NBS TECHNICAL NOTE 733

Methods of

Measurement for

Semiconductor

Materials, Process

Control, and Devices

Quarterly Report

January 1 to March 31, 1972
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and

technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measure- ment system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety.

The Bureau consists of the Institute for Basic Standards, the Institute for Materials

Research, the Institute for Applied Technology, the Center for Computer Sciences and

Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United States of a complete and consistent system of physical measurement; coordinates

that system with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics—Electricity—Heat—Mechanics—Optical, Physics—Linac

Radiation2—Nuclear Radiation2—Applied Radiation2—Quantum Electronics3—

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. THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research lead- ing to improved methods of measurement, standards, and data on the properties of

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1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

2 Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.
'»- — '- - kj Lf *ii**&* _ J

J 1 1 1972

tf^d^

"7 =5 Methods of Measurement for

Semiconductor Materials,

Process Control, and Devices

Quarterly Report, January 1 to March 31, 1972

W. Murray Bullis, Editor

Electronic Technology Division

Institute for Applied Technology

[/ \ . National Bureau of Standards

Washington, D.C. 20234

Jointly Supported by:

The National Bureau of Standards,

The Defense Nuclear Agency,

The U.S. Navy Strategic Systems Project Office, The U.S. Navy Electronic Systems Command,

The Air Force Weapons Laboratory,

The Air Force Cambridge Research Laboratories,

The Advanced Research Projects Agency,

The Atomic Energy Commission, and

The National Aeronautics and Space Administration.

*m

***a,u °* *

U.S. DEPARTMENT OF COMMERCE, Peter G. Peterson, Secretary

NATIONAL BUREAU OF STANDARDS, Lawrence M. Kushner, Acting Director,

Issued September 1972
National Bureau of Standards Technical Note 733

Nat. Bur. Stand. (U.S.), Tech. Note 733, 59 pages (Sept. 1972)

CODEN: NBTNAE

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.46:733). Price 60 cents.
TABLE OF CONTENTS

METHODS OF MEASUREMENT FOR SEMICONDUCTOR

MATERIALS, PROCESS CONTROL, AND DEVICES

PAGE

1. Introduction 1

2. Highlights 3

3. Semiconductor Materials

3.1. Resistivity 8 3.2. Gold-Doped Silicon 13

3.3. Infrared Methods 17 3.4. References 22

4. Semiconductor Process Control

4.1. Die Attachment Evaluation 24

4.2. Wire Bond Evaluation 27 4.3. References 33

5. Semiconductor Devices

5.1. Thermal Properties of Devices 34

5.2. Microwave Device Measurements 38

5.3. Carrier Transport in Junction Devices 41

5.4. References 46

Appendix A. Joint Program Staff 47

Appendix B. Committee Activities 48

Appendix C. Solid-State Technology & Fabrication Services 50

Appendix D. Joint Program Publications 51
LIST OF FIGURES

1. Instrument for measuring loading force on tips of four-probe array

with close up of the probe assembly and probe position locator

mechanism 10

2. Percent difference between actual capacitance and capacitance as measured with series resistors of 100, 300, and 1000 Q 12

3. Resistivity as a function of gold concentration in p-type silicon .... 14

4. Resistivity as a function of gold concentration in initially

n-type silicon ._ 14

5. Infrared response spectra obtained from five lithium-drifted

germanium diodes fabricated from crystals from five different

sources and four diodes fabricated from specimens of another

crystal treated in known manners . 18

6. Infrared response spectra of a commercial lithium-drifted

silicon detector obtained before and after irradiation with

1.6-MeV electrons 20

7. Infrared response spectra of a lithium-drifted silicon

detector obtained before, immediately after, and about

2 months after irradiation with 1.6-MeV electrons 20

8. The product of the peak chip thermal resistance between the center

of the top surface and the isothermal bottom surface and the thermal conductivity as a function of length of chip side for a square

chip with a centrally located square heat source for different chip

thicknesses and heating source sizes 25

9. The product of the temperature difference between a point along a perpendicular bisector of the edge on the top surface and the thermal conductivity as a function of distance from the center of the

chip for a chip 200 mils square and 5 mils thick, a power dissipation of 1 W, and different heating source sizes 26

10. Average pull strength of first bonds as a function of distance

between the first bond and the hook used to pull the bond loop 29
LIST OF FIGURES

PAGE

11. Wire-bond flexure, AH , as a function of power dissipation,

P , for triangular and circular arc bond loops with various

values of initial loop height, H 32

12. Comparison of emitter-base voltage for a measuring current of

6.5 mA as determined with emitter-only and collector-andemitter switching for a 35-W, triple diffused, silicon n-p-n

transistor for delay times of 10 and 100 us 35

13. Emitter-base voltage as a function of time after cessation of the

power pulse for a measuring current of 6.5 mA for a 35-W, triple

diffused, silicon n-p-n transistor for emitter-and-collector and

emitter-only switching 36

14. Emitter-base voltage as a function of applied power for a 35-W,

triple diffused, silicon n-p-n transistor operating in the currentconstricted mode for emitter-only and emitter-and-collector

switching 36

15. Mixer output circuit for incremental modulation measurements

of conversion loss and i-f conductance 40

16. Sandia bridge showing extraneous voltage generators 42

17. Non-reactive plug-in elements used to calculate magnitude

and phase of extraneous voltage generators 42

18. Delay time of an R-C plug-in network as measured as a function of frequency on the Sandia bridge constructed at NBS , the correction term appropriate to this bridge

for this network, and the corrected value of delay time 42

LIST OF TABLES

1. Preliminary results of multilaboratory test of four-probe

method for epitaxial resistivity measurement 9

2. Characteristics of infrared response spectrum types 18

3. Summary of crystal and diode characteristics of germanium

specimens 20
FOREWORD

The Joint Program on Methods of Measurement for Semiconductor Materials, Process

Control, and Devices was undertaken in 1968 to focus NBS efforts to enhance the performance, interchangeability , and reliability of discrete semiconductor devices and

integrated circuits through improvements in methods of measurement for use in specifying materials and devices and in control of device fabrication processes. These

improvements are intended to lead to a set of measurement methods which have been

carefully evaluated for technical adequacy, which are acceptable to both users and

suppliers , which can provide a common basis for the purchase specifications of government agencies, and which will lead to greater economy in government procurement.

In addition, such methods will provide a basis for controlled improvements in essential device characteristics, such as uniformity of response to radiation effects.

* The Program is supported by the National Bureau of Standards , the Defense

t § Nuclear Agency, the U.S. Navy Strategic Systems Project Office, the U.S. Navy Elec- +

' 11 tronics Systems Command, the Air Force Weapons Laboratory, the Air Force Cambridge

# x Research Laboratories, the Advanced Research Projects Agency, the Atomic Energy

** f f Commission, and the National Aeronautics and Space Administration. Although

there is not a one-to-one correspondence between the tasks described in this report

and the cost centers through which the Program is supported, the concern of certain

sponsors with specific parts of the program is reflected in planning and conduct of

the work.

Through Research and Technical Services Cost Centers 4251126, 4251127, 4252128, and

4254115.

Through Order EA072-810. (NBS Cost Center 4259522).

Administered by U.S. Naval Ammunition Depot, Crane, Indiana through Project Orders

PO-2-0023 and PO-2-0054. (NBS Cost Center 4259533).

Through Project Order PO-2-1034. (NBS Cost Center 4252534).

Through Delivery Order F29601-71-F-0002. (NBS Cost Center 4252535).

Through Project Order Y72-873. (NBS Cost Center 4251536).

X ARPA Order 1889 Monitored by Space and Missile Systems Organization under

MIPR FY76167100331. (NBS Cost Center 4254422).

Division of Biology and Medicine. (NBS Cost Center 4254425).

tt Through Order S-70003-G, Goddard Space Flight Center. (NBS Cost Center 4254429^-
METHODS OF MEASUREMENT

FOR SEMICONDUCTOR

MATERIALS, PROCESS CONTROL, AND DEVICES

QUARTERLY REPORT

JANUARY 1 TO MARCH 31, 1972

This quarterly progress report, fifteenth of a series, describes

NBS activities directed toward the development of methods of measurement for semiconductor materials, process control, and devices. Significant accomplishments during this reporting period include development of a procedure to correct for the substantial differences of

transistor delay time, a device characteristic frequently used as a screen in radiation hardness assurance tests, as measured with different instruments or with the same instrument at different frequencies;

association of infrared response spectra of poor quality germanium

gamma-ray detectors with spectra of detectors fabricated from portions

of a good crystal that had been degraded in known ways; and confirmation of the excellent quality and cosmetic appearance of ultrasonic

bonds made with aluminum ribbon wire. Work is continuing on measurement of resistivity of semiconductor crystals; study of gold-doped

silicon; development of the infrared response technique; evaluation of

wire bonds and die attachment; and measurement of thermal properties of

semiconductor devices , delay time and related carrier transport properties in junction devices, and noise properties of microwave diodes.

Supplementary data concerning staff, standards committee activities,

technical services, and publications are included as appendixes.

Key Words: Aluminum wire; base transit time; carrier lifetime; die

attachment; electrical properties; epitaxial silicon; gamma-ray detectors; germanium; gold-doped silicon; infrared response; methods of

measurement; microelectronics; microwave diodes; nuclear radiation detectors; probe techniques (a-c) ; resistivity; ribbon wire bonding;

semiconductor devices; semiconductor materials; semiconductor process

control; silicon; thermal resistance; thermographic measurements;

ultrasonic bonding; wire bonds.

1. INTRODUCTION

This is the fifteenth quarterly report to the sponsors of the Joint Program on Methods of Measurement for Semiconductor Materials, Process Control, and Devices.

It summarizes work on a wide variety of measurement methods that are being studied

at the National Bureau of Standards. Since the Program is a continuing one, the

results and conclusions reported here are subject to modification and refinement.
INTRODUCTION

The work of the Program is divided into a number of tasks, each directed toward

the study of a particular material or device property or measurement technique. This

report is subdivided according to these tasks. Highlights of activity during the

quarter are given in section 2. Section 3 deals with tasks on methods of measurement

for materials; section 4, with those on methods of measurement for process control;

and section 5, with those on methods of measurement for devices. References for each

section are listed in a separate subsection at the end of that section.

The report of each task includes the long-term objective, a narrative description of progress made during this reporting period, and a listing of plans for the

immediate future. Additional information concerning the material reported may be

obtained directly from individual staff members connected with the task as indicated

throughout the report. The organization of the Joint Program staff and telephone

numbers are listed in Appendix A.

An important part of the work that frequently goes beyond the task structure is

participation in the activities of various technical standardizing committees. The

list of personnel involved with this work given in Appendix B suggests the extent of

this participation. Additional details of current standardization activities not as- sociated with a particular task are given in section 2.

Background material on the Program and individual tasks may be found in earlier

reports in this series as listed in Appendix D. From time to time, publications are

Drepared that' describe some aspect of the program in greater detail. Current publi-

:ations of this type are also listed in Appendix D. Reprints or copies of such publications are usually available on request to the author.
2, HIGHLIGHTS

Significant accomplishments during this reporting period include development

of a procedure to correct for the substantial differences of transistor delay time,

a device characteristic frequently used as a screen in radiation hardness assurance

tests, as measured with different instruments or with the same instrument at different frequencies; association of infrared response spectra of poor quality germanium

gamma-ray detectors with spectra of detectors fabricated from portions of a good crystal that had been degraded in known ways; and confirmation of the excellent quality

and cosmetic appearance of ultrasonic bonds made with aluminum ribbon wire. Highlights of these and other technical activities are presented in this section; details

are given in subsequent sections of the report. This section concludes with a summary of standardization activities not associated with a specific task being carried

out by program staff members.

Resistivity — Principal effort was concentrated on study of the capacitancevoltage method; detailed investigations of the discrepancies between results obtained

with this and the four-probe method were initiated with experimental study of the

influence of series resistance on the capacitance measurement. An instrument was designed and constructed to measure the loading of the probes in a four-probe array.

Standardization efforts undertaken in cooperation with ASTM Committee F-l on Electronics included completion of preliminary analysis of the interlaboratory test of measurement of the resistivity of epitaxial layers by the four-probe method. Work continued on development of procedures for analysis of the results obtained to date in

the study of current and probe-force dependence of the four-probe method.

Gold-Doped Silicon — The model which includes a gold-related shallow acceptor

state was used successfully to predict the dependence of resistivity on gold concentration in p-type silicon of both higher and lower resistivity than had been studied

previously and, at high gold concentrations, in initially n-type silicon. In the

study of the properties of interstitial gold, electrical measurements on specimens

diffused with gold for very short times at high temperatures gave results similar to those on specimens with the same gold concentration obtained from longer diffusions

at lower temperatures. This result can be explained even though the electrical properties of substitutional and interstitial gold are dissimilar if it is assumed that

the rate for the conversion of interstitial to substitutional gold is rapid compared

with the diffusion time. Investigation of diode recovery methods for measuring carrier lifetime was resumed. Preliminary results suggest that the previously observed

differences in lifetime as determined by the reverse recovery and open circuit voltage decay methods may arise because of variations in the dependence of the storagetime-to-lifetime ratio on the forward-to-reverse-current ratio in the reverse recovery method.
HIGHLIGHTS

Infrared Methods — Study of infrared response (IRR) spectra obtained from lithiumdrifted germanium detectors showed that spectra observed to date can be grouped into

five types: one representative of good quality detectors and four representative of

poor quality detectors. Three of the latter have been matched by spectra from diodes

fabricated from a good quality crystal that had been degraded in a known manner. Thv

substantial progress has been made in identifying the causes of poor crystal quality

from IRR spectra.

Die Attachment Evaluation — Analysis of heat flow to determine the limitations

of the thermal response technique for detecting poor die adhesion in semiconductor

devices continued. Curves were generated from which a lower limit of the transient

thermal response normalized to heating power needed for maximum sensitivity to voids

in the die attachment of semiconductor devices can be determined. It was also es- tablished that the thermal response technique is relatively insensitive to voids that are located in areas laterally removed from the junction used to sense the chip tem- perature. Initial experiments to apply the transient thermal response technique to transistors with poor die adhesion indicated that the transistors could be operated

in a diode mode with the collector shorted to the base without exceeding the current

handling limitations of the base lead and metallization.

Wire Bond Evaluation — In the continuing study of ultrasonic bonding of aluminum

ribbon wire, pull strength measurements were made on magnesium-doped wire. The re- sults confirmed the previous work on silicon-doped wire in which the use of a bonding

schedule with longer time and lower power than normally used resulted in increased

bond strength and improved cosmetic appearance. Measurements of pull strength of

single-level round wire bonds as a function of pulling hook position were repeated on several different substrates. In all tests, the second bond was epoxied so that the

dominant failure mode was rupture at the heel of the first bond. There was considerable variability in pull strength exhibited by groups of bonds on different substrates, although the changes in pull strengths with hook position, normalized to that found at the center position of the pulling hook, were, in each case, in agreement with those calculated by resolution of forces- Calculations showed that the

temperature distribution in a transistor bond loop under conditions of slow thermal

cycling is linear rather than exponential as heretofore assumed. Recalculation of

flexure as a function of loop height yielded small but detectable changes from the

original calculations . A considerable effort during the quarter was directed toward

standardization activities and dissemination of information on wire bonding and bond

evaluation.

Thermal Properties of Devices — Work continued to determine the origins of the

frequently observed difference between the straight line extrapolation to zero power

4
HIGHLIGHTS

of the measured value of the temperature sensitive parameter as a function of power

and the d-c calibration value at the zero power level. Work on diodes and diodeconnected transistors indicated that the cause might lie partially in the charge distribution disturbance caused by switching off the collector voltage when measuring

thermal resistance. Measurements made by switching only the emitter suggested that

this method yields values of junction temperature that are more nearly the same as the values measured with an infrared microradiometer than are values measured by

switching both the emitter and the collector. Work also continued in the investigation of the physical mechanisms of thermal hysteresis, the temperature and current

density dependence of common- emitter current gain, hTM , and hot-spot initiation and

stabilization. It was found that in an area of sharp hot-spot formation the temperature sensitivity of h was greater at lower collector currents than at higher col- H

lector currents and that h,,^ was also larger at hot-spot formation at lower currents

than at hot-spot formation at higher currents. These observations are in accord with

the need for greater power to form current constrictions as collector current is increased.

Microwave Device Measurements — Several changes have been made in the X-band

mixer measurement system in order to improve the repeatability of the incremental

conversion loss measurement, and to allow the direct measurement, without calculation,

of the mixer i-f output conductance by a load perturbation method that also uses the

mixer output voltage change resulting from the incremental modulation. Several factors have been suspected of causing the lack of repeatability: mechanical changes in

the waveguide system due to forces used in changing diodes, drifts in microwave power

or other system characteristics due to temperature or line voltage changes or to aging

of components, diode-to-holder contact imperfection, and changes in the diodes themselves due to mechanical shock (moving the whisker) or to temperature changes. Each

factor is being examined independently.

Carrier Transport in Junction Devices — A general approach has been developed

for determining the location and magnitude of delay-time error-producing sources

arising from extraneous pickup at the measurement frequency, and this approach has

been tested by applying it to the NBS model of the Sandia bridge. With these sources

so characterized, their influence on subsequent transistor measurements can be re- duced greatly. The results of this work can explain previously observed differences

in measurements of delay time of transistors. Several preliminary steps have been

taken toward an interlaboratory comparison of transistor scattering-parameter measurements. Measurements were made as a function of frequency with various bias conditions for three transistor types, the effects of case temperature on the Sparameters were determined qualitatively, the equation widely used to relate h_ to

5
HIGHLIGHTS

S-parameters was verified, and the S-parameters of an R-C network on a transistor

header were measured to determine its suitability for use as a calibration standard.

Standardization Activities — Many of the standardization activities undertaken

by program staff are broader than the technical tasks described in the following sections. These activities involve general staff support in committees, liaison between

committees, coordination of efforts which may encompass a variety of tasks, and participation in areas where no direct in-house technical effort is underway. Standardization activities directly related to particular task areas are reported with the

appropriate tasks.

Nine program staff members attended the regular winter meeting of ASTM Committee

F-l on Electronics in New Orleans. Three staff members chaired section meetings; the

Interconnection Bonding Section continued to show vitality with 40 attendees representing 31 organizations (see section 4.2.). Documents on methods of measuring carrier lifetime by photoconductive decay and by surface photovoltage were presented to the Lifetime Section. The previously prepared draft procedure, for measuring thickness of thin oxide and metal layers (NBS Tech. Note 475, p. 24) was discussed by the

Dielectrics Section; with only minor revisions the document is being used as the basis

of a round robin test coordinated by a program staff member.

The modified collaborative reference program to provide silicon wafer resistivity

standards (NBS Tech. Notes 717, p. 7, and 727, p. 8) was described to the F-l Advisory

Committee. The manner and extent to which this program is carried out is contingent

on the response of the Committee. Other standards work on resistivity methods is re- ported in section 3.1.

A request from the Defense Nuclear Agency for assistance by Committ-ee F-l in the

development of standard methods of measurement in support of assurance of radiation

hardness of semiconductor devices was presented to the F-l Advisory Committee. The

Committee expressed its interest in this work and its intent to expand the activities

of Committee F-l to include it.

As a result of discussions and interest by the Hermeticity Section, the helium

mass spectrometer leak test procedure was further modified and extended. Procedures were developed for determining the limits of leak size for which the test is applicable; these are particularly significant for large volume packages, such as those of

hybrid microcircuits , which cannot be subjected to high pressures. In addition, a go, no-go test was included explicitly in the procedure.

After the F-l meeting, four documents were edited at the committee level.
HIGHLIGHTS

Most activity in connection with EIA-JEDEC Committees was in the thermal mea- surements area (see section 5.1.). In addition extensive comments were made on the

Proposed Standard for the Measurement of Small-Signal Transistor Scattering Parameters, being prepared by Task Group 5 of Committee JC-24 on Low Power Transistors.

A program staff member attended the Second Conference on the Definition of Pressing Problems and Projected National Needs in Radiometry and Photometry at NBS in

February. This group, now known as the Council for Optical Radiation Measurements

(CORM) , includes within its scope standards for emission from light emitting diodes.

This important area is of considerable relevance to semiconductor device users; close

liaison is being maintained with CORM.

A new group concerned with assembly and testing of components intended for non- military applications has been established in the IEEE Manufacturing Technology Group.

A program staff member attended a meeting of this group in March; continuing liaison

is expected to provide both identification of serious problem areas for program planning considerations and a forum for getting results of the work to the industry.
3, SEMICONDUCTOR MATERIALS

3.1. RESISTIVITY

Objective : To develop methods suitable for use throughout the electronics industry for measuring resistivity of bulk, epitaxial, and diffused silicon wafers.

Progress: Principal effort was concentrated on study of the capacitance-voltage

method; detailed investigations of the discrepancies between results obtained with

this and the four-probe method were initiated with experimental study of the influence of series resistance on the capacitance measurement. An instrument was designed

and constructed to measure the loading of the probes in a four-probe array. Standardization efforts undertaken in cooperation with ASTM Committee F-l on Electronics

included completion of preliminary analysis of the interlaboratory test of measurement of the resistivity of epitaxial layers by the four-probe method. Work continued

on development of procedures for analysis of the results obtained to date in the study

of current and probe- force dependence of the four-probe method.

Standardization Activities — Lower order polynomial curve fit approximations to the temperature coefficient for silicon resistivity (NBS Tech. Note 727, pp. 7-8) were presented for discussion at the January meeting of the Resistivity Section of ASTM

Committee F-l. The consensus favored the higher order curve fits previously reported

(NBS Tech. Note 560, pp. 6-7) which cover the entire range of resistivity. In re- sponse to a request by the section, work was initiated on the development of tables

derived from these polynomials which are suitable for linear interpolation.

(J. R. Ehrstein)

The round-robin experiment to determine interlaboratory precision of four-probe

measurements of the resistivity of silicon epitaxial wafers deposited on opposite

conductivity type substrates being conducted in cooperation with ASTM Committee F-l

has been completed by five of the nine participating laboratories. Measurements are being made on each of six specimens at a single specified current level and probe

loading and on three resistance boxes each containing a standard resistor and, in each

of the four measuring arms, a resistor of value 2000 times that of the standard. Preliminary analysis of the data summarized in table 1 suggests that the variability in

this test is substantially smaller than that observed in similar earlier experiments.

(F. H. Brewer)

Measurements were made as part of a round-robin experiment on capacitance-voltage

(C-V) measurements in cooperation with ASTM Committee F-l. Measurements of capaci- ' tance as a function of voltage and measurements of device area were made on two diodes

on each of five epitaxial specimens. In addition the capacitance of four reference

capacitors was measured. (R. L. Mattis)
RESISTIVITY

Table 1 - Preliminary Results of Multi laboratory Test

of Four-Probe Method for Epitaxial Resistivity Measurement"

Specimen

No. Type - b

P c

s

la p/n 1 3 . 33 n • cm 3.44 %

2 p/n 0.8879 3.03

3a p/n 0.4473 3.49

4 n/p 1.336 3.73

5 n/p 0.1494 0.93

6 n/p 7.761 1.53

Resistor

Box No. R d

s c

1 2.010 a 0.27 %

2 100.01 0.01

3 998.77 0.31

Five laboratories reporting

Multilaboratory resistivity grand average

c . Relative sample standard deviation

Multilaboratory resistance grand average

Probe Loading — Although it is necessary to control the probe loading when measuring the resistivity of silicon with a four-probe array, no standard procedures

exist at present to measure or adjust the loading force. In response to the need for

such procedures, an instrument has been designed and built to facilitate the measurement and adjustment o.f the tensioning springs which are used to load the probe tips.

The instrument, pictured in figure 1, was designed to minimize two significant sources

of error in such measurements: friction in parts linking the probe tips to the force

gage and inaccuracy in simulating the working position of the probe tips while measuring the spring tension. Recirculating bearing sleeves to reduce friction in the

push rod assembly and a set of electrical contacts to sense the position of the probe

points were employed.

To adjust the instrument, the position locator tips (A) are set so that they

just touch the tensioning springs (B) when the probes are depressed to their working

position. To measure the probe loading, the push rod (C) is lined up with a probe
RESISTIVITY

Overall front view. b. Close up

A Position Locator Tips

B Probe Tensioning Springs

C Push Rod

D Probe Tips

E Probe Assembly

F Operator Lever

G, Indicator Light

H Dynamometer Force Gage

I Position Locator Adjustment Screws

J Probe Tensioning Spring Adjustment Screws

K Vertical Position Adjustment

L Horizontal Position Adjustments

M Position Locator Tip Selector Switch

N Reference Position Indicator (for test purposes only)

Figure 1. Instrument for measuring loading force on tips of four-probe

array with close up of the probe assembly and probe position locator

mechanism.

tip (D) and the probe assembly (E) is advanced against it by means of the operating

lever (F) until the light (G) goes on, indicating that the probe is depressed to its

working position, and the probe loading is read from the force gage (H) . Tests are now being run to characterize the operation of the instrument.

(F. H. Brewer and G. P. Spurlock)

Capacitance-Voltage Methods — Efforts were directed toward investigation of the causes of observed differences between doping density values deduced from four-probe

resistivity measurements and doping density VS. depth profiles measured by the C-V

method. The failure of the C-V doping density profile to reach a plateau and the

10
RESISTIVITY

consistently lower doping density measured by the C-V method were discussed previously (NBS Tech. Note 727, pp. 9-10).

In the first phase of the study the specimen was considered to be composed of

a two-element network consisting of a contact resistance R in series with the depletion capacitance C. Such a network is equivalent to a capacitance C' in parallel with

a resistance R' . To measure R a previously profiled diode from each of the six slices

which had been profiled was measured as a function of bias voltage using a capacitance

bridge. The bridge measures C' and R' whereas the meter which is normally used measures only capacitance. The values of C' and R' measured by the bridge were transformed into the equivalent series capacitance and resistance, C and R. The capacitance values were altered only slightly in this transformation. The transformed

equivalent R values for the six diodes ranged from 48 to 778 fi and were constant as a function of bias voltage within about ±15 percent for five of the six specimens.

The fact that for a given diode the transformed R value is relatively constant with

bias voltage suggests that the two-element model which was selected represents a rea- sonably good approximation to the actual specimen behavior. That this series contact

resistance is the only significant resistive component that needs to be included in

the model is also borne out by the fact that parallel leakage resistance calculated

from the measurement of applied reverse bias and the reverse leakage current is typically tens or hundreds of megohms as compared with a capacitive reactance at 1 MHz as- sociated with the depletion capacitance for the six specimens of from 2.6 to 300 kfi.

In the second phase of the study the effect of the series resistance on measurements made with the capacitance meter which is normally used in the C-V work was considered. A low-leakage standard capacitor was measured over a range of values with

series resistances, R , of 10, 30, 100, 300, and 1000 tt. The capacitance measured

by the meter was significantly less than the standard capacitor value on many readings

taken with R equal to 300 and 1000 Q. These resistance values overlap the range of ser actual resistances R which have been encountered in the diodes. Results are shown in

figure 2 for R of 100, 300, and 1000 Q. For smaller R and for capacitance 300 pF

ser ser

or less the difference between the measured capacitance and the actual value was always

less than 2 percent. Since the doping density is approximately proportional to C , low

capacitance readings in the presence of series resistance lead to an erroneously low

doping density such as has been observed in comparison with four-probe data. With ref- erence to figure 2, the difference is observed to be a function of capacitance with a minimum around 10 to 30 pF. If diodes behave similarly to the series R-C circuit, such

an effect would lead, on uniformly doped specimens, to measured doping density profiles

which are either increasing or decreasing depending on whether the capacitance values

in the C-V data were greater than or less than the capacitance at which the minimum

11
RESISTIVITY

Figure 2. Percent difference between

actual capacitance and capacitance as measured with series resistors of 100

(A), 300 (O) , and 1000 () Q. Capacitance values encountered in test specimens studied to date for reverse

bias between 2 V and breakdown fall to the left of the vertical dashed line.)

10 30 100 300 CAPACITANCE (pF)1000

difference occurs. However, the measured profiles do not always conform to this predicted behavior.

Preparation of additional specimens for measurement by the four-probe and C-V

methods continued. The processing procedure was altered to include aluminum metallization of the finished diodes in order to reduce the resistance in series with the

junction depletion capacitance. , (R. L. Mattis and D. R. Ricks)

Plans : On the completion of the development of procedures for statistical analysis of data already acquired, experimental study of the current and probe-force dependence of the four-probe method will resume on thin silicon layers epitaxially deposited on opposite conductivity type substrates. Work on vapor-etched bulk silicon

wafers will be deferred until the, study of epitaxial layers is completed.

Reports on the tabulation of the temperature coefficient of silicon resistivity

and on the round-robin experiment to test four-probe resistivity measurements of epitaxial layers will be prepared and presented at the June meeting of ASTM Committee

F-l.

i Characterization of the instrument for measuring probe loading will be completed..

Further work will be undertaken to seek explanations for the discrepancies between four-probe and C-V measurements . Measurements will be made on the metallized

devices. Other processing alterations such as sintering of the metallization, sandblasting the back contact and wire bonding will be instituted as seems appropriate. j The effect of stray capacitances on the C-V measurement will be considered.

12
3.2. GOLD-DOPED SILICON

Objective : To characterize n- and p-type silicon doped with gold and to develop

a model for the energy-level structure of gold-doped silicon which is suitable for use in predicting its characteristics.

Progress : The model which includes a gold-related shallow acceptor state was

used successfully to predict the dependence of resistivity on gold concentration in

p-type silicon of both higher and lower resistivity than had been studied previously

and, at high gold concentrations, in initially n-type silicon. In the study of the

properties of interstitial gold, electrical measurements on specimens diffused with

gold for very short times at high temperatures gave results similar to those on specimens with the same gold concentration obtained from longer diffusions at lower

temperatures. This result can be explained even though the electrical properties of

substitutional and interstitial gold are dissimilar if it is assumed that the rate

for the conversion of interstitial to substitutional gold is rapid compared with the

diffusion time. Investigation of diode recovery methods for measuring carrier lifetime was resumed. Preliminary results suggest that the previously observed differences in lifetime as determined by the reverse recovery and open circuit voltage decay

methods may arise because of variations in the relationship between the lifetime and

the storage time in the reverse recovery method.

Resistivity Measurements — Resistivity and Hall effect measurements were made

at 25 ± 1°C on Hall bars cut from 0.53-, 1100- , and 2700-fi'cm p-type wafers diffused

with gold at temperatures of 850, 950, 1050, 1150, and 1250°C. The resistivity results are shown in figure 3 along with theoretical curves based on a solution to the

charge balance equation to find the Fermi level and, hence, the hole concentration.

In these calculations the energies of the gold donor and acceptor states were taken

as 0.35 eV and 0.58 eV, respectively, above the valence band [1]. A degeneracy factor of 0.125 was assumed for both the donor and the acceptor states of gold, the

energy gap was taken as 1.1057 to fit the intrinsic resistivity data, and the effective masses were taken from the work of Barber [2] . Also included was the additional

acceptor impurity state located 0.033 eV above the valence band proposed by Bruckner

[3] with a concentration which varies as the third power of the gold concentration.

Its concentration was taken as 4.5 x 10 15 cm

-3 at a gold concentration of 10 17 cm

-3

. Lattice mobility [4] and impurity mobility [5] were combined reciprocally to obtain

the hole mobility used in the calculation of the resistivity. The same model was also

used to calculate the resistivity as a function of gold concentration for initially

n-type silicon with gold concentration greater than the donor concentration. With

the exception of a few points, the calculated curves generally fit the previously

reported experimental data (NBS Tech. Note 717, pp. 10-12) as shown in figure 4.

13
GOLD-DOPED SILICON

10'

10'

"I I 1 I Mill

10"

Initial Resistivity Theory Exp. 0.53ft •cm o

10' 3 10,e 10

'' GOLD CONCENTRATION (atoms/cm 3

)

Q: 10*

10'

:

i i i i M 1

1

1 i --,— i

1 T 1

l 1 i III .

- A b :

- /-"V^"

-,x - \

r / / \

_

: ^

s \

U

i

*•-'^ / \ .

' y \

-

\

:

-

: Initial

-

Resistivity Theory Exp. -

- 1100 ft -cm 2700 ft -cm

I i . ,i , i ,

,

1 D

1 1 1 1 I ,,i 10

,:> 10,B 10'' GOLD CONCENTRATION (atoms/cm 3

) Figure 3. Resistivity as a function of

gold concentration in p-type silicon.

(The parameters used to generate the theoretical curves are given in the text. Error bars are not given for the experimental points. The experimental resistivity values are reproducible within about

5 percent and the standard deviation of

the gold determination is estimated to be

10 percent.)

i 1

1 1 i —i—

i

1 1 II 1

|

1 1 1 1 1 1 1 1

|

o "~-v/ " ^^' ^

/£lN

> /N i" XN jf X

,o' /

u / / v \ '> - AA \ :

: /

:/

To nn

/ \

/ \\ 1'-.' Y

\r

Resistivity

Initial Theory

Exp.

n p V\ " 5.3 a cm \ - 75 acm A* \ " 380 a- cm 2300 a- cm — \ r

V? 1 1 1 1 M il 1 I , K>" 10" 10,e 10 17

Figure 4. Resistivity as a function

of gold concentration in initially

n-type silicon. (Open symbols represent specimens which remained ntype after gold diffusion; solid

symbols , specimens which were con- verted to p-type by the addition of

gold. The theoretical curves were

generated using the model and parameters discussed in the text. Error

bars are not given for the experimental points. The standard deviation of the gold determination is

estimated to be about 10 percent.

Because of the large temperature dependence of the resistivity of nearintrinsic silicon, resistivity values

are estimated to be reproducible only

to within about 10 percent.)

GOLD CONCENTRATION (•toms/cm')

14
GOLD-DOPED SILICON

To examine the effect of interstitial gold on the electrical properties of silicon, diffusions of short duration were done at 950 and 1150°C in both n- and p-type

wafers to obtain gold concentrations much lower than the saturation value. Electrical

measurements were made on the diffused material and the results compared with data obtained on the same or similar starting material which was diffused to saturation for

long times at lower temperatures. The data points for the initially 2300-fi*cm, n-type

silicon at the two lowest gold concentrations in figure 4 were short-time diffusions

and are either in agreement with nearby points from long-time diffusions or are close

to the calculated curve. The resistivity data on the p-type silicon diffused for

short times were in agreement with those obtained on other wafers with the same gold

concentration but diffused for much longer times. Thus the resistivity depends only

on the gold concentration and not on whether saturation was achieved. Schuman [6]

studied the electrical characteristics of silicon diffused with gold for short times

and concluded that interstitial and substitutional gold affect the electrical properties of silicon in the same manner. This similarity need not be invoked if the rate

for the conversion of interstitial to substitutional gold is very rapid compared to the diffusion time so that the ratio of substitutional to interstitial gold is independent of diffusion time.

The study of the effect of heat treatment on the electrical properties of silicon indicated that resistivity and type changes do occur in high-resistivity crystals

subjected to heating at the same times and temperatures used for the gold diffusions.

Results on 1100-£2*cm, p-type wafers, which converted to high-resistivity n-type after

a treatment at 1250°C, suggest that about 10 13 compensating donors per cubic centimetre are introduced by the heat treatment. These results are consistent with the

data in figure 3 for 1100- and 2700-fi*cm specimens where the experimental points have

higher resistivity than the calculated curves

.

Carrier Lifetime Measurements — Measurements of carrier lifetime by the surface

photovoltage (SPV) method [7] continued on bars cut from a number of silicon wafers

diffused with gold. In several cases the specimens had been diffused with gold at 850°C for times of 2 to 33 hours. The SPV plots for these specimens did not show a significant linear region using either of two sets of absorption coefficient data.

One set, measured on silicon that was not stress relieved [8], has given the more

linear plots on wafers of the as-grown crystals. The other set, measured on stressrelieved silicon [9], has given linear plots on specimens that had been gold-diffused

to saturation. Apparently a gold diffusion of short duration at 850°C relieves only

a portion of the stress which is initially present in the slice while the heating

cycle associated with the usual gold diffusion is sufficient to relieve essentially

all of the stress in the surface. (W. R. Thurber , A. W. Stallings , and W. M. Bullis)

15
GOLD-DOPED SILICON

The investigation into the relationship between the lifetime of minority carriers

in semiconductor structures as measured by the reverse recovery (RR) and open circuit

voltage decay (OCVD) techniques was resumed. Apparatus that was used in previous ex- periments was reassembled and tested. The RR apparatus was similar to that described

by Owen and Wilkinson [10] ; the OCVD apparatus was similar to that described by Lederhandler and Giacoletto [11].

In addition, other, different RR and OCVD apparatus were built to investigate the

possibility that previously observed differences between these methods (NBS Tech. Note

560, p. 13) might have been associated with the equipment. The new OCVD equipment

utilized a reed relay to isolate the diode under test. The new RR apparatus utilized

batteries to forward bias the diode, and was designed so that the bias batteries were

isolated by a resistance of a few thousand ohms from the pulse generator which was

used to reverse bias the diode. Both new apparatus were isolated from ground so as

to eliminate the possibility of ground loops.

RR and OCVD transients of a germanium power diode were studied with the old ana new equipment. There were no significant differences in the transient waveforms observed on the old and new RR or OCVD equipment; these results indicated the transient

waveforms were related to properties of the diode and were not artifacts of a particular test circuit.

The dependence of the RR and OCVD transients on different values of currentlimiting resistance, forward current, and the ratio of the forward to reverse cur- rents was studied to determine those factors which could perturb, or appear to perturb, the lifetime, t, of the minority carriers in the diodes tested. The results

of RR experiments run on four commercial diodes showed that although the same OCVD

waveform was observed in all cases, there was no single relationship between the

storage time, t , and the ratio of the forward and reverse currents, I

f /I > which

could be applied to all diodes. For three of the four diodes tested the functional

relationship between the ratio of the storage time and lifetime and the ratio of the

forward and reverse currents was

t h = In [1 + (I f /I )] , s r r characteristic of the lumped constant model, but for the fourth diode the functional

relationship was

as derived by Lax and Neustadter [12] for an abrupt junction. Since the ratio of

the storage time and carrier lifetime for a particular ratio of forward and reverse

current is considerably different in the two cases, the origin of the differences

16
GOLD-DOPED SILICON

in functional relationship are being investigated as a possible cause of the discrepancy between the RR and OCVD methods. (D. C. Lewis)

Plans : Sets of p-type wafers with initial resistivities of 9.08, 1, 300, and

2400 fi'cm, which were diffused with gold this quarter, will be characterized at room temperature by resistivity and Hall effect measurements. Gold diffusions will be

started on wafers cut from several n-type silicon crystals. Hall effect and resistivity will be measured as a function of temperature on selected gold-doped silicon

specimens to gain information on the nature of the shallow, gold-coupled acceptor

states which appear to be influencing the resistivity at high gold concentrations.

The annealing experiments to study gold precipitation will be analyzed after

activation analysis results for gold concentration are obtained. Measurements will

be made on high-resistivity p-type silicon specimens that were heat treated in an oxygen or argon atmosphere at 950 and 1250°C in a new diffusion tube, prior to its use for gold diffusions, to provide information on the effects of heat treatment

without interference from gold contamination. Similar heat treatments will be done

on n-type specimens

.

Measurements of carrier lifetime by the surface photovoltage method will continue

with emphasis on p-type specimens with initial resistivities of 0.08 and 1 fl-cm. Other absorption coefficient data will be used in the analysis to attempt to obtain

straight line plots in specimens diffused for relatively short times.

Additional RR and OCVD measurements will be made in an effort to determine those

factors which can alter, or appear to alter, the lifetime of minority carriers in diode structures. Experiments will be undertaken to try to establish the origins of the

differences in the functional relationship between the ratio of the storage time and

lifetime and the ratio of the forward and reverse currents. The correlation between

the carrier lifetime as measured by RR and OCVD will be further investigated.

3.3. INFRARED METHODS

Objective : To study infrared methods for detecting and counting impurity and

defect centers in semiconductors and, in particular, to evaluate the suitability of

the infrared response technique for this purpose.

Progress : A study of infrared response (IRR) spectra obtained from lithiumdrifted germanium detectors showed that spectra observed to date can be grouped into

five types: one representative of good quality detectors and four representative of

poor quality detectors. Three of the latter have been matched by spectra from diodes

17
INFRARED METHODS

Figure 5. Infrared response spectra obtained

from five lithium-drifted germanium diodes

fabricated from crystals from five different

sources [(1) - (5)] and four diodes fabricated

from specimens of another crystal treated in

known manners [(A) - (D) ] . (Spectral features

of interest as indicated by short vertical

lines are identified in table 2. The diodes

are identified in table 3.)

0.60 0.65 0.70 0.60 ENERGY («V)

0.65 0.70

Table 2 — Characteristics of Infrared Response Spectrum Types

Spectrum Energy of

Minimum (eV

(1), (A) 0.66

(2), (B) 0.65

(3), (C) 0.68

(4), (D)

0.66

Energy of

Major Features (eV)

0.65, 0.64, 0.63',

0.61 , .0.59, 0.58

0.63, 0.61

0.66, 0.65, 0.64

0.67, 0.66, 0.65

0.65, 0.63, 0.61

Comments

Rises sharply into bandedge above

0.66 eV. Rises sharply into bandedge above

0.65 eV. Rises into bandedge above 0.68 eV; spectrum decreases below 0.66 eV. No prominent minimum; this type

shows the least number of spectrum

features (fairly featureless from

0.58 to 0.65 eV).

Small minimum; sharply rises into

bandedge above 0.66 eV.

as shown in figure 5

18
INFRARED METHODS

fabricated from a good quality crystal that had been degraded in a known manner. Thus

substantial progress has been made in identifying the causes of poor crystal quality

from IRR spectra.

The IRR of two lithium-drifted silicon diodes was measured; the silicon detectors were studied both before and after electron irradiation, and energy states present before and after irradiation were tentatively identified. Long term annealing effects were observed on one of the devices.

Infrared Response Measurements on Germanium — Measurements of infrared response

(IRR) continued on lithium-drifted germanium diodes fabricated from specimens of the

85 crystals collected for the germanium study (NBS Tech. Note 472, p. 16). Crystals

obtained from a number of different suppliers have been examined. These crystals

yield detectors of varying quality; the comparison of their IRR spectra with those of

specimens with known crystal parameters has begun.

Figure 5 shows a comparison of IRR spectra obtained from five Ge(Li) detectors

fabricated from crystals from five different suppliers [curves (1) through (5)] with

spectra obtained from four specimens of one crystal, each treated in a different man- ner [curves (A) through (D) ] . From previous studies and a review of pertinent literature, it has been concluded that, in general, the energy region covered in figure 5,

0.57 to 0.70 eV, is that in which crystalline imperfections are detected; most of the

features arising from impurities fall below 0.57 eV in the IRR measurements. The

spectra of these detectors exhibit distinct features that serve as the basis for dividing IRR spectra into five types. The characteristics of each type of IRR spectrum

are given in table 2.

Of particular interest are the four IRR spectra on the left side of figure 5; each is set opposite the spectrum type on the right for which similar features are

shown. These spectra are of diodes all fabricated from specimens of germanium crystal NBS 83. NBS 83-3 (Spectrum A) was a good quality detector, subjected only to the

usual lithium-fabrication procedures, which has been used for some time as the standare of comparison with various other diodes measured in the IRR studies. NBS 83-8

(Spectrum B) was irradiated with neutrons from a plutonium-beryllium source at a flu- ence of approximately 1.3 x 10 10 cm

-2 (NBS Tech. Note 727, pp. 17-19). NBS 83-6

(Spectrum C) was stored at room temperature after lithium diffusion for a considerable

period of time and is thus thought to contain lithium precipitates (NBS Tech. Note

717, pp. 13-14). NBS 83-4 (Spectrum D) was heated to 800° C, quenched to room temperature, and is thought to contain approximately lO 14 cm

-3 thermal defects [1] (NBS

Tech. Note 598, pp. 14-16). That the IRR spectrum of a germanium specimen of known

high quality can be modified by specific treatment of a crystal to yield spectra

19
INFRARED METHODS

Table 3 — Summary of Crystal and Diode Characteristics of Germanium Specimens

Spectrunf

(1)

(2)

(3)

(4)

(5)

Di ode

No.

NBS 91

NBS 13

NBS 302

Material

Prooerties

NBS 619 Oxygen cone.

< 10 14 cm" 3

NBS 76 High Li mobility

Low Li mobility,

oxygen cone.

:

8 x 10 14 cm" 3

High Li mobility,

high disloc.

density

Detector Corresponding Spectrum

Properties Reference

Di ode

Good diode, slight NBS 83-3: (A)

electron trapping lithium drift

only, good

detector

Good diode, severe NBS 83-8: (B)

hole trapping neutronirradiated

NBS 83-6:

lithium

precipitates (C)

Very unstable, NBS 83-4:

diode could not be thermal

maintained defects (D)

Poor diode, —

electron trapping

as shown in figure 5,

0.90 ENERGY (eV)

1

'

1 1 -s 1 a o

1

1

o yftjk o \/

o

CO o /

1 /

1 Jr

o

1 / tooL

' I \\ *» \\ ° V- \ \\ ° A \ \\l ! \\^vJ/c

^^~^\ / o

1

1

NBS1

7S

1 1 0.90 ENERGY (eV)

Figure 6. Infrared response spectra of a commercial lithium-drifted silicon detector

obtained before (A) and after (B) irradiation with 1.6-MeV electrons.

Figure 7. Infrared response spectra of a lithium-drifted silicon detector obtained

before (A) , immediately after (B) , and

about 2 months after (C) irradiation with

1.6-MeV electrons.

20
INFRARED METHODS

similar to that of lesser quality crystals, as shown in figure 5, indicates for the

first time that the energy region from about 0.57 to 0.70 eV can be used to yield

qualitative information on crystalline imperfections in germanium. Table 3 summarizes some of the measured material and detector characteristics of the diodes studied. While it is not possible at present to state with certainty that the known defects can be linked to the test crystals by the IRR spectra alone, it appears likely

that further study of these spectra will yield specific information on the causes of

poor quality in germanium intended for detector fabrication.

(A. H. Sher, W. J. Keery , and H. E. Dyson)

Infrared Response Measurements on Silicon — Measurements of IRR on electronirradiated, lithium-drifted silicon detectors continued. Figure 6 shows IRR spectra

obtained from a commercial detector, NBS-8S, fabricated from 1000-fi'cm, float-zoned,

boron-doped silicon. Before electron irradiation (curve A) , the spectrum exhibits

features at 0.77, 0.86, 0.93, 0.97, and 0.99 eV. The first three of these features

can be attributed to energy states of divacancies located 0.4 eV below the conduction

band, and 0.31 and 0.24 eV above the valence band, respectively [2-4]. The feature

at 0.99 eV in the spectrum corresponds to a state 0.18 eV below the conduction band

arising from the vacancy-oxygen complex [3]; the feature at 0.86 eV might also be evidence of a lithium-defect interaction [5], while the feature at 0.97 eV has, as yet,

not been identified. After room temperature irradiation with 1.6-MeV electrons at a fluence of about 2 x 10 11+ cm

-2 (curve B) , the features at 0.77 and 0.86 eV are no longer evident, and additional features are observed at 0.80 and 0-90 eV. An energy

state 0.38 eV below the conduction band (0.80 eV) has been attributed to the divacancy [4], while a state located 0.27 eV above the valence band (0.90 eV) is thought

to arise from lithium precipitates [6]

.

Figure 7 shows IRR spectra obtained from a lithium-drifted silicon detector before, shortly after, and about 2 months after 1.6-MeV electron irradiation. This detector, NBS-7S, was fabricated in this laboratory from a crystal with essentially

similar characteristics to that of NBS-8S. Before irradiation (curve A), the same

features are observed as for NBS-8S, except the feature at 0.97 eV appears to be absent. Shortly after irradiation (curve B) , the same features are exhibited in the

IRR spectrum of NBS-7S as in NBS-8S , except the two peaks at 0.97 and 0.99 eV are not

well resolved. Curve C shows the IRR spectrum obtained from NBS-7S after the detector

had been stored for about 2 months after the post irradiation measurement without bias

at room temperature. That the feature at 0.80 eV has disappeared and those at 0.77

and 0.86 eV are again observed as before irradiation may indicate that some annealing

effects took place during room temperature storage, presumably due to the presence of

mobile lithium ions. It is also interesting to note that the features at 0.97 and

21
INFRARED METHODS

ill resolved in curve C as in the spectra of NBS-8S. Features in the

spectrum at 1.04 and 1.06 eV are also observed; the upper feature may be related to an energy state about 0.1 eV below the conduction band which has been reported in

the literature [7] but not identified with a particular defect.

Since the commercial lithium-drifted silicon detectors used in this study had

gold p-contacts, approximately 15 to 20 nm thick, through which the infrared radiation

must pass, it was necessary to determine what effects, if any, this thin contact had

on the measurement of IRR. The infrared transmission of a 0.14-mm thick silicon filter, the thickness used in the measurement of the IRR of silicon detectors, was measured using a thermocouple detector both before and after evaporation of approximately

15 nm of gold onto one side of the filter. No effect was observed in the spectral

distribution of the radiation; however, the overall transmission was reduced by a factor of about three as compared to the uncoated filter.

(A. H. Sher, W. J. Keery, Y. M. Liu, and H. E. Dyson)

Plans : The studies of IRR on germanium and silicon diodes will continue with

emphasis this next quarter on germanium detectors fabricated from crystals on hand.

Efforts to extend the technique to transistors and diodes will continue.

3.4. REFERENCES

3.2. Gold- Doped Silicon

1. Collins, C. B., Carlson, R. 0., and Gallagher, C. J., Properties of Gold-Doped

Silicon, Phys. Rev. 105 , 1168-1173 (1957).

2. Barber, H. D., Effective Mass and Intrinsic Concentration in Silicon, Solid-State

Electronics 10_, 1039-1051 (1967).

3. Bruckner, B., Electrical Properties of Gold- Doped Silicon, Phys. Stat. Sol. (a)

±, 685-692 (1971).

a

4. Ludwig, G. W., and Watters , R. L. , Drift and Conductivity Mobility in Silicon,

Phys. Rev. 101, 1699-1701 (1956).

5. Brooks, H., Theory of the Electrical Properties of Germanium and Silicon, Advances m Electronics and Electron Physics, Vol. VII, L. Marton, ed. , pp. 156-160

(Academic Press, New York, 1955).

6. Schuman, V. B., Diffusion of Gold in Silicon, Sou. Phys. -Semiconductors 1, 790-791 (1967-68). " -

7, Thurber, W. R. , and Bullis , W. M. , Resistivity and Carrier Lifetime in Gold-Doped Silicon, Air Force Cambridge Research Laboratories Report AFCRL-72-0076 (January

„7' lg 72). Available from National Technical Information Service, Springfield, Virginia 22151, Accession Number AD 740592.

22
REFERENCES

8. Dash, W. C. , and Newman, R. , Intrinsic Optical Absorption in Single-Crystal

Germanium and Silicon at 77°K and 300°K, Phys. Rev. 99_, 1151-1155 (1955).

9. Runyan ,. W. R. , A Study of the Absorption Coefficient of Silicon in the Wave

Length Region between 0.5 and 1.1 Micron, Southern Methodist University Report

SMU 83-13 (1967). Available from National Technical Information Service, Springfield, Virginia 22151, Accession Number N68-16510.

10. Owen, D. B. B., and Wilkinson, E. L. G., Junction Recovery and Trapping in

Silicon p

+-n Junctions, Int. J. Electronics 20_, 21-29 (1966).

11. Lederhandler , S. R. , and Giacoletto, L. J., Measurement of Minority Carrier Lifetime and Surface Effects in Junction Devices, Proc. IRE _43, 477-483 (1955).

12. Lax, B., and Neustadter , S. F. , Transient Response of a p-n Junction, J. Applied

Phys. 25_, 1148-1154 (1954).

3.3. Infrared Methods

1. Logan, R. A., Thermally Induced Acceptors in Germanium, Phys. Rev. 101 , 1455-

1459 (1956).

2. Young, R. C. , and Corelli , J. C, Infrared Photoconductivity of ElectronIrradiated Phosphorous -Doped Silicon, Air Force Cambridge .Research Laboratories

Final Report (August 1, 1971). Available from National Technical Information

Service, Springfield, Virginia 22151, Accession Number AD 729477.

3. Lappo, M. T., and Tkachev, V. C, Divacancies in Silicon Irradiated with Fast

Neutrons, Sov. Phys .-Semiconductors 4_, 1882-1884 (1970-71).

4. Konozenko, I. D. , Semenyak , A. K., and Khivrich, V. I., Radiation Defects Created

by Co 60 Y-Rays in p- and n-Type Silicon of High Purity, Phys. Stat. Sol. 35 , 1043-1052 (1969).

5. Young, R. C.j Westhead, J. W., and Corelli, J. C, Interaction of Li and with

Radiation-Produced Defects in Si, J. Appl. Phys. 40_, 271-278 (1969).

6. Smirnova, I. V., Chapnin, V. A., and Vavilov. V. S., Radiation Defects in LithiumDoped Silicon, Sov. Phys .-Solid State _4, 2469-2474 (1962-63).

7. Vorobeichik, V. A., Konopleva, R. F. , Novikov, S. R. , and Rubinova, E. E., Photoelectric Properties of Si Irradiated with 660-MeV Protons, Sov. Phys. - Semiconductors 5, 394-401 (1971-72).

23
4. SEMICONDUCTOR PROCESS CONTROL

4.1. DIE ATTACHMENT EVALUATION

Objective : To evaluate methods for detecting poor die attachment in semiconductor devices with initial emphasis on the determination of the applicability of thermal

measurements to this problem.

Progress : The analysis of heat flow to determine the limitations of the thermal

response technique for detecting poor die adhesion in semiconductor devices was continued. Curves were generated from which a lower limit of the transient thermal re- sponse normalized to heating power, AT /P, needed for maximum sensitivity to voids

in the die attachment of semiconductor devices can be determined. It was also es- tablished that the thermal response technique is relatively insensitive to voids

that are located in areas laterally removed from the junction used to sense the chip

temperature. Initial experiments to apply the transient thermal response technique

to transistors with poor die adhesion indicated that the transistors could be operated in a diode mode with the collector shorted to the base without exceeding the current handling limitations of the base lead and metallization.

Analysis — Calculations of the steady-state temperature distribution on the top

surface of an idealized diode configuration were made to determine (1) the effect of

chip and heating source size on the peak temperature difference and (2) the dependence

of surface temperature on lateral distance from the source of heat. The threedimensional Laplace equation [1]

:

V

2 T = 0,

was solved for boundary conditions appropriate to a rectangular semiconductor chip

with a rectangular heat source centered on the top surface to represent the power dissipated in the active region. The bottom surface was assumed to be isothermal. All

other surfaces were assumed to be adiabatic.

The solutions were obtained in the form of an infinite series. The degree of ac- curacy depends on the number of terms used to approximate the infinite series. Typical results of the calculation of the peak temperature difference, at the center of

the slab, are given in figure 8. The results are presented as the product of peak chip

A thermal resistance and thermal conductivity for square chips as a function of chip

The so-called peak chip thermal resistance is the steady-state temperature difference between the temperature of the hottest spot, in this case the center of the top

surface, and the temperature of the isothermal bottom surface divided by the applied

power. Note that this quantity does not include thermal resistance associated either

with the die attachment material or with the case.

24
DIE ATTACHMENT EVALUATION

i i

1 ' ' ' ' '

' "1 1 1 1 1 1 1

I 1.

'E

I N. HEATING SOURCE SIZE = 5 mil s squire -

- ^**^^ - >

1->

1-

- . HEATING SOURCE SIZE lOmlls squirt "

i 10 o r

~

HEATING SOURCE SIZE ?0mils squirt - < -

5.

Uiz

-

o

< 1.0

i-

;

HEATING SOURCE SIZE 50mlls squire -

cn

inw

<Z

a:

uiX

ti i i

W\J I 1

| 1 I 1 1 1 1 1 1

|

1 I 1 1 I I l L ^ - _ E _ w _ _ >-

i->

- -

l-o

DQ

Zo

too T I

o - HEATING SOURCE SIZE = Smils squsrt - <

- - 2

orUJI

1- V HEATING SOURCE SIZE lOmlls squirt

X

" -

UJo

z

<

10 r

HEATING SOURCE SIZE 20mll» squirt

1- - ^•^.^ -

<7> UJ

-

_

a. - - _j<2

a.UJI

i1

HEATING SOURCE SIZE -50mila square

i i i i > i i LENGTH OF CHIP SIDE (mils) Chip thickness: 5 mils.

10 100 LENGTH OF CHIP SIDE (mils) Chip thickness: 10 mils,

Figure 8. The product of the peak chip thermal resistance between the center of

the top surface and the isothermal bottom surface and the thermal conductivity as

a function of length of chip side for a square chip with a centrally located square

heat source for different chip thicknesses and heating source sizes.

size for various sizes of square heat sources and for chip thicknesses of 5 and 10

mils. Since the thermal conductivity of silicon is about 1 W/cm ,0 C, the ordinate

can also be read as peak chip thermal resistance in degrees Celsius per watt. For

maximum sensitivity to voids in the die attachment, the heating power pulse width

for transient thermal response measurements should be long enough that the resulting value of AT /P is greater than the peak chip thermal resistance.

Figure 9 shows plots of the product of the temperature difference between

points along a perpendicular bisector of an edge on the top surface and the isothermal bottom surface of the chip and the thermal conductivity as a function of position

on a 200 mil square chip for various heat source sizes. A chip thickness of 5 mils,

power dissipation o'f 1 W, and a square, centrally located heat source were assumed.

The plots indicate that if a void is located only a small distance outside the heat

source area it would have little effect on a temperature indicator located at the

center of the chip upper surface. Alternatively, if a void caused the temperature

distribution indicated, a temperature sensing element located in an area other than near the void would be insensitive to its presence. This type of analysis indicates

the difficulties in trying to use the temperature sensitivity of individual elements

on integrated circuit chips to indicate the presence of voids in the die attachment

material. (R. L. Gladhill and F. F. Oettinger)

Lateral dimensions of semiconductor chips are customarily given in mils;

40 mils z 1 mm.

25
DIE ATTACHMENT EVALUATION

Figure 9. The product of the temperature

difference between a point along a perpendicular bisector of the edge on the top

surface and the thermal conductivity as a function of distance from the center of the

chip for a chip 200 mils square and 5 mils

thick, a power dissipation of 1 W, and different heating source sizes.

o '

HEATING SOURCE SIZE = 5mils squire J I I I I

1

HEATING SOURCE SIZE = 20mils squirt

I I I I I

HEATING SOURCE SIZE = 40mlls squ«r«

.1- _L _L

20 40 60 80 K)0 DISTANCE FROM CENTER OF CHIP (mils)

The analysis of heat flow to determine the limitations of thermal response techniques for detecting poor die adhesion in the diodes under investigation was continued. A mesa diode, similar to the ones used in the study of diode die attachment

evaluation (NBS Tech. Note 727, pp. 25-27), was modeled for calculations with the

TRUMP thermal analysis program [2] of the effects of die bond voids of various sizes

on the device junction temperature. Initial computations, limited to centrally located voids on an iron-nickel-cobalt alloy, glass-backed TO-5 header and on a solidsteel TO-5 header, indicated that further device modeling modifications are needed

if satisfactory results are to be achieved. (W. E. Phillips)

Transistors — Experiments to apply the transient thermal response technique to transistors with poor die adhesion were continued. Based on previous measurements of

thermal resistance of a 35-W, triple-diffused transistor connected as a diode (NBS

Tech. Note 727, pp. 49-50), it was decided that the diode operating modes suitable

for transistor die attachment evaluation might be obtained either by connecting the

emitter to the base and using the base-emitter junction. Initial thermal response

measurements made on two diode-connected 800-mW transistors indicated that when the

transistor was operated in the second of these modes sufficient power can be dissipated in the device without overloading the base lead and base metallization. In this

26
DIE ATTACHMENT EVALUATION

mode, less than 2 percent of the total current went through the base while 15 to 30

percent of the current went through the base when the emitter and base were connected.

Complete curves of thermal response normalized to heating power were then made

on 13 transistors with maximum power ratings ranging from 500 to 800 mW using both

the transistor and the emitter-base diode operating modes. In all cases, a sufficient temperature difference was attained in the diode operating mode without exceeding the maximum ratings of the transistors tested. In general, it was found that the

difference in the measured thermal response for the two techniques increased as the

heating power pulse width was decreased. This is to be expected since thermal re- sponse measurements with shorter heating power pulses are more sensitive to temperature and current distribution variations on the chip.

(F. F. Oettinger and R. L. Gladhill)

Plans: The analysis of heat flow to determine the limitations of thermal re- sponse techniques for detecting poor die adhesion in the diodes previously investigated will continue using the TRUMP thermal analysis program. The design and fabrication of a die attachment evaluation test circuit capable of measuring thermal response

of both diodes and power transistors (in both the diode and transistor operating modes)

will be undertaken. Further measurements of thermal response of diode-connected transistors will also be made in order to better understand the usefulness and limitations

of this mode of operation for transistor die attachment evaluation.

4.2. WIRE BOND EVALUATION

Objective : To survey and evaluate methods for characterizing wire bond systems

in semiconductor devices and, where necessary, to improve existing methods or develop

new methods in order to detect more reliably those bonds which will eventually fail.

Progress : In the continuing study of ultrasonic bonding of aluminum ribbon wire,

pull strength measurements were made on magnesium-doped wire. The results confirmed

the previous work on silicon-doped wire in which the use of a bonding schedule with

longer time and lower power than normally used resulted in increased bond strength

and improved cosmetic appearance. Measurements of pull strength of single-level round

wire bonds as a function of pulling hook position were repeated on several different

substrates. In all tests, the second bond was epoxied so that the dominant failure

mode was rupture at the heel of the first bond. There was considerable variability

in pull strength exhibited by groups of bonds on different substrates, although the

changes in pull strengths with hook position, normalized to that found at the center

position of the pulling hook, were, in each case, in agreement with those calculated
WIRE BOND EVALUATION

by resolution of forces. Calculations showed that the temperature distribution in a transistor bond loop under conditions of slow thermal cycling is linear rather than

exponential as heretofore assumed. Recalculation of flexure as a function of loop

height yielded small but detectable changes from the original calculations . A con- siderable effort during the quarter was directed toward standardization activities and

dissemination of information on wire bonding and bond evaluation.

Ribbon-Wive Bonding — Work on ultrasonic bonding of aluminum ribbon wire equivalent in cross-sectional area to 0.001-in. (25-um) diameter round wire has continued.

The pull strength and cosmetic appearance of bonds made with aluminum (1% magnesium)

wire with a tensile strength of about 14 gf (137 mN) and cross-sectional dimensions

of 0.0005 by 0.0015 in. (13 by 38 ym) were investigated using single-level bonding

substrates. The bonding force was held constant at about 25 gf (245 mN) throughout

the investigation, and a loop height of 0.012 in. (300 ym) was used as in previous

round wire studies. The same tool was used in all tests.

Measurements of pull strength of bonds made with various values of ultrasonic

power and time were in agreement with previous studies using silicon-doped wire (NBS

Tech. Note 727, pp. 32-35) that showed that the use of a low-power, long-time bonding

schedule resulted in higher pull strengths and better cosmetic appearance than a more

conventional high-power, short-time bonding schedule. The present data show that the

highest average pull strengths and smallest variations are obtained at tool tip vibration amplitudes of 27 to 32 yin. (0.68 to 0.81 ym) and times of 140 to 285 ms. The

groups of bonds made at low power and long time show mean pull strengths of from 11.0

to 12.2 gf (108 to 120 mN) with an average standard deviation of only ±4 percent

about the mean. (H. K. Kessler)

Pull Test Evaluation — The study of variables that might affect the sensitivity

of the pull test continued. Further analysis* of the data obtained previously (NBS

Tech. Note 727, pp. 38-42) was carried out, and the experiment to determine the effect

of the position of the pulling hook on single-level bonds with the second bond epoxied was repeated. Measurements were performed on bonds made on six different substrates. On one substrate, two independent sets of measurements were performed. As

in the past, the wire bonds tested were 0.001-in. (25-ym) diameter aluminum (1% silicon) wire ultrasonically bonded to aluminum pads. These single-level bonds had a bond-to-bond spacing of approximately 0.040 in. (1 mm) and a loop height of approximately 0.012 in. (0.3 mm). Three power settings were used to yield three groups of

bonds on each substrate. Bonds made with low power had relatively undeformed heels,

those made with intermediate power were deformed but did not exhibit excessive deformation, and those made with a higher power exhibited the greatest deformation.

Unless otherwise noted, the average pull strengths discussed are for breakage at the

heel of the first bond.

28
WIRE BOND EVALUATION

I

O 1-2 Z

w

a.

3

Q.Q

UJ 8 N

_l<

s

q:O 0.6 z

"l I n n 1 1 1 n n 1 1 1 r a b

• c

0.125 0.375 0.625 0.125 0.375 0.625 0.125 POSITION OF HOOK (mm)

0.625

Figure 10. Average pull

strength of first bonds as a function of distance between

the first bond and the hook

used to pull the bond loop

.

(The bond spacing was 1 mm;

the second bond was held with

an epoxy drop to eliminate

failure of the second bond.

Bonds were made with low [a]

,

intermediate [b] , and high [c]

power. The average pull

strength at the center of the

loop was 12.1, 7.9, and 7.2 gf

[118.6, 77.4, and 70.6 mN]

,

respectively. Values for the sets with the highest [•] and

lowest [A] pull strengths at each position are presented

to indicate the range of re- sults obtained. The solid

curves are calculated by res- olution of forces.)

Figure 10 shows pull strength data for these bonds normalized to the value obtained at the center of the loop as a function of pulling hook position. In the 1-mm

spacing between the first and second bonds, seven hook positions, 0.125 mm apart, were

used. The solid line in each case is the curve calculated by resolution of forces

(NBS Tech. Note 555, pp. 31-35) for the variation in pull strength as a function of

hook position for single-level bonds. For bonds made at all three power levels, the

agreement is generally good. However, as the hook is moved toward the first bond,

scatter in the data increases substantially.

When the average pull strengths obtained at each position of the pulling hook

for the same power group were compared for all seven sets examined (including the one reported last quarter), wide variations were observed. The average difference between

highest and lowest for the lower power group was 2.6 gf (25 mN) , for the intermediate

power group, 3.1 gf (30 mN) , and for the high power group, 4.7 gf (46 mN) . In general, the bonds made on a particular substrate that exhibited the lowest pull strengths

at all hook positions at lower power, also showed the lowest strengths at the other

two power settings. Similar results obtain for the group exhibiting the highest pull

strengths, etc. It is interesting to note, however, that for the two series of bonds

made on the same substrate, the average differences for low, intermediate, and high

power settings were 0.5, 1.6, and 0.9 gf (4.9, 15.7, and 8.8 mN) , respectively.

Although only bonds in which the break occurred at the heel of the first bond

were used in calculating the pull strengths reported, other modes of bond failure were

29
WIRE BOND EVALUATION

observed during the course of the experiment. For bonds made at the lowest ultrasonic

power setting, many first bond lift-offs were observed at the two hook positions nearest the first bond; at the two hook positions nearest the second bond virtually all

bond failures were due to breakage of the wire in the segment near the second bond.

For the two . higher power settings, no lift-offs were observed, and breakage occurred

at the heel of the first bond in nearly every case except for bonds pulled at the

three hook positions nearer the second bond, where breakage of the wire was observed

in the segment near the second bond. This is consistent with the resolution-of-forces

calculation of the ratio of forces in the segments of the wire from the position of

the hook to each bond as the hook is moved from the first bond to the second. With

the hook near the first bond, the force in the wire is greater for that segment of

the wire loop nearest the first bond, and breaks at the heel of the first bond should

be almost always observed. At the center position, equal forces are exerted on each

bond, but with the second bond held by epoxy, breakage at the first bond heel should

be observed. As the hook is placed nearer the second bond a decreasing number of

breaks should occur at the heel of the first bond. At a hook position of 0.75 mm, bond

failures for the two higher power groups were distributed mainly between first bond

heel breaks and wire breaks. At the position nearest the second bond, breakage in the

wire at the epoxied second bond accounted for a large proportion of the observed failures. The calculated increase in the force on the segment of wire near the second

bond is considerably greater as the hook position is changed from 0.75 to 0.875 mm

than it is as the hook position is changed from 0.5 to 0.75 mm.

(A. H. Sher, K. 0. Leedy, and C. A. Main)

Bond Failure from Slow Thermal Cycling — Calculations of wire bond flexure due

to thermal cycling to determine the dependence of flexure on bond loop height have

been reported previously (NBS Tech. Note 717, pp. 23-25). In these calculations it was assumed that the temperature in the wire decreased exponentially with distance

from the chip bond. To test this assumption, the aluminum bonding wire was modeled

to compute its temperature distribution by means of the TRUMP program [1] . The model

was idealized in that the wire and post were assumed collinear with a butt joint. At junction temperatures up to 180°C, corresponding to the rated power input of 500 mW,

it was found that the temperature distribution was linear within the wire and that

the temperature drop along the wire was two- thirds the junction-to-case temperature

difference whether the case was heat sinked or not. Practically all of the remaining

third of the temperature drop occurred in the post very close to the butt joint. The

physical interpretation of the linear temperature distribution is that heat transfer

occurs predominantly by conduction. Radiation is less than a thousandth of the conduction; heat flow by convection is much less than that by radiation. Since the

30
WIRE BOND EVALUATION

temperature distribution is linear rather than exponential, four of the previous

equations must be changed.

The equation for the wire length, S, in the circular arc model becomes

S = S

XCUgLil, O, -LL1 LUC L. IfLUldl ClL I

[

1 + aAl(^3^- 23 )] •

where S is the length of the wire at 23°C, a is the thermal expansion coefficient

for aluminum, T is the junction temperature, and T is the case temperature. The

equations for the length of the die half, a, and the post half, b, of the wire in

the triangular model become,

and

s a = -^~

So

b =

2

The modified equation for post height, Y

Y = Y

where Y is the post height at 23°C and a is the thermal expansion coefficient of the

post alloy, gives a post expansion less than 0.01 percent larger than that obtained

previously with the assumption that the post is at the case temperature.

Revised curves of flexure (change in loop height, AH ) plotted against power

Li input to the junction, P , (or, equivalently , junction temperature, T ) are shown in

figure 11 for initial loop heights, H , of 50, 100, 250, and 500 urn for the circular

arc model (dashed lines) and the triangular model (solid lines) . The plots resemble

.those of the previous graph, being nearly linear, and have the same values at low

power, but indicate a greater flexure at rated power. The curve for the very high

loop height of 500 pm which is also added here shows, as expected, only a small de- crease in flexure from that associated with a loop height of 250 um. (W. E. Phillips)

Standardization and Dissemination — A third successful meeting of the Interconnection Bonding Section of ASTM Committee F-l was held in connection with the January

meeting of the Committee in New Orleans. In addition to developing standard nomenclature and measurement methods, this section is providing a forum for the exchange

of ideas and new testing technology. Three invited papers on various aspects of

bonding technology were presented. (K. 0. Leedy and G. G. Harman)

The use of phased-burst bonding schedules (NBS Tech. Note 727, pp. 35-38) was discussed at the Drexyl University Microcircuit Interconnection Workshop in February.

31
WIRE BOND EVALUATION

Figure 11. Wire-bond flexure, AH

L> as a function of power dissipation, Pp, for

triangular (solid lines) and circular

arc (dashed lines) bond loops with various values of initial loop height, H^

Q

. (The analysis was made for 1-mil diameter post-to-die aluminum wire bonds in

a 500-mW, 50-mA silicon transistor in a TO-18 can with post and header of the

usual iron-nickel-cobalt alloy. The

junction temperature, T , ±a also indicated. )

20 40

T, (°C) 55 85 180

Hlo=500, 500 1000

An extensive tour of west coast facilities was undertaken in March. Talks on the bond evaluation work carried out at NBS over the past few years were given before

local sections of the International Society of Hybrid Microelectronics in San Diego,

Los Angeles, San Jose, and Mountain View, California. The results of this work were

also discussed extensively at seven semiconductor manufacturing facilities covering

discrete device, monolithic integtated circuits, and both thick film and thin film

hybrid circuits. (G. G. Harman)

Bibliography and Critical Review — The critical survey paper is in the final

stages of editorial review. The preparation of an annotated bibliography of limited

distribution reports was temporarily deferred. (H. A. Schafft)

Plans: Evaluation of ribbon wire for ultrasonic bonding will be completed with

a comparison of bond strengths obtained using round and ribbon wires made at the same

range of time and power settings. Improvements will be made to equipment used in the

work on electronic mixing of bonding tool ultrasonic signals with local oscillators

for better understanding and control of bonding; experimental study, deferred this

quarter, will resume on completion of these improvements. Experimental and statistical analysis of significant factors affecting the wire bond pull test on single-level

substrates will continue. Procedures for the publication of the critical survey paper

32
WIRE BOND EVALUATION

will be completed. The preparation of an annotated bibliography of limited distribution reports on wire bonds will be resumed. Standardization and dissemination efforts will continue as appropriate.

4.3. REFERENCES

4.1. Die Attachment Evaluation

1. Lindsted, R. D. , and Surty, R. J., Steady-State Junction Temperatures of Semiconductor Chips, IEEE Trans. Electron Devices ED- 19 , 41-44 (1972).

2. Edwards, A. L. , TRUMP: A Computer Program for Transient and Steady-State Temperature Distributions in Multidimensional Systems, Lawrence Radiation Laboratory, Univ. of California, Livermore , California, UCRL-14754, Rev. I, 1 May

196 8. Available from the National Technical Information Service, Springfield,

Virginia 22151.

4.2. Wire Bond Evaluation

1. Edwards, A. L. , TRUMP: A Computer Program for Transient and Steady-State Temperature Distributions in Multidimensional Systems, Lawrence Radiation Laboratory, Univ. of California, Livermore, California, UCRL-14754, Rev. I, 1 May

1968. Available from the National Technical Information Service, Springfield,

Virginia 22151.

33
5, SEMICONDUCTOR DEVICES

5.1. THERMAL PROPERTIES OF DEVICES

Objective : To evaluate and improve electrical measurement techniques for determining the thermal characteristics of semiconductor devices.

Progress : Work continued to determine the origins of the frequently observed

difference between the straight line extrapolation to zero power of the measured value

of the temperature sensitive parameter (TSP) as a function of power and the d-c calibration value at the zero power level. Work on diodes and diode-connected transistors

indicated that the cause might lie partially in the charge distribution disturbance

caused by switching off the collector voltage when measuring thermal resistance. Measurements made by switching only the emitter suggested that this method yields values

of junction temperature that are more nearly the same as the values measured with an infrared microradiometer than are values measured by switching both the emitter and

the collector. Work also continued in the investigation of the physical mechanisms

of thermal hysteresis, the temperature and current density dependence of common- emitter current gain, h , and hot-spot initiation and stabilization. It was found

r Ji that in an area of sharp hot-spot formation the temperature sensitivity of h„„ was greater at lower collector currents, 0.09 A, than at higher collector currents,

0.11 A, and that at hot-spot formation h„„ was also larger at lower currents than at higher currents . This observation is in accord with the need for greater power to form current constrictions as collector current is increased.

Standardization Activities — The devices for the preliminary round robin on thermal resistance being conducted in cooperation with EIA-JEDEC Committee JC-25 on Power Transistors have been measured by the third and fourth participants. Work was completed on revising portions of a document on power transistors dealing with thermal characteristics and their measurement that was recently letter balloted in Committee JC-25. The revised document has been submitted to the committee for review

and eventual re-balloting.

A test fixture and heat sink to be used for thermal resistance measurements of

axial lead signal diodes was designed and fabricated. The measurements are being

undertaken as part of a preliminary thermal resistance round-robin experiment being

conducted- by EIA-JEDEC Committee JC-20 on Signal Diodes.

(F. F. Oettinger and S. Rubin)

Thermal Resistance Methods — Investigation continued into the cause of the frequently observed difference between the straight line extrapolation to zero power of

the measured value of the TSP as a function of power and the d-c calibration value at the zero power level (NBS Tech. Note 598, pp. 30-33). Initial measurements on diodes

34
THERMAL PROPERTIES OF DEVICES

630 r- 1 1 r

dc i

610

590 -

dc

570

550

530

EMITTER AND COLLECTOR SWITCHED

100/iS

EMITTER ONLY SWITCHED

4 8 12 16 20 COLLECTOR -EMITTER VOLTAGE VCE (V)

Figure 12. Comparison of emitter-base

voltage for a measuring current of 6.5 mA

as determined with emitter-only and

collector-and-emitter switching for a 35-W,

triple diffused, silicon n-p-n transistor

for delay times of 10 and 100 us. (During

the power pulse the collector current was

500 mA. Case temperature was held at 25°C.

The d-c calibration values for collector

connected and disconnected, respectively,

are also shown for comparison.)

and diode-connected transistors (NBS Tech. Note 727, pp. 47-50) suggested that electrical switching transients are responsible for this difference which occurs for mea- surements made with delay times less than 50 us. Additional measurements on diodes

and diode-connected transistors further supported this suggestion. As a consequence

the grounded-base measuring circuit was modified by shorting out the collector switching transistor so that only the emitter was switched during test. Measurements made

using this method resulted in differences about half the magnitude of those using

the former collector-and-emitter switching method for a delay time of 10 us. As an example, the results of measurements made on a 35-W, triple-diffused, silicon n-p-n

transistor are shown in figure 12. Oscilloscope traces of the emitter-base waveform

as a function of elapsed time after the cessation of the power pulse, shown in figure

13, show that the magnitude of the electrical transients of the emitter-base voltagewaveform at a delay time of 10 us is smaller for emitter-only switching than for

emitter-and-collector switching.

In the course of measuring thermal resistance with emitter-only switching, measurements were made on the triple-diffused transistor discussed above in the thermal

hysteresis region. As shown in figure 14, the indicated temperature rise at hot-spot

initiation (V = 180 V) for emitter-and-collector switching is 37 °C while the indicated rise for emitter-only switching is 104°C. Measurements made previously using

an infrared microradiometer showed that at hot-spot initiation the temperature of the

hot-spot region rises approximately 180°C. The emitter-only switching method thus

appears to afford some improvement over the emitter-and-collector switching method

for measuring the temperature of a hot spot in a transistor operating in a currentconstricted mode. (S. Rubin and F. F. Oettinger)

35
THERMAL PROPERTIES OF DEVICES

Figure 13. Emitter-base voltage as a function of time after cessation of the

power pulse for a measuring current of

6.5 mA for a 35-W, triple diffused,

silicon n-p-n transistor for emitterand-collector (a) and emitter-only (b)

switching. (During the power pulse

the collector current was 500 mA and

the collector-emitter voltage was 20 V. Case temperature was held at 25°C. The

horizontal scale is 10 microseconds per

division, and the vertical scale is 5 millivolts per division.)

Figure 14. Emitter-base voltage as a function of applied power for a 35-W, triple

diffused, silicon n-p-n transistor operating in the current-constricted mode for

emitter-only (solid line) and emitter-andcollector (dashed line) switching. (During

the power pulse the collector current was

100 mA. Measurements were made with a measuring current of 6.5 mA, 10 us after ces- sation of the power pulse. The sharp voltage drop at V s 180 V is indicative of

the initiation of a hot spot; the magnitude

of the drop is directly related to the change

in effective junction temperature.)

600

1 1 1 1 1 1

> 550

E ~~~~- —

> 'V"*"-*- Ul 500 Vs^ ^S —

f.

S *x

v \ o

>

<m£400 _

E

350 -

300

9sn 1 1 1 1 1 1

-

60 100 120 140 160 COLLECTOR -EMITTER VOLTAGE VCE (V)

36
THERMAL PROPERTIES OF DEVICES

Screen for Hot Spots — Investigation of the physical mechanisms of the thermal

hysteresis effect (NBS Tech. Note 520, pp. 49-52) has been intensified. A better

understanding of these mechanisms is expected to aid in the improvement of the previously developed screen for hot spots [1] . Experiments were initiated to determine

what roles the temperature and current density dependence of h play in the initiation and stabilization of hot-spots and in the thermal hysteresis cycle.

Two possible models have been proposed previously for the observed increase of

Yi with decreasing collector-emitter voltage, V , while a device is operating with

FE Lb

a hot spot or current constriction (NBS Tech. Note 520, pp. 51-52). Briefly, the

device could be operating in a region where the gain increases with decreasing temperature (NBS Tech. Note 727, p. 50) or the current density in the constricted region is

large enough that the device is operating in a region where h increases with decreasing current density. The latter model requires that as V is decreased, the

current density in the constricted region is decreased also.

An experiment was performed which demonstrated for a triple-diffused, 35-W, silicon n-p-n transistor that the temperature dependence of h could not account for

the observed variation of h with V . While the device was operated in the hot-spot

or current-constricted mode the hot-spot temperature was maintained constant by gradually increasing the case temperature as V was decreased. For constant hot-spot

temperature, h.^ increased at a faster rate with decreasing V than when the hot spot

FE Lb

temperature was allowed to decrease naturally as V decreased. This indicates that

L£j in the hot-spot mode, hTM is still increasing with temperature for this device.

r hi The role of the temperature variation of h„„ in the initiation of a hot spot was

r £. also studied. Temperature measurements were made with an infrared microradiometer and

thermographic phosphors at a point on a device where a severe hot spot was known to form at values of V just a few tenths of a volt below the value at which the hot

spot forms . These measurements were made for a narrow range of collector currents

(0.09, 0.10, and 0.11 A) to minimize the effect on hTM of varying current density and

current distribution within the device. It was found that the temperature immediately

before the hot spot formed was lower for the smaller collector currents. Both the rate

of increase of h with increasing temperature and the magnitude of h„„ were larger for rh r E the smaller collector currents. This decreasing temperature dependence of h with

r Ei increasing collector current may account for the generally higher power needed to form

hot spots at higher collector current levels. (D. L. Blackburn)

Plans : Work on standardization activities related to thermal measurements will

continue. The literature search and work on the bibliography on thermal measurements

of semiconductor devices will continue. Studies will continue to determine the

37
THERMAL PROPERTIES OF DEVICES

validity of the emitter-only switching technique as an improved method for the measurement of transistor thermal resistance for cases of moderate as well as severe

current crowding. Experiments will be performed to explore the role of the variation

of h with current density, to measure the temperature gradient present immediately

FE before the hot spot forms , and to relate this gradient to the magnitude and temperature variation of h immediately before the hot spot forms.

rE

5.2. MICROWAVE DEVICE MEASUREMENTS

Objective : To study the problems and uncertainties associated with the measurement of electrical properties of microwave diodes , and to improve the techniques of

these measurements.

Progress : Several changes have been made in the X-band mixer measurement system

in order to improve the repeatability of the incremental conversion loss measurement

on point-contact mixer diodes and to allow the direct measurement, without calculation, of the mixer i-f output conductance by a load perturbation method that also

uses the mixer output voltage change resulting from the incremental modulation.

The general requirements for conversion loss measurement repeatability may be seen from the following. Mixer diodes are commonly marketed with standard overall

average noise figure limits denoted by suffix letters in 0.5-dB increments; 0.5 dB

is considered to be a significant difference as far as system performance is concerned

and can drastically affect diode cost. Standard overall average noise figure, ex- pressed in decibels, is equal to the conversion loss plus a term which combines mixer

noise and i-f amplifier noise, so that 0.5-dB differences in conversion loss appear

as 0.5-dB differences in overall noise figure.

The classification of diodes by these 0.5*-dB increments requires a total measurement uncertainty considerably less than this. A total uncertainty of ±0.1 dB

with at least 90 percent confidence would seem to be about the largest that could

reasonably be considered for such classification. Since these limits could allow a fairly high percentage of the diodes to be misclassified, tighter limits would cer- tainly be desirable, although they would be very difficult to meet. Preliminary

measurements have indicated a repeatability for the NBS X-band system somewhat poorer

than this for weekly measurement intervals, even excluding obviously unstable diodes.

The systematic error uncertainty, which must be added to this, has not yet been es- tablished.

Several factors have been suspected of causing the lack of repeatability: mechanical changes in the waveguide system due to forces used in changing diodes, drifts

38
MICROWAVE DEVICE MEASUREMENTS

in microwave power or other system characteristics due to temperature or line voltage

changes or to aging of components, diode-to-holder contact imperfection, and changes

in the diodes themselves due to mechanical shock (moving the whisker) or to temperature changes. Except for contact imperfection, which may be impossible to separate

from diode instability, each of these factors has been observed as significantly affecting some measurements, but because of their largely unpredictable nature it is not known how much each contributes to the observed lack of repeatability of the con- version loss measurements as a whole. Each factor is being studied to reduce to the

extent possible its influence on the measurement.

Attention has been given to mechanical stability by rigid clamping of the waveguide and, during this reporting period, by transferring the waveguide system to a rigid bench formed from welded L-beams and having an inch-thick aluminum top. Shockmounted castors allow the bench to be easily moved for rear access with a minimum of

distortion of the bench framework due to floor unevenness.

To increase the stability of the local oscillator power, the operational amplifier formerly used in the leveling loop was replaced by one with the lowest temperature

coefficient of offset voltage (0.25 uV/°C) currently available for a chopperless type.

In addition the operational amplifier power supply was replaced by a more stable type.

The other major source of power drift in the leveling loop is the diode detector; an attempt to replace it with a dry calorimeter (thin-film thermocouple) was made, but was unsuccessful because of the exceedingly low voltage developed by the calorimeter.

The dry calorimeter is, instead, now being tried as a power monitor to which the r-f

power may be directed by a highly repeatable waveguide switch. The power may thereby

be checked during a measurement run without removing the mixer.

The mixer output circuit shown in figure 15 has been constructed for use with

incremental modulation measurements to permit the direct measurement of i-f conductance and matched-load output voltage. For the i-f conductance measurement, two types

of load perturbation methods have been incorporated. One is Mance's method (NBS Tech.

Note 592, pp. 57-59), in which a calibrated variable resistor is switched in series

with the generator, whose internal resistance is to be determined, and its load. The

other is an apparently new method in which the calibrated variable resistor is switched

in shunt with the generator and its load. This new circuit was originally developed

in order that, for a small load perturbation, the calibrated variable resistance,

which at balance is proportional to the generator resistance, would be larger than

the latter rather than smaller, as with Mance's method. For a 10:1 arm ratio, this

circuit reduces the effect of switch contact resistance in the calibrated variable

resistor arm by a factor of 100 as compared with Mance's circuit. For a unity arm

ratio i which was needed to increase the sensitivity sufficiently to overcome the

39
MICROWAVE DEVICE MEASUREMENTS

Figure 15. Mixer output circuit for incremental modulation measurements of conversion loss and i-f conductance.

observed voltage instability, the only advantage is that the load is perturbed by a smaller factor (1.5) than in Mance's circuit (2), but at the expense of a larger re- sistance uncertainty. There is thus a trade off between these circuits, with Mance's

offering better repeatability and the new circuit offering less susceptability to systematic error caused by mixer nonlinearity . To test for the significance of the

latter, both circuits were incorporated within the i-f chassis. In addition to increasing the sensitivity, the unity arm ratio simplifies the operating procedure by

allowing the calibrated variable resistor used in measuring the i-f conductance to be

switched across the mixer as the matched load, rather than requiring that its setting

be transferred to a second resistor with the decimal point shifted, as was required

by the 10:1 arm ratio.

This output circuit has four switches: Switch S is used to select the correct

load for each operating step and is ordinarily sequenced from position 1 to position

6. Switch S is used to select the series (Mance's) or shunt method for the conductance measurement. Switch S is used in operating whichever of the methods is selected by switch S . Switch S is normally left in position 6, which allows the correct

voltmeter connection to be selected by switch S for each load. In other positions,

the voltmeter connection is independent of the load switch setting. This allows, for

example, the total mixer output voltage to be monitored as the load is changed. Ease

of operation for normal use is thus combined with flexibility for experimental purposes . 40
MICROWAVE DEVICE MEASUREMENTS

Plans : Charts will be prepared to facilitate the rapid and precise conversion

of incremental output voltage and i-f conductance data to conversion loss. Using

these charts, the repeatability of all previously made diode loss measurements will

be evaluated. Further measurements will be made to ascertain the sensitivity of mea- sured quantities to system characteristics such as local oscillator power and output

circuit null accuracy to help estimate systematic error uncertainty.

5.3. CARRIER TRANSPORT IN JUNCTION DEVICES

Objective : To improve methods of measurement for charge carrier transport and

related properties of junction semiconductor devices.

Progress : A general approach has been developed for determining the location

and magnitude of delay-time error-producing sources arising from extraneous pickup

at the measurement frequency, and this approach has been tested by applying it to the NBS model of the Sandia bridge. With these sources so characterized, their influence on subsequent transistor measurements can be reduced greatly. The results

of this work can explain previously observed differences in measurements of delay

time of transistors. Several preliminary steps have been taken toward an interlaboratory comparison of transistor scattering-parameter measurements. Measurements

were made as a function of frequency with various bias conditions for three transistor types, the effects of case temperature on the S-parameters were determined qualitatively, the equation widely used to relate h f to S-parameters was verified, and

the S-parameters of an R-C network on a transistor header were measured to determine

its suitability for use as a calibration standard.

Determination of Delay-Time Error Sources — Previously it has been shown that

extraneous coupling at the measurement frequency between various portions of the mea- surement circuit may have a profound effect on the delay time observed (NBS Tech.

Note 727, pp. 53-56). In this quarter, a technique was developed to identify these

error sources and to minimize their effects. In the analysis, the measurement circuit, constructed of n connected loops, is treated as known except for the magnitude

and phase of an extraneous error-source generator in each loop . These generators

are added to the circuit to characterize the effects of undesired signal coupling.

The magnitude and phase of these error sources may be calculated if a network of

known composition is assumed to be in the transistor socket. With n known networks,

the magnitude and phase of all n error-source generators may be determined. These

are a property of the particular measurement system and remain constant unless the

system is changed. Once the individual error sources are so specified, the delaytime error introduced by these sources can be calculated for any network in the

41
CARRIER TRANSPORT IN JUNCTION DEVICES

Figure 16. Sandia bridge showing extraneous voltage generators .

50*1 \ A/W 1 \ 50il \COLLECTOR LINE

| DETECTOR""]

son \ ^ SEMITTER LINE son

a. Schematic diagram

b. Equivalent circuit.

a. Through b. Series c. Series and

short. resistance. parallel resistance.

Figure 17. Non-reactive plug-in elements used to calculate

magnitude and phase of extraneous voltage generators

.

Figure 18. Delay time of an R-C plug- in

network as measured as a function of frequency on the Sandia bridge constructed

at NBS (A) , the correction term appropriate to this bridge for this network

(B) , and the corrected value of delay

time (C). 10 15 20 FREQUENCY (MHz)

I 25

42
CARRIER TRANSPORT IN JUNCTION DEVICES

socket. A correction term, the negative of this error, can then be added to the measured delay time to obtain the correct value. The analysis also permits monitoring

the quality of the measurement circuit design, construction, and modifications, since

ideally this delay-time error should be zero. To make the calculation for a transistor it is necessary to determine the appropriate equivalent circuit.

As an example, consider the application of this technique to the Sandia bridge.

The operation of the bridge, shown schematically in figure 16, is as follows: the

controlled signals V and V are obtained from half-turn loops coupled to a primary

c e loop driven by an r-f generator. The primary loop is adjusted to vary the magnitude

of the ratio V /V . The phase angle between V and V is varied by adjusting the

lengths of the collector and emitter delay lines. These lines are provided with

scales calibrated in picoseconds. A reference delay time is first obtained by taking

the difference between the scale times on the collector and emitter lines when there is

a null at the detector with the emitter terminal of the socket shorted to the collector

terminal. The apparent delay time for a network, or transistor, in the socket is then

obtained by taking the difference between scale times on the collector and emitter

lines and reducing this difference by the reference delay time.

If any of the error sources, Vaba , V, , V,, is non-zero, a different combination

of the amplitudes and phases of V and V are required for balance than would be rec e quired for the ideal case where V = V, = V, = 0. If this different balance can be

a b d achieved only by changing the phase between V and V by varying the length of the

emitter or collector lines, an error in the measurement of delay time is introduced.

The imaginary components of the error sources referred to the phase of the controlled

source V are the components which cause delay-time errors. Only for the case of a delay-time network such as a high-gain transistor with negligible base current at the

measurement frequency does the emitter-collector short-circuit technique for establishing a delay-time reference remove a major portion of the error introduced by

these extraneous generators.

The Sandia bridge constructed at NBS was analyzed with the three non-reactive

plug- in units shown in figure 17. The results of this analysis as applied to the

measurement of a plug-in R-C delay-time element with a time constant of 665 ps are shown in figure 18. Curve A shows delay time as measured on the bridge, as a function of frequency, using an emitter-collector short for the delay-time reference.

Curve B is the correction term calculated from the measurements on the three nonreactive elements. Curve C is the corrected delay time for the network. The latter

is nearly independent of frequency, as it should be, but is about 15 percent larger

than the value calculated for the network. The nature of this disagreement is being

investigated.

43
CARRIER TRANSPORT IN JUNCTION DEVICES

For the same measurement system, both theory and experiment show that the severity of the frequency-dependence of the measured delay time depends on the amount of

signal current in the base lead. Delay-time measurements are conveniently made by

referring them to a delay-time reference established with an emitter-collector short

circuit. For the Sandia bridge, this is tantamount to assuming that the correction

to be applied is strictly a function of the sum of V and V, , and not a function of

V and V, individually. This assumption is not applicable when base current is appreciable; the simple through-short reference technique becomes less and less satisfactory as base current is increased, or as the transistor gain decreases. This is

believed to be the reason why the delay-time data previously reported on one transistor showed a significant frequency dependence, while others did not (compare figures

27 and 28 of NBS Tech. Note 727) . Since there is also base current in the R-C plug-in

networks , the through-short reference technique does not remove all the deleterious

effects of the error-source generators V and V, . a b For a transistor idealized in that the emitter-base voltage is assumed to be

zero and that the only delay time is that associated with the transport of minority

carriers across the base and collector junction, it can be shown that the delay time

read by the bridge, t , is given by

T m= T a" ( J /Wh

fe} Tm <VV •

I

where x is the true delay time of the transistor. The error term is proportional to

1/h,. . This suggests that anomalous changes in apparent transistor delay-time may be

observed following gain degradation such as might occur after neutron irradiation.

The changes quite likely include the signature of the particular measuring setup;

this signature can be expected to vary in magnitude and frequency dependence among

delay-time instruments. In the case of a real transistor, knowledge of the elements

of the equivalent circuit is required to develop an accurate correction term but the

general form would be expected to be similar to that of the idealized case.

(D. E. Sawyer)

Interlaboratory Comparison of S-Parameter Measurements — Several preliminary

steps have been taken in preparation for an interlaboratory comparison of scattering

parameter measurements to be conducted in cooperation with a sponsor.

S-parameters of typical transistors of types 2N709, 2N918, and 2N3960 have been

measured to determine optimum biases and frequencies for the determination of f from

S-parameter data. These transistors were selected as representative types of interest

to the sponsor. Present plans are to operate the transistors at a collector-emitter

voltage of 5 V, and at eight emitter currents from 1 to 10 mA. For the intercomparison, the participants are to measure the 2N709's and the 2N918's over the frequency

44
CARRIER TRANSPORT IN JUNCTION DEVICES

range from 100 MHz to 1 GHz and the 2N3960's from 100 MHz to approximately 2 GHz. From these measurements, for each type, hfe can be obtained as a function of frequency

in the transition region where h varies from its low-frequency value to a vlaue approaching unity.

The equation for calculating h f from S-parameters

,

- 2 ^T

h

fe

= (1 - S n) (1 + S 22

) + S 12

S n

is derived from the S-parameter definition of h by expressing the input and output

currents in terms of the incident and emergent waves of the scattering parameters,

and substituting values for these waves obtained by solving the scattering equations

for the network when the output of the network is short-circuited. Examination of

this equation shows that the sensitivity of h to any of the scattering parameters

depends upon the value of all the other S-parameters.

This relationship has been determined empirically for typical transistors of

the three types. In general, for the bias conditions stated above, the magnitude of

h

f varies directly with changes in the magnitude of S„ , while changes in the phase

of h vary from 1 to 1-1/2 times the change in the phase of S„ . Changes in either

magnitude or phase of S „ have a small effect (Ah < 0.1 AS „) on either the magnitude or phase of h

f

. Changes in either the magnitude or phase of S-. , and S 22 produce an inverse change in h f whose magnitude varies between 25 and 100 percent of

the change in the S-parameter.

S-parameter measurements made on an R-C network with a delay-time of 665 ps mounted on a TO-5 transistor header have disclosed series resonances in the vicinity

of 200 and 400 MHz caused by the inductance of the leads. An improved version mounted

on a TO-72 header is planned for use as a control in the interlaboratory comparison.

The resonant frequencies obtained for the T0-5 header device are reasonably close to those computed from a knowledge of the fabrication steps. Measurements such as these

made using the S-parameter equipment facilitate the analyses of delay-time instruments, such as the Sandia bridge, designed to operate at lower frequencies, e.g. 1 to

30 MHz, by providing a more accurate representation of the equivalent circuit for the

R-C plug- in element.

The effect of case temperature on S-parameter measurements has been determined

qualitatively. The results indicate that the power dissipated at any of the proposed

bias levels is low enough that temperature effects are not a problem.

(G. J. Rogers and F. H. Brewer)

45
CARRIER TRANSPORT IN JUNCTION DEVICES

Plans : The work specifying the error sources in delay-time measurement instruments will continue. The format specifying the laboratory procedure for intercomparison of S-parameter measurements will be completed and tested using an automatic network

analyzer facility. It is anticipated that the portion of the project work concerned

with the probe measurement of high-frequency characteristics of transistors while the

devices are at the wafer stage of preparation will receive increased attention.

5.4. REFERENCES

5.1. Thermal Properties of Devices

1. Oettinger, F. F. , and Rubin, S., The Use of Current Gain as an Indicator for the

Formation of Hot Spots Due to Current Crowding in Power Transistors, Proceedings

of the 1972 IEEE Reliability Physios Symposium, Las Vegas, Nevada, April 5-7,

1972.

46
APPENDIX A JOINT PROGRAM STAFF

Coordinator: J. C. French

Consultant: C. P. Marsden

Semiconductor Characterization Section

(301) 921-3625

Dr. W. M. Bullis, Chief

F. H. Brewer

M. Cosman

Mrs. K. E. Dodson

Dr. J. R. Ehrstein

Dr. D. C. Lewis

R. L. Mattis

Dr. W. E. Phillips

Miss D. R. Ricks

H. A. Schafft

A. W. Stallings

Mrs. M. L. Stream

W. R. Thurber

W. K. Croll

Mrs. S. A. Davis

H. E. Dyson

Mrs. J. M. Guy ton

Semiconductor Processing Section

(301) 921-3541

Dr. A. H. Sher, Acting Chief

G. G. Harman

H. K. Kessler

J . Krawczyk

T. F. Leedy

Y. M. Liu

L. M. Smith

G. P. Spurlock

D. L. Blackburn

C. F. Bolton+

Mrs Miss B. S. Hope

R. L. Gladhill

F. R. Kelly

Electron Devices Section

(301) 921-3622

J. C. French, Chief

J. M. Kenney

Mrs. K. 0. Leedy , § Miss C. A. Main

F. F. Oettinger

M. K. Phillips

G. J. Rogers

S. Rubin

D. E. Sawyer

L. R. Williams

Part Time

Secretary

Telephone

:

(301) 921-3625

47
APPENDIX B COMMITTEE ACTIVITIES

ASTM Committee F-l on Electronics

C. F. Bolton, Committee Assistant Secretary

W. M. Bullis, Editor, Subcommittee 4, Semiconductor Crystals; Leaks, Resistivity,

Mobility, Dielectrics, and Compound Semiconductors Sections

J. R. Ehrstein, Chairman, Resistivity Section; Epitaxial Resistivity, and Epitaxial Thickness Sections

J. C. French, Committee Editor

G. G. Harman, Secretary, Interconnection Bonding Section

K. 0. Leedy, Chairman, Interconnection Bonding Section

T. F. Leedy, Photoresist and Dielectrics Sections

C. P. Marsden, Committee Secretary

R. L. Mattis, Lifetime Section

W. E. Phillips, Chairman, Lifetime Section; Secretary, Subcommittee 4, Semiconductor Crystals; Crystal Perfection, Dielectrics, Encapsulation, Thin Films,

and Thick Films Sections

A. H. Sher, Germanium Section

W. R. Thurber, Mobility, Germanium, Compound Semiconductors, and Impurities in

Semiconductors Sections

ASTM Committee E-10 on Radioisotopes and Radiation Effects

W. M. Bullis, Subcommittee 7, Radiation Effects on Electronic Materials

J. C. French, Subcommittee 7, Radiation Effects on Electronic Materials

Electronic Industries Association: Solid State Products Division, Joint Electron

Device Engineering Council (JEDEC)

J. M. Kenney , Microwave Diode Measurements, Committee JC-21 on UHF and Microwave

Diodes

F. F. Oettinger, Chairman, Task Group JC-11.3-1 on Thermal Considerations for

Microelectronic Devices, Committee JC-11 on Mechanical Standardization;

Technical Advisor, Thermal Resistance Measurements, Committees JC-22 on Rectifier Diodes and Thyristors, JC-20 on Signal Diodes, JC-25 on Power

Transistors, and JC-30 on Hybrid Integrated Circuits

S. Rubin, Chairman, Council Task Group on Galvanomagnetic Devices

D. E. Sawyer, Task Group JC-24-5 on Transistor Scattering Parameter Measurement

Standard, Committee JC-24 on Low Power Transistors

H. A. Schafft, Technical Advisor, Second Breakdown and Related Specifications,

Committee JC-25 on Power Transistors

48
APPENDIX B

IEEE Electron Devices Group:

J. C. French, Standards Committee

J. M. Kenney , Chairman, Standards Committee Task Force on Microwave Solid-State

Devices II (Mixer and Video Detector Diodes)

H. A. Schafft, Chairman, Standards Committee Task Force on Second Breakdown

Measurement Standards

IEEE Magnetics Group

S. Rubin, Chairman, Galvanomagnetic Standards Subcommittee

IEEE Parts, Hybrids, and Packaging Group

W. M. Bullis , New Technology Subcommittee, Technical Committee on Hybrid

Microelectronics

Society of Automotive Engineers

J. C. French, Subcommittee A-2N on Radiation Hardness and Nuclear Survivability

W. M. Bullis, Planning Subcommittee of Committee H on Electronic Materials and

Processes

IEC TC47, Semiconductor Devices and Integrated Circuits:

S. Rubin, Technical Expert, Galvanomagnetic Devices; U.S. Specialist for Working

Group 5 on Hall Devices and Magnetroresistive Devices

NMAB ad hoc Committee on Materials and Processes for Electron Devices

W. M. Bullis

NMAB ad hoc Committee on Materials for Radiation Detection Devices

D. E. Sawyer

49
APPENDIX C SOLID-STATE TECHNOLOGY & FABRICATION SERVICES

Technical services in areas of competence are provided to other NBS activities

and other government agencies as they are requested. Usually these are short-term,

specialized services that cannot be obtained through normal commercial channels.

Such services provided during the last quarter are listed below and indicate the kinds

of technology available to the program.

1. Channel Electron Multipliers (Y . M. Liu)

A study was begun for the NASA Goddard Space Flight Center to evaluate the

performance and long-term stability of channel electron multipliers to be used in

low-energy charged-particle experiments in the NASA Atmosphere Explorer Satellite

series

.

2. Active Probe (T. F. Leedy)

Fabrication was begun on a field effect transistor to be used as an active

probe with a relatively flat band-pass from d-c to 1 MHz with an oscilloscope for

the NBS Microwave and Mechanical Instrumentation Section.

3. Thin Metal Films (J. Krawczyk)

Thin films of gold, nickel, or chromium were vacuum evaporated onto various

polymeric films for fabrication into piezoelectric and pyroelectric detectors by the

NBS Instrumentation Applications Section.

4. Radiation Detectors (A. H. Sher and H. E. Dyson)

A lithium-drifted germanium gamma-ray detector fabricated for the study of

germanium materials characteristics was loaned to the NBS Photonuclear Physics Section.

+

5. Electrical Burnout Studies (W. K. Croll and H. A. Schafft)

Electrical tests and visual and scanning electron microscope examinations were conducted on 24 devices (SN54L00 and SN54H00 quadruple 2-input positive NAND

gates) for the Harry Diamond Laboratories to assist in establishing degradation and

failure modes.

NBS Cost Center 4254429

NBS Cost Center 4251541

50
APPENDIX D JOINT PROGRAM PUBLICATIONS

Prior Reports : A review of the early work leading to this Program is given in Bullis , W. M. , Measurement Methods for the Semiconductor Device Industry — A Review of NBS Activitiy,

NBS Tech. Note 511, December, 1969.

Quarterly reports covering the period since July 1, 1968, have been issued under

the title Methods of Measurement for Semiconductor Materials, Process Control, and

Devices. These reports may be obtained from the Superintendent of Documents (Catalog

Number C.13.46:XXX) where XXX is the appropriate technical note number. Microfiche

copies are available from the National Technical Information Service (NTIS) , Springfield, Virginia 22151.

Quarter Ending

September 30, 1968

December 31, 1968

March 31, 1969

June 30, 1969

September 30, 1969

December 31, 1969

March 31, 1970

June 30, 1970

September 30, 1970

December 31, 1970

March 31, 1971

June 30, 1971

September 30, 1971

December 31, 1971

NBS Tech. Note

472

475

488

495

520

527

555

560

571

592

598

702

717

727

Date Issued

December, 1968

February, 1969

July, 1969

September, 1969

March, 1970

May, 197Q

September, 1970

November, 1970

April, 1971

August, 1971

October, 1971

November, 1971

April, 1972

June, 1972

NTIS Accession No,

AD 681330

AD 683808

AD 692232

AD 695820

AD 702883

AD 710906

AD 718534

AD 719976

AD 723671

AD 728611

AD 732553

AD 734427

Current Publications : As various phases of the work are completed, publications are prepared to summarize the results or to describe the work in greater detail. Copies of most of such

publications are available and can be obtained on request to the editor or the author.

The following publications appeared during the quarter covered by this report or subsequently.

Schafft, H. A., Wire-Bond Electrical Connections: Testing, Fabrication and Degradation, A Bibliography - 1957-1971, Technical Note 593 (January, 1972).

Thurber, W. R. , and Bullis, W. M. , Resistivity and Carrier Lifetime in Gold-Doped

Silicon, AFCRL Report AFCRL-72-0076 , January 31, 1972.

Sher, A. H. , Keery, W. J., and Dyson, H. E., Improved Infrared Response Measurements

in Semiconductor Nuclear Radiation Detectors, IEEE Trans. Nucl. Sai. NS-19 , No. 1, 341-

344 (February, 1972).

51
APPENDIX D

Sher, A. H. , and Keery, W. J., Improved Infrared-Response Technique for Determining

Impurity and Defect Levels in Semiconductors, Appl. Phys. Lett. 20_, 120-122 (1 February 1972)

.

Harman, G. G. , and Leedy, K. 0., An Experimental Model of the Microelectronic Ultrasonic Wire Bonding Mechanism, Proceedings of the 1972 IEEE Reliability Physics Symposiums Las Vegas, Nevada, April 5, 1972.

Oettinger, F. F. , and Rubin, S., The Use of Current Gain as an Indicator for the

Formation of Hot Spots Due to Current Crowding in Power Transistors, Proceedings of

the 1972 IEEE Reliability Physics Symposium, Las Vegas, Nevada, April 5, 1972.

Sher, A. H., Liu, Y. M. , and Keery, W. K. , Infrared-Response Measurements on Radiation-Damaged Si(Li) Detectors, IEEE Trans. Nucl. Sci. NS-19 , No. 3, 312-

317 (June, 1972)

.

Liu, Y. M. , and Coleman, J. A., Radiation Damage Effects by Electrons, Protons and

Neutrons in Si(Li) Detectors, IEEE Trans. Nucl. Sci. NS-19 , No. 3, 346-352 (June,

1972).

Marsden, C. P., Tabulation of Published Data on Electron Devices of the U.S.S.R.

Through December 1971, NBS Technical Note 715 (June, 1972). (Supersedes NBS Technical Note 526)

.

Schafft, H. A., Testing and Fabrication of Wire-Bond Electrical Connections, NBS

Technical Note 726 (August, 1972).

Phillips, W. E. , Interpretation of Steady-State Surface Photovoltage Measurements

in Epitaxial Semiconductor Layers, to be published in Solid-State Electronics.

52
^orm NBS-114A (i-7i) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS-TN-733

2, Gov't Accession No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Methods of Measurement for Semiconductor Materials, Process

Control, and Devices. Quarterly Report, January 1 to March

31, 1972.

5. Publication Date September 1972

6. Performing Organization Code

7. AUTHOR(S) W. Murray Bullis, Editor

8. Performing Organization

10. Project/Task/Work Unit No. See Item 15

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

11. Contract/Grant No.

See Item 12

12. Sponsoring Organization Name and Address NBS , DNA (EA072-810) , Navy

Strategic Systems Project Office, NAD, Crane, Indiana

(PO-2-0023 and PO-2-0054), NAVELEX (PO-2-1034) , AFWL

(F29601-71-F-0002), AFCRL (Y72-873) , ARPA (MIPR FY76167100331)

AEC, NASA (S-70003-G).

13. Type of Report & Period Covered Interim January

1 to March 31, 1972

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

4251126, 4251127, 4252128, 4254115, 4259522, 4259533, 4252534, 4252535, 4251536.

4254422, 4254425, 4254429.

!4. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) This quarterly progress report, fifteenth of a series, describes NBS activities

directed toward the development of methods of measurement for semiconductor materials,

process control, and devices. Significant accomplishments during this reporting period

include development of a procedure to correct for the substantial differences of transistor delay time, a device characteristic frequently used as a screen in radiation

hardness assurance tests, as measured with different instruments or with the same instrument at different frequencies; association of infrared response spectra of poor

quality germanium gamma-ray detectors with spectra of detectors fabricated from portions of a good crystal that had been degraded in known ways; and confirmation of the

excellent quality and cosmetic appearance of ultrasonic bonds made with aluminum ribbon

wire. Work is continuing on measurement of resistivity of semiconductor crystals;

study of gold-doped silicon; development of the infrared response technique; evaluation

of wire bonds and die attachment; and measurement of thermal properties of semiconductor devices, delay time and related carrier transport properties in junction devices,

and noise properties of microwave diodes. Supplementary data concerning staff, standards committee activities, technical services, and publications are included as appendixes . Key Words (cont.): electronics; microwave diodes; nuclear radiation detectors; probe

techniques (a-c) ; resistivity; ribbon wire bonding; semiconductor devices; semiconductor materials ; semiconductor process control; silicon; thermal resistance; thermographic measurements; ultrasonic bonding; wire bonds.

7. KEY WORDS (Alphabetical order, separated by semicolons) Aluminum wire; base transit time; carrier

lifetime; die attachment; electrical properties; epitaxial silicon; gamma-ray detectors; germanium; gold-doped silicon; infrared response; methods of measurement

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