SPECIAL PUBLICATION 400-3

D

7

30-3

U.S.

tTMENT

OF

IMERCE

National

Bureau

of

andards

S('nii( (fti(J iiclor McdsnremaU I cchnoloij'v:

ARPA/NBS Workshop I.

Measurement Problems in

Integrated Circuit Processing

and Assembly
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standaids' 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 measurement 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 Institute 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 — Mechanics — Heat — Optical Physics — Nuclear

Sciences ^ — Applied Radiation ^ — Quantum Electronics ^ — Electromagnetics ' — Time

and Frequency ' — Laboratory Astrophysics ^ — Cryogenics ^ THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of well-characterized materials needed by industry, commerce, educational institutions, and Government; provides

advisory and research services to other Government agencies; and develops, produces, and

distributes standard reference materials. The Institute consists of the Office of Standard Reference Materials and the following divisions: Analytical Chemistry — Polymers — Metallurgy — Inorganic Materials — Reactor

Radiation — Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote

the use of available technology and to facilitate technological innovation in industry and

Government; cooperates with public and private organizations leading to the development of

technological standards (including mandatory safety standards), codes and methods of test; and provides technical advice and services to Government agencies upon request. The Institute consists of a Center for Building Technology and the following divisions and offices: Engineering and Product Standards — Weights and Measures — Invention and Innovation — Product Evaluation Technology — Electronic Technology — Technical Analysis — Measurement Engineering — Structures, Materials, and Life Safety* — Building

Environment * — Technical Evaluation and Application ' — Fire Technology.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effec- tiveness in the conduct of their programs through the selection, acquisition, and effective

utilization of automatic data processing equipment; and serves as the principal focus within

the executive branch for the development of Federal standards for automatic data processing

equipment, techniques, and computer languages. The Institute consists of the following

divisions: Computer Services — Systems and Software — Computer Systems Engineering — Information Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal Government; promotes the development of the National Standard Reference Data System and

a system of information analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services to ensure that the NBS staff has optimum

accessibility to the scientific information of the world. The Office consists of the following

organizational units: Office of Standard Reference Data — Office of Information Activities — Office of Technical

Publications — Library — Office of International Relations.

>• Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address

vVashington, D.C. 20234.

' Part of the Center for Radiation Research,

a Located at Boulder, Colorado 80302.

* Part of the Center for Building Technology.
Semiconductor Measurement Technology:

ARPA/NBS Workshop I.

Measurement Problems in

Integrated Circuit Processing and Assembly

Harry A. Schafft

Electronic Technology Diyision

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

This activity was supported by

The Defense Advanced Research Projects Agency

under ARPA Order 2397, Program Code 4D10

National Bureau m' Statidard; MAY 6 1974

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secrefory

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, D/rec/or

Issued February 1974
I

I

Library of Congress Catalog Number: 74-600001

National Bureau of Standards Special Publication 400-3

Nat. Bur. Stand. (U.S.), Spec. Publ. 400-3, 24 pages (Feb. 1974)

CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1974 For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.10:400-3). Price 60 cents.
TABLE OF CONTENTS

Page

INTRODUCTION 1

MORNING SESSION 2

AFTERNOON SESSION 2 Silicon ^ 3 Oxides - 3 Photolithography 4 Assembly 5 Information Dissemination 6

APPENDIX 1 7

APPENDIX 2 9

APPENDIX 3 13

lil
PREFACE

The Semiconductor Technology Program serves to focus NBS efforts to enhance the performance, interchangeability , and reliability of discrete semiconductor devices and integrated

circuits through improvements in measurement technology for use in controlling device fabrication processes and in specifying materials and devices in national and international commerce. Its major thrusts are the development of carefully-evaluated and well-documented

test procedures and associated technology for use on production lines, and the dissemination

of such information to the electronics community. Application of the output by industry is

expected to contribute to higher yields, lower cost, and higher reliability of semiconductor

devices. In addition, the improvements in measurement technology will lead to greater economy in government procurement and will provide a basis for controlled improvements in fabrication processes and in essential device characteristics.

The Program receives direct financial support principally from three major sponsors:

the Defense Advanced Research Projects Agency (ARPA),* the Defense Nuclear Agency (DNA),''' and the National Bureau of Standards (NBS).^ The ARPA-supported portion of the Program,

Advancement of Reliability, Processing, and Automation for Integrated Circuits with the

National Bureau of Standards (ARPA/ IC /NBS ) , addresses critical Defense Department problems

in the yield, reliability, and availability of integrated circuits. The DNA-supported portion of the Program emphasizes aspects of the work which relate to radiation response of

electron devices for use in military systems. There is considerable overlap between the

interests of DNA and ARPA and both interests parallel the measurement-oriented mission of

the NBS.

Cooperation with industrial users and suppliers of semiconductor devices is achieved

through N2S participation in standardizing organizations; through direct consultations with

device and material suppliers, government agencies, and other users; and through periodically

scheduled symposia and workshops. This report describes the results of the first workshop

in this series which was an activity of the ARPA/ IC /NBS program.

*Through ARPA Order 2397, Program Code 4D10 (NBS Cost Center 4259555).

^Through Inter-Agency Cost Reimbursement Order 74-811 (NBS Cost Center 4259522).

'^Through Scientific and Technical Research Services Cost Centers 4251126, 4252128, and

4254115.

iv
ARPA/NBS Workshop I

Measurement Problems in Integrated Circuit Processing and Assembly

by

Harry A. Schafft

The dual purpose of the workshop was (1) to announce and describe the new

effort, "Advancement of Reliability, Processing, and Automation for Integrated

Circuits with the National Bureau of Standards," sponsored by the Defense Advanced Research Projects Agency (ARPA) , and (2) to obtain additional input on

critical measurement problems in integrated circuit processing and assembly to assist in planning future work in the effort. More than 130 engineers representing 61 organizations from the electronics industry and government participated in the workshop. The measurement problems in silicon, oxides, photolithography, and assembly and the problems in information dissemination that were

identified by the participants are summarized. Included as appendices are summaries of two talks given: one which described the results of earlier direct

contacts with a cross section of industrial representatives on major measurement

problems in integrated circuit processing and assembly and the other which described the initial plans for work in the new effort.

Key Words: Die bonding; hermeticity; integrated circuits; measur^ient methods;

microelectronics; oxides; photolithography; process control; reliability; semiconductor devices; silicon; wire bonding.

Introduction

The first workshop under the new effort. Advancement of Reliability,

Processing, and Automation for Integrated Circuits with the National Bureau

of Standards, sponsored by the Defense Advanced Research Projects Agency

(ARPA), was held on September 7, 1973, in Palo Alto, California. The dual

purpose of this ARPA/NBS workshop was to announce the start of the new

effort and to supplement input obtained from earlier direct contacts with

a cross section of industrial representatives on the major measurement

needs for improved control of process and assembly procedures for integrated

circuits. A total of 137 scientists and engineers representing 61 organizations attended.

The new effort is being developed in concert with the reliability

laboratories of the three military services. The major thrusts of the

effort are the development of well-documented test procedures and measureiment technology for use on semiconductor devices production lines and the

pdissemlnation of such information to the electronics community. A feature

Ilof the effort will be the contracting of selected work to industrial,

iuniversity, government, and independent research laboratories. Application

lof the output of the program by industry is expect;ed to contribute to higher

[(yields, lower cost, higher reliability, and greater availability of special

first

workshop

purpose of workshop

major thrusts

of effort

contract work

goals

1
Semiconductor

Technology

Program

devices such as those needed by the Department of Defense. This effort is

part of the NBS Semiconductor Technology Program which is described briefly

in Appendix 1.

rationale

for effort

aims of effort

NBS approach

field visits

plans

Appendices

Morning Session

During the morning session. Dr. C. M. Stickley, Director of ARPA's

Materials Sciences Office, outlined the rationale for a new program which is

intended to make it possible for the Defense Department to obtain in the

future the large-scale integrated circuits that it needs to meet its special-,

ized applications with low-volume purchases at acceptable costs and reli-

| ability. He pointed out that the cost to the Department of Defense for elec-(|

tronics equipment maintenance alone is reported to be five billion dollars

yearly. The long term goal is to reduce these costs through enhanced inte- ^ grated circuit reliability. As a new thrust of the Department of Defense to this goal, the ARPA/NBS effort will be concentrated in process understanding

and control. Dr. Stickley described the role of NBS as primary agent for the

five-year effort.

Representing the NBS Electronic Technology Division, where the effort is

to be conducted, J. C. French described the Division's involvement in and

approach to measurement methods in the area of semiconductor technology,

H. A. Schafft discussed the results of field visits made to identify major

measurement problems in process and assembly control that interfere with

achieving greater yield and reliability, and W. M. Bullis outlined plans for

the various technical areas to be covered in the development of improved measurement methods and associated technology for controlling and automating key

processing and assembly procedures. Summaries of the talks given by H. A. Schafft and W. M. Bullis are included in Appendices 2 and 3, respectively.

Afternoon Session

problem areas In the afternoon, discussion groups met to identify non-proprietary measurement problems related to silicon, oxides, photolithography, and assembly

and problems of information dissemination for consideration in planning work

corroboration in the ARPA/NBS effort. In general, the input from these groups and the workshop as a whole corroborated the findings obtained from earlier direct convariances tacts and reviewed by H. A. Schafft in the morning session. Two variances were noted, however. There appears to be greater interest in the use of test

patterns for silicon process control and in tests for metallization step cover

, ,^ J,. age than was indicated in consultations made earlier in the year with repre-

- • sentatives of the electronics community.

The measurement needs expressed by the discussion groups are summarized

in the following paragraphs:
Silicon

Measurements of thickness and riesistivity of thin (<2 ym) epitaxial layers

were singled out as the greatest need; adequate techniques for these measurements do not now exist. Other high priority areas mentioned were methods for

determining impurity profiles, particularly in the base region of bipolar tranthickness measurement

profiles

sistors, and rapid methods for inspecting wafers and other materials for par- particular

contamination

ticulate contamination which could cause defects to be introduced during growth

of epitaxial films.

The usefulness of test patterns for determining impurity profiles in the test patterns

base region was widely recognized. However, it was emphasized particularly

in the case of small-geometry devices that test patterns may not adequately

reflect the true characteristics; in fact, it was noted that the device itself

makes the best test vehicle, if it is accessible.

A problem noted in connection with the use of test patterns was associated

with the large volume of data which must be acquired and analyzed. Automated automated procedures

procedures and standardized reporting formats would be welcomed.

Other problem areas identified where measurement procedures need improve- defects, flatiment were characterization of swirl defects, determination of surface flatness '^geneities^^

before and during processing, and evaluation of resistivity homogeneity on both

(microscopic and macroscopic scales.

There was much discussion of the relationship between the properties of process

j control and jsilicon and device reliability. There was general agreement that it was nearly reliability

jimpossible to ascribe the causes of failed devices to silicon defects> Howjever, because the characteristics of a device can be more accurately predicted

jif the parameter distributions are narrow, the ability to control all processes.

Including wafer fabrication does impact device reliability. To this end, rapid

jand simple measurements of appropriate accuracy and precision are required at

'many points.

^Oxides

tests for oxide

Although a number of problem areas were discussed by the oxide group, it

^had difficulty in defining specific measurement needs. Oxide stability is of

jconsiderable concern. The bias-time-temperature stress tests are commonly

lused to test stability of oxides but there is great variability in the stress stability

conditions used, even within a given company. While the needs for standardIfLzlng the stress conditions and for more quantitative tests of stability were

Implicit, no specific mention of such needs was expressed. Another area of

bonsiderable concern is in regard to assuring that contact windows are free tests for oxide

jjf oxides and photoresist; oxides are the primary concern. While it is common removal

ijto over-etch somewhat in an attempt to remove all residues, this procedure is

I 3
not always successful. Therefore there is a need for an inspection method

that can be used immediately after the etching procedure to detect the

presence of any oxide or other residues in contact windows.

The control and rapid measurement of oxide thickness is of moderate conthickness cern. A number of methods are used to measure oxide thickness; a common

standards? method is to use interference colors. The use of standard oxide films of

specific thickness was felt by the group to be impractical because of problems experienced with oxide film instabilities.

test patterns The use of test patterns was discussed and the group concluded that the

device itself is ultimately the best test pattern. The problems with handling

and interpreting the data from test patterns mentioned in the silicon group were repeated in this group.

Photolithography

mask inspection Problems with mask inspection were considered to be more critical than

vs. photoresists

those with photoresists. This is because more time and material are apt to have been invested before a defective mask is detected and replaced than if a problem with photoresist is discovered.

mask inspection Therefore, of primary concern was the need to improve the methods for

inspecting glass masks for defects and dimensional correctness. Specifically

there is a need for a method co detect automatically the location and number

of mask imperfections, such as pin holes, and to provide a quantitative measure of mask quality. Methods are needed to measure both linear and angular

dimensions. Methods for linear dimensions should be capable of measuring

lengths of up to 3 inches (7.6 cm) with an accuracy of ±20 yin. (0.5 ym)

.

Referee methods are needed to establish the degree of registration between

levels of a mask set and to measure both die and array dimensional errors

["runout"].

photoresist It was felt that because the photoresist literature is so extensive and

bibliography

scattered in many journals, it would be extremely useful if NBS could compile

a bibliography of work done in the field. This would then facilitate an awareness of at least what work has been,done.

Many information deficiencies in the characterization and use of photophotoresist resist are felt to exist. Paramount is the need to determine the interrelacharacter ization

and use tion of the physical properties of the photoresist material, such as specific

gravity, viscosity, and solids content, with the properties of the photoresist

film, such as resolution, etch resistance, and adhesion. Also, concern was expressed about how developers, thinners, and impurities can affect the properties of the photoresist film and its removal. Specific problems were mentioned with regard to defining and measuring the photosensitivity and

4
cleanliness of photoresists. While processes can be altered to compensate

for different levels of cleanliness no quantitative procedures appear to be

available.

There is also a critical need for light standards for use in controlling and in assuring constancy of the exposure of photoresist fMms. Problems exist with aging of the lamps used to illuminate photoresists.

Finally, interest was expressed in the need for research in electron

beam lithography, in general, and in particular for range-straggle information for ions and electrons in metals, polymers, and semiconductors because

of th,e growing use of ion implantation and electron beam lithography.

light standards

electron beam lithography

Assembly

Concerns were expressed in three areas: bonding, packaging, and

hermeticity

.

In wire bonding there is a need for 100% testing for bond integrity to eliminate random, weak wire bonds. Tests for the bondability of wires and wire bonding

metallizations are also needed to avoid the more systematic bonding problems that are encountered. Tests for surface cleanliness were of particular

interest because dirty metal was felt to be responsible for many bonding

problems. To isolate problems associated with the bonding machine itself,

a need was expressed for a test surface to which wires could be bonded.

A strong desire was expressed to have NBS disseminate information and

make available surveys on bonding. In particular, a manual on procedures

for bonding and the characteristics of materials used in bonding was desired.

Discussions devoted to die bond problems were directed primarily to the

importance of having methods for determining the purity and bondability of

materials used in the die bond.

In packaging there is a need to develop methods to characterize package packages

materials, particularly of g] ass-frit ceramic packages, to assure package

consistency. Problems were mentioned in methods for determining the quality

of glasses, the retention of the binders, and the detection of voids. Furthermore, while there is a desire for a "high-reliability" package, there

is no clear concept of how to define such a package nor how to test and

evaluate it. A.dditionally , there is generally poor communication between

the suppliers and users of packages in regard to their respective needs.

In hermeticity there is a need to evaluate the capabilities of the hermeticity

methods used in order to obtain assurance in the measurements made and to

be able to intercompare methods. Also theri is a need to correlate the

bonding surveys

and manuals

die bonding

5
penetration of moisture and other undesirable vapors into the package with the; measured leak rates for tracer gases used and therefrom determine needs for

leak rate rejection levels.

Information Dissemination

It was unanimously agreed that the proliferation of information in the

semiconductor electronics field is a very serious problem. There is a desperate need to order, evaluate, and disseminate information th'at already

exists. This need was corroborated in the other discussion groups. Filling

in the gaps of information, although very significant appeared to be of

secondary importance.

It was proposed that NBS could play a valuable role in addressing this

need by serving as a center for collecting information and distributing it in

usable form. Publications which the group thought to be most widely read by

the engineering community were discussed.

Vehicles for disseminating information were discussed. Bibliographies,

reviews, and state-of-the-art surveys which are periodically updated are

needed. Workshops and other meetings were felt also to be useful.

Meetings involving primarily information input and exchange on a particular topic should include experts by invitation; the location of these meetings

was not considered to be overly important. However, meetings involving primarily information output require care in the selection of the meeting location to maximize the number of people reached. With regard to the latter

meeting type it was suggested that NBS explore the use of video tape presentations with an expert in attendance to answer questions.

Acknowledgement

The efforts of the many people involved in this workshop are acknowledged here with

pleasure. The following acted as discussion group leaders during the afternoon session and

provided me with summary statements on the results of their respective groups which were

used in the preparation of this report: James R. Ehrstein and W. Murray Bullis for the

silicon group, Martin G. Buehler and Willie E. Phillips for the oxide group, Thomas F. Leedy

and Donald B. Novotny for the photolithography group, George G. Harman and Stanley Ruthberg

for the assembly group, and David L. Blackburn for the information dissemination group.

Behind the scenes but vitally important to the success of the workshop were the efforts of

Kathryn 0. Leedy who played a major role in the publicity and preparations for the workshop

and in the operation of the workshop itself. Also involved in this work were Charles P. Marsden, Bonnie S. Hope, and David C. Lewis. Thanks go to C. Martin Stickley, Judson C. French, and W. Murray Bullis for their talks in the morning session which helped set the

stage for the afternoon group discussions. Finally, thanks go out to the attendees who with

their active participation made it the useful and interesting workshop that it was.

My special thanks go to Marilyn L. Stream who typed the camera-copy draft.

6

inf ormacion

proliferation

NBS role

vehicles for disseEinacion

meetings
Appendix 1

THE SEMICONDUCTOR TECHNOLOGY PROGRAM

in the

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

PROGRAM OBJECTIVES

The Semiconductor Technology Program serves to focus NBS efforts to enhance the performance, interchangeability , and reliability of discrete semiconductor devices and integrated

circuits through improvements in measurement technology for use in controlling device fabrication processes and in specifying materials and devices in national and international commerce. Its major thrusts are the development of carefully-evaluated and well-documented

test procedures and associated technology for use on production lines, and the dissemination

of such information to the electronics community. Application of the output by industry is

expected to contribute to higher yields, lower cost, and higher reliability of semiconductor

devices. In addition, measurement technology will provide a basis for controlled improvements in fabrication processes and in essential device characteristics, such as uniformity

of response to radiation effects.

SPONSORS

The Program receives direct financial support principally from three major sponsors:

the Defense Advanced Research Projects Agency (ARPA) , the Defense Nuclear Agency (DNA) , and

the National Bureau of Standards (NBS). The ARPA-supported portion of the Program addresses

critical Defense Department problems in the yield, reliability, and availability of integrated circuits. The DNA-supported portion of the Program emphasizes aspects of the work

which relate to assurance of radiation hardness of electron devices for use in military

systems. There is considerable overlap between the interests of DNA and ARPA, and both

interests parallel the measurement-oriented mission of NBS.

INDUSTRY TIES

Cooperation with industrial users and suppliers of semiconductor devices is achieved

through NBS participation in standardizing organizations; through direct consultations with

device and material suppliers, government agencies, and other users; and through periodically scheduled symposia and workshops. New ties to the industry will develop through the

contracting out of selected parts of the program activity. Direct input from these contracts is expected to augment the valuable assistance that has traditionally been received

from the industry through cooperative experiments and technical exchanges.
REPORTS

Progress reports are regularly prepared by the Electronic Technology Division for

issuance as NBS Special Publications under the new title Semiconductor Measurement Technology, continuing the series of reports issued as NBS Technical Notes during the past five

years. More detailed reports such as state-of-the-art reviews, literature compilations, and

summaries of technical efforts conducted within the Program are issued as these activities

are completed.

Selected Activities

in the

^ ": Semiconductor Technology Program

-'r:.'::^^}. . MATERIALS

'

' . ; - STANDARD WAFERS FOR FOUR-PROBE RESISTIVITY

:. - SPREADING RESISTANCE MEASUREMENTS

. ; M - CAPACITANCE-VOLTAGE MEASUREMENTS

^- r - THERMALLY STIMULATED CURRENT AND CAPACITANCE

' :- .'

. - INFRARED RESPONSE MEASUREMENTS

- v -;'.c - CHARACTERIZATION OF GOLD-DOPED SILICON

. : .'-.V . ; - CARRIER LIFETIME

- OXIDE AND INTERFACE STATE CHARACTERIZATION

PROCESS CONTROL

- TEST PATTERNS FOR PROCESS CONTROL

' .;>..

•

' - PHOTORESIST MEASUREMENTS

- MASK INSPECTION

- BEAM LEAD AND WIRE BOND EVALUATION

- DIE ATTACHMENT EVALUATION

- HERMETICITY

DEVICES

- SCANNING ELECTRON MICROSCOPY

- THERMAL RESISTANCE AND THERMAL RESPONSE

- HOT SPOT DETECTION

- MICROWAVE MIXER DIODE EVALUATION

- NUCLEAR RADIATION DETECTORS

- CONTROLS AND SCREENS FOR HARDNESS ASSURANCE

- STANDARDIZATION ACTIVITIES

- SEMINARS AND WORKSHOPS

- TECHNICAL CONSULTATION

8
Appendix 2

Process- and Assembly-Control Problems and Measurement Needs

by

Harry A. Schafft

National Bureau of Standards

Washington, D. C. 20234

To assist in the selection of activities to be undertaken in the ARPA/NBS

effort, initial consultations were completed with representatives of the electronics community to identify the more important measurement problems in process control for silicon wafer fabrication and device assembly primarily of

bipolar and MOS digital integrated circuits. These consultations were made

during visits to ten device manufacturing companies and one private research

organization where discussions were conducted with a total of almost 100 technical people.

The generalized need expressed by many of the respondents was for more

nondestructive, direct, and rapid production-type methods for measurement in

order to reduce the appreciable number of silicon slices that would have to be sacrificed if all the necessary measurements were to be made, to avoid the

need to infer product characteristics from tests on sample silicon slices, and

to provide the faster feedback necessary to allow the desired degree of control

on the production line.

The most critical measurement needs identified during the visits are

ranked and reviewed briefly in the following paragraphs. The relative importance of the problem areas identified was gauged according to the number of

organizations that mentioned the area as being important and the degree of

interest or ranking that was indicated by the respondents.

Process control measurement problems of primary concern are with: (1)

the starting silicon material in regard to defects and resistivity; (2) silicon diffused and epitaxial layers in regard to thickness, doping profiles,

and sheet resistivity; (3) oxide films in regard to stability and impurities;

(4) photolithographic and associated procedures in regard to mask inspection,

photoresist materials and their use, and pin holes; and (5) hermeticity.

source of review

general needs

ranking

criteria

five areas

of primary

concern

9
silicon —

defects

silicon —

resistivity

profiles

epi thickness,

junc tion depth

With regard to the starting silicon material and silicon layers there are

a number of problem areas that were mentioned. There is a general need for

improved methods for detecting and characterizing defects and contaminants,

and for determining the effect they have on device yield and reliability.

Among the defects mentioned were swirling patterns of vacancies, crystallographic defects, ion-implantation-induced defects, defects in epitaxial material, and back-lap defects.

Improvements in the measurement of resistivity of bulk silicon and of

diffused and epitaxial silicon layers and in the measurement of resistivity

profiles of these layers were of broad interest. A number of organizations

expressed concern about resistivity inhomogeneities in silicon slices and the

need for adequate methods for measuring them. Concerns were also expressed

about the correction factors used with the spreading resistance method, about

insufficient reproducibility of the four-probe method on a routine basis, and

about the problems associated with the use of the four-probe method for measuring thin epitaxial layers. Those using the capacitance-voltage method for

measuring profiles were generally not satisfied with the method.

Much concern was expressed in most of the organizations about the inadequacy of the methods for measuring junction depth and epitaxial film thickness. The concern is most acute for layers less than about 1 ym thick, which

are now being more widely used. While many use the lap-and-stain method, no

one expressed satisfaction with this method. Complaints about the need to interpolate between fringes and the difficulty of staining, especially for

thin layers, were common. Problems mentioned with nondestructive methods

,

- ; i employing infrared interf erometry involved insufficient sensitivity and precision, inability to make quantitative measurements on epitaxial layers over buried layers because of out-diffusion, and inability to measure epitaxial

layer thicknesses less than about 1 ym.

A strong need was expressed for improved means for characterizing oxides

oxide <= I characterization in MOS devices. Dissatisfaction with the capacitance-voltage method used to test for oxide stability was widespread. A typical comment was that the method could not be used to identify the reason for oxide instability. Some felt

the method to be too slow and others felt it to be insufficiently sensitive to the level of impurities encountered that would have an effect on stability.

Many felt that there should be a standard method for judging oxide stability

by specifying temperature-bias-time stress conditions and the allowed shift

in threshold voltage as a result of these stresses. Fewer expressed a need

to have adequate means of determining the kinds and levels of impurities that can lead to instability.

photolithography Great interest was expressed in the need for improved control of the

photolithographic and associated procedures. There was a general and strong

10

I
need to have a better means of inspecting photomasks. Present visual inspection methods are inadequate because of the time needed to make the inspection,

the operator subjectivity, and the inability to detect all defects. The inability to detect random defects is of particular concern. It is felt that

better inspection methods would significantly improve yield.

Great concern was expressed over the need to improve the methods for

characterizing photoresist materials and photoresist films in use-type tests. Needs varied from knowing what characteristics to measure to how to measure

these characteristics. Also, many were frustrated about the inadequacy of

their methods for calibrating and determining the uniformity of the light

used in photoresist exposure.

On the slice itself there is a desire to detect smaller pin holes (or

defects) and potential pin holes, and to do this by nondestructive means if

possible. Also, an often expressed need was one of being able to even define

what a pin hole is.

Still another problem is the disagreement of the filar eyepiece and the

double image shear methods for measuring oxide window dimensions, line width

and line separation on the mask and slice. Some suggested the need for

length standards that could be used for calibration purposes to resolve the

differences

.

While control of the various assembly procedures is important to yield

and reliability, and processes such as die attachment and. wire bonding con- tinue to be of major importance, the problems with hermeticity testing appear

to be particularly frustrating to many in the industry. Common complaints

are: the lack of agreement between the helium leak detector and the radioisotope methods and the problems that ensue at the vendor-user interface;

the lack of adequate leak-rate standards; and the subjectivity of the

bubble test for detecting large leaks.

There are a number of other problem areas of somewhat less relative

Importance by virtue of the smaller number of times they were mentioned.

These process control measurement problems are in the control of incoming

materials, ion implantation processes, and surface cleanliness.

With regard to incoming materials there is a need for fast, productionline methods to test for impurities in acids, solvents, dopants, etc. In

general, there is the additional problem of not knowing which impurities

may have a potentially harmful effect on device yield and reliability and

•at what concentrations. Maintenance of piece-part quality is also a common

problem.

mask

inspection

photoresists

pin holes

length measurements

assembly

hermeticitj'

three areas

of moderate

concern

incoming

materials

11
ion Of the organizations involved in the use of ion implantation, there is

implantation ^, , j i ^ • ^, i ^ . . i -, -, . the commonly expressed need to improve methods for monitoring and controlling

the number and uniformity of the ions implanted.

Finally, there is no satisfactory method for the production line that can

surface determine if selected localized areas on the device slice are sufficiently

clean and free of residues for metallization deposition or for wire bonding. -cleanliness

other areas The following measurement areas were also mentioned: temperature in epimentioned taxial reactors and diffusion furnaces; gas flow rati 3; metallization and

oxide thickness; phosphorus concentration in glass overcoats; dielectric breakdown characteristics of oxides; characteristics of silicon nitride layers;

wafer warpage; metallization step coverage; and test pattern usage.

12
Appendix 3

Initial Plans for the ARPA/NBS Program

by

W. Murray Bullis

National Bureau of Standards

Washington, D. C. 20234

A preliminary and planning phase of the new program was undertaken early

this year. Principal emphasis during this period was placed on selection of

task areas to be undertaken at the beginning of the program and on formulation

of specific objectives for these areas. This was accomplished through field

visits to over 25 groups in industry, government, universities, and research

institutes; contacts with or visits from more than 10 other such groups; intensive review of ongoing project activities; work with the Quality and Hardness Assurance Subcommittee of ASTM Committee F-1 on Electronics; and review

of recommendations developed in related studies by other organizations, such

as the Navy program on Integrated Circuit Reliability and Manufacturing

The information developed in these various activities was combined with

I past experience to formulate the initial progran task areas. These areas fall

generally into two classes: One class consists of ongoing or new areas in

1 which the needs are well defined and the resouces are on hand or soon to become available so that significant output can be anticipated during the first

(1 year, whether through in-house or contracted efforts. The second class consists of areas where the broad need is apparent but the details are not. In these areas, further definition of the specific work to be undertaken remains

j to be completed. This definition may be done in-house, by contract, or both

' waj's

.

I Specific task areas are listed below together with a brief discussion

f of the approach and an indication of the progress expected to be made during

fiscal year 1974. The ordering of the tasks in the list follows the progression from raw materials to final assembly; it should not be taken as an indijl cation of priority.

preliminary

and planning

phase

formulation

of task areas

specific

task areas

13
resistivity Resistivity; Dopant Characterization — Dopant atoms are intentionally

added to semiconductor materials to control the electrical properties and to form device structures. The resistivity, usually measured by the four-probe

method, which is the basic electrical parameter by which semiconductor materials are specified, is governed directly by the density of dopant atoms. To

assist the industry in further improving the measurement of resistivity,

resistivity standard reference wafers with provisionally certified resistivity values were

standard reference wafers developed, with partial support from the NBS Standard Reference Materials

Program, for sale to the industry. Experimental efforts will be undertaken to

stability of determine the stability of these standards and to establish a basis for the

standards full certification of each standard wafer. This will be a voluntary, cooperative effort with selected owners of standard wafers; the industrial portion

relationship will be undertaken at no cost to the government. Recently considerable interbetween resistivity and dopant est has developed in the industry with regard to updating and improving the

density relationships between resistivity and dopant density particularly in specific

ranges where existing information appears to be suspect. To this end, a re- view of the available data will be undertaken; this may lead to initiation of

a redetermination of resistivity and carrier mobility in silicon as a function

of dopant density later in the year. Knowledge of doping densities in thin

epitaxial, diffused, or implanted semiconductor layers is critical to the control and understanding of device performance. Frequently it is necessary to

impurity profiles know the density profile as a function of position within the layer. Work on

several methods appropriate to these measurements has been underWay; efforts

will be concentrated on evaluating and comparing the spreading resistance and

junction capacitance-voltage methods. To support the above activities an ex- panded mathematical study of the dopant profiles resulting from diffusion

will be continued with emphasis on the redistribution of boron during heat

treatments

.

impurities and Crystal Defects and Contaminants — Other chemical impurities and crystaldcf Gc c s lographic defects can also affect device characteristics. Although these impurities and defects are usually unintentionally introduced during crystal

growth or subsequent wafer processing, they may also be added intentionally

to control material parameters such as minority carrier lifetime and related

device characteristics. Since these impurities and defects do not usually afnew, powerful feet the resistivity, their presence must be detected by other means. Study

methods of new, powerful methods for identifying and counting defects and impurities

in junction structures, based on measurement of thermally or optically stimudefect catalog lated current and capacitance will continue; in addition to research on the

methodology and interpretation, a defect catalog will be generated and simplified instrumentation, suitable for use in a production environment, designed

and constructed. These tools will permit the rapid identification of defects

and contaminants in junction structures on the fabrication line. In addition,

14
several of the more traditional carrier lifetime measurements will be compared

with each other and with the results of the thermally stimulated current and

capacitance measurements, and work will be completed on the energy level model

for gold, an impurity widely used to control minority carrier lifetime, in

silicon. Some of the mathematical modeling effort discussed above will also

relate to the interpretation of experiments in this area.

Oxide ov Other Insulator Films — Stable insulator characteristics are essential to the reliable operation of metal-insulator-semiconductor (MIS)

integrated circuits. A new effort on insulator characterization will begin

with a detailed review of the existing state of the art in control procedures

used during fabrication of MIS devices. At the same time, experimental investigations of methods for determining stability and other characteristics

of insulator films will be initiated and several surface analysis methods

including ion microprobe analysis. Auger spectroscopy, and ESCA will be

studied in cooperation with both the NBS Institute for Materials Research

and the NRL group on radiation-hardened MIS devices; the latter is funded

through DNA as a consequence of the realization that problems in assuring

stable insulator films are particularly acute in the more exotic combinations

used in radiation-hardened MIS devices

.

Test Patterns — Special device configurations are widely used to monitor

process procedures. These configurations are designed so that the measured

electrical characteristics can be interpreted in terms of the desired process

parameters. These patterns may be included on each circuit chip or spotted

at various selected points on the wafer. Earlier this year, a test pattern

was designed and fabricated primarily for use in evaluating in-house processing procedures. This pattern contains 20 individual structures; both

p-n junction and MOS capacitor structures are represented. Although it is

substantially larger than would be appropriate for use in connection with

device production, it will furnish a reference for analysis of test patterns

used in the industry. The first phase of an effort to collect, review, and

analyze test patterns used in the industry, will be completed. In addition,

work on a CCD test vehicle for evaluating certain characteristics of MOS

devices is expected to be funded at the Naval Electronics Laboratory Center

under this program and efforts on test patterns for use in electrical tests

of metallization step coverage will be initiated.

Photolithography — There are a number of problems associated with the use of ultraviolet-sensitive emulsions in the photolithographic steps of

device fabrication. Generally these relate to contamination, application,

exposure, and removal. This problem area will be reviewed in detail and

if appropriate experimental work will be initiated either in-house or by

contract

.

carrier

lifetime

gold in silicon

oxides

characterization

MIS devices

surface analysis

radiationhardened MIS devices

test patterns

CCD test vehicle

photolithograpny

photoresists

15
mask inspection

film and layer

thickness

wafer

inspec ti on

and test

SEM

flying spot scanner

other methods

die and wire

bonding

beam-lead

hermeticity

Length calibrations and automated procedures for locating and identifying

flaws in masks used in the photolithographic processes would be a boon to the

industry. Much effort has been expended by the military and NASA on systems

for accomplishing automated pattern inspection on wafers. It may be that

these systems are too ambitious for the present state of the art; it is hoped

to undertake a contract effort to determine whether, within the present state

of the art, an automated mask inspection system is feasible.

Film and Layer Thickness — Another quantity of critical importance to device performance is the thickness of the various layers and films which com- prise devices. In particular, rapid, noncontacting procedures are needed for

determining thickness of epitaxial layers, oxide films, diffused layers, implanted layers, and the like. This area will be reviewed with the aim of intiating appropriate experimental work later in the program.

Wafer Inspection and Test — Visual inspection procedures have long been

used to screen wafers and individual dice. More recently, inspection by

scanning electron microscopy has been found to reveal in circuits flaws which

cannot be detected by visual inspection. In addition, both the scanning electron microscope (SEM) and a flying spot scanner with a laser source have been

shown to be feasible instruments for use in exercising relatively complex integrated circuits to permit essentially noncontacting testing of circuits or circuit elements. A flying spot scanner will be constructed, tested, and used

in conjunction with an existing SEM to study effects of electron beam induced

damage to circuits inspected or tested with the SEM. Plans for further study

of inspection methods and noncontacting test procedures will be developed in

parallel with this work, probably with out-of-house assistance.

Bonding — Bonding of the die to the header and bonding of the electrical

interconnection leads are critical steps in the assembly of devices. Dissemination of the results of past efforts on ultrasonic aluminum wire bonding,

pull-test procedures, and die attachment evaluation by the thermal response

technique will continue. Development of an in-process monitoring system for

ultrasonic wire bonding will also continue. Facilities for beam-lead bonding

will be established, procedures for testing beam-lead bonds will be reviewed,

and detailed study of evaluation of beam-lead bonds will be initiated if

appropriate.

Hermeticity and Residual Gas Analysis — Tests for detection of both fine

and gross leaks in -hermetic packages have been of concern to the military for some time. The theoretical bases for the analysis of two of the most widely

used methods, helium mass spectrometry and radioisotope, are not in accord.

Further, it would be desirable to have a single, noncontaminating test sensitive to all sizes of leaks. Accordingly a new effort is being established to

16
develop procedures for intercomparison of leak rates; transfer standards;, and,

eventually, Improved test procedures. A further extension of this type of

activity is analysis of the residual gas content in production equipment such

as vacuum evaporation and sealing stations.

Thermal Properties — Thermal characterization of certain types of integrated circuits is essential to assure that they have appropriate power handling capabilities. Electrical tests involving the use of the substrate isolation diode will be investigated to determine their applicability in this area. Also in a related effort, work on high current devices, and r-f power devices

will be undertaken, and study of current crowding effects in power devices

will continue.

Processing Faoility — Some improvements in the in-house processing facility are essential to permit it to be utilized efficiently in providing devices

and test patterns for evaluation of various measurement procedures and in preliminary establishment of the feasibility of selected process control procejdures. Steps are now being taken to upgrade these facilities in order to,

5 reduce background contamination levels to an extent which will permit fabriIcation of low-leakage p-n junctions and satisfactory MOS capacitors and

transistors.

Planning — The general review of process control procedures and collection of information regarding relationships between processes and reliability

will continue for the next few years to assure timeliness and completeness of

the program. This activity will include field visits, literature reviews and

interaction with coordination groups and standards groups. Workshops and

symposia in the early stages of the program will also be used for gaining

additional insight into industry problems. Both in-house and contract

efforts will be employed.

Contract Efforts — In addition to state-of-the-art reviews to assist

in program definition and planning, contract research may be undertaken for

the purposes of development or evaluation of test methods, development or fabrication of test specimens, and development of test instruments. Supporting research activity in selected areas may also be undertaken by contract. As part of the overall effort it is also expected that funds will

be made available for direct ARPA support of related activities in the

ij-various military services; these projects will not come under the direct

: purview of NBS.

Dissemination — initially, the workshops and symposia will be organized

jto emphasize the input of information to the program; later, the emphasis

will shift to dissemination. Other means of dissemination are expected to (include technical publications, direct interactions with industry and

residual

gasanalysis

thermal characterization

Integrated

circuits

power devices

processing

facility

planning

information

input

methods

contract research

information

dissemination

17
government organizations, and interactions with standards conimittees. As

appropriate, artifacts, such as standard reference materials, will be developed to assist in utilization of the measurement technology. Eventually,

measurement assurance programs may be developed to enhance further the utilization of the technology developed.

18
14A (REV. 7-73)

S. DEPT. OF COMM. LIOGRAPHIC DATA

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1. FUnLILAllUN UK KhPUKl NO. NBS-SP-400-3

2. Gov't Accession

No.

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TLE AND SUBTITLE

Semiconductor Measurement Technology

:

ADT5A /\7T5C T.T^-*-!*- V»^-r^ T AKxA/JNiio worKsnop 1. Measurement Problems in Integrated

Circuit Processing and Assembly

5. Publication Date February 197^

6. Performing Organization Code

JTHOR(S)

Harry A. Schafft

8. Performing Organ. Report No.

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B STRACT (A 200-word or less factual summary of most si^ificant information. If document includes a significant

'jibliography or literature survey, mention it here.)

The dual purpose of the workshop was (1) to announce and describe the new

jffort, "Advancement of Reliability, Processing, and Automation for Integrated

jircuits with the National Bureau of Standards," sponsored by the Defense Adjanced Research Projects Agency (ARPA), and (2) to obtain additional input on

ritical measurement problems in integrated circuit processing and assembly to

jssist in planning future work' in the effort. More than 130 engineers repreenting 61 organizations from the electronics industry and government particiated in the workshop. The measurement problems in silicon, oxides, photolithoraphy, and assembly and the problems in information dissemination that were

identified by the participants are summarized. Included as appendices are

jiummaries of two talks given: one which described the results of earlier direct

contacts with a cross section of industrial representatives on major measurement

roblems in integrated circuit processing and assembly and the other which described the initial plans for work in the new effort.

jiEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper

}ame; separated by semicolons)

ie bonding; hermeticity; integrated circuits; measurement methods; microelectronics;

|,xides; photolithography; process control; reliability; semiconductor devices;

ilicon; wire bonding.

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