented in Figures 1 and 2. The data

in control chart format is shown with typical MEAN and=RANGE values representing

progress made as a result of various evaluations. The X and control limits for the

various time periods was obtained from actual data.

It is seen that X and control limits are decreasing for each successive time period.

The Jul/Sep plot represents a projection from a limited sample, completed near the

end of the 2nd quarter. Actual start-up in the 3rd Quarter proved to be better than

this projection, as can be seen in later figures.

Assembly Process Monitor

In addition to the unit sampling for mass spectrometer moisture measurement, manufacturing performs read and record monitors on a daily or shift basis (see Table 3).

24
Process Capability

We found in the evaluation studies that experiments would need to be repeated over

time to arrive at a statistically valid conclusion that a change had been effected.

It was decided that control chart methodology was the best tool to provide long-term

assessment of changes, and to provide a process control monitor. One half of the

weekly product and dummy sample was sent to a DESC certified lab for cavity moisture

measurement. The product and dummy measurement sample size was three units each with

the remaining two units acting as spares. The other five units were archived.

At the initiation of ramped volume production in the 3rd Quarter 1981 the original

process capability study was made (see Table 4). With the passage of time the process appeared to move into tighter control than that of the original study. A

second study was performed in the 3rd and 4th Quarters of 1982. In the second study

X, the average of the MEANS, remains constant. However, the average RANGE has

decreased to less than one-half its original value.

The process stability in this period of time is shown in Figures 3 and 4. These are

the MEAN and the RANGE control charts of product for 1982. The RANGE chart shows

only two points above the upper control limit (weeks 24 and 50). One of these, week

24, is coincident with a MEAN point above its upper control limit.

The MEANS chart shows four points above the upper control limit and five points below

the lower control limit. The 1982 process period generally appears to be in control

with signs of instability.

Whenever data that plotted above the upper control limits occurred, process information and other monitors were reviewed by engineering for assignable causes. No as- signable causes were found in the four instances where the upper control limit was exceeded.

A histrogram was generated from individual product measurements. This histogram includes all data from the initiation of ramped product volume through the first

quarter of 1983.

The plot, shown in Figure 5, is skewed. It has a tail of high values, usually asso- ciated with sports. The low value side is truncated because of a measurement limit

at one-hundred PPMV. Overlayed on the bar graph is a plot of an ideal normal Gaussian

distribution, created from the MEAN and Sigma of the actual distribution. It can be seen that, the tail of the distribution has shifted the Gaussian MEAN to a value

higher than the Median.

The population statistics of product sampled from 1981 through third quarter 1983 are summarized in Table 5. The number of observations in this distribution is 372. The

MEAN is 623 PPMV. This should be compared with tjie Median of 555 PPMV and the 1982

Control Chart X of 555 PPMV. The Control Chart X is derived from process capability

studies. Sport measurements were excluded from these studies. The 1982 upper control

limit of 712 PPMV (555 + 157 PPMV) is defined as three Sigma of the sample X above X. For large populations the following relationship is valid.

25
Assuming: 3oj = 157 PPMV,

then, a

x

= 91 PPMV

The actual spread (a

x

) with sports included is 300 PPMV. The question then arises,

on what basis can we determine that a measurement is a sport. The DESC lab estimates,

based on calibration data and internal controls, that they should be able to reproduce + 100 PPMV at an average moisture level of 500 PPMV. If we assume this 100 PPMV

reproducibil ity value is between the la and 2a limit of the measurement then the

conrol chart may be reflecting only measurement scatter. The sports could be then,

either measurement error or actual moisture level change in the package. There does

not appear to be any way, at the present, to discriminate between the two alternatives.

Shift of the Process Average

As the process continued into 1983 a sudden change of the average of the MEANS in the

product samples was observed. It appeared, based on the measurements after this

change that the process average had shifted up from 555 PPMV to 815 PPMV.

A view of this change is given by a 26 week period from 4th Quarter 1982 to 2nd

Quarter 1983. It is shown with an arbitrary weekly numbering system (see Figure 6).

The product RANGE chart (Figure 7) shows two values above the upper control limit

from week 18 through 26. From this there is no evidence to reason that most of the

above control limit points on the MEANS chart are invalid.

The dummy samples monitor shows a similar pattern to the product sample, in that^the

pattern seems to have shifted upward after week 17 (see Figure 8).

It was observed that the RANGE values here (Figure 9) typically stayed below the

upper control limit, with only one of 8 points above the RANGE upper control limit

after week 17. Again, there are no indications that the data was not valid.

After a pattern was established we began to resample product and dummy date codes.

We first resampled date codes after week 17. The MEANS data showed general agreement

with the original data. We then resampled date codes before week 17. The ranges of

all resampled data showed fewer points above the upper control limit than original

data (see Figure 11).

The MEANS of resampled product and dummy after week 17 confirmed a shift to a higher

value. However, the resample of the MEANS before week 17 showed a gradual trend

upwards in marked contrast to the original data (see Figures 10, 12).

Now the Dilemma

The initial product monitor and its back-up the dummy monitor, indicated a sudden

change in moisture level had occurred. The resample of this out of control period

seemed to confirm this out of control regime. However, when we resampled the "IN"

control period just prior to week 18, it too was shifted up and out of control.

26
The question was then raised what changed, the process or the measurement, since all

measurements made after week 17 seemed to have moved up by 200 to 300 PPMV.

During this period of resampling, a thorough engineering review was undertaken both

in Santa Clara and in the Korean manufacturing facility. Attempts were made to cor- relate the apparent change in moisture to a specific change or to a drift in monitors,

production practices, and a variety of other probable and improbable factors. The

combination of resample results and our own failure to turn up any clue as to the cause of a change, led us to consult with the DESC lab performing the measurements.

The data and our general lack of success was discussed with the lab. They then re- viewed their own methodology to see if anything had changed.

An elaborate software program is used by the DESC lab to acquire data and to perform

quantitative calculations of the data. On April 16, 1983, a typographical error was inadvertently introduced into the quantitative portion of the program. The error re- sulted in occasional incorrect background subtraction. The error was in a section of

the program that is only accessed when quantitating for small packages that have about

500 PPMV moisture. Because of the limited use of this portion of the program, the error may have gone undetected for an indefinite length of time had not a review been

requested.

Background effects are usually insignificant because the signal from testing the

sample is orders of magnitude higher than the background signal. But for small

packages, the background is significant and background subtraction errors can produce

substantial errors in the quantitation for moisture.

Calibration for moisture is done at 500 PPMV by the lab, but not routinely. The

major interest in the industry is between 1000 PPMV and 6000 PPMV. Calibration outside this range is only performed occasionally. A method to obtain tighter tolerances at 500 PPMV would be more frequent calibration. Another method would be the

calibration procedure using the RADC standards now in development.

After requantitation of the data, the corrected values were plotted with the original

and resampled measurements. This correction of data affected some but not all of the

control chart points after the apparent sudden change (shown in Figure 13).

There was no correction for the very high peak of week 18. There were two small cor- rections for weeks 19 and 21. Starting in week 22 there were more substantive changes.

There were no corrections to the resample data prior to the week 18 "sudden change".

However, week 19,20,21 have very substantial decreased values, bringing that portion

of the chart below the upper control limit.

The original dummy data (Figure 14) shows no change in the high peak for week 18.

There are substantial corrections for week 21, 22, and 24. The resampled data has

no correction for week 18, but for weeks 19 through 21 there are substantial shifts

downward.

27
SUMMARY

A single consistent phenomena throughout this analysis is that week 18 is consistently

a high s above control limits measurement. It is less clear whether the period from

week 19 through 25 is in or out of control. The general appearance of the process is

that there is evidence of instability. This indicates the process is probably subject

to a complex combination of causes. One possible cause is the substantial correction

factor used to arrive at measurement values. For example, the average difference

between data before and after correction is 383 PPMV out of a 555 PPVM process average

These control charts show a process in control for a period of 85 weeks, followed by

a period of increased instability. A sudden change in X during this period of instability was the result of at least one assignable cause. This was a data quantitation

problem caused by a typographical error.

Control Chart Methodology has been useful to monitor and assess a cerdip seal process.

Measurements, less than 1000 PPMV, for small cavity packages can be subject to the instabilities from complex causes. These instabilities could be reduced by: increased

frequency of calibration and further refinements in measurement technique. This will

provide the necessary confidence for further process improvements, or proper responses

to apparent out of control points.

References

1. "Statistical Quality Control Handbook", by Western Electric Co., Inc., second

edition 1958, fourth printing 1970.

Acknowledgements

1. The authors wish to "thank" Mr. Tom Rossiter and Oneida Research for their cooperation and contribution to this paper.

28
TABLE 1 : Cerdip Package Part Materials

1 .

2 .

3.

4.

5.

6 .

Pressed alumina ceramic base and cap.

Commercially available low temperature non-devitri Tying

glass.

Gold/glass frit die-attach pad

Alloy 42 lead frame with spot aluminum bonding fingers.

Cavity desiccant

Cavity volume -.06 cubic centimeters

Table 2: Assembly Process Flow

1. Infrared frame attach

2. Die attach

3. Lead Bond

4. Desiccant

5. Seal

6. Tin plate

7. Mark

8. Temperature cycle

9. Lead trim and tube load

29
Table 3: Manufacturing Monitors

1 . Air dryer dew point

2. Seal furnace dew point

3. Seal furnace gas flow rate

4. Work area relative humidity

5. Work area temperature

TABLE 4

Process Capability Studies

ORIGINAL STUDY SECOND STUDY

PRODUCT DATA PRODUCT DATA

10 WEEKS 20 WEEKS

1981 1982

PPMV PPMV

R = SUM R/N 400 154

UCLR = D4R 1020 396

LCLR = D3R 0 0

X = SUM X/N 550 555

UCLX = X+A2R 970 712

LCLX = X-A2R 130 398

30
TABLE 5

CERDIP MOISTURE MONITOR

BASIC STATISTICS

NUMBER OF SAMPLES 372

STANDARD ERROR OF THE MEAN 15.6

MEAN 623

COEFFICIENT OF VARIATION 48 %

VARIANCE 89966

STANDARD DEVIATION 300

SKEWNESS 1.6

KURTOSIS 7.4

95% CONF. INTERVAL FOR THE MEAN UPPER 654

LOWER 592

MEDIAN 555

.25 QUANTILE 441

.75 QUANTILE 706

31
FIGURE 1 CERDIP MOISTURE MONITOR

DEVELOPMENTAL PHASE MEANS

m~m may-jun

isei tesi im

PPMV

FIGURE 2 CERDIP MOISTURE MONITOR

DEVELOPMENTAL PHASE RANGE

JAM-APR MAY-JUN JUL-SEP

1081 1981 1981

PPMV

32
FIGURE 3 CERDIP MOISTURE MONITOR

MEAN

\m

MONITOR

PPMV

FIGURE 4 CERDIP MOISTURE MONITOR

RANGE

legs MONITOR

PPMV

33
FIGURE 5 NO %

CERDIP MOISTURE MONITOR

LIMs

X BAR (PPMV)

o O o O

O

CD

CDO

o o CD

CD

o O CD CD CD CM "tf CO 00

o CM CD GO rH iH tH

FIGURE 6 CERDIP MOISTURE MONITOR

MEAN

PRODUCT

MONITOR

PPMV

34

2000
FIGURE 7 CERDIP MOISTURE MONITOR

RANGE

PRODUCT

RANGE

PPMV

figure 8 CERDIP MOISTURE monitor

DUMMY MEANS

DUMMY MONITOR

PPMV

35
FIGURE 9 CERDIP MOISTURE MONITOR

DUMMY RANGE

aesw

MONITOR

PPHV

FIGURE 10 CERDIP MOISTURE MONITOR

MEAN

PRODUCT RESAJffTiD MONITOR PRODUCT

PPMV

36
FIGURE 11 CERDIP MOISTURE MONITOR

RANGE

PRODUCT KSM

MONITOR PRODUCT

PPMV

FIGURE 12 CERDIP MOISTURE MONITOR

DUMMY MEANS

DUWC ^SAMPLED

MONITOR mm

PPMV

37
FIGURE 13 CERDIP MOISTURE MONITOR

MEAN

product m&fmm eearaniD

monitor pram pram resaisie

PPMV

FIGURE 14 CERDIP MOISTURE MONITOR

DUMMY MEANS

DUMMY RESAMPLED CORRECTED CORRECTED MONITOR DUMMY DUMMY RESAMPLE

PPMV

38
2.5 A MOISTURE STANDARD FOR IC PACKAGE GAS ANALYSIS*

Ray F. Haack

Jet Propulsion Laboratory

California Institute of Technology

4800 Oak Grove Drive, M/S 122-123

Pasadena, CA -9110-9

ABSTRACT

Moisture determination of IC package gas by the mass spectrometric method requires a carefully controlled analysis sequence and a standard for calibration. The availability of a well-defined standard at present is lacking. In order to provide a re - producible and reusable standard package, an investigation was begun to test a prototype model.

The standard consists of a stainless steel body with threads, brass cap, and aluminum disc. A pressure seal is made by applying suitable torque to the assembled

unit. The disc enables one to access (via puncture) the trapped gas. The body of

the unit has a machined cavity which provides the desired calibration volume of

"moist" gas for a respective IC package gas analysis. The standard not only duplicates the sample volume, but also allows the introduction of the standard gas for

calibration in identical manner to that for the IC package gas. Therefore, the dynamic method for moisture analysis of IC packages using the mass spectrometer has

comparable calibration and analysis parameters.

Feasibility of the standard has been determined in two stages: (1) hermeticity

testing and (2) analysis of the trapped gas. Gas analyses of sealed units having a leak rate less than 1x10“® atm*cm®/s He have shown no helium. Since the unit can be

assembled and sealed at ambient conditions in an atmosphere of known moisture content, the trapped sample represents the total of surface moisture and moisture in

the gaseous state at the time of sealing. For the prototype used, the effects of

sealing disc material, surface area, and gas used are being investigated. Ambient

air has been the principal sealing environment since its moisture content can be accurately determined and the package can be readily assembled under such conditions.

INTRODUCTION

Water and other contaminants have been of concern to the electronics industry and to

customers requiring high reliability integrated circuit packages for applications in

space, defense, and medicine. As a consequence of necessary qualification, test

methods and procedures for analysis of moisture have been developed, as outlined in

MIL -STD -883B , Method 1018.2. The mass spectrometric method and its development have

been described by Thomas [1] and also outlined by this author [2,3]. Considerable

progress has been made in standardization among different testing laboratories usin i empty parts from the electronics industry, but availability, consistency, and necessary sealing equipment have been limiting factors. A reusable moisture standard

package promises reproducible volumes, ambient sealing conditions, and avai lab i 1 1 1-

y

for the analyst. This investigation was undertaken to provide such a standar 1.

This paper represents one phase of research performed by the Jet Propu) s i on i.,i: - oratory, California Institute of Technology, sponsored by the National Acron.vi‘

and Space Administration, Contract NAS7-918.

39
EXPERIMENTAL

Fabrication

The fixture designed for containing a fixed volume of moist gas is shown in Figure

1 . Stainless steel #304 was selected as the material for the body containing the

cavity volume and for the cap. Initial tests produced galling of the threads, and

consequently brass was used for the cap material. Cold-rolled aluminum alloy #1100

of 5 mil thickness was used for the pressure seal when torque was applied to the

body, disc, and cap assembly. The type B fixture (courtesy of Pernicka Corp., Mountain View, CA) utilizes a variation of the pressure seal. It differs from type A in

that it contains a stainless steel washer which transmits the pressure from the cap

to the sealing disc.

Part Surface Treatment and Assembly

The surface of the cavity and the seal area were progressively worked with finer

grades of grit paper and finally polished with alumina (1 micron); all steps were performed using the concentric motion of a lathe. The parts of the package were cleaned with sonication in a series of washes (water, 2 -propanol, methanol). A

microscopic examination of the seal area was made in order to check for any transverse scratches. The parts were dried in air at IIO'C for two hours and then equilibrated at ambient conditions for 1 hour prior to assembly in the same area.

Leak Test

Within hours of assembly, the unit was subjected to a 20 -hour helium pressurization

test at 15 psig (2 atmospheres absolute), and then fine leak tested using a model

MS-170 VEECO tester. The test method is described in MIL -STD -883B , Method 1014.4,

and its application to large hybrid packages is detailed by Ruthberg [4].

Package Gas Analysis

A 20 -hour high vacuum bakeout at 100°C was used prior to puncture and analysis at

100°C. The batch inlet system is interfaced with a Finnigan Model 3200 quadropole

mass spectrometer. This system, which is also used for IC package gas analysis for

moisture and other volatile contaminants, has been discussed in detail with respect

to operation, procedure, and results, as mentioned earlier [2,3].

RESULTS AND DISCUSSION

Hermeticity Testing

The first criteria for a candidate moisture standard is a level of hermeticity which

prevents a change in its composition due to external conditions. Since the package

contents must maintain their integrity during a vacuum bakeout at 100*C and also at

ambient conditions, helium pressurization was used so that the mass spectrometer

would detect any inward diffusion of helium. Initially, this would assist in determining the minimum leak rate necessary for this type and size of package. Representative leak rates and helium analyses are shown in Figure 4. A variation of torque

values was used for assembling the package and achieving an optimum seal. Although

150 in-lbf torque produced some good seals, the torque is mechanically excessive for

the thickness (5 mil) of the disc, the threads, and for the metal hex. Several

trials are not shown here because either the leak rate was of the order of 10“'

atm’cm /s or helium in the package gas was in excess of 10% (volume/volume) . A decreased torque of 50 in-lbf was not sufficient to produce a seal significantly hermetic in the case of helium. Finally, for the 75-90 in-lbf torque, several units

40
had measured leak rates better than 1x1 0

“8 atm a cm3 /s He. Gas analysis for these

packages did not indicate any presence of helium (<0.1%).

Gas Analysis

It is possible to reasonably predict the pressure produced upon puncture of the

sealed package (burst pressure) when the cavity dimensions, sealing conditions, and

batch inlet chamber volume are known. The measured and predicted values for burst

pressures of some units are shown in Table I. For packages of ~0 . 1 cm

3 volume, good

agreement was found except for S/N 4-0216 (gas analysis indicated 7% helium!). Unit

S/N 6- was sealed at two different times and gave almost identical burst pressures.

The units of ~0.0 2 cm

3 volume were generally higher in burst pressure than expected.

It has been observed that the sealing disc (5 mil) does not always remain flat while

the unit is assembled. The curvature of the "cap" for the 0.02 cm

3 cavity would

significantly affect the total volume.

A complete set of physical measurements and gas analysis results for several packages (all sealed in air at ambient conditions) are shown in Table II. The following

items should be recognized:

8 8 • For leak rates <1x10“ atm*cm /s He, helium was not detected ( <0 . 1 % ) upon analysis of the package gas.

• The analyzed %H20 represents the total moisture (includes adsorption), whereas

the %H20 (theory) is calculated using the relative humidity measurement of ambient air.

• S/N 26 - and 27 - were sealed on the same date but analyzed by two different

laboratories. Agreement with regard to helium and to moisture levels measured

is encouraging.

The gases evolved upon puncture of the package are analyzed in their entirety by the

mass spectrometer. Since the burst pressures for known cavity volumes are measured

in a calibrated volume at 100°C, it is possible to express the moisture content in

units of total micrograms (Mg). These moisture units are more pertinent than % volume/volume when some variation in burst pressure is observed and when discussing adsorbed surface moisture effects. The measured moisture (Mg) for packages of 0.1 cm

: and 0.02 cm

3 volumes is given in Table III. The corresponding theoretical values

listed do not include adsorbed moisture. An approximation of this factor can be

made using physical constants for the water molecule [5,7]. If the geometric area

for S/N 24-0816 has a surface monolayer, the total yg would be 0.0338. At ambient

conditions (40-50% relative humidity) , 10 monolayers could actually be present, and

furthermore, the geometric area would be considerably less than the true surface

area. Consequently, even a geometric area with 10 monolayers of water for this

package could contain at least 0.338 yg of water.

The preceding discussion concerning adsorbed moisture has applications in this re- spect: the stability and measured value of moisture for a sealed package must be

established. A reusable package (known history and surface condition) should decrease significantly the surface characteristics influence as discussed by Singh et

al. [6] .

CONCLUSIONS

These packages can be sealed and analyzed at a later time. Results thus far i n i cate that the hermeticity and moisture can be maintained until time of analysis

41
after a conventional vacuum bakeout at 100°C for a period of 20 hours. Although the

degree and role of adsorbed moisture has not yet been determined, changes in sur - face/volume ratios and sealant gas should elucidate these factors. Inward diffusion

of moisture and/or argon which could occur upon storage of the packages at ambient

conditions can be adequately determined later when nitrogen is used as the sealant

gas. The detection at low 'ppm level for inward argon diffusion is reasonably accurate .

More precise moisture control, as is possible with a gas humidification system and

an optical dew-point hygrometer, will aid in determining the adsorbed moisture effect. Further study involving either nitrogen or air at the 0.5% volume/volume level and an effort to control the surface finish should provide a basis for determining the stability of the amount of total moisture present.

The second phase of this investigation will consider some of the aforementioned

aspects such as surface nature and lower, more precisely controlled moisture levels.

Presently, it appears that the package is also suitable as a container for testing

outgassing characteristics of small amounts of materials at temperatures from ambient to 100°C.

The author wishes to acknowledge the assistance of W.W. Reilly with the design of

the package and B. Moore of RADC for suggestions and analysis results.

42
REFERENCES

1. Thomas, R.W., "Moisture Myths and Microcircuits", Proceedings 26th Electronic

Components Conference, pp. 272-276, San Francisco, CA, April 1976.

2. Haack, Ray F. and Shumka, Alex, "Microcircuit Package Gas Analysis", ARPA/NBS

Workshop V, Moisture Measurement Technology for Hermetic Semiconductor Devices,

NBS SP 400-69, May 1981, pp. 43-57.

3. Haack, Ray F. and Shumka, Alex, "Moisture and Other Contaminants in Hybrid

Packages", 1978 International Microelectronics Symposium, September 25-27,

1978, Minneapolis, MN, pp. 128-133.

4. Ruthberg, Stanley, "Hermetic Testing of Large Hybrid Packages", 1982 International Microelectronics Symposium, November 15-17, 1982, Reno, NV, International Journal for Hybrid Microelectronics, Vol. 5, No. 2, November 1982, pp. 215-

232.

5. Brunauer, Stephen, The Adsorption of Gases and Vapors, Volume I., Physical Ad - sorption , Princeton University Press, Princeton, NJ, 1945.

6. Singh, Avtar et al, "Effects of Surface Condition on Nucleation and Boiling

Characteristics", Technical Report prepared for Army Research Office, National

Technical Information Service, AD/A -007 275 (Report No. ARO-9731 .2 -E) , September 1974.

7. Vasofsky, R. , Cyanderna, A., Cyanderna, K. , "Mass Changes of Adhesives During

Curing, Exposure to Water Vapor, Evacuation, and Outgassing. Part I: Abelfilms 529, 535, and 550", IEEE Transactions on Components, Hybrids, and Manufacturing Technology, Vol. CHMT-1, No. 4, December 1978.

43
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CO

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Eh

— ““

COU 2 CO D >1 <T> LO LO LO in 'd* 'd' •H CO

ffl P 04 CN 04 04 00 00 00 TJ 0 -d1 N< o 0 0 tr

Q) • • • • 9 9 9 On cd rC © o o O o O O C 4* P •H u

rH <d

<d

0)CO

<d P rH

(d cd rH 0) s 14—1 CO H

Eh 43 tr>

p

G 0 W CO CN CO CO <T> r» O' O- •H <p O >1 r- T— g < cd 9 CO 2 T) CO 0) o CO p

Eh cd

G

CO CO T3 CO G a)

<d

p

cu

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si 0 D g

PI •H O cd LO lO in ID in in in T3 > 04 04 04 CN 04 04 04 no T— T— T— r— 04 04 04 9 >H f—

*—

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tr> 0 G -H •H P

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(0 -H G T)G

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CD

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r—C £

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44
1 1

| CAVITY

|

| VOLUME

|

S/N | ( cm

3

) |

1 1

STORAGE

TIME

( days)

1 1

| BURST |

| PRESSURE |

I (torr) |

1 1

FINE LEAK

RATE

atm*cm 3 /s

He x 1 0 '8

1

| ANALYSIS

(% volume/volume

He 1 H 0 1 Ar

>1 1 2

1 1

%h 2 o

| actual/

( theory) c | theory

1

1 1

1 -0316|0.1 125|

1 1 90

1 1

| 0.410 |

1 | 0.14

1 1

| NDa

|

I 1 1.48 | 0.94 0.93 | 159

1 1

1 -0725 | 0 . 1 125 |

1 1

l 1

2

1 I

| 0.414 |

1 1

1 |

0.15

1 1

1 nd |

! 1

1 1

1.70 j 0.94 1.25 | 136

1 1

1 1 3-0221 | 0 . 1 1 25 |

|

I 3

1 1

1 I

| 0.474 |

1 | 0.14

1 1

1 1

1 nd I

1 1

1 .40 | 1.05 0.86 | 163

1 1 6-0221 | 0 . 1 1 25 |

1 1

1 1

3

1 1

| 0.459 |

1 1

1 |

0.54

1 1

1 nd 1

1 1

1 1

1.14 | 1.10 0.86 | 133

1 1

1 1 23-0816|0. 0232|

1 | 2

1 1

1 1

| 0.134 |

1 | 0.70

1 1

1 1

1 <0 . 02 1 3.05 | 1.04 1.28 | 238

1 1 24-O816|0. 1125|

I 1 2

1 1

| 0 ? 490 |

1 | 0.16

I 1

| <0 . 02 1 1.24 | 0.95 1.28 1 97

1 1 25-0816|0. 0225|

1 1

1 |

2

1 1

| 0.123 |

1 1

1 i

0.70

1 1

| <0 . 02 1

1 1

1 |

3.42 | 1.07 1.28 | 267

1 1

1 1 26-0823|0. 0225|

1 | 17

1 1

1 1

| 0.118 |

1 | 0.30

1 1

1 1

1 <0.1 I

1 1 1.19 | 0.94 1.39 1 87

1 1 27 -0823|0. 0225|

1 1

| |

~30

1 1

| NAd

|

1 1

| |

0.30

1 1

| ND 1

1 1

1 |

1 . 424e

| 0.887 1.39 | 102

1 1

1 1 26-0913|0. 0225|

| | 17

1 1

1 1

| 0.122 |

1 | 0.05

y

1 1

1 <0.1 |

1 1 1.90 | 1 .05 1.33 | 143

1 1 28-0913|0. 0225|

1 1

17

1 1

| 0.093 |

1 1 0.03

1 1

1 <0.1 1

1 1

2.41 | 1.01 1.33 | 181

a ND = None Detected

b The results for oxygen and nitrogen are omitted for the purpose of simplicity,

c Calculated from ambient relative humidity measurement (psychrometer ) ; does not

include surface adsorbed moisture

d NA = Not Available

e Analysis courtesy of PADC, Griffiss AFB, Rome, N.Y.

Table II. Gas Analysis of Prototype Moisture Standards

45
•4-1 3D

EhWHS

-G 0 co 0 4-> 03 • a •H

d •H 0 CO >

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CM P rH d>

tJ3 P 9 P •H

3.

G

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p

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Cn >1 CD

G 0 rH £ Q CD £ 0

co & CD P id P IT) 3 G -G <D 0 CN CN OX U p

CD • © • 4-> (d e* >. rd jC r- O O G CD 00 03

4-> a)

p • ft CN

d cd in

g c o

g> o 03 4-3 ft •H •H G co

CDp (D CO

d

p rH s

*—

a) CD CD T- < s * P •

id 0 O

G T— 2 CD CD •H 03 <2 P Cn c <d • x

rd (N CTi <3* id <D CD pa — O d s £ g U CN co in CD CD CD 3 < £ • • • u 4-> G rH

T— CN o (d fd 4-3 0 K — CM rH d •H > D P G CD d

to

G

U rH CO

03 rH rH

gg

id 0 3 u

CD

up 4a

G CO P p cn P CD 0 CD CD 00 ,c cm > P o

CD 4-> •H

G • 4a CD 4-3 4-> o

pa Eh £ U rd 03 s 0 <d rH •H D P CM CD 0 4-> • CM P P 6 P o in r- r— <d G CD > — CN CO t— CD d 03 -P d 03 no t— CN CN P rH • G CD G

>H 6 T— o O id 0 -P 1 1 CD 43 •H ^ 0 • • • CM C uo •H P H o o o o CD '— ,Q 0 rH > •H rH P g 03 id <c p rd rd ID (3 d u u -p P ft <d •H

CD CD ft

p £ > rd d £ CD d 0 CD CD 0 u c

CD 03 \ 4-3 P rd •H

tP CD rd CM CM rH O > p d >i rd •H d G u o CD P »M 4-3 CD 03 tD to CD -P rd P u -P 03 T— id > G rd rd CD G z 00 00 00 rH 03 rH d -H ? ? ? GO

n3rH CD rH

GO 3

rH

rd

4-1 CO ID rH G G rH o G

CN CN CN id 0 P CD rd

G o U 0 4-3 G u •H o

r-|O

CD0>

rd o

46

Table

III.

Package

Cavity

Moisture

"{

micrograms)

and

Surface

Area
OUTGASSING

0.020" DIA

ALUMINUM DISC

0.005" THICKNESS

0.010" CAVITY

LAND 0.187" DIA x 0.250" DEPTH

0.2101

SERRATIONS

45° x 0.010" DEEP

1/2-20 THD

3/8" HEX (FLATS)

Figure 1 . Package Schematic - Type A

47
48

Figure

2.

Package

-

Type

A

-

Photograph
9/16" HEX (FLATS)

*

Figure 3 . Package Schematic - Type B

49
TORQUE

(in-lb)

I

O

<r\

E

o

o

cr* O I in in o h n in • < O

CNJ o

O

>

o

o

o

LTv

wnmH*.

50

Figure

4.

Seal

Permeability:

Gas

Analysis

(helium)

vs.

Assembly
2.6 THE EVOLUTION OF METHOD 1018

Robert W. Thomas

Rome Air Development Center

Griffiss AFB NY 13441

(315)330-3730

Method 1018.2 is not, at present, a universal test method which can be applied

to any hermetic enclosure. It was specifically tailored to measure the internal

ambient of packages such as CERDIPs and metal TO-5 cans that contain eutectically

bonded chips. A further constraint is that the package interior be free of any

outgassing sources (epoxies, desiccants, devitrifying sealing glass). Since this covers 95% of the 3AN military qualified parts that are produced, the specification can then

be judged as successful at least as far as the military is concerned.

With the introduction of MIL-STD-1772 to qualify hybrid manufacturers there

will have to be major revisions to Test Method 1018.2 that will take into account the

variety of outgassing sources used to fabricate hybrid parts.

In order to better understand the necessary changes, it may be useful to review

what a test method is and what it isn’t.

What is Method 1018?

The method defines the desired measurement as the amount of moisture in the

vapor state at 100°C under equilibrium conditions. The parts must be prepared for

analysis so that the results are reproducible. The method describes a method of

puncture that will give valid results and specifies the range of volume and moisture

levels that may be measured accurately based on current availability of standards. It

offers three methods that may be used for analysis and sets the temperature of

analysis.

51
What it isn't!

It is not a detailed cookbook that anyone can use to do moisture analysis. There

is a great deal of technique and hardware required in order to meet the Defense

Electronic Supply Center (DESC) suitability requirements. As was said before, it is

not universally applicable to all hermetic enclosures. For instance, it does not apply

to parts which are either evacuated or filled with an inert liquid.

There are many questions left unanswered by the test method that must be

established by the analytical laboratory customer. The customer must specify

whether the sample being tested to Method 1018 is a JAN, DESC controlled,

Qualified Parts List (QPL) test sample. The analytical lab is required to record and

report the results of these tests in a specific way. Essentially, for JAN samples the

data from a Method 1018 test becomes the property of the government. DESC

controls, through the application of MIL-STD-5004, how and when the samples are

pulled from the production line, establishes the failure criteria and grants suitability

status to the analytical labs by quality source inspection (QSI). For JAN parts, DESC

requires the test apparatus to be set up to maximize ambient moisture as opposed to

someone performing a epoxy adhesive study to optimize bakeout time. The latter

might require extended bakeout prior to analysis to insure that moisture has been

adequately removed from the adhesive material.

To summarize the problems associated with invoking Test Method 1018 for non- JAN devices, refer to Figures 1 and 2. In the case of JAN devices, the system is a

self-contained set of specifications, and surveillance that is controlled at each stage

by a government agency. For non-JAN analysis, any lab can do a 1018-like analysis,

however, the vendor must perform his own suitability evaluation and laboratory

surveillance to assure that the laboratory complies with Method 1018 requirements.

The vendor must also call out specific sections in Method 5004 or 5008 depending on

the device type (ie, IC or Hybrid). This will insure that the method of sampling and

52
the failure criteria are adequately specified. It is also the responsibility of the parts

supplier or systems house to indicate in the service contract information which will

assist the analytical lab in setting up the analytical conditions that will insure

optimum results. It is required that the manufacturer notify the laboratory if the

samples submitted are to be analyzed for JAN qualification. It is also necessary to

notify the laboratory that the samples contain either desiccants or known

absorbing/desorbing material such as polyimides, epoxy or silicone. Information on

unusual ambients, such as 100% helium, greater than 20% oxygen, or greater than 1%

condensible gases like freon, alcohol, MEK or acetone should be supplied to the

analytical laboratory. It is also helpful to indicate a preferred or optimum place to

puncture the package. If the package is fragile (like some cerpacks) this will prevent

unnecessary seal breakage which invalidates the analysis. The point of puncture may

be necessary to control in samples that are destined for failure analysis where an

undamaged chip is high priority requirement. Giving the analytical laboratory as

much information about the analysis as is possible will insure that the customer gets

his money's worth.

Recent Changes

To insure that organic containing parts are analyzed in a uniform manner,

RADC is in the process of establishing a moisture standard that will contain an

absolute amount of water. The purpose of the total water standard will be to provide

a correlation standard that will allow the analytical laboratories to establish their

own data analysis procedures that will include the majority of the outgassing

moisture in the final analysis. RADC has proposed that the laboratories be required

to adjust their data reduction routines so that 75% of the total moisture in the known

standard be reported. This requirement should not affect the results for currently

qualified vendor packages which do not contain absorbing species (i.e., epoxy,

polyimides, and porous sealing glass). The new moisture standard will require a

53
change in the definition of internal moisture so that some of the desorbing moisture

is included in the reported moisture. It is also RADC's intention to lower the

moisture limit for all packages to 2000 ppm

v as a part of a progressive program to

eliminate moisture from electronic packaging systems. As a preliminary requirement

to lowering the moisture content, RADC will conduct further round robin correlation

studies at 1000, 2000, 5000 and 10,000 ppm

y

. Concurrently with the correlation

tests, RADC will request DESC (Defense Electronic Supply Center) to sample current

data from JAN suppliers to insure that all of them can meet the new limit before it is

imposed. Recent Data indicates that 95% of the JAN qualified packages can now

meet a 2000 ppm

y

requirement.

ON THE LIGHTER SIDE

On many occasions over the past eight years, RADC has been called upon to

resolve disputes regarding questionable water vapor analysis. A summary of the

reasons for why moisture analysis on identically produced parts is sometimes

radically different is presented below.

The Government has determined that the following can affect the moisture

content of identically sealed parts:

1. A blister decided to outgas

2. The glass- feed through had internal surface cracks

3. Some one slipped in a dessicated lid

4. Purge gas tank ran dry

5. Some parts missed bakeout

6. Bulk liquid ^ tank was filled during sealing

7. Compressed gas solenoid valve leaked inside dry box

8. Operator couldn't get seal - so removed from dry box and blew it off with

clean dry air

54
9. One package had twice the amount of epoxy

10. Gas supply lines were contaminated

11. Used Neoprene gloves on the dry box

12. CERDIP lids were tossed into a container for bakeout

13. KC-1M super dry was allowed to sit out on the bench

14. Inspectors didn't check the super dry packaging for leaks

15. QC opened the dessicated DC-1M super dry containers to inspec them

16. The sealing furnace was cracked

17. When the furnace was moved, there was a pool of water on the floor

18. After solder dipping, the packages were cooled down in a water bath

19. Lids were mixed from overseas operation

20. Mass spec had a hiccup

21. Operator misplaced the decimal point

22. The percent constituents added up to 80% by volume

23. There was little gas left in the package

24. Wet epoxy parts were baked out next to clean empty controls

SUMMARY

The moisture specification of 5000 parts per million was chosen to eliminate

gross corrosion in ICs and hybrids. It was based on many years of moisture research

at RADC and elsewhere seeking scientific answers to practical problems involving

measurement techniques, the use of organic materials and dessicants, and the

establishment of bakeout schedules. Military parts are unique in that they must be

demonstrated reliable over wide temperature extremes. These conditions have

proven to accelerate moisture related failure mechanisms. The cost of part failure in

an operational military system can be enormous. Continued system reliability

problems have created a constant pressure to control the moisture content of

55
military electronic parts. Prior to 1976, many JAN parts had moisture contents

greater than 10,000 parts per million. This coupled with analytical facilities which

were orders of magnitude apart in correlation prompted the Defense Department to

prepare MIL-STD-1018 and specify, for JAN parts, a maximum moisture content of

5000 ppmv.

For the past eight years RADC has pursued a program of improving calibration

procedures and correlation standards. As a positive effect of this program, the

moisture content of both hybrids and integrated circuits has been steadily dropping as

an awareness of moisture and the problems it causes becomes known more widely.

This has increased system reliability and reduced the cost of logistic support resulting

in the savings of untold millions of taxpapers dollars. Unfortunately, the task is not

complete. The test method is still only applicable on a pass or fail basis to military

IC packages that do not contain organic materials. For other package types or

moisture limits, the use of this test method should be for general guidance only

realizing that the results from the Mil-Qualified test labs will generally not give the

same answer at lower moisture limits or if the parts contain organic die attach

materials. It is expected that this situation will be corrected shortly with the

distribution of 1000 ppmv moisture standards and the rewriting of the specification to

include organic containing packages. In the interim, manufacturers should still be

concerned about packages with high moisture content and efforts should continue on

product improvements that will produce dry hermetic packages.

Our common goal is to eliminate moisture as a cause of failure. To do this,

limits have been set consistent with the ability to measure accurately. Changes have

been programmed several years ahead of time. This will solve basic problems before

restricting component availability.

56
SURVEILLANCE

57
58

HOW

THE

MIL

SPEC

&

STDS

WORK

FOR

JAN

PARTS
3. SESSION II MOISTURE MEASUREMENT METHODS

3.1 SELECTING MOISTURE MEASUREMENT METHODS FOR SEMICONDUCTOR DEVICES

R. K. Lowry

Harris Semiconductor

P. 0. Box 883

Melbourne, FI. 32901-0101

305-724-7566

Package assembly and reliability engineers, failure analysts, and quality

assurance personnel all struggle with various aspects of moisture control

in IC packages. Choosing the best analytical method can be governed by a variety of factors, including cost, resources, point at which moisture information is needed, etc. The best testing method in one situation, e.g. screening

incoming package lots for moisture content, may not be the best method for a different situation, e.g. analyzing a field failure. This paper will compare

and contrast both developed methods and emerging (though not necessarily readily

available) methods to assist in selecting package moisture measurement methods.

59
A COMPARISON OF PACKAGE MOISTURE MEASUREMENT METHODS

METHODS DESTRUCTIVE TO THE SPECIMEN PACKAGE METHODS NON-DESTRUCTIVE TO THE SPECIMEN PACKAGE

Mass

Spectrometry

Derivative

Infrared Spectroscopy

Chemical Ionization Mass

Spectrometry

Surface

Conductivity Sensor

Volume -

Effect Sensor

Capacitance Raoo

Test

A. GENERAL AVAILABILITY OF THE METHOD

Wide Limited Limited Wide Wide Wide

B OVERALL DIFFICULTY OF THE METHOD

Difficult Moderately EasyDifficult Easy Easy Easy

C.

AVAILABILITY OF COMMERCIAL MEASURING

SERVICES

4

Certified Laboratories None None None None None

D. COST OF

COMMERCIAL

SERVICES

$70-100

Per Specimen - - - - -

E.

AVAILABILITY OF COMMERCIAL

INSTRUMENTATION

Pemicka Corp., Many Others None at

this time

PCP. Inc. Hams Panametncs Intended

F. COST OFINSTRUMENTATION

Very High. >150K$

Moderately High. 25K$

High, 100KS

Low. <2K$

Moderate

< 5K$ Low <2K$

G. COMPLEXITY OF INSTRUMENTATION Complex

Moderately Comdex Complex Simple Simole Simme

H. PROVISION FOR TEST METHOD

IN MIL-SPEC

Method

1018.2 Procedure 1

Candidate

tor Procedure 1

Method 10182

Procedure 1

Method

1018.2 Procedure 3

Method

1018.2 Procedure 3

Candidate

lor Procedure 3

1 . PHYSICAL STATE OF

SPECIES DETECTED

Gas Gas Gas Liquid Gas Liquid

J. PARAMETER MEASURED AMU 18. H2 0

Ootical Absorbance

at 6.05um AMU 73. h-(h20),

Leaxage

Current. -10-9 AmpsImpedance Capacitance

K. TEST VEHICLE REQUIRED IN SPECIMEN PACKAGE

No No No Yes Yes No

L. CALIBRATION

Three Volume

Calibration Valve

in Conjunction

with NBS Dewpoint Hygrometer

Analyze Gas

Mixture of Known Water

Content at Various Pressures

Iniection of Controlled Vdlume

of Room Air of Known Relative Humidity

Correlate to Mass Spectrometer

Measure Response

of Each Test ChiD in Known

Ambients

Measure Capacitance Ratio at 75% RH 22X and

Correct to -6°C

M. ACCURACY Hardware lor use as Standard" packages, re-tillable in known

moisture ambients and circulatable to participating measurement laboratories, is presently being developed

N. PRECISION =20% at 5000 PPMV

= 1% at 0.05 sec. Time

Constant tor 55ul Sample

Not

Well Defined

Total Condensate

Obtained

Within =2°C

ol Dewpoint

=8.5% at 1000 PPMV

Standard

Deviations - =30%

0. ANALYSIS CONDITIONS

1 0—6 Tort 105’C 1<r3 Torr 25Y;

1 ATM

25

! C

Cooling Profile

ot Specimen. 25"C to -35°C

Specimen at 25’C

Specimen at 0 to -6'C

P. ANALYSISTIME 15-45 Mins. 10 Mins. 15 Mins. 10 Mins. 10 Mins. 5 Mins.

Q. SENSITIVITY 100-1000 PPMV

100-10.000 PPMV in 50ixl Specimens

200 PPMV 500 PPMV 10 PPMV -4000 PPMV.

or 45% RH at 0‘C

R. SKILL LEVEL OFTHOSE PERFORMING

ANALYSES

Skilled \fecuum

Technologist DegreedChemist Degreed

Chemist

Technician Technician Technician

S. INFORMATION ON

SPECIES BESIDES MOISTURE

All Volatiles No All Volatiles No No No

T. APPLICABILITY TO PACKAGE TECHNOLOGY

DEVELOPMENT

Excellent Good Excellent Excellent Excellent Good

U. APPLICABILITY TOPACKAGING QUALITY CONTROL

Good, with Limited Sample

Size Good Poor Excellent Excellent Good

V. APPLICABILITY TOQUALITY CONTROL

OF INCOMING PARTS

Good, with Limited Sample

Size Good Poor Onlv if Sensors arein Subject Parts

Only it Sensors are

in Subject Parts

Excellent

W. APPLICABILITY FOR

FAILURE ANALYSIS

Excellent Good Good None None Good tor Relatively Wet Specimens

X. REFERENCES 1.2.3 4 5 6 7.8 9.10

60
REFERENCES

1. "Mass Spectrometer Moisture Measurements," Session I, Proceedings of

ARPA/NBS Workshop on Moisture Measurement Technology for Hermetic

Semiconductor Devices, March 22-23, 1978, Gaithersburg, MD, pp. 3-68.

2. Thomas, R.W., and Meyer, D.E., "Moisture in Semiconductor Packages,"

Solid State Technology , Vol. 17, No. 9, September, 1974.

3. Pernicka, J.C., and Raby, B.A., "The Paradox of Moisture Measurement: A

Modern Tetralogy," in Proceedings of NBS/RADC Workshop on Moisture

Measurement Technology for Hermetic Semiconductor Devices, II, November

5-7, 1980, Gaithersburg, MD, pp. 3-7.

4. Bossard, P.R., and Mucha, J.A., "Dynamic Measurement of the Water Vapor

Content of Integrated Circuit Packages Using Derivative Infrared Diode

Laser Spectroscopy," in Proceedings of the 19th Annual Reliability

Physics Symposium, April 7-9, 1981, Orlando, FL, pp. 60-64.

5. Stimac, R.M., Cohen, M.J., and Wernlund, R.F., "Water Vapor Measurements

in Small Volumes Using Atmospheric Pressure Chemical Ionization-Mass

Spectrometry," in Proceedings of the 20th Annual Reliability Physics

Symposium, March 30 - April 1, 1982, San Diego, CA, pp. 260-263.

6. Lowry, R.K. , Miller, L.A., Jonas, A.W. , and Bird, J.M., "Characteristics

of a Surface Conductivity Moisture Monitor for Hermetic Integrated

Circuit Packages," in Proceedings of the 17th Annual Reliability Physics

Symposium, April 24-26, 1979, San Francisco, CA, pp. 97-102.

7. Kovac, M.G., Chleck, D., and Goodman, P., "A New Moisture Sensor for

'In-Situ' Monitoring of Sealed Packages," in Proceedings of the 15th

Annual Reliability Physics Symposium, April 13-15, 1977, Las Vegas, NV.

8. Finn, J.B., and Fong, V., "Recent Advances in A^O^ 'In-Situ' Moisture

Monitoring Chips for Cerdip Package Applications," in Proceedings of the

18th Annual Reliability Physics Symposium, April 8-10, 1980, Las Vegas, NV,

pp. 10-16.

9. Merrett, R.P., Sim, S.P., and Bryant, J.P., "Using the Die of an Integrated Circuit to Measure the Relative Humidity Inside Its

Encapsulation," in Proceedings of the 18th Annual Reliability Physics

Symposium, April 8-10, 1980, Las Vegas, NV, pp. 17-25.

10.

Bakker, N., "In-Line Measurement of Moisture in Sealed IC Packages,"

Philips Telecommunication Review, Vol. 37, No. 1, March, 1979, pp . 11-19.

61
3.2 THE INFLUENCE OF FILLING GAS OF HERMETICALLY SEALED PACKAGES ON

DEVICE RELIABILITY: A DEMAND FOR A COMPLETE RESIDUAL GAS ANALYSIS

Luc J. Van Beek and Hendrik A. Loos

Bell Telephone Mfg. Co.

Gasmeterlaan 1 06

9000 Gent

Belgium

91/35 12 25

Abstract: A large number of reliability studies have proved that corrosion

phenomena and device instabilities are merely caused by residual moisture and

ionic species. Life testing (125°C, no bias, 1000 h) on different types of

packages showed also a stringent relationship between the nature of the

filling gas, composition of the chip metallisation and the rate of corrosion.

Devices hermetically sealed in air are more sensitive to corrosion, not only

by a general higher moisture content but also by the catalytic effect of the

oxygen in air. Aluminum chip metallisation with 1% of silicon showed a higher degree of corrosion compared with pure aluminium.

In order to further quantify the observed phenomena, corrosion simulation

tests are being performed. A broad range of surface sensitive analytical

techniques, together with RGA-mass spectrometry, are being used in order to

obtain a clear insight in the corrosion process.

Although these experiments are not in final state yet, preliminary results

confirm the tendency of the earlier observed data: the presence of air

(oxygen) in an hermetic package, together with moisture and ionic species,

will give rise to corrosion artifacts much faster and more severe than a 100%

nitrogen fill does.

Therefore, not only moisture-measurement, but a complete analysis of the

package atmosphere should be an essential part of quality control in a high

reliability manufacturing.

1 . INTRODUCTION

In the past few years, various investigations (1,2,3) and failure analyses of

microelectronic circuit packages have identified moisture, together with

ionic species, trapped within the hermetically sealed packages, as a major

reliability problem, throughout the industry. Consequently, research centers

and manufacturers of hermetically sealed IC's have developed a number of

techniques to accurately measure the moisture content (4,5) of these

components

.

Mass Spectroscopy has proved to be very successful in this manner, though for

small partial pressures the technique still incorporates a number of

difficulties. An important benefit of this technique is the information that

can be obtained from the residual gas as a whole (its components), rather

than just the result of an internal water vapor content to which the other

techniques are limited.

62
The composition of a sealing gas can be very different: analyses of a large

number of various packages revealed a great variety of residual gases. The

main abundant appeared to be nitrogen or air, though various mixtures of both

were also frequently encountered; in certain ceramic packaging techniques,

oxygen or air is added to the furnace gas in order to get a better adhesion

of the different parts (6).

Inertness of the encapsulated gas is a demand for reliable working conditions

of the assembled product. Consequently, one should assess the possibility of an adverse effect in using an oxygen rich (air) atmosphere compared to a nitrogen one. This paper describes the status of a research work in this

topic, which is being carried out at Bell Telephone Mfg. Cy. , Gent, Belgium.

The work is focussed specifically on Al - 1% Si metallization and wire bond

material.

2. EXPERIMENTAL TECHNIQUES

2.1 RESIDUAL GAS ANALYSIS BY MASS SPECTROMETRY ( RGA-MS

)

The apparatus used to measure the internal water vapor content and the other

gaseous components present in hermetically sealed packages is a modified

Micromass 601 system of V. G. Gas Analysis Ltd. This type of mass spectrometer is equipped with a 6 cm radius electromagnet which is able to scan from amu 1 to 500. An adjustable expansion chamber and leak valve

ensure that a molecular flow is produced to the ionisation chamber.

An electron current of 40 pA was used to generate the ions. The background

pressure before puncturing a package was typically 2.10~ b mbar. Detection of mass spectra is done by an ion multiplier coupled to a 3 decades fast U.V.

recorder. Spectra were taken in automatic scan mode from mass 12 to 22 with

a 10 sec. scan time. One overall spectrum from mass 1 to 50 was taken after

1.5 minutes measurement time. The complete system was operated at elevated

temperature being 100°C for the sample holder and 150°C for the expansion

chamber and analyzer. A degassing and heating time of the charged system of one hour was applied. A calibration for moisture measurement was done for a baked 0.04 CC stainless steel calibration volume using a PANAMETRICS MG 101

moisture generator.

2.2 SURFACE - AND MICROANALYTICAL TECHNIQUES

- Electron spectroscopy

Auger Electron Spectroscopy (AES-SAM) measurements were performed with a Physical Electronics Industries - 590 Scanning Auger electron

spectrometer.

The X-ray Photoelectron Spectroscopy (XPS-ESCA) apparatus is part of the same PHY 590 system.

- Electron microscopy

The electron probe micro-analysis ( EPMA) is a JEOL 733 superprobe, TRACOR

114-2000 automated.

The Scanning Electron Microscope (SEM) is a JEOL 35 microscope.

63
- Mass Spectrometry

A Leybold-Heraeus Laser Microprobe Mass Analyser (LAMMA), type 5000 was used for analysis of small particles.

Secondary Ion Mass Spectrometry (SIMS) was performed with the CAMECA IMS - 300 ion analyser/ion microscope.

3. EXPERIMENTAL RESULTS

3.1

REAL DEVICES

3.1.1

R.G.A. - M.S .

A number of metal can transistors were life tested for 1000 hours at 125°C,

without bias. The samples were electrically tested, before residual gas

analysis was performed, according to the upper described methodology. The

results for moisture and oxygen content at given in table 1 for 5 manufacturers. The composition is clearly different for each process.

It is shown by this table that the lower moisture contents are generally

found for nitrogen sealed devices. This phenomenon has been reported

earlier, at the Reliability Physics Symposium 1982 (7), to be the effect of

oxidation of free silicon by competition of oxygen and water.

After decapsulation the corrosion phenomena were observed. Samples of

manufacturers D and E with electrical catastrophic failure revealed wire bond

interruption. Most of the corrosion was found on the wire bonds.

3.1.2 Study of Corroded Bond Wire

The material of the bond wires was A1 - 1% Si, the same alloy as the

metallisation of the chip. The Si dispersion in the wire was homogeneous,

which is shown by a SEM backscattered electron image of an etched wire

(figure 1 )

.

Electron microscopy showed that the corrosion products are amorphous with a layered structure and are loosely bound to the wire (figure 2).

LAMMA analyses of about 40 corrosion particles originating from the footfeedthrough area, with a diameter of 1 to 10 pm were performed. The

identification of the species of which these patterns are indicative was done

by comparison with spectra of pure reference compounds. Figure 3 gives an example of a corrosion particle which corresponds with Al^, ( S0

4 ) ^ aq. Five

different compound-groups were found: Al(Si) oxides, Al(Si) hydroxides and

oxyhydroxides, Al - sulphates, Al(Si) chlorides and alkali (Na,K) and earth

alkali (Mg,Ca) rich particles. These contaminants are coming from the glass

bead of the bottom lid.

3.2 CORROSION SIMULATION EXPERIMENTS

In order to derive a proper understanding of the observed phenomena, and to assess the different parameters responsible for the corrosion process, short

term direct corrosion simulation tests (8) were performed, in rigorously

controlled laboratory conditions.

64
First, planar metallisation structures were chosed to fulfill the dimensional

criteria of the analytical instruments.

Secondly, wire bonds were used in the simulation experiments.

The simulation tests selected were chosen according to the experience with

the life testing experiments on the metal can transistors.

3.2.1 Planar Structures

The material used was silicon wafer on which a 1 ym thick Al - 1% Si or Al

99.99% layer was vacuum deposited. In all the experiments the temperature

was kept at 95 ± 1% and the relative humidity at 100%. NaCl was added in the

test desiccators as a corrosion stimulator and activator. Its concentration

at the surface of the samples was adjusted to 0.8, 8 and 80 yg NaCl/cm^

.

Tests were performed in either environmental air or under nitrogen gas.

Exposure periods ranged between 5 and 200 hours.

It was hoped to derive more information about the corrosion mechanism,

especially the influence of the environmental gas. A broad range of

analytical techniques was employed for this subject; their respective results

will be discussed in turn.

- Al - 1% Si

Figure 4 shows an electron micrograph of two samples exposed to air and

nitrogen, for 18 hours and 8 yg NaCl/cm^. No attack of the surface was observed for the nitrogen exposed samples, but for the air exposed samples,

pits are clearly visible.

It was found that the number of pits formed after prolonged exposure is

linearly related to the NaCl concentration (figure 5).

XPS showed the presence of Al^Ch , Al(OH) or AlO(OH), for both atmosphere,

though for nitrogen, under severe concentrations of NaCl, an Al-Si-Na-0

ionic network was found. Scanning Auger Spectroscopy was used to determine

the repartition of Al, Si and 0 on the corroded surfaces. Si enrichment - as oxide - was found at the first monolayer of the surface.

Secondary Ion Mass Spectrometry in depth profiles confirmed the results of

electron spectroscopy. Also Na enrichment was found at the oxide-metal

interface.

- Al 99.99%

Simulation experiments were carried out for Si-free Al to assess for its

influence. Figure 6 shows the number of pits per unit area (arbitrary

scale) in function of exposure time for 8 yg NaCl/cm2 on Al and Al - 1 % Si

(reference) in air and nitrogen atmospheres. It is clearly shown that Si

presence stimulates the corrosion phenomenon of Al. Its granules were too

small to be visualized by SEM (< 200 A).

Electron Spectroscopy showed for all samples a pure Al ; 0 layer on Al, for

the air exposed samples reaching a thickness of 0.7 ym after 100 hours.

65
3.2.2 Al - 1% Si Bond Wire

Bond wire of Al - 1% Si metallisation underwent the same contamination and

aging procedure as the planar structures did, the atmosphere being air.

Figure 7 shows an electron micrograph of a 1 .000 X magnified piece of wire

bond after 25 hours exposure time. Small deformations of the surface are visible (diameter 0.1 pm 0 1 pm). These flatted spheres appeared to have

grown out of the wire. Since the background of the wire contains a Si

signal, it was not straightforward to geographically correlate these points

with Si abundance. Therefore Al - 1 % Si wire bonds were heat-treated for 12

hours at 500 ± 20°C. This treatment concentrates Si into larger volumes. It was shown by SEM (after etching in 2% HF) that these heat-treated wires

contained Si particles with a diameter of 2 to 6 pm. A number of these wires were exposed to the described corrosive atmosphere for some hours. The early

phase of the corrosion phenomenon could be visualized by SEM. It is believed

that the observed phenomena were due to grain boundary corrosion.

4.

CONCLUSIONS

The internal water vapor content of hermetically packaged devices decreases

with lower oxygen contents. This phenomenon has been reported earlier by

Bell Labs, to be the effect of the chemical reaction of moisture - in

competition with oxygen - with free silicon of the chip.

Silicon presence in aluminum accelerates strongly the corrosion rate of the

alloy.

Corrosion of aluminum and aluminum - 1% silicon in moist atmospheres is

stimulated by the presence of oxygen. Therefore, one may consider being less

tolerant to moisture content for oxygen containing or air sealed devices.

REFERENCES

1. P. Courtney-Saunders, Reliability Physics Brief, 7, 01 (1978).

2. R. W. Thomas, Proc. of the 26th Electronic Components Conference, edited

by the Institute of Electrical and Electronics Engineers Inc., San

Francisco, CA., April 26-28, 272 (1976).

3. B. Drotman, 19th Ann. Proc. Reliab. Phys., 19, 188 (1981).

4. R. Schubert, J. Vac. Sci. Technol. 14, 227, (1977).

5. R. K. Lowry et al., 17th Ann. Proc. Reliab. Phys., 17, 97 (1979).

6. C. A. Harper, Handbook of Electronic Packaging, Edited by C. A. Harper,

McGraw Hill Inc., New York (1969).

7. M. L. White et al. , 20th Ann. Proc. Reliab. Phys., 20, 253 (1982).

8. K. Barton, "Protection Against Atmospheric Environment, Theories and

Methods", edited by John Wiley & Sons, chapter 6, New York (1976).

ACKNOWLEDGMENT

The authors wish to thank Prof. Dr. F. Adams, Prof. Dr. R. Gijbels of the

University of Antwerp (U.I.A.) and Dr. M. Van Craen, as well as Mr. R.

Vlaeminck and Dr. R. Vanden Berghe of Bell Telephone Mfg. Cy., for allowing

the use of analytical infrastructure and fruitful cooperation.

66
Manufacturer Moisture content Oxygen content

A < 500 pprnV < 1 % in

B 500 - 1.000 ppmV 5 % in N.p

C 1.500 - 3.000 pprnV 5 % in N

2 D 3 .000 - 8.000 ppmV 21 % (AIR)

E 2 . 000 - 4.000 pprnV 21 % (AIR)

'abel 1 moisture and oxygen content for 5 different manufacturers.

Figure 1 : backscattered electron image

of an etched ( 2 % HF 2 min.)

A1 - 1 % Si wire.

Figure 2 : backscattered electron image

of a corroded bond wire.

32 (.3 59 tU 80 97

U8

- IL. . -aI -

119

139 157

- U- * ^JL-wSIl 1

Figure 3 (Top) : LAMMA negative massspectrum of a 2 - 4 yum diameter corrosion particle, identified as aluminium sulphate by comparison with the pure Al^fSO ) .aq Spectrum (below).

67
Figure 4 : backscattered electron image

of two samples, respectively exposed to air and nitrogen , for 18 hours and 8 ug

NaCl/cm 2

. ^

Figure 5 : the number of pits per unit area

in function of NaCl concentration

for 100 hours exposure.

•U

1<n

inCl Ii'i -*-• VI C

-I o

O 'T*

• 1« J_J jIj t= ’*- D •T*

0 25 50 75 100

Time (hrs)

Fiqure 6 : the number of pits per unit area

2 in function of exposure TIME FOR 8 yug NaCl/cm .

Figure 7 : backscattered electron image

of a 1.000 X magnified piece of wire bond

after 25 hours exposure time.

68
3.3 EXTENDED COMPUTER CONTROL AND DATA PROCESSING FOR THE MASS

SPECTROMETRIC ANALYSIS OF PACKAGE ATMOSPHERES

Leslie J. Rigby

Standard Telecommunication Laboratories,

London Road, Harlow, UK CM17 9NA

In the comparative analysis of small gas volumes, the precision achieved by

microprocessor controlled data acquisition can be significantly improved by an automated system for sample transfer. This objective has been partially

realised with a Pernicka gas analyser with automated solenoid valves, which

facilitates routine system equilibration to moisture by a pre-exposure and

pump down cycle, and also guarantees a completely synchronous analysis

procedure for all samples. Software has been developed which allows multiple

ion data acquisition of all relevant ion currents for mass 2 to 50 and an early mass scan to cover the range from 51 to 97. Maximum sensitivity has

been maintained by the automatic selection of the most appropriate isotopes

for the major constituents, and possible interferences are taken into account

by the use of previously determined ion current ratios. Helium pre-bombing at

elevated temperatures exposes real and potential leaks to below 10 “^ atm. ml/sec

during the subsequent gas analysis.

Calibration curves for water will be presented and typical results with

package volumes of from 5 to 0.001 ml will be used to illustrate the limits of

detection that have been achieved.

INTRODUCTION

The mass spectrometric method of package analysis attempts to compare sample

gas with suitable standards under carefully chosen and hopefully reproducible

conditions. The sequence of the analysis and the subsequent data manipulation

depends on the capabilities of the instrumentation and the availability of

software. The choice of parameters used during data acquisition are usually

the result of a fairly pragmatic approach to the difficulties associated with

the transport of water vapour through the system and may have a considerable

influence on the final result particularly when low water levels are being

measured

.

System Design

A Pernicka gas analysis system has now been in semi -continuous operation for over 18 months. The use of computer controlled solenoid valves closely

defines the cycle time of the analysis and a sequence of rapid data

acquisition and signal averaging improves data reliability. The following

objectives define the computer software:

1. Complete gas analysis without prior knowledge of the major

gaseous components

2. A sensitive test for hermeticity to below lO

- -*-^ atm. ml/sec

3. Fail safe and reproducible.

Gas transfer to a mass spectrometer is unfortunately a destructive process but

is the only method which affords a complete assessment of the gas ambient in an encapsulated device and provides a high degree of accuracy for water

measurement

.

Computer control and measurement becomes essential for reliable measurement.

The software can be designed to make rapid decisions regarding system failure

and choice of ion currents to maximise sensitivity without multiplier

saturation. Vacuum valves can be precisely switched and repetitive

measurements made in an exact timing sequence so that reliable comparisons can

69
be made between sample and standard gases. Area computations for all the

measured ion currents can include corrections for overrange data. Delay in

sample puncture, and slow sample transfer caused by either incomplete puncture

or absorptive materials within the package are potential errors which can be

minimised by area measurement.

Fig. 1 shows a schematic of the system in which valves V1-V4 are controlled by

a dedicated Midas IV microprocessor. V2 is a needle valve which is preset

according to an estimate of the package volume. The Balzers Q311 quadrupole

mass spectrometer has very good mass stability and the resolution is adjusted

to provide broad peaks with high sensitivity and consistent ion current

sensitivity. Pressure and ion current data are read by Midas at 1 ms intervals and subsequently transferred to an HP 9825T computer which uses a millisecond clock to record time increments.

Table 1 shows the transfer parameters which were measured for values of V2

which were chosen to keep the initial pressure in the mass spectrometer to

below 2 x lO

-

^ torr. For gas volumes below 0.2 atm. ml, V2 is at its widest

setting and total transfer of the sample gas through the mass spectrometer ion source is achieved in about 3 minutes. During that time, five data readings

are taken of sample chamber pressure and each of 32 ion currents in one cycle

of an operation which is repeated 20 times. The 32 ions routinely monitored

(Table 2) includes all the ions of interest up to mass 50. A single mass scan

from mass 51 to mass 97 covers the remaining ions available for analysis. The

analysis time is readily extended for smaller values of V2 by increasing the

number of repeat readings of each ion current from 5 to a maximum of 80.

Experimental Procedure

All packages are prebombed in 2 atm. of helium (or krypton) for a minimum

period of 16 hours at 150°C. Subsequent detection of 10 ppm of this tracer

gas in a gas content of 0.1 atm. ml. yields an estimated leak rate of 5 x

10

- H atm. ml/sec. Up to 12 similar packages are then loaded into the

heated carousel and evacuated for a period of 16 hours. The value of V2 is

then chosen to suit the package volume and the following sequence of analysis

is carried out using standard gas with a similar volume to that of the sample

volume

.

1. Dry nitrogen (for water background)

2. Moist air (for water calibration)

3. Samples

4. Repeat moist air

5. Calibration for other gases as required

6. Correct sensitivity and interference factors

7. Print out final report

Interference factors are calculated from standard gases for the list of major

interferences given in Table 8.

Tables 3-5 show similar sequences of software procedures for nitrogen, air and

sample gases respectively; the relevant differences are underlined. Ion

currents of the naturally occurring isotopes of the major gases which can be

selected to avoid saturation are shown in Table 6. Helium is the only gas

which creates a difficulty and helium filled devices usually require a by

difference estimate of the true helium content. Table 7 details the multiple

ion detection (MID) sequence common to all standards and samples.

RESULTS

Standard water samples were obtained by either mixing laboratory air with dry

nitrogen or by the use of a pressurised humidifier. The resultant gas stream

of about 500 ml /min was sampled by a 3 volume control valve and monitored by a recording dew point hygrometer (Fig. 1). Typical water calibration curves for

70
this system (Fig. 2) indicate good reproducibility at the 1000 ppm level

(0.12). The decay curves for T05 packages (Fig. 3) indicate how quickly the

water transmission curves approach the background curve as the water content

was reduced towards zero. At a level of 100 ppm the area under the water

curve was about twice the area obtained in dry nitrogen.

The latter curve has a small positive slope which results from the slow

release of water molecules pre-adsorbed on the surfaces of the sample chamber

during the initial preconditioning by air exposure. Area measurements for all curves are terminated at the last measured point in an admittedly arbitrary

fashion. However the alternative is to extrapolate to a suitably chosen

baseline and may introduce errors, particularly when non-ideal curves are encountered. The present method does readily discriminate between different

levels of moisture even though accuracy may become more difficult at

concentrations below say 500 ppm.

Table 8 summarizes the estimated sensitivities and detection limits for the common gases. These values represent a best case situation and may be

adversly influenced by the listed interferences.

Examples of final reports are reproduced in Tables 9-11. In these reports,

the percent compositions of the usual components are obtained from the

selected ion currents and their relative sensitivity factors listed in Table 8

after taking into account possible interferences from other species as discussed above. A list of detected other masses is produced by assuming unit

sensitivity of all of these with respect to nitrogen. Table 1 indicates that

some of these could result from the isotopes of major impurities (e.g. HD0 + at mass 19, a t mass 30), spill over of large peaks (e.g. an apparent

mass 31 from a very large mass 32 ion current), or doubly charged ions

(C0^+ at mass 22) and these effects are usually denoted by a <notation.

The remaining ions of significance usually indicate the presence of other

species which are normally organic in origin.

Table 4 shows comparative results from two T05 packages. The present U.S.

883B Military Specification for water in encapsulated devices is 5000 ppm.

The top set of data in Table 4 show a comfortably lower water content of 1150

ppm, low oxygen and carbon dioxide. The absence of helium indicates that

hermeticity is better than 1.1 x 10“^ atm. ml/s. A helium content of 0.68%

in the lower data correspond to a fine leak of 6.6 x 10"9 atm. ml/s. This is

confirmed by higher levels of oxygen, water, argon and carbon dioxide which

may all be attributed to air ingress.

Table 5 shows similar sets of data from very small diodes of less than

0.

002.atm. ml. All detection limits are higher than those quoted in Table 8

but leak rates down to 7 x 10“^ atm. ml/s are still measurable. These

devices had been subjected to a damp heat test so it is not surprising that 9%

water had accumulated in a non-hermetic device.

Table 6 shows data which were obtained for two larger packages containing

epoxide adhesive. Large quantities of water with detectable amounts of

ammonia and several organic species can be attributed to a poorly cured

adhesive. These results illustrate the need for a careful choice of a high

quality resin and prolonged post baking procedures to minimise a potential

source of harmful vapours in hybrid packages.

CONCLUSIONS

1. Rapid analysis system with high reliability

2. Good reproducibility at 1000 ppm water for sample volumes of 0.1 atm. cc

3. Minimum operator intervention

4. Automatic data presentation including simple hermeticity calculation.

ACKNOWLEDGEMENTS

The author thanks STL Ltd for permission to publish this paper.

71
parameters

for

quantities

up

to

30

atm.

ml

1 *0 3

05 (3

a) 00 co C M3 O AJ a

4J .2 g

100 ON '66 CM00

CMUO 21 c 1-4 *H

S'? -H CO 4J

co •h e 05

05 *H

So

05 AJ \D CM O o <r

i—I a)

CO E

3

C

CO CO

• vO CT\

• ON cO

• H

e

C

io in

co3 05 44 4-1 CO .53 o00 30 -O’

1-4 05 r—

<

o

44 co w ^

i-Hs d o

• o

• o o

05

ctj e

00 *H O 05 05 AJ E "4-1 0) 13 3 O r-i C

aj 05 a o

B c ss? e o

• H O o co 05 H o m 05 05

CN

CM 00

00 m

oo

CM00 001

CO

CM

053

O

a)

05

CO O

05 Cm

o a) <—i CL i-i to 3 3

•H V-l 05 JJ 13 05 •r4 (0 a) a 3 u m a* a- •

u

m oo M3 vO

M3

CO H

I-I 00

a) c

•I-I >H CM C 4.1 > V4 4-1 a) a) <4-i > 05 O

o

o om

m

CM

05

1—4 I-I I-I CO 05 3

•-4 M3 05

4-5 E 05

•i-t CO 0) C £ H h o a

oSo

CM 00 O

co

S u H

i - 5

*S S B

CO 3 co X <r

CMo

m

o

o

CM o

00 o

co

72
2 o4

7 12

15

16

17

18

19

20

22

26

29

30

31

32 i

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

Table 2 - list of 32 ions routinely monitored to

the possible sources of each ion:

H* or HD

+ from hydrogen

He

+ from helium

+ , . N ion from nitrogen

C

+ from carbon monoxide and carbon dioxide

(nitrogen), NH

+ (amines), CH^ (hydrocarbons)

+ +*f / N + /

v + . , v + 0 or 0 (oxygen), 0 (water), NH^ (amines), CH^ methane

+ / N + / . N OH (water), NH^ (ammonia)

H^O (water)

HOD (water), F (fluorocarbons and HF)

Ne

+ (neon), Ar

++ (argon), HF+ (hydrogen fluoride)

Ne

+ (neon), CO** (carbon dioxide)

CLH^ (hydrocarbons)

1 i . . 14 15 + , . C^H^ (hydrocarbons), N N (nitrogen)

C0 H. (ethane), ^N^N+ (nitrogen), CH.NH (amines)

+ + CH^O (alcohols), CF (fluoro compounds)

0^ (oxygen), S (sulphur compounds)

Cl (chlorine compounds)

^A (argon), HC1

+ (hydrogen chloride)

Cl

+ (chlorine compounds)

HC1 (hydrogen chloride), C^H* (Unsaturated hydrocarbons)

C^H^ (aromatics and unsaturated aliphatics)

A (argon)

C^H+ (hydrocarbons), CH^CN

+ (nitrides)

CH^CO (lactones, furans , ketones, aldehydes)

C^H^ (hydrocarbons), CH^CD

+ (ketones)

CO

2 (carbon dioxide), C^H^NH^ (amines)

C^H^O (alcohols, diols , ethers)

+ . , . # # NO^ (nitrogen dioxide, organic nitrates)

CC1 (chloro compounds), OOF , C^H^F (fluoro compounds)

SO (sulphur compounds)

CC1

+ (chloro compounds)

CF^ (fluoro compounds), C^H^ (aromatics)

73
74
Table

4

-

Second

analysis,

water

calibration

n

03a

o

•uo

03

60

0) c

4J • H

<0 x

r—

•

Ci 3 o o 3 H

to 03 o 0) C

i-^ •H

T5 a E c E M

tO CO 03

03 -3- 4-1 C > 0) 03 Cl T3

4-1 •H 03

03 cd 03 O

E O 4-1 c r-^ • 4J •H o T3 c s •H Cl i CO o 03

0-1 4J >>

CO E 4-1 3 3 CO •H

(S >4-1 03C • iH o 4J Xi Ci r—

4

T5 CO 03 CO 0) > to E C • H 4-1 03 cc 03 • r4 C > to

a) 06 cc C E •H 3

03 •H cd Ci O 1 E o o o

^"4 r-4 E U-l 4-1 o o o

4—) !—

1

Ci 4-1 •H 4-1 M-l 03 4-1 c > 03 •H

• r4 •H 4-1 Ci 0) 4-1 >

E 4—1 o 1—4 • H

•H Cl 03 cc 0 4J O CO o r-4 Cl E 4-1 • H

T—

4

c CO Cl co CO a; Cl 3 0 03 c C

CO Cl 13 4-1 CO a;

4-1 o • i—

1

4-1 u CO

c/o 4-1 03 00 •r4 03 u S 03 1 E to o CO Ci o 13 03 C u 1

1—

CO 03 *H

r CO o 03 CO HD

Ci a; r—

1

Ci X E D

Ci 4J 3 0) i—4 o X 03 i—4 U-l o

4-1 CO a CC 0) c

0) *—

\

03 E •H • M

0 1—4 UO E o CO w a • 4-1 03 CO o E o •

«

4J

i—

4

CO CM 0) Ci CO

03 + > 4-1 o CO 13 X CO CM •H 4-1 C4 1—1 C 3 4J c H /-v •H a 03 o -X • M 03 CO

(—1 cd N-X O- CO 03 c E •M o > CO •M 03 4-1 + 4J a) C4 M

CO a 13 13 •> 3 c

1—1 •r4 C 13 C .X 03 •H a Q rH CO c co o CO £ •H CO o 4J

4-J ^"4 i—4 r-4 g 3

V4 g > t—

!

> o 0

a o 3 > 0)

M-4 03 03 4J r-l 4—1 c a 03 c 03 Ci cc c

0) & g 0 03 O CO E •M

rj 9 i—4 a 1—1 4-1 CO Ci 5 P-. o o o CO CO CC

<—i cm co -o- in ao

75
co

ata

o

A)o

co

• w

V

3

AJ

CJ3

3

a

a;oo

CO CO

4*: C

cj CO •H

co C s a •HE o r\ rH

CO >> o

•H 4-J i-H A

co

• H A

AJ >

•H AJ U

co 4J o

c

•H U CO

CO rH CO o u > C co u

TO

cu

<

u

u 3 o

U CO u TO AJ •H CO o •H

4= O CO AJ CO 00

Eh l““l E CU 1 CJ 00 o

1 4J 1 rH

" CO o 0) Vj (1) rH u X

m Vj 4-J 3 O X CO rH

a) AJ * CO a rH <D (U 43

co H

00

i oa

E

COco

m

o

+

o

AJ CN>

TOc

CO

<uo

•H>

0) TO

0)u

3

AJo

c

3

(Xl

a

S

CO

CO

VjO

4*! m

co

00c

•Ha

o

3

a)c

• H

S

u

0) AJ

<v to

8

J-l 43

CO TO

•H

r—

<

cx

' c

<u 3

CJ

i 4J

• r4 c rH

cd rH cd -w

'

a CO vt c •H + 4J V o >

<u > •H u

4-J

• H

r—

1

a

CO

• H

g

a

rH

•

and ”0£

TO u

0

CO

«d

c

•H A Vj 4J6

rH>

TO

f-H rH>

o

rHo

s out cxc

0)

O

4-1 3

CX

0)

COO

>

c

<u

a)

COo

4Ju

<d

CL) rHa

E

4Jc

• H $ Pj

f—

1

o

a

o

rHu

4-J CO

co cn A

Pj

•3

O

ua

co

C0

<uoo

co 4*O

COa

TO

ai ajC

3

COC

u

<u ajX

<uuo

a;u

3

TO

<DO

o

Vaa

cn on m vo 00 CTn

CO

76
Table

6

-

Isotopes

of

major

gases

Ph

w

o

w

C3

oo cm o

CM CO <t

w

o

ft g

CO

O'* r—

4

T3 CM

CM

H

C/5

CT\ CM <T

co

\£> co 00

8

O

QdQ J

td3S

O

H

O

S

O

CM g 8

&

Id<:3

77
78
Table 8 - Typical sensitivity factors and

detection limits in a 0.1 atm. ml gas sample

Gas Mass Relative Detection Interference

sensitivity 1 imi t in

ppm

Hydrogen 2 45 10 Wa ter

Hel ium 4 19 10 None

Carbon monoxide 12 15 50 C0

2

Me th ane 16 95 5 °2’ N

2

Ammon ia 17 40 200 Wa ter

Water 18 58 500 °2

Nitrogen 28 100 5 CO, CO^, Organics

29 0.74 Organics

Oxygen 32 87 5 None

Argon 40 84 5 None

Carbon Dioxide 44 190 5 °2

79
GAS ANALYSIS BY iMASS SPECTROMETRY MODE: Batch

SAMPLE I DENT I F I ClATIOH : T05 encipiu 1 at ed t r ans i s t or

MS file no = 12 DATE = 06:10: 32 T I ME = 11:45

RESULTS : TOTAL GAS CONTENT = 0.23 at m . m 1 Needle valve set COMPONENT COMPOSITION OTHER MASSES M REL.COMP.

Wat er 0.14'? 30 <0.0034

Hydrogen 0.062 19 <0.0031

He 1 iuii < 0.0040 2d 0.0017

N i t r o g e n 99. 31 0.0011

Oxvaen 0.0012 42 0.0011

Argon 0.0008 43 0.0014

Methane < 0.0003

Caroon monoxide < 0.0060

C ar o o n dioxide 0.0197

NOTES Helium content i s egu t val ent t 0

-r *

a 1 eak r at e of 1.1E-11 atm.ml/iec.

GAS ANALYSIS BY MASS SPECTROMETRY MODE: Batch

SAMPLE IDENTIFICATION: T05 encapsulated transistor

MS file no = 13 RESULTS : DATE = 06: 10: 32

TOTAL CAS CONTENT = 0.C2 atm. ml

T I ME= 11:38

Needle value set to 100 COMPONENT V. COMPOSITION OTHER MASSES JJ REL.COMP. Water 1.16 22 <0.0019

Hydrogen 0.040 30 <0. 0040

He 1 i urn 0.68 19 <0.0035

N i t rogen 96

.

20 <0.0115

Oxygen 0.93 26 0.0013

Argon 0.061 31 <0.0095

Met hane 0.0032 35 0.0011

Carbon monoxide < 0.0040 39 0.0015

C ar bon dioxide 0.070 4 1 0. 0009

42 0.0011

43 0.0022

45 0.0015

91 0.0016

NOTES Helium content i s egu i val ent to a 1 e ak rate of 6

.

,6E-09 atm. mix

Table: 9 Tabulated Data for 0.2 ml Packages

80
GAS ANALYSIS BY MASS SPECTROMETRY MODE: Batch

SAMPLE IDENTIFICATION: VY d i o d e 3 3 ns file no " 21 DATE * IS: 05:132 TIME- 09:31

RESULTS : TOTAL GAS VOLUME* 0.0019 itn.nl -Needle valve set to 100

COMPONENT COMPOSITION OTHER MASSES rel.comp.

Wat er 1 . 82 19 <0.024

Hydrogen l . 67 26 0.023

He 1 iun < a.03ee 39 0.0172

N i t rogen 96. 43 0 . 0168

Oxygen 0.031 50 0.022

Argon < 0.0068 51 0.037

Met hane < 0.0050 73 0.070

Carbon aonoxide < 8.0300

Carbon dioxide 0.58

NOTES He 1 i uo content is equivalent to a leak rate of 6. 3E-10 atm.ml/sec.

»•**•  + ** ************************** + *******»** + ********• '»•** ***•***»••*•»•*«*•

CAS ANALYSIS BY MASS SPECTROMETRY MODE: Batch

SAMPLE IDENTIFICATION: VY diode-'bS

MS file no - 22 DATE - 19:05:82 TIME- 09:52

RESULTS : TOTAL GAS VOLUME* 0 .8013 itn.nl Needle valve set to COMPONENT 7. COMPOSITION OTHER MASSES * REL.COMP.

Uat er 9.8 22 <0.0099

Hydr ogen 1.35 19 <0.030

He 1 iui 8.35 20 <0.058

N i t rogen 83. 26 0.021

Oxygen 4.6 39 0.037

Argon 0.47 43 0.032

Met hane < 0.0030 45 0.0170

Carbon monoxide < 0.0200 50 0 . 026

Carbon dioxide 1.31 78 0.071

NOTES Helium content is equivalent to a leak rate of 1 . 3E-08 aim.nl/sec.

1 0£i

Table: 10 Tabulated Data lor.'0.0&2 ml Packages

81

<L‘

l\>
r -r -

Gr. : n N h Li SI 3 £ V MR S3 SPECTROMETRY MODE : Cyclic

: ri M P L E IDENTIFICRTIONTsit P aC k aQc with cured epoxide adhe

i

i ve

MS file no = 25 DATE = 22/ 10-32 T I ME = 13 34 RESULTS : TOTAL GAS CONTENT = 1.72 a ’ in . in 1 Needle valve

COMPONENT :: COMPOSITION OTHER MASSES : REL.COMP.

h rn m o n i a 0 . 0096 22 <0. 0007

Water 0.56 30 < 0. 0044

Hydrogen < 0.3007 19 < 0.0056

He 1 i urn < 0.0020 20 0.0013

Hi t r o g e

n

99. 26 0.0179

Oxygen 0. 0023 31 0 . 0066

Argon 8.0O79 35 0.0015

Methane 0. 0070 36 0. 0008

Carbon monoxide 0.0130 37 0.0013

Carbon dioxide 0.032 33 0.0015

39 0.004

1

41 0. 0060

42 0.0050

43 0.0112

45 0.0022

46 0.0008

47 0.0017

4S 0.0011

49 0.0054

58 0.0013

64 0.0190

66 0.0059

GAS ANALYSIS BY MASS SPECTROMETRY MODE: Cyclic

SAMFLE I DENT I F I CRT I ONTes t Package with cured epoxide adhesive

MS file no = 2? DATE = 22-10/82 T I ME= 14 3'? RESULTS : TOTAL GAS CONTENT = 1.75 atm. ml Needle valve set to COMPONENT '. COMPOSITION OTHER MASSES 1: REL.COMP.

A in Hi o n l a 0.093 O o <0.0057

Wat er 4 . £. 30 <0. 0074

H 1 d r o g e n < 8.0010 l ?

t 20

0.0051

H e 1 i u m < 0.0030 0. 0051

N l trogen 97. 26 0.0155

Oxygen 0.0024 31 0.0183

Argon 0.0185 37 0. 0034

M e t h an e 0.0200 38 0.0053

Carbon monoxide 0.043 39 0.8193

Carbon dioxide 0. 28 4 1 0.04 1 42 0.041

43 0.043

45 0. 012S

46 0.8034

49 0.0009

58 0 . 0036

55 0.0123

Table : 11 Tabulated Data for 2 ml Packages

82
3VCV

Figure 1 - Schematic diagram of sample chamber (SC) and Baratron gauge (b)

with gas supply from a 3 volume calibration valve (3VCV) . The quadrupole mass spectrometer (MS) and the sample chamber are maintained at 100°C and are pumped

by two turbomolecular pumps (TP) monitored by ion gauges (IG) and backed by a common

rotary pump (R2). R1 is used to initially pump SC via V3. D is a direct reading

mirror hygrometer.

83
Mass 18

area

(relative

units)

Fig. 2 Water calibration for gas volumes of 0.1 and 0.01 atm. ml

and needle valve settings of 50, 100 and 160.

84
Typical

nitrogen

decay

(mass

29)

oo

aP ap aP aP aP

ap in CO CM o O) T“ o

o 0) 00 lO CO o o o

CM o o o d o o o

CD O Q LU LL o X t—

H

o

o

CMO

00

CO O

c

o

o

0

CO s—

0

E

i—

oo

Ux -• 3 co Q. «- 3 §

85

Water

transmission

from

T05

packages

with

V2

=

100
3.4 GASEOUS COMPOSITIONS OF HERMETIC PACKAGE CAVITY AMBIENTS, II

R . K . Lowry

Harris Semiconductor

P. 0. Box 883

Melbourne, FL 32901

305-724-7566

ABSTRACT

Mass spectrometric measurement of internal water vapor content also yields

data on other volatiles in the package cavity. Identity and concentration

of all species present can supply useful information for packaging technology

improvements to produce cleaner and drier parts. In an extension of the

discussion presented at the 1980 Workshop, and utilizing data from more than

1300 packages, this paper describes some of the relationships found between

levels of moisture and associated levels of N^, 0^, H

2

» CO^, and Ar in

several different package styles.

INTRODUCTION

Mass spectrometry provides a measure of all volatile species within a hermetic package enclosure. Knowing the interrelationships between various

volatiles in particular package styles gives clues to hermetic package

ambient chemistry. This has proved important not only for moisture control [1]

but also for other aspects of hermetic packaging problems [2], In particular,

effects of hydrogen on device electrical stability [3] and on metallization

integrity [4-5] have been reported. Deleterious effects of ammonia in hybrid

parts have also been described [6],

In the previous Workshop paper [1], some relationships between package moisture

content and certain other volatiles were discussed for three different package

styles. Trends identified were as follows, where n indicates the number of

specimens:

Cerdip, n = 146 Braze Seal, n = 56

h

2 o - o2 h

2 o oc h

2 H

2 0 oc Ar H

2 0 4 C0

2 H„0 4 CO

2 2 a means exhibits dependency. 4 means no apparent dependency.

Some species in some packages were present only in the fractional volume

percent range. But with the ever-diminishing limits on moisture content

(2000ppmv by June 1984 plus initial work on a 500ppmv standard) [7] combined

with the profound effects of hydrogen on device physics, the relationships

discussed are technically important for advanced packaging technology.

In the time elapsed since the previous paper, many more parts have been

analyzed. The above trends are re-examined here using data from over 1300

specimens analyzed since 1979. Data comes from assembly engineering studies

as well as product qualifications. It is thus indicative of broad trends to

be expected in a package type but is not entirely representative of typical

delivered product.

TO, n = 36

h

2 o * H

2

h

2 o « C0

2

86
DISCUSSION

Cerdip

Ceramic dual- in-line packages are composed of an alumina base and cap. The

IC chip is attached to the base, and the lid is sealed to the base using a solder glass to enclose the chip in a hermetic cavity. Sealing is done in a temperature profiled furnace. Seal ambient is either natural or synthetic air.

Where natural air is the sealing ambient, the trend of cavity constituents is

typically

> £ £

N

2

> 0^ > Ar < CO^ > H^O > and other species

Where synthetic air is the sealing ambient, the trend is typically

N

2 >> 0^ > CO^ > H^O > Ar , , and other species

In these sequences items indicated by * are often not detected.

Table I summarizes relationships found between H_0, 0_ , Ar, and CCL within 694

Cerdip specimens. The data here is, first of all, a testimony to the ability

to build dry Cerdips. Only 19 of 694 parts were > 5000ppmv. Thirteen of these

19 represented non-standard processing. It should be further noted that 2 of

every 3 Cerdip packages contained < lOOOppmv moisture.

Oxygen . Ref. [1] alluded to the ability to control package moisture by

controlling 0^ in the seal ambient. From Table 1 and Fig. 2 this is clearly

established. An : 0^ ratio of about 18:1 is optimum for producing parts

containing < lOOOppmv moisture.

Carbon Dioxide . Carbon dioxide within Cerdips is derived from organic

binders and vehicles used in glazing the alumina piece parts. Variability in

burn-out of these organics prior to and during seal causes substantial variation

in the CO content of individual Cerdip packages. Concentrations usually range

from 2,000 to 20,000ppmv. A few specimens may contain significantly more or less than these amounts. From Fig. 1 there is a trend towards declining amounts

of CO^ with increasing H^O above 5000ppmv. This may reflect the greater amounts

of 0^ available to help pyrolyze the organics from the glass during the seal

process as one progresses down Table 1.

Argon . Ref. [1] reported an apparent relationship between Ar and H^O.

An examination of the data from the 694 specimens reveals three distinct

categories of Ar content. Parts contain either:

a. No detectable Ar

b. 100- lOOOppmv Ar

c. > 8000ppmv Ar

Category (a) is made up of parts sealed in synthetic air. Category (c) is

parts sealed in natural air, which itself contains 9340ppmv Ar. Category (b)

is presumably an extension of (a) in which minor quantities of Ar were derived

from the glass binders during seal, there is a small leakage of ambient air into

the seal furnace being fed from ^

and C>2 cylinders, or the packages at some time in their history were fine leakers and let in atmospheric Ar. Note that the

tendency for parts to contain amounts of Ar inherent from air increases with

their moisture content.

87
There is another interesting observation not reflected in Table 1. Parts sealed

in synthetic air contain just as much CC^ as those sealed in natural air. CC^

derived from the sealing glass overwhelms the 330ppmv naturally available from

air (Table 4) .

Hydrogen A new relationship identified is that of H

2 in Cerdips.

Between 100 and 5000ppmv, about 1 of every 5 Cerdips contain in the 0.1-0.2v%

range with no apparent dependency on the H2 O content. Very dry and very wet

(>5000ppmv) Cerdips do not contain H

2

.

Variabilities in H^ are, like CO^, presumably due to variabilities in behavior

of the glass during seal. content is observed to be glass lot oriented.

Presence of in the 0.2v% range in Cerdips is not trivial in light of Ref. [3].

Such quantities may be of» no consequence to metallization integrity [4-5], however.

Braze Seal

These packages consisted of a ceramic body with a cavity for the chip onto

which a gold-plated lid is brazed at low temperature to form a seal. The parts

studied here were sealed in an ^

atmosphere. The concentration of cavity

species are typically:

^2 >> ^2 > ^2^ > ^2 > ot^er sP e°ies * *"other species" seldom detected

Hydrogen Ref. [1] reported a definite trend of moisture with H

2

. That

trend is clearly evident in Fig. 4 , with significant quantities of H increasing

from 1 to 7v% with H^O up to 7500ppmv H^O.

The greater availability of H^ increases ^0 content within the cavity. This

may be due to internal chemical reactions where H^ reactant yields H^O as a product. Excessive H^ atoms may occupy sorption sites on the gold surfaces

within the cavity where ^0 might otherwise have been tied up. Out-diffusion

of H^ incorporated within the gold plating may be accompanied by desorption of

H^O as well. It is clear, though, that by correct choice of gold plating and

pre-package sealing parameters both moisture and YL content in braze seal parts

can be controlled.

CO

^ Previous studies on a limited number of specimens indicated no relationship between package moisture and CO

2

. From Fig. 4, however, there is

a weak dependency, with CC^ rising slightly as H-O increases. This is unlike

the trend in Cerdip, where CCk tends to fall as increases. There is

chronological importance to the data in Table 2 . In total, 10% of the samples

analyzed contained >5000ppmv ^0. A check of sample date code records reveals

that only 4 of these were sealed after December 1980, corresponding to a <2%

incidence of excessive moisture in braze seal over the past 34 months.

TO

These are the "can" style packages having the chip mounted on a gold-plated

header and encapsulated inside a cylindrical lid welded to the header. The TO

package lids in this study are nickel. Weld sealing is done in an ^

atmosphere

near room temperature. The concentration of species in the cavity is typically:

88
< * N

2 >> H

2 0 > C02 > H

2 > °2 > other sPecies

often not detected.

Table 3 and Fig. 6 summarize data for 369 TO specimens.

CO The tendency for C0

2

to increase with H

2 0 is clearly evident,

similar to the trend for side braze (though amounts of C0

2 are greater) and

opposite to that of Cerdip.

H

2 There are two categories of TO cans with regard to those which

contain a few hundred parts per million H

2 and those which contain no detectable

H

2

« Incidence of H

2 appears to be lot-oriented , perhaps reflecting the manufacturing process treatments of the plated headers and/or the nickel caps. Highmagnification examination of the caps reveals the nickel to be extremely porous.

Where parts do contain H

2

, there is no particular trend with H

2 0 content if the

100-500ppmv HO data point is ignored. This agrees with that previously reported

in Ref. [1].

L

H O Distribution A disturbing proportion of TO cans contain >5000ppmv HO;

see histogram. It is important to understand a subset of the data not reflected

in the histogram. Three-fourths of those >5000ppmv, as well as many of those in

the 2500-5000ppmv category, were engineering test parts at an offshore assembly

facility. Perfect moisture control was not anticipated in those particular parts.

CONCLUSION

Mass spectrometry yields useful information about all gaseous components within

a hermetic package, in addition to its H

2 0 content. This paper has identified

significant related trends in Cerdip between H

2 0 - 02 and H2 0 - C0 2> in braze

seal between H

2

0 - H

2

» and in TO between H

2 0 - C02> Knowing levels of species

other than moisture will continue to be important, not only in the drive to

lower moisture levels, but also in efforts to control hermetic cavity gas compositions for overall device reliability.

REFERENCES

1. Lowry, R. K. , "Gaseous Compositions of Hermetic Package Cavity Ambients",

NBS Special Publication 400-72, Workshop on Moisture Measurement Technology

for Hermetic Semiconductor Devices II, November 5-7, 1980, pp. 15-18.

2. Ebel, G. H. , "Application of 1018.2 to Hybrid and VLSI Devices", Proc. 21st

Inti. Rel. Phys. Symp., Phoenix, AZ., April 5-7, 1983, pp. 274-281.

3. Sun, R. C., Clemens, J. T. , and Nelson, J. T., "Effects of Silicon

Nitride Encapsulation on MOS Device Stability", Proc. 18th Inti. Rel. Phys.

Symp., Las Vegas, NV. , April 8-10, 1980, pp. 244-251.

4. Shih, D. , and Ficalora, P. J. , "The Reduction of Au-Al Intermetallic , Proc.

16th Inti. Rel. Phys. Symp., San Diego, CA. , April 18-20, 1978, pp. 268-272.

5. Murthi, A. K. , and Shewchun, J. , "The Effect of Hydrogen Ambients on Failure

Mechanisms in CMOS Metallization", Proc. 20th Inti. Rel. Phys. Symp.,

San Diego, CA. , March 30 - April 1, 1982, pp. 55-65.

89
6. Ebel, G. H., Minnowbrook Coordination Meeting on Hybrid Packaging, Sept. 18-

21, 1983.

7. Moore, B. A., "Mass Spectrometric Measurement of Moisture Workshop", Proc.

21st Inti. Rel. Phys. Symp . , Phoenix, AZ. , April 5-7, 1983, p. 261.

TABLE I

CERDIP DATA SUMMARY

Moisture Mean Mean Hydrogen

c Argon

b Not < . 04v% >0. 7v%

level, ppmv n v% o

2 v% co2

3. n Mean v% H

? Detected range range

<100 24 1.86 1.91 i 0.01 16 8 0

100- 500 140 6.62 1.27 10 0.14 58 66 16

500- 1000 293 14.30 0.99 20 0.17 31 79 183

1000- 2500 186 14.52 0.97 27 0.21 25 53 108

2500- 5000 32 15.54 1.12 10 0.10 2 8 22

5000- 7500 2 17.90 0.47 0 - 0 0 2 7500-10000 2 19.70 0.21 0 - 0 0 2 >10000 15 19.45 0.11 0

- 0 0 15

a. n = number of specimens in which was detected.

b. mean value of for the n specimens.

c. n = number of specimens containing Ar in each indicated range.

PPTV MOISTURE Figure 1

RELATIONSHIPS OF OXrbQt, oIRBuN DIOXIDE, AND

TO LEVELS OF WATER VAPOR IN CERDIP PttKAGES

MOISTURE CONTENTS, PPMV Figure 2 90
TABLE 2

BRAZE SEAL DATA SUMMARY

Moisture Range

ppmv n

<100 16

100- 500 46

500- 1000 39

1000- 2500 66

2500- 5000 46

5000- 7500 15

7500-10,000 2 >10,000 8

Figure 3

Mean Mean

v%H

2

v%C0

2

1.08 0.023

1.16 0.049

2.52 0.047

2.89 0.062

3.72 0.079

6.24 0.077

1.91 0.083

0.86 1.51

RELATIONSHIPS OF OTKXBI MG CAABCN DIOXIDE

TO LE^LS OF WTER IN EMI SEAL PACXAffiS

M0ISTU€ CWIENTS, Pf*V

Figure 4

91
TABLE 3

TO DATA SUMMARY

loisture Range

ppmv n

Hydrogen

n

a Mean

v% H

2

Mean

v% C0

2 <100 10 0 _ 0.063

100- 500 22 1 0.025 0.070

500- 1000 62 13 0.147 0.070

1000- 2500 135 60 0.041 0.104

2500- 5000 99 41 0.044 0.211

5000- 7500 27 13 0.021 0.250

7500-10000 6 3 0.017 0 467

>10000 8 2 0.022 0.450

a. n = number of specimens observed to contain H . b. mean value of for the n specimens. 1

fTW TMSTURC

PELATICKSHIPS OF WOROGEN AN) CAPBOH DICK HE

TO LEV€LS OF HATER IN TO PACKAGES

MDISHJE CONTENTS, PFW

Figure 5 Figure 6
TABLE 4

NATURAL COMPONENTS OF ATMOSPHERIC AIR EXCEEDING 0.1 PPMV

(exclusive of water vapor)

No 78.084 + 0.004 v%

°2 20.946 + 0.002 v%

Ar 0.943 + 0.001 v%

C°2 0.033 + 0.001 v%

Ne 18.18 + 0.04 ppmv

He 5.24 + 0.004 ppmv

ch42 ppmv

Kr 1.14 + 0.01 ppmv

h2 0.5 ppmv

from CRC Handbook of Chemistry and Physics, 53rd edition.

93
3.5 IMPROVEMENTS ' IN THE CAPACITANCE-RAT10 METHOD OF USING AN IC DIE

AS A MOISTURE SENSOR

R P Merrett

British Telecom Research Laboratories

Martlesham Heath

IPSWICH

IP5 7RE

United Kingdom

Tel 0473 642309

ABSTRACT Changes have been made to the method of using the frequencydependence of pin-to-pin capacitance to assess the water content of an

IC encapsulation. The modified test is easier to perform and the

results obtained can be directly compared with those of mass spectrometry

.

1 . INTRODUCTION

The assessment of the water content of hermetic semiconductor packages by mass spectrometry (1) is now an accepted method for product evaluation and failure

analysis. However, despite considerable improvements in the operating

procedures and precision of this test method, it seems unlikely that it will

become simple enough for manufacturers to adopt for quality monitoring. Thus

although it is difficult to control all of the factors which determine water

content, manufacturers still lack a means of obtaining a rapid assessment of

this aspect of their encapsulation process. In an attempt to fill this gap, the

capacitance-ratio test was developed (2). Based on the use of an IC die to assess the relative humidity (RH) inside its package, the capacitance-ratio test

is a rapid go/no-go procedure which is sensitive enough to detect 5000 ppm

moisture contents.

/

If the internal atmosphere of an hermetic encapsulation contains moisture, there

will be a conducting film of water adsorbed on the surface of the IC die, and

this film introduces fringing fields which increase the pin-to-pin capacitances

of the IC (2). The moisture induced capacitance is highly frequency dependent

and can thus be measured by comparing the capacitance between a pair of pins of

an IC at two test frequencies (100 Hz and 1 kHz). From these measurements it is

possible to derive a parameter, called the capacitance-ratio, which is directly

related to the moisture induced surface conductivity of the die.

In the capacitance-ratio test a measurement of the moisture induced capacitance

is made when the die is cooled to a temperature at which the maximum permitted

water content would cause a RH of 75% to exist at the die surface, and the

package is rejected if the capacitance-ratio exceeds that corresponding to such

a condition. The failure criterion is derived from a calibration procedure

which involves opening a representative sample of the packages under

investigation and subjecting them to a RH of 75%.

94
Since the test was first described it has been used for product evaluation and

failure analysis of a wide variety of MSI and LSI circuits in both CERDIP and

sidebrazed packages. From the experienced gained, several modifications have

been made to the operating procedures. Specifically, the test method has been

restructured in order to simplify the equipment required, and to make the

results easier to compare with those obtained by mass spectrometry. Also,

methods have been developed to minimise the contributions that both the package

and circuit elements make to the measured frequency dependence. In addition,

studies have been performed which show how outgassing of water from package

materials can affect correlation between the results of the capacitance-ratio

test and those from mass spectrometry. After these factors have been discussed,

it will be shown that there may be an additional use for the information

obtained when the calibration procedure is used to assess the sensitivity of an

IC to moisture.

2. CHANGES IN THE METHOD OF MEASURING CAPACITANCE

Water adsorbed on the surface of an IC die forms a conducting film which

provides additional capacitance coupling between the metallisation and the

substrate (2). Although many ICs lack a direct connection to the substrate it

is always possible to observe the influence of moisture, because the capacitance

between any two pins includes the series combination of the metallisation to

substrate capacitances associated with each pin. In order to maximise the

response to moisture, one of the pins chosen should have a large capacitance to

the substrate, ie be connected to an extensive area of metallisation. A pin

connected to a supply rail satisfies this criterion and, subject to a reservation to be discussed, any input or output pin will be suitable for the

other connection. (Although the response to moisture depends on the amount of

metallisation connected to the second pin (3), the response for a 100 x 100 jum

bond pad leading to 300 fim of 10 /urn wide track is only twice as great as that

for the same size bond pad connected directly to a diffusion.) Finally, because

a pin-to-pin capacitance can be less than 1 pF, it is convenient to increase the

capacitance presented to the test equipment, and thereby improve the resolution,

by measuring the capacitance between a supply rail pin and a pair of other pins.

In order to separate the moisture measurement term from the intrinsic

capacitance, the frequency dependence of the pin-to-pin capacitance is assessed

by making measurements at 100 Hz and 1 kHz. The difference, d, between these

two values of capacitance can be expressed as:-

d = 100 Hz capacitance - 1 kHz capacitance ( 1 )

d = d + d + d m c p

( 2 )

where d is the contribution due to moisture,

m

d^ is the contribution due to circuit elements linking the tracks

connected to the chosen pins. This term can be minimised by using j low amplitude measurement signal (20 mV rms).

95
d is a package contribution which is primarily due to water forming a

P conducting film on the internal surface of the package and thereby

providing resistive coupling to adjacent pins.

For the capacitance-ratio test it is necessary to find a combination of pins for

which d is the dominant term. In order to make the selection procedure as simple as possible, and to increase the probability of success, capacitance

measurement equipment has been developed which enables the unwanted terms to be

minimised. The two main units in this equipment are a 20 mV rms oscillator and

a virtual earth amplifier having a reference capacitor as its feedback element.

These are connected to the chosen pins of the IC under test as shown in Figure

1. Because most of the applied voltage appears across the IC, the capacitive

component of the current flowing into the amplifier is directly related to the

pin-to-pin capacitance of the IC, and thus the latter can be measured by using a phase sensitive amplifier to monitor the in-phase component of the output

voltage of the amplifier. The contribution of the package to the frequency

dependence of this capacitance is eliminated by earthing the pins either side of

those connected to the amplifier. These 'guard pins' prevent currents flowing

on the internal surface of the package from reaching the amplifier.

A substantial reduction in the contribution due to circuit elements can often be

obtained by superimposing a DC bias voltage on the sinewave and thereby reverse biasing active elements connected to the chosen pins. This bias voltage is

added, via a 4.3MI2 resistor, at the input to the amplifier and is prevented

from reaching the latter by an 0 . l^F blocking capacitor. The success of this

technique can be assessed by measuring d when an exposed die is held at the test

temperature and exposed to dry nitrogen. If the value thus measured is greater

than the failure criterion derived from the 75% RH calibration, then the IC is

suitable for the capacitance-ratio test.

3. AMENDED CALIBRATION PROCEDURE

A RH value of 75% was chosen for the calibration because it is high enough to

produce a detectable moisture induced capacitance, while also being low enough

to be within the range which will be found when packages having an unacceptable

water content are cooled during the capacitance-ratio test.

Ideally, the calibration should be performed at the low temperatures (typically

0°C) needed to produce a 75% RH inside a package having a barely acceptable

water content. However, because the temperature dependence of the moisture

induced capacitance can be predicted, it is possible to simplify the equipment

required by performing the calibration at room temperature.

Moisture enhances the pin-to-pin capacitance by an amount which is given by the

equation ( 2 ) : -

Moisture induced capacitance ( 2 )

where f is the frequency of the measuring signal and o is the surface

conductivity. The temperature dependence of a has been studied experimentally

96
4) and, for RH values of greater than 70%, it has been found that the

dependence can be described by an activation energy of 0.4 eV. Thus by using

this value, and equation 2, it is possible to derive the relationship between

(i) the value of d measured for a calibration condition of 75% RH at room

temperature, and (Ti) the value corresponding to 75% RH at the lower temperature

specified for the capacitance-ratio test. However, although this extrapolation

technique is suitable for most ICs, it should not be used when the sensitivity

to moisture is such that surface conductivity becomes large enough to make the

100 Hz value of the parameter (a/f) fall outside the range of validity of

equation 2 (2). For this reason, it is now recommended that, instead of

measuring d at the calibration condition, the response to this condition should

be characterised by determining the amount, D, by which the 1 kHz capacitance

increases when the RH is raised from <5% (a dry gas) to 75%. Using equation 2,

and 0.4 eV activation energy for surface conductivity, it can be shown that the

value of d at the test temperature is given by:-

d m

) T = 2.16 D exp (2321 (±

t c

(3)

where T

C °K is the calibration temperature and T^°K is the temperature specified

for the capacitance-ratio test.

When the calibration is performed on specimens bearing the same date code,

die-to-die variations in the value of d are observed. These variations are presumably due to differing amounts of ionic surface contamination, since the

latter is known to have a strong influence on surface conductivity. Thus, the

failure criterion for the capacitance-ratio test must be based on the average

value of d for several specimens: A sample size of 5 should normally be

sufficient, because the die-to-die variations are usually less than 2:1.

From studies performed on different production batches from a given

manufacturer, it is apparent that the batch-to-batch variations in the average

value of d are usually much less than the die-to-die variations for a single

batch. Thus, it may be possible to dispense with the need to perform regular

calibrations once confidence in the stability of the manufacturing process has

been established.

A minority of the CERDIP encapsulated components subjected to the calibration

procedure had abnormally high values of d . In order to compare these

calibration results with those obtained for other types of IC in CERDIPs, it is

convenient to normalise the value of d by dividing it by the value of the 1 kHz

capacitance. The normalised parameter,^, is called the capacitance-ratio and

it can be related to the surface conductivity by the equation:-

d> (4)

where the value of k depends on the thickness of the dielectric layers, and on the dimensions of the metallisation tracks^ For a typical IC (3), k will be

approximately equal to 2.5 x 10 (mhos/d) 2

. The values of for two types of

97
LSI components having abnormally high sensitivities to moisture are listed in

Table 1, where they are compared with results (for UVEPROMs) which are representative of the majority of components in CERDIPs.

Although the cause of the abnormally high values of tf) of some components has not

yet been identified, the fact that such behaviour is confined to CERDIPs is an indication that it may be a consequence of the surface conductivity being

enhanced by ionic contamination released by sealing glasses during the high

temperature sealing operation associated with this type of package. Because the

rate of moisture induced degradation is related directly to surface

conductivity, it seems reasonable to postulate that components having abnormally

high values of <D are more likely to fail in service. The results of a failure

analysis study which gives some credence to this hypotheses will be presented

after discussing the changes made to the test temperature specified for the

capacitance-ratio test.

TABLE 1

SUMMARY OF THE CALIBRATION RESULTS OBTAINED FOR VARIOUS TYPES OF IC IN 24 PIN

CERDIPS

j Type of IC

Value of (p for

at 0°C

a 75% RH

Range Average

Manf A 0.023 to 0.027 0.025

64k B 0.023 to 0.028 0.025

UVEPROM C 0.039 to 0.067 0.051

Custom D 0.030 to 0.150 0.085

| LSI E 0.050 to 0.145 0.100

4. REVISION OF THE TEST TEMPERATURE

All of the moisture induced failure mechanisms of ICs depend on the moisture

induced surface conductivity of the die. From the functional dependence of this

conductivity on both temperature and RH, it can be shown that the risk of

failure from a given amount of water is greatest at low operating temperatures

(2). Thus, it follows that the maximum permitted water content can be specified

in terms of the largest RH which can be tolerated at the lowest expected

operating temperature. The value of this RH can be quantified by extrapolating

experimentally derived surface conductivity data and, for the case of a requirement of a 20 year life at a lowest expected operating temperature of 0°C,

it has been shown that the maximum permitted water content would produce a 45%

RH at 0°C (2). Although this RH is too low for detection by a measurement of

the moisture induced capacitance, such a measurement can be used to assess the

98
water content if localised cooling of the die is used: The package is held at

0°C and the temperature of the die is reduced to the value (-6.5°C) at which the

maximum permitted water content would produce a RH of 75% at the die surface.

The advantages of this test procedure is that, because the RH inside the package

is assessed at the worst case operating temperature, the criterion against which

the package is judged is unaffected by the way in which the total water content

of the ''internal cavity is shared between (i) the gas phase and (ii) physical

adsorption on the walls of the cavity. The influence of adsorption is greatest

at low temperatures and, for a given water content, the RH inside a package

having highly adsorbent walls will be much less than that inside one which does

not (eg a CERDIP and a metal package respectively). Thus, it can be argued that

methods, such as mass spectrometry, which give the total water content of a package do not provide a fair indication of the risk of moisture induced

failure. Nevertheless, because mass spectrometry has been adopted as the

standard against which the accuracy of alternative techniques are judged, the

test temperatures specified for the capacitance-ratio test have been changed in

order to make the results comparable with those of mass spectrometry.

The generally accepted method of reporting the total water content measured by mass spectrometry, namely ppm , does not uniquely define the partial pressure of

water, although the latter determines the rate of moisture induced degradation.

For a given ppm value, the partial pressure depends on the total internal

pressure of the package, and this quantity varies significantly between package

types. For a CERDIP, where the total internal pressure is typically 0.5 bars, a water content of 5000 ppm corresponds to a partial pressure of 2.5 mbars, while

for a welded metal package, where the total internal pressure is typically 1 bar, it corresponds to 5 mbars. Despite this discrimination against packages

having a low internal pressure, the fact remains that the maximum water content

requirement for general purpose components is usually expressed as 5000 ppm

regardless of package type. Accordingly, the same procedure will be followed

for the capacitance-ratio test.

In order to calculate the test temperature required to produce a 75% RH inside a package containing 5000 ppm , it is necessary to estimate the amount of water

adsorbed by the walls of the internal cavity at the test condition. Thus it is

desirable to minimise adsorption by keeping most of the package hot (>15°C),

whilst using probe cooling to hold the die at the required temperature. The

equipment needed to establish these conditions is thus similar to that (5) used

for the 'dew-point 1 tests which involve plotting a moisture sensitive parameter

(such as leakage current or capacitance) against die temperature.

The test temperatures corresponding to a 5000 ppm failure criterion are listed

in the first column of Table 2. For completeness, the last two columns of this

Table give the temperatures appropriate to a maximum permitted water content

expressed in terms of (i) a partial pressure of 5 mbars (ii) a RH of 45% at 0°C.

99
TABLE 2

DIE TEMPERATURES SPECIFIED FOR THE CAPACITANCE-RATIO TEST ON THE BASIS THAT THE

PACKAGE LID IS MAINTAINED AT >15°C

Package

Type

Test Temperature (°C) for a Maximum Permitted

Water Content of : —

5000 ppm

V

5 mbars 45% RH at 0°C

Metal 0 0 -5

Side-Brazed -5

-2 0

CERDIP -13

The accuracy of the revised test is primarily dependent on that of the estimate

of the value of the physical adsorption term in the package model which is

described in the Appendix. In order to check the predictions based on this

model it is necessary to compare the results of the capacitance-ratio test with

those obtained from mass spectrometry. A limited exercise of this type has been

conducted on CERDIPs, and the capacitance-ratio test has been shown to be able

to distinguish between packages having large (>8000 ppm ) and those having small

(<2000 ppm ) water contents. However, the results presented in the next Section

have shown that it can be difficult to make a fair comparison of the two tests

when the water content is near the barely acceptable level.

The equipment needed to perform the revised test is simpler than that originally

described (2) and, because it is not necessary to wait for the whole package to

cool down, the test can be completed within 2 minutes.

5. LIBERATION OF TIGHTLY HELD WATER DURING MASS SPECTROMETRY

As part of the exercise to assess the accuracy of the revised capacitance-ratio

test, an examination was made of CERDIPs having devitrifying glass seals. These

packages, which had been in store for 7 years, were thus from an era when most

such packages had unacceptable water contents (6).

From these studies it was evident that, in addition to the water in the gas

phase and that physically adsorbed ( ie 'loosely held 1 ) on the walls of the

internal cavity, it is necessary to consider water which is 'tightly held' (eg

chemisorbed) by packaging materials. The characteristic features of these two

types of adsorption is that, whereas equilibrium between the gaseous state and

physical adsorption is achieved in only a fraction of a second, it can take many

100
months for the amount of tightly held water to reach a steady state at room

temperature (7).

The tests on the CERDIP packages showed that the elevated temperatures

associated with mass spectrometry (1) were able to drive tightly held water out

of packaging materials, and into the internal cavity. An example of this

behaviour is shown in Figure 2 which gives the plot of capacitance-ratio versus

temperature for a CERDIP before and after this had been baked at 100°C for 1 hour - a condition which is comparable with that which will be experienced prior

to mass spectrometry analysis. Before the bake, the capacitance-ratio was barely measurable over the range 10 to -20°C, and the value at the test

temperature (-13°C) corresponding to a maximum permitted water content of 5000

ppm was well below the failure criterion indicated by the cross. However,

after the bake the capacitance-ratio test indicated that the water content was unacceptable, and this was later confirmed by mass spectrometry.

From the vintage and type of construction of these packages there is no reason

to doubt that a capacitance-ratio test and mass spectrometry would have both

indicated an unacceptable water content had they been performed shortly after

manufacture of the packages. It must, therefore, be concluded that the amount

of tightly held water gradually increased during the period of room temperature

storage, and thereby caused a reduction in the amount of water which was able to come into contact with the die (and be detected by the capacitance-ratio test).

When the temperature was subsequently raised to 100°C, some of the tightly held

water would have been desorbed and this would explain the increase in the

indicated water content.

Clearly, it is essential to use standardised pre-treatments during any attempts

to demonstrate correlation between mass spectrometry and the capacitance-ratio

test, and it must be recognised that there will be circumstances where mass spectrometry will overestimate the amount of water which presents a long term

hazard to the encapsulated semiconductor.

6. THE CAPACITANCE-RATIO TEST AS A FAILURE ANALYSIS TOOL

When the capacitance-ratio test is used for product evaluation, the calibration

can be performed on specimens from a homogenous population, and it is thus

possible to obtain a reasonably accurate estimate of the response to moisture of

the specimens to be evaluated. This procedure cannot usually be followed during

failure analysis studies which typically involve components from many production

batches, but in such cases it is nevertheless possible to use the test to

rapidly identify the packages having the largest water content. To illustrate

this application, results will be given for a LSI circuit which had a metallisation corrosion failure rate of about 700 FITs within 2 years of being

used in equipment having a very low power dissipation.

Figure 3 is a histogram of the values of tf> obtained when 190 of the in-service

failures were subjected to a capacitance-ratio test based on a die temperature

of -13°C. From this data it is evident that there are two distinct groups of

specimens, namely those having a value of d> less than 0.01 and those for which

it is greater than 0.1. Because the lower of these values is typical of the

contribution due to circuit elements, it was concluded that specimens in the

101
other group had the higher water content. This supposition was confirmed byshowing that only 44/190 of the specimens had corroded metallisation and that,

without exception, these came from the group having the high value of d> . (The

remaining specimens were either classified as having failed due to a persistent

substrate bond problem or as 'wrongly removed from service').

A calibration at a 75% RH on a representative sample of these CERDIP

encapsulated specimens revealed that the response to moisture was significantly

greater than that usually observed for ICs in this type of packages: The

average value of d> for a 75% RH at 0°C was 0.09 whereas the corresponding value

for a typical CERDIP encapsulated die will be in the range 0.02 to 0.05.

However, Auger electron spectroscopy did not reveal abnormally high levels of

ionic surface contamination which could explain the 75% RH calibration results,

and there seemed to be no connection between the latter and the hermeticity

problem which was the cause of the unacceptable water content.

From the square law relationship between d> and surface conductivity it can be

concluded that, for a given water content, the surface conductivity of the

service failures would have been at least 4 times greater than that for ICs

having a normal sensitivity to moisture. Because the rate of moisture induced

degradation is directly linked to surface conductivity, such a difference could

account for the fact that corrosion failures occurred within less than two years

of the components entering service. It is, therefore, likely that future

failure analysis exercises will confirm that the results of the 75% RH

calibration procedure are an indicator of the susceptibility of a component to

moisture induced failure mechanisms.

7. CONCLUSIONS

The capacitance-ratio test procedures have been improved, and the results can now be more readily compared with those of mass spectrometry. A comprehensive

assessment of the accuracy of the capacitance-ratio test has yet to be

performed, but the results of a limited study have demonstrated the potential of

the test as a means of rapidly detecting unacceptable water contents. Although

the capacitance-ratio test is unlikely to match the precision of mass spectrometry, there are applications such as in-line monitoring, failure

analysis, and incoming goods inspection, where the regular application of the

capacitance-test would provide greater protection than would the spasmodic use of mass spectrometry.

By comparing the results obtained from the capacitance-ratio test with those

from mass spectrometry it has been shown that, because the latter can release

water which is normally tightly held by packaging materials, it will not always

give a true indication of the amount of water which presents a long term hazard

to the encapsulated die.

The results a failure analysis exercise, and of an assessment of CERDIP

packages, suggest that the 75% RH calibration procedure associated with the

capacitance-ratio test may provide an indication of the susceptibility of an IC

to moisture induced degradation.

102
8. ACKNOWLEDGEMENT

Acknowledgement is made to the Director of the British Telecom Research

Laboratories for permission to publish this paper.

9. REFERENCES

1 R W Thomas and D E Meyer, 'Moisture in SC packages', Solid State Technology,

17, 1974.

2 R P Merrett, S P Sim and J P Bryant, 'A Simple Method of Using the Die of an

IC to Measure the Relative Humidity Inside its Encapsulation' , Reliability

Physics Symposium Proceedings, 18, 17, 1980.

3 R P Merrett, J P Bryant and R Studd, 'An Appraisal of High Temperature

Humidity Stress Tests for Assessing Plastic Encapsulated Semiconductor

Components', Reliability Physics Symposium Proceedings, 21, 73, 1983.

4 R P Merrett, S P Sim, 'Assessment of the use of Surface Conductivity as a Means of Determining the Moisture Content of Hermetic Semiconductor

Encapsulations', ARPA/NBS Workshop on Moisture Measurement Technology for

Hermetic Semiconductor Devices, 1978; NBS Special Publication 400-69, 94.

5 N Bakker, 'In-Line Measurement of Moisture in Sealed IC Packages', Philips

Technical Review, 37, 11, 1979.

6 R W Vasofsky and R K Lowry, 'Moisture Evolution from Sealing Glasses: Dry

CERDIP Packages', Reliability Physics Symposium Proceedings, 18, 1, 1980.

7 C Hayashi , 1957 Vacuum Symposium Transactions, 13, Pergamon Press, New York

1958.

8 DM Young and A D Crowell, 'Physical Adsorption of Gases' Butterworths 1962.

APPENDIX

MODEL OF THE INTERNAL ATMOSPHERE OF AN HERMETIC PACKAGE

The fractional relative humidity (R) inside an internal cavity of volumn V and

surface area A at a temperature T, is a solution of the equation:-

P

o

R Vt + (Al)

where P is the partial pressure of water when the temperature is high enough

for all the physically adsorbed water to be in the gas phase, P ) is the

saturation vapour pressure of water at the temperature T, and mf

R is the amount

of physically adsorbed water per unit area when the fractional relative humidity

is R. The value of m) can be calculated by fitting a BET isotherm (8) to

measured adsorption data (4):—

103
0.21 R x

m ( A2)

)

R (0.11 + 0.98R - 1.09R 2

)

where the 'roughness factor' x is the the quotient of (i) the true adsorbing

surface area and (ii) the geometric surface area of the internal cavity. For

ceramics x will depend on the degree of sintering and values in the range 10 to

100 are likely, whereas for pressed metals it will vary from 1 to 3 (8). On the

basis that the metals and ceramics used in package construction have x equal to

2 and 10 respectively, it is estimated that the value of (Ax/V) for completely

metal packages is 3 while for ceramic or partly ceramic packages it is 10.

In the capacitance-ratio test, the die is held at the temperature T^_ required

for a barely acceptable water content to cause a fraction relative humidity (R)

of 0.75 at the die surface, while the rest of the package is maintained at a temperature which is high enough to minimise adsorption on the walls of the

internal cavity. By combining equation A1 and A2 it can be shown that can be

calculated by using:-

P = 0.75 P ) m + 0.68 A B x (A3) om s r t V

where P is the partial pressure of water corresponding to the maximum

permitted water content, and B is a factor which makes allowances for the

reduction in adsorption due to localised cooling of the die (B = 1 for the case of uniform cooling of the package and the die).

When calculating the value of P corresponding to a maximum permitted water

content expresses in terms of ppm , the total internal pressure of the package

must be taken into account. For metal packages, side-brazed packages and

CERDIPs, the total internal pressures will typically be 1, 0.8 and 0.5 bars

respectively

.

The value of (BAx/V) will depend on the way in which localised cooling is

achieved. If a heater is used to keep the top of the package at 15°C while a cold probe, having dimensions comparable with that of the die, is pressed

against the underside of the package, most of the adsorption will be confined to

the die which will have a roughness factor x of near unity. In such a case the

value of (BAx/V) is estimated to be 0.7 for metal packages and 1.5 for

side-brazed packages and CERDIPs. These values have given fair agreement

between the results of mass spectrometry and the capacitance-ratio test.

A direct measurement of (BAx/V) can be made by plotting the capacitance

parameter d versus die temperatures, and then using the 75% RH calibration data

to determine the die temperature at which the RH inside the package passed

through 75%. By substituting this temperature into equation A3 it is possible

to express the water content in terms of (BAx/V). The latter can thus be

calculated if the water content is subsequently measured by mass spectrometery

.

To date this technique has been performed on a five 14 pin CERDIP packages, and

104
an average values of (BAx/V) was found to be 2.3, ie a slightly higher figure

than the 1.5 used to calculate the test temperatures listed in Table 2. If

further studies confirm that the higher value is more appropriate, these test

temperatures will need to be revised.

FIG 1 The general features of the equipment used to measure the pin-to-pin

capacitance of an IC

FIG 2 The temperature dependence of

d> for an IC in a CERDIP (i) after

several years in storage at 20°C,

and (ii) after baking for 1 hour

at 100°C

FIG 3 Histogram of the value of d> for 190 in-service failures. The

shaded area defines the specimens

which had corroded metallisation

105
3.6 COMPARISON OF VARIOUS MOISTURE MEASUREMENT

METHODS: TEST PROCEDURES AT RADC

N.K. Annamalai*

Rome Air Development Center (RBRE)

Griffiss AFB NY 13441

(315)330-4055

ABSTRACT

A description of test procedures being developed at RADC to compare various

moisture measurement methods is given. The capacitance ratio test is suitable for go

or no-go test while the Philips capacitance technique is suitable for use with the

automatic test equipment. Other methods will be tested for sensitivity, convenience

of use in view of the coming 2000 ppm

y

requirement and the requirement for in-line

testing for moisture.

INTRODUCTION

Mass spectrometry is an accepted method

^

to determine the moisture content in

hermetically sealed packages, but this method is not simple or convenient for

manufacturers to use in quality control. Mass spectrometry is destructive and

requires skilled personnel, hence other simple methods suitable for in-line testing are being considered.

MOISTURE MEASUREMENT METHODS

A test facility to study and to compare the various moisture measurement methods is being developed at RADC. The various methods being considered are:

a. Philips Capacitance Technique

b. Capacitance Ratio Test

*Prof Annamalai is on a year sabbatical from Clarkson College of Technology,

Postdam NY 13676

106
c. Harris Corporation Sensor Chips

d. a^ 2^3 Sensor Chips

( 2 ) Philips Capacitance Technique

This is similar to the surface conductivity type. One advantage of this method is that

it does not use extra pins. The capacitance between two leads - theoretically any

two leads - of the IC chip is measured as a function of temperature.

(3)

Dry Package Circuit Model

Let us consider a dry package, i.e., no moisture present in the package. Let us also

consider any two pins of the IC chip that are connected to the metallization tracks.

Between the metal track and substrate a capacitance Ct exists. The cross section of

the IC die, when the package is dry, is shown in Figure la. The circuit model of the

die when the package is dry is shown in Figure lb. The measured capacitance is 2 Ct.

(3) Wet Package Circuit Model

Let us now consider a wet package, i.e., a certain amount of moisture present in the

package. When the package is cooled, water will condense on top of the overglaze at a specified temperature depending upon the moisture content. This moisture

introduces a capacitance CW^ between the metal track and the water layer and

another capacitance CW

2

between the water layer and the substrate (Figure 2a). The

water layer has a resistance value of RW. The circuit model for a wet package is shown in Figure 2b. Comparing Figure 2b with Figure lb one realizes that the water

layer increases the capacitance meaured between the two pins. The water layer also

introduces a resistance. Hence, as we decrease the temperature of the die, moisture

condenses and shows up as an increase in capacitance. Dew point is a function of the

moisture content.

Capacitance Plot

A capacitance plot as a function of temperature reported by N. Bakker

^ is reproduced in Figure 3. For a certain moisture content which corresponded to a dew

point of +12°C, the capacitance vs temperature plot is shown in Figure 3a. The

107
capacitance plot is shown for both cooling and heating. During cooling, at dew point,

water condenses on the overglaze and capacitance increases. The capacitance

remains high until water freezes. The water can remain in the liquid state (in the

supercooled state) up to -20°C. The capacitance drops abruptly at the freezing point,

due to the sudden freezing of the water film on the chip. Ice has a much lower

conductivity than the water film. During heating capacitance increases since the ice

melts. Upon further heating the water evaporates and the capacitance decreases. At

above the dew point, all but a few monolayers of water has evaporated. Above the

dew point, the heating curve coincides with the cooling curve. For an IC package

with a moisture content too low to be detected, having a frost point below -25°C, the

capacitance plot is shown in Figure 3b.

Moisture Determination Using Nomograph

( 4 ) A nomograph for dew points and moisture content in ppm

y

as a function of

pressure is shown in Figure 4. The dew point for a package is determined from

capacitance vs temperature plot. Knowing the package seal pressure and

temperature, pressure at dew point could be obtained by using the relationship Pj/T^

= where Pj = pressure at time of seal, Tj = absolute temperature at time of

seal, P^ = pressure at dew point, T

2

= Absolute temperature at dew point. Having

determined the pressure at dew point, moisture content of the package in ppm

v can

be obtained from the nomograph.

Capacitance Plot at Various Frequencies

The capacitance plot as a function of temperature is shown (Figure 5) at various

(3) frequencies . The capacitance peak increases sharply at low frequencies. This can

be understood by referring to Figure 2b.

Capacitance Ratio Test

The capacitance ratio test is an extension of Philips capacitance test. The fact that

the capacitance of a wet package is frequency dependent is the concept used in this

test. Measuring a difference in pin-to-pin capacitance at two frequencies while

holding the die at constant temperature (dew point for a given moisture level) and

comparing to a calibration value yields a pass/fail determination. This method is

sensitive enough to determine 5000 ppm

y

moisture level. This method may not be

sensitive enough to determine low levels of moisture. This can be understood by

108
referring to Figure 2b and noting that low levels of moisture means a large RW.

Large RW implies an open in that branch and the capacitance measured between pins

is only 2 Ct (dry package capacitance). A detailed description of this method can be

(3 5) found elsewhere ’

.

Harris Corporation Sensor Chips

Harris Corporation sensor chips are the surface conductivity type. These are in the

form of chips fabricated in the integrated circuit production line by vacuum

depositing aluminum over oxidized silicon followed by a photoresist step to delineate

closely spaced interdigited metal strips. The chips are mounted inside the package to

be tested, bonded for external electrical connection and the packages are then sealed

by the applicable process to be investigated or monitored. When the temperature of

the package is reduced, water condenses on the sensor chip and leakage current

increases. Sensor chips are shown in Figure 6a and the corresponding leakage current

as a function of temperature is shown in Figure 6b. Moisture content in ppm

y can be

determined by using the nomograph if all the moisture can be condensed on the chip.

A diode is also fabricated on the chip for use in temperature determinations. The

chip uses up two extra pins not counting diode leads, but it is a non-destructive test.

Al^Oj Sensors

This is a volume effect sensor. The number of water molecules absorbed on the pore

walls of A^O^ determines the total complex impedance of the sensor. A detailed

description of this device can be found in references 6 and 7. This sensor requires a

calibration procedure. The sensor uses up two extra pins. A major concern is the

aging problem associated with this sensor.

TEST PROCEDURES

A setup is being established at RADC to change the moisture content and test

the device under various moisture conditions. This setup will be useful to test various

methods and sensor chips. An environmental chamber made by Tenney Jr. equipped

with wet a bulb and dry bulb thermometer controls is used for establishing a desired

moisture content in the chamber. A small plexiglass box as shown in the photograph

(Figure 7) is mounted inside this environmental chamber. The IC chip (1) with the top

109
lid removed will be mounted inside the small plexiglass box (2). The plexiglass box

along with the bottom plate (3) is the simulated package. A small window (4), with

the lid (5) open, keeps the small plexiglass box in equilibrium with the environmental

chamber. During the test the window is closed to prevent any further moisture

getting into this small box from environmental chamber. This small box is not

hermetically sealed. To take care of this problem, testing has to be done at a

sufficiently fast rate. An alternative design is to provide "O" ring seals at all openings. The coaxial connectors (6) are used for electrical connections.

Temperature of the die is monitored by the diode on the chip. The die is cooled by a thermoelectic cooler (7). A heat sink (8) is provided for heat dissipation. The

thermoelectric cooler is capable of cooling the die to -30°C. The environmental

chamber dry bulb is kept at 40°F and the wet bulb is set at a temperature

corresponding to a desired moisture level in ppm

y

. Dew point sensors require cooling

of the device and analyzing their response. The temperature and the response

(capacitance or sensor current, etc) will be recorded by a Bascom Turner Instruments

8120R programmable plotter. The plexiglass box is quite large (infinite moisture

source) compared to the IC package. Hence, the test does not represent a real life

situation but is useful to compare the sensitivity of various moisture measurement

methods. Another set of experiments planned are to seal CERDIP packages with

known moistures and test them by various methods.

CONCLUSIONS

The two set of experiments planned will permit us to compare various moisture

measurement methods and choose the appropriate one for in-line moisture

measurement and also for use with automatic test equipment. The selection will be

guided by the present 5000 ppm

y requirement and also by the future requirement of

2000 ppm

v

- REFERENCES

1. Benjamin A. Moore, "Mass Spectrometer Measurements at RADC," Proceedings

of the ARPA/NBS Workshop V held at the National Bureau of Standards, Gaithersburg

MD, March 22-23, 1978.

2. N. Bakker, "In-line Measurement of Moisture in Sealed IC Packages," Philips

Telecommunication Review, Vol 37, No 1, March 1979.

no
3. R. P. Merrett, S.P. Sim, 3. P. Bryant, "A Simple Method of Using the Die of an

Integrated Circuit to Measure the Relative Humidity Inside its Encapsulation,"

Proceedings of the 18th Annual Relaibility Physics, Las Vegas NV, April 8-10, 1980.

4. Nomograph reproduced from Harris Corporation Moisture Sensor Chip Technical

Bulletin.

5. R. P. Merrett, "Moisture Measurement by the Capacitance-Ratio Test," RADCTM-82-2.

6. Kovac, M.G., Chleck, D., and Goodman P., "A New Moisture Sensor for In Situ

Monitoring of Sealed Packages," Proceedings of the 15th Annual Reliability Physics

Symposium, Las Vegas NV, April 13-15, 1977.

7. Finn, 3.B. and Fong, V., "Recent Advnaces in Ai ? 0^ In Situ Moisture Monitoring

Chips for CERDIP Package Applications," Proceedings of the 18th Annual Reliability

Physics Symposium, Las Vegas NV, April 8-10, 1980.
METALLIZATION TRACK

OVERGLAZE

Ct Z- —c,OXIDE

CONDUCTING SUBSTRATE

Figure la

Cross section of an IC die (no water layer)

AC SOURCE

Circuit Model (no water layer)

112
ABSORBED WATER LAYER

ZZZZZ7ZZ3

CW

CW 2

OVERGLAZE

Ct OXIDE

SUBSTRATE

Figure 2a

Cross section of an IC die with absorbed

WATER LAYER

Figure 2b

Circuit model (with water layer)
Figure 3 (A) Capacitance plot as a function of temperature

DEWPOINT AT +12C

(B) IC OF THE1 SAME TYPE WITH VERY LOW MOISTURE

CONTENT

Device Tested: m Pin DIL Package

114
100,000 80,000 60,000

50.000 40.000

30.000

20.000

10000 8000

6000

5000 4000

3000

2000

1000

800

600

500 400

300

200

100

8060

5040

30

20

10

UJ2

3

=jO

>

>

COS

Q.

Q>H

Z

UJK

Z

o

O

UJQC3

H

W

O

s

+60-

+50-

+40-

+30-

+ 20 -

+ 10 -

0-

O

©

- 10—

- 20 —

-30—

5

CL

C/3 o

cc -50— _

§

UJ -60- 3

+ 140

+ 120

- + 100

•+80

+60

.+50 .+40

-+30

-+20

-+10 0 . .-10

.-20

--30

.-40 —50

-60

-70 —80 —90

-70

-80

H

Z

5

a.

C/3 o

0C2

UJQ

—100 —110

Figure 4 - Nomograph for Dewpoints a Moisture Content in ppm

AS A FUNCTION OF PRESSURE

115
! dew point 1

-40 -20 0 20°C -40 -20 0 20 -40 -20 0 20°C

aJ b) cl

Figure 5 Capacitance versus temperature curve of

AN IC AT (A) 1 KHz (B) 10 KHz AND (C) 100 KHZ

Device Tested: 1M Pin DIL Package
CELL 4 CELL 3 The HI-55001 has four electrically identical moisture sensing cells on chip.

The user may apply any one cell or

parallel combination of cells in a moisture measuring experiment. The in- dividual cells are accessed as follows:

Cell 1 - pads 2 & 3; Cell 2 - pads

4 & 5; Cell 3 - pads 8 & 9; Cell 4 -

pads 10 & 1 1 respectively.

Chip Si/te: 95 mils by 50 mils.

Figure 6(a) - Harris Corporation Sensor Chip

Figure 6(b) Sensor chip current as a function of temperature.

Dew point -18°C for a package cavity pressure of

0.45 ATMOSPHERE. THIS TRANSLATES TO APPROXIMATELY

2800 PPM

U

MOISTURE
Figure 7 Set up for various moisture measurement methods

INSIDE THE ENVIRONMENTAL CHAMBER

118
3.7 LASER SPECTROMETRIC DETERMINATION OF MOISTURE

John A. Mucha

AT&T Bell Laboratories

Murray Hill, New Jersey 07974

(201) 582-3659

ABSTRACT

Derivative infrared diode laser spectrometry has been used for the rapid and precise (±50 ppmv) determination of

trace (^ 50 ppmv ) moisture in small (10-100 ^L) samples of gas as they expand into an evacuated absorption cell. Adsorption/desorption of the analyte is the most serious potential source of error in the measurement; however, the

application of time-resolved (resolution < 10 msec) techniques permits accurate analyses in the presence of these

dynamic processes. The experimental method and a simple model which accurately reproduces the concentrationtime behavior observed during analysis are presented which permits the analyst to ensure the accuracy of any given

measurement or detect and correct erroneous ones. The method has been applied to the determination of moisture in hermetically sealed integrated circuit (IC) packages and is shown to be capable of determining both gaseous and

adsorbed moisture within the sample. Results for the packages sealed under controlled moisture conditions exhibit

large discrepancies when compared to those obtained by mass spectrometry. Potential sources of the deviations are

discussed along with advantages of this new method for IC moisture measurements.

INTRODUCTION

In this age of miniaturization, analysts are being pressed to develop new methods and tools with sufficient sensitivity

to analyze smaller and smaller samples. Nowhere is this more true than in the microcircuit industry where the establishment of reasonable specifications for moisture in hermetic integrated circuit packages has been hampered by

the inability to accurately and reliably make the measurement. For the past 10-15 years mass spectrometry has been

the method most widely used El]; however, its history has been checkered by lack of agreement and correlation

between laboratories [2,3]. This is not so surprising when one considers that the volume enclosed within a memory

device package typically contains between 0.01 atm-cm3 and 0.1 atm-cm3 of gas which at a typical moisture

specification level (5000 ppmv) contains an amount of moisture that would form monolayer coverage of a few (3-10)

cm2 of surface. Since the surface area of most laboratory apparatuses into which the sample is injected are generally an order of magnitude or more greater (i.e., several hundred cm2), accuracy is limited by adsorption and desorption

phenomena associated with the sampling process.

Here, we report a new method [4] for the determination of moisture in hermetically sealed integrated circuit packages using a derivative infrared diode laser spectrometer. Because of the high resolution (—0.00 1 cm

-1

) and

sensitivity of the instrumentation, trace analyses can be made on small volumes of gases with high selectivity.

Furthermore, the time resolution ( —0.01 sec) capabilities allow a complete history of the adsorption/desorption

dynamics to be obtained in such a way that the analytical accuracy of results for each sample can be verified. EXPERIMENTAL

A commercial Spectra Physics (Laser Analytics Division) semiconductor diode laser spectrometer was used for the

studies presented here. Injection current modulation at 1 kHz was employed for derivative detection of water vapor

spectra and the basic instrument has been modified for line locking. The modified spectrometer and a more detailed

description of the gas handling and vacuum systems has been published elsewhere [5]. As before, the Pb S| ,Sc, diode laser was locked to the 2^ lot transition of water vapor at 1653.29 cm

-1 and second harmonic detection was employed to detect the second derivative of sample absorption.

Additional modifications [4] were made to the spectrometer and sample cell in order to make the s\stcm suitable for the analysis of small volumes of gases. The time constant on the lock-in amplifier used for sample detection w.:

119
reduced to 3-10 msec in order to accurately monitor the time dependence of water vapor in the sample cell during

analysis. The output of the lock-in amplifier indicating sample absorption was continuously monitored as a function

of time using a chart recorder and microcomputer.

The most extensive modifications were those made to the 16 cm Pyrex sample cell in order to calibrate the

instrument and open the integrated circuit (IC) packages for analysis. A diagram of the modified sample cell is shown in Figure 1. The calibration arm shown in the upper right portion of the figure contained a leak valve and

small (55 yiL) simulator volume for continuous and grab sampling, respectively, of a flowing standard water vapor

mixture. The simulator volume, equilibrated with the standard (nominally 3000 ppm H20 in N2) under flow

conditions, provided a convenient method for obtaining fresh (age < 100 msec) samples of known composition for elucidating the kinetics of the analytical method. In addition, the accuracy could be verified by direct comparison to results obtained using the leak valve to admit a continuous flow of standard at an identical pressure as that observed

for the simulator.

The center arm of the sample cell contained a device for opening the IC packages and an MKS Baratron capacitance

manometer for pressure measurements. The whole opener assembly was wrapped with heating wire and insulated. In this way the samples could be analyzed at any temperature while other parts of the system (cell, calibration arm, and pressure transducer) remained at room temperature.

The basic procedure consisted of evacuating the cell to the base pressure (4 x 1CT6 — 1 x 10

-5 Torr), isolating the

cell from the vacuum pumps and monitoring moisture as a function of time before and after the contents of the

simulator volume or packages were released. The second-derivative signal which had been previously calibrated in units of absolute partial pressure of moisture (using continuous flows of standard at known pressures) along with

total pressure measurements were sufficient to determine the moisture composition in the cell at any time.

MEASUREMENTS WITH THE SIMULATOR VOLUME

Because of the unique design of the calibration arm it is possible to make a direct comparison of measurements on

the same gas as a small parcel in a closed system and as a continuous flow at steady state conditions in an open

system. One such comparison is shown in Figure 2 for a moisture standard containing 3200 ppmv H20 at room

temperature. The solid curve represents the experimental time dependence of the calibrated second derivative

infrared signal when the contents of the simulator volume are released (t=0) into the sealed-off cell. In this instance

the cell was isolated from the vacuum pumps 1 3 sec before the release and in the interim, a noticeable increase in the

signal is observed due to desorption of residual moisture from the cell walls until a stable equilibrium is achieved.

The amount of moisture desorbed depends, of course, on the previous history of cell with respect to previous exposures to moisture and the duration of evacuation between experiments. Usually, at room temperature it is relatively easy to obtain an insignificant amount of desorption by pumping the cell for several minutes between

releases after an initial pump-down of several hours. However, as will be shown later, the presence of desorption in no way limits the experimental accuracy and pump down procedures do not have to be stringently controlled. Opening the simulator volume (t=0) produces a dramatic, transient increase (T) in the moisture content of the cell which is followed by a rapid decay as moisture adsorbs to the surfaces of the system. After a period of time (40-50

sec) a new final equilibrium level is observed corresponding to (in this instance) —15% of the moisture content of the simulator volume on the walls of the system at the end of the experiment.

One might be tempted to use the final equilibrium moisture content for analytical purposes; however, such an approach requires carefully prescribed evacuation cycles to ensure reproducible moisture sorption on the walls of the

analytical system. This was judged to be too cumbersome for practical analyses.

The logical alternative then is to use the transient observed at opening for analyses. The accuracy of this approach

can be determined using a continuous flow of standard at the same pressure as that noted for the parcel (0.229

Torr). Since we are dealing with trace analysis where the bulk of the sample gas (N2 or air) is non-adsorbing, the

final pressure is an accurate (usually ± 1%) indicator of the total amount of gas in the sample. The dashed line at the top of Figure 2 represents the experimentally observed steady-state signal amplitude when a continuous flow of

standard is passed through the cell at 0.229 Torr. As can be seen the steady-state signal amplitude (CF) is identical

to the transient signal (T). Thus, the transient is not only an accurate indicator of moisture content, but also is an

exact replicate of the standard with no interference or error due to adsorption or desorption. Of course, this
condition is only present for a brief period of time and the ability to achieve this condition depends critically on the

dynamics of the ongoing processes in the closed system.

KINETIC MODEL

The fact that the transient signal exactly matches the continuous flow signal is a very gratifying circumstance;

however, such an observation places severe restrictions on the interplay of the dynamic processes which are an

integral part of the analysis. To gain some insight into these restrictions and to establish guidelines for ensuring

analytical accuracy, a simple model has been constructed based on a series of first-order processes. Consider the

simple sequence of steps

k, k2 A — B ^ C (1)

k-2

as a representation of the microvolume sampling process. Here A represents gaseous moisture in a sample at atmospheric pressure; B is gas phase moisture in the cell at reduced pressure; and C is moisture on the walls of the

cell. The rate constants k l5 k2 , and k_2 are first-order rate constants associated with sample opening and expansion

into vacuum, adsorption, and desorption, respectively. As noted previously, the amount of moisture in a typical

i sample is sufficient for monolayer coverage of only a few cm2 of the 625 cm2 surface area of the system.

Furthermore, in a cell evacuated to between 2 x 10

-5 and 5 x 10-6 Torr a simple calculation based on the kinetic theory of gases [6] indicates the surface coverage due to physical adsorption is on the order of 2 x 10~3 to .04 monolayers prior to introduction of the sample. As a result, desorption and adsorption (the second step of eq. (1))

are likely to be nearly first-order processes since the surface coverage by analyte before and after sample introduction

will remain small. The use of first-order kinetics to describe the first step is tenuous since the expansion of gas into

the cell is a flow problem varying continuously from turbulent to transitional flow complicated by diffusion through

connection tubes. Its justification rests solely on the simplicity of the resulting model and the ability to fit experimental data relatively accurately.

The general equations for the time dependence of the concentration of each species for the above kinetic model were

given by Rakowski [7]. Here we are only interested in the temporal behavior of B, the analyte in the gas phase after opening, since it is the only measurable quantity. We desire to know under what conditions Bmax, the maximum

concentration detected (more specifically the difference between the observed maximum and moisture level immediately prior to opening, Bo), approaches A0, the concentration of moisture in the original sample (i.e., (Bmax-BoVAo-l).

The simplest cases occur when the initial concentrations B0 and C0 are either zero or represent an equilibrium

situation (Bo/C0 = K

_1

, K = k2/k_2). For these cases the time dependence of B normalized to A<, is given by

B(t)-B0 A0

k2 k2 +k_2

ki(k2+k_2)e (M' k-,)t

k2 + k_2 — ki

(kj-k-2) (k2 +k_2) -[k.-Ocj+k.ph

k,k2 e ( 2)

Locating the maximum is simply a matter of differentiating eq. (2) with respect to time and leads to the time to reach a maximum concentration,

ln[ (k! —k_2)/k2 l k,-<k2 +k_2>

with that concentration given by

1 21
(4) Bmax Bp

1+K

1 + K e

(kj+k-j)l~

Similarly, at long times eq. (2) yields the equilibrium concentration

®eq B0 1 A0 = 1+K (5)

which by experimental observations permits the analyst to obtain the ratio k2/k_2 (=K) directly. Examination of eq.

(3) indicates that a maximum will always be observed when k[>k_2.

Since most of the analyses of packages and simulator volumes exhibit a time dependence like that shown in Figure 2, eqs. (3) and (4) are most useful for assessing the analytical accuracy of using the transient signal. One can

arbitrarily choose relative rate constants for the system dynamics and determine the effect on (Bmax—BJ/Aq. Figure 3 shows one appropriate representation of the results of such a procedure. Here the transient observed

concentration (Bmax—Bj relative to the true sample concentration (Aj is presented as a function of the relative rate constants of opening and expansion to adsorption (k x /k2) for several different overall cell adsorption characteristics governed by the equilibrium constant, K. When K is small the cell is essentially non-adsorbing and large values of K

refer to strongly adsorbing walls. Solid lines indicate conditions for which a concentration maximum is observed

while dashed lines indicate no detectable maximum—only a rise in concentration to the equilibrium level (i.e., ki<k_2

). First one sees that when kx /k2 >100, the transient signal and thereby the detected concentration is always

within 5% of the true value regardless of the adsorption characteristics of the cell. Second, when a maximum is observed experimentally the determined concentration can never exceed the true value. Thus, the absolute accuracy

of any given measurement can be determined by obtaining K and the relative rate constant k!A2

- In most cases, k x /k2

is sufficient. Since the time history has been recorded for both standard and sample such a determination can

be made by computer simulations using eq. (2) and judicious choices of the three system rate constants. The use of Figure 3 to assess and accurately correct errors in analysis rests on the validity of the model in describing

the system dynamics and experimentally achieving the initial constraints (Bo=Co =0 or B0/C0=K_1

) imposed

previously. Since the analyst can monitor moisture prior to opening the sample or simulator volume to the system, he can easily determine that the latter does indeed apply. Actually, this constraint can be relaxed by modifying the model slightly to include desorption prior to opening the sample. This simply requires that one allows the second step

in process (1) to proceed with Co ^0 for the period of time from when the cell is isolated from the vacuum pumps

until the sample is opened. For the case 0,^0 and Bo =0 (no gas phase moisture detectable during active evacuation) the return to equilibrium when pumping is terminated should be first order with an effective rate constant (k2 +k_2). Thus, one can model the whole time dependence from the time the cell is sealed until equilibrium

is established after the sample is introduced.

This provides a very important test for the validity of the model in describing the dynamics of the system. Based on

the initial model presented one knows that since the transient signal is identical to the flow signal, k!»k2

. Furthermore, as observed in Figure 2 the cell is strongly adsorptive, k2 »k_2

. This suggests that kj»(k2 +k_2). Thus, desorption before introducing a sample and adsorption after should obey the same kinetics if the model is a reasonable representation of the dynamics of the system. The dots in Figure 2 are a theoretical fit to the data which

confirms the validity of the model.

This fit was obtained using the rate constants k x =7 5 s_l

, k2 =0.25 s_l and k_2 =0.086 s-1

. The only other adjustable

parameter was the relative amount of moisture on the walls (C(t=— 13s)/Ao) when the cell was sealed. This was

determined in the fit to be 0.4 A<>. Since A<j is known for the simulator volume of standard it is a simple matter to compute the actual number of molecules of H20 on the walls when the cell was sealed. This along with the geometric surface area of the system indicated an initial surface coverage of 0.03 which is in excellent agreement

with the kinetic theory surface coverage noted earlier for a cell at 10~5 Torr. A noticeable deviation between experiment and theory is present during the decay in Figure 2; however, this deviation is only 10%. Observations of numerous decays following the release of standard gases (concentrations from

1 22
1000 to 10,000 ppmv ) into the cell have indicated that the decay is actually bi-exponential with the two decay

constants differing by approximately an order of magnitude. This is not surprising considering that metal and glass are present in the system and would likely exhibit different adsorption characteristics. The ability to obtain as good

a fit as that shown is Figure 2 speaks well for the model chosen to represent the system. In addition, initial decay

rates observed for the first 0.5 sec of the decay were accurately first order (±5%) for the above concentration range.

APPLICATIONS OF THE MODEL

In practice, we are more concerned with the few seconds in time before and after the sample is opened for establishing the accuracy of any given measurement. Extensive measurements on releases of the simulator volume

over a year and a half have indicated that k2 =0.5 ± 0.1 s-1 and k_2 =0.05 ± .02 s-1 provide an excellent fit (±2%)

to this critical portion of the analytical time dependence. It should be noted that these two rate constants represent

properties of the cell and are relatively invariant for any analysis (so long as modifications to the system are not made). Since opening an IC package involves a mechanical device and an operator, it is likely to exhibit some

variability. Therefore, one cannot arbitrarily assume that kj for a sample will be the same as kj (75 ± 15 s -1

) for releasing the simulator contents. Determining k! for a sample is simply a matter of measuring t max using the high

resolution time data stored in the microcomputer and eq. (3). From the ratio k^2 and Figure 3 (a more extensive

grid for K values can be computed) corrections can be applied to samples which are not opened rapidly enough to satisfy the condition k] » k2

. Rise times observed for typical samples using our opening procedure were typically 111 ± 22 msec compared to 66 ± 10 msec for releases from the simulator volume. This indicates that concentrations obtained for samples are likely to be lower than the true values. For the worst case tmax =133 msec,

ki/k2 = 60 indicating the maximum error to be less than 7%. The important conclusion here is that based on measurements of k! for the simulator volume, moisture detected in the cell is always within 3% of the true value

established by continuous flow calibration and simulator volume releases can be used by themselves to calibrate the instrument. However, simulator volume releases cannot calibrate the kinetics of sample opening. Fortunately, the method of opening packages here is adequate for accuracy to within 5%. More importantly it is correctable.

Although the discussion of this type of error has relied heavily on real samples where the sample concentration is unknown, it is possible to conduct experiments using the simulator volume and continuous flow standards to test the

ability of using the model to correct experimental results. For example, increasing the time constant of the detection

electronics to 1 sec artificially changed k! and yielded a transient signal less than 50% of the continuous flow signal.

From the observed t max, k! was accurately determined to be the reciprocal of the lock-in time constant (12 dB roll- off) and data correction using Figure 3 retrieved the continuous flow signal value to within 2%. Similarly, by

installing a wax plug with a 90 nm hole in the bore of the simulator volume an initial error of 1 7% (continuous flow

vs transient) was observed; however, rate constants were easily extracted and results were correctable to better than

1%.

SENSITIVITY AND REPRODUCIBILITY

Figure 4 shows the time dependence for the analysis of a 0.066 atm cm3 sample of gas containing 325 ppmv moisture

Based on the signal-to-noise ratio observed, the detection limit (S/N=l) corresponds to approximately 2 ppmv per atm cm3 sample of gas. Noting that the detection limit scales linearly with the volume to path-length ratio (VJb) of

the cell, a general detection limit of

D.L. (ppm) = 0.2 (Vc/b) (PS VS)-' (6)

should be applicable to any system when derivative diode laser spectroscopy is used for the determination of moist u re in samples containing PS VS atm-ern3 of gas. The apparent noise in Figure 4 is quite regular and is associated with

optical interferences due to vibration (3 Hz) of the laser in the cold head of the closed cycle refrigerator used to control the temperature of the laser. Improved vibration isolation is available [8] which should virtu alls eliminate

this problem and reduce detection limits correspondingly. This and incorporating an optimized cell (smaller \ m

are likely to increase sensitivity by at least two orders of magnitude.

1 23
Measurements on simulator volumes of standard are exceptionally reproducible. Over a typical 8 hr. period (see

Figure 5) precision was found to be on the order of 1.5% (Rel. Std. Dev.) for a 3100 ppmv standard.

COMPARISON WITH MASS SPECTROMETRY

Table I shows the results [9] for TO-18 packages tested as part of a recent Method 1018.2 recertification program.

The mass spectrometric (MS) results (columns 2-5) are seen to exhibit generally good agreement with each other

and the target values for the test packages. The infrared (IR) results at 25 "C also agree well; however, IR analyses

at 100°C (the ones that should agree) indicate much higher moisture contents — well above statistical variations. It

is not surprising the 25 °C IR analyses are in reasonable agreement with target values since the test packages were

equilibrated and sealed at approximately this temperature. The higher results at 100°C are not surprising either since one would anticipate an equilibrium amount of moisture to be present on internal package surfaces. This

moisture goes undetected at room temperature since desorption is a much slower process than expansion of the gas

into the IR cell. At 100°C the equilibrium within the package shifts strongly to favor desorption of surface moisture

and the IR analyses give the new total gaseous moisture (original surface & original gaseous).

On the other hand, internal correlation studies [10] on Western Electric production packages and controlled packages

prepared by the methods of White and Sammons [11] have consistently exhibited agreement between the IR method

and the MS spectrometric method employed at Bell Labs/Allentown [12]. These studies do indicate that the MS

and IR methods can agree quantitatively and that the observed discrepancies in Table I are either due to an

accelerated aging effect [9] that perturbs the moisture content of the package or a systematic error in the MS

results. The former interpretation [9] has merit since all analyses of these packages made nine or more months after the

original certification study were in agreement with the 100°C IR data. These subsequent results included MS data

from RADC which disagreed with the IR data originally and Bell Labs/Allentown which has always been in agreement with the IR data. Since the IR results were obtained on packages that were at temperature for more than an hour longer than those of the MS studies a thermally accelerated aging effect cannot be ruled out. However, the

possibility of a systematic error in the MS data cannot be overlooked either. Simple calculations [6] of the moisture

that might be anticipated on the internal surfaces ( —-0.4 cm2

) of the TO-18 packages indicates that multilayer

adsorption is expected, even for packages sealed at the lowest concentration (1000 ppm). Furthermore, only five monolayers are required to explain the largest discrepancy observed. This quantity of moisture is not unlikely based on experimental studies [13] of physisorption of moisture on package materials. Thus, concentrations considerably

higher than the target values are quite likely at an analysis temperature of 100°C. Model calculations on the MS

method, similar to those presented earlier for the IR method, were carried out in order to gain some insight into systematic error as a possible origin of the observed discrepancies. Using such an approach, two possibilities were

identified that could lead to errors in the MS data of the correct direction and magnitude. First, if the package

opening procedure leads to a mass transport rate out of the package that is comparable to the transport rate into the

ionization chamber, then burst mode calibration will fail to adequately "calibrate" the dynamics of the MS mass

transport. This would certainly make MS results based on "peak" measurements systematically low; however, it should not effect integrated measurements unless background moisture levels are comparable to that detected for the package. Second, if mass transport between the antechamber and ionization chamber is rate limiting, there may be

sufficient time for moisture to desorb from the walls of the 3-volume calibrator and be included in the calibration

measurement. This additional moisture would have the net effect of reducing the apparent moisture content of a package for both "peak" and integrated MS determinations. Although, such modeling is by no means exact, it does

suggest areas for closer scrutiny of the MS procedures.

CONCLUSIONS

A rapid and accurate analytical method utilizing derivative infrared diode laser spectrometry has been demonstrated

for the determination of trace moisture in small volumes of gases which are opened to a larger, adsorbing analysis

system. Since its inception time-resolved determinations of moisture using the diode laser spectrometer have never been in error by more than 5% based on a kinetic analysis of the data.

1 24
For the analysis of moisture in hermetic integrated circuit packages the infrared method offers a superior alternative

to M1L-STD Method 1018.2 based on mass spectrometry. In addition to higher sensitivity and precision, the timeresolved method is highly insensitive to the moisture exposure history of the system walls and does not require

cumbersome procedures for evacuating and conditioning that are inherent to the mass spectrometric determinations.

The sample throughput of the laboratory instrument demonstrated here (50 packages per day) is almost a factor of

two greater than that by the MIL-STD Method and should easily scale to a system capable of analyzing several hundred per day. Furthermore, since it is not necessary to heat the walls of the analytical system it is possible to study the effects of temperature on the sample package. By comparing results on representative samples at room

temperature and 100°C one can estimate the amount of moisture on the internal surfaces of the package. This

quantity is probably more important in assessing moisture effects on IC reliability. Since failure modes such as corrosion and current leakage are likely to involve ion mobility through a fluid surface medium (H20) the amount of

moisture on surfaces is an extremely important piece of information.

REFERENCES

1. Thomas, R. W., IEEE Trans., Parts, Hybrids and Packaging, 1976, PHP-12, 176.

2. Thomas, R. W. and Meyer, D. E., Solid State Tech., 1974, 17 (9), 56.

3. Perkins, K. L., in "Semiconductor Measurement Technology: ARPA/NBS Workshop V, Moisture

Measurement Technology For Hermetic Semiconductor Devices", H. A. Schafft, S. Ruthberg, and E. C. Cohen, eds. (NBS Spec Pub 400-69, 1981) p. 58.

4. Bossard, P. R. and Mucha, J. A., IEEE Proceedings of the 19th Annual International Reliability Physics

Symposium, 1981 , p. 60.

5. Mucha, J. A., Appl. Spectrosc., 1982, 36, 393.

6. Robinson, N. W., "The Physical Principles of Ultra-high Vacuum Systems and Equipment" Chapman and Hall

Ltd: London, 1968; Chapter 7.

7. Rakowski, A., Zeit fur Physik. Chemie 1907, 57, 321.

8. Spectra Physics, Laser Analytics Division, Bedford, MA 01730.

9. Moore, B. A., IEEE Proceedings of the 21st Annual International Reliability Physics Symposium, 1983. 10. White, M. L., Mucha, J. A. and Angst, D. L., unpublished results, 1981.

11. White, M. L. and Sammons, R. E., "NBS/RADC Workshop: Moisture Measurement Technology for Hermetic Semiconductor Devices, II", NBS Spec. Publ. 400-72, p. 49 (April, 1982).

12. Although most of the results are unpublished a general idea of the level of correlation can be gain by

comparing refs 4, 11 and Gale, R. J., "NBS/RADC Workshop: Moisture Measurement Technology for Hermetic Semiconductor Devices, II", NBS Spec. Publ. 400-72, p. 19 (April, 1982).

13. Ammons, J. M., Hoff, G. R. and Kovac, M. G., "The Physisorption of Water onto Integrated Circuit Package

Components," this monograph.
Table 1: Results* of R.A.D.C. TO-18 Round Robin (December, 1981)

-MASS SPEC. AT 100 C- -INFRAREDTARGET RADC LAB 1 LAB 2 LAB 3 100 X 25 C

1000 2663 2800 1981 3067 5420 2639

3000 4535 5730 3317 8690 8630 3724

5200 5327 5670 5631 4453 10,250 4753

6000 6040 7800 5417 6676 12,257 5460

10,000 10,060 9900 8301 9170 14,220 8897

* All values in ppm by volume moisture

Figure 1: Infrared sample chamber.

126
K = 0

Figure 2: Time dependences for a simulator volume

release ( ) and continuous flow ( ) of

standard along with theoretical fit (•) to the data.

Figure 4: Time dependence for the analysis of an IC

package containing 325 ppm moisture.

Figure 3: Relative kinetic parameters for assessing

analytical accuracy.

3100 ppm

IN 55/z£ CALIBRATED

VOLUME

T = 25°C

Figure 5: Typical reproducibility over an 8 hr period. Data includes variations in instrumcnt.il

sensitivity due to laser retuning and variations in I.i( bubbler standard due to minor changes in r<*>m temperature.

127
3.8 TOTAL WATER MEASUREMENT EQUIPMENT AND PROCESS

USING METHOD 1018, PROCEDURE 2

Craig Sloneker and Bruce Church

Medtronic, Inc.

6951 Central Avenue

Minneapolis, MN 55432

(612) 574-4761

ABSTRACT

Several systems are currently used for the determination of total water content of

a hermetic package. Each has its advantages and disadvantages. The analysis sytem

to be presented was originally designed and developed, observing Method 1018 of

MIL-STD-883B, to provide a flexible, economical method of determining water content

of implantable pulse generators (pacemakers) for process control.

Unique features of this water analyzer include its ability to accommodate various

package sizes and perform an analysis with little technical attention.

Typical operational procedures for sample runs, calibration of the instrument, and

equipment description with a descriptive layout will be presented. Both assets and

drawbacks of this system will be discussed in an attempt to provide the audience

with enough information to assess whether this would be a viable moisture

measurement technique for their process applications.
4. SESSION III MOISTURE SENSORS

4.1 FUNDAMENTAL STUDIES OF THE AGING MECHANISM IN ALUMINUM OXIDE THIN FILMS

Chen-Hsi Lin and Stephen D. Senturia

Department of Electrical Engineering and Computer Science, and

Center for Materials Science and Engineering

Massachusetts Institute of Technology

Cambridge, MA 02139

ABSTRACT

This paper reports the results of electrical and infrared studies of aging mechanisms in hydrated aluminum oxide films of the type typically found in aluminum oxide

moisture sensors. Three distinct components of the water of hydration are identified, referred to as "physisorbed", "chemisorbed”, and "fixed". The physisorbed

component adsorbs and desorbs reversibly at room temperature. Furthermore , presence of the physisorbed layer is found to enhance the aging rate at room temperature. The direction of aging is such as to indicate successively lower apparent

moisture levels. Isotope exchange experiments show that the chemisorbed component

is stable in dry room temperature ambients, but exchanges rapidly with the physisorbed layer in moist ambient. Annealing studies show that the fixed component is

stable up to 250°C, above which a gradual phase transition of the hydrated oxide

from pseudoboehmite to an a-alumina phase is observed. This result suggests that

exposure of aluminum oxide moisture sensors to eutectic die attach and Cerdip

sealing temperatures should be expected to produce an irreversible shift in moisture sensor characteristics.

INTRODUCTION

The use of thin porous hydrated aluminum oxide films for moisture measurement

inside integrated circuit packages is well known [1]. Difficulties in maintaining

device calibration during die attach and packaging operations [2] and long-term

stability and hysteresis problems have been reported [3,4], This paper reports

results of a research program directed toward a fundamental understanding of the

physical mechanisms that may be responsible for such phenomena as aging and hysteresis [5].

In typical aluminum oxide moisture sensors, the moisture-sensitive aluminum oxide

(referred to as "AlOx" hereafter) is formed by aqueous anodization or hydration of

a planar aluminum electrode, followed by the deposition of a porous gold upper

electrode to make a parallel-plate capacitor structure. Moisture induced variation

of the electrical conduction on the surfaces of the AlOx grains (and possibly in

microcapillary-condensed water between the grains) gives rise to an effective

interelectrode capacitance that increases with increasing ambient moisture. In

order to eliminate any possible role of the gold upper electrode in our studies,

all of the research reported here was done using characterization methods in which

no upper electrode is required. These include measurement at low frequencies of

the AC sheet resistance of the AlOx film using the charge-flow transistor [6]

,

and

infrared ref lectance—transmission spectroscopy of the AlOx film in conjunction with

D

2O-H2O isotope exchange techniques. Film properties were studied as functions of

time after preparation, and after a sequence of low- and high-temperature anneals.

129
EXPERIMENTAL METHODS

Sample Preparation

The AlOx films were formed by the standard technique of immersing an aluminum layer

into boiling deionized water [7], Substrates consisted of (1) silicon wafers, some with only a thermal oxide on them, others with complete charge-flow transistor

devices fabricated in them. Pure aluminum (.99995) was deposited in a tungstenfilament evaporator, in thicknesses ranging from 50-150 nm for electrical studies

to l-1.5|Jtm for infrared studies. The electrical samples were completely converted

to AlOx, with an increase in thickness to 100-300nm, while the infrared samples were converted only part of the way, leaving some aluminum to serve as a reflector.

Figure 1 illustrates the difference between (a) conventional samples, (b) our CFT

samples, and (c) infrared samples. SEM, TEM, and X-ray diffraction studies

revealed the structural and morphological characteristics of pseudoboehmite , which

is similar to boehmite, A10(0H) , but with additional hydration, as reported by Sun,

et. al. [8].

Electrical Measurements

The electrical properties were measured in the range 1-4000 Hz with charge-flowtransistor (CFT) devices, as described by Davidson and Senturia [9]. Using this

technique, the AC sheet resistance of the film, Rg, can be determined with an accuracy of ±5% in the range 10 -10 ohms/square. During measurement, the device

is kept at room temperature, typically 22-24°C, and in a chamber through which a dew-point-controlled air stream flows, as monitored by a General Eastern dew point

hygrometer

.

Infrared Measurements

A dispersive double-beam Beckman spectrometer was used for infrared measurements,

with samples mounted on a 60° reflection accessory. Since the samples consist of

AlOx on aluminum, the infrared beam passes through the AlOx film, is reflected by

the aluminum, and passes again through the AlOx film, producing, in effect, a transmission spectrum. The sample was housed in a dew-point-controlled cell during

measurement, nominally at room temperature, but actually at about 30°C due to

infrared-beam heating.

Sample Aging and Anneal iwfl

Following preparation and between experiments, samples were generally stored in

clean, closed wafer holders in normal lab ambient (22-24°C, and nominal 40% R.H. ).

Some samples were stored in nitrogen dry boxes. Room temperature "aging" refers to

the results of periodic observations of electrical and infrared properties of

samples stored either under lab ambient or dry nitrogen conditions. Annealing

experiments of two types were performed. Low-temperature (T<;250°C) anneals were

done by placing the samples in an air-ambient lab oven for intervals of 10 or 20

minutes, returning the sample to room temperature for characterization measurements. High-temperature anneals (T2250°C) were also done in an air-ambient oven,

but for intervals as long as several hours.

130
RESULTS

Sheet Resistance Variation with Aaibient Mois tore

The room temperature sheet resistances of freshly prepared AlOx films vary by six

orders of magnitude as the ambient dew point changes from -55 to +20°C, an agreement with earlier work [3]. An approximately linear relationship is observed

between log Rg and dew point, both for aged and annealed samples. Typical data for

two cases are shown in Figure 2. The major effect of aging and annealing (except

for a few cases of annealing in a temperature-humidity chamber at high R.H. ) is to

shift the entire curve upward, which would correspond in a sensor to a shift of

calibration toward lower apparent moisture level, consistent with reported results

[2], For purposes of following the aging and annealing processes, the measured Rg

at a 10°C dewpoint at room temperature is used as a reference point.

Infrared Spectrum and Isotope Exchange

The infrared spectrum of a typical AlOx film (Fig. 3(a)) is characteristic of

pseudoboehmite [7,10], which resembles boehmite (Fig. 3(b)), but with additional

hydration. We choose to focus on the absorption peaks at 3300 and 3100 cm

-

-*-, which

) and symmetric ( g correspond to the asymmetric ( , b) stretching modes of the

linear OH chains in boehmite [111. Superimposed on these boehmite-like bands is a broad absorption attributable to ^0 [12]. These features can be revealed by

isotope exchange with D

2

O, as explained below.

By replacing the water in the dew point system with D

2O, isotope exchange reactions

can be followed. The sequence of events is illustrated in Fig. 4. Immediately

following preparation, the AlOx spectrum in moist ambient consists of the complete

OH absorption [P(0H)+C(0H)+F(0H)] and no OD absorption. When exposed to dry

ambient (dew point ~ -45°C), the component labeled P(0H) disappears. This is the

"physisorbed" component, which is fully reversible at room temperature. The C(0H)

and F(0H) components (denoting "chemisorbed" and "fixed" OH, respectively) , are stable at room temperature, even in dry ambients. However, when exposed to a moist

D

2

O ambient, the C(OH) component disappears, and the complete OD line [C (OD) +P(0D)

]

appears. That is, the physisorbed D

2

O exchanges with the chemisorbed ^0, producing both chemisorbed and physisorbed 0D. Then, on re-exposure to dry ambient, the

physisorbed 0D component P(OD) disappears, but the chemisorbed 0D component C(0D)

remains. The cycle is reversible, in that exposure to moist ^0 ambient re- exchanges the OD spectrum back to the original total OH spectrum. The fixed

component F(0H) is attributed to water of hydration within the pseudoboehmite

grains

.

Aging

The room temperature aging behavior of the AlOx film was monitored by periodic

measurement of Rg at room temperature in a 10°C dew point (see Fig. 5). Following

an incubation period of approximately 30 hours, Rg exhibited an approximately

square-law increase with time, suggestive of the nucleation and growth of circular

domains. Circular domains of diameter 25 pm were observed in many SEM photographs,

but thus far, it has not been possible to demonstrate correlation between SEM

observed domains and sheet resistance aging.

We observe a significant decrease in aging rate for samples stored in dry ambient,

hence, samples not having the physisorbed water component (see Fig. 6 ). However,

once exposed to a moist ambient, a very rapid increase in sheet resistance is

observed. Note, in particular, the sharp increase in the solid dot data at 70

131
hours. This increase occurred within 30 minutes during the Rg measurement.

Restoring the sample to the dry ambient did not decrease Rg; it simply restored the

slower aging rate.

Anneal iwfl

Low temperature annealing (T<250°C) consisted of a sequence of 20 minute exposures

to each successively higher temperature in lab air, with the sample returned to room temperature between anneals for sheet resistance and infrared measurement.

Based on the overall characteristics of the infrared spectrum, in conjunction with

isotope exchange, we find that the film retains its pseudoboehmite structure during

low-temperature anneals. The three water components behave differently with

anneal. The physisorbed component decreases steadily with increasing annealing

(Fig. 7) . The chemisorbed component is stable until 120°C is reached, above which

it decreases. The fixed component is stable at least to 250°C. The corresponding

behavior of R^ is shown in Fig. 8. Note the increase in slope above about 120°C.

The increase in Rg below 120°C is attributable to the loss of physisorbed moisture,

and the additional rate of increase above 120°C suggests that the chemisorbed

component does contribute somewhat to the overall moisture sensitivity of electrical properties. High temperature annealing (T>250°C) produces a gradual loss of

fixed water, and a gradual phase transformation to a-alumina, as evidenced by the

infrared spectrum.

CONCLUSIONS AND DISCUSSION

There are three principal findings in this work. The first is that three distinct

components of water of hydration can be observed with isotope exchange techniques,

in conjunction with infrared spectroscopy. The second is that the aging rate of

AlOx samples is dependent on the presence of the physisorbed component, which may

actually be microcapillary condensed liquid water between the AlOx grains. The

third is that the pseudoboehmite structure of the AlOx film is stable at temperatures below 250°C, but exposure to higher temperatures will produce a gradual phase

transformation to a-alumina.

The primary implication of these results for the use of AlOx sensors for moisture

measurement inside integrated circuit packages is in explaining the effect of hightemperature exposure that may result either from eutectic die-attach or typical

Cerdip sealing- processes. Such high temperature exposures can be expected to

produce the irreversible shifts in device calibration during packaging that are typically observed.

ACKNOWLEDGEMENT

This work was supported in part by the Thermal Processes Division of the National

Bureau of Standards under Grant NB80-DADA-1004 and by the National Science Foundation under Grant ECS-8114781. The authors wish to thank Dr. Kenneth G. Kreider and

Dr. S. Hasegawa of the National Bureau of Standards, and Dr. David R. Day of MIT

for their suggestions and assistance. Devices used in this work were fabricated in

the MIT Microelectronics Laboratory, a Central Facility of the Center for Materials

Science and Engineering, which is sponsored in part by the National Science Foundation under contract DMR-81-19295 . Thanks are also due to Norman F. Sheppard for

his device fabrication assistance. Some of the measurement instrumentation used in

this work was purchased under NSF contract ENG-7717219 and under programs sponsored

by the Office of Naval Research.

132
REFERENCES

1. M. G. Kovac, D. Chleck, and P. Goodman, A new moisture sensor for in-situ

monitoring of sealed packages, Proc . IEEE 15th Reliability Phvs ics Symposium , Las Vegas, April 1977, pp. 85-91.

2. M. Brizoux, J. Perdrigeat, D. Kane, and R. Gauthier, Recent evaluations of

A1

2 0^ in situ moisture sensors for industrial electronic hermetic components

application, Proc . Moisture Measurement and Control for Semiconductor Devices

,

III , Gaithersburg, MD, November, 1983, this volume.

3. S. Hasegawa, Performance characteristics of a thin-film aluminum oxide humidity sensor, Proc . 30th Electronic Components Conference , San Francisco, April,

1980, pp. 386-391 .

4. M. Sze, an anodized aluminum humidity sensor, Sc.D. Thesis, Department of

Chemical Engineering, Massachusetts Institute of Technology, 1970, unpublished.

5. See also C.-H. Lin and S. D. Senturia, The aging of hydrated aluminum oxide

thin films, Proc . Solid State Transducers '83 , Delft, June, 1983; Sensors and

Actuators , in press.

6. S. L. Garverick and S. D. Senturia, An MOS device for AC measurement of

surface impedance with application to moisture monitoring, IEEE Trans . Electron Devices , ED-29 (1982) 90-94.

7. W. Vedder and D. A. Vermilyea, Aluminum + water reaction, Trans . Faraday

Soc iety , 65 (1969) 561-584.

8. T. S. Sun, J. D. Venables, and J. M. Chen, Effects of surface morphology and

chemical composition on the durability of adhesively bonded aluminum structure, Martin Marietta Laboratories Annual Technical Report MMR TR 80-34C,

Baltimore, MD, p. 8.

9. T. M. Davidson and S. D. Senturia, The moisture dependence of the electrical

sheet resistance of aluminum oxide thin films with application to integrated

moisture sensors, Proc . IEEE 20th International Rel iab il itv Physics Sympos ium , San Diego, April, 1982, pp. 249-252.

10. R. S. Alwitt, The aluminum-water system, in Oxides and Oxide Films , Vol

.

4,

John W. Diggle and Ashok K. Vijh, Eds., Marcel Dekker, New York, p.169.

11. K. A. Wickersheim and G. K. Korpi, Interpretation of the infrared spectrum of

boehmite, J. Chem. Physics . 42 (1965) 579-583.

D. Eisenberg and W. Kauzmann, The Structure and Properties of Water . Oxford,

New York, 1969, p. 230.

1 33

12 .
POROUS GOLD

(a) CONVENTIONAL

ALUMINUM

OXIDE

(b)

(c) INFRARED

Figure 1 Schematic illustration of the cross-sections of (a)

ture sensors, (b) the CFT electrode structure, and

samples

.

1 34

conventional mois

(c) the infrared
15

Figure 2

Figure 8

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120 °C

10 MIN

ANNEAL

200 C

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I I I 0 1 *- -40 -30 -20 -10 0 10 20

DEW POINT (°C)

Sheet resistance of two annealed samples, showing the approximately

linear relationship between log Rg and dew point.

15

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The low-temperature annealing effect of Rg

135
TRANSMISSION

WAVENUMBER, CM

'

1 4000 3000 2000 1500 1000

WAVENUMBER, CM

'

1

Figure 3 IR spectra of (a) AlOx film and (b) boehmite.

136
WAVELENGTH (Mm)

2.5 3 3.5 4 4.5 5

WAVENUMBER, CM

Figure 4 Three OH and OD components of the infrared spectrum.

1 37
14

Figure 5

Figure 6

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MEASURED AT TEMP: 22 °C DEW POINT: IO°C / /0

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The time dependence of the sheet resistance of five samples prepared at

two different times and aged in room ambient.

14

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CO2

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The time dependence of the sheet resistance of four samples prepared at

the same time. Two were aged in room ambient, the others in dry air

ambient.

MEASURED AT TEMP: 22 °C DEW POINT: I0°C

STORED IN Q

ROOM AMBIENT

/ STORED IN ' A

DRY AIR AMBIENT /\

/ A

i

>•'

138
NORMALIZED

ABSORPTION

COEFF.

Figure

<

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= 1.0 H

PHYSISORBED WATER 1 i

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0.5 -

l

l

CHEMISORBED WATER

i.o- i-f i i f

0.5

FIXED WATER

1

40 80 120 160 200

TANNEAL (°C)

The low-temperature annealing effect on the IR normalized absorption

intensities of the three components of the OH absorption spectrum.

139
I I I. 2 RECENT EVALUATIONS OF AL

20

3 IN SITU MOISTURE SENSORS FOR

INDUSTRIAL ELECTRONIC HERMETIC COMPONENTS APPLICATION

M. Brizoux and J. Perdrigeat

Thomson - CSF/SCTF

Domaine de Corbeville

91401 - Orsay, France

(6) 019-70-00

D. Kane and R. Gauthier

I.N.S.A. Lyon

20, av. Albert Einstein

69621 - Villeur banne , France

(7) 893-31-12

Presented by Judith Weiner

Thomson-CSF, White Plains, New York

ABSTRACT

The sensitiveness of Al

203 sensors to the assembly procedures (used in the

hybrid and semiconductor manufacturing) demands a recalibration of the sensor

for each different process used. This operation is costly and timeconsuming, and the precision is not acceptable for industrial standards.

A better knowledge of the assembly dispersion parameters (namely,

temperature, and time) and the mathematical model of the sensor response will

make their usage possible for industrial application.

KEY WORDS:

Accuracy, aluminum, assembly process, effect, mathematical model, moisture

measurement, oxide sensor.

It is now well documented that excessive moisture trapped within sealed

electronic packages has become of major concern due to its ability to degrade

the electrical performance with time.

Actually the specifications MIL-STD 883B and MIL-M 38510E require, in

hermetic packages, a moisture level lower than 5000 ppmV for class B and 3000

ppmV for class S.

Among the several methods, the nondestructive electrical test of dew point sensors is really attractive; but in accordance with these standards, it is

necessary to demonstrate the ability of these sensors for an industrial

application goal.

Interest in this study was to provide an in situ sensor response mathematical

model, taking into consideration housing die parameters. This mathematical

model is intended to provide improvement in measurement accuracy, less

expensive cost, and calibration time.

Response Versus Assembly Process

The experimental setup for the sensor susceptibility to assembly process is

the following.

INTRODUCTION

ai

2o

3 SENSOR SENSITIVITY TO ASSEMBLY PROCESS

140
Twenty-one sensors are assembled in 24 pins side-brazed packages by adhesive

(H20E) or eutectic perform (Au-Si) before and after sealing (for eutectic die

attach)

.

Figure 1 shows the different magnitudes of sensor calibration curves,

obtained from our test equipment (picture 1 ) , as it could be

expected 1 > 2 • 3

.

Effect of Assembly Parameters Dispersion on Calibration Curve

The calibration curves reproducibility from one wafer run to another and the

necessity of assembly parameters control present a great high interest in

determining the opportunity of an industrial use in our semiconductor

division.

To bring the sensors' sensitivity close to the above parameters:

- Many sensors (63 in all) from three wafer runs were assembled in 24 pins

side-brazed packages with eutectic perform (Au-Si) die attach and U.S. Al

bonding, and calibrated with our test equipment (results in figure 2).

- Keeping the same package and assembly process, we have just changed the

eutectic time and the time control accuracy; calibrations made on 14 sensors are shown in figure 3.

The results presented above have shown:

- The great dispersion of sensors' response due to the assembly process 3. - Necessity to calibrate each run of wafer.

- Accuracy of eutectic condition (acceptable results for eutectic time

dispersion of 2 seconds).

We used many assembly processes (hybrid package, CERDIP, side-brazed, TO

...), and we found out that it is difficult to measure the humidity level

that way because of the cost (high price of these sensors, minimum of five sensors for each calibration) and the delay ( time-temperature effect on sensor response implies time recalibration, monosource manufacturer).

However, the fidelity of such sensors, measured at different times in the

same condition, allows us to say that it can be used industrially, providing

that a mathematical response model is possible (recalibration after the timetemperature effect would not be useful in this case).

MATHEMATICAL MODELISATION OF SENSOR RESPONSE

Test Program

An important test program has been established to validate the assumption

advanced in the above paragraph. Experiments are made on the same wafer run

of sensors in 24 pins side-brazed packages, die attach by ambient cure

adhesive (araldite AV 138) and U.S. Al bonding (cold process). None of the

packages was sealed.

A network taking into account time and temperature of sensor storage

(commonly used in our different assembly divisions) was defined as follow-,

(see Table 1 )

.

141
Table 1 : Test Program

Temperature Time

Ambient

(reference)

o

ii

1 50°C 24 h 168 h 500 h lOOOh

350°C 10 s 30 s 2 mn 10 min

430°C 5 s 10 s 1 mn 10 min

5 sensors in each (a, t) conditions

Calibration curves were obtained from humid gas generation at eight dew/frost

point (-47°C, -43°C, -37°C, -°27C, -21 °C, -1 3°C, -2°C, +8°C)

.

Note : Sensors baked at (150°C/ 500 h and 500°C, 1000 h) failed. Visual

inspection passed; the failure analysis has not yet shown the origin.

Results

Figures 4 and 5 are two examples of the weak dispersion of the 5 sensors in

each couple (a, t) . After that only average curves will be considered.

All the results of this study are summarized in figure 6; at first glance it can be noticed that all the curve variations agree with time- temperature

evolution.

MODELISATI ON

Calibration Curve Modelisation at Constant (g, t)

We tried to identify a response curve by an exponential and polynomial

function; an example is given in figure 7 (reference sensors). Third order

polynomial function, in this example, is a good model (accuracy better than

one dew/frost point) . The same model has been applied on the couples a, t

(figures 8, 9, and 10) and pointed out that the polynomial function was acceptable for the different temperatures until 430°C, only if the polynomial

was of fifth order (accuracy better than two dew/frost points).

V = A.T5 + B.T4 + C.T3 + D.T 2 + F.T + F

V: sensor response

T: dew/frost point (see Table 2).

142
Table 2: Polynomial Curve Parameters

parameters

M

A B C Reference 0 0 9,2 10- 8 150°C, 24 h 2,9 10- 8 3,7 10- 6 1,7 10- 4 150°C , 168h 3,8 10- 8 4,5 10- 6 1,9 10- 4 350°C , 10 s 1,1 10- 8 1,2 10- 6 5,io- 3 350°C , 30 s 1,1 10- 8 1,5 10- 6 7,1 10- 5 350°C , 2 mn 1,4 10- 8 1,9 10- 6 9,1 10- 5 350°C , lOmn 8,2 10- 9 1,1 10- 6 5,1 10- 5 430°C , 5s 1,4 10- 8 1,9 10- 6 9,6 10- 5 430°C , 10s 6,5 10- 9

9 10- 7 4,8 10- 5 430°C , lmn 7,3 10- 9

1 10- 6 5,4 10- 5 430°C, lOmn 3,7 10- 1C 7 10- 8 4,9 10- 6

Parameters

(£-, t) D E F

Reference 1,2 10- 3 5,8 10- 2 1,4

150°C , 24 h 4,2 10- 3 7,5 10- 2 1,3

1 50°C , 168h 4,2 10- 3 7,5 10- 2 1,3

350°C , 10s 1,2 10- 3 2,7 10- 2 6,6 10- 1 350°C , 30s 1,7 10- 3 2,6 10- 2 5,4 10- 1 350°C , 2mn 2,1 10- 3 2,8 10- 2 4,7 10- 1 350°C , lOmn 1,1 10- 3 1,4 10- 2 3,3 10- 1 430°C , 5 s 2,4 10- 3 3,5 10- 2 5,9 10- 1 430°C , 10s 1,3 10- 3 2,3 10- 2 4,9 10- 1 430°C , 1 mn 1,4 10- 3

2 .10- 2 4,1 10- 1 430°C , 10 mn 1,7 10-4 3,6 10- 3 2,3 10- 1

Table 2 : polynomial curves parameters

Drift Calibration Curve Model Versus Time-Temperature Storage ( o , t) and

Dew/Frost Point (T)

Time effect: the graphs for the equation shown in figures 11, 12, and 13

plotted in log-log coordinates at constant T and a.

log V = -a • log t + b

or

V wi th ak

f (T)

g (T)

Temperature effect: we propose an Arrhenius model, very often used in

semiconductor field, such as:

* j ~d V = c • exp -

ka

where k is the Boltzmann constant.

are

143
Consequently, a mathematical model of Al

2 03 sensor response can be given

as:

TT j

V = P (T) • — • c exp f——

1

t a

where P (T) is the fifth order polynomial function.

Remark : An industrial assembly process is characterized by several couples

(o, t) . Frequently, a values are very different, in this case, for close t

values, we have to consider only the maximum temperature; and for close a values (A0 < 30°C ) we make a linear summation of time values.

HIGH TEMPERATURE STORAGE EFFECT

It is interesting to be able to follow the component's behavior (included a sensor) with respect to the humidity level and hermetici ty*4 ' 5 during

electronic board and equipment functioning.

To approach this kind of behavior, we baked the sensors at 85°C and 125°C,

which are assembled in 24 pins side-brazed packages with eutectic perform

(Au-Si) die attach and U.S. Al bonding, during 168 h, 500 h, and 1000 h.

Results (figure 14) obtained for 125°C bake point out that the calibration

curve's drift is stopped after 168 h (same results are obtained for the 85°C

bake

)

.

At present time, we have not seen any incompatibility between the use of the sensors and the knowledge of component's humidity level and hermeticity drift

on equipment.

It should be necessary to confirm these first results by a temperature

cycling test for 500 to 1000 cycles.

CONCLUSION

This study has proved the semiconductor and hybrid industrial application of

Al^O^ sensors in moisture measurement.

This would be possible by tightening assembly process parameters and

modelisation of calibration curve.

The high quality and reliability demand of the complex VLSI semiconductor

will be satisfied by the control of several physical parameters, like

humidity level.

ACKNOWLEDGMENTS

The authors wish to thank the internal working group "METAVAPEAU" for their

help in the definition of the test program and Dominique Osselin and Michel

Bordas for their contribution to these experiments. They also thank Alain

Godefroy and Jean-Pierre Plantard for their technical advice. The authors

are also grateful to Mr. Fong from Panametrics, Inc. for fruitful

discussions

.

144
REFERENCES

1

.

V. Fong, "The Effects for Assembly Techniques Upon the Performance

Characteristics of A1

2 03 In-Situ Moisture Sensors", NBS/RADC Moisture

Workshop, March 22-23, 1978.

2. A. Koudounaris and E. E. Wargo, "Detection and Control of Moisture in

Space Hybride", Technical Report, November, 1980.

3. M. L. White and R. E. Sammons, "A Procedure for Preparing Hermetic

Packages with Known Moisture Levels", NBS/RADC Workshop, November 5-7,

1980.

4. D. R. Fancher and R. G. Horner, "Hybride Package Hermeticity and Moisture

Level Monitoring", ISHM Proceedings, September, 1978.

5. J. B. Finn and V. Fong, "Recent Advances in Al^O^ In-Situ Moisture

Monitoring Chips for Cerdip Package Applications", IEEE, 1980.

Picture 1 . Test equipment description

1 - hygrometer model 771 for MINI-MOD HT sensors

2 - humidity generator MG 101

3 - calibration box

145
PPM V AT 1 atm

100 1000 10,000

DEW/FROST POINT (°C)

Figure 1. The assembly process effect on calibration. (-) an average calibration curve before sealing with adhesive die

attach and curve cycles at 180°C for one hour. (-•-) calibration curve before sealing with Au/Si eutectic die attach at

420°C for 15 s. ( ) calibration curve after sealing with

Au/Si eutectic die attach and sealing at 370°C for 4 s in

N

2 /12% H2

.

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1.0 0.9 0.8

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0.3

0.2

0.1 -110 -90 -70 -50 -30 -10 10 30 DEW/FROST POINT (°C)

t

1 1 1 r

J I I L. 1 L

Figure 2. Dispersion in sensor response between 3 lots for

side brazed packages with Au/Si eutectic die attach and ultrasonic Al wire bonding. Twenty-one sensors per run.

146
DEW/FROST POINT (°C)

Figure 3. Dispersion in sensor response with eutectic attach

heating time. (-) average values for 9 s. (-•-) average values for 15 s. Maximum and minimum values are also shown for

each condition.

PPMV AT 1 atm

100 1000 10,000

Figure 4. Sensor calibration after 350°C at 2 min with average, minimum, and maximum values indicated.

147
PPMV AT 1 atm

100 1000 10,000

Figure 5. Sensor calibration after 450°C for 5 s with average,

minimum, and maximum values indicated.

PPMV AT 1 atm

100 1000 10,000

DEW/FROST POINT (°C)

Figure 6. The effect of time and temperature on sensor calibration curves. The uppermost curve represents initial values.

In descending order the curve represents the effect of 24 h at

1 50°C, 168 h at 150°C, 10 s at 350°C, 5 s at 430°C, 30 s at

350°C, 10 s at 430°C, 2 min at 350°C, 1 min at 430°C, 10 min at

350°C, and 10 min at 430°C.

148
TEMPERATURE (°C)

Figure 7. Response in reference sensors. a. exponential function, b. polynomial function (3rd order).

TEMPERATURE (°C)

Figure 8. Response after 150°C, 168 h: a. 3rd order polynomial function, b. 5th order polynomial function.

1 49
TEMPERATURE (°C)

Figure 9. Response after 350°C, 10 min: a. 3rd order polynomial function, b. 5th order polynomial function.

r

TEMPERATURE (°C)

Figure 10. Response after 430°C, 10 min: a. 3rd order polynomial function, b. 5th order polynomial function.

150
T

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• Reference points

I I i

1 24 168 500

TIME (h)

Figure 1 1

.

a function

= -48, -2,

Drift in response for given dew/frost points (T) as of temperature-time storage (0,t). 0 = 150°C and T

+8 °C.

TIME (s)

Figure 12. Drift in response for given dew/frost points (T) as

a function of temperature- time storage (0,t). 0 = 350°C and T

= -48, -2, +8 °C.

151
Figure 13. Drift in response for given dew/frost points ( T) as

a function of temperature- time storage (0,t). 0 = 430°C and T

= -48, -2, +8 °C.

PPMV AT 1 atm

100 1000 10,000

DEW/FROST POINT (°C)

Figure 14. High temperature storage effect on calibration.

(-) initial values. ( ) after 168 h at 125°C. (• — •)

after 500 h at 125°C (

) after 1000 h at 125°C.

152
1.3 MOISTURE SOURCES AND MEASUREMENT IN CERDIP PACKAGES WITH ALUMINA SENSORS

Harold Joss

Coors Porcelain Company

17750 W. 32nd Avenue

Golden, Colorado 80401

(303)277-4057

Abstract: This paper discusses the findings of work completed

at the Coors Porcelain Company in the electronic package program.

The alumina sensor, as manufactured by Panametrics, Inc. (Waltham,

Mass.) is chosen from the several commercially available moisture

test methods due to its ease of use and non-destructive character.

The non-destructive nature allows not only moisture level determination, but also a monitoring system that can observe internal

moisture levels as a function of time, adding insight to the

effect of aging on package moisture levels. Experimental procedure and routine chip calibration is described, as is the

method by which this device is correlated to mass spectrometry. An equation defining this relationship is offered.

Several CERDIP manufacturing processes and material variables

are examined in general terms, and the alumina sensors are used

to establish the fixed level contributions of each variable to

total package moisture. Results indicate that each moisture

source may not be dependent on the other sources. Throughout

the test, a systematic decrease in package moisture with

respect to time was noted in all samples. A hypothesis is

proposed to explain this occurrence.

1 . INTRODUCTION

When the low moisture program at Coors Porcelain Company was started, the first

major objective defined was to establish a method by which the moisture inside

a hermetically sealed package could be accurately measured. At that time

several commercial test methods were available. Two of these were nondestructive techniques, both involving use of chips that could be placed

inside of the package before sealing.

The first chip type was a surface conductivity moisture sensor, HI0-55001-6,

purchased from Harris Corporation, Semiconductor Products Div., Melbourne,

Florida. Initial testing with the Harris cell was unsuccessful, as no peak was clearly discernable. Later work with this method did yield some information, but it was decided that an easier to use technique would be desirable.

The next nondestructive technique investigated utilized an alumina moisture

sensor chip, MM-HT, purchased from Panametrics, Inc., Waltham, Massachusetts.

The first work done with this chip (and the Panametrics model 771 hygrometer

which is used to read it) was generally successful. Some of the first readings were scattered, probably due to less than perfect assembly techniques.

153
I

Assembly is a sensitive operation, as the alumina portion of the sensor chip

undergoes some physical changes in pore size, surface area, or density when

heated during die attach, lead imbed, bakeouts, and seall. The chip can be

recalibrated after this thermal cycling, but the process must be constant for

the recalibration to be meaningful. Then, when the alumina sensor test procedure is stabilized, the test values can be calibrated to mass spectrometry,

the reference test procedure chosen for this program.

2 . PROCEDURE

In order to minimize the effects of heat treatment, a standard CERDIP sample

preparation and assembly procedure was developed. All CERDIP materials were processed on the same day under the same conditions (unless one was deliberately changed as a variable). Package assembly, including die attach, wire bond,

and lead imbed were standardized. Since most vitreous sealing glasses on the

market today seal at similar temperatures 3

t all samples were glazed using the same temperature profile and atmosphere (air dried to -35°C dewpoint).

Some of the original packages containing the alumina sensors were sent to

Panametrics for calibration. Briefly, their procedure involves de-lidding the

packages and bringing them back to their original (dry) condition under flowing

dry nitrogen. The samples are then placed in a humidity-controlled chamber and

the sensor readings recorded at various dewpoints. From this data a calibration

curve is constructed, an example of which is shown in figure 1.

To maintain the same calibration curve throughout the study, a series of steps were taken to insure that any replacement lots would be the same as the first

lot. Initially the replacement chips were matched to the originals on a best

fit basis by Panametrics. To verify this calibration at Coors, test samples were assembled. One chip from the new lot and one chip from the old lot were placed in each of six packages and sealed. In this way the two lots were evaluating exactly the same environments. If the readings for the two sets were the same, as was always the case, the replacement lot would be used.

The moisture sensor chips still needed to be correlated to mass spectrometry.

Initially, equation (1), established by Finn and Fong 3 was used to convert sensor moisture levels to mass spectrometry values.

M = (6.42 * 10 6 * A)* 397 (1)

where M = mass spectrometry value in ppm

A = alumina sensor value in ppm

For the first correlation test, 11 packages previously tested for moisture with

alumina sensors were chosen to represent the entire spectrum of moisture values

from below 500 ppm to 9000 ppm. Each 24 lead CERDIP was marked and delivered

to Oneida Research Services, New Hartford, New York. Once the samples were

returned, each of the two readings, representing the two test methods, were analyzed on a specimen by specimen basis.

154
3. RESULTS AND DISCUSSION

Sensor moisture values were compared to the mass spectrometer values using a standard correlation and linear regression analysis. However, it was found

that the meter readings themselves provided a better correlation coefficient

(0.96) than the predicted moisture values derived through use of equation (1).

The results of this second analysis, listed in Table I, are shown graphically

in figure 2. Using this information, a linear relationship between the alumina

sensor meter reading and actual mass spectrometer moisture was derived. The

result was equation (2).

M = (150890 * R)- 21629.5 (2)

where M = expected mass spectrometer value, ppm

R = Panametrics 771 hygrometer meter reading

Equation (2) will go negative at a very low meter reading. However, some packages tested in this range were also evaluated by mass spectrometry and found

to have moisture levels below 750 ppm. Since this project was not concerned in

particular with accurate measurement and correlation in this range, and since

packages with those moisture levels are generally considered acceptable, it was

not considered a problem.

During the course of this study, several interesting observations were made.

First, all of the curves have a generally decreasing moisture level with time.

Examples of this can be seen in figures 3-6. Although no mathematical study

was made, the plots seem to fall on a straight line if moisture level is plotted

against log time.

This moisture level reduction could have been caused by a drift in the response

of the sensor itself. However, tests have been performed whereby samples 2 weeks

to 2 months old have been measured by mass spectrometry and their readings

verified. This not only supported the belief that the moisture decrease was

real, but also that the 100°C preconditioning of the sample for mass spectrometry

was not driving extraneous moisture into the cavity.

A better explanation for the moisture level reduction is that the package itself

may take a long time to reach equilibrium after seal. This is not a new concept^ >5 > and moisture may be reabsorbed onto lids, walls, and bases after thermal

cycling. Since the chip surface is the most insulated area from the package

exterior, it is probably the last to observe any condensation^ » ? . This is

obviously the case with an alumina sensor chip, as condensation on it would

raise the moisture readings, rather than decrease them.

Another interesting discovery in this work concerned changes in moisture level

when variables in the CERDIP manufacturing process were altered. This in itself was not surprising, but by cross-checking variables, it seemed that moisture

introduced (or removed) by each process variable was independent of the other

variables. If this was the case, moisture sources are additive and more than a

155
few key factors require attention when low moisture levels are necessary. An

example of one of these comparisons is illustrated in figure 3. In this figure,

three different glazing atmospheres are investigated. The results illustrate

that much lower moisture levels can result through use of atmospheres B and C

during the CERDIP glazing cycle.

Although atmospheric moisture absorption by the raw materials can be a source of

package moisture, humidity doesn't seem to be a problem once the CERDIP is

properly glazed. As a test, six Coors low moisture CERDIP sets were stored at

-40°C dewpoint and six other sets stored at +27°C dewpoint for 16 hours. When

sealed, these parts had the same moisture levels (figure 4). The test was repeated with a different sealing glass and the results were essentially the same (figure 5).

A final characteristic noted was that sealing glasses have intrinsic moisture

levels. This concept is well known in the industry and has been previously

cited8>9. Results indicated that these moisture levels are independent of other

moisture sources, like the process variables. The variation between glasses is

consistent enough to classify them into three general categories (figure 6). Of

the four glasses displayed here, LS 0113 is representative of a high moisture

glass, KC 401 and SG 202 are moderate moisture level glasses, and LS 0803 is an example of a low moisture glass. Devitrifying type glasses were not included in

this study, but would be expected to fall into the high category. It is worth

noting that a dry process CERDIP has a moisture level that is a function of the

glass specified by the customer, as processing can only remove processing

moisture sources.

The moisture in sealing glasses is created, at least in part, by the starting

materials and smelting (manufacture and refining) process. The raw materials

used to manufacture glasses usually include nitrates, hydrates, sulfates, and

carbonates. During the smelting process, these materials decompose to form the

oxides that make up the glass itself. The gasses given off are mostly removed

during the firing process, where the molten glass can bubble like any other

liquid. However, some of these dissolved gasses are never removed and subsequently incorporate themselves into the glass structure. Once these materials

have been chemically absorbed by the glass, quite a bit of heat may be required

to drive them off 10. It is therefore proposed that package sealing temperatures

are high enough to drive some of these impurities out of the glass structure.

These can then combine to form an atmosphere which includes H 2 O , CO 2 , N2 , and

SO 2 within the CERDIP itself.

4. CONCLUSIONS

Several conclusions may be drawn as a result of this study.

1. The alumina moisture sensor chip does correlate to mass spectrometry.

2. The moisture levels seen by a chip sealed within a CERDIP package become

systematically lower with time.

3. Moisture sources emanate from both processing parameters and the materials

used, particularly the sealing glass. These sources are not interdependent.

156
REFERENCES

1. Kovac, Michael G., "Performance Characteristics of AI 2 O 3 Moisture Sensors

Inside Sealed Hybrid Packages", ISM Symposium , Baltimore, Md . , October

1977, pp. 249-252.

2. Pittman, J. D. and Joss, H. D. , "Low Temperature Sealing Glass Strength

Study for Type C Chip Carriers", Proceedings 32nd Electronic Components

Conference , 1982, pp. 277-281.

3. Finn, J. B. and Fong, V., "Recent Advances in AI 2O 3 'In Situ' Moisture

Monitoring Chips for CERDIP Package Applications", Pr oc e e d i ngs 1 8th Annua. 1 International Re lia.bill ty Physics Symposium , Las Vegas, Nevada, April 1980.

4. Kovac, Michael G., "Cross Correlation Experiments on Different Types of

Sensors", Proceedings NBS/RADC Workshop, Moisture Measurement II , November,

1980, pp. 79-89.

5. Davy, J. Gordon, "Thermodynamic and Kinetic Considerations of Moisture

Sorption Phenomena", Proceedings NBS/RADC Workshop , Moisture Measurement II , November, 1980, pp. 184-200.

6 . Kovac, Michael G., "Microenvironments and Accelerated Testing", Proceedings

NBS/RADC Workshop, Moisture Measurement II , November 1980, pp. 165-174.

7. Hale, John C. and Fong, Victor, "Moisture Sensors, Mass Spectroscopy, and

Mil Standards", Pr oceedings NB S/RADC Workshop , Moisture Measurement II , November 1980, pp. 90-97.

8 . Shukla, Rama K. et. al., "Moisture Content of Solder Glasses", Proceedings

NB S/ RADC Workshop L Mois ture Measurement II , November 1980, pp. 213-219.

9. Lowry, Robert K., "Dry Sealing Glasses - A

NBS/RADC Workshop Moisture Measurement II ,

10.

"Vacuum Hot Gas Extraction Apparatus", P & Glass Co., Ltd., Otsu City, Japan (1980).

Summary of Research" , Pr oceec November 1980, pp. 220-233.

P 9, pp. 28-29, Nippon Electric

157
CALIBRATION

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158
Moisture,

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5,000

4,000

3,000

2,000

1,000

0

a Glazing Cycle A o Glazing Cycle B a Glazing Cycle C

1,000

Time After Seal, hours

Figure 3: Comparison of Cerdip Glazing Cycles

4.000

3.000

2.000

1,000

0

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1 10 100 1,000 3,000

Time After Seal, hours

Figure 4: Effect of Glazed Cerdip Storage Conditions on Sealed Package Moisture - LS0803

159
Moisture,

PPM

Time After Seal, hours

Figure 5: Effect of Glazed Cerdip Storage Conditions on Sealed Package Moisture - LS0113

160
CORRELATION 4 LINEAR REGRESSION

VARIABLE X: METER READING VARIABLE V: MASS SPEC PPM

NUMBER OF PAIRS <N> =11

CORRELATION COEFFICIENT <R) = 96

DEGREES OF FREEDOM CDF) =9

SLOPE CM) OF REGRESSION LINE = 150890

V INTERCEPT CB) FOR THE LINE = -21629. 5

Table I: Correlation Between Panametrics 771 Hygrometer Reading and

Mass Spec PPM

161
4.4 MOISTURE CONTENT CONTROL USING ALUMINA SENSOR

Carl M. Roberts, Jr.

Analog Devices

Semiconductor Division

804 Woburn Street

Wilmington, MA 01887

(617) 935-5565

ABSTRACT

Implementation and use of an aluminum oxide sensor to control header

sealed package moisture content at the point of package seal in an

integrated circuit assembly process is discussed. Correlation of

moisture content readings between the aluminum oxide sensor and mass

spectrometr ic determination of moisture is also addressed.

INTRODUCTION

MANUFACTURING SITUATION

At Analog Devices Semiconductor (ADS), volume assembly of hermetic

header product (Figure 1) is typically done at assembly locations in

Korea, Taiwan, and the Philippines. Once received at ADS, the

finished product is sampled and inspected at Incoming Quality Control

(IQC) for conformance to ADS assembly specifications. Samples from

each received assembly lot are also sent to an outside lab for mass

analysis of internal moisture content. Product

5000 PPM moisture content is released for electrical

product in excess of 5000 PPM is rejected, impounded,

disposit ioned to scrap.

spectrometr ic

acceptable to

testing while

and typically

IQC data during the first half of 1981 indicated that too many header

assembly lots were rejecting for high moisture content. The MASS

SPEC data for moisture content was usually available for analysis and

IQC decisions typically five to six weeks after the product was

sealed at the assembly location. With such a time lag from the point

of seal to the actual receipt of moisture data, reject lots created

several difficult situations: first, the rejected product was

impounded and typically disposi tioned to scrap; second, rejected

product severely disrupted product schedules; third, work- in-process

might be similarly affected with high moisture content.

ADS AND MOISTURE

ADS ' SPEC

moisture content is certainly in response to MIL

but, more specifically, is founded in the product

ADS produces. Most ADS product includes thin film resistors (Figure

2) in addition to the normally used aluminum.

attention to

requirements

The presence of thin film resistors in any IC product tends to, in

theory, increase the probability that device corrosion may occur

under the right circumstances: as in the case of high moisture

content in the sealed package. The thin film resistor itself has a possibly greater tendency to suffer from corrosion in the presence of

high moisture simply because of its thinness of construction,

approximately 100 Angstroms, versus the thickness of typically

162
deposited aluminum, approximately 15000 Angstroms. Aside from the

thinness, interconnects between thin film resistors and aluminum are

often sites of high voltage potential relative to adjacent device circuitry, which again coupled with high moisture increases the tendency

of device corrosion. Additionally, wafer product fabricated at ADS

is typically laser trimmed through the top layer glassivation to the

thin film resistor to electrically modify certain resistors (Figure

3). The laser trim process has the potential to crack the top layer

glassivation at the point of trim which, within limits, is visually

acceptable to 883B MIL SPECS. The crack, if present, is a site of

possible moisture ingression right to the thin film resistor and thus

is another construction factor contributing to corrosion tendency.

The mere presence of thin film resistors, the interconnects from thin

film resistors to aluminum, and the possible glassivation cracks at

laser trim sites are not singularly or collectively reasons for

device corrosion. They are conditions of construction that, when

coupled with high moisture, would tend to make such constructed

devices more susceptible to experience and exhibit corrosion. At

ADS, the attention to moisture and control of such is therefore more

specifically centered in product concerns rather than in conformance

to MIL SPEC requirements. In fact, the 5000 PPM moisture control

limit is applied to both military and commercial product at ADS.

CORRECTIVE CONTROL ACTION

The occurrence of moisture rejections at IQC in early 1981, the time

delay between product seal and available MASS SPEC data, and ADS'

attention to moisture because of product construction prompted a need

to effect measurement and control of moisture during the assembly

process at the point of actual package seal. The procedure selected

to effect such measurement and control was the implementation and use

of the Panametrics MINI-MOD-HT moisture sensor chip.

Discussed herein is the implementation procedure done at ADS to

effect use of the moisture sensor chip, results from two years of

such a control procedure, and, finally, a comparative examination of

moisture PPM readings for identical units measuring moisture with the

moisture chip and then as measured through MASS SPEC. The theory of

operation of the Panametrics MINI-MOD-HT moisture sensor chip is well

documented in many available papers and will not be addressed.

MOISTURE SENSOR CHIP: PRODUCTION USE

INITIAL EVALUATION

Effecting the use of the moisture chip as a control measurement in

production is a relatively straightforward procedure that must

carefully be tailored to the assembly process steps of the package to

be controlled. The response of the moisture chip to indicate any

particular moisture level is affected by the thermal exposures that

the moisture chip experiences. In this regard, to effect control at

a particular point in the assembly process, the moisture chip should

be exposed only to the normal assembly thermal excursions that actual

production product would experience up to the point in the process

that the control measurement is effected for the package to b*' controlled

.

163
In implementing the control procedure at the point of seal for ADS

header product, the assembly thermal excursions are only two and

include eutectic die attach and oven baking of product prior to seal

(Figure 1 )

.

Moisture sensor chips were acquired from Panametrics. A small

quantity of approximately twenty units were eutectic die mounted,

aluminum ultrasonic wire bonded, baked and sealed exactly as normal

production assemblies. The units were serialized and then

individually connected to a Panametrics Model 771 meter and the

reading for each unit was recorded.

The meter reading at this point is essentially meaningless without a means to convert the reading to moisture PPM. This is done by

generating a calibration curve at Panametrics (or some alternative

source) with a moisture generator traceable to the National Bureau of

Standards. Prior to sending units to Panametrics to generate the

calibration curve, all twenty units were first sent to an outside lab

for MASS SPEC readings of moisture content. The MASS SPEC moisture

readings were recorded against the original serialized Panametrics

meter readings with the intent of comparing the moisture PPM readings

of the two measurement techniques once the Panametrics calibration

curve was available.

Ten units were sent to Panametrics and all ten were used to develop

the calibration curve. The procedure is relatively simple. Each

unit is individually mounted to the moisture generator and different

known levels of moisture are generated and exposed to the moisture

chip (the header lid was punctured during MASS SPEC testing thus

allowing a path for moisture access to the moisture chip). Each unit

is simultaneously connected to a Panametrics Model 771 meter and as

each known level of moisture is generated the corresponding meter

reading is recorded. All the recorded meter readings are then

averaged at each known moisture level. The averaged meter readings

are then plotted against the known corresponding generated moisture

level where the vertical axis is the meter reading and the horizontal

axis is the known generated moisture level. A curve is then drawn on

the graph fitting the plotted points. The drawn curve is the

calibration curve. An example of a completed calibration curve is

shown in Figure 4.

The twenty units that had originally been read on the Panametrics

meter were now converted to moisture PPM using the calibration curve.

Comparing the moisture data unit for unit with both measurement

techniques did not show a one for one correlation. However, the

comparison did indicate a sufficient enough relationship such that a maximum allowable moisture chip meter reading could be selected as a process measurement control point, while applying some factor of

safety. The selected maximum reading, with the applied factor of

safety, would allow the control process to be used to effect the

desired intent while not tightening down too much so as to cause

unnecessary production stoppages.

Having done the above process once, the entire set of actions was

repeated again and again so as to examine the consistency of the

moisture chip system as well as test the validity of the selected

164
moisture meter control point as compared to MASS SPEC moisture

readings for the same units. The results of such evaluations

confirmed the original findings: the moisture chip and the selected

control point sufficiently related to the corresponding MASS SPEC

readings to at least use the moisture chip as a tool for process

control. Precise correlation studies at this early time in the use

of the moisture chip was not appropriate as the initial intent was to

control a seal process with a tool, specifically the moisture chip.

More precise statistical correlation studies would be considered long

after the moisture chip was well established as a production control

tool and more data of comparative readings of moisture was available.

It should be noted here that MASS SPEC is done at 100°C and looks

at vapor phase, adsorbed and absorbed moisture while the moisture

chip is operated or read at 25°C and measures only vapor phase

moisture. Given a specific application of control for a specific

package with unchanging process conditions, a statistical study might

relate the two techniques. Again, this analysis was deferred until

more data was available.

One additional evaluation was completed prior to production

implementation. All of the samples generated for the above tests

were done in the ADS Wilmington facility with a process defined as in

control so none of the sealed units exceeded 5000 PPM as tested

through MASS SPEC. Many groups of units were now processed through a variety of sealing conditions to generate: extraordinarily dry parts

by multiple baking, normal parts by the normal specified process, and

various increasingly wet groupings of parts by progressively reducing

and finally eliminating the prebake step. All units were read on the

Panametrics meter and segregated into acceptable and rejectable

groupings. The units were all tested through MASS SPEC. Although

the unit for unit readings did not directly correlate in PPM, the

groupings did correlate all the reject units were rejects, and all

the acceptable units were acceptable. As a tool for control, the

test data indicated the process was ready to be implemented.

PRODUCTION IMPLEMENTATION

Panametrics meters were purchased for each assembly location and a long term volume order for moisture chips was issued. Procedural

specifications were written including a log book for recording the

moisture chip meter readings. Serialized library parts were created

in the ADS Wilmington facility. These parts were read on each meter

to assure meter consistency. Some of the serialized library parts

were also shipped with each meter to each assembly facility, but

without any readings as taken in ADS Wilmington.

Each facility was first requested to report by telex their reading of

the serialized parts. This identified that each meter was working

correctly, and that each facility was also using the meter properly.

The serialized library units have continued to be periodically read

and reported back to ADS Wilmington to maintain a system check. Two

years after implementing the procedure, virtually all of the

serialized library units read exactly as when originally read.

Each facility then assembled several moisture chips per the

documented procedures. Each was serialized, read and returned to ADS

165
Wilmington. These were then read in ADS Wilmington to assure the

accuracy of the readings from each facility.

With user understanding in place and accuracy established, production

control was implemented as follows. The using facility would

assemble through wirebond an inventory of moisture chips and store

these in a nitrogen dessicator. At the start of each shift of header

seal, the first lot to be sealed would be baked with five moisture

chips from the prepared inventory. Both the baked production parts

and the five moisture chip assembled units are then moved into the

sealing chamber. The moisture chips are then sealed and removed from

the sealing chamber. Readings are then taken

Panametrics meter. If the readings are

sealing proceeds. If any one reading is

sealing does not proceed until the process is corrected and another

complete bake, seal and read sequence is acceptably completed. Used

moisture chips are serialized, read, logged, and returned for ADS

reading to assure facility use accuracy and possible MASS SPEC

reading for correlation studies.

for moisture using the

acceptable, production

rejectable, production

TWO YEARS LATER

The described control procedure has been in effect for header

assemblies for slightly more than two years. Having used more than

in the previous two-year period and, having

15,000 production units through MASS SPEC

17,000 moisture chips

also measured another

during the same period, several points are clear:

Production seal stoppages have occurred where moisture chip

readings were high;

2 . Rejections at IQC for moisture content

has been reduced significantly, almost

rejections that do occur are typically

plus 20% of the 5000 PPM control point;

as measured by

to noise level

borderline or

MASS SPEC

, and the

within a

3. An enhanced moisture awareness has been effected at each assembly

facility;

4. The moisture

accomplishing

point of seal

sensor chip

the desired

and reducing

has, as a process tool, assisted in

goal of controlling moisture at the

rejections at ADS IQC.

MOISTURE SENSOR CHIP VERSUS MASS SPEC

Having used approximately 17,000 moisture chips in the past two

years, and having also measured moisture by MASS SPEC on another

approximately 15,000 actual production units over the same time

period, reading moisture by the moisture chip and then reading the

same units by MASS SPEC seemed an appropriate analysis for assessing

correlation between the two techniques. Figure 5 presents a graphic

representation comparing the PPM moisture content of 108 moisture

chip units as measured first by the Panametrics meter (horizontal

axis), and then as measured by MASS SPEC (vertical axis). The small

dots are actual MASS SPEC PPM readings for any individual unit at a particular moisture chip PPM reading. The large dots are averages of

166
the MASS SPEC PPM readings for the group of units at a particular

moisture chip PPM reading. Some comments on the data follow: 1.

The moisture chip readings are always lower than the

corresponding MASS SPEC readings. This is generally attributed

to the already discussed measurement differences of the two

techniques: MASS SPEC is done at 100°C and examines vapor

phase, adsorbed and absorbed moisture while the moisture chip

reading is done at 25°C and examines only vapor phase moisture.

2.

The wide range of MASS SPEC moisture readings at any one

particular moisture chip reading could be argued to be affected:

first, by MASS SPEC inaccuracies or tolerances; or second, by

moisture chip inaccuracies or tolerances; or third, by variations

in sorption and desorption characteristics within the sealed

package that would affect the amount of vapor phase moisture

available to be detected when read by the moisture chip. All

probably contribute, but the variations in the process of

eutectic die attach or mounting of the moisture chip probably

contributes most to the variation. The moisture chip response is

affected by heat exposure: for a given calibration curve in the

ADS application, moisture chips deliberately exposed to twice the

normal time at the die attach temperature as those used to

establish the calibration curve will read a lower moisture level

when baked and sealed simultaneously with those units used to

establish the calibration curve. This is a point of possible

further investigation.

3.

In the ADS application, the die attach process is manual so

operator variation is present with each assembled moisture chip.

Looking at the situation from a broad perspective and assuming

this is a relatively constant variation, as is typically the

situation with most repetitive manual manufacturing operations,

then this variation when examined over many months is probably no

more significant then the reading accuracy of MASS SPEC or the

accuracy of the moisture chip as affected by the Panametrics

fabrication process. Assuming this to be generally appropriate,

using both techniques to then measure the moisture of a group of

moisture chips generated over a broad period of time would tend

to average into any correlation study all the possible

variations. Essentially, the affect of the variations would be

minimized in their impact, positive or negative, on a correlation

examination

.

4.

Using regression analysis, Figure 5 shows within the 50 to 1000

PPM moisture chip range, or the corresponding approximately 500

to 4500 PPM MASS SPEC range, that there is little correlation

between the two techniques at this range as evidenced by thecalculated 0.188 correlation coefficient.

5.

Conversely, the same regression analysis within the 500 to 2000

PPM moisture chip range, or the corresponding 1500 to 7000 PPM

MASS SPEC range, shows an excellent level of correlation between

the two techniques at this range as evidenced by the calculat'd

0.871 correlation coefficient.

167
SUMMARY

In considering

chip as an

attention must

to be effected

which the ac

properly, the

control tool.

the use of the Panametrics MINI-MOD-HT moisture sensor

in-line process control measurement tool, careful

be given to identifying the point at which control is

for the specific application and the procedure through

tual control process is established. Implemented

MINI-MOD-HT is an excellent and repeatable process

As applied at ADS for the header package sealing process, the

MINI-MOD-HT does demonstrate a promising correlation to MASS SPEC

moisture readings. Additional study and evaluation may be necessary

to understand the variations affecting the moisture chip response

with the intent of further improving the accuracy and correlation to

MASS SPEC particularly as relates to an apparent pending MIL SPEC

change to a 2000 PPM moisture control limit.

168
FIGURE 1

ADS use of moisture

chip was for control of

metal can or header

type TO- 9 9 and TO-lOO

packages. Assembly and

sealing of the moisture

chip exactly replicates

normal production

process

.

FIGURE 2

Portion of typical ADS

device is shown.

Bottom half of picture

shows many thin film resistors connected to

al umi num

.

169
FIGURE 4

Typical calibration

curve as generated by

Panametrics. The reading of moisture as reported by the

Panametrics meter is on

the vertical axis and

is converted to moisture via the generated

calibration curve and

down to the horizontal

PPM axis.

FIGURE 5

Graph of moisture content readings of units

measured first with the

moisture chip and then

by MASS SPEC.

(—CORR COEFH

= .188

[—CORR C0EF=.871—

|

;

i ft t t

:

170
4.5 MONITORING OF MOISTURE IN CERDIP DEVICES

USING A MOISTURE SENSOR CHIP

John Hunter

Advanced Micro Devices, Inc.

901 Thompson Place

P.O. Box 3453

Sunnyvale, CA 94088

(408) 749-3089

ABSTRACT

For effective process control over moisture levels in cerdip package assembly a method of measurement is needed which can (a) be readily set up within the manu- facturing facility and (b) provide results in a time short compared with the

assembly cycle time.

A moisture sensor which can be incorporated into an actual cerdip package satisfies

these requirements. The paper will describe some practical aspects of implementing

such a sensor (the Panametrics Mini-Mod HT) in a production situation. The aspects

covered will be:

(a) calibration against mass spectroscopy (previous publications on this topic were

found not to be applicable in all cases);

(b) effects of assembly processing variables on the sensor readings (in particular,

sensor readings are not stabilized until some time after seal; also any high tempera

ture operations after seal impact the readings obtained) ; and

(c) recommended method of use.

The conclusion is that this moisture sensor chip is an effective in-line monitor;

moreover it can also be used as a powerful problem solving tool, in which respect

it is superior to mass spectroscopy because it is (a) quick and (b) non- destructive.

However, calibration against mass spectroscopy is not straightforward; and this

calibration is affected by changes in assembly process flow and package configuration.

INTRODUCTION

There have been several publications about alumina moisture sensor chips, including

References 1 and 2, which describe the principal of operation of the Panametrics

Mini-Mod HT and how its electrical impedance can be calibrated against moisture

levels inside cerdip packages. These references conclude that the Mini-Mod HT can

be a reliable means of determining pass or fail to the limit of 5000 ppmy as defined

in Method 1018 of Mil-Std-883B. They also make the point that such a moisture

sensor chip can provide a valuable means of in-line process control, since a measure

ment can be taken quickly and easily shortly after seal.

This facility for process control is especially useful in an offshore assembly

operation remote from laboratories capable of performing moisture analysis by mass

spectroscopy. Therefore it was decided to implement the Panametrics Mini -Mod HT

as an in-line process control for cerdip assembly in AMD’s offshore assembly plant . This paper will describe some of the difficulties which were encountered during 1 1 .

i

implementation, and how they were resolved. All of the measurements were carrini

out on 24 lead cerdip packages.

171
DESCRIPTION OF PREVIOUS CALIBRATION PROCEDURE

It is necessary to summarize some of the calibration data contained in References 1 and 2 in order to clarify the situation.

The first stage in calibration is to determine the relationship between chip imped- ance (as measured by a meter manufactured by Panametrics called the Model 770MM) and

the amount of moisture per unit volume in the atmosphere adjacent to the chip's

surface. This calibration is actually unique for each individual chip; however the

variation from chip to chip within one batch is small enough that one calibration

curve can accurately represent all chips within that batch. This calibration is

performed as a service by Panametrics and Figure 1 shows an example of one such

calibration, which is obtained by decapping a package containing a sensor chip and

measuring its impedance as it is exposed to atmospheres of various known humidities.

Now there are two components to the moisture inside a sealed cerdip package. One

component is the moisture present as vapor. This is the component which is detected

by the sensor chip. The other component is the moisture which is adsorbed (more or less tightly) on the internal surfaces of the package. This component is not

detectable by the sensor chip, but part of it will contribute to a mass spectroscopy

measurement. This is because some of the adsorbed moisture will desorb from the

internal surfaces under the high temperatures and lower pressures obtained during

the measurement. So, in order to obtain a value which can be referred to as an "equivalent mass spectroscopy value", it is necessary to do a direct calibration of

ppmvsc (ambient vapor moisture level as directly detected bv the chip) against

ppmvms (moisture level obtained by actual mass spectroscopy) . The experimental

relationship obtained in Reference 1 was:

PPmvms =

t 6 - 42 x 106 (ppmvsc)] 0 * 3 9 7 (D

This relationship (which is shown graphically in Figure 2) is a best fit to a series

of measurements on 14/16 lead cerdips sealed by various manufacturers using various

sealing glasses. The relatively large amount of moisture desorbable from the

internal surface can be seen from Figure 2. For example, the Mil-Std-883B limit of

5000 ppnv as measured by mass spectroscopy (ppmvms) corresponds to only 300 ppnfy as detected by the sensor chip (ppmvSC )

.

This completes the calibration procedure, enabling a meter reading to be converted

to an equivalent mass spectroscopy value. It would seem relatively straightforward

to implement this procedure directly; however, in practice, some difficulties

appeared. One concerned calibration against mass spectroscopy; the second changes

in meter reading after seal.

CALIBRATION AGAINST MASS SPECTROSCOPY

Initial measurements used equation (1) to calculate the equivalent mass spectroscopy

values, and the results were suspiciously high. Therefore, it was decided to do an independent calibration against mass spectroscopy. Twenty-eight units sealed with

sensor chips were sent to Oneida Research Services, Inc. for mass spectroscopy

analysis. The crosses shown in Figure 3 are the actual calibration points; the

continuous curve is calculated as a best fit to the experimental points; and for

comparison, the dotted curve is a graphical representation of what would be obtained

by applying equation (1). It is apparent that the value of ppmvms calculated from

equation (1) is appreciably higher than that which was actually obtained.

172
[It should be noted that Figure 3 is a direct plot of meter reading against mass

spectroscopy, whereas the procedure previously described involves two stages; first,

meter reading to ppirvsc; second, ppmvsc to ppnvms- The reason for this two-stage

method is that the relationship between meter reading and ppmvsc varies from batch

to batch, so a separate calibration curve should be used for each batch of sensor

chips. However, the calibration curves for the first three batches received were

so close that it was decided, in the interests of simplicity, to use a constant

calibration curve, enabling a direct translation of meter reading to equivalent

mass spectroscopy.]

It is assumed that the prime reason for the discrepancy between the two curves shown

in Figure 3 is the fact that the experimental vehicles used were 24 lead devices,

which have a much larger internal volume than the 14/16 lead devices used to generate equation (1). With 24 lead packages, the ratio of moisture adsorbed on the

internatal surfaces should be smaller than for 14/16 lead packages, due to the lower

surface area: volume ratio. This explanation is consistent with the results

obtained. In any case, it is clear that the correlation to mass spectroscopy is a function of internal package configuration, and a separate calibration should be

carried out for each new package configuration.

CHANGES IN MEASURED VALUE AFTER SEAL

It was observed that the meter reading of a given sensor chip fell significantly

during the first few days after seal, simply during storage at room temperature.

An actual example is shown in Figure 4, where the fall after 3 days corresponds to

an apparent decrease of ever 1000 ppm. The presumed explanation for this effect is

that a few days are required for equilibrium to be reached in the process of

adsorption of moisture on to the internal surfaces of the package. The problem

introduced by this effect is that it is necessary to wait a few days after seal

before a valid reading can be taken - this detracts somewhat from the immediacy of

feedback necessary for effective process control.

This phenomenon was not the only type of change in meter reading after seal. All

cerdip production lots are processed through the standard Mil-Std-883B screens,

including High Temperature Storage (24 hours at 150°C). After this bake, the sensor chip meter readings were lower than before (see example shown in Figure 5).

The fall in meter reading during this baking operation is typically greater than

that which occurs simply by storage at room temperature.

To investigate this effect further, an experiment was done to see if similar

results could be detected by mass spectroscopy. A random sample of 20 units was pulled from a production lot directly after seal. The lot was then processed

through High Temperature Storage, after which a further random sample of 20 units

was pulled. The sampls were sent together to Oneida Research Services for moisture

measurement by mass spectroscopy. Histograms of the results are shown in Figures

and 7, which show a fall in measured moisture which parallels that detected by the sensor chip. This shows that during the bake some of the moisture vapor is

adsorbed "permanently" on some of the internal surfaces of the package - permanent 1 enough, at any rate, to survive the temperature and pressure conditions of the ma spectroscopy measurement

.

Permanent moisture reduction inside hermetic packages through chemical react i i has been described (Reference 3). It may be that such reactions occur during 1 1

*

•

High Temperature Storage process;

e.g. Si + 2H2 0 -> Si02 + 2H2 173
However, hydrogen was not detected consistently in all the packages analyzed, so this may not be the complete explanation, especially since the lowest temperature

evaluated in Reference 3 was 250°C, and maybe 150°C would be too low a temperature

for appreciable chemical reaction to occur. It could be that the adsorbed moisture

is bound with various degrees of tightness from loose physical bonding to a full

chemical reaction. This seems to be an area worthy of more investigation, for

this reason:

Any baking operation done after seal, but before moisture measurement, can affect

that measurement. The 150°C, 24 hour bake just described caused a reduction; on

the other hand, a lower temperature or shorter bake might increase measured values

by causing desorption of moisture from internal surfaces. Better understanding of

the mechanisms involved would enable more meaningful measurements to be done.

Anyway, this particular investigation was limited to establishing a sensor chip

measurement method that could be reliably related to mass spectroscopy. It was concluded that this could only be done by measuring the sensor chip after exposure

to all high temperature post seal operations (which in this case included not only

High Temperature Storage but also Temperature Cycling)

.

(Incidentally, all the data points in Figure 3 were obtained on units which had

been processed through these high temperature operations.)

This means that a measurement cannot be taken until a few days after seal. As

previously shown, such a delay is necessary anyway, even if no high temperature

operations were carried out, due to the effect shown in Figure 4. However, even

though this delay somewhat reduces the efficiency of feedback, the technique

remains an effective process control, since it is still a lot more immediate than

sending samples out to a mass spectroscopy laboratory.

CONCLUSIONS

The Panametrics Mini-Mod HT is a useful and reliable means of in-line moisture

monitoring. However, the following precautions should be taken.

1) Calibrate each individual package configuration individually against mass

spectroscopy.

2) Take a reading only after the level of moisture vapor has stabilized itself

through all post- seal processes.

REFERENCES

1) "Recent Advances in A1 2 0 3 "In- Situ" Moisture Monitoring Chips for Cerdip Package

Applications" by J.B. Finn (Mostek) and V. Fong (Panametrics) - March 1980 IEEE.

2) "Moisture Sensors, Mass Spectroscopy and Mil Standards" by John C. Hale (Texas

Instruments) and Victor Fong (Panametrics)' - National Bureau of Standards,

November 5, 1980.

3) "Attaining Low Moisture Levels in Hermetic Packages" by White, Striny and

Sammons of Bell Telephone Laboratories - July 1982 IEEE/Proc. IRPS.

174
FIGURE 1

CALIBRATION OF METER READING TO PP%

MINI - MOD®

CALIBRATION CURVE

PANAMETRICS, INC.

Waltham, Ma. 02154

(617) 899-2719 x 227

DATE 8 / 1 6/82

TYPE/WAFER amb

CUSTOMER

SEALED

DEW/FROST POINT (®C) UNITS

1.0 10 100 1,000 10,000

PPMy AT 1 ATM

175
ppm

(Moisture

Chip)

FIGURE 2

CHIP CALIBRATION VERSUS MASS SPECTROSCOPY

10 2 10 3 10 4 10 5

ppm (Mass Spectroscopy)

176
FIGURE 3

Meter Reading

177
FIGURE 4

CHANGES IN MEASURED VALUE AFTER SEAL

20 -

U)

G

•HG

CD Qi

G

<D •P

.18 -

.16 -

.14 4

.12 -

X

X

X

.10

Days After Seal

20 -

18 -

16 -

FIGURE 5

CHANGES IN MEASURED VALUE AFTER SEAL

(24 Hours at 150°C)

X

14 -

12 -

10 -

X

X X X

Post- Seal Post

Stabilization

Bake

T

1

1 2 Days After Seal

1 78
FREQUENCY

FREQUENCY

FIGURE 6

PPMy (After Seal)

FIGURE 7

PPMy (After High Temperature Storage)

179
5. SESSION IV HERMETICITY AND MOISTURE

5.1 HOW TO RAISE THE PERMISSIBLE LEAK RATE BY FOUR ORDERS OF MAGNITUDE

J. Gordon Davy

Westinghouse Manufacturing Systems and Technology Center

9200 Berger Rd., Columbia, MD 21045

ABSTRACT

This paper presents bad news and good news. The bad news is that the maximum

permissible leak rates specified in the military standards are too large by at

least 3 orders of magnitude, and that for microelectronic packages the true

values are too small to measure. The good news is that the true permissible

leak rate can be raised by four orders of magnitude or more by the inclusion

in the package of means to capture water as it enters through small leaks.

Three different means are presented. Experimental studies are needed before

any specification changes are made. Also, expressing moisture level in terms

of dew point and ppmV is misleading and should be avoided.

WHAT IS THE TRUE PERMISSIBLE LEAK RATE?

Expressing the Moisture Level

The maximum permissible leak rate for semiconductor devices is determined by

the maximum permissible increase in the internal moisture level. Since

moisture level is expressed in many ways, it is desirable to begin by

considering how best to express the level for this application.

Moisture levels based on measurement by an in-situ device (alumina capacitor,

interdigitated structure) are often reported as a measured or calculated dew

point. Dew point is an inappropriate measure because it neglects the primary

role of adsorption during the cooling process. In fact, the behavior of water

in small sealed packages cannot be understood or explained apart from an understanding of adsorption. For such packages, a dew point may not even exist [1] . 180
Moisture levels based on measurement by a gas analysis system are often

reported in parts per million by volume (ppmV). The unit ppmV is

appropriately used as a measure of relative abundance of the gas of interest

(water in this case) to the total of all gases. It is the ratio of the

partial pressure of water vapor to the total pressure, or the ratio of the

number of water molecules per unit volume to the number of all gas molecules

per unit volume. Relative abundance is what is determined by a mass

spectrometer, but it does not indicate how wet a package is. For example, in

most vacuum systems the background pressure is due primarily to water vapor.

The relative abundance of water in such systems is well above 500,000 ppmV,

but that does not mean they are wet.

Sometimes the relative abundance is multiplied by the total internal pressure,

to correct to one atm, but the resulting number is still reported as ppmV.

This is actually a misleading way of referring to the partial pressure of

water vapor in millionths of an atmosphere or microbars. As an example, 5000

ppmV (referred to 1 atm) equals 5000 microbars, alternately expressed as -3

5 millibars or 5 X 10 atm. (The partial pressure of a gas in a mixture of

gases is the pressure that would be measured for that gas if it were the only

gas present .

)

Since it is the partial pressure and not the relative abundance of water vapor

that indicates the wetness of the package, partial pressure is what should be

used. Conversion is simple. The partial pressure in microbars is numerically

equal to the relative abundance in ppmV, multiplied by the internal pressure.

If the internal pressure is unknown, it can be calculated by dividing the gas

3 content found during gas analysis (in pressure-volume units such as atm cm ) by the gas volume of the package.

Another useful way of referring to moisture level is the relative humidity.

RH is the ratio of the partial pressure of water vapor at a given temperature

to the saturation vapor pressure (maximum possible partial pressure) at that

temperature. The saturation vapor pressure of water increases exponentially

with temperature. Its value is well established for all temperatures and can

be found in reference handbooks. The alumina capacitor responds (with a temperature coefficient) to the relative humidity [1].

181
The moisture level can also be expressed as a vapor density in units of grams

or moles per unit volume of vapor space. The mass of water vapor per unit

_3

volume, m/V, corresponding to a partial pressure of 5 x 10 atm can be

calculated by assuming ideal gas behavior, which is reasonable for pressures

less than the saturation vapor pressure (i.e. for relative humidity less than

100%): m/V = PM /RT, where R, the gas constant, is expressed as 82.06 atm

3 -1 -1 W

-1 cm mole K and M is the molar mass of water, 18 g mole . Taking

W

-3

T = 298K for room temperature, m/V is 3.7^g cm

Dividing the mass of water vapor present in the package by the internal

surface area gives a measure of susceptibility to surface leakage currents if

it were to condense or be adsorbed: one monolayer is approximately 30 ng

-2 cm

It is also important to distinguish which moisture level in the package is

being referred to [1]. Total moisture includes not only water vapor, but also

adsorbed and absorbed water. Total moisture is determined by those gas

analysis techniques which integrate the moisture signal over time after

package puncture. It includes water which until puncture was not water vapor

and thus was unavailable for adsorption or condensation upon cooling. The

appropriate way to measure the moisture level by gas analysis is to soak the

package at the maximum anticipated storage temperature long enough for

equilibrium to be established (this sets the maximum level of water vapor),

and then measure the moisture level based on the peak value as quickly as possible after puncture.

The reason for this method is that puncturing upsets the equilibrium; it is

important to minimize the contribution due to water which enters the vapor

phase after puncture. The time to reach equilibrium depends on materials

present in a package. The time constant for phys i sorption (metal surfaces) is

less than a second; for chemisorption (ceramics) and absorption (organic

materials such as adhesives) it may be days. If such latter materials are present, in significant quantities, the required time should be determined by

experiment

.

1 82
Calculating the Permissible Leak Rate

The maximum permissible leak rate is calculated by calculating the rate at

which water vapor can be permitted to enter the package. It involves:

3

1. The package internal cavity (gas) volume V cm .

2. The required lifetime (including storage) of the package, t^ years.

3. The maximum permissible moisture level. Though generally taken as _3

5 X 10 atm (5000 ppmV) at 100°C, the actual physical requirement

is that there not be a layer of surface water thick enough to allow

enough current flow for dendritic growth or corrosion. The surface

water need not be condensed (in fact the RH can be as low as about 35%

at room temperature); adsorbed water a few molecular layers thick has

been found sufficient [2], even down to temperatures of -20°C. (At

lower temperatures, ion mobility is slow enough to not be a problem.)

It should be understood that if the water vapor is at the maximum

permissible level, then no further increase in moisture level is

acceptable, and the permissible leak rate is zero. This of course

cannot be measured. In other words, the moisture level at the time

the package is sealed should be low enough to give a safety margin for

water vapor which enters through the maximum permissible small leak

during the lifetime of the package.

For the calculation which follows, it is assumed that the

room-temperature moisture level in the package can be allowed to

_3 increase during the package lifetime by an amount^p^ of 5 X 10

atm. It should be remembered that microelectronic packages have a large ratio of internal surface area to internal volume, so that the

level of water vapor drops sharply as the package is cooled. Thus the

tolerable partial pressure of water vapor in the package at room temperature (for example) is higher than at 0°C.

183
4. The average ambient moisture level p , which varies with the wa

intended application of the device. To bracket the range, two values

are considered:

a. pwa = 0.043 atm, corresponding to 100% RH at 30°C or 75% RH

at 35°C. This applies to the tropics.

b. p = 0.011 atm, corresponding to an air-conditioned

wa

office. For this case, an unacceptable rise in the RH and

amount of adsorbed water would be a possibility if the package were taken outdoors on a cold day.

The maximum rate at which water vapor can be allowed to enter a package is

given by

Rwmax - ^Pw

V/t« | (1)

! 1 I

This rate, which is not directly measurable, needs to be converted to an air-equivalent standard leak rate L - the rate at which air at 1 atm pressure

(absolute) on one side of the leak would escape into vacuum on the other. The

relationship between rates under different conditions depends on the type of

gas flow that exists. For this calculation, it will be assumed that the

transport mechanism for gas molecules through a leak is molecular flow. This means that the leak rate for any gas is proportional to the difference in

partial pressure for that gas between inside and outside the package, and to

the inverse square root of the molecular mass of the tracer gas.

The maximum permissible air-equivalent standard leak rate is given by

kmax ~ ^wmaxd &tm/pwa ) (Myf/Mg^)

j

(2)

where M and M are the molecular masses of water (18) and air (28.7). w a Use of the values discussed forAp and p leads to

184
(3a)

(3b)

It is sometimes felt that the entry of water vapor could be reduced if the

package were pressurized. The intuitive impression is that the gas inside

would prevent the entry of air by blowing it back. However, for small leaks,

the principal flow mechanism is molecular flow, in which encounter between gas

molecules along the leak path is rare. Note that the equations above do not

depend on the initial pressure in the package. Thus, pressurization does not

reduce the rate of entry of water molecules.

Comparison with Military Standards

Figure 1 compares the permissible leak rate based on maximum allowable

internal moisture level (tropics and office) for an assumed 10-year lifetime

with the rates specified in the military standards. The conclusions to be

drawn from this figure are:

1. There are no specifications directly applicable to large packages.

2. The existing specifications are somewhat inconsistent.

3. The existing specified leak rates do not increase linearly (as they

should) with package volume.

4. The existing specifications exceed the limits calculated by the

techniques presented here by three to four orders of magnitude. In

other words, the existing specifications can only guarantee

protection from moisture for lifetimes of days or weeks, not years.

—8 3 It is important to note that measuring leak rates smaller than 10 atm cm

sec

1 presents formidable difficulties. Also, very small leaks tend to plug

by themselves from ordinary handling. The plug may prevent tracer-gas flow

during leak testing but be permeable to water. If leak test results are to be

reliable, packages must be handled carefully to prevent this plugging until

after the final test. One way around the problem of needing to measure small

leak rates is to look for increases in the internal moisture level, using an in-situ moisture sensor or a bare integrated circuit chip [2].

185
Standard

Air

Leak

Rate,

atm

cm 3

sec

10" 5 r

10

10

10

10

10

10

10

10

Figure 1

MIL-STD-202, Method 112D

Test Condition Ilia, IIIc (helium)

Test Condition Illb, (krypton)

MIL-STD-7508, Method 1071.1

Test Condition H (helium)

Test Condition G (krypton)

MIL-STD-883B , Test Method 1014.4

Test Condition Al, A2 (helium)

Test Condition B (krypton)

10- 1 1 10 100

GAS VOLUME IN PACKAGE, cm3

1000

Calculated Maximum Permissible Leak Rates Compared with Military

Specifications Rates by Various Methods

186
A reasonable question to raise is why, with such dif f icult-to-meet

requirements, aren't there more moisture-related failures? There are several

possible answers.

1. Many packages are truly hermetic - they don't leak at all.

2. Many packages live in environments where chilling never occurs(e.g., an office).

3. Water condenses, but not where it can cause problems, because the

vulnerable areas run hotter than the ambient.

4. Water gets used up as fast as it gets in by reacting chemically

with the materials of construction of less-critical parts (e.g.,

rusting of package walls).

5. There are no vulnerable areas (chip passivation is defect-free,

surfaces are conformally coated, etc.).

6. The failure is not recognized by service personnel as moisture-related

and thus is not reported.

There have been many published reports of attempts to establish a statistical

correlation between moisture level or leak rate and failure rate of

microelectronic devices. Unfortunately none has been successful, in spite of

the fact that the role of water in causing failures is well established.

HOW CAN THE PERMISSIBLE LEAK RATE BE INCREASED?

The strategy for increasing the permissible leak rate is to capture the water

as it leaks in, so it does not become available as water vapor. This can be

done by the use of a suitable dessicant, or by decomposing the water in a chemical or electrochemical reaction.

Molecular Sieve Dessicant

Molecular sieve is the term used to refer to a class of synthetically produced

crystalline alumino-silicates that have been "activated" for absorption by

removing their water of hydration. This dehydration does not cause a change

in structure. Instead, it leaves pores approximately the size of water

187
molecules. The name derives from its property of being open to small

molecules while excluding large ones. While dessicants are often said to

adsorb water, in the case of molecular sieve it is actually a bulk rather than

a surface process, and thus the term absorption is more appropriate.

Molecular sieves can be purchased inexpensively from chemical supply houses in

the form of small pellets or spheres.

Molecular sieve dessicant allows the permissible leak rate to be increased,

because a far greater mass of water can exist in the package for a given

relative humidity. Figure 2A shows for four temperatures how the relative

humidity varies with the weight percent of water which type 4A molecular sieve

has absorbed [3]. The curves for types 3A and 5A sieve are similar.

As a package containing the sieve is heated, a very small fraction of the

absorbed water desorbs, causing the relative humidity to increase somewhat.

The results can be predicted adequately by assuming that the weight percent of

absorbed water does not change. It can be seen that if the dessicant contains

10 weight percent water and is at equilibrium with the ambient in the package,

the relative humidity is very low - not only at 0° and 25°C, but even when

heated to 100°C or 125°C. In fact, the partial pressure at these latter

-3 -3

temperatures is 6 x 10 and 12 X 10 atm respectively. When the hot

package is allowed to cool, re-absorption reduces the partial pressure again,

although it may take some time to return to complete equilibrium. The

superiority of molecular sieve to other dessicants in retaining water when

heated is shown in Figure 2B.

The specific gravity of the sieve is about 0.75. If 50% of the gas volume in

the package is occupied by molecular sieve, and this sieve is allowed to

absorb 10% of its weight in water, the mass of water per unit volume is m/V = 0.75 X 0.5 X 0.1 = 37 mg cm

-3

.

In the assumptions upon which equations 3a and 3b are based, the pressure was

_3 allowed to increase by 5 X 10 atm. The mass of water causing this pressure

_3 increase was calculated above as 3.7^g cm . The ratio of m/V <50% sieve)

4 to this figure is 10 , which means that the use of molecular sieve in the

package increases the permissible leak rate by this factor. This increased

rate is large enough to measure even for small packages (see Figure 3).

188
WEIGHT % WATER ADSORBED

OR ADSORBED

Figure 2. A. Isotherms for Type AA Molecular Sieve; B. Isobars for Three

Dessicants: Silica Gel and Alumina (p = 0.013 atm) and Type AA Molecular

wa

Seive (p = 0.005,0.013 atm). wa

WITH DESSICANT

L (tropics)

max

2.9 X 10

5 V/t,

L (office) = 1.1 X 10

4V/t

max j

(Aa)

(Ab)

The results are proportional, of course, so that if 25% of the gas volume is

occupied by molecular sieve, the permissible leak rate is increased by 5,000

instead of 10,000.

The dessicant could be in a paper bag, or held in place as a covering on the

underside of the cover with an adhesive which was permeable to water vapor.

Unfortunately, the use of dessicants in microcircuit packages is prohibited

(without express approval) by MIL-STD-A5AG , Requirement 6A and MIL-M 38510E.

189
There are two reasons for the military’s reluctance to accept dessicants [4]:

1. There have been some bad incidents when another type (not molecular

sieve) was used. Upon heating, the dessicant released a significant

amount of water which upon chilling condensed before it could be

re-adsorbed (see Figure 2B) . Molecular sieve differs from other

desiccants in that is must be heated far above normal maximum

storage temperatures before it releases its water.

2. Every new material added to a package has to be evaluated for

unexpected secondary effects, and may require screening by the

manufacturer or user for purity.

Chemical Reaction

Another way of capturing the water that leaks into the package is by an irreversible chemical reaction, referred to as gettering. Here are several

possibilities

:

1. Reaction with an unplated area on the underside of the Kovar lid of

a hybrid package, burnished just before sealing to expose fresh

metal, to form rust and hydrogen gas. This reaction may be partially

reversible and thus not as reliable as the other possibilities [5].

2. Reaction with silicon in gold-silicon eutectic, melted in a dry

environment to expose fresh silicon, to form silicon oxide and

hydrogen [6].

3. Reaction with a suitable metal such as barium that is evaporated

after the package is sealed - a method that has been widely used for vacuum tubes, to form metal oxide and hydrogen.

4. Reaction with suitably packaged calcium carbide to form calcium

hydroxide and acetylene gas [71.

The carbide reaction is

CaC2 + 2 H2 0 > Ca(OH) 2 + C2 H2

calcium carbide water calcium hydroxide acetylene

190
Standard

Air

Leak

Rate,

atm

cm 3

sec

10 -i 3

l 1 1 1 1 1 1 10- 3 10- 2 10- 1 1 10 100 1000

GAS VOLUME IN PACKAGE, cm3

Figure 3 Maximum Permissible Leak Rate for Packages Containing 50% Molecul

Sieve Dessicant Compared to Rates Without Dessicant
The equation above shows that one mole of carbide reacts with two moles of

water. Assuming (as for molecular sieve) a 50% filling of the package and

given a specific gravity of 2.22 and molecular mass of 64.1 for calcium

3 carbide this means that 623 mg of water per cm of gas volume can be allowed

to leak in. This is about 17 times greater than for molecular sieve.

The acetylene partial pressure rises only until a steady-state condition

develops, at which one molecule of acetylene escapes for every two molecules

of water entering. Using a method similar to that shown in the next section,

the corresponding partial pressure is calculated to be 0.62 p , a few wa hundredths of an atm.

An example of suitable packaging might be a paper bag or a thin coating of an adhesive which would be permeable to water vapor and acetylene. Since calcium

carbide is not really a dessicant, it is not clear whether there is any

military restriction on its use.

Electrochemical Reaction

Water can be decomposed into its elements hydrogen and oxygen by the

electrochemical reaction called electrolysis:

2 H

2 0 — 2 H2 + 0

2

Required is a suitable medium for carrying water molecules to the electrodes,

and a potential difference of at least two volts.

This could be done in a sealed package by use of a special material referred

to as a solid polymer electrolyte, a sulfonated fluorocarbon with the DuPont

trade name Nafion. This material has been used for moisture analysis in gas

streams [8], using it in the form of a thin-wall tube with platinum wire

spirals inside and out for electrodes.

A sheet of Nafion could have an electrode grid applied to each side, say by

evaporation (see Figure 4). A piece could be mounted in a sealed electronic

package with the electrodes attached to two feedthrough pins. Most electronic

packages are connected to a DC power supply of at least 3 volts anyway, so no additional external wiring would be involved.

192
Solid Polymer Electrolyte

(DuPont Nation Sulfonated Fluorocarbon)

o2

For diagnostic purposes an electrometer could be used. This would allow

determination of moisture content, using the Faraday constant of 96,500

coulombs per gram-equivalent (10,700 coul per gram of water).

With voltage applied to the electrodes, water molecules diffuse into the

polymer and hydrogen and oxygen molecules diffuse out. With continuous power,

a slight overpressure will develop, since two molecules of reactant (water)

yield three molecules of product (hydrogen and oxygen). The partial pressure

of hydrogen and oxygen inside will increase until at steady state, the escape

rate equals the production rate.

This can be expressed

Rw

= rH2 = 2R02

Expressing the rates in terms of the various partial pressures, molar masses,

and the air-equivalent standard leak rate, the steady-state partial pressures

can be expressed in terms of the ambient moisture level pwa

193
PH2 = Pwa “ Pwa^

APO2 = Pwa = 2pwa/3

PH2 + 4P02 = Pwa

The partial pressure of oxygen in the package rises until it exceeds that of

air (0.21 atm) by^p . During this time the partial pressure of fill gas

drops. These results are independent of the size of the leak over the range

for which molecular flow is a suitable assumption. For larger leaks, the

limit is set by the rate at which the Nafion could support electrolysis of the

water

.

SUMMARY

This paper has presented the case for allowing the use in hermetic packages of

materials which can capture water vapor and thus prevent it from increasing to

a level which could cause damage to the devices inside. Without the use of

such materials, the true maximum permissible leak rate is too small to measure.

Three such materials have been presented: molecular sieve, which absorbs

water, calcium carbide, which reacts with water, and Nafion polymer

electrolyte, which support the electrolysis of water. Since new materials

added to a package may cause unanticipated effects, the ideas presented here

should be studied experimentally before any specification changes are made.

The case has also been presented for abandoning the expression of moisture

level in terms of dew point and relative abundance (ppmV) in favor of

expressions in terms of partial pressure (atm, millibars, etc.), relative

humidity, and mass per unit volume and area.

1 94
REFERENCES

1. J. G. Davy, "Thermodynamic and Kinetic Considerations of Moisture Sorption

Phenomena", pgs. 184-200, Moisture Measurement Technology for Hermetic

Semiconductor Devices. II . NBS/RADC Workshop, November 5-7, 1980, NBS Special

Publication 400-72.

2. R. P. Merrett et al , "A Simple Method of Using the Die of an Integrated

Circuit to Measure the Relative Humidity Inside Its Encapsulation", Proc

.

International Reliability Physics Symposium . 1980, 17-25.

3. "Molecular Sieve Type 4A Pellets, Water Adsorption Isotherms", Union

Carbide Data Sheet F-43A-1.

4. Robert W. Thomas, private communication

5. W. E. Swartz et al, "The Adsorption of Water on Metallic Packages", Proc

.

International Reliability Physics Symposium , 1983, 52-59.

6. M. W. White et al , "Attaining Low Moisture Levels in Hermetic Packages",

Proc. International Reliability Physics Symposium . 1982, 253-259.

7. W. M. Hickam and W. R. Morgan, "Mass Spectrometr ic Moisture Analysis of

Hermetic Semiconductor Devices by Acetylene Conversion", pgs. 153-158 in

Moisture Measurement Technology for Hermetic Semiconductor Devices , ARPA/NBS

Workshop V, March 22-23, 1978, NBS Special Publication 400-69.

8. D. D. Lawson, "Long-Lasting Solid-Polymer Electrolytic Hygrometer",

Technical Support Package for NASA Tech Brief . Vol. Ill, No.l, Item 52, June

1978.

195
5.2 HERMETICITY AND MOISTURE INGRESS

A. DerMarderosian

Raytheon Company

Sudbury, MA 01 776

(61 7) 443-9521 , x2791

Abstract

This discussion will deal with the basic concepts of hermeticity testing and

current practices and procedures in the electronics industry. The talk will

also include work related to moisture ingress rate as a function of air leak

rate and offer a rationale which can be used as a guideline for hermeticity

specifications.

Summary

Hermeticity testing has a long and somewhat tarnished history in the electronics industry. When electronic components were large and somewhat simply

structured, the bubble tests and crude helium leak tests of yesteryear seemed

reasonable and "comfortable." With the increasing use, however, of smaller

semiconductors and finally integrated circuits, the value of those test techniques became questionable and several new ones were quickly developed and

adopted by the industry. For a time, everyone had his favorite test procedure, including some of the older ones. The basic principle of each of these

types of tests will be discussed, highlighting their advantages and disadvantages. In addition, specifications will be discussed, particularly the maximum allowable leak rates and how they affect our own industry. Moisture

ingress rates as a function of air leak rate will be examined from an experimental and theoretical viewpoint. Finally, these data will be examined with

particular attention on the maximum allowable moisture content of packages as specified in Method 1018 of MIL-STD-883.

196
OUTLINE

• HERMETICITY TESTING

• MOISTURE INGRESS

• LEAK RATE GUIDELINES

DEFINITION OF HERMETICITY

“THE STATE OR CONDITION OF

BEING AIRTIGHT”

NEED FOR HERMETICITY

TO CONTAIN THE MATERIAL IN A PACKAGE

AND EXCLUDE ALL OTHERS OUTSIDE OF

THE PACKAGE I.E., GENERALLY LIQUIDS

AND GASES IN THE HOPE OF OBTAINING A

MORE RELIABLE PART

197
STANDARD LEAK RATE IN THiS DISCUSSION IS

DEFINED AS THE QUANTITY OF DRY AIR @ 25°C PER

UNIT TIME WHICH PASSES THROUGH A LEAK WITH A

PRESSURE OF 14.7 PSIA ON ONE SIDE OF THE LEAK

AND A PRESSURE OF LESS THAN 1 TORR ON THE

OTHER SIDE. UNITS ARE GIVEN AS FOLLOWS:

ATMOSPHERIC CUBIC CENTIMETERS/sec, AIR

OR

ATM cc/sec, AIR

STANDARD CONDITIONS FOR

LEAK RATE DETERMINATION

198
TYPICAL RANGE OF LEAKAGE

G.W.N.

L.R. WGT (AIR) WGT (H20) # OF MOLECULES

1 X 10 * cc/sec 1.3 X 10 12 gms 8 X 10~ 13 gms 2.7 X 10

10

1 X 10

-7

1.3 X 10

_1° 8 X 10" 11 2.7 X 10

12

1 X 10* 1.3 X 10'® 8 X 10* 2.7 X 10

14

1 X 10~ 3 1.3 X 10

® 8 X 10

-7

2.7 X 10

1 ®

1 cc = 1.3 X 10 * gms 1 cc = 8 X 10 4 gms

M.G.W.N.

L.R. TIME TO LEAK ONE (1) cc

1 X 10 cc/sec 10 SECONDS

1 X 10

-3

1000 SECONDS

1 X 10"®

1.2 DAYS

1 X 10~7 4 MONTHS

1 X 10~®

33 YEARS

199
HISTORY OF LEAK TESTING

FIRST MILITARY SPEC CALLED OUT (MIL-S-19500)

• JOY BOMB TEST (1956) • HELIUM LEAK TEST (~ 1960)

MAX. ALLOWABLE LEAK RATE ALL

PARTS = 1 X 10

-8

cc/sec

FOLLOWED SHORTLY THEREAFTER BY IMPROVEMENTS IN

GROSS LEAK TEST (EARLY 1960’s)

BUBBLE

• HOT WATER, OIL, ETHYLENE GLYCOL, GLYCERINE

(GENERALLY 90-150°C)

• VACUUM (AP ~ 10-14 PSI) • BACKFILL (PRESSURIZED IN FLUID, THEN HOT BATH)

OTHER

• WEIGHT TEST, OMNI-SEAL, FREON L.D.

FINE LEAK TEST WAS FURTHER REFINED IN

MIL-STD-883 (1968)

• TEST CONDITION WAS MORE CAREFULLY DEFINED WITH

ADDITION OF TERRIFYING EQUATION WHICH ACCOUNTED

FOR PERTINENT PARAMETERS

200
PRESENT POPULAR

TEST METHODS

GROSS LEAK (MIL-STD-883, 750, 202)

• BUBBLE TEST — OPERATOR OBSERVES FOR

BUBBLES ESCAPING FROM THE PART

PRINCIPLE: AP IN PACKAGE FORCES

GAS OUT INTO FLUID

METHOD: HEAT/A PRESSURE

BUBBLE TEST (CONT’D)

PERTINENT FACTORS:

• SURFACE TENSION OF FLUID

• TEMPERATURE OF FLUID

• LEAK SIZE/DIMENSION

• PRESSURE DIFFERENTIAL

BUBBLE FORMATION

LIQUID

(SURFACE TENSION)

e

LEAK PATH (PRESSURE)

P

CAPILLARY

(DIAMETER)

d

201
INCREASE IN PACKAGE PRESSURE AS

A FUNCTION OF TEMPERATURE OF FLUID

DIFFERENTIAL PRESSURE REQUIRED TO INITIATE

BUBBLING VS LEAK RATE IN VARIOUS FLUIDS

(CAPILLARY LEAK)

BUBBLE TEST

CHART

MIL-STD

PROC./CONDITION TYPE AP SENSITIVITY

202/COND. A (HOT MIN. OIL) 4.9 5 X 10~4 cc/sec

202/COND. B (SIL. VACUUM) 13 2 X 10

s

202/COND. D (HOT FC-40) 4.9 3 X 10-

750/COND. Ct (HOT FC-43) 4.9 3 X 10-

750/COND. Ca (BACKFILL) UP TO ~ 80 1 X 10“*

750/COND. D (HOT ETH. GLY) 3.7 8 X 10

-4

750/COND. F (HOT MIN. OIL) 4.9 5 X 10~ 4

883/COND. C (BACKFILL) UP TO ~ 80 1 x io202
NON-BUBBLE GROSS LEAK TESTS

• WEIGHT TEST (MIL-STD-883, COND. E. DEV. AT RAYTHEON

DURING APOLLO PROGRAM)

• OMNI-SEAL HERMETICITY (WESTINGHOUSE MID 1960’S)

• FREON LEAK DETECTOR (G.E.)

ALL TESTS USE THE BACKFILLING TECHNIQUE

WHICH FORCES A LIQUID INTO A LEAKING

DEVICE UNDER HIGH PRESSURE (>30 PSIG)

WEIGHT TEST

• WEIGH THE PART

• PRESSURIZE UNDER FC-77

• RE-WEIGH

• WEIGHT INCREASE > 0.5 Mg IS REJECT

COMMENTS:

1. MOST RELIABLE OF ALL GROSS LEAK TESTS

RANGE: 5 X 10 ® =* > 10

1 cc/sec

2. NOT POPULAR; MAIN COMMENT IS LOST

203
OMNI-SEAL HERMETICITY TEST

• PRESSURIZE UNDER ALCOHOL

• MEASURE ELECTRICAL LEAKAGE BETWEEN COVER

AND LEADS, ETC.

• REJECT IF LEAKAGE IS EXCESSIVE (<200 MEGOHM)

COMMENTS

• NOT A MIL-SPEC; NOT IN COMMON USE

• POTENTIAL HAZARD IF OPERATOR/EQUIPMENT

MISSES LEAKER, i.e., IONICS IN PACKAGE

• MAY ONLY PARTIALLY FILL AND CAUSE POSSIBLE

LONG TERM RELIABILITY PROBLEM

FREON LEAK DETECTOR

• PRESSURIZE UNDER FREON T.F.

• “SNIFF” PACKAGE WITH LEAK DETECTOR

• REJECT IF SIGNAL EXCEEDS BACKGROUND LEVEL

COMMENTS

• NOT A MIL-SPEC; NOT IN COMMON USE

• HAS HIGH “OVERKILL” RATE

• QUESTION RELIABILITY OF PART IF MISSED

204
FINE LEAK TESTING

HELIUM LEAK TEST

• THE HELIUM LEAK TEST IS PERFORMED BY ALLOWING THE

LEAKING GASES OF THE TEST ITEM TO PASS THRU A SPECIAL

MASS SPECTROMETER WHICH IS DESIGNED TO BE SENSITIVE TO

HELIUM GAS

• PART MUST HAVE HELIUM GAS PRESENT INTERNALLY, i.e., INITIAL

FILL OR BACK PRESSURE TECHNIQUE (“BOMBING”)

1. INITIAL FILL 2. BACK PRESSURE (“BOMBING”)

UNDER

PRESSURE

VACUUM

TEST CHAMBER

205
HELIUM LEAK TEST (CONT’D)

• 90/10 MIXTURE IS SIMPLEST. READING ON METER IS

MULTIPLIED BY 10 TO OBTAIN STANDARD HELIUM LEAK

RATE. THE VALUE IS THEN DIVIDED BY 2.7 TO OBTAIN

THE STANDARD AIR LEAK RATE

EXAMPLE:

METER READING = 1 X 10

-7

cc/sec

HELIUM L.R. = 1 X 1<Te cc/sec He

AIR L.R. = 3.7 X 1(T7 cc/sec AIR

LEAK RATE DETERMINATION

• BACK PRESSURE (“BOMBING”)

1. REQUIRES CALCULATION OF THE TRUE LEAK RATE USING

SCARY EQUATION:

Ri

LPe

Po

'v c

J \

WHERE

R1 - THE MEASURED LEAK RATE OF TRACER GAS (He) THROUGH THE

LEAK IN ATM cc/s

L = THE EQUIVALENT STANDARD LEAK RATE IN ATM cc/s

PE = THE PRESSURE OF EXPOSURE IN ATMOSPHERES ABSOLUTE

P0 = THE ATMOSPHERIC PRESSURE IN ATMOSPHERES ABSOLUTE (1)

Ma = THE MOLECULAR WEIGHT OF AIR IN GRAMS (28.7)

M = THE MOLECULAR WEIGHT OF THE TRACER GAS (He) IN GRAMS (4)

t 1 = THE TIME OF EXPOSURE TO PE, IN SECONDS

t2 = THE DWELL TIME BETWEEN RELEASE OF PRESSURE AND LEAK

DETECTION, IN SECONDS

V = THE INTERNAL VOLUME OF THE DEVICE PACKAGE CAVITY IN CUBIC CENTIMETERS

206
RADIOISOTOPE TESTING

• AN ALTERNATE METHOD OF FINE LEAK TESTING

WHICH UTILIZES Kr

®5 GAS AS A TRACER INSTEAD

OF HELIUM

• THIS TECHNIQUE USES THE BACK PRESSURE

METHOD ONLY AND HAS ESSENTIALLY THE SAME

EFFECTIVE RANGE AS HELIUM

• DETECTION OF GAS PENETRATION INTO LEAKY

PACKAGES IS ACCOMPLISHED BY MEASURING THE

COUNT RATE OF EMITTED GAMMA RAYS THRU THE

WALLS OF THE PACKAGE

207
RADIOISOTOPE TESTING (CONT’D)

• AS WITH THE HELIUM TEST, THIS METHOD ALSO

HAS A METHOD OF CALCULATING LEAK RATE AS

SHOWN BELOW

Q.

R

SKTPt

Q8 = THE MAXIMUM CALCULATED LEAK RATE ALLOWABLE, IN ATM cc/secKr.

R = COUNTS PER MINUTE ABOVE THE AMBIENT

BACKGROUND.

8 = THE SPECIFIC ACTIVITY, IN MICROCURIES PER

ATMOSPHERE CUBIC CENTIMETER.

k = THE OVERALL COUNTING EFFICIENCY OF THE

SCINTILLATION CRYSTAL IN COUNTS PER MINUTE PER

MICROCURIE OF KRYPTON-85.

T = SOAK TIME, IN HOURS.

P = P.*-P|a

, WHERE P. IS THE ACTIVATION PRESSURE IN ATMOSPHERES ABSOLUTE AND P, IS THE ORIGINAL

INTERNAL PRESSURE OF THE DEVICES IN ATMOSPHERES

ABSOLUTE.

t = 3600 SEC/HOUR

ANOMALOUS BEHAVIOR OF LEAKS

• MAJORITY OF LEAKS ARE NOT CAPILLARY HOLES;

CRACKS WHICH MEANDER

• PHYSICAL NATURE TENDS TO MAKE THEM — PRESSURE SENSITIVE — TEMPERATURE SENSITIVE — PRONE TO CAPILLARY ABSORPTION AND PLUGGING

i.e. FLUIDS AND HUMIDITY

2Q8
GEOMETRY OF REAL LEAKS

SIDE

A

B

PURPOSE

• TO DETERMINE THE RATE OF WATER VAPOR

PENETRATION INTO PACKAGES WITH KNOWN

LEAK RATES.

• TO PROVIDE DATA WHICH MAY BE USED AS

A GUIDELINE IN ESTABLISHING MAXIMUM

ALLOWABLE LEAK RATES FOR SEMICONDUCTOR

PARTS.

• TO ESTABLISH AN ANALYTICAL METHOD FOR

USE IN CALCULATING WATER VAPOR PENETRATION RATES FOR ANY DEVICE TYPE WITH A

KNOWN LEAK RATE.

209
COMPLETED ASSEMBLY OF CALIBRATED

GROSS LEAKER AND TO-5 CAN

CALIBRATED GROSS LEAKER

CONSTRUCTED FROM 1 MM CAPILLARY

SURFACE CONDUCTIVITY

MOISTURE SENSOR

MOLECULAR FLOW

MOLECULAR FLOW OCCURS WHEN THE MEAN FREE

PATH OF THE GAS IS GREATER THAN THE LONGEST

CROSS-SECTION DIMENSION OF THE PHYSICAL

LEAK.

Q

30.48 r £

210
WATER

VAPOR

LEAK

RATE

IN

CC/SEC

AIR LEAK RATE VS WATER VAPOR LEAK RATE

AIR LEAK RATE VS WATER VAPOR LEAK RATE

AIR LEAK RATE IN CC/SEC
LEAK RATE SPECIFICATION

• THE INGRESS OF MOISTURE IS THE MAJOR

CONCERN FOR SEMICONDUCTORS. PROCESS

LIQUIDS ALSO A CONCERN

• MAXIMUM ALLOWABLE MOISTURE LEVEL IN

PACKAGES AT PRESENT IS 5000 PPM. 500 PPM

FOR THE FUTURE

.011V

T

Qa

r = TIME CONSTANT IN HOURS FOR H20 VAPOR

INGRESS (63%)

V = INTERNAL VOLUME OF DEVICE IN CC

Qa = AIR LEAK RATE IN CC/SEC

WATER VAPOR PENETRATION TIME AS A FUNCTION

OF AIR LEAK RATE FOR VARIOUS TYPE PACKAGES

212
MOISTURE PENETRATION TIMES

TIME TO EXCEED

MAX. ALLOW. L.R (1018) VOLUME METHOD <U AVG. AMB. = 10,000 PPM

5000 PPM 500 PPM < 0.4 CC

R-I5X10 ‘ CC/SEC)

FIXED He

(.01 CC) 4.5 X 10'* CC/SEC 71 DAYS 5 DAYS < 0.4 CC

R - (5 X 10'* CC/SEC)

FIXED He

[J39 CC) 2.8 X 10~7 CC/SEC 1.2 YEARS 32 DAYS > 0.4 CC

R - (2 X 10~ 7 CC/SEC)

FIXED He

(.4 CC) 6 X 10~ 7 CC/SEC 214 DAYS IS DAYS

2: 0.4 CC

R - (2 X 10

-7 CC/SEC)

FIXED He

(5 CC) 2 X 10~* CC/SEC Z2 YEARS 57 DAYS

MOISTURE PENETRATION TIMES

TIME TO EXCEED

MAX. ALLOW. L.R. (1018) VOLUME METHOD (L) AVG. AMB. = 10,000 PPM

5000 PPM 500 PPM < 0.01 CC FLEXIBLE (He)

(.009 CC)

5 X 10'* CC/SEC 58 DAYS 4.1 DAYS

-01-.4 CC FLEXIBLE (He)

(.01 CC) 1 X 10~ 7 CC/SEC 32 DAYS 2.3 DAYS

( 4 CC) 1 X 10' 7 CC/SEC 3.5 YEARS 92 DAYS > 0.4 CC .41 CC 1 X 10 * CC/SEC 132 DAYS! 9.4 DAYS

5 CC 1 X 10~* CC/SEC 4.4 YEARS! 114 DAYS

MOISTURE PENETRATION TIMES

TIME TO EXCEED

MAX. ALLOW. L.R. (1018) VOLUME METHOD <L) AVG. AMB. - 10,000 PPM

< 0.01 CC FIXED (K”)

(.009 CC)

1 X 10'* CC/SEC

5000 PPM

.79 YEARS

500 PPM

21 DAYS

;> 0.01 FIXED (K”)

(0.01 CC)

5 X 10“° CC/SEC 64 DAYS 4.8 DAYS

(0.4 CC) 6 X 10'* CC/SEC 7.0 YEARS .5 YEAR

(5 CC) 5 X 10 * CC/SEC 86 YEARS 6.3 YEARS
EFFECTS OF PACKAGE VOLUME

MODEL FOR CALCULATIONS

X

INTERNAL

AREA

CM CM2

.17 .08

.37 .38

.79 1.7

1.7 8.2

3.7 38

7.9 175

INTERNAL

VOLUME

CM3

AREA

RATIO VqL

.001 80

.01 38

.1 17

1 8.2

10 3.8

100 1.75

MAXIMUM ALLOWABLE PACKAGE

MOISTURE CONTENT

VOLUME

MOISTURE

CONTENT

MOISTURE

CONTENT

APPARANT

SURFACE AREA

WATER

THICKNESS

NUMBER

OF MONLAYERS

CM3 PPM, GRAMS CM2 CM —

.001 13,000 9.6 X 10

" .08 1.2 X 10

7 3

.01 6,200 4.6 X 10~* .38 1.2 X 10

7 3

0.1 2,700 2 X 10~ 7 1.7 1.2 X 10“ 7 3

1.0 1,300 9.8 X 10“ 7 8.2 1.2 X 10' 7 3

10 600 4.6 X 10

"* 38 1.2 X 10

7 3

100 280 2.1 X 10~ B 175 1.2 X 10

7 3

214
PERCENT

MOISTURE

CONTENT:

PPM

MAXIMUM ALLOWABLE MOISTURE CONTENT AS A

FUNCTION OF INTERNAL PACKAGE VOLUME

.01 0.1 1.0 10 VOLUME IN CM3

MOISTURE INGRESS TIME

T 2T

TIME CONSTANT

215
TIME TO REACH ONE(1) T AND 3 MONOLAYERS OF H20

VOLUME

C.C.

L.R. ATM CC/SEC

T

HOURS

TIME TO REACH

3 MONOLAYERS

HOURS (AVG AMB

13K PPMV ) .001 1 X 10 8 11 55

.001 1 X 10~ 7 110 550

.001 1 X 10

8 1,100 5,500

.01 1 X 10

8 110 66

.01 1 X 10“ 7 1,100 660

.01 10~ 8 11,000 6,600

.1 10

8 1,100 257

.1 10- 7 11,000 2,570

.1 10~ 8 110,000 25,700

1 10'* 11,000 1,283

1 10

-7

110,000 12,830

1 10~ 8 1,100,000 128,300

10 10~ 8 110,000 5,500

10 10~ 7 1,100,000 55,000

10 10“* 11,000,000 550,000

100 10~8 1,100,000 27,500

100 10- 7 11,000,000 275,000

100 10

-8

110,000,000 2,750,000

TIME TO REACH 3 MONOLAYERS OF H20

AS A FUNCTION OF PACKAGE INTERNAL

VOLUME AND AIR LEAK RATE.

(AVO AMBIENT - 13.000 PPM.)

216
5.3 LEAK DETECTION, GROSS AND FINE, USING HELIUM AS A TRACE GAS

Paul R. Forant

Varian/Lexi ngton Vacuum Division

121 Hartwell Avenue

Lexington, MA 02173

(617) 861-7200

In this moisture measurement workshop you have been discussing various methods of

determining moisture content in electronic devices and the means of controlling

moisture. A key ingredient in the prevention of moisture contamination is proving

the hermeticity of the devices.

MIL-STD-883B , Method 1014.2, details the test of semiconductor devices to determine

the effectiveness of the seal. Two fine leak methods using helium or a radioisotope

trace gas are prescribed. Three gross leak test methods using a fluorocarbon immersion test or weight gain and a dye penetrant test are also prescribed.

These gross leak tests must be made after fine leak testing as the liquids used

would plug small leaks and make them impossible to find in the fine leak methods.

It is desirable to test for gross leaks first and remove those from the lot being

tested. Current experience indicates that gross leaks represent over 50% of the

leakers present in production lots. It is probable that the fine leak method re- jects many gross leaks, particularly if the internal volume is large. The best

solution would be to develop a test method that would create the maximum overlap

and extend the range of useful testing in the gross leak mode to find as many

leakers as possible.

The first illustration is the equation for relating the leak rate indicated to the

actual leak rate when using the bombing technique. Figure 2 is derived by solvina

the equation for the parameters in the fixed conditions for test, i.e., 60 psig for

2 hours and testing within 1 hour.

The vertical line is drawn at 5 x 10~8 atm. cc/sec. which is the fine leak reject

limit for devices with a volume of less than .4 cc. If the volume exceeds .4 cc the reject limit is 2 x 10 - ? std. cc/sec.

If we assume that the gross leak test will find leaks as small as 5 x 10" atm.

cc/sec., which appears to be an optimistic assumption, then there are leaks which

will escape detection by either the fine or gross leak method if the component has

an internal volume of .0001 cc or .001 cc. This is the problem we are addressing.

At .01 cc, actual leaks between 3 x 10"? and 1 x 10 - 6 atm. cc/sec. will be rejected.

Larger leaks will have to be found by gross leak testing methods. As the internal

free volume increases, the gross leaks that are as large as 1 x 10-4 atm. cc/sec.

for .1 cc volumes and as large as 3 x 10~3 atm. cc/sec. for 1 cc volumes will be

identified by the helium fine leak method.

It is the gap that exists on the smaller devices that are a problem for overall

hermeticity testing. There is a solution to this situation, and it has been described in detail in a paper written by Stanley Ruthberg entitled, "A Rapid Cycle

Method for Gross Leak Testing with the Helium Leak Detector". This was published

in the IEEE Transaction on Components and Hybrids and Manufacturing Technology,

Vol , CHMT3, No. 4, Dec. 1980. Mr. Ruthberg has built a rapid-cycle helium leak

tester for testing devices over a range of 1 x 10 - 5 to more than 1 atm. cc/sec.

If we were to contemplate a test system that combined the gross and fine leak test

21 7
in close sequence, the gap currently existing between gross and fine leak testing

would be eliminated, and furthermore, the signal from the leaks would be increased

to assure that apparently marginal tests would become definitely defined.

The following conditions are proposed for gross and fine leak testing:

1. The parts are first to be pressurized to 60 psig for 2 hours and tested within

1 hour.

2. The rapid-cycle gross leak test calls for a booster pressurization at 30 psig

for 6 seconds.

3. Transfer to the second chamber in 1 second.

4. Evacuating and testing the part in 5 seconds.

These parameters are readily achievable in current leak testing equipment when operated with the appropriate material handling, fixtures and microprocessor controls.

If the part is gross leak tested in this manner, the part can then be fine leak

tested in the same test mode by transferring it to a third chamber for removal of

surface helium, so that when transferred to a fourth chamber, the fine leak test at

5 x 10_ 8 std. cc/sec. can be conducted.

The leak test is made with the above parameters of a long-term bombing and a booster bombing just prior to test. For gross leak testing the reject limit of

5 x 10 _b std. cc/sec. can be achieved,

the following:

Volume cc

.01

.10

1.0

We can now reject leaks in accordance with

Leak Detection Range

5 x 10~5 to .5 atm. cc/sec.

2.5 x 10-6 to 10 atm. cc/sec.

1 x 10"6 to 10 atm. cc/sec.

The prebombing of the part to 60 psi for 2 hours plus the short-term booster bombing covers the marginal area as well as the gap on the smaller devices. The indicated signal is substantially greater than that available from the long-term bombing

and dwell currently used.

Equipment to accomplish the test has been built and is described in detail in

Stanley Ruthberg's paper referenced above. Varian has conducted surveys of the

industry to see if such equipment would be saleable. The interest was high with

the stipulation that the test method be added to the gross leak test methods permitted by MIL-STD-883

.

Using the formulae for flow rates for filling the part with helium and the rate of

loss of helium during the handling and test phases, a minicomputer could be programmed to adjust the bombing cycle for higher pressures or longer dwell, decreasing

dwell time after bombing or decreasing pump down and test time. These variables

are readily calculated and the test setup can be adjusted to assure rejection of

defective parts.

The high-quality levels currently achieved by manufacturers indicate that several

parts can be tested at once and the rejects can be handled in small lots or indi218
vidua! screening. With multiple parts testing the problems of sorbed and residual

helium would probably cause problems if a single chamber were used for both bombing

and testing. Tv/o chambers for gross leak testing and two for fine leak testing are indicated to achieve good correlation to standard control devices of known leak rate

We will be actively pursuing a change to the MIL-STD so that we can go ahead with a development program that will make commercial hardware available for this test

method.

21

9
St = P[ 1 - c-’* 00 ar

] [c- a'l L

where:

St - indicated leak rate (standard cm*/s),

F = bombing pressure of helium (atmospheric pressure, absolute),

T - bombing time (h),

/ = waiting time (s) between bombing and

testing,

L = actual leak rate (standard em’/s atmospheric pressure, absolute) (Note 1),

a = L/V where V - internal volume (cm*),

and

e =2.71 (natural logarithm).

Ill ustration 1

.

220
ACTUAL

LEAK

STQ

CC/StC

ACTUAL

LEAK

»OM* TIKE - 2 HRS. BOMB PRESSURE - SO PSIA

DWELL - 3000 SEC.

30MB TIME - 2 HRS. PRESSURE - 60 PSIA

DWELL - 3000 SEC.

BOMB TIME - 2 HOURS PRESSURE - 60 PSIA

DWELL - 3000 SEC.

ROMS TIME - 2 HRS. PRESSURE - 60 PSIA

DWELL - 1000 SEC.

Illustration 2.

221
Reprinted from Applied Optics, Vol. 21 , page 3702. October 15, 1982 Copyright © 1982 by the Optical Society of America and reprinted by permission of the copyright owner.

SESSION IV, Paper 5.4

Using optical correlation to measure leak rates in

sealed packages

James W. Wagner, Louis C. Phillips, Edward P. Mueller, and Robert E. Green, Jr.

An optical correlation technique has been shown to accurately measure leak conduction rates from 10_1

to 10~6 atm cc/sec. Using hybrid microcircuit packages with glass capillary leakers, dimensional changes were

detected as the packages were exposed to a small increase in external pressure. The rate at which the package returns to its original dimensions is measured and used, along with the internal free volume, to compute

the leak rate. Because of its several advantages, the technique has the potential to become a useful nondestructive quality-control technique.

I. Introduction

Of the several techniques available for detecting and

measuring leak rates in sealed electronics packages,

most quantitative techniques use a tracer gas which is either sealed into the package or forced in under pres- sure and then measured as it escapes through leaks in the package. Owing to the rapid loss of gas from

packages with leak rates larger than ~1 X 10-5 atm

cc/sec, these tracer-gas techniques suffer a loss in sensitivity in the gross-leak range unless special techniques

are employed. 1 Unfortunately, present gross-leak test methods typically are nonquantitative, have limited

ranges and detection efficiencies, may not detect very

large leaks or porosities, and may contaminate and

thereby accelerate failure of leaky devices not accurately

screened by the test. 2 A new optical technique, which measures minute

displacements of the package walls as a result of applied

external pressure, has been studied as a potential means

for measuring leak rates in sealed packages. The

technique has been shown to provide a high degree of

accuracy in measuring leak rates over the entire test range from 10-1 to 10-6 atm cc/sec. Since no tracer gases or detection fluids are used, the optical technique

offers several advantages over conventional leak test methods. The optical method is noncontaminating,

directly measures the air leak rate (no conversion re- quired), is immune to interferences from absorption of

tracer gas by polymer components on or in the package,

James VYagner and Edward Mueller are with U.S. Food & Drug

Administration. Bureau of Medical Devices, Center for Medical Device Analysis. 8757 Georgia Avenue. Silver Spring, Maryland 20910;

the other authors are with Johns Hopkins University, Department

of Materials Science & Engineering, Baltimore, Maryland 21218. Received 14 May 1982.

and is potentially faster since no back pressurization

with tracer gases or fluids is required. In addition, it is virtually impossible for a very large leaker to be

missed using the optical method. Although the data

presented here were generated from tests on a series of

hybrid microcircuit packages, the method may be applied to transistor and microcircuit packages, food

containers, or even larger packages as long as the in- ternal free volume of the package is known.

II. Theory

If a leaky package stored for some time at atmospheric pressure Pa is then subjected to a step increase

in pressure to some pressure Pe on equilibration, the

change in standard volume within the package will be

where Vf = internal free volume, and AU = difference

between final standard gas volume and Vf. The time dependence of the internal-pressure in- crease is described for the various modes of flow through

the leak as a function of leak size, geometry, and pres- sure difference.3 For the case of purely molecular flow,

the volume of gas which passes through the leak, can be expressed as a simple exponential relationship

Ui(t) = AU[1 - exp(— f/r)], where r is a function of

temperature, leak geometry, molecular weight of air, and the internal free volume of the package. The leak

rate for a fixed pressure difference, Pe — Pa , is simply

the slope of this curve at t = 0:

, , d „ AV' Vf (Pe \ leak ratep, = — Vi(t )|,= 0 = — = — — - 1

• dt t t \P0 I Therefore, given Pe and Vf, the leak rate can be computed exactly for the molecular flow case and approximated for viscous laminar flow. The optical method discussed here measures r by

detecting displacement and subsequent relaxation of

222
Twi Chjmb#f

(c)

Fig. 1. Package-lid displacement resulting from increased chamber

pressure.

REFERENCE

(PLANE WAVE)

NOTE 8 • F | bti.ylj

(a)

Fig. 2. (a) Constructing the matched filter; (b) generating the correlation signal.

Fig. 3. Optical setup used to measure leak rate.

the package surface as a result of an applied externalpressure change. Figure 1 illustrates the displacement/relaxation phenomenon showing a package placed

in a sealed chamber whose pressure may be controlled.

Initially, the air within the chamber and the package are both at atmospheric pressure [Fig. 1(a)]. At some time,

t o, the chamber pressure is increased to some value

above atmospheric pressure. Figure 1(b) shows that, at this time, the package lid is immediately displaced

and air begins flowing into the package through the leak. As time passes, the pressure within the package equilibrates with that in the chamber, allowing the package

lid to return to its original position [Fig. 1(c)].

To detect package-lid displacement, a coherent optical-processing technique known as optical correlation or matched Filtering4

is used. Before the chamber

pressure is increased, a Fourier transform hologram5is made of the package lid viewed through a window in the

front of the test chamber. This hologram, once developed, has an amplitude transmittance proportional to the complex conjugate of the Fourier transform of the

object surface [see Fig. 2(a)]. To detect displacement

of the package surface, this hologram can now be used

as a matched filter as shown in [Fig. 2(b)]. The output

of the correlator system is a symmetric pattern of light

whose amplitude is proportional to the cross-correlation

of the object surface at the time the hologram was ex- posed and the object surface in real time after the hologram is developed. If the object surface remains

unchanged ( Pe = P( ), the output of the system is the

autocorrelation of the surface with a very high peak

intensity in the center (0,0). As correlation of the sur- face with its original state decreases, this (0,0) intensity

will also decrease. By using a pinhole to select only this

(0,0) value, a photomultiplier tube can be used to detect

and record the degree of correlation.

The actual system configuration as shown in Fig. 3

does not project an accurate cross-correlation signal in the output plane. This results from the fact that to improve the efficiency of the system, neither the object nor the film plate are located as they should be in the

front focal planes of the transform or correlation lenses,

respectively. Fortunately, it has been shown that the

(0,0) value of the output signal remains unchanged by

this configuration.6 When pressure is applied to the package, the surface

deflects inward and the correlation signal drops to a minimum value. Once the pressure within the package

equilibrates with that of the chamber, the correlation

signal will return to its original value. The time re quired for the signal to return to its original value is used

to compute the time constant, r, for the package leak

III. Procedure

To test the application of optical correlat ion to leak

testing, twelve hybrid microcircuit packages were fitted with glass capillaries of known leak rati The pai k ig< measure 33 X 18 X 4 mm, are made of Kovar, and have

a measured internal free volume of 1.11 mliter

4). The sidewalls and bottom are 1 mm t h - k and t 1

223
Using a nonleaker, the relationship between the

leak-rate time constant, r, and the recovery time for the

correlation signal was established. By precisely controlling the pressure within the chamber, it was dis- covered that the knee at the point where recovery of the

correlation signal begins corresponds to a pressure

difference of ~0.8 cm H2 O across the package lid. This

is 1.6% of the initial applied-pressure difference.

Therefore, the time between the drop in the correlation

signal and the knee at the beginning of recovery corre- sponds to the time required for the interior-package

pressure to come within 98.4% of its final equilibrium

value. This means that the recovery time corresponds

to approximately four time constants since 1 — exp(—4) = 0.982 or 98.2%.

50cm of H2 0

External Pressure The recovery time for a test 10_1 atm cc/sec leak is ~2

sec. However, the recovery time for the test packages

with leaks finer than 7 X 10-4 atm cc/sec is >5 min. For

these packages a slightly different procedure is used.

Instead of waiting for the recovery time, the pressure

within the chamber is slowly backed off after some

known dwell time, typically 5 min. The pressure within

the chamber when the correlation signal returns indi- cates the package pressure after the dwell time at the

increased chamber pressure. Using these data, the time

constant may be computed as follows:

Vmax r

Td

In (1-^) \ Pe - Pal

where Td - dwell time.

J

J | | | l_

1 2 3 4 5

r

Correlation Signal From PM Tube

Fig. 5. Dependence of correlation signal on chamber and package

pressure.

lid, which is either welded or soldered in place, is 0.4 mm

thick.

The package is placed in the test chamber located in the object plane of the optical correlator (refer to Fig.

2). A film plate (Agfa 10E56) is exposed using equal

object and reference beam intensities. To ensure precise alignment of the developed hologram, the film plate

is placed in a liquid-cell holder throughout the exposure,

development, and playback. During playback, the

reference beam is blocked and the photomultiplier

(PMT) and pinhole are aligned in the back focal plane

of the correlation lens for maximum signal. A stripchart recorder is used to record the output of the PMT.

When the pressure in the test chamber is increased by

50 cm H^O (4.9 kPa), the correlation signal drops to a minimum value. As the pressure within the package

equilibrates with that of the chamber, the recorder

correlation signal returns to its autocorrelation value

(see Fig. 5).

To verify the accuracy of the experimental results using the optical technique, each package leak was di- rectly tested using a water manometer method shown

schematically in (Fig. 6). A hole was drilled through the

lid of each test package over which a glass burette was

secured using a cyanoacrylic cement. The burette

constitute one arm of a U-tube water manometer in which a 50-cm difference in water height was established. As air leaked into the package, the columnheight difference was reduced. The time required for the column heights to change indicating that 0.1 cc of

air had passed through the leaker was recorded. The

leak rate was then computed by simply dividing the

recorded time into the volume and normalizing to standard conditions.

IV. Results and Discussion

A comparison of the leak rates measured using the

optical techniques and the direct manometer method

is shown grapically in (Fig. 7). The 98% recovery time was measured for packages leaking at a rate of 10-3 atm

cc/sec or greater. The leak rates of the remaining

packages were computed by measuring the internal

pressure after a 5-min dwell time at increased chamber

pressure.

224
Optical

Measurement

(atm

cc/sec)

Fig.

Fig. 7.

Package Under Test

i. Water manometer technique for measuring leak rates.

io _ 1

io- J

10-3

10- 4

10- 5

•y

•/

/V

/ -—Ideal Correlation

•y

J l_ J I 10 -6 10 -5 10 -4 1 0

- 3 10 - 2 10 -1 , "Direct" Leak Measurement (atm cc/sec) Leak-rate data obtained optically and directly using the water manometer technique.

It should be noted that leak-rate data are usually

assumed to indicate the rate of gas flow into a package

at a 1-atm differential pressure. Bear in mind that the

data presented here represent the leak conduction at only 50 cm H2 O or 1/20 of 1-atm differential pressure.

To obtain standard leak values, the optical tests must

be run again using Pe - Pa + 1 atm or Pe = Pa — 1 atm.

Because of changes in flow modes at increased differential pressure, the lower-pressure data may not in general be directly extrapolated to give the higherpressure values. Having previously obtained standard

leak-rate values for all of the test packages, however, it was interesting to note that for those packages with leak rates below 10-4 atm cc/sec, there was a direct proportionality between the leak rate and the applied pressure. That is to say that leak rate of these packages at 1/20

atm (50 cm HoO) was very nearly 1/20 of their measured

leak rates at 1-atm differential pressure. This was not

true for packages with larger leaks. This variation in leak-conduction rate with applied pressure may be

useful in discerning flow modes, thereby providing some

indication of the leak geometry.

Optical correlation has been shown to provide an

accurate means for detecting and measuring leak rates

in hybrid microcircuit packages in the range from 10“ 1 to 10-6 atm cc/sec. Using higher pressures and/or

longer dwell times, there is every reason to believe that

its accuracy may be extended further into the fine-leak

range. With erasable, rapid-process storage media

(thermoplastic film or Prom, etc.) and simple control

circuitry, a fast automated system might be configured

for quality-control applications.

References

1. S. Ruthberg, “A rapid cycle method for gross leak testing with the helium leak detector” IEEE Trans, Vol. CHMT-3 (4) December

1980.

2. S. B. Banks, R. E. McCullough, and E. G. Roberts, “Investigation of Microcircuit Seal Testing,” Rome Air Development Center

Tech. Rep. RADC-TR-75-89 (April 1975).

3. D. A. Howl and C. A. Mann, Vacuum 15, 347 (1965).

4. J. W. Goodman, Introduction to Fourier Optics (McGraw-Hill,

New York, 1968), pp. 171-184.

5. R. J. Collier, C. B. Burckhardt, and L. H. Lin, Optical Holography

(Academic, New York, 1971), pp. 206-210.

6. J. W. Wagner, “Final Segment Report: Characteristics of Optical

Correlation Systems” CMDA80-009 (U.S. GPO, Washington. D C.. 1980).

225
6. SESSION V MOISTURE PHYSICS

6.1 THE PHYS I SORPTION OF WATER ONTO

INTEGRATED CIRCUIT PACKAGE COMPONENTS

by

James M. Ammons, Gerald R. Hoff and Michael Kovac

Center for Engineering Development and Research

and

Jack H. Linn and William E. Swartz, Jr.

Department of Chemistry

University of South Florida

Tampa, Florida 33620

ABSTRACT

The phy s i sor p t i on of water onto various substances commonly

encountered in integrated circuits has been studied. Data were

obtained for iron, nickel, gold, Si 02 , and polyimide using a commmercially available quartz crystal thickness monitor. Test gas

mixtures containing 100 to 23000 ppmV water were prepared using

nitrogen, hydrogen or helium. It was found that adsorption isotherms

for most of the materials were similar to BET curves. Exceptions were those obtained for materials such as polyimide, which absorbs as well

as adsorbs water. The quartz crystal oscillator technique is a powerful tool for characterizing the physisorption/disorption of water

to package component materials. Since the technique can be employed

at atmospheric pressure and not at high vacuum as do other surface

techniques, it is possible to study materials under conditions which

they may experience during processing and use.

INTRODUCTION

It is well known that moisture inside a mi cr oel ectroni c package can

reside at a number of different locations. At any given time the

location of the water is determined by the local temperature

differential between competing surfaces and their

adsorption/desorption properties as defined by their chemical and

physical nature (1). This study is aimed at elucidating the intrinsic

moisture uptake properties of materials commonly employed in component

parts of microelectronic packages. These include gold, iron, nickel

(2), silicon dioxide, silicon nitride, polyimide and others.

Previous work (2) has demonstrated that it was possible to study the

physisorption of water onto various substances via a quartz crystal

thickness monitor operated at atmospheric pressure. After coating

with the substance of interest, the test crystal was exposed to a

226
controlled test gas containing known amounts of moisture. Water

adsorption onto the various substrates was monitored by noting the

resultant deviation of the frequency of the crystal. Most materials

characterized appear to exhibit Brunauer, Emmett, and Teller (BET) (3)

type adsorption curves.

EXPERIMENTAL

A. Equipment

Quartz crystal oscillator data were obtained using equipment described

previously (2). This system consists of a Kronos QM 311 thickness

monitor and FTT 350 detection head. The detector head was modified so

as to allow unrestricted gas flow to both sides of the crystal and was housed in a stainless steel test chamber measuring 2" wide x 4" long x 1" deep. The analog output of the QM 311 was fed to a Hewlett-Packard

3465S strip chart recorder.

Controlled test gas mixtures containing 200 to 23000 ppmV water were prepared by dilution of water saturated carrier gas. Nitrogen (N ) (boil off from liquid), dry compressed hydrogen (H ) or helium (He)

served as the carrier gas. The water content of the test gas stream

was monitored via a General Eastern National Bureau of Standards (NBS)

traceable optical dewpoint hygrometer.

B. Materials

Quartz crystals were obtained in plano-plano configuration from0 Phelps

Electronics. These crystals were supplied with a nominal 1 000A layer

of gold over a 1000A chromium, both thermally evaporated. The base

oscillating frequency was 4.73 MHz.

Nickel and iron were obtained with a minimum purity of 99.99)? from

Research Organics/Research Inorganics. Silicon monoxide was obtained

from a commercial coating firm. DuPont Pyralin PI 2550 polyimide was

obtained from Honeywell.

C. Procedures

Quartz Crystal Preparation . Water adsorption studies on gold were performed both on "as received" crystals and crystals which had been

rinsed in isopropanol, or trichloroethylene under a blanket of dry

nitrogen. Extreme care was exercised in removing all traces of

isopropanol since, as was determined during the experiments, that the

presence of even traces of a polar solvent led to greatly enhanced

water uptake. Nickel and iron surfaces were prepared by thermal

evaporation of the metal onto the existing gold/chrome surface of

crystals which had been previously characterized for moisture uptake

properties. Thicknesses ranged from 800A to 1500^. Silicon dioxide

surfaces were prepared by thermal evaporation of silicon monoxide in a Drumheller (4) style source. Polyimide surfaces were prepared by spin

coating the crystals and curing the material according to the

procedures given by the manufacturer. All surfaces were analyzed vi i

227
X-ray photoelectron spectroscopy (XPS) for purity and composition.

Nickel and iron were found to be present as a partially hydrated

oxide. Evaporation of silicon monoxide yielded a surface which

analyzed as silicon dioxide.

Water Adsorption Testing . Preliminary data were obtained by switching

the moisture generation system to "drydown", waiting for the system to

reach dry conditions as determined by the dewpoint hygrometer, zeroing

the thickness monitor, and then stepping to a preselected moisture

value of from 200 to approximately 24,000 parts per million by volume

(ppmV). After equilibration (W 5 min.) and subsequent data

recording, the system was switched back to dry conditions. It was found that more rapid equilibration occured and equivalent data could

be obtained if the system were first brought to dry conditions and

then stepped sequentially from low to high moisture values. At the

conclusion of the data gathering, the system was stepped dry. Drift

in the instrumentation was found to be negligible over the time span

of an experiment (^3 hours). The latter procedure was used in the

majority of the work reported herein. Crystals were also stepped from

wet to dry in order to demonstrate that the same equilibrium points

were obtained. Generally, each crystal was allow to sit in "drydown"

overnight before testing in order to ensure that equilibration with

the dry carrier gas was achieved.

RESULTS AND DISCUSSION

A. Water Adsorption on Gold . Figure 1 presents data obtained for a typical gold coated crystal using nitrogen, hydrogen, and helium

carrier gases containing up to approximately 23,000 ppmV water. The

data have been corrected to account for only one side of the crystal

(both sides are coated). This was done to facilitate comparison to

other surfaces which were coated on only one side of the crystal. As can be seen, the shapes of the adsorption isotherms are quite similar

for the three carrier gases and appear to be characteristically BET in

nature. The onset of multilayer formation appears to occur at

approximately 6000 ppmV. It would be expected that multilayer

formation would start at the completion of one monolayer of water —

about 4.5 X in thickness. Examination of the isotherms reveals that

the inflection point at 5000-6000 ppmV water occurs at a 4 to 5 A

increase in thickness referenced to the zero point under dry

conditions. The implication from this data is that a monolayer is

formed at approximately 5000 ppmV water. It is possible that

microcapillary condensation also occured, and, since this technique

measures a change in mass , this water would also be detected. This seems unlikely since examination of the surface via scanning electron

microscopy did not confirm the presence of significant numbers of

capillaries. Therefore, fractional monolayers of water are probably

present at ambient water concentrations less than 5000 ppmV. It

should be noted that heating the crystal at 100° C for several hours

in or N^ had no effect on the shape or position of the isotherms.

The data have not been corrected for the difference in zero observed

when changing from one carrier gas to another. This phenomenon is

228
believed to arise from a difference in packing and therefore condensed

phase density of the carrier gas and not in water displacement or accumulation. The same relative change was observed no matter which

order was used. With H as zero reference, He was observed to have an

^ o ° increase in mass of 4.5A, while was 22-24A. The hydrogen carrier

was found to have slightly more trace water contamination than the

other two carriers. Repetitive experiments using only dry carriers

failed to demonstrate that the differences arose from water

adsorption/ desorption.

Figure 2 shows a comparison between two gold crystals using an H2 carrier. Similar curves were obtained with N2 and He. As can be

seen, there is a significant difference in moisture uptake between the

two crystals. Scanning electron micrographs of the two crystals

revealed that crystal 2, with the larger uptake, had an obvious and

significantly rougher surface than crystal 1. Therefore, it is

believed that the difference observed between these two crystals

arises from a difference in surface areas. The crystal with the

larger surface area exhibited greater water adsorption. While results

from crystal to crystal varied, it was found that adsorption for a given crystal could be well characterized and was quite reproducible.

Figure 3 is a plot of water adsorption on iron (iron oxide) using H^

and N^. The plots have been corrected for a zero offset between the

two carrier gases of approximately 24A. As can be seen, there is a significant enhancement in water adsorption especially in the area of

less than 3000 ppmV relative to the gold. This is undoubtedly due to

the presence of the oxide. It was found during the course of this

study, that oxide on a surface always lead to enhanced water

adsorption. This confirms earlier observations (2). The shape of

these curves is the same as that found in classical multilayer systems

(5). The knee of the curve at approximately 2000 ppmV is

traditionally taken to be the completion of one monolayer (5). In the

present case, this corresponds to approximately 12-15A of equivalent

water. Since the instrumentation reading corresponds to mass, and not

true thickness, this suggests that the oxide structure has

significantly greater surface area than the gold.

An enlargement of the region of these curves from 0 to 6000 ppmV is

shown in Figure 4. The displacement of the isotherm obtained in H^

towards higher water content was reproducible on iron. A similar

effect was not found in the case of gold or nickel. For iron, it

appears that H^ can cause a suppression of water adsorption at low

moisture values. At values less than 3000 ppmV, the partial pressure

of water does not seems to be sufficient to cause a full displacement

of hydrogen.

Figure 5 shows the difference in water adsorption for two differin'

iron crystals. The crystal termed "light oxide" was prepared by

exposing a freshly evaporated iron film to air at room tempera' ,r , and is the same data as that presented in Figure 4. The "heavy •

229
crystal was prepared by exposing a freshly deposited iron film to air

while the crystal was at approximately 50

d

C for 10 minutes. The

latter treatment resulted in a visible layer of oxide, which analyzed

as an iron oxy-hydroxide by XPS. This is an extreme example of a trend noted repeatedly — the more oxide present, the more water there

is adsorbed, and the longer the time that is required for the surface

to release the water and reach equilibrium with a dry carrier gas.

Figure 6 shows the results obtained for a nickelo

( oxide ) surface.

Again the nitrogen zero has been adjusted by 23A. Both curves are classical multilayer formation curves. The similarity to the iron

curves can be explained on the basis of the presence of an oxide

surface. It should be noted that this crystal, like the iron crystal

in Figure 3 was exposed only to air at room temperature. XPS analysis

of this surface indicated that nickel is present as a mixture of

oxide and water as was the case for iron. It has been shown

previously (2) that this water is probably water of hydration and is

therefore not physically adsorbed. Its presence, however, undoubtedly

influences the physisorptive properties of both the nickel and iron

surfaces. Oxides and water both are capable of forming associative

bonds with water. Both the nickel and iron isotherms may be

interpreted in terms of the BET equation or a modification (6).

The results obtained for SiO^ are shown in Figure 7. The large amount

of equivalent angstroms of water adsorbed illustrates the well known

hydorscopicity of SiO^ and glass in general. Undoubtedly the water

significantly penetrated the upper layers of SiO^ and formed a hydrated SiO structure (7). Hydrated gel layers are known to exist

o © and vary in depth from 50A to 1000A depending on the glass. As can be seen from the figure, relatively high levels of moisture 1 OOOOppmV)

are required to initiate multilayer formation.

Figure 8 presents data obtained for polyimide. As can be seen, this

material exhibits an enormous tendency to absorb and adsorb moisture.

There is probably a multilayer structure formed on the surface but the

amount of water required is insignificant compared to the amount of

water absorbed by the polymer. Therefore the character istically

shaped BET curve is not readily seen overlaid onto the bulk adsorption

curve. The time required for equilibration this material was

approximately the same as for the other materials examined. This is

probably due to the thickness of the material -- a few thousand

angstroms.

SUMMARY AND CONCLUSION

It has been demonstrated that a quartz crystal thickness monitor may

be adapted to measure the adsorption/desorption properties of various

materials. It was found that materials which adsorb moisture through

interaction with the surface atomic layer only (i.e., gold), exhibit

a character i stic BET style adsorption isotherm. Materials which

interact through the uppermost atomic layers, such as oxides, exhibit

230
enhanced BET type isotherms. It has been demonstrated that the amount

of moisture adsorbed in this case is proportional to the amount of

oxide present. Materials which absorb water as well as adsorb water

(polyimide) are capable of retaining large amounts of water even under

relatively dry conditions. In all cases studied, the adsorption

process was found to be freely reversible in as much as desorption

readily occured.

ACKNOWLEDGEMENT

This work was supported by Rome Air Development Center (RADC) under

contract No. F30602~80~C~Dl69.

REFERENCES

1. G. A. Samorjai, Chemistry in Two Dimensions; Surfaces , Cornell

University Press, Slhaca/London, 1981.

2. W. E. Swartz, Jr., J. H. Linn, J. M. Ammons, M. Kovac, and K.

Wilson, Reliability Physics 21st Annual Proceedings, 52 (1983).

3. S. Brunauer, P. H. Emmett, and Teller, T. Amer. Chem. Soc., 60 , 309 (1938).

*4. C. E. Drumheller, Trans. 7th AVS Symp. , Pergamon Press, New York,

p. 306 (I960).

5. A. W. Adamson, Physical Chemistry of Surfaces, t h Ed., Wiley

Interscience, New York, Chapter 26 (1982).

6. L. E. Drain and J. A. Morrison, Trans. Faraday Soc., 49, 654

(1953).

7. H. H. Willard, L. L. Merritt, and J. A. Dean, Instrumental Methods

of Analysis , 5th Ed., D. Van Nastrand Co., New York, Chapter 20

(1974).

231
Gold Crystal - Various Ambients

o

<1)

u.o

CO "O<

<x> 4-< CC

coE

o

COO)c<

ppmV Water (xIO -3

)

Figure 1. Water adsorption on Gold

Gold Crystal Comparison

ppmV Water (xIO -3

)

Figure 2.

232
Water Adsorption on Iron

ppmV water (xIO -3

)

Figure 3.

Water Adsorption on Iron

-a

<

ppmV Water (xIO* 3

)

Figure 4.

233
TJ Iron Surface Comparison

ppmV Water (xIO -3

)

Figure 5.

Water Adsorption on Nickel

ppmV Water (xIO -3

)

Figure 6.

234
Angstroms

Water

Adsorbed

Water Adsorption on Si0 2

Figure 7.

Water Adsorption on Polyimide

Figure 8.

235
6.2 SIMULATING THE CORROSION THRESHOLD OF LSI/VLSI DEVICES

USING MOISTURE SENSITIVE TEST PATTERNS

John B. Kiely

Intel Corp.

3065 Bowers Ave., Santa Clara CA 95051

ABSTRACT

Two moisture sensitive test die, the Panametrics MM-HT

and an Intel built interdigitated structure of aluminum

lines on silicon oxide, have been used to simulate

corrosive conduction. It is shown that an absolute

minimum bound on the moisture limit for high reliability

can be set by examining the moisture concentration

threshold for conduction. A definitive value cannot be

determined from this work, but for VLSI size CERDIP

packages the industry standard 5000 ppma limit is

probably sufficiently conservative. More investigations

are necessary to quantify this value.

INTRODUCTION

The interest in measuring moisture concentrations of hermetic

semiconductor packages arises from corrosion and metal migration

reliability concerns. Both failure modes require a voltage, a reactive ion, and surface moisture to be present. Of these

three, the package moisture level is the only realistically

controllable attribute. Thus a measurement technology has been

developed to quantify and control hermetic package moisture. The

premier technique is residual gas analysis (RGA) . Though powerful, it is expensive, slow, difficult to do, and under some

circumstances the measurement can be erroneous [1,2]

.

Another

technique is using surface moisture sensitive test die which can

be calibrated to RGA [3,4]. The die are inexpensive and

straightforward to use, but are not as sensitive as is RGA and

are considered somewhat suspicious because of sensitivity,

repeatibility , and calibration concerns.

It is interesting to compare the "measure and control the number

of water molecules" philosophy of the hermetic package reliability workers to similar work in plastic package technology.

Plastic semiconductor packages are often used in mild environments, such as air conditioned offices, or when high reliability

is not a concern. The plastic technologists concern themselves

with maximizing the time to failure of the packages in a harsh

environment, eg. biased 85/85 (85°C/85% RH) or unbaised steam

chambers. These difficult conditions act as accelerating factors

236
allowing the lifetime under normal use to be estimated. Test die

are regularly used in plastic package corrosion investigations.

This approach is not used with hermetic packages, mainly on the

assumption that highest reliability is desired and (assuming the

package is truely hermetic) accelerating techniques cannot be

found to sufficiently quicken the failure rate for measurement in

a reasonable time.

The only major pitfall with the philosophy of measuring package

moisture is that the moisture level at which reliability is

jeopardized is unknown. The industry standard of 5000 ppma is at

best a guess [5]

,

not an experimentally determined value. It is

quite easy to make arguments that the standard has inaccuracies.

For example, 5000 ppma of moisture in a 0.02 cc ceramic packaqe

corresponds to about two monolayers of water on the cavity surfaces. In a 10 cc metal 'package , 5000 ppma corresponds to tens

of monolayers L 6 J

.

The discrepancy here is glaringly visible.

The main problem with the fixed standard is that the industry

uses a wide variety of hermetic packages, from large hybrid

metal cans to small ceramic VLSI DIPs. A single reliability

standard cannot span such a huge range. Unfortunately, there is

no systematic way of determining the reliability standard moisture level for each package style; thus a best guess seems to be

the only available solution.

This paper will illustrate a technique that can estimate the

maximum moisture level for long term reliability in hermetic

packages. The method involves low level leakage current measurements of moisture sensitive test die.

CORROSION CURRENTS

Corrosion or metal migration failure occurs due to a destructive

ionic current. Estimates of current levels which jeopardize

reliability can be made. For bondpad corrosion, failure will be

defined as depletion of 30% of the bondpad metal. Also, the

bondpad is assumed to be aluminum and there are three electrons

conducted for each aluminum atom removed by corrosion. The time

to failure (T) is

T > (V x d x 3 x 0 . 3 ) /I = (V x d)/l

where V is the bondpad volume (cc)

,

d the aluminum density

(atoms/cc) , and I the corrosion current. For a bondpad (100 x 100 x 1) microns with a 1 pA corrosion current

T > 3 years.

Thus, to assure a 30 year lifetime, the corrosion current should

be below 0.1 pA in this example.

The significance of this example is that if one measured a current from the bondpad of this packaged device, THIS CURRENT IS

GREATER THAN OR EQUAL TO THE CORROSION CURRENT. Consequently, an

upper bound on the corrosion current can be determined. There237
fore, if the bondpad to bondpad current measured less than 0.1 pA

in the above example the device would be reliable for the thirty

year time period. One never knows the true corrosion current by

this method, only the upper bound. Measuring less than one

picoampere can be done, but it is not easy.

THE AMPLIFICATION FACTOR - PROPER TEST DIE DESIGN

It is quite simple to design a test die that amplifies the total

current that would be observed on a real device. Take the

previous example of the bondpad as an illustration. With a constant voltage, a larger current could be produced by using a pad with a larger perimeter. An example of exploiting geometries

to amplify the measurable current are given in Figure 1.

This can be stated more analytically for both volume and surface

conduction. An example of volume conduction is a current between

two plates of a capacitor. Surface conduction is typified by

current passing on a surface between two metal lines. Ohm's Law

can be written for each case;

Volume Conduction; I = O" x (A/L) x V

Surface Conduction; I = O’ x (P/L) x V

where Q~ is a conductivity, A an area, P a perimeter length, L

the distance between conductors, and V the voltage. The conductivity is fixed by the type of material between the conductors,

the moisture level, the number of available charge carriers, ect.

The amplification factor is (A/L) or (P/L) . Examples for surface

conduction are given in Figure 1. Note that to quantify the

amplification, the geometric configuration must be kept constant

and only the dimensions scaled. Otherwise, the effects of fringing fields must be taken into account.

The methodology used in this work can now be stated. If one is

concerned that 0.1 pA of corrosion from a bondpad is the maximum

for adequate reliability, but the measurement accuracy is only

1.0 pA, then a test die with greater than 10X amplification using

the same manufacturing technology can be used instead. It must

be assembled and the leakage current measured under otherwise

identical conditions.

CORROSION CURRENT THRESHOLDS

One more concept must be introduced before the experimental

results are reviewed. If the material between the conductors is

an "insulator” and no moisture is adsorbed, the conductivity is

zero. As water molecules are added the conductivity does not

immediately become nonzero. One water molecule cannot support

conduction, nor can one monolayer (one monolayer of water is

strongly bonded and has no mobility) . As the number of water

molecules increases a minimum conduction supporting film is

238
eventually formed. This is the moisture concentration threshold

for conduction. Again, because the total current has the

corrosion current as one of its components the previous rule can

be restated as follows:

THE THRESHOLD FOR CORROSIVE CONDUCTION IS ALWAYS

EQUAL TO OR GREATER THAN THE THRESHOLD FOR THE

TOTAL CONDUCTION.

Physical models for conductive threshold effects abound in the

literature such as percolation theories or domain growth

theories. A very interesting physical example is the insulator to

metal transition of doped polyacetylene. As the dopant concentration is increased a sharp increase in the conductivity occurs

at 2 - 3 % doping. Resistivity changes of twelve orders of

magnitude have been observed [7]. Which model most closely models

the data in this work is indeed interesting, but not necessary

for a first order analysis. An analogy will be given, though, to

illustrate the dependent variables.

Consider a fish tank with perfectly insulating sides and bottom

except for two opposite sides. An ohmmeter is connected to the

two conductive sides such that there is an open circuit between

the two plates. Now the tank is filled with glass balls. The

measured resistance is infinite. Then remove all the balls and

replace one glass ball with an iron ball, mix the balls up well

and replace them in the tank. If this one ball replacement step

is repeated over and over eventually the iron balls will form a closed circuit between the conductive sides and a finite resistance will be measured. During the step where the circuit is

first completed the resistance will jump from infinite to some

finite value. This quantum change marks the iron/glass percentage threshold for conduction.

This simple analogy illustrates the variables that determine the

threshold value. The first variable is the percentage of water in

the package, equivalent to the iron to glass ratio in the

example. It is the water between the conductors that is important, but for both volume and surface conduction this should be

proportional to the total moisture content. The second variable

is a materials property, such as the hydrophobic or hydrophilic

nature of the surface. The anology in the fishtank example is

whether the iron balls tend to form clumps (they are magnetic) or

be randomly but uniformly distributed.

The last variable is not obvious and requires some explanation.

The threshold may or may not depend on the distance between the

conductors. In the fishtank example, if the balls are infinitely

small the threshold occurs at the same perce ntage regardless of

the conductor spacing. However if the ball diameter is a significant fraction of the spacing, then varying the distance would

change the threshold point. The similar situation for a device

is whether the moisture builds up in consistent monolayers as the

package moisture increases, or whether it forms nucleated clumps

and the clumps grow with the increasing ambient moisture. If the

239
moisture forms clumps with mean diameters that are significant

fractions of the conductor spacing then a distance dependence

will occur.

The purpose of this paper is not to form a detailed theory of the

conduction process. However in order to interpret the data a few

assumptions must be made. Therefore, assume the threshold dependent variables are the moisture percentage in the package, the

type of material between the conductors, and further assume the

dependence on spacing is either weak or nonexistent. Consistent

with these assumptions, one would expect the leakage current

versus moisture content (as measured by RGA) to behave as in

Figure 2.

EXPERIMENTAL TECHNIQUES

Two test die were used in these studies. The first was a structure of interdigitated aluminum lines on a thermal oxide grown on

a silicon substrate. The conduction path was from one metal

line, across the surface of the oxide to the other aluminum line.

The line spacing was eight microns and the line periphery was

about 64,000 microns. If compared to a typical device design

rule for bonding pads of 100 x 100 microns with a 100 micron pad

to pad spacing, the test die had an amplification factor of about

8000.

The second test die used was the Panametrics MM-HT Sensor die

[8], Because it is a gold/aluminum oxide/aluminum capacitor, it

approximates a volume conductor more closely than a surface

conductor. The conduction should be along the granular boundaries of the aluminum oxide; therefore it only closely approximates a true volume conductor. For the purposes of this paper,

it is assumed that the die is a volume conductor with an

area/thickness ratio around 10,000 cm. Relating this to a surface amplification factor is impossible. The Panametrics die had

a capacitance about 50X greater than the interdigitated structure. One would expect that the amplification factor would be

larger due to the more intense electric field. This is

speculation though, subject to experimental verification. The

conductor spacing of the MM-HT is roughly 100X smaller than the

interdigitated spacing.

The most sensitive leakage current measurement technique is by DC

methods. The capacitances involved are large enough that hundreds to thousands of picoamperes of displacement current will be

conducted if AC detection is used. For example:

Die capacitance C = 50 pf

at a frequency f = 10 Hz

and a voltage V = 25 V

the displacement current is 80 nA . Measurement resolution of better than 10 - 100 pA is difficult

under these AC conditions, but with DC techniques resolution of

240
around one picoampere is possible.

There are serious disadvantages to using DC methods. Figure 3

illustrates the worst drawback - the leakage current has an

initial decay that can last for many minutes. Figure 3 demonstrates the situation for the interdigitated die, the Panametrics

die has a similar behavior. This decay phenomena is probably due

to polarization effects. Any bound dipoles on the surfaces will

align with the applied electric field. The long time decay is

caused by the binding: the electric field has to break this force

before the dipole can align. The primary evidence that this is

the proper model is that the decay shows hysteresis with the

field polarity. If the voltage is applied in one polarity and

then repeated with the same polarity after a short rest, the

second decay is very small. This is because the dipoles were

already properly aligned for the second try. If the second

application is at opposite polarity a large decay again occurs;

the dipoles must be flipped this time. Though this polarization

current is moisture dependent, it has nothing to do with the

corrosion current. The corrosion current cannot cause serious

damage if it is only a transient. The measuring circuit is

shown in Figure 4. * Another DC measurement problem is parallel conduction in the

package or the measurement circuit. If a leaky switch is used or

the package material is slightly conductive, this circuit is in

parallel with the die leakage route and could cause erronous

readings. It is actually quite difficult to keep pin to pin

leakage in a ceramic package below one picoampere at greater than

60% relative humidity in the room. The method for control under

these circumstances is to do a differential measurement: measure

the leakage for equivalently spaced open pins and then measure

the pins attached to the die. For this work, units with open pin

currents above one picoampere were discarded (less than 5% of the

units were discarded) . Therefore the differential technique was

not needed. The resolution was 1-2 pA for the interdigitated

die and 4-5 pA for the Panametrics die. The reason for the

increased noise and drift on the Panametrics sensor was never

determined, but both noise levels were sufficiently low for these

investigations

.

The packages used were 24 lead CERDIPs (a ceramic dual-in-linepackage using a solder glass for the lid to base seal) with a vitreous sealing glass. The package was desiccant free and the

die attach was gold/silicon eutectic. The method for generating

wet packages was crude but effective. Packages were sealed on a heaterblock in room air at 40 - 50 % relative humidity. A nozzle

of heated nitrogen was used to modulate the amount of room air in

the package cavity, done by positioning the nozzle at varying

distances from the package during seal. Though extremely crude,

a range of 2000 to 16,000 ppma moisture could be obtained. More

controlled methods exist [9] and are recommended over this technique for more quantitative studies. The Intel standard sealing

process ( << 5000 ppma) was too dry for use in this work.

241
All the packages were electrically measured and sorted into

groups of nearly identical leakage currents. A standard time of

one and a half minutes was used for decay settling after a bias

was applied to the die (ie. the reading was made 90 seconds after

the bias was applied) . The bias voltage was 25 VDC for the MM-HT

and 40 VDC for the Intel interdigitated die. Random samples from

each group were then measured for internal package moisture using

the Intel RGA. The RGA readings were corrected as necessary for

the oxygen content to null any spurious signals generated in the

equipment [2] . After RGA, all die were tested for

functionality.

EXPERIMENTAL RESULTS

The results are given in Figure 5 for the leakage current versus

package moisture (as measured by RGA) for both test die. Figure

6 shows a more detailed analysis with linear and quadratic fits

for the Panametrics die. The fits were done by ignoring the zero

current data points. Therefore, the conduction thresholds shown

in the fits are from fitting the nonzero data points. Note that

the thresholds determined by fitting agree quite well with the

"by eye" threshold. The Panametrics die was by far the most sensitive of the two. As shown in Figure 6, the threshold for

conduction is 2100 ppma for the linear fit, 2800 ppma for the

quadratic fit, and about 3000 ppma "by eye". The interdigitated

structure is not sensitive below 9000 - 12,000 ppma and the data

quality is not as good due to lower sensitivity to the moisture

levels above 10,000 ppma (ie. the amplification factor is

smaller)

.

Qualitatively the data fits the simple model quite well. The

slope of the nonzero portion of the MM-HT plot is 30X - 60X

larger than the equivalent interdigitated die region. This is

strikingly close to the ratio of capacitances. Also, the 100X

difference in conductor spacings made only a 3X - 4X shift in the

threshold point. Finally, note that for the Panametrics die (the

test die for which the data is the best) the threshold is a very

sharp phenomena. If the conduction is supposed to suddenly turn

on, as the model predicts, then this sharpness is expected.

Detailed quantitative conclusions cannot be drawn from these

data. It was noticed that the MM-HT sensor leakage current

increased with age as much as 50 % in a three month period for

packages in the 4000 - 7000 ppma range. Whether the Panametrics

die decreases in sensitivity with aging or the packages slowly

getters moisture was not determined. All packages were verified

to be hermetic, so ambient gas leakage could not be the cause.

No similar shift with time was observed with the interdigitated

structure, but the sensitivity cannot compare to the MM-HT.

C-.QN.CIt.USX-Q-NjS

The most outstanding conclusion of this work is that the conduc242
tion threshold for moisture dependent conduction does exist. The

consequences of this fact is that a lower limit on the corrosion

threshold for the test die can be set: it must be at a moisture

level equal to or greater than the level for the total conduction

threshold. This is because the total conduction must include the

corrosion current as one of its many components. If the test die

is properly designed (ie. it is identical in manufacturing technology as the real device, except that the conductor geometries

are changed to take advantage of amplification) then the corrosion threshold of a real device can also be bounded.

To prove the model for the conduction threshold and amplification

factor quantitatively, more detailed experiments are needed. The

best experiments would characterize the threshold and amplification with a number of similar test die geometries where only the

P/L or A/L of the die are allowed to change. A comparison with

identical geometries but differing insulators (for example,

thermal oxide versus silicon nitride versus CVD oxide) would also

be useful.

An important conclusion shown by this work is that the Panametrics MM-HT is an extremely sensitive design. It is more

sensitive in fact than anything normally designed into a semiconductor chip. These data have shown the MM-HT in CERDIPs to be

insensitive below 3000 ppma. Furthermore, the Intel die more

closely simulates MOS technologies and the threshold in this case

is around 10,000 ppma. The result from all this is that the 5000

ppma reliability limit in CERDIPs is most likely correct or too

conservative, but not too loose. The author's personal opinion

is that for VLSI CERDIPs the proper reliability limit is in the

5000 to 15,000 ppma range. The "best guess" of 5000 ppma is

probably a good safe limit. Note that more test die designs would

have to be run to confirm this and that these results only hold

for IC DIP type packages. The surface to volume ratio of the

package cavity should greatly affect the results due to moisture

gettering by the walls. Therefore, although the principles

should be the same for larger packages, such as large hybrid circuits, the threshold point may shift for those packages. For

LSI/VLSI type devices the safety of the 5000 ppma limit seems

sensible: both makers and users of LSI/VLSI semiconductors have

stated that moisture related reliability issues have evaporated

since the enforcement of MIL STD 883 Method 1018. Regardless of

whether the issues were real or perceived, the 5000 ppma limit

seems to have settled them.

ACKNOWLEDGEMENTS

I would like to thank Dr. Richard Blish of Intel for many useful

discussions over the last two years on the topics of corrosion

and moisture sensors. Without his suggestions and tutoring I doubt the results would have been as successful as they were. I would also like to thank Dr. Paul Flinn of Intel for many

similarly useful discussions.

243
REFERENCES

1. J.C. Pernicka; "Moisture Measurement By Mass Spectrometry"

,

ARPA/NBS Moisture Measurement Workshop (March 1978) , NBS

Spec. Publ. 400-69; pg. 45.

2. J. Kiely, P. Flinn, and B. Sun; "A Mechanism For The Dependence Of Moisture Detection Sensitivity On Gas Composition

In RGA", Int. Rel. Physics Symp. (April 1981); pg. 167.

3. M.G. Kovac; "Cross Correlation Experiments On Different

Types Of Sensors", NBS/RADC Moisture Measurement Technology

Workshop (Nov. 1980), NBS Spec. Publ. 400-72; pg. 76.

4. J.C. Hale and V. Fong; "Moisture Sensors, Mass Spectrometry,

And MIL Standards", ibid (ref. 3); pg. 90.

5. R.W. Thomas; "Problems In Specifying And Measuring Moisture

Content In Electronic Devices", ibid (ref. 1); pg. 179.

6. For example: assume a square volume, a microscopic roughness

factor of 2-4, and a 5 A

0 water molecule spacing in a monolayer. These numbers are only approximate but they do

illustrate the surface to volume effect quite well.

7. C.K. Chaing, C.R. Fincher, Y.W. Park, A.J. Heeger, H. Shirakawa, E.J. Louis, S.C. Gau, A.G . MacDiarmid; "Electrical

Conductivity In Doped Polyacetylene", Phys. Rev. Lett.

Vol . 39 (Oct. 1977); pg. 1098.

8. Panametrics Inc., 221 Crescent St., Waltham MA 02154

9. M.L. White and R.E. Sammons; "A Procedure For Preparing Hermetic Packages With Known Moisture Levels", ibid (ref. 3);

pg. 49.

244
-;r r P

JL k at L

hi

(A) (B)

Figure 1 An example of using die design to amplify

a surface current. In case (A) the conduction

path size is P/L, whereas in (B) it is about

16 (P/L). Therefore, the current is amplified

16X by using (B) design.

245
LEAKAGE

CURRENT

Figure 2 The expected behavior of the die leakage

current versus ambient moisture in a hermetic package. Two examples are shown

to illustrate the effect designed in

amplification has. The lower curve

has less amplification.

246
DC

LEAKAGE

CURRENT

(pA)

Figure 3 The measured DC leakage current

versus time for the Intel

interdigitated die. Time zero is

when the DC bias is applied. The

relevant current for this work is the

value after about 90 to 100 seconds.

247
Figure 4 The measurement circuit used in this

experiment
LEAKAGE

CURRENT

(pA)

Figure 5 The DC leakage current versus internal moisture

(measured by RGA) for the Panametrics MM-HT

and the Intel die. The dotted lines are guides for the eye. The lowest current points for

each die were actually fluctuating around zero.

All such points were plotted at the measured

noise level for graphical clarity.

249
0 2000 4000 6000 8000

• MOISTURE CONTENT (pptna)

Figure 6 The same Panametrics MM-HT data as in Figure 5 with linear and quadratic data fits. The thresholds

from the fits are 2100 ppma for the linear

fit and 2800 ppma for the quadratic fit. A

reasonable RGA uncertainty is ±500 ppma (shown).

250
6.3 THE EFFECT OF HUMIDITY ON CERMET RESISTORS

Thomas R. Homa and Phillip P. Vadala

IBM Corporation

Endicott, NY 13760

ABSTRACT

The development of the thin film Cermet (Cr A1 0) resistor material

involves assessing the product's stability to typical commercial computer

environments. The standard package for these resistors is not hermetic.

An epoxy seal is used in this package but it only delays moisture

transport into the interior of the module. Therefore the resistors must

be capable of operating in equilibrium with nominal humidity.

During accelerated testing it was learned that high resistance drift was occurring on samples that were operating at nominal power levels. Modules

exhibited the problem after 400 hours in 85°C/80%RH with a 25mW load. The

thin film resistors were visually changing from an opaque material to a transparent one around the laser trim areas. The conversion was accelerated when a conformal coating was applied to the resistors. When a thin epoxy layer from the encapsulation process seeped onto the resistors,

or a corrosion inhibiting polyimide film covered the resistor, more drift

was observed than when the resistors were uncoated. Stress tests on modules with normal ionic contamination levels were not significantly

different than ultraclean samples. Different humidity levels were tested

to see if a humidity acceleration factor could be defined, however, no drift was observed on modules exposed to conditions less severe than

85°C/80% RH. When a silicone coating was placed over the resistors, no drift or attack was observed at any humidity level. Parts with the

coating stressed through 2000 hours of 85°C/80% RH had drifted less than

0.2%. All functional requirements were achieved with this coating.

INTRODUCTION

The rapid advance of computer technology is demanding novel packaging

approaches to satisfy the aggressive system requirements. Each new generation of systems has become physically smaller, uses less power, and

is significantly faster than its predecessor. One of the major items

affecting performance in many computers is the time needed for a signal to

pass from one chip to another. [1] In order to reduce this delay the chips

must be placed closer together, or each chip must have more functions.

When more circuits are placed on a single chip, more electrical contacts

are usually required, but that is feasible only if the package can accommodate more connections and signal paths.

Paste thick film resistors have been successfully used in the computer

industry for many years, however paste resistors require paste conductor

lines which have density limitations. Line-to-line and 1 i ne-to-contact

tolerances are not sufficient to allow large output chips to be

251
connected in the most efficient pattern. In order to reduce package size

and increase density, evaporated copper conductors have been developed.

With this technology, high density packages can be achieved in a minimal

amount of space. [2]

In high density packages there is still a critical need for precision

resistors. The resistors must conform to the overall package strategy in

that they must be compact, very reliable, capable of active adjustment,

and indigenous within the package to minimize delay time. Finally,

because of the intense competition, the package must be cost effective.

For use in commercial systems, the trend has been away from strict

hermetic packages and into various types of plastic encapsulations.

Plastic packages should be classified as "quasi-hermetic" in that there are no direct free paths for contaminants to reach the chip area, but

diffusion, especially of water vapor, readily passes through the seal area. [3]

PACKAGE DESCRIPTION

The package used by IBM many in its low and intermediate end systems may

have between one to three chips in a pin grid array design whose physical

dimensions range in size from 12 to 36mm. All connections are on the top

surface and there are no interplanes within the ceramic. The chip is

connected by reflow solder joints [4], then a conformal topseal is applied.

The topseal on copper conductors is an amide-imide copolymer (AIP). The

entire assembly is placed into an aluminum cap and sealed with epoxy.

The details regarding fabrication of the Cermet resistor have been

published [5] previously and only a brief description of the product will

be given here. Product specifications for this resistor are ±1% from trim

to end-of-life. To facilitate testing, 0.6% has been apportioned between

end-of-process and end-of-life. End-of-process is defined as a completed

module mounted on a printed circuit carrier. Cermet is defined in the

context of this paper as a thin film resistor originating from a Cr/SiO

target. Depending upon deposition method and subsequent thermal

treatments, the resistor material transforms to varying proportions of

chrome silicides, Si0

2

, and alpha chromium.

The bare alumina substrate is entirely covered by sputtering from a 62/38

(mole %) Cr-SiO target. Then the conductor material (copper) is blanket

deposited over the surface. Circuitry is produced using conventional

photoresist and etch wet processes. The copper conductor is then removed

from the resistor locations to give a completed substrate. A schematic of

the typical part is shown in Figure 1. Contact resistance is reduced

because the resistor material underlays the circuitry and therefore

provides a broad contact area in contrast to that for a paste resistor. A

typical part is shown in Figure 2.

The substrates are next sent to chip join for the reflow operation. After

this step the assembly is ready for trimming. The resistors can either be

passively trimmed to a specific value or the chip powered and the signal

adjusted to a specific output. Topseal and backseal are applied and the

module is complete.

252
STRESS TESTS

When a new technology is proposed, a number of accelerated reliability

tests are performed to determine how the product will withstand field

conditions. The tests are similar to the MIL standards but are modified

for commercial products.

Accelerated Stress Tests

Thermocycle 0-100°C 3 cycles/hr.

Thermal age 130°C

Temperature & Humidity 85°C/80%RH

Wet Sulfur

Corrosive Gas

Shipping Stress -40 +65°C

The temperature and humidity stress is performed with and without a 10V

bias to examine susceptibility to migration and corrosion failure

mechanisms respecti vely . All tests were successfully completed.

During the 10V temperature and humidity bias test, an anomaly occurred on the Cermet package. There are instances where resistors are deleted from

the circuit by simple complete cuts with the laser. To test the

possibility of metal migrating back across the gap and shorting the

deleted circuit, a simple bias test was initiated. The part was produced

in the standard manner, and the AIP topseal placed on all resistors.

After 1000 hours of stress with a 10 volt bias no shorting was observed

and the test was considered successful. However, when the module was examined, the laser delete appeared to have grown in size as shown in

Figure 3. Parts without bias were unaffected.

Such a change is not a problem with a delete, but what about a real part

that has been trimmed? Figure 4 shows voltage distribution of a resistor

trimmed to 7352 ohms with a total drop of 10 volts. At the start of

either plunge cut, there is a potential of about 8 volts across the trim

gap. Therefore if a resistor was trimmed to that specification and run at

a nominal load, degradation may be observed.

CONTAMINATION

At this point it was felt that contamination was the major cause for the

degration. In any failed temperature and humidity test, contamination is

always suggested as the cause, especially when a bias is involved. A

sample of 20 substrates were identically prepared with 10K ohm resistors,

except 10 were cleaned in an ionograph [6] bath. [An ionograph bath is a mixture of isopropyl alcohol and water. The alcohol suppresses the

ionization constant of water and allows the solution to remove more ionic

material than possible with pure water]. The parts were then stressed for

1000 hours in 85/80 with a 10 volt bias. During readouts, the load was removed and the modules taken from the humidity chamber to a 100°C drying

oven for 24 hours. Failed resistors are those that changed more than 0.6‘

from the initial value. The results are given in the following table

253
OUT OF TOLERANCE PERCENT vs. TIME

STRESS TIME (Hrs.) 528 816 1000

NORMAL HANDLING 3% 22% 25%

IONOGRAPH CLEANED 0 18% 25%

There appears to be a very slight difference between normal and ionograph

cleaned parts, but clearly not enough to categorically define

contamination as the major cause of resistor drift. Whatever is causing

the resistance drift did not appear to be strongly dependent upon ionic

contamination level and no further testing was performed with

contamination as a variable.

A careful examination of the parts that had significant positive drift

shows an attack of the resistor in the region of highest potential drop as mentioned earlier. An example of the degradation is shown in Figure 5.

The resistor material appears to have been transformed in the attacked

region from an opaque to a transparent film.

POLY AMIDE- IMIDE COATING COVERAGE

The next item to be investigated was the extent of topseal coverage. A

test matrix was developed to stress parts with 100%, 50%, and no AIP

topseal coverage with a 5 volt drop across the resistor. The 50%

resistors were prepared by simply painting about half of the resistors

with a brush. All resistors on the package were trimmed to 15K ohms, and

backsealed with epoxy. After 1000 hours of 85°C/80%RH significant drift

was observed on all modules. For each readout the modules were removed

from the humidity chamber to a 100°C drying oven for 24 hours. The damage

was again found in the regions predicted by the voltage distribution

model

.

Figure 6 shows a cumulative plot of the percent drift. From the graph we

can see that the uncoated resistors drifted the least, 100% coverage

drifted more, and the resistors with 50% coverage drifted the most. If

the 0.6% drift criteria were used, the failure rate for 0, 50%, and 100%

coverage would be 6%, 17% and 10% respectively.

POWER LEVELS

A test was designed to examine the effect of drift as a function of power

level, temperature, humidity, and time. We were looking for a critical

temperature and humidity below which no reaction occurred. Parts were produced to the following specifications: Six resistors per module, all

trimmed to 15K ohms, no topseal or backseal , four modules per cell. Our

intent was to start substrates at a low humidity level and record any

changes for a fixed time, then the humidity would be increased and the

process repeated. All readings were performed in-situ. A description of

the matrix is given in Figure 7.

The results of the step stress may be summarized as follows. No

significant drift was observed in the 70°C test regardless of relative

humidity or power setting. In the 85°C cell, no drift was observed at

relative humidities less than 80% RH. Modules were held at 80% RH for 400

254
hours to see if any drift could be induced. The results of this test are shown in Figure 8.

There was a slight increase in resistance for cells stressed in 85/80 as can be seen in Figure 8b. The results are not uniform because the most

drift was observed on the 50mW cell and not the 200mW one. This may be

explained by the fact that the surface temperature of a 200mW resistor is

about 40°C above ambient which will reduce the relative humidity in the

vicinity of the resistor from 80% to less than 20% RH. Conversely a 25mW

load will only raise the surface temperature about 5°C. Therefore the 200

mW cell should not drift as much as the 25mW one if the reaction is

dependent upon relative humidity.

RESISTOR LOCATION

When the raw data was examined it was observed that certain locations

seemed to be more susceptible to drift than others even though all

resistors were trimmed to the same final value. The high drift resistors

were near the edge of the substrate on parts without topseal coating but were backsealed with epoxy. Closer examination showed that some of the

epoxy had wicked between the cap and substrate and wet the suspect

resi stor.

SILICONE TOPSEAL

A number of moisture studies on this particular package have been

performed [7] which revealed that the encapsulated package stored at

85°C/80%RH can contain about six times more moisture than expected from

free volume condensation. This moisture must be adsorbed on the interior

surfaces of the package. Because the drift was greater on coated

resistors, more moisture must be adsorbed on them rather than on uncoated

resistors. Very little drift was observed on coated resistors powered to

130°C.

A protective coating was necessary to prevent formation of the moisture

film [8]. A simple barrier coating is not sufficient because given enough

time, moisture will diffuse through the film, just delaying the inevitable

reaction. The coating should form high energy bonds with the conductor

which are not broken by moisture. It should have a low solubility of

water to suppress conductivity in the film itself. The film should have a low sorption coefficient and be chemically stable. The material which can meet these requirements is a silicone topseal. Corrosion studies have

been performed on other systems where the corrosion properties of AIP [9]

and epoxy [10] were compared to silicon topseal s. It was found that the

electrodes were protected by the silicone but attacked under the other

material s

.

Several tests were initiated to study the effect of Cermet resistors under

silicone in our particular IBM package. The first test was run with 10K

ohm resistors at 85/80 with a 25mW bias. One set of substrates had

silicone over all the resistors while the other set was not intentionally

coated. All modules were sealed with epoxy. After stressing the results

could be divided into three groups, 1) coated with silicone, 2) uncoated.

255
3) coated with accidental run-in from epoxy backseal process. The results

are presented in below:

PERCENT FAILS AFTER 500 HOURS

Silicone Coating Epoxy Coating Uncoated

0% 27 % 5%

As can be seen, there were no fails under the silicone. On the uncoated

resistors, very few resistors failed, while on the resistors where run-in

occurred, a much larger fraction was affected. The reduction of drift by

the silicone on modules sealed with epoxy can be simply explained by the

fact that when silicone is put on first, it will prevent wetting of any

subsequent materials.

A second test was performed to determine what effect manufacturing

variability and stress conditions would have on resistor reliability. It was intended to simulate the effect of partial resistor coverage due to

equipment malfunction by producing samples with partial resistor and

partial substrate coverage. These two parameters would simulate the effect

a random bubble on the coating, or a malfunctioning dispense nozzle

respectively. Epoxy backseal coating was used on all parts. Stress time

was 500 hours except for the silicone parts which was 1000 hours. -The

stress conditions were the usual 85°C/80%RH, with 70°C/70% RH and

90°C/60°RH as additional conditions. The humidity cells were added to

determine if there is any threshold for the corrosion reaction. Failures were considered to be those resistors which had drifted more than 0.6%

percent. The results are summarized in the following table.

PERCENT FAILURES vs. STRESS

85/80 70/70 90/60

Silicone 0 0 0

Partial Substrate 11 7 4

Partial Resistor 28 13 4

Uncoated 27 58 17

The above results repeat the previous observations that the silicone

sealant prevents resistor drift.

DISCUSSION OF RESULTS

From the results of the previous tests one item is very clear. When

silicone topseal is applied to Cermet resistors no significant drift is

observed when the resistors were powered in a humid environment.

The picture is not quite as clear when the results of the resistors

powered without silicone coating are reviewed. Where resistor drift was

256
observed during a particular test, it is not uniformly distributed in

extent or location. The skewed behavior could not be attributed to

resistor variation because all test samples were accurately trimmed to

either 10K or 15K ohms and powered in parallel. Most of the drifted

resistors appeared to be near the outer edge of the substrate. Resistors

in this location are susceptible to epoxy run-in. The run-in is very

light and has never affected performance of other products. The edge

resistors are also more susceptible to inadvertent condensation. The

aluminum cap is a good heat conductor and if a cool current of air struck

the module during chamber entry, condensation would occur on the cap and

run down to the perimeter of the module. In the presence of liquid water

and biasm, the corrosion reaction occurs very rapidly.

The procedure followed during readouts may also contribute to the observed

drift. The only test not to show any significant drift was the power step

stress test. These were the only parts that were read in-situ, the rest were taken from the stress chamber and baked before measuring. This

procedure has been used for many years without problems on other products

but it may inadvertently contribute to damage on Cermet resistors.

The corrosion reaction seems to take place without much driving force.

The as-deposited base material was identified as a mixture of chrome

silicides, silicon dioxide, and alpha chrome. It is dark gray, which

optically supports the composition argument because both chrome silicide

(Cr-,Si) and chrome are gray. After the resistor has been attacked, a transparent glassy film remains. This indicates that chrome is being

depleted from the clear region. Electron microprobe analysis confirms

that fact because the clear region has less chrome that the opaque one. The transparent region would be the residual SiCL which is an insulator.

Resistance on parts that are attacked always increases supporting the

above arguments.

CONCLUSION

The addition of sputtered Cermet resistors on a metallized ceramic

substrate provides a versatile packaging strategy. High circuit density

modules can be produced with laser trimmed precision resistors.

During a powered high humidity stress test, a corrosion reaction was observed on trinmed Cermet resistors. There was an attack of the resistor

material at the point of maximum voltage drop across the trim region.

Microprobe analysis of the resistor show that there is a depletion of

chrome in the afflicted region. The corrosion reaction must involve

transport of free chrome, and chrome from the chrome silicide compounds on the resistor, leaving the glassy silicon dioxide phase unaltered. The

resistance of the damaged resistor increases over its initial value,

supporting this argument.

Resistor attack was definitely affected by topseal coverage. The

amide-imide polymers and epoxy coatings accelerated damage while silicone

prevented it. Data analysis was difficult because of the possible

interaction of test variables that initially were not believed to affect

results, but in retrospect may have contributed to the observed erratic

257
behavior. Inadvertent epoxy run-in and temperature-humidity fluctuations

are candidates for stringent control in future experiments.

The application of complete silicone topseal coating on the Cermet

resistors prevented any drift during testing. The coating stopped epoxy

run-in by preventing the epoxy from wetting the resistor and also

prevented water from forming a continuous film across the substrate

surface. When a water film is present on the resistor, the corrosion rate

is highly accelerated. The combination of Cermet resistors and silicone

topcoating is a very stable resistor system that will perform well in

commercial applications.

Acknowledgements

The authors would like to thank J. Elmgren, H. S. Hoffman, R. T. Howard,

R. T. Huebner and A. J. Posocco for their critical contributions.

REFERENCES

1.) A. J. Blodgett, "Microelectronic Packaging", Scientific American,

July 1983, p. 86.

2) T. R. Homa, "Substrate Reliability of Fine Line Metallized Ceramic

Packages" , Proceedings of the 1982 Electric Component Conference , San Diego, Ca. , p. 9l.

3) H. C. Barron, F. R. Moses, and J. Susko, "Internal Moisture

Measurement of IBM Integrated Circuit Memory Package, Moisture

Measurement and Control Workshop , National Bureau of Standards,

Gaithersburg, Md. , 1980.

4) L. S. Goldman, "Geometric Optimization of Controlled Collapse

Interconnections," IBM Journal of Research and Development,

May 1969, p. 251.

5) H. S. Hoffman and E. Stephans, "Cermet Resistors on Ceramic

Substrates" , Proceedings of the 1981 Electronic 1 s Components

Conference , Atlanta, Ga.

'

6) J. G. Ameen, "Ion Extraction Method Improves Reliability, ibid., p. 401.

7) F. R. Moses and H. C. Baron, "Internal Moisture Measurements of

IBM Circuit Packages," Proceedings of the 1981 Electronic's

Component Conference , San Diego, Ca . , p. 406.

8) R. T. Howard, "Electrochemical Model for Corrosion of Conductors on Ceramic Substrates," Proceedings of the 1981 Electronic Components

Conference. Atlanta, "Sal

258
9) M. L. White, "Encapsulation of Integrated Circuits," Proceedings of

the IEEE , Vol . 57, No. 9, September 1969, p. 1610.

10) S. F. Sim and R. W. Lawson, "The Influence of Plastic Encapsulation

and Passivation Layers in the Corrosion of Thin Aluminum Films

Subject to Humidity Stress," 1979 International Reliability Physics

Symposium .

LASER CUT COPPER CONDUCTOR

Figure 1. Schematic of Cermet Resistor on an Alumina Substrate

Figure 2. Photograph of a Typical Cermet Part. 2 . 7X

259
Figure 3 Damage Observed on a Deleted Cermet Resistor After 1000

Hours of 85/80

A 10.0 9.6 B 9.6 9.0

C 9.0 8.4 D 8.4 7.3

F 7.3 6.3 T H 6.3 5.1

J 5.1 4.0 L 4.0 3.0

N 3.0 1.3 0 1.3 0.7

R 0.7 0.2 S 0.2 0.0

Figure 4. Voltage Distribution on a 7352 Ohm Cermet Resistor

Ten Volt Total Drop

260
N

Figure 5.

Figure 6.

Cermet Degredation at Laser Trim 70X

Percent Drift of Cermet Resistors Covered with AIP

0.6% Drift is Considered a Failure

261
i/ICIO

~X>

>—

ir-rno

Test Description

15 K ohms Six Resistors / Module

No Topseal or Backseal Stress Temper ature 70°C, 85°C

Stress Times At Each Humidity (hours)

.Relative Humidity

Power Level(mw) 35% 5 0% 65% 80%

25 96 96 96 400

50 96 96 96 400

200 96 96 96 400

Figure 7

.

Power Drift Step Stress Test

Figure 8. Drift with Time for 70/70 and 85/80 Stress. No Drift was Observed at Lower H umid it ies . Scatter in Readings is Due

to TCR Effect During Powering. The 85/80 Cell Shows a Slight

Increase with Time

262
6.4 MOISTURE EFFECTS ON PASSIVE HYBRID COMPONENTS

- 4 YEAR EXPOSURE TEST -

Jacob A. Ronning and Allan H. Jevne

Medtronic, Inc.

3055 Old Highway Eight

P.O. Box 1453

Minneapolis, Minnesota 55440

Abstract : Thick film hybrids with passive circuitry were constructed for humidity

exposure tests at 0, 10, 30, and 85% RH at room temperature (21°C) . Circuit com- ponents tested were thick film resistors, capacitors, and insulation resistances

of various conductive paths and crossovers. Glass passivated circuits were com- pared with unglassed circuits. The effectiveness of coating the circuits with

Parylene ’C’ or Silicone QCF-3-6550, was compared to no coating. The circuits

were powered at 3.5V D.C. Glass passivated circuits compared better than unglassed. Coated circuits exhibit real, but subtle advantages over uncoated

circuits

.

TEST DESCRIPTION

To evaluate conformal coatings as a protection for hybrid circuits against

moisture and contamination, representative substrates were built.

Electrical resistor and capacitor shifts, insulation resistance reductions, and

conductor path integrity were monitored for potential failure or changes during

the four-year test period.

Design, materials, and processing of the test substrates were representative of

actual product.

Test Conditions

1. Exposure to 0, 10, 30, and 85% relative humidities at room temperatures.

2. Circuit voltage, 3.5V D.C.

3. Controlled contamination.

Test Substrate Construction

The test substrates were ceramic (AI 2 O 3

), 1.12" x .63" x .025" thick. Eleven

connector pins were attached for voltage application during the test and for

parameter measurements.

Resistors were thick film, laser trimmed. Conductors were gold and platinum/

gold. Solder dipping was used for capacitor attach pads, solder test pads, .inci lead frame attach points. One-half of the substrate (pins 1 through 6 ) was

glass passivated over the resistors and conductor paths. The circuits from

pins 6 to 11 were not glassed.

Figure 1 shows a schematic layout of the test substrate to better illustrate

the physical nature of the test circuits under evaluation.

263
Circuit Coatings

Each of the test substrates had glass passivated and nonglassed construction to

allow that comparison in the coated or uncoated condition. Initial screening

resulted in the selection of two circuit coatings for the evaluation test. Forty

substrates were uncoated; forty were coated with Parylene * C', a Union Carbide

vapor deposited product; and forty substrates were coated with QCF-3-6550, a Dow

Corning silicone product.

The Parylene coating was clear and changed the appearance of the substrate very

little. The Dow Corning QCF-3-6550 coating was grey and opaque. After coating,

all resistor values, insulation resistance readings and capacitor values were recorded as reference parameters.

Cont aminat ion

Each of the test substrates was then contaminated by placing one cc of an NaCl/H20 solution on the circuit surface to dry. The contamination level was

0.44 equivalent microgram NaCl per cm^ of surface.

Humidity Test Setup

Room temperature humidities for the exposure test were generated in sealed

containers by the use of saturated salt solutions.

Salt

% Relative Humidity

20° - 50°C

ZnC^ ° x H

2 O 10 - 10

MgCl

2

° 6H

2 0 33 - 30

KC1 86 - 81

The containers used were one-half gallon wide-mouth jars with 250 ml of the

saturated salt solution in each jar. Feedthrough connector blocks were mounted

on the jar lids.

The test circuits were plugged inside the jar lids for exposure to the humidity

levels established by the saturated salts.

The 0% humidity test chamber was constructed in the same manner, but instead

of a saturated salt solution, 250 grams of a desiccant (14 x 30 mesh, 4A

LINDE-molecular sieve) plus 10 grams of a color indicating Drierite were put

in the jar.

Ten uncoated circuits, ten coated with Parylene ' C', and ten coated with

QCF-3-6550 silicone were exposed to each of the humidities (0, 10, 30, 85%) and

powered at 3.5V D.C. for four years.

264
SUMMARY OF RESULTS

Abnormalities Over Four Years - 0, 10, 30% R.H.

Reference readings were taken after cleaning and coating applications. Zero

time ("0") readings were taken after the contamination step, and placing in

the controlled humidity jars.

Changes from the original reference readings for insulation resistance and

resistors are plotted as "abnormalities" in Figures 2 and 3. A resistance

change greater than 1/4% is defined as an abnormality. A reduction in insulation

resistance to less than 2 x 10 12 ohms (2Tfi) is defined as an abnormality.

Figure 2 shows abnormalities in the insulation resistance of conductive paths

and the resistors plotted together for 0% and 10% humidities. Figure 3 shows

abnormalities for the 30% humidity exposures over the four years. There are

80 readings for each point plotted in Figure 2 (0%, 10% R.H.) and 40 readings

for each point in Figure 3 (30% R.H.).

These figures show fewer abnormalities for coated circuits. They also show an advantage for glass passivation over unglassed circuits. The initial number of

abnormalities measured at the zero year point (after contamination and placing

in the controlled humidity jars) did not change significantly over the years.

Insulation Resistance, Coating, and R.H. % at Three-Year Point

To look at performance of coatings versus relative humidity, data on insulation

resistance at the three-year point was taken for all the coatings and humidity

exposures

.

Figure 4 shows the trend towards more abnormalities as humidity increases.

For insulation resistance, glass passivation is comparable to the coatings in

the range 0-30% R.H. Both coatings were an improvement over uncoated circuits

at 85% R.H.

Readings for the 0, 10, and 30% R.H. points were taken with the test substrates

remaining in the humidity jars. The 85% R.H. jars were so wet that insulation

resistance breakdowns in the connectors became a problem. For the 85% R.H.

readings, the substrates were therefore removed from the connectors and air

dried so that comparative readings could be taken.

A dendrite growth was found under the Parylene ? C' coating in an 85% R.H. test

substrate. It was surrounded by a white appearing spot indicating spot con- tamination and loss of adhesion. The insulation resistance reduced to 2K. at

three years and the dendrite was found. Visual examination of all 0, 10, and

30% R.H. test samples revealed no dendrites nor a start of dendrite growth on any substrate.

Capacitors

A . lyf ceramic capacitor and a 22 yf tantalum capacitor were mounted on each test

substrate. The capacitors are constructed to be reflow solder attached to

solder pads.

265
The ceramic capacitors exhibited value shifts less than 1% over time for most

units. A few shifted up to 3h%. No correlation could be established between

parameter shift and coating. Likewise the tantalum capacitors showed value

shifts that did not correlate with coating. One manufacturer's lot shifted from

-2-2% to +3% as humidity increased and others shifted less than ±%% with humidity.

It appears that redundant coatings do not improve ceramic or tantalum capacitor

stability over uncoated capacitors.

CONCLUSIONS FROM THE 4 YEAR TEST, CIRCUIT COATING

EVALUATION FOR MOISTURE AND CONTAMINATION

1. Glass passivation is comparable to other coatings at relative humidities

less than 30%.

2. Some contamination can be tolerated by glassed and coated circuits up to

30% R.H., (21°C) for the voltage and materials used.

3. The Silicone QCF-3-6550 and Parylene 'C' coatings improved insulation

resistance stability in the 85% R.H. (21°C) test.

4. In the 0, 10, 30% R.H. tests, initial abnormalities did not increase significantly with time.

REFERENCES AND ACKNOWLEDGMENT

1. Complete data set is referenced in Medtronic Laboratory Notebook Number 341,

page 51, Allan Jevne, Energy Technology Division.

2. A sincere thank you is extended to Eileen Schultz for her untiring dedication

to the data acquisition and reduction efforts of this project.

266
GLASS PASSIVATED NOT GLASSED

HYBRID TEST SUBSTRATE

4 YEAR COATING EVALUATION TEST FOR HUMIDITY AND CONTAMINATION

+3.5V D.C. PINS 1-5 AND 7-11 (Pin 6 NEG.)

FIGURE 1

267
o I 2. 3 4

YEARS

KEY:

UNGLASSED

o-—

o

GLASSED

YEARS

COMPONENT ABNORMALITIES

12

RESISTORS >hZ, INSULATION RESISTANCE <2x10 ft(2Tft)

0% & 10% RELATIVE HUMIDITY, 80 COMPONENTS PER DATA POINT

FIGURE 2

268
ABNORMALITIES

ABNORMALITIES

ABNORMALITIES

11 1! I O / £ 3 4

YEARS

KEY:

UNGLASSED

o o

GLASSED

D

Si

SILICONE QCF-3-6550 COATED

T ' T 1 —~T—'

YEARS

COMPONENT ABNORMALITIES

RESISTORS >*5%, INSULATION RESISTANCE <2xlo'“ (2! ) 30% RELATIVE HUMIDITY, 40 COMPONENTS PER POIM

FIGURE 3

269
ABNORMALITIES

ABNORMALITIES

ABNORMALITIES

% RELATIVE HUMIDITY

GLASSED

a

8-i PARYLENE ’ C

’ COATED

4

OH =D

Jr

4-

^o C3)

Zk-t„6<s —

D

lo 30 85

"

% RELATIVE HUMIDITY

INSULATION RESISTANCE ABNORMALITIES

(Number <2x10 or 2Tfi)

20 COMPONENTS PER DATA POINT, 3h VOLTS D.C., 3 YEARS, 21°C

FIGURE 4

270
7. SESSION VI MOISTURE AND ORGANICS

7.1 MASS SPECTROMETRIC EVALUATION OF EPOXY ADHESIVE SYSTEMS

William Bardens

Beckman Instruments, Incorporated

2500 Harbor Boulevard

Fullerton, California 92634

(714) 773-8666

Abstract: Epoxy adhesives used for die and/or substrate attachment in

hermetically-sealed packages can be a source of significant amounts of

moisture, as well as other atmospheric contaminants. The level of moisture experienced is the result of many variables such as epoxy formulation, preseal bake conditions, sealing process, and post-seal temperature

exposure. It is difficult to predict the magnitude of the effect of

different combinations of these variables. The effects can, however, be

determined by utilizing selected combinations of these variables.

This paper discusses the results of mass spectrometric analysis of the

internal atmosphere of hermetically-sealed packages using various combinations of epoxy types, preseal bakes, and post-seal temperature exposure.

The results demonstrate that this technique is, in fact, useful for

evaluating such variables.

1. INTRODUCTION

In the course of manufacturing hybrid microcircuits, failure to conform to the

water vapor requirement of 5,000 ppm maximum required by MIL-M-38510 occurred

on a sporadic basis. Experience had shown that the epoxy adhesive used in the

construction of the hybrids was the primary water vaoor contributor. Furthermore, this same experience had shown that the problem occurred on units which

utilized epoxy adhesives for attachment of the substrate to the header, but

did not occur on units which omitted this attachment material even though both

types of units utilized epoxy adhesive die attachment. It was thus assumed

that the epoxy substrate attachment material was the primary source of the

internal water vapor content, a not unreasonable conclusion considering the

fact that the amount of substrate attachment material contained within the

package cavity is generally 5 to over 100 times the amount of die mount material. It was felt that, at the present limit of 5,000 ppm water vapor content,

effort should be concentrated on investigating the behavior of the substrate

attachment adhesive to achieve the maximum benefit. The data discussed in this

paper is the result of work done on the effects of material and process

variation on water vapor content and illustrates the use of mass spectrometn

:

analysis as an evaluation tool.

2 . EXPERIMENTAL DESIGN

The goal of the experiment was to investigate a new material for substrate

attachment, the use of which could result in a lower water vapor content.

For the past several years, Ablefilm 529 adhesive has been the stan iar : na' —

rial in use for substrate attachment. As water vapor content limits were

271
imposed and proceeded to be lowered, it was found to be more and more difficult

to meet these requirements. The experiment was designed to provide comparative

data on the water vapor levels resulting from the use of Ablefilm 529 and a new attachment material, Ablefilm 555. Additionally, the effect of varying the

preseal bake-out process was investigated as well as the effect of post-seal

storage time.

The variables included in the experiment are as follows.

1. A comparison of two materials - Ablefilm 529 and Ablefilm 555.

2. Post-seal high temperature storage at two temperatures - 125°C. and

150 °C. - for up to 1,000 hours.

3. Preseal bake-out at 150°C. for two different time periods - 16 hours

prebake + 4 hours vacuum bake and 48 hours prebake + 4 hours vacuum

bake

.

This third variable needs some clarification as to the bake combination used.

It is generally accepted that in order to obtain a dry package, some level of

preseal baking is required. This bake could be performed entirely in the

vacuum oven which is attached to and directly opens into the sealing chamber.

A number of years ago, before the imposition of internal water vapor requirements, the preseal bakes used were only 1 or 2 hours in length and the vacuum

oven was used exclusively. However, the need for longer preseal bake cycles to

meet current requirements has made it impractical to utilize the vacuum oven

for the total time period because of the severe limitation it places on production thru-put. Several alternative methods were considered, but the one adopted utilizes a first bake in a forced convection oven with nitrogen

gas flow into the oven. This bake is identified by the term "prebake" and

this identifier will be used in the balance of the paper. Following prebake,

the parts were transferred to a vacuum oven for completion of the preseal

bake cycle. Since the transfer between ovens results in exposure to room

air- the transfer time was limited to a period of ten minutes to minimize

diffusion of moisture back into the epoxy.

In order to make the results as meaningful as possible, conditions other than

those previously discussed were kept as nearly identical as possible. To this

end, the following conditions were observed for all samples.

1. The package style used was a TO-8 metal header with a nickel-plated body

and gold-plated leads utilizing a solid nickel lid.

2. The lids were attached to the header by single shot resistance welding

inside a glove box containing dry nitrogen atmosphere with less than 10 ppm

water vapor content.

3. Internal volume of the sealed unit was approximately 0.65 cc.

4. The epoxy used to attach the substrate to the header came from the same manufacturing lot for each of the two materials tested.

5. The same volume of epoxy material was used in each test sample.

272
6. Blank alumina substrates the same size as the epoxy preform (.330" square) were bonded to the header using the same procedure and cure schedule for

all samples of the same material.

7. All water vapor analysis was done by the same facility (Oneida Research

Service) utilizing the procedure specified in MIL-STD-883, Method 1018.1

except that the bake required prior to puncture was specified as 20-24

hours at 125°C.

8. Timing of the testing of the samples was controlled such that puncture

for mass spectrometric analysis occurred 4-7 days after the parts were

removed from the post-seal bake oven.

3

.

EXPERIMENTAL RESULTS

Figures 1 and 2 show the results of the mass spectrometric data in graphic

form. Each point on the curve represents the average of at least two samples

which were tested after post-seal bake exposure of 168, 500 and 1,000 hours.

The interval of 168 hours was chosen as the first test point because this

represents the time required for burn-in on units built to conform to MIL-STD883, Method 5008, and normally the requirement for water vapor content must

be met after burn-in.

In order to avoid wasting test time and the unnecessary expense of analyzing

units which exhibited unacceptable leak rates, a seal test per the requirements

of MIL-STD-883, Method 1014, Conditions A, and C, was performed immediately

after seal and just prior to sending any units to the testing facility. Even

with this precaution, three of the units tested out of 128 samples were found

to be leak test escapes when the mass spectrometric results were analyzed; i.e.,

units containing measurable helium or fluorocarbon materials were assumed to be

leaking units. The results from these three units were considered invalid and were not included in the data presented.

In addition to the water vapor results shown in Figures 1 and 2, the mass

spectrometric analysis gives information on other components of the gas con- tained within the sample. Although there is no specification limit on these

other components, their presence or absence can aid in a better understanding

of the systems under investigation. Out of the total 128 samples, the following

was noted.

1. No ammonia was detected in samples built with either material.

2. Oxygen was detected in only 10 samples and had a high value of 800 ppm in

those where it was detected.

3. M.E.K. was detected only in those samples utilizing Ablefilm 529 and was present up to a maximum level of 15,000 ppm.

4. Methane was detected in most samples, but was less than 4,000 ppm.

5. Other solvents and hydrocarbons were not detected or were less than 3 iin.

273
6. Hydrogen was detected in approximately half of the samples, but at levels

less than 400 ppm.

7. CC>2 was detected in varying amounts depending on the time and temperature

of exposure the units had seen after sealing. Figure 3 gives the range

of CC>2 seen for each substrate mount material.

4.

CONCLUSIONS

Referring to Figure 1 and Figure 2, several conclusions can be reached concer- ning the performance of the two epoxy systems when stored at 125 °C. or 150 °C.

after seal.

1. Storage at 150°C. after seal greatly increases the internal water vapor

content over that reached at the same time period at 125 °C. storage for

both materials.

2. Given equivalent conditions of processing and temperature exposure, Ablefilm 555 resulted in a significantly lower internal water vapor level than

did Able film 529.

3. Increasing the duration of preseal baking will result in lower internal

water vapor levels for both materials at all time periods tested. The

degree of reduction is similar for both materials.

4. After 1,000 hours of post-seal baking, the internal water vapor content is

still increasing at a rate which could have been predicted by the 168-hour

and 500-hour levels and shows no indication of having reached a maximum

value

.

5. As indicated by the nearly identical slope of the curves, the rate of

water vapor evolution is similar for both materials. The lower levels

achieved by the use of Ablefilm 555 at any time during post-seal temperature

storage appear to be the result of a lower level of absorbed water vapor

remaining after preseal baking.

6. Given the observations as stated in paragraphs 4 and 5 preceding, it would

be unreasonable to conclude that the increase in water vapor content is due

solely to the release of absorbed or adsorbed water. When the additional

observation concerning the large increase in CO2 experienced during postseal bake is considered, it would be reasonable to assume that some portion

of the increase in water vapor is due to thermal breakdown of the epoxy

system which results in the formation of water. It would require additional

testing to prove this assumption, but analysis of the results of work to

date point heavily in this direction.

274
5. SUMMARY

The work described in this paper has utilized the technique of mass spectrometric analysis as a method to provide useful data concerning the effect of

multiple variables on the level of internal water vapor content inside

hermetically-sealed packages. The data developed has proven useful and can

be summarized as follows.

1. Differences in internal water vapor levels can result from the usage of

different epoxy materials in the internal package cavity.

2. Of the two materials tested, the usage of Ablefilm 555 in place of

Ablefilm 529 will result in lower water vapor levels.

3. When units contain epoxy materials, it is important to minimize the postseal temperature exposure, particularly if the temperatures are in excess

of 125°C.

4. The curves developed during the experiment have proven useful in predicting

the internal water vapor levels for various combinations of post-seal

elevated temperature exposure.

5. Materials with improved thermal resistance are needed if internal water

vapor levels below 5,000 ppm are required after extended post-seal

exposures to elevated temperatures of 125 °C. or above.

275
WRTER

WPOR

LEMEL

,

PPM

Fib. I

I 1 3 4 5 £ 759 10 2 3 45^769 10 //D X70z

T//Wf /97 /2S°C.(hRS)

276
WATER

VAPER

LEVEL

,

PPM

F/6.2

mi/mus iu water vapor vs post seal temp, storage

TIME AT I 50°c. (msT)

277
MATER/Ai T/ME E)7 TEMFERATUEE

/ZS°c STORAGE /sot STORAGE

I68HOURS IOODHOU& K&HOURS IQDO HOURS

523 2)30 2530 4330 72>AOO

sss 4346 6600 7000 70,500

xauae of_ tzg2 QQUTeuT, ppm

F/G.3

278
7.2 MOISTURE TRANSPORT IN POLYIMIDE FILMS:

IMPLICATIONS FOR MOISTURE MEASUREMENT IN IC PACKAGES

Denice D. Denton, David R. Day, and Stephen D. Sentnria

Department of Electrical Engineering and Computer Science, and

Center for Materials Science and Engineering

Massachusetts Institute of Technology

Cambridge, MA 02139

ABSTRACT

As organics such as polyimide (PI) find increasing use in integrated circuits, the

fact that such materials can absorb moisture must be carefully examined. We have

made in-situ measurements of the moisture uptake and moisture diffusion rate in

thin PI films on integrated circuits. The samples consist of parallel-plate capacitors with aluminum electrodes in which the PI film (typical thickness 1 pm) is

the dielectric. By comparing weight change with change in capacitance, it is found

that the capacitance change on exposure to ambients with varying moisture can be

used as a quantitative measure of moisture absorption. Furthermore, by monitoring

capacitance versus time, the rate of diffusion of moisture in the thin sheet of PI

between the electrodes can be determined. Principal experimental results are: (1)

Moisture diffusion in PI is Fickian, with an activation energy of^about 0.25 eV,

and a diffusion constant at room temperature of about 5 x 10 cm /sec. For this

activation energy, the diffusion constant at 100°C is about an order of magnitude

larger than that at room temperature. (2) The equilibrium absorption of moisture

in the temperature range 20 - 80°C follows the ambient relative humidity, although

the adsorbed moisture - R.H. relation is not perfectly linear. Based on these

results, a model is developed which shows that absorption of moisture by a PI

passivation layer could greatly reduce the apparent dewpoint inside an IC package.

In addition, attempts to measure moisture inside packages in which PI is used as an interlevel dielectric must take into account the time scale of absorption and

desorption both from covered and uncovered PI regions, and must be related to the

actual geometry of the device structure being tested. For example, a 1 nm thick PI

film covered by 10 pm wide metal izations would required about 10 minutes to equilibrate, but a typical bonding pad of dimension 250 pm would require about one day.

INTRODUCTION

Because of its excellent thermal and chemical stability and its good dielectric

properties, polyimide (PI) is finding increasing use in integrated circuit technology both as a passivant and as an interlevel dielectric. One property of PI

that has caused concern is its strong interaction with moisture [1]. As part of a comprehensive study of the electrical properties of PI, we have developed a capacitance-based technique for the in-situ measurement of moisture uptake and transport

in thin PI films. This paper presents the first quantitative results from this

work, and discusses those results with particular emphasis on the problems of

moisture measurement in integrated circuit packages within which PI may be used

either as a passivant or as an interlevel dielectric.

EXPERIMENTAL PROCEDURES

Sample Preparation

All samples consisted of aluminum-PI-aluminum parallel-plate capacitance structures

on silicon substrates. The fabrication procedure was as follows: A 0.75 pm pure

aluminum lower electrode was evaporated onto a cleaned silicon wafer. Then a PMDA

279
ODA polyamic acid precursor in NMP solvent was spin-coated onto the wafer. No

adhesion promoter was used; spinning conditions were 6000 rpm for 30 sec in dry

air. A cure sequence of 90°C for 30 minutes in dry air followed by 300°C for 1 hour in ambient air (i.e., nominal 40% R.H.at room temperature) was used for the

results reported here. Other cure schedules have been examined, and will be

reported in a later publication. Final PI thickness was 1 |un. Following cure, a 0.75 pm thick pure aluminum upper electrode was evaporated onto the sample, and

patterned photol ithographically using Shipley 1470J resist and 351 developer. The

overall size and configuration of the upper electrode was varied. Figure 1 illustrates a typical sample used for electrical measurements. The stripe electrode

pattern is chosen to permit moisture access, and to create a dominant shortest

diffusion path for moisture equal to half the width of each stripe. This diffusion

path is in the PI beneath the stripe, as illustrated schematically by the arrows in

Fig. Kb). Three different stripe and spacing widths, 13, 51, and 102 |im, were used for electrical measurements.

Two types of samples were used in weight uptake experiments. A1-PI-A1 samples

having 51 pm A1 stripes and 5 pm spacings covering an entire wafer were used. In

addition, some weight uptake experiments were performed on samples with PI on both

sides of a silicon wafer, and no metallization.

Capac itance- and We ight-Versus-Moisture Measurements

Wafers were placed on a vacuum chuck covered by an enclosure through which air of

known dew point was flowed. The air source was a compressor, followed by oil and

water traps, a particle filter, and a molecular sieve dryer. The dry air stream was divided into two streams, one of which passed through a saturating bubbler at

22°C. Dewpoint control was achieved by mixing wet and dry streams. A General

Eastern dew-point hygrometer was used to measure dew point of the final air stream.

Capacitance was monitored as a function of time using a GenRad 1689 Digibridge. It was determined that the capacitance did not depend on frequency in the range 1-100

kHz. Below 1 kHz, there was deterioration of signal-to-noise ratio. Except where

noted, the results reported here are for measurements at 2 kHz.

Weight uptake versus time was followed in two different ways. First, a sample

initially equilibrated at + 18°C dewpoint was placed on the pan of a Mettler balance

in lab air, and the weight loss monitored as a function of time, while a similarly

equilibrated and exposed capacitance sample was monitored electrically. The second

approach was to place a sample coated on both sides with PI and with no metallization in a Cahn microbalance, and heat it to above 100°C in dry air (dewpoint -11°C)

until its weight stabilized. Then air of -20°C dewpoint was introduced into the

microbalance and the weight and temperature of the sample were monitored during

cooling, in effect monitoring the weight change against relative humidity.

EXPERIMENTAL RESULTS

Capacitance Versus Time

Figure 2(b) shows typical capacitance-versus-time behavior for a sample with

L = 51 (im at room temperature for the dewpoint sequence of Fig. 2(a). The capacitance increases with increasing dewpoint, and the effect is large , ~15%. The

moisture cycle is reversible on a time scale of hours, as shown. No investigation

of long-term changes has been made.

280
That the capacitance change can be attributed directly to moisture uptake is shown

in Fig. 3, in which the capacitance of a capacitor is plotted against weight of a similarly exposed sample, with time as a parameter. The nearly perfect linear

correlation shows that the capacitance change is proportional to moisture uptake.

The shape of the capacitance transient can be analyzed with a simple one-dimensional model. We assume that vertical moisture transport in the exposed PI regions

is so rapid compared to the lateral diffusion beneath the stripe that the exposed

PI regions establish a constant moisture concentration boundary condition that is

in equilibrium with ambient. Solution of the diffusion equation beneath the stripe

is then obtained from standard methods [2], to yield an expression for the fractional moisture uptake versus time for the sample as a whole. Since capacitance

and moisture uptake are proportional, we can write for the time dependent capacitance C(t ) :

where D is the diffusion constant, L is the stripe width, and and C£ are the

initial and final capacitances, respectively.

Equation 1 is fitted to the capacitive transient data as follows: First, and C^. are found from the endpoints of the transient. Then, the transient portion is

plotted against t 1 ' with a scale factor that is adjusted so that the midpoint of

the capacitive transient agrees with the midpoint of Eq. 1. The scale factor,

then, for the known experimental value of L, yields the diffusion constant D.

Figure 4 illustrates a typical fit between theory and experiment for a dry-to-wet

transient. There are some minor shape discrepancies which may mean that a somewhat

more elaborate model may be required for a fully detailed interpretation of the

data. That the diffusion model is reasonable, however, can be seen from Table I,

which shows the D extracted at room temperature for samples of different L. The

times to reach mid-point of the transient t vary by a factor of 40, but the D

values agree to within 25%.

The diffusion constant was also determined at 40, 60, and 80°C. The results are plotted in Ahrrenius form in Fig. 5. If this is a thermally activated process,

then the activation energy is small, ~.25 eV. That is, D increases by only a factor of 10 between 22 and 80°C.

The amount of moisture uptake, however, as indicated by (C^ - C^) , does depend on temperature via the relative humidity. Figure 6 shows AC/C versus ARH for dewpoint

changes from -20°C to +20°C but measured at different temperatures. At 22°C, the

ARH is 75%, while at 80°C, the same dewpoint change produces a ARH of only 3%. An

approximately linearly correlation between AC/C and ARH is observed. However,

corresponding studies of weight uptake versus temperature measured during a cooling

experiment, show a small but distinct nonlinearity.

The implications of these results for moisture measurement in integrated circuit

packages are several. When PI is used only as a passivant, and is therefore not

covered by other layers, equilibration times with the ambient are quite rapid, on

CO D(2n+1) 2 n

2 t L

2 C(t ) = c_ + (C. - C,)

f if ( 1 )

DISCUSSION

281
the order of a few seconds for a 1 [us thick film. In test sequences in which the

package is heated, moisture will desorb from the PI. If the package is then

punctured while warm, the moisture can be expected to leave the package quantitatively. However, if the package is subsequently cooled, for example, in a dewpoint

type test, rapid reabsorption of the moisture will occur, and the dewpoint measured

will be an underestimate of the total moisture content of the package. That this

could be a large effect is illustrated by the following simplified model:

Let p^(T) be the parts per million by volume (PPMV) of moisture in the air inside a TO-8 transistor package at temperature T, and let P§(T) be the corresponding PPMV

at saturation. We have observed that the absorption of moisture by the PI is

approximately proportional to relative humidity, i.e., to p^(T)/pg(T). Based on

this observation, it is easily shown that

pA

(oo) VT) - ~K (2)

mPg (T)

where p^ 00

) is the high-temperature PPMV when all moisture is fully desorbed from

the PI, K is a constant representing the maximum mass of water the PI can absorb at

100% R. H. , and m is the mass of air inside the package.

If we assume that a 4x4 mm chip is covered with a 1 pm thick PI film, use the data

of Fig. 3 to evaluate K, and use the volume of a TO-8 can at one atmosphere to

evaluate m, we can plot p^(T) versus either T or pg(T) , as in Fig. 7, for various

values of p^(°°) . Note that at high T, all of the moisture is desorbed, but as the

package is cooled, the PPMV in the package drops significantly due to absorption in

the PI. In fact, this simple model predicts that for the geometry used in this

example, the PPMV measured at high temperature would have to be about 20,000 before

one would ever observe a dewpoint, as indicated by the crossing of the condensation

line in Fig. 7. For less moisture than this value inside the package, the PI

absorbs enough to prevent condensation at any temperature. While this fact could

significantly interfere with the problem of measurement of moisture using dewpoint

methods, the fact that PI suppresses condensation in all but the wettest packaging

conditions may actually be a reliability benefit, provided that the moisture in the

PI does not itself create reliability problems. These issues are the subject of our current research.

When PI is used as an interlevel dielectric, the diffusion of moisture beneath the areas covered by metal makes the overall rates of absorption and desorption depend

strongly on geometry. For example, at room temperature, alp thick PI film

covered by aluminum stripes 10 pm wide would require about 10 min to equilibrate

with ambient, but a typical bonding pad of dimensions 250 pm wopld require about

one day. At 100°C these times decrease by about a factor of 10, but bonding pads

would still require several hours to equilibrate. Therefore, a puncture test with

a heated package would yield too low a result unless adequate time at elevated

temperature had been allocated for desorption. Furthermore, a heating cycle followed by cooling toward the dewpoint could yield extremely confusing results,

depending on the precise intervals used for heating and cooling. If, for example,

a 1 pm interlevel dielectric is 50% covered by metal, and the cooling is so rapid

that even though adequate time was allowed for desorption on heating, re-equil ibration with the 50% of the PI that is covered does not take place during cooling.

Under these circumstances (full desorption followed by only partial reabsorption),

each of the curves in Fig. 7 would have to be shifted up, meaning that condensation

282
on rapid cooling would occur for less total moisture in the package than on slow

cool ing.

ACKNOLVEDGEMENT

This work was sponsored in part by IBM. Extensive use ws made of the MIT Microelectronics Laboratory, a Central Facility of the Center for Materials Science and

Engineering, which is supported in part by the National Science Foundation under

Contract DMR-81-19295

.

REFERENCES

1. E. Sacher and J. R. Susko, "Water Permeation of Polymer Films. III. HighTemperature Poly imide s", J. Appl . Polymer Science, 26. 679-686 (1981).

2. J. Crank, The Mathematics of Diffusion . Oxford, Clarendon Press, 1956,

p. 54

.

TABLE I

Room Temperature Moisture Diffusion Constant Determined from

Capacitance Transients for Three Different Stripe Widths

Width Time to midpoint Diffusion Constant

L (pm) t mid ^ sec ^ D (cm /sec)

13 20 3.8 X 10

51 300 4.0 X 10

102 867 5.6 X 10

283
TOP VIEW CROSS SECTION

(Q )

( b )

Figure 1. (a) Schematic top view of aluminum upper electrode with stripe width

L2 = 56 pm and surrounded by guard ring of width L3 = 102 pm. An alternate electrode structure with stripe width LI = 13 pm is not shown. (b)

Schematic cross section through one stripe of the upper electrode or guard ring showing the aluminum-poly imide-aluminum parallel plate capacitor structure on a silicon substrate, where L = LI, L2, or L3 . Arrow

shows direction of dominant moisture diffusion assumed by the model.

DEWPO

+ 20

NT (°C)

-20

i 1 1 0 12 3 TIME

C ( P F ) (HOURS)

Figure 2. (a) Dewpoint versus time for a dry-wet -dry-wet cycle. (b) Typical

capacitance versus time response to the dewpoint cycle of (a) for

L = 51 pm.

284
Figure 3

Figure 4

C ( P F)

7.5

7.0 -

6.5 -

/ /•

/ •

.r

• / • / • / / /

CORRELATION

COEFFICIENT = 0.99

6.0 .60530

_l I I L. .60535 .60540 .60545

WEIGHT (GRAMS)

Correlation between capacitance change and moisture uptake measured

gravimetric ally

.

C(t) - c f Cj-Cf

( Dt/i2 )' 72

Comparison between experimental capacitance plotted versus (time)'

and the one-dimensional diffusion model for a dry-to-wet transient

(data at 100 kHz). The diffusion constant can be extract|d ? fi time axis scale factor required to fit the slope in the t “ re.-i n.

285
- 7.5

Figure 5

Figure 6

Log( D )

- 8.0

- 8.5

2.5

. «

\

• \

•\

3.0 3.5

~jr x 10"

Arrhenius plot of moisture diffusion constant versus temperature,

Correlation between the fractional capacitance change and the change

in relative humidity.

286
50.000

30.000

22.000

5,000

1,000

100

PA (o=)

5

-60 ^5 -20 0 20 40

TEMPERATURE T(°C)

Figure 7. Effect of moisture absorption on the apparent PPMV of moisture inside

a typical T0-8 transistor package, based on the assumptions of Equation 2.

287
7.3 MOISTURE CONTENT VARIATION IN A HIGH POLYMER CONTENT PACKAGE

Bruce Church

Medtronic, Inc.

6951 Central Avenue

Minneapolis, MN 55432

(612) 574-4761

The cardiac pacemaker is a high polymer content electronic package. Hermetically sealed, it must reliably operate for years in a hostile environment. Extensive testing has been performed to determine the moisture content

and its distribution within the device.

This paper summarizes the moisture testing performed on Medtronic's pacemakers. The effects of configuration and processing are shown. Methods

employed include analysis of total water per MIL-STD 883B, total water observed at other temperatures, and the use of AI 2 O 3 sensors.

288
7.4 VOLATILITY BEHAVIOR OF ORGANIC DIE ADHESIVE MATERIALS

R. K. Lowry and K. L. Hanley

Harris Semiconductor

P. 0. Box 883

Melbourne, FL. 32901

ABSTRACT

Mass spectrometry, surface conductivity moisture sensors, and thermal analysis

were used to study volatility of three organic adhesives. Mass spectrometry and

in-situ sensors verified the moisture-gettering property of polyimide material

but not of epoxy. Agreement on general levels of moisture in package groups

occurred between mass spectrometry and the in-situ sensors, though specific

agreement on individual packages was poor. Mass spectrometry reveals interesting

differences in the quantities of volatiles other than water vapor outgassed by

the different adhesives.

INTRODUCTION

Organic adhesives for die attach of semiconductor devices in some hermetic

package styles can offer distinct advantages in certain applications. These

include:

'device thermal limits precluding eutectic die attach

'heat sinking

'cost reduction

Optimizing package technology with regard to moisture content has yielded some interesting data on the performance and properties of three adhesive systems

chosen for study. All three were silver filled materials, as follows:

'Ablebond 843-1 epoxy

'Amicon C-860 XLC epoxy

'Ablebond 71-1 polyimide

EXPERIMENTAL

The test vehicle used for these adhesive studies was a 24-lead side braze

package. This package utilizes a gold-filled die cavity and gold-plated Alloy 42

lids. One Harris surface conductivity moisture sensor chip [1-2] was mounted in

each package with one of the adhesives of interest. Adhesives were then cured

with slightly less than the optimum cures specified for each adhesive type. The

purpose of less-than-optimum cure was to identify any unique characteristics

imparted to the package cavity by the particular adhesive. Packages were then

sealed and leak-tested. They were then analyzed for internal water vapor via

Test Method 1018.2 Procedure 3 using the sensor chip. They were subsequently

analyzed for moisture via Test Method 1018.2 Procedure 1 using mass spectrometry.

No pre-analysis bake was used. Several packages with eutectically attached

sensor chips were also prepared for reference.

In separate experiments, thermo gravimetric analysis (TGA) was run on each

adhesive. Carefully-weighed samples of cured material were baked at 150°C,

cooled to room temperature, exposed to moist air, and then this cycle was repeated several times. Increases and decreases in adhesive mass were monitored

as a function of time and temperature.

289
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290
DISCUSSION

Moisture cell and mass spectrometric findings are given in Table I. Discussion

points include:

(1) Baseline H^O in these packages is around 4000ppmv.

(2) Four of five 71-1 parts contain no H_0 detected by the sensor, which

depends for its operation on controlled cooling of the specimen to -35°C.

The first 71-1 specimen is discounted because it contains air.

(3) The five non-air containing 71-1 parts have 3500-7900ppmv H

? 0 when

measured by mass spectrometry at 105°C.

(4) Inability to detect H^O at the cold temperatures coupled with detection

of H

? 0 at elevated temperatures in the same package suggests a temperaturedependent HO property of 71-1 polyimide. This is further explored in the

TGA studies.

(5) 843 epoxy parts contained 2-4v% H^O by sensor and 5-6v% H^O by mass

spectrometry.

(6) 860 epoxy contained either >4v% or l-2v% H^O by sensor and like amounts

when measured by mass spectrometry.

(7) Sensor/mass spectrometer agreements are

‘lacking in 71-1, see item (4)

‘good for 860 epoxy

‘poor for 843 epoxy

(8) The eutectic mount and the 843 samples contain about the same amount of H 0

, about 5.2v%. But the 71-1 parts contain an average of 4.9v% H„ , and the“

860 specimens contain an average of 2.5v% H^. It is possible that these two

materials consume from the package ambient.

(9) Relative to organic species

*843 and 860 display about the same amounts of amu's 15 and 45.

'They display substantially more amu's 15 and 45 than 71-1 polyimide

*71-1 polyimide is not much different than eutectic in organics content

(10) With regard to carbon dioxide, the adhesives parts usually contain more than

eutectics, but amounts are highly variable.

(11) There are some interesting interlab measurement comparisons in the data in

Table I. Specimens marked "H" in the left-hand column were analyzed in

Harris' mass spectrometer. Those marked "C" were analyzed at a commercial

DESC-certif ied laboratory. Again, none of the specimens received a preanalysis bake.

‘Eutectic mount: The relative difference between sensor and mass spec

occurred at both H and C.

‘71-1 polyimide: C agreed exactly with the sensor on the one wet package.

C and H alike found H^O in all other parts measured dry by sensor.

'843 epoxy: C had one serious discrepancy with the sensor. Otherwise

both C and H measured somewhat higher than the sensor.

291
’860 epoxy: Both C and H agree with the sensor.

’H^: C and H are in general agreement on levels in the various groups.

'Organics: C reported no amu 15 but higher amu 45 than H in 71-1.

These data, which have been reproducible over years of experimentation with these

adhesives, indicate the desiccating properties of some organic materials. Note

the inability of the moisture sensor to detect >5000ppmv quantities of H«0 in the

polyimide parts. In the 843 epoxy parts the sensor seems to detect substantially

less H^O than mass spectrometry. Only in the 860 epoxy did the sensor and mass spectrometry agree. Of these three materials, 71-1 epoxy is consistently dry at

dewpoint temperatures. This has important implications, both for the operating

reliability of the part as well as the moisture measurement technology used. Preanalysis baking and/or measurement at elevated temperatures may cause detection of

1^0 which, because of sorption properties of adhesives, is never available at the

much lower dewpoint temperatures.

Thermal analyses were carried out to further investigate this behavior of organic

adhesives. Fig. 1 is a thermogravimetric analysis (TGA) scan of a fully-cured

50mg sample of 71-1 polyimide. The left-hand vertical axis is weight percent,

plotted by the solid line. The right-hand vertical axis is °C plotted by the

dashed line. The polyimide, when baked in N„ at 150°C for 20 mins., offgasses

about 0.04% of its mass. When cooled to 35°C in. ^

the weight remains almost

constant. But when moist air is admitted to the cell, the polyimide regains back

to its original value in about 40 mins. Behavior is the same when this cycle is

repeated, and is indicative of its moisture uptake properties at low temperatures.

Similar scans of 843 and 860 epoxy, combined in Fig. 2, showed similar behavior.

However, offgassed and resorbed quantities varied. These are summarized in Table 2.

The polyimide lost the least mass on initial heat. And on subsequent coolings and

heatings the sample mass change was exactly the same. Epoxies lost 4-6 times as much on initial heating, and resorbed only about one-fifth the offgassed material

when exposed to moist air. Subsequent re-heating caused the epoxy to lose additional mass. However, by the second cooling step the epoxies have begun to resorb

similar amounts as they out-gassed during the second heating.

Table 2

TGA HEAT /COOL CYCLES

INITIAL 1ST HEAT 1ST COOL 2ND HEAT 2ND COOL

SAMPLE MASS TO 150°C TO 35°C TO 150°C TO 35°C

71-1 50.45MG - 0.03MG + 0.02MG - 0.02MG + 0.02MG

843 62.85MG - 0.18MG + 0.03MG - 0.06MG + 0.05MG

860 52.50MG - 0.13MG + 0.03MG - 0.04MG + 0.04MG

The TGA data confirm the mass spectrometry data in these respects:

'71-1 epoxy in general gave the driest packages, agreeing with the Table 2

information showing it had the least volatiles.

'H^O was undetectable in the cooled 71-1 specimens by sensor. Uptake of all

volatiles in 71-1 on cooling was confirmed by TGA.

"H^O was detectable in the cooled 843 and 860 specimens by sensor. TGA

snowed these materials did in fact not resorb all offgassed products.

292
CONCLUSION

A unique combination of three diverse analytical techniques was used to study

the volatility behavior of several organic adhesives intended for semiconductor

packaging. Relative moisture-producing potential of sub-optimum cured material

was defined, as was the potential to produce significant amounts of hydrogen

and organic species. Mass spectrometry provides significant useful information

on species other than moisture. The moisture sorption behavior of the polyimide

suggests that when properly processed it will not contribute significant amounts

of available moisture to a package cavity at dewpoint temperatures <10°C, while

the epoxy materials do not exhibit the same desirable moisture removal effect.

REFERENCES

1. Lowry, R. K. , Miller, L. A., Jonas, A. W. , and Bird, J. M. , "Characteristics

of a Surface Conductivity Moisture Monitor for Hermetic Integrated Circuit

Packages," in Proceedings of the 17th Annual Reliability Physics Symposium,

April 24-26, 1979, San Francisco, CA. , pp. 97-102.

2. Product Information Bulletin, Harris Semiconductor HIO-55001-6, Moisture

Sensor Chip, March, 1979.
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DuPont

1090
Sample:

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14-Sep-83

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8. SESSION VII MOISTURE CONTROL

8.1 MOISTURE IN ELECTRONIC PACKAGING: MEASUREMENT AND THE CONTROL OF IT

George B. Cvijanovich

AMP Incorporated Research

3441 Myer Lee Road

Winston-Salem, NC 27102

Summary

Review of origins of moisture in a sealed package. Systems of measurement of

entrapped water (invasive and noninvasive ) . Possible new technologies for

dealing with moisture residues. Suggested methods: accept some moisture,

but immobilize ion migration by activation of getters; or use water consuming

organic compounds. Use of catalysts to redirect the moisture activity.

Importance of ultra-dry rooms in process control. Prospects for VLSI

packaging.

Introduction

Several years of experimentation and of operation in the field, with sealed

electronic packages, have clearly demonstrated the failure of these systems

when the presence of moisture within the package was above a certain limit.

The failure is defined here, as the final or the intermittent interruption of

the functioning of the system due to the corrosion or electromigration of

aluminum from the leads, pads, and other conducting sites located on the

chip, or the chip-holding assembly. To quantify this type of failure, most

of the people involved in the study of it are looking for some kind of

accelerated life test under fairly exotic conditions. Then, most of the

effort to solve this problem is directed toward designing a package for a so- called 85/85 test, rather than for a proper long lasting functioning of the

device. In many instances not only aluminum is involved in the migration,

but also other material such as silver, copper, etc. According to

experimental evidence we must strive for essentially two principal design

criteria in the packaging of ICs: thermohydrolytic stability and, if

necessary, a predetermined oriented diffusion, i.e., a quasi directed

corrosion (that will not affect the functioning of the device). The aim is

to package a chip for a functional expectancy of about 10 1 * 2 years. This

is an average time interval of the "modernity" of a device (about 15 years).

This could mean a life time of about 20 years. To quantify such a concept we will adopt what is known as a Mean Time to Failure (MTF ) concept. The MTF,

at a given site, is proportional to the divergence (or the strength) of the

migration ion flux. This definition is based on the assumption that, almost

exclusively, the interruption of the functioning of a device occurs when a large portion or the whole of a conductor is removed from its design

location. We have, therefore, to relate MTF to moisture and its corroding

activity.

Design Criteria

At present we package, within a very small space, a plethora of elements and

compounds together: silicon, aluminum, silver, gold, silicon oxide, nitride,

assorted series of alloys, polymers, and special organic compounds. Some of

296
it is in monomolecular layer quantities. In addition, very limiting

geometries are imposed on most of the structures designed, such as leads as small as one micrometer square, separated by completely heterogeneous

materials. This necessarily complex design generates very hard physical and

chemical conditions.

Some operational conditions are determined by the necessity to transport not

more than 18,000 to 50,000 electrons from one place to another, or then to

handle 10 ^ to 10 & Coulombs/sec/cm 2 per active site. This means that

occasionally an active site must dissipate enormous quantities of heat. As

an example, the volume resistivity of Al, at 100°C, is about 3.5 x 10 ^ ohm

cm. At a current density of 10 6 A/ cm 2 such systems must dissipate almost

3.5 x 10 6 W/cm 3

. In addition, very often, such leads have significant local

microdefects. Therefore, local temperature and concentration gradients are extremely large.

Chemical parameters for the operation of VLSI are even more stringent. Due

to high local temperatures and concentration gradients, electrochemical

environment becomes especially severe. These conditions, coupled to a large

surface to volume ratio, are of central importance in the role of

electromigration and the ultimate failure of the device. We believe that, in

essence, the local high "chemical temperature islands" represent the ultimate

failure factories.

Observed consequences of these conditions are such processes as self

diffusion, surface diffusion, electromigration, fast ion transport,

thermohydrolysis and local corrosion, growth of material due to stress

gradients, whiskers, etc. Almost all of these processes are greatly

enhanced in the presence of moisture and operational currents. But, the main

question is at what percentage of moisture, the really dramatic and final

enhancement occurs. Based on several years of experimental work and field

data (and after few thousand pages of print) a tentative limit of 4000 to

6000 ppm of H^O at STP may be acceptable. We believe that even this amount

of moisture, in certain restricted cases, may not be acceptable in local

electromigration.

The methods of measurement of entrapped water in a chip are numerous and well

known. All of these methods deal, however, with water extracted from the

chip. Not all of that moisture is necessarily active in electromigration.

We have seen that the ion flux strength is a highly local affair. After

inspection of some chips, it was discovered that some of them, even slightly

corroded, did not fail.

A new method to discover such active sites consists of using a tritium

saturated water (1) in a 85/85 environment, and then take a radio assay print

of the chip. The radio assay test, in conjunction with other measurements,

shows that the water presence is due mostly to curing of epoxies, molecular

flow through cracks and pinholes, to a possible surface catalysis of 2H + 2H

2 0, and very often to careless manufacturing processes (need for ultr.i dry

rooms). Also the main location of the corrosive action, almost always, is at

the surface boundaries between the elements of the IC device and the pottn.i

compound, and at very sharp edges, where extremely high local fields can exist.

297
Directed Electromigration

The type of failures, leading to the interruption of functioning of the

device, is recognized almost universally as due, at least to some degree, to

electromigration.

Electromigration, at an active site, is a form of directed diffusion. A

number of people (2, 3) have investigated microdiffusion processes, using

Na22 and C136 isotopes (figures 1 and 2) in alkyd coatings. When a small

potential was applied, an order of magnitude increase in diffusion occurred,

as shown in figure 2. If we now consider the MTF as a consequence of locally

directed electromigration (through self and surface diffusion), then let us establish the following:

1 . The removal of material from a location on the chip through directed

diffusion, and

2. The exchange of atomic species at a site is due to ion flux

proportional to the current density at the site. In that case, the MTF

becomes conveniently measurable, at least to some degree. Thus, we can write MTF a 1 /current density = 1/J.

This model, corrected for temperature effects (temperature gradients), leads

to an expression of the form MTF a 1/J 3

.

Experiments have shown that for aluminum these relations hold:

MTF a 1/J for J = 10 3 -10 5 A/cm2 MTF a 1/J 3 for J = 10 5 -10 6 A/cm 2

Why this discrepancy for different J?

The inspection of A1 leads shows that the crystalites are relatively small

compared to the lead structures. Therefore, the contribution to the

diffusion from lattice grain boundaries and the surface geometry must be

taken into account with more weight. Especially in the case of VLSI where

the surface to volume ratio is very large for all leads, and therefore the

conventional laws of conduction must be modified. In such cases the limit of

bulk to grain conductivity must be redefined. Thus the overall diffusion

coefficient D must be written as D = D

s

• Dgr. D^, i.e., as a combination of surface, grain, and lattice diffusion coefficients. The current

density itself, in that case, becomes a nonlocal (4) quantity, as defined in

figure 4. In this context, the problem of moisture and its impact on MTF can

be related to two stages: the nonlocal, mostly Ds dependent migration, and

at a later, catastrophic, stage, to the bulk electrolytic properties of the

water

.

Tentative Conclusion and Possible Remedies

Based on experimental work and some theoretical models, we think that a highly local electromigration occurs, under minimal presence of moisture,

(less than 5000 ppm). According to that model it is more efficient to act

locally to modify Ds , and to immobilize water, rather than to try to make a package absolutely dry. Thus, it seems that the use of titanates, or

298
vanadites is preferred to getters. Such compounds consolidate the local

orientation of hydrophylic-hydrophobic chains. The use of these compounds

does not exclude the application of moisture binding chemistry. However, the use of titanates makes the functioning of the chip less dependent of its

state of dryness.

The application of "water-consuming" organic catalysts is another very

intriguing idea.

To ascertain the functioning of a chip packaged in the above fashion, it is

best to establish the presence of activity sites and their character by

obtaining a tritium radio assay print for each design. Such a print will

show the critical points in the packaging system.

Bibliography

1. A. Baily, G. B. Cvijanovich, to be published.

2. J. Bass, The Phil Magazine, Vol. 15, No. 136, pp 717, April, 1967.

3. J. Black, IEEE Trans. Electron Devices, Vol. ED-16, pp 338-347, April,

1969.

J. Black, "Physics of Electromigration" RADC Tech. Rept. TR-68-243,

October, 1969.

4. G. B. Cvijanovich, NBS/RADC Workshop, NBS Special Publication 400-72,

April, 1982.

299
Fig. 1 Chlorine uptake as a function of time for seven different

alkyd coatings. Measurements made

using Cl-36 isotope, a to g from

25 to 58 microns.

Fig„ 2 Sodium uptake as a function of time for alkyd coatings

(as in Fig. 1), exposed to 0.5 M

NaCl. Measurements were made

using Na-22 isotope. The values

increase by an order of magnitude

when only -0.8 V vs. SCE are applied. (H.0+ may play an important role.)

cb

LL)

Z

o

I-<

oc

UJo

z

o

o

CO z

300
(S-inon)

z(2

wo/sdwe)

(dXlAI) 2r

,

uuo

V

180 100 80

1.8 2.0 2.2 2.4 2.6 2.8 1000/T (°K)

Fig. 3 MTF of some Al alloys as a function of J, T and cross sec- tional area A.

Fig. 4 Current density J as a function of a non-local field

E distribution.

301
8.2 EVALUATION OF MOISTURE SOURCES IN HYBRID MICROCIRCUITS

George H. Ebel, Harry Hammer and Stephen Herman

Singer Company

Kearfott Division

1150 McBride Ave

Little Falls, NJ 07424

(201) 785-2982

ABSTRACT

An effort to evaluate polymeric substrate attachment in hybrid microcircuits

led to the study of moisture sources in these devices. Over ten hybrid

suppliers, three polymeric material suppliers, three package manufacturers,

ROME Air Development Center and several users combined resources to gather

the data reported in this paper. The observations presented are based on

a comprehensive study of RGA tests on over 500 hybrids. Most of the packages

in the study were square metal flatpacks in the range of 1 to 1-1/4 inch

on a side.

Possible moisture sources discussed in this paper have been categorized into

three basic areas. These are the packages, polymeric materials and processing.

Packaging items that will be covered include 1) glass/lead/metal interfaces,

2) base-sidewall junction, 3) internal plating and 4) the advantages of using

a special one piece package with no leads for evaluation of polymeric materials.

A review of problems associated with the use of polymeric materials include

1) long time thermal decomposition, 2) trapped gasses under large area devices,

3) excess of curing agent, 4) moisture gettering and 5) material handling

prior to the curing operation. Areas related to processing that will be

discussed are 1) insufficient bakeout (time/temperature), 2) exposure to

atmosphere between bakeout and sealing, 3) sealer drybox conditions, 4) rework

of poor package seals and 5) necessity for having a flowing atmosphere during

the curing and post curing bakes.

Throughout the testing programs, the RGA data were carefully studied to

evaluate this method of locating possible moisture sources. Based on these

data, recommendations for minimum sample sizes, the use of empty packages

and other observations related to bake times and temperatures will be discussed.

Also the effect of "rest time" between bake and RGA tests will be covered.

POSSIBLE SOURCES OF MOISTURE

In an earlier paper [1], the details of three groups of round robin testing

were reported. Those tests led to the highlighting of most of the possible sources of moisture as listed above. As a result of these tests, particularly

the observations made after dye penetrant tests of glass, lead, metal interfaces,

it became apparent that a leadless one piece package was the best vehicle

for evaluation of polymeric materials used in hybrid packages. Such a package

was used for all tests starting with the third round robin. In that round

robin, excessively high moisture levels were measured on an epoxy substrate

evaluation group of electrolytic nickel plated packages. Moisture levels

greater than 1 percent were measured for the evaluation group and for the

302
empty packages used as control samples in the evaluation. The data is shown

in Figure [1] under the heading 'First Seal'. This round robin test included

over 120 samples sealed by 8 hybrid suppliers who had, in the previous two

round robins, demonstrated their ability to construct hybrid microcircuits

with low moisture content. It was theorized that the high moisture levels

measured were caused by the electrolytic plating on the cases. A decision

was made to run an experiment using empty packages with different plating

combinations to evaluate the effects of plating. At the same time, the

electrolytic plated packages were delidded, baked out and resealed. RGA

(residual gas analysis) measurements were repeated and the moisture content

data is shown in Figure [1] under the heading 'Second Seal'. All the reseals

showed significantly lower moisture levels. The empty package measurement

of 400 PPMV reinforced the theory that plating was at least a partial cause

of the 'First Seal' high moisture content.

Empty Package Experiment

Three package suppliers made up one piece leadless packages using the following

plating configurations:

1) Electroless nickel over a kovar base

2) Electrolytic nickel over a kovar base

3) Gold plating over electroless nickel on a kovar base

4) Gold plating over electrolytic nickel on a kovar base

The packages were all processed and sealed as a batch. The parts were vacuum

baked at 150 degrees C for 24 hours in an oven attached to the package sealer

and were transferred directly to the sealer dry box without exposure to

the external atmosphere. The packages were sealed as rapidly as possible

to minimize dwell time in the dry box. Cell size for RGA analysis was set

at eight parts. Two packages from each vendor lot by plating type formed

the cell (one vendor had submitted two sets of packages). Four cells were baked at 150 degrees C for 24 hours and four cells were baked at 175 degrees C

for 24 hours before RGA was performed. Figure [2] shows the results of

this testing. Each line shows the range of moisture content for the eight

packages in that cell. The range for the 64 parts was 25 to 600 PPMV.

There was little difference between the parts baked at 150 degrees C and

the parts baked at 175 degrees C. The best parts, both from the average

moisture content and range were the gold over electrolytic nickel packages.

The poorest were the electrolytic nickel plated parts. Looking back at

the empty package data of Figure [1], the moisture level for the second

seal falls in the range of the data for electrolytic nickel plated packages

shown in Figure [2].

303
Round Robin No. 4 Epoxy Evaluation

Based on the empty package experiment, the gold over electrolytic nickel

package was selected for a new evaulation round of epoxies for substrate

attachr. Each participant in this test was supplied with all the materials

including a sufficient number of packages to seal some empty packages as control samples. The test results are shown in Figure [3]. Evaluation

packages as well as empty control samples showed high moisture content with

the notable exception of the empty package that had been pre-seal baked in an oven not used for organics. That package showed a moisture level

consistent with the data obtained in the empty package experiment.

Review of the data of Figure [3] with both round robin and general industry

participants in the epoxy evaluation effort led to the theory of cross contamination (or "epoxy rain") for empty packages baked in the same oven used for curing the packages containing epoxy. The remaining data for the

empty packages substantiates the "epoxy rain" theory since two units with

flowing N2 in the bake out oven showed appreciably lower moisture levels

(0.23 and 0.39 percent) than the all vacuum bake parts (1.08 percent). The

parts with the 0.23 percent level were also capped with a loose lid during

the bake which adds additional credence to the "epoxy rain" theory. The

data for packages with epoxy mounted substrates still shows units with very

high moisture levels. A comparison of the Ablestick 555 results in Figures 2 and 3 shows similar moisture levels. The data for Ablestick 555 with reduced

curing agent in Figure 3 shows how excess curing agent can affect moisture

levels. Comparision of these data in Figures [2 and 3] suggests the need

to reduce curing agents in the epoxy formulations to the minimum consistent

with maintaining the physical properties required for adhesion. A combination

of limited curing agent and appropriate bakeout conditions should minimize

this source of moisture in the package. The long term thermal decomposition

was considered to be another major source for the high moisture levels measured

in this group of tests. IR spectroscopy tests were performed on samples

of uncured, cured and cured and baked 550 epoxy. The initial results qualitatively confirm thermal decomposition as a source of moisture. Further testing

is necessary to define the effects of long term thermal decomposition. In

the general discussions during the data review, the question of how precure

handling of an epoxy (shipment, storage, exposure) affected post seal moisture

levels was raised. It will be necessary to set up experiments to evaluate

possible effects of handling. It should also be noted that in the first

two round robins, the test cell size was set at 4 units while in the subsequent

tests, cell size was 2 units. The cell sizes were set to provide qualitative

results indicating direction of the measured test results.

Summary

While plating can be a source of moisture in a hybrid package, it can be

controlled so that package contribution to moisture levels is negligible.

Minimizing curing agent in the epoxy should also reduce moisture levels.

The effect of long term thermal breakdown needs further evaluation as does

the "epoxy rain" theory and the effects of pre-cure handling.

304
Acknowledgements

The work reported on in this paper represents the efforts of many people

in the hybrid industry working together on a common industry problem. The

authors wish to particularly acknowledge the following people for their

efforts in the empty package experiment.

For supplying packages:

F. Vaccaro - Hermetite

S. Tower - Isotronics

R. Chalman - Tekform

For constructing the test samples:

C. Totten - Rockwell

For testing and control of the experiment:

W. Stewart - RADC

Many others were previously acknowledged [1] and we reaffirm our appreciation

for their efforts. With so many participants, it is easy to err and omit

a name. We therefore, wish to acknowledge all who have offered help, information

and raised many questions in the pursuit of this effort.

REFERENCES

[1] G.H. Ebel, Application of 1018.2 to Hybrid and VLSI Devices Proceedings,

Reliability Physics 1983, pp. 274-281.

COMPARISON OF MOISTURE CONTENT IN ELECTROLYTIC NICKEL PLATED PACKAGES SEALED TWO TIMES

Substrate Attachment Moisture Content.% Ratio

Material

First Seal Second Seal

Amicon T-86 4.57 .94 4 86

Semi Alloys 915 9 20 309 2 98

Ablestik 555 5 88 1 60 3 67

Empty 255 004 58 6

Note: All pans baked for 100 hrs at 175 degrees C

before running RGA.Figure 1

305
600

X

>

a.

a.2

500

400

300

200

100

o

o- -

MOISTURE CONTENT OF

3 CM x 3 CM * 0.5 CM

EMPTY HYBRID FLATPACKS

AS A FUNCTION OF PLATING

AND POST SEAL BAKE

o

o

ELECTROLESS ELECTROLYTIC GOLD OVER GOLD OVER

NICKEL NICKEL ELECTROLESS ELECTROLYTIC

NICKEL NICKEL KOVAR PACKAGE FINISH Figure 2

COMPARISON OF MOISTURE LEVEL IN PACKAGES

CONTAING SUBSTRATES MOUNTED WITH EPOXY

WITH EMPTY PACKAGES PROCESSED AT THE SAME TIME

Substrate Attachment Moisture Content.% Preseal

Material

With Substrate

Ablestik 555 4 24

Ablestik 555 with reduced curing agent

2 68

Ablestik 550 1.12

Amicon TG-86 046

Amicon TG-86 095

Bake

Empty Package

1 08 All Vacuum

0 06 * All Vacuum

0 39 Flowing N2

followed by Vacuum Bake

0 23 Flowing N2

followed by Vacuum Bake

0 77 Vacuum with N2 bleed followed by Vacuum Bake

* Part baked in a separate oven not used for organics

Note: All parts baked 300 hours at 150 degrees C before RGA

Figure 3

306
8.3 IMPLANTABLE PACEMAKER MOISTURE CONTROL

Jacob A. Ronning, Jr.

Medtronic , Inc

.

3055 Old Highway Eight

P.O. Box 1453

Minneapolis, Minnesota 55440

Abstract : Since 1976 Medtronic has been producing hermetically sealed heart

pacemakers as its mainline product. The units operate for years in the totally

wet environment of the human body, monitoring and pacing the heart when

necessary. Moisture concerns for the hermetically sealed designs were different

from moisture concerns for the epoxy encapsulated pacemakers that preceeded

them. Hermetic sealing processes, cleanliness practice, and internal moisture

levels are reviewed along with moisture related product performance.

EPOXY POTTED IMPLANTABLE PACEMAKERS

Older implantable pacemakers consisted of discrete component circuits and

batteries encapsulated in epoxy resin potting compounds. After implantation in

the body, the epoxy would become 100% moisture saturated.

Moisture related failures could occur if a sufficient size fluid film would

develop between portions of the circuit so that unintended leakage currents would

result. Adhesion of the epoxy resin to components and interconnecting wires

would prevent this type of failure. Components were tested for suitability for

epoxy potting. Semiconductors were hermetically sealed components. Moisture

related failures were minimized by good cleaning practices to promote epoxy

adhesion, by visual examination and rework to prevent circuit bridging bubbles

or foreign material, and by maintenance of circuit spacing standards within the

epoxy. See Figure 1, Epoxy Potted Pacemaker.

Batteries used were of Mercury-Zinc construction and outgassed hydrogen as their

energy was depleted. The hydrogen permeated through the epoxy, minimizing

pressure buildup.

HERMETICALLY SEALED IMPLANTABLE PACEMAKERS

Lithium iodide batteries, which are themselves hermetically sealed, brought the

improved features of long life and no hydrogen outgassing. Pacemaker designs

were then changed to incorporate hermetically sealed enclosures as protection

for the circuits and battery. Design considerations in developing the hermetic

enclosure were selecting a material to minimize corrosion as a concern and developing a design and processes that would allow virtually no moisture ingress

over the life of the pacemaker. See Figure 2, Titanium Hermetically Sealed

Pacemaker

.

Hermetic Enclosure

Titanium was selected for the pacemaker enclosure. It is a body compatible

material with negligible corrosion. In addition, titanium is light weight

(about 55% the weight of stainless steel) and can be fabricated by stand.ir :

307
metalworking techniques. The titanium enclosure consists of high precision

parts welded together. The parts are inspected and pre-tested to meet design

requirements necessary for functionality and for high reliability welding

practice. All welds are done with qualified processes and equipment. Each weld seam is visually inspected. Sample weld parts from each weld run are analyzed

for acceptable weld penetration and hardness.

The pacemakers are backfilled with helium before the final weld so they can be

reliably leak checked in a helium leak detector. They are tested to a 1 x 10“8

standard cc. helium/sec. fine leak rate. Because of helium sorbing materials

within the pacemaker, a gross leak will not dissipate the entrapped helium

immediately. A fine leak test done within a specified time of the helium backfill and final weld point will detect a gross leak as well as a fine leak.

This helium leak test approach is a reliable quality control tool that has been

used successfully for years at Medtronic.

Since going to the titanium welded design, over 500,000 units have been built

with no returns for hermetic failure. This record was made possible by the use

of high reliability parts and processes monitored by an excellent quality

assurance program.

Internal Moisture Control

Hermetic sealing of the pacemaker locks in residual moisture and contaminants

for the life of the device. "How clean?" and "how dry?" are reliability questions

that are addressed by the design and processing of the pacemaker.

The most moisture sensitive circuit elements are the integrated circuit chips.

These are mounted in I.C. chip carriers, hermetically sealed and high temperature

processed. The I.C. chip carriers become part of a hybrid circuit. They are solder reflow attached to a ceramic thick film circuit along with capacitors

and component attach pins. This subassembly is an open hybrid, with hermetically

sealed I.C. carriers, ready for cleaning, electrical check and then assembly into

pacemaker circuits. (Figure 3)

At the pacemaker assembly stage, discrete components such as a reed switch, diode,

crystal oscillator and antenna are added by resistance spotwelding to the hybrid

component pins. (Figure 4) Soldering is avoided to minimize flux contamination

and cleaning problems. The hybrid with components attached are placed in a plastic assembly carrier with the battery for final cleaning. (Figure 5) The

proprietary cleaning process used has been developed to improve cleanliness of

parts and assemblies. A key to improving the cleaning process and monitoring

its performance is the Ionograph. This measures the quantity of ions extracted

from samples and gives a measure of cleanliness expressed in equivalent micrograms of NaCl. The cleanliness levels maintained then, are based on a process

capability that has been optimized and monitored with the Ionograph.

After cleaning, the battery and pacemaker connections are made by resistance

spotwelding, the titanium shields are closed around the assembly and the seam

weld is made. A hole is left in one shield to allow vacuum drying. The

vacuum bake process first heats the pacemakers to their maximum storage temperature range (125°F) and then applies a vacuum. After a vacuum of a

308
sufficiently low level is reached, the required vacuum bake time is started.

At the end of the cycle, the vacuum chamber is filled with dry helium. This

backfills the pacemakers with helium also. The pacemaker's fill hole is welded

shut in a helium environment, preserving the helium within the pacemaker for

subsequent hermetic ity checking in a helium leak detector.

The effectiveness of the drying process is monitored on samples by measuring total

water content in micrograms. Method 1018.2, Procedure 2 is used [1], By

experimentation it has been shown that the total water moisture content of a pacemaker varies with prior moisture exposure, vacuum bake time and temperature,

and polymer (epoxy) content of the design [2].

Internal Moisture and Product Performance

Since the adoption of vacuum bake processing to achieve an acceptable internal

dryness level, no internal moisture caused failures have been identified as a problem. Over 500,000 units have been produced with this process.

The Medtronic four year test of "Moisture Effects on Passive Hybrid Components"

predicts no failure modes for the 0-30% relative humidity range [3]

.

Vacuum

bake process results are in this range.

CONCLUSIONS - PACEMAKER MOISTURE CONTROL

1. The titanium welded hermetic enclosure meets reliability goals with thorough

process and quality control.

2. The vacuum bake and proprietary cleaning processes have given the pacemakers

sufficient cleanliness and dryness to meet reliability goals.

3. The acceptable dryness range within the pacemaker is probably 0% - 30%, or over, depending on materials and cleanliness.

4. New designs may have different acceptable dryness levels.

5. Internal relative humidity in relation to total water content is useful in

comparing dryness between different designs and processes.

REFERENCES

[1] "Total Water Measurement Equipment and Process Using Method 1018.2,

Procedure 2". C. Sloneker and B. Church, Moisture Measurement and Control

Conference Paper, NBS, November 2-4, 1983.

[2] "Moisture Content Variation in a High Polymer Content Package". L>. Church,

Moisture Measurement and Control Conference Paper, NBS, November 2-4, 1983.

[3] "Moisture Effects on Passive Hybrid Components". Jacob A. Ronnin,., Jr. c Allan H. Jevne, Moisture Measurement and Control Conference Paper, NBS,

November 2-4, 1983.

309
FIGURE 1 EPOXY POTTED PACEMAKER

FIGURE 2 TITANIUM HERMETICALLY SEALED PACEMAKER

310
FIGURE 3

HYBRID CIRCUIT SHOWING I.C. CHIP CARRIERS

FIGURE 4

HYBRID CIRCUIT SHOWING COMPONENTS

ATTACHED BY RESISTANCE SPOTWELDING

311
FIGURE 5

HYBRID, BATTERY AND PLASTIC CARRIER

312
8.4 DETERMINATION OF RELIABLE MOISTURE LEVELS

Bruce Church

Medtronic, Inc.

6951 Central Avenue

Minneapolis, MN 55432

(612) 574-4761

The determination of a reliable moisture level for an electronic package is a difficult process. Many variables enter into the equation. This paper

describes an undergoing procedure to evaluate the reliability of a hermetically sealed pacemaker regardless of the configuration. The method

uses technology in a step-by-step approach.

The steps involve:

1. The use of an Al^O^ sensor mounted inside the device to determine the

relative humidity seen by the components at the operating temperature.

2. The determination of the total free water available by heating the device

to its maximum storage temperature.

3. Replacing the Al

203 sensor with a surface conductivity sensor and

subjecting the device to temperature excursions. This will demonstrate

the possibility of reaching humidity levels that allow surface

conductivity to begin.

4. Quantify the total water present in the device by Method 1018 or equivalent. This is used to determine the specification for moisture and

for process control.

313
9. WORKSHOP PARTICIPANTS

9.1 List of Speakers

Moisture Measurement Workshop

November 2-4, 1983

James M. Ammons

University of South Florida

4202 Fowler Avenue

Tampa, FL 33620

(813) 974-3532

Naga K. Annamalai

RADC/RBRE

Griffiss Air Force Base

Rome, NY 13440

(315) 330-4055

Bill Bardens

Beckman Instruments, Inc.

2500 Harbor Boulevard

P.O. Box 3100

Fullerton, CA 92634

(714) 773-8666

Bruce Church

Medtronic Inc.

6970 Central Avenue N.E.

Findley, MN 55432

(612) 574-4486

George B. Cvijanovich

AMP Inc.

Myer-Lee Road

Winston-Salem, NC 27102

(919) 727-5298

J. Gordon Davy

Westinghouse

9200 Berger Road

Columbia, MD 21045

(301) 995-5825

Ray Denton

Naval Weapons Support Center

Code 6052

Crane, IN 47522

(812) 854-1338

Aaron DerMarderosian*

Raytheon Co.

528 Boston Post Road

Sudbury, MA 01776

(516) 443-9521 x2791

Diane Feliciano-Welp£

Oneida Research Services, Inc.

3 Ellinwood Court

New Hartford, NY 13413

(315) 736-3057

Victor Fong*

Panametrics, Inc.

221 Crescent Street

Waltham, MA 02254

(617) 899-2719

Paul R. Forant

Var ian/Lexington Vacuum Div.

121 Hartwell Avenue

Lexington, MA 02173

(617) 861-7200

Fred Gollob

Gollob Analytical Service

47 Industrial Road

Berkeley Heights, NJ 07922

(201) 464-3331

Ray F. Haack

Jet Propulsion Laboratory

4800 Oak Grove Drive, MS 122-123

Pasadena, CA 91109

(213) 354-6568

Harry Hammer

Singer Company

1150 McBride Avenue

Little Falls, NJ 07424

(201) 785-2982

* Session Chairman

314
Kristen Hanley

Harris Semiconductor

P.0. Box 883

Melbourne, FL 32901

(305) 729-5535

Tom Horn

a

IBM

Dept. T66, Bldg. 619-1

1701 North Street

Endicott, NY 13760

(607) 755-8639

John Hunter

Advanced Micro Devices

901 Thompson Pi.

Sunnyvale, CA 94088

(408) 749-3089

Harold Joss

Coors Porcelain Company

17750 W. 32nd Avenue

Golden, CO 80401

(303) 277-4057

Dr. John Kiely*

Intel Corp.

3065 Bowers Avenue

Santa Clara, CA 95051

(408) 496-9549

Robert K. Lowry*

Harris Semiconductor

Box 883, MS PT-40

Melbourne, FL 32901

(305) 724-7566

Arnie M. Massoletti

American Microsystems, Inc

3800 Homestead Road

Santa Clara, CA 95051

(408) 246-0330 x2758

Robert Merrett

British Telecoms Research

Martlesham Heath

Ipswich, IPS 7RE

United Kingdom

473642309

Benjamin A. Moore

RADC/RBRE

Griffiss Air Force Base

Rome, NY 13440

(315) 330-4055

John A. Mucha

Bell Laboratories

Room ID-249

600 Mountain Avenue

Murray Hill, NJ 07940

(201) 582-3659

Louis C. Phillips

Johns Hopkins University

1024 E. 38th Street

Baltimore, MD 21218

(301) 338-8679

Dr. L. J. Rigby

Standard Telecommunication

Laboratories Limited

London Road

Harlow, Essex, England CM17 9NA

0279-29531

Carl Roberts, Jr.

Analog Devices, Semiconductor

804 Woburn Street

Wilmington, MA 01887

(617) 935-5565

Jack A. Ronning, Jr.

Medtronic, Inc. Company

6951 Central Avenue, N.E.

Minneapolis, MN 55432

(612) 574-4761

Thomas J. Rossiter*

Oneida Research Services, Inc.

3 Ellinwood Court

New Hartford, NY 13413

(315) 736-3057

. Stanley Ruthberg*

National Bureau of Standards

A3 31/Bldg . 225

Washington, DC 20234

(301) 921-3625

Labs Stephen D. Senturia

Massachusetts Institute of Technology

Room 13-3010, 77 Mass. Avenue

Cambridge, MA 02139

(617) 253-6869

Robert W. Thomas*

Rome Air Development Center

Griffiss AFB , NY 13441

(315) 330-3730

315
Judith Weiner

Thomson-CSF

2 Gannett Drive

White Plains, NY 10604

(914) 694-4450

Phillip Vadala

IBM Corp,

1701 North Street

Endicott, NY 13760

(607) 755-8638

Luc Van Beek

Bell Telephone Manufacturing Co.

Gasmeter laan 106

900 Gent

Belgium

91/35 12 25

316
Moisture Measurement Workshop

November 2-4, 1983

ATTENDEES LIST

James M. Ammons

University of South Florida

4202 Fowler Avenue

Tampa, FL 33620

(813) 974-3532

David Angst

Bell Laboratories

555 Union Boulevard

Allentown, Pa 18103

(215) 439-5676

Naga K. Annamalai

RADC/RBRE

Griffiss Air Force Base

Rome, NY 13440

(315) 330-4055

Bernhard A. Bang

Westinghouse Electric Corp.

P.O. Box 1521

Baltimore, MD 21203

(301) 765-7340

Bill Bardens

Beckman Instruments, Inc.

2500 Harbor Boulevard

P.O. Box 3100

Fullerton, CA 92634

(714) 773-8666

Sandra Baumler

Westinghouse Electric Corp.

P.O. Box 746, MS V-14

Baltimore, MD 21203

(301) 765-4036

J. J. Bindell

Bell Laboratories

555 Union Boulevard

Allentown, PA 18103

(215) 439-6827

James C. Burrus

Texas Instruments Inc.

P.O. Box 6448, MS 3003

120 and FM Road, 1788

Midland, TX 79711

(915) 561-6944

Steve Carvellas

Gollob Analytical Service

47 Industrial Road

Berkeley Heights, NJ 07922

(201) 464-3331

E. L. Chavez

Sandia National Laboratories

P.O. Box 5800

Albuquerque, NM 87185

(505) 844-8593

Bruce Church

Medtronic Inc.

6970 Central Avenue N.E

.

Findley, MN 55432

(612) 574-4486

George B. Cvijanovich

AMP Inc.

Myer-Lee Road

Winston-Salem, NC 27102

(919) 727-5298

Demetrios Dalietos

Northrop Electronics Corp.

2301 West 120th Street

Hawthorne, CA 90250

(213) 418-4788

J. Gordon Davy

Westinghouse

9200 Berger Road

Columbia, MD 21045

(301) 995-5825

Gary Dency

Harris Corp.

Govt. Aerospace Systems Div.

P.O. Box 37, MS 16-308

Melbourne, FL 32901

(305) 727-4033

Ray Denton

Naval Weapons Support Center

Code 6052

Crane, IN 47522

(812) 854-1338

317
Aaron DerMarderosian

Raytheon Co.

528 Boston Post Road

Sudbury, MA 01776

(516) 443-9521 x2791

William D. Dorko

National Bureau of Standards

B-364/Bldg. 222

Washington, DC 20234

(301) 921-2886

Michael Ebert

Medtronic Inc.

6970 Central Avenue, N.E.

Fridley, MN 55432

(612) 574-6475

Leonid Finkelshteyn

Technology Glass (Div. of Demetron)

1643 27th Avenue

San Francisco, CA 94122

(415) 566-2928

Bill Fitch

Motorola

4629 E. Sunset Drive

Phoenix, AZ 85028

(602) 897-3331

Diane Feliciano-Welp£

Oneida Research Services, Inc.

3 Ellinwood Court

New Hartford, NY 13413

(315) 736-3057

Victor Fong

Panametrics, Inc.

221 Crescent Street

Waltham, MA 02254

(617) 899-2719

Paul R. Forant

Var ian/Lexington Vacuum Div.

121 Hartwell Avenue

Lexington, MA 02173

(617) 861-7200

Dr. Gary T. Forrest

Spectra Physics

25 Wiggins Avenue

Bedford, MA 01730

(617) 275-2650

Peter J. Frasso

Var ian/Lexington Vacuum Div.

121 Hartwell Avenue

Lexington, MA 02173

(617) 861-7200 x324

Fred Gollob

Gollob Analytical Service

47 Industrial Road

Berkeley Heights, NJ 07922

(201) 464-3331

Vince Green

Solid State Equipment Corp.

1015 Virginia Drive

Fort Washington, PA 19034

(215) 643-7900

T. F. Gukelberger

IBM Corp.

B/908 D/A29, Box 590

Poughkeepsie, NY 12602

(914) 432-7238

Ray F. Haack

Jet Propulsion Laboratory

4800 Oak Grove Drive, MS 122-123

Pasadena, CA 91109

(213) 354-6568

David C. Hackburth

Cardiac Pacemaker, Inc.

4100 N. Hamline

P.O. Box 43079

St. Paul, MN 55408

(612) 631-3000 x4376

Ronald Hall

Kyocera International Inc.

8611 Balboa Avenue

San Diego, CA 92123

(619) 576-2600

Orjan Hallberg

RIFA AB

S-16381

Stockholm, Sweden

Harry Hammer

Singer Company

1150 McBride Avenue

Little Falls, NJ 07424

(201) 785-2982

318
Kristen Hanley

Harris Semiconductor

P.0. Box 883

Melbourne, FL 32901

(305) 729-5535

Ronald Kersey

Fairchild Camera & Instrument Corp

333 Western Avenue

South Portland, ME 04106

(207) 775-8112

George G. Harman

National Bureau of Standards

B344/Bldg. 225

Washington, DC 20234

(301) 921-3621

Dr. John Kiely

Intel Corp.

3065 Bowers Avenue

Santa Clara, CA 95051

(408) 496-9549

Lewis G. Harriman, III

Cargocaire Engineering Corp.

79 Monroe Street

Amesbury, MA 01913

(617) 388-0600

Allen I. Kine

Northrop Electronic Division

2301 W. 120th Street

Hawthorne, CA 90250

(213) 418-5469

Sab Hasegawa

National Bureau of Standards

B250/Bldg. 221

Washington, DC 20234

(301) 921-3748

Terry Layton

Burroughs Corp.

P.O. Box 28810

San Diego, CA 92128

(619) 451-4379

Tom Homa

IBM

Dept. T66, Bldg. 619-1

1701 North Street

Endicott, NY 13760

Kemp Lehmann

G. M. Kemp Mfg. Co.

Glen Burnie, MD 21061

(301) 761-5100 x333

(607) 755-8639

Charles Hopkins

Cordis Corp.

10555 W. Flagler Street

Miami, FL 33174

Thomas E. Libbey

Fairchild

333 Western Avenue

South Portland, ME 04106

(207) 775-8012

(305) 551-2520

Nick Horvath

Cardiac Pacemakers, Inc.

4100 Hamline Avenue, North

St. Paul, MN 55164

Eric W. Lindsay

ITAS-West Coast Technical Service

17605 Fabrica Way

Cerritos, CA 90701

(213) 921-9831

(612) 631-4410

John Hunter

Advanced Micro Devices

901 Thompson Pi.

Sunnyvale, CA 94088

Mike C. Loo

Advanced Micro Devices

901 Thompson Place

Sunnyvale, CA 94088

(408) 749-3847

(408) 749-3089

Harold Joss

Coors Porcelain Company

17750 W. 32nd Avenue

Golden, CO 80401

(303) 277-4057

Robert K. Lowry

Harris Semiconductor

Box 883, MS PT-40

Melbourne, FL 32901

(305) 724-7566

319
Nigel MacDonald

Cordis Corporation

- Implantables Div.

P.0. Box 525700

Miami, FL 33152

(305) 551-2232

John A. Mucha

Bell Laboratories

Room ID- 249

600 Mountain Avenue

Murray Hill, NJ 07940

(201) 582-3659

Raymond M. Maffey

General Electric Co.

1 Neumann Way, M/S E7

P.0. Box 156301

Cincinnati, OH 45215

(513) 243-7291

Ed Murphy

Pall Pneumatic Products Corp.

4647 S.W. 39th Avenue

Ocala, FL 32674

(904) 237-1220

Margie Marlow

Naval Surface Weapons Center

White Oak

Silver Spring, MD 20910

(202) 394-1 295

Wolfgang H. Penzel

Honeywell, Inc.

MN1 1-1812

600 2nd St., N.E.

Hopkins, MN 55343

(61 2) 931-6983

Arnie M. Massoletti

American Microsystems, Inc.

3800 Homestead Road

Santa Clara, CA 95051

(408) 246-0330 x2758

Louis C. Phillips

Johns Hopkins University

1024 E. 38th Street

Baltimore, MD 21218

(301) 338-8679

Richard Merrell

ITAS-West Coast Technical Service

17605 Fabrica Way, Suite D

Cerritos, CA 90701

(213) 921-9831

Arthur L. Poole

Pall Pneumatic Products Corp.

4647 S.W. 39th Avenue

Ocala, FL 32674

(904) 237-1 220 xl 26

Robert Merrett

British Telecoms Research Labs

Martlesham Heath

Ipswich, IPS 7RE

United Kingdom

473642309

Frank C. Quinn

10911 Bornedale Drive

Adelphi, MD 20783

(301) 937-6435

Dr. L. J. Rigby

Standard Telecommunication

Dr. John Meyer

Raychem Corporation

66 Blauvelt Road

Monsey, NY 10952

(914) 425-9452

Laboratories Limited

London Road

Harlow, Essex, CM1 7 9NA

United Kingdom

0279-29531

Benjamin A. Moore

RADC/RBRE

Griffiss Air Force Base

Rome, NY 13440

(315) 330-4055

Carl Roberts, Jr.

Analog Devices, Semiconductor

804 Woburn Street

Wilmington, MA 01887

(617) 935-5565

Hayden Morris

U. S. Naval Surface Weapons Center

Code U-1 4, Rm. 2-249

Silver Spring, MD 20910

(202) 394-1295

James A. Rogowski

Analog Devices Semiconductor

804 Woburn Street

Wilmington, MA 01887

(61 7) 658-831 3 x2362

320
Jack A. Ronning, Jr.

Medtronic, Inc.

6951 Central Avenue, N.E.

Minneapolis, MN 55432

(612) 574-4761

Harry P. Ross

Western Electric

Dept. 1910

555 Union Boulevard

Allentown, PA 18052

(215) 770-3169

Thomas J. Rossiter

Oneida Research Services, Inc.

3 Ellinwood Court

New Hartford, NY 13413

(315) 736-3057

Stanley Ruthberg

National Bureau of Standards

A331/Bldg. 225

Washington, DC 20234

(301) 921-3625

Stephen D. Senturia

Massachusetts Institute of Technology

Room 13-3010, 77 Mass. Avenue

Cambridge, MA 02139

(617) 253-6869

Christopher T. Shillito

Defense Electronics Supply

Center - EQM

Wilmington Pike

Dayton, OH 45444

(513) 296-6259

Rod Steward

Honeywell Inc.

13350 U.S. Highway 19S

Clearwater, FL 33546

(813) 531-4611 x2676

Milton Stoll

Research Instrument Co., Inc.

36 Mascolo Road

So. Windsor, CT 06074

(203) 528-9625

Bradford L. Sun

Intel Corp.

2896 Kingsgate Ct.

San Jose, CA 95132

(408) 263-3076

Robert W. Thomas

Rome Air Development Center

Griffiss AFB, NY 13441

(315) 330-3730

Phillip Vadala

IBM Corp.

1701 North Street

Endicott, NY 13760

(607) 755-8638

Leopoldo Valero

Zilog Corporation, Dept. 537

1315 Dell Avenue

Campbell, CA 95008

(408) 370-8000 x4866

Luc Van Beek

Bell Telephone Manufacturing Co.

Gasmeterlaan 106

900 Gent

Belgium

91/35 12 25

Danny R. Walker

Defense Electronics Supply

Center - EQM

Wilmington Pike

Dayton, OH 45444

(513) 296-6259

Tom Walker

Naval Weapons Support Center

Code 6052

Crane, IN 47522

(812) 854-1338

Judith Weiner

Thomson-CSF

2 Gannett Drive

White Plains, NY 10604

(914) 694-4450

Stephen Weisskoff

Phys-Chemical Research Corp.

36 West 20th Street

New York, NY 10011

(212) 924-2070

Bob Woodwell

Honeywell Inc.

13350 U.S. Highway 19S

Clearwater, FL 33546

(813) 531-4611 x3 516

321

N BS-1 ) 4 A RE V. 2-8C! U.S. hEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 84-2852

2. Performing Organ. Report No. 3. Publication Date May 1984

4. TITLE AND SUBTITLE

RADC/NBS Workshop, Moisture Measurement and Control for Semiconductor Devices , III

5. AUTHOR(S)

B. A. Moore and S. Ruthberg, Editors

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8.

Type of Report & Period Covered

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

10.

SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

The workshop, one of a series concerned with measurement problems in integrated circuit

processing and assembly, served as a forum to examine the continuing progress that has

been made in the measurement and control of moisture in hermetically packaged semiconductor devices. Thirty-four presentations are included which contain detailed

information for securing hermetic packages with low moisture content. Agreement in

measurement has been obtained with the mass spectrometer for cerdip and metal packages

at the 5000 ppmv level of moisture through the use of suitable moisture generators, a 3-volume calibrator, calibrated dewpoint hygrometers, and appropriate operational

procedures. An approach is given for a reproducible and reliable transfer package.

However, the increased use of organic materials in new and rapidly expanding

technologies such as VLSI/VHSIC and hybrid packaging presents new and more complex

challenges to accurate measurement of interior moisture.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words ty Analysis of moisture content; hermetically packaged semiconductor devices; hybr .

• ; i - ages; mass spectrometer measurements; moisture; organic package materials; mo i

;

re- generators; moisture sensors; quality control; reliability of semiconductor devices;

-Semi condnr-t- nr ripvi rpg,

13. AVAILABILITY

_X_ Uni i mi ted For Official Distribution. Do Not Release to NTIS _ Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

IX] Order From National Technical Information Service (NTIS), Springfield, VA. 2216 i

14. NO. OF

PRINTED PAGES

328

15. Pr

> 26 . 50

* U.S. GOVERNMENT PRINTING OFFICE: 1984-420-997:149