UNITED STATES PARTMENT OF JMMERCE

JBLICATION

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MAR n 1970

NBS TECHNICAL NOTE 520

U.S.

PARTMENT

OF

IOMMERCE

National

Bureau of

Standards

Methods of

Measurement for

Semiconductor

Materials, Process

Control, and Devices

Quarterly Report

July 1 to September 30, 1969
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards ' was established by an act of Congress March 3, 1901 . Today,

in addition to serving as the Nation's central measurement laboratory, the Bureau is a principal

focal point in the Federal Government for assuring maximum application of the physical and

engineering sciences to the advancement of technology in industry and commerce. To this end

the Bureau conducts research and provides central national services in four broad program

areas. These are: (1) basic measurements and standards, (2) materials measurements and

standards, (3) technological measurements and standards, and (4) transfer of technology.

The Bureau comprises the Institute for Basic Standards, the Institute for Materials Research, the

Institute for Applied Technology, the Center for Radiation Research, 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 an Office of Measurement Services and the following technical

divisions:Applied Mathematics—Electricity—Metrology—Mechanics—Heat—Atomic and Molecular Physics—Radio Physics -—Radio Engineering - —Time and Frequency -—Astrophysics '-' —Cryogenics.2 THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to im- proved methods of measurement standards, and data on the properties of well-characterized

materials needed by industry, commerce, educational institutions, and Government; develops,

produces, and distributes standard reference materials; relates the physical and chemical properties of materials to their behavior and their interaction with their environments; and provides

advisory and research services to other Government agencies. The Institute consists of an Office of Standard Reference Materials and the following divisions: Analytical Chemistry—Polymers—Metallurgy—Inorganic Materials—Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote

the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations in the development of technological

standards, and test methodologies; and provides advisory and research services for Federal, state, and local government agencies. The Institute consists of the following technical divisions and

offices: Engineering Standards—Weights and Measures — Invention and Innovation —Vehicle Systems Research—Product Evaluation—Building Research—Instrument Shops—Measurement Engineering—Electronic Technology—Technical Analysis.

THE CENTER FOR RADIATION RESEARCH engages in research, measurement, and application of radiation t0 the solution of Bureau mission problems and the problems of other agencies and institutions. The Center consists of the following divisions: Reactor Radiation—Linac Radiation—Nuclear Radiation—Applied Radiation. THE CENTER FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and

provides technical services designed to aid Government agencies in the selection, acquisition,

and effective use of automatic data processing equipment; and serves as the principal focus

for the development of Federal standards for automatic data processing equipment, techniques,

and computer languages. The Center consists of the following offices and divisions: Information Processing Standards—Computer Information — Computer Services — Sys- tems Development—Information Processing Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal government; promotes the development of the National Standard Reference Data System and a system of information analysis centers dealing with the broader aspects of the National Measure- ment System, and provides appropriate services to ensure that the NBS staff has optimum accessibility to the scientific information of the world. The Office consists of the following

organizational units:

Office of Standard Reference Data—Clearinghouse for Federal Scientific and Technical Information ; ' —Office of Technical Information and Publications—Library—Office of Public Information—Office of International Relations.

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

1 Located at Boulder. Colorado 80302.

1 Located at 52R5 Port Royal Road, Springfield, Virginia 22151.
UNITED STATES DEPARTMENT OF COMMERCE

Maurice H. Stans, Secretary

NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

NBS TECHNICAL NOTE 520

ISSUED MARCH 1970

Nat. Bur. Stand. (U.S.), Tech. Note 520, 69 pages (Mar. 1970)

CODEN: NBTNA

Methods of Measurement for Semiconductor

Materials, Process Control, and Devices

Quarterly Report

July 1 to September 30, 1969

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, and the

National Aeronautics and Space Administration

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 13.46:520). Price 65 cents.

CONTENTS

PAGE

1. Introduction and Highlights 1

2. Resistivity 6

3. Carrier Lifetime 13

4. Inhomogeneities 17

5. Infrared Methods 20

6. Hall Effect 21

7. Deep-Level Studies 22

8. Gold-Doped Silicon 23

9. Specification of Germanium 24

10. Metallization Evaluation 27

11. Processing Facility 30

12. Wire Bond Evaluation 32

13. Die Attachment Evaluation 46

14. NASA Measurement Methods 47

15. Second Breakdown 48

16. Thermal Properties of Devices 49

17. Thermographic Measurements 53

18. Microwave Device Measurements 55

19. Silicon Nuclear Radiation Detectors 56

Appendix A. Joint Program Staff 59

Appendix B. Committee Activities 60

Appendix C. Solid-State Technology and Fabrication Services ... 61

Appendix D. Joint Program Publications 62

Appendix E. Notes on Procedures for Use of a Capacitor Microphone to Measure Vibration Amplitude of the Tool

Tip of an Ultrasonic Wire Bonder 6 3 iii
LIST OF FIGURES

PAGE

1. Resistivity profiles of Specimen 2062 along two perpendicular diameters

located 9 deg from an arbitrary reference mark 8

2. Resistivity (ft-cm) vs. angle (deg) for Specimen 2062 showing effects of misorientation of wafer and probes when measuring specimens with large radial

resistivity gradients 10

3. Nomograph relating gamma-ray energy (EY

), effective Fano factor (F'), and detector resolution 25

4. Typical scratch test plate showing failure points of a 0.56-um thick vacuum

evaporated aluminum film on a quartz substrate viewed by transmitted light .... 27

5. Precision ultrasonic transducer-microphone mounting apparatus 33

6. Unusual wave form obtained with the ultrasonic microphone 33

7. Vibrational amplitude of a long ultrasonic bonding tool (two extensions) 35

8. Vibrational amplitude of a long ultrasonic bonding tool (same extension) 35

9. Vibration amplitude of a short ultrasonic bonding tool 36

10. Mechanical resonance curves for the same tool set at two different extensions

below the horn 37 11. Vibration amplitude of a long ultrasonic bonding tool which demonstrates unusually

large resonance effects 39 12. Hole in an ultrasonic transducer horn through which the bonding tool is inserted . 39

13. Oscilloscope trace of a laser interferometer fringe measurement of bonding tool

tip motion 42

14. Oscilloscope trace of a laser interferometer fringe measurement of bonding tool

tip motion with improved signal-to-noise ratio 42

15. Diagram of apparatus used to plot automatically curves of transistor base current

as a function of the collector-emitter voltage with the collector current held

constant 50

16. Automatically plotted curve of base current versus collector-emitter voltage for

a planar, diffused, power transistor 51

17. Test equipment for observation of hot spots on the surface of a decapped trans- istor 53

18. Relative counting efficiencies of two silicon avalanche detectors as a function- of electron energy 57

19. Cross-sectional view of a high-resolution extension tip for the capacitor microphone 64

LIST OF TABLES

1. Results of Four-Probe Resistivity Round Robins 7

2. Specimens for Determination of the Precision of the Revised PCD Procedure .... 14

3. Specimens for Intercomparison of Lifetime Values by Various Measurement Methods . 14

4. Critical Loads for Failure of Aluminum Film 28

5. Pull Strength of Aluminum - 1 Percent Silicon Wire 44

6. Nominal Values of Several Parameters of Silicon Crystals for Si(Li) Detectors . . 57
METHODS OF MEASUREMENT FOR

SEMICONDUCTOR MATERIALS, PROCESS CONTROL, AND DEVICES

Quarterly Report

July 1 to September 30, 1969

ABSTRACT

This quarterly progress report, fifth of a series,

describes NBS activities directed toward the development

of methods of measurement for semiconductor materials

,

process control, and devices. Principal emphasis is

placed on measurement of resistivity, carrier lifetime,

and electrical inhomogeneities in semiconductor crystals;

evaluation of wire bonds ; and measurement of thermal properties of semiconductor devices . Other tasks involve

study of infrared measurement methods , deep-lying impurities

in InSb, and gold in silicon; establishment of a processing

facility; evaluation of aluminum metallization and wafer die

attachment; review of NASA measurement methods; and measurement of Hall effect in semiconductor crystals , second breakdown in transistors, and properties of microwave devices.

Related projects on silicon nuclear radiation detectors and

specification of germanium are also described. Supplementary

data concerning staff, committee activities, technical

services, and publications are included as appendixes. Laboratory procedures for use and calibration of a capacitor

microphone to measure vibration amplitude of the tool tip

of an ultrasonic wire bonder are also described in a separate

appendix.

Key Words: Alpha detectors; aluminum wire; carrier lifetime;

die attachment; electrical properties; epitaxial silicon;

gamma detectors; germanium; gold-doped silicon; indium antimonide; metallization; methods of measurement; microelectronics; microwave devices; nuclear radiation detectors; resistivity; semiconductor devices; semiconductor materials;

semiconductor process control; silicon; thermal resistance;

thermographic measurements; ultrasonic microphone; wire bonds,

1. INTRODUCTION AND HIGHLIGHTS

This is the fifth quarterly report to the sponsors of the Joint

Program on Methods of Measurement for Semiconductor Materials , Process
INTRODUCTION AND HIGHLIGHTS Control, and Devices. The report is subdivided according to tasks which

have been identified as parts of the Program. Sections 2 through 9 deal

with methods of measurement for materials; sections 10 through 14, with

methods of measurement for process control; and sections 15 through 19,

with methods of measurement for devices

.

Resistivity is the principle parameter used in specifying both bulk

silicon wafers and epitaxial layers. Detailed study of the traditional

four-probe method for application both to bulk silicon wafers and epitaxial layers continued. Results of a round robin conducted with Committee F-l of the Americal Society for Testing and Materials (ASTM) to

extend the range of applicability of the standard four-probe method to

silicon wafers with higher and lower resistivity values were analyzed.

For measurements on inhomogeneous. wafers , the importance of making the

measurement at the center of the wafer was demonstrated. In addition,

serious errors were shown to be introduced into the measurement of radial

resistivity variations when the specimen is slightly mispositioned.

Renewed interest in carrier lifetime measurements in ASTM Committee

F-l led to the reactivation of the carrier lifetime section at the September meeting. This interest was stimulated by the need to review for

possible improvement the existing standard method for measuring carrier

lifetime by the photoconductive decay method and by the increased desire

to be able 'to determine carrier lifetime in epitaxial layers. There is

also considerable interest among the radiation effects community for additional understanding of the carrier lifetime in transistors. To re- flect the renewed interest work has been initiated on measurements of

carrier lifetime in epitaxial layers in addition to continuing the

efforts on measurements in bulk crystals, diodes, and transistors.

The emphasis in the inhomogeneities task has been reoriented toward

establishment of the suitability of applying the photovoltaic method to

the measurement of radial resistivity variations in germanium and silicon

wafers. The apparatus has been improved to permit the measurement to be

made on lapped wafers with room temperature resistivity 1 fi-cm or higher.

The feasibility of extending these measurements to specimens with room

temperature resistivity down to 0.1 fi-cm has been demonstrated. The

quantitative evaluation of the relationship between photovoltage and

resistivity gradient along the diameter of a wafer is continuing.

Substantial progress was made in the characterization of ultrasonic

bonding systems by means of the capacitor microphone technique. Results

of the studies demonstrate that the vibration amplitude of the tip of

the bonding tool can be very different even when the bonder is set up

in ostensibly the same fashion. Difficulties with absorption of ultrasonic power into the transducer mount were noted. In addition variations

in both standing wave pattern and vibration amplitude of the tip were observed when the bonding tool was removed from the horn and reinserted
INTRODUCTION AND HIGHLIGHTS either to the same or a slightly different extension. These studies further _ confirmed that the unloaded Q of the transducer, which is normally

specified, has no significance insofar as the vibration characteristics of

the bonding tool are concerned. Some dependence of both resonance frequency and tip vibration amplitude on the temperature was also observed.

This work to date has been a laboratory characterization of ultrasonic systems employed in bonding wire leads to silicon devices. However

preliminary studies suggest that the capacitor microphone technique can

be used to determine tip vibration amplitude during bonding. This approach is expected to provide a more direct means for obtaining both

optimum and reproducible conditions for bonding. This phase of the program which is directed toward the development of procedures for obtaining

reproducible bonds has been restricted to the aluminum- aluminum ultrasonic

process . The bonds produced are intended for use as the vehicle for

evaluating the effectiveness of various test methods in identifying weak

bonds. It is intended that the test methods under consideration will

also be applicable to other bonding systems such as the gold thermocompression system.

Studies of both the pull test and the high-current pulse test methods continued during the quarter. A scanning electron microscope is

now available for use by the wire bond evaluation and other tasks in this

program. Initially, work with this machine was devoted principally to

operator training, but its usage is expected to increase considerably in

the near future

.

Preparation of the bibliography and critical review survey paper on wire bond evaluation has taken longer than anticipated. It is now ex- pected that these will be completed before the end of this fiscal year.

Measurements of d-c current gain as a function of collector emitter

voltage have shown a hysteresis effect which appears to be subject to

interpretation on the basis of hot-spot formation. If the collectoremitter voltage of a transistor operated in a normal non-constricted

current mode is increased to the point where a hot spot is formed and

then reduced, the transistor will operate in a stable constricted current

mode. It is thought that both the reversal of the current gain-temperature dependence at high current density and temperature levels as well as the dependence of current gain on current density are responsible for the

hysteresis effect. These studies together with related studies in which

the surface temperature distribution is being investigated by means of

thermographic phosphors are continuing.

Work on the high field effects task has been suspended indefinitely

in order that the staff originally assigned part time to it may devote

all their efforts to the wire bond evaluation task. A preliminary survey

to determine the problems associated with measurements of the properties
INTRODUCTION AND HIGHLIGHTS

of the silicon oxide films used in the semiconductor device industry was undertaken. Many different problems were encountered particularly in

connection with thin silicon oxide and silicon nitride films used on MIS

devices. The establishment of a task to study these measurement problems

is being considered.

Further plans were made for the Symposium on Silicon Device Processing to be held in June, 1970, under the joint sponsorship of ASTM Committee F-l and National Bureau of Standards. Acceptances have been obtained from all the speakers invited to participate in the opening ses- sion of the symposium which will consist of papers on silicon oxides

and nitrides. A large number of papers is being considered for presentation at subsequent sessions which will deal with the unit processing

operations, epitaxy and diffusion, from the standpoint of the required

techniques and facilities as well as the properties and characterization

of their product, surface preparation, and interdependence of the unit

processing operations. Final selection of these papers will be made

early in 1970.

The Joint Program was undertaken last year to focus NBS efforts to

enhance the performance, interchangeability , and reliability of discrete

semiconductor devices and integrated circuits through improvements in

methods of measurement for use in specifying materials and devices and

in control of device fabrication processes. These improvements are intended to lead to a set of measurement methods which have been carefully

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

suppliers, and which can provide a common basis for the purchase specifications of government agencies. 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 [l], the National Aeronautics and Space Administration [2], the Defense Atomic Support Agency

[3], and the U. S. Navy Strategic Systems Project Office [4]. Because

of the cooperative nature of the Program, there is not a one-to-one corre- spondence between the tasks described in this report and the projects

by which the Program is supported. Although all sponsors subscribe to

the need for the entire basic program for improvement of measurement

methods for semiconductor materials, process control, and devices, the concern of certain sponsors with specific parts of the Program is taken

into consideration in planning.

Additional background information on the Program and individual

tasks may be found in earlier reports in this series (see Appendix D).

A summary of early work which led to the establishment of the Joint Program has been prepared [5]. This summary contains a bibliography which

includes all NBS publications which were issued in this field prior to

the formation of the Joint Program.
INTRODUCTION AND HIGHLIGHTS

Besides the tasks sponsored under the Joint Program, this report

contains descriptions of activity on related projects supported by NBS

or other agencies. Although the specific objectives of these projects

are different from those of the Joint Program, much of the activity

undertaken in these projects will be of interest to Joint Program sponsors. The sponsor of each of these related projects is identified in

the description of the project.

1. Through Research and Technical Services Projects. 4251120, 4251123,

4251126, 4252128, 4254111, 4254112, and 4254115.

2. Through Order ER- 11897, Electronics Research Center. (NBS Project

4259523)

3. Through Inter-Agency Cost Reimbursement Order 808-70. CNBS Project

4259522)

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

Cost Reimbursement Order P09-0016. (NBS Project 4259533)

5. W. M. Bullis , "Measurement Methods for the Semiconductor Device

Industry — A Review of NBS Activity," NBS Tech. Note 511, December,

1969.
METHODS OF MEASUREMENT FOR SEMICONDUCTOR MATERIALS

2. RESISTIVITY

Objective : To develop improved methods , suitable for use throughout

the electronics industry, for measuring resistivity of bulk, epitaxial,

and diffused silicon wafers.

Progress : Detailed study of the four-probe method for application

to bulk silicon wafers and epitaxial layers continued. Effort was increased on both the spreading resistance and capacitance-voltage methods

which are intended primarily for use on epitaxial layers. In addition,

capability was established for measuring impurity concentrations in bulk

silicon by an infrared reflectance method which can be applied to meas- urement of the resistivity of epitaxial substrates. At present it is

possible to measure impurity concentration in the range between 5 x 10 *° cm

-3 and 2 x io 20 cm-3

.

Four-Probe Method — Results of a round robin based on the ASTM fourprobe method for measuring resistivity of silicon wafers [l] completed in

cooperation with ASTM Committee F-l have been tabulated and reported at

the September meeting of the committee. The round robin was conducted to

determine the precision of the method for wafers with resistivity down

to 0.001 ft-cm and up to 2000 ft-cm. The analysis was based on data submitted by six of the eight participants; two participants failed to

follow certain aspects of the test procedure. It was found that the precision specified in the present version of the method (3-sigma limit of

±2 percent) can be achieved for measurements on the low-resistivity

wafers. Wider limits were found for wafers with resistivity above 120

fl-cm. These results and the results of several previous round robins are summarized in Table 1.

There is some indication that wafer inhomogeneity was at least

partially responsible for the increased scatter in the higher resistivity

ranges. Because this had been suspected from earlier results an inhomogeneous 1 ft- cm wafer was included in the present round robin. The

scatter in resistivity reported for this wafer was considerably greater

than would have been expected for a homogeneous wafer of this resistivity.

The resistivity profile of this wafer as determined by four-probe measurements along two perpendicular diameters is shown in Fig. la. To

demonstrate the importance of correctly centering the wafer when making

a radial profile, a second set of data was taken with the center of the

wafer displaced about 0.9 mm to the right along each of the perpendicular

measurement diameters. If this were done inadvertantly, the off-diameter

correction factors [2] would be applied as though the center of the wafer

had been correctly placed with the result illustrated in Fig. lb. If the

displacement distance is known, of course, appropriate correction factors

can be used. In this case as would be expected, the result, illustrated

in Fig. lc, is quite similar to that of Fig. la.
RESISTIVITY

Table 1 - Results of Four-Probe Resistivity Round Robins'

Specimen Specimen Number of Average s(%)

D Number Type Laboratories Resistivity

Participati ng (n-cm)

B-2 P 4 10.023 0.20

B-4 P 4 10.133 0.36 K-4 P 4 14.494 0.13

605603-3 n 5 0.008367 0.69

601333-2 n 5 0.08514 0.51 71983-2 n 5 101.08 0.63

71166-2a n 3 838.0 0.48

71166-2b n 3 932. 9.24 600200-2 P 5 0.0077426 0.41 607075-2 P 5 0.10881 0.34

70877-3 P 5 7.909 0.65

49445-2 P 5 11.877 0.63

66969-1 P 5 112.59 0.91 16603-2 P 5 944.8 1.02

1165-2 n 6 .000992 0.29

612552-1 P 6 .000979 0.56 2062 n 6 1.057 1.18

K.0.1 n 6 342.3 0.89

48290-2 P 6 1223.6

[1195. l]

c 5.86 [1.54] c 40/1-1 n 6 1915.1 , [1884. 9] d 4.04 . [1.37]d

All results recomputed from raw data

Relative sample standard deviation

Lab 8 omitted from computation

Lab 2 omitted from computation
RESISTIVITY

24 r

20

16 C 12

<*. 8 <

4-

a

24,-

• 0-45°

0=-45°

12 -8-4 4 8 12 DISTANCE FROM CENTER (mm)

12 -« -4 4 8 |2

DISTANCE FROM CENTER (mm)

• 0-45°

0-45°

"^DISTANCE FROM ASSUMED CENTER (mm)

Fig. 1. Resistivity profiles of Specimen

2062 along two perpendicular

diameters located 8 deg from an arbitrary reference mark.

a) Wafer centered, geometrical

correction factors applied

properly

.

b) Wafer off-center; geometrical

correction factors applied as though wafer were centered.

c) Wafer off-center; geometrical

correction factors applied

properly.
RESISTIVITY

The effect of improper centering on the measurement of the resistivity at the center of inhomogeneous wafers was also investigated. In

each of five cases, resistivity measurements were taken at ten equiangular intervals as the pedestal on which the wafer was mounted was rotated

through 360 deg. In case 1 both the probe and the wafer were carefully

centered on the pedestal. In case 2 the probe was centered over the

pedestal but the specimen was moved 1.5 mm (about one probe spacing) off

center. As the pedestal was rotated, the measurement was always made

about the same off-center point on the specimen but due to the inhomogeneity the effective resistivity as seen by the probe changed. In cases

3 and 4 the specimen was centered on the pedestal and the probe was displaced 1.5 mm from the center of rotation. In case 3 the displacement

was in a direction perpendicular to the line of probes so that the probes

were tangent to the circle of rotation, while in case 4 the displacement

was in a direction along the line of probes so that the probes always

lay along a radius. In case 5 both the probe and the specimen were displaced from the center of rotation by 1.5 mm in such a way that the probe

position varied from the center of the specimen to a point 3 mm from the

center. No corrections for off-center displacement were made. The re- sults, summarized in Fig. 2, confirm that failure to place both the

wafer and the probe at the center of rotation can degrade measurement

precision if the wafer is not homogeneous. Interlaboratory precision

would therefore be degraded if different operators measure the specimen

at slightly different locations. It should be noted that, in this

study, no allowance is made for the intrinsic errors introduced into

the four-probe readings by the inhomogeneity of the specimen. This

problem has been discussed by Swart zendruber [3] and, more recently,

by Vieweg-Gutberlet and Schonhofer [4],

ASTM Committee F-l is considering a round robin to extend the fourprobe resistivity method to the 10,000 Q-cm resistivity range. Since

higher resistivity material tends to have poor homogeneity which leads to

degradation of measurement precision, resistivity profiles were measured

on several 6,000 to 10,000 fi-cm specimens to determine the best specimens

for use in the round robin.

The study of the current and probe-force dependence of the resistivity as measured by the four-probe method, preparatory to the use of

this method for measuring the resistivity of epitaxial layers deposited

on opposite conductivity type substrates, is continuing. Both resistivity

and contact resistance are to be measured on wafers with lapped, mechanically polished, and chemically polished surfaces. Some measurements

made on lapped surfaces ahve shown an erratic behavior, hence they are being repeated before proceeding to polished surfaces

.

(F. H. Brewer, W. M. Bullis , and D. R. Ricks)

Spreading Resistance Methods — This technique is widely used for

measuring resistivity of epitaxial layers. Several different methods for
RESISTIVITY

Fig. 2. Resistivity (fl-cm) vs. angle (deg) for Specimen 2062 showing effects

of misorientation of wafer and probes when

measuring specimens with

large radial resistivity

gradients

.

Case Symbol a b Probes Specimens

1 o 1.0445 0.13 Centered Centered

2 1.135 1.53 Centered Displaced

3 1.111 0.63 Displaced Centered

4 • 1.142 0.74 Displaced Centered

5 A 1.132 3.73 Displaced Displaced

a) Resistivity corrected to 23°C (n-cm)

b) Relative sample standard deviation [%)

c) 3 robes perpendicular to radius

d) Probes along radius

10
RESISTIVITY

measuring spreading resistance have been reported [5, 6]. Although each

of these is reported to yield highly reproducible results, the factors

which must be controlled to enable adequate precision on intercomparison

of the various methods are not fully recognized. The present effort is

directed toward identifying these factors and establishing the technical

basis necessary to the development of a standard procedure for this

measurement. This effort is being carried out in cooperation with ASTM

Committee F-l which is now developing such a standard 'procedure.

A basic spreading resistance apparatus has been assembled. The

purpose of this assembly is to evaluate and solve anticipated measurement

problems relating to high electrical isolation of the measurement system,

reduction of electrical noise below 1-yV , and attainment of reproducible

contacts between probes and specimen with minimum damage to each. At

present noise has been reduced to the 10-yV level by checking the circuit

stage by stage for thermally sensitive contacts, proper grounding, and

shielding. Isolation between stages has been maintained higher than

10 ^ ft. The question of reproducible contacts has not yet been investigated. (_J. R. Ehrstein)

Capacitance-Voltage Method — This is an alternate technique for

measuring resistivity of epitaxial layers. Initial measurements of ca- pacitance have been made as a function of voltage with a lr-MHz capacitance

bridge on shallow diffused mesa diodes fabricated on 8 ft-cm p-type silicon. These measurements are intended to evaluate the available measurement equipment and to test the adequacy of a program used to compute

doping density from the data [7]. (G. N. Stenbakken)

Plans : The investigation of the current and probe-force dependence

of resistivity as measured by the four-probe method will continue. After

consistent" results have been achieved on wafers with lapped surfaces,

measurements will be made for the same range of variables on mechanically

and chemically polished surfaces

.

Further four-probe resistivity measurements will be made on the

6,000 to 10,000 ft-cm wafers in order to evaluate the single laboratory

precision which can be expected on such wafers.

At the request of ASTM Committee F-l, a round robin will be coordinated on the measurement by the four-probe method of the resistivity of

silicon epitaxial layers deposited on opposite type substrates.

Necessary steps will be taken to reduce further the electrical noise

in the spreading resistance apparatus. After verifying that the probe

impressions made with the apparatus are of a quality appropriate to the

procedure being developed by ASTM Committee F-l, the effect of probe

pressure and current levels will be investigated on several well-profiled,

mechanically-polished, bulk wafers. The initial point material will be

11
RESISTIVITY

an osmium alloy and a three-probe measurement configuration will be

used. Later these results will be compared against those obtained from

a two-probe configuration.

Acquisition of capacitance data for the study of the capacitancevoltage technique will be expedited by the installation of a directreading capacitance meter which will be interfaced with a scanner-digital voltmeter system which is already operational. With such a system

the entire range of bias voltages required to allow profiling the material under the fabricated diodes will be generated sequentially and

recorded automatically along with the corresponding capacitance values.

Planar and mesa diodes will be fabricated on bulk silicon wafers for

which the radial resistivity uniformity has been determined by the

four-probe method. Capacitance-voltage measurements on these diodes will

be made to identify and investigate the importance of factors which

affect the precision of the method.

1. "Method of Test for Resistivity of Silicon Slices Using Four

Pointed Probes" (ASTM Designation: F84-68T), 1969 Book of

ASTM Standards , Part 8, November, 1969. This document covers the

range 0.05 to 120 ft- cm.

2. L. J. Swartzendruber, "Correction Factor Tables for Four-Point

Probe Resistivity Measurements on Thin, Circular Semiconductor

Samples," NBS Tech. Note 199, April, 1964.

3. L. J. Swartzendruber, "Four-Point Probe Measurement of Non-Uniformities in Semiconductor Sheet Resistivity," Solid State Electronics

7_, 413-422 (1964).

4. F. Vieweg-Gutberlet and F. X. Schonhofer, Grenzen der Anwendbarkeit

des 4-Spitzen-Gleichstrom-Messverfahrens an Silicium-Proben, Teil I u. II," Arehiv fur techn. Messen. No. 369, 237-240 (October, 1966),

No. 370, 259-262 (November, 1966).

5. R. G. Mazur and G. H. Dickey, "A Spreading Resistance Technique

for Resistivity Measurements on Silicon," J. Electrochem, Soo. 113,

255-259 (1966).

6. J. M. Adley, M. R. Poponiak, C. P. Schneider, P. A. Schumann, Jr.,

and A. H. Tong, "The Design of a Probe for the Measurement of the

Spreading Resistance of Semiconductors," Semiconductor Silicon3 R. Haberecht and E. Kern, Eds., The Electrochemical Society, New

York, 1969, pp. 721-731.

7. J. Hilibrand and R. D. Gold, "Determination of the Impurity Distribution in Junction Diodes from Capacitance-Voltage Measurements,"

RCA Review 21, 245-252 (1960).

12
3. CARRIER LIFETIME

Objective : To determine the fundamental limitations on the precision and applicability of the photoconductive decay method for meas- uring minority carrier lifetime and to develop alternate methods for

measuring minority carrier lifetime in germanium and silicon which are more precise, more convenient, or more meaningful in the specification

of material for device purposes.

Progress : Renewed interest in carrier lifetime measurements in ASTM

Committee F-l led to the reactivation of the Carrier Lifetime Section

with Dr. W. E. Phillips as chairman at the September meeting of the

Committee. This interest was stimulated by two factors: (l) the need to

review for possible improvement the existing standard method [1] for

measuring carrier lifetime by the photoconductive decay method, and (2)

increased desire to be able to determine carrier lifetime in epitaxial

layers. In addition there is increased interest among the radiation

effects community for additional understanding of the carrier lifetime in

transistors. To reflect this renewed interest the task has been organized into four areas according to the kind of structure being studied.

Bulk Crystals — Work is continuing on the evaluation and improvement

of the photoconductive decay (PCD) method. To study the effect of a high density light source, nine specimens were measured by the PCD method

with a HeNe (20 mW at 1.15 um) laser as the light source. Comparison of

these measurements with those made with a tungsten filament light source showed that a longer lifetime value is measured with laser illumination.

That the longer lifetime could be attributed to the higher excess carrier

density which is present with the laser was demonstrated by making several measurements with the laser defocused. With the resulting reduction

of excess carrier density, the lifetime values agreed with those made

with the tungsten filament light to better than 10 percent.

As reported last quarter (NBS Tech. Note M-95, pp. 9-10), the apparatus for one of the contactless techniques [2] is operational. Additional work both on the extension of the PCD measurement capability to

shorter lifetimes and on the two contactless methods for measuring lifetime by PCD has been postponed at least until work is completed on the

revision of the existing ASTM method for PCD measurements [l] which is now being prepared. An experiment has been designed to establish a multi-operator, single-laboratory precision for the revised PCD procedure,

The specimens of various resistivity, type, and geometry listed in Table

2 have been selected for study. Each specimen is to be measured four

times by each of five operators following the revised procedure.

Another important task is the intercomparison of bulk lifetime meas- urements made by the various methods on the same set of specimens. The

initial group, tabulated in Table 3, has been measured by the PCD method

and surface photovoltage (SPV) measurements have been begun. The optical

system of the SPV equipment was modified to permit a measurement of light

13
CARRIER LIFETIME

Table 2 - Specimens for Determination of the Precision of the Revised PCD Procedure

Specimen

SSI 7

16444/2

HS-1

SS6

SS10

NG-13#4

SS11

SS12

Type and Material

n Si

P Si

n Si

P Si

n Ge

P

Ge n Ge

n Ge

Resistivity Nominal Lifetime

(ft-cm)

1

1500

7

270

8

18

8

8

lys)

400

3600

(trapping^

35

350

600

100

100

Width

(cm)

Thickness

(cm)

Length

(cm)

0.5 0.5 2.5

diameter: 1.99 5.0

1.0 0.56 3.4

0.25 0.25 2.5

1.0 1.0 2.5

0.5 0.5 2.5

0.5 0.5 2.5

0.25 0.25 2.5

Table 3 - Specimens for Intercomparison of Lifetime Values

by Various Measurement Methods

Specimen

609651B

29021

605971B

31150

HG-1

44853A

612225

322J

HS-2

678C

Type and Material

n Si

P Si

P

Si

P Si

n Ge

P

Si

n Si

n Ge

n Si

P

Ge

Resistivi

(n-cm)

ty Nominal B Lifetimeulk

(us)

Width

(cm)

0.1 50 0.25

1.0

60 1.0

4 85 1.0

9.2

75 1.0

45

75 1.0

185 60 0.5

9.6

8 1.0

7.4

33 1.0

11 220 1.0

10

> 350 1.0

Thickness

(cm)

Length

(cm)

0.25 2.5

1.0 1.9

1.0 2.5

1.0 1.9

0.28 3.3

0.5 2.5

1.0 2.5

1.0 2.3

0.5 3.7

1.0 2.5

14
CARRIER LIFETIME intensity without moving the illumination area on the specimen. The re- producibility of data was improved significantly.

(R. L. Mattis and W. E. Phillips)

Epitaxial Layers — Two methods for measurement of carrier lifetime

in epitaxial layers are being studied: the metal-oxide-semiconductor

(MOS) capacitance method [3] and the SPV method. The instrument requirements and limitations of the MOS capacitance technique are being investigated through study of published literature on the method to establish

the appropriate directions for further investigation. (R. L. Mattis)

SPV measurements have been made on 9 to 11- ym thick epitaxial layers

of 6 and 0.6 fi-cm p on p

+ and 5 and 0.6 ft-cm n on n

+ silicon. Although

the data yield straight-line plots of photon intensity against penetration depth, or reciprocal absorption coefficient, as predicted [4] for

a thick specimen, the theoretical consequences of the violation in thin

epitaxial layers of the condition that the diffusion length be less than

the thickness of the sample must be investigated before the intercept can be interpreted in terms of minority carrier diffusion length as it can in

the case of a thick specimen. CW. E. Phillips)

Diodes — Emphasis during this period continued on studies intended

to lead to a determination of the relevance of carrier lifetime of bulk

silicon crystals to the characteristics of nuclear radiation detectors

fabricated from the crystals". Present effort is concentrated in the area

of the voltage decay technique for measuring carrier lifetime. Computer

calculations were made of the dependence of the carrier lifetime as measured by this technique upon injection levels. In these calculations,

various approximations used in computing the voltage decay following a forward current pulse were tested to determine their limits of validity.

In particular the dependence of the shape of the voltage decay curve on the amplitude of the forward pulse and on the nature of the diode barrier

was studied. Some conditions were found in which the linear voltage decay found for p-n step- junction diodes also occurs for surface barrier

diodes. Several surface-barrier diodes have been fabricated and tested,

but the conditions needed to obtain the linear voltage decay have not been

achieved.

Fabrication of alloyed and shallow-diffused diodes for voltage decay

and reverse recovery measurements of carrier lifetime in p-n stepjunction diodes was begun. The mathematical analysis of the reverse re- covery method was also continued. Additional data were taken in the ex- perimental study of the reproducibility over an extended period of time

of the reverse recovery technique for measuring carrier lifetime in p-n

step- junction diodes. (A. J. Baroody)

Transistors — In order to make meaningful carrier lifetime measurements for device purposes , the interrelationships of the device structure

15
CARRIER LIFETIME

and the measurement procedures must be more fully understood. A study of

the literature on methods for measuring minority carrier lifetime in

transistor structures was begun. (W. E. Phillips)

Plans: Work on the PCD method will continue with emphasis on the

establishment of its multi-operator, single-laboratory precision. Further

intercomparisons of PCD measurements with SPV measurements will be made.

The review of the MOS capacitance technique will be completed and

areas will be selected for detailed investigation. The analysis of the

SPV results will be studied to determine the relationship between the

value of the intercept and the diffusion length for various ratios of

diffusion length to epitaxial layer thickness.

To provide a firmer basis for understanding the various diode re- covery techniques, emphasis will be shifted toward additional study of

p-n step- junction diodes. Voltage decay measurements will be made on the

junction diodes now being fabricated. Further understanding will be

sought of the dependence of the voltage response on the amplitude of the

forward pulse to the injection level. The experiment to study the re- producibility of the reverse recovery technique will be concluded and the

results analyzed. The dependence of the measured reverse recovery time

on the frequency of the input square wave will also be studied, and

efforts to modify the reverse recovery circuitry to permit continuous

variation of the ratio of forward to reverse current will continue.

The study of the literature on the measurement of carrier lifetime

in transistor structures will also continue.

1. Method for Measuring the Minority-Carrier Lifetime in Bulk Germanium

and Silicon (ASTM Designation F28-66), 1969 Book of ASTM Standards,

Part 8, November, 1969.

2. J. Nishizawa, Y. Yamoguchi, N. Shoji, and Y. Tominaga, "Application

of the Siemens Method to Measure the Resistivity and the Lifetime of

Small Slices of Silicon," Ultrapurification of Semiconductor Materials, MacMillan Co., New York, 1962, pp. 636-6414.

3. C. Jund and R. Poirier, "Carrier Concentration and Minority Carrier

Lifetime Measurement in Semiconductor Epitaxial Layers by the MOS

Capacitance Method", Solid-State Electronics 9_, 315-319 (1966);

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

From the Transient Response of an MOS Capacitor", IEEE Trans.

Electron Devices ED-14 , 781-784 (1967); S. R. Hofstein, "Minority

Carrier Lifetime Determination from Inversion Layer Transient

Response", IEEE Trans. Electron Devices ED-14, 785-786 (1967).

4. F. Bergman, C. Fritzsche, and H. D. Riccius , "Bestimmung der

Diffusionslange in Halbleiten Mit Hilfe des Oberflachenphotoeffektes,"

Telefunken Ztg. 37, 186-193 (1964).

16
4. INHOMOGENEITIES

Objective : To develop improved methods for measuring ihhomogeneities responsible for reducing performance and reliability of germanium

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

as a means of measuring radial resistivity variations in germanium and

silicon circular wafers.

Progress : The present method [l] commonly used to detect radial re- sistivity variations in germanium and silicon circular wafers involves

six four-probe measurements of resistivity: two at the center of the

wafer and four, 3.2 mm from the edge, 90 deg apart. Several problems are encountered with this method. First, there are basic limitations in the

application of the four-probe method to the measurement of resistivity

variations, as has been discussed by Swartzendruber [2], More recently,

Vieweg-Gutberlet and Schonhofer [3] have examined this limitation in more

detail and have recommended that to obtain more exact information about

the radial resistivity profile along a diameter of a wafer, two-probe

measurements should be made on a bar cut along that diameter of the wafer,

They have shown in two cases that the discrepancy between profiles obtained by the two- and four-probe methods is as large as 10 to 20 percent. Second, the off-center correction factor [M-] is very sensitive to

the location of the probe near the wafer edge. As a result, small un- certainties in the probe placement produce large errors in the apparent

resistivity (see Section 2). Finally, information about resistivity

variations between the center and the measurement points near the edge

cannot be obtained with this method.

Because of limitations in the four-probe method, the measurement of

radial resistivity variation has caused considerable confusion between

silicon suppliers and users. Preliminary evaluation of the photovoltaic

method has indicated that it may in principle be used for this determination. Some interest in the use of this method as a replacement for

the four-probe method has been indicated. Thus, the immediate objective

of the task has been reoriented toward establishment of the suitability

of applying the photovoltaic method to the measurement of radial resistivity variation on a practical scale.

Resistivity is a factor in determining the equipment required for

the photovoltaic method. For a given light probe intensity, the voltages

that must be measured are larger for higher resistivity specimens and

so the measurement requirements are less severe. Several measurement

techniques useful at different resistivity levels have been tested. The

resistivity range investigated extends from about 100 fi-cm to 0.1 fl-cm.

For the higher resistivity specimens a 150-W incandescent lamp can serve as the light source for the light probe and a low-noise (*0.5-uV

peak-to-peak input noise) d-c amplifier can be used as the detector. The

specimen surface need only be lapped with 5-um alumina; no other surface

preparation is needed. Specimens with a nominal resistivity as high as 100 ft-cm and as low as about 40 ft-cm have been measured in this way.

17
INHOMOGENEITIES

For lower resistivity specimens, to about 10 £2-cm, use of this apparatus requires that the specimen surface be specially treated to reduce

the surface recombination velocity and thereby increase the signals

to a level measurable by the d-c amplifier. With the use of a more sensitive detecting scheme, measurements can be made on wafers with still

smaller resistivities, to about 1.0 ft-cm, without treatment of the specimen surface other than lapping with 5-um alumina. In this case a chopped

light probe and phase-sensitive lock-in detector are used. The longer

integrating time required to detect the smaller signals lengthens the

scanning time. As much as several minutes, depending on the signal level,

may be required for a single scan compared with the 20 s which is typical

when signals are large enough to use the d-c amplifier.

To make measurements on lapped specimens with resistivities less

than about 1.0 ft-cm requires a more intense light source for the light

probe. A cw He-Ne laser with an output of 20 mW at a wavelength of 1.15

um was used to make measurements on a silicon specimen with a nominal re- sistivity of 0.1 fl-cm. The intensity of the light probe required results

in thermal gradients in the specimen. However, by chopping the light and

using a lock-in detector it is possible to extract the photovoltage signals from the thermally induced voltages

.

The surface preparations to reduce the surface recombination velocity, s, in germanium and silicon have been inconvenient to use and inconsistent in their ability to reduce s. Further use of these preparations was abandoned and the use of the light chopper and the lock-in

detector was extended to the higher resistivity range so that lapped

specimens may be used throughout. At present, the incandescent lamp is

being used as the primary source. Consideration is being given to incorporation of a laser source permanently into the system because of its

advantages in the reduction of scanning time, in the extension of the

resistivity range to at least 0.1 ft-cm, and the assurance that the distribution of the photo-induced electron-hole pairs is uniform in the

specimen bulk intercepted by the light probe. For the uniform distribution condition to be satisfied to a good approximation when the incandescent lamp is used, the specimen thickness must be less than the car- rier diffusion length, a boundary condition which is not always conve- nient to satisfy.

Two problem areas were encountered while continuing to evaluate the

relationship between photovoltage and the resistivity gradient (NBS Tech.

Note 488, p. 9) by comparing the resistivity profile obtained using this

equation with the resistivity profile obtained from four-probe measurements . They have to do with making contact to the specimen and the precision of the four-probe method and are discussed below.

This equation was derived with the assumption that the electrical

contacts to the wafer are ohmic and semicircular in shape. To avoid the

18
INHOMOGENEITIES

inconvenience of making semicircular notches in the wafer, small flats were made at the edge of the wafer and contacts made to them. It was

felt that sufficiently small, flat contacts (1.0 to 1.5 mm) would provide

a good approximation to the semicircular contact specified in the theory.

Experiments have indicated this to be the case when measuring the photovoltage and photo-induced change in resistance of the specimen, but not

when measuring the total specimen resistance. This resistance enters

as the square in the equation for the resistivity gradient. Means for

preparing wafers with semicircular contacts were developed for use in

further evaluating the equation.

Discrepancies between the resistivity profile obtained by the photovoltaic method and that obtained by the four-probe method were observed

especially near the edge of the wafer. Part of this difficulty may lie

both with the basic limitation of the four-probe method to measure re- sistivity variations and the sensitivity of the off-center correction

factor to location of the probe near the wafer edge as noted above. Both

these limitations are absent if the two-probe method is used to make the

profile. However, since this requires cutting a rectangular bar from the

wafer along the measurement diameter, it cannot be undertaken until all

photovoltaic studies of the wafer have been completed.

Plans : The evaluation of the relationship between photovoltage and

radial resistivity gradient in germanium and silicon circular wafers will

be completed. Profiles determined by both the four-probe and two-probe

methods will be compared with the photovoltaic resistivity profile obtained on selected wafers. To help establish the priority of continuing

work on this measurement method, the interest of additional suppliers of

silicon wafers in the photovoltaic method for detecting radial resistivity

gradients will be sought in telephone contacts.

1. Methods of Test for Bulk Semiconductor Radial Resistivity Variation

(ASTM Designation F81-67T), 1969 Book of ASTM Standards, Part 8,

November, 1969.

2. L. J. Swartzendruber, "Four-Point Probe Measurement of Non-Uniformities in Semiconductor Sheet Resistivity," Solid-State Eleotronics

]_, 413-1+22 (1964).

3. F. Vieweg-Gutberlet and F. X. Schonhofer, "Grenzeh der Anwendbarkeit

des 4-Spitzen-Gleichstrom-Messverfahrens an Silicium-Proben, Teil

I. u. II," Arohiv fur teohn. Messen, No. 369, 237-240 (October, 1966);

No. 370, 259-262 (November, 1966).

4. L. J. Swartzendruber, "Correction Factor Tables for Four-Point

Probe Resistivity Measurements on Thin, Circular Semiconductor

Samples," NBS Tech.. Note 199, April, 1964.

19
5. INFRARED METHODS

Objective: To evaluate impurity photoconductivity as a method for

detecting low concentrations of deep-lying impurities such as copper,

gold, iron, and nickel in silicon and germanium, and to assist ASTM

Committee F-l in extending the applicability of infrared absorption as a method for detecting impurities such as oxygen and carbon in silicon and

germanium.

Progress: Photoconductivity measurements as a function of wavelength have been made on copper-doped germanium specimens cooled to about

10 K. Specimens doped with 10 15 copper atoms cm

-3 were prepared from both

n- and p-type starting material. Photoconductivity was observed at

wavelengths less than 30 ym, the long wavelength limit associated with

the copper impurity level at 0.0M- eV above the valence band. The

apparatus is now available for photoconductivity measurements on germanium specimens (see Section 9).

Work on the summary report describing the determination of oxygen

in silicon and germanium by infrared absorption methods was postponed to

next quarter. (W. R. Thurber)

An editorial and technical review was completed of four ASTM draft

documents concerning determination of impurity concentrations by infrared

absorption. Calculations were made to establish the conditions under

which the present and revised methods for determining oxygen content of

silicon yield a similar value for absorption coefficient. These results

are particularly important in view of the fact that the calibration re- lating oxygen concentration to absorption coefficient is different in the

new method. (W. M. Bullis)

Plans: The literature will be reviewed and an informal summary prepared on the relevance of photoconductivity measurements for determining

deep impurities and their concentrations in silicon and germanium.

The summary report describing the determination of oxygen in silicon

and germanium by infrared absorption methods is expected to be completed

next quarter.

20
6. HALL EFFECT

Objective : To establish a facility for making measurements of Hall

coefficient as a function of temperature between 4 and 350 K and to improve methods for collecting and interpreting Hall effect data.

Progress : The Hall effect apparatus was modified to permit automatic measurement with the digital voltmeter on high-resistivity materials. A new specimen holder was fabricated and the. necessary wiring

completed.

The report on the use of a time-shared computer to control the Hall

experiment has been completed [1]. (W. R. Thurber and W. M. Bullis)

Plans : The new Hall apparatus will be tested and then used for

measurements on high-purity germanium (see Section 9) and gold-doped

silicon (see Section 8).

Work will continue on the report concerning Hall measurements and

their interpretation.

1. W. M. Bullis, W. R. Thurber, T. N. Pyke, Jr., F. H. Ulmer, and

A. L. Koenig, "Use of a Time-Shared Computer System to Control a Hall

Effect Experiment," NBS Tech. Note 510, October, 1969.

21
7. DEEP-LEVEL STUDIES

Objective : To determine the nature and origin of the deep-lying

centers in high-resistivity indium antimonide.

Progress : Although it has been concluded from comparisons of the

results of resistivity, Hall coefficient, and carrier lifetime measurements on high-resistivity indium antimonide before and after the introduction of lithium that the residual deep-lying energy level is associated with a lattice vacancy, some ambiguity remained in the interpretation of the lifetime data (NBS Tech. Note 495', pp. 15, 16). Consequently, the photomagnetoelectric-photoconductivity apparatus was improved and additional lifetime measurements on both untreated and

lithium-treated specimens were made. Because lithium tends to precipitate from indium antimonide crystals, Hall coefficient and resistivity

measurements were repeated on a treated specimen to establish that the

lithium concentration had not changed during the time between these and

earlier measurements. Hall coefficient and resistivity measurements were

also made on a previously unmeasured, untreated specimen to establish the

characteristics of the wafer prior to introduction of lithium.

Analysis of the data obtained in these measurements was begun.

(J. L. Scales)

Plans : Analysis of the data will continue with emphasis on resolving the previously noted ambiguity in the interpretation of the lifetime

data. On completion of the analysis, a final report will be prepared

for publication.

22
8. 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 : Specimens for initial electrical and activation analysis

studies were prepared by diffusing gold into 10 fl-cm boron-doped silicon

wafers at temperatures of 850, 950, 1050, 1150, and 1250°C. The wafers were lapped with 5-um abrasive to a depth of 25 urn on each side to remove

the surface layer of gold which remains after diffusion. Diffusion con- ditions were selected so that each wafer was intended to be saturated with

gold. Since the solid solubility is an exponential function of reciprocal

temperature, gold concentrations were expected to vary from about 10* 5 to

10 17 cm

-3 [l]. The wafers were cut and a portion sent to the Analytical

Chemistry Division for determination of the actual gold concentration by

neutron activation analysis. Hall bars were cut ultrasonically from the

remainder, and a 0.5-um layer of aluminum was evaporated onto the contact

areas of the Hall bars. Measurements of the resistivity and Hall coefficient at room temperature were begun to determine the effect of the

gold on the free carrier concentration and the hole mobility.

A program to solve the charge balance equation for various models

[l] which describe the energy level structure of gold in silicon was prepared for use on the NBS computer. It had been observed previously that

the resistivity increases until the gold concentration of about 10 16 cm

-3

is reached and then decreases as still more gold is added. It was thought

that the higher values for the degeneracy factor of the lower, or donor,

level might explain this maximum in the room temperature resistivity

against gold concentration curve for p-type silicon. A series of com- putations was made in which the degeneracy factor for this level was varied between 2 and 16. The computed curves did not show a maximum even

for the highest values of degeneracy factor; it was concluded that some other mechanism must be responsible for the observation.

(W. R. Thurber and W. M. Bullis)

Plans: The initial electrical measurements will be completed and

analyzed. The results of the neutron activation analysis will be used to

verify that diffusion procedures such as those employed in the initial

run provide the expected quantity of gold in the wafer. After establishment of satisfactory preparation procedures, the work will continue with

studies of wafers cut from crystals of both n- and p-type silicon crystals

with a room temperature resistivity of about 1 fl-cm.^

1. W. M. Bullis, "Properties of Gold in Silicon," Solid-State Electronics 9, 143-168 (1966).

23
9. 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 : The study [l] which shows that the effective Fano factor,

F' , in germanium at 77 K is less than the previously determined value of

0.13 [2-4] has been completed with the analysis of gamma-ray pulse-height

data obtained with the collimated gamma-ray beam on Ge(Li) detector 213

in the manner previously described for Ge(Li) detector 83-2 (NBS Tech.

Note 4-95, pp. 18-20). Trapping of charge carriers in detector 213, a 6-cm 3 detector fabricated for use as a reference detector in the con- tinuing program of evaluation of materials and devices for gamma-ray

spectroscopy, was observed to be less than in previous detectors. Excellent collection of charge over a compensated region of 8.5 mm appears

to occur at about 1400 V/cm as compared to the previous test detector,

83-2, where a field of 2400 V/cm was required to obtain good charge

collection over a compensated region of only 5 mm. Consistent with the

observation of less electron trapping in detector 213 than in detector

83-2, F' is about equal to 0.10 over a larger region, approximately 4-mm

wide, in the former device. This region was shifted towards the center

of the compensated region which is also consistent with the observation

of more nearly equal collection of electrons and holes in detector 213.

The value of F' = 0.107 found in the present study is considered to be

the upper limit of the intrinsic Fano factor, F. The measurement, however, appears to be limited by the lack of knowledge of the width of the

secondary ionization sheath in the compensated region created by the

collimated gamma-ray beam. The data also suggest that the use of lower

noise electronic components , such as an FET preamplifier with the input

FET operated below room temperature , would help in improving the quality

of the measurement. (A. H. Sher and W. J. Keery)

The Germanium Section of ASTM Committee F-l is examining methods of

measuring the quality of lithium-drifted germanium gamma-ray detectors

and the single crystal, p-type germanium from which these detectors are fabricated. Round-robins are currently being conducted on the measurements of lithium-ion drift mobility in germanium and trapping times in

detectors. As part of the lithium-ion drift mobility round robin, measurements of lithium-ion drift mobility on specimens of germanium were

completed according to the test procedure being considered by this Section. A computer program was written to analyze this data. NBS was requested to analyze with this program all subsequent data collected by

the participants in the round robin. Measurements of trapping lifetimes

in Ge(Li) detectors were also performed for specimens distributed in the

second round robin. (A. H. Sher)

Additional nomographs are being prepared which relate to fabrication

and testing procedures of Ge(Li) detectors. These nomographs permit

24
SPECIFICATION OF GERMANIUM

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y of 662 keV yields an F' of approximately 0.1.

rapid evaluation of various parameters used during the fabrication,

and testing of lithium-drifted germanium. Nomographs completed thus far

include those relating drifted depth, time, temperature, and voltage

during drift [NBS Tech. Note 488, p. 15]; junction area, capacitance, and

drifted depth for planar detectors; detector resolution, total system

resolution, and pulser resolution; and gamma-ray energy, effective Fano

factor, and detector resolution. The last of these is shown in Fig. 3.

A straight line between a value of the detector contribution to linewidth and the gamma-ray energy, E (keV), at which the measurement was performed will intersect the center scale at the value for the effective

25
SPECIFICATION OF GERMANIUM

Fano factor, F'. Also, by connecting values of E with F = 1, the

statistical width, A^ (keV), for the particular gamma-ray energy will be

obtained on the right-hand scale. Nomographs in preparation are those

relating oxygen concentration, lithium mobility, and acceptor concentration; and, capacitance, area, and drifted depth for coaxial Ge(Li) detectors. The publication of these nomographs in the form of a Technical

Note is anticipated. (A. H. Sher)

Infrared response measurements (IRR) on Ge(Li) detector structures

are being investigated as a possible procedure for screening p-type,

single-crystal germanium to be used in fabricating Ge(Li) detectors as suggested by Armantrout [5]. A prototype of the low-noise preamplifier

to be used in these measurements was assembled. The operating characteristics of this preamplifier unit are not yet suitable for use in the

measurement system. (R. T. Dalton and W. J. Keery)

Plans: Lithium mobility studies, detector performance measurements

(with emphasis on the use of a collimated gamma-ray beam to study carrier

trapping phenomena), and the treatment of gamma-ray pulse-height data by

computer techniques will be continued. Construction and testing of a cooled FET preamplifier for use "in the spectroscopy system and the preamplifier for the IRR measurements will also proceed. An investigation

of the relationship between the distribution of etch pits on crystals of

p-type germanium and the characteristics of Ge(Li) detectors fabricated

from the crystals will be begun. Impurity photoconductivity measurements

(see Section 5) will be made on germanium crystals which show a high degree of charge- trapping. Measurements of IRR will also be made at 77 K on Ge(Li) structures fabricated from this material.

t Supported by the Division of Biology and Medicine, U. S. Atomic

Energy Commission. (NBS Project 4259425)

1. A. H. Sher and W. J. Keery, "Variation of the Effective Fano Factor

in a Ge(Li) Detector," presented at Nuclear Science Symposium, San

Francisco, October, 1969; to be published in IEEE Trans. Nucl. Soi. 3 February, 1970.

2. H. R. Bilger, "Fano Factor in Germanium at 77°K," Phys. Rev. 193 , 283-253 (1967).

3. J. M. Palms, P. Venugopalo Rao, and R. E. Wood, "The Characteristics

of an Ultra-High Resolution Ge(Li) Spectrometer for Singles and

Coincidence X-Ray and Gamma-Ray Studies," IEEE Trans. Nuel. Sci

NS-16 , 34-46 (January, 1969).

4. A. H. Sher and B. D. Pate, "Determination of the Fano Factor in

Germanium at 77°K," Nuol. Instr. and Metk. 71, 251-255 (1969).

5. G. A. Armantrout, "A Highly Sensitive Photoresponse Technique for

Determining Impurity and Defect Energies and Concentrations in Germanium," presented at Third International Conference on Photoconductivity, Palo Alto, August, 1969; available from the Clearinghouse

for Federal Scientific and Technical Information, Springfield, Va.

22151, UCRL-71625.

26
METHODS OF MEASUREMENT FOR SEMICONDUCTOR PROCESS CONTROL

ID. METALLIZATION EVALUATION

Objective : To improve methods for measuring the properties of thin

metal films with initial emphasis on adhesion of aluminum metallization

deposited on non-metallic substrates.

Progress : A scratch test for characterizing the adhesion of metal

films to non-metallic substrates based on the work of Karnowsky and

Estill [l] is being evaluated. This study is being coordinated with a proposed procedure for the method which is presently under consideration

by ASTM Committee F-l. The film-substrate system being used for the

preliminary work is aluminum on quartz because of the importance of this

system to microelectronics.

Experiments were begun on controlled scratching of .aluminum films

with a rudimentary apparatus consisting of a polished diamond stylus

mounted on the arm of a dynamometer. Preliminary work was directed toward investigation of criteria for film failure, precision of a crude

scratch test apparatus, and ways to determine the tip radius. Test films

were prepared by evaporating about 0.5 urn of aluminum onto polished

fused-quartz plates. The stylus was lowered onto the film to be scratched

until the desired load was attained as determined by the dynamometer.

The substrate was moved for a distance which gave a linear scratch path

length of at least 1 cm. The nominal radius of the styli used for these

familiarization studies varied from 18 to 127 urn. The total load on the

stylus was varied on successive

scratches in the range from 5 to 14

grams

.

Both reflected light from the

scratched aluminum film and light

transmitted through the regions

where the film was removed were used

to observe the scratches. The critical load Wc at which the film fails

is defined as that load for which

light can be observed at 40X magnification to be transmitted through

a scratch. As shown in Fig. 4, the

aluminum film was removed by the Fig

stylus intermittently along the

scratch path in all cases. As the

stylus load was increased, removal

of the metal was more complete.

However, in no case was a length of

film longer than about 8 mm removed.

4. Typical scratch test plate showing fail- ure points of a 0.56-um thick vacuum evaporated aluminum film on a quartz

substrate viewed by transmitted light.

Region A has noticeably higher adhesion

than the adjacent areas. The scratch

test path is 1 cm long.

27
METALLIZATION EVALUATION The intermittent failure is believed to result from variations in both

film adhesion and substrate hardness.

The repeatability of the determination of the critical load W for

film failure was also studied. A series of scratches was made with a

76 -urn radius tip in which tip loads were varied by 0.1 g between adjacent scratches. This procedure was repeated several times in different

regions on an aluminum- coated quartz plate. It was found that in areas

of a few millimeters in width, the critical load for film failure was constant within ±0.1 g of its mean value of 1.9 g. This observation

illustrates the sensitivity of the scratch test and indicates its use- fulness as a quantitative method for comparing the relative adhesion of

aluminum films on quartz substrates.

The difference in the critical load for two styli with different

radii used to scratch the same aluminum film was determined. According

to a derivation by Benjamin and Weaver [2], the shear force, F (g/cm2 ),

required to strip a metal film from a non-metallic substrate may be ex- pressed as F = P/[ -fp . xp. t (1)

c

where r is the radius of the stylus tip (cm), Wc is the critical load

(g), and P is the indentation hardness of the non-metallic substrate,

(g/cm2

). If the film adhesion to a given substrate is assumed to be

constant, the force, F, should be constant and the critical load, W , should vary as the square of the tip radius, r. From measurements with

two styli, it was found as shown by the data in Table 4- that this relationship was not followed if r is taken as the nominal tip radius . This

anomalous result has not yet been explained.

Table 4 - Critical Loads for Failure of Aluminum Film

Tip radius, r Critical load, W

(pita) (g)

18 5.0 to 5.5

76 2.0 to 2.5

Attempts were made to use photomicrography to determine the radius

of curvature of the stylus profile in the region of the stylus tip which

comes in contact with the aluminum film. Shadowgraph techniques previously used had not provided enough detailed information about the tip

28
METALLIZATION EVALUATION

radius. Photomicrography was found to be tedious, time consuming, and

without any advantage over the shadowgraph techniques. Neither of these

methods provides information about the critical portion of the tip.

A normal pull test for determining the adhesion of aluminum films

is also being considered. A hot-melt resin glue was used to attach a dozen steel "common" pins to a 0.5-um thick film of aluminum on a glass

slide. When the pins were pulled off, the fracture always occurred at

the interface between the glue and the top of the pin head and not at the

aluminum-substrate interface. This failure is thought to be associated

with improper positioning of the pin during gluing which can be mini*

mized with proper jigging.

The literature search on adhesion testing of metallic films is

continuing. (W. K. Croll and J. Oroshnik)

Plans: Evaluation of the scratch test method will continue using

a form of balanced lever system in order to achieve better versatility,

sensitivity, and a uniform stylus force on the film while the scratch is

being made. A draft method for determining the adhesion of metal films

to hard translucent substrates by the scratch test now being considered

by ASTM Committee F-l will be evaluated. Determination of stylus tip

radius and topography will be attempted using interferometric methods.

A test jig will be designed for use in the normal pull test method to

hold the pins during their attachment to the metal film with a hot-melt

resin glue. The literature search will be continued but limited to adhesion testing of thin metallic films on non-metallic substrates.

1. M. M. Karnowsky and W. B. Estill, "Scratch Test for Measuring Adherence of Thin Films to Oxide Substrates," Rev. Soi. Instrvm. 35_,

1324-1326 (1964).

2. P. Benjamin and C. Weaver, "Measurement of Adhesion of Thin Films,"

Proa. Roy. Soo. (London) 254 A , 163-176 (1960).

29
11. PROCESSING FACILITY

Objective : To establish a microelectronic fabrication laboratory

capable of producing specialized monolithic silicon devices for use in

research on measurement methods.

Progress: Photomasking, computer-aided device design, and fabrication of test structures have been emphasized. The present status of

the remainder of the processing facility may be described by considering

each of the operations necessary to produce a monolithic device. At

present diffusions may be produced with characteristics suitable for

transistor fabrication. Boron-diffused layers have sheet resistances of

100 ^/square with a Gaussian distribution of boron and surface concen- trations of approximately 3 x 10 18 cm

-3

. Phosphorus-diffused layers,

suitable for an emitter, may be produced with error function phosphorus

distributions and sheet resistances of 10 to 15 ft/square. Both types of

diffusions have depths of approximately 2 urn in 1 °,-cm silicon.

The steam oxidation process yields 0.5-um thick oxides which are adequate for masking both phosphorus and boron diffusions . To demonstrate

that these oxides are free of pinholes an aluminum film is evaporated over

the oxide and raised to a temperature sufficient to alloy the aluminum

into silicon. Alloying occurs where pinholes are present; triangular pits

can be observed where alloying has occurred. Vacuum-evaporated aluminum

metallizations 0.5-um thick are being routinely produced. This thickness

is sufficient to permit good ultrasonic aluminum wire bonding and yet

allow easy photoengraving. No problem has been experienced with excessive pinholes or poor adhesion of the metal films. A photographic system

for producing emulsion masks was developed. Mask sets suitable for making

1-mil (25.4-ym) line widths can be produced by means of a pinhole camera.

The system has an overall reduction of the artwork of 200 times. The

basic chip size used is 50-mils (1.27-mm) square. The camera efficiently

produces masks with sufficient resolution to satisfy present needs for

simple devices for test method development. Difficulties have been ex- perienced in the adhesion of a positive-working photoresist on the oxidized wafers during etching. Poor photoresist adhesion and the attendant

undercutting of the oxide is a common problem in the semiconductor industry. It is usually solved by careful attention to surface conditions

during and after oxidation. Better control of these conditions is being

attempted.

A set of "design rules" has been developed for device structures

which can be produced by the fabrication facility. These rules define

the technical objectives of the shallow-diffusion process which has been

established and provide a starting point for the design of monolithic

devices. To further assist in the design of circuit structures, a com- puter program, SNAP- I I [l], has become operational on the NBS computer.

The program has the capability of analyzing an electronic circuit and

predicting its performance and has already been useful in designing a filter network for the wire bond study. In addition, a proposed

30
PROCESSING FACILITY silicon transistor structure was investigated to predict its feasibility

as a test device. This structure was designed within the constraints of

the design rules mentioned above. A preliminary analysis based on a simple hybrid-pi model [2] indicates that the bipolar structure will have

an upper frequency limit (f^) of approximately 2.5 MHz.

A large-area diode structure, 40-mils (1,0-mm) square, was designed

and the mask set produced. After mounting on TO-5 headers these diodes

are intended for use as heat-generating devices in the study of voids

introduced into the die bonds (see Section 16).

Metallized arrays of square bonding pads have been produced for wire

bond evaluation studies (see Section 12). The bonding pads are typical

of those found on actual devices using aluminum metallization. Their

size is nominally 5-mils (127-ym) square over 0.5-ym thick silicon oxide.

The aluminum film was approximately 0.5-ym thick. Various heat treating

techniques have been employed to optimize their suitability for wire

bonding. (T. F. Leedy and J. Krawczyk)

A new reflector for the substrate heater has been completed for use

in the evaporator. This quartz reflector is rectangular in cross-section

rather than cylindrical. The rectangular design is expected to provide

a more uniform substrate temperature distribution than the cylindrical

type. A modified version of the platinum-bonded silicon sandwich substrate temperature sensor (NBS Tech'. Note 475, p. 21) is presently under

construction. (W. K. Croll)

Plans : Diodes for die attachment evaluation studies will be produced. Small planar diodes (area about 10 mil2

) will be produced for

both epitaxial resistivity and lifetime studies. Work will continue on the electronic circuit analysis program ECAP [3] for use in evaluating

devices and circuits under transient conditions. A contact printer will

be installed in the photomask area to allow production of emulsion photomasks with greater contrast and better edge definition.

1. T. D. Price, C. M. Kimme, and J. R. Gliessman, "User's Manual for

SNAP-II Computer Program," prepared for U. S. Naval Weapons Center,

China Lake, Calif. 93555, by ARINC Research Corporation, Western

Division, Santa Ana, California. Publication 474-01-1-909. (Available from Naval Weapons Center.)

2. P. E. Gray, D. DeWitt, A. R. Boothroyd, and J. F*. Gibbons, Physical

Electronics and Circuit Models of Transistors, John Wiley 6 Sons,

Inc., New York, 1964, p. 168.

3. IBM Application Program, "1620 Electronic Circuit Analysis Program

(ECAP) (1620-EE-02X) User's Manual," (Available from either IBM,

Technical Publications Department, 112 East Post Road, White Plains,

N. Y. 10601 or UNI VAC Division of Sperry Rand Corporation, P. 0. Box

8100, Philadelphia, Pa. 19101.)

31
12. WIRE BOND EVALUATION

Objective : To survey and evaluate methods for characterizing wire

bond systems in semiconductor devices and where necessary to improve

existing methods or develop new methods in order to detect more reliably

those bonds which eventually will fail.

Progress : Experimental efforts during this period were directed

primarily toward further characterization of ultrasonic bonding systems

by means of the capacitor microphone described last quarter (NBS Tech.

Note 495, pp. 25-30). Preliminary work on the high-current pulse method

for non-destructive wire bond testing was begun and the study of aluminum

wire ball formation continued. The investigation of the tensile strength

of both round and ribbon aluminum wire after burn- in at various temperatures was resumed. A scanning electron microscope has become available

to the project; initial efforts were devoted to operator training. Evaluation of factors affecting the pull test for wire bond stregnth has begun. Improvements are being made to the wire bond puller to increase the

speed and convenience of operation. Work on the comprehensive bibliography and critical review is continuing.

Characterization of Ultrasonic Bonding Systems — Extensive studies

of the motion of ultrasonic bonding tools have been made with a capacitor

microphone. Both equipment and techniques have been refined; the improved 0.007-in. diameter resolution makes it possible to measure the

relative vibrational amplitude along the entire length of the tool.

The apparatus shown in Fig. 5 was assembled in order to position the

microphone precisely with respect to the transducer and tool. In addition to micrometer-controlled translational movement along three perpendicular axes, the microphone can be rotated ±135 deg. around the tool

in order to observe sideways and other modes of vibration. Details of

the design of the high resolution microphone extension tip and the various measurement techniques are discussed in Appendix E.

The effects of mount design on the vibration amplitude of the bonding tool tip were examined. As can be seen in Fig. 5, the transducertool assembly is suspended from a fixed mount. It is essential that this

be done in such a way that absorption of ultrasonic energy into the mount

is minimized. Transducer manufacturers have provided tables [1] of design dimensions which should be avoided for various materials because of

the possibility of resonant absorption.

Different transducer mounts were made and tested with several com- mercial transducer-tool assemblies. In one poor mounting condition, the

wave form shown in Fig. 6 was observed. This wave form apparently results when some of the transducer energy is absorbed into the mount at a subharmonic rate. Under another condition, hash was observed at the peak

of an otherwise normal sine wave. It was also observed that an improperly mounted transducer can have different resonance frequencies as the

32
"WIRE BOND EVALUATION

Fig. 5. Precision ultrasonic transducer-microphone mounting

apparatus

.

Ultrasonic transducer

Transducer mount Tapered horn

Bonding tool

Capacitor microphone

Tapered tip

Graduated scale for rotation

Y-axis micrometer

X-axis micrometer

Z-axis micrometer

Fig. 6. Unusual wave form obtained with the ultrasonic

microphone. This wave form results from a trans- ducer mount resonance that absorbs power at half

the excitation rate (30 kHz rate when the trans- ducer is driven at 60 kHz).

33
WIRE BOND EVALUATION

applied power is changed. It was noted that energy can be absorbed by screws on the mount, near but not necessarily contacting the transducer.

Since polytetrafluoroethylene (PTFE) is an excellent ultrasonic damping

material, this absorption can be reduced by wrapping each screw with PTFE

plumbing pipe tape before insertion.

These problems do not depend solely on the mount characteristics

,

but rather on the mount-transducer combination. In a particular transducer mount that was carefully designed to avoid resonant dimensions , one commercial transducer lost energy by subharmonic absorption, but another

transducer from a different commercial source functioned satisfactorily.

It was also noted that most of the above characteristics are power sen- sitive. Below some threshold there is usually no evidence of absorption

into the mount. As the power control setting is increased, more power

may be absorbed into the mount instead of driving the tool. At still

higher levels this power absorption may decrease. As a result, it cannot

always be assumed that the vibration amplitude of the tool tip will always increase at the same rate as the power control setting.

It has also been verified that these effects can exist on commercial

ultrasonic bonding machines. Direct observation of the vibration amplitude of the tool tip by means of an ultrasonic microphone has been shown

to be an effective means of establishing whether or not power is being

transmitted to the tool [2].

The amplitude of vibration along the length of the bonding tool was

studied on many different tools inserted to various extensions. Several

different transducers were used to drive the tools. In Figs. 7 to 9 are shown typical examples of the relative amplitude (peak-to-peak) of the

tool vibration as a function of distance along the tool. The relative

amplitude (peak-to-peak) of the vibration of the end of the transducer

horn is also shown. The horn tip vibration amplitude, which is typically

less than the tool motion, is not constant across the face of the horn

tip. The reason for the variation is not understood.

The standing wave patterns in Fig. 7 demonstrate the changes which

may occur when the same tool is removed from the horn and reinserted to a slightly shorter tip extension. The relative amplitude of the tip vibration changes substantially as does the relative amplitude of the

standing wave pattern above and below the horn tip. The same power setting was used for both measurements , and the driving frequency required

to maximize the tip vibration amplitude changed only slightly (about

50 Hz).

As shown in Fig. 8 similar changes occur when the tool is removed

and reinserted to within 0.002 in. of the same extension. The results of

a similar experiment carried out on a short tool driven by a different

transducer and type of power supply are shown in Fig. 9. In this case

34
WIRE BOND EVALUATK

0.8

0.7

0.6 -

•0.5

0.4

0.3

0.2 -

0.1 -

4 3 2 10 12 3 4 VIBRATION AMPLITUDE (RELATIVE VALUES)

4 3 2 10 12 3 4 5 6 7 VIBRATION AMPLITUDE (RELATIVE VALUES)

Fig. 7. Vibrational amplitude of a long ultrasonic bonding tool.

A: A bonding tool extended 0.350

in. below the horn.

B: The same bonding tool ex- tended 0.335 in. below the

horn. The vibrational amplitude of the horn,

which is in general less than that of

the tool, is shown in the center part

of the figure. Data for both ex- tensions were taken with the same power setting on the ultrasonic supply,

although the frequency was retuned

slightly for maximum amplitude in each

case. Amplitudes are relative values

consistent within a given figure;

they cannot be compared directly with

other figures. The amplitudes and position of the horn are measured at its tip. Since the horn is tapered,

the tool extension cannot be measured from the figure. Rounding of the amplitude curve at the tool extremities

can be attributed to edge effects.

Fig. 8. Vibrational amplitude of a long ultrasonic bonding tool. The tool was re- moved after taking the data for A and then replaced to the same extension

for taking the data for B. The vibrational amplitude of the horn, which

is in general less than that of the

tool, is shown in the center part of

the figure. Data for both extensions were taken with the same power setting

on the ultrasonic supply, although the frequency was retuned slightly for maximum amplitude in each case. Ampli- tudes are relative values consistent

within a given figure ; they cannot be

compared directly with other figures. The amplitudes and position of the horn are measured at its tip. Since

the horn is tapered, the tool ex- tension cannot be measured from the

figure. Rounding of the amplitude

at the tool extremities can be attributed to edge effects.

35
WIRE BOND EVALUATION

Fig. 9. Vibration amplitude of a short ultrasonic bonding tool. The tool was removed after taking the data A and

then replaced to the same extension

for taking the data B. The vibrational amplitude of the horn, which

is in general less than that of the

tool, is shown in the center part of

the figure. Data for both extensions were taken with the same power setting

on the ultrasonic supply, although the frequency was retuned slightly for

maximum amplitude in each case. Amplitudes are relative values consistent

within a given figure; they cannot be

compared directly with other figures.

3 2 10

VIBRATION AMPLITUDE 12 3 RELATIVE VALUES)

the tip amplitude remains essentially the same; however the standing wave pattern changes markedly.

A dramatic indication of the effect of tool extension can be seen

from the frequency dependence of the tip vibration amplitude as shown in

Fig. 10. For the 0.375-in. tool extension there is a double resonance

peak with very large amplitudes in the resonance. After removal and re- insertion of the tool to an extension of 0.325 in., the curve with a single peak which has amplitude, resonant frequency, and shape typical of

normal bonding is observed. The standing wave pattern observed with the

bonder tuned to the 59 kHz resonance peak of Fig. 10 is shown in Fig. 11.

In this case the relative vibration amplitude at the tip of the tool is

the largest of any amplitude measured during this series of experiments

,

but the vibration amplitude of the upper portion of the tool is smaller

than usually observed. The power control setting was that normally used

for bonding 0.001-in. aluminum wire.

The results of these studies further confirm that the normally

stated, unloaded electrical Q of a transducer has no significance insofar

as the vibration characteristics of the bonding tool are concerned. Not

only does the mechanical (acoustical) Q of a particular transducer-tool

combination depend on the method of electrical drive (NBS Tech. Note 495,

p. 28), but is also depends on the length and extension of the bonding

tool. Mechanical Q can be changed as much as a factor of 2 by varying

the tool extension within the range specified by the manufacturer. It was found to be relatively difficult to tune-up the system if the length

of the tool is greatly different from that specified by the manufacturer

of the transducer. In particular, a long (0.828-in.) tool employed in a

36
WIRE BOND EVALUATION

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37
WIRE BOND EVALUATION

0.8 —

0.7

0.6

• 0.5

2 0.4

30 25 20 15 10 5 VIBRATION AMPLITUDE (RELATIVE VALUES)

Fig. 11. Vibration amplitude of a long ultrasonic bonding tool which demonstrates

unusually large resonance effects.

These measurements were made with

the ultrasonic system tuned to the

59 kHz resonance peak of Fig. 10, curve A. Amplitudes are relative values consistent within a given figure;

they cannot be compared directly with

other figures. The amplitudes and position of the horn are measured at its tip. Since the horn is tapered,

the tool extension cannot be measured from the figure.

38
WIRE BOND EVALUATION transducer designed for a short (0.437-in.) tool often results in a multiple resonance curve similar to that shown in Fig. 10.

It is to be expected that the vibration characteristics of the tool

tip will be modified somewhat during bonding because of the loading on the tool; the mechanical Q of the loaded transducer-tool combination will

be lowered. Preliminary measurements of the standing wave pattern of

tools under a simulated bonding load of 24 grams show that the position

of the lowest node shifts only a small amount (^0.007 in.). In general

the shift was found to be in a direction away from the end of the tool

but for some conditions of tool length and extension the node moved toward the tool tip. The observed shift was sufficiently small that it

alone does not appear to affect the maximum vibration amplitude at the

tip. This shift may or may not be mirrored in the node near the top of

a long tool. Under this load condition, the resonance curve broadens and

the effective Q is reduced. Thus the ability to maintain the tool tip

amplitude constant under load conditions depends to some degree on feedback in the transducer power supply. Further studies are necessary to

establish more firmly the effects of loading on the tool tip motion.

Efforts are now under way to establish the origin of the variability

in the vibration amplitude of the tool tip. A series of experiments was

conducted to ensure that the variability is not due to faulty measurement

equipment or techniques. Measurements on tools which have not been moved

can be repeated to within about 5 percent. At the present time, it is

felt that the principal difficulty rests with irregularities in the shape

of the hole in the horn through which the bonding tool is inserted and

with irregularities in the set screw which clamps the tool in

place. A typical hole in a relatively new transducer horn is shown

in Fig. 12. Irregular ridges with

a height about 0.001 in. can be

readily seen, and there is evidence

of metal bent outward during the

threading of the set screw hole.

Because of these irregularities,

the tool would not necessarily be

driven from the same point on its

length each time it was reinserted

and clamped. This is equivalent to

inserting the tool to a different

depth each time even though the

gage block measurement between horn

and tool tip remains unchanged.

Furthermore , the set screw wears

with use and probably will not always clamp the tool in the same position. Some improvement could

Fig. 12. Hole in an ultrasonic transducer horn

through which the bonding tool is in- serted. The dark area on the left is

a part of the thread for the set screw. The photograph clearly shows

irregularities on the wall of the

hole. Magnification is approximately

18 times.

39
WIRE BOND EVALUATION

probably be obtained if the tool holes were drilled, reamed, and polished.

Even under such conditions, a tolerance of ^0.0002 in. is about the best

that could be expected, and this, combined with a similar tolerance which

is customarily specified on the radius of tungsten carbide bonding tools,

can result in a misfit many times the vibration amplitude. Wear of the

edge of the hole by the tool after prolonged use may also increase the

problem.

Several possible solutions to these problems are being considered.

A simple remedy for set screw wear is to replace it every time the bonding tool is changed. It was also found that more reproducible contact

between such screw and tool can be achieved if the end of the set screw

is filled with a soft material such as lead. Such a screw, or at least

the lead tip, would still have to be replaced periodically. Various

methods of rigidly holding the tools, such as epoxying or brazing, appear

to be impractical. The misfit problem might be reduced by filling the

empty regions with a glue-like material that transmits 60 kHz vibration

and that can be put in and removed easily. It is also possible that a replaceable metal-foil gasket could be used for this purpose or that the

geometry of the hole could be modified by enlarging it everywhere except

directly behind the set screw so that all motion originates from a much

smaller area.

Another problem associated with tolerance is the length of bonding

tools which is specified to ±0.015 in. This means that two supposedly

identical tools could differ in total length by 0.03 in. If the motion

of the top portion of a long tool is coupled to that of the bottom of the

tool then the tip motion can be seriously affected when a tool is re- placed by another of different but supposedly identical length. All ex- cept one of the tools used in this study were well within the specification limits. This suggests that tool length tolerances could be tightened considerably.

It appears that once a tool is firmly clamped in position, it will

maintain the same tip motion for given drive and temperature conditions

until removed. This suggests that it would be feasible to establish the

bonding schedule from measurements of tip vibration amplitude after each

tool change or adjustment. Either an ultrasonic microphone or a simple

laser interferometer could be used for this purpose.

The temperature dependence of the resonant frequency and tip vibration amplitude of tools driven by transducers with both ferrite and

nickel magnetostrictive ultrasonic elements was measured to determine if

a change in ambient temperature could detune a system. A linear temperature coefficient of 17 to 20 Hz/deg C over a range of 18 to 30°C was measured for both types of transducers. This value is consistent with

the expansion characteristics of the stainless steel parts and is satisfactorily low for normal bonding machine use. However, some modern bond40
WIRE BOND EVALUATION

ing machines have multiple high intensity lamps that illuminate an ex- tended area around the work. Since these lamps are capable of heating

the transducer by 5 or 10 deg C, it is possible that changes in the

illumination could cause measurable changes in the operating characteristics of the transducer.

To date this work has been a laboratory characterization of the

ultrasonic systems employed in bonding wire leads to silicon devices.

These measurements and techniques are to be applied to studies of bonding

machines during the bonding operation in order to establish the significance of individual findings with respect to the bonding process.

Laser Calibration of the Capacitor Microphone — Although much use- ful information can be obtained by means of the ultrasonic microphone

system, its major limitation is that absolute amplitude measurements can- not be made without independent calibration of the system. The only

known accurate method for this calibration is a laser interferometer system such as that described by Martin [2]. Figures 13 and 14- show typical

oscilloseope traces obtained by laser interferometer measurement [3] of

the vibration amplitude of the tip of a highly polished bonding tool.

Each fringe represents 0.316-ym (12.5-Uin.) displacement. The end or turn-around points are clearly broader than the intermediate motion

fringes; this defines one cycle of motion. Because the measurement system was not refined, it was difficult to measure accurately the fraction

of a fringe represented at the end points. Although the signal is

relatively weak, the trace in Fig. 13 represents a normal type of interferometer curve. For the trace in Fig. 14 the signal-to-noise ratio was improved by adjusting the interferometer. The turn-around points in this

trace appear to be unusual, but because of the preliminary nature of the

measurement, no interpretation has been attempted.

The power setting required to produce the tool tip motion shown in

Fig. 14 (about 60 Uin. peak-to-peak) is on the low side of the range

normally employed for bonding 0.001-in. aluminum wire. At slightly

higher power levels still within the normal bonding range, the turnaround point showed high frequency modulation which could not be ex- plained unless some harmonic motion was present which was not observable

by the ultrasonic microphone which has a maximum response of 80 kHz.

Additional measurements are required to complete the calibration of the

ultrasonic microphone and to explain the nature of the traces

.

(G. G. Harman)

High-Current Pulse Test Method — During the quarter, work began on

a high-current pulse method of testing finished bonds and interconnections on integrated circuits. It had been noted on a previous project

that during high-current pulsing of laser diodes, weak wire bonds would

separate at the bond-device interface or give an irregular current wave

shape pattern during the pulse. Based on this experience, this bond

41
WIRE BOND EVALUATION

Fig. 13. Oscilloscope trace of a laser interferometer fringe measurement of bonding tool tip motion. The turn- around points are broader than the intermediate

fringes. Each complete fringe represents a 12.5-

uin. displacement of the tool.

Fig. 14. Oscilloscope trace of a laser interferometer fringe measurement of bonding tool tip motion with im- proved signal-to-noise ratio. The turn-around

points are more difficult to interpret than those

of Fig. 13.

42
WIRE BOND EVALUATION

evaluation method is being adapted to test wire bonds on silicon devices

and integrated circuits

.

Various probes and micromanipulators were acquired and modified to

be used in the test. It was established that the direct current capability of typical aluminum metallization stripes, 0.001 to 0.002 in. wide

and 0.5 Urn thick, on silicon exceeds that of 0.001- in. diameter aluminum

wire which can carry about 1 A. Presumably this is because of the better

heat sinking of the aluminum stripes. A four-terminal method has been

used to determine contact resistance between probe and metallization

under pulsed current conditions. Because of burnout of the metallization

immediately under the probes , tungsten probes with relatively flat tips are being made. These new probes have been designed to have a very low

inductance in order to avoid ringing in making the low impedance measurements .

Aluminum Wire Ball Formation — Some aluminum balls made by the

spark-discharge method previously described (NBS Tech. Note 495, p. 30) were examined under a scanning electron microscope. They were found to

be sufficiently uniform in size and sphericity to be acceptable for bonding purposes. An improved method for making aluminum balls involves the

passing of a tiny jet of hot inert gas (^1000°C) across the wire. An

apparatus for making aluminum balls in this way is being assembled.

(H. K. Kessler)

Tensile Strength After Burn-In — To help in determining how much of

the bonding strength decrease after burn-in is due to a decrease in the

strength of the wire, a comparison was made between the breaking strength

of aluminum (with 1 percent silicon) ultrasonic bonding wire before and

after heat treatment at 150°C for 168 hours. Both 0.001-in. diameter

round wire and 0.0015-in. by 0.00055-in. ribbon wire were tested. The

wires were cut to approximately bond length and mounted vertically in

epoxy. They were pulled to breaking with the hot-melt-glue bond puller.

The results of this test, which are summarized in Table 5, suggest that

the ribbon wire is degraded less by the heat treatment than the round

wire.

Scanning Electron Microscopy — Scanning electron microscopes have

been widely used in studying wire bonds and other parts of semiconductor

devices because of the high resolution and large depth of focus. Near

the end of this quarter, NBS acquired a new scanning electron microscope

which is available for use by this and other tasks in the program. Initial activity was devoted principally to operator training; a number of

bonds were scanned in addition to the study of aluminum balls reported

above.

Pull Test Evaluation — A series of experiments was begun to determine the effect of pull angle on the strength of wire bonds as measured

43
WIRE BOND EVALUATION

Table 5 - Pull Strength of Aluminum - 1 Percent Silicon Wire

Spool No heat treatment After heat treatment Percent

No. Avg.

b

s c No. Avg.

b

s c difference

?fl - Round 19 11.2 g 0.95 g 23 8.2 g 0.66 g 26.8

#2 - Round 26 11.7 0.80 16 7.7 0.42 34.2

#1 - Ribbon 15 10.7 0.60 15 8.8 0.70 17.7

#1 - Ribbon 35 11.2 0.72 43 8.9 0.70 19.9

a Heat treated at 150°C for 168 hours

Average force required to break wire

c Sample standard deviation

d Percent difference of averages

by the pull test. A group of similar loop-type bonds is prepared by

first gently straightening the wire with a regular hook-type puller and

then cutting each loop at its highest point. Each bond is then pulled

to destruction at the desired angle with the hot-melt-glue puller. The

initial tests involve pulling at angles of 30, 45, 60, and 90 deg from

the plane of the wafer surface. (K. 0. Leedy)

Improvements in the bond pulling apparatus are being made in order

to speed up both the process of bond pulling and the recording of data.

These improvements include a means of recording on a graph the force at

which the bond fails. (H. K. Kessler)

Bibliography and Critical Review — The collection of papers for the

bibliography has taken longer than anticipated. The search for reports

of government-sponsored work is essentially complete except for scanning

Scientific and Technical Aerospace Reports for NASA reports that may not

have been included in the list previously provided by DDC. Research of

journal literature is about 95 percent complete. Almost 70 articles and

reports that are considered significant have been assigned keywords for

their inclusion in the bibliography.

A detailed outline of the critical review survey paper has been

completed, and the writing of the paper itself has begun. The paper is

expected to include the following sections:

I

.

Introduction

II. Types of Wire Bond Systems and their Fabrication (thermocompression, ultrasonic)

44
WIRE BOND EVALUATION

III. Factors in the Fabrication of Highly Reliable Wire Bond

Systems (wire bond system components and fabrication processes, associated assembly processes)

IV. Stress- Induced Degradation and Failure (functional, temperature, electrical, radiation stresses)

V. Evaluation of Wire Bond Systems (purpose, description,

evaluation, and correlation of observational, mechanical,

in-process, electrical. thermal, and radiation tests)

(H. A. Schafft and E. C. Cohen)

Plans : Methods for studying bonding tool motion in the laboratory

will be extended to studies of the actual bonding process on bonding

machines. A simple, inexpensive milliwatt laser interferometer will be

constructed and used in conjunction with the microphone for studying tool

and transducer motion. Additional methods of ultrasonic bond monitoring

will be studied. A linear sweep frequency drive will be introduced to

permit monitoring of the resonance Q spoiling during the actual bond

formation.

Investigations of the effect of burn- in at various temperatures on the tensile strength of wire and wire bond systems will continue. Experiments will be designed statistically to evaluate the importance of

various factors in the pull test. Work on the modifications to the bond

puller will continue.

The scanning electron microscope and microprobe will be used for

studying sections of bonding wire including some with which good bonds

could not be made in an effort to evaluate inhomogeneities which might

be responsible for its poor performance.

The study of the effect of metallization sintering time and temperature on ultimate wire bond strength will resume. Work on ribbon wire

will resume when new, harder wire is received. A search for damage to

the silicon substrate by this hard wire will be made; damage will be

detected by means of preferential etching of the silicon after removal of

the bond and aluminum metallization pad.

Work on the bibliography and critical review will continue, are expected to be completed by next summer. Both

1. P. M. Uthe, UTI Tech. Newsletter 1, No. 5 (August, 1968).

2. Other methods such as the laser interferometer system described by

B. D. Martin at the New York State Technical Services Symposium on Lasers and their Application in Local Industries (Binghamton, N. Y.,

April 14, 1969) may also be used to observe the tool tip vibration.

3. These measurements were carried out with a laser interferometer of

the NBS Vibration Measurements Section temporarily adapted to this use with the cooperation and assistance of L. D. Ballard.

45
13. DIE ATTACHMENT EVALUATION

Objective : To evaluate methods of detecting poor die attachment in

semiconductor devices with initial emphasis on the determination of the

applicability of thermal measurements to this problem.

Progress : The literature search for techniques utilized in evaluating the uniformity and quality of semiconductor device die attachment

has provided general guidelines for the progress of the program to date.

Due to the small number of relevant papers, continued emphasis on this area does not appear justified other than to maintain an awareness of

current progress. Therefore, no comprehensive bibliography or review

paper is planned at this time.

To determine the sensitivity of radiographic techniques for detecting voids in the die attachment of the devices to be studied, radiographs were taken of TO- 5 headers with holes drilled through the 3.0-um

thick gold layer that is plated over the header shell. The TO-5 headers

used in this study consist of a gold plated iron-nickel-cobalt alloy

shell with a compatible borosilicate glass seal around the leads. This

type of header was chosen for its compatibility with the power rating of

the diodes being fabricated on chips 50-mils square. Initial results indicate that radiographic techniques will be of use in determining the

size and location of the voids that are deliberately incorporated into

the die attachment system.

The processing of the diode chips to be used in the die attachment

studies was initiated (see Section 11). (F. F. Oettinger)

Plans: The processing of diode chips to be used in the void detection studies will be continued and die-bonding equipment will be assembled. Following these, bonding of diode chips to headers with controlled

voids will begin.

46
14. NASA MEASUREMENT METHODS

Objective : To review existing semiconductor test method standards

for materials and process control measurements and to prepare interim

test methods in a standard format as may be appropriate.

Progress : Additional assistance was given ASTM Committee F-l in the

development of standard procedures for leak testing. The procedure for

determining the hermeticity of electron devices with a helium mass spectrometer leak detector was extensively revised. (W. M. Bullis

)

Plans : Assistance to ASTM Committee F-l in the development of

standard procedures for leak testing will continue. Further revisions

will be made in the helium mass spectrometer procedure; the bubble test

procedure will also be modified. Further review of NASA test methods [1]

will be undertaken with emphasis on the correlation of the precision

needed to meet the requirements for microcircuit line certification [2]

and the precision to be expected from these test methods.

1. "Test Standards for Microcircuits ," Draft of NASA-STD-XX-3, October

1, 1969.

2. "Microcircuit Line Certification," Draft of NASA-STD-XX-6, December,

1968.

47
METHODS OF MEASUREMENT FOR SEMICONDUCTOR DEVICES

15. SECOND BREAKDOWN

Objective : To maintain an awareness of progress in the field of

second breakdown and to assist both manufacturers and users of semiconductor junction devices in the development and use of meaningful

specifications for maximum operating conditions free from second breakdown.

Progress: Assistance was given to Task Group 1 of the JEDEC Committee JS-6, on Power Transistors, in the revision of the initial drafts

of the chapter, "Users Guide for Power Transistors" which is to be part

of a JEDEC suggested standard titled "Recommended Standards for Power

Transistors." (H. A. Schafft)

Plans : Assistance to the JS-6 task group in the writing of "Users

Guide to Power Transistors" will be continued. The section on failure

modes will be revised to include suggestions made during the task group

and committee meetings.

48
16. THERMAL PROPERTIES OF DEVICES

Objective ; To evaluate and, if necessary, improve electrical meas- urement techniques for determining the thermal characteristics of semiconductor devices.

Progress : The literature search and the review of the methods of

measurement of thermal resistance and transient thermal response of semiconductor devices was continued. The writing of the first draft of the

review paper was initiated. (M. Sigman and F. F. Oettinger)

Previously reported, preliminary measurements of thermal resistance,

Rq, and d-c current gain, h EE , made on a limited number of transistors

(NBS Tech. Note 495, pp. 33-36) indicated that a correlation exists between the onset of lateral thermal instability and the abrupt changes in

Rq and hpE which occur at a particular value- of collector-emitter voltage,

Vp£. This result suggested the possibility that hpE measurements can be

used as a screen to detect the onset of thermal instabilities. To facilitate the study of the use of hpE

as a screening technique it was decided to design and build an hEE measuring system that would allow

greater speed and flexibility in the measurements.

A block diagram of this apparatus , which generates and plots base

current, Ig, as a function of collector-emitter voltage VqE

, for constant

collector current, is shown in Fig. 15. A constant-voltage power supply

with a small (2-ft) current-sensing resistor in its negative lead is con- nected across the collector-emitter terminals of the device under test,

DUT. This 300-mA, 500-V power supply has been modified so that the output voltage can be swept at rates which are limited to 5 Hz or less by

the mechanical response of the x-y recorder. The I B

- VqE characteristics

showed no noticeable difference between manual operation which required

5 minutes to complete the sweep, and electrically generated sweep rates

of up to 5 Hz. The x-axis of the x-y recorder is made proportional to

the collector-emitter voltage. The operational amplifier (Ig SERVO) com- pares the potential across the current sensing resistor with that of a negative reference voltage and adjusts Ig to keep Ip constant. A portion

of the voltage drop across the 100-fi resistor in series with the base of

the DUT is applied to the y-input of the x-y recorder. Since I c is held

constant, I R is proportional to the reciprocal of h

EE . The case temperature of the DUT is kept constant at 25°C by means of a servo controller

not shown in the figure.

A representative Ig - VqE characteristic is depicted in Fig. 16. As

VpE

is increased from V. to Vg the transistor operates in a normal mode.

If Vce exceeds Vg, a hot spot forms and Ig increases abruptly. If V

CE

is increased further, Ig increases until second breakdown occurs. If, on the other hand, VpE

is varied between Vg and the lower voltage Vq, the

transistor operates in a stable, current-constricted mode. If VpE is

further reduced, the transistor returns to the normal non-constricted

mode at Vp and remains in this mode for lower values of VqE

. If V^E

49
THERMAL PROPERTIES OF DEVICES

p

c

0)uu

d

a)en

01 XtU • p p

w c

•H m

VI p

c 01 m c M o p o

IP. T) O H

a) 0) £

a)> p

u c

? a) O h

m

>, 3 H o

f-im u

o H p

•P a m (i) H -H O H

P O

3 O

m

at p x:

T) (1) a> 1X1 m ffl 3 -pH

tfl 3 > P

m M

s- 0)

rt p

a, -p ex •H

tfl h

(1) Ip. 1 o U

(> H P

m o

^ a) M H

(fl H (> O O

50
THERMAL PROPERTIES OF DEVICES

CO

CASE TEMPERATURE (Tc)-25'C

COLLECTOR CURRENT (I c

) • CONSTANT

V» Vc

COLLECTOR- EMITTER VOLTAGE- V

CE (V)

Ve

Fig. 16. Automatically plotted curve of base current versus collector-emitter voltage

for a planar, diffused, power transistor.

is then increased, the transistor operates along the line AEB unless V

fi is again exceeded.

To interpret the characteristics traced in Fig. 16 consider the

following: Initially, there is an increase of hpE with temperature due

to the increased power dissipation [l] as VpE is increased from V. to Vg.

At V

CE = VR

, a hot spot develops, and hpE abruptly decreases. This may

be explained by making use of the observation of Whittier and Tremere [2]

that in thin-base, diffused, planar transistors, the base transport

factor drops abruptly when a critical current density is exceeded. Since

hot-spot formation is caused by a rapid current constriction, the associated increase in current density can account for the rapid decrease in

the base transport factor which in turn causes hpE to decrease. After

the formation of the current constriction, a decrease in VqE causes an increase in hpE as indicated by that portion of the curve between C and

D in Fig. 16. The reason for this hysteresis effect is not wholly understood, but effects being considered as possible causes are the reversal

of the current gain-temperature dependence at high current density and

51
THERMAL PROPERTIES OF DEVICES

temperature levels [1], as well as the dependence of hpp on current density [2], At point D, the hot spot is sufficiently cool that the current

density in the constriction is not maintained above the critical value

noted above and hp£ increases abruptly to its value prior to the onset

of the current constriction (point E). Initial studies in which thermographic phosphors are used to display visually and measure transistor sur- face temperature distributions (see Section 17) substantiate the

existence of the hot spots postulated in this interpretation of the hpE

- V

CE traces. (S. Rubin, R. L. Gladhill, and F. F. Oettinger)

Plans ; The literature search will continue. The format and key

words for the bibliography will be established. The first draft of the

review paper on methods for measurement of thermal resistance and transient thermal response is expected to be completed in the next quarter.

Construction of an apparatus to automate the measurement of Rfl by

the emitter-base voltage technique will be studied, and if warranted,

undertaken. Further measurements of hpp as a function of Vqt, will be

made on transistors representing different production techniques and

geometries with the automated hpE apparatus. The results of these measurements will be compared with thermographic phosphor measurements (see

Section 17) to determine the suitability of the h p measurement to detect

the onset of thermal instabilities and to study further the physical

basis for the noted curves.

D. Buhanan, "Investigation of Current-Gain Temperature Dependence in

Silicon Transistors," IEEE Trans. Electron Devices ED- 16 , 117-124

(1969).

R. J. Whittier and D. A. Tremere , "Current Gain and Cutoff Frequency

Falloff at High Currents," IEEE Trans. Electron Devices ED-16 , 39-57

(1969).

52
17. THERMOGRAPHIC MEASUREMENTS

Objectives : To evaluate the utility of thermographic techniques for

detection of hot spots and measurement of temperature distribution in

semiconductor devices.

Progress : The photometric equipment for recording the temperature

distribution on the surface of a transistor chip has been used to confirm

that the sudden change in hp^ illustrated in Fig. 16 (see Section 16)

coincides with the formation of a hot spot on the transistor. Furthermore, once the hot spot has formed, the collector-emitter voltage must be

lowered considerably before the hot spot disappears , again following the

pattern indicated in Fig. 16. Work in under way to measure changes in

the area of the hot spot as collector-emitter voltage is reduced from the

voltage at which the hot spot occurs.

MOTOR DRIVEN

FIBEROPTIC PROBE ^

MICROSCOPE

U.

^r\

PHOTOMULTIPLIER

4 444*

PHOTOMETER

PROBE POSITION

• xX-Y RECORDER

I HEAT SINK "1

MECHANICAL STAGE

DECAPPED TRANSISTOR UNDER TEST

COATED WITH THERMOGRAPHIC PHOSPHOR

Fig. 17. Test equipment for observation of hot spots on the surface of a decapped

transistor.

53
THERMOGRAPHIC MEASUREMENTS

The equipment used in this study is shown schematically in Fig. 17.

The fiber optic probe is located effectively in the image plane of the

microscope and views an area approximately 60 urn in diameter on the sur- face of the transistor. The transistor can be moved by means of the

mechanical stage so that any desired area on the surface can be brought

under observation of the probe. The probe can also be driven along a diameter of the image plane to scan a path across the surface of the

transistor. An electrical output coupled to the probe drive provides an input to the x-axis of the recorder that is indicative of the position

of the probe. The input of the y-axis of the recorder is obtained from

the output of the photometer. Because of the thermographic phosphor

coating on the surface of the transistor, the light radiated from the

transistor is proportional to temperature, so that the photometer output

is proportional to temperature , and the recorder can be used to plot temperature as a function of position.

(G. J. Rogers, F. F. Oettinger, and L. R. Williams, Jr.)

Plans: Studies of hot-spot formation in transistors will be con- tinued to determine the extent and temperature of the hot spot as collector-emitter voltage is varied. Studies of the spatial resolution and

temperature resolution of the phosphors will be resumed.

54
18. MICROWAVE DEVICE MEASUREMENTS

Objective : To study the problems and uncertainties associated with

measurement of microwave device properties and to improve the methods of

measurement for related characteristics.

Progress : Continuing discussions have been held with representatives

of the Electronic Industries Association, the IEEE Microwave Theory and

Techniques Group, the Naval Electronic Systems Command, the Naval Applied

Science Laboratory, and others to determine further the nature and priorities of the work which is to be undertaken on microwave diodes and

transistors. Work was started on error analysis for transistor scattering parameter measurements in cooperation with the JEDEC Committee JS-9,

on Low Power Transistors, Task Group on Transistor "S" Parameter Measurement Standards which held its initial meeting at the National Bureau

of Standards on September 30

.

Radio frequency bridge network apparatus has been assembled for

measurement of microwave transistor and diode impedance as a function of

bias from 0.05 to 1 GHz. Slotted line and reflectometer or network

analysis equipment is being assembled. The primary purpose of this type

of measurement is to determine and compare errors associated with the

most common techniques for determining transistor scattering parameters

.

An X-band sweep-frequency generator has been incorporated into

equipment for measuring reflection and transmission of two-port waveguide

devices from 8 to 12.4 GHz. The primary purpose of this set up is to

determine the r-f characteristics of diode mounts and therefore the impedances associated with microwave mixer diodes. This is necessary for

evaluating mixer-diode, noise-measurement techniques. (R. C. Powell)

Plans : Detailed studies are expected to begin next quarter. Apparatus for measurement of mixer diode noise will be assembled to the

extent that equipment is available. Evaluation of errors associated with

microwave transistor scattering parameters will continue. The equipment

for measuring reflection and transmission at X-band will be refined by

the addition of such elements as precision attenuators and calibrated

power meters to obtain the improved accuracy which will be needed.

55
19. SILICON NUCLEAR RADIATION DETECTORSt

Objective: To conduct a program of research, development, and device evaluation in the field of silicon nuclear radiation detectors with

emphasis on the improvement of detector technology, and to provide con- sultation and specialized device fabrication services to the sponsor.

Progress : As part of the continuing effort to .evaluate largediameter silicon crystals for Si (Li) detectors, two dislocation-free

crystals were characterized. The nominal values of the parameters measured on these crystals are given in Table 6 together with results of

measurements on two 67-mm diameter crystals reported earlier (NBS Tech.

Note 475, p. 30). The long-time component of carrier lifetime given in

column 3 of Table 6 is an indication of single or multiple trapping of

charge carriers. Note the interesting fact that only crystal 2 which had

no long lifetime component has a low oxygen concentration. All crystals

exhibited excellent junction rectification properties after the diffusion

of lithium. During the compensation process, the lithium drift-rate was very low on the three crystals which contained large amounts of oxygen.

The principal problem with large-diameter silicon crystals appears

to be the relatively high concentration of oxygen found in most crystals

not grown by the vacuum float-zone process. The oxygen impedes the process of charge compensation by reducing the lithium-ion mobility. Efforts

to develop silicon crystals with diameters greater than 50 mm are being

made at several laboratories in this country. However, the principle

objective of these efforts is to provide larger silicon crystals which

would increase the yield of semiconductor devices and circuits on a single wafer rather than to develop high-resistivity, low-oxygen, detector-grade material.

The investigation of charge carrier lifetime in detector-grade silicon has continued with the collection of eleven silicon crystals with

differing values of resistivity and lifetime. The nominal range of re- sistivity is 150 to 3000 fi-cm as determined by a two-probe measurement

made on the long dimension of the crystals; the range of PCD lifetime is

12 to 12000 us. Methods to be used in this study for measuring the lifetime in diodes made from these crystals are being developed (see Section

3).

Switching circuits were developed for the detector test system

which will automatically monitor the current and noise of several hundred

detectors both on bias and in storage. It was found that the contribution of noise from the switching network was not a significant portion

of the total detector system noise. This system is expected to be in

operation within four months. (B. H. Audet)

Silicon avalanche-type radiation detectors [1] are being evaluated

for possible use in space radiation experiments for the high-rate counting of low-energy electrons or protons. Two detrimental characteristics

56
SILICON NUCLEAR RADIATION DETECTORS

Table 6 - Nominal Values of Several Parameters of Silicon Crystals for Si(Li) Detectors

Crystal

1

2

3

4

iameter Re sistivity Lifetime

mm n-cm us

50 c 54 225, 6250

31 c 500 470, none

67 77 210, 1950

67 66 180, 2350

Oxygen

cm" 3

1.3 x 10 18

<2.4 x 10 15

1.8 x 10 18

1.7 x 10 18

Carbon

cm" 3

<].2 x 10 16

<1.2 x 10 16

<1.2 x 10 16

<1.2 x 10 16

70

60

50

30

20

iophotoconductive decay lifetime, short-time and long-time components

determined from infrared absorption measurements

dislocation-free crystals

DETECTOR -A

DETECTORB

B

10 15 20 ELECTRON ENERGY (keV)

25 30

Fig. 18. Relative counting efficiencies of two silicon avalanche detectors as a function of electron

energy. Optical scans of each detector at the right show that the bright areas, which indi- cate a high electric field, are more extensive in A than in B, thus accounting in part for

the higher counting efficiency of A at low energies. (Photographs provided through the courtesy of G. C. Huth.)

57
SILICON NUCLEAR RADIATION DETECTORS of avalanche detectors which reduce the counting efficiency of low-energy

electrons are the dead layer of about 1 urn at the sensitive surface, and

the relatively large regions of significantly lower electric fields at

the junction. The relative counting efficiencies for electrons as a function of electron energy are shown for two representative detectors in

Fig. 18. Also shown are the areal scans of each detector using visible

light with the detectors at their normal operating bias [2]. The regions

of high electric field are represented by the bright areas while lowfield regions appear darker. The largest avalanche multiplication takes

place in the high-field regions of the junction. The lower counting

efficiency of detector (B) for the lower electron energies can, in part,

be explained by the smaller total area of high electric field than occurs

in detector (A). (Y. M. Liu)

Plans: The study of electron damage in silicon surface-barrier detectors was not pursued in this reporting period because the Van de

Graaff accelerator was not available. The effects on detector performance of 400-keV electrons, which have energies too small to displace a silicon atom, will be studied next. The concern here is with the very

intense flux of low-energy electrons which occur in the radiation belts

around the earth.

The current, noise, and counting response of detectors made from

dislocation-free silicon crystals 1 and 2 (see Table 6) will be determined. Logic circuits for the automated system for monitoring detector

performance during storage, in high- vacuum, or while undergoing temperature cycling will be developed. The counting characteristics of additional avalanche detectors will be evaluated, and efforts will be made

to improve their performance for detecting low-energy particles

.

t Supported by Goddard Space Flight Center, National Aeronautics and

Space Administration (NBS Project 4254429).

1. G. C. Huth, "Recent Results Obtained with High Field, Internally

Amplifying Semiconductor Radiation Detectors," IEEE Trans. Nuo. Soi.

NS-13 , No. 1, 36-42 (1966).

2. G. C. Huth, "A Flying Spot Scanner for Direct Measurement of Electric

Field-Resistivity Profiles in Silicon p-n Junctions," presented at

the New York meeting of the Electrochemical Society, May, 1969.

58
Appendix A

JOINT PROGRAM STAFF

Coordinator: J. C. French

Consultant: C. P. Marsden

Semiconductor Characterization Section

Dr. W. M. Bullis, Chief

A. J. Baroody, Jr.

D. L. Blackburn

F

.

H . Brewer

Mrs. E. C. Cohen"

M. Cosman

Dr. J. R. Ehrstein

G

.

G . Harman

F. R. Kelly 5

H. K. Kessler

Mrs. K. 0. Leedy

R. L. Mattis

Dr. W. E. Phillips

Miss T. A. Poole"1"

Miss D. R. Ricks'" Dr. J. L. Scales^

H. A. Schafft

A. W. Stallings

G. N. Stenbakken

W. R. Thurber

Miss R. E. Young+

Semiconductor Processing Section

Dr. J. A. Coleman, Chief

Miss D. A. Adamson 5

B. H. Audet

H. A. Briscoe

R. D. Clayton 5

W. K. Croll

R. T. Dalton"*" Mrs. S. A. Davis"1"

H. E. Dyson"

H. A. Gitelson 5

W. J. Keery

Mrs

.

C. F. Bolton" Mrs. R. Y. Cowan

Miss B. S. Hope

+

R. L Gladhill

F. F Oettinger

M. K Phillips

.'. Part Time

t Guest Worker

fe Summer

+ Secretary

Electron Devices Section

J. C. French, Chief

E. I. Klein

J. Krawczyk

T. F. Leedy

Y. M. Liu

J. Oroshnik

R. C. Schaevitz''' Dr . A . H. Sher

L. M. Smith

G. P. Spurlock

Miss E. D. Willis 5

R. C. Powell

G. J. Rogers

S. Rubin

M. Sigman

L. R. Williams

59
Appendix B

COMMITTEE ACTIVITIES

ASTM Committee F-l; Materials for Electron Devices and Microelectronics

F. H. Brewer, Resistivity Section

W. M. Bullis, Editor, Subcommittee IV, Semiconductor Crystals

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

Materials

J. R. Ehrstein, Epitaxial Resistivity and Epitaxial Thickness

Sections

J. C. French, Chairman, Subcommittee VIII, Editorial

T. F. Leedy, Photomasking Section

J. Oroshnik, Thin Films, Thick Films, and Photomasking Sections

W. E. Phillips, Crystal Perfection, Encapsulation, Thin Films,

and Thick Films Sections; Chairman, Lifetime Section

A. H. Sher, Germanium Section

M. Sigman, Editor, Subcommittee V, Semiconductor Processing Materials

W. R. Thurber, Germanium and Impurities in Semiconductors Sections

Electronic Industries Association:

MED 32, Active Digital Circuits: F. F. Oettinger, Associate Member

MED i+l, Physical Characterization Requirements: F. F. Oettinger,

TG 41.6, Thermal Considerations

Joint Electron Device Engineering Council (EIA-NEMA):

F. F. Oettinger, Technical Advisor on Thermal Resistance Measurements, JS-9, Low Power Transistors; Technical Advisor, JS-14,

Thyristors.

R. C. Powell, Microwave Diode Specification Problems, JS-3, UHF and

Microwave Diodes; Task Group on Transistor Scattering Parameter

Measurement Standards, JS-9, Low Power Transistors.

H. A. Schafft, Consultant on Second Breakdown Specifications, JS-6,

Power Transistors.

IEEE: Nuclear Science Group: J. A. Coleman, Administrative Committee;

Nuclear Instruments and Detectors Committee; Editorial Board,

Transactions on Nuclear Science; Chairman, 1970 Nuclear

Science Symposium

Magnetics Group: S. Rubin, Chairman, Galvanomagnetic Standards

Subcommittee

IEC TC47, Semiconductor Devices and Integrated Circuits:

F. F. Oettinger, U. S. Experts Advisory Committee

S. Rubin, Technical Expert, Galvanomagnetic Devices

NAS-NRC Semiconductor Detector Panel:

J. A. Coleman

60
Appendix C

SOLID-STATE TECHNOLOGY £ FABRICATION

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 — (A. H. Sher and B. H. Audet)

a. Assistance with mounting and testing a germanium gammaray detector was provided for the Nuclear Spectroscopy

Section.

b. The assembly of special silicon detectors for nuclear

radiation studies by the Photonuclear Physics Section

was begun.

2. Special fabrication — (G. P. Spurlock)

High- voltage, precision resistance cards were prepared for

the High Voltage Section.

3. Quartz and glass fabrication — (E. I. Klein)

a. The absorption cell of a methane laser was modified and

improved for the Engineering Metrology Section.

b. Improvements in ion and thermocouple gauges were made for

the Vacuum Measurements Section.

c. Quartz cells used for studying the ultraviolet exposure

characteristics of plastics and paints were constructed for

the Materials Durability and Analysis Section.

4. Hall effect specimens — (H. K. Kessler)

Ohmic contact dots were applied to several cross-shaped

Hall effect specimens cut from both single and epitaxial

gallium arsenide crystals for the Harry Diamond Laboratories."'"

NBS Project 4251424

61
Appendix D

JOINT PROGRAM PUBLICATIONS

Prior Reports :

"Methods of Measurement for Semiconductor Materials, Process Control, and Devices, Quarterly Report, July 1 to September 30, 1968," NBS

Tech. Note 472, December, 1968. (AD 681330)

"Methods of Measurement for Semiconductor Materials , Process Control, and Devices, Quarterly Report, October 1 to December 31, 1968,"

NBS Tech. Note 475, February, 1969. (AD 683808)

"Methods of Measurement for Semiconductor Materials , Process Control, and Devices, Quarterly Report, January 1 to March 31, 1969," NBS

Tech. Note 488, July, 1969. (AD 692232)

"Methods of Measurement for Semiconductor Materials , Process Control, and Devices, Quarterly Report, April 1 to June 30, 1969." NBS

Tech. Note 495, September, 1969. (AD 695820)

Current Publications:

W. M. Bullis and R. I. Scace,* "Measurement Standards for Integrated

Circuit Processing," Proa. IEEE 57_, 1639-1646 (September, 1969).

W. .M. Bullis, W. R. Thurber, T. N. Pyke, Jr., 1" F. H. Ulmer, 1" and

A. L. Koenig, "Use of a Time-Shared Computer to Control a Hall Effect

Experiment," NBS Tech. Note 510, October, 1969.

W. M. Bullis, "Measurement Methods for the Semiconductor Device Industry — A Review of NBS Activity," NBS Tech. Note 511, December, 1969.

A. H. Sher and W. J. Keery, "Variation of the Effective Fano Factor in

a Ge(Li) Detector," to appear in IEEE Trans. Nuol. Soi. , February, 1970.

* General Electric Company, Auburn, N. Y. 13021 (Chairman, ASTM Committee F-l.)

t Information Processing Technology Division,, NBS Center for Computer

Sciences and Technology.

62
Appendix E

Notes on Procedures for Use of a Capacitor Microphone

to Measure Vibration Amplitude of the Tool Tip

of an Ultrasonic Wire Bonder

Introduction — Measurements of the amplitude of the motion of the

ultrasonic bonding tool by means of a capacitor microphone sensitive to

ultrasonic vibrations have been used to characterize ultrasonic bonding

systems. Most of the experiments were carried out on the apparatus shown

in Fig. 5 (see Section 12). In this apparatus the transducer-horn- tool

assembly is mounted on a micrometer stage and can be moved horizontally.

The capacitor microphone is mounted on a second micrometer stage which

can be moved horizontally and vertically and rotated about an axis. The

entire assembly is used inside a small, bench-top anechoic chamber lined

with two layers of black darkroom felt.

Microphone Extension Tip Design — To improve the spatial resolution

of the capacitor microphone system, a tapered extension tip is added to

the microphone. Bonding tool motion is generally studied at some fixed

frequency except for determinations of mechanical Q of the transducer.

For fixed-frequency measurements , the microphone-taper system need not

have a flat frequency response; a reinforcing standing wave at the test

frequency would tend to enhance the sensitivity. Therefore, almost any

hard, continuous, tapered material may be used for the tip. The plastic

cone from a tube of silicone rubber has been found to be satisfactory.

To determine the mechanical Q of a typical transducer, the frequency

must be changed over a range of 4- or 5 kHz. In this case- standing waves

in this frequency range must be eliminated. The simplest way to do this

is to construct a tip that has the dimensions desired for making the

measurements and insert appropriate material into the tip to damp any

possible standing waves.

A cross section of a high resolution microphone taper is shown in

Fig. 19. This 1%-in. long taper has a pick-up area 0.007 in. in diameter

and is suitable for studying the motion of the bonding tool over its

entire length. The pick-up area is defined by the hollow needle, A. The

stainless steel sleeve, E, must not protrude more than about 0.04-0 in.

past the diaphragm of the microphone. To eliminate standing waves over

the range of interest around 60 kHz, the damping material, D, should ex- tend within 0.040 in. of the diaphragm. In this form the tip provides at

least 90-percent rejection of all signals not entering through the hole.

The rejection can be increased to more than 95 percent by wrapping a layer of %-in. thick urethene foam around the large end of the taper,

overlapping the microphone to prevent sound from entering from the rear.

A slight additional improvement can be obtained by spiral-wrapping the

cone with polytetrafluoroethylene tape or increasing the thickness of the

outer coating, B, with a different silicone rubber.

63
APPENDIX E

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(0 P •H CU cu rd o ft E 43 p

P. e Ph p c

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Ph CU 13 rH o rH o C rd Lp

<u •H

Ph

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bO P. c Ph ft cu cu •H

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rd o

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(0 rd G o P co c •H 43

<u M 13 ft rd >> c u p C c £ H cu o

X •H 3 id cu P cu

CU P 13 > 43 3 p

rd s rd bO P 43 c 0) O E 43 •rH CU bo o rH o ^^ ra H-> 43 •H

•rH T3 .M P

4-> 111 rH o <P P T3 i3 3 CU 0) •rH 0) C CU H C 43 43 o 43 P o 43 O 43 H-> •H P P 43

co O 3 P M •H P

0> -rH ?H (0 C 4h fc CO

Ph E C rd CU CU 3

1 S^ CU •H H rH 0) 43 E

43 cu c 1 ft > 43 M 13 CN a CU 3 13 •H O o O 01 •H CU Ph a

x: o, •H O rH rH o

>,rH • o 13 co a) 43 m £ •H o ft a) (3

CO 1 p rH o CO Mh [> c CO CU o •H o CN &0 rH cu 3 CU •H 43 * c •H a •1—

P H E-- '—

s

s •H o •a co •H fc_S • 0) 4h H CM ra CO cu H 0) fc CO C3 > > E o c CO U • O

C cu 3 o d) O O 43 rH H a c •H H ft m •H 1 •H o P a >• O

c P Mh e p o •rH X. CD Ph o O rH 3 p 0) rd o c o

•H 01 cu ^H rH p a o H

P cu (/) CO < o 4h C/5 H o E

O c

<U o

00 43

1 ft< m o a w in

co

CO h o o

f4 rH o e

64
APPENDIX E

Many specialized cones and tips have been designed. In one case the

steel tip was seated at right angles to the cone axis so that the microphone could be placed at the side of the bonding tool and horn. In

another case a compression tip was made in which the entrance to the cone

was sealed with a 0.002-in. thick layer of high-performance silicone rubber. Although this offered complete environmental freedom from standing

waves and noise, the pick-up area could not be decreased below a 0.02- in.

diameter circle. This was adequate for many measurements. In operation

the tip was brought into direct contact with the tool or horn. It produced a good signal and did not mechanically load the system noticeably.

Acoustical Properties of Materials Simple arrangements of the

microphone and horn may be used to determine, with accuracy suitable for

these studies, the reflectivity or absorption of plastic foams or other

materials in the frequency range of interest. For reflectivity measurements, place the microphone (without the tapered tip) so that its axis

is parallel to that of the transducer horn and both point in the same direction. The microphone and transducer should be about 3/8 in. apart

with the microphone diaphragm placed in a standing wave node . The sample to be tested is placed with a flat area facing the microphone and

transducer. For measurements of transmission through materials which are

not highly reflective, place the microphone in front of and pointing toward the transducer horn, and place the sample between the two.

Most materials, such as paper, metal foil, and rubber plastic foams,

are highly reflective. However darkroom felt, soft paper tissues, and a few open-pore plastic foams have almost no reflection. A single layer of

black darkroom felt was found to absorb about half of the 60-kHz energy

with almost no reflection. Two layers of this material used to line the

bench-top anechoic chamber were sufficient to reduce reflections to a negligible level. One open-pore plastic foam which had almost no re- flection was found to have relatively low absorption. This material was

used for damping standing waves inside extension tips. The length of the

foam was adjusted for about 50 percent absorption, the calculated absorption required to critically damp a standing wave in the tapered tip.

Measurement Techniques — The actual measurement technique depends

on the quantity of interest. For recording a resonance curve to determine the mechanical Q of the system, for observing the motion of the

bonding tool during swept-frequency operation, or for peak-tuning the

system, the entrance hole to the tapered tip can be 0.025 in. or more in

diameter. The center of the hole is placed in front of and about 0.001

in. away from the bonding tool and about 0.035 in. above the tip toward

the horn. Under these conditions, a signal of several millivolts is

obtained directly from a capacitor microphone with a ^-in. diameter

diaphragm. These measurements are relative and can be made with the

microphone in one position. The amplitude reproducibility when the

microphone was removed and then replaced carefully in the same position

65
APPENDIX E

was found to be better than ± 15 percent. This limits the precis'ion of

comparisons between different runs

.

For studying the vibration along the entire length of a bonding tool,

an amplitude reproducibility of about ± 5 percent is essential to obtain

meaningful results . This was achieved by means of the following procedure. The steel tip of the microphone is advanced until it just touches

the vibrating tool. The point of contact is clearly evident because hash

appears on the oscilloscope wave form of the microphone output. Then the

tip is retracted, about 0.0001 in., until the hash just disappears. A

large and reproducible signal is obtained with this technique.

Such measurements are tedious . The bonding machine power supply

must be shut off during the time the microphone is raised to its next

position. This prevents heating of the transducer which would eventually

detune it. Nevertheless a complete measurement such as those in Figs. 7 to 11 (see Section 12) can be taken in about 20 min.

Information about sideways vibration modes of the bonding tool can also be obtained. In this case the system is tuned to resonance with

the microphone in front of the tool. The microphone is then rotated 90

deg to the side of the tool, and the system is retuned to a new maximum

if one exists. Sideways modes with amplitudes about one fourth that of

the main resonance have been observed about 250 Hz from the main resonant

frequency. These characteristics are dependent on the length of the tool

extension. In order to observe weaker sideways modes it is necessary to

compare resonance curves from the front of the tool to those taken at the

side. For practical purposes such weak modes would appear to have little

effect on bonding.

Calibrati-on — For absolute measurements it is necessary to calibrate the microphone response by an independent means. A laser interferometer system can be used for this calibration. Although preliminary

measurements have been made (see Section 12) detailed calibration procedures have not yet been worked out.

66
THE NATIONAL ECONOMIC GOAL

Sustained maximum growth in a free market economy, without inflation,

under conditions of full employment

and equal opportunity

^TOrcQv \

SfATES O* h

THE DEPARTMENT OF COMMERCE

The historic mission of the Department

is "to foster, promote and develop the

foreign and domestic commerce" of the United States. This has evolved, as a result of legislative and administrative

additions, to encompass broadly the re- sponsibility to foster, serve and promote

the nation's economic development and

technological advancement. The Department seeks to fulfill this mission through

these activities:

MISSION AND

FUNCTIONS

OF THE

DEPARTMENT OF

COMMERCE

"to foster, serve and

promote the nation's

economic development

and technological

advancement"

lipating with government

ies in the on of national

, through the

lent's Cabinet

:s subdivisions.

inet Committee

iconomic Policy

an Affairs

ncil

ironmental

lity Council

Promoting progressive business policies and

growth.

• Business and Defense Services

Administration

• Office of Field Services

Assisting states, communities and

individuals toward economic progress.

• Economic

Development

Administration

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Commissions

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This schematic is neither an organization chart nor a m outline for budget purposes. It is a general statement Department's mission in relation to the national goal nomic development.

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