U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards^

Semiconductor

ent Technology

Progress Report

July 1 to September 30, 1976
NATIONAL BUREAU OF STANDARDS

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

Bureau's overall goal is to strengthen and advance the Nation's science and technology and

facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is performed by the National Measurement Laboratory, the National Engineering Laboratory,

and the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

Agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities^ .— Radiation Research Thermodynamics and

Molecular Science — Analytical Chemistry — Materials Science. THE NATIONAL ENGINEERING LABORATORY provides technology and technical

services to users in the public and private sectors to address national needs and to solve national problems in the public interest; conducts research in engineering and applied science

in support of objectives in these efforts; builds and maintains competence in the necessary

disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities; provides engineering measurement traceability services; develops test methods and proposes engineering standards and code changes; develops and

proposes new engineering practices; and develops and improves mechanisms to transfer

results of its research to the utlimate user. The Laboratory consists of the following centers:

Applied Mathematics — Electronics and Electrical Engineering^ — Mechanical

Engineering and Process Technology- — Building Technology — Fire Research —

Consumer Product Technology — Field Methods.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal Agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal Agencies; and

provides the technical foundation for computer-related policies of the Federal Government.

The Institute consists of the following divisions:

Systems and Software — Computer Systems Engineering — Information Technology.

'Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234. 'Some divisions within the center are located at Boulder, Colorado, 80303.

The National Bureau of Standards was reorganized, effective April 9, 1978.
Semiconductor Measurement Technology

Progress Report, July 1 to September 30, 1976

W. Murray Bullis and J. Franklin Mayo-Wells, Editors

Electron Devices Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Washington, D.C. 20234

Jointly supported by: The National Bureau of Standards The Navy Strategic Systems Projects Office The Defense Nuclear Agency

The Defense Advanced Research Projects Agency

The Space and Missile Systems Organization, and The Energy Research and Development Administration

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued July 1978
National Bureau of Standards Special Publication 400-36

Nat. Bur. Stand. (U.S.), Spec. Publ. 400-36, 78 pages (July 1978) CODEN; XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1978

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

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(Add 25 percent additional for other than U.S. mailing).
TABLE OF CONTENTS

SEMICONDUCTOR MEASUREMENT TECHNOLOGY

PAGE

List of Figures iv

List of Tables v Preface vi

Abstract and Key Words 1

1. Introduction 1

2. Highlights 2

3. Materials Characterization by Electrical Methods 7 3.1. Four-Probe Method 7 3.2. Spreading Resistance Methods 7 3.3. Capacitance-Voltage Methods 18

3.4. Hole Mobility in p-Type Silicon 18

3.5. Indium-Doped Silicon 19

3.6. Thermally Stimulated Current and Capacitance 20

3.7. Extended-Range MIS C(V) Method 24

4. Materials Characterization by Physical Analysis Methods 26

4.1. Calibration Standards for Ion Microprobe Mass Analysis 26

4.2. Electron Spectroscopy Techniques 30

4.3. Techniques for Chemical Diagnostics in Semiconductor Processing 33

4.4. Nuclear-Track Technique 36

4.5. Optical Test for Surface Quality of Sapphire Substrates 37

5. Materials and Procedures for Wafer Processing 40

5.1. Ion Implantation Parameters 40

6. Photolithography 42

6.1. Optical Imaging for Photomask Metrology 42

7. Test Patterns 45

7.1. Cross-Bridge Sheet Resistor 45

7.2. Advanced Planar Silicon Test Structures 46

7.3. Test Pattern Design and Analysis for SOS/LSI 48

8. Assembly and Packaging 49

8.1. Correlation of Moisture Infusion, Leak Size, and Device Reliability 49

8.2. Loose-Particle Detection 52

9. Device Inspection and Test 54

9.1. Scanning Acoustic Microscopy 54

9.2. Thermal Properties of Transistors 57

9.3. Integrated Circuit Die-Attachment Evaluation 59

10. References 60

Appendix A Semiconductor Technology Program Staff . . - 64

Appendix B Semiconductor Technology Program Publications . 65

Appendix C Solid-State Technology & Fabrication Services 70

Index 71

iii
LIST OF FIGURES

PAGE

1. Typical calibration relations between measured spreading resistance and

resistivity of silicon for selected cases 9

2. Spreading resistance, Rsp, measured as a function of probe spacing, s, for

three boron-diffused wafers with different sheet resistances 10

3. Profiles constructed by superposition of multiple uniform layers 12

4. Conductivity profiles for selected diffused layers 15

5. Comparison of traces from high-speed dragging-stylus and conventional

stepping-type spreading resistance measurements in two orthogonal directions on an n-type silicon slice with an average resistivity of 2.5 fi'cm

and a (111) surface orientation 16

6. Preliminary results of hole mobility calculations in boron-doped silicon

at 300 K 19

7. Thermally stimulated current and capacitance response of the gold donor in

p-type silicon at various heating rates 20

8. Scaled Arrhenius plot of hole emission probability, ep/T^, as a function of

inverse temperature for the gold donor in p-type silicon . 22

9. Typical thyristor structure 24 10. Cross section of mesa diode test structure 24 11. Composition profiles of metal-silicon dioxide interfaces by Auger-sputter

techniques 31 12. Broadening of a step interface resulting from the effects of electron escape

depth 32 13. Interface state density, Nss» as a function of carbon concentration at the

interface 35}

14. Resistivity of p-type silicon as a function of boron density as determined

by the nuclear-track technique 37

15. Correlations between optical and x-ray determinations of the quality of a sapphire surface and the corresponding deposited silicon film 38

16. Illustration of implanted random-equivalent and channeled depth distributions and comparisons with ideal amorphous and perfectly channeled distributions, respectively 41

17. Fraction of implanted ions outside ideal distributions as a function of the

critical angle for channeling, 41

18. Calculated image profiles of a 10-ym wide clear line for an optical microscope with a constant 2X of spherical aberration and defocus varying in

steps of A/4 from 0 to -2X, 0.22 NA condenser and 0.65 NA objective, illumination wavelength of 560 nm, and a 0,13-vim wide scanning slit 43

19. Experimental image profiles of one edge of a nominally 10-pm wide clear line

for an optical microscope with white light and filtered light at 560 nm, 0.60

NA condenser and 0.90 NA objective, and 0.13-ym wide scanning slit 43

20. Comparison of theoretical and experimental image profiles of clear lines

for an optical microscope with 0.60 NA condenser and 0.90 NA objective,

illumination wavelength of 560 nm, and 0.13-ym wide scanning slit 43

21. Schematic diagram of the cross-bridge sheet resistor 45

22. Cross-sectional view of a diffused region showing various dimensions 46

23. A simple equivalent circuit model for the bridge sheet resistor 46

24. Normalized width error, E, anticipated from measurements on a bridge test structure 46

iv
LIST OF FIGURES

PAGE

25. Schematic diagrams of advanced test structures 47

26. Hybrid test packages for use in the moisture infusion study 50

27. Typical calibration data for oxide moisture sensor 50

28. Typical dew-point sensor response curve of leakage current as a function

of substrate temperature 51

29. Average response of three driven acoustic emission transducers A, B, and

C attached to driving acoustic emission transducer by couplant K, L, or

M, as a function of driving frequency 52

30. Response of three acoustic emission transducers A, B, and C attached to driving acoustic emission transducer by couplant K, L, or M and driven at 150 kHz, as a function of number of removal and remounting cycles 53

31. Scanning acoustic and optical photomicrographs of a portion of a C-MOS-onsapphire integrated circuit 54

32. Variations in intensity of reflected pulsed acoustic signal as the lens-tospecimen spacing is changed 56

33. Scanning acoustic micrographs of a 300-nm thick gold film grid pattern on silicon at various lens-to-specimen distances 56

34. Effect of sub-surface void on intensity of acoustic signal reflected from

an aluminum film on silicon dioxide 57

35. A schematic representation of the thermal instability and second breakdown

limits of safe power transistor operation in the forward-biased region 58

36. Curves of emitter-base voltage and its time derivative as a function of time

following removal of power 58

37. Simplified schematic of circuit for measuring time derivative of emitterbase voltage 59

LIST OF TABLES

PAGE

1. Sheet Resistance of Three Boron-Diffused Wafers 11

2. Characteristics of Diffused Layered Structures Used for Spreading Resistance

Measurements 1^

3. Performance Characteristics of New Thermally Controlled Chuck 22

4. Capacitance Measurements on SOS Specimens Obtained with Guarded and Unguarded

Electrodes 25

5. Normalized Sputtering Rate of an Ion Microprobe Mass Analyzer Operated with a 19.4-keV 02''" Beam and a Nominal Raster Size of 125 ym by 100 pm 28

6. Calibration Data for Ion Microprobe Mass Analysis of Phosphorus-Doped Silicon

Based on Integral Ratio Method 29

7. Preliminary Results of Auger-Sputter Profiling Measurements on CVD and Dry

Thermal Oxides

8. Oxide Sensor and Dew-Point Determination of Moisture Content of Sealed-Off

Test Packages

9. Response of Test Transducers at 150 kHz Obtained Using Viscous Liquid

Couplant M

V
PREFACE

The Semiconductor Technology Program serves

to focus NBS research to enhance the performance, interchangeability , and reliability

of discrete semiconductor devices and integrated circuits through improvements in measurement technology for use in specifying

materials and devices in national and international commerce and for use by industry in

controlling device fabrication processes.

This research leads to carefully evaluated

and well-documented test procedures and as- sociated technology. Special emphasis is

placed on the dissemination of the results

of the research to the electronics community. Application of these results by industry will contribute to higher yields , lower

cost, and higher reliability of semiconductor

devices. Improved measurement technology also leads to greater economy in government

procurement by providing a common basis for

the purchase specifications of government

agencies, and, in addition, provides a basis

for controlled improvements in fabrication

processes and in essential device characteristics .

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

the Defense Advanced Research Projects Agency (ARPA)* and the National Bureau of Standards (NBS)."^ In addition, the Program receives support from the Defense Nuclear Agency (DNA) , § the Air Force Space and Missiles

Systems Organization,^ the Navy Strategic

Systems Project Office,^'' and the Energy Research and Development Administration.^ The

ARPA-supported portion of the Program, Advancement of Reliability, Processing, and

Automation for Integrated Circuits with the

National Bureau of Standards (ARPA/IC/NBS)

,

addresses critical Defense Department problems in the yield, reliability, and availability of digital monolithic integrated circuits . Other portions of the Program emphasize aspects of the work which relate to the

specific needs of the supporting agency.

Measurement-oriented activity appropriate to the mission of NBS is an essential aspect in

all parts of the Program.

Essential assistance to the Program is also

received from the semiconductor industry

through cooperative experiments and technical

exchanges. NBS interacts with industrial users and suppliers of semiconductor devices

through participation in standardizing organizations; through direct consultations with device and material suppliers, government agencies, and other users; and through periodically scheduled symposia and workshops. In addition, progress reports, such as this one, are regularly prepared for issuance in the NBS

Special Publication 400- sub-series. More de- tailed reports such as state-of-the-art reviews, literature compilations, and summaries

of technical efforts conducted within the Pro- gram are issued as these activities are completed. Reports of this type which are published by NBS also appear in the Special Publication 400- sub-series.

Another means of interaction with the electronics community is by direct contact. In particular, comments from readers regarding

the usefulness of the results reported herein

or relating to directions of future activity

in the Program are always welcome.

Disalaimer'

Certain commercially available materials or instruments are identified in this publication for the purpose of providing a complete

description of the work performed. The experiments reported do not constitute a complete

evaluation of the performance characteristics

of the products so identified. In no case does such identification imply recommendation

or endorsement by the National Bureau of Stan- dards nor does it imply that the items identified are necessarily the best available for

the purpose.

" Through ARPA Order 2397, Program Code TDIO ; (NBS Cost Center 4257555), Unless otherj wise noted, work was funded from this source

,

t Principally through the Electronic Technology Program (Cost Center 4257100), Additional funding through the Dimensional Metrology Program (Cost Center 2131119) and

the Nondestructive Evaluation Program (Cost

Center 4253130),

§ Through Inter-Agency Cost Reimbursement Order 7T-812 (NBS Cost Center 4259522),

H Through MIPR FY76 167600366 (NBS Cost Center 4259560).

# Code SP-23, through project order N00164-

76-MP-04511 administered by Naval Weapons

Support Center, Crane, Indiana (NBS Cost

Center 4251533).

X Through ERDA Contract No. E(49-l)-3800

,

Modification 3 (NBS Cost Center 4259561).

vi
SEMICONDUCTOR MEASUREMENT TECHNOLOGY

PROGRESS REPORT

July 1 to September 30, 1976

Abstract: This progress report describes NBS activities directed toward the development of methods of measurement for semiconductor materials, process control, and devices. Both in-house and contract efforts are included. The emphasis is on silicon device technologies. Principal accomplishments

during this reporting period included (1) completion of the systematic study of the effects of surface preparation and probe material on the empirical calibration between specimen resistivity and spreading resistance of n- and p-type silicon; (2) initial evaluation of the nuclear-track technique for quantitative determination of trace amounts of boron in silicon; (3) development of procedures for using an optical research microscope to make accurate measurements of the width of a clear line as narrow as 0.5 um in a completely or nearly opaque background; (4) design of a compact cross-bridge test structure

for electrical measurement of line width and sheet resistance in minimum line-width geometries; (5) completion of the initial phase of the study of the particle-impact noise detection test for screening

devices for the presence of loose particles in the package; (6) demonstration of a greater-thanexpected line resolution capability for the scanning acoustic microscope; and (7) development of a non- destructive technique to measure the onset of second breakdown in forward-biased, medium-power transistors. Also reported is other ongoing work on materials characterization by electrical and physical

analysis methods, Tiaterials and procedures for wafer processing, photolithography, test patterns, and device inspection and test procedures. Supplementary data concerning staff, publications, and technical services are included as appendices.

Key Words: Auger electron spectroscopy; capacitance-voltage methods, dew-point sensing; dragglngstylus probe; electrical properties; electronics; four-probe method; hermetlcity; hole mobility; infrared reflectance; ion implantation; ion microprobe mass analysis; line-width measurements; nuclear-track

technique; particle-impact noise detection; photolithography; photovoltaic method; p-n junction; power- device grade silicon; resistivity; resistivity variations; resistors, sheet; safe operating area, tran- sistor; scanning acoustic microscope; second breakdown; semiconductor materials; semiconductor process

control; s iiicon; silicon dioxide; silicori-on-sapphire ; spreading resistance; test patterns; test structures; thermally stimulated current and capacitance; thermal properties, transistor; thermal response; thyrlstors; transistors, power; ultraviolet reflectance; x-ray pbotoelectron spectroscopy.

1 . INTRODUCTION

This is a report to the sponsors of the Semiconductor Technology Program on work carried

out during the thirty-third quarter of the

Program. The report summarizes research on a wide variety of measurement methods for semiconductor materials, process control, and devices that are being studied at the National

Bureau of Standards. The Program, which em- phasizes silicon-based device technologies,

is a continuing one, and the results and con- clusions reported here are subject to modification and refinement.

The work of the Program is divided into a num- ber of tasks, each directed toward the study

of a particular material or device property

or measurement technique. This report is subdivided according to these tasks. Highlights

of activity during the reporting period are given in section 2. This section provides a management-level overview of the entire effort. Subsequent sections deal with each

specific task area. References cited are listed in the final section of the report.

The report of each task includes a narrative

description of progress made during this re- porting period. Additional information con- cerning the material reported may be obtained directly from individual staff mem1

bers identified with the task in the report.

Organizational locations and telephone numbers for Program staff members are given in

Appendix A.

Background material on the Program and individual tasks may be found in earlier progress reports as listed in Appendix B. From

time to time, publications are prepared that

describe some aspect of the program in

greater detail. Current publications of

this type are also listed in Appendix B. Reprints or copies of such publications are usually available from the author on request.

In addition, tutorial videotapes are being

prepared on selected measurement topics for

dissemination to the electronics community.

Currently available videotapes and procedures

for obtaining them on loan are also listed in

Appendix B.

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 ser- vices that cannot be obtained through normal

commercial channels. To indicate the kinds

of technology available to the Program, such

services provided during the current reporting period are listed in Appendix C.
2 . HIGHLIGHTS

Highlights of progress in the various technical task areas of the Program are listed in

this section. Particularly significant ac- complishments during this reporting period

included

(1) completion of the systematic study

of the effects of surface preparation and

probe material on the empirical calibration

between specimen resistivity and spreading

resistance of n- and p-type silicon;

(2) initial evaluation of the nucleartrack technique for quantitative determination of trace amounts of boron in silicon;

(3) development of procedures for using

an optical research microscope to make accurate measurements of the width of a clear

line in a completely or nearly opaque background or of an opaque line in a clear background down to 0.5 ym;

(4) design of a compact cross-bridge

test structure for electrical measurement of

line width and sheet resistance in minimum

line-width geometries

;

(5) completion of the initial phase of

the study of the particle-impact noise detection test for screening devices for the

presence of loose particles in the package;

(6) demonstration of a greater-thanexpected line resolution capability for the

scanning acoustic microscope; and

(7) development of a nondestructive

technique to measure the onset of second

breakdown in forward-biased, medium-power

transistors

.

New tasks were undertaken to investigate

techniques for characterizing extrinsic silicon for use in infrared detector arrays, to evaluate the application of x-ray photoelectron spectroscopy to diagnostic measurements

in semiconductor device processing, and to develop test structures with integral signalprocessing circuitry to allow high-speed measurement of small currents. The work on the

high-speed, dragging-stylus spreading resistance instrument and instrumentation for measurement of capacitance and conductance at applied voltages of up to 25 kV was completed.

Work on development of techniques for measuring trace impurities in process chemicals

(such as diffusion sources and carrier gases) was terminated because available measurement

techniques proved to be too insensitive for

conclusive experiments.

Unless another organization is identified,

the work described in the following paragraphs was performed at the National Bureau

of Standards.

Materials Chavaotevization hy Eleotrioal Methods — Work continued on the study of correction factors for four-probe resistivity measurements on slices of small diameter and intermediate thickness. The results of measurements on a 5/8-in. (15.9-mm) diameter,

10-n*cm, p-type silicon slice suggest that

conditions may exist for which it is not valid to use as an overall geometric correction

factor the product of the individual thickness and diameter factors, as is assumed in

the standard four-probe method.

The first phase was completed of the study of

the effects of specimen surface preparation

and probe material on the empirical calibration between specimen resistivity and spreading resistance. The calibration relation was

found to depend on both surface preparation

and probe type for n-type silicon; no probe

dependence was found for p-type silicon.

Nearly linear relationships between spreading j resistance and resistivity could be obtained \ on p-type silicon with (111) surface orienta- i tion and, for certain probe materials, on n- | type silicon with (100) surface orientation. I For no combination of surface preparation and

probe materials studied could a linear relationship be obtained for n-type silicon with

(111) surface orientation.

Preliminary evaluations of simplified , correction-factor algorithms for rapid calculation of resistivity or dopant density pro- I files from spreading resistance measurements

on beveled sections of semiconductor structures have been completed at Solecon Laboratories. Algorithms based on the local slope

of the measured spreading resistance profile were found to give satisfactory results un- | der certain conditions. [

In work at RCA Laboratories , conditions were li found which permit reasonable probe life in > the high-speed, dragging-stylus apparatus | for measuring spreading resistance. Results j of measurements along the diameter of a heav- f ily striated silicon slice demonstrated that i resolution comparable with that of a conven- I tional stepping-type instrument could be obj tained with the high-speed instrument in

j l/20th of the time. |

i
HIGHLIGHTS

A new method was developed to extract background dopant densities in the range from

10^^ cm"^ to about 10^^ cm~^ from junction

capacitance-voltage data. This method is

[ intended for use in cases in which the juncI tion is prepared by diffusion into a uniformi ly doped substrate. In the method, it is as-

! sumed that the diffused layer profile in the

region of the junction is approximately Gaussian. In addition, work is continuing on the

I examination of the range of validity of the

Schottky equat ions , which are commonly used

to extract dopant density from capacitancevoltage data.

The methods for developing theoretical expressions for carrier mobility in silicon as a

I function of temperature and dopant density

I are being extended to cover the case of hole

mobility in p-type silicon. In this case,

I there is an additional complication because

of the complexity of the valence band structure. Initial work is being concentrated on boron-doped silicon; silicon doped with other

acceptor impurities will have slightly different mobility-versus-dopant density characteristics because of differences in fractional

ionization occurring in the dopant density

' range from 10^^ to 10^^ cm~^ even at room

I temperature.

A new task was initiated to study methods for

characterizing extrinsic silicon for use in

infrared imaging arrays. Initial efforts are

I being concentrated on evaluation of various

I techniques to detect extraneous levels in

indium-doped silicon; particular emphasis is

being placed on study of the so-called Xlevel, which appears to be an acceptor state

located between the indium level and the valence band. The presence of this level at concentrations considerably lower than the indium concentration reduces the performance of

I indium-doped infrared detectors and makes it

necessary to operate them at lower temperatures to obtain the best signal-to-noise ratio.

• Measurements of thermally stimulated current

and capacitance were made on a gold-doped ptype silicon MOS capacitor. As expected from

theoretical considerations, the phase I therjl mally stimulated current and capacitance re- sponses of the gold donor center in this

|l structure were the same as responses of the

|i center in an n^p junction diode. In addition,

I measurement procedures were established to characterize defect centers by analyzing

! their transient capacitance response under

' isothermal conditions.

Progress was made toward all three of the objectives of the task to evaluate the use of

thermally stimulated current and capacitance

measurements as a means for characterizing

defects in power device-grade silicon wafers.

An improved thermally controlled chuck and

automatic wafer prober was assembled and

tested; the characteristics of this apparatus meet or exceed all design targets. A processing sequence, in which all temperatures are AOO°C or less, was selected for

fabricating MOS capacitors for making thermally stimulated current and capacitance measurements. Initial steps were taken toward

defining procedures for fabricating test structures which provide access to both sides

of the p-n junction in a partially or com- pletely fabricated thyristor structure.

Development and initial evaluation of the 25-

kV system for capacitance and conductance measurements as a function of voltage was com- pleted at RCA Laboratories. This system permits application of the extended-range

capacitance-voltage technique to metalinsulator-semiconductor structures with insulator thickness in excess of 130 ym.

The automated data collection and analysis

system for making photovoltaic measurements

to determine resistivity variations along the

diameter of circular slices was assembled and

is being tested. This system is Intended to be used to measure slices suitable for fabrication of high-power thyristors and rectifier

diodes. The system allows a user to measure

the resistivity profile along a slice diameter in about 2 min with some user-system interaction after the measurement run has begun.

Although measurement of the average resistivity by the van der Pauw technique has not yet

been automated, this measurement can routinely be made using the same probes as in the

automated system to make contact to the slice.

Materials Characterization by Physiaal Analysis Methods — Development of calibration

standards for ion microprobe mass analysis

continued at Texas Instruments with the com- pletion of the analysis of the silicon specimens implanted with phosphorus. The relative

sample standard deviation of the calibration

coefficient (ratio of average phosphorus density to instrument signal) for 18 profile

measurements made on four slices over a period of several months was 25 percent; this is a measure of the long-term reproducibility of

ion microprobe mass analysis to be expected

when measuring phosphorus in silicon.

3
HIGHLIGHTS

The study of the effects of various experimental conditions on Auger-sputter profiling of

oxide-silicon structures continued at Stanford University. In order to avoid effects

of beam asymmetry, procedures were developed

for aligning the ion beam employed to sputter

etch the crater produced in carrying out depth-profiling measurements with Auger electron spectroscopy. Measurements of ion stimulated Auger emission were completed to the

point of defining the possible interferences

which this process might produce in profiling work; it was found that negligible interference from this mechanism occurs at a primary ion energy of 1 keV, which has been

found on other grounds to produce the best

depth resolution in the silicon dioxidesilicon system. Initial work in the study of

limitations on the resolution of interface

width in Auger-sputter profiling were concentrated on the effects of finite electron escape depth; an extension to include the effects of ion knock-on mixing is also under

development. Preliminary Auger electron spectroscopy measurements were made on interfaces

between pure and doped aluminum metal films

sputtered onto thermally grown silicon dioxide. Work has also been undertaken to study

the relationships between the microscopic

properties (such as width and stoichiometry)

of silicon-silicon dioxide interfaces, the

electrical characteristics of the interfaces,

and the methods of preparing the oxide films.

Preliminary results indicate that the interfaces between silicon and oxides prepared by

chemical vapor deposition are 2 to 2.5 nm

wider than interfaces between silicon and

"good" thermal oxides.

A new task was undertaken at the Jet Propulsion Laboratories to evaluate the application

of x-ray photoelectron spectroscopy to diagnostic measurements in semiconductor device

processing and to establish and document appropriate methods for specimen preparation

and data collection, analysis, and interpretation. Instrumental modifications were made

to improve the ability to determine binding

energies referenced to a known potential for

species being examined; spectral shifts which

resulted from specimen surface charging and

which could be misinterpreted as chemical

shift variations had been observed. The effects of residual carbon on interface-state

density were investigated; it was tentatively concluded that different chemical forms

of carbon at the silicon interface show different retention ratios after oxidation and

that both the final amount and chemical

state of carbon at the interface significantly affect the interface state density. A

significant effort has been devoted to exami4

nation of data-reduction algorithms. A number of algorithms have been developed which

give varying degrees of recovery of original

information in the presence of asymmetric

Gaussian instrument functions. Similar algorithms have been developed to treat the problem of recovering signal from noise.

Boron cannot be detected by conventional neutron activation analysis, and most chemical

techniques are not sufficiently sensitive to detect trace concentrations. Therefore, alternative techniques are necessary. Work was begun to evaluate the applicability of the

nuclear-track technique to the determination

of trace amounts of boron in silicon slices.

Preliminary results suggest that a boron density of about 10^^ cm~^ is the lower limit

for routine measurements by the technique;

good agreement with various electrical techniques was obtained in a variety of cases.

Work on a rapid, nondestructive reflectance

method for determination of surface quality

of sapphire substrates and the characteristics of silicon films epitaxially grown on sapphire continued at RCA Laboratories. Good

correlation was obtained between the surface

quality of the substrate as determined by infrared reflectance, the quality of the epitaxial film as determined by ultraviolet reflectance, and crystalline quality of the

film as determined by x-ray diff ractometry

.

The reflectance method has the potential of

providing a quantitative measure of substrate

surface quality which could be used to monitor the amount of polishing of substrates.

Reduction of the amount of polishing required,

with consequent substantial cost savings,

could result. | Materials and Proaedures for Wafer Proaessing — Development of measurement technology for

critical ion implantation parameters continued at Hughes Research Laboratories. A study was made of the effects of variations in the

angle between ion beam and crystallographic

axes of the wafer being implanted on both

random-equivalent and channeled-implantation

depth distributions in silicon. It was found | that in implantation of low-velocity ions

(ions with high atomic number, or low energy, or both) , a significant channeling tail can exist when the implantation is conducted at the commonly employed randomization angle.

On the other hand, accurately channeled distributions can only be achieved with a high

degree of angular control when high velocity

ions are being implanted; much less control

is required if low velocity ions are being im-l planted

.

[,
HIGHLIGHTS

I Pho toli thography — Additional investigations

I were completed of the effects of the operating conditions of an optical microscope on line-width measurements in transmitted illumination. In particular, the effects of a finite transmittance of the opaque background

and of spherical aberration and defocus were

studied. The optical and mechanical performance characteristics of equipment required

for accurate measurement of the width of photomask lines as narrow as 0.5 ym have been defined. The NBS optical microscope system

can presently measure line widths to within

an uncertainty of ±0.2 pm; if the scan is re- peated at the same position on the line, the

the uncertainty is less than ±0.05 ym. It appears that the edge quality of the line

may be the limiting fi.ctor in the accuracy

and repeatability of the measurement of the

width of lines narrower than 10 pm.

Test Patterns — A new test structure which

incorporates both a cross-type van der Pauw

' sheet resistor and a conventional bridge-type

1 sheet resistor into a single unit was de-

] signed. Both the width of the bridge structure and the sheet resistance can be obtained

from electrical measurements on this test

j structure, which requires only six contact

I pads. The results of a preliminary analysis

I suggest that the structure is suitable for

ij making measurements on the widths of the

I smallest lines occurring in the pattern.

j A new task was undertaken at the Westing-

' house Defense and Electronics Systems Center

jj to determine the operational limitations and

il potential applications of integral electromeI ter amplifiers in specialized circuits used

I for the measurement of small currents in

I times compatible with use in high-speed auto-

! matic testers.

I Additional measurements have been made to study the characteristics of individual structures in the SOS/LSI test pattern being deI veloped at RCA Laboratories. One result of

general interest is that the cross-type van

der Pauw sheet resistor test structure, which

is present in a number of devices in this pattern, was shown experimentally to be appropriately treated as a simple van der Pauw resistor.

1 Assembly and Paakaqing — Additional studies

I were carried out in efforts to calibrate the

dew-point moisture sensors being used in the

investigation of the correlations between

moisture infusion, leak size, and device re- liability at Martin Marietta Aerospace. Although consistent results were not obtained.

adequate calibration may be possible by use

of appropriate test conditions.

Additional studies were completed on selected

aspects of the particle-impact noise detection test for screening devices for the presence of loose particles in the package. Principal emphasis was placed on the determination of the effectiveness of various cou- plants for transmitting the high-frequency vibrations generated by the impact of a particle against the package wall. At this stage

of the investigation it appears that, of the

three couplants recommended in the test procedure being proposed for inclusion in MILSTD-883, the liquid couplant is most effective in transmitting acoustic emission signals and provides most repeatable results.

Devioe Inspeation and Test — Investigations

at Stanford University and Hughes Research

Laboratories of the scanning acoustic microscope for examining semiconductor devices and

integrated circuits were directed primarily

toward examination of a variety of test specimens in order to establish an improved understanding of the information which can be obtained with this instrument. The microscope

was found to be as easy to operate as an optical microscope; its simple controls and lack

of a vacuum system render it more convenient

to operate and maintain than a scanning electron microscope. Moreover, it is superior to the scanning electron microscope for the non- destructive examination of insulating surfaces because charge does not accumulate and

degrade the image. Edges and material differences can be displayed with greater clarity than with either optical or scanning

electron microscopes. Various levels in an integrated-circuit structure can be displayed in a series of micrographs readily by

making small changes in the lens-to-specimen

spacing. The microscope has a resolution ca- pability greater than expected, about 1/3 of

a wavelength at 10-percent contrast. The microscope has also been successfully utilized

to view gallium arsenide field-effect and

transferred-electron devices, aluminum fuse

links, and clad and unclad optical fibers.

The predictions of a physical model which was proposed to explain the relationship between

thermal instability and second breakdown in

forward-biased medium-power transistors were

confirmed experimentally. A method for non- destructive determination of safe-operatingarea limits based on the exclusion of current: constrictions and hot spots was developed.

This method is also suitable as a nondestructive test for second breakdown under condi5
HIGHLIGHTS

tlons in which the limits of thermal instability and second breakdown nearly coincide.

Work has resumed on the investigation of

methods for using the temperature response of

a substrate diode for determining the integrity of the die attachment of integrated circuits. Initial measurements on devices

bonded to headers with intentionally introduced voids indicate that dc methods are not

sufficiently sensitive. Pulse-heating and

time-delay measurement methods are expected

to provide the required sensitivity.

6
3. MATERIALS CHARACTERIZATIO

BY ELECTRICAL METHODS

3.1. Four-Probe Method

Work continued on the study of correction

factors for four-probe resistivity measurements on slices of small diameter and intermediate thickness (NBS Spec. Publ. 400-25,

pp. 7-8). The results of measurements on a 5/8-in. (15.9-mm) diameter, lO-fi'cm, p-type

silicon slice suggest that conditions may ex- ist where it is not valid simply to combine

the thickness and diameter correction factors multiplicatively as is assumed in the

standard four -probe method [1].

For slices of finite thickness but infinite

lateral extent, the resistivity is given by*

^

2irs

-p -p ^ G7(w/s) sp T I (1)

where V is the potential difference between

the inner pair of probes, I is the current

through the outer pair of probes, s is the

average probe spacing, w is the thickness of

the slice, F = 1 + 1.25[1 - (so/s)], so is sp the spacing between the inner pair of probes.

F^ = 1 - C^(T - 23), is the temperature

coefficient of resistivity [1,2], T is the

temperature, in degrees Celsius, of the slice

at the time of measurement, and GyCw/s) is a correction factor [3] (NBS Spec. Publ. 400-

25, table 1).

For infinites imally thin slices of finite

diameter, the resistivity as measured at the

center of the slice is given by

p =.wF2F^pF^- (2)

where F2 is a correction factor for diameter

[1,4] and, now, F = 1 + 1.082[1 - (sa/s)].

sp

If Smits' correction factor F(w/s) [5], is

used to correct eq (2) for finite thickness

[1] , one obtains"^

p = WF2 F(w/s) FgpF^ Y

(3)

Since F(w/s) = 2 ln2 s/[w G7(w/s)], the re- sistivity which is calculated from eq (1) is

Tr/(F2*ln2) times that calculated from eq (3),

provided that the difference in probe spacing

correction factors, F^ , is ignored.

For w >> s, the value of G7(w/s) approaches 1, so that the resistivity calculated by eq (1),

which is applicable for large diameter slices,

should be independent of thickness; for w/s > 8, GyCw/s) differs from unity by less than

0.2 percent. At the other extreme, where w << s, the resistivity calculated by eq (3)

should be independent of thickness. If the

voltage-current ratio is measured on a uniform slice as a function of slice thickness,

the resistivity calculated from either eq (1) or eq (3) should approach constant values at both large and small values of w/s; these values should differ by |1 - [tt/ (F2 • ln2) ] | which

ranges from 1.4 percent for slice diameter

D = 25s to 8 percent for D = 10s. Measurements on 1-in. (25-mm) diameter slices (NBS

Spec. Publ. 400-25, fig. 1) followed this ex- pected behavior. However, the more recent

measurements on 5/8-in. (15.9-mm) diameter

slices failed to approach a constant value

for w/s as small as small as 0.24.

This result could arise from tne presence of

longitudinal resistivity variations in the

crystal or from failure of the multiplicative

approximation for combining the thickness and

diameter correction factors in eq (3).

(J. R. Ehrstein and D. R. Ricks)

3.2. Spreading Resistance Methods

Surface Preparation Effects — The first

phase was completed of the study of the effects of specimen surface preparation and

probe material on the empirical calibration

between specimen resistivity and spreading re- sistance. The principal intent of this

phase of the study (NBS Spec. Pubis. 400-25,

pp. 8-12, and 400-29, pp. 9-11) was to determine the extent to which surface preparation

and probe material affect the reproducibility and linearity of the calibration for se-

'This equation is the one which was actually

used to calculate the solid data points in

figure 1 of NBS Spec. Publ. 400-25; the fac- tor G7(w/s) was erroneously omitted in the

original discussion of that figure.

^This equation is identical with eq (2d) of

NBS Spec. Publ. UOO-25 which was used to ob- tain the open data points of figure 1 of

that report. In eq (2d), F2 is designated

as C.

7
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

lected low-index planes of both n- and ptype silicon. Without reproducibility, a calibration curve has little meaning; if the

calibration relation is nonlinear, the number of data points required to define the

calibration curve is increased.

The present data were taken at probe loads

of 440 mN (1 gf = 9.8 mN), which is higher

than is generally used for depth profiling applications; such loads are typical of radial

profiling applications. These data also provide an important background for work at lighter loads for which experience indicates

a pronounced increase in measurement scatter

will be seen.

For each surface preparation used, measurements were taken with five sets of probes

made from four different alloys. While it

is difficult to assure that the correct

amount of material was removed for each preparation cycle so that the surface treatments

used give results typical of the intended

surface type, the following steps were taken

to aid in obtaining the most representative

data

(1) specimens of like orientation and

conductivity type were mounted in sets on a common block with the oriented face upward

and arranged so that the resistivity was a somewhat random function of position;

(2) the order in which the probes were

used for any surface preparation was ran- domized ; and

(3) fresh surfaces were prepared each

time a different set of probes was used to make the measurement.

The calibration relation was found to depend

both on surface preparation and probe set

used for n-type silicon; there was no probe

dependence for p-type silicon. Strong nonlinearities were observed for all orientations of n-type silicon with lapped surfaces

and for (111) n-type silicon with mechanically polished surfaces; smaller nonlinearities

have also been observed for lapped surfaces

of (111) p-type silicon. Such nonlinearities have been modeled in terms of barrier

effects at the metal-semiconductor spreading resistance contact [6,7]. Results are presented below for the three major classes

of silicon conductivity type and surface

orientation [8]

.

The simplest case is that of p-type silicon

with (111) surface orientation. As shown in

figure la, nearly linear relations between

spreading resistance and resistivity are obtainable on surfaces of specimens with resistivity above 0.01 fi*cm either by polishing in

aqueous media followed by thermal treatment

(bakeout) or by polishing in nonaqueous media

The nonlinearity observed for lower resistivity specimens is believed to be due to a com- bination of power-supply limitations and se- ries resistance effects.

The effect of specimen preparation on (100)

n-type silicon is nearly as well defined as that described above. Results from probes

with relatively low average spreading resistance, osmium or tungsten-osmium, show nearly

linear responses with resistivity for all sur- face conditions as illustrated for tungstenosmium in figure lb. Data from the other two probe sets, tungsten carbide and tungstenruthenium, tend to show slight deviations

from this linear behavior. Since these alloys have higher average spreading resistance

and smaller areas of contact than the osmium

and tungsten-osmium probes, the anomalous results cannot be explained in terms of alloy

composition alone. Results, illustrated in

figure Ic, obtained with tungsten carbide

probes are consistent with a residual barrier effect of a type which is more prominent on lapped surfaces of n-type specimens

for all probe materials. The tungstenruthenium probe data exhibit some of the same apparent barrier effect and, on the whole,

provide the most erratic results of all the

probe sets in terms of local scatter about

the general resistivity.

The situation for (111) n-type silicon is

much more complex. For no combination of

polishing preparation and probe material do

the resultant data show an absence of the

barrier-type nonlinearities so prominent on lapped specimens. The form of the resistivity dependence has a strong dependence on the

probe set used. The higher the average

spreading resistance for a given probe set,

the larger are the deviations from linearity

attributable to barrier effects, regardless

of the polishing procedure used. Tungstenruthenium probes were in most frequent need

of probe reconditioning, and the data ob- ; tained with these probe materials are the

most erratic of any taken. The plots of figures Id and le exhibit surface-preparation

dependence for two sets of probe materials —

tungsten- osmium and tungsten carbide; data

from other probe materials (except for

tungsten-ruthenium) generally fall within

the bounds loosely defined by these plots.

For chem-mechanically polished surfaces , the

deviations from linear behavior are reduced

8
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

b. n-type silicon, (100) orientation,

tungsten-osmium probes.

RESICTIVITY ;o-cm|

a. p-type silicon, (111) orientation,

tungsten-osmium probes.

I

' ' '""I TTTi 1—r0.1 1.0 RESISTIVITY (o-cm|

0.1 1.0 RESISTIVITY lo-cinl

c. n-type silicon, (100) orientation,

tungsten carbide probes.

d. n-type silicon, (111) orientation,

tungsten-osmium probes.

O.QOI 0.01 0.1 1.0 10

RESISTIVITY lo-cmj

e. n-type silicon, (111) orientation,

tungsten carbide probes.

Figure 1. Typical calibration relations between measured spreading resistance and resistivity of silicon for selected cases..

9
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

for all probes by use of the bakeout cycle.

For specimens mechanically polished in an aqueous medium, barrier nonlinearities were

noticeably more in evidence than for chemmechanically polished specimens and in general were centered at a somewhat higher resistivity; following a bakeout cycle, the non- linearities attributable to barrier effects were reduced, and drastic differences in the

responses of the several probe sets appeared.

For specimens mechanically polished in the

nonaqueous medium the results were in general agreement with those obtained on surfaces

polished in an aqueous medium after bakeout,

but additional changes in spreading resistance occurred when the surfaces polished in

a nonaqueous medium were baked.

(J. R. Ehrstein and D. R. Ricks)

Computational Algorithms for Depth-Profile

Measurements — This task was undertaken to develop simplified correction factors for

rapid calculation of resistivity or dopantdensity profiles from ' spreading resistance

measurements on beveled sections of semiconductor structures (NBS. Spec. Publ. 400-19,

p. 10). In this regard, certain simplifying

assumptions concerning the nature of the

spreading resistance measurement are made;

the quality of the resultant abbreviated algorithm is being tested in several ways [9].

Initial efforts were directed at using the

formalism of incremental sheet resistance

measurements [10] to extend the concepts applied to the analysis of thin uniform slabs

to the analysis of regions of nonuniform re- sistivity on nonconducting substrates. For

a two-probe configuration, the measured

spreading resistance, Rgp> of ^ thin uniform

slab of resistivity, p, on an insulating substrate is [11]

sp za TTt a

(4)

where F = (2a/7rt)ln(s/a) is the correction

factor for finite slab thickness, s is the

probe spacing, a is the effective contact

radius of the probe tips, and t is the slab

thickness. The factor p/t is the sheet re- sistance, R^, of the slab. If the spreading

resistance is measured as a function of probe

spacing and R is plotted as a function of

sp log s, a straight line results with a slope

equal to 0.733 and an (extrapolated) intercept (at R =0) where s = a. sp

To test the validity of eq (4) on thin, non- uniform layers, spreading resistance was measured as a function of probe spacing on three

thin diffused layers formed by diffusing boron into phosphorus-doped silicon. Plots of

the resulting data, with s on a logarithmic

scale as shown in figure 2, are linear and

have a common intercept at s " 2 ym. As indicated in table 1, reasonable agreement was obtained between the values of sheet resistance calculated from these plots and those

measured directly with a four-probe array.

These results are taken as confirmation of

the validity of using this form of the cor- rection factor in conjunction with measurements on thin, nonuniform isolated regions.

The experiment also provides a useful procedure for determining the effective contact

radius.

For profiling, one seeks the functional relationship between measured spreading resistance and resistivity or dopant density at all positions within the layer. The conductivity profile of a thin, nonuniform slab on

an insulating substrate can be represented

by that of N very thin layers each having a uniform conductivity incrementally different

800

500 -

9. 400

t; 300

200

too

T 1 I I I lll| I I I I 1 I I I I Ml|

o /

^

10 100

PROBE SPACING iMml

1000

Figure 2. Spreading resistance, Rsp, measured as a function of probe spacing, s, for

three boron-diffused wafers with different

sheet resistances. (The slope of the curve, • the ratio of ARgp to Alog s, is equal to 0.773 Rs, where Rg is the sheet resistance

of the layer; the extrapolated intercept,

Rsp ~ 0, occurs for s = a, the effective contact radius.)

10
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

Table 1 - Sheet Resistance of Three Boron -Diffused Wafers

Specimen

Surface Density, Junction Depth, Sheet Resistance, o. Sheet Resistance,

cm

-3

ym (Four-Probe Method) (Slope of Rgp vs. log s|

-^^1019

'V1019

'x.lQis

0.9

1.7

4.8

284

126

54.6

250

126

52

from that of the adjacent layers as shown in

figure 3a. If the total thickness of the

slab, t, is less than the contact radius, a, the contacts may be considered as cylinders

extending throughout the entire slab. Since

the conductive regions are in parallel, the

conductances add. The conductance between

the cylindrical contacts is given by

^0 ln(s/a) E

i=l

a . (z . - z . T

)

1 1 1-1 (5)

If the top layer is removed, the conductance

becomes

n

^ , . ^

^ a. (z. - z. s/a) JLj 1 1 x-1

i=2

^1 ln(s/a)

(6)

= -

0 ln(s/a) '^l ^^1 ^0^

This equation may be rewritten in terms of an arbitrary jth layer to yield an expression

for its conductivity

0 . = -

2

ln(s/a) AG.

t: Az . (7)

where AG. G. - G. ^ and Az . = z. - z. ^.

J J J-1 J 3 J-1

in the limit as Az tends to zero, eq (7) becomes

a(z) = - ln(s/a) dG

TT dz

(8)

This analysis can be extended to the case of

layers thicker than a contact radius [9]. In

the extended analysis, additional terms are present containing the second derivative of

the inverse correction factor, H(t) (NBS Spec.

Publ. 400-19, p. 10). As a first approximation, terms containing the second derivative

of H(t) are assumed to be negligible compared

with the right-hand side of eq (8) . Equation

(8) is used by first applying it to the deepest point of the layer being profiled (be- cause here the correction depends only on the

boundary condition and not on the nature of

the profile itself) and then working out to- ward the surface. It is therefore convenient

to shift the coordinate system so that the

origin is at the junction, that is, at the in- terface between layer and insulating substrate :

^ ln(s/a) dG

IT dC (9)

where ? = t - z, and t is the total layer

thickness

.

The concept of parallel superposition was

tested by calculating the correction factors

for a variety of three-layer structures and

comparing the results with correction factors

calculated from the Schumann-Gardner solution

[12], which is assumed co be exact. It was concluded [9] that the parallel superposition

approximation yields results which agree to within 10 percent only in a limited number of cases. It appears to be most satisfactory

for cases in which the conductivity decreases

as the insulating substrate is approached or

in which the thickness of the layer is less

than the effective contact radius.

At the other extreme, one can consider a relatively high-resistivity region on a lowresistivity substrate, again modeled in terms

of N very thin layers, as shown in figure 3b.

If the thickness, t, of the region is small,

the current travels through the highresistivity region to the substrate in a direction nearly perpendicular to the surface.

The resistance measured at the surface can

be approximated as the sum of the incremental

resistances, p^, in the cylinder below each

contact disk and the spreading resistance between the virtual contacts on the substrate

is given by

11
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

0 2, Z5 t

DISTANCE FROM SURFACE C,0

DISTANCE FROM JUNCTION

Conductivity profile of thin, nonuniform

slab on an insulating substrate.

00

00

0 Zi Z5 t z—

DISTANCE FROM SURFACE

DISTANCE FROM INTERFACE

b. Resistivity profile of thin, highresistivity region on low-resistivity

substrate

.

Figure 3. Profiles constructed by superposi

tion of multiple uniform layers.

^0= 2 2^ Pi —^2—+

s 2a (10)

i=l

where p is the resistivity of the substrate,

s If the top layer is removed, the new resistance will be smaller by the first term

in the sum:

Rq - = 2 (11)

As before, this equation may be rewritten in

terms of an arbitrary jth layer to yield an expression for its resistivity:

Tra^ AR.

2 Az. (12)

where AR. = R. - R. -, and Az. = z. - z.

J J J-1 J J J-1

In the limit as Az tends to zero, eq (12) becomes

P(z) = - na^ dR

2 dz (13)

It is again convenient to shift the coordinate system so that the origin is at the interface between the layer and the substrate,

at z = t, with the result that:

P(C) =

Tra^ dR

2 dc '

where !; = t - z

.

(14)

In practice the data are obtained in terms

of the common logarithm of the spreading re- sistance as a function of distance. Since

dy

dx

d In y dx

In y = In 10 log y,

and

dG

d? R2 dc '

it is possible to rewrite eqs (9) and (14) in

terms of the spreading resistance, > the local slope, m, of the raw spreading re- sistance data:

o(c) = - In 10 ln(s/a) m

TT R

(9a) sp

12
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

and

2 p(0 = In 10 ^- R m , Z sp

where

(14a)

d log R

sp m = . dC

For thin layers on an insulating substrate,

eq (9a), m is large and negative; for thin

layers on a conducting substrate, eq (14a), m is large and positive.

In general, the basic equations which relate

conductivity or resist"vity to spreading re- sistance are written

F/2aR

and

P =

sp

2aR /F

sp

(9b)

(14b)

Comparing eqs (9a) and (14a) with (9b) and

(14b) one finds that for the limiting case

of the insulating substrate

p _ 2a ln(s/a)

k m - - Kim , (9c)

and for the limiting case of the conducting

substrate

F =

Tra m

K2m

(14c)

Each of these correction factors, if used

separately, would be applied to the raw data

as follows to obtain the true spreading re- sistance :

R (corr) = R (raw)/F

sp^ sp

(15)

Since the forms given are limiting forms, the

proper method for combining them to describe

more general device structures is not intuitively clear. The trial combination

F = (16)

reduces to the limiting forms above for m >> 0 and m << 0. It can be seen, however,

that for the trivial case of profiling a uniform bulk specimen, for which m = 0, the correction factor reduces to (KiK2)'S, or

(8 ln(s/a)/Tr2)'2, rather than to unity, as it

should for such a specimen. This form of the

correction factor was tested against the Schu mann and Gardner solution [12] in a manner

similar to that described above for parallel

superposition. For this purpose, the term

K1K2, which makes a negligible contribution

to F when m >> 0, but contributes more and more to F as m approaches zero, was arbitrari

ly set to unity.

The first step in the computer evaluation of

correction factors from the local slope for

selected three-layer configurations was the

determination of local-slope values for each

configuration. This evaluation was accom- plished by numerical integration of the cor- rection factors due to Schumann and Gardner

[12] for top layer thicknesses slightly small

er and slightly larger than the arbitrary

thickness value selected in the evaluation,

for each configuration considered, and using

the differences to compute the local slope

[9] . It was found that for the case of two thin layers on a third thick layer, the local

slope method with K1K2 = 1 provides results

that are nearly as accurate as those obtained

by parallel superposition for most conditions

when the conductivity decreases with depth.

For the same configuration, the local-slope

method provides substantially more accurate

results than parallel superposition when the

conductivity increases with depth. For con- figurations in which all the layers are thick

er than the existing contact probe radius,

the agreement of this form of local-slope

evaluation with the results from Schumann and

Gardner is very satisfactory.

For three layers of equal thickness , the

local-slope method provides results that are marginally inferior to those from parallel

superposition for cases where the conductivity decreases with depth. However, the local

slope evaluation agrees better with the

Schumann-Gardner results for a much wider

range of layer configurations than the

parallel-superposition method.

Following the computer evaluation, an experimental test of the local-slope algorithm was

conducted. Profiles on six diffused specimens as determined from spreading resistance

measurements corrected by the ]ocal-slope

method were compared with the profile as measured by the incremental sheet resistance

method as described previously (NBS Spec.

Publ. 400-29, pp. 11-13).

13
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

Table 2 — Characteristics of Diffused Layered Structures Used for Spreading Resistance

Measurements

oUDSira te Sheet Resistance, n Juncti on Depth, pm

Specimen^ Resistivity,

• cm Four-Probe Spreadi ng Resi stance

(Integrated

Profile)

Incremental

Sheet Resistance

(Extrapolated)

Spreadi ng Resi stanceAngle Lap

and Stain

PB03 0.03 77 81 1.5 1 .6 2.0

PBl 1 59 63 1 .9 2.0 2.6

PBIOO 100 54 57 2.0 2.5 2.8

BP03 0.03 132 195 1 .6 1.5 2.0

BPl 1 88 125 1 .9 2.0 2.5

BP150 150 88 112 2.8 2.7 2.8

The first letter represents the dopant in the diffused layers and the second letter represents the dopant in the substrate; B = boron; P = phosphorus.

The test specimens were fabricated with twostep diffusions to give approximately Gaussian profiles of dopant density with depth,

with dopant density at the surface of about

10-^^ cm~^ and junction depths between 2 and

3 um. The characteristics of the test specimens are listed in table 2. The incremental

sheet resistance measurements were made on 1-in. (25-mm) diameter disks which were cut ultrasonically from 2-in. (51-mm) diameter

starting wafers; the spreading resistance mea- surements were made on beveled chips taken

from adjacent sections of the same wafers.

Results were compared in terms of conductivity profiles to minimize differences resulting from conversions between conductivity and

net dopant density.

The spreading resistance analysis was carried

out as follows: raw spreading resistance

data were taken for a shallow bevel section

of each specimen chip and were then stored in

the calculator memory. The bevel angle for

each chip was measured by an optical smallangle measurement technique [13]. The spreading resistance data were corrected using eq

(15) beginning with data at the junction and

proceeding outwards toward the top surface of

the structure. For this test, the form of

the correction factor was modified slightly

from that used for the initial testing of

the local-slope model; the term K1K2 in eq

(16) was replaced by

= 1 +

81n(s/a)

tan" Rlog ^s (17)

where ROSand R are the measured spreading f b resistances at the site being corrected and

at a site one contact radius deeper, re- spectively; if the site one contact radius

from the site being corrected is on the opposite side of an isolating junction from

that site, then the value used for R is the

s value of spreading resistance as measured at the junction. The arctan function is ex- pressed in radians , and the value of a is determined from a measurement of spreading re- sistance as a function of probe separation

on the top surface of the structure to be

profiled; for the diffused specimens being

analyzed, a was found to be 1.1 and 2.1 pm

for the n~ and p-type layers, respectively.

The form used here for K3 allows F to approach the same limiting values as previously

for the cases of insulating and conducting

substrates, m >> 0 and m << 0, and in addition gives a value of F = 1 when the layers

are uniform, m = 0.

Conductivity values were then calculated using eq (9b) . The results are shown in figure

4. Table 2 lists the sheet resistance values

determined by four -probe measurements [14] at

the beginning of the incremental sheet resistance measurement procedure and those obtained by integrating the conductivity pro14
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

Figure 4. Conductivity profiles for selected diffused layers. (N,P: from spreading resistance data, corrected by the local-slope method. Solid curves: from incremental sheet resistance measurements. See table 2 for specimen identification.)

15
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

Figure S- Comparison of traces from higirspeed dragging-stylus and conventional steppingtype spreading resistance measurements in two orthogonal directions on an n-type silicon

slice with an average resistivity of 2.5 fi'cm and a (111) surface orientation. CThe upper

pair of traces was taken along the line A to B, as shown in the plan view, and the lower

pair along C to D. In each pair, the upper trace is from the high-speed dragging-stylus

instrument and the lower trace is from the conventional instrument stepping at intervals

of 25 um.)

16
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

file obtained from corrected spreading resistance profiles. It also lists junction

depths obtained from (1) extrapolation of the

incremental sheet resistance data to the re- sistivity of the substrate, (2) the corrected

spreading resistance data, and (3) direct measurement by the angle lap and stain method

[15]. Sources of error which would account

for the observed discrepancies are being investigated . (D. H. Dickey§ and J. R. Ehrstein)

High-Speed Dragging-Stylus Probe — This task

was undertaken to develop an instrument which

could collect spreading resistance data at least 10 times faster than the commercially

available stepping- typ? instrument with little or no degradation of that instrument's

electrical performance and probe life characteristics (NBS Spec. Publ. 400-19, pp. 10-

13). A variety of probe tip materials and

tip sizes was selected and tested. All probe

tip materials listed previously (NBS Spec.

Publ. 400-25, pp. 12-13) were tested with

the exception of tungsten. Both lO-pm and

25-pm radius tips were fabricated in many of

these alloys; for some, only 25-ym radius

tips were available. The tip radius of the

tungsten-ruthenium alloy probes was 19 ym. Based on judgments of overall quality of the

data obtained with the various probe materials together with lifetimes over which the

probes could be effectively reconditioned,

Carballoy 895^ tungsten carbide alloy [16]

with a tip radius of 25 ym and tungstenruthenium alloy^ [17] with a tip radius of 19

ym appear to be the preferred probe materials.

Both alloys have very fine grain size, and of

the tungsten carbide alloys, 895 has the highest abrasion resistance and highest rated com- pressive strength. These judgments were made

on the basis of a probe loading of 530 mN for

the blunter probes and probe loading of 150

to 210 mN for the sharper probes. The loading for the sharper probes was selected with

the intent of maintaining approximately constant force per unit area at the contact and was modified in part by the observed width

of the track pattern left on specimens by the

probes

.

probe holders from the measurement instrument

and touching each probe tip for a few seconds

to a rotating hard (steel or sapphire) disk

covered with 3-ym diamond paste. A rocking

motion is imparted to the tip so as to abrade

and recondition the entire radius of the tip.

The probe tip is subsequently sv/abbed with

f luorocarbon-methylene chloride, reattached

to the probe station, and drawn across the

surface of a polished silicon wafer for 50 to 100 mm while under normal operating load.

The whole operation takes just a few minutes

and the calibration graphs were found not to be changed by the reconditioning.

The previously used lubricant , butoxy ethoxy

ethanol, was replaced because it had an un- pleasant odor. Evaluation of a number of other lubricants (methanol, propylene glycol,

synthetic engine oil, and several fluorocarbons) showed No. 43 f luorocarbon^ [18] to be

the only one of these which met the desired

conditions of low volatility, lack of toxicity, low conductivity, and low viscosity; it can be readily removed with methanol. It was

used for all recent wear tests and calibration evaluations.

To illustrate the resolution and signal-tonoise ratio obtainable with the high-speed

dragging-stylus instrument when using Carballoy 895^ probes, spreading resistance measurements were made on a freshly polished 2-in.

(51-mm) diameter n-type silicon slice with an average resistivity of about 2.5 n*cm and a (111) surface orientation. The results are presented in figure 5 together with the re- sults obtained at NBS with a conventional instrument which was stepped at intervals of

25 ym. The profile was obtained in about 4 min using the high-speed instrument, as com- pared to 80 min for the profile with the con- ventional stepping-type instrument. It can

be seen that the striation patterns measured

by the two instruments are essentially the

same. (A. Mayer'i' and N. Goldsmith^)

The two preferred alloys exhibit point life

in excess of 9 m of cumulative specimen probing with reconditioning every 2 to 3 m. A different reconditioning procedure appears

to be preferable to the previously used procedure of dragging the probes over an abraded

silicon surface or over a plate covered with

diamond polishing paste. The new procedure,

not yet automated and hence subject to operator variability, consists of removing the

Work performed at Solecon Laboratories under

NBS Contract No. 5-35881.

See disclaimer, p. vi

.

Work performed at RCA Laboratories under NBS

Contract No. 5-35914. NBS contact for additional information: J. R. Ehrstein.

17
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

3.3. Capacitance-Voltage Methods

A new method was developed to extract background dopant densities in the range from

10^^ to about 10^^ cm~^ from junction

capacitance-voltage (C-V) data [19]. When

the conventional method of calculation [20]

is applied to specimens having large

5 X 10^^ cm~^) background dopant density,

the dopant density profile may not level off

at the background dopant density because in

such specimens the depletion region cannot be

driven out of the compensated region which exists near the junction.

In the new method, the C-V data are corrected for peripheral effects, and a profile un- corrected for diffused layer effects is calculated [20] . Given the junction depth for

a Gaussian diffusion, a succession of estimates of the ratio, R, of effective surface

dopant density to background dopant density

is made until the correct value is found.

For each such estimate the dopant density profile of the uncompensated background is calculated. When the slope of a least-squares linear fit through the points of this profile is

sufficiently small, the estimate of R is

judged to be correct, and the average of the

most recently calculated background dopant

density values is taken as the average background dopant density.

The method has been evaluated using both

real and theoretical C-V data [19]. Satisfactory results were achieved when the method was applied to specimens with background

dopant density as large as lO-'-^ cm~^; above

this value, the depletion region lies only in

the linearly graded region. Since an average value rather than a profile of discrete

points is calculated, the method is not suitable for measuring regions of varying dopant

density, such as epitaxial layer-substrate

interfaces. The method is not appropriate

for use on specimens with low 10-^ ^ cm~^)

background densities in which an appreciable

real gradient exists. In such instances, no diffused layer correction exists which can cause the uncompensated background dopant

density profile to be flat.

(R. L. Mattis and M. G. Buehler)

3.4. Hole Mobility in p-Type Silicon

It was noted previously (NBS Tech. Note 806,

pp. 20-24) that the calculated conductivity

mobilities for holes in p-type silicon were considerably larger than the measured mobilities for dopant densities greater than

lO-'-" cm~^. This discrepancy may be attributed in part to the inadequacy of the theoretical model used in the mobility calculations and in part to the use of total dopant

density to derive the mobility values from

resistivity data, without properly taking into account the deionization of the acceptor

impurities with increasing dopant density.

Experimental data obtained by Wagner [21] on boron-implanted silicon showed significant

differences from Irvin's classical curves

[22] for acceptor densities greater than

10^6 ^^-3 (j^Bs Spec. Publ. 400-4, pp. 13-14).

The present task was undertaken to derive

theoretical expressions for mobility in ptype silicon that would yield satisfactory

agreement with experimental data.

The total hole mobility can be obtained by

weighting the mobilities of the holes in the

heavy, light, and the spin-orbit splitting

bands, yi, V2, and IJ3, respectively, according to their relative populations in each

band

(18)

The density-of-states effective masses, m^,

m2 , and m3, for the three bands were obtained

from the results given by Barber [23]. At

300 K, one finds that mi* = 0.537mo, mg* = 0.43mo, and m3''' = 0.134mQ, where mg is the

free electron mass , so that eq (18) reduces

to

Up = 0.543(pi + 0.7165y2 + 0.1247^3) . (19)

The hole mobility of each band is calculated

separately by taking into account the contributions of lattice, ionized impurity, and

neutral impurity scattering. In addition,

the effect of hole-hole scattering is included for dopant density greater than

10^^ cm~^. The lattice mobility, was based on the results of Costato and Reggiani

[24] which give a lattice mobility value of

475 cm^/V's at 300 K. For ionized impurity

scattering, the Brooks-Herring [25] formula

was used for the calculation of impurity mo- bility, and the formula given by Sclar

[26] was used for the calculation of neutral

impurity mobility, y . The detailed proce18
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

400

300

200

100

G

- ' ' 1 1

1 1 liiiij 1 1 i iinij ) niiii| Mil ill! 1 1 llilllj 1 1 llllll ^ -

— - \

\

'\

3

5

1 1 1 mill 1 1 iiiiiil 1 1 iiiiiil 1 1 mill 1 1 1 iiiii ; ; iiinil , i imii

10'^ 10" 10" 10'° 10" 10° to" 10'" TOTAL BORON DENSITY Icm"')

Figure 6. Preliminary results of hole mobility calculations in boron-doped silicon at 300 K. (Curve 1: calculated without holehole scattering; curve 2: calculated v;ith hole-hole scattering at higlier hole densities; curve 3: calculated with hole-hole

scattering over entire range; curve k: Wagner curve [21]; curve 5: Irvin curve [22];

O, , experimental data; see text for discuss ion .

)

dures for computing y^, y^, and P3 are similar to those used previously in calculating

the electron mobility in n-type silicon [27]

(NBS Spec. Publ. 400-29, pp. 17-19). The

results of the calculation, which are shown

in figure 6, are in reasonable agreement with

preliminary data taken in cooperation with

ASTM Committee F-1 (NBS Spec. Publ. 400-4,

pp. 13-14). These data were obtained by com- bining four-probe resistivity measurements

with measurements of Hall coefficient of ultrasonically machined bar-shaped specimens

(solid squares) and of capacitance-voltage

characteristics of ungated diodes (solid triangles). In the dopant density range from

10^5 to 3 X 10^^ cm~2 the results of the calculations also agree fairly well with more re- cent data (NBS Spec. Publ. 400-29, pp. 15-17).

These data (open circles) were obtained with

the planar square array four -probe and gated

junction test structures of test pattern NBS3 [28]. However, these data appear to fall

closer to Wagner's curve in the dopant density range above 10^^ cm" 2; revisions to the

calculated curve are being made to improve

the fit to the more recent experimental results .

It should be noted that there are differences in the mobility relationships for silicon doped with different acceptor impurities

[29] because of differences in fractional

ionization in the dopant density^ range from

10^^ to 10^^ cm"-^ even at room temperature.

At the present time, additional experimental

data are being collected on boron-doped silicon specimens, and these theoretical formulations for the mobility and resistivity calculations in boron-doped silicon are being

extended to cover the temperature range from

77 to 400 K. (S. S. Li)

3.5. Indium-Doped Silicon

The intrinsic, across-the-band-gap photosensitivity of silicon is limited to wavelengths

shorter than about 1 ym. However, the photoresponse of silicon can be extended to longer

wavelengths by photoionizing dopant states

which have been deionized by cooling to cryogenic temperatures. Silicon doped with antimony, arsenic, boron, gallium, indium, or phosphorus has been used for the detection of

infrared radiation in various wavelength re- gimes out to almost 30 ym [30-31]. Extrinsic

silicon detectors have a large potential application in infrared imaging systems in

which a large number of detector elements are combined into a two-dimensional m.osaic that can be fabricated monolithically along with

the necessary read-out circuitry. Integrated

image sensors of this kind are being developed for remote-sensing reconnaissance and

surveillance applications [32-34].

Although silicon may be one of the most studied elements and the one for which the technology is the most advanced, it is a relative

ly new arrival on the infrared detector scene Extrinsic infrared detection in silicon depends critically on properties that are often

of only secondary importance to other silicon

devices. Therefore, a need has arisen to develop suitable methods of measuring these

properties to characterize silicon with re- spect to its ability to detect infrared radia

tion. For example, it is necessary to measure and control the impurity compensation,

which plays a critical role through its effect on the lifetime of the photoexcited ex- trinsic carriers. The existence of any "extra" levels introduced by impurity pairing

or impurity-defect complexes could complicate

the compensation scheme, affect the lifetime,

and thus become very important. The situation is further complicated by the stringent

requirements for uniformity of the infrared

photoresponse imposed on mosaic arrays of imaging elements intended for thermal imaging

applications. A single point measurement is

inadequate for characterization of silicon

photoresponse for mosaic applications because of the requirement for uniformity of

19
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

response over the extended area of the mosaic. In addition, it would be highly desirable to be able to perforin uniformity measurements nondestructively on wafers before

they are processed into mosaics. Such a procedure would screen out the rejects at an early stage and, by eliminating unnecessary

processing of bad wafers, result in reduced

costs and increased ultimate yield.

The present task was undertaken to identify

and develop appropriate measurement technology to address these problems. Initial efforts are being concentrated on evaluation of

various techniques to detect extraneous levels in indium-doped silicon. One such level

has been named the X-level and appears to be

an acceptor state located about 0.11 eV above

the valence band [35]. The presence of this

level in amounts considerably smaller than

indium density reduces the performance of

indium-doped infrared detectors and makes it

necessary to operate them, at lower temperatures than otherwise in order to obtain the

best signal-to-noise ratio. (R. D. Larrabee)

3.6. Thermally Stimulated Current and Capacitance

Gold Donor in p-Type Silicon MOS Capacitor —

Measurements of thermally stimulated current

and capacitance were made on a gold-doped ptype silicon MOS capacitor. Theoretical models [36] suggest that the phase I thermally

stimulated current and capacitance response

of the gold donor center in this structure

should be the same (except for a geometric

scale factor) as the response of the same center in an n^p junction diode [37]; this

expectation was confirmed in the measurements.

The capacitor used was structure 3.8 of test

pattern NBS-3 [28]. The peripheral gate was biased into accumulation (-25 V) and the main

gate was deeply inverted (+12 V) during cooling and biased into deep depletion (+22 V)

immediately before the heating cycle was begun. The current and capacitance are shown

in figure 7 as a function of temperature for

various heating rates , The measured peak

emission temperatures occurred within 0.1 K

of the peak temperature calculated from the

theoretical model. For this device, the gold

donor density and the dopant density were calculated to be 4.2 X 10^3 3 and 8.3 x 10

cm~^, respectively.

Analysis of Isothermal Capacitance Transient

Response — Measurement procedures have been

established for the characterization of defect centers from analysis of the isothermal

240 260 280 300

TEMPERATURE |K|

Figure 7- Thermally stimulated current and

capacitance response of the gold donor in ptype silicon at various heating rates. (The

indicated heating rates are evaluated at the

temperature of the current peaks. The bottom

curve is a 5x expansion of part of the center

capacitance response curve. Device Au 13 P8.8.)

capacitance transient response. To illustrate the procedures, consider the case of

the gold donor center in a gated r&p silicon

diode, structure 3.10 of test pattern NBS-3

[28]. Measurements were made on a diode

from the same wafer which contained the above

MOS capacitor and also one from a wafer with

less gold.

The procedure used was as follows: With re- verse bias, V^, applied to the diode and the

peripheral gate biased into accumulation, the

diode is cooled to one of a series of temperatures in an appropriate range. When an iso20
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

thermal condition is achieved, the reverse

bias is removed (changed to zero bias) momentarily, then reapplied while the capacitance

of the diode is recorded as a function of

time on an x-y recorder. The transient decay

of the junction capacitance that occurs when

the bias is changed from zero bias to the re- verse bias is controlled in p-type silicon by

the hole density on defect centers which lie

below the mid-gap energy.^ Under one-sided

step-junction conditions, the junction capacitance, C, is given by

2V„

--2 = C, qeA2(N - p^)

' (20)

where is the zero-bias capacitance, A is

the junction area, s is the dielectric con- stant (permittivity) of silicon, N is the total acceptor density, p^ is the hole density

on the defect center, and q is the electronic

charge. This equation can be solved for p to obtain

Equation (22) can be rewritten

Pt - Ptf

Pti - Ptf

= exp[-(e + e )t] n p (23)

The capacitance C is at the beginning of

the transient when p = p . . After an

t ti

elapsed time equal to several time-constant

intervals, t may be taken as infinite: p^

becomes and C becomes . If p^, P^^j

and p^^ are each written in terms of eq (20a)

and the resulting expressions are substituted

into eq (23) and simplified, the left-hand

side of eq (23) is seen to be a ratio of ca- pacitances, denoted :

(C^2 _ c.2)(C^2 _ c2)

(C, 2 - C2)(C.2 - C.2)

b r 1

= exp[-(e + e )t]

,

n p

(24)

p = N - 2V^C^2c2

t - qeA2(C, 2 - C2)

D (20a)

The net rate of change of the hole density on

the defect center is given by [37]

dp. —r- = e n - e p , dt n t p'^t ' (21)

where e is the electron emission rate, e is n p the hole emission rate, and n^, is the electron density on the defect center. By use

of the totality condition, n^ + p^ = N^,

where is the total defect density, one can eliminate n and solve eq (21) with the re- sult that

Pt = Ptf + (Pti - Ptf)e^P[-(e^ + , (22)

where p . is the hole density on the defect

^ti

center at the start of the transient (t = 0)

and p^^ is the hole density on the defect

center at the end of the transient (t = «>)

.

The analysis can be extended to the case of

defect centers in the upper half of the band

gap in n-type silicon by a suitable exchange

of variables: holes with electrons; acceprors with donors

.

or.

e + e = (In C )/t . n p r (25)

Eight to twenty points are read from the

transient capacitance curve, scaled, and re- plotted as log against time. A leastsquares fit of the calculated points to a straight line is made, and the time constant

is calculated. For cases such as the gold

donor center in silicon, for which e << e , n p the reciprocal of the time constant is the

hole emission probability, e^. This procedure is repeated at a number of temperatures

in the appropriate range, 120 to 145 K in the case of the gold donor center. Since e =

P B^t2 exp [AEp/ (kT) ] (where is the emission

coefficient for holes, T is the absolute temperature, AEp is the difference in energy

level between trap and valence band, and k

is the Boltzmann constant) , an Arrhenius plot

of ln(e /T^) against T"-^ has a slope of AE^/k

and an intercept of In B . The data obP tained in the present experiment are plotted

in figure 8. These data were fitted by least

squares to obtain the values of AE = 0.343 ±

^ P 0.0045 eV and B = 2.22 x lO^iO-l^S^ good

P agreement with the results of diode measurements by Buehler [37], who obtained 0.33 ±

21
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

O.OOt 0.007 0.008 O.OOS INVERSE TEMPERATURE IK'^j Figure 8. Scaled Arrhenius plot of hole emission probability, ep/T^ , as a function of in- verse temperature for the gold donor in ptype silicon. (Devices Au 13 PIO.ll and

Au 10 PIG. 1 .)

Table 3 — Performance Characteristics of New

Thermally Controlled Chuck

Characteristic Value

Low temperature limit of top -185°C

surface (88 K)

Low temperature limit of chuck -190. 9°C

body (82.9 K)

High temperature limit^ 'v350°C

Minimum elapsed time from room tern- . <4 ml n perature to low temperature limit

Minimum elapsed time from low tern- perature limit to room temperature

Maximum heating rate from low tem- perature limit to room tempera- +5.5 K/s ture Minimum elapsed time from room tern- perature to 300°C ^'^ ^

Minimum elapsed time from 300 °C to

<i"io s room temperature

0.03 eV and 4.2 x lo8±1.2. g^j,

[38], who obtained 0.345 + 0.0029 and (2.7 ± 0.66) X 10^, respectively.

(W. E. Phillips and M. G. Buehler)

Applioation to Power-Grade SiliQor& — This

task was undertaken to evaluate the use of

measurements of thermally stimulated current

and capacitance (TSM) as a means for characcerizing defects in power-device-grade silicon wafers. The following were established

as the objectives of the first phase of the

task:

(1) To design and construct apparatus

for performing TSM on large-diameter silicon

wafers in the appropriate temperature range;

(2) In order to preserve the characteristics of the starting material, to develop

and evaluate a low-temperature process for

fabricating MOS capacitors suitable for use

in performing TSM on high-resistivity n-type

silicon; and

(3) To determine the feasibility of using and interpreting TSM on thyristors after

each step in the device fabrication process.

Progress was made during this reporting period toward all three of these objectives.

Principally funded by the Energy Research

and Development Administration, Division of

Electric Energy Systems

,

estimated

A. Whole-Wafer Apparatus . The new ther

mally controlled chuck and automatic wafer

prober (NBS Spec. Publ. 400-29, pp. 20-22) was assembled and functionally tested. The

results of these tests are summarized in ta- ble 3. These characteristics meet or exceed

the design targets which were selected to permit measurement of thermally stimulated

current and capacitance of centers throughout most of the band gap with adequate

signal-to-noise ratio and performance of bias

temperature stress tests (to determine mobile ion density in oxide films) with minimal ramp times. Rapid measurements of many

devices on the same wafer can be made automatically with the automatic wafer prober,

thus facilitating the mapping of characteristics across the wafer (NBS Spec. Publ, 400-

25, pp. 43-44).

All systems within the dry-box enclosure are fully operational. The gold acceptor has

been identified in both a p^n diode and an n-MOS capacitor on the same gold-doped wafer,

using the thermally stimulated current technique. In this environment currents as small

as 0.1 pA can be easily detected.

Although the apparatus has been successfully

tested, two deficiencies have appeared.

First, the stainless-steel bellows used to transport the liquid nitrogen to the chuck

22
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

has been found to be too flexible. Although

the bellows assembly is mechanically re- strained, violent expansion of the liquidnitrogen vapors causes excess strain in the

assembly. Second, the excessive lowering of

the ambient temperature within the box

causes an intermittent mechanical malfunction

of the prober. The low temperature in the

box is believed to result from the large cold

surface exposed by the bellows system. Both

deficiencies are being corrected by the addition of a telescoping tube arrangement to con- fine the bellows motion and to insulate the

bellows assembly from the interior ambient.

Other improvements to the apparatus are also

being considered. These include the addition

of instrumentation to control the temperature

and a liquid-nitrogen handling system to minimize loss of coolant fluid. (R. Y. Koyama)

B. Low-Temperature Processing of MPS

Capacitors . Further studies have confirmed

that the most satisfactory low-temperature

process for fabricating MOS capacitors for

TSM involves the chemical vapor deposition

of silicon dioxide, 150 to 250 nm thick, at 400°C; electron-beam evaporation of aluminum,

400 to 600 nm thick; patterning; backsurface metallization; and microalloying in

dry nitrogen for 20 min at 400°C (NBS Spec.

Publ. 400-29, pp. 23-25) .

Further work on tantalum pentoxide and anodically formed silicon dioxide dielectric

films has been discontinued. Anodically

formed films were found not to give reproducible results. A second lot of capacitors fabricated with tantalum pentoxide approximately

80 nm thick formed leaky capacitors similar

in behavior to most of those in the first lot

which were only about 35 nm thick.

(R. Y. Koyama and T. F. Leedy)

C. Process Monitoring . Although different manufacturers use different processes to fabricate thyristors, there appears to be a "basic" process which is common to the industry. Typical starting material for power

thyristors is n-type, (lll)-oriented , lapped

silicon slices with resistivity in the range

from 50 to 200 Q'cm and cut from a floatzoned crystal. For a given product device,

the two most important requirements which the

starting slices must meet are for resistivity

and slice thickness. These parameters are critical to the reverse blocking voltage of

the completed device.

Following appropriate chemical clean-up, the

slice undergoes a closed-tube p-type diffusion at about 1250°C for 24 to 72 h to form

the deep junctions bounding the n-type region.

At this stage, a p-n-p structure is formed,

but the two junctions are shorted at the edge. As a result of this processing, the lifetime

of the original n-type region has been degraded from several milliseconds to a few

microseconds. Although the exact reason for

this lifetime degradation is not known, heavy

metal contamination from unknown sources is

suspected. The time for this diffusion determines the final thickness of the interior n- type region; this thickness, along with the

dopant density determines the reverse blocking characteristics of the device.

The next major processing step is the emitter

diffusion, an open-tube n-type diffusion

which forms the cathode region of the thyristor. The cathode configuration is defined

photolithographically . At this point some form of lifetime control treatment is used to tailor the switching characteristic of the device. The tailoring can be completed at this

stage or stopped short of completion in preparation for final adjustment to be made with

electron irradiation just prior to packaging.

The processed wafer is then bonded to a metal

backing plate for rigidity and ease of handling in the packaging steps, and then the metallization for the gate and cathode is defined by evaporation through a transmission

mask or photolithographically. The edge of

the wafer-backing plate assembly is contoured

to separate the junctions, and the junction

edge is appropriately passivated to withstand

the rated reverse blocking voltage. The re- sulting structure is illustrated in figure 9 together with a schematic indication of major

processing stages. At this stage the switching characteristic is further adjusted, if

necessary, and the packaging steps are car- ried out.

The most interesting points in this process

for evaluation by TSM are immediately before

the first diffusion, after the first diffusion, after the emitter diffusion, and after

the lifetime-control step. To make TSM at these steps, appropriate test structures must

be formed to provide electrical contact to the regions of interest. Prior to the first

diffusion, the low-temperature MOS capacitor

(see above) is the appropriate structure.

Following the diffusion, there is a p-r. junction, but the n-type region is not accessible

unless a special test structure is used to provide the contact. This structure must

provide the following:

(1) contact to the p-type region;

(2) contact to the n-type region;

23
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

lb)

METALLIZATION nZJ PASSIVATION

Figure 10. Cross section of mesa diode test structure.

GATE

PASSIVATION- BOND

BACKING PLATE

CATHODE

liii^^ii tump a" V"fy"f'J"[

'. METALLIZATION

P

nP

ANODE

Figure 9- Typical thyristor structure.

Shown schematically are (a) typical n-type

slice starting material, (b) slice after ptype diffusion, (c) slice after n-type emitter diffusion, and (d) cross section of un- packaged device.

(3) definition and isolation of the p-n

junction, and

(4) passivation of the junction so that

leakage currents are acceptably small.

Ideally, fabrication of the structure must be

possible with low-temperature (<400°C) steps.

It must be possible to make the measurements

in wafer form.

tion details of the specimen holding system

[39], the bias-protection circuit [40], and

the linear-sweep, high-voltage supply [41].

This system permits application of the ex- extended-range technique to metal-insulatorsemiconductor (MIS) structures with insulator

thickness in excess of 150 pm.

Among the objectives for the current phase of

the work were improvement in the signal-tonoise ratio above that achievable with the

initial 10-kV system and development of a guard-ring contact.

The increased signal-to-noise ratio was obtained through improvements incorporated in

the new high-voltage power supply [41] and

by increasing the magnitude of the 1-MHz

test signal in the modified PAR 410 capacitance meter**[42] from 15 to 100 mV.+"'" These

changes increased the signal-to-noise ratio

by about two orders of magnitude. The improved signal-to-noise capability has been

employed in all recent measurements made with

the system.

The structure, in its simplest form with

front-surface contacts to both regions, is depicted in figure 10. The mesa diode is

formed by chemical or physical etching, mechanical sawing, or a combination of techniques , and passivated with chemical-vapordeposited silicon dioxide. It is likely that

a doped-gold metallization or a lower resistivity diffused region may be required to form the contact to the high-resistivity ntype region. (R. Y. Koyama and M. G. Buehler)

3.7. Extended-Range MIS C(V) Method

Development and initial evaluation of the

25-kV system for measurements of capacitance

and conductance as a function of voltage was

completed. Separate reports have been prepared which describe the design and construcA new test structure with a guarded electrode

was evaluated. Guard-ring structures were de

fined on the free surface of the sapphire in

three SOS specimens which previously had been

used for measurement employing unguarded elec

trodes. Extended-range MIS C(V) measurements

were then carried out on these specimens using the same bias-sweep conditions (range and

sweep rate) as those used for the previous

measurements, A comparison of some of the

results is shown in table 4.

See disclaimer, p. vi. tt

It is

. believed . that a similar modification ...

can be made to one of the other capacitance

meters previously investigated (the Boonton

72B), but not to the third (the Boonton

71A).

24
Table 4 — Capacitance

El ectrodes

MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

Measurements on SOS Specimens Obtained with Guarded and Unguarded

Silicon Film Interfacial Charge Density

Electrode Insulator ———

Configuration Capacitance, pF Conductivity Dopant At Flat At Weak

Type Density, cm"3 Band, ciTi"^ Inversion

Threshold, cm"^

2463E Unguarded 25 45 n 4 85 X 1015 -6 5 X IQii -0 5 X 1011

Guarded 23 90 n 3 85 X 1015 -6 2 X 1011 -0 3 X 1011

2523A Unguarded 29 20 P 6 30 X 1015 9 9 X IQii 4 6 X 1011

Guarded 27 30 P4 55 X 1015 9 4 X IQii 4 7 X 1011

2523D Unguarded 26 9 P 6 15 X 1015 8 0 X 1011 3 1 X 1011

Guarded 25 15 P

4 45 X 1015 7 7 X 1011 3 2 X lOii

It is seen that the measured value of the insulator capacitance, C^, is larger for the

unguarded-electrode measurement, resulting in an overestimate of the dopant density of the

silicon film for this measurement. The effect on the interfacial charge densities at FB flat-band, N , and at the weak inversion

WI

threshold, N , is relatively small and non- systematic. Qualitatively, the curves of

AC(V) are slightly less "smeared out" when

the guarded-electrode configuration is used,

but the differences are not large.

(A. M. Goodman§§)

The extended-range MIS C(V) measurement technique has been used to study the dependence

of the charge density at the silicon-sapphire

interface of silicon on sapphire (SOS) as a function of (1) irradiation dose and (2) applied electric field in the sapphire. Results to date show that the radiation-induced

charge is always positive for either zero or positive voltage on the metal-gate electrode.

Negative voltage applied to the metal-gate

electrode (E ^ 2 x 10^ V/cm) resulted in a negative radiation- induced charge. The functional dependences of the radiation-induced

charge upon applied electric field are different for the positive and negative applied

voltages. The observed behavior is not as yet understood [43].

(A. M. Goodman§§ and A. G. Kokkas^^)

The technique is also being used to study the

effects of various sapphire substrate preparation techniques on the silicon-sapphire interfaces of SOS layers. Initial measurements

indicate that the effects are (1) measurable

and (2) likely to have a profound effect upon

devices fabricated in these layers. This application will provide a valuable adjunct to the development of an optical test for the

surface quality of sapphire (see sec. 4.5.).

(A. M. Goodman§§ and C. E. Weitzel^M

Work performed at RCA Laboratories under

NBS Contract No. 5-35912. NBS contact for

additional information: R. Y. Koyama.

Work performed at RCA Laboratories under

Contract No. F19528-76-C-0183 , Rome Air De- velopment Center, Deputy for Electronic

Technology, Hanscom AFB, HA 01731.

25
^ . MATERIALS CHARACTERIZATIOiJ BY

P [| Y S I C A L ANALYSIS f 1 E T H 0 D S

4.1. Calibration Standards for Ion Microprobe Mass Analysis

This task was undertaken to develop methods

for preparation of standard specimens for em- pirically calibrating an ion microprobe mass

analyzer (IMMA) so that quantitative measurements can be made of selected impurities in

silicon and silicon dioxide regions of semiconductor devices. Implicit in this objective is the evaluation of the applicability

of the IMMA for this type of analysis and

the determination of the instrumental parameters best suited for these analyses. Ion implantation was chosen as the most appropriate

method for preparation of the specimens (NBS

Spec. Publ. 400-19, pp. 20-22) because of the

inherent ability of this method to implant

specific impurities at specific depths at se- lected concentrations. The standard specimens thereby simulate actual device structural layers and exploit the in-depth profiling

capability of the IMMA. The versatility of

ion implantation impurity selection makes it

possible (dependent on the ion implantation

equipment) to produce standard specimens for

virtually every element in the periodic table.

Analysis of the silicon specimens implanted

with phosphorus has been completed. These

specimens were implanted with 80-keV ^^P to nominal doses of 10^^^ IQI'*, 10^5, or 10^^

cm~^ or with 60- or 100-keV ^^P to a nominal

dose of 10^^ cm~^. The specimens were analyzed using neutron activation analysis and

then were in-depth profiled using the IMMA. The reproducibility of both the sputter rate

and the calibration coefficient was examined

in some detail.

Neutron Activation Analysis — Implant doses were determined by neutron activation analysis using the unpatterned half of the implanted wafer. The half wafers as received

from microvolume fabrication were loaded into

a high-purity quartz dish with an inside diameter of 2-1/8 in. (54 mm). High-resistivity

boron-doped silicon wafers were used as spacers between each half wafer and on the top

and bottom of the stack. These spacers com- pletely fill the inside diameter of the dish.

They are routinely cleaned and etched before

each irradiation. The same spacers have been

used for more than four years in over 50 irradiations ; they do not appear to be a source

of contamination. Three separately weighed

phosphorus monitors, each sealed in its own high-purity quartz ampoule, were also placed

in the dish. A quartz top was attached using

pressure-sensitive tape, and the dish was

sealed in an aluminum can for the irradiation

which was carried out for 14 h at a nominal

neutron flux of 1 x 10^^ cm~^'s~^ in the

Texas A&M reactor.

After irradiation, the aluminum can was opened and discarded. The half wafers were

removed from the quartz dish and cleaned

three times in a mixture of hydrochloric

acid and methanol, rinsed in acetone, and

dried in air. The wafers were then placed

on x-ray film to check the dopant uniformity

by autoradiography.

Using the autoradiogram as a guide, a uniformly implanted region was scribed from

each wafer and this specimen mounted in an aluminum planchet [44] for beta counting.

Normally, the specimen consisted of the entire half wafer except for those portions of

the corners that would not fit into the

planchet. Prior to scribing, the mass of

each wafer to be counted was determined with

an analytical balance, and the wafer thickness was measured with a micrometer. A surface area was calculated using 2.32 g/cm^ as the density of silicon.

The planchets containing the half wafers

were loaded into holders for counting. The

counting sequence included three background

specimens (filter paper on planchets) at the

beginning, end, and middle, as well as nine

monitor specimens (aliquots of monitors carried by filter paper on planchets) evenly dis

tributed among the half-wafer specimens. All

specimens were counted for the shorter of 50

min or 2 X 10*^ preset counts with the beta

plateau adjusted for ^^F . The counter was operated in the net mode with a discriminator

set for elimination of electronic noise. The

background count rate averaged 4.34 ± 0.58

cpm, where the uncertainty is 1.96 standard

deviations from the mean for multiple determinations of 50 min each.

Two different chemical compounds were tested

as phosphorus monitors, ammonium phosphate

(26.927-percent phosphorus) and diammonium

phosphate (23.455-percent phosphorus). A matrix of three weighings of each compound was

packed, irradiated, and processed following

the standard procedure for monitors. No difference between these compounds in either the

specific activity or the standard deviation

from the mean specific activity was observed
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

in a comparison test. The compound diammonium phosphate was chosen for routine work be- cause of the ease in handling its small crystals for determinations of mass. No mass

loss was observed for this compound after 6 h in a vacuum oven at 100°C.

The ampoules containing monitors were cleaned

in aqua regia and deionized water. They were

then cracked in clean beakers, and the monitor was dissolved in deionized water. For each monitor, the solution and three subsequent rinses were transferred to a 10-mL volumetric flask and deionized water added to bring the flask contents to volume. Three

aliquots (typically 100 \iL) were taken from

each flask and mounted in planchets on filter

paper. The mounting procedure is designed to approximate the counting geometry of the sili- con wafers as closely as possible. Tests in

which phosphorus in silicon dioxide was counted in wafer form and then as a solution

indicated that there is no measurable mismatch in counting geometry. Reproducibility

in pipetting monitors was checked by beta

counting aliquots of the monitor solution.

Reproducibility between 1 and 2 percent at the 95-percent confidence level was generally

achieved

.

Specimens were allowed to decay for 10 to 12 days after irradiation before beta counting.

Thereafter, they were counted at 60- to 72-h

intervals over a period of 20 or more days.

The elapsed time for each counting of a set

of specimens (backgrounds, monitors, and half

wafers) was typically 8 h and rarely exceeded

12 h, even after long decay periods.

Phosphorus counts from both the monitors and

wafers were corrected for background and decay (t, = 14.4 days), and the total number of

phosphorus atoms in each wafer was calculated

directly using the specific activity of the

monitors. The constancy of the total number

of atoms for a wafer as a function of decay

time is confirmation that the correct halflife value is being used, since this value is

proportional to the ratio of beta activity

from the monitors and a wafer. A long-lived

component in some low-dose implants was identified as ^^Cr by half-life measurements and

gamma-ray spectroscopy. Phosphorus results were determined for these wafers when the decay curve could be resolved graphically into

components of 2 7.7 days and 14.4 days.

Investigations of sources of error were aimed

principally at determining reproducibility of

a determination from a single irradiation; re- producibility between irradiations was not in27

vestigated. In summary, all factors considered proved to be sm,all compared to the standard deviations for the mean number of atoms

per wafer (typically less than 1 percent) and

the mean specific activity determined from

the monitors (typically 3 to 5 percent). The

uncertainty of the determination of Implant

doses was estimated by summing the variances

from these two means.

Sputtering Rate — The sputtering rate in an ion microprobe is a critical parameter both

during calibration and during subsequent application as an analytical tool. In work re- ported here, it was assumed that the sputtering rate remained constant during each analysis. The question of reproducibility between

analyses was examined for the case where the IMMA was operated with a nominal raster size

of 125 urn by 100 pm, a beam current between

18 and 30 nA, inclusive, and a 19.4-keV 02'*' beam. Since the specimens were held at a potential of +1.5 kV, the energy per incident

oxygen atom in this case was approximately

9 keV. The implanted silicon slices were sputtered for long tim^es to obtain in-depth

profiles of counts per sputtering interval

(channel) vs. channel number. Crater depth,

crater area, beam current, and sputtering

time were measured, and both the actual sputtering rate and the sputtering rate normalized to unit current density were calculated.

The results are given in table 5. It is es- timated that the crater depth can be determined (with the microscope) to ±10 percent

and the crater area (also with the microscope) to ±20 percent. The beam current was

read to the nearest ranoampere. As can be seen from the data in table 5, the relative

sample standard deviation (coefficient of

variation) of the measured crater area was

7.7 percent; the relative sample standard deviation of the normalized sputtering rate

was 11 percent.

Calibration Coefficient — Two data analysis

techniques can conveniently be used to obtain

calibration curves of phosphorus density as

a function of signal. In one method, the

signal is taken as the ratio of the phosphorus counts in one channel of the profile to the silicon counts in the same channel; in

the present work only the peak channel was

used. In the second method, the signal is

taken as the ratio of the sum of the phosphorus counts over a substantial fraction of the

profile to the sum of the silicon counts over

the same region of the profile. Although the

silicon signal is typically flat over the region of integration, normalization of the

phosphorus to the silicon counts is intended
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

Table 5 - Normalized Sputtering Rate of an Ion Microprobe Mass Analyzer Operated with a 19.4-keV 02''" Beam and a Nominal Raster Size of 125 m by 100 ym

Sputtering Crater Sputtering Beam Crater Normalized

Specimen Time

,

s Depth,

nm Rate

,

nm/s

Current,

nA Area

,

Sputtering Rate,

nm-ym^/nA'S

1 2640 532 0.20 29 15730 109

2 2816 602 0.21 29 13030 96

3 2442 465 • 0.19 28 14200 97

4 2684 461 0.17 30 12800 73

5 2684 567 0.21 30 11370 80

6 2442 314 0.13 18 13890 99

7 2420 368 0.15 21 11810 85

8 2794 324 0.12 18 13890 89

9 2794 355 0.13 18 13310 94

10 2024 290 0.14 18 13310 106

11 2420 273 0.11 19 13310 79

12 3498 426 0.12 19 1 3530 87

13 3366 450 0.13 20 13390 90

14 3498 465 0.13 20 13900 92

Mean 0.15 1 3390 91 Sample Std. Dev. 0.04 1040 10

Coef. of Var 23% 7.7% 11%

to circumvent variations in sputter-detection

efficiency which can be caused by fluctuations in a myriad of machine and sample parameters. The second, or integral ratio,

method was used to obtain the results discussed below.

The shape of the in-depth phosphorus distribution for the ion implantations was observed

to be approximately Gaussian. Consequently,

a pseudo-Gaussian approximation was employed

in mathematical modeling; the assumption that

the profile shape can be approximated by a Gaussian makes it possible to select limits

of integration for both the number of atoms

and the signal (counts) which encompass a known fraction of the area under the curve. For a Gaussian, the standard deviation, a, is related to the full width at half maximum

(FWHM)

a = FWHM/2.35.

The center of the profile was taken as the

center point of the FWHM. To minimize roundoff errors which arise because of the repre28

sentation of the profile as a histogram, the

integration was carried out over 90 percent

of the pseudo-Gaussian (±1.68o). Thus, the

contents of the "limit channels" were usually

small compared to the sum. Round-off in se- lecting the limit channels had a small impact ' on the resultant integral. The ratio between

sputtering rate and multiscaling dwell time

has a sharp impact on the precision of this

method; errors in integration are enhanced

when too few channels define a peak. The

trade-off between a sufficient number of

points to define a profile shape and the

statistical significance of each point is a familiar problem in nuclear spectroscopy.

The background under the profile peak can cause problems in analysis when the signal- ; to-background ratio is low. The origin of

tailing on the "deep side" of the profile is complex. Possible contributing factors include neutral beam sputtering of a high phos- | phorus concentration outside the raster area, i channeling during implantation, contributions from the crater wall during sputtering,

traces of water in the vacuum system near
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

Table 6 - Calibration Data for Ion Microprobe Mass Analysis of Phosphorus-Doped Silicon

Based on Integral Ratio Method

Specimen^

Implanted

Phosphorus

Dose, cm~2

Beam

Current,

nA

IMMA

Signal

Number

of

Channel

s

Average

Phosphorus

Density, cm"^

Cal ibration

Coefficient,

cm"

80-16 1 .23 X 1016 30 0. 021 34 6 8 X 1020 3 24 X 1022

28 0. 026 35 7 1 X 1020 2 73 X 1022

30 nU

.

35 rD

rD X

1 n 9 n

1 O'^'^ 1

"7 /I /4 X 30 0. 043 34 5 8 1020 11 58 1 022

30 0. 029 35 6 5 11 020 yJ 2 28 1022

80-15 1 .27 X 1015 30 0. 0032 40 0 59 X 1020 1 84 X 1022

34 nu

.

UUoO 35 nU oU X 1 Q9QL X 32 0. 0029 40 0 56 1 020 1 93 1 022

32 0. 0030 39 n 57 1 020 11 90 1 022

32 nU no?7 40 0 11 020 \J ?C 07 11 022 \J

100-16 1.17 X 1016 32 0 021 47 0 44 X 1020 2 10 X 1022

32 0. 021 49 0 42 X 1020 2 00 X 1022

oU 0. 020 0 42 X 1020 2 10 X 1022

30 0. 023 49 0 45 X 1020 1 96 X 1022

60-16 1.17 X 1016 24 0 030 26 10 5 X 1020 3 50 X 1022

22 0 032 31 9 .6 X 1020 3 00 X 1022

24 0 033 29 9 .5 X 1020 2 88 X 1022

19 0 034 37 9 .4 X 1020 2 .76 X 1022

The left-hand number in the specimen designation represents the implantation energy in kilovolts; the right-hand number represents the order of the nominal implant dose.

the specimen, knock-on during sputtering or implantation or both, and SiH"*" (mass 31) background. For implantation doses of 10l5 or

10 1^ cm~2 the background was small enough to be ignored over the integration range of

±1.68a; data on specimens with lower implantation doses were not included in the present

analysis. Results of 18 profile measurements

made on four slices are summ.arized in table 6. All analyses were performed on 760-ijm by 760-

um microvolumes (NBS Spec. Publ. 400-19, pp. 20-22) with a nominal raster size of 125 ym

by 100 ym; similar results were obtained on "infinite" planes. The specimens analyzed

are identified in column 1 of the table. The

implanted phosphorus dose, as determined by

neutron activation analysis, is given in column 2. The beam current is given in column 3; variations in beam current over the range

employed were not expected to affect the results. The signal, S, given in column 4 is

the ratio of the sum of the phosphorus counts

to the sum of the silicon counts from the num

ber of channels, indicated in column 5, over which the profile was integrated. The sputtering time, in seconds, was 22 times the num

ber of channels. The average phosphorus density, given in column 6, was obtained by dividing 90 percent of the implanted dose (col- umn 2) by the crater depth which was found

from the mean normalized sputtering rate (table 5) , the beam current (column 3) , the nomi

nal raster area, and the sputtering time.

The calibration coefficient, the ratio of the

average phosphorus density to the signal, is

given in column 7; this coefficient is ex- pected to be constant if a linear relationship exists between the signal and the phosphorus density.
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

The deviations from the mean were shown to be

random by an analysis of residuals. The relative sample standard deviation of the calibration coefficient for these 18 analyses made

over a period of several months was 25 percent; this is a measure of the long-term re- producibility of the IMMA when measuring phosphorus in silicon.

Other Impurities — Specimens of phosphorus implanted in silicon dioxide and arsenic, sodium, antimony, and gold in both silicon and

silicon dioxide have been prepared and both

neutron activation and ion microprobe mass analyses are underway.

(R. Dobrott" and G. B. Larrabee""')

4.2. Electron Spectroscopy Techniques

Ion-Beam Eastering — In order to achieve a large ion current density (>40 yA/cm^) uniform to better than 0.5 percent over a square

of edge 250 pm (the size of the scanned electron beam) , it is necessary to raster the ion

beam which is employed to sputter-etch the

crater in carrying out depth profiling measurements with Auger electron spectroscopy

(AES). The unrastered beam has an FWHM of

about 3 mm. With typical rastering, this is

broadened by a factor of from 1.2 to 2.5.

The beam maximum may, however, undergo a shift in position on rastering if the beam is

initially asymmetric, because the beam raster

is of the same order as the beam dimensions.

In addition, the voltages applied to the deflection plates must be strictly of triangular waveshape. If the waveform is at all

rounded, more time is spent in some regions

than others, and it is possible with large

'scans to obtain craters which are deeper at the edges than at the center. Measurements

of these effects were made, and an alignment

technique was developed to avoid the effects

of beam asyariietry

.

The alignment technique consists of the following procedure:

(1) Allow the ion gun to come to thermal equilibrium at the emission current to be used by running it in the absence of both

argon and beam voltage for at least 30 min

prior to beginning profiling. In addition, run the gun for 15 min under operating conditions while the ion current density is measured using a Faraday cup.

(2) Position the Faraday cup at the focal point of the analyzer and focus and deflect the unrastered ion beam for a maximum

current into the cup.

(3) Measure the profile of the rastered

beam in two directions, one parallel to the

axis of the cylindrical mirror analyzer (CMA)

and the other perpendicular to this axis, taking current readings as a function of micrometer setting. If the distribution is not centered, adjust the deflection to center the

distribution at the analyzer focal point.

The ion gun controls and electron gun deflection controls are then held constant for a series of comparative profiles. Before and

after these profiles are made, the ion-beam

current density is checked by bringing the

Faraday cup to the focal point of the analyzer using the scanning sample positioner and

the 2000-eV elastic peak. The former positions the aperture on the axis of the analyzer; the positioning adjustment along the CMA

axis can be made with the aid of the latter.

(W. E. Spicer''", N. J. Taylor § , and Y. E. Strausser^)

Ion-Stimulated Auger Transitions — Measurements of ion-stimulated Auger (ISA) emission

(NBS Spec. Publ. 400-25, p. 17) have been

completed to the point of defining the possible interferences which this process might

produce in profiling work. ISA emission has

a very strong primary ion energy dependence

which leads to problems with 3-keV argon-ion

bombardment of either aluminum or silicon

but produces negligible interferences at a primary ion energy of 1 keV. Fortunately,

it has been found on other grounds that the use of ion energies near 1 keV produces the

best depth resolution in the SiO /Si system

x

(NBS Spec. Publ. 400-25, pp. 16-17).

The ISA electrons are produced in

transitions in silicon and aluminum. There

are no ISA KLL transitions in either silicon

or aluminum; therefore, these transitions

give good profiling data even at 3-keV ion

energy. ISA emission has also been observed

from silicon dioxide and alumina, although

Work performed at the Central Research Laboratories of Texas Instruments Incorporated

under NBS Contract No. 5-35917. NBS contact

for additional information: K. F. Galloway.

^Work performed at Stanford Electronics Laboratories of Stanford University under NBS

Contract No. 5-35944. NBS contact for additional Information: K. F. Galloway.

§ Work performed at the Surface Analysis Laboratory of Varian Vacuum Division under subcontract to Stanford University.
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

the yield is greatly reduced from that in

the spectra of pure silicon and aluminum.

The characteristics of the ISA spectra, such

as line shapes, lead to the speculation that

the ISA electrons originate from single ions

or small clusters leaving the specimen surface. (W. E. Spicer+,

N. J. Taylor^, and Y. E. Strausser§

)

Profiling of Metal-Siliaon Dioxide Interfaces — Preliminary AES measurements have been made

on interfaces between sputtered pure or doped

aluminum metal films and thermally grown sili- con dioxide. In the case of pure aluminum,

the primary interest was in the solid-state

reactions which might occur with the alumi- num reducing the silicon dioxide to form

alumina and silicon. This reaction did occur;

a typical profile measurement, illustrated in

figure 11a, exhibited a surprisingly broad interface. Measurements on such films following post-deposition annealing showed results

similar to those of measurements on films

without post-deposition annealing.

In the measurement of sputter-deposited alumi- num films doped with 6-percent copper, the

interesting result is the distribution of the

copper through the film; a typical result is

shown in figure lib. This double peaking

with the majority of the copper sitting near

the silicon dioxide has been measured in

films produced at different times and in mea- surements with significant differences in the

profiling conditions, but at present there is no explanation for the observed segregation

effects. (Y. E. Strausser^

,

P. E. Luscher^ , and J. S. Johannessen"'"^

)

Limitations on Interface Width Resolution in

Auger-Sputter Profiling — Although numerous

effects that cause broadening of an interface

during Auger-sputter profiling can be reduced

to negligible levels by selecting appropriate

measurement conditions [45], two broadening

effects inherent in the technique remain:

t.Work performed at Stanford Electronics Laboratories of Stanford University under NBS

Contract No. 5-35944. NBS contact for additional Information: K. F. Galloway.

^Work performed at the Surface Analysis Laboratory of Varian Vacuum Division under subcontract to Stanford University.

Partially supported by Electronics Research

Laboratory, University of Trondheim and the

Royal Norwegian Council for Scientific and

Industrial Research.

1 1 1 1 1

1

1

Si in SiOj

—-1**''

1 1

Si in Free Si

1

1 1

Profile through interfacial region be- tween aluminum and silicon dioxide films

b. Profile through copper-doped aluminum

film on silicon dioxide. A 200-nm thick

portion of the film was doped with ^4- percent copper and 2-percent silicon; th remainder of the film was doped with 6- percent copper.

Figure 11. Composition profiles of metalsilicon dioxide interfaces by Auger-sputter

techn i ques

.

31
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

electron escape depth and ion knock-on mixing.

Initial efforts have been concentrated on the

electron escape-depth problem.

Consider an interface profile given by a function, f (x) ; at a distance, d, away from the

interface, the emission strength of a transition is given by the convolution of f(x) with

the escape-depth function:

e

""'^^ f(x'-d) dx' =

-d/L

(26)

-x/L . , e f (x) dx .

As an example, consider the case where

f(x) = 0 for X < 0, and

f(x) = 1 for X > 0,

shown in figure 12 as the solid curve. For

d > 0, the interface [which is defined as the point a, where f (a) = 0.5] has not yet

been reached, and eq (26) reduces to

-d/L

0

-x/L ^ -d/L

e dx = e (26a)

For d < 0 the interface has been passed, and

eq (26) reduces to

e dx = 1 . (26b)

The measured interface profile, as given by

eqs (26a) and (26b) is shown in figure 12 as the dashed curve. The measured interface position is given by d = L In 2 = 0. 7 L closer

to the surface than the actual interface, so that for transitions of different escape

depth the measured interface position is not

identical. If the measured interface width,

W^,, is defined as that thickness for which

M

the signal strength rises from 10 to 90 percent of its maximum value, W„ can be found

M

from eq (26a)

:

W,, = L{ln 10 M

In 1,11} = 2.2L (27)

In the case of silicon and oxygen, the es- cape depths have been estimated to be 0.5 nm

for the silicon LVV transition, 1 nm for the

oxygen KLL transition, and 3 nm for the silicon KLL transition. For the silicon LVV

transition, the minimum broadening of a step

W„= 2.2L

I* 0.7L

DEPTH Figure 12. Broadening of a step interface

resulting from the effects of electron escape

depth

.

interface is 1.1 nm. Since the measured interface width is a convolution of the escape

depth with the actual profile, the broadening

of an interface with finite real width is not additive but behaves more nearly as an rms average. For example, an interface 1.0 nm wide would appear to be about 1,5 nm wide

when measured with the silicon LVV transition.

The calculated shift in the interface position due to escape depth as indicated in eq

(27) provides a means for determining the actual interface position. In this analysis,

however, the effects of sputter knock-on mixing have been neglected. This process alters

the actual interface profile, so that f(x) is

a more complex function with dependence on

the argon ion range and the distance between

the surface and the interface, A quantitative formulation similar to eq (26) which includes both the effects of electron escape

depth and ion knock-on mixing is under development. Ideally, this will alow a deconvolution of the real interface profile from the

measured one.

(C. R. Helms+ and W. E. SpicerT)

Auger-Sputter Profiling of MOS Oxides — Work

has been undertaken to investigate the relationships between the microscopic properties

(such as width and stoichiometry) of siliconsilicon dioxide interfaces, the electrical

characteristics of the interfaces, and the

methods of preparing oxide films. Initial efforts are being concentrated on measuring the

Work performed at Stanford Electronics Laboratories of Stanford University under NBS

Contract No. 5-35944. NBS contact for additional information: K. F. Galloway.

32
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

Table 7 - Preliminary Results of Auger-Sputter Profiling Measurements on CVD and Dry Thermal Oxides

Interface Width, nm

;: — Sputtering Specimen From Oxygen KLL Transition From Silicon LVV Transition Rate,

Measured Corrected Measured Corrected nm-cm^min-pA

Average dry

thermal oxide

CVD oxide

without anneal

CVD oxide

with anneal

3.3 ± 0.3 2.5 ± 0.5 2.5 ± 0.3 2.25 ± 0.35 0.11 ± 0.005

5.5 5.0 4.1 4.0 0.145

5.35 4.9 4.5 4.4 0.121

microscopic properties of silicon-silicon dioxide interfaces with interface state density

(as determined by capacitance-voltage measurements [46]) in the range from 10^ to 10^^

cm~^. Oxides prepared in four ways are being

investigated: dry thermal oxidation, steam

thermal oxidation, chemical vapor deposition

(CVD) , and vacuum evaporation. Growth or deposition time and temperature are being varied in the study, but post-growth processing

is being held to a minimum in order to avoid

additional complications from other processing steps.

Preliminary results on oxides prepared^-* by

CVD are compared in table 7 with the properties of oxides thermally grown^'* in dry oxygen. The interface width was measured between the 10-percent and 90-percent values

of the Auger signal. A correction for electron escape depth was applied as discussed

above to obtain the corrected width. The

values for the "average dry thermal oxide" were obtained by averaging runs for a number

of different specimens. These results indicate that the CVD oxide-silicon interfaces

are 2 to 2.5 nm wider than the interface in

a "good" thermal oxide. The bulk of these

oxides also appears to have higher density as can be seen from the higher sputtering rates

obtained for them. Following the anneal, the

interface state density of the CVD oxide was approximately one order of magnitude higher

than that of a good thermal oxide. A meaningful value of surface state density could not

be obtained for the CVD oxide without anneal

because the C-V curve was very broad, indicative of the much poorer electrical characteristics of this oxide.

(C. R. Helms^ and W. E. Spicer")

4.3. Techniques for Chemical Diagnostics in Semiconductor Processing

This task was undertaken to evaluate the application of x-ray photoelectron spectroscopy (XPS)^ to diagnostic measurements in semiconductor device processing and to establish

and document appropriate methods for speci- men preparation and data collection, analysis

and interpretation. The work is being carried out at the Surface Analysis Facility of

the Jet Propulsion Laboratory [46]. This

facility has been organized to provide as broad a spectrum of surface data as could be

integrated into one experimental system. Although the emphasis is on XPS, the facility

also includes instrumentation for Auger electron spectroscopy (AES) , a scanning lowenergy electron probe (SLEEP), secondary ion mass spectroscopy (SIMS) , and ion scattering

spectroscopy (ISS) , and various chambers for

specimen preparation, wet chemistry, plasma

cleaning, and in situ oxidation.

The e-lectron methods chamber is based on a modified HP 5950A ESCA spectrometer"^ with a

These oxide films were grown and electrically characterized by Dr. Noble Johnson at Xerox Palo Alto Research Laboratories who

also performed the ellipsometric measurements from which the sputtering rates were determined

.

"^Work performed at Stanford Electronics Laboratories of Stanford University under^ NBS

Contract No. 5-35944. NBS contact for additional information: K. F. Galloway.

XAlso known as Electron Spectroscopy for Chem

ical Analysis (ESCA).

"*"See disclaimer, p. vi

.

33
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

monochromatic aluminum Ka x-ray source. The

electron kinetic energy analyzer is a fixedenergy 180-deg hemispherical electron monochromator with a retarding-potential lens

system for scanning the electron energy spectrum. The detector system uses a channelplate electron m.ultiplier with phosphor and a vidicon for data readout. The use of the

monochromatic source and high-resolution

spectrometer results in narrower line widths

and consequently more chemical information.

However, the enhanced signal-to-background

ratio for the monochromatic system (400 to 800:1) as compared to the standard source

(7 to 10:1) is even more important for the

study of low-level impurities in the siliconsilicon dioxide system. For application to the study of the silicon-silicon dioxide interface, it was necessary to determine the

escape depth of photoelectrons in silicon dioxide. This escape depth for silicon 2p photoelectrons was determined to be about 4.2

nm; therefore, XPS can be used to study interfaces under 4 to 10 nm of silicon dioxide.

Determination of Binding Energies — The interaction of electrons, ions, and photons with

the silicon-silicon dioxide system was studied, and both electron- and ion-stimulated decomposition were observed. Serious difficulties were encountered because of charging effects. Silicon dioxide specimens tended to build up large positive charges on the surface after x-irradiation. This resulted in a binding energy uncertainty greater than or equal to the chemical shift. In order to stabilize these measurements, an interactive

SLEEP system for referencing spectra to the vacuum level was developed to permit direct

measurement of surface potentials and therefore make possible a series of biascontrolled experiments. These experiments

demonstrated the existence of several chemical states of sodium which are involved in

trapping at the silicon-silicon dioxide interface [48] . The importance of surface potential control was also demonstrated in differential charging experiments. Several speci- mens were prepared with a thin surface oxide

covering a highly conductive layered substrate. Peaks in the XPS spectrum were observed from the oxide region as well as the

substrate. The oxide peaks showed a strong

charging shift to high binding energy, which

was a function of the electron flux density

to the surface. The substrate peaks showed

no such effect. Consequently, the recorded

spectrum varied markedly as a function of

secondary electron flux. Such physical

charge differences could be misinterpreted as chemical shift variations. To improve the

capability of determining binding energies in terms of a known potential, considerable effort was directed toward decreasing the size

of the interrogating electron spot in the

SLEEP experiment. The original implementation of this system employed an electron gun

from a comm.ercial vidicon, the gun being electrostatically focused and deflected. This

gun was modified in two ways: 1) a lOO-ym

diameter aperture was substituted for the

1-mm diameter aperture in the original gun,

and 2) a ground-plane extension tube was fabricated onto the end of the gun to bring the

ground plane as close as possible to the specimen surface. The first modification resulted in smaller beam diameters and lower current densities, both of which are desirable

in the present application. The second modification seems to have significantly reduced

surface patch effects on the irradiated portion of the insulator.

Preliminary results with the modified gun indicate that 1) peak positions can be stabilized to ±0.2 eV with little difficulty, 2)

line widths of observed peaks (for insulators) can be reduced significantly by minimizing the surface potential distribution over

the specimen, and 3) the surface of an insulator charges to a limiting value and then

discharges in a time scale of milliseconds.

This last behavior seems to be related to patch effects on the surface and results in

a 2- to 4-eV peak-to-peak contribution to the line shape.

Interface Studies — Studies of the siliconsilicon dioxide interface involve two fundamentally different experimental procedures:

1) grow oxides thicker than 10 nm and etch

back (by ion milling or wet chemistry) to the

interface while studying changes in silicon

and oxygen chemistry or silicon-oxygen ratios,

or 2) grow very thin oxides (1 to 10 nm thick)

and follow chemical changes as a function of

increasing thickness. The object of both of

these approaches is to determine the chemical

structure of the interface and boundary oxide.

This understanding, together with a study of

how contaminants affect the interface region,

is a key prerequisite for the control of electrical characteristics of MOS devices.

It has been observed previously that cleaning

procedures used in silicon wafer processing

do not remove organic surface contamination

[49]. To study the effect of residual car- bon on the interface state density, pairs of

silicon wafers were cleaned by a variety of

procedures and, without removing them from

the analysis facility, oxidized in dry oxygen

34
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

I _J L

0! 12 16

IHTENSmr RATIO |C Is/Si 2p|

Figure 13- Interface s'-ate density, Ngs, as

a function of carbon concentration at the in- terface. (The carbon concentration is ex- pressed as the intensity ratio of the Is photopeak of carbon at 0.7 eV (o) or 0.55 eV (a) to the 2p photopeak of silicon.)

at 800°C to produce a 6-nin thick oxide. On one wafer from each pair the intensity of the

carbon Is XPS peak was measured. The other

wafer of each pair was metallized with alumi- num by a shadow-mask procedure, and the interface state density was determined by means of

capacitance-voltage measurements. Several

pairs of wafers were held in a nitrogenfilled dry box until XPS could detect no species other than silicon. One of these pairs was directly oxidized and measured; several

others were intentionally contaminated with

controlled amounts of well-defined organic

chemicals before oxidation. Six pairs were

cleaned using six different cleaning recipes

just prior to oxidation. These cleaning procedures resulted in differing amounts of car- bon contamination at the interface, as determined by the carbon Is photoelectron peak

intensities

.

The relationship between interface state density and the carbon Is intensity (normalized

to the silicon 2p intensity) is shown in fig- ure 13. There are three regions of behavior.

The initial region shows a rise in interface

state density with increasing carbon Is intensity. This is followed by a region over which interface state density is not strongly

dependent on carbon Is intensity. At still

higher values of carbon Is intensity, the interface state density decreases sharply. The

carbon signal strength was also measured for

these specimens before oxidation. There was

no correlation of interface state density

with the carbon intensities before oxidation.

In addition, several wafers were coated v;ith the same thickness of organic compound (1

nm) , and the carbon intensity was monitored

as a function of final oxide film thickness.

Initially, a rapid change in the carbon spectrum was interpreted as evidence of both oxidized and reduced species. There was a 10-

to 20-percent loss of carbon at the beginning

No further change was observed in amount of

carbon after formation of a silicon dioxide

film 1.5 to 2.0 nm thick. The ratio of oxidized to reduced carbon stabilized after approximately 6 nm of silicon dioxide had been

grown

.

From these results, it is tentatively con- cluded that different chemical forms of car- bon added at the silicon surface show different retention ratios after oxidation. The

final amount and chemical state of carbon at the interface significantly affect the interface state density. Additional experiments

designed to establish the mechanism of the ef

feet are now under way.

Data Reduation Procedures — A significant

part of the work was devoted to examination

of data-reduction algorithms. Approximately

40 different methods have been described in

various elements of the periodical literature

These methods have all been reduced to algorithms and implemented in a computer program.

This program was used to study the effects of

methods of noise removal and deconvolution on simulated test spectra with signal and noise

characteristics virtually identical to those

observed in XPS and AES. These spectra con- sisted of 2:1 intensity-ratio Lorentzian doublets which have a peak splitting larger than

the natural line width and which are con- volved with an asymmetric Gaussian instrument

function. Asymmetry was introduced because

of the severe complications this adds to the

problem and because instrument functions in

electron optics are asymmetric. For example,

a 2:1 doublet with a full width at half

maximum of 0.1 eV, split by 0.6 eV closely ap- proximates the natural 2p photoelectron spectrum of silicon. A typical instrument function width was chosen as 0.6 eV. A series of test spectra was prepared with signal-tonoise ratios varying from 1000:1 to 0.001:1.

Two types of noise were employed: Gaussian

and experimental; the latter was stripped

from a large sample of XPS spectra and included a significant 1/f component.

In order to evaluate the methods, two criteria were chosen: 1) magnitude of the residuals and 2) mean square error. All the meth35
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

ods were reduced by means of linear algebra

into a common form and it was determined

that all deconvolution methods could be re- duced to two variations. It was observed

that the differences between published algorithms are generally manifested in the way

the noise terms are manipulated. Virtually

all iterative deconvolution routines employ

a minimization of residuals as a criterion

for convergence. Large changes were observed

among the various methods in the magnitude of

the final residuals but there was little observable effect on the magnitude of the mean

square error.

A publication is being prepared [50] in which

existing methods are evaluated and a number

of algorithms are developed which give varying degrees of recovery of original information. Similar algorithms are being developed

to treat the problem of recovering signal

from noise with explicit recognition of the

implications of the compromises made.

A major difficulty in applying such methods

is that of characterizing the instrument function, the basic signal, and the noise component. In the course of this research a num- ber of algorithms have been developed; these

results and the methods used to obtain them

will be the subject of a later publication.

(F. J. Grunthaner""'

)

4.4. Nuclear-Track Technique

Neutron activation analysis [44,51] is a useful technique for determining total densities

at trace levels of most impurities in silicon.

However, boron cannot be detected by conventional neutron activation analysis. Therefore, alternative methods are necessary for

measurement of total boron density. The

nuclear-track technique had previously been

used to detect trace concentrations of boron

in glass [52]. Work was begun to evaluate

the applicability of this method to the determination of boron density in silicon

slices

.

The analysis for boron in silicon by the

nuclear-track technique is based on the num- ber of radiation-damaged areas produced in a dielectric material in contact with the silicon. These radiation-damaged areas are a result of the predominantly 1.44-MeV alpha particles that are produced during thermal neutron bombardment of the boron by the reaction

^ °B (n,a) ''Li . The latent radiation-damaged

areas or tracks are made visible in the dielectric detecting material by chemically

etching the detecting material. The number

of tracks per unit area in the detector is

proportional to the boron density in the sili

con. The path length of the 1.44-MeV alpha

particle in silicon is 2.5 ym, and the product of this length and the area yields the

volume sampled.

Specimens for analysis were obtained from

boron-doped silicon crystals grown by the

Czochralski process with a [111] growth axis.

Some of the specimens were prepared from polished slices supplied by commercial vendors

and others from slices cut and polished inhouse. Prior to scribing or cutting the

slices into sections for analysis, fourprobe resistivity measurements were made on

the back surface of each slice in accordance

with the standard procedure [1] . For most of

the slices, the uniformity of the resistivity

across the slice was measured, or had been

previously measured on a companion slice cut from the same crystal, by one of the following measurements: four-probe measurements at 100-mil (2.5-mm) increments along two perpendicular diameters, four-probe measurements at center and half-radius positions, or spreading resistance measurements at 25-ym increments along a diameter. All slices had radial resistivity variations of less than 5 percent between center and half-radius positions

and no fine structure was observed in the

spreading resistance.

In the analysis procedure, a specimen, typically square (7.5 mm on a side) and 0.3 mm

thick, was sandwiched between two pieces of

cellulose acetate butyrate plastic, vacuumheat-sealed in polyethylene, and irradiated

in the NBS reactor. The irradiation time and

location in the reactor were dependent on the

anticipated boron density. Time in the reactor ranged from 1 s to 10 min for a typical

1 3 1

neutron flux density of 1.33 x 10

Following irradiation, the detectors were sep

arated from the silicon specimens, chemically

etched in 6.5-normal sodium hydroxide for 5 min at 72°C, and rinsed with distilled water.

After drying in air, the detectors were

mounted on microscope slides, and the alpha

particle tracks were counted with the aid of an optical transmission light microscope.

The counting area was 1500 ym^; usually sever

al such areas on the detector were counted

and averaged to arrive at a value for each

Work performed at Jet Propulsion Laboratory

under NBS Order No. 611377. NBS contact

for additional information: K. F. Galloway

36
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

I

! Illlllll I Mlllll{ I llllllll I I llllllll I llllllli

' Figure 14. Resistivity of p-type silicon as

a function of boron density as determined by

the nuclear-track technique. (Experimental

data shown as open circles; the solid curve

i s from Wagner [21 ] .

)

specimen. Standard Reference Materials, SRM

619 Trace Element Glass Standards, containing

known densities of boron in glass [52], were

irradiated simultaneously with the silicon

specimens. The boron density in the silicon

was obtained by comparing the track count

from the silicon with that from the boron-inglass standards with a correction to account

for the difference between the range of the

alpha particles in silicon and that in glass.

In later irradiations, a silicon specimen,

previously analyzed with respect to the SRMs

,

was used as a reference.

Initial results, plotted in figure 14 together with Wagner's empirical relationship between resistivity and boron density, are quite encouraging. Agreement between the

nuclear-track data and Wagner's curve is generally better than 10 percent, although a few

data points agree much less well. Repeated

nuclear-track measurements on the same speci- men generally agreed to within a factor of

1.-5, but again data on a few slices show

wider variation. At this stage in the investigation it is not clear whether the large

variations are due to nonuniformity of the

boron distribution in an individual slice,

to a variability in the nuclear-track measure- ment, or to some combinations of these conditions. The error associated with trackcounting statistics did not exceed 3.5 percent. In some cases, however, the boron density within a single counting area was observed to vary as much as ±10 percent; this

variability is indicative of the boron distribution only within a 2.5-Mm depth from the

surface

.

The most reproducible nuclear-track determination occurred on a slice with a boron density

of 2.26 X lO^S cm~^. The total variability,

based on three separate irradiations and

counts, was 1 x 10 ^'^ cm"^. Electrical mea- surements of net carrier density and resistivity were made on a companion wafer on which

test pattern NBS-3 [28] had been fabricated.

The net carrier density (equal to the boron

density less the compensating donor impurity

density) was found to be 2.22 ± 0-05 x 10^^

cm~^ from junction C-V measurements on five

diodes. A value of about 2.1 x 10^^ cm~^ was obtained from MOS C-V measurements on a single capacitor. A resistivity of 0.72 n^cm

was measured by means of the four-probe collector resistor [53] in the region of interest on the fabricated wafer compared to 0.73

fi'cm for the specimen used for the track technique measurements. Additional measurements

are being carried out to determine whether

this excellent agreement is characteristic of

the method.

The lower limit of detectability is governed

by the background count level. For the present geometry the level is about 5 x 10 cm~^;

thus, a boron density of about 10^^ cm~^ appears to be a lower limit for routine measurements by the nuclear-track technique.

(W. R. Thurber, B. S.

Carpenter"*""^ , R. L. Mattis , and D. R. Ricks)

4.5. Optical Test for Surface Quality of

Sapphire Substrates

Much of the cost of sapphire substrates for

silicon-on-sapphire devices is incurred in

the polishing operation. Up to now there has

been no quantitative measure of surface quality; both the substrates and the silicon

films epitaxially grown on them are passed to subsequent processing steps on the basis of

qualitative visual inspections. As a result,

sapphire substrates are polished until evidence of surface damage is no longer visible.

If devices with satisfactory electrical properties could be produced on substrates with

an intermediate degree of polishing, substantial cost savings could result.

NBS Activation Analysis Section, Analytical

Chemistry Division.

37
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

1 1 1

- \\

1 1 1 1 1 1

\

-• \.

^ •

• •

1 1 1 1

1

1 1 1 1 1 oil 0 16 0 18 0 20 0 22 0 24 0 26 0 26 0 30 0 32 0 54 C 36 REFLECTANCE RATIO Heoo cm'l /I450 cm'i ] Ultraviolet reflectance ratio at A = 2'tO nm (x) or A = 280 nm (•) for epitaxial

silicon films as a function of tfie infrared reflectance ratio of the corresponding sapphire substrate.

• 1 1 1 1 1 1 1 1 1 1 \

\

- \

\\

\ \

\\

\\

•

1 1 1 1 0 14 0 16 0 18 0 20 0 22 0 24 0 26 0 28 0 30 0 32 0 34 0 36 REFLECTANCE RATIO (Ijoo cm"'/I<50 cm"']

b. Half-width of the x-ray diffraction peak

of epitaxial silicon films as a function

of the infrared reflectance ratio of the

corresponding sapphire substrate.

Half-width of the x-ray diffraction peak

of epitaxial silicon films as a function

of the ultraviolet reflectance ratio at

A = 240 nm (x) or A = 280 nm (•)

.

Figure 15. Correlations between optical and x-ray determinations of the quality of a sap- phire surface and the corresponding deposited silicon film.

This task was undertaken to develop rapid,

nondestructive, quantitative, optical tests

for the surface quality of sapphire substrates and for the quality of epitaxial silicon films on sapphire and to establish correlations between the results of these tests

and the electrical properties of devices fabricated in the silicon films.

Specimen sapphire substrates were variously

polished with diamond grit (of different particle sizes), Linde § , and silica sol [54] to introduce different amounts of damage in

the different substrates. Infrared multiplereflectance spectra were then obtained from

the substrates to determine surface character. The quality of each surface was ex- pressed as the ratio of the reflected intensity at wavenumber 600 cm~-^ to that of wavenumber 450 cm~l (NBS Spec. Publ. 400-25, pp. 20-23) [55]. The higher the value of this

ratio, the better is the surface quality.

Epitaxial silicon was grown on all substrates

simultaneously, and ultraviolet reflectance

measurements performed on each silicon film

See disclaimer, p. vi

.

38
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

to determine the quantity log(R^/R) at the

wavelengths 240 and 280 nm (NBS Spec. Publ.

400-29, pp. 29-31). The lower the value of

this term, the better is the quality of the

epitaxial film. The results, which are plotted as log(R^/R) as a function of infrared reflectance ratio in figure 15a, indicate that there is a good correlation between the surface quality of the substrate

and the epitaxial film characteristics as determined optically.

The epitaxial silicon films were then analyzed by x-ray dif f ractometry to determine

crystalline quality. Figure 15b shows a plot of diffraction peak half-width (6:29 scan, (400) diffraction plane) for the various silicon films as a function of the infrared reflectance ratio for the corresponding

substrates. The larger the value of the diffraction peak half-width, the poorer is the

film quality. There is also a good correlation between film character as determined by

x-ray dif f ractometry and the initial substrate character as determined by infrared

reflectance. The shape of the curve in figure 15b is very similar to those in figure

15a, suggesting a correlation between the ultraviolet reflectance and x-ray measurements.

This correlation is confirmed by the nearly

straight-line plots of figure 15c, in which

the x-ray data are plotted as a function of

the ultraviolet reflectance data. It has also been observed that when silicon film

quality is adversely affected by growth con- ditions, this condition can be detected by

optical reflectance measurements in the ultraviolet. Thus, ultraviolet measurements

conveniently can be used to monitor the crystalline character of films independently of

whether this character is influenced by substrate character or by growth conditions.

The sensitivity required to ensure satisfactory device operation has not yet been established, but some clarification of this point

can be expected from the performance of devices which are being fabricated using the

SOS/LSI test pattern (NBS Spec. Pubis. 400-

19, p. 47, and 400-25, pp. 44-45).

(M. T. Duffy^",

P. J. Zanzucchi^^, and G. W. Cullen''^)

Work performed at RCA Laboratories under

NBS Contract No. 5-35915. NBS contact for

additional technical information: K. F. Galloway

.
5 . n A T E R I A L S A i'J D PROCEDURES FOR

WAFER PROCESSING

5.1. Ion Implantation Parameters

This task is concerned with the identification of critical parameters associated with

ion implantation and with the development of

methods for the measurement and control of

these parameters. A study [56] was conducted

to investigate how the control of depth distributions in silicon depends on the control

of the angle between the ion beam and the

crystallographic axis of the wafer being implanted. In implantation of low-velocity

ions (high atomic number, Z, or low energy,

E, or both), a critical angle for channeling,

^'^ [57], which is proportional to (Z/E)i (approximately inversely proportional to the

square root of the ion velocity) , can approach the commonly employed randomization

angle, This situation would seem to imply that channeling should be difficult to prevent for intended random implants of lowvelocity ions, or at least that large channeling tails should exist. This hypothesis was

tested by measuring and comparing the depth

distributions of electrical activity of ran- dom and accurately channeled ion implantations and studying the sensitivity of the

depth distributions to angular control. For

the cases studied, which are representative

of most current laboratory implantations, \p'c varied from 2 to 7 deg. Learning how implantation distributions change when ^'^ was com- parable to the practical laboratory value of

7 or 8 deg for ii was of particular interest.

Another important consideration was the accuracy with which the angle must be controlled

to guarantee a particular quality of channeled distribution.

The technique used in this investigation was

to implant ions at various angles away from

the accurately channeled alignment and toward

the direction of random equivalent (RE) orientation up to the RE angle of about 7 deg, and

to record the changes in depth distribution

of electrical activity measured by differential C-V profiling (NBS Spec. Pubis. 400-25,

pp. 31-32, and 400-29, p. 40). The implantation dose was 1.5 x 10 '^ cm~^ and the implants were annealed at 800°C for 20 or 30

min in a dry nitrogen atmosphere. This an- neal was shown to fully activate these low

dose im.plants. The implantation angle, ii, was controlled to less than 0.1 deg for small

angles away from axial alignment by a combination of Rutherford backscattering of protons [58] and a goniometer capable of controlling the angle to about 0.02 deg. From 3 deg

to the implantation angle was obtained by

tilting a standard target mount. This degree

of angle control was considered adequate for

the larger, and hence likely to be less sensitive, angles. Boron and arsenic ions were implanted at 300, 75, and 20 keV and indium and

aluminum ions were implanted at a few se- lected energies to fill in the range of

from 2 to 7 deg. In addition, the analysis

of a detailed set of published profiles for

phosphorus implanted at 450 keV [59] was included in the study.

It was necessary to devise a method to characterize the change in implanted profile with

angle. The technique selected was to measure

the differential integrals of the areas between the various depth distributions as illustrated in figure 16. From these one can obtain

:

1. the fraction, ^, of dechanneled ions

that are not within the channeled distribution of a perfectly aligned implant, and

2. the fraction, c,, of ions in a random

implant that are deeper than the calculated

amorphous distribution which, in accordance

with common practice, was computed as a Gaussian approximation using published range and

straggle data [60]. Although it is recognized that a more complete theory for describing the implanted profile may attribute some

of the tail deviation to other mechanisms,

the fraction i; was taken in this work as the measure of the magnitude of the channeling

tail.

Two important results of the study are sum- marized in figure 17. There is an approximately quadratic dependence of the channeling

tail on \p'^, as shown in figure 17a. This

plot provides a means to estimate the magnitude of channeling for any given implant into

bare crystalline silicon performed in the ran- dom equivalent orientation at a dose below

the onset of amorphization.

The fractions of implanted ions not in channeled distributions are plotted against

in figure 17b. These plots show that the

fraction of ions dechanneled increases ra- pidly as ^' decreases and increases approxi40
MATERIALS AND PROCEDURES FOR WAFER PROCESSING

mately linearly as the implantation angle deviates from the normal to the crystal plane.

Accurately channeled distributions can only

be achieved at low ^'^ with a high degree of

angular control, whereas as ^p'^ approaches

much less control is required.

(R. G. Wilson" and D. M. Jamba")

AMORPHOUS

DISTRIBUTION RANDOM EQUIVALENT

(MINIMUM CHANNELING)

DISTRIBUTION MISALIGNED (CHANNELED)

DISTRIBUTION — OPTIMALLY OR "PERFECTLY" CHANNELED DISTRIBUTION

AREA INTEGRAL EQUALS

FRACTION OF IMPLANTED

IONS NOT WITHIN CHANNELED DISTRIBUTION

AREA INTEGRAL EQUALS

FRACTION OF IMPLANTED

IONS DEEPER THAN AMORPHOUS DISTRIBUTION, OR THE MAGNITUDE OF THE CHANNELING TAIL

DEPTH, iim

Work performed at Hughes Research Laboratories under NBS Contract No. 5-35891. NBS

contact for additional information: K. F. Galloway

.

Figure 16. Illustration of implanted randomequivalent and channeled depth distributions

and comparisons with ideal amorphous and per- fectly channeled distributions, respectively.

0 2 4 6 8 10

a. Fraction, of ions channeled in a ran- dom equivalent distribution, 7 deg off

the crysta 1 1 ograph i c axis. The solid

line represents the empirical relation:

C = 0.018 (i|/c " i/o)^) where \pQ is an em- pirical constant

.

Figure 17- Fraction of implanted ions outside

cal angle for channeling, rp^. 41

0 2 4 6 8

V'c Ideg)

b. Fraction, E, , of ions dechanneled in a channeled distribution misaligned by an angle tj; from the crysta 1 log raph i c axis

for = 1 (o) , 2 (a), and 5 (0) deg. The

solid lines represent the empirical relation: ^ = {2^iJ -

Ideal distributions as a function of the criti
6 . PHOTOLITHOGRAPHY

6.1. Optical Imaging for Photomask Metrology

Additional investigations of the effects of

the operating conditions of an optical microscope on line-width measurements in transmitted illumination were completed [61]. In particular, the effects of non-zero transmittance of the opaque background and of spherical aberration and defecus , not included in

the previously reported work (NBS Spec. Publ.

400-29, pp. 50-52), were studied.

It was found that the 25-percent transmittance criterion for edge location in a microscope with diffraction-limited optics and nu- merical apertures of 0.60 for the condenser

lens and 0.90 for the objective lens is appropriate for clear lines as narrow as 0.5 ym in

a completely opaque background. If the background has finite transmittance , Tq, the

transmittance , T^, which corresponds to the true edge location, is given by

= 0.25 (1 + /t^)2 . (28)

As an example of this correction, consider a black chromium photomask with Tq = 0.004;

the transmittance at the true edge location

would be 0.36 rather than 0.25. If the measurement were made with an objective lens

with a numerical aperture of 0.90, and if the

25-percent transmittance criterion were used,

the line would appear to be 0.08 pm wider

than its actual width. This error would in- crease with the presence of either defocus or spherical aberration.

The focus setting, or the resulting amount of

defocus, in an optical microscope is known to significantly affect the line image. Another

optical aberration, which is similar to defocus, is spherical aberration. This aberration is usually present to some degree in

most microscopes and can be partially compensated by the focus setting. Figure 18 shows

the calculated image profiles of a lO-pm wide

clear line for a microscope with constant 2A of spherical aberration and defocus varying

in steps of A/4 from 0 to -2X. The numerical aperture is 0.22 for the condenser and

0.65 for the objective. The choice of 2X of

spherical aberration corresponds to using an incorrect tube length between the objective

and eyepiece (i.e., 160 mm instead of 220 mm) or using a sample cover glass whose thickness does not match the thickness for which

the objective was designed (i.e., 0.12 mm instead of 0.17 mm). The curve for -IX of defocus has the steepest slope, whereas the curve with -X/2 of defocus has the maximum

contrast between the edge and the first dark

band (ringing) . Since the optimum focus setting is usually selected on the basis of the

image profile exhibiting 'the steepest slope

or the maximum ringing, a focus setting between those corresponding to -X/2 and -IX of

defocus would correspond to optimum focus.

For any of these settings, the true edge location corresponds to the 25-percent transmittance if the background is assumed to be

completely opaque.

The effect of making the measurement with

white light rather than quasi-monochromatic

light was also considered. If white light

rather than monochromatic illumination is

used in an optical microscope, additional mea

surement errors may be introduced. Figure 19

shows typical experimental profiles of one edge of a nominally 10-ym wide clear line for

white light and filtered light at 560 nm. The significant differences between the edge

profiles are due to both chromatic aberrations and variation in image magnification

with illumination wavelength. All of the microscope objectives examined exhibited this

difference to a varying degree indicating the

presence of varying amounts of longitudinal

chromatic aberration and chromatic difference

of magnification.

Based on the present results, it appears that

recommended equipment for accurate measurement of the width of photomask lines as narrow as 0.5 ym with an optical microscope using transmitted light will include the following: 1) the ratio of the numerical aperture of the condenser lens to the numerical

aperture of the objective lens should be

less than 1.0 (a reasonable choice is 0.60/

0.90.); 2) the sample illumination should be

filtered to give monochromatic light peaked

at the best response of the optical system;

3) the focus mechanism should be adjustable

to within a tolerance of X/4 (corresponding

to ±0.35 ym for an objective lens with a nu- merical aperture of 0.90 and illumination

wavelength of 560 nm) ; and 4) a transmittance of 25 percent corrected for the finite

transmittance of the opaque area of the photomask should be used as the criterion for

line-edge location. The only presently

known method of determining the 25-percent

threshold value on the image profile is from

a photometric scan of the image. The NBS

' optical-microscope system can presently measure line widths to within an uncertainty of

42
PHOTOLITHOGRAPHY

3.0 4 0 5.0

DISTANCE \fj.m]

Figure 18. Calculated image profiles of a 10-ym wide clear line for an optical microscope with a constant 2A of spherical aberration and defocus varying in steps of X/h

from 0 (a) to -2X (b) , 0.22 NA condenser and

0.65 NA objective, illumination wavelength

of 560 nm, and 0.13"um wide scanning slit.

DISTANCE [fJ.m\

Figure 19- Experimental image profiles of one edge of a nominally 10-ym wide clear line

for an optical microscope v/ith white light

(A) and filtered light at 560 nm (B) , O.6O NA condenser and 0.90 NA objective, and 0.13"Mfn

wi de scann i ng slit.

DISTANCE l/im

a . L i ne wi dth : 3 • 1 5 yni

DISTANCE \fjLm]

Line width: 0.5 ym.

Figure 20. Comparison of theoretical and experimental image profiles (wave trace and smooth

line, respectively) of clear lines for an optical microscope with O.6O NA condenser and O.9O

NA objective, illumination wavelength of 560 nm, and 0.13-ym wide scanning slit.

43
PHOTOLITHOGRAPHY

±0.2 Mm, If the scan is repeated at the same position on the line, the uncertainty

is less than +0.05 ym.

The remaining source of error is the edge

quality of the photomask. The test specimens used were the best obtainable with less

than O.l-ym variations barely observable optically at high magnification. However, variations in the edge response were observed.

Some indication of the variation may be found

in comparison of the right and left edges of

the typical experimental profiles given in

figure 20, although these profiles have been

selected for edge sharpness in order to get

the best possible correspondence with theory.

It appears that the edge quality may be the

limiting factor in the accuracy and repeatability of line-width measurements below 10 pm

The largest contributions to variations in

the measurement appear to be due to 1) non- repeatability of focus at high numerical aper

tures with conventional fine-focus adjustments found on microscopes; 2) variation in

edge quality, which has a noticeable effect

for state-of-the-art photomasks; and 3) the

effect of chromatic aberrations in the microscope when white light is employed. These sources of error will affect measurements

made with filar and shearing eyepieces as well as photometric image scans, although

they may be masked because of larger variations contributed by visual factors,

(D, Nyyssonen")

NBS Optical Physics Division.

44
7. TEST PATTERNS

7.1. Cross-Bridge Sheet Resistor

The sheet resistance of a conducting layer is

usually measured with a four-terminal conducting sheet in the shape of a bridge [62] or a van der Pauw [62,63] test structure. Since

the sheet resistance as measured with the

bridge structure depends on the length-towidth ratio of the bridge while that as measured with the van der Pauw structure does

not, it is possible to determine the width

of the bridge structure as well as the sheet

resistance by combining electrical measurements from these two structures provided the

actual sheet resistances of both structures

are identical (NBS Spec. Publ. 400-17, pp. 22- 24) [28].

A new structure has been designed which incorporates both bridge and van der Pauw elements.

This structure requires six probe pads instead of the eight required by separate struc- tures. The greater compactness of the new structure assures that the sheet resistance

of the van der Pauw element is more nearly

equal to the sheet resistance of the bridge

element than would be the case for separate

structures. In designing the van der Pauw

part of the new test structure, the cross design (NBS Spec. Publ. 400-29, pp. 64-65) [64] was used; hence, the name: cross-bridge

sheet resistor.

The new structure, shown in figure 21, was de- signed to be compatible with the NBS 2 by N probe pad array, where N is an arbitrary integer (NBS Spec. Publ. 400-25, pp. 41-43). The

sheet resistance, , is calculated from measurements on the cross part of the resistor:

ii.ir

1 Ui—

1

5 /im —I ^|~0.2 mil)

I h3.2 mil) , r 80 /^m ^

D

SYMMETRY TAB

V2.V,

Figure 21. Schematic diagram of the crossbridge sheet resistor. (Metallized contact

to a diffused sheet resistor is shown by the

cross-hatched areas. For a metal sheet re- sistor, the entire structure is metallized.)

= (V/I)(TT/ln 2) , (29)

where the potential difference, V, is Vi - V2

for a current, I, passed into and out of

I2. The effective width of the currentcarrying channel, W^, is calculated from measurements on the bridge part of the sheet re- sistor :

W = R L (I*/V*) (1 - E) , e s m

where the potential, V*, is Vj* - V2* for a current, I*, passed into Ij* and out of I2*;

L is the distance, determined from the photo- m ' ^ mask, between the voltage taps of the bridge;

and E is a correction term which takes into

account the error due to the perturbation of

the voltage taps. If the test structure is a diffused layer, the effective width, as shown

in figure 22, is given by:

Wem= W + aX. + W , J oe

where W is the width of the line on the phom

tomask, W accounts for the over-etching,

oe

X. is the iunction depth, and a is the later3 al diffusion factor [65]. If the test structure is a metallized layer, the effective

width is given by:

w = w - w

e m oe

The cross part of the sheet resistor was designed to permit the sheet resistance to be

45
TEST PATTERNS

W,

2

We •

W

Figure 22. Cross -sect ional view of a diffused region showing various dimensions.

-m

2 2 I

2 I 2

R,/2 R,/2

'-WV-' R,/2 R,/2

Figure 23. A simple equivalent circuit model for the bridge sheet resistor. (The cur- rent lines penetrate the side arms at a distance Ag .

)

E 0.05

calculated from eq (29) witliout requiring additional correction factors for contact shorting or asjTnmetry. Previous analysis has

shown that the contact-shorting error for a Greek cross is less than 0.1 percent if the

length of the arm is greater than the width

of the arm (NBS Spec. Publ. 400-29, pp. 64-

65) [64]; therefore, this structure was designed so that the length of each arm of the cross is much greater than the width of the

arm. Symmetry tabs were introduced at the

ends of the three arms denoted Ij, I2, and

in order to compensate for the effect of the

bridge structure which is extended from the arm denoted V2.

Because this structure is expected to be suitable for making measurements on minimum linewidth geometries, the effect of the contact

arms on the bridge structure must be consider

ed. It can be shown, with the aid of the simple equivalent circuit model in figure 23,

that

Figure 24, Normalized width error, E, antici

pated from measurements on a bridge test structure. (y = Ag/Dg-)

D / W /D V'l

m \ e e/

(30)

where the symbols represent dimensions given

in the figure. The value for y ^ A /D is

e e not known. The worst case, where the contact

arms effectively short the current flow so that Ri = R2 = 0, is represented by y

~ °°' A

more realistic, but still conservative, case

is the case in which y = 1, An evaluation of

the error as a function of D /L is given in em

figure 24 for these two values of y. For the

minimum line-width case, D is equal to W ; ' e e if Y

= 1 and D /L =0.05 (as is the case for

e m

the structure illustrated in fig. 21), E is

equal to or less than 2.5 percent. A more de

tailed analysis, either experimental or mathe

matical, is needed to establish better estimates for y, and hence E, but a width error correction of this magnitude is of little con cern in many applications. (M. G. Buehler)

7.2. Advanced Planar Silicon Test Structures

This task was undertaken to determine the operational limitations and potential applications of integral electrometer amplifiers

used as test devices in specialized circuits.

The use of an electrometer amplifier in a sampling circuit configuration allows for the

measurement of small currents . It is an ideal high-impedance device that can be inter

faced with an active circuit element such as

46
TEST PATTERNS

Vb

J

1 iJ

BURIED CHANNEL , MOSFET

' J

'OUT

Gated diode sampling circuit with' electrometer for measurement of junction

leakage current, bulk lifetime under diffused region, and surface recombination

vel oc i ty

.

b. Buried channel MOSFET circuit with electrometer for measurement of bulk lifetime

and surface recombination velocity.

If

I

»B J

"OUT

MOS capacitor with electrometer for dc

measurements of oxide (insulator film)

th i ckness

.

J

o J

'OUT

^ C»Cp

MOS capacitor with electrometer for tran- sient measurements of lifetime under gate

oxides and surface state studies.

Vg

J

'VoUT

Rl

e. Basic electrometer circuit.

Figure 25. Schematic diagrams of advanced test structures,

47
TEST PATTERNS

a diode or capacitor to measure small charge

fluctuations. It is intended to perform and

evaluate both ac and dc measurements.

A number of quantities which relate to parameters that must be controlled in various

phases of silicon device and integrated circuit manufacturing are very small in magnitude and require exacting time-consuming measurements with sensitive apparatus. With an appropriately calibrated electrometer amplifier very small currents can be sampled and

measured with high-speed testers to provide

data rapidly and in sufficient quantity for

statistically significant results.

The different electrometer output configurations being considered are illustrated in

figure 25. A test pattern array is being designed, using the 2 by N modular concept, so that each of these can be evaluated and that

the results obtained can be compared with

those from more conventional test structures.

( D . McCarthy" , B . Godwin" , M . L . Lonky" , and

D. S. Herman")

7.3. Test Pattern Design and Analysis for

SOS/LSI

This comprehensive test pattern is being developed to provide a means for characterizing

and monitoring a large-scale integration

(LSI), silicon-on-sapphire (SOS) process and

other mask compatible processes (NBS Spec.

Publ. 400-25, pp. 44-45). The mask set has

been designed and produced, and a number of

test wafers have been fabricated. During

this period, the principal emphasis has been

on development of test programs for selected

structures and measurement of the characteristics of individual structures of types I and

II which are intended to test design and process parameters and spatial variations, re- spectively (NBS Spec. Publ. 400-19, p. 47).

A result of general interest was obtained in

connection with the measurements on the cross- shaped van der Pauw sheet resistor (NBS Spec

Publ. 400-29, pp. X, 64-65). This essential

structure is present in a number of devices

in the type I pattern. Analysis of this

structure [64] indicated that the geometrically introduced error should be much less than

could be observed by the measurement system,

fleasurements of structure 1-53 (p"*" poly) indicated that at a current level of 1.00 mA,

the applied voltage of about 142 mV produced

a measured signal of about 17 mV for a ratio

of about 12 percent and a sheet resistance of

75.8 fi/D . On conventional bridge-shaped

structure 1-63 a nominal value of 76.8 fi/D was measured; in this structure a current level of 1.00 mA produced a measured signal of

about 45 mV, or approximately three times

that of the van der Pauw device. The agreement between the two measurements is well

within the uncertainties of the geometry of

the bridge-shaped specimen. It is therefore

quite clear that the cross-shaped van der

Pauw structure has no serious design or theoretical problem. This structure allows a very fine spatial resolution of resistivity.

(W. E. Ham+)

Work performed at Westinghouse Defense and

Electronic System Center under NBS Contract

No. 6-35766. NBS contact for additional information: M. G. Buehler.

^Work performed at RCA Laboratories under NBS

Contract No. 5-35916. NBS contact for additional information: M. G. Buehler.

48
8 . ASSEMBLY AND PACKAGING

8.1. Correlation of Moisture Infusion, Leak

Size, and Device Reliability

Of crucial importance to the examination of

moisture infusion as a function of leak size

is the accuracy of the dew-point measurement

method intended for the determination of the

amount of moisture present in the test packages. Previously, the precision of this

method was evaluated through exposure of the

dew-point sensors directly to controlled mac- roenvironments of an environmental chamber

(NBS Spec. Publ. 400-25, p. 56). Environments ranged in dew-point value from about

-10°C to 30°C, which covers the range from

0.25- to about 4.3-percent water vapor. Dispersion in the dew-point measurements appeared to be about 5°C, with most of the error related to the techniques involved in

calibrating a microsensor in a macroenvironment

.

Two procedures were selected for evaluation

of the dew-point method as applied to package interiors, where ionic content and sorption kinetics can be expected to affect the

measurements. One procedure was to seal test packages containing atmospheres of known

water-vapor content introduced through a miniature valve and then to determine this content with the dew-point measurement. The

second procedure was to include a miniature

aluminum oxide dielectric moisture sensor, now in the developmental stage [66], into the

test package for use as a transfer reference.

A special series of 27 hybrid test packages

was fabricated for the evaluation. Each 24-

pin dual-in-line metal package was equipped

with one or two miniature valves, a temperature sensor, and a dew-point sensor, as pictured in figure 26. Seven of the packages were also equipped with an oxide sensor [67].

For the calibration of the oxide sensors, one package was delidded, the valve stems were removed from the others, and PTF tubing was inserted into the valve stem, guides.

Test air was introduced at a slight positive

pressure and of known moisture content as obtained with a two-pressure generator [68].

When the sensor outputs steadied, readings were made. Calibration was performed over

the range from 100 to 12 000 ppm water vapor

by volume at room temperature (25°C) and at 85°C. Moisture points were generated in increasing and decreasing sequences to provide

a test for hysteresis. Moisture content of

the test air was known to an accuracy of ±3

percent for the range from 100 to 250 ppm

and to ±0.5 percent for the range from 250

to 12 000 ppm. Typical results are given in

figure 27.

The packages were then divided into sets with

an electric hygrometer sensor in each set. Moist air was flushed into the packages as before until steady-state conditions were

reached. Valve stems were replaced and the

packages were sealed while in the moisture

generator. The packages contained 96.2,

442, 994, 6263, or 12 020 ppm water vapor by

volume as obtained with the two-pressure generator.

Dew-point measurements were made by chilling

the test package while monitoring both the in

ternal temperature and the leakage current be

tween the interdigitated electrodes of the

dew-point sensor. A sample result, taken

with a bias of 10 V, is shown in figure 28.

The dew point was taken as the temperature at which the leakage current indicated a 20-

percent change from the baseline leakage of

the circuit (NBS Spec. Publ. 400-25, p. 56). In the case illustrated, the measured dew

point was about 4°C, which corresponds to about 10 000 ppm water vapor by volume . The reason for the maximum at about -7°C and the

decrease in leakage current at lower temperatures is not yet understood.

The effect of bias voltage on the measured

dew point was also examined. A 20-percent de

viation occurred at 10°C (12 000 ppm) with

50 V applied and at 13°C (15 000 ppm) with

100 V applied. The 50- and 100-V biases saturated the amplifier so that peak values

could not be obtained. The results obtained

on all the packages measured are summarized

in table 8. It can be seen that these results are not very complete or consistent.

It is hoped that suitable agreement between

the oxide sensor and the dew-point sensor may

follow from suitable choice of dew-point sensor bias voltage and point of deviation from

baseline current.

(R. Sulouff" and S. Ruthberg)

"work performed at Martin Marietta Aerospace,

Orlando Division, under NBS Contract No. 5-35880.

49
ASSEMBLY AND PACKAGING

^ <^ m

Photograph of the thick film substrate

showing the i n terd i g i ta ted electrode

structure (A) for the dew-point measurement, bonding pads (B) for the oxide

moisture sensor mounted in selected packages, and the bonding pad and interconnect system (C) for the 7^1 operational

amplifier to be used in subsequent tests

to monitor effect of moisture on circuit

operation. A temperature sensing diode

is mounted at point D,

b. Photograph of exterior of test package

showing miniature valve (E) .

Figure 26. Hybrid test packages for use in the moisture infusion study.

100 1000 10000 WATER VAPOR CONCENTRATION |ppni|

Figure 27. Typical calibration data for oxide moisture sensor. (Sequence of tests: At 25°C: •, increasing moisture level; o, increasing; , decreasing; a, increasing. At 85°C: A, decreasing; o. increasing.)

50
ASSEMBLY AND PACKAGING

60

50

40

% 30

tu

CD2 20

15

0

TEMPERATURE TCj

Figure 28. Typical dew-point sensor response curve of lealoge current as a function of substrate temperature. (Applied voltage: 10 V.)

Table 8 — Oxide Sensor and Dew-Point Determination of Moisture Content of

Sealed-Off Test Packages

Water Vapor Concentration, ppm

Test

Package^ Sealed-Inb Oxide Sensor

Dew

Peak

Point Sensor

20% Deviation

5 96.2 c d d

6 994 6 200 d d

7 6 263 6 200 3 300 10 050

8 12 020 12 300 10 000 20 000

9 422 3 500 c c 22 6 263 e 17 000 22 000

27 96.2 e 330 1 050

28 12 020 e 6 000 18 000

a Sixteen other packages without oxide sensors which had sealed-in water

vapor concentrations in the range from 96.2 to 12 020 ppm gave no change

in leakage of the dew-point sensor as the temperature was lowered.

b Measured with an electric hygrometer sensor.

c Electrical failure; no result obtained.

d No change in leakage of the dew-point sensor observed.

e Package did not contain an oxide sensor.

25 20 15 10 5 0 5 10 -15 -20 -25 -30 -35

51
ASSEMBLY AND PACKAGING

8.2. Loose-Particle Detection+

Additional studies were completed on se- lected aspects of a draft procedure for the

particle impact noise detection (FIND) test which is being considered for inclusion in

MIL-STD-883, Test Methods for Microcircuits

,

as Method 2020/20xx. In previously reported

work (NBS Spec. Publ. 400-29, pp. 75-76) it was shown that each of the three recommended

couplants was satisfactory for transmitting

the exciting mechanical vibrations to the microcircuit. In addition, the impact frequencies of metal spheres (of representative

sizes and materials) with an elastic wall

were calculated and shown to be considerably

higher than the frequency of maximum response

of the FIND test system, which ranges from

about 130 to 170 kHz. Nevertheless, it was concluded that particle impacts should be

able to shock and excite the ultrasonic detection transducer, thus producing a measurable output.

During the present period, the effectiveness

of the couplants for transmitting highfrequency vibrations was investigated in detail. In the FIND test, the high-frequency

(acoustic emission) signals which are generated when a particle vibrated at a low frequency hits the wall of the package must be

transmitted through the package wall,

through the couplant, and into the ultrasonic transducer in order to be detected. The

transmissibility of the couplant therefore

has a considerable influence on the effectiveness of the FIND test in detecting the

presence of small loose particles in the

package

.

In these tests, the amplitude frequencyresponse characteristics of three acoustic

emission transducers (A, B, and C) of the same type were determined when they were

driven through the couplant by a wide-band

acoustic emission transducer, and swept over a frequency range from 5 to 500 kHz in

about 30 s. The wide-band (5 kHz to 2 MHz)

transducer was mounted in a fixture with its

sensing surface facing up and in the horizontal plane. The test transducer with its

sensing surface down was placed on the driver. A single layer of tape couplant, typically 0.4 mm thick for tape K and 0.1 mm

thick for tape L, or a 0.1-mm thick layer of

viscous liquid couplant M bonded the two surfaces together. The wide-band transducer was driven electrically by a low- impedance,

constant-voltage function generator at an output of 5.0 ± 0.1 V rms.

FREQUENCY |kHz|

Figure 29. Average response of three driven

acoustic emission transducers A, B, and C at- tached to driving acoustic emission transducer by couplant K (a) , L (o) , or M (o) , as a function of driving frequency.

Five runs were performed for each couplant

with each test transducer, and the averaged

results are shown in figure 29. Since the

frequency response of the driving transducer

is reported by its manufacturer as being

smoothly rising about 10 dB from about 10 kHz

to 2 MHz, it is likely that the peaks in the

response shown in the figure are those of the test transducers. These peaks occurred roughly at 150, 200, 325, and 400 kHz. The maxi- mum response for all test transducers and

couplants occurred at about 150 kHz.

It should be noted that the response with

the viscous couplant was larger than that obtained using the tape couplants for all

three test transducers. At about 150 kHz

output amplitudes for all transducers averaged 863 mV with this couplant, 627 mV for

Principally funded by SAMSO with additional

funding from the NBS Nondestructive Evaluation Program.

52
ASSEMBLY AND PACKAGING

Table 9 — Response of Test Transducers at

150 kHz Obtained Using Viscous

Liquid Couplant M

20 30 NUMBER OF TEST CYCLES Figure 30. Response of three acoustic emission transducers A, B, and C attached to driving acoustic emission transducer by

couplant K (^) , L (o) , or M (o) and driven

at 150 kHz, as a function of number of re- moval and remounting cycles.

tape L, and 292 mV for tape K. Differences

between the couplants at 325 kHz were more pronounced than those at 150 kHz; again the

viscous couplant resulted in the largest output .

One application of couplant is expected to be used to test a number of microcircuit packages. In the case of tape K, for example,

the proposed test method specifies replacement of the couplant at least once for each

50 parts tested.

The amplitude-frequency responses of transducers A, B, and C were determined as above

after the test transducer was first mounted

using a fresh application of couplant, and

after the transducer had been removed and re- mounted 10, 20, 30, 40, and 50 times, respectively. In this case, one run was made using

each of the three couplants with each of the

three transducers. The output signal amplitude at 150 kHz is plotted in figure 30 as

a function of the number of remounts. The

results show a decided decrease in this signal amplitude when using tape couplant L;

Transducer

Average of

5 Runs

,

mV

Relative Sample

Standard Deviation,

a

%

A 777 4.7

B 858 7.3

C 803 10.2

Grand

Average

812 5.1

a The variability includes the couplant in

all cases.

the amplitudes after 50 remounts were 57, 43,

and 61 percent of the initial amplitudes for

transducers A, B, and C, respectively. Tests

with tape couplant K showed a change of less

than 10 percent in output during the 50 mountings and removals, and viscous liquid couplant M less than 15 percent, for all three

transducers. It should also be noted that

the output with viscous liquid couplant M at the end of the test was 2 to 4 times that of

the tape couplants in all cases.

The above data can be used to compare the

performance of the three test transducers.

Since viscous liquid couplant M produced the

most repeatable results, test results from,

its use were analyzed. The results at 150

kHz, summarized in table 9, suggest that the

three test transducers have very similar performance as far as sensitivities and resonances are concerned. However, the sample is too small for these results to be regarded as indicative of the characteristics of all

transducers of this type.

At this stage of the investigation it appears

that the viscous liquid couplant M is more effective in transmitting acoustic emission signals than the two tape couplants. In addition, couplant M appears to give more repeatable results during the testing procedure.

(P. S. Lederer^, J. S. Hilten^ , and

C. F. Vezzetti^)

NBS Components and Applications Section,

Electronic Technology Division.

53
9. DEVICE INSPECTION AND TEST

9.1. Scanning Acoustic Microscopy

During this period, efforts on the development of the scanning acoustic microscope for

application to examination of semiconductor

devices and integrated circuits have been directed primarily toward examination of a variety of test specimens and study of the images

in order to begin to acquire a greater understanding of the information which can be obtained by this technique [69,70].

Stanford Project — In this part of the work,

the microscope has been used in both the

transmission mode and the reflection mode.

Thinned wafers have been used as specimens in

connection with the transmission mode of operation. Although these wafers do not corre- spond to production items, they permit comparison of the image in transmission with the

image in reflection. The ability to make

this comparison proves to be of value in interpreting the results for the reflection

mode

.

With the reflection mode, short pulses some

4 ns in length at a frequency near 850 MHz

are being used. The images obtained are improved over those reported previously (NBS

a. Scanning acoustic micrograph. Magnification: '^300x. Note the edge definition, contrast changes at crossovers,

and resolution of the metallization

lines which are about 5 ym wide.

Spec. lubl. 400-29, pp. 30-Hj; , because the

signal-to-noise ratio has been improved by

increasing the amplitude of the pulse at the

input transducer. With the transmission mode,

both the change in amplitude and the change

in phase through the thinned wafer have been

monitored. It is now evident that there is

much more information available in the "phase

only" images in those regions where the amplitude remains nearly constant.

In this work, time gating techniques have

been used to delineate different levels in

the specimen. It has been found possible to focus on both the front and back surfaces of

a wafer thinned by mechanical polishing to about 6 ym.

Figure 31 provides a comparison between acoustic images (fig. 31a) and an optical photomicrograph taken with interference contrast

(fig. 31b). The specimen* is a portion of a C-MOS-on-sapphire integrated circuit. The

lines on this circuit are about 5 ym wide.

These images illustrate the resolution now

Supplied by P. Hagon , Autonetics Division of

North American Rockwell.

b. Optical photomicrograph taken with interference contrast. Magnification: '\'250x.

Figure 31- Scanning acoustic and optical photomicrographs of a portion of a C-MOS-onsapphire integrated circuit.

54
DEVICE INSPECTION AND TEST

available with the acoustic microscope. The

differences in contrast at the crossovers of

various elements in the acoustic image are noteworthy. (C. F. Quate"^')

Hughes Project — In this part of the work,

the microscope is operated at a frequency of

about 400 MHz; operation in both the cw and

pulsed reflection modes (NBS Spec. Publ. 400-

29, pp. 79-81) is possible. From study of an Ealing 22-8635 High Resolution Test Target,

§

photolithographically replicated with gold

metallization, 300 nm thick, on silicon-onsapphire wafers, it was concluded that the

resolution capability of the microscope operated at 370 MHz in the pulsed reflection mode

can approach 1.5 ym at 10-percent contrast

[71]. The resolution at 50-percent contrast

is 2.8 ym. The acoustic wavelength in water

is 4.0 ym at 370 MHz; thus it appears that

the resolution capability of the microscope

is about one-third of the acoustic wavelength

in water. If linear extrapolation remains

valid, a decrease of lens radius to 0.1 mm

and an increase of operating frequency to 1500 MHz, both reasonable values, would be

expected to increase the resolution of the

system by a factor of 4, or to about 0.3 or

0.4 ym.

Since the completion of the pulsed reflection

system, many integrated circuits and other

specimens have been examined. The images of

the integrated circuits and other planar specimens have shown surprising detail that results from the greater-than-expected line res- olution capability of the instrument, noted

above. In all of these imaging experiments

another interesting behavior of the acoustic

system has come to light. This behavior appears in all types of specimens examined and

is, therefore, thought to be characteristic

of the acoustic lens system itself. Repeated

intensity maxima and minima are observed as the specimen is moved through the lens focal

plane parallel to and toward the lens, as illustrated schematically in figure 32. These

intensity extremes are manifested as contrast

inversions as the distance between the lens

and the specimen is changed. These contrast

inversions are illustrated in figure 33,

which shows the image of a 300-nm thick gold

mesh on silicon obtained with the pulsed reWork performed at Microwave Laboratory,

Stanford University, under NBS Contract No. 5-35899. NBS contact for additional information: R. I. Scace.

§ . . See disclaimer, p. vi.

flection mode at the various lens-to-specimen

positions between 0 and -4 as indicated in figure 32. As can be seen, the greatest reso lution occurs at the -2 position; this is the

position used in the resolution studies described above.

Based on experience with the scanning acoustic microscope to date, it is believed that

the instrument, when fully developed, will be

capable of detecting two distinctly different

kinds of structural features. One such feature constitutes a local defect, such as a void beneath a metallization as shown in figure 34. In this figure, a portion of a 0.5-

ym aluminum surface metallization makes no contact with the layered structure beneath

it. The complex reflection coefficient (only

the magnitude is shown) changes abruptly as the acoustic beam crosses the buried void, re suiting in distinct image intensity changes

which can be readily resolved by the microscope.

The second and equally important diagnostic

measurement which the microscope is in principle able to make quantitatively is of the

thickness of the layered regions of an integrated circuit, consisting of many overlay

processing steps. It is envisioned that a map of an integrated circuit can be con- structed in which the image is a complex num- ber (amplitude and phase) that represents the sum of all layered regions. At any point on

the circuit the image information therefore

contains the expected complex reflection coef- ficient, computed from known material properties (density, velocity, and layer thickness)

occurring locally. A stable microscope, on

the other hand, yields a two-dimensional description (map) of the actual total complex

reflection coefficient of the integrated circuit being diagnosed. If, for example, an er- ror in metallization thickness has been inadvertently made in the fabrication process,

this error would be, in principle, readily de- tectable when the actual and computed maps

are superimposed.

The following general and specific observations can be made at this time as a result of

experience gained operating the scanning

acoustic microscope:

(1) The instrument is easy to operate,

and a new specimen can be examined in a matter of a few minutes. Operation is more com- parable to that of an optical microscope

than to that of a scanning electron microscope. The indications are that, independent of whether unique observations can al55
DEVICE INSPECTION AND TEST

-4 -3 -2 -1 0 +1 +2

|-« 40/im^-|

Figure 32. Variations in intensity of re- flected pulsed acoustic signal as the lensto-specimen spacing is changed. (When sur- face contrast exists, the reflected signal

intensity varies over the surface; the dashed

curve depicts the intensity variations (with

spacing) of a region of opposite contrast to that of the solid curve. The lens-tospecimen spacing increases to the right.)

a. Position 0; focal point of lens at speci- men surface; maximum signal obtained.

Position -1 c. Position -2; note contrast inversion

increased sharpness of image.

and

d. Position -3. 'Lmmu. **s»*4

e. Position -k; focal point of lens inside

silicon; contrast same a^s in position 0.

Figure 33- Scanning acoustic micrographs of a 300-nm thick gold film grid pattern on silicon at various lens-to-specimen distances. (The elements are 2.5 ym wide on 25-VJm centers.)

56
DEVICE INSPECTION AND TEST

REFLECTED

INTENSITY 69% 99%

(0.5^m)

(0.15 pm) — (0.7 ;um)

Figure 3^- Effect of sub- surface void on intensity of acoustic signal reflected from

an aluminum film on silicon dioxide.

9.2. Thermal Properties of Transistors'^

The predictions of a physical model which

has been proposed to explain the relationship between thermal instability and second

breakdown [72] were confirmed experimentally.

As shown in figure 35, there are two intersecting loci in the collector current (1q) » collector emitter voltage (V^^^,) plane that

define limits on the operation of forwardbiased power transistors. One locus defines

the limit of thermal instability and the other the limit of second breakdown. For cur- rents greater than I*, stable hot spots exist

in the region between the two limits. For

currents below I*, the two limits nearly coincide, but thermal instability was found always to precede second breakdown.

ways be made, the instrument is suitable for

in-line operation in a processing or circuit

manufacturing line. Its simple controls and

lack of a vacuum system render it more con- venient to operate and maintain than a scan- ning electron microscope or an ion implantation system.

(2) The instrument is superior to the

scanning electron microscope for nondestructive examination of insulating substrates or integrated circuits with (a) isolated regions or (b) large areas of insulating material, for example SOS integrated circuits,

because charge does not accumulate on the

surface.

(3) Scanning acoustic images appear to show edges (especially shallow steps) in materials and material differences better than

images from either optical or scanning electron microscopes.

(4) The instrument has been shown to be

capable of differentiating the various levels of processing in an integrated circuit

(in a series of micrographs) through the simple operation of making small changes in the

lens-to-specimen spacing.

(5) The instrument has been demonstrated

to exhibit better-than-expected resolution

capability.

The scanning acoustic microscope has also

been successfully utilized to view gallium

arsenide field-effect and transferredelectron devices, aluminum fuse links, and

clad and unclad optical fibers. It appears

to offer interesting possibilities for examining these structures in addition to con- ventional and SOS integrated circuits.

(R. G. Wilson^ and R. D. Weglein^

)

57

Based upon the premise that the extremely

high junction temperatures reached in the

stable hot-spot region (usually greater than

200°C, often greater than 300°C) make operation in that region unsafe, it is proposed

that the limit of thermal inst.ability should

be used as the safe-operating-area (SOA) limit for power transistors. A measurement

method to determine the locus of thermal instability quickly and accurately and a non- destructive test for second breakdown have

been developed.

The model predicts that thermal instability

will occur when the temperature difference

between the device chip and the outside case reaches some critical value. When an instability occurs, the current begins to con- strict to a smaller and smaller area of the

device. For values of I^ greater than I*,

the current stabilizes in some small finite

area, but for values of I^ less than I*, the

current continues to constrict until second

breakdown occurs.

Experiments have been performed to estimate

how rapidly the current constricts and to determine how closely in time the measured

increase in junction temperature coincides

with the constriction. The estimates of the

time variation of the current-carrying area

Work performed at Hughes Research Laboratories under NBS Contract No. 35898. NBS

contact for additional information: R. I. Scace

.

^Funded through the NBS Electronic Technology

Program

.
DEVICE INSPECTION AND TEST

Stable Hot Spots

I*--

Second Breakdown Limit

Thermal Instability Limit

Ln V CE

Figure 35. A schematic representation of

the thermal instability and second breakdown

limits of safe power transistor operation in the forward-biased region. (Below I*, the

two limits nearly coincide, but second breakdown always occurs after the onset of thermal

i nstab i 1 i ty .

)

were made using the cooling-curve technique

previously described [73]. Although this

technique is strictly applicable only for

cooling from steady-state conditions, the

slight errors introduced into the calculated

area by transient heating effects do not affect the validity of the observations re- ported here. The temperature was measured us ing a technique analogous to the standard

method for measuring steady-state thermal re- sistance [ 74 ]

.

In each instance, it was observed that the

current-carrying area begins to decrease before any significant change in heating rate

is noticed, and that the rate of heating con- tinues to be quite high even after the current has effectively stabilized to a small ar

ea. The characteristic shape of the rate of

temperature increase is exploited in the tech

nique developed to detect the onset of thermal instability. The on-state, emitter-base

voltage, V , is used as the temperature

Ed (on ) monitor. Frequently, the change in V , , ho (, on ) when instability occurs is very small; to per

mit easy observation of the change in these

cases, the time derivative of V^^ , , is monl

EB (on)

tored. Curves of V^^ , , and its time deriva

EB (on)

tive are illustrated for two actual cases in

figure 36. In the case illustrated in figure

36b, the onset of instability is difficult to discern in the V„„ , , curve, but it can be

EB(on) ' seen clearly in the derivative curve.

EB(on)

EB (on)

^

t

i-- -- , I J

EB(on)

a. Onset of instability clearly evident in both curves. Onset of instability barely evident in VEB(on) ^^""veFigure 3G. Curves of emitter-base voltage and its time derivative as a function of time following removal of power. (In these curves, the emitter-base voltage is negative and de- creases in magnitude as time increases.)

58
DEVICE INSPECTION AND TEST

DIFFERENTIATOR

Figure 37- Simplified schematic of circuit

for measuring time derivative of emitter-base

vol tage

.

A schematic of tl:ie circuit for this measurement is shown in figure 37. The circuit is

very similar to that used for making thermal

resistance measurements [75]. The differentiator uses an operational amplifier as its

active element. The pulse applied to the

transistor under test is typically varied

from 1 to 150 ms for measurements. It was established that the limits determined for

pulses of 100 ms or longer were identical;

thus, as far as thermal instability is concerned, 100-ms pulses are the equivalent of

dc power.

The reason that pulses of 100 ms or longer

are equivalent to dc is that the instability

is related to the temperature difference between the case and the junction. For pulses

longer than 100 ms or so, the base begins to heat, but the temperature difference between

it and the junction does not increase significantly. Therefore, to generate even the dc

SOA using the onset of thermal instability as the limit, no heat sinking would be required,

and only pulses of 100 ms or shorter are re- quired.

Since thermal instability always precedes second breakdown, this circuit can be used to develop the region of the curve for values of

I^ less than I* where the limit is governed

by the onset of second breakdown. By triggering a turn-off circuit on the rising slope

of the derivative curve, the device can be

turned off before damage from second breakdown can occur [75].

(D. L. Blackburn and S. Rubin)

9.3. Integrated Circuit Die-Attachment Evalu

ation

Work has resumed on the investigation of meth

ods for using the temperature response of a substrate diode for determining the integrity

of the die attachment of integrated circuits.

For the first stage of this work, integrated

circuit chips, nominally 0.050 in. (1.25 mm)

square, were obtained. These chips are tran- sistor arrays normally used for display drivers. By means of eutectic bonds one group of

chips was attached to headers that each had a 0.038-in. (1.0-mm) diameter void ultrasonical

ly machined into the surface [76]. Another

group was bonded to headers with a 0.025-in.

(0.6-mm) void, and a third group was bonded

with no voids. The void area, expressed as a percentage of the total bonded area, was 45

percent for the large void and 20 percent for

the small void.

Initial temperature measurements were made by

applying a square wave to the device under

test. For one-half of the cycle the transistors in the circuit were dissipating 1 W of

power, and for the other half, the transistors were off and the substrate diode was for

ward biased so that the voltage drop across

this junction could be measured. A precision

rectifier circuit was used to measure the

average dc voltage developed across the substrate diode during the forward-biased portion of the cycle. A repetition rate of 1 kHz was found not to allow much change in the

temperature between the heating and measuring

parts of the cycle. The results obtained to date have shown that the change in temperature produced in the chips bonded to the head

ers with the large void is 5 to 10 percent

greater than that produced in the chips

bonded to headers with no intentional void.

This result suggests that a more sensitive

technique would be desirable. New pulse heat

ing and time delay measuring techniques are being developed in an attempt to find a more sensitive method.

(D. V/. Berning and D. L. Blackburn)

59
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6. Keenan, W. A., Schumann, P. A., Tong, A. H., and Phillips, R. P., A Model for the

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9. Dickey, D. H. , and Ehrstein, J. R.

,

Semiconductor Measurement Technology:

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NBS Special Publication 400-48 (in preparation) .

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.

11. Dickey, D. H. , Two Point Probe Correction Factors, Semiconductor Measurement

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.

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Technique, ASTM Designation F 110, Annual Book of ASTM Standards, Part 43 i (November 1977)

.

16. Carballoy Division, General Electric

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18. Fluorinert Brand Electronic Liquid, EC

43, Commercial Chemical Division, 3 M

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.

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.

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,

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.

36. Buehler, M. C, and Phillips, W. E. , A

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,

Tasch, A. F., Jr., and Tole, A. B.,

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39. Goodman, A. M. , Semiconductor Measurement Technology: Suppression of Premature Dielectric Breakdown for HighVoltage Capacitance Measurements, NBS

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Unit for 1-MHz Capacitance and Conductance Measurements, NBS Special Publication 400-40 (September 1977).

41. Kuczer, P., Hook, H. 0., and Goodman, A.

M. , Semiconductor Measurement Technology

A Versatile High-Voltage Bias Supply for

Extended Range MIS C(V) and G(V) Measure

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.

42. Goodman, A. M. , Semiconductor Measurement Technology: The Safe Operation of

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43. Kokkas, A. G. , Silicon/Sapphire Interface Radiation Study, RADC-TR-77-138

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44. Kant, A., Call, J. P., and Thompson, H. D., Determination of Impurities in Sili61
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.

47. Grunthaner, F. J., and Lewis, B. F.,

Measurement of Energy Level References

in X-ray Photoelectron Spectroscopy,

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.

48. Grunthaner, F. J., and Maserjian, J.,

Sodium Ions at Defect Sites at Si02/Si

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Vegas, Nevada, April 1-3, 1975, pp. 15-

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49. Grunthaner, F. J., Lewis, B. F. , and

Maserjian, J., Effect of Residual Organic Contamination on MOS Devices at Si02/Si Interfaces, Presented at IEEE

Nonvolatile Semiconductor Memory Workshop, Vail, Colorado, August 1976. (Unpublished)

50. Grunthaner, F. J., Klein, J., and

Barton, J. J., Resolution Enhancement,

Deconvolution and Noise Removal in Xray Photoelectron Spectroscopy (XPS)

(in preparation)

.

51. Heinen, K. G. , and Larrabee, G. B., Analysis of Silicon by Conventional Carrier

Separations and by Computer Reduction of

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: Carlson, H. G. , Activation Analysis in

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Special Publication 337, pp. 375-383

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.

52. Carpenter, B. S., Determination of Trace

Concentration of Boron and Uranium in

Glass by the Nuclear Track Technique,

Anal. Chem. 44, 600-602 (1972).

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Planar Four-Probe Test Structure for

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Quality of Sapphire, Report No. 2, NBSGCRt76-61 (September 1976). (Available

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56. Wilson, R. G. , Dunlap, H. L. , and Jamba,

D. M. , Angular Sensitivity of Controlled

Implanted Doping Profiles, Semiconductor

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.

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,

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.

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.

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,

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63
APPENDIX A SEMICONDUCTOR TECHNOLOGY PROGRAM STAFF

(on September 30, 1976)

Significant changes in assigned organizational units and telephone numbers have

occurred between the end of the reporting period and the issue date of this report. Call the Electron Devices Division, (301) 921-3786, for current information.

Electronic Technology Division

(301) 921-3357

J. C. French, Chief

Miss B. S. Hope*

J. F. Mayo-Wells

Dr. W. M. Bullis, Asst. Chief for

Semiconductor Technology

Mrs. M. L. Stream

Mrs. E. J. Walters*+

F. F. Oettinger, Acting Asst. Chief

for Operations

R. L. Raybold

Mrs. T. J. Talbott

Mrs. E. Y. Trager*

Semiconductor Characterization Section

(301) 921-3625

F. H. Brewer

Dr. W. M. Bullis, Chief

Mrs. F. C. Hoewing'' Mrs. C. A. Cannon

Mrs. E. C. Cohen+

Dr. J. R. Ehrstein

Dr. K. F. Galloway

W. J. Keery

Dr. R. D. Larrabee

Mrs. K. 0. Leedy

Dr. S. Mayo

Mrs. B. A. Oravec'

Miss D. R. Ricks

R. I. Scace

H. A. Schafft

L. R. Williams

Semiconductor Processing Section

(301) 921-3541

Dr. M. G. Buehler, Chief

R. L. Frisch

Mrs. A. D. Glover''

S. D. Grant § Miss D. L. Mines''

Dr. R. Y. Koyama

J. Krawczyk

T. F. Leedy

Dr. S. S. Li

Y. M. Liu

R. L. Mattis

D. A. Maxwell

Dr. W. E. Phillips

Dr. T. J. Russell

Dr. D. E. Sawyer

W. R. Thurber

Electron Devices Section

(301) 921-3621

Dr. J. H. Albers

D. W. Berning

D. L. Blackburn

G. W. Burdette

W. A. Cullins

t

Mrs. K. E. Dodson'

H. E. Dyson

G. G. Harman

H. K. Kessler

M. D. Nelllgant

Dr. D. B. Novotny

F. F. Oettinger, Chief

G. J. Rogers

S. Rubin

S. Ruthberg

Secretary

+ Part Time

§ Summer

64
APPENDIX B SEMICONDUCTOR TECHNOLOGY PROGRAM PUBLICATIONS

B.l. Prior Reports

A review of the early work leading to this Program is given in Bullis , W. M. , Measurement

Methods for the Semiconductor Device Industry — A Review of NBS Activity, NBS Tech. Note

511 (December 1969)

.

Progress reports covering the period July 1, 1968, through June 30, 1973, were published

as NBS Technical Notes with the title. Methods of Measurement for Semiconductor Materials,

Process Control, and Devices:

Quarter Ending

September 30, 1968

December 31, 1968

March 31, 1969

June 30, 1969

September 30, 1969

December 31, 1969

March 31, 1970

June 30, 1970

September 30, 1970

December 31, 1970

March 31, 1971

June 30, 1971

September 30, 1971

December 31, 1971

March 31, 1972

June 30, 1972

September 30, 1972

December 31, 1972

March 31, 1973

June 30, 1973

Quarter Ending

September 30, 1973

December 31, 19731

March 31, 1974 j June 30, 1974

September 30, 1974

December 31, 1974

March 31, 1975)

June 30, 1975 ( September 30, 1975)

December 31, 1975 / March 31, 1976)

June 30, 1976 f

INiJO lecn, iNOLc Ds-te Issued iNlJ-tj rt.L.(_fc:oo-LUll

iJcCcIIlDcir X^DO

475 February 1969 AD 683808

488 July 1969 AD 692232

495 September 1969 AD 695820

520 March 1970 AD 702833

527 May 1970 AD 710906

555 September 1970 AD 718534

560 November 1970 AD 719976

571 April 1971 AD 723671

592 August 1971 AD 728611

598 October 1971 AD 732553

702 November 1971 AT) l'\UU')l

717 April 1972 An 7Anfi74 727 June 1972 AD 744946

733 September 1972 AD 748640

743 December 1972 An 7S'ifi42 754 March 1973 AD 757244

773 May 1973 An 769840

788 August 1973 AD 766918

806 November 1973 AD 771018

reports were issued in the NBS Special Publication 400 sub

Dnductor Measurement Technology:

NBS Spec. Publ. Date Issued NTIS Accession No

400-1 March 1974 AD 775919

400-4 November 1974 COM 74-51222

400-8 February 1975 AD/A 005669

400-12 May 1975 AD/A 011121

400-17 November 1975 AD/A 017523

400-19 April 1976 PB 251844

400-25 October 1976 PB 258555

400-29 April 1977 AD/A 040011

65
APPENDIX B

B.2. Current Publications

As various phases of the work are completed, publications are prepared to summarize the

results or to describe the work in greater detail. Publications of this kind which have

appeared between February and September 1977 are listed below:

Jerke, J. M. , Hartman, A. W. , Nyyssonen, D. , Rosberry, F. W. , Swing, R. E. , Swyt, D. A., and

Young, R. D., Accurate Linewidth Measurements at the National Bureau of Standards, Proa. Miaroeleotronias Seminar — Interface '76, Monterey, California, October 3—5, 1976, pp. 51—59.

Leedy, K. 0. , A Bibliography on the Electron Beam Induced Current Analysis of Semiconductor

Devices, Solid State Technology 20,(2), 45-48 (February 1977).

Sawyer, D. E. , and Berning, D. W. , Semiconductor Measurement Technology: A Laser Scanner

for Semiconductor Devices, NBS Special Publication 400-24 (February 1977).

Buehler, M. G. , and Sawyer, D. E. , Microelectronic Test Patterns Aid Production of Custom

ICs, Circuits Manufacturing 11_ (2), 46-56 (February 1977).

French, J. C, The Measurement Challenge in Electronic Technology, Dimensions/NBS 61 (2),

12-15 (February 1977).

Li, S. S., and Thurber, W. R. , Semiconductor Measurement Technology: The Dopant Density and

Temperature Dependence of Electron Mobility and Resistivity in n-Type Silicon, NBS Special

Publication 400-33 (March 1977).

Bullis, W. M. , Ed, Semiconductor Measurement Technology, Progress Report, January 1 to June

30, 1976, NBS Special Publication 400-29 (April 1977).

Devaney, J. R. , Leedy, K. 0., and Keery, W. J., Semiconductor Measurement Technology: Notes

on SEM Examination of Microelectronic Devices, NBS Special Publication 400-35 (April 1977).

Buehler, M. G. , and Thurber, W. R. , Measurement of the Resistivity of a Thin Square Sample

with a Square Four-Probe Array, Solid-State Electronics 20_, 403-406 (May 1977).

Blackburn, D. L. , Koyama, R. Y. , Gettinger, F. F., and Rogers, G. J., Measurement Techniques for High Power Semiconductor Materials and Devices: Annual Report, January 1 to December 31, 1976, NBSIR 77-1249 (May 1977). [Also published as ERDA Report CONS/3800-2 (May

1977).]

Mayo, S., and Evans, W. H. , Development of Sodium Contamination in Semiconductor Oxidation

Atmospheres at 1000°C, J. Electroahem. Soc. 124 , 780-785 (May 1977).

Bullis, W. M. , and Vieweg-Gutberlet , F. G. , Current Trends in Silicon Characterization Techniques, Semiconductor Silicon/1977 , H. R. Huff and L. Sirtl, Eds, pp. 360-366 (Electrochemical Society, Princeton, New Jersey, May 1977).

Ehrstein, J. R. , Effect of Specimen Preparation on the Calibration and Interpretation of

Spreading Resistance Measurements, Semiconductor Silicon/1977 , H. R. Huff and L. Sirtl, Eds,

pp. 377-386 (Electrochemical Society, Princeton, New Jersey, May 1977).

Jerke, J. M. , Hartman, A. W. , Nyyssonen, D. , Swing, R. E. , Young, R. D. , and Keery, W. J.,

Comparison of Linewidth Measurements on an SEM/Interf erometer System and an Optical

Linewidth-Measuring Microscope, SPIE, Vol. 100, Semiconductor Micro lithography II, 37-45

(June 1977). #

Leedy, K. 0., Measurement of Transistor Collector-Emitter Saturation Voltage, NBSIR 77-1231

(June 1977).

Ruthberg, S., Hermetic Test Procedures and Standards for Semiconductor Electronics, Nondestructive Testing Standards - A Review, ASTM STP 624, H. Berger, Ed, pp. 246-259 (American

Society for Testing and Materials, Philadelphia, Pennsylvania, June 1977).

66
APPENDIX B

Ruthberg, S., Gas Infusion into Doubled Hermetic Enclosures, IEEE Trans. Parts, Hybrids,

and Packaging PHP-13 , 110-116 (June 1977).

Harman, G. G. , The Use of Acoustic Emission in a Test for Beam-Lead, TAB, and Hybrid Chip

Capacitor Bond Integrity, IEEE Trans. Parts, Hybrids, and Packaging PHP-13 , 116-12 7 (June

1977).

Sawyer, D. E. , Berning, D. W. , and Lewis, D. C, Laser Scanning of Active Integrated Circuits and Discrete Semiconductor Devices, Solid State Technology 20 (6), 37-41 & 48 (June

1977).

David, J. M. , and Buehler, M. G. , A Numerical Analysis of Various Cross Sheet Resistor Test

Structures, Solid-State Electronics 20, 539-543 (June 1977).

Nyyssonen, D. , Optical Linewidth Measurements on Silicon and Iron-Oxide Photomasks, SPIE,

Vol. 100, Semiconductor Microlithography II, 127-134 (June 1977).

Scire, F. E., New "Piezo-Flex" Micropositioning Stage Permits Smooth Control of Displacements in Microscope Systems, Dimensions/NBS 61 (7), 26 (July 1977).

Young, R. D., Length Calibrations in the Micrometer and Sub-Micrometer Range, Ann. CIRP 25 , 245-250 (July 1977).

Li, S. S., and Thurber, W. R. , The Dopant Density and Temperature Dependence of Electron

Mobility and Resistivity in n-Type Silicon, Solid-State Electronics 2Q_, 609-616 (July 1977).

Schafft, H. A., Ed, Semiconductor Measurement Technology: Reliability Technology for Cardiac

Pacemakers II — A Workshop Report, NBS Special Publication 400-42 (August 1977).

Nyyssonen, D., Linewidth Measurement with an Optical Microscope: The Effect of Operating

Conditions on the Image Profile, Appl. Optics 16_, 2223-2230 (August 1977).

Swyt, D. A., and Rosberry, F. W. , A Comparison of Some Optical Microscope Measurements of

Photomask Linewidths , Solid State Technology 20 (8), 70-75 (August 1977).

Blackburn, D. L. , Semiconductor Measurement Technology: Safe Operating Area Limits for Power

Transistors —Videotape Script, NBS Special Publication 400-44 (September 1977).

B.3. Contractor Reports and Publications

Work carried on under contract to NBS is reported in both NBS publications and in the open

literature as various phases are completed. Recent publications and reports of this kind

are listed below:

Souloff, R. E., and Zatz, S., A Study of Leak Rates and Moisture Transport as Related to Reliability, Proc. 2976 Intemat. Microelectronics Symp. , Vancouver, B. C, October 11-13,

1976, p. 337.

Goodman, A. M. , Safe Operation of Capacitance Meters Using High Applied-Bias Voltage, RCA

Rev. 37., 491-514 (December 1976).

Kern, W. , and Comizzoli, R. B. , New Methods for Detecting Structural Defects in Glass Passivation Films, J. Vac. Sci. Teah. 14, 32-39 (January/February 1977).

Kern, W. , and Comizzoli, R. B. , Semiconductor Measurement Technology: Techniques for Measuring the Integrity of Passivation Overcoats on Integrated Circuits, NBS Special Publication

400-31 (March 1977) .

67
APPENDIX B

Dobrott, R. D. , Keenan, J. A., and Larrabee, G. B., Ion Microprobe Analysis of Integrated

Circuit Structures, ISth Annual Proceedings, Reliabilitij Physios 2977, Las Vegas, Nevada,

April 12-14, 1977. pp. 54-60.

Weglein, R. D., Wilson, R. G. , and Bonnell, D. M. , Scanning Acoustic Microscopy — Application

to Fault Detection, 16th Annual Proceedings^ Reliability Physics 1977, Las Vegas, Nevada,

April 12-14, 1977, pp. 37-43.

Quate, C. F., A Scanning Acoustic Microscope, Semiconductor Silicon/1977 , H. R. Huff and E. Sirtl, Eds, pp. 422-430 (Electrochemical Society, Princeton, New Jersey, May 1977).

Wilson, R. G. , Weglein, R. D. , and Bonnell, D. M. , Scanning Acoustic Microscopy for Integrated Circuit Diagnostics, Semiconductor Silicon/1977 , H. R. Huff and E. Sirtl, Eds, pp. 431-440 (Electrochemical Society, Princeton, New Jersey, May 1977).

Wilson, R. G. , Dunlap, H. L. , and Jamba, D. M. , Angular Sensitivity of Controlled Implanted

Doping Profiles, Semiconductor Silicon/1977 , H. R. Huff and E. Sirtl; Eds, pp. 1023-1034

(Electrochemical Society, Princeton, New Jersey, May 1977).

Johannessen, J. S., Helms, C. R. , Spicer, W. E., and Strausser, Y. E. , Auger Depth Profiling

of MNOS Structures by Ion Sputtering, IEEE Trans. Electron Devices ED-24 , 547-551 (May 1977).

Goodman, A. M. , Semiconductor Measurem.ent Technology: Suppression of Premature Dielectric

Breakdown for High-Voltage Capacitance Measurements, NBS Special Publication 400-37 (July

1977) .

Jamba, D. M. , Semiconductor Measurement Technology: Some Aspects of Dose Measurement for

Accurate Ion Implantation, NBS Special Publication 400-39 (July 1977).

Goodman, A. M. , and Weitzel, C. E. , The Effect of Electron Beam Aluminization on the SiSapphire Interface, Appl. Phys. Letters 31, 114-117 (July 1977).

Comizzoli, R. B. , Nondestructive, Reverse Decoration of Defects in IC Passivation Overcoats,

J. Electrochem. Soo. 124 , 1087-1095 (July 1977),

Helms, C. R. , Garner, C. M. , Miller, J., Lindau, I., Schwarz, S., and Spicer, W, E. , Studies of Si/Si02 Interfaces by Auger Sputter Profiling and Photoelectron Spectroscopy Using

Synchrotron Radiation, Proc. 7th Intern. Vac. Congr. & 3rd Intern. Conf. Solid Surfaces,

Vienna, September 1977, pp. 2241-2243.

Goodman, A. M. , Semiconductor Measurement Technology: A 25-kV Bias-Isolation Unit for 1-MHz

Capacitance and Conductance Measurements, NBS Special Publication 400-40 (September 1977).

B.4. Videotapes

Color videotape cassette presentations on improvements in semiconductor measurement technology are being prepared for the purpose of more effectively disseminating the results of

the work to the semiconductor industry. These videotapes are available for distribution

on loan without charge on request to Mrs. E. C. Cohen, Room A327, Technology Building, National Bureau of Standards, Washington, D. C. 20234. Copies of these videotapes may be

made and retained by requestors. As an added feature, arrangements can be made for the

authors to be available for a telephone conference call to answer questions and provide

more detailed information, following a prearranged showing of any of the videotapes.

The following videotapes are available for distribution:

1. Defects in FN Junctions and MOS Capacitors M. G. Buehler August 1974

Observed Using Thermally Stimulated Current and

Capacitance Measurements

68
APPENDIX B

2. Laser Scanning of Active Semiconductor Devices D. E. Sawyer and October 1975

D. W. Berning

3. Safe Operating Area Limits for Power Transistors D. L. Blackburn September 1977

B.5. Availability of Publications and Videotapes

In most cases reprints of articles in technical journals may be obtained on request to the

author. Current information regarding availability of all publications and videotapes issued by the Program is provided in the latest edition of NBS List of Publications No. 72 which can be obtained on request to Mrs. E. C. Cohen, Room A327, Technology Building, Na- tional Bureau of Standards, Washington, D. C. 20234.

69
APPENDIX C SOLID-STATE TECHNOLOGY & FABRICATION SERVICES

Technical services in areas of competence are provided to other NBS activities and other

government agencies as they are requested. Usually these are short-term, specialized ser- vices that cannot be obtained through normal commercial channels. Such services provided

during this and the previous reporting period, which are listed below, indicate the kinds

of technology available to the program.

C.l. Semiconductor Device Fabrication (T. F. Leedy and J. Krawczyk)

Silicon samples were prepared for the Naval Surface Weapons Center for use in studies of

radio-frequency burnout.

MOS devices were prepared for the Harry Diamond Laboratories for use in studies of the optical stimulation of charge carriers.

Further special silicon samples were prepared for the NBS Mechanics Division for use in

metal-vacuum-metal experiments designed to provide direct observation of tunneling phenomena between metals and to measure properties of Fermi surfaces. The fabrication of these

samples included thermal and chemical vapor preparation of silicon oxide layers, chemical

vapor deposition of polycrystalline silicon layers, evaporation of chromium pattern using

photomask techniques, evaporation of aluminum and copper films, and use of preferential

plasma etching, scribing, and mounting chips onto headers.

Oxide-coated silicon wafers were prepared for the NBS Polymers Division for studies of the

attachment of lipid structures to well characterized surfaces in connection with surgical

implantation

.

C.2. Metal Evaporation (T. F. Leedy)

Special emulsion reticles were fabricated for the NBS Physical Chemistry Division for use

in a photogrammetric apparatus.

C.3. Micromechanlcal Work (T. F. Leedy)

A special high-frequency, high- impedance , low-capacitance probe previously fabricated for

the NBS Product Engineering Division was repaired at the request of that Division.

70
INDEX

Auger electron spectroscopy (AES) 30-34

binding-energy measurements 34 boron density 18-19; 36-37

capacitance-voltage methods 18-19; 24-25

carbon concentration 34-35

channeling 40-41

chemical diagnostic methods 33-36

complex reflection coefficient 55-57

contrast inversion, scanning acoustic microscope 55-57

couplant, loose-particle detection 52-53

cross-bridge sheet resistor 45-46

cylindrical mirror analyzer (CMA) 30

defocus , optical microscope 42-44

defocus , scanning acoustic microscope 54-57

depth-profile measurements 10-17; 30-33

dew-point measurements 49-51

dragging-stylus probe 16-17

modular test structures 45-48

moisture infusion 49-51

neutron activation analysis 26-27

nuclear-track technique 36-37

numerical aperture, optical microscope 42-44

optical image profiles 42-44

optical microscope focus 42-44

optical test for surface quality 37-39

oxide hygrometer sensor 49-51

particle-impact noise detection (FIND) techniques 52-53

phosphorus density 26-30

photolithography 42-44

planar silicon test structure 46-48

power-grade silicon 22-24

process monitoring 23-24; 48

pulsed reflection mode, scanning acoustic

microscope 54-57

electrometer circuits 46-48

electron spectroscopy techniques 30-36

extrinsic silicon detectors 19-20

four-probe method 7

gold donor 20-22

hermeticity 49-51

hole emission probability 20-22

hole mobility 18-19

indium-doped silicon 19-20

integrated circuit die attachment 59

interface studies 31-35

ion implantation 40-41

ion microprobe mass analysis 26-30

ion-beam raster ing 30

ion-scattering spectroscopy (ISS) 33

ion-stimulated Auger transitions 30-31

Irvin curve 18-19

isothermal capacitance transient response

20-22

KLL transitions 30-31

large-scale integration (LSI) 48

line-width measurements 42-44

local-slope algorithm 12-15; 17 loose-particle detection 52-53

low-temperature processing 23

mesa diode test structure 23-24

metal-insulator semiconductor (MIS) 24-25

metal-oxide semiconductor (MOS) capacitors

20-23; 37; 46-48

metal-oxide semiconductor (MOS) electrometer

circuits 46-48

« U. S. GOVERNMENT PRINTING OFFICE : 1978 —261-238/161

resolution, scanning acoustic microscope

55-57

safe-operating-area (SOA) limit, transistor

57-59

sapphire substrates 37-39; see also silicon

on sapphire

scanning acoustic microscopy 54-57

scanning low-energy electron probe (SLEEP)

33-34

second breakdown 57-59

secondary-ion mass spectroscopy (SIMS) 33

silicon on sapphire (SOS) 24-25; 27; 37-39;

48; 54

spreading resistance methods 7-17

sputtering rate 27 stable hot spots, transistor 57-59

surface preparation, silicon 7-10

test patterns 45-48

test structures 46-48

thermal properties of transistors 57-59

thermally controlled chuck 22-23

thermally stimulated current and capacitance

20-24

thyristor 23-24

transistor thermal instability 57-59

tungsten carbide spreading resistance probes

8-10

tungsten-osmium spreading resistance probes

8-10

van der Pauw structure 45-46

wafer prober, automatic 22-23

Wagner curve 37

X-level, silicon 20

x-ray photoelectron spectroscopy (XPS) 33-36

71
i

i

1
|p.ll4A (REV. 7-73)

' U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUHI.K A I ION OR K1-:P0RT NO. NBS Spec. Publ. 400-36

2. Gov't Acfcssion

No.

3. K< ( i|)ietit's Aiccssiori

TITLK AND SUBTITLI-; Semiconduotor Measurement Technology: Progress Report

July 1 to September 30, 1976

5. Publication Date July 1978

6. Performing Organization Gode

AUTHOR(S)

W. Murray Bullis and J. Franklin Mayo-Wells , Editors

8. Performing Organ. Report No.

PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Ta.sk/Work Unit No.

11. f ontract/Grant No. see item 15

. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

NBS - Washington, D.C. 20234; SSPO - Washington, D.C. 20376;

DNA - Washington, D.C. 20305; ARPA - 1400 Wilson Boulevard,

Arlington, VA 22209; SAMSG - P.O. Box 92960, Worldway Postal

Center, Los Angeles, CA 90009; ERDA - Washington, D.C. 20545

13. Type of Report & Period Covered Interim: July 1 to Sept. 30, 1976

14. Sponsoring Agency Code

. SUPPLEMENTARY NOTES

ARPA Order 2397, Program Code TDIO; DNA lACRO 7T-812; SSPO Order NOO164-76-MP-04511

;

MIPR FY76 167600366; ERDA Contract E(49-l)-3300, Mod. 3. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

lis progress report describes MBS activities directed toward the developnent of methods of measurement

)r semiconductor materials, process control, and devices. Both in-house and contract efforts are in-

|.uded. The emphasis is on silicon device technologies. Principal accomplishments during this report-

|ig period included (1) complption of tht systematic study of the effects of surface preparation and

}:obe material on the empirical calibration betxveen specimen resistivity and spreading resistance of

i[- and p-type silicon; (2) initial evaluation of the nuclear-track technique for quantitative determ.inalon of trace amounts of boron In silicon; (3) development of procedures for using an optical research

Ucroscope to make accurate me'iGureinents of the width of a clear line as narrow as 0.5 um in a comjLetely or nearly opaque background; (4) design of a compact cross-bridge test structure for electrical

jaasurement of line width and sheet resistance in minimum line-width geometries; (5) com.pletion of the

bitial phase of the study of the particle-impact noise detection test for screening devices for the

llresence of loose particles in the package; (6) demonstration of a greater-than-expected line resolu-

|lon capability for the scanning acoustic microscope; and (7) development of a nondestructive technique

b measure the onset of second breakdown in forward-biased, medium-power transistors. Also reported is

>jther ongoing v7ork on materials characterization by electrical and physical analysis methods, materials

Ed procedures for wafer processing, photolithography, test patterns, and device inspection and test ocedures. Supplementary data concerning staff, publications, and technical services are included as

(Ippendices

.

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

? name; separated by semicolons ) pger electron spectroscopy; capacitance-voltage methods, dew-point sensing; dragging-stylus probe;

iLectrical properties; electronics; four-probe method; hermeticity; ho]e mobility; infrared reflectance;

;Dn implantation; ion microprobe mass analysis; line-width measurements; nuclear-track technique;

'^rticle-impact noise detection; photolithography; photovoltaic method; p-n junction; power-device grade

tllicon; resistivity; resistivity variations; resistors, sheet; safe operating area, transistor; scanning

Icoustic microscope; second bieakdown; semiconductor materials; semiconductor process control; silicon;

jilicon dioxide; silicon-on-sapphire; spreading resistance; test patterns; test structures; thermally

Ikimuiated current and capacitance; thermal properties, transistor; thermal response; thyristors; tranjlstors , power; ultraviolet reflectance; x-ray photoelectron spectroscopy.

18. AVAILABILITY fx^ Unlimited

I ! For Official Distribution. Do Not Release to NTIS

[x2 Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Stock No. SNOO^Oj -

IT] Order From National Technical Information Service fNTIS)

Springfield, Virginia 22151 (Microficho Only)

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

78

22. Price $2. 75

USCOMM-DC 29042-P74

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BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-awareness and literature-survey bibli- ographies are issued periodically by the Bureau: Cryogenic It,ata Center Current Awareness Service. A litera- ture survey issued biweekly. Annual subscription: Domestic, $25.00; Foreign, $30.00.

Liquified Natural Gas. A literature survey issued quarterly.

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Superconducting Devices and Materials. A literature survey

issued quarterly. Annual subscription: $30.00. Send subscription orders and remittances for the preceding bibliographic

services to National Bureau of Standards, Cryogenic Data

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