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ED STATES TMENT OF MERCE

ICATION Ref

NBS TECHNICAL NOTE 592

Methods of

Measurement for

Semiconductor

Materials, Process

Control, and Devices

Quarterly Report

October 1 to December 31, 1970
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—

Electromagnetics3—Time and Frequency3—Laboratory Astrophysics3—Cryogenics3

. THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research lead- ing 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 pro- mote 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 also monitors NBS engineering standards activities and

provides liaison between NBS and national and international engineering standards

bodies. The Institute consists of the following technical divisions and offices: Engineering Standards Services—Weights and Measures—Flammable Fabrics

Invention and Innovation—Vehicle Systems Research—Product Evaluation

Technology—Building Research—Electronic Technology—Technical Analysis

Measurement Engineering.

THE CENTER FOR COMPUTER SCIENCES AND TECHNOLOGY conducts re- search and provides technical services designed to aid Government agencies in improving cost effectiveness 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 Center

consists of the following offices and divisions: Information Processing Standards—Computer Information—Computer Services —Systems Development—Information Processing 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,

and directs the public information activities of the Bureau. The Office consists of the following organizational units: Office of Standard Reference Data—Office of Technical Information and

Publications—Library—Office of Public Information—Office of International

Relations.

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.
|0O

/

'•

UNITED STATES DEPARTMENT OF COMMERCE

Maurice H. Stans, Secretary

NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

NBS TECHNICAL NOTE 592

ISSUED AUGUST 1971

Nat. Bur. Stand. (U.S.), Tech. Note 592, 72 pages (Aug. 1971)

CODEN: NBTNA

Methods of Measurement for Semiconductor

Materials, Process Control, and Devices

Quarterly Report

October 1 to December 31, 1970

Edited by W. Murray Bullis

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 Atomic Support Agency, the U.S. Navy

Strategic Systems Project Office, the U.S. Navy Electronic Systems

Command, the Atomic Energy Commission, and the

National Aeronautics and Space Administration

*< °?<v

"*f«u of

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C, 20402.

(Order by SD Catalog No. C 13.46:592). Price 50 cents.

Stock Number - Cl3. 46:592

CONTENTS

PAGE

Foreword vi

1. Introduction 1

2. Highlights 3

3. Methods of Measurement for Semiconductor Materials

3.1. Resistivity 8 3.2. Carrier Lifetime 14

3.3. Inhomogeneities 17

3.4. Gold-Doped Silicon 18

3.5. Specification of Germanium 21

3.6. References 27

4. Methods of Measurement for Semiconductor Process Control

4.1. Metallization Evaluation 29

4.2. Die Attachment Evaluation 32

4.3. Wire Bond Evaluation 34

4.4. Processing Facility 45

4.5. References 48

5. Methods of Measurement for Semiconductor Devices

5.1. Thermal Properties of Devices 49

5.2. Thermographic Measurements 56

5.3. Microwave Device Measurements 57

5.4. Carrier Transport in Junction Devices 59

5.5. Silicon Nuclear Radiation Detectors 60

5.6. References 62

Appendix A. Joint Program Staff 63

Appendix B. Committee Activities 64

Appendix C. Solid-State Technology £ Fabrication Services .... 66

Appendix D. Joint Program Publications 67

in
LIST OF FIGURES

PAGE

1. Gold concentration as determined by neutron activation

analysis as a function of diffusion time for 10- and 20-ft»cm,

boron-doped, gold-diffused, silicon wafers 20

2. Oxygen concentration, Nq, obtained from lithium precipitation

(•) and infrared absorption measurements (O) plotted against

Nq obtained from lithium mobility measurements 23

3. Infrared response (IRR) spectra obtained using a 3-mm thick

germanium filter 24

4. IRR spectra in the incident photon energy range of 0.30 to

0.70 eV obtained using a 1-mm thick germanium filter 26

5. Transient thermal response curve for a typical mesa diode on

a 0.95-mm square chip mounted on a TO-5 header 33

6. Laser system set-up for calibrating ultrasonic bonding tool

vibration amplitude 38

7. A comparison between the output of a microphone and a laser

interferometer simultaneously monitoring the same loose

bonding tool 38

8. Schematic representation of mixer system for monitoring ultrasonic bond formation 40

9. Measurement of the self-induced vibrations of a bonding

machine 42

10. Improved electromagnetic displacement sensor and wire indentation tester 44

11. Electron-beam evaporation system 45

12. Schematic representation of silicon dioxide vapor deposition

system ' 46

13. Apparatus for measuring dynamics of photoresist spinners ... 46

14. Base current as a function of collector-emitter voltage for

a 35-W, triple-diffused, silicon transistor 50

15. Base current as a function of collector-emitter voltage for

a 20-W, triple-diffused, silicon power transistor 51

IV
16. Data used to calculate the thermal resistance of a 35-W,

triple-diffused, silicon transistor as a function of applied

power 52

17. Common-emitter current gain, hpjr, and thermal resistance,

Rq, of a 35-W, triple-duffused, silicon transistor as a function of applied power, P 54

18. Thermal resistance, RQ

, as a function of case temperature, Tq,

for a 35-W, epitaxial-diffused, silicon transistor 54

19. Spatial resolution of fiber optic probes compared by scanning

a black-white boundary on a Microcopy Resolution Test Chart

and recording the changes in photometer output 56

LIST OF TABLES

1. Thickness Correction Factor, F(w/S) 11

2. Results of Scratch Tests on a 1.0-ym Thick Aluminum Film

with an 18-ym Diamond Stylus 31
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 care- fully 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 Atomic Support Agency," the U. S. Navy Strategic Systems Project

Office, § the U. S. Navy Electronics Systems Command, the Atomic Energy

Commission, # and the National Aeronautics and Space Administration. x There is not a one-to-one correspondence between the tasks described in

this report and the projects by which the Program is supported. Although

all sponsors subscribe to the need for the entire basic program for improvement of measurement methods for semiconductor materials, process

control, and devices, the concern of certain sponsors with specific parts

of the Program is reflected in planning and conduct of the work.

t Through Research and Technical Services Projects 4251120, 4251123,

4251126, 4252114, 4252119, 4252128, 4254111, 4254112, and 4254115.

* Through Order EA071-801. (NBS Project 4259522).

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

Project Orders PO-1-0030 and PO-1-0041. (NBS Project 4259533)

+ Through Project Order PO-1-1057. (NBS Project 4252534)

# Division of Biology and Medicine. (NBS Project 4259425)

x Through Orders S-70003-G, Goddard Space Flight Center, and H-76553A,

Marshall Space Flight Center. (NBS Projects 4254429 and 4251449)

VI
METHODS OF MEASUREMENT FOR

SEMICONDUCTOR MATERIALS, PROCESS CONTROL, AND DEVICES

Quarterly Report

October 1 to December 31, 1970

ABSTRACT

This quarterly progress report, tenth 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 successful application of the surface

photovoltage technique, a non-contacting method, to the measurement of carrier diffusion length in silicon epitaxial layers

and development of a novel, but simple, method for measurement

of acceleration and terminal angular velocity of a photoresist

spinner. Work is continuing on measurement of resistivity,

carrier lifetime, and electrical inhomogeneities in semiconductor crystals; specification of germanium for gamma-ray detectors; evaluation of wire bonds, metallization adhesion, and

die attachment; measurement of thermal properties of semiconductor devices, transit-time and related carrier transport

properties in junction devices, and electrical properties of

microwave devices; and characterization of silicon nuclear

radiation detectors. Supplementary data concerning staff,

standards committee activities, technical services, and publications are included as appendixes.

Key Words: Alpha-particle detectors; aluminum wire; base

transit time; carrier lifetime; die attachment; electrical

properties; epitaxial silicon; gamma-ray detectors; germanium;

gold-doped silicon; metallization; methods of measurement;

microelectronics; microwave devices; nuclear radiation detectors; probe techniques (a-c); resistivity; semiconductor devices; semiconductor materials; semiconductor process control;

silicon; thermal resistance; thermographic measurements; ultrasonic bonder; wire bonds.

1. INTRODUCTION

This is the tenth 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 re- ported here are subject to modification and refinement.
Fourteen tasks , each directed toward a particular material or device property or measurement technique, have been identified as parts

of the Program. The 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.

An important part of the work which 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 efforts in this area are given in Section 2.

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 con- cerning 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.

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 that describe some aspect of the program in

greater detail are prepared. Current publications are also listed in

Appendix D.
2. HIGHLIGHTS

Highlights of the technical activity during this reporting period

are presented in this section; details are given in subsequent sections

of the report.

Resistivity — The study of current and probe force dependence of

resistivity as measured by the four-probe method was interrupted while

several changes in the experimental procedure were evaluated. It was observed that the sensitivity of the four-probe apparatus to mechanical

vibrations is lowest when the line of probe points is perpendicular to

the supporting boom. A simplified computation procedure for the correction factor appropriate to four-probe resistivity measurements on wafers

with finite thickness was developed in a form suitable for use on a programmable desk calculator. Investigation of temperature coefficient data

for silicon showed that the published coefficients can be used to correct

resistivity measurements to any referenced temperature in the range 20 to

25°C with the same limitations that apply to correction to the standard

reference temperature, 23°C. Development work on procedures for examining probe damage in silicon with the scanning electron microscope is con- tinuing in connection with work on the spreading resistance method. A

change in the diffusion procedure has enabled diodes with more abrupt

junctions and higher breakdown voltages to be fabricated. Although preliminary capacitance-voltage measurements were made on some of these

diodes, the analysis was not completed.

Carrier Lifetime — Increased emphasis was placed on the study of the

photoconductive decay method for measuring bulk carrier lifetime. The

dependence of the measured value of lifetime on signal level, applied

electric field, chopped-light turn-off time, and series resistance was established. An experiment to determine the relative single laboratory

precision of the time-mark method and the null method for measuring photoconductive decay time was begun. The measuring circuit for the metaloxide-semiconductor capacitance method for measuring carrier lifetime in

epitaxial layers was calibrated, and initial measurements were carried

out.

Previously observed variability in the measurement of the surface

photovoltage was greatly reduced by shielding the specimen and thermopile

from ambient room light. This new, non-contacting technique was successfully applied to the measurement of carrier diffusion length in silicon

epitaxial layers.

Irihomogeneities —Correlation studies between photovoltaic, fourprobe, and two-probe resistivity profiles were continued. An investigation into the various factors which may affect precision and accuracy of

the profiles determined by each of the three measurement methods was initiated.
HIGHLIGHTS

Gold-Doped Silicon — The effect of gold accumulated at the rim of

gold-diffused wafers on the determination of concentration by neutron

activation analysis was established. Rim portions of wafers diffused at

temperatures below 1250°C showed erroneously high concentrations. Borondoped silicon wafers diffused in an oxygen atmosphere showed higher con- centrations of gold than similar wafers diffused in an argon atmosphere.

Several relatively high resistivity p-type specimens showed the same un- explained decrease in resistivity with increasing gold concentration at

high gold concentrations that had been observed earlier elsewhere.

Specification of Germanium — Analysis of the data obtained in the

comparison study of the lithium mobility and the lithium precipitation

methods for determining oxygen concentration in germanium has been com- pleted. The improved infrared response technique was used to examine

several additional lithium-drifted germanium diodes. The results show

that this technique is capable of detecting impurities such as copper

and identifying them by means of their observed energy levels. It can detect such impurities at concentrations below 10 ll+ cm*" 3 and appears to be

more sensitive than other methods which have been used.

Metallization Evaluation — Study of the scratch test was extended to

the case of thicker aluminum films deposited on quartz substrates. Some

differences in failure characteristics were observed, but they could not

be attributed unambiguously to the change in thickness. Preliminary

study of methods for detecting threshold adhesion failure in the aluminumsilicon dioxide-silicon system was begun.

Die Attachment Evaluation — Increased emphasis was placed on this

area during this reporting period. Detailed studies of the reproducibility of the thermal response curve for batches of diodes were initiated.

The reproducibility achieved in the first experiments suggests that the

measurement of thermal response after a heating power pulse a few milliseconds long is a sensitive indicator of die attachment quality. Development of procedures for bonding diodes with voids of controlled area

and location in the die attachment continued.

Wire Bond Evaluation — Study of the effect of pull rate on the pull

test revealed that, when the failure occurred at the heel of the first

bond, pulling at a rate equivalent to the "jerk" test used by some device

manufacturers gives results statistically equivalent to those obtained by

pulling at the much slower rates previously studied. The importance of

specifying all details of the pull test including the wire bond failure

mode has become apparent.

Tests carried out to simulate the high temperature environment en- countered by bonds during glass sealing operations for ceramic packages

showed that the reduction in pull strength can be accounted for principally by the reduction in wire tensile strength caused by the high temperature anneal.
HIGHLIGHTS

Improvements in the laser mount for interferometry measurements

were made to increase the ease of optical alignment and use of the interferometer on bonding machines. The length of the laser was increased in

order to limit all emission to the TEM nn mode.

New experiments directed toward development of measurement methods

applicable to in-process bond quality determination were started. The

initial effort consisted of electronically mixing the 60-kHz output

from the capacitor microphone directed at the bonding tool with a local

oscillator. Preliminary results showed that the bonding process can be

heard through an audio speaker and recorded on magnetic tape. Harmonics

of the tool motion can be individually treated in the same manner. It

is expected that the harmonic characteristics can be correlated with bond

quality.

An improved electromagnetic displacement sensor was designed and

built for the wire indentation tester which is nearing completion.

Processing Facility — Installation of a 10-kW electron beam evaporation system for the deposition of aluminum was completed. A system was built to deposit vitreous silicon dioxide films from silane so that silicon dioxide layers may be produced at low temperatures.

A novel, but simple, method was developed for the measurement of the

acceleration and terminal angular velocity of a photoresist spinner.

Thermal Properties of Devices — Work continued on the preparation of

a suitable measurement procedure and data collection format for the preliminary thermal resistance round robin being conducted by JEDEC Committee

JC-25 on Power Transistors.

It was found that the increase in the thermal resistance which is

sometimes observed as case temperature increases can be related to the

increase in thermal resistivity of the silicon chip with temperature.

The results of a study undertaken to determine the effect of variations

in device case temperature on the voltage at which a hot spot was formed

suggest that hot-spot formation is governed by the magnitude of a localized temperature increase in the transistor rather than by the absolute

temperature of the chip. This result appears to be consistent with a previously proposed model for thermal hysteresis.

Microwave Device Measurements — The audio i-f portion of the X-band

mixer diode measurement system was built and tested. A measurement technique was developed to enable the mixer i-f resistance to be read directly

from a calibrated precision decade resistance box. A feedback loop was constructed to improve the amplitude stability of the local oscillator

in the r-f portion of the X-band system.
HIGHLIGHTS

Carrier Transport in Junction Devices — Construction, initial testing, and adjustment of the Sandia-type delay-time bridge were completed.

A preliminary analysis of the bridge showed that the delay time read from

the line stretchers can be made to be independent of mismatch between the

line stretchers and the transistor being measured. Field and literature

surveys of both delay time measurement and probing techniques were con- tinued.

Silicon Nuclear Radiation Detectors — Preflight bench-testing of

lithium-drifted silicon radiation detectors continued. Initial experiments were conducted in the ambient exposure test program to determine

the effects of hazardous ambients on the performance characteristics of

lithium-drifted and surface-barrier silicon detectors. Eight lithiumdrifted silicon detectors were acquired for radiation damage experiments.

Standardization Activities — Many of the standardization activities

undertaken by program staff members are broader than the technical tasks

described in the following sections. These activities involve both co- ordination of efforts which may encompass a variety of tasks and participation in areas where no direct technical effort is presently underway.

W. M. Bullis and D. E. Sawyer attended an intersociety meeting on Need for Standards in Radiation Effects on Electronic Parts, Materials,

and Devices organized by R. S. Shane of American Society for Testing and

Materials (ASTM) Committee E-10 on Radioisotopes and Radiation Effects

in October. The Institute of Electrical and Electronics Engineers (IEEE),

Society of Automotive Engineers (SAE J, Americal Institute of Aeronautics

and Astronautics (AIAA), American Nuclear Society (ANS), and ASTM were represented in addition to various government agencies. At this meeting

the various requirements for standard test methods, nomenclature, formats,

and specifications in this field were discussed and various areas were referred to the groups represented for appropriate action. The development of test methods appropriate to electronic materials and devices was referred to both IEEE and ASTM. Program staff members are active participants in all appropriate groups in these societies.

W. M. Bullis presented an invited discussion of standardization

efforts in the integrated circuit field at the fall meeting of SAE Committee H on Electronic Materials and Processes. This committee is presently undergoing reorganization in an effort to focus appropriate attention on the development of specifications of parts and materials for

integrated circuits

.

At the invitation of the chairman, J. C. French attended a meeting

of SAE Subcommittee A-2N on Radiation Hardening and Nuclear Survivability.

He was asked to provide continuing liaison to the subcommittee in regard

to test method development in ASTM Committee F-l.
HIGHLIGHTS

Mrs. K. 0. Leedy organized the Washington Area Scanning Electron

Microscopists in order to provide a medium of exchange for local workers

in this important diagnostic field. The organization has over 80 members

and conducts monthly meetings arranged around a technical presentation in

the field.

The reorganization of the Electronic Industries Association Committees on Semiconductor Devices (JEDEC) and Microelectronics (MED) into

the Solid State Division has caused some rearrangement of activity by

Program staff members. Participation in the new committee structure is

indicated in Appendix B. F. F. Oettinger has been requested to chair a Task Group on Microelectronics Thermal Considerations of Subcommittee 3 on Microelectronic Devices of Committee JC-11 on Mechanical Standardization. Other activities directly related to thermal resistance measurements are reported in Section 4.1.

Although there was no meeting of ASTM Committee F-l during this

quarter, considerable round robin and editorial review activity continued

in a variety of areas.
3. METHODS OF MEASUREMENT FOR 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 : Data collection activity on the study of current and

probe force dependence of resistivity as measured by the four-probe

method was interrupted while several changes in the experimental procedure were evaluated. A simplified computation procedure for the correction factor for finite wafer thickness was developed in a form suitable

for use on a programmable desk calculator. Investigation of temperature

coefficient data for silicon showed that the published coefficients can

be used to correct resistivity measurements to any reference temperature

in the range 20 to 25°C from a measurement temperature up to 5°C different from the reference temperature. Work on the spreading resistance

method has been concentrated on development of procedures for examining

probe damage in silicon with the scanning electron microscope. Diodes

with more abrupt junctions and higher breakdown voltages have been fabricated and studied by the capacitance-voltage method.

Four-Probe Method — The study of current and probe force dependence

of resistivity as measured by the four-probe method on mechanically

polished slices is being delayed pending investigation of certain changes

in experimental procedure. The object of this study has been to distinguish changes in measured resistivity with change in probe force, current

level, or surface preparation at or near the one-percent level. Difficulties in the statistical analysis of measurements previously made on wafers with lapped surfaces could be attributed to the fact that for a number of measurements the average resistivity or measured precision was

too far outside the consensus for the wafer. A plot of average resistivity against standard deviation was made for all data taken on each

wafer. The data for each wafer which included all combinations of cur- rent level and probe loading used in the test (NBS Tech. Note 555, pp.

6-7) tended to cluster except for a few scattered points. In general,

data taken at the intermediate probe load of 50 g produced the largest

number of data points lying outside the cluster. The cause of this observation has not yet been identified.

Heretofore measurements at each probe loading have been made with a different four-probe head. Because the probe point quality might vary

from head to head, this has introduced an extra variable into the procedure. A short investigation showed that probe springs can be preset

for a given probe head and reproducibly inserted when needed if care is

taken to lock the adjustment screws in place and if small shims are added

to the probe head so that each spring block can be repeatably mounted at
RESISTIVITY

the same height within the probe head. With the use of these procedures

the same probe head and probe point set can be used for all future meas- urements .

Microscopic examination of polished wafers after a series of meas- urements indicated a considerably larger probability of placing the probe

points in previously damaged spots than had been anticipated. Earlier

work on lapped surfaces had shown an increased scatter of data when measurements are taken in previously damaged locations. These two observations suggest that data should be taken at all current values before

changing wafer orientation rather than at a single current value for all

wafer orientations as has been done so far. Several specimens are being

investigated to determine the effect on precision if the data-taking

sequence is modified in this way. (F. H. Brewer and D. R. Ricks)

During the course of the investigation, some unreasonably random

data were encountered on a 100-ft* cm p-type slice, A subsequent check

disclosed that the four-probe apparatus is quite sensitive to vibrations,

both transmitted through the laboratory and air-borne, such as voice.

This microphonic signal is directly proportional to the current level

used for measurement of any one wafer; for equal current levels on different wafers it is proportional to the power supply compliance voltage.

This microphonic signal was observed on all wafers with resistivity above

0.01 ft*cm. It is believed that vibration results in oscillation of the

probe point and of its associated contact resistance to the wafer and

that the output of the power supply then oscillates as it tries to maintain constant current. Indeed, current oscillations were measured as a result of these fluctuations. Spurious signals as large as 5 V could be

induced across the current probe points by footfalls near the equipment

bench when operating at a 150-V compliance voltage on 100-Q*cm material.

This produced as much as 60 mV of spurious normal mode signal with a distorted, modulated 60-Hz wave form at the voltage measuring probes. An erroneous measurement of slice resistivity may result from such a signal

since it may not be fully rejected by the digital voltmeter. The use of

acoustic padding under the probe stand and a vacuum chuck to hold the

wafer reduced the "noise" by about 40 percent. It was also observed that

changing the orientation of the probe head in its clamp could also result

in about a 50-percent change in the susceptibility of the probe to vibrations. An orientation with the line of probe points transverse to the

axis of the supporting boom was found to be preferable. (J. R. Ehrstein)

Correction Factor for Finite Thickness — The resistivity p of a thin

sheet of material of thickness w as measured by a collinear four-probe

array of spacing S is

P

= I

' W ' IH2 ' F(w/§) (1)
RESISTIVITY

where I is the current passing through the outer probes and V is the potential difference between the inner probes. The correction factor

F(w/S) has been calculated by Smits [1] using methods developed by Uhlir

[2]. If the sheet is surrounded by insulating media

F(w/S) = 2 In 2 • - • ^—

(2) W

G 7 (w/S)

where [3]

00 G 7 (w/S) = 1 + 2 I {[(1/4) + (nw/S) 2 ]"^ - [1 + (nw/S

)

2 ]"^} . (3)

n=l

For a thin wafer, the summation in GyCw/S) converges very slowly; great

care must be taken and multiple precision arithmetic must be used to prevent a significant accumulation of round-off errors.

The problems of subtracting two nearly equal terms can be avoided

by expanding each of the terms in a binomial series and performing the

subtraction before computing the terms . In this case

n , Wx v r 3 r$ , 3 45 ,S x 5 315 ,S , 7

-. ,, *

S n=l

Convergence here requires that nw > S; if nw < 2S, additional terms in

the binomial expansion are required for satisfactory precision.

In calculating the functions from which he computed his correction

factors, Uhlir [2] suggested switching from one series to the other at an appropriate value for n. This technique is satisfactory in the present case if the switch is made after M terms where M is the smallest

value of n for which the inequality n > 2S/w applies. The series (4) is

truncated after N terms where the Nth term is required to have a value

less than a specified residual. Then eq (3) becomes:

M

I n=l

M 9 1 7

(2L) = i + 2 I {[i t (2L) ]-* - [i + (SL)*!*)

+ I d (—

)

3 - 1| (—

)

5 + 1# (—) ]. (5)

n=M+l

'4 nw 64 nw 512 nw

The computation may be carried out easily on a programmable desk

calculator or larger computer. The results are given in table 1. The

values quoted by Smits [1] are listed in column 2 while interpolated

values [4] for even tenths are listed in column 3. Results of the present

10
RESISTIVITY

calculation agree to within 1 part in 10 4 with those of Smits except for

w/S =1. In this calculation the Nth term was required to be smaller

than 10- 5

.

Table 1 -Thickness Correction Factor, F(w/5)

w/S Smits' [1]

F(w/S)

F84 [4]

F(w/§)

Present Calculation

M N F(w/5)

0.1 1.0000 21 422 1.0000

0.2 1.0000 11 211 1.0001

0.4 0.9995 6 106 0.9996

0.5 0.9974 0.997 5 85 0.9975

0.5555 0.9948 4 76 0.9949

0.6 0.992 4 71 0.9920

0.625 0.9898 4 68 0.9899

0.7 0.982 3 61 0.9818

0.7143 0.9798 3 60 0.9799

0.8 0.966 3 53 0.9664

0.8333 0.9600 3 51 0.9601

0.9 0.944 3 47 0.9460

1.0 0.9214 0.921 3 43 0.9216

The same results are obtained (to within 3 parts in 10 5

) and the same number of terms are required if M is held constant at 20. Exact

agreement with Smits is achieved if the condition that the residual be

smaller than 10 -6 is imposed but this more than doubles the number of

terms in the series (4); the calculation is degraded if the seventh order

terms are not used or if a residual larger than 10" 5 is permitted.

For a finite specimen, the overall correction is estimated by

multiplying together correction factors for the thickness (derived on the

basis of an inf initesimally thin sheet of finite diameter). In view of

the second-order error inherent in this procedure, refinement of the calculation for G7CW/S) beyond that given in eq (5) with a residual of 10~ 5 does not seem justified. (W. E. Phillips)

11
RESISTIVITY

Temperature Coefficient of Resistivity — The applicability of previously calculated temperature coefficients [5] to correcting silicon re- sistivity data to reference temperatures other than 23°C was investigated

at the request of the Resistivity Section of ASTM Committee F-l. From

the data taken previously [5] on n-type and boron-doped silicon wafers,

values of resistivity were calculated for each wafer at 1°C intervals for

wafer temperatures from 15 to 30°C inclusive. These values were then cor- rected to 20, 23, and 25°C with the use of 1) the temperature coefficient

experimentally determined for the wafers and 2) the temperature coefficient computed from the polynomial fit (NBS Tech. Note 560, pp. 6-8) to

the temperature coefficient-resistivity curve. The corrected values were

then compared with the calculated values derived from the data.

In all cases the corrected values of resistivity computed from the

experimentally determined temperature coefficient for that wafer were

within 0.1 percent of the calculated value if the difference between

wafer temperature and reference temperature did not exceed 5°C. Similar

results were found when the temperature coefficient computed from the

polynomial fit was used. In the few cases where greater differences were

observed between resistivities derived from the experimentally determined

and computed coefficients , the errors introduced for correction to reference temperatures of 20 and 25°C were not significantly different from

those for correction to a reference temperature of 23°C. In all of these

cases the experimentally derived temperature coefficient did not fall on the smooth curve joining most of the data points.

From these results it was concluded that the temperature coefficient

data derived for a reference temperature of 23°C could be used to correct

resistivity measurements to 20 and 25°C without significantly reducing

the precision of the measurement provided that the wafer temperature

(during measurement) did not differ from the reference temperature by

more than 5°C. (W. M. Bullis)

Standardization Activities — Data has been received and analyzed

from six of the eight participants in the round robin sponsored by Committee F-l to measure the resistivity of ultra-high resistivity silicon

by the four-probe method. As expected, the greatest interlaboratory

variation occured in general for the specimens which had the highest re- ported resistivity values. One laboratory had particular difficulty with

p-type specimens, measuring well below the multi-laboratory average on all three p-type specimens. Difficulty with measuring the conductivity

type of the specimens was encountered only on the specimen which had the

highest average resistivity of all specimens considered. For this specimen, two laboratories measured both n- and p-type character.

Additional work done on this particular specimen indicates an ex- treme dependence of measured resistivity upon the method of surface

cleaning and the time lapse between cleaning and measurement. Average

12
RESISTIVITY

resistivities between 9100 and 18400 ft* cm were obtained after various

cleaning procedures. Details of the cause of this variability have not

yet been established.

Data have been received and analyzed from six of the twelve participating laboratories in the round robin to measure the resistivity of

epitaxial layers on opposite conductivity type substrates by the fourprobe method. (F. H. Brewer)

Spreading Resistance Methods — The study of spreading resistance con- tacts has concentrated on use of the scanning electron microscope to ex- amine probe damage in silicon. Although some high quality electron microscope pictures have been made on silicon surfaces after the surface re- ceived a gold deposition or was alloyed with gold, the results have not

been fully reproducible. Because of lack of understanding of the re- lation between the conditions of probe lowering and the quality of the

resulting spreading resistance- contact, a search is being made for a suitable piezoelectric or other form of transducer that can be used to measure

the impact momentum of the spreading resistance probe so that this quantity can be related to specimen surface damage and electrical stability

of the contact. (J. R. Ehrstein)

Capacitance-Voltage Method — Diodes with more abrupt junctions than were attainable with previous procedures have been obtained by using a

3 h single-step boron diffusion at 1050°C. With this diffusion procedure,

junction depth was increased from a planned value of 1 \xm to about 2 urn, and breakdown voltages up to 40 V and 60 V, respectively, were obtained

for 0.76-mm and . 5-mm diameter diodes fabricated in nominal l-fi'cm,

n-type silicon.

Capacitance measurements on these wafers are limited to diodes with

high breakdown voltages. A comparison of the resistivities of three of

the diffused n-type wafers as measured by the four-probe and capacitancevoltage (C-V) methods showed the C-V resistivity values to be consistently

higher. The best agreement between four-probe and C-V resistivity meas- urements was obtained on the wafer that had the smallest radial resistivity variation. (G. N. Stenbakken and T. F. Leedy)

Plans: The evaluation of the effects of changes in the experimental

procedure for the current and probe force dependence of resistivity as measured by the four-probe method will be completed and appropriate

changes made. Data collection on mechanically polished wafers will be

resumed. When the wafers from the ultra-high resistivity round robin are returned, work will begin on a comprehensive study of the effect of specimen surface preparation on the resistivity measurement of these wafers.

Data from this and the epitaxial resistivity round robin will be com- puted and tabulated as received. Because of difficulties encountered in

the resistivity inhomogeneity study (see Section 3.3) when wafers are

13
RESISTIVITY

mis centered by only 100 ym or so during a four-probe resistivity profile,

a mechanical wafer centering apparatus will be built.

A detailed study will be made on the necessary treatment to get re- peatable, high-resolution SEM photomicrographs of probe damage in silicon.

It will include a range of silicon resistivities of both conductivity

types with both evaporated coatings and alloyed surfaces . Work will con- tinue on finding a satisfactory transducer for measuring the impact mo- mentum of the spreading resistance probe under various experimental situations. Results of this work will be related to the electrical stability

of the contacts

.

Four-probe resistivity profiles will be repeated on previously diffused l-°,*cm wafers to investigate whether the long diffusion times have

in any way altered the radial resistivity profile on the original material. Diodes will be fabricated on additional l-fl'cm, n-type wafers with

more uniform resistivity profiles than wafers previously used. Diodes

will also be fabricated in 15-fl«cm, float zoned, n-type wafers. These

two groups of wafers will be used to investigate the diagreement between

C-V and four-probe measurements of resistivity already observed for dependence on resistivity level and effect of resistivity profile.

3.2. CARRIER LIFETIME

Objective; To determine the fundamental limitations on the precision

and applicability of the photoconductive decay method for measuring minority carrier lifetime and to develop alternative methods for measuring

minority carrier lifetime in germanium and silicon which are more precise,

more convenient, or more meaningful in the specification of material for

device purposes.

Progress: Increased emphasis was placed on the study of the photoconductive decay method for measuring bulk carrier lifetime. The dependence of the measured value of lifetime on signal level, applied

electric field, chopped-light turn-off time, and series resistance was established. An experiment to determine the relative single laboratory

precision of the time-mark method and the null method for measuring photoconductive decay time was begun. The measuring circuit for the metaloxide-semiconductor capacitance method for measuring carrier lifetime in

epitaxial layers was calibrated, and initial measurements were carried

out

.

Previously observed variability in the measurement of the surface

photovoltage was greatly reduced by shielding the specimen and thermopile

from ambient room light. This new, non-contacting technique was success- fully applied to the measurement of carrier diffusion length in silicon

epitaxial layers

.

14
CARRIER LIFETIME

Bulk Crystals — To expedite the revision of the PCD method, increased emphasis was placed on this measurement. A group of experiments

was designed to study the dependence of the measured value of lifetime on PCD signal level, applied electric field, chopped-light turn-off time,

and series resistance. The objective of these experiments was to accu- mulate data to determine appropriate test conditions for the revised PCD

procedure, including methods of selection of the specimen current and

signal level.

The decay time was studied as a function of the magnitude of the

series resistance to determine a lower limit of the series resistance

.

For a group of four samples, decay time was measured for different values

of the ratio of the series resistance to the specimen resistance (R /R)

in the range . 1 to 20 . It was found that (1) the signal level decreased

as Rs /R decreased, (2) the decay times measured at low Rs /R differed from

those measured at high Rs /R s and (3) the magnitude of the standard deviation of the measurement varied randomly over the range of R /R despite

the lower signal-to-noise ratio at low R

s /R. To ensure that the measured

decay time is independent of the series resistance, a series resistance

at least ten times the specimen resistance must be employed.

The dependence of the measured decay time on the turn-off time of

the chopped light was studied to determine an upper limit for the turn-off

time in terms of the ratio of turn-off time to PCD lifetime (tgrr/ippn).

Five samples were measured for different values of t ff/ipcp in the range

one to four. It was found that for ^ ff/T-prr) < ^, the measured decay time

was approximately constant and equal to the PCD lifetime. To avoid influencing of the result by the time-off time, measurements must be made

in this range. A transition occurred for t ff/ippj) between 2 and 3; for

^off/^prT) > 3, the measured decay time increased with increasing turn-off

time. For the conditions of measurement the transition point was calculated to be between 3.9 and 4. The discrepancy between the calculated

and observed values is attributed to lack of sharpness in the turn-off

characteristic of the light.

To study the effect of sweep-out, photoconductivity scans of eight

specimens were made on the apparatus developed for studies of resistivity

inhomogeneity (see Section 3.3). Since the photoconductivity is proportional to the lifetime, the fall of the photoconductivity near the end

contacts is a measure of sweep-out. The electric field was normally

selected to produce a drift length of about 1 mm. It was found that

sweep-out always occurred within five drift lengths of the end contacts.

Thus, masking the specimen so that the region within five drift lengths of

the contacts is not illuminated is expected to avoid sweep-out.

The lifetime is a function of the injected carrier density, which

can be related to the signal level. For a group of six samples, lifetime

was measured at various signal levels. For these lifetime measurements

15
CARRIER LIFETIME

the relationship of the standard deviation to the signal level was studied.

These data showed a clearcut difficulty in making precise lifetime meas- urements at lower signal levels, but a method for specification of the

appropriate signal level has not yet been formulated.

Since sweep-out, ohmic heating of the specimen, current level, and

signal level are interrelated, an experiment to study the effect of sam- ple heating was initiated. The dependence of specimen temperature on the

specimen current is being investigated for a group of fourteen samples.

These samples cover the resistivity range 1 to 1500 ft«cm.

To determine the relative precision of two methods for measuring the

specimen voltage decay time two operators are measuring a single sample

by both methods for an extended period. The first method consists of

matching the PCD curve to an exponential curve inscribed on the oscilloscope graticule. The time constant of the matched curve is measured with

a time-mark generator. In the second method, a differential oscilloscope

input is used to match the PCD curve with an electronically-generated

exponential curve. When the time constants of the two curves are equal,

a null in the form of a straight line is formed. This experiment is still

in progress. (A. J. Baroody and R. L. Mattis

)

Epitaxial Layers — The measuring circuit for the MOS capacitance

method was constructed, calibration was carried out, and initial lifetime

measurements were begun. A computer program was written so that the transient capacitance data could be analyzed by the methods of Zerbst [1],

Heiman [2], and Salama and Holmes [3], Whereas the lifetime values calculated by the methods of Zerbst [1] and Salama and Holmes [3] were in

agreement, the values calculated by the method of Heiman [2] differed

from the others. This prompted a reexamination of the theory. It was found that the methods of Zerbst [1] and Heiman [2] are not entirely

mutually consistent as suggested earlier (NBS Tech. Note 560, p. 12).

Although the method of Zerbst [1] reduces to the method of Heiman [2] for

the case in which surface charge generation is negligible, the two methods

give different lifetime values when this condition is not met.

The first measurement was made on a l-fi'cm, n-type silicon wafer

with an array of 1.25-mm diameter aluminum dots evaporated onto a 0.12-um

thick oxide layer. The capacitance vs. time data were fitted to polynomials of several orders, and the best polynomial fit was used to derive

the lifetime value. It was found that the lifetime value depends on the

bias voltages from which and to which the device is switched to produce

the capacitance transient. This dependence is strongest in very shallow

inversion. (R. L. Mattis)

The SPV equipment was modified to shield the specimen and the

thermopile from ambient room light. Preliminary measurements indicate

that the previously reported variability in the SPV measurements has been

greatly reduced.

16
CARRIER LIFETIME

Analysis of the SPV method [4, 5] was extended to the case of an epitaxial layer on a thick substrate. The general boundary conditions

[6, 7] at the epi-substrate interface simplify to a Boltzmann equation for

n-on-n+ or p-on-p+ layers; the other boundary conditions are unchanged.

The calculated expression for the SPV plot yields an approximately linear

dependence of intensity on reciprocal absorption coefficient. For epitaxial layers thicker than several diffusion lengths, the results are the same as for thick specimens and the intercept gives a measure of the

diffusion length in the epitaxial layer. For very thin epitaxial layers,

less than half the diffusion length in the substrate, the intercept is

equal to the diffusion length in the substrate. For layer thickness intermediate between these limits , the intercept is influenced by the layer

thickness as well as the diffusion lengths both in the epitaxial layer

and in the substrate. In principle, in this intermediate region the

diffusion length in the epitaxial layer could be obtained from SPV measurements on both sides of the wafer. However, at present, the precision

of the method is not adequate for such a determination. (W. E. Phillips)

Plans : Revision of the PCD procedure will continue. The data from

the sweep-out, signal level, and current dependence studies will be evaluated to determine specifications for specimen current and signal level.

The precision comparison between the time mark and the null methods of

determining the specimen voltage decay time will continue. To compare

these two techniques over a range of lifetime, a group of samples will be

measured with both techniques.

Several specimens, including bulk and epitaxial specimens, will be

measured by the MOS capacitance method. Agreement of repeated measurements will be evaluated for one or two specimens. Lifetime values derived

from different polynomial fits to the same capacitance vs. time data will

be compared. Lifetime values from two devices from different locations

on the same wafer will be compared.

Additional SPV measurements will be made on other epitaxial layers

to extend the experimental verification of the analysis. Also, SPV measurements will be made on several single-crystal specimens which have been

previously measured by the PCD method in order to compare results obtained

by the two methods.

3.3. INHOMOGENEITIES

Objective : To develop improved methods for measuring inhomogeneities

responsible for reducing performance and reliability of germanium and

silicon devices, and in particular, to evaluate a photovoltaic method as

a means for measuring radial resistivity gradients in germanium and silicon circular wafers without contacting the flat surfaces of the wafers.

17
INHOMOGENEITIES

Progress : The analysis of the correlation between photovoltaic,

four-probe, and two-probe profiles by the statistical techniques discussed

previously (NBS Tech. Note 571, p. 13) was continued. Since correlation

coefficients calculated by these techniques range from good (0.9) to

quite poor (0 or negative), emphasis in the photovoltaic study is being

placed on determining why the correlation of the resistivity profiles

made by the different methods of measurement is very good for some wafers

and very poor for others. In addition, greater care is now being taken

to assure that both the photovoltaic and four-probe measurements are being

made on the same wafer diameter and that the bar on which the two-probe

measurement is made is cut accurately from that diameter.

An investigation into various factors which may affect the precision

and accuracy of the resistivity profiles determined by each of the three

measurement methods has been initiated. Two factors which strongly influence the four-probe method are the size of the resistivity gradient

and the location (or the mis-location) of the probes on the specimen.

Detailed study of the effect of uncertainty of probe location was begun.

It was found that the calculated profile shape can be profoundly affected

if the assumed probe positions are shifted by 100 urn or so.

Factors which may affect the photovoltaic resistivity profile are

the resistivity gradient magnitude, the electrical quality (degree of non- ohmicity) of the measurement contacts, and non-radial resistivity gradients in the specimen. (D. L. Blackburn)

Preliminary work was begun on a plasma resonance technique that might

be suitable for measuring resistivity inhomogeneity in low-resistivity

wafers . The initial approach is either to modulate the specimen temperature or to place the specimen in a modulated magnetic field and use a phase sensitive lock-in detector to detect the plasma resonance minima.

(D. L. Blackburn and W. R. Thurber)

Plans : The study of the effect of uncertainty of probe location on the four-probe method will be completed. Investigation of the effects

of large resistivity gradients, non-radial resistivity gradients, and

measurement contact quality on the photovoltaic method will continue.

Further work on the plasma resonance technique will be deferred un- til the present stage of the photovoltaic study is concluded and a report

has been prepared.

3.4. 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.

18
GOLD-DOPED SILICON

Progress: Total gold concentrations, as determined by neutron activation analysis, in 10- and 20-ft*cm, boron-doped, silicon wafers diffused

at 850 and 1250°C are plotted in figure 1 as a function of diffusion time.

These experiments were done to study the effects of crystal quality (dislocation density) and atmosphere on the gold diffusion process. The effect of a variety of atmospheres has been reported in the literature with

conflicting claims as to the most suitable one. The results of the present study suggest that gold diffuses slightly faster in an oxygen atmosphere than it does in an argon atmosphere. It was also found that gold

diffused faster in the high-dislocation wafers (20 fl'cm) than it did in

the low-dislocation wafers (10 ft •cm). This result is in agreement with

that of other workers

.

It is known that gold accumulates at surfaces of wafers [1], To

avoid contributions to the neutron activation analysis determinations

from such gold, both wafer surfaces are routinely lapped prior to activation analysis. Because some previously measured specimens were determined to have gold concentrations which could not be correlated with re- sistivity measurements (NBS Tech. Note 571, p. 15), contamination from

gold accumulated at the rim of the wafer was suspected. Therefore in the

present group, the rim was cut away from each of the specimens prior to

activation analysis.

To test the hypothesis that gold accumulated at the rim might have

influenced previous activation analysis results, the gold concentration

was measured in both the center and rim portions of two specimens , one diffused at 1250°C for 4 h and the other at 850°C for 0.5 h. The con- centrations measured in both the center and rim portions of the 1250°C

specimen were about equal to the solid solubility of gold. In the

case of the 850°C specimen, the rim portion had an average concentration

4 times the solid solubility while the center portion, because of the

short diffusion time, had a concentration about l/50th of the solid

solubility. Because the rim portion also contained a significant volume

in which the gold concentration was characteristic of the center portion

it is clear that very large amounts of gold can accumulate at the rim.

These results suggest that for specimens diffused at temperatures below

1250°C gold concentrations considerably in excess of the solid solubility

at the diffusion temperature can be measured if the rims are not removed

even though the gold concentration in the interior of the wafer may be

equal to or less than the solid solubility. They also support the con- clusion that the difficulties encountered previously in correlating measured gold concentration with electrical properties were probably due to

the fact that the wafer rims had not been cut away prior to activation

analysis

.

To determine the lateral uniformity of the diffused gold, several

specimens were examined by x-ray fluorescence with the scanning electron

microscope. Examination of a diffused specimen from which the surface

19
10

17

10

16

COE

o

toE

o+J

re

10'

10

13

GOLD-DOPED SILICON

I | I

I l I I l| II I I I

I I I |I I—I I

I II l

SOLUBILITY AT 1250°C -

<

h-

-^. LUO

I0

15

zz 20-0--HDD

oC_3 - + 20-A-HDD

o

- A

10-A-LDD

SOLUBILITY AT 850°C -

' .0 10.0 DIFFUSION TIME (hours)

100.0

Figure 1. Gold concentration as determined by neutron activation analysis as a function

of diffusion time for 10- and 20-n«cm, boron-doped, gold-diffused, silicon wafers. (The upper curves are for wafers diffused at 1250°C, the lower for wafers diffused at 850°C.

The diffusion atmosphere was either oxygen (0) or argon (A). The 10-f2«cm wafers had low

dislocation density (500 cm

-2

) and are indicated by LDD; the 20-fJ-cm wafers had high dislocation density (17.,,000 cm"2

) and are indicated by HDD. As diffused, the wafers were 1.1 mm thick; before activation analysis, 0.12 mm was lapped from each side and the rim was

removed.)

2Q
GOLD-DOPED SILICON

gold had been removed by aqua regia showed clusters of gold, 2 to 5 urn in diameter near the surface. A piece of the same material with 0.06 mm

lapped from the surface did not show any clusters.

Resistivity and Hall effect measurements were made at room temperature on Hall bars cut from the 10- and 20-ft # cm, boron-doped wafers diffused with gold at 1250° C. For the set of four 20-ft»cm specimens diffused

in an oxygen atmosphere the gold concentrations ranged from 3.0 to 9.9 x 10 16 atoms/cm3 and the resistivity decreased with increasing gold concentration. Likewise for two 20-ft*cm specimens diffused in argon the one with the higher gold concentration had the lower resistivity. Also of

two 10-fi'cm specimens diffused in argon, the one with the higher gold

concentration had the lower resistivity. This retrograde behavior of the

resistivity is not in agreement with theoretical predictions (NBS Tech.

Note 520, p. 23) but is in agreement with previous measurements made elsewhere (W. M. Bullis, unpublished).

Spreading resistance measurements of a preliminary nature were made

on an angle-lapped specimen to study the distribution of gold from the

surface into the bulk of the wafer. Since gold was evaporated on both

sides of the wafer before diffusion, a symmetrical profile was expected,

but not observed. More work on this technique is needed before meaningful

profiles can be obtained. (W. R. Thurber, T. F. Leedy, and W. M. Bullis)

Plans: More 10- and 20-fi*cm, boron-doped silicon wafers will be

diffused with gold to complete the range of gold concentrations for these

resistivities. Also 90-fi*cm, p-type wafers will be diffused at temperatures from 850 to 1250°C. Additional n- and p-type material will be se- lected and slices prepared for diffusion. Following gold diffusion, a part of each wafer will be sent for activation analysis and a Hall bar

for electrical measurements will be cut from the remainder. Work will

continue on the study of lateral and depth uniformity of the gold diffusions. Hall effect measurements as a function of temperature will be

made on two or three of the 20-fl*cm, gold-diffused, boron-doped specimens

to determine if the ionization energy of the gold donor level at 0.35 eV

above the valence band varies with gold concentration. If the ionization

energy decreases as the gold concentration increases, then this may be a possible explanation for the retrograde behavior of the resistivity.

3.5. SPECIFICATION OF GERMANIUM

Objective : To measure the properties of germanium crystals and to

correlate these properties with the performance of germanium gamma-ray

detectors in order to develop methods for the early identification of

crystals suitable for fabrication into lithium-compensated gamma-ray detectors .

21
SPECIFICATION OF GERMANIUM

Progress : Analysis of the data obtained in the evaluation of the

lithium mobility and lithium precipitation methods for determining oxygen

concentration in germanium has been completed, and the sensitivity and

expected precision for each method determined. Several additional

lithium-drifted germanium diodes have been examined by the improved infrared response technique. Response spectra have been obtained for a copper-doped specimen and another specimen without copper cut from the same crystal. The results show that the improved technique is capable

of detecting impurities such as copper and identifying them by means of

their observed energy levels.

Characterization of Germanium. — Analysis and interpretation of data

obtained in the study of lithium mobility and lithium precipitation as a means of measuring oxygen concentration in germanium have been completed.

In figure 2 oxygen concentration as determined from both lithium precipitation and infrared absorption measurements is plotted against oxygen

concentration determined from measurements of lithium-ion drift mobility.

The error bars shown for some of the points represent an estimate of the

precision of the measurements: ±30 percent for lithium mobility, ±45

percent for lithium precipitation, and ±60 percent for infrared absorption.

The precision statements for the lithium mobility and lithium precipitation

methods are based on repetitive, single-lab oratory measurements on speci- mens with oxygen concentration between 0.2 and 20 ppb atomic (10 13 and

10 15 atoms/cm 3

) and represent three relative standard deviations. The

precision statement for the infrared absorption method is based on a multi-laboratory round-robin test [1] and also represents three relative

standard deviations

.

The straight line was obtained from a least squares fit to the solid

data points where the three solid points for which error bars are shown

have been ignored in the fitting procedure. These three points were re- jected as outliers with a 1 percent risk that they really belonged to the

group [2]. The slope of the line shows that determination of oxygen con- centration by measurements of lithium precipitation and lithium mobility

differ by less than 10 percent. Since the infrared absorption technique

relates to oxygen in germanium through the unique 11.7-um absorption band,

the measurements of oxygen concentration by infrared absorption were used

to verify that the lithium mobility and precipitation methods were also

measuring oxygen concentration. That this is so can be seen by the co- incident relationship between the solid and open data points.

(A. H. Sher, W. K. Croll, and W. R. Thurber)

Ge(Li) Detector Measurements — Infrared response (IRR) spectra [3]

of three Ge(Li) detectors at about 100 K have been studied in detail.

Spectra of these diodes obtained with a 3-mm thick germanium filter are shown in figure 3. Ge(Li) 13Cu was fabricated from a portion of crystal

NBS-13 which had been diffused with copper at 750°C for 12 days, Ge(Li)

13 was fabricated from a portion of crystal NBS-13 which had been

22
SPECIFICATION OF GERMANIUM

- 10

SLOPE- 1.09

E

o

O

CO

Qd

I0

1 10" 2XI0

15

-3'

N BY LITHIUM MOBILITY (cm

-1

)

Figure 2. Oxygen concentration, Nq, obtained from lithium precipitation (•) and infrared absorption measurements (O) plotted against Nq obtained from lithium mobility

measurements. (The error bars represent three standard deviations (see text).)

subjected only to the normal detector fabrication procedure, and Ge(Li)

83-3 was fabricated from crystal NBS-83. The IRR spectrum taken with a 1-mm thick germanium window on another detector fabricated from crystal

NBS-83 was indicative of many discrete levels in the forbidden energy gap

(NBS Tech. Note 571, pp. 16-18).

According to previous studies [3], the interpretation of the prominent features displayed by the spectra would indicate that both Ge(Li)

83-3 and Ge(Li) 13 contain "lithium-defect" electron traps as indicated

by the shelf-like response at 0.50 eV. Another Ge(Li) detector fabricated

23
200

SPECIFICATION OF GERMANIUM

i—

i

a:

0.55 0.65 0.75

ENERGY (eV) . Figure 3. Infrared response (IRR) spectra obtained using a 3-mm thick germanium filter.

(Specimen Ge(Li) 13Cu contains approximately 10 11* copper atoms/cm3

. All spectra have

been normalized to the band-edge response peak (at 0.71 eV) ; a portion of the IRR spectrum

for Ge(Li) 83-3 is shown at a sensitivity level increased by a factor of 200 over the others. Specimen temperature was about 100 K. A 640 line/mm grating was employed.)

24
SPECIFICATION OF GERMANIUM

from crystal NBS-83 showed slight electron trapping in previous studies

made with a collimated gamma-ray beam (NBS Tech. Note 495, pp. 18-19).

Ge(Li) 13 is expected to show much more severe electron trapping due to

the increased magnitude of its IRR response, but this has not as yet been

confirmed experimentally.

The IRR spectrum of Ge(Li) 13Cu shown in figure 3 does not exhibit

any features which were previously attributed to the presence of copper

in a crystal [3]. On the basis of this IRR spectrum one would predict

that the detector should exhibit severe carrier trapping characteristics,

but the type of trap cannot be specified. Measurements of resistivity of

the copper- diffused portion of crystal NBS-13 showed that approximately

10 14 cm

-3 electrically active impurities had been introduced into the

crystal.

The results become less ambiguous when a 1-mm thick germanium filter

is used. Figure 4 shows IRR spectra for incident infrared radiation of

energy between 0.30 and 0.70 eV obtained for the three cyrstals under

study. These curves are plotted on a semi-log scale to indicate the relative signal magnitudes obtained during a spectrum scan, and to aid com- parison of data between specimens . The curves have been plotted so that

the levels of the minimum detectable signal coincide. The semi-log display of the spectra, it should be noted, causes some of the features to

be seen in less detail than on the linear scale as normally obtained from

the experiment. The overall signal level of Ge(Li) 13Cu is an order of

magnitude greater than that of Ge(Li) 13. The spectrum of Ge(Li) 13Cu

exhibits two features not seen in the other curves, a peak at 0.38 and a knee at 0.49 eV. These correspond to the copper levels at Ev

+ 0.33 eV

and E

c

- 0.22 eV, respectively. Analysis of the gamma-ray characteristics

of Ge(Li) 13Cu determined using a collimated gamma-ray indicated severe

hole trapping as is predicted for copper-doped germanium [3],

IRR studies previously performed using a 3-mm thick germanium filter

[3] gave no indication of the presence of a number of the discrete energy

levels observed with a 1-mm thick filter in the present work. This can probably be accounted for by differences in the spectral transmission of

the 1- and 3-mm thick germanium filters; there is a much decreased absorption of radiation of energy between 0.65 and 0.70 eV when the 1-mm

thick filter is used. Scattering of this high energy radiation in the

monochromator may make it possible for optical pumping of the levels to occur when using a 1-mm thick filter [5], and so many of the discrete

levels unobserved using the 3-mm thick filter can be excited. The peaks

at 0.36 eV and others observed at 0.24 and 0.18 eV (not shown) appear to

result from band-edge radiation which occurs because of higher order diffraction. Although radiation of this energy is filtered out, it is not

completely eliminated. Since the detectors are much more sensitive to it

than to lower energy radiation, a signal can be seen.

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

25
SPECIFICATION OF GERMANIUM

O.OI0.30 0.40 0.50

ENERGY (eV)

0.60 0.70

Figure 4. IRR spectra in the incident photon energy range of 0.30 to 0.70 eV obtained using a 1-mm thick germanium filter. (The spectra are plotted so that the

levels of minimum detectable signal coincide. To the right of the vertical dashed

line a 640 line/mm grating was used; to the left, a 240 line/mm grating. In all cases specimen temperature was about 100 K.)

Plans: Study and correlation of measurements of lithium driftability

,

infrared response, and distribution of etch pits, three of the most promising methods for characterizing germanium for Ge(Li) detectors, will

continue with the aim of developing one or more of these methods into a meaningful test for the rapid and proper specification of detector-grade

germanium. Additional specially doped crystals will be examined along

26
SPECIFICATION OF GERMANIUM

with heat-treated and radiation-damaged crystals in order to aid the interpretation of the results of the improved IRR measurements. Measurement and interpretation of Ge(Li) detector characteristics with standard

experimental methods and theoretical models will be continued.

3.6. REFERENCES

3.1. Resistivity

1. Smits, F. M., Measurement of Sheet Resistivities with the Four-Point

Probe, Bell System Tech. J. 37_, 711-718 (1958).

2. Uhlir, A., The Potentials of Infinite Systems of Sources and Numeriial Solutions of Problems in Semiconductor Engineering, Bell System

Tech. J. 34, 105-128 (1955).

3. Valdes , L. , Resistivity Measurements on Germanium for Transistors,

Proa. IRE 42_, 420-427 (1954).

4. These values, obtained by linear interpolation from Smits' table, are those quoted in Method for Measuring Resistivity of Silicon Slices

with a Collinear Four-Probe Array (ASTM Designation: F84-70), Annual

Book of ASTM Standards t Part 8, Am. Soc. for Testing and Materials,

Philadelphia, Pa., 1970, pp. 746-760.

5. Bullis , W. M. , Brewer, F. H. , Kolstad, C. D., and Swart zendruber,

L. J,, Temperature Coefficient of Resistivity of Silicon and Germanium near Room Temperature, Solid-State Electronics 13_, 639-646

(1968).

3.2. Carrier Lifetime

1. Zerbst, M., Relaxationseffekte an Halbleiter-Isolator-Grenzflachen,

Z. Angew. Phys _22, 30-33 (1966).

2. Heiman, F. P., On the Determination of Minority Carrier Lifetime from

the Transient Response of an MOS Capacitor, IEEE Trans. Electron

Devices ED-14 , 781-784 (1967).

3. Salama, C. A. T. , and Holmes, F., On the Determination of Surface

Recombination Velocity from the Transient Response of MIS Structures,

Solid-State Electronics 13, 1204-1205 (1970).

4. Goodman, A. M. , A Method for the Measurement of Short Minority

Carrier Diffusion Lengths in Semiconductors, J. Appl. Phys. 32 , 2550-2552 (1961).

27
REFERENCES

5. Bergmann, F., Fritzsche, C, and Riccius , H. D., Bestimmung der

Diffusionslange in Halbleitern mit Hilfe des Oberflachemphotoeffektes, Telefunken Ztg. 37_, 186-193 (1964).

6. Harrick, N. J., Characteristics of Junctions in Germanium, J. Appl.

Phys. 29_, 764-770 (1958).

7. Fletcher, N. H. , General Semiconductor Junction Relations, J. Electronics 2, 609-610 (1957).

3.4. Gold-Doped Silicon

1. Boltaks, B. I. , Kulidov, G. S., and Malkovich, R. Sh. s The Effect of

Gold on the Electrical Properties of Silicon, Soviet Physios — Solid

State 2, 167-175 (1960).

2. Sprokel, G. J., and Fairfield, J. M. , Diffusion of Gold into Silicon

Crystals, J. Electrochem. Soo. 112 , 200-203 (1965).

3. Martin, J., Haas, E., and Raithel, K. , Radiochemische Untersuchungen

zur Diffusion von Gold in Silizium, Solid-State Electronics 9, 83-85

(1966).

3.5. Specification of Germanium

1. Quoted in Method of Test for Interstitial Atomic Oxygen Content of

Germanium by Infrared Absorption (ASTM Designation F 122-70T).

Annual Book of ASTM Standards 3 Part 8, Am. Soc. for Testing and

Materials, Philadelphia, Pa., 1970, pp. 885-886.

2. Natrella, M. G., Experimental Statistics, NBS Handbook 91, rev. ed.,

p. 17-3, 1966.

3. Armantrout , G. A., Infrared Evaluation Techniques for Ge(Li)

Detectors, IEEE Trans. Nucl. Sci. NS-17 , No. 2, 16-23 (1970).

4. McElroy, J. M. , Photoconductivity —An Instructive Exercise in

Semiconductor Theory, IEEE Trans. Ed. E-ll , No. 1, 12-18 (1968).

5. Armantrout, G. A., private communication.

28
METHODS OF MEASUREMENT FOR SEMICONDUCTOR PROCESS CONTROL

4.1. METALLIZATION EVALUATION

Objective : To improve methods for measuring the properties of thin

metal films with initial emphasis on the adhesion of aluminum metallization deposited on various substrates.

Progress ; An exhaustive series of scratch tests was made on a 1.03-um thick aluminum film deposited on a quartz substrate. This was

one of a batch of substrates which was prepared by vacuum evaporation

immediately after cleaning and degreasing. Previously, substrates had

been prepared similarly except that they were heat treated in air at

about 850°C just prior to vacuum evaporation. Three diamond styli with

nominal tip radii of 18 urn, 45 urn, and 120 um were used to scratch the

films

.

With the 45-ym stylus, the load was varied from 40 to 300 g. Threshold adhesion failures of the type described earlier (NBS Tech. Note 527,

p. 23) were not observed here. Although the initial measurements were

made when the film was but two days old, the film behaved as though it were a fully aged, highly adherent film. Under the stylus the film was progressively thinned with increasing stylus load, as evidenced by the

translucency of the scratch. Although translucency was detectable under

microscopic observation at a magnification of 200 X after scratching with

an 80-g load, there was no optical evidence of the transparency indicative

of complete film removal, even at loads as high as 300 g. Similar observations, considered to be characteristic of highly adherent aluminum

films, have been reported by Collins [1].

Scratches with the 120-ym stylus were made with the loads ranging

from 100 to 1100 g. As the loads increased, there was a corresponding

increase in track width, but thinning of the film was insufficient to

allow detectable translucency. Only a few clear threshold adhesion failures were observed at loadings of 1000 and 1100 g. Crescents (NBS Tech.

Note 571, p. 22) were observed in abundance. Frequently thin, translucent

spots or breaks were closely associated with these crescents, but the

breaks were not of threshold failure type.

The crescents observed here were first observed in a 0.5-ym thick

aluminum film in scratches also produced by a 120-ym stylus. It was suspected that these might be due to fracture of the quartz substrate.

Experiments were carried out to estimate the lowest load required on a stylus so that it would leave an identifiable permanent deformation on a quartz substrate, without an interposing aluminum film. For a 120-ym

stylus on fused quartz, it was found that crescent shaped fractures were produced along the scratch path with a 140-g load. With increasing stylus

loads, many groups of crescent fractures were generated, frequently

29
METALLIZATION EVALUATION

including complete circular fractures. This tends to confirm the belief

that a crescent observed in a scratch on an aluminum film is associated

with a crescent fracture in the substrate directly beneath it. It would

seem possible then, that crescent-shaped thin spots or breaks observed in

aluminum films at higher loads may be brought about by the thinning down

and deformation of the film over the crescent fracture in the substrate.

Since the scratch test fails when the substrate suffers destruction, it

follows that breaks or failures in the film connected with crescents are not threshold adhesion failures and are therefore set aside. It is interesting to note that Hamilton and Raws on [2] have very recently reported

similar fracture results for a steel ball sliding on a glass plate.

Loads in the range 30 to 120 g were used for scratching with the

18-um stylus. Progressive thinning with increasing load took place, and

translucency was detectable at a stylus loading of approximately 40 g.

It is to be emphasized that (l) any one scratch is not uniformly translucent, (2) the number of translucent streaks usually varies from track

to track and often within a single track, (3) the light transmission of

any one translucent streak may change d is continuously along its length,

and (4) the scratch is not transparent.

However, within the scratches made with the 18-um stylus many spots

appeared to be significantly brighter than the surrounding translucent

region, and to have a distinct white-light diffraction pattern associated

with them. These spots strongly resembled previously observed threshold

adhesion failures, although they were decidedly smaller. Since the diffraction pattern was present, it is believed that these tiny brighter

spots are transparent holes in the film and represent failure points.

That the occurrence or non-occurrence of these spots was quite distinct

with rather small differential stylus loadings is taken as further evidence of this conclusion.

On the basis that the tiny bright spots within associated diffraction

patterns are threshold adhesion failures, a number of measurements were

made to determine sensitivity and reproducibility. The usual up-and-down

method was applied. From the results, shown in table 2, it can be seen

that the test can distinguish between failure and non-failure for a differential load of 2.0 g. Thus for this particular 1-um thick film a sensitivity of approximately 4 percent is indicated. Although this sen- sitivity is considerably lower than values reported previously for 0.5-um

films, it by no means represents the ultimate value expected for 1-um

thick films

.

Runs 1 through 3 were interdigitated. If the differences in load

increment are ignored and the data are treated together, the grand mean

failure load is 57.6 ± 1.8 g. If the data are assumed to be normally

distributed it can be asserted with 95 percent confidence that the relative standard deviation for the entire population is less than 14 percent.

30
METALLIZATION EVALUATION

Table 2 - Results of Scratch Tests on a 1.0-ym Thick

Aluminum Film with an 18-um Diamond Stylus

Run No. Load Increment

(g)

Mean Failure

Load

(g)

Sample

Standard

Deviation

(g)

1

2

3

4

5.0

2.0

2.0

2.0

59.2

57.9

55.7

59.0

10.0

4.4

2.0

4.8

Grand Mean (1-3 only) 57.6 1.8

Although aging effects had been observed in earlier measurements,

measurements on specimen B6-S1 (l ym thick) revealed no evidence of any

increase in adhesion due to aging. It is possible that omission of the

heat treatment may be responsible for the lack of aging effects and for

the high adherence of this film. It is also possible that the lack of

observed aging effects may be characteristic of films of this thickness;

previously reported [3] aging effects were observed on much thinner films.

A parallel effort was undertaken to determine the feasibility of detecting threshold adhesion failure in the aluminum-silicon dioxide-silicon

system. One approach is to use an infrared sensor such as PbS. Built

into a suitable microscope system, the PbS detector probably holds the

best promise for an automated apparatus. A second approach which was investigated in more detail involves the use of an infrared imaging tube

combined with a microscope.

All commercial infrared imaging tubes use the same sensing element

known as an S-l photocathode. Although its sensitivity falls off very

rapidly in the silicon transmission band (1.2 to 1.5 ym) it appears that

sufficient signal is available in that range to permit viable instrumentation. An important advantage of the imaging tube approach is that the

detector is very sensitive in the visible, thereby allowing an investigator to inspect the specimen from above and make necessary mechanical

adjustments.

The capabilities of an infrared imaging tube were explored using a 0.5-um thick aluminum film on quartz with known failure points and a glass

filter which filtered out all the visible radiation. An available

imaging tube was mounted on the microscope ordinarily used for scratch

31
METALLIZATION EVALUATION

test observations (objective magnification: 11 X ) . Under these experimental circumstances, only the larger failure points were observable.

Very small failure points, several micrometers or less in diameter, were

not resolvable.

A similar film specimen and a silicon wafer for use as an infrared

filter were submitted to a vendor of an infrared imaging microscope

to determine the capabilities of his instrument. He reported no difficulty in seeing even the smallest holes in the aluminum film.

(J. Oroshnik and W. K. Croll)

Plans : Future work on this task will be redirected toward other

techniques more directly applicable to the determination of adhesion of

bonding pads and the effect of bonding-pad adhesion characteristics on bond strength.

4.2. 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 : Measurements of thermal resistance and transient thermal

response were made on 20 diode chips bonded to TO- 5 headers. In all the

devices a silicon-gold eutectic die bond was employed. Heat for the

measurements was generated by pulses of power ranging from 100 us to 5 s in duration. The duty cycle was such that the junction temperature cooled

to an equilibrium value, the case temperature, during the period that

heating power was not applied. A typical transient thermal response curve for these devices is shown in figure 5. The curve depicts the diodejunction-to-case temperature difference, ATjq, normalized to its steadystate value as a function of the heating-power pulse width. The junction

temperature was determined during the heating-power off period from the

forward voltage, Vp, of the diode at 1 mA. The calibration of Vp with

respect to temperature was obtained by externally heating the case and

measuring the forward voltage at 1 mA . This current is not sufficient to

alter the junction temperature significantly.

A mesa diode on a 0.95-mm square chip mounted on a TO-5 header

typically exhibited 60°C/W thermal resistance. The theoretical thermal

time constant for a device with junction-to-bonding-surface thickness of

0.25 mm (0.01 in.) is approximately 500 us [1]. Since it is necessary

for the chip to reach thermal equilibrium to maximize the amount of heat

reaching the chip bonding surface, a heating pulse of 5 to 10 thermal

time constants is expected to result in maximum sensitivity of the thermal

response measurement to voids in the die attachment (NBS Tech, Note 555,

32
1—I I 1 1 II I

DIE ATTACHMENT EVALUATION

1—I I I I Mil I—I I I M ill 1—I l| Mill 1—I I M il •

pi i i i I mil i i mill I I I I Mill I I I llllll I I I Mill

10 4

I0" J

10" * 10'

I 10 WIDTH OF APPLIED POWER PULSE (s)

Figure 5. Transient thermal response curve for a typical mesa diode on a 0.95-mm square

chip mounted on a TO-5 header. (Diode junction-to-case temperature difference normalized

to its steady-state value is plotted as a function of the heating power pulse width. The

case temperature was held at 25°C. For this diode the steady-state value corresponds to

a thermal resistance of 60°C/W.)

pp. 25-27). Initial measurements of the transient thermal response indicate that a 5-ms , 800-mA heating power pulse produces an increase in

the junction temperature of 10 to 12°C which is sufficient for reproducible measurements of ATjq. Increased sensitivity is attainable by increasing the heating current. However, under such conditions the accuracy of the measurement is reduced because of an increased delay in the

measurement of the forward voltage drop which is necessary since the

higher heating. current being switched increases the masking effect of

electrical transients on the thermal transients.

Preliminary measurements of both thermal resistance and transient

thermal response were made on five devices to check the reproducibility

33
DIE ATTACHMENT EVALUATION

of the die attachment evaluation equipment. The results of two runs of

measurements taken 10 and 50 ys after the termination of the 5-ms heating

power pulse indicate that the measurements of ATjq for both transient and

steady state operating conditions can be reproduced to within 1°C for any- particular device. The variation in ATjq between the five devices for a particular run was a maximum of 2°C. As expected, the longer the delaytime before measuring Vp, the smaller is the ATjp for a given heating

current due to conduction cooling of the diode junction.

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

Plans : Long-term, single -operator measurements will be undertaken

to check the reproducibility of the die attachment evaluation equipment.

Measurements of thermal response will be undertaken to determine the

sensitivity of the junction-to-case temperature change as a function of

the magnitude of the diode heating current for a heating power pulse

width of 5 ms and for direct current.

The fabrication of diodes with various diameter controlled voids will

continue and measurements of thermal resistance and transient thermal re- sponse will be undertaken on these diodes.

4.3. 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 eventually will fail.

Progress: Study of the effect of pull rate on the pull test re- vealed that when the failure occurred at the heel of the first bond pulling at a rate of 77 gf/s (764 mN/s), equivalent to the "jerk" test used

by some device manufacturers, gives results statistically equivalent to

pulling at a rate of 1 gf/s (9.8 mN/s). The importance of specifying all

details of the test including the wire bond failure mode has become apparent. Tests carried out to simulate the high temperature environment en- countered by bonds during glass sealing operations for ceramic packages

showed that the reduction in pull strength can be accounted for principally by the reduction in wire tensile strength caused by the high temperature anneal.

Improvements in the laser mount for interferometry measurements were made to increase the ease of optical alignment and use of the interferometer on bonding machines-. The length of the laser was increased in order

to limit all emission to the TEMQ0 mode.

34
WIRE BOND EVALUATION

New experiments directed toward development of measurement methods

applicable to in-process bond quality determination were started. The

initial effort consisted of electronically mixing the 60-kHz output from

a capacitor microphone directed at the bonding tool with a local oscillator. Preliminary results showed that the bonding process can be heard

through an audio speaker and recorded on magnetic tape. Harmonics of the

60-kHz tool motion can be individually treated in the same manner.

The wire indentation tester is nearing completion.

Vull Test Evaluation — It was previously reported (NBS Tech. Note

560, p. 31) that the rate of pull had no significant effect on the meas- ured values of pull strength for pull rates of from 1.0 to 12.5 gf/s

(9.8 to 122 mN/s) for single-level bonds, where the mode of failure is a break at the heel of the first bond. Two extensions of this work were

considered: (1) faster rates of pull to simulate a "jerk test" which is

used by some device manufacturers, and (2) the effect of pull rate on measured bond strength for the case where the failure mode is peeling or lift off of the bond.

Groups of 60 bonds each were pulled at rates of 1.0 and 77 gf/s

(9.8 and 764 mN/s) resulting in times of approximately 9 and 0.12 s to

pull a bond, respectively. The bonding schedule was such that failure

occurred at the heel of the first bond in all cases. There was no statistically significant difference in the measured pull strengths for either

pull rate.

For bonds that fail by peeling, it is assumed that the rate of pull

would have a large effect on the measured pull strength since peel failure is known to be rate sensitive. However, it is very difficult to show

this statistically, using the measured pull strengths. Up to the present,

a procedure to make bonds reproducibly that will fail by peeling has not

been found. Attempts thus far have resulted in bond strengths that have

greater variations from bond to bond, at a given pull rate, than between

groups pulled at different rates. Therefore, a pull-rate experiment as just described cannot now be performed for bonds that fail by peel.

Nevertheless it seems clear that the nature of the failure mode may affect

comparisons of pull strength and rates of pull.

It should be noted that all pull rate tests previously described were performed on single-level bonds. These results have not yet been verified

for the case of two-level bond pairs such as are usually made on packaged

devices. There may be some differences because the resolution of forces

for this configuration tends to favor the lift-off failure mode for one

of the bonds. (K. 0. Leedy and C. A. Main)

Bond pull tests are often made on devices which are heat treated

during the package sealing operation. For the case of glass-sealed,

35
WIRE BOND EVALUATION

ceramic packages, bonds are subjected to about 500°C for relatively long

periods of time. In order to evaluate the effect of this process on the

bond strength an experiment was designed to test both the bond and the

wire before and after exposure to high temperatures

.

Two hundred single-level bond pairs were made in sequence by one operator using identical machine settings. Every other bond pair was pulled to destruction and the average pull strength determined. The substrate with the remaining bonds was placed in a nitrogen atmosphere at

505°C for 20 min. These annealed bond pairs were then pulled to destruction. Before heating, the average pull strength was 9.3 ± 0.66 gf*

(91 ± 6.5 mN); 87 percent of the bonds broke at the heel of the first

bond, and none broke in the span of the wire. After heating, the average

pull strength was only 4.5 ± 0.38 gf* (44 ± 3.9 mN), and 80 percent of

the bonds broke in the span of the wire while only 16 percent broke at

the heel of the first bond.

To see how much influence annealing the wire has on the bond pull

strength, unbonded wire samples were given heat treatment identical to

that of the bond pairs described above. The ends of the wire samples

were mounted in epoxy cement and the wire was pulled to destruction using

the hot-melt glue puller (NBS Tech. Note 488, pp. 22-23). The unheated

wire had an average breaking strength of 12.0 ± 0.33 gf:

': (117 ± 3.2 mN).

The annealed samples had a breaking strength of only 3.5 ± 0.3 5 gf:

': (34 ± 3.4 mN). (K. 0. Leedy and C. A. Main)

A resolution-of-forces calculation (NBS Tech. Note 555, p. 33) of

the tensile force, T, applied to the annealed wire bond pair during

pulling was made using the measured loop height, 0.017 to 0.018 in.

(0.43 to 0.46 mm), and bond length, about 0.04 in. (1.0 mm). t The calculated tensile force, Twt , in the wire of a bond pair having a pull

strength of 4.5 gf (44 mN) was 3.4 gf (33 mN), almost identical to the

separately measured wire breaking strength. This suggests that if a wire

bond loop breaks in the wire span, the breaking strength can be used to

'* The range given is one sample standard deviation.

t For accuracy in the calculation for the case of annealed wire, it is

essential that the loop height be measured after the wire is stretched

by the pull test apparatus because wire annealed at high temperatures

undergoes very large elongation that may add as much as 0.002 or 0.003

in. (0.05 to 0.07 mm) to the loop height. This may result in a different distribution of forces at the time of breakage. One way of forming

the loop so that it can be measured after stretching is to push the

wire segments carefully back together after the bond is pulled. Another is to tension the bond on the pulling apparatus to a force

slightly less than that required to break adjacent bonds.

36
WIRE BOND EVALUATION

calculate the tensile strength of the wire on the basis of a simple

resolution of forces. This is in contrast to the result found when the

bond fails at the heel of the first bond (NBS Tech. Note 571, pp. 24-26).

Using the same resolution of forces equations, the loop height that

will result in Twt = T was calculated for the various bond lengths that

are normally used in this laboratory. Use of the appropriate loop heights

in the future would permit the reported bond strength to be the same as the tension in the unannealed wire at the moment of rupture. As an ex- ample, for a bond length of 0.04 in. (1 mm) in a single level bonding

situation the loop height for Twt = T is approximately 0.012 in. (0.3 mm).

(G. G. Harman)

Some organizations seal at temperatures slightly below 500°C. Under

these conditions it is observed that the majority of the bonds break at

the heel rather than in the span. This was confirmed by performing a second experiment with 10 single-level bond pairs. After pulling every

other bond pair to destruction the substrate with the remaining bonds was placed in a nitrogen atmosphere at 490°C for 20 min. The annealed bond

pairs were then pulled to destruction. The average pull strengths and

standard deviations before and after heating were similar to those found

in the 505°C experiment. Again, before heating, no bonds failed in the

span. However, after heating only 40 percent of the bonds broke in the

span as compared with 80 percent for the 505°C experiment. This result

is in accord with the known fact that metallurgical changes take place

in the wire around 500°C. (K. 0. Leedy and C. A. Main)

Characterization of Ultrasonic Wire Bonding Systems — The laser interferometer system [1] previously described (NBS Tech. Note 555, p. 30,

and NBS Tech. Note 571, p. 31), has been greatly improved in its con- venience of operation. The laser is now mounted on a stabilized, portable

tripod platform having x- , y-, and z-axis micrometer positioners as shown

in figure 6. The interferometer output is displayed on an oscilloscope

not shown in the figure. The system can now be set up and optically

aligned on any bonding machine or experimental setup in approximately 5 min. A complete calibration of bonding tool tip vibration amplitude as

a function of power supply control setting takes only 5 to 10 min of

additional time.

When using the above system the laser is not physically attached to

the bonding machine. Therefore, small, low-frequency room vibrations,

of the order of several microinches, will change the output waveshape at

the end or turn-around points of the tool motion. This makes it impossible to display the laser output as a repetitive oscilloscope pattern and

requires that data be taken from a single sweep of the oscilloscope beam.

This can be done either with a memory oscilloscope that has a sufficiently

fast writing rate or by photographing the interferometer pattern from a standard oscilloscope trace. Photographs have been used in the present

study. No room motion is evident during the short (approximately 40 us)

37
WIRE BOND EVALUATION

C -

HEAVY DUTY TRIPOD WITH STA- BILIZED BASE

X-, Y-, AND Z-AXIS MICROMETER ADJUSTMENTS FOR AIMING THE LASER LASER SYSTEM WITH FOCUSING OPTICS

NEAREST THE BONDING MACHINE AND DETECTOR AT THE REAR BONDING MACHINE

LASER POWER SUPPLY

Figure 6. Laser system set-up for calibrating ultrasonic bonding tool vibration

amplitude.

AM/VW

Figure 7. A comparison between the output of a microphone and a laser

interferometer simultaneously monitoring the same loose bonding tool. (The upper curve is the output of the capacitor microphone and encom- passes about five repetitive traces with approximately 5.5 tool motion

cycles in each trace. Distortion of the wave shape is obvious. The

horizontal scale is approximately 10 ps/div. The lower curve is the laser interferometer output resulting from a single oscilloscope trace covering approximately 1.5 tool motion cycles. The horizontal scale is approximately 2 us/div.)

38
WIRE BOND EVALUATION

sweep period, and the photograph serves as a record that may be preserved

for future reference

.

In addition to measuring the absolute displacement of bonding tools

,

the laser has been compared with the capacitor microphone for trouble

shooting bonding machines. Since it is not practical to design many different mounting systems to attach the laser rigidly to the variety of

bonding machines available, measurements are restricted to a single oscilloscope sweep and thus are not suitable for trouble shooting and diagnostic work. An example is shown in figure 7, a single exposure photograph

from a double beam oscilloscope. The upper pattern is a repetitive trace

from a capacitor microphone, the lower is a single trace from the laser

interferometer. Both instruments were directed toward the same bonding

tool on which the set screw had been loosened. The microphone pattern

clearly indicates that a problem exists, but the single interferometer

trace below does not encompass enough bonding tool vibration cycles to

detect the problem. Thus at present the laser is used exclusively for

absolute calibration of bonding tool motion. This calibration can then

be transferred to more portable measuring devices, such as magnetic

pickups and capacitor microphones.

One problem with the small commercial laser employed in the present

interferometer is that it is primarily a multimode device. For interferometry, the unit must lase in a single mode. Previously the laser had

been tuned to maximum output and then detuned to the TEMqq mod. This is a critical adjustment and subject to both amplitude and mode drifting.

Even a few percent of a higher mode creates an unacceptable noise level

in the detector output. At the suggestion of the manufacturer, special

end plates were made to increase the laser cavity length by 1 cm. The

mirrors were reinstalled and the laser retuned. This resulted in a laser

that operates only in the TEMqq mode and therefore makes a better interferometer. (G. G. Harman and A. W. Stallings

)

New experiments were undertaken to investigate the process of making

aluminum ultrasonic wire bonds and, if possible, to develop measurement

methods that may be applicable to in-process bond quality determination.

The initial experiment consisted of mixing a local oscillator with the

output of a 1/8-in. (0.3-mm) diameter capacitor microphone with a frequency response greater than 200 kHz. The microphone was positioned to

pick up sound emitted from the tip of a bonding tool during actual wire

bonding. The mixer output was fed into an audio amplifier and from there

into a speaker, an oscilloscope, and a tape recorder as shown in the block

diagram of figure 8. This apparatus allows the bonding machine operator

to listen to an audible beat frequency sound during actual bond formation.

Typically, with 0.001-in (25-ym) diameter aluminum wire, an ultrasonic

bond is made in about 100 ms which is too short a time for the human ear

to distinguish small amplitude or frequency changes. Therefore the audio

beat frequency was recorded on magnetic tape and later played back at a

39
WIRE BOND EVALUATION

A - TRANSDUCER E - AMPLIFIER

8 - BONDING TOOL F - OSCILLOSCOPE

C - CAPACITOR MICROPHONE G - TAPE RECORDER

D - DOUBLE BALANCED MIXER H - SPEAKER

Figure 8. Scheaat i c representation of mixer system for monitoring ultrasonic bond formation.

slower rate so that d tails of bonding could be both heard and carefully

examined on an oscill "cope. Such recordings were made for the fundamental tool vibration ^equency through the fourth harmonic, the extreme

response limit of the microphone. A brief investigation extending from

the fundamental to the fifteenth harmonic was also carried out using the

voltage or current from ,he power supply driving the transducer. At times

the bonding machine schedule was modified by reducing the ultrasonic power

and increasing the bonding time so that the bonding process could be heard

more clearly in real time.

This initial study was made using two ultrasonic power supplies of

different manufacture. However, only one bonding machine, one transducer,

and one bonding tool extension were used in the tests. Some variation in

the results could be expected if the latter two had also been changed.

The initial results were dependent on the particular power supply used to

drive the transducer. For power supply A there was a few percent decrease

in the 60-kHz tool vibration amplitude during bonding, while for power

supply B there was a 20 percent increase. These differences are presumably a function of the electronic feedback systems in each power supply

as well as the method of transducer drive, which is constant current in one and constant power in the other. As would be expected, differences

were observed in the various harmonic relationships during bonding. The

second harmonic of both supplies increased in amplitude but in the case

of power supply B an unusual frequency warbling was heard during part of

each bond. The third harmonic of power supply A was pure tone that increased 5 to 6 dB in amplitude as the tool touched the wire or substrate.

There was an additional increase of several decibels during actual bonding.

40
WIRE BOND EVALUATION

These large increases in amplitude of the third harmonic which is generated by pressure on the tool are in general agreement with a simple

theory of a damped mechanical mass-spring system [2].

The results for both power supplies were a function of the microphone pickup position with respect to the vibrating bonding tool. The

largest amplitude changes were obtained with the microphone near the

bottom of the tool, close to the wire. The smallest changes were observed

with the microphone in front of the transducer horn. Similarly small

changes were also observed when the mixer was connected directly to the

electrical drive of the transducer.

This experiment was originally intended to be run with a special

power-supply-transducer combination in which the transducer is a selftuning element of the oscillator circuit. Thus as the tool motion is

affected by the bonding process both the frequency and the amplitude can

be expected to change, producing two measurable variables during bond formation. The power supply and transducer did not arrive during the quarter

and thus the initial experiments were performed with a fixed frequency

system. (G. G. Harman)

Eight magnetic pickups were assembled and eight bonding machine

mounts for either magnetic pickups or capacitor microphones were made at

the request of a sponsor. A procedure for using both this equipment and

capacitor microphones to tune and trouble shoot ultrasonic wire bonding

equipment has been prepared [3].

(G. G. Harman, H. K. Kessler, G. P. Spurlock, and A. W. Stallings)

Additional studies of extraneous machine-induced motions were carried

out with the electromagnetic displacement sensor during actual bonding on

a second bonding machine. Results were similar to those reported previously (NBS Tech. Note 571, p. 27). A typical oscillogram of the motion between the machine housing, which supports the bonding tool, and the work

stage with the motor and all cams operating is shown in figure 9. The

indicated motion is in the side-to-side direction. The trace in figure

9a covers the part of the bonding cycle from first search through bonding

to loop formation. The upper trace displays the ultrasonic bonding pulse

to indicate when the bonding occurs. The lower curve, on the same time

scale, shows the various self-induced bonding machine vibrations. Maximum

vibration amplitudes about 13 ym peak-to-peak during and immediately after

the bond is formed are indicated by fluctuations in the envelope amplitude

.

As the machine goes into the loop position, a displacement of the entire

housing is indicated where the envelope of the oscillogram decreases to

about one fifth its initial value. Vibrations with amplitude of 5 ym or more peak-to-peak continue into this portion of the cycle, with several

peaks approaching 13 ym.

41
WIRE BOND EVALUATION

1 HjMBM

PmipldlP Mil PIhHIhI

iflisi vifr

" mWM'! "PI*

|l

1 illttll liW|»|

If if"

i li lliill till ill tii, L

iiil

il!ii

J iBiiMa r^

a: First search position through bonding and into loop position.

b: Second search position through bonding and wire cutoff to reset position.

Figure 9. Measurement of the self-induced vibrations of a bonding

machine. (The upper trace displays the ultrasonic bonding pulse. The lower trace gives the output of the displacement sensor. The vertical

scale for the lower trace (displacement) is 2.5 ym/div. The horizontal

scale for both traces is 100 ms/div.)

42
WIRE BOND EVALUATION

The trace in figure 9b covers the next part of the bonding cycle

from second search, through second bonding and wire cutoff, to the

starting or reset position. The temporary displacement of the housing

which occurred earlier in the cycle continues until wire cutoff, at which

time the housing moves back to its original position accompanied by large

vibrations with initial amplitudes approaching 13 urn. After this, the

vibrations damp out as the machine reaches its starting position and the cam motor stops.

A vibration study was made of each individual cam, including the

operating levers , in order to determine in which cam and lever combination

the vibration originated. It was found that each cam and lever has its own vibration characteristics, but if all cams and levers are operating,

some vibrations are amplified and others are cancelled out.

(H. K. Kessler)

Equipment Improvements — An improved electromagnetic displacement

sensor was built for the wire indentation tester previously described

(NBS Tech. Note 560, p. 37). As shown in figure 10a, it is shielded both

magnetically and electrostatically to prevent pickup from the motor and

other moving parts of the tester. The system shown in Figure 10b, is designed to simulate the bonding operation insofar as the wire characteristics are concerned. The wire is indented at regular intervals by a wedge with dimensions similar to those of a bonding tool. A fixed force

is applied to the wire each time and the depth of impression is measured

by the displacement sensor and recorded. Any irregularities can be readily observed and hence the indentation qualities of the wire assessed.

Modifications were made to the bond-pulling apparatus to allow

pulling of bonds at a maximum rate of 77 gf/s (764 mN/s )

.

(H. K. Kessler and A. W. Stallings

)

Bibliography and Critical Review — The first draft of the bibliography on microelectronic wire bonds was completed. The bibliography is

a compilation with key words of over 200 published articles, U. S. Government reports, U. S. Patents, standards, and conference talks relevent

to wire bonds in three areas: testing, fabrication, and degradation. An

attempt was made to make the collection of papers on test and evaluation

methods complete while the collection of articles in the areas of fabrication and degradation is meant to be representative. While articles

published as early as 1957 are included, the bibliographic search concen- trated on the period from 1965 to 1970, inclusive. The selection of

papers was generally limited to those that were pertinent to wire bonds

where the wire diameter is less than about 50 pm (0.002 in.) and where

the wire is bonded by either thermocompressive or ultrasonic means.

Revision of the first draft of the critical survey paper was begun.

(H. A. Schafft and E. C. Cohen)

43
WIRE BOND EVALUATION

.^nn^mHiu^lll^ ' A.^1

ffffiutuntjj

oooooooooo^

SZZZZZZZZZ2ZZZ2ZZ

IVV"^^!

12./ f A

A - LAMINATED MU-METAL CORE

B - COIL FORM

C - COIL

D - MU-METAL SHIELD

E - BRASS SHIELD

F - OUTPUT CONNECTOR

G - GROUND CONNECTION

Improved pickup coil for electromagnetic displacement sensor.

A - LIFTING ARM CAM

B - LIFTING ARM

C - DRIVE MOTOR

D - SENSOR HEIGHT ADJUSTMENT

E - DISPLACEMENT SENSOR, PICKUP COIL

F - ELECTRICAL INPUT CONNECTOR

G - SAPPHIRE ANVIL

H - ELECTRICAL OUTPUT CONNECTOR

I - WIRE FEED CAPILLARY

J - WIRE FEED SPOOL

K - WIRE TAKE-UP SPOOL

L - WIRE FEED HEIGHT ADJUSTMENT

M - DISPLACEMENT SENSOR, DRIVER HEAD

N - LOCATION OF WEIGHT - WIRE FEED RATE CONTROL

b: Indentation tester.

Figure 10. Improved electromagnetic displacement sensor and wire in- dentation tester.

44
WIRE BOND EVALUATION

Plans : Laser interferonstry calibration and other studies of bonding tool motion will continue. Further work on electronic mixing of

bonding tool signals with local oscillators will resume after the special

power- supply- transducer combination is received. Evaluation of ribbon

wire for ultrasonic bonding will resume after the ribbon wire bonder is

received following modification by its manufacturer. Work on the wire

indentation tester will continue. Experimental and statistical analysis

of significant factors in the wire bond pull test will continue. Further

assistance will be given to sponsors in connection with problems en- countered on device production lines. Preparation of the bibliography

and revisions of the first draft of the survey paper will continue.

4.4. PROCESSING FACILITY

Objective: To establish a microelectronic fabrication laboratory

with the facilities and procedures necessary for the production of

specialized silicon devices for use in research on measurement methods,

Progress : A. 10-kW electron beam evaporation system was installed

for the deposition of aluminum. A photograph of the system, with the

planetary substrate drive (NBS Tech.

Note 560, p. 39), is shown in figure

11. This system offers several advantages over the tungsten filament

system it replaces because it makes

possible very high aluminum deposition rates (in excess of 0.5 ym/min),

Since the evaporation rate is high,

the contamination of the metal is

lower. Experience with p-channel,

metal-oxide-semiconductor structures

revealed that contaminated aluminum

was one of the primary contributions

to excessive threshold voltages.

(T. F. Leedy, J. Krawczyk, and

L. M. Smith)

A system was built for the purpose of depositing vitreous silicon

dioxide films from silane. This

process is useful when a silicon

dioxide layer must be produced at

low temperature. Such oxide films

may be used for diffusion masks, for

thin film components, or to protect

Figure 11. Electron-beam evaporation system. ^ chip from abrasion. The SOUTCe

45
PROCESSING FACILITY

FLOWMETERS

JL

1%

SiH4

N2

<y

REGULATOR

^ ^>

^

N* 2

o

TEMPERATURE

CONTROLLER

A

REACTION

CHAMBER

-•*«-

HOT

PLATE Fn

THERMOCOUPLE

POWER

Figure 12. Schematic representation of silicon dioxide vapor deposition system.

n E)

DUAL BEAM

OSCILLOSCOPE

LAMP - DIAPHRAGM

LENS-^^-PHOTOCELL

4

E

ROTATING WAFER

TIME MARK

GENERATOR

PHOTO RESIST

SPINNER

a: Schematic representation.

b: Silicon wafer with seg- mented oxide pattern

(shaded region)

.

Figure 13. Apparatus for measuring dynamics of photoresist spinners.

46
PROCESSING FACILITY

material is commercially available high-purity silane. A silicon dioxide

deposit is produced by reacting nitrogen-diluted silane with oxygen at

temperatures ranging from 200 to 500°C. The system constructed is shown

schematically in figure 12. The wafers to be coated are placed on a heated glass plate in the reaction chamber. Since the deposition rate is

temperature sensitive the wafer temperature must be held constant to ±1°C

[1], The design of the reaction chamber is critical to the production of

uniform films at high deposition rates. Of prime concern are the total

flow rate, the geometry of the chamber, and the uniformity of the substrate temperature.

Preliminary infrared analysis of films deposited at 350°C shows the same features as observed for thermally produced oxides with one exception,

The absorption peak at 9.2 urn associated with thermally produced oxides

was shifted to a higher wavelength (9.7 um), characteristic of less dense,

deposited oxides [2], The dielectric constant of the deposited oxide

measured at 1 MHz was observed to be 4.50, again characteristic of such

films, as opposed to 3.8 5 for the thermally grown films.

(T. F. Leedy, J. Krawczyk, and E. I. Klein)

A simple method was developed for the measurement of the dynamics of

photoresist spinners. The final thickness of a photoresist film depends

on the angular acceleration of the wafer as well as its terminal angular

velocity [3]. To assure that a photoresist spinner will produce films

of consistent thickness, the acceleration must be measured and controlled.

The angular acceleration is dependent on the torque, the moment of inertia

of the rotating components, and other physical parameters of the spinner

system.

The measurement apparatus is shown in figure 13. A silicon wafer

of the same size that is used in a standard monolithic process is oxidized to appear dark blue in color (oxide approximately 0.13 pm thick).

The oxide is etched in a radial pattern as shown in figure 13b. Utilizing

the difference in the reflectivity in the pattern, a light source and a photocell sense the passage of the pattern as the wafer rotates on the

spinner. The output of the photocell, representing the passage of each

segment, is displayed on the oscilloscope. By incorporating an accurate

timing pulse into the oscilloscope display, both the acceleration and

final velocity can be determined. (T. F. Leedy and J. Krawczyk)

Plans : A three-tube furnace will be installed and made operational

to permit the increased diffusion capability presently needed. Additional measurements on thin dielectric films will be started to determine

their characteristics more precisely. Study of the techniques to determine spinner dynamics will be continued.

47
4.5. REFERENCES

1.1. Metallization Evaluation

1. Collins, L. E., Perkins, J. G., and Stroud, P. T., Effect of Ion

Bombardment on the Adhesion of Aluminum Films on Glass, Thin Solid

Films 1, 41-45 (1969).

2. Hamilton, B., and Rawson, H., Fracture of Glass Under a StationaryBall with Combined Normal and Tangential Loading, Brit. J. Phys. D:

Appl. Phys. 3_» L40-L44 (1970).

3. Weaver, C, and Hill, R. M., Adhesion of Evaporated Aluminum Films,

Phil. Mag. 3_, 1102-1110 (1958).

4.2. Die Attachment Evaluation

1. Daniels, R. G., Thermal Management of Integrated Circuits — Part I. Heat Transfer and Integrated Circuits, Electro-Technology 83_, No. 1,

22-30 (January, 1969).

4.3. Wire Bond Evaluation

1. Martin, B. D., and Wilson, A. D., An In-Line Laser Interferometer

Applied to Ultrasonic Bonder Characterization, 1970 IEEE Ultrasonics

Symposium, San Francisco, California, October 21, 1970.

2. Salzer, T. , Raytheon Co., Sudbury, Mass., private communication.

3. Harman, G. G. , and Kessler, H. K.

,

Application of Capacitor Microphones and Magnetic Pickups to the Tuning and Trouble Shooting of

Microelectronic Ultrasonic Bonding Equipment, NBS Tech. Note 573,

May, 1971.

4.4. Processing Facility

1. Goldsmith, N., and Kern, W. , The Deposition of Vitreous Silicon

Dioxide Films from Silane, RCA Review 28_, 153-165 (1967).

2. Pliskin, W. A., Lehman, J. S. , Structural Evaluation of Silicon

Oxide Films, J. Electrochem. Soc. 112, 1013-1019 (1965).

3. Damon, G. F., The Effect of Whirler Acceleration on the Properties

of Photoresist Film, Proc. Second Kodak Seminar on Microminiaturization, 34-36 (1966).

i-J. 8

* U.S. GOVERNMENT PRINTING OFFICE : 1971 181-323/15
5. METHODS OF MEASUREMENT FOR 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 : The literature search for methods to measure thermal re- sistance and transient thermal response of semiconductor devices was continued. Work on the bibliography and review paper was continued, although in a lower priority status. (M. Sigman and F. F. Oettinger)

Work has continued on the preparation of a suitable measurement procedure and data collection format for the preliminary thermal resistance

round robin being conducted by JEDEC Committee JC-25 on Power Transistors.

The capabilities of on-line measuring equipment available to participants

are being determined in order to obtain information to assist in establishing the conditions to be used for the test. (F. F. Oettinger)

A study was conducted to determine what effect variations in device

case temperature would have on the voltage at which a hot spot was formed.

Measurements of base current, Ig, as a function of collector-emitter

voltage, Vp E

, at constant collector current, Iq, for case temperatures of

25 to 100°C indicated that the voltage level at which the hot spot was formed is nearly independent of the case temperature. Typical curves,

shown in figures 14 and 15, indicate that the change in the value of Vqe

at which the hot spot occurs as the case temperature increased from 25 to

100°C, although small, is greatest under low Iq, high Vq£ operating con- ditions. The change under these conditions may be due to the temperature

dependence of the avalanche characteristic of the collector-base junction

at Vpp levels close to the breakdown voltage. Preliminary infrared microradiometer measurements indicated that at the voltage at which the hot

spot was formed, the maximum hot-spot temperature increased approximately

75°C when the case temperature was raised from 25 to 100°C. These results

suggest that hot-spot formation is governed by the magnitude of a localized temperature increase (.which may occur as a result of device geometry,

device construction, or junction or bulk anomalies) rather than by the

absolute temperature of the chip, but that the maximum temperature in the

hot spot is affected by the case or chip temperature. The results appear

to be consistent with both the lateral thermal instability model [1] and

the previously proposed thermal hysteresis model CNBS Tech. Note 520,

pp. 49-52).

To allow study of the thermal inertia of the hot spot during thermal

hysteresis, modification of existing equipment was considered. It was found that such modification would compromise its usefulness for the

study of thermal resistance and common emitter current gain. The design

and fabrication of a second, more flexible measuring system was started.

(S. Rubin, G. J. Rogers, and F. F. Oettinger)

49
THERMAL PROPERTIES OF DEVICES

0.73

40 60 80 100 120 140 COLLECTOR -EMITTER VOLTAGE- V CE (V)

Figure 14. Base current as a function of collector-emitter voltage for a 35-W, triplediffused, silicon transistor. (The curves were generated for various values of case teaperature, T

c

, and collector current, Iq, as noted. Since each curve was generated

while holding I^ constant, 1/Ig is proportional to hp.£. The peak Junction temperature,

Tj, as aaaaured with an infrared microradiometer at the voltage at which the hot-spot was

formed is also given for some cases. The hot spots form' at the minima of the curves*)

50
THERMAL PROPERTIES OF DEVICES

20 40 60 COLLECTOR- EMITTER VOLTAGE- YCE (Y)

120 140

Figure 15. Base current as a function of collector-emittar voltage for a 20-W, triplediffused, silicon power transistor. (Tha curvea were generated for various values of case temperature, T

c

, and collector current, Iq, as noted. Since each curve was gen- erated while holding I c constant, 1/Ig ia proportional to hp£. Hot spots form at the minima of the curves.)

51
THERMAL PROPERTIES OF DEVICES

* 8 12 16 20

POWER (W)

24 28 32

Base-emitter voltage as a function of applied power. The base-emitter voltage

was measured 12 ys after switching to the measuring base current of 6.5 mA. During the application of power the collector current was 1 A; case temperature was held at 25°C.

100

o

0^ 90

o

i 80

LJ

(£ 70 D

5 60 <ZUJ

0. 50 2

UJ

»- 40

UJa 30

u

JUNCTION

TEMPERATURE

CASE

TEMPERATURE

20

440 480 520 560 600 640

BASE-EMITTER VOLTAGE - VBE (mV)

b. Dynamic calibration curve. In this calibration procedure the transistor under

test was switched from the operating condition VCE - 4 V, Ig " 1 A to the measuring condition IB

- 6.5 mA. The base-emitter voltage was measured 12 ys after

switching to the low level measuring current.

Figure 16. Data used to calculate the thermal resistance of a 35-W, triple-diffused,

silicon transistor as a function of applied power.

52
THERMAL PROPERTIES OF DEVICES

Electrical measurements of thermal resistance, Rg, and common emitter

current gain, hpp» as a function of Vp E and applied power for I„ up to

200 mA have been reported (NBS Tech. Note 571, pp. 35-42). The base

switching circuit and the operational amplifier used to control the collector current were modified to increase the collector current capabilities of the measuring system to 1 A. Typical raw data used to calculate

thermal resistance as a function of applied power are shown in figure 16.

Base-emitter voltage, VBE , measured for a metering current of 6.5 mA,

12 ys after the end of the power pulse, is plotted as a function of power

in figure 16a (solid line). Below 4 W, (Ip = 1 A, VpE

= 4 V), an increase

in non-thermal switching effects, probably due to the decrease in hpE as Vp£ is decreased, causes the VBE curve to become nonlinear. Although the

non-thermal effects become less significant if the delay in measuring the

temperature sensitive parameter is increased from 12 to 17 ys, the additional cooling of the junction that occurs during the longer delay time

may result in falsely low values for Rg. If the non-thermal switching

effects were not present, VgE at low power levels would have the values

represented by the dashed line in figure 16a provided the linear relationship between power and junction temperature, Tj, extends to zero power

dissipation where Vgj? = 621 mV.

The dynamic calibration, shown in figure 16b, was carried out by

switching from a low-power condition (V-p = 4 V, T_ c

= 1A) to the measuring condition (Vp E open, Ig = 6.5 mA) for case temperature, T

p

, from

25 to 100°C. Under these conditions Tj is somewhat greater than Tp. The

junction temperature may be found by displacing the case temperature line

to the right so that the extrapolated value of V

fiE for zero power dissipation (621 mV) corresponds with 25°C as shown by the arrow in figure 16b.

The thermal resistance, Rq, can be found from the slopes of the

linear plots in figure 16a and b: RQ

= (ATC /AVBE ) (AVBE /AW). For the

example shown in figure 16, RQ

= (0.514°C/mV) (4.23 mV/W) = 2.2°C/W. In

figure 17, hEE for this device, a 35-W, triple-diffused, silicon transistor, is plotted together with Rg as computed above. The data indicate

that there is no decrease in hpp as Vp Eis increased so apparently no appreciable current constriction occurs. It should be noted that if Rq

had been calculated from the non-linear portion of the Vg E against power

curve there would have been an apparent increase in Rg with decreasing

power. It has been shown previously (NBS Tech. Note 571, pp. 35-42) that

for this type of device structure the actual chip thermal resistance does

not increase with decreasing power. (S. Rubin and F. F. Oettinger)

In some cases, an increase in RQ

is observed as Tp is increased. An

experiment was conducted to determine if this increase in Rg can be re- lated to the increase in the thermal resistivity of the silicon chip with

temperature. Measurements were made on a 35-W, epitaxial diffused, silicon transistor with a silicon-chip-to-total-device thermal resistance

ratio of approximately 1 to 3. The thermal resistance measured for

53
THERMAL PROPERTIES OF DEVICES

5 4 L

o

o

Q>

tr 2

I 1 —

T~"

1 1 1 1 1

I 1 1 T 1 1 1

- hFE^ -

- R*

-

1 1 . 1 1 1 1 1 1 i i i 1

i i I 2 4

120

100

80

UJ 60 u- 40

20

8 10 12 14 16 18 20 22 24 26 28 30 32

P(W)

Figure 17. Common-emitter current gain, h-pg, and thermal resistance, Rg, of a 35-W,

triple-diffused, silicon transistor as a function of applied power, P. (Case temperature was held at 25°C. The thermal resistance was calculated from the data shown in Figure 16.)

Figure 18. Thermal resistance, RQ

, as a function of case temperature, T^, for a 35-W, epitaxial-diffused, silicon transistor. (During the application of power the collector cur- rent was 500 mA. The base-emitter voltage, from which the thermal resistance was derived, was measured 12 ys after switching to a low level measuring current.)

54
THERMAL PROPERTIES OF DEVICES

I~ = 500 mA and for V„p from 1 to 16 V at case temperatures ranging from

25 to 100°C is plotted as a function of To in figure 18. In these experiments dynamic calibration curves were run beginning at case temperatures

of 25, 50, 75, and 100°C so that the effect of the temperature dependence

of Rq on the relationship between To and Tj under calibration conditions

(power dissipation of 4 W) could be neglected.

The measured increase in Rq for an increase in Tq of from 25 to

75°C was 9 percent while the calculated increase in Rq expected because

of the increase in the thermal resistivity of the silicon chip over the

same temperature range was approximately 12 percent. The calculation was based on thermal conductivity data reported by Shanks [2] and the assumption that about one third of the temperature drop from junction to mounting surface is due to the thermal resistance of the silicon chip [3].

Although it has been known that the thermal resistivity of silicon is

temperature dependent, most experimenters have not considered this effect.

The results of this study indicate that, for devices with a significant

chip-to-total-device thermal resistance ratio, the effect of the variation of the thermal resistivity of silicon with temperature should be

considered when determining the test conditions for Rq measurements. For

such devices, either R

fl should be measured at maximum—rated junction temperature or a derating factor should be generated to allow for the in- crease in thermal resistance of silicon with temperature.

(S. Rubin, G. G. Rogers, and F. F. Oettinger)

Plans : The literature search, work on the bibliography and writing

of the first draft of the review paper on thermal resistance and transient

thermal response measurements will continue.

The measurement procedure and data collection format for the Committee JC-25 preliminary thermal resistance round robin will be established.

The measurements will be made on TO-66 and TO-3 encased power transistors.

It is expected that Rq will be measured after operation of transistors in

high-current, low voltage conditions: Ip = 0.5 to 1 A and VC £

= 10 to

20 V.

Design and fabrication of the more flexible system for measuring Rq

and hFE will continue. Various studies pertaining to hot-spot formation

and thermal hysteresis can be carried out with the new system.

Measurement of Rq and hpp as a function of power for various collector

current levels will continue. Infrared microradiometer and thermographic

phosphor measurements will then be made on the same devices to investigate

relationships which may exist between the peak temperature and the electrically-measured temperature.

55
5.2. THERMOGRAPHIC MEASUREMENTS

Objective ; To evaluate the utility of thermographic techniques

for detection of hot spots and measurement of temperature distribution

in semiconductor devices.

Progress : A 50-um diameter fiber optic probe and scanning eyepiece

for the photometric microscope was received but had to be returned for

correction of defects in the mechanical drive. The spatial resolution

of the 50-um and 150-um diameter probes were compared by scanning a blackwhite boundary on the 1.0 pattern of a Microcopy Resolution Test Chart*

with each and recording the output of the photometer. The results are shown in figure 19.

10

CO

CD

BLACK WHITE

T

150/jn PROBE

50/im PROBE

50 100

DISTANCE (/im)

ISO 200

Figure 19. Spatial resolution of fiber optic probes compared by scanning a blackwhite boundary on a Microcopy Resolution Test Chart and recording the changes in

photometer output. (The 7 to 1 reduction in light output from the 50-pm probe was compensated by increasing the amplification of the photometer and recorder.)

Available as SRM-lOlOa from the Office of Standard Reference Materials,

National Bureau of Standards, Washington, D. C. 20234. The 1.0 pattern consists of 0.5-mm wide alternate black and white stripes.

56
THERMOGRAPHIC MEASUREMENTS

A preliminary evaluation of the performance of the infrared microscope indicated that temperatures can be measured with a precision of

±1°C. If emissivity is known, the accuracy is about ±2°C for temperatures above 45°C; the accuracy is poorer between 25 and 45°C. The infrared microscope was used to make temperature profile and spot temperature measurements on several power transistors whose thermal properties

had previously been checked electrically. Further work was interrupted

by a malfunction which prevented zeroing the output of the microscope.

(G. J. Rogers, F. F. Oettinger, and L. R. Williams)

Plans : Temperature calibration of the thermographic phosphors will

be completed. When the IR microscope is again available, comparison of

the relative capabilities of the IR microscope and thermographic phosphors will be resumed.

5.3. MICROWAVE DEVICE MEASUREMENTS

Objective : To study the problems and uncertainties associated with

the measurement of microwave devices, and to improve the techniques of

these measurements.

Progress : The audio i-f portion of the X-band mixer diode measurement system (NBS Tech. Note 571, pp. 43-45) was built and tested. It uses an automatic switch and an oscilloscope to compare a precision d-c

reference voltage with the peaks of the 1-kHz square wave used as the

i-f signal. This i-f voltage is used to calculate mixer conversion loss.

The need for better shielding became apparent when measurements closer

than ±0.5 mV were attempted. Bypassing the load and bias circuits and

using a 50-ft coaxial load to terminate the mixer for both a-c and d-c

allowed measurements to better than ±50 uV. This represents a precision

of ±0.1 percent for a 6-dB conversion loss mixer output voltage of about

50 mV peak-to-peak across a normal 400-ft i-f load, using a 1-dB peak-totrough modulation which is equivalent to a single-frequency r-f signal of

about -28 dBm. The precision was limited by the presence of a non-random,

non-sinusoidal noise whose origin has not been determined. Based upon

the resolution obtained with the reference voltage trace, another orderof-magnitude improvement in precision would be possible if this noise

were eliminated (leaving only the random oscilloscope and system noise).

Mixer conversion loss is based upon available, rather than delivered,

output power, and its determination thus requires a knowledge of the source immittance which is predominately resistive at audio i-f's.

Equations were developed which enable the mixer i-f resistance and opencircuit i-f voltage to be calculated from measurements of normal mixer

i-f voltages across two different finite values of load resistance. This

obviates the use of an external i-f signal source as with a bridge measurement. The closer the two resistance values are to each other, the

57
MICROWAVE DEVICE MEASUREMENTS

less is the dependency on mixer linearity and the less is the effect of

circuit reactances, but the precision is lower.

Preliminary measurements, which were limited by available equipment

to a ±0.5 mV precision for load voltages of about 20 to 90 mV, seemed to

indicate that the mixer i-f resistance as calculated from these equations

is independent of load resistance over the range from 100 ft (equal to the

d-c load) to 100 kft (essentially an open circuit for the mixer). Extensive proof of this independence, however, using higher precision and a variety of diodes, is required before complete mixer output linearity can be assumed for the measurement.

To obtain i-f resistance and open-circuit i-f voltage without calculation, a measurement technique was developed to enable the mixer i-f

resistance to be read directly from a calibrated precision decade re- sistance box. Setting the i-f load to this value then enables half the

open-circuit mixer i-f voltage to be measured directly. A search of the

literature revealed that this method was invented in the nineteenth cen- tury by Sir Henry Mance to measure battery resistance [1-3]. This method

is related to a method invented by Kelvin [3] for measuring galvanometer

resistance and appears to have been lost to later generations. It seems well worth reviving, since the apparently unrecognized ability of the

method to enable a source immittance to be measured with only small load

variations makes it potentially valuable for non-linear and high-power

devices, where the load immittance must be maintained close to a nominal

value at all times. It appears that the method can be applied to microwave and other high-frequency sources by using a hybrid junction, which

is the high-frequency analog of the Wheatstone bridge whose theory forms

the basis for Mance' s method.

In further study of the r-f portion of the X-band system the local

oscillator power occasionally drifted in amplitude to a small but objectionable extent, even though the klystron is frequency stabilized by

being phase-locked to a crystal-controlled oscillator. Stable local

oscillator power is of fundamental importance to all mixer measurements,

but is particularly important to modulation-type conversion loss measurements, where the r-f test signal is proportional to the power of the local

oscillator from which it is derived. In order to improve the local oscillator amplitude stability, a preliminary, experimental version of a feedback loop (leveler) was built and tested. The circuit contained a diode

detector, an operational amplifier, and the PIN modulator that had been

previously connected for amplitude modulation. The leveler successfully

maintained the power available to the mixer independent of klystron power

variations (over the several decibel range of excess power available) to

within the discernible limits of measurement (less than ±50 yV). Power

to the mixer was set by means of the operational amplifier offset voltage

control, thus obviating a d-c reference supply. A clean, stable modulation was obtained by applying a low-level square wave to the noninverting op-amp input terminal. (.J. M. Kenney)

58
MICROWAVE DEVICE MEASUREMENTS

Plans: The r-f portion of the circuit will be rebuilt to permit

leveling during actual mixer measurements. This will require extensive

and difficult alterations of the r-f circuit because of the need to

isolate the monitoring detector from power reflected by the mixer and to

relocate the precision attenuator used to measure the degree of modulation to a point following the feedback loop. The modulator which is now

connected by a switch and a coaxial line will be moved to the main wave

guide line to avoid loss and leakage, a harmonic-absorption pad will be

added to minimize the reflection, back to the mixer, of local oscillator

and signal harmonics generated by the mixer, and several other components

will be relocated to improve circuit performance.

Following this, the audio i-f circuit will be redesigned, to incorporate Mance's method for measuring mixer i-f resistance, and rebuilt

on a shielded chassis. Mance's method, and its application to low and

high frequency measurements, will be reported upon at a later date after

some experience with it is obtained.

5.4. CARRIER TRANSPORT IN JUNCTION DEVICES

Objective : To improve methods of measurement for charge carrier

transport and related properties of junction semiconductor devices.

Progress : On the recommendation of sponsors, visits were made to

discuss methods for making transistor delay time measurements with the

participants in previous comparative tests of the measurement: the Air

Force Weapons Laboratory, Sandia Laboratories, and Bendix Research Corporation. Similar discussions were held with personnel from the Boeing

Aircraft Company (a contractor of a sponsor ) and from the NBS Electromagnetics Division.

Laboratory work was focused on assembly, initial testing, and adjustment of the Sandia-type delay-time bridge [1] constructed in this

laboratory. Measurements on the bridge were made between 3 and 30 MHz.

It was found that the signal generator to be used with the bridge must be

carefully shielded and have a power output of 0.2 W or greater and that

connections between the bridge, signal generator and radio receiver (used

for null detection) must also be well shielded. Lack of adequate shielding shows up, in the extreme case, by nulls not being obtained, or in less severe cases, by a strong frequency dependence of the delay zero-set value,

The transistorized current amplifier which interfaces between the transistor under test and the null detector in the original Sandia version of the

bridge was duplicated with minor modifications to increase its stability ,*

* The amplifier was modified and constructed by D. J. Brenner of the NBS

Measurement Engineering Division.

59
CARRIER TRANSPORT IN JUNCTION DEVICES

A preliminary analysis of the Sandia bridge" showed that if the

line stretchers (which are used as the adjustable delay elements) are terminated — as they are — in their characteristic impedance of 50 ft at

the attenuator-phase splitter ends, the delay time read from the line

stretchers should be independent of any mismatch between the other ends

of the line stretchers and the transistor being measured. This is an important result since it is known that transistor impedance varies with

frequency, bias conditions, and type. It was also stated that electrical

isolation of 40 dB between the "test" and "reference" line stretchers is

required to achieve a negligible (< 1 percent) measurement error. A

measurement at 10 MHz yielded isolation of 57 dB on the bridge constructed in this laboratory.

As a continuation of the literature survey and evaluation of existing

techniques for application to the project goals, literature citations

demonstrating that an analysis of transistor noise spectra should yield

values of base transit time are being compiled. References on the problem of making electrical probe measurements on transistors prior to metallization are also being collected. (D. E. Sawyer)

Plans : Delay-time measurement and probing techniques will continue

to be discussed with appropriate workers in the field. The compiling of

literature citations on these two topics, and on the application of noise

spectrum analysis to the determination of device internal processes, will

continue.

Pertinent measurement data will be obtained on the Sandia delay-time

bridge including interchannel isolation frequency, the ratio (which should

be unity, ideally) of "test" to "reference" line delay for null as a function of frequency, and initial delay. Construction of the socket and

biasing arrangement for the vector voltmeter will be completed. It is

planned that work on both the Sandia bridge and the vector voltmeter

system will be completed at about the same time so that initial transistor delay time measurements can be made on both instruments to provide,

comparative data.

5.5. SILICON NUCLEAR RADIATION DETECTORS

Objective: To conduct a program of research, development, and device evaluation in the field of silicon nuclear radiation detectors with

emphasis on the improvement of detector technology, and to provide con- sultation and specialized device fabrication services to the sponsor.

Carried out by D. Ellerbruch of the NBS Electromagnetics Division,

60
SILICON NUCLEAR RADIATION DETECTORS

Progress : Pre-flight bench-testing of lithium-drifted silicon

radiation detectors continued. Initial experiments were conducted in

the ambient exposure test program to determine the effects of hazardous

ambients on the performance characteristics of lithium-drifted and sur- face-barrier silicon detectors. Eight lithium-drifted silicon detectors

were acquired for radiation damage experiments.

Testing and Evaluation — Commercial lithium-drifted, surface-barrier

and diffused-junction silicon radiation detectors were given extensive

bench tests as part of a continuing program of detector evaluation and

acceptance for the sponsor.* An unusually high percentage of the large area (850 mm2

) devices failed to meet performance specifications when

operated under high-vacuum conditions. Several ultra-thin (2 to 5 um)

surface-barrier detectors were evaluated for use in a particle identification telescope. While the general performance of these detectors

was found to be satisfactory, their excessive capacitances resulted in

a large electrical noise contribution by the charge-sensitive preamplifiers used. In addition a few diffused- junction detectors were examined.

In general these devices performed poorly, due primarily to bad electrical contacts and thick particle entrance windows.

(Y. M. Liu and J. M. Morrison)

Radiation Damage — A literature search of radiation effects in

lithium-drifted silicon detectors and related subjects was completed. In

preparation for the next phase of the radiation damage study, eight

lithium-drifted silicon detectors were purchased from a commercial source,

(Y. M. Liu and B. H. Audet)

Ambient Exposure Tests — Silicon radiation detectors on future

spacecraft, notably Pioneer F, are expected to be subjected to certain

harmful gases and vapors, both before and after launch, which may have a detrimental effect on their performance. Prior to launch such solvents

as methyl alcohol and certain commercial leak detector fluids used to

check pressure and vacuum seals are used in and around the spacecraft.

After launch, the attitude in space of the satellite will be periodically

adjusted by hydrazine thrusters, which are expected to leave a low pressure shroud of ammonia gas around the spacecraft. The pressure of this

shroud is not known at this time.

A glass vacuum system was assembled to provide an appropriate en- vironment for determining the effect of such gases and vapors on detector

performance. Initial experiments revealed problems with excessive outgassing within the system. The glass system was disassembled, and con- struction of a new system, utilizing a stainless steel test chamber and

* Detailed data resulting from these tests have been transmitted to the

sponsor.

61
SILICON NUCLEAR RADIATION DETECTORS

stainless steel, greaseless ultra-high vacuum valves was begun. The new completely bakeable system is expected to solve the outgassing problem.

(E. I. Klein and B. H. Audet)

Plans : Exhaustive pre-flight bench-testing of detectors will con- tinue as required by the sponsor. The radiation damage study in lithiumdrifted silicon detectors will begin. The new ambient exposure test

system will be completed and tested.

5.6. REFERENCES

5.1. Thermal Properties of Devices

1. Scarlett, R. M., and Shockley, W., Secondary Breakdown and Hot Spots

in Power Transistors, IEEE International Convention Record 11, Part 3,

3-13 (1963).

2. Shanks, H. R. , Maycock, P. D. , Sidles, P. H. , and Danielson, G. C.

,

Thermal Conductivity of Silicon from 300 to 1400°K, Phys. Rev. 130 , 17i+3_1748 (1963).

3. Daniels, R. G., Thermal Management of Integrated Circuits, Part I,

Heat Transfer and Integrated Circuits, Electro-Technology 83, No. 1,

22-30 (January, 1969).

5.3. Microwave Device Measurements

1. Price, W. A., A Treatise on the Measurement of Electrical Resistance,

Clarendon Press, Oxford, 1895, pp. 153-155, 192-196 (Appendix V).

2. Roller, F. W., Electric and Magnetic Measurements and Measuring

Instruments, McGraw Publishing Co., N. Y. , 1907, pp. 146-148.

3. Northrup, E. F., Methods of Measuring Electrical Resistance, McGraw

Hill Book Co., N. Y. , 1912, pp. 218-219 (Art. 1106). Also: Kelvin's

Method, ibid, p. 235 (Art. 1117).

5.4. Carrier Transport in Junction Devices

1. Sullivan, W. H., A New Technique for the Direct Measurement of

Minority Carrier Base Transit Time in the Junction Transistor, Sandia

Laboratories Report SC-DR-69-419, September, 1969. Available from

the National Technical Information Service, Springfield, Va. 22151.

62
Appendix A

JOINT PROGRAM STAFF

Coordinator: J. C. French

Consultant: C. P. Marsden

Semiconductor Characterization Section

(301) 921-3625

A. J. Baroody, Jr.

D. L. Blackburn

F. H. Brewer

Mrs. E. C. Cohen*

M. Cosman

Dr. J. R. Ehrstein

Dr. W. M. Bullis, Chief

G . G . Harman

Mrs. R. E. Joel+

H. K. Kessler

Mrs. K. 0. Leedy

Miss C. A. Main

R. L. Mattis

Dr. W. E. Phillips

Mrs. T. A. Schultz"*"

Miss D. R. Ricks

H. A. Schafft

A. W. Stallings

G. N. Stenbakken

W. R. Thurber

Semiconductor Processing Section

(301) 921-3541

B. H. Audet

H. A. Briscoe

W. K. Croll

Mrs. S. A. Davis"1"

H. E. Dyson"

Dr. A. H. Sher, Acting Chief

W. J. Keery

E. I. Klein

J. Krawczyk

T. F. Leedy

Y. M. Liu

Miss J. M. Morrison

J. Oroshnik

L. M. Smith

G. P. Spur lock

Electron Devices Section

(301) 921-3622

Mrs. C. F. Bolton+ Mrs. R. Y. Cowan

Miss B. S. Hope"1"

R. L. Gladhill

J. C. French, Chief

J. M. Kenney

F. F. Oettinger

M. K. Phillips

R. C. Powell"

G. J. Rogers

S. Rubin

D

.

E . Sawyer

M. Sigman

L. R. Williams

* Part Time

+ Secretary

x Dr. J. A. Coleman is on temporary assignment to Office of Associate

Director for Programs

.

63
Appendix B

COMMITTEE ACTIVITIES

ASTM Committee F-l on Electronics

A. J. Baroody, Lifetime Section

C. F. Bolton, Committee Assistant Secretary

W. M. Bullis, Editor, Subcommittee 4, Semiconductor Crystals; Leaks,

Resistivity, Mobility, Dielectrics, and Compound Semiconductors

Sections

J. A. Coleman, Secretary, Subcommittee 5, Semiconductor Processing

Materials

J. R. Ehrstein, Resistivity, Epitaxial Resistivity, and Epitaxial

Thickness Sections

J. C. French, Committee Editor

T. F. Leedy, Photoresist Section

C. P. Marsden, Committee Secretary

R. L. Mattis, Lifetime Section

J. Oroshnik, Thick Films and Photomasking Sections; Chairman, Thin

Films Section

W. E. Phillips, Crystal Perfection, Encapsulation, Thin Films, and

Thick Films Sections; Chairman, Lifetime, Section

A. H. Sher, Germanium Section

M. Sigman, Editor, Subcommittee 5, Semiconductor Processing Materials

W. R. Thurber, Mobility, Germanium, and Impurities in Semiconductors

Sections

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 on Thermal Considerations for

Microelectronic Devices, Committee JC-11 on Mechanical Standardization; Technical Advisor, Thermal' Resistance Measurements,

Committees JC-22 on Thyristors, JC-20 on Signal Diodes, and

JC-25 on Power Transistors

R. C. Powell, Microwave Diode Measurements, Committee JC-21 on UHF

and Microwave Diodes; Task Group on Transistor Scattering

Parameter Measurement Standards, Committee JC-24 on Low Power

Transistors

S. Rubin, Chairman, Council Task Group on Galvanomagnetic Devices

H. A. Schafft, Technical Advisor, Second Breakdown and Related

Specifications, Committee JC-25 on Power Transistors

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)

64
APPENDIX B

H. A. Schafft, Chairman, Standards Committee Task Force on Second

Breakdown Measurement Standards

IEEE Nuclear Science Group:

J. A. Coleman; Administrative Committee; Nuclear Instruments and

Detectors Committee; Editorial Board, Transactions on Nuclear

Science; Chairman, 1970 Nuclear Science Symposium

A. H. Sher, Technical Program Committee, 1970 Nuclear Science

Symposium

IEEE Magnetics Group

S. Rubin, Chairman, Galvanomagnetic Standards Subcommittee

Society of Automotive Engineers

J. C. French, Subcommittee A-2N on Radiation Hardness and Nuclear

Survivability

IEC TC47, Semiconductor Devices and Integrated Circuits:

S. Rubin, Technical Expert, Galvanomagnetic Devices; U. S. Specialist

for Working Group 5 on Hall Devices and Magnetoresistive Devices

NMAB ad hoc Committee on Materials and Processes for Electronic Devices:

W. M. Bullis

65
Appendix C

SOLID-STATE TECHNOLOGY S 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. Radiation detectors — (B. H. Audet)

Design and development of fabrication procedures were begun

for a 52 element lithium-drifted silicon detector array to

be used as an electron scattering hodoscope by the Electronuclear Physics Section.

2. Sectioning and plating — (H. A. Briscoe)

Transistors were sectioned, polished, and stained to reveal

cross-sectional geometries, small piece parts were gold or nickel plated, and silver films were vacuum evaporated on several types of plastics for other groups in the Electronic

Technology Division.

3. Quartz and glass fabrication — (E. I. Klein)

A quartz closure for a vacuum system for growing barium

crystals was fabricated for the Physical Chemistry Division.

4. Ultrasonic fabrication — (J. Krawczyk)

Four pieces of piezoelectric ceramic were ultrasonically

machined for the Naval Ship Research and Development Center.

66
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 IndustryA Review of NBS Activity, 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.

Quarter Ending NBS Tech. Note Date Issued DDC Accessio

September 30, 1968 472 December, 1968 AD 681330

December 31, 1968 475 February, 1969 AD 683808

March 31, 1969 488 July, 1969 AD 692232

June 30, 1969 495 September, 1969 AD 695820

September 30, 1969 520 March, 1970 AD 702833

December 31, 1969 527 May, 1970 AD 710906

March 31, 1970 555 September, 1970 AD 718534

June 30, 1970 560 November, 1970 AD 719976

September 30, 1970 571 April, 1971

Current Publications:

Bullis, W. M., Measurement Methods for Microcircuits, Spaae Simulation,

J. C. Richmond, Ed., NBS Spec. Publ. 336, October, 1970, pp. 449-455.

Silicon Device Processing, C. P. Marsden, Ed., NBS Spec. Publ. 337,

November, 1970.

Sher, A. H., Carrier Trapping in Ge(Li) Detectors, IEEE Trans. Nucl. Sci.

NS-18 , No. 1, 175-183 (February, 1971).

Liu, Y. M. , and Coleman, J. A., Electron Radiation Damage Effects in

'Silicon Surface-Barrier Detectors

192-199 (February, 1971).

IEEE Trans. Nucl. Sci. NS-18, No. 1,

Rubin, S., and Oettinger, F. F., Thermal Hysteresis and its Possible

Effect on Restriction of the Hot-Spot Free Operating Range of Some Power

Transistors, to be published in IEEE Trans. Electron Devices, June, 1971.

Harman, G. G. , and Kessler, H. K. , Application of Capacitor Microphones

and Magnetic Packups to the Tuning and Trouble Shooting of Microelectronic

Ultrasonic Bonding Equipment, NBS Tech. Note 573, May, 1971.

67

FORM NBS-U4A (1-71) r PUBLICATION OR REPORT NO.

HBS-TN-592

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

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, October 1 to December

31, 1970

5. Publication Date August 1971

6. Performing Organization Code

7. AUTHOR(S)

W. Murray Bullis, Editor

8. Performing Organization

10. Project/Taslc/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 Natl. Bureau of Standards,

DAS A (EA071-801), Navy Strategic Systems Project Office, NAD,

Crane, Indiana (PO-1-0030 and P0- 1-0041), Navy Electronics

System Command (P0-1-1057), AEC, and NASA (S-70003-G and

H-76553A)

13. Type of Report & Period Covered t j. Interim

Oct. 1 to Dec. 31. 1970

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES 4251120, 4251123, 4251126, 4251449, 4252114, 4252119, 4252128,

4252534, 4254111, 4254112, 4254115, 4254429, 4259425, 4259522, 4259533.

16. 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, tenth 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 successful application of the surface photovoltage technique, a non-contacting

method, to the measurement of carrier diffusion length in silicon epitaxial layers and

development of a novel, but simple, method for measurement of acceleration and terminal

angular velocity of a photoresist spinner. Work is continuing on measurement of resistivity, carrier lifetime, and electrical inhomogeneities in semiconductor crystals;

specification of germanium for gamma-ray detectors; evaluation of wire bonds, metallization adhesion, and die attachment; measurement of thermal properties of semiconductor devices, transit-time and related carrier transport properties in junction devices, and electrical properties of microwave devices; and characterization of silicon

nuclear radiation detectors . Supplementary data concerning staff, standards committee

activities, technical services, and publications are included as appendixes.

Key Words (cont.): measurement; microelectronics; microwave devices; nuclear radiation

detectors; probe techniques (a-c); resistivity; semiconductor devices; semiconductor

materials; semiconductor process control; silicon; thermal resistance; thermographic

measurements; ultrasonic bonder; wire bonds.

17. key words (Alphabetical order, separated by semicolons) Alpha-particle detectors ; aluminum wire;

base transit time; carrier lifetime; die attachment; electrical properties; epitaxial

silicon; gamma-ray detectors; germanium; gold-doped silicon; metallization; methods of

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