taW ^ M "' ^''^^«UoN

NBS TECHNICAL NOTE 702

Methods of

Measurement for

Semiconductor

Materials, Process

Control, and Devices

Quarterly Report

April 1 to June 30, 1971
NATIONAL BUREAU OF STANDARDS

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

technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measure- ment system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology, the Center for Computer Sciences and

Technology, and the Office for Information Programs.

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

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

Radiation2—Nuclear Radiation2—Applied Radiation2—Quantum Electronics3—

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

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

2 Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.
10NAL BUREAU Of STANDARDS

l\0O

71

OEC 1 W

UNITED STATES DEPARTMENT OF COMMERCE

Maurice H. Stans, Secretary

L,,;, NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

NBS TECHNICAL NOTE 702

ISSUED DECEMBER 1971

Nat. Bur. Stand. (U.S.), Tech. Note 702, 45 pages (December 1971) CODEN: NBTNA

Methods of Measurement for Semiconductor

Materials, Process Control, and Devices

Quarterly Report

April 1 to June 30, 1971

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 Air Force Weapons Laboratory, the Air Force Cambridge Research

Laboratories, the Advanced Research Projects Agency, the Atomic Energy Commission,

and the National Aeronautics and Space Administration

S«»EAU °<

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 13.46:702). Price 50 cents.

CONTENTS

PAGE

Foreword vi

1. Introduction 1

2. Highlights ..... 3

3

Methods of Measurement for Semiconductor Materials

3.1. Resistivity 7 3.2. Gold-Doped Silicon 9 3.3. Specification of Germanium 11

3.4. References 14

4. Methods of Measurement for Semiconductor Process

Control

4.1. Die Attachment Evaluation 15

4.2. Wire Bond Evaluation 17

4.3. Processing Facility 21 4.4. References 22

5. Methods of Measurement for Semiconductor Devices

5.1. Thermal Properties of Devices 23 5.2. Thermographic Measurements 28

5.3. Microwave Device Measurements 29 5.4. Carrier Transport in Junction Devices 31

5.5. Silicon Nuclear Radiation Detectors 32

5.6. References 34

Appendix A. Joint Program Staff 36

Appendix B. Committee Activities 37

Appendix C. Solid-State Technology S Fabrication Services .... 39

Appendix D. Joint Program Publications 40

Appendix E. Standardization and Dissemination Activities .... 41
LIST OF FIGURES

PAGE

1. Resistivity as a function of gold concentration in p-type

silicon 10

2. Infrared response spectra of three germanium gamma -ray

detectors 13

3. Thermal response curves for diodes with 20-percent void

areas and their controls 16

M-. Thermal response curves for diodes with 40-percent void

areas and their controls 16

5. SEM photomicrograph of typical ribbon-wire first bond .... 18

6. SEM photomicrograph of typical ribbon-wire second bond. ... 18

7. SEM photomicrograph of overbonded ribbon-wire first bond. . . 18

8. SEM photomicrograph of lift-off pattern of ribbon-wire

first bond 18

9. SEM photomicrograph of lift-off pattern of ribbon-wire

second bond 18

10. Circuits for measuring thermal resistance of transistors. . . 24

11. Base current (solid curves) and base-emitter voltage (dashed)

curves) as a function of power for a silicon power transistor

showing onset of hot spots 26

12. Circuits for measuring common-emitter current gain of

transistors 26

IV
LIST OF TABLES

PAGE

1. Results of Repetitive Junction-to-Case Temperature

Difference Measurements after Equipment Improvements 16

2. Comparison of Thermal Resistance Measured using

Common-Emitter and Common-Base Circuits 26
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 Air Force

Weapons Laboratory, 1" the Air Force Cambridge Research Laboratories.* the

Advanced Research Projects Agency, x the Atomic Energy Commission,"" and

the National Aeronautics and Space Administration. tt Although there is

not a one-to-one correspondence between the tasks described in this re- port and the projects by which the Program is supported, the concern of

certain sponsors with specific parts of the program is reflected in

planning and conduct of the work.

Through Research and Technical Services Projects 4251120, 4251123, 4251126 4252114

4252119, 4252128, 4254111, 4254112, and 425115.

"t" Through Order EA071-801. (NBS Project 4259522).

5 Administered by U. S. Naval Ammunition Depot, Crane, Indiana through Project Orders PO-1-0030, PO-1-0041, and PO-1-0067, and Naval Avionics Facility through~Work Reauest

WR-1-1038. (NBS Projects 4259533 and 4254432).

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

' Through Delivery Order F29601-71-F-0002. (NBS Project 4252535).

# Through Project Order Y71-906. (NBS Project 4251535). ARPA Order 1889 Monitored by Space and 'Missile Systems Organization under MIPR

FY76167100331. (NBS Project 4254422).

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

++Through Orders S-70003-G, Gs>ddard 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

April 1 to June 30, 1971

This quarterly progress report, twelfth 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 a demonstration of the high sensitivity of the infrared response technique by the identification

of gold in a germanium diode doped to a level of 10 11 gold

atoms per cubic centimeter, verification that transient thermal

response is significantly more sensitive to the presence of

voids in die attachment than steady-state thermal resistance,

and development of a simplified circuit for screening transistors for susceptibility to hot-spot formation by the currentgain technique. Work is continuing on measurement of resistivity of semiconductor crystals; study of gold-doped silicon;

specification of germanium for gamma-ray detectors; evaluation

of wire bonds 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 sili- con 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; 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 twelfth quarterly report to the sponsors of the Joint

Program on Methods of Measurement for Semiconductor Materials, Process

Control, and Devices. It summarizes work on a wide variety of measurement methods that are being studied at the National Bureau of Standards.

Since the Program is a continuing one, the results and conclusions reported here are subject to modification and refinement.
INTRODUCTION

The work of the Program is divided into a number of tasks, each directed toward the study of a particular material or device property or measurement technique. This report is subdivided according to these

tasks. Highlights of activity during the quarter are given in Section 2. Section 3 deals with tasks on methods of measurement for materials;

Section 4-, with those on methods of measurement for process control; and

Section 5, with those on methods of measurement for devices. References

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

section.

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

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

structure is participation in the activities of various technical standardizing committees. The list of personnel involved with this work given

in Appendix B suggests the extent of this participation. Additional details of current efforts in this area are given in Section 2.

Background material on the Program and individual tasks may be found

in earlier reports in this series as listed in Appdndix 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

Significant accomplishments during this reporting period include a demonstration of the high sensitivity of the infrared response technique

by the identification of gold in a germanium diode doped with 10 ^ gold

atoms per cubic centimeter, verification that transient thermal response

is significantly more sensitive to the presence of voids in die attachment than steady-state thermal resistance, and development of a simplified circuit for screening transistors for susceptibility to hot-spot

formation by the current-gain technique. Highlights of these and other

technical activities are presented in this section; details are given in

subsequent sections of the report. This section concludes with a summary

of standardization activities being carried out by program staff members

.

Resistivity — Principal emphasis was placed on completion of measurements on wafers with chem-mechanically polished surfaces in the study of

the effect of current level and probe force on resistivity as measured by

the four-probe method and on coordination and analysis of round-robin

interlaboratory tests being conducted in cooperation with ASTM Committee

F-l on Electronics. In other work evidence was found that suggests that

the difficulty encountered in fabrication of diodes that are satisfactory

for capacitance-voltage measurements is due to crystal defects that might

be introduced during wafer preparation or during subsequent processing

steps. Work in support of spreading resistance methods continued with

further efforts to obtain high-quality scanning electron microscope photomicrographs of probe damage to silicon.

Gold-Doped Silicon — Work continued in preparing and evaluating golddoped silicon wafers in the extensive series of measurements of resistivity, Hall coefficient, and gold concentration outlined previously.

Results of the study of lateral uniformity of diffused gold as determined

by x-ray fluorescence with the scanning electron microscope suggest

either that the clusters which form do not extend as much as 5 ym below

the surface or that the gold concentration in a cluster decreases rapidly

with distance below the surface. Preliminary measurements of carrier diffusion length by the surface photovoltage technique were attempted on gold-diffused Hall bars

.

Specification of Germanium — Study of methods for measuring lithiumion drift mobility in germanium has been completed with the adoption by

ASTM Committee F-l of a method based on technology developed under this

task. Work on methods for measuring carrier trapping in germanium is

continuing in cooperation with the committee. The improved infrared re- sponse technique was used to study eight germanium diodes; gold was identified in one doped with 10 11 gold atoms per cubic centimeter. Application of the infrared response method to silicon devices continued.

Die Attachment Evaluation — Modification of the die attachment evaluation equipment to increase its power handling capabilities and to in- crease the resolution of the digital voltmeter resulted in improved reproducibility of the measurements. Transient thermal response measurements
HIGHLIGHTS

for a heating-power pulse width of about 10 ms were shown to be more sen- sitive to the presence of 10-, 20-, and 40-percent void areas in diode

die attachment than steady-state thermal resistance measurements. For

the particular device tested, a 10-percent void area appears to be the

smallest that can be detected by the thermal response technique; it is

not detectable by the thermal resistance technique.

Wire Bond Evaluation — Work progressed on ribbon-wire bonding despite several continuing problems with both the bonding machine and the

dimensional uniformity of the wire. Scanning electron microscope photomicrographs were made of bonds and of lift-off patterns. Evaluation of

the pull test on round-wire bonds was deferred due to serious mechanical

problems with the round-wire bonding machine. A simple technique for

refinishing used bonding tools was devised. Assistance in the organization of a section on microelectronic bonding was provided in conjunction

with the summer meeting of ASTM Committee F-l at NBS. More than 40 representatives of device manufacturers, materials and equipment producers,

and consumers attended. Ten areas in bonding were selected and task

forces were set up to take the necessary action.

Processing Facility — Principal effort is being expended on development of a process for fabricating diffused silicon diodes required in the

resistivity task for the purpose of measuring resistivity as a function

of depth in bulk and epitaxial silicon wafers by the capacitance-voltage

method.

Thermal Properties of Devices — Prototype circuits to measure transistor thermal resistance and common-emitter current gain in the common- base operating mode were designed and built. These circuits are much

simpler than the common-emitter circuits used up to this point because

it is not necessary to use special circuitry in the base lead to control

the collector current. In the case of the current-gain measurement, the

simplicity of the common-base circuit makes feasible a productionoriented current-gain screening technique for hot-spot formation. In a comparison of the effectiveness of the use of base-emitter junction voltage as a screen for thermal instabilities with that of the common-emitter

current gain it was found that while both were sensitive to constricted

hot-spots that exhibit thermal hysteresis the former was much less sen- sitive to the more amorphous, less constricted hot-spots that occur under

higher current, lower voltage operating conditions. A study was undertaken to determine the pulse conditions under which a power transistor

could be put into a constricted operating mode with thermal hysteresis.

With repetitively pulsed collector excursions, it was found that hysteresis occurred when the product of the magnitude of the voltage pulse

and the duty cycle was equal to the steady-state voltage change required

to initiate hysteresis from a given collector voltage below the d-c hotspot initiation voltage. For singly pulsed excursions from various

collector voltages below the d-c hot-spot initiation voltage, it was found that, for a given pulse width, the combined d-c and pulsed voltage
HIGHLIGHTS

required to initiate hot-spots increased as the starting d-c collector

voltage level was reduced.

Microwave Device Measurements — To improve the precision of the low

intermediate- frequency conversion-loss measurements, an incremental modulation technique has been introduced. With this technique, precision

is limited principally by the resolution of the attenuator used to set the increment and by the mechanical stability of the waveguide system

rather than by the output resolution.

Carrier Transport in Junction Devices — The vector-voltmeter measure- ment system for measuring transistor delay time was assembled and preliminary tests were carried out . Two systems for the measurement of transistor S-parameters in the 200 to 800 MHz range are being assembled. An analysis of the equivalent circuits of a transistor in the Sandia-type

bridge and in the vector-voltmeter circuit has been made to relate the

measured delay time to various time constants and delay times within the

device

.

Silicon Nuclear Radiation Detectors — Study of radiation damage effects of 600-keV electrons and fast neutrons in lithium-drifted silicon

detectors has been completed. A preliminary study of the effect of ex- posure of a silicon surface -barrier detector to dry ammonia and methanol

vapor at various pressures was completed. No significant changes in

noise level were observed during these tests.

Standardization Activities — Many of the standardization activities

undertaken by program staff are broader than the technical tasks described in the following sections. These activities involve general staff

support in committees, coordination of efforts which may encompass a va- riety of tasks , and participation in areas where no direct in-house technical effort is underway. Standardization activities directly related

to particular task areas are reported with the appropriate tasks. A sum- mary of standardization and dissemination activities during FY 1971 is

given in Appendix E

.

Twelve program staff members attended the regular summer meeting of

ASTM Committee F-l held at the NBS facilities in Gaithersburg . Program

staff members were instrumental in the organization of a new section on Bonding of Subcommittee 7 on Hybrid Microelectronics (see Section 4.2).

Mrs. K. 0. Leedy was appointed chairman of the section. J. R. Ehrstein

was appointed chairman of the Resistivity Section of Subcommittee 6 on Electrical Measurements. W. M. Bullis was appointed member-at-large to the Advisory Committee

.

Activity in connection with JEDEC committees of the Electronic Industries Association continued in a variety of areas. Contact with the

chairman of Committee JC-21 on UHF and Microwave Diodes was renewed in

order to discuss the status of the microwave diode task and its relation
HIGHLIGHTS

to current industry requirements. F. F. Oettinger attended meetings of

Committees JC-22 on Rectifier Diodes and Thyristors and JC-11 on Mechanical Standardization.

W. M. Bullis was appointed to the Planning Subcommittee of SAE Committee H on Electronic Materials and Processes. F. F. Oettinger was appointed to the Technical Program Committee for the March 1972 IEEE Inter- national Convention.
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 : Principal emphasis was placed on completion of measurements on wafers with chem-mechanically polished surfaces in the study of

the effect of current level and probe force on resistivity as measured by

the four-probe method, and on coordination and analysis of round-robins

being conducted in cooperation with ASTM Committee F-l on Electronics

.

In other work, evidence was found that suggests that the difficulty en- countered in fabrication of diodes that are satisfactory for capacitancevoltage measurements is due to crystal defects that might be introduced

during wafer preparation or during subsequent processing steps . Work in

.support of spreading resistance methods continued with further efforts to obtain high-quality scanning electron microscope photomicrographs of

probe damage to silicon. Study of probe impact momentum was deferred.

Fouv-Pvobe Method — Measurements were completed on seven silicon wafers with chem-mechanically polished surfaces in the study to determine

the effect of current level and probe force on the measurement of resistivity by the four-probe method. The procedure used was identical to that used previously on mechanically polished slices (NBS Tech. Note 598,

p. 7). Preliminary examination of the results suggests that the form of

the current and probe-force dependence is virtually unchanged between the

mechanically polished and chem-mechanically polished surfaces on the same wafer. However, at any specific combination of current and probe force

the resistivity measured on the chem-mechanically polished surface is

lower by 0.5 to 1.5 percent than that on the same wafer with a mechanically polished surface. (J. R. Ehrstein and D. R. Ricks)

An auxiliary study has been started to try to relate variation in

measured resistivity with surface condition to amount of subsurface damage. Measurements of carrier diffusion length by the surface photovoltage

method have been found to be sensitive to the nature of the specimen surface. Surface photovoltage measurements are being made on specimens with

lapped, mechanically polished, and chem-mechanically polished surfaces to determine qualitatively the extent of subsurface damage.

(J. R. Ehrstein, W. R. Thurber, W. E. Phillips, and A. W. Stallings

)

Standardization Activities — A summary of the results of the roundrobin on four-probe measurement of resistivity of silicon wafers in the

5,000 to 20,000 fi'cm range was presented to the Resistivity Section of

Committee F-l at its June meeting. Seven laboratories participated in

the round-robin. Five silicon specimens and two analog resistance boxes

[1] were circulated. Two p-type and one n-type specimen had room- temperature (23°C) resistivity around 6,000 fi-cm; one n-type and one
RESISTIVITY

p-type specimen were supplied as 14,000 to 15,000 ft-cm. The analog boxes

used for calibration had standard resistors of 1,000 and 10,000 ft.

No serious difficulties were encountered in measurement of the analog

boxes; with resistors 300 times as large as the standard resistor in both

the current and potential leads, the maximum error in measuring the resistance of the standards was ±1.5 percent. Measurement of the 6,000-ft'cm

n-type specimen resulted in a sample standard deviation of 3 to 4- percent,

well within the precision quoted for wafers with resistivity between 500

and 2,000 ft-cm [1]. Measurement of the 6 ,000-ft-cm p-type specimens and

of both the higher resistivity specimens resulted in sample standard deviations greater than 10 percent, considerably outside the precision

quoted for lower resistivity specimens [1].

Preliminary results of the round-robin on four-probe measurement of

resistivity of silicon epitaxial layers deposited on substrates of opposite conductivity type were also presented at the June meeting of Committee F-l. Seven of the eleven scheduled participants had completed their

measurements. The results are being reviewed to determine whether they

are sufficient to support a precision statement in the standard method

now under development for this test.

Capacitance-Voltage Method —Arrays of 20- and 30-mil (0.5- and

0.75-mm) diameter diodes were fabricated in four 15-°,* cm, n-type silicon

slices by means of the same fabrication procedure previously used on

1 fi'cm slices (see Section 4.3). Measurements of capacitance as a function of voltage (C-V) were inhibited by insufficiently large breakdown

voltages on most of the diodes on the four wafers. Microscopic inspection of these wafers revealed a pattern of linear defects present on most

of the devices on all four wafers . On each wafer all the linear defects

were oriented in the same direction over the entire wafer surface. It was also found that those few diodes which had acceptable breakdown voltages were characterized by an absence of these defects. All diodes that

could be measured were located near the edge of the wafer where the fourprobe resistivity values were not reliable so that valid comparison between C-V and four-probe data was impossible. At present the various

steps in the processing procedure are being scrutinized to try to determine the origin of these defects so that the procedure can be modified

to produce a greater yield of diodes with acceptable breakdown voltages.

Preliminary measurements were made on 20-mil (0.5-mm) diameter,

0.5 um deep, boron-diffused, mesa diodes obtained from another laboratory

in order to gain additional information concerning the C-V measurement

system and to provide data for use during the development of analytical

procedures. (R. L. Mattis)

Spreading Resistance Method — Attempts to use the scanning electron

microscope to study probe impact damage on silicon are continuing. Em- phasis is on reproducible production of photomicrographs of extremely

high quality similar to those which were made on a recent occasion with
RESISTIVITY

magnifications up to 17,000 X. Work is in progress to produce a silicon

specimen that can be used to qualify the SEM system for high resolution

on any given occasion before proceeding with the specimen of interest.

(J. R. Ehrstein)

Plans : Study of the current-level and probe-force dependence of

four-probe resistivity measurements will be continued with measurements

of resistivity on lapped surfaces in order to compare with resistivity

values measured on this surface finish at the beginning of the study.

Detailed statistical analysis of the data obtained in the study will begin. To estimate the amount of subsurface damage caused by various processing steps the series of surface photovoltage measurements on wafers

with the same surface preparations as in the four-probe resistivity study

will be continued. An attempt to relate the extent of surface damage to the observed shift in measured resistivity between mechanically and chemmechanically polished slices will be made. Experiments will be performed

to determine whether the surface defects which reduce diode yield are caused by the wafer processing. Development of procedures for analyzing

the results of C-V measurements will continue. Work on use of the scan- ning electron microscope to examine probe damage on silicon will continue. Work will also resume on use of a piezoelectric transducer to study

impact momentum of various configurations of a spreading resistance probe.

3.2. GOLD-DOPED SILICON

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

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

Progress: Hall effect and resistivity measurements were made at room temperature on Hall bars cut from 10-, 20-, and 90-fi-cm p-type wafers diffused with gold at various temperatures. The gold concentration

in each of the wafers had been determined by neutron activation analysis.

These results are shown in figure 1. For comparison, resistivity calculated from an energy-level model for gold-doped silicon is also shown in

figure 1. In these calculations the energy of the gold donor and acceptor were taken as 0.35 eV above the valence band and 0.54 eV below the

conduction band, respectively [1], the degeneracy factors were taken as 0.25 for the donor and 1.5 for the acceptor [2], and the energy gap and

effective masses were taken from the work of Barber [3]. The calculation

is based on a solution to the charge balance equation to find the Fermi

level and, hence, the hole concentration. Lattice mobility [4] and impurity mobility [5] were combined reciprocally to obtain the hole mobility

used in the calculation of the resistivity.

Experimentally it was found that the resistivity decreases at large

gold concentration, whereas the theory predicts a continual increase in

resistivity with gold concentration. Hall measurements confirm that the
GOLD-DOPED SILICON

Figure 1. Resistivity as a function of gold concentration in p-type silicon.

(Error bars are not indicated. The

standard deviation of the gold determination is estimated to be about 10 percent. Resistivity values are re- producible within about 5 percent).

1 ' 111] 1 J>il 1 1 1 1 1 ,

I ."* J^ -

\ /* /'s s^ '

/

/ / / o

-

r / o / /o °

oa _ / /

° I / / / /° °

: / &/ -

- / / S " / / /

o / / Theory Exp. _

10 il cm o _ /' / 20 n cm *

-

; / / 90flcm a - s' /

s" / _ --S"°

I

, i 0'4

I0 a I0 1* 10" GOLD CONCENTRATION ( atoms/cms

)

retrograde behavior is due to an increase in the charge carrier density

rather than an increase in the carrier mobility. A suitable model to ex- plain this behavior has not yet been found.

Gold diffusions were c room temperature (23°C) res diffusion temperatures and

144- h, 1050°C for 72 h, 115

centrations were determined

previous sets of wafers ind

1250°C are long enough that

ity limit [6], but the time

enough for this to occur.

(W. R. Thurber, A

ompleted on sets of n-type wafers with initial

istivity of 5, 80, 400, and 2,200 ft-cm. The

times used were: 850°C for 288 h, 950°C for

0°C for 24 h, and 1250°C for 8 h. Gold con- by activation analysis. Data on these and

icate that the diffusion times at 1150 and

the concentration of gold reaches the solubils at the lower temperatures are not long

W. Stallings, T. F. Leedy, and W. M. Bullis)

A layer 5-ym thick was removed by lapping from the surface of the

Hall bar previously shown to have gold clusters by x-ray fluorescence

(NBS Tech. Note 598, p. 13). In all regions, including those where clusters had been observed, the x-ray intensity of the gold peak did not ex- ceed the background level. This suggests either that the clusters do not

extend as much as 5 pm below the surface or that the gold concentration

in a cluster decreases rapidly with distance below the surface.

(W. J. Keery and W. R. Thurber)

Preliminary measurements of carrier diffusion length were attempted

on gold-diffused Hall bars. Difficulties were encountered that can be

traced to lew signal levels; modifications to the equipment to overcome

these limitations were begun. (W. R. Thurber and W. E. Phillips)

10
GOLD-DOPED SILICON

Plans: A new specimen holder will be fabricated for room temperature Hall effect and resistivity measurements. Improved means for maintaining and measuring the specimen temperature will be included. Electrical properties of the Hall bars cut from the 5-, 80-, 400- , and

2,200-fi-cm n-type, gold-diffused wafers will be measured using the new

holder. Sets of p-type wafers with room temperature (23°C) resistivities

of 0.5, 1,000, and 2,000 fi*cm will be diffused with gold at five temperatures from 850 to 1250°C.

To further study the retrograde behavior of the resistivity in ptype material, 10-fi*cm wafers will be diffused at 1250°C for 8, 16, 32,

and 64 hours. These times are all long enough for the gold concentration to stabilize at its maximum solubility, but if foreign impurities

are diffusing in, their concentration should increase with time and

change the measured resistivity.

Additional measurements of carrier diffusion length will be made by

the surface photovoltage method on the same Hall bars used for electrical

measurements after .modifications to the equipment have been completed.

3.3. 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 .

Progress: Study of methods for measuring lithium-ion drift mobility

in germanium has been completed with the adoption by ASTM Committee F-l

of a method based on technology developed under this task [1]. Work on methods for measuring carrier trapping in germanium is continuing in co- operation with the committee. The improved infrared response technique

was used to study eight germanium diodes: gold was identified in one that

had been doped to a concentration of 10 lJ gold atoms per cubic centimeter.

Application of the infrared response method to silicon devices continued.

Characterization of Germanium — Measurements of carrier trapping

have been completed on specimens from two germanium crystals. The procedure used was that specified for the second round-robin being conducted

in cooperation with the Germanium Section of Committee F-l. Detector

resolution was measured as a function of applied electric field using the

1333-keV gamma ray of 60 Co. The position (channel number) of the center

of the 1333-keV peak in the pulse-height spectrum was also determined as

a function of bias; independent values of effective carrier lifetime in

the test specimens can be obtained from the results of the two measurements. An additional series of measurements was carried out using a collimated beam of 662-keV gamma rays from 137 Cs in order to calculate

11
SPECIFICATION OF GERMANIUM

values of carrier lifetime using a model for carrier trapping developed

in this laboratory [2] and compare them with the values obtained according to the round-robin procedure

.

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

Germanium Detector Measurements — Detectors fabricated from specimens

of eight germanium crystals were examined by the infrared response (IRR)

technique. IRR spectra obtained from three germanium diodes for the

energy range 0.5 to 0.7 eV using a 640-line per millimeter grating and a 1-mm thick germanium filter window are shown in figure 2. The spectrum

of NBS-83-3, a lithium-drifted diode, is typical of germanium used for

fabricating good quality gamma -ray detectors of this type. The spectrum

of NBS-112, a diode" fabricated from a specimen of high-purity germanium

with an initial net donor concentration of approximately 7 x 10 I0 cm" 3 is in general similar to that of NBS-83-3 except that structure is not

observed at either 0.50 or 0.52 eV. Since lithium compensation was not

used in the fabrication process (NBS Tech. Note 598, pp. 15-16) one would

not expect to observe the 0.50-eV level due to the lithium-defect interaction. Structure in the region near 0.52 eV has been noted in all

lithium-drifted diodes \ some of the defects responsible for this structure

apparently are not present in the high-purity crystals . The spectrum of

NBS-113, a diode" fabricated from a crystal of high-purity germanium intentionally doped with approximately 10 ll gold atoms per cubic centimeter,

exhibits peaks at 0.57 and 0.66 eV. Gold introduces a donor level at an energy of E + 0.05 eV, and acceptor levels at E + 0.15, E - 0.20 and

E - 0.04 ev in germanium [3]; the peaks in the spectrum can be related

to the excitation of electrons from the donor level and the lowest acceptor level to the conduction band. These results demonstrate the high sen- sitivity of the IRR technique to the presence of gold in germanium.

Attempts were made by measurement of IRR to confirm the presence of

iron in NBS-301, fabricated from a germanium crystal 1 intentionally con- taminated with stainless steel during growth from the melt. Levels in

germanium arising from iron impurity are at energies of E + . 34 eV and

E - 0.27 eV [3]. Observed broadening on the high energy side of the

0.36 eV peak might possibly be due to excitation of electrons from the

lower iron level to the conduction band; however, because of the still un- known origin of peaks observed in germanium at 0.36, 0.24, and 0.18 eV, no definite conclusions can be drawn from the IRR data at present.

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

A study was carried out to determine the feasibility of obtaining IRR

spectra in digital form for analysis by computer and ease of comparison

of spectral runs made at different settings. A method that involved use

t

Supplied by R. D. Baertsch, General Electric Company, Cchenectady, N. Y. 12301.

Supplied by I. L. Fowler, AECL Chalk River Kuclear Laboratories, Chalk River,

Ontario, Canada.

12
SPECIFICATION OF GERMANIUM

Figure 2. Infrared response spectra of three germa- nium gamma-ray detectors.

0.6 ENERGY(eV)

of equipment currently on hand was found to be successful. The varying

voltage output' from the phase-sensitive amplifier is converted linearly to

a variable frequency and fed to a miltichannel analyzer that is operating

in the multiscaling mode in which total counts occuring in a given time

interval are stored in one channel. A switch mounted on the' monochromator

grating drive to give 100 closures per drum turn advances the analyzer address so that the recorded count is proportional to the IRR signal at any

given energy. The IRR spectrum in digital form is then punched onto paper

tape and analyzed by a computer. (W. J. Keery and A. H. Sher)

Further efforts were made to apply the IRR technique to microelectronic devices (NBS Tech. Note 598, p. 16) by studying a 37-mil (0.94-mm)

square silicon mesa diode mounted on the special right -angle holder at the

end of the cryostat cold finger. Although considerable improvement in

signal-to-noise ratio was obtained by using a 0.1-mm thick silicon filter

and by replacing the 4-5-W light source with a 150-W light source, only the

band-edge peak was observed. The present effort was also hampered by development of mechanical problems in the motorized monochromator grating

drive. (W. J. Keery)

13
SPECIFICATION OF GERMANIUM

Plans: Emphasis will continue on the interpretation of results obtained using the improved IRR technique. Efforts to extend this technique to silicon microelectronic devices will continue. As this effort

has been expanded to include silicon devices and detectors as well as those of germanium, subsequent results will be reported in a section of

future quarterly reports devoted to infrared methods. The monochromator

drive mechanism will be repaired, the 100-closure per drum turn switch

installed, and the monochromator recalibrated. Measurements of IRR on lithium-drifted silicon detectors subjected to electron and neutron radiation damage (see Section 5.5) will begin.

3.4. REFERENCES

3.1. Resistivity

1. Standard Method for Measuring Resistivity of Silicon Slices with a Collinear FourProbe Array (ASTM Designation: F84-70), Annvjal Book of ASTM Standards, Part 8, (Available from American Society for Testing and Materials, 1916 Race St., Philadelphia, Pa. 19103).

3.2. Gold- Doped Silicon

1. Collins, C. B., Carlson, R. 0., and Gallagher, C. J., Properties of Gold-Doped Silicon, Phys. Rev. 105 , 1168-1173 (1957-).

2. Bullis, W. M, and Strieter, F. J., Flectrical Properties of w-Type Silicon Doped

with Gold, J. Appl. Phys. 39_, 314-318 (1968).

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

Electronics 10_, 1039-1C51 (1967).

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

Rev. 101 , 1699-1701 (1956).

5. Brooks, H. , Theory of the Electrical Properties of Germanium and Silicon, Advances

in Electronics arid Electron Physics, L. Marton, ed. (Academic Press, New York,

1955) Vol. VII, pp. 156-160.

6. Bullis, W. M.. Properties of Gold in Silicon, Solid-State Electronics 9_, 143-168

(1966).

3.3. Specification of Germanium

1. To be published, November, 1971, as Method for Measuring Lithium-Ion Drift Mobility

in Germanium Singls Crystals (ASTM Designation: F334-71T) Annual Book of ASTM Standards, Part 8 (Am. Soc. for Testing and Materials, Philadelphia, Pa., 1971).

2. Sher, A. H., Carrier Trapping in Ge(Li) Detectors, IEEE Trans. Nucl. Sci. NS-18 , No. 1, 175-183 (1971).

3. Neuberger, M. , Germanium Data Sheets, Electronic Properties Information Center

(EPIC) Report DS-14 3 (February, 1965); Available from National Technical Informa- tion Service, Springfield, Virginia 22151, Accession Number AD 610828. 14
4. METHODS OF MEASUREMENT FOR SEMICONDUCTOR PROCESS CONTROL

H.l. 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 steady-state and transient thermal re- sponse of diodes with various size voids reported previously (NBS Tech.

Note 598, p. 20) indicated that the increase in sensitivity to voids of

the transient thermal response measured for power pulses of 7 and 9 ms over that of the steady-state thermal response ranged in most cases from

90 to 700 percent. The data also indicated that although the spread in

measured junction-to-case temperature difference, AT , was small for

devices with no intentional voids this was not the case with the intentionally voided devices. These conclusions are illustrated in figures 3 and 4 which show transient thermal response curves for a representative

sampling of devices. Curves denoted by the symbol HV or IV are for diodes

that had a void area introduced into the die attachment while curves denoted by the symbol H or I are for control diodes without voids. The

voided devices, series HV and IV, were bonded on gold-plated T0-5 headers

with 20- and 29-mil (0.51- and 0.74-mm) diameter dimples ultrasonically

machined into the bonding surface (NBS Tech. Note 560, pp. 27-29) to produce voids that are approximately 20 and 40 percent of the total chip

bonding area, respectively. The results presented in these figures generally confirm the anticipated thermal response behavior (NBS Tech. Note

555, pp. 25-27). At heating-power pulse widths greater than about 10 to

20 ms , the increase in AT due to poor die adhesion is essentially constant; the maximum sensitivity occurs for pulse widths of about 10 ms

.

The spread in thermal response of the devices with the 20-percent void

areas was significantly larger than in that of devices with 40-percent

void areas. Radiographs and sectioning of both types of voided devices

indicated that the unbonded region around the smaller dimples was larger

than expected and varied from diode to diode, while better control was in

evidence for devices with the larger dimples.

The die attachment evaluation equipment was modified to increase its

power handling capabilities and to increase the resolution of the digital

voltmeter that indicates the forward voltage used as the temperature sen- sitive parameter. Long-term, single-operator measurements were made to recheck the reproducibility of the equipment. Over a period of 9 days,

steady-state and transient thermal response for heating-power pulse widths

of 7, 10, and 14 ms were measured 12 times each on three commercial silicon mesa diode chips bonded to TO-5 headers. The average value and sample

standard deviation of AT obtained for each measurement condition are listed in table 1. The diode heating current was 800 mA for the pulsed

measurements and 300 mA for the steady-state measurements. In all cases

15
DIE ATTACHMENT EVALUATION

ff, 30-

uj 20

- y-~— HV3 yS y— HV7

- y/ S^ /^/~~HI 8 H4

-

/ yS y^

/\^^^ /c*^

WIDTH OF APPLIED POWER PULSE (s) Figure 3. Thermal response curves for diodes with 20-percent void

areas and their controls.

ob0 K

1 ' <

I UJ50 ,^IV3

,£»-IV9alV2 . O

2 ^y-—IVI4

UJ

UJ x€s y—-i4 y/y/ yy^- 1 " fc«o y/y^^yy^

a

UJ

a.?30 y^y^yy^^

<

<r UJ

Q-5 20 y-Z^^yy<' 1-UJ

in<V

l0 ^^^\y*^^

zK

O 1 =»

1 ill

WIDTH OF APPLIED POWER PULSE (s) Figure A. Thermal response curves for diodes with 40-percent

areas and their controls.

void

Table 1 -Results of Repetitive Junction-to-Case Temperature

Difference Measurements after Equipment Improvements

Pulse Width Device No.

7 ms 10 ms 14 ms Steady State

II12

13

10.44±0.28°C

10.86+0.19 9.84±0.10

11.86±0.23°C 12.26+0.20

11.25±0.09

13.31+0.20°C

13.64±0.18

12.63±0.15

14.74±0.15°C 15.36±0.19

14.39±0.24

16
DIE ATTACHMENT EVALUATION

the forward voltage was measured 50 ys after termination of the heating

current. The maximum sample standard deviations of ±0.28°C for the transient thermal response measurements and ±0.24°C for the steady-state meas- urement were substantially lower than the values of ±0.54°C and ±0.69°C

obtained before the circuit was modified (NBS Tech. Note 598, pp. 18-19).

Measurements of steady-state thermal response and transient thermal

response for heating-power pulse widths ranging from 1 to 100 ms were

made on 10 diodes bonded to gold-plated TO-5 headers with 15-mil (0.37-mm)

diameter dimples ultrasonically machined into the bonding surface and on

10 control diodes with no intentional voids. This size dimple produces

voids that are approximately 10 percent of the total chip bonding area. The average steady-state thermal response and transient thermal response

for a heating-power pulse width of 10 ms , measured on the control diodes

50 ys after the termination of the heating pulse, were 15.53°C and

12.01°C, respectively. A heating current of 800 mA was used for the transient response measurement while 300 mA was used for the steady-state

measurement. The sample standard deviations in measured AT under steady

state and transient conditons were ±0.73°C and ±0.90°C, respectively.

This was significantly larger than was observed previously (NBS Tech.

Note 598, pp. 19-20) on control devices associated with diodes having 20-

and 40-percent void areas. For half of the diodes with voids, AT measured under transient conditions fell within the scatter obtained on the controls, while the other half showed significantly larger values of

AT . Nevertheless, it can be asserted with 95 percent confidence [1]

that the average of the thermal response readings for the diodes with 10-

percent void area exceeds the average of the controls , although there is no reason to believe that the average of the steady-state temperature rise

exceeds the average of the controls. It appears then, that a 10-percent

void in the chip bonding area is the smallest size void that can be detected in this particular device using the transient thermal response technique while a measurement of thermal resistance would not be sensitive to void areas this small. (F. F. Oettinger and R. L. Gladhill)

Plans: Measurements of steady-state and transient thermal response

on diodes with controlled void areas of various diameters will continue.

Analysis of the accumulated steady-state and transient thermal response

data for diodes with the various size voids and their controls will continue. A study will also be undertaken to ascertain, theoretically, the

limitations of thermal response techniques for detecting poor die adhesion

in the devices under investigation.

4.2. WIRE BOND EVALUATION

Objective: To survey and evaluate methods for characterizing wire

bond systems in semiconductor devices and where necessary to improve existing methods or develop new methods in order to detect more reliably those

bonds which eventual±y will fail.

17
WIRE BOND EVALUATION

Figure 5. SEM photomicrograph of typical ribbon-wire first bond. (Magnification

350 X)

Figure t>. SEH photomicrograph of typical

ribbon-wire second bond. (Magnification

390 X)

Figure 7. SEM photomicrograph of overbonded

ribbon-wire first bond. (Magnification 400 x)

Figure 8. SEM photomicrograph of lift-off pattern of ribbon-wire first bond. (Magnifi- cation 350 x)

Figure 9. SEM photomicrograph of lift-off pattern of ribbon-wire second bond. (Magni- fication 350 X)

18
WIRE BOND EVALUATION

Progress : Work progressed on ribbon-wire bonding despite several

continuing problems with both the bonding machine and the dimensional uniformity of the wire. Scanning electron microscope (SEM) photomicrographs

were made of bonds and of lift-off patterns. Evaluation of the pull test

on round-wire bonds was deferred due to serious mechanical problems with

the round-wire bonding machine. A simple technique for refinishing used

bonding tools was devised. Assistance in the organization of a section

on microelectronic bonding was provided in conjunction with the summer meeting of ASTM Committee F-l at NBS. More than 40 representatives of

device manufacturers , materials and equipment producers , and consumers

attended. Ten areas in bonding were selected and task forces were set

up to take the necessary action.

Ribbon-Wire Bonding — Limited work continued on the ribbon-wire bonder received last quarter. Several problems remain to be solved before

optimum performance and reliability can be achieved. The most serious

problem is that the bonding tool jumps straight up and bounces after the

first bond is made. Such motion bends the wire at the heel, causing a crack which weakens the bond. Correction of this motion can only be ac- complished by designing a new transducer-motion cam. A considerable var- iation in bond tail length has also been observed. This must be con- trolled before the machine can be used to bond devices. In addition the

available ribbon wire does not have uniform dimensions. The variations

observed on a single spool of wire with cross-sectional dimensions 38 by

12 urn were large enough to cause significant changes in bonding characteristics. Efforts were begun to determine if closer manufacturing tolerances can be achieved.

SEM photomicrographs were made of several ribbon-wire bonds' and liftoff patterns in order to understand the way such bonds are formed. A photomicrograph of a typical first bond made with normal power, time, and

force settings is shown in figure 5. The crack in the heel can be largly

attributed to the vertical jumping motion of the tool after the bond is

made. A photomicrograph of a typical second bond also made with normal

settings is shown in figure 6. The deformation for each bond is negligible. Figure 7 shows a photomicrograph of a grossly overbonded first bond

that was made with twice the power setting of the bond in figure 5. Even

in this extreme case the deformation is only about 25 percent. This small

deformation is assumed to be the reason why most ribbon bonds break in a pull test at a tensile force in the wire of between 0.7 and 0.9 times the

tensile strength of the wire, whether a heel crack is present or not. This is comparable with very good quality round-wire bonds (NBS Tech. Note

598, p. 26). Photomicrographs of lift-off patterns of a Jirst bond and a second bond are shown in figures 8 and 9, respectively. Such patterns are made by reducing the value of one or more bonding parameters, in this case

the power, below the point where the wire will stick to the pad. The bonding pattern is similar to those obtained for round-wire (NBS Tech. Note

560, pp. 33-34); the main bonded area is around the perimeter of the bond.

19
WIRE BOND EVALUATION

As bonding progresses to the point that the wire will stick, the bonded

area extends inward toward the center. (H. K. Kessler)

Technique for Polishing Bonding Tools — A simple and convenient technique for repolishing the bonding face of flat bonding tools has been devised. This technique requires no specialized equipment. A flat copper

plate loaded with a diamond-bort paste is placed in the work stage of the

bonding machine. The tool is lowered onto this polishing plate by adjusting the search-height control. The tool is polished by moving the work

stage under the stationary tool in a general figure-eight pattern while

the tool is applied with a force comparable with that used in bonding.

During polishing, it is essential that the bonding tool be perpendicular

to the diamond-loaded copper plate. The tool is removed and cleaned

ultrasonically before it is used for bonding. A preliminary evaluation

of the method was carried out with diamond paste commerically specified

as 3-ym particle size and by polishing for 1 min. This produced such a highly smooth finish that the tool had to be broken in by making a number

of bonds at high power until the finish was slightly degraded. Use of a coarser diamond-bort paste should eliminate the need for this step.

(H. K. Kessler)

Standardization Activities — Considerable time was devoted to an or- ganizational meeting to form a new section of Subcommittee 7 of ASTM Committee F-l to develop standard test methods for microelectronic bonding.

The purpose of the initial meeting was to identify areas in bonding where

meaningful standards can be written, considering the present technology.

More than 40 representatives of device manufacturers , materials and equipment producers, and consumers attended the meeting, chaired by K. 0. Leedy.

Ten areas in bonding were selected for further investigation: the destrucive pull test, wire deformation, visual inspection, the temperature cycling test, the air blast test, bonding wire specifications, bonding tool

specifications, tests for metallization adherence and bondability, tests

for beam-lead adherence, and a glossary of terms used in microelectronic

bonding. Tests discussed but not chosen for further investigation at this

time include the non-destructive pull test, the centrifuge test, the ultrasonic vibration test, the mechanical shock test, the shear test, and tests

for evaluating flip-chip bonds. Anyone interested in participating in

this activity should contact the chairman. (K. 0. Leedy and G. G. Harman)

Bibliography and Critical Review — The bibliography was cleared for

publication as an NBS Technical Note [1]. Work on the final draft of the

critical survey paper continued. (H. A. Schafft)

Plans: Theoretical and experimental work on electronic mixing of

bonding tool ultrasonic signals will resume in the effort to better understand and control the bonding process. Advancement of the experimental

work is contingent upon acquisition of specialized equipment . Attempts

will be made to secure ribbon-wire with closer dimensional tolerances and

its evaluation for ultrasonic bonding will continue. Further work on the

20
WIRE BOND EVALUATION

wire indentation tester will be deferred until a suitable method for calibration is found. Experimental and statistical analysis of significant

factors in the wire bond pull test will resume when the round-wire bonder

is repaired and brought under control. Further assistance will be given

to sponsors in connection with problems encountered on device production

lines. Cooperation with the newly formed Section on Microelectronic Bonding in Committee F-l will continue with work on defining and evaluating

the parameters of the destructive wire bond pull test. Efforts will be

made to coordinate this ASTM activity with associated activity in the

Society of Automotive Engineers and the Electronic Industries Association.

The final draft of the critical survey paper will be completed and prepared for publication as an NBS Technical Note. The preparation of a bibliography of limited distribution reports will be initiated.

4.3. PROCESSING FACILITY

Objective: To establish a microelectronics fabrication laboratory

with the facilities and procedures necessary for the production of specialized silicon devices for use in research on measurement methods.

Progress: A process is being developed for fabricating diffused silicon diodes required by the resistivity task for the purpose of measuring

the resistivity as a function of depth in silicon bulk and epitaxial wafers by the capacitance-voltage method. The requirements of the diffusion

are that its surface concentration be high (>10 zD cm

-3

), that the junction

be abrupt, and that the junction be shallow (<1 ym). Preparation of the

diodes begins with the thermal oxidation of 1- to 10-fi-cm n-type wafers to

a thickness of '0.5 urn to form a diffusion mask. Circular windows, 0.5 mm

in diameter, are defined in the oxide with conventional photolithographic

techniques . After cleaning , the wafer is diffused for 1 h with boron from

a boron nitride source at 1050°C. Sections of the diodes are beveled and

stained to verify the junction depth. Typically the sheet resistance is

10 fi/D, giving a surface concentration of approximately 6 x 10 20 atoms

per cubic centimeter. Diodes prepared in this manner on l-^*cm wafers

have exhibited breakdown voltages less than 20 V in contrast to an expected value of 60 V. The origin of the low breakdown voltages is being investigated in cooperation with the resistivity group (see Section 3.1).

Additional effort was expended attempting to improve the quality of

aluminum films produced by the electron beam evaporation system. A cryogenic coil was installed in an unsuccessful attempt to getter the gas produced during evaporation. The coil consisted of 5 turns of %-in. (6.2-mm)

diameter copper tubing bent into a 16-inch (450-mm) diameter helix. Liquid nitrogen passed through the coil served as the refrigerent. No de- crease in pressure was noted as the coil was cooled. Efforts were also

hampered by leaks around the low temperature connectors

.

(T. F. Leedy and J. Krawczyk)

21
PROCESSING FACILITY

Plans: Groups of diodes required by the resistivity task will be

fabricated in an attempt to determine where the defects originate. After

each step of the process a wafer will be withdrawn and etched in a preferential etch to display the crystallographic damage induced by that

step. Both commercially polished wafers and wafers from a crystal cut

and polished in-house will be used.

In the future, fabrication work will be reported with the appropriate

task. This section on Processing Facility will not be included unless

significant inprovements or additions to the capabilities of the facility

have been made.

4.4. REFERENCES

4.1. Die Attachment Evaluation

1. Natrella, M. G., Experimental Statistics, NBS Handbook 91 (August 1, 1963) pp. 3-36 to 3-37.

4.2. Wire Bond Evaluation

1. Schafft, H. A., Wire Eond Electrical Connections- Testing, Fabrication and De- gradation- A Bibliography 1957-1971, NBS Tech. Note 593, to be published.

22
METHODS OF MEASUREMENT FOR SEMICONDUCTOR DEVICES

5.1. THERMAL PROPERTIES OF DEVICES

Objective: To evaluate and, if necessary, improve electrical measure- ment techniques for determining the thermal characteristics of semiconductor devices.

Progress: Prototype circuits to measure transistor thermal resistance and common-emitter current gain in the common-base operating mode were designed and built. These circuits are much simpler than the common- emitter circuits used up to this point because it is not necessary to use special circuitry in the base lead to control the collector current . In the case of the current-gain measurement, the simplicity of the common- base circuit makes feasible a production-oriented current-gain screening

technique for hot-spot formation. In a comparison of the effectiveness

of the use of base-emitter junction voltage as a screen for thermal instabilities with that of the common-emitter current-gain it was found that

while both were sensitive to constricted hot-spots that exhibit thermal

hysteresis the former was much less sensitive to the more amorphous, less

constricted hot-spots that occur under higher current, lower voltage operating conditions. A study was undertaken to determine the pulse con- ditions under which a power transistor could be put into a constricted

operating mode with thermal hysteresis. With repetitively pulsed collector excursions, it was found that hysteresis occurred when the product

of the magnitude of the voltage pulse and the duty cycle was equal to the steady-state voltage change required to initiate hysteresis from a given collector voltage below the d-c hot-spot initiation voltage. For

singly pulsed excursions from various collector voltages below the d-c

hot-spot initiation voltage it was found that, for a given pulse width,

the combined d-c and pulsed voltage required to initiate hot-spots increased as the starting d-c collector voltage level was reduced.

Standardization Activities — The measurement procedure and data collection format were established for the preliminary round robin on thermal

resistance being conducted in cooperation with JEDEC Committee JC-25 on Power Transistors. Fourteen test specimens were measured in accordance

with the prescribed test procedure and sent to the first of the 10 industry participants. (F. F. Oettinger and'S. Rubin)

Thermal Resistance Methods — The study to compare other frequently

used thermal resistance measuring methods with the common-emitter baseand-collector switching technique now under investigation (NBS Tech. Note

598, pp. 33 -34) was continued. A prototype circuit to measure transistor

thermal resistance, R , in the common-base operating mode was designed

and built. The simplified circuit diagrams in figure 10 show both the

previously built common-emitter R measuring circuit and the newly designed and fabricated common-base R measuring circuit. In each case the tran- sistor Ql is initially on, and the sample-and-hold unit is holding. In

23
THERMAL PROPERTIES OF DEVICES

Figure 10. Circuits for measuring thermal resistance of tran- sistors .

CLOCK PULSE

a. Common-emitter V> Q1

10 «a MHrr~ "

D1 ^ L

1« /~> 1 PHOTO 1

| COUPLER | |-« r \r>T < <

-92 n 1

S 1H t\ dvm 1 CLOCK PULSE ^>J 1,

" —

X-Y

, RECORDER

, ' ll.' At

•" * *

b. Common-I ase the common-emitter circuit, the transistor Q3 is initially on and the tran- t h " f

1^1^ °ff S ° that °nly the

to h — is Providing currn the base of the transistor under test. In

B transistor

the common-base circuit the Q2 is off and the diode Dl in the emitter lead is conducting

sxstor

In both cases when the 120-ys wide, 4-pps clock pulse is applied the trail Ql is turned off by means of the photo cooler, and^ aampll-andt£

U

^

lt/

am^

es *he emitter-to-base voltage for approximately 1 ys at

l,LT

d

, " ' ?° _> 5 °~' °r 10 °- yS d6lay- TMs del^ in measuring the temperature sensitive parameter is set by an external switch. In thf common-emitter circuit, the clock pulse also initiates a 4-ys delay, then reverses the states of transistors Q3 and Q4 to turn the T servo off and the V

BB measuring current supply on for a period of 110 us* after which transistors Q3 and Q4 revert to their original state. In the common-base

out

Cthe'v " nVf

ted

, ClOCk

. PUlSe tUrnS the transistor Q2 on, shorting °"* ^ V

EE.

8,*P1y

and causin§ the diode Dl to become nonconducting so T^lel'^^lotTeTTy T^ ** ^"^ase JTM*I« - the measuring

7 BB'

in. lo^,

CT°K_baSe R

§ m

? ing

asurinS ci ^uit does not require the base switch- logic which is used

1 with the common-emitter configuration. Further when measuring transistors with betas of 50 or more, the change in collecce^thuf the'"

6 C °^leCt0r V° ltage 1S VariSd 1S USUa11^

le- than 1 perthP nnn ; constant-current servo in the base lead to control

and voliT

CU^

nt

- and

1S n0t necessa^- B oth circuits have heating current voltage capabilities of to 2 A and to 250 V, respectively.

24
THERMAL PROPERTIES OF DEVICES

Measurements of R using both circuits were made on four 35-W,

triple-diffused, silicon power transistors. The results of these preliminary measurements, given in table 2, indicate that there is substantial

agreement between the two techniques. Further studies are needed to determine the relative merits of the two techniques when the transistor un- der test is operated under a wide variety of operating conditions.

An attempt was also made to make common-base R measurements using

the collector-base junction voltage as the temperature sensitive parameter.

Problems were encountered with the sample-and-hold unit as well as with

spurious signals affecting the voltage reading. A study was undertaken

to determine what corrective measures should be taken to solve these problems. (F. F. Oettinger and S. Rubin)

Screen for Hot Spots — The use of d-c current gain, h , as an indicator for the formation of hot spots and thermal hysteresis in power transistors has been discussed previously (NBS Tech. Note 520, pp. 49-52).

In this technique, the base current, I , is measured as a function of

power dissipated in the transistor while holding the collector current

and case temperature constant. Under these conditions h is proportional

to the reciprocal of I . Hot-spot formation is indicated by a distinct

increase in I with increasing power. The increase may be abrupt, as with

constricted hot spots that exhibit thermal hysteresis, or more gradual, as may occur under higher current, lower voltage operating conditions. Typical examples are shown as solid curves in figure 11.

The effectiveness of the more commonly used base-emitter junction

voltage, V , as a screen for thermal instabilities [1] was compared with

that of the base current. In the V technique, the junction voltage is

measured as a function of transistor power under conditions of constant

collector current and case temperature. Typical results, shown as dashed

curves in figure 11, indicate that while V is sensitive to hot spots

that exhibit thermal hysteresis it is relatively insensitive to the more amorphous, less constricted hot spot that occurs under higher current,

lower voltage operating conditions. On the other hand, the I technique

clearly indicates the formation of hot spots at high as well as low cur- rent levels

.

The measurements described above were made in the common-emitter mode.

Similar indications of the formation of hot spots and thermal hysteresis

are obtained if I is measured in the common-base configuration. By com- paring the common-emitter and common-base circuits , shown in figure 12 it can be seen that, as was the case with the common-base R measuring circuit, the common-base h circuit does not require a constant-current servo

in the base lead to control the collector current as the collector voltage

is varied. It is therefore evident that a production-oriented screening

technique for hot-spot formation is feasible using the common-base h

circuit. (F. F. Oettinger and S. Rubin)

25
THERMAL PROPERTIES OF DEVICES

Table 2 -Comparison of Thermal Resistance Measured

using Common-Emitter and Common-Base Circuits

Transistor

0-60

0-61 0-62

0-69

Conrnon -Emitter 1.99°C/W

1.72

1.55 1.79

Common-Base

2.00°C/W

1.77 1.76

1.77

Figure 11. Base current (solid curves)

and base-emitter voltage (dashed

curves) as a function of power for a silicon power transistor showing onset

of hot spots.

2 6

2.7

26

<lea

i.o

0.9

8 12 16 20 24 28 32 POWER (W)

700 7

IdO

675 <

655 >

600

r DVH JW\

TUT £

X Y .RECORDER. T

t

a. Common-emitter b. Common-base

Figure 12. Circuits for measuring common- emitter current gain of transistors.

26
THERMAL PROPERTIES OF DEVICES

Thermal Hysteresis under Pulsed Operation — A study was undertaken

to determine the pulse conditions (voltage transients or surges) under

which a power transistor would be put into a constricted operating mode

with thermal hysteresis. Both singly and repetitively pulsed collector

excursions were performed with the common-base h circuit. The voltage

pulses were generated by utilizing the collector-switching transistor nor- mally used with the common-base R measuring circuit to short out a variable resistor in the collector circuit of the transistor under test. It was determined from the singly pulsed measurements that the transistor

would go into thermal hysteresis if the collector voltage pulse had the

proper combination of magnitude and width for a given starting point below the hot-spot initiation voltage. For example, a 20-V pulse, 5-ms

wide caused a particular 35-W, triple-diffused, silicon power transistor

to go into the constricted mode when the pulse was added to a d-c collector

voltage level 6 V below the hot-spot initiation voltage. When the d-c

collector voltage level was decreased to a point 12 V below the hot-spot

initiation voltage the 5-ms wide pulse had to be increased to 30 V to cause a hot spot to form; a 10-V increase in pulse height is therefore

required for a 6-V decrease in d-c collector voltage level. This non- linear difference in energy needed to cause the device to go into thermal

hysteresis might be explained by the fact that as the device operating

point approaches the hot-spot initiation voltage it takes a proportionally

smaller amount of energy to produce the needed temperature gradient due

to that which already exists (NBS Tech. Note 592, pp. 49-51).

Work with repetitively pulsed collector voltage excursions demonstrated that hysteresis occurred when the product of the magnitude of the voltage pulse and the duty cycle equalled the steady-state change required to initiate hysteresis from a given collector voltage below the hot-spot initiation voltage. This was evidenced most clearly by the action of a moving coil voltmeter that was used to read the collector voltage. The voltage magnitude at which the device went into thermal hysteresis as indicated by the meter was the same for a d-c collector voltage as for a combined

d-c and pulsed voltage. (S. Rubin and F. F. Oettinger)

Plans : The literature search and work on the bibliography on thermal

resistance and transient thermal response measurements will be resumed.

Design changes will be incorporated into the existing common-base R meas- uring circuit to facilitate the use of the collector-base junction voltage

as the temperature sensitive parameter for transistor thermal resistance

measurements. Comparison of measurements made with this and other techniques for measuring R will then be made. Work will also continue on the

determination of the thermal inertia characteristics of the thermal hysteresis phenomenon. Thermographic measurements will be made on devices to investigate the relationships which may exist between the peak temperature

on the chip and the electrically measured junction temperature.

27
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 : An improved heat sink was fabricated to facilitate the calibration of the temperature sensitivity of the phosphors. The heat sink is suitable for calibrations in the temperature range 20 to 315°C which includes the temperature range of the three lowest temperature phos- phors and the lower end of the temperature range of the highest tempera- ture phosphor.

further work was done with a slurry-settling technique for coating a specimen with phosphor. In this technique the specimen is placed in a slurry composed of 0.45 to 0.9 g of phosphor in 60 cm 3 of ethyl alcohol and the phosphor is allowed to settle onto the specimen surface for sever- al minutes. The slurry is then drained and the specimen allowed to dry for 1.5 to 2 h while it is protected from air currents and kept at approx- imately room temperature. Preliminary results suggest that the thickness of the phosphor coating thus obtained can be controlled by controlling the depth beneath the surface at which the specimen is placed or by con- trolling the phosphor concentration in the slurry. The slurry-settling technique appears to give a more uniform phosphor coating than the water- floatation technique (NBS Tech. Note 488, p. 31), but conclusive experi- ments have yet to be made.

It was determined that the microcopy resolution chart (NBS Tech. Note 592, p. 56) used to establish the spatial resolution of the fiber-optic probe does not have strictly well defined white and black lines . Upon viewing the charts under a microscope (approximately 300 X magnification)

a gray transition region between the black and white lines can be seen The gray region consists of black dots randomly spaced on a white field- the density of dots decreases toward the center of the white line For' the smallest line width available on the test chart (0.028 mm), the dots extend across the entire width of the white line. This gray region may account for the apparently poor agreement between the spatial resolution of the 50-um fiber-optic probe as determined with the aid of the microcopy resolution charts and the spatial resolution stated by the manufacturer.

It has not yet been established whether it is possible, by accounting for the gray region, to determine the resolution of the fiber-optic probe bv

means of these charts.

mi urn. Silicon wafers with surfaces of silicon dioxide, aluminum, gold, chroi, and polished silicon have been obtained for use in a study to estab- lish coating uniformity and to determine whether a reaction occurs between the phosphors and the various surfaces.

(D. L. Blackburn and L. R. Williams)

Plansj The temperature calibration of the phosphors coated by the previously used water- floatation technique will be concluded. The proce28
THERMOGRAPHIC MEASUREMENTS

dure for coating wafers and devices by the slurry-settling technique will

be further developed to allow a controlled coating thickness to be obtained. If it is not feasible to use the microcopy resolution charts to es- tablish the spatial resolution of the fiber-optic probe, metallization

strips on a silicon substrate will be tested for this determination. Calibration and temperature sensitivity measurements on specimens coated by

the slurry-settling technique will begin. Measurements will be made to establish the comparative uniformity of coatings made by the waterfloatation and slurry-settling methods.

5.3. MICROWAVE DEVICE MEASUREMENTS

Objective: To study the problems and uncertainties associated with

the measurement of selected microwave device properties, and to improve

the techniques of these measurements.

Progress: To improve the precision of the low intermediatefrequency conversion-loss measurements, an incremental modulation technique has been introduced. With this technique, precision is limited

principally by the resolution of the attenuator used to set the increment

and by the mechanical stability of the waveguide system rather than by

the output resolution.

In the new arrangement, the square-wave a-f generator used to modulate the local oscillator has been replaced by a stable d-c power supply.

The change in r-f power caused by this d-c supply can be made identical

to that caused by the square-wave generator. As before, power calculations are based upon sinusoidal modulation yielding the same peak and

trough values. In effect, this change to incremental modulation reduces

the intermediate frequency to zero. In addition to the advantage of

using a more stable modulation source, this change also allows the use of a narrower signal bandwidth, which reduces- system noise. It also allows the use of the precision rotary-vane attenuator, currently used to calibrate the PIN modulator, directly as the incremental modulator itself. A further advantage of incremental modulation is that the output

voltage "peaks" can be read directly on a digital voltmeter, rather than

by the more complex, less precise switched-oscilloscope d-c comparison

method used with a-f modulation.

Incremental modulation has been used in the past for absolute mixer

conversion loss measurements of the highest accuracy. As this method has

been used [1], a microammeter is used to read an output current increment

that occurs with an incremental change in power. The equation used to convert the current increment to conversion loss was not based on the

modulation concept. In developing the periodic modulation method, commonly known as the modulation method, as an absolute rather than a relative

method, it has become apparent that the incremental modulation method,

29
MICROWAVE DEVICE MEASUREMENTS

commonly known as the incremental method, remains the superior technique

for high precision measurements. Now, however, the availability of

precise digital voltmeters makes a method based on a voltage increment

preferable to one based on a current increment. The equation used to

convert the voltage increment is based on the modulation concept; this

equation can be shown to be equivalent to the earlier one used with

measurements of current increment.

Using a table of attenuation as a function of vane angle [2], it was calculated that the 0.0001-inch (2.5-ym) resolution of ordinary vernier

micrometer heads used as adjustable stops to position the rotary-vane attenuator at precise locations should contribute less than 0.01 dB uncertainty to the conversion loss measurement. To obtain a commensurate ac- curacy, however, requires calibration of the attenuator to ±0.001 dB for

a 1 dB modulation increment, which is several times better than can presently be obtained. Preliminary consideration was given to the use of a precision r-f power meter to establish the increment. To determine theincrement within an uncertainty of ±0.001 dB requires independent power

measurements within an uncertainty of ±0.0005 dB or ±0.01 percent of the

power level. This is within the total bridge uncertainty (substituted

d-c power, exclusive of voltmeter) of ±0.0061 percent reported [3] for

the NBS Type II Power Measurement System for measurement of a power level

of 10 mW using a 200-fi, -12.3-ft/mW thermistor. Bolometer instability appears likely to severely degrade this small uncertainty, so it is questionable that the use of a power meter to establish the increment would prove

to be superior to the use of a calibrated attenuator. If the power meter

were, in fact, superior, this would suggest that the attenuator calibration would be improved by using the same power meter.

An immediate limitation on precision is a lack of rigidity in the

waveguide system. It was observed that the r-f power to the mixer is affected by mechanical forces on the waveguide. There is sufficient hysteresis in these power changes to suggest that interchanging the bolometer

(mount) and the mixer (holder) or removing and reinserting a diode would

result in a significant permanent power change. Although power changes

may also result because of lack of reproducibility of diode contacts and

position and of waveguide flange connections, these aspects of the problem

cannot be investigated until the waveguide system is made more rigid.

(J. M. Kenney)

Plans : To facilitate use of the precision attenuator as the incremental modulator, it will be modified by the addition of precise mechanical

stops (vernier micrometer heads with locks), a modification that is also

required in order to improve the calibration accuracy beyond that possible

using the relatively coarse dial. It will then be sent to NBS Boulder for

calibration. A support system to add rigidity to the waveguide will be

developed. A review of the current status of measurement requirements

for microwave mixers will be undertaken to assure that task goals continue

to be appropriate to these needs.

30
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: The vector-voltmeter measurement system for measuring transistor delay time was assembled and preliminary tests were carried out. Two systems for the measurement of transistor S-parameters in the 200 to

800 MHz range are being assembled. An analysis of the equivalent circuits

of a transistor in the Sandia-type bridge and in the vector-voltmeter circuit has been made to relate the measured delay time to various time con- stants and delay times within the device.

Expevimental — A delay-time measurement circuit was constructed to interface between the transistor under test and a vector voltmeter. This

allows measurements made using the Sandia-type bridge to be compared with

those made using the completely independent, vector-voltmeter system.

Preliminary measurements made with the vector-voltmeter on a silicon

n-p-n transistor with f of about 350 MHz yielded delay times which were relatively independent of measurement frequency, as they should be, over the range 3 to 30 MHz. In order to resolve questions of absolute accuracy, a tiny R-C network of known delay time (423 ps) was assembled on a TO-18 transistor header. A spot measurement at 13 MHz on this network

in the transistor socket yielded a measured delay time of 460 ps . This

is within the specified accuracy of the vector voltmeter; greater accuracy requires the use of more refined techniques for calibration that are now being developed. The present approach Is derived from the R-C network plug-in idea, and uses networks of known phase shift to calibrate

the vector voltmeter to the accuracy needed for the transistor measurements. (D. E. Sawyer and F. R. Kelly)

Two systems are being assembled for the measurement of transistor Sparameters in the 200 to 800 MHz range. In both systems the r-f source

is coupled to the transistor under test through a dual directional coupler

and provisions are made to measure the amplitide and phase of both the signal reflected from the transistor terminals and the signal transmitted by

the transistor. One system, being obtained commercially, is self-contained

and requires only a bias supply and connections to the signal source and

the device being measured. The second system is being assembled of individual components to provide an independent check on measurements made

with the first. (G. J. Rogers)

Analytical — The equivalent circuits of a transistor in the Sandiatype bridge and in the common-base delay-time measurement circuit based

on the use of a vector voltmeter have been analyzed to relate the measured

delay time to the various time constants and delay times within the device.

With the generally applicable measurement condition dot << 1 prevailing,

where to is the test angular frequency and i the delay time obtained from

the vector voltmeter, the transistor delay time expression simplifies to

t =i + i -i/h_. vv a e c fe

31
CARRIER TRANSPORT IN JUNCTION DEVICES

In this equation t is the delay time due to the transit of minority carriers across the base and collector "junction regions, i = r C , and

e e e

t cbc= r, C , where r is the small-signal emitter-base input resistance, r, e . b

is the base resistance, C and C are the emitter-base and base-collector

e c transition-region capacitances, respectively, and h is the commonemitter small-signal current gain. Analysis of the Sandia-type bridge

yields a similar delay-time equation except that correction factors appear

as a consequence of the fact that for this bridge the signals applied to the transistor are obtained from quasi-voltage sources, and not from current sources, and so the transistor bridge input currents depend on the

values of the transistor elements such as r and r„ . A memorandum is bee B ing written to record all the details of the analysis of these two measurement circuits so that applicable delay-time expressions can be obtained

for a wide range of measurement conditions.

In considering the delay-time error that might be introduced into

measurements made on the Sandia-type bridge by moving the primary loop in

the attenuator and phase-splitter box relative to the two secondary coils

(NBS Tech. Note 598, p. 4-0), it was concluded that this would not be a dominant source of error under present conditions of measurement.

( D . E . Sawyer

)

Plans: Techniques for rapid, and accurate calibration of the Sandiatype bridge and the vector-voltmeter delay-time measurement equipment will

be investigated. The memorandum relating measured delay times with device

internal processes will be completed. Two systems for the measurement of

delay time by means of S-parameter measurement will be set up. Measurements on the same transistors will be made by all four methods and the results compared.

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 consultation

and specialized device fabrication services to the sponsor.

Progress : The studies of radiation damage effects of 600-keV electrons and fast neutrons in lithium-drifted silicon detectors have been

completed. The effects of exposure of a silicon surface-barrier detector

to dry ammonia and methanol vapor at various pressures have been studied.

Testing and Evaluation — Testing of commercial lithium-drifted silicon detectors for evaluation before acceptance by the sponsor has been

concluded. These pre-flight bench-tests were carried out at the NASA-GSFC

site for the IMP-H and Pioneer-F programs. (A. J. Baroody)

32
SILICON NUCLEAR RADIATION DETECTORS

Radiation Damage — A commercial, 2-mm thick, lithium-drifted silicon

detector was irradiated with 600-keV electrons at fluences between 1 x io 12 and 5 x 10 15 cm

-2 on the particle entrance window (p-contact). The irradiation was carried out at room temperature and with an applied detector

bias of 400 V. The average range of electrons of this energy in silicon

is approximately 850 ym; therefore, defects are expected to be localized

in the region extending from the entrance window to a depth of approximately 640 ym [1]. Damage effects similar to those observed using 1.5 meV

electrons were seen (NBS Tech. Note 598, pp. 41-42), but to a lesser ex- tent. After irradiation at a fluence of 5 x 10 15 cm

-2

, a dead layer approximately 600 ym thick was observed at a bias of 400 V at the irradiated

p-contact. No change in detector response was observed when counting particles incident on the unirradiated rear contact.

A second detector was irradiated through the rear n-contact under the same conditions as above. The results indicated that the increase of

noise with increased fluence was much faster than that obtained from front

p-contact irradiation while the variation in current was about the same.

Two detectors were irradiated at room temperature with fast neutrons

from a calibrated plutonium-beryllium neutron source. One detector was reverse-biased at 400 V; the other, at 100 V. With the detectors located

5 cm from the source, the neutron flux is approximately 5 x io 4 cm

-2

s -1. The gamma-ray background was small and damage effects due to gamma rays

could be neglected. Fast neutron irradiation produces many secondary interactions in silicon because the energy acquired by the silicon atoms

from elastic neutron collisions is quite high. Dearnaley [2] has estimated that each neutron-silicon atom collision produces a minimum of 150

Frenkel pairs. Such interactions are characterized by small, highlydamaged regions of complex defect clusters separated by considerable

amounts of undamaged material due to the large mean free path between collisions. The degradation of detector performance observed due to neutron

damage was similar to that observed in the charged particle irradiations

with significant increases in noise and current above fluences of

10 10 cm

-2

. After the final irradiation at a neutron fluence of approximately 6 x 10 1C cm

-2

, a dead layer of about 40 ym was evident on the irradiated front contacts of both detectors. This result indicates that the

pulse-height defect was relatively independent of bias. The increase in

noise and current with increased fluence was faster at the higher bias.

Annealing effects have not been observed during a two week period following the irradiation. (Y. M. Liu)

Ambient Exposure Tests — The effects of exposure of dry ammonia and

methanol vapor on the noise of a silicon surface-barrier detector were measured. The detector was placed in the vacuum manifold and pumped at 0.06 mTorr for 24 hours before dry ammonia was admitted into the test

chamber. The detector noise level as measured using an oscilloscope and

RMS voltmeter [3] varied from 33.5 to 39.7 keV before the tests were begun.

33
SILICON NUCLEAR RADIATION DETECTORS

Dry ammonia was leaked into the system through a variable micro-leak

valve and unless otherwise noted, the system was maintained at pressure

for 5 min before noise level measurements were made. Four measurements

were made at pressures from 0.03 to 0.7 mTorr; detector noise remained

fairly constant at 39.1 keV. Twenty measurements were made at pressures

from 1 to 100 mTorr; noise varied from 39.3 keV to 40.9 keV. The ammonia

pressure was held at 5 mTorr for 2 h; noise varied between 39.3 to 40.1 keV At the end of the 100 mTorr run, the pressure was lowered to 50 mTorr and

held there for 17 h; the noise level of the detector changed from 40.1 to 37.7 keV. The system was then pumped down to 0.03 mTorr overnight, and

tests were subsequently made from 100 to 1000 mTorr in six steps, holding

for 5 min at each step. The noise level remained near 36.9 keV.

Methanol exposure tests were run in a manner similar to the ammonia

runs. The spectro-grade methanol source consisted of a reservoir of the

solvent fed directly to the face of the leak valve. Ten measurements

were made at pressures ranging from 0.05 to 100 mTorr; detector noise

levels ranged from 36.2 to 36.9 keV . The pressure was held at the 1000

mTorr level for 2 h with no change in noise. (W. K. Croll)

Plans: The study of 2-MeV proton radiation damage effects in lithiumdrifted silicon detectors will be completed. Detectors irradiated with

electrons, neutrons, and protons will be studied by means of the infrared

response technique in order to seek correlation between the results observed and energy levels introduced into the silicon by irradiation. Ambient exposure tests will be carried out on lithium-drifted silicon detectors supplied by the sponsor. Dry ammonia, methanol, and commerical

leak detector fluid will be used.

5.6. REFERENCES

5.1. Thermal Properties of Devices

1. Scarlett, R. M., Shockley, W. , and Haitz, R. H., Thermal Instabilities and Hot

Spots in Junction. Transistors, Physios of Failure in Electronics, M. F. Goldberg

and M. Vaccaro, Eds. (Spartan Books, Inc., Baltimore, Md., 1963), pp. 194-203.

5.3. Microwave Device Measurements

1. Method 4101.3, Conversion Loss, Test Method A (Incremental), MIL-STD-750B, Test Methods for Semiconductor Devices, Notice 1, 13 May 1970.

2. Larson, W. , Table of Attenuation as a Function of Vane Angle for Rotary-Vane At- tenuators (A = -40 log 10 cos 8), NBS Tech. Note 229 (Jan. 7, 1965); see also Larson,

W., Table of Attenuation Error as a Function of Vane-Angle Error for Rotary Vane Attenuators, NBS Tech. Note 177 (May 20, 1963).

3. Design Package Available for NBS RF Power Measurement System, NBS Tech. News Bull.

55_, 117 (1971).

34
REFERENCES

5.5. Silicon Nuclear Radiation Detectors

1. Liu, Y. M. , and Coleman, J. A., Electron Radiation Damage Effects in Silicon

Surface-Barrier Detectors, IEEE Trans. Nual. Sai. NS-18 , No. 1, 192-199 (1971).

2. Dearnaley, G. , and Northrop, D. C. , Semiconductor Counters for Nuclear Radiations

(E. £ F. N. Spon Ltd., London, 1964), pp. 268-269.

3. USA Standard and IEEE Test Procedure for Semiconductor Radiation Detectors (for

Ionizing Radiation), USAS N42 .1(1969)/IEEE No. 300, pp. 5-6.

35
Appendix A

JOINT PROGRAM STAFF

Coordinator: J. C. French

Consultant: C. P. Marsden

Semiconductor Characterization Section

(301) 921-3625

Dr. W. M. Bullis, Chief

A. J. Baroody, Jr.

F. H. Brewer+

M. Cosman

Dr. J. R. Ehrstein

R. L. Mattis

Dr. W. E. Phillips

Miss D. R. Ricks

H. A. Schafft

A. W. Stallings

Mrs . M . L . Stream"1" W. R. Thurber

Semiconductor Processing Section

(301) 921-3541

Dr. A. H. Sher, Acting Chiefx

H. A. Briscoe

W. K. Croll

Mrs. S. A. Davis+

H. E. Dyson"

G . G . Harman

W

.

J . Keery

H. K. Kessler

J. Krawczyk

T. F. Leedy

Y. M. Liu

Miss J. M. Morrison

Mrs. E. A. Simmons^

L. M. Smith

G. P. Spurlock

Electron Devices Section

(301) 921-3622

J. C. French, Chief

D. L. Blackburn

Mrs. C. F. Bolton+

Miss B. S. Hope+

R. L. Gladhill

F. R. Kelly11

J . M . Kenney

Mrs. K. 0. Leedy§

Miss C. A. Main§

F. F. Oettinger

M. K. Phillips

G. J. Rogers

S. Rubin

D

.

E . Sawyer

L. R. Williams

* Part Time

t On Leave

+ Secretary

U Summer

x Dr. J. A. Coleman is on temporary assignment to Office of Associate

Director for Programs

.

§ Teleohone: (301) 921-3625

36
J. C. G. G. K. 0. T. F. C. P.

R. L. W. E.

A. H. W. R.

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, and

Subcommittee 7, Hybrid Microelectronics; Leaks, Resistivity,

Mobility, Dielectrics, and Compound Semiconductors Sections

J. A. Coleman, Secretary, Subcommittee 5, Semiconductor Processing

Materials

J. R. Ehrstein, Chairman, Resistivity Section, Epitaxial Resistivity,

and Epitaxial Thickness Sections

French, Committee Editor

Harman, Bonding Section

Leedy, Chairman, Bonding Section

Leedy, Photoresist Section

Marsden, Committee Secretary

Mattis, Lifetime Section

Phillips, Chairman, Lifetime Section; Secretary, Subcommittee

4, Semiconductor Crystals; Crystal Perfection, Encapsulation,

Thin Films, and Thick Films Sections

H. Sher, Germanium Section

R. Thurber, Mobility, Germanium, Compound Semiconductors, and

Impurities in Semiconductors Sections

ASTM Committee E-10 on Radioisotopes and Radiation Effects

W. M. Bullis, Subcommittee 7, Radiation Effects on Electronic

Materials

J. C. French, Subcommittee 7, Radiation Effects on Electronic

Materials

Electronic Industries Association: Solid State Products Division, Joint

Electron Device Engineering Council (JEDEC)

J. M. Kenney, Microwave Diode Measurements, Committee JC-21 on UHF

and Microwave Diodes

F. F. Oettinger, Chairman, Task Group JC-11.3-1 on Thermal Considerations for Microelectronic Devices, Committee JC-11 on Mechanical Standardization; Technical Advisor, Thermal Resistance Measurements; Committees JC-22 on Thyristors , JC-20 on Signal Diodes, JC-2'5 on Power Transistors, and JC-30 on Hybrid

Integrated Circuits

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)

37
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, Transaction on Nuclear

Science

IEEE Magnetics Group

S. Rubin, Chairman, Galvanomagnetic Standards Subcommittee

IEEE Parts, Hybrids, and Packaging Group

W. M. Bullis, New Technology Subcommittee, Technical Committee on Hybrid Microelectronics

Society of Automotive Engineers

J. C. French, Subcommittee A-2N on Radiation Hardness and Nuclear

Survivability

W. M. Bullis, Planning Subcommittee of Committee H on Electronic

Materials and Processes

IEC TC47, Semiconductor Devices and Integrated Circuits:

S. Rubin, Technical Expert, Galvanomagnetic Devices; U. S. Specialist for Working Group 5 on Hall Devices and Magnetroresistive

Devices

NMAB ad hoc Committee on Materials and Processes for Electron Devices

W. M. Bullis

NMAB ad hoc Committee on Materials for Radiation Detection Devices

D. E. Sawyer

38
Appendix C

SOLID-STATE TECHNOLOGY 6 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 (W. K. Croll and J. Krawczyk)

Five lithium-drifted silicon detectors measuring 50 by 20 by

0.75 mm and divided by 0.38 mm wide ultrasonically-machined

slots into 12 parallel active regions measuring 46 by 1.5 mm were fabricated for the Electronuclear Physics Section for

use in an electron-scattering hodoscope.

2. Thin-Film Deposition (T. F. Leedy)

Thin-film silver contacts were vacuum evaporated on piezoelectric polymer sheets for the Instrumentation Applications

Section.

3. Capacitor Fabrication (T. F. Leedy)

Three quartz guard ring capacitors were fabricated for the

Pressure Measurements Section. Each capacitor consisted of

a quartz disc 1.5 cm in diameter by 2 mm thick. Aluminum

contacts were applied to each face and the edge. Guard

rings were photolithographically defined in the faces of the

disc so that three-terminal capacitance measurements can be

made

.

39
Appendix D

JOINT PROGRAM PUBLICATIONS

Prior Reports

:

A review of the early work leading to this Program is given in

Bullis, W. M. , Measurement Methods for the Semiconductor Device Industry—

A Review of NBS 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. These reports may be obtained from

the Superintendent of Documents (Catalog Number C. 13.46: XXX) where XXX

is the appropriate technical note number. Microfiche copies are available from the National Technical Information Service (NTIS), Springfield,

Virginia 22151.

Quarter Ending NBS Tech. Note Date Issued NT IS Accession

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 AD 723671 December 31, 1970 592 August, 1971 AD 728611 March 31, 1971 598 October, 1971

Current Publications:

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, IEEE Trans. Electron Devices ED-18 , '393-394 (June, 1971).

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, April, 1971.

Sher, A. H. , and Thurber, W. R. , Minority Carrier and Lithium-Ion Drift

Mobilities and Oxygen Concentration in p-Type Germanium, to be published

in J. Appl. Phys. (September, 1971).

Sher, A. H. , Croll, W. K. , and Thurber, W. R. , Determination of Oxygen in

Germanium Below 20 ppb by Measurements of Lithium Mobility and Precipitation, to be published in Anal. Chem. (November, 1971).

Brenner, D„ J.," A Technique for Measuring the Surface Temperature of Tran- sistors by Means of Fluorescent Phosphor, NBS Tech. Note 591, July, 1971.

NBS Measurement Engineering Division

40
APPENDIX E

STANDARDIZATION AND DISSEMINATION ACTIVITIES

(A summary for fiscal year 1971)

STANDARDS ACTIVITIES:

.- Interim standards

- ASTM editorial review (15 documents)

- Verification of standards

- Round robin experiments

ASTM (F-l) 5 methods

EIA (JEDEC: JC-22, 25) 3 methods

- Committee participation

ASTM (F-l, E-10)

EIA (JEDEC: JC-11, 20, 21, 22, 24, 25, 30)

IEC (TC-47)

IEEE (G-ED, MAG, MTT, NS , PHP) NMAB

SAE (A2N, H)

About 20 staff members active

About 15 offices held by staff members

Over 50 task forces or committees attended

Benefits are (1) guidance to program, (2) rapid utilization of

results by industry

FIELD VISITS: 54 to industrial, university, and government installations

PUBLICATIONS AND TALKS: 22; in addition QPR ' s go by request to nearly

600 persons on initial distribution

VISITS TO NBS: About 75 formal; 105 for specific information exchange

CONSULTATIONS: 170

41
FORM NBS-114A (WD

U.S. DEPT. OF COMM. -

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS-TN 702

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 , April 1 to June 30

,

1971

5. Publication Date November. 1 971

6. Performing Organization Code

7. AUTHOR(S)

W. Murray Bullis , Editor

8. Performing Organization

9. PERFORMING ORGANIZATION NAME AND ADDRESS NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/ Work Unit No. See Ttem 1

5

11. Contract/Grant No.

See Item 12

12. Sponsoring Organization Name and Address NBg ^ MgA (EA071-801), Navy

Strategic Systems Project Office, NAD, Crane, Indiana

(PO-1-0030, PO-1-0041, and PO-1-0067), NAFI (WR-1-1038), NAVELEX

(P0-1-1057), AFWL (F29601-71-0002), AFCRL (Y71-906), ARPA

(MIPR FY76167100331), AEC, NASA (S-70003-G and H-76553A).-

13. Type of Report & Period Covered Interim April 1 to June 30, 1971

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES 4251120j

^^^ ^

112 ^ ^251

^, 4251535) k25211^ ^ 25211%

4252128, 4252534, 4252535, 4254111, 4254112, 4254115, 4254422, 4254429, 4254432

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, twelfth 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 a ^demonstration of the high sensitivity of the infrared response technique by

the ^identification of gold in a germanium diode doped to a level of 10 n gold atoms peri cubic centimeter, verification that transient thermal response is significantly more

sensitive to the presence of voids in die attachment than steady-state thermal resistance, and development of a simplified circuit for screening transistors for susceptibility to hot-spot formation by the current-gain technique. Work is continuing on measurement of resistivity of semiconductor crystals; study of gold-doped silicon; specification of germanium for gamma-ray detectors; evaluation of wire bonds and die attachment;

measurement of thermal properties of semiconductor devices, transit time and related car- rier 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.): 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) at^u-, „-,„.,- • -, j i , Alpha-particle detectors; aluminum wire; base transit time; carrier lifetime; die attachment; electrical properties; epitaxial silicon;

gamma-ray detectors; germanium; gold-doped silicon: methods of Msnrp^t

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NBS TECHNICAL PUBLICATIONS

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Comprehensive scientific papers give complete details of the work, including laboratory data, experimental

procedures, and theoretical and mathematical analyses. Illustrated with photographs, drawings, and

charts.

Published in three sections, available separately:

• Physics and Chemistry

Papers of interest primarily to scientists working in these fields. This section covers a broad range of physical and chemical research, with major emphasis on standards of physical measurement, fundamental

constants, and properties of matter. Issued six times a year. Annual subscription: Domestic, $9.50; $2.25

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• Mathematical Sciences

Studies and compilations designed mainly for the mathematician and theoretical physicist. Topics in mathematical statistics, theory of experiment design, numerical analysis, theoretical physics and chemisty, logical design and programming of computers

and computer systems. Short numerical tables. Issued

quarterly. Annual subscription: Domestic, $5.00;

$1.25 additional for foreign mailing.

• Engineering and Instrumentation

Reporting results of interest chiefly to the engineer

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data processing, and development of test methods.

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Domestic, $5.00; $1.25 additional for foreign mailing.

Applied Mathematics Series. Mathematical tables, manuals, and studies.

Building Science Series. Research results, test methods, and performance criteria of building materials, components, systems, and structures.

Handbooks. Recommended codes of engineering

and industrial practice (including safety codes) de- veloped in cooperation with interested industries, professional organizations, and regulatory bodies.

Special Publications. Proceedings of NBS conferences, bibliographies, annual reports, wall charts, pamphlets, etc.

Monographs. Major contributions to the technical

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