NBS SPECIAL PUBLICATION 400-29

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards
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 pro- mote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology, the Institute for Computer Sciences and Technology, the Office for Information Programs, and the

Office of Experimental Technology Incentives Program.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United States of a complete and consist- ent system of physical measurement; coordinates that system with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of the Office of Measurement Services, and the following center and divisions: Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Center for Radiation Research — Laboratory Astrophysics- — Cryogenics* — Electromagnetics" — Time and Frequency'.

THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measurement, standards, and data on the properties of well-characterized materials needed by industry, commerce, educational insti- tutions, and Government; provides advisory and research services to other Government agencies; and develops, produces, and

distributes standard reference materials. The Institute consists of the Office of Standard Reference Materials, the Office of Air and Water Measurement, and the following divisions:

Analytical Chemistry — Polymers — Metallurgy — Inorganic Materials — Reactor Radiation — Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services developing and promoting the use of avail- able technology; cooperates with public and private organizations in developing technological standards, codes, and test methods; and provides technical advice services, and information to Government agencies and the public. The Institute consists of

the following divisions and centers: Standards Application and Analysis — Electronic Technology — Center for Consumer Product Technology: Product

Systems Analysis; Product Engineering — Center for Building Technology: Structures, Materials, and Safety; Building

Environment; Technical Evaluation and Application — Center for Fire Research: Fire Science; Fire Safety Engineering.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and provides technical services designed to aid Government agencies in improving cost effectiveness in the conduct of their programs through the selection, acquisition, and effective utilization of automatic data processing equipment: and serves as the principal focus wthin the executive branch for the development of Federal standards for automatic data processing equipment, techniques, and computer

languages. The Institute consist of the following divisions: Computer Services — Systems and Software — Computer Systems Engineering — Information Technology.

THE OFFICE OF EXPERIMENTAL TECHNOLOGY INCENTIVES PROGRAM seeks to affect public policy and process

to facilitate technological change in the private sector by examining and experimenting with Government policies and practices in order to identify and remove Government-related barriers and to correct inherent market imperfections that impede

the innovation process. THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and accessibility of scientific information generated within NBS; promotes the development of the National Standard Reference Data System and a system of in- formation analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services

to ensure that the NBS staff has optimum accessibility to the scientific information of the world. The Office consists of the following organizational units:

Office of Standard Reference Data — Office of Information Activities — Office of Technical Publications — Library —

Office of International Standards — Office of International Relations.

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

2 Located at Boulder, Colorado 80302.
Semiconductor Measurement Technology

Progress Report, January 1 to June 30, 1976

W. Murray Bullis, Editor

Electronic Technology Division

Institute for Applied Technology

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. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued April 1977
National Bureau of Standards Special Publication 400-29

Nat. Bur. Stand. (U.S.), Spec. Publ. 400-29, 119 pages (Apr. 1977) CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1977

For sale by the Superintendent of Documents, U.S. Government Printing OlTice Washington, D.C. 20402 - Price $1.85

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TABLE OF CONTENTS

SEMICONDUCTOR MEASUREMENT TECHNOLOGY

PAGE

List of Figures iv

List of Tables vii

Preface viii

Abstract and Key Words ix

1. Introduction 1

2. Highlights 2

3. Materials Characterization by Electrical Methods 7 3.1. Four-Probe Method 7 3.2. Spreading Resistance 9 3.3. Spreading Resistance Profiles 11

3.4. Photovoltaic Method _ 13

3.5. Reevaluation of Irvin's Curves 15

3.6. Electron Mobility in Phosphorus-Doped Silicon 17 3.7. Thermally Stimulated Current and Capacitance Measurements 19

3.8. Extended Range MIS C-V Method 26

4. Materials Characterization by Physical Analysis Methods 29 4.1. Optical Test for Surface Quality of Sapphire Substrates 29 4.2. Impurities in Sapphire Substrates 30

4.3. Electron Spectroscopy Techniques 31

5. Materials and Procedures for Wafer Processing 36

5.1. Sodium Contamination in Oxidation Furnaces 36 5.2. X-Ray Dose in Electron-Beam Evaporators 36 5.3. Ion Implantation Parameters 40

5.4. Passivation Integrity 42

5.5. Process Materials Characterization 47

6. Photolithography 48

6.1. Line-Width Measurement with Spatially Filtered Coherent Optical Radiation .... 48

6.2. Optical Imaging for Photomask Metrology 50

6.3. Calibration Standards for Photomask Metrology 53

6.4. Line-Width Measurements on Chromium-on-Glass Artifacts 53

6.5. Line-Width Measurement by Diffraction Pattern Analysis 57

6.6. Automated Photomask Inspection 58

7. Test Patterns 62

7.1. Square Array Collector Resistor 62

7.2. Sheet Resistor Test Structures 64

7.3. Test Pattern for TTL Technology 65

7.4. Test Pattern Design and Analysis for SOS/LSI 65

8. Assembly and Packaging 69 8.1. Nondestructive Test for Beam-Lead Bonds 69

8.2. In-Process Bond Monitor 72 8.3. Radioisotope Leak Test 74 8.4. Loose Particle Detection ..... 75

9. Device Inspection and Test 77 9.1. Dual-Laser, Flying-Spot Scanner 77 9.2. Scanning Acoustic Microscopy 79 lii
TABLE OF CONTENTS

PAGE

9.3. Scanning Electron Microscopy 84

9.4. Automated Scanning Low Energy Electron Probe 88

9.5. Thermal Properties of Transistors 88

10. References 90

Appendix A Semiconductor Technology Program Staff 96

Appendix B Semiconductor Technology Program Publications 97

Appendix C Workshop and Symposium Schedule 102

Appendix D Standards Committee Activities 103

Appendix E Solid-State Technology & Fabrication Services 105

Index 106

LIST OF FIGURES

PAGE

Frontispiece x

1. Photomicrographs of typical tungsten carbide probe tips 8

2. Ratio of spreading resistance, Rsp > to resistivity, p , as a function of

resistivity as measured with a variety of probe tips on (111) n-type

silicon surfaces mechanically polished with 0.5-um alumina in aqueous slurry ... 10

3. Ratio of spreading resistance, Rgp, to resistivity, p, as a function of

resistivity as measured with relatively sharp osmium-tungsten alloy

probe tips on (100) n-type silicon surfaces prepared in various ways 10

4. Dopant density profiles in boron-diffused n-type silicon as measured by

the spreading resistance and incremental sheet resistance methods 12

5. Prototype automated data collection and analysis system for resistivity profiling of circular silicon slices by the scanning photovoltaic method 14

6. Conductivity mobility of electrons at 300 K as a function of total donor

density for phosphorus-doped silicon 18

7. Resistivity as a function of temperature for seven phosphorus-doped silicon

slices 18

8. Resistivity of phosphorus-doped silicon at 300 K as a function of total

donor density 18

9. Thermally controlled chuck and automatic wafer prober 20

10. Thermally stimulated current response of gold-doped silicon MOS capacitors

in wafer form 22

11. Thermally stimulated current response of gold-doped n-MOS capacitor with an oxide film contaminated by mobile ions 22

12. Capacitance-voltage characteristics of various oxides on n-type silicon with

aluminum gate electrodes 24

13. Capacitance-voltage characteristics of four MOS capacitors on each of four

n-type silicon wafers with dry thermal oxides, about 90 nm thick, and e-gun

evaporated aluminum gates microalloyed at different temperatures 25

14. Temperature of peak value of thermally stimulated current response as a function of heating rate for selected defect centers in silicon 26 iv
LIST OF FIGURES

PAGE

15. Specular reflection data for four bulk silicon substrates polished with

different polishing grits and for six silicon-on-sapphire composites 29

16. Scanning electron micrographs of silicon-on-sapphire surfaces 30

17. Schematic of experimental conditions for ion sputter etching and Auger

electron spectroscopy 32

18. Spatial distributions of ion beam current 32 19. Additional broadening of an interface at a depth of 100 nm due to flux inhomogeneity in rastered ion beams of normal and defective ion guns . . 33

20. Oxygen KLL profiles showing effects of electron-beam enhancement of

sputtering rate 33

21. Geometry for crater-edge profiling using Auger electron spectroscopy 34

22. Line scans for oxygen, phosphorus, and elemental silicon across the interface

between the points C and on the x-axis 35

23. Sodium density in the atmosphere of a fused silica oxidation tube as a function

of water vapor content at temperatures from 1200 to 1400 K 37

24. Total ionizing dose deposited in the oxide layer as a function of electronbeam energy during the electron-gun evaporation of aluminum and chromium 38

25. Total ionizing dose deposited in the oxide layer as a function of the deposition rate of aluminum and chromium in an e-gun evaporation system 38

26. Total ionizing dose deposited in the oxide layer as a function of chromium

layer thickness during the e-gun evaporation of chromium or of a 1-ym thick

aluminum layer over chromium 38

27. Absorbance of x-ray energy in nylon film dosimeter as a function of the

electron charge incident on aluminum in an e-gun evaporator 38

28. Typical target configuration for characterizing the ion beam before ion implantation 41

29. Scanning geometry and overscan efficiency 41

30. Target current densities as a function of scanner voltage for an 84-keV

boron ion beam 41

31. Schematic representation of corona charging with positive ions 43

32. Schematic representation of the reverse decoration of defects with negative

ions following corona charging with positive ions 44

33. Photomicrograph of reverse decorated device showing high contrast which

renders microscopic evaluation fast and sensitive . 45

34. Photomicrograph of portion of reverse decorated device aluminum-etched at

50° C for 5 min without removing carbon black 45

35. Schematic representation of microscope modified to obtain image of source field

stop at the photocell to measure the integrated light reflected from defective

areas 46

36. Sketch of the single-lens coherent optical system, showing the locations of

various planes and coordinate systems 48

37. Calculated images of clear lines in opaque backgrounds for coherent illumination of wavelength 500 nm, and a 0.95 numerical aperture objective lens 49

38. Sketch of a possible configuration for a microscope system with spatial

filtering 50

39. Diagram of optical research microscope as modified for line-width measurement ... 51

V
LIST OF FIGURES

PAGE

40. Calculated and experimentally determined line-image profiles for clear

lines in opaque backgrounds 52

41. Photomicrograph of chromium-on-glass artifact, viewed under bright-field

transmitted light at a magnification of about 85x 53

42. Photomicrograph of four 1-pm wide clear spaces each separated by 1 un in

an iron-oxide-on-glass artifact viewed under bright-field transmitted light

at a magnification of about 2000x 53

43. Schematic representation of optical system for diffraction pattern analysis .... 57

44. Schematic flow diagram for estimation of width of gaps 58

45. Arrangement of a square four -probe array on the diagonals of a thin, square

specimen 63

46. Normalized sheet resistance error resulting from shorting contacts on the

periphery of a square test structure 64

47. Normalized sheet resistance error resulting from shorting contacts at the

ends of the arms of a Greek cross test structure 64

48. Junction-isolated TTL test pattern 66

49. Two-level interaction test structure 67

50. Two-level alignment structures 67

51. Distribution of p-channel saturation current in heavy inversion along the

diameter of an SOS wafer perpendicular to the edge flat 68

52. Acoustic emission signals from single beams pulled to destruction 70

53. Beam-lead device with poor mechanical integrity 70

54. Apparatus for applying upward or downward force on beam-lead devices and

for detecting any resulting acoustic emission 71

55. Acoustic emission signals from individual beams subjected to downward force

on the horizontal portion of the beam 71

56. Beam-lead bonding tool modified to apply force to the beams without contacting

the chip 72

57. Typical signatures of ultrasonic aluminum-aluminum wire bonds made with different power settings 74

58. Photoresponse of a typical 2N4431 UHF transistor to 0.633- and 1.15-ym radiation as a function of temperature 77

59. Temperature profile of hot spots in typical 2N4431 UHF transistors as measured

with a laser scanner and an infrared microradiometer 78

60. Photograph of the reflected light plus photoresponse image obtained when the

0.633-)jm laser beam scans the back side of a silicon-on-sapphire C-MOS 4007

inverter 78

61. Photograph of the reflected light image obtained when the 1.15-Mm laser beam

scans the back side of a flip-chip C-MOS 4007 inverter 78

62. Photographs of the reflected light plus photoresponse image obtained when the

1.15-pm laser beam scans the back side of a flip-chip 709 operational amplifier . . 78

63. Comparison of operating modes of the scanning acoustic microscope 80

64. Acoustic images showing differences between cw and pulsed operation 80

65. Comparison between 940-MHz acoustic images of a bipolar microwave transistor

with 2-pm wide metal fingers and a photomicrograph taken with interference

contrast 81

vi
LIST OF FIGURES

PAGE

66. Frequency dependence of attenuation of acoustic waves in water at various

temperatures 82

67. Acoustic images, reflection mode at 940 MHz, of an integrated circuit showing

the effect of non-normal incidence 82

68. Spherical tool grinder for making acoustic lenses 82

69. Photograph of the tool grinder 82

70. Frequency response of return and transmission loss of two sputtered glass

acoustic anti-reflection coatings (AARC) 83

71. Change in threshold voltage, AV.j.^, of a p-channel MOS transistor as the result

of exposure to an electron flux of 5.2 x 10^'^ cm~^*s~-^ at energies of 17.5 and

30 keV in a scanning electron microscope 84

72. Electron-beam-induced-current response of a copper-doped silicon p-n diode .... 85

73. Electron-beam-induced-current response of a silicon npn transistor 86

74. Portion of a quad 2-input NAND gate including a functional gate on the left

and a nonfunctional gate on the right 87

75. Portions of a low-power, four -bit shift register 87

LIST OF TABLES

PAGE

1. Results of Four-Probe Pilot Study on Thin Layers 7

2. Test Specimens for Four-Probe Matrix Experiment 8

3. Characteristics of Probes Used in Matrix Experiment 8

4. Preliminary Electron Mobility Data for Phosphorus-Doped Silicon at 300 K 16

5. Preliminary Hole Mobility Data for Boron-Doped Silicon at 300 K 16

6. Emission Coefficients and Activation Energies of Selected Defects in Silicon ... 27

7. Elemental Content of Sapphire Substrates 31

8. Equilibrium Partial Pressures and Sodium Densities in Fused Silica Oxidation

Tubes at 1300 K as a Function of Water Content in Flowing Oxygen 37

9. Equilibrium Partial Pressures and Sodium Densities in Tubes at 1300 K With an Atmosphere of 10% Hydrogen Chloride or Chlorine in an Inert Carrier Gas 37

10. Zinc Content in Glass Transferred from Doped Borosilicate Glass to Silicon at 1000°C 47

11. Apparent Widths of Lines and Spaces Measured in Bright-Field Transmitted

Illumination 54

12. Effect of Focal Position on the Apparent Width of a Clear Space Measured

with an Image-Shearing Eyepiece in Bright-Field Transmitted Illumination 54

13. Apparent Widths of Clear Lines in an Opaque Field 56

14. Representative Data for Line-Width Measurements Repeated on the Same Line 56

15. Characteristics of Automated Photomask Inspection Techniques 59

16. Correction Factors for a Square Four-Probe Array on a Thin Square Specimen .... 63

17. Pull Test Results for Monitored Ultrasonic Wire Bonds 73

18. Representative Response Frequencies of Metal Spheres Impacting a Steel

Surface 76

vii
PREFACE

The Semiconductor Technology Program serves

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

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

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

controlling device fabrication processes.

Its major thrusts are the development of

carefully evaluated and well documented test procedures and associated technology and the

dissemination of such information to the

electronics community. Application of the

output by industry will contribute to higher

yields, lower cost, and higher reliability

of semiconductor devices. The output provides a common basis for the purchase specifications of government agencies which will

lead to greater economy in government procurement. In addition, improved measurement

technology will provide a basis for con- trolled 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 re- ceives 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 detailed reports such as state-of-the-art re- views, literature compilations, and summaries

of technical efforts conducted within the Pro

gram are issued as these activities are com- pleted. Reports of this type which are published by NBS also appear in the Special Publication 400- sub-series. Announcements of

availability of all publications in this subseries are sent by the Government Printing Of

fice to those who have requested this service

A request form for this purpose may be found

at the end of this report.

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

.

Disclaimer

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

description of the work performed. The exper

iments 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 identi

fied are necessarily the best available for

the purpose.

" Through ARPA Order 2397 , Program Code 6D10

(NBS Cost Center 4257555). All contract

work was funded from this source

.

t Principally through the Electronic Technology Program (Cost Center U257100). Additional funding through the Dimensional Metrology Program (Cost Center 2131119) and

the Nondestructive Evaluation Program (Cost

Center 4253130).

§ Through Inter-Agency Cost Reimbursement Order 76-806 (NBS Cost Center 4259522).

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

# Code SP-23, through project order

N0016475P070030 administered by Naval Weapons Support Center, Crane, Indiana (NBS

Cost Center 4251533).

X Division of Electric Energy Systems,

through ERDA Contract No. E( 49-l)-3300

,

Modification 3 (NBS Cost Center 4259551).

viii
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) development

of theoretical expressions for electron mobility in silicon based on combinations of scattering mechanisms; (2) successful low-temperature

processing of MOS capacitors to permit measurement of thermally stimulated current and capacitance without subjecting the specimens to potentially degrading heat treatments; (3) completion of a study of

the thermodynamics of reactions in an oxidation furnace tube which

provides a basis for models of the effect of water vapor, chlorine,

and tube wall conditions on sodium contamination levels; (A) development of a rapid, nondestructive method for reverse decoration of defects in passivation overcoats; (5) development of the theoretical

basis for accurate measurement of small line widths by analysis of a spatially filtered image of the line; (6) extension of the acoustic

emission technique to the nondestructive testing of tape-bonded chips

and hybrid components; and (7) analysis of the results of a first

exploratory inter laboratory evaluation of the radioisotope method for

testing hermeticity of semiconductor devices. New tasks were undertaken to develop techniques for measuring resistivity uniformity of

and non-dopant impurities and defects in power-device grade silicon

and to investigate the particle impact noise detection method for

screening devices for the presence of loose particles in the package;

initial results were obtained in the study of scanning acoustic mi^

croscopy which was begun during the previous reporting period. Be- cause of technical limitations in the methods, efforts to develop

both the automated scanning low energy electron probe and in-process

ultrasonic bond monitor were terminated. Also reported is other on- going work on materials characterization by electrical and physical

analysis methods, materials and procedures for wafer processing,

photolithography, test patterns, and device inspection and test procedures. Supplementary data concerning staff, publications, workshops

and symposia, standards committee activities, and technical services

are also included as appendices.

KEY WORDS

Acoustic emission; Auger electron spectroscopy; beam-lead bonds;

capacitance-voltage methods; carrier mobility; C-MOS circuits; dopant

profiles; electrical properties; electronics; four -probe method; hermeticity; ion implantation; Irvin's curves; leak tests; line-width

measurement; measurement methods; microelectronics; optical flyingspot scanner; oxidation particle impact noise detection test; passivation overcoats; photovoltaic method; power-device grade silicon;

radioisotope method; resistivity; resistivity variations; safe operating area, transistor; scanning acoustic microscope; scanning electron microscope; scanning low energy electron probe; semiconductor

devices; semiconductor materials; semiconductor process control;

silicon; silicon dioxide; silicon on sapphire; sodium contamination;

spreading resistance; surface roughness; test patterns; thermally

stimulated current; transistors, power; TTL circuits; ultrasonic wire

bonding; ultraviolet reflectance; x-ray damage.

ix
a. Greek cross. b. Greek cross with minimum line width.

X
SEMICONDUCTOR fl E A S IJ R E M E N T TECHNOLOGY

PROGRESS REPORT

January 1 to June 30, 1976

1 . I n T R 0 D [I C T I 0 r

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

the thirty-first and thirty-second quarters

of the Program. It summarizes work 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 number of tasks, each directed toward the study

of a particular material or device property

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

of activity during the 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 members 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 on request to the author.

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.

Communication with the electronics community

is a critical aspect both as input for guidance in planning future program activities

and in disseminating the results of the work

to potential users. Formal channels for such

communication occur in the form of workshops

and symposia sponsored or co-sponsored by NBS

At the present time, no such seminars and

workshops are scheduled. However, the availability of proceedings from past workshops

and seminars is indicated in Appendix C.

An important part of the work that frequently goes beyond the task structure is participation in the activities of various technical

standardizing committees. The list of personnel involved with this work given in Appendix D suggests the extent of this participation. In most cases, details of standardization efforts are reported in connection

with the work of a particular task.

Technical services in areas of competence are provided to other NBS activities and other

government agencies as they are requested.

Usually these are short-term, specialized services that cannot be obtained through normal

commercial channels . To indicate the kinds

of technology available to the Program, such

services provided during the current reporting period are listed in Appendix E.

1
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) development of theoretical expressions for electron mobility in silicon based

on combinations of scattering mechanisms;

(2) successful low-temperature processing of MOS capacitors to permit measurement

of thermally stimulated current and capacitance without subjecting the specimens to potentially degrading heat treatments;

(3) completion of a study of the thermodynamics of reactions in an oxidation fur- nace tube which provides a basis for models

of the effect of water vapor, chlorine, and

tube wall conditions on sodium contamination

levels

;

(4) development of a rapid, nondestructive method for reverse decoration of defects in passivation overcoats;

(5) development of the theoretical basis for accurate measurement of small line

widths by analysis of a spatially filtered

image of the line;

(6) extension of the acoustic emission

technique to the nondestructive testing of

tape-bonded chips and hybrid components; and

(7) analysis of the results of a first

exploratory interlaboratory evaluation of

the radioisotope method for testing hermeticity of semiconductor devices.

New tasks were undertaken to develop techniques for measuring resistivity uniformity

of and non-dopant impurities and defects in

power-device grade silicon and to investigate

the particle impact noise detection method

for screening devices for the presence of

loose particles in the package; initial results were obtained in the study of scanning

acoustic microscopy which was begun during

the previous reporting period. Because of

technical limitations in the methods, efforts

to develop both the automated scanning low

energy electron probe and in-process ultrasonic bond monitor were terminated.

Unless another organization is identified,

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

of Standards,

Materials Charaoterization by Eleotriaal Methods — An extensive matrix experiment was carried out to examine the conditions under

which the sheet resistance of very thin

junction-isolated silicon layers could be mea

sured reliably by the four-probe method. In contrast to the less promising results of an interlaboratory pilot study, conducted in co- operation with ASTM Committee F-1 on Electron

ics , the results of these experiments showed

that better than 1 percent repeatability is

feasible if measurement conditions are properly controlled. An interlaboratory test to verify this conclusion is now being con- ducted in cooperation with Committee F-1.

Experiments to determine the effects of speci men surface preparation and probe material on

the empirical relation between silicon resistivity and spreading resistance were continued with the study of surfaces mechanically

polished with O.S-ym alumina in an aqueous

slurry. Anomalous results were obtained on

(111) n-type surfaces; satisfactory results were obtained on (111) p-type and (100) n- type surfaces.

As a preliminary test of the application of a simplified calculational procedure being developed at Solecon Laboratories for correcting spreading resistance data on layers with

nonuniform resistivity, the dopant profiles

of two boron-diffused layers were measured by

the spreading resistance method and by the in

cremental sheet resistance method and compared. The relatively close agreement obtained suggests that the parallel superposition approach is quite accurate for diffused

structures with junction depths less than 2 \m. Discrepancies near the surface could be

accounted for on the basis of bevel-rounding

effects

.

Work on the bulk photovoltaic effect was resumed with emphasis on the application of

this method to evaluation of resistivity vari

ations along the diameter of circular slices

suitable for fabrication of high-power thyris

tors and rectifier diodes. An automated data

collection and analysis system for making pho

tovoltaic measurements has been designed and

assembled. Subsequently, the resolution of

this technique is to be compared with that of

the spreading resistance method.

As part of the continuing experimental redetermination of the resistivity-dopant density

relation (Irvin's curves), additional measure

ments of room-temperature resistivity and dopant density were made on silicon wafers

doped with phosphorus. In addition, initial

measurements were made on silicon wafers

2
HIGHLIGHTS

doped with boron. In general the electron

mobilities lie above those based on Irvin's

data and the hole mobilities lie below those

reported by Wagner.

Theoretical expressions have been developed

for the electron mobility in silicon as a function of temperature and dopant density.

By incorporating the effects of electronelectron scattering and scattering anisotropies in the mobility calculation, good agreement between theory and experiment for both

electron mobility and resistivity was obtained for phosphorus-doped silicon slices

with dopant density of 10^'* to 3 x 10 cm"^ over the temperature range 100 to 500 K.

A new task has been undertaken to examine the

range of validity of the Schottky equations

(based on the depletion approximation) which

are commonly used to extract dopant density

from capacitance-voltage data. Numerical solutions to boundary value problems based on

the carrier transport equations or Poisson's

equation are being developed to provide a basis for comparison. Among the questions to be studied explicitly are: (1) what is the

correct definition of junction capacitance in terms of field quantities such as electric

field and electron and hole densities? and

(2) is the quantity derived from capacitancevoltage data by the Schottky equation the

equilibrium free-carrier density or the impurity (dopant) density in the depletion region?

A new task has been initiated to study the

thermally stimulated current and capacitance

response of silicon suitable for fabrication

of high-power thyristors and rectifier diodes.

Initial work has concentrated on the development of apparatus for making the measurements

at the wafer level and evaluation of lowtemperature procedures for fabrication of MOS

capacitors. Early results suggest that capacitors fabricated with a silicon dioxide layer

chemically vapor deposited at 400°C and an electron-beam (e-gun) evaporated aluminum

gate, microalloyed in dry nitrogen at 400°C,

are suitable for thermally stimulated current

and capacitance measurements. It was also

demonstrated that such measurements can be

made at the wafer level in a dry nitrogen atmosphere; an improved apparatus for this purpose was designed and assembled. The results

of preliminary experiments suggest that it

may be possible to make thermally stimulated

current and capacitance measurements even in

capacitors with oxides contaminated by relatively large densities of mobile ions provided that the mobile ions are driven to the

metal-oxide interface. In related work, the

emission temperatures of a number of impurity

and defect centers in silicon were calculated

as a function of heating rate with the use of

data compiled from literature.

Design and evaluation work on the modified in

strument for measuring capacitance at applied

voltages of up to 10 kV has been completed at RCA Laboratories . In an application of this

instrument, it was shown that a large positive charge develops at the silicon-sapphire

interface during e-gun metallization and that

annealing at 400°C in forming gas does not en tirely remove this charge. The extendedrange measurement technique was refined to permit measurement of conductance as a function of voltage in addition to capacitance.

Materials Charaaterization by Physiaal Analysis Methods — Development of calibration

standards for ion microprobe mass analysis

continued at Texas Instruments with emphasis

on improved masking techniques for fabricating the microvolume structures, preparation

of implanted specimens , and collection of data. Preliminary results suggest that the

smaller microvolume structures may be inappro

priate for calibration artifacts. Consequent

ly, future activities in this task are being

directed toward collection of reproducibility

data and refinement of procedures for calibrating standard specimens.

The rapid, nondestructive reflectance technique being developed at RCA Laboratories to determine the surface quality of sapphire sub

strates was extended to ultraviolet wavelengths. The reflectance characteristics of

silicon-on-sapphire composites could be cor- related with the surface roughness of the sil

icon film.

The impurity content of three sapphire substrates was determined by neutron activation

analysis. These measurements were made to assist an Air Force contractor in studying

the influence of impurities transferred from

the sapphire substrate to the silicon film on

the properties of silicon-on-sapphire devices

A study was made of the effect of ion flux

distribution in Auger/ion beam profiling of

oxide-silicon structures at Stanford University. It was shown that ion flux inhomogeneities can be a limiting factor in depth resolution. Apparent interface broadening was

also shown to occur in the presence of en- hanced sputtering as a result of electronstimulated desorption of oxygen from the oxide. In related work at Varian Associates,

the crater edge formed by ion beam etching

3
HIGHLIGHTS

was shown to provide a bevelled surface which

allows depth profiling by Auger electron spectroscopy with a spatial resolution significantly greater than that obtainable on a me- chanically lapped bevelled surface.

Materials and Proaedures for Wafer Processing — A thermodynamic study of the reactions

in a fused silica oxidation furnace tube was

completed. Equilibrium sodium concentrations

in the furnace atmosphere were determined at several temperatures for varying amounts of

water vapor and chlorine; it was assumed that

the sodium was transferred from the tube wall

in which it occurs at a level of about 10

parts per million by weight. The presence of

water vapor was shown to increase the sodium

density significantly because of the formation of sodium hydroxide. The results of the

calculations also provide insight into the

mechanisms which occur when cleaning furnace

tube walls with hydrogen chloride or chlorine.

Additional calculations were made to estimate

the x-ray dose received by oxide films in MOS

structures from the deposition of aluminum or chlorine gate electrodes by electron-beam

evaporation. It was found that the dose re- ceived is essentially independent of the electron beam energy. A significant reduction in

dose absorbed in the oxide occurs if a chromium film is used as the gate electrode rather than an aluminum film, primarily because

the power necessary to deposit a chromium

layer is significantly less than that re- quired to deposit the same thickness of aluminum .

Dose measurement studies were completed as part of the development of measurement technology for critical ion implantation parameters at Hughes Research Laboratories. Evaluation of a group of typical current integrators revealed inaccuracies at high implantation currents, a situation which may cause problems as the industry moves to higher cur- rents for production use. Procedures for measuring the ion beam current density distribution were also developed. In addition several metallization systems were evaluated for use as Schottky barrier contacts on n-type

silicon; gold yielded the best results in

this study.

A new, simple, nondestructive reverse decoration technique was developed at RCA Laboratories to detect defects in insulating films

over metal portions of semiconductor devices.

The defects are clearly visible for microscopic examination because the decorating medium,

carbon black, covers only portions of the surface on which defects are absent. The decrating medium can be completely removed following examination.

Studies of the transfer of zinc from simulated boron nitride diffusion sources to the

glass which forms on the surface of a silicon

wafer during a typical diffusion process were carried out at the Pennsylvania State University. The results of these experiments were

inconclusive because available measurement

techniques proved to be too insensitive. In addition, calculations were made of equilibri

um partial pressures of the chemical species

in both hydrogen chloride and trichloroethylene oxidation systems. It was found that the

partial pressure of water vapor in the trichloroethylene system was about an order of

magnitude lower than that in the hydrogen

chloride system.

Pho to li thography — Application of the spatial filtering technique to line-width measurement with an optical microscope was analyzed. In this method, the observer uses an optical transformation of the line in which

the image no longer resembles the original

line but in which the location of the line

edges is marked by two narrow, dark lines

within a bright surround. Although implementation of this technique will require redesign of microscope objectives with large nu- merical apertures so that the plane of the ob

ject spectrum occurs outside the physical dimensions of the lens system, this technique

appears in principle to alleviate many of the

problems associated with the use of filar and

image-shearing eyepieces.

A research microscope was modified to make

photometric scans of line-image profiles for

comparison with theoretically computed profiles. The agreement obtained between measured and calculated profiles for transparent

lines as narrow as 1 pm in an opaque background was quite satisfactory; the results to date suggest that the system can be used to measure the width of transparent lines nearly

as narrow as the impulse response of the objective lens, and thus that it may be possible to calibrate line-width artifacts as narrow as 0.5 ym without resorting to scanning

electron microscopy. Work is underway to investigate causes for the remaining discrepancies between theory and experiment.

Chromium-on-glass artifacts with opaque and

transparent lines as small as 1 ym wide have

been fabricated successfully. Additional mea

surements of line widths on such artifacts

confirmed the previously predicted discrepan4
HIGHLIGHTS

cies between measurements made with filar

and image-shearing eyepieces.

A study was undertaken at Recognition Systems

to delineate the tolerances, design criteria,

and limitations associated with the measurement of line widths by analysis of the diffraction pattern. It was shown that the system response depends on the uniformity and

phase of the aperture illumination and on the

numerical aperture and quality of the objective lens as well as on the characteristics

of the line or gap being measured. The re- sults suggest that additional analysis and

instrumental improvements will be necessary

to interpret accurately widths of lines less

than 2.5 ym wide; such modifications are being considered.

Detailed analyses were completed of a variety

of automated technologies applicable to the

inspection of photomasks for defects. It was

concluded from this work, conducted at Law- rence Livermore Laboratory, that the search

for dimensional errors and the search for defects differs significantly, and that these

functions should be performed independently.

Speed is the most important consideration in

the automated search for visible defects;

the analyses showed that few systems as presently envisioned have economically attractive

inspection times. To provide a means for fu- ture evaluation of various automated mask inspection systems, work was initiated on the

design and fabrication of a test mask with

intentionally added defects.

Test Patterns — The square array four-probe

collector resistor structure was further analyzed for the case in which the probe spacing

is comparable with the lateral dimensions of

a square specimen. It was found that simple,

limiting expressions for the specimen resistivity provide 1 percent or better accuracy

except for ratios of the length of the specimen side to the probe spacing between 1.35

and 23.

Additional analysis of cross-type sheet re- sistor test structures was completed. The

Greek cross structure, which can be treated

as a simple van der Pauw resistor, is particularly interesting because it affords the possibility of measuring sheet resistance in a region whose dimensions are limited only by

the minimum line width achievable by the photolithographic technology utilized in device

fabrication.

A test pattern for TTL technology is being developed as an illustration of a microelectronic test pattern for use in implementing the

process validation wafer concept. The pattern consists of a variety of modularized

test structures which are intended to be

tested electrically with a high-speed dc wafer tester. Included are test structures

which can identify variations in process and

circuit parameters and the presence of random

faults

The mask set for the SOS/LSI test pattern being developed at RCA Laboratories was com- pleted and two lots of wafers were processed.

Preliminary results were obtained on several

test structures designed for visual examination and on the distribution of the characteristics of the p-channel transistor across

a wafer.

Assembly and Paokaging — Detailed experiments were carried out to evaluate the acoustic emission test as a nondestructive means

for evaluating the quality of beam-lead bonds

Distinct differences were obtained in the

emission signals for various types of bond

weaknesses. In addition it was shown that

this technique could also be applied to evalu

ation of other types of gang-bonded devices.

An extensive series of experiments was car- ried out to complete the evaluation of the in

process ultrasonic bond monitor. These exper

iments showed that the characteristic signal

observed at the node of the bonding tool did

not correlate with bond quality but occurred

as a result of transients in the power supply

transducer-tool system. Because of the negative results of these experiments , further

development of this technique has been abandoned .

The first exploratory interlaboratory evaluation of the radioisotope method for testing

hermeticity of semiconductor devices has been

completed and the results analyzed. This ex- periment demonstrated conclusively that an interlaboratory comparison of hermetic packages can be accomplished under appropriate

metrological guidelines . The ten measurements on each of 100 test packages yielded

leak rates which agreed to within a fraction

of the decade in which the mean value occurred, in contrast to order of magnitude dif

ferences observed in earlier, less formal industrial comparisons. In addition the experiment provided information necessary to develop an improved measurement procedure.

Detailed evaluation of the particle impact

noise detection test has been initiated as a first phase of a study of loose particle de5
HIGHLIGHTS

tection methods. Initial results showed the

feasibility of using seeded cans as reference

standards for evaluation of equipment for

this test and demonstrated the equivalence of

three commonly used couplants used to attach

the device to the test apparatus.

Device Inspection and Test — Procedures were

developed to obtain a quantitative measure

of localized temperatures within operating

bipolar silicon devices using the dual-laser,

flying-spot scanner. The feasibility of using the laser scanner to observe a device

through the back side was demonstrated; this

has implications for examining face-mounted

chips which cannot be inspected in the usual

fashion. A color display was added to the scanner in order to assist in separating various responses and, with suitable mixing, to display contours of equal photoresponse.

Detailed studies of scanning acoustic microscopy as a technique for the inspection of

semiconductor devices and integrated circuits

continued at Stanford University and Hughes

Research Laboratories. At Stanford, a 940-

MHz microscope has been constructed which appears to have a resolution of about 0.5 pm. The feasibility of observing temperature differences with the microscope was demonstrated

by means of a brief investigation, and initial work was undertaken to evaluate the use

of an acoustic beam with non-normal incidence

in reflection- type imaging. Hughes has con- structed a second-generation microscope which

operates at 400 MHz. This microscope has

demonstrated good day-to-day mechanical and

electrical stability, and appears to have a resolution of better than 2.5 pm. In addition an acoustic anti-reflection coating was developed to reduce signal losses at the lenswater interface by up to 18 dB.

Experiments and analyses were conducted to demonstrate the relevance of total dose as the key parameter in device degradation as a result of inspection with a scanning electron

microscope. It was shown that if the device

is expected to function electrically after

such examination, it is essential to develop

a careful analysis strategy before the device

is placed in the specimen chamber of the scan ning electron microscope. Some of the limita

tions of a simple method of calculation for

estimating the magnitude of electron-beaminduced currents in device structures were demonstrated by measurements on a diode doped

with copper and on an npn transistor. In addition the usefulness of electron-beaminduced-current images in the comparative

mode was demonstrated.

The automated "scanning low energy electron

probe being developed at the Naval Research

Laboratory was used to view a typical integrated circuit structure. These observations were impaired by surface charging effects

which degraded resolution below acceptable

levels. Significant instrument alterations

will have to be made to remove this obstacle

to routine examination of integrated circuit

chips. No further work is planned.

A model for the formation of stable hot spots

in transistors was developed in cooperation

with Westinghouse Research Laboratories. The

model explains the thermal hysteresis effects

of transistor current gain as a function of

collector voltage which had been observed

previously and provides a sounder basis for

the determination of safe operating conditions for power transistors.
3 . MATERIALS CHARACTERIZATION

BY ELECTRICAL METHODS

3.1. Four-Probe Method

An extensive matrix experiment was carried

out to examine the conditions under which

the sheet resistance of very thin junctionisolated silicon layers (0.2 to 1.5 ym thick)

could be measured reliably by the four-probe

method. The experiment was undertaken because of the wide variations in sheet resistance values reported in a pilot study conducted under the auspices of ASTM Committee

F-1 on Electronics.

In the pilot study, sets of four wafers were

measured by NBS and by another of the six

other participating laboratories. Each set contained one diffused, one polysilicon,

and two implanted specimens selected from

those listed in table 1. All layers were

prepared on opposite type substrates and had

sheet resistance, R^, in the range 4 to 1100 Q/O and thickness in the range 0.2 to

2 ym. Each laboratory used, and reported,

the test conditions it considered to be best

suited for the measurements. Current levels

chosen were generally based on those specified for measurements on thicker epitaxial

layers [1], and a wide variety of probe tip

radii and loads were utilized. These ranged

from 4-mil (0.1-mm) radius tips loaded with

50 to 175 g to 1.6-mil (0.04-mm) radius tips

loaded with 50 gf (0.49N). The degree of re- producibility can be estimated roughly from

the pooled relative sample standard deviations listed in the last two columns of table

1. Each specimen was measured twice, once by

NBS and once by another laboratory. In some

cases the several specimens of a given type were similar enough that they could be characterized by a single mean and standard deviation; in these cases the relative sample

standard deviation, s, listed in column A, was found from the usual formula

s = 100

2n zi = l

x)

where 2n is the number of measurements, x. is

the i th measured value of sheet resistance,

and x" is the average sheet resistance. In other cases the specimens could not be characterized by a single mean and standard deviation; in these cases s, listed in column B, was found from

s =

j = l

where n is the number of specimens and s . is

J the relative sample standard deviation associated with a pair of measurements on the j th

specimen. Since the various specimens were

Table 1 - Results of Four-Probe Pilot Study on Thin Layers

Specimen Type

Nominal

Layer

Thickness

,

ym

Nominal

Sheet

Resistance,

Number

of

Specimens

Pooled Relative

Sample Standard

Deviation, %

A B

n-type polysilicon 0.3 1000 3 1 .4 p-type polysilicon 0.3 250 3 0.9

n-type diffusion 2.0 4 3 2.5 1.9

p-type diffusion 1.0 300 3 3.6 3.6

p-type implant 0.2 250 2 2.0 2.0

p-type implant 1.5 750 2 32 39

p-type implant 0.2 1000 2 11 11.5

n-type implant 0.2 350 2 53 73

n-type implant 1.5 600 2 2.0

n-type implant 0.2 1100 2 7.3

7
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

Table 2 — Test Specimens for Four-Probe Matrix Experiment

Specimen Layer Type Dopant

Nominal

1 11 1 L.f\flfc:bb ) pin Nominal Sheet

Ktrb 1 b LariLc ) io/LJ

1 Diffusion Boron 1 270

2 Epitaxy Arsenic 1.5 3600

3 Epitaxy Boron 1.5 3100

4 Implant Boron 1.5 850

5 Implant Boron 0.2 1150

6 Implant Phosphorus 0.2 350

7 Implant Phosphorus 0.2 1250

Table 3 — Characteristics of Probes Used in Matrix Experiment

Probe Probe Material Probe Tip

Shape

Probe

Radius, in. (mm) Probe Loads Used, gf (Nl

A Tungsten Carbide Round 0.002 (0.05) 15,30,45 (0.15,0.29,0.4

B Tungsten Carbide Round 0.004 (0.10) 30,60,90,150 (0.29,0.55

0.88,1.47)

C Tungsten Carbide Flat 0.001 (0.02) 30 (0.29)

D Tungsten Carbide Flat 0.002 (0.05) 30 (0.29) i

E Tungsten Carbide Flat 0.0035 (0.09) 30 (0.29)

F Tungsten Carbide Flat 0.05 (1.3) 30 (0.29)

G Osmium Round 0.002 (0.05) 18,30,45 (0.18,0.29,0.'

H Osmium (current)

Tungsten Carbide (voltage)

Round

Round

0.004

0.004

(0.10)

(0.10) 30,60,90 (0.29,0.59,0.!

I Osmium (current)

Tungsten Carbide (Voltage)

Round

Round

0.002

0.004

(0.05)

(0.10) 30,90 (0.29,0.88)

J Osmium (voltage)

Tungsten Carbide (current)

Round

Round

0.002

0.004

(0.05)

(0.10) 30,90 (0.29,0.88)

Lapped flat, 2.6-mi1 (65-iJm) radius

circle; magnification: '^JSx--

a. As supplied, ^-mil (lOO-ym) radius of

sphere; magnification: '^75x.

b.

Figure 1. Photomicrographs of typical tungsten carbide probe tips.

8
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

not exposed to the same set of probe conditions, it is not possible to draw firm con- clusions regarding the relative sensitivity

of wafer types to various measurement conditions .

For the matrix experiment, a set of specimens

representative of various layer characteristics — method of fabrication, thickness, and

sheet resistance — was selected from available stock; the properties of the specimens

used are listed in table 2. In each case the

substrates were of opposite type with room- temperature resistivity in the range 0.5 to

5 fi'cm. Because the initial screening showed

that the several available wafers of any one

layer type, thickness, and sheet resistance

combination were not in general good enough

replicates (2 percent or less variation in

sheet resistance) , it was decided to use only

one wafer of each type to generate the com- parison of the various measurement conditions.

The principal intent of the study was to identify as wide a range of measurement con- ditions as possible for which valid results

could be obtained in order to make any re- sulting recommended procedures applicable to as wide a variety of commercial resistivity

test sets as possible, as well as to identify

a safe guard band to allow for degradation of

equipment as commonly experienced in a production environment. A secondary purpose was

to provide a basis for unifying, where possible, the several differing sets of conditions

recommended for use in making sheet resistance and resistivity measurements by the

four-probe method [1,2].

Table 3 lists the characteristics of the

probes used in the study. Some probes were

used exactly as bought, with a hemispherical

tip at the end of the stylus as shown on the

left in figure 1. For this case, the size

given is the radius of the tip. Other probes

started with a hemispherical tip, but were

lapped flat prior to use as shown on the

right in figure 1. For these probes the size

given is the radius of the flatted probe tip.

Blunted probes were included in the study

because they were expected to minimize stress

concentration in the silicon under the probe.

Such stresses appear to be responsible for

junction leakage to the substrate, particularly in thin epitaxial wafers. The risk entailed with blunted probes is that the contact resistance to the specimen may become un- desirably large, due, for example, to stray

dirt accumulating under a probe tip or to a surface oxide film. Such contamination appears, in general, to be less easily penetrated with a flatted than with a rounded

probe tip. Except for the bottom three sets

in the table, all four probes in a given set were identical. Probe loads used in the

study are also listed in the table.

To establish a baseline, measurements were

made for each measurement condition using a value of current which provided a voltage of

10 to 15 mV between the inner probes of the

four-probe array; this is a somewhat more re- strictive condition than that recommended in

the standard method [1]. In general, measurements made at currents one-tenth of this

nominal value yielded sheet resistance values

which were the same as those made with the

baseline current within the estimated measure

ment error of 1 percent. Except in a few

cases, measurements made at currents five

times this nominal value also yielded the same sheet resistance value.

To compare measurements made under various

conditions, six measurements were made for

each case using the nominal value of current.

For each specimen, the correct sheet resistance value was taken as the highest stable

reading. Excessive leakage to the substrate,

which occurred when sharp points or large

loads were used, caused the measured sheet

resistance to be smaller than the correct val

ue. Use of lightly loaded, blunt tips fre- . quently resulted in erratic contacts which

caused an increase in the variability of a six-measurement data set. Typically a good

set of data had a relative sample standard

deviation less than 1 percent.

The results of these experiments suggest that one should use hemispherical or flatted

probes with a radius of at least 0.004 in.

(0.1 mm) with a load in the range 30 to 80 gf

(0.29 to 0.78 N) . Although current levels

to provide a voltage of 10 to 20 mV between

the inner probes are preferred , the current

levels specified in the standard method [1] are also satisfactory. An interlaboratory

test to verify these conclusions is now being

conducted in cooperation with Committee F-1.

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

3.2. Spreading Resistance

Experiments to determine the effect of speci- men surface preparation and probe material

on the empirical relation between silicon re- sistivity, p, and spreading resistance, ^^^^

(NBS Spec. Publ. 400-25, pp. 8-12) were con- tinued with the study of surfaces mechani9
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

4500

4000 -

3500

3000

2500

2000

1500

1000

rmm—I 1 1 iiiiii I I 1

1 1 II

I I I mill i l I I I mill I I I mill 1 I I I III!

4500

0 001 0.01 0.10 1 0 10 100 RESISTIVITY 10. cm)

I M I iiii|—I I 1 1 —I I 1 1 —I I I iiiii|—I I 1 1 nil

1000

0.001

ml I I iiiiiil I I 1.1 mil I I I mill—I 1 1 1 III!

0 01 0.10 10 RESISTIVITY (o.cnij

100

a . Not baked out

.

Baked out,

Figure 2. Ratio of spreading resistance, Rjp, to resistivity, p, as a function of resistiv

ity as measured with a variety of probe tips on (111) n-type silicon surfaces mechanically

polished with 0.5-ym alumina in aqueous slurry. (1: tungsten-ruthenium alloy; 2: tungsten

carbide; 3: osmium; k: relatively sharp osmium-tungsten alloy; 5: relatively blunt osmiumtungsten a 1 1 oy .

)

4000

3500

3000

E

" 2500

2000 -

1500

1000

-1 1—I Mill T I I I I II 1

1

J I I I I 0.001

n—I—I I 1 1 1

1

J

I 1 I 1

1

0.1 1.0 RESISTIVITY (ii'cm)

100

Figure 3- Ratio of spreading resistance, R^p, to resistivity, p, as a function of resistiv

ity as measured with relatively sharp osmium-tungsten alloy probe tips on (100) n-type sili con surfaces prepared in various ways. (Circles: lapped with 5-pm alumina; squares: chemmechanically polished with silica sol; triangles: mechanically polished with 0.3~yrn alumina

in nonaqueous slurry; diamonds: mechanically polished with 0.3-Mm alumina in aqueous slurry

solid points: not baked out; open points: baked out.)

10
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

cally polished with 0.3-tJin alumina in an aqueous slurry. Measurements were made both

immediately after polishing and after a subsequent bakeout in room air at 160°C for

20 min.

Results obtained on the specimen block with

(111) p-type surfaces were very similar to those obtained previously on either of the

types of chem-mechanically polished surfaces

[3], silica sol [4] or zirconium silicate

[5]. The spreading resistance response was very erratic for freshly prepared surfaces

of specimens with resistivity greater than

0.2 n-cm, but a very stable, nearly linear

relationship between resistivity and spreading resistance was obtained after bakeout.

On the specimen block with (111) n-type surfaces, the data obtained on the mechanically

polished surfaces were unlike those obtained

j on either lapped or chem-mechanically pol1 ished surfaces. Prior to bakeout, data

taken with all five probes exhibited very nonI linear p-R relationships as shown in figure

sp

2a. After bakeout, both sets of osmiumtungsten tips, one relatively sharp, one relatively blunted with use, showed a noticeably

more constant ratio between spreading resistance and resistivity than did the other

three probes shown in figure 2b. This probe

material dependence was not seen for any other surface orientation or finish.

I On the specimen block with (100) n-type surI faces the response obtained on the mechanically polished surfaces was similar to that on chem-mechanically polished surfaces. This is

, illustrated in figure 3 for the case of the

I relatively sharp osmium-tungsten probes.

Note that there is no significant difference

I between data taken before and after bakeout.

1 Generally, similar results were obtained with

I the other probes; a somewhat more linear reI lationship between R and p was obtained

P with the relatively blunt osmium-tungsten

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

3.3. Spreading Resistance Profiles

As a preliminary test of the application of

a simplified calculational procedure for cor-

. recting spreading resistance data on layers

I with nonuniform resistivity (NBS Spec. Publ.

I 400-19, p. 10), the dopant profiles of two boron-diffused layers were measured both by

the spreading resistance method [6,7] and by

the incremental sheet resistance method [8]

and compared.

As previously described (NBS Spec. Publ. 400-

19, p. 10), the diffused layers had a boron

surface density slightly less than 10^^ cm~^

and nominal junction depths of 1 and 2 ym. The incremental sheet resistance measurements

were made by repetitively anodically oxidizing the specimen surface, stripping the oxide,

and measuring the sheet resistance by the

four-probe method [1]. The specimens measured were 1-in. (25-min) diameter disks, ultrasonically machined from the center of each

wafer. Prior to anodization, the disks were

cleaned first in a mixture of approximately

5 parts (by volume) deionized water, 1 part

hydrogen peroxide, and 1 part ammonium hydroxide; then in a mixture of approximately 6 parts deionized water, 1 part hydrogen peroxide, and 1 part concentrated hydrochloric acid; rinsed in deionized water; and dried in

air [9].

The anodizing solution was made up by dissolving 3.86 g of potassium nitrate in 19.5 ml of

deionized water and adding 946 ml of reagentgrade ethylene glycol which yields a 0.04 molal solution of potassium nitrate with 2 percent water. During anodization the back side

of the disk is held against a carbon block

electrode by means of a vacuum through a tube

which surrounds the electrode. The disk was lowered face down into the anodizing solution

which was contained in a glass beaker; a second tube around the electrode supplied a downdraft of nitrogen to keep the dielectric so- lution from creeping over the lip and oxidizing the back side of the disk.

A constant current, about 10 mA/cra^ , was passed through the disk; the positive electrode was a platinum wire spiral at the bottom of the beaker. To reduce forward resistance, the junction was illuminated by a 500-

W projection lamp aimed through the side of

the beaker. To offset the heating effect of

the projection lamp, the anodizing solution

was continually pumped through tubing resting

in an ice bath. Upon returning to the main

beaker, the solution was directed at the center of the wafer front surface through a nozzle about 1/8 in. (3.2 mm) in diameter about

1 in. (25 mm) away from the surface. The

purpose of the jet was to clear the surface

of bubbles and to provide refreshed solution

at the wafer surface.

Current was supplied until the current supply was forced to provide about 270 V more

than the turn-on voltage. The oxide thickness was determined from the measured voltage difference using a calibration curve es- tablished empirically with the use of ellip11
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

sometric determination of oxide thickness

[10] over the range 40 to 160 nm. Typically

the thickness of the oxide layer grown was about 145 nm. It was assumed that the amount

of silicon removed was 0.44 times the oxide

thickness [11]

.

Following each oxide growth step, the wafer

was agitated in hydrofluoric acid for about

5 min to remove the oxide, rinsed in a flowing deionized water bath for about 10 min,

and blown dry with nitrogen. Sheet resistance was measured with a collinear fourprobe array with equally spaced probes, with

62-mil (1.59-mm) separation and tungsten car- bide tips lapped flat (see sec. 3.1.) to give

a contact radius of about 2 mils (50 ym)

.

The probes were loaded with a force of 30 gf

(0.29 N) each. Sheet resistance was measured

over 20 to 25 increments, converted to sheet

conductance, plotted as a function of position, and smoothed graphically. The local

slope of the smoothed sheet conductance curve

was calculated at 40-nm intervals to yield

the bulk conductivity as a function of position. Bulk conductivity was converted to net

acceptor density on the basis of Wagner's em- pirical relation [12].

Spreading resistance measurements were made

with a commercial two-probe instrument (NBS

Spec. Publ. 400-25, p. 9) on wafers diffused

at the same time as the replicate wafer measured by the incremental sheet resistance

method. Chips cut from the wafer were beveled at an angle of 0.50 deg with a silica

sol solution [4] on a methyl methacrylate

plate. Measurements were made at lateral intervals of 5 pm with well conditioned osmiumtungsten alloy probes loaded with a force of

20 gf (0.2 N) and spaced 50 pm apart.

Data were corrected by the local slope method

This is based on the assumption that, from

parallel superposition (NBS Spec. Publ. 400-

19, p. 10):

ln(s/a) dG(x)

TT dx

a(x) ' = (1)

where a(x) is the local bulk conductivity, s is the probe spacing, a is the effective radius of probe contact, and G(x) is the con- ductance measured at a distance x from the

insulating back surface of the layer. Equation (1) can be transformed into

p(x) = 2a R(x)/F, (2)

1.0 1,5 2.0 DEPTH [fim]

2.5 3.0

a. Specimen No. 1

10'

10'V

10'

^ 10'^

« 10'^

10'

10'

pN

G.O 0.5 1.0 1.5 DEPTH

2.0 2.5 3.0

b. Specimen No. 2,

Figure k. Dopant density profiles in boron-diffused n-type silicon as measured by the

spreading resistance and incremental sheet resistance methods. (P and N: net boron and

phosphorus density, respectively, from spreading resistance measurements, corrected by the

local slope method; dashed curves: net boron density from incremental sheet resistance mea- surements .

)

12
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

where p(x) = l/a(x), R(x) = 1/G(x) and 3.4. Photovoltaic Method^

F =

2a ln(s/a) d[lnR(x)]

dx (3)

where now x is the depth from the top surface

of the specimen.

The effective contact radius was determined

from measurements on a block of 15 homogeneous specimens (NBS Spec. Publ. 400-25, p. 9)

which had been surfaced in the same way as

the beveled surface of the diffused wafers.

For homo|eneous specimens the resistivity is

given by

p = 2aR. (4)

Since both R and p are known, a may be calculated as a function of R. In the present

case, a linear relation between p and R was

found and a constant value of the effective

contact radius was used.

The local slope of the InR(x) vs. x curve at

the ith point was found by taking the differ- ence (logR._|_j - logR._p and multiplying by

the factor [lnlO/(x.

end of the curve, differences were taken between adjacent readings and the interval was

taken as Ax = x. , , - x.. The correction fac1 + 1 I tor F was computed on a point-by-point basis

using eq (3), and the resistivity was calculated at each point using eq (2) . The re- sistivity was converted into net acceptor

density on the basis of Wagner's empirical

relation [12].

The results of these comparisons are shown

in figure 4. The relatively close agreement

suggests that the parallel superposition approach is quite accurate on this type of

structure. One reason for some of the discrepancy near the surface on both specimens

may be a bevel-rounding effect. It was assumed for the calculations that each successive point was a constant distance further

below the original surface, but slight rounding of the bevel at its apex makes this a

, poor assumption for the first few points on

the profile. Extensive additional comparisons will be required before conclusions can

be drawn regarding the range of validity of

this approach for determining correction

factors. (J. R. Ehrstein,

F. H. Brewer, and D. H. Dickey"'")

)] At either

Interest in a rapid, nondestructive method

for measuring resistivity uniformity of silicon slices has been rekindled because of the

importance of this parameter in determining

the characteristics and the perform.ance of

high-power semiconductor devices. Variations

in the resistivity of a slice of starting material not only cause variations in the characteristics of devices fabricated from that

slice, but can also contribute to poor yields

in the manufacturing process and adversely affect the reliability of finished devices.

For instance, poor junction geometry and non- uniform current distributions are frequent

in devices fabricated from inhomogeneous material. Also, localized regions of low resistivity limit the ultimate device operating voltage to a value below that possible from the

remainder of the device.

The feasibility of utilizing the bulk photovoltaic effect [13] for measuring resistivity

variations along the diameter of a circular

silicon slice has been demonstrated [14].

The technique was shown to be nondestructive

in that no contact is required with the surface area on which devices are to be fabricated and to have better spatial resolution

than the widely used four-probe method [15,

16]. However, to take advantage of the potential speed of this scanning technique it

is necessary to automate the data collection

and analysis procedures. Further, improvements in slice mounting and contacting are necessary to facilitate rapid exchange of

specimens, and comparison of the spatial res- olution with that of the spreading resistance

method [6,7] is desirable.

The present task was undertaken to refine and

automate the method and to compare its reso- lution with that of the spreading resistance

method. A new specimen holder, illustrated

in figures 5a and 5b, was designed to accommodate 2-in. (51-mm) diameter slices. This

t,

Note that in the limit of uniform, thick

specimens eq (2) does not yield eq (U);

this is because eq (2) is appropriate only

for very thin layers with insulated back

surfaces

Work performed at Solecon Laboratories under

NBS Contract No. 5-35881.

Principally funded by the Energy Research

and Development Administration, Division of

Electric Energy Systems.

13
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

Block diagram.

A Adjustment ring

B BCD input

C Stepp ing-motor controller

E Knife-edge holder (k)

H Specimen holder base N

1 Pin (if) P

J Lever (k) R K Knife edges (2 in each holder) S

L Motor-driven linear stage T

M Mi rror W

Locking adjustment knob

Pivot (k)

Motor-driven rotary stage

Double-pole, double-throw switch

Guide notch (k)

S 1 i ce under test Figure 5. Prototype automated data collection and analysis system for resistivity profiling

of circular silicon slices by the scanning photovoltaic method.

14
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

' holder centers the slice with sufficient accuracy for the measurement. Each knife edge,

K, has a single leaf spring behind it which

permits enough freedom of movement to allow

each to make contact with the circumference

of the slice, W, at the same time. The four

knife-edge holders, E, each holding two knife

edges, move in concert at the ends of the levers, J. The levers are each pivoted at points, P, at 90-deg intervals from one another on the circumference of a common circle.

As the adjustment ring. A, is moved by means

of the locking knob, N, the pin, I, rides in

a notch, T, of each lever which forces the

I four holders in or out together. Note that

the knife-edge holders do not rotate at the

ends of the levers. Thus, the four knife

edges do not strike the wafer exactly at the

ends of perpendicular diameters for slices of

all diameters; however, the variation about

the design slice diameter permitted by the

SEMI specifications [17] is small enough that

the locations at which the knife edges make

contact do not change enough to affect the

measurements significantly.

An automated data collection and analysis sys- tem was designed and assembled. A block diagram of this system is shown in figure 5c. The slice under test, W, is mounted in the

holder, H, described above. For clarity,

only two of the knife edges, K, are shown at opposite ends of a diameter. The holder is

oriented on two stages so that the slice is

concentric with the platform of the rotary

stage, R, and so that the movement of the

linear stage, L, is parallel to one of the

diameters determined by the knife edges. The

rotary stage has a rotational resolution of

2 min of arc and bidirectional rotational re- peatability of 0.2 min of arc. The linear

stage has a total travel of 85 mm, a linear

resolution of 10 pm, and bidirectional linear

repeatability of 5 ym.

Photo-excitation at a wavelength of 1.15 ym

is furnished by a 2-mW helium-neon laser,

which is mounted horizontally. The laser

beam is directed to the center of the rotary

stage by a mirror, M. The slice is scanned

by moving the stage along a slice diameter

under the stationary laser beam. The control

card, C, for the stepping-motor in the multiprogrammer, operating under the direction of

the calculator /controller , generates a pulse

train of the appropriate length to move the

stage the desired number of 10-pm steps in

either direction; the pulse train is supplied

to the motor windings through the selector

switch, S, and the translator. End-point

protection is provided by microswitches and

appropriate relays at both ends of the stage

travel.

During the first scan, the photovoltage is

measured at predetermined points along the

diameter. To improve the signal-to-noise

ratio, the beam is chopped and the photovoltage is measured by a lock-in amplifier which

is synchronized to the frequency of the

chopped light beam. The chopping frequency

is adjusted so that the on time of the laser

beam is long enough to achieve a steady-state

excess-carrier distribution. Following the

first scan, a second scan along the same diameter is made; during this scan the photoconductivity is measured at the same points

along the diameter as before. This measurement requires that a small current be passed

along the diameter of the slice (using one pair of knife edges) while the potential drop across the diameter is measured as a function

of light position (using the other pair of

knife edges). The calculator/controller,

which together with its associated multipro

grammer and printer/plotter is compatible

with standard interface requirements [18], re- cords the data, performs the calculations re- quired to obtain the resistivity, and plots

the results. After measurements along the

first diameter are completed, the wafer can

be rotated 90 deg and the measurements repeated. (G. J. Rogers and D. L. Blackburn)

3.5. Reevaluation of Irvin's Curves

Additional room-temperature resistivity and

dopant density measurements were made on silicon wafers doped with phosphorus as part of

the continuing experimental redetermination

of the resistivity-dopant density relation

first reported by Irvin [19]. In addition,

initial measurements were made on silicon wafers doped with boron.

The measurements were made on test structures

included in test pattern NBS-3 [20]. Dopant

density values were obtained from measurements on structure 3.10, base-collector diode

(NBS Spec. Publ. 400-17, pp. 27-28), by the

junction capacitance-voltage (C-V) method.

The gate on the structure was biased at the

flat-band voltage as determined from measurements on structure 3.8, MOS capacitor over collector (NBS Spec. Publ. 400-17, pp. 26-27).

Instead of calculating discrete dopant density values from the C-V data as has been

done previously (NBS Tech Note 788, pp. 9-11),

a new data reduction procedure was developed

in which a Gaussian shape is assumed for the

base diffusion near the junction and the sur15
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

Table 4 — Preliminary Electron Mobility Data for PhosphorusDoped Silicon at 300 K

Wafer No.

No. of p, Q • cm

N cm"

•3 cm^/V • 5

B38Ph-l 6 31 .3 1 4^ IQi'* 1409 ± 69

B12Ph-l 10 13.5 3. 34 101'+ 1392 ± 70

D2.4Ph-l 8 2.46 1 93 X 10^5 1314 ± 33

A2.0Ph-2 4 2.14^ 2 25 X 1015 1300 ± 47

Bl .4Ph-3 7 1 .36 3 57 X 1015 1295 + 87

Al .OPh-1 4 1 .05 4 78 X 1015 1240 ± 24

B0.77Ph-l 6 0.801 6 36 X 1015 1227 ± 33

B0.47Ph-l 8 0.504 1 08 X 1016 1156 ± 51 A0.27Ph-2 4 0.278 2 13 X 1016 1052 ± 29

B0.099Ph-l 4 0.101^ 7 59 X 1016 815 ± 9 B0.051Ph-2 4 0.052^ 2 17 X 1017 556 ± 25

B0.050Ph-l 4 0.051 2 25 X 10^7 544 ± 7 A0.022Ph-2 4 0.022^ 1 17 X 1018 248 ± 4

Resistivity as measured by mechanical four-probe array

before processing.

Table 5 — Preliminary Hole Mobility Data for Boron-Doped

Silicon at 300 K

Wafer No. No. of

diodes

P , n-cm

Ncm

-3 V, cmVV s

A12B-1 4 11.5 1 18 X 1015 457 ± 7 B5.7B-3 4 5.85^ 2. 42 X 1015 440 ± 3

Bl .18-3 4 1.16 1 29 X 1016 415 ± 8 B0.72B-3 5 0.72 2 22 X 1016 391 ± 14 D0.58B-2 6 0.587 2 92 X 1016 364 ± 2 B0.090B-2 7 0.0903 3 40 X 1017 204 ± 3

Resistivity as measured by mechanical four-probe array

before processing.

16
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

face concentration of the diffusion and the

background dopant density are adjusted to give a best fit to all the C-V data. This

procedure has made it possible to obtain results on wafers with dopant densities greater

than 10^^ cm"^ which could not be measured by

the previous approach.

The bulk resistivity of the wafers was determined from measurements on structure 3.17,

four-probe collector resistor (NBS Spec. Publ.

400-17, pp. 25-26) [21]. For a few wafers,

the resistivity used for calculating mobility

was that determined prior to processing by

mechanical four-probe measurements [2].

These were either wafers for which the emitter diffusion was omitted in order to obtain

a shallow base junction depth or wafers processed with the large-pipe base mask which

introduced a significant uncertainty in the

pipe spacing [21].

The carrier mobility, p, was calculated from

U = (Nqp)~l, (5)

where N is the carrier, or net dopant, density, q is the electronic charge, and p is the

resistivity corrected to 300 K. To minimize

the influence of resistivity variations over

the wafer the average of the resistivities

measured on the two collector resistors^ on

either side of the diode used to determine N was used for p in eq (5). The procedure was

repeated for a number of good diodes in the

same general area of the wafer, usually near

the center, to arrive at an average mobility

as listed in tables 4 and 5. The uncertainty

in the mobility for each wafer is the sample

standard deviation calculated from the individual mobility values. Table 4 contains re- vised electron mobility values based on repeated and additional measurements for wafers

reported on previously (NBS Spec. Publ. 400-

19, p. 25). In general the electron mobility

values lie above those which would be predicted using the Caughey-Thomas fit [22] to Irvin's data (see sec. 3.6.), and the hole

mobility values lie below those which would

be predicted using Wagner's empirical relation [12]. (W. R. Thurber, R. L. Mattis

,

Y. M. Liu, M. L. Doggett, and M. G. Buehler)

3.6. Electron Mobility in Phosphorus-Doped

Si 1 icon

;| Traditional analysis of electron mobility in

] n-type silicon has neglected the effects of

electron-electron scattering and scattering

anisotropy in the mobility calculations. As

a result, theory fails to conform with experi

ment when dopant density exceeds 2 x 10^^

cm~3 (NBS Tech. Note 806, pp. 20-23). By

incorporating these effects , good agreement

between theory and experiment for both electron mobility and resistivity was obtained

for phosphorus-doped silicon slices with dopant density from lO^"* to 3 x 10^^ cm~^ over

the temperature range 100 to 500 K [23,24].

Initially the lattice mobility, taken from

the work of Norton et at. [25], was combined

with the ionized impurity mobility, as calculated by Long [26], according to the sineand-cosine-integral method [27] to obtain the

mixed scattering mobility. This was combined

reciprocally with the neutral impurity mobility, as calculated by Sclar [28], to obtain

the total electron mobility. The mobility

calculated in this way for 300 K is shown as curve 1 of figure 6. For comparison, the

Caughey-Thomas fit [22] to Irvin's data [19]

is shown as curve 2 and more recent experimental results are shown as data points ; the

solid dots are taken from the mobility data

of Mousty et al. [29] and Baccarani and Ostoja [30], corrected to 300 K, and the open

circles are NBS data obtained on processed

phosphorus-doped silicon wafers (see sec. 3.5.).

It can be seen that the calculated result

fits the more recent experimental data for

dopant density less than 2 x 10^^ cm~^. However, at higher dopant density, the calculated curve is higher than the experimental

data.

While electron-electron scattering does not

affect the current density directly since it

cannot alter the total momentum, it tends to randomize the way in which this total momentum is distributed among electrons with different energy. When the scattering mechanism is such as to lead to a nonuniform distribution, electron-electron scattering gives

rise to a net transfer of momentum from electrons which dissipate momentum less efficiently to those which dissipate momentum

more efficiently, resulting in an overall

greater rate of momentum transfer and lower

mobility. The size of the effect of electron

electron scattering on the mobility is a func

tion of the energy dependence of the relaxaWhen the resistivity was measured by means

of a mechanical four-probe array, the re- sistivity value for the location was based

on radial profiles along two diameters of

the wafer.

17
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

1400

1200

1000

800

600

400

200

010'

1 1 i Minj 1 1! ! mill 1 1 iMiii 1

1 mill 1 1 iiiiii

-

\ \ \ \ '\ \ \ \ — \^\\

2

34

1 1 1 Hill Ill 1 il nil 1 1 I 1 1 1 Mini

10'^ ID'S 10' 10" 10" 10'

TOTAL DONOR DENSITY (cmFigure 6. Conductivity mobility of electrons

at 300 K as a function of total donor density

for phosphorus-doped silicon. (Curve 1: Theoretical, not including electron-electron

scattering; Curve 2: Caughey and Thomas repre- sentation of Irvin's curve [22]; Curve 3' Theoretical, including maximum electron-electron

scattering over entire range; Curve h: Theoretical, including electron-electron scattering in intermediate and high dopant density ranges; open data points: experimental,

this work; solid data points: experimental

[29,30].)

100 150 200 250 300 350 400 450 500 TEMPERATURE (K)

Figure J. Resistivity as a function of tem- perature for seven phosphorus-doped silicon

slices. (Solid points are measured values

and curves are calculated assuming the phosphorus density, curves .

)

D' as indicated on the

10'* 10'= 10'i= 10'^ 10"

TOTAL DONOR DENSITY Icm-M

Figu.re 8. Resistivity of phosphorus-doped

silicon at 300 K as a function of total donor

density. (Solid curve: this work; dashed

curve: from Irvin [19].)

18
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

tion time. Thus, for neutral impurity scattering where the relaxation time is independent of energy, the mobility is not affected

by electron-electron scattering. Ionized impurity scattering would be expected to be

much more affected than lattice scattering

since in the former case the relaxation time

is proportional to the three-halves power of

the energy, while in the latter it is inversely proportional to the square root of

the energy.

From the work of Luong and Shaw [31], it is

found that electron-electron scattering reduces the ionized impurity mobility in uncompensated n-type silicon by a maximum of 36.8

percent while, from other published work [27,

32], it can be shown that electron-electron

scattering reduces the lattice scattering mobility by a maximum of 12 percent. Using

these values, the electron mobility at 300 K

was calculated as above; the result is shown

as curve 3 in figure 6. This agrees well

with the experimental data for dopant density

above about 2 x 10^'' cm~^. In the intermediate region, for dopant density between

2 X 10^5 and 2 x 10 1^ ^m"^ it was assumed

that the reduction factor, F, for both ionized impurity and lattice mobility varies

linearly with dopant density

F = F

N _ I

max ] .8x1017 9

'

where F =0.12 for lattice mobility and max

0.368 for ionized impurity mobility. With

this assumption, and assuming F = 0 for dopant density less than 2 x 10^^ cm~3 and

F = F for dopant density greater than max ^

2 X 10^^ cm~^, one obtains curve 4 in figure

6 for the electron mobility at 300 K. This

curve agrees with the experimental data to within about ±5 percent over the entire dopant density range shown. Note that the electron mobility in lightly doped n-type silicon

appears to be significantly higher than that

based on Irvin's data [19,22].

At other temperatures, it is not convenient

to measure the carrier density directly. The

resistivity, p, is given by

p = (nqy^)~l,

where n is the electron density. Therefore,

if M is obtained as a function of dopant

n

^ density and temperature as described above

and n is obtained as a function of dopant

density and temperature by solving the chargebalance equation [23,24], it is possible to

obtain calculated resistivity-temperature

curves for various values of dopant density

which can be compared with experimentally

determined curves.

To make this comparison, resistivity measurements were made between 100 and 500 K on seven phosphorus-doped silicon wafers, using

the four-probe collector resistor (NBS Spec.

Publ. 400-17, pp. 25-26) [21]. The test cells used for the resistivity measurements were taken from the same silicon wafers used

for the mobility measurements at 300 K. For

the temperature dependent measurements, a test cell containing the four-probe structure

was mounted on a TO-5 header; a temperaturesensing diode was also mounted next to the

cell for measuring the temperature. The TO-5

header was then mounted inside a speciallydesigned cryostat where temperatures can be

varied from 82 to 673 K with a maximum heating rate of 7 K/s (NBS Tech. Note 788, pp. 12

14) [33]. The results are illustrated in fig ure 7; the solid dots are measured values and

the curves are values calculated with the as- sumption of the phosphorus density, Np, indicated on the curve. Over the entire range

shown, the theoretically predicted resistivity values are within ±7 percent of the ex- perimental data. The values of were found

from the measured resistivity at 300 K using

the calculated resistivity-dopant density

curve shown as the solid line in figure 8. (S. S, Li and W. R. Thurber)

3.7. Thermally Stimulated Current and Capacitance Measurements

jj Application to Power-Grade Silicon — Many

critical properties of high-power silicon

thyristors and rectifier diodes are significantly affected by the presence of impurity

and defect centers which cannot be counted or identified by ordinarily used electrical measurements such as resistivity and lifetime.

Many such centers, such as heavy metal contamination, irradiation-induced defects, and

process- induced defects have been identified

by means of their thermally stimulated cur- rent or capacitance response, and each defect is noted to have a particular signature.

Although the measurement of thermally stimulated current and capacitance is reasonably

well established [34-38], several aspects

need further development before the techPrincipally funded by the Energy Research

and Development Administration, Division of

Electric Energy Systems.
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

a. Chuck assembly - exploded view.

1 Chuck assembly

2 Copper top plate

3 Insulator plate

k Heating module

5 Cool ing module

6 Wafer index stop

7 Thermocouples

8 Wafer

9 Probe ring

10 Probe manipulator

11 Multiprobe assembly

12 Bellows

13 Prober bridge

\k Probe ring support

15 Wafer alignment manipulator

16 Vacuum pick-up probe

17 Transfer si ide

18 Front viewport

19 Top viewport

20 Probe ring height manipulator

21 Prober control panel

22 Microscope and mount

23 Heaters

b. Heating module - inverted view.

c. Automatic wafer prober - interior view.

20- to —

d. Prober assembly - exterior view.

Figure 3. Thermally controlled chuck and automatic wafer prober.

20
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

nique can be widely applied to the characterization of power-grade silicon; a new task area has been initiated to carry out this

work.

To date, all measurements reported in the

literature have been made in cryostats on

small, individual devices scribed from processed wafers bonded onto headers. One objective of the task is the development of apparatus suitable for performing thermally

stimulated current and capacitance measurements on whole large ' diameter silicon wafers.

Use of such an apparatus would eliminate the

need for further processing of test specimens

after wafer fabrication and allow relatively

fast feedback of information to the process

line.

Since it is desired to characterize starting

material as well as material at various

stages in the device fabrication process, it

is necessary to develop procedures for fabricating test structures which preserve the

characteristics of the starting material.

The possibility of examining starting material was made feasible by the observation

that MOS capacitors were just as suitable for

the measurement as p-n junctions (NBS Spec.

Publ. 400-1, pp. 16-19) and that both structures yielded the same information (NBS Spec.

Publ. 400-4, pp. 27-33). However, to exploit

this possibility, low- temperature processes

for forming the capacitor oxide and gate electrode must be refined; this is a second objecI tive of the task.

j Application of the measurement technique at various stages in thyristor fabrication is

I complicated by the fact that not all regions

I of the device are accessible externally; a third objective of the task is to determine

I the feasibility of using monitor wafers and

! special process steps to enable the measurement after all key thyristor fabrication

steps.

i Initial work has concentrated on the first

two objectives:

A. Whole-Wafer Apparatus . One major

component of the whole-wafer apparatus is

i[

a large-area, thermally-controlled chuck,

i optimized for its thermal response and capaj ble of operation over a temperature range

1 from about 80 to over 600 K. The chuck consists of several components as shown in the

,1 exploded view in figure 9a. The copper top

!' plate (2) has vacuum grooves for specimen

holddown and a hole for attachment of a thermocouple. A thin insulator plate (3) sepa21

rates the top plate from the heating-cooling

module (4,5). The copper upper section (4)

of this module consists of an integral cooling cavity and heater elements. An inverted

view of this module with the four 150-W heater elements exposed is shown in figure 9b.

The stainless steel lower section (5) encloses the cooling cavity and forms the manifold for the liquid nitrogen coolant; this

section is heli-arc welded to the copper section. The lower section has been designed

with a thin rib on the circumference to minimize heat transfer to the supporting hardware

The top surface of the heating module (4) is

identical (including vacuum grooves and thermocouple hole) to the top plate; for measurements which do not require electrical isolation, the top plate and insulator can be re- moved to permit faster thermal response. The

wafer index-stop (6) may be attached either

to the top plate or the heating module.

Parallel to the development of the chuck is

the modification of an automatic wafer prober

to accommodate the chuck and to allow the

capability to screen individual devices on a wafer for mobile ion contamination and for ox

ide integrity before making the thermally

stimulated current or capacitance measurements. The prober and associated parts are illustrated in figure 9c together with the

mounted chuck assembly (1) ; the bridge structure (13) to which the probe ring support

(14) is mounted has been moved back to separate the probe ring (9) from the chuck assembly (1) for greater clarity of presentation

in the photograph. The probe ring, which nor mally accommodates up to 60 probe manipulators (10) has been modified to allow mounting

of an optional multiprobe assembly (11)

,

which can be used in place of separate manipulators to apply bias and temperature stresses to 36 devices on a wafer simultaneously as

a convenient method for detection of mobile

ion contamination. The separate manipulators

are commercial units modified to allow the use of miniature 50-fi shielded coaxial cables

which is necessary to minimize the stray ca- pacitance in the input leads to the 1-MHz ca- pacitance bridge. The bellows (12) are used

to transport liquid nitrogen to and from the

cooling module. In operation, the wafer is

placed on the transfer slide (17) and pushed

into the enclosure; by means of the vacuum

pick-up probe (16), the wafer is transferred

from the slide to the top plate of the chuck

(2) and aligned to the raised stop (6). Fine

adjustment of the wafer alignment with the

prober axes can be made by rotating the chuck

about a vertical axis through its center by means of the alignment manipulator (15).
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

120 140 160 180 200 220 240 260 280 300 320

TEMPERATURE |K1

Gold acceptor in n-type silicon. Specimen 59B -7 V cooling; -15 V heating;

heating rate, '^2 K/s,

— 0.2

120 140 160 180 200 220 240 260 280 300 320

TEMPERATURE |K)

Gold donor in p-type silicon. Specimen

60 B Al-8; +10 V cooling; +20 V heating;

heating rate, '\^2 K/s.

Figure 10. Thermally stimulated current response of gold-doped silicon MOS capacitors in wafer form.

The prober assembly is surrounded by a hermetic enclosure, shown in the exterior view

in figure 9d. All components are sealed to the wall of the enclosure with rubber 0-rings

or gaskets. The top (19) and front (18) viewports have light-tight covers (not shown) to provide a dark measurement environment. The

enclosure is constantly flushed and slightly

pressurized (6 psig or 40 kPa) with dry nitrogen to minimize room air penetration; the ex- haust gases can be monitored for moisture.

A microscope (22) is provided to check wafer

alignment through the top viewport (19) ; the

microscope support assembly was modified to allow clearance for the viewport between the

microscope objective and the wafer (8). The

relative spacings and orientation of the

probes must be preset, but the vertical adjustment is accessible by means of the probe

ring manipulator (20) . All the utility ports

are located in the back of the enclosure;

these include vacuum, heater power, liquid

nitrogen, thermocouple signals, prober control interfacing, and dry nitrogen pressurization.

Prior to completion of this apparatus, an earlier version (NBS Spec. Publ. 400-4, pp. 39-40) was modified to increase its maximum

heating rate to 2 K/s in order to conduct

preliminary feasibility studies of measuring

thermally stimulated current and capacitance

at the wafer level as described below.

(R. Y. Koyama and L. M. Smith)

MOS capacitors were made with aluminum gates

and thermal oxides on n- and p-type silicon

doped with gold. The thermal responses of

the gold acceptor in n-type silicon and the

gold donor in p-type silicon were measured

on the unscribed wafers and, as illustrated

in figure 10, were found to be the same as those reported in the literature for measurements on packaged diodes and capacitors [38,

34] except in the case of one n-type wafer

where the oxide was inadvertently contaminated, probably with sodium. These results

demonstrated that measurements at the wafer

level were feasible. However, because mo- bile ion contamination may be encountered in

the low-temperature oxides, it was decided to investigate the contaminated wafer more fully

In the as-processed condition, the response,

shown as curve a in figure 11, includes a cur rent peak near the expected temperature of

the phase I emission (vertical arrow) but the

0.8

0.6

0.4

0.2

0.0

- ' 1 '

1

' 1

~ r

;

' i'a'

'

i / / \ :

; ' / \

// \

' / \

-

; c// \

-

y /

- / ^^^^

T 1 y:

' V

/y .- ^ J ^^-^"^ 1 , 1 . 1 1 1 I 1

20.0

15.0

10.0

5,0

0.0

200 220 240 260 280 300

T (K)

Figure 11. Thermally stimulated current re- sponse of gold-doped n-MOS capacitor with an oxide film contaminated by mobile ions. (a: as processed; b: ions moved to metal gate;

c: ions moved to Si-Si02 interface; heating,

both cases: '^1 K/s; specimen 62B 8^1-8.

22
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

phase II response, at higher temperatures, is

very different from the normal response. Following a bias-temperature stress on the capacitor which drove any mobile ions which were

present in the oxide to the metal-oxide interface, the normal response of the gold acceptor was obtained. This response, shown as curve b in the figure, has the phase I shoulder at about 225 K and the phase II peak at about 280 K. If the mobile ions are driven

to the silicon-oxide interface by a second

(reversed) bias-temperature stress, the re- sponse again becomes abnormal as shown in curve c of the figure. These results suggest

that the presence of mobile charge at or near

the silicon-oxide interface greatly affects

the charge state or the emission process of

the traps in the depletion region of the semiconductor. However, by removing such mobile

charge from the interface, it is still possible to obtain the proper thermally stimulated current response. The presence of mobile ion contamination can be determined by means of bias-temperature stress measurements

(NBS Spec. Publ. 400-19, pp. 29-30) prior to the thermally stimulated current measurement.

(R. Y. Koyama)

B. Low-Temperature Processing of MOS

Capacitors . Typical processing of MOS capacitors for thermally stimulated current and

capacitance measurements includes the following principal steps:

1. Clean wafer,

2. Grow silicon dioxide 0.1 to 0.2 ym

thick in dry oxygen at 1000°C,

3. Deposit aluminum,

4. Photoengrave gate contacts with

level 4 (METAL) mask of test pattern

NBS-3 [20],

5. Metallize back side, and

6. Microalloy at 500°C for 20 min in

dry nitrogen.

The high-temperature oxidation step in this

process is likely to cause alterations in the

impurity and defect characteristics of the material. Hence the suitability of three types

of oxides for forming the capacitor dielectric at lower temperatures is being considered: anodic oxidation of silicon at room

temperature, chemical vapor deposition (CVD)

of silicon dioxide at 400°C, and oxidation at 400°C of evaporated tantalum to form tantalum pentoxide.

Capacitance-voltage (C-V) characteristics

were used to determine the properties of the

various oxides . The C-V curves were analyzed

to find the oxide thickness, X ; the average

dopant density, N^, in the depletion region;

and the flat-band capacitance, C^|_^. The flat

band voltage, V^^, was determined directly

from C . t b Figure 12 displays typical measured C-V characteristics for each of the oxide types fabricated. For comparison purposes, the C-V

characteristics of a standard, dry thermal

oxide are also shown in the figure. Theoreti

cal curves [39] are shown as dots for compari

son; these points have been shifted along the

voltage axis to allow coincidence of V^^^ with

the experimental value. The silicon-dioxide

capacitors (curves a, b, and d) show similar

characteristics; the flat-band voltage is

negative (indicating a positive surface-state

charge) and the experimental curve lies above

the theoretical one in the inversion region

(indicating interface-state dispersion) . By

contrast, the tantalum pentoxide capacitor

(curve c) shows a negative surface-state

charge and the theoretical curve shows higher

dispersion than the experiment. Since the di

electric film in this case was very thin, it

is not clear that the C-V measurement is valid because of the possibility of leakage in

the oxide; hence, it is possible that the devices goes into deep depletion rather than to true inversion. (F. H. Brewer, R. Y. Koyama,

J, Krawczyk, Y. M. Liu, and D. A. Maxwell)

At this time, due to the ease of in-house

fabrication, the CVD silicon dioxide is the

most promising low-temperature oxide. However, neither the anodic silicon dioxide or

the tantalum pentoxide processes have been

eliminated from consideration.

The low-temperature processing procedure was

further tested with a lot of n-type, 10 ^'cm

<100> silicon wafers which had been diffused

with gold to introduce the known gold acceptor [38]. These simulated starting wafers

were then processed with a 400°C CVD oxide

and 500°C microalloyed aluminum metallization

Thermally stimulated current measurements were made on these capacitors in the modified

wafer apparatus. Although detailed measurements were not made, the response of the gold

acceptor was clearly observed. These results

indicate that the MOS structure used in these

measurements is a suitable vehicle for detect

ing defects in the starting material.

(R. Y. Koyama)

The microalloy step is necessary with aluminization using an e-gun source. This is done
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

-4 0 -3.0 -2.0 -1.0 0.0

a. Chemical vapor deposited silicon dioxide,

1^2.5 nm thick.

Nq = 6.6 X iol'+ cm-3;

Vf.^ = -2.07 V.

) l_i I I I I J I I I I 1 1 1 : L

0.0 0.5 10 1.5 VglV)

c. Oxidized tantalum (13205), 33.1 nm thick.

No = 1.6 X 10^5 cm" 3; Vfb = 0.62 V.

to remove the interface state dispersion in

the C-V curve due to the x-ray damage [40]

(see sec. 5.2.) from the e-gun. In addition,

it promotes better adhesion of the aluminum

to the oxide surface. Since this microalloy

temperature was higher than the maximum oxide

fabrication temperature, a series of measurements was conducted to e ^.e if a lower microalloy temperature would be acceptable.

Four wafers were processed with dry thermal

oxides, about 90 nm thick. After gate metallization (e-gun aluminum) and photoengraving,

each of the four wafers was microalloyed at

a different temperature for 20 min in dry nitrogen. Capacitance-voltage measurements

were made on MOS capacitors of the resulting

wafers; the results are displayed in figure

13. Four C-V measurements from each of the

four wafers are shown. Wafer 74A was not mi5 0 -

-4.0 -3 0 -2 0 -H 0 0 VglVl

b. Anodically grown silicon dioxide, I8O.8 nm thick.

Hq = 6.7 X 10^^+ cm~3;

Vfb = -2-12 V.

-4.0 -3 0 -2.0 -1.0 0.0 VolVI

d. Thermally grown silicon dioxide (dry),

1 05- 3 nm th i ck. Nq = 6.7 lO^"* cm"3;

Vfb = -1.77 V.

s i 1 i con wi th values, discroalloyed at all. It shows the typical interface state distortion accompanied by the

higher than normal flat-band voltage shift

due to the x-ray damage. The other three wafers, 74C, 74B, and 74D, alloyed at 300°,

400°, and 500°C, respectively, show very lit- ' tie interface-state distortion but varying

quantities of oxide charge. From the standpoint of thermally stimulated current and ca- pacitance measurements , the presence of oxide

charge is not a problem, providing that it re-i mains stable. Since this is usually the case,

the 400°C microalloy step would be appropriate for use. (R. Y. Koyama and J. Krawczyk)

Tentative Catalog of Defect Centers — In or- der to utilize the thermally stimulated cur- rent or capacitance response of a diode or capacitor to identify impurities and defects

in silicon, it is necessary to know the charFigure 12. Capacitance-voltage characteristics of various oxides on n-type

aluminum gate electrodes. (Solid lines: measured values; dots: theoretical

placed to account for flat-band voltage.)

24
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

tron emission in ?7-type silicon (NBS Spec.

Publ. 400-4, p. 31) was rearranged and generalized as :

-2.0 -1.5 -1.0-0.5 0.0 0.5 1.0 1.5 Vg ivi

Figure 13. Capacitance-voltage characteristics of four MOS capacitors on each of four

n-type silicon wafers with dry thermal oxides,

about 90 nm thick, and e-gun evaporated aluminum gates microalloyed at different tem- peratures. (For clarity, the curves are displaced vertically; the oxide capacitance is about h2 pF. Curves a: no alloy treatment;

curves b: microalloy temperature, 300°C; curves c: microalloy temperature, kOO°C; curves d: microalloy temperature, 500°C.)

acteristic responses of such impurities and

defects. Emission coefficients and activation energies of 31 impurity or defect centers in silicon were compiled from the literature and used as the basis of plots of the

temperature, T^, at which the majority carrier (phase I) thermal emission from charged

defect centers produces a discharging current

peak as a function of the heating rate, 6, at

T^. Impurities include cobalt [41], gold

[42], irridium [43], platinum [44], rhodium

[43], silver [45], sulfur [46], tantalum [47],

and zinc [48]. Defects include those induced

by 1-MeV electron irradiation [49], 1.5-MeV

electron irradiation [50], 1-MeV neutron irradiation [50], and high-temperature processing [50,51].

To obtain the T -g curves, the relationship

e between 3 and previously derived for elecB T 3exp(-AE /kl ) X e X e

2 + AE /kl

X e

(6)

where B is the emission coefficient, the

X subscript x is n or p designating the conductivity type of the silicon, k is Boltzmann's

constant, AE is E

n c E^, and AE is E^t' p t (energy levels of the defect center as measured from the conduction and valence bands

respectively) . The model used for this calculation [38] assumes a single emission process (minority carrier emission rate is trivial at the majority carrier emission current

peak) and a small defect density relative to the dopant density. Violation of the former

condition, as in the case of the mid-gap gold

center, shifts the peak temperature only by

1 K. Uncertainties in the emission coefficients and activation energies of the order

generally reported in the literature result

in uncertainties of only a few degrees in

the peak temperature. The model also assumes

that the emission rates are described by the

empirical equation of Arrhenius of the form:

e^ = B^T^exp (-AE^/kT) . Because the temperature variation of the emission is so dominated by the exponential term when AE^/kT is

fairly large, experiments cannot distinguish

between various values of s in the more general form: e = AT^exp (-AE^/kT) [52]. The

value 2 for s was adopted since it corresponds to a temperature independent cross sec

tion.

The curves are given in figure 14; the

values of emission coefficient and activation energy used to construct these curves

are listed in table 6. The intersection of a horizontal line at the peak temperature with

a vertical line at the heating rate should oc

cur on the curve of the center responsible for the response.

It would be desirable to add to this catalog

centers which result from other impurities

including copper, iron, lithium, manganese,

mercury, molybdenum, nickel, neodymium, oxygen, thalium, thulium, tungsten, and vanadium

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

25
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

60

I I 1 1 iiii|—I I 1 1 nil

I I 1 1 mil I I Mini _LLJ

HEATING RATE (K/s)

a. Hole emission in p-type silicon.

0.01 0.1 1

HEATING RATE |K/sj

Electron emission in n-type silicon

10

Figure 14. Temperature of peak value of thermally stimulated current response as a function

of heating rate for selected defect centers in silicon. (See table 6 for identification of

symbol s .

)

3.8. Extended Range MIS C-V Method

Design and evaluation work on the modified

instrument for measuring capacitance, C, at applied voltages, V, of up to 10 kV (NBS

Spec. Publ. 400-19, pp. 31-32) has been com- pleted [53].

Detailed studies have be<=n carried out on

the influence of bias-protection circuitry

on the accuracy of the capacitance measurement. It was found that with the three com- mercially available 1-MHz capacitance bridges

tested (NBS Spec. Publ. 400-25, p. 28), the

bias-protection circuitry introduces an error of less than 1.1 percent when measuring

capacitances up to 130 pF.

A modified specimen holder and probe assem- bly were designed and constructed to permit

the use of a guard ring around the electrode

on the free surface of the sapphire. Use of an unguarded electrode resulted in a nonuniform electric field at the silicon surface in

the region opposite the edge of the unguarded

electrode. The total edge capacitance in the

vicinity of the nonuniform electric field is on the order of a few percent. WWhile this is

not important for the determination of the

flat-band charge and silicon doping, it may

introduce an undesirable error in a determination of the interface-state density.

The measurement technique has been extended

to permit the measurement of the conductance,

G, as a function of voltage. The results of

measurements on two specimens indicate that

there is reasonable agreement between the

value for the density of interface states at

the silicon-sapphire interface obtained from

the peak value of the C-V characteristic at one point in the forbidden-band gap and the

26
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

Table 6 — Emission Coefficients and Activation Energies of Selected Defects in Silicon

Emission

Defect Center Symbol Coefficient,

s-i-K-2

Activation

Energy, eV SourceReference

a. Centers Which Emit Holes in p-Type Silicon

1 MeV electron induced EI-1 1.53 X 107 0.239 [49]

Zinc ZnO 7.53 X 107 0.297 [48]

Gold Au"^ 2.70 X 108 0.345 [42]

Cobalt Co" 2.73 X 108 0.352 L41J

Silver Ag^ 2.89 X 108 0.378 [45]

Platinum Pt 1 .77 X 1 0° 0. 320 [44]

1 MeV neutron induced NI-1 2.2 X 10 =

r\ oil

0. 31

[50]

1.5 MeV electron induced EI-2 2.3 X

1 n S

1 0. 31 3 L50J

1 MeV electron induced EI-3 2.63 X 1 0° n 'D IT /I 0. 354 [49]

1 MeV neutron induced NI-2 17 U.COl [50]

Zinc Zn" 1.57 X 1 U° A c: "7 1 U. b/ 1 [48]

Process induced PI-1(++) 3.06 X

ft CO c

1.) . bob [51

b. Centers Which Emit Electrons in n--Type Silicon

1 MeV neutron induced NI-3 5.8 X 105 0.201 [50]

1.5 MeV electron induced EI-4 1.75 X 106 0.216 [50]

Platinum Pt 7.06 X 107 0.260 [44]

Tantalum Ta-1 1 .67 X 105 0.232 [47]

Iridium Ir-1 4.22 X lOii 0.385 [43]

Rhodium Rh-1 2.85 X 109 0.353 [43]

Process induced PI-2(0) 6.13 X 105 0.264 [51]

Process induced PI-3 4.1 X 103 0.226 [50]

Sulfur S" 1 .82 X 105 0.276 [46]

1 MeV neutron induced NI-4 2.7 X 106 0.401 [50]

1.5 MeV electron induced EI-5 1.54 X 106 0.397 [50]

Sulfur s" 1.14 X 107 0.528 [46]

Si Tver Ag" 3.34 X 107 0.544 [45]

Process induced PI-4(+) 1.67 X 107 0.542 [51]

Gold Au" 1 .97 X 107 0.547 [42]

Cobalt Co" 8.52 X 106 0.534 [41]

Rhodium Rh-2 1.46 X 108 0.591 [43]

Iridium Ir-2 4.3 X 108 0.629 [43]

Tantalum Ta-2 1.63 X 105 0.472 [47]

27
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

corresponding value previously obtained from

point-to-point differentiation of the C-V

curve. (A. M. Goodman^)

It is well known that e-gun aluminization of

MOS devices produces positive charge at the

silicon-oxide interface. The effect is presumably due to the generation of soft x-rays

by the electron beam striking the melted aluminum. The charge is generally removed during a 400° to 500°C annealing step in a neutral or reducing atmosphere that follows the

metallization.

It was found that a large positive charge develops at the silicon-sapphire interface during the e-gun aluminization and that conventional annealing at 400°C in forming gas does

not entirely remove it. Further annealing

does not help to remove the radiation induced

charge and, in fact, increases the interfacial positive charge. This information has

strong implications not only for current device processing but also for possible contemplated use of soft x-ray lithography in the

fabrication of C-MOS/SOS devices and device

arrays on sapphire.

(A. M. Goodman'^ and C. E. Weitzel^)

The developed apparatus is useful for a variety of applications . The present ±10 kV capability of the system appears sufficient for

all anticipated applications with the exception of the characterization of silicon-onsapphire substrates thicker than 150 ym. For sapphire substrates 2 in. (51 mm) or more in

diameter which are about 325 ym thick it

would be necessary to have a higher bias voltage capability. Design considerations are now being given to develop a system which

would allow characterization of thick sapphire substrates at applied voltage up to ±25

kV. This new system includes a linear-sweep

high-voltage supply, high-voltage biasprotection circuitry, and a high-voltage specimen holding system. (A. M. Goodman^)

Work performed at RCA Laboratories under

NBS Contract No. 5-35912. NBS contact for

additional information: R. Y. Koyama.

28
. MATERIALS CHARACTERIZATION BY

PHYSICAL ANALYSIS f1 E T H 0 D S

4.1. Optical Test for Surface Quality of

Sapphire Substrates

Specular reflection measurements offer the

possibility of providing useful information

on both silicon and sapphire surfaces. The

infrared surface analysis of sapphire (NBS

Spec. Pubis. 400-19, pp. 22-24, and 400-25,

pp. 20-23) [54] is a useful means of detecting lattice-related damage, but is much less

sensitive to surface scratch lines which may

remain after the associated lattice damage

is removed by substrate annealing. This is

not surprising since the dimensions of these

imperfections are much smaller than the infrared wavelength used, and scattering losses

are relatively small. Thus, reflectance measurements at ultraviolet wavelengths comparable in magnitude with the dimensions of surface defects should be more effective in detecting surface irregularities. Similar con- siderations apply to epitaxial silicon films

which, in turn, exhibit a surface texture re- lated to the substrate surface condition and

deposition parameters. Epitaxial silicon

films are generally accepted or rejected on

the product line in a subjective manner by

visual inspection for light scattering effects or "haze." Films which exhibit haze

appear to have inferior electrical properties;

a fast, nondestructive, quantitative test is

needed to place the screening on an objective

basis

fleeted component can be ignored and eq (7)

becomes

log R^R = [(itpa)2/2.303](l/A2). (8)

This technique requires that the surface be

highly reflecting with R greater than about

40 percent. This requirement can be met by

using a wavelength that is heavily absorbed.

Since silicon is highly absorbing in the ultraviolet region of the spectrum, while sapphire is transparent, reflection measurements

can be easily made on silicon-on-sapphire com- posites to evaluate the quality of the silicon.

Initial experiments were performed on four

bulk silicon substrates which had been variously polished in order to introduce different degrees of surface roughness. The re- flectance for each surface was measured as a function of wavelength and the ratio R /R at

240 and 280 nm was determined for each specimen; an aluminum mirror was used to determine

R . The logarithm of this ratio is plotted

against (on a logarithmic scale) the size of

the polishing grit in figure 15. The values

of a to be used in eq (8) are not known but

are related to the size of the polishing grit.

Scanning electron micrographs show that surface roughness is related to the polishing

grit size. It appears from these preliminary

An optical technique which measures the surface roughness of a reflective surface has

been described by Bennett and Porteus [55].

This technique is based on published work on

the reflection of radar waves from rough surfaces [56]. According to the theory presented in these publications, the reflectance

of a surface is related to the root mean

square roughness, a, of a surface and to the root mean square slope of the profile of the

surface, m. The complete expression is [55]

R = R^{exp[-(:5i£^)^ + 3^(^)\Ae)2}, (7)

where R is the reflectance of a nonideal surface, R is the reflectance of an ideal suro face (without roughness) , A is the wavelength

of the incident radiation, and AG is an instrumental acceptance angle. The second terra takes into account diffusely reflected light.

If the reflectance measurement is made at wavelengths much longer than the root mean

square surface roughness, the diffusely re0.1 1.0 ID POLISHING GRIT SIZE [fim] Figure 15. Specular reflection data for four

bulk silicon substrates polished with different polishing grits and for six silicon-onsapphire composites. (a: bulk silicon at 240 nm; d: bulk silicon at 280 nm; o: siliconon-sapphire at 280 nm.)
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

results that this plot may be used as a ref- erence curve for determining silicon surface

roughness on this scale and at these wavelengths .

On this basis, reflection data were obtained

at 280 nm on six silicon-on-sapphire composites. These results are also plotted in figure 15; in this case the point for each speci- men is placed on the 280-nm line at the measured value of reflectance. The horizontal

position, then, should give some indication

of the surface roughness. Scanning electron

micrographs of two typical specimens are presented in figure 16. Specimen 1, which is

equivalent to a very small grit size, had an acceptable surface smoothness which compares

favorably with a highly polished bulk silicon

surface, while specimen 4, which is equivalent to a significantly larger grit size, had

been rejected from the product line because^

of haze. ^ (M. T. Duffy']', P. J. Zanzucchi" , and G. W. Cullen"

)

4.2. Impurities in Sapphire Substrates

Besides surface roughness (see sec. 4.1.),

the presence of impurities introduced unintentionally in the epitaxial silicon film can

also affect the performance of silicon-onsapphire devices. One potential source of

these undesired impurities is the polished

sapphire substrate. In order to determine

the typical impurity levels in commercially

available polished sapphire substrates, three

substrates from different sources were characterized by neutron activation analysis.'''

Two specimens cut from half of each of the

circular polished sapphire slices, one near

the center and one near the edge , were

weighed, sealed in polyethylene, and irradiated in a neutron flux of 6 x 10^^ cm~2-s~^

for 30 min in the NBS reactor. After a cooling period of about 30 min, the specimens were cleaned with a 50-percent hot nitric acid solution to remove possible external contamination. The specimens were then counted

using a Ge(Li) detector-multichannel analyzer

system. Calcium was determined 30 min after

irradiation and manganese was determined 120

min after irradiation. Standards were aqueous solutions of the two elements.

Subsequently, the specimens were again irradiated, this time for 10 h. After a cooling

period of about 10 days to eliminate ^'^Na matrix activity, the specimens were counted to determine the amounts of molybdenum, chromium,

antimony, scandium, iron, cobalt, and zinc

which were present. This counting was also

Work performed at RCA Laboratories under

NBS Contract No. 5-35915. NBS contact for

additional technical information: K. F. Galloway

.

^The substrates were supplied by Rockwell In- ternational Corporation which had obtained

them from various sources for evaluation under Air Force Contract F19628-75-C-0108

.

30
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

Table 7 — Elemental Content of Sapphire Substrates

Element

Concentrations of Element, ppb (by weight)

Substrate 1 Substrate 2 Substrate 3

Center Edae Center Edge Center Edge

Mol vbednum 1000 1000 <10 <10 400 400

Chromi um 100 100 'X-IO 40

Antimony 0.2 n ? 1 2 Scandium 0.1 0.1 0.1 0.1 0.3 0.02

Iron 70 60 300 300 60 60

Cobalt 0.1 0.1 0.1 0.1 0.1 0.1 Manganese 0.5 0.5 1 1 0.5 0.5

Calcium <10000 < 10000 <10000 <10000 <10000 <10000

Zi nc <10 <10 <10 <10 <10 <10

done with the Ge(Li) system. Standards for

all elements were either aqueous solutions

or small amounts of metal foils dissolved

and aliquoted after irradiation.

The results are presented in table 7. The

chromium content of substrate 2 could not be

determined because it contained a small

amount of iridium which interfered with this

determination. (S. Mayo and G. J. Lutz§

)

4.3. Electron Spectroscopy Techniques

Effect of Ion Flux Distribution on Depth Resolution in Auger/Ion Beam Profiling of OxideSiliaon Structures — As a result of an electrical breakdown and attempts to repair the

ion gun in the Auger spectrometer, the depth

resolution was seriously deteriorated. This was manifested by an apparent widening of the

oxide-silicon interface in a previously measured specimen, and also by a large scattering in the data. In order to clarify the

I source of the deterioration, the flux distriI bution in the primary ion beam was measured

1 for typical operating conditions,

i

[ Consider the experimental conditions outlined

I in figure 17. The specimen surface, S, is poI sitioned at the focal point, F, of the cylindrical mirror analyzer, CMA, with its normal

j at an angle 9 30 deg with respect to the

' primary electron beam, EB. The ion beam, IB,

J is incident on the surface at an angle n and

is carefully aligned so that it strikes the

specimen at the focal point, as depicted in

the figure. The ion current (flux) distribution at the specimen surface is also shown in

the figure; the electron beam is considered

inf initesimally narrow since it is much

smaller than the ion beam. During the alignment procedure there is a finite possibility

of displacing the specimen along the z (CMA)

axis . The broken lines and curves show how

the ion and electron beams are displaced.

Notice that in this case the electron beam

strikes the wall of the ion sputtered crater

in the specimen. A similar situation arises

when the ion beam is not perfectly aligned

and hits off the focal point of the CMA, or

if a shift occurs in the ion beam alignment

during sputtering. The latter effect may result from thermal drift in the ion optics,

particularly when the total sputtering time

is long (hours)

It can be shown that the fractional change

in apparent interface width is equal to the

fractional change in ion flux density provided that fluctuations in sputtering yield

and specimen density can be neglected [57]. To demonstrate the importance of reducing

flux inhomogeneities to a minimum, a 100-nm

thick oxide thermally grown on a (100) silicon surface was profiled using two ion guns,

NBS Activation Analysis Section, Analytical

Chemistry Division.

31
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

IB

Figure 17. Schematic of experimental conditions for ion sputter etching and Auger electron spectroscopy. (Note that the angles 6 and n are not In the same plane.)

one normal and one with a defect in the optics. With the former the observed width of

the oxide-silicon interface was 4.3±0.3 nm. With the defective gun there was a large

scatter in the observed width but the average

of the minimum width was about 6 nm.

The spatial distribution of the ion beam cur- rent in the two guns was measured by moving a

1.05

>-

tozo

X

z>

-1.4 -1.0 -0.6 -0.2 0 02 0.6 1.0 DISTANCE ALONG CMA AXIS (mm)

a. Normal gun.

Figure 18. Spatial distributions of ion beam

focal point (z=0) ; a: rastered beam with ion ^ tionary value.)

square Faraday cup 250 ym on a side, along

the z-axis. The results are plotted in figure 18 as ion flux density normalized to its

value at the CMA focus (z=0) . Both stationary and rastered beams were measured; the additional broadening of an interface at a depth of 100 nm, due to the flux inhomogeneities in the rastered beams are shown in fig- ure 19. The predicted broadening is consistent with the experimental results described

above. Note that in figure 19 there is a region of zero broadening for the normal gun

if the specimen is located about 0.4 mm behind the CMA focus. This occurs as a result

of the flat distribution of the normal rastered ion beam in this region (see fig. 18a).

These results show that in Auger/ion depth

profiling it is of critical importance to check routinely the ion current distribution

and to apply a suitable rastering with com- pensation for small displacements of the rastered beam with respect to the focal point of

the CMA. In most of the depth profiling work

reported in the literature neither of these measures is taken. It is, therefore, reasonable to conclude that flux inhomogeneities

have been a limiting factor in the depth reso- lution [58] and that interface broadening

larger than several nanometers can occur. Such effects have probably been important in

producing the idea that the depth resolution

in sputter milling is 5 or 10 percent of the

DISTANCE ALONG CMA AXIS (mm)

b. Defective gun.

current. (•: stationary beam directed at CMA

urrent at z=0 reduced to one-half of Its sta32
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

total depth milled. The present work shows

that this is not the case if proper techniques are used. (J. S. Johannessen^^

,

W. E. Spicer^, and Y. E. Strausser**)

Interface Broadening by Electron Stimulated Desorption — Another important parameter in Auger analysis and in Auger/ion depth

profiling is the primary electron fluence.

To demonstrate the adverse effect of a high

primary electron current density on depth reslution, the interface profile of the oxide

film described above was measured with a 5-yA

primary electron beam. In one case the beam

was held stationary; in the other case it was rastered over an area 250 pm by 250 ym. The

defective ion gun was used to obtain both

profiles. The two 502-eV 0 profiles are KLL

shown in figure 20. The horizontal axis is

based on the sputter rate associated with the

ion beam and the sputtering time. The interface appears to be significantly wider when

measured with the stationary electron beam.

The early reduction in the measured oxygen

density occurs because the sputtering rate in

the area where the specimen is exposed to both ions and electrons is greatly enhanced

[59] as illustrated in the inset of the figure. As a result the electron beam reaches

the interface ahead of the main portion of

0

(nm)

u— ION BEAM *| 1—J e-BEAM J-r—

\ *^ / T \. 1 SiOj 100

Si -.one nm ^ 75 nm

- Okll

1 1 1 1 1 1 1 I SPUTTERED DEPTH IN UNITS OF 2 nm

Figure 20. Oxygen KLL profiles showing effects of electron-beam enhancement of sputtering rate. (a: stationary electron beam; o: rastered electron beam; the Inset shows the

sputtered crater profile for the case of a stationary electron beam.)

the sputtered crater. The reason for the en- hanced sputtering rate is probably due to electron stimulated desorption of oxygen.

The enhancement of the sputtering rate is not

uniform across the area of the electron bea'm; therefore, the measured oxygen density decreases gradually until the main portion of

the sputtered crater reaches the interface.

(J. S. Johannessen^'^ and W. E. Spicer^)

L4 -1.0 -0 6 -0 2 0 02 0 6 LO DISTANCE ALONG CMA AXIS (mm)

Crater Edge Profiling

formed by appropriate

vides a beveled surfa

produced by more usua

[60]; however, the an duced by three orders

much greater spatial

shallow angle is made

controlled and chosen

beam and practical by

depth ('^1 nm) of Auge

To carry out a crater

— The crater edge

ion beam etching proce analogous to that

1 mechanical lapping

gle can be usefully reof magnitude, allowing

resolution. The very

possible by the well

parameters of the ion

the shallow analysis

r electron spectroscopy,

edge profile analysis

Figure 19. Additional broadening of an inter-

^ face at a depth of 100 nm due to flux inhomogeneity in rastered ion beams of normal (a)

and defective (o) ion guns.

33

Work performed at Stanford Electronics Laboratories of Stanford University under NBS

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

Partially supported by Electronics Research

Laboratory, University of Trondheim and the

Royal Norwegian Council for Scientific and

Industrial Research.

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

of an oxide-silicon (or other) interface, the

electron beam is slowly swept in a line scan

across the portion of the crater edge which

exposes the interface. The Auger spectrum

of an element is recorded. In this manner

elemental profile shapes essentially identical to those of conventional depth profiling

are obtained. The abscissa now becomes the

position of the electron beam on the crater

edge, which, from a knowledge of the ion beam

current density and the associated sputter

rates, can be transformed to a distance nor- mal to the specimen surface. Although a num- ber of elements can be recorded simultaneously by using a multiplexer, repetitive scans can be made across the interface, so there is no absolute need to multiplex.

This procedure was tested by carrying out ex- periments on specimens with 50- to 150-nm

thick oxides thermally grown on phosphorusdoped silicon [61]. The experimental geometry is shown in figure 21a; the rastered 1-

keV ion beam (current density ~ 40 yA/cm^)

formed an approximately elliptical crater as shown in figure 21b. The ion beam current

density as measured along the x-direction

through the focal point of the analyzer by

using a small Faraday cup is shown in figure

21c.

In an experiment to compare conventional

depth profiling with crater edge profiling

the specimen was depth profiled to the interface as determined by monitoring oxygen, and

then for an additional time equal to 20 percent of the time that it took to reach the interface. The additional sputtering time was chosen such that the position of the interface was along a relatively linear part of

the ion current density distribution of figure 21c. The width of the crater at the interface along the x-direction should then be

given by the width of the current density distribution at five-sixths of maximum. This was checked and the widths were found to be

identical within the experimental error of

0.1 mm, giving confidence that the slope of

the crater wall may also be obtained from the

slope of the current density distribution provided that the sputtering rate is known and

that it remains unchanged across the interface .

Line scans for oxygen, phosphorus, and elemental silicon were made along a 530-ijm

length of the crater edge which included the

oxide-silicon interface (C to in fig. 21b).

The results are shown in figure 22. From the

slopes of the oxygen curve and the crater

wall (fig. 21c) the interface width was found

to be 5.0 ± 0.2 nm as compared with the value

of 4.9 ± 0.2 nm obtained from the initial

depth profile. Of chief significance, however, is the relative ease with which the

measurement could be repeated.

\

c'

'1

\

X

b \

W "

\

X-TRANSLATION Imml

Orientation of electron and ion beams relative to the specimen normal, n. The axis, yj,

is the projection of the y-axis on the specimen surface. The projection of the ion beam

on the specimen surface is at an angle 57 deg from the x-axis.

Ion-beam crater in the plane of the specimen surface. The ellipse represents exposed in- terface; to obtain the profile, a line scan is made along the x-axis between C and C^.

Ion current density measured along the x-axis through the focal point of the cylindrical

mirror analyzer (z=0) . The interface occurs at a position equal to five-sixths of the

crater depth.

Figure 21. Geometry for crater-edge profiling using Auger electron spectroscopy.

34
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

ASYMPTOTE

X-SCAN (mm)

Figure 22. Line scans for oxygen, phosphorus,

and elemental silicon across the interface between the points C and on the x-axis.

The crater edge interface analysis technique

can be used as an adjunct to the conventional

technique but in many cases it can be used

quite independently since the positioning of

the electron beam during sputtering is much

less critical when the monitoring of Auger

peaks is mainly to determine the time to an interface. In many cases the sputtering may

be rapid and fears of bulldozing too rapidly

through the interface are no longer important,

since the sputtering and analysis are now

separated. When there is difficulty in measuring details of the ion beam shape, the

slope at an interface can be calibrated by

making a good conventional depth profile from

which the interface width can be measured.

Obviously not all interface analysis lends

itself to this technique since in a macroscopic sense the films must be uniform, parallel to the surface over a distance of a few

millimeters. However, films of this degree

of uniformity are not uncommon.

(N. J. Taylor"^,

J. S. Johannessen^^ , and W. E. Spicer^)

II

#

Work performed at Stanford Electronics Laboratories of Stanford University under NBS

Contract No. 5-359^4. NBS contact for additional information: K. F. Galloway.

Partially supported by Electronics Research

Laboratory, University of Trondheim and the

Royal Norwegian Council for Scientific and

Industrial Research.

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

35
5. MATERIALS AND PROCEDURES FOR

WAFER PROCESSING

5.1. Sodium Contamination in Oxidation Fur- naces*

Additional experiments verified the previous

conclusion (NBS Spec. Publ. 400-25, pp. 29-

30) that despite its great sensitivity for

free sodium, the resonance fluorescence method [62] is not adequate to detect sodium con- tamination in furnace tube atmospheres at 1000°C which produce oxide films with sodium

contamination of the order of 10^^ cm"^.

To investigate the relationship between free

sodium density and the density of sodium com- pounds , thermodynamic calculations were made

to determine the equilibrium partial pressures of various species at 1300 K in oxygen

flowing at 4 cm^/s. It was assumed that the source of sodium was the contamination in the

fused silica tube wall which generally has an average sodium content of about 10 ppm by

weight [63]. Because of errors and approximations in thermodynamic data involved in

these equilibrium calculations an absolute

error of ±50 percent could be expected, although the relative error of these results is

believed to be much smaller.

The results of these calculations [64] are summarized in table 8 which lists both partial pressures, P, of appropriate compounds

and density of sodium, N, for several levels

of water content in the oxidation atmosphere.

For dry oxygen, free sodium is the predominant species but as water vapor is added the

total sodium density increases significantly

because of the formation of sodium hydroxide.

The effect of the added water vapor is illustrated in figure 23 which also includes the

results of calculations made for 1200 and

1400 K.

Calculations were also made to establish the

effect of cleaning the furnace tube walls

with hydrogen chloride or chlorine. The results of these calculations for the case of a 10-percent mixture of hydrogen chloride or chlorine in an inert carrier gas at atmospheric pressure are summarized in table 9. The

calculations indicate a pressure of NaCl higher than the vapor pressure for NaCl(liq), but

under the actual conditions, equilibrium is

probably not reached. Micro-droplets of liquid sodium chloride, if formed on the tube

wall hot zone, would be removed by the gas

stream and transported to a cooler tube zone where sodium chloride precipitates on the

wall (sodium chloride melts at 801°C) . This

"transpiration mechanism" continues while

sodium in the hot zone is available for reaction. The sodium chloride formed in the gas phase also is carried down the flushing gas stream and condenses at the cooler zone where

the temperature is appropriate.

The cleaning action can thus be interpreted

in terms of relocating sodium in the tube, depleting the hot zone and enriching the ex- haust tube end where the wall temperature is

appropriate for condensation. Assuming that

the oxidation tube wall surface is about 4000

cm^ and that the reaction depth in the wall

is 10~^ cm, the total reacting volume is 4 cm^ or about 10 g of fused silica, which at a 10-ppm sodium content in the fused silica is

equivalent to 0.1 mg of sodium which requires

the use of only 0.15 mg of chlorine or hydrogen chloride for complete reaction equilibrium.

At a typical flow rate of 4 cm^/s (0.5 scf /h)

,

this quantity of chlorine or hydrogen chloride is transported through the oxidation

tube in about 1/2 s. Thus the reaction rate

is regulated by diffusion of sodium in the

fused silica. When sodium in the wall bulk

diffuses to the wall surface, the reaction

with chlorine effectively takes place immediately. In practice, a cleaning period of

about 20 h is generally used to deplete the

tube wall. The longer the cleaning period,

the longer the oxidation tube can later be

operated under acceptable oxide growing con- ditions. (S. Mayo and W. H. Evans''') \<

5.2. X-Ray Dose in Electron-Beam Evaporators*

Additional calculations were made to estimate

the x-ray dose received by the oxide films in

MOS structures with aluminum or chromium gate

electrodes deposited by electron-beam evaporation [40]. Results of a preliminary experiment (NBS Spec- Publ. 400-17, pp. 20-21) suggested that an absorbed dose of the order of

1 kGy (0.1 Mrads(Si02)) is received during

the evaporation of a 1-ym thick aluminum film over silicon dioxide with 10-keV electrons.

Principally funded by the Defense Nuclear

Agency.

"^NBS Chemical Process Data Evaluation Section,

Physical Chemistry Division.
MATERIALS AND PROCEDURES FOR WAFER PROCESSING

Table 8 - Equilibrium Partial Pressures and Sodium Densities in Fused Silica Oxidation Tubes

at 1300 K as a Function of Water Content in Flowing Oxygen

Water Content, ppm: 0 1 30

Compound P, atm N, cm'^ P, atm N, cm'^ P, atm N, cm'^

Na 2. 5 X 10-12 1. 4 X 107 2. 5 X 10- 12

1 . 4 X 107 2 5 X 10- 121 . 4 X 107

NaO 9. 1 X 10-13 5. 2 X 106 9. 1 X 10- 13 5. 2 X 106 9 1 X 10- 1 3 5. 2 X 106

NaaO 2 X 10-•24 2 X 10-24 2 X 10- 24

NaOH 8. 6 X 10-11 4. 9 X 108 6 1 X 10- 10 3. 5 X 109

Na2(0H) 3. 7 X 10- 20 1 1 X 10- 18

Total Sodium 3. 4 X 10-12 1. 9 X 107 8. 9 X 10- 1 1 5. 1 X 108 6 1 X 10- 10 3. 5 X 105

Figure 23. Sodium density in tiie atmosphere

of a fused silica oxidation tube as a function of water vapor content at temperatures

from 1200 to ]kOO K.

0 10 20 30 40 SO 60 70

WATER CONTENT IN OXIDATION ATMOSPHERE |ppni|

Table 9 — Equilibrium Partial Pressures and Sodium Densities in

Tubes at 1300 K With an Atmosphere of 10% Hydrogen

Chloride or Chlorine in an Inert Carrier Gas

Compound

10% HCl 10% CI2

P, atm N, cmP, atm N, cm

NaCl 5 5 X 10-3 3.1 X 1016 1 4 X 10-2 7.9 X 1016

Na^Cl2 8 2 X 10-'^ 9.3 X 1015 5 4 X 10-3 6 X 10I6

H2O 3 6 X 10-3

O2 6 3 X 10-3

37
MATERIALS AND PROCEDURES FOR WAFER PROCESSING

ALUMINUM

kTOTAL

»Ka CONTRIBUTION

0 5 10 15 0 5 10 ELECTRON BEAM ENERGY (keV)

Figure 2k. Total ionizing dose deposited in the oxide layer as a function of electronbeam energy during the electron-gun evaporation of aluminum and chromium. (See text for

evaporation conditions.)

0.1

GO!

CHARGE INCIDENT ON MELT (C) 100 I0_ p ill

10 100 METAL DEPOSITION RATE (nm/s)

1000

Figure 25. Total ionizing dose deposited in the oxide layer as a function of the deposition rate of aluminum and chromium in an e- gun evaporation system. (See text for evaporation conditions.)

100 200 300 400 500 CHROMIUM THICKNESS (nm)

20 40 60 80 100

INCIDENT CHARGE |C|

Figure 26. Total ionizing dose deposited In the oxide layer as a function of chromium

layer thickness during the e-gun evaporation

of chromium or of a 1 -pm thick aluminum layer over chromium. (See text for evaporation cond i t i ons .

)

Figure 27. Absorbance of x-ray energy in nylon film dosimeter as a function of the

electron charge incident on aluminum in an € gun evaporator. (Solid points are measured

values, dashed curve is the signal due to optical radiation, solid curve is the cor- rected x-ray dose.)

38
MATERIALS AND PROCEDURES FOR WAFER PROCESSING

The results of calculations of absorbed dose

as a function of electron-beam energy are shown in figure 24. The calculations were

made for the case of the deposition of alumi- num or chromium metallization on a 50-mm diameter silicon wafer covered with a 100-nm

thick layer of silicon dioxide and located

about 500 mm from the melt. The wafer subtends a solid angle of 0.01 sr. For each energy, the beam current was taken as the value

necessary to maintain the beam power at 5 kW.

A typical deposition rate of 3 nm/s was assumed and the process was continued for 333 s in order to deposit 1 ym of metal.

With this set of parameters, the absorbed ra- diation dose in the oxide due to both K and

a and bremsstrahlung radiation was calculated

as a function of beam energy. In the range

5 to 15 keV the absorbed dose was found to be

essentially independent of energy; approximately 60 kGy (6 Mrads(Si02)) was absorbed in

the oxide film for aluminum evaporation and

approximately 20 kGy (2 Mrads(Si02)) for

chromium evaporation. It should be noted

that no significant dose reduction can be ex- pected during aluminum evaporation by operating the electron gun at energies below 1.5

keV, where the aluminum K x-ray is not exa cited, because of the increase in the bremsstrahlung radiation at low energies.

To evaluate the shielding effect of the metal

film, the x-ray dose in the oxide was evaluated under the assumption that the metal absorbed no energy. At a beam energy of 10 keV

the aluminum film was found to reduce the absorbed dose in the oxide by a factor of 1.7

while the chromium film was found to reduce

it by a factor of 3.9. The calculation of

energy deposited per unit thickness of oxide

indicated that the dose is essentially uniformly distributed throughout the oxide, being only approximately 10 percent higher at

the metal-oxide interface than at the oxidesilicon interface.

The effect of absorbed dose on the metal deposition rate was calculated for several beam

energies. The radiation dose for depositing

1 ym of metal as a function of deposition

rate is given for a beam energy of 10 keV and

a beam current of 0.5 A in figure 25; the results of calculations at other beam energies

were similar.

These calculations are based on the assumption of the deposition of equal thicknesses

of aluminum and chromium for comparison.

However, in actual practice, a chromium gate

layer of 100 to 300 nm would probably have

about 1 ym of aluminum deposited over it.

Assuming the depositions are carried out with

a 10-keV electron beam at 0.5 A yielding a deposition rate of 2 nm/s, figure 26 illustrates both the dose absorbed in the oxide as

a function of chromium thickness and the dose

absorbed in the oxide with a 1-ym thick aluminum layer deposited over the chromium layer

of thickness as shown on the abscissa. The

shielding effect of the chromium is clearly

illustrated.

The discrepancy between the results of these

calculations and the preliminary experimental

results is thought to arise from the sensitivity of the thermoluminescent dosimeters (TLBs)

used to measure the absorbed dose to in-situ

annealing effects caused by optical radiation

and thermal effects due to the kinetic and

condensation energies of the metal film being

deposited. A temperature rise during irradiation causes a well known glow peak indicative of release of trapped electrons. The

net reading after irradiation of TLBs in contact with a silicon wafer therefore corresponds to only a fraction of the actual dose

deposited

Therefore to test the accuracy of the calculations, a second experiment was performed in

which polychlorostyrene and nylon film dosimeters, 50 and 60 ym thick respectively, were

used to measure the x-radiation produced in

the electron-beam evaporator. These films were exposed at the normal wafer location in

the evaporator while a 10-keV electron beam

impinged on aluminum. The beam intensity was reduced to 0.04 A while the crucible containing the metal was cooled to prevent any alumi- num evaporation. Under these conditions, an x-ray exposure equivalent to that obtained in

a typical metal deposition is obtained provided the total number of electrons (or total

charge) incident on the metal is equivalent

to that during metal deposition. Two groups

of nine 1-cm^ polychlorostyrene and nylon

films were exposed in pairs to increasing x- ray doses. Figure 27 shows the measured nylon film absorbance as a function of total

electron charge incident on the metal. The

measured absorbance must be corrected to account for the sensitivity of the film to optical radiation which is present in the evaporator chamber during metallization. The fig- ure also shows the correction factor and the

corrected results for nylon films. Similar

results were obtained for the polychlorostyrene films which, however, required no correction for optical sensitivity.

39
MATERIALS AND PROCEDURES FOR WAFER PROCESSING

The dose absorbed in the dyed films when a 10-keV electron beam deposits 100 C in alumi- num was calculated. This charge incident on

the metal at an appropriate current is typical of that necessary to deposit 1 \im of aluminum at a rate of about 3 nm/s. The calculalations yielded an absorbed dose of 3.9 kGy

(0.39 Mrads (nylon) ) and 4.5 kGy (0.45 Mr ads

(poly)) in the nylon and polystyrene films,

respectively. This compares reasonably well

with the experimentally measured values of

4.3 kGy (0.43 Mrads (nylon) ) and 3.0 kGy (0.30

Mrads (poly)) and provides the basis for the

deposited dose calculations in silicon dioxide films during the metallization process.

Discrepancies can be explained in terms of er- rors introduced by the extrapolation of the

aluminum bremsstrahlung below 2 keV.

The trends in x-ray dose due to the electronbeam metallization process illustrated by

these calculations can be used in examining

experimental results on device hardness. The

deposition of chromium results in reduced

dose when compared to aluminum deposited under equivalent evaporation conditions. However, since chromium sublimates, less electron beam injection power is necessary to deposit a chromium layer of the same thickness

as an aluminum layer. This can result in a significant reduction in dose absorbed in the

oxide. Several experimenters have noted that

MOS devices fabricated with electron-beam

evaporated chromium gates exhibit a higher

degree of radiation resistance than devices

fabricated with aluminum gates [65-68].

Gates formed by depositing aluminum over chromium result in a reduced x-ray dose in the

oxide when compared to gates formed only of

aluminum as illustrated in figure 26.

An attempt to reduce the electron-gun damage

in the fabrication of aluminum gates by re- ducing the electron-gun beam energy to about

3 keV and reducing the deposition time to about 20 s has been reported [69]. The authors concluded that even under these conditions considerable damage to the oxide occurred. Figure 24 illustrates that reducing

the electron-beam energy does not reduce the

x-ray dose absorbed in the oxide; however,

figure 25 indicates that reducing the evaporation time (increasing the deposition rate) can reduce the oxide dose.

(S. Mayo, K. F. Galloway, and T. F. Leedy)

5.3. Ion Implantation Parameters

This task is concerned with the development

of methods for measuring critical parameters

associated with ion implantation. Study of

Schottky barriers for use in measuring implanted profiles by the capacitance-voltage

(C-V) technique was extended to the case of

phosphorus-implanted silicon. As part of a comprehensive study of methodology for dose

measurement, the characteristics of various

current integrators were investigated.

Schottky -Barrier Capaaitanae-Voltage Technique — The four metallization systems, aluminum, gold, titanium-gold, and molybdenumgold, previously studied on boron-implanted

silicon (NBS Spec. Publ. 400-25, pp. 31-32), were studied on cleaned, natural (unetched)

surfaces. Gold consistently yielded rectifying current-voltage (I-V) characteristics,

stable C-V profiles, and barrier capacitance

in the range 200 to 400 pF/mm^. Results with

aluminum were mixed; some barriers yielded

fairly rectifying I-V characteristics, satisfactory but slightly unstable C-V profiles,

and barrier capacitance in the range 200 to 300 pF/mm^ while others yielded non-rectifying I-V characteristics, unstable C-V profiles, and barrier capacitance in the range

2000 to 3000 pF/mm2. Molybdenum-gold yielded

poorly rectifying I-V characteristics, barrier capacitance of the order of 1600 pF/mm^,

and C-V profiles which were only occasionally

satisfactory. Titanium-gold yielded resistive I-V characteristics, barrier capacitances in the range 1000 to 5000 pF/mm^, and no C-V curves

.

The quality of the I-V characteristic and the

value of the barrier capacitance are good indicators to determine whether the C-V curve

is going to be good or bad. For satisfactory

C-V profiles, the I-V characteristics should

be at least fairly rectifying and the barrier

capacitance should be in the range from 200

to 400 pF/mm^. It was found that the metal

film thickness should be about 100 nm; films

less than 50 nm or more than 200 nm thick were likely to yield poor or time-dependent

C-V curves. In the case of silicon implanted

with phosphorus, gold is the preferred metallization material.

Dose Measurement Technology — Measurement of

the ion beam current density and integration

of the beam current for dose measurement are important parts of the implantation process

and must be done before as well as during implantation. Figure 28 shows a typical target

configuration for characterizing the ion beam

before beginning an implantation step. The

suppressor electrode is maintained at a slightly negative potential with respect to the target collector in order to reduce er-
MATERIALS AND PROCEDURES FOR WAFER PROCESSING

WAFER TO

BE IMPLANTED

Figure 28. Typical target configuration for

characterizing the ion beam before ion implantation.

rors in current measurement which would result from the emission of secondary electrons

for the target collector. The dimensions and

spacing of the various electrodes can be

changed to provide for differences in target

area.

In order to obtain a uniform ion current over

large areas, the beam must be scanned in both

vertical and horizontal directions or the target must be scanned mechanically. The ratio

of scanning frequencies is usually such that

a series of high frequency pulses is superimposed on a series of low frequency pulses . . Since the area of the ion beam itself is much

smaller than the area of the circular defining aperture, this type of scanning produces

a square pattern over the aperture as shown

in the inset of figure 29. The ratio of the area of the circular defining aperture, -nd^/k, to the area covered by the beam scan, D^,

which is known as the overscan efficiency, is

a measure of the utilization of the ion beam.

As shown in figure 29, this is about 80 percent for the case where the scanned area

fills the entire circular aperture (D=d) and

•drops to less than 10 percent when the beam

is overscanned one full target diameter on

each side of the aperture (D=3d) , a condition

necessary if the beam shape is badly distorted.

During the set-up procedure, the Faraday cup

current (see fig. 28) can be compared with

the total target collector current to determine when the current density is equal in the

two areas. Typical plots of the two current

80 100 120 OVERSCAN (%)

200

Figure 29. Scanning geometry and overscan

efficiency. (Overscan {%) = [(D/d)-1] x 100; overscan efficiency {%) = (-nd^/kD^) x 100.)

densities are shown in figure 30. With no scanning voltage applied, the steady-state

ion beam is centered on the cup and shows a relatively high reading depending on how well

the beam can be focused. As the scanner voltage is increased, the beam starts to move back and forth across the cup, resulting in a series of pulses which are averaged by the

current measuring instrument (an electrometer,

for example). The target collector current- measured by a current integrator remains es- sentially the same, increasing slightly as less current is collected by the cup, until

200 400 600 800 SCANNER VOLTAGE, PEAK-TO-PEAK TRIANGULAR WAVE (V)1000

Figure 30. Target current densities as a function of scanner voltage for an SA-keV

boron ion beam. (•: collector cup current;

total target current.)

41
MATERIALS AND PROCEDURES FOR WAFER PROCESSING

the scan voltage is high enough to deflect

the beam past the aperture defining the tar- get collector area. As the voltage increases

beyond this point the target collector cur- rent drops proportionately. When the collector cup current density and the total collector current density are equal, a satisfactory

beam uniformity is obtained over the entire

target area. For the case shown in figure 30 the current densities are equal for current

densities less than about one-fourth of the

unscanned target current density. From fig- ure 29 , this is seen to be equivalent to an overscan of about 80 percent so that the beam

deflection distance, Q, is equal to 1.8 times

the diameter of the defining aperture.

Measurement of target area is critical in determining the correct implantation dose. It is advisable to use a calculated area based

on the physical measurements of the system

being used. The distances between the scanner, target, and defining aperture must be

known to calculate the ratio by which the

target area is larger than the defining aperture. Other methods, such as measurement of

visible implanted areas, are subject to a number of uncontrolled factors, such as beam

size and instability, that cause shifting of

the beam and uncertainty in the location of

the implanted region boundaries.

As the beam passes over the center of the

target in the typical 80-percent overscan

case, the integrator measures a square-wave

type pulse. The pulse width gradually de- creases with distance from the center, becoming zero during the maximum levels of the

slower sweep voltage. The integrator must

therefore be capable of measuring this wide

variety of pulse widths which ranges from

zero to about 250 vis.

In order to investigate the accuracy of the

current measurements, two sources of current were used: 1) actual ion beam currents obtained from an implantation system and 2) simulated current pulses obtained from a pulse

generator. The currents in both instances were measured and compared using five different commercially available current integrators as well as other current measuring instruments. The readings were taken from dc

conditions to 10,000 pulses per second (pps)

.

The ion beam scanner unit used was variable

over a range from 20 to 2000 pps; the remaining frequencies were covered with the pulse

generator. Average current readings were

checked over the range from about 50 nA to 500 yA, usually with a 50-percent duty cycle.

42

The preliminary conclusions from these studies are that all the current integrators

agree within a few percent for dc conditions,

but only three of the five agreed within 10

percent for 10 to 2000 pps. Inconsistencies

were observed between 1 and 10 pps; above

2000 pps other anomalies were observed.

Therefore, both scan frequencies must be between 10 and 2000 pps; a ratio of 100 to 1 between the horizontal and vertical scan frequencies was found to be most desirable.

Errors related to measuring current pulses

with peak amplitude greater than the full

scale setting of certain of the current integrators were also encountered. All the

current integrators performed well for peak

ion beam currents less than 1 \iA; four were

satisfactory to 10 yA; three, to 100 yA; and

two to 1 mA. Significant inaccuracies were

observed for higher beam currents. Since the

present trend in industry is toward the use

of implantation currents of the order of 1 mA or more, these inaccuracies may cause difficulties in defining and comparing implantation conditions.

(R. G. Wilson^ and D. M. Jamba §

)

5.4. Passivation Integrity

A corona discharge, reverse decoration technique was found to be a rapid, nondestructive

method for decorating cracks and other defects in passivation overcoats [70]. This is

a two-step method. In the first step, ions

from a glow discharge at atmospheric pressure

are deposited on insulating regions of the

device. Then the device is immersed in a sus- pension of oppositely charged particles which

are attracted to the charged insulating regions .

The charging process is illustrated schematically in figure 31. The source of ions is an array of seven parallel 40-ym diameter

nickel alloy wires, 1.8 cm apart, held in a plastic frame in a horizontal plane. The

power supply is typically an rf-type, highvoltage, dc source capable of ±10 kV and a maximum current of 10 mA. A grounded plate,

not shown in the figure, is placed about 3 cm above the wires to provide greater current

uniformity and to protect the wires from mechanical damage. The specimen is placed on a

Work performed at Hughes Research Laboratories under NBS Contract No. 5-35891. NBS

contact for additional information: K. F. Galloway.
MATERIALS AND PROCEDURES FOR WAFER PROCESSING

Ammeter

Corona grid wire array

I ons Ground plate

Power supply

Vol tmeter Specimen wafer

Figure 31- Schematic representation of corona charging with positive ions.

grounded plate about 2 cm below the wire array.

The charging process is carried out in a nitrogen atmosphere with about 25-percent relative humidity for 15 to 20 s. It was established that the charging of a surface containing closely spaced insulator and conductor re- gions is limited mainly by the conducting areas. As the ions collect on the insulator

they generate an electric field in the gas

ambient which causes subsequent ions to move

toward the regions of ground potential before

arriving at the wafer surface. Thus, for the

geometries of interest, the oxide surface

voltage is not determined by the oxide breakdown potential (except for very thin oxides

or very large insulator areas)

The ambient relative humidity must be con- trolled to prevent lateral current flow on

the oxide surface. The oxide surface voltage decay rate after charging was found to increase greatly above 30-percent relative

humidity for thermal silicon dioxide, chemical vapor deposited (CVD) silicon dioxide,

and CVD phosphosilicate glass (PSG) . The sur- face voltage also decays rapidly if the bulk

conductivity of the oxide is high due to the

presence of moisture. Thus, for some wafers

with CVD silicon dioxide or PSG passivation,

it was found to be necessary to remove adsorbed or absorbed water before charging.

This can be done by heating in room air at 200°C for 5 min. This has been made the initial step in the standard charging process to

ensure retention of surface charge on the

specimen.

This procedure is satisfactory for devices or wafers in which the passivating layer has

been etched open at the contact pads or at the grid lines. These etched regions furnish

the closely spaced insulator-conductor geometry required so that the surface voltage does

not depend on whether the passivating layer

is over metal or over insulator. It is also

possible to decorate wafers before the passivation layer is etched open by placing a grounded conducting grid over the wafer during the charging. A convenient grid consists

of an aluminum sheet 0.8 mm thick with 3.5-cm

diameter holes on 5.0-cm centers. When the

grid is used, charge is deposited only at openings in the grid . The entire wafer can be charged if the grid is moved laterally during charging while it is spaced about 0.3 mm

above the wafer.

An important consequence of the charging properties is that the surface charge and potential around a defect decreases laterally as the defect is approached. This is due to ion

deflection during charging and to surface con- duction. Thus, there is a margin around a crack or pinhole of lower surface potential

than the potential in nondefect areas. This

effect increases detectability since it magnifies the defects as far as charged particle

deposition is concerned. The margin is of

the order of 25 ym in lateral dimension.

It is necessary that the surface charge re- main on the passivating layer for a time

about equal to the delay between charging and

deposition plus the deposition time. The dielectric relaxation time of the passivation

layer must be longer than the total time

which is about 20 s. Thus, a bulk resistivity of about 10^^ Q'cm or greater is needed.

The resistivity of an insulating layer may be

increased by reducing its temperature; this can be accomplished by cooling the specimen

during charging and by the use of cooled particle suspensions.

The reverse decoration process is illustrated

schematically in figure 32. This process has

been accomplished with various powders including phosphors, zinc oxide, and lead aluminosilicate glass, but the best results in terms

of sensitivity and contrast were obtained using carbon black suspensions. Various carbon

black suspensions were tested. These included commercially available electrophotographic toner concentrates and specially prepared formulations.

43
MATERIALS AND PROCEDURES FOR WAFER PROCESSING

Figure 32. Schematic representation of the reverse decoration of defects with negative

ions following corona charging with positive

ions

.

Best results were obtained with a carbon

black suspension prepared in the following

way.^ First, a concentrate is made from 17.0

g Raven 1255 carbon black [71], 100.0 ml tolu- ene, 10.0 ml of a mixture of 50 g Lubrizol

894 [72] in 50 ml toluene, and 10.0 ml of a mixture of 50 g A-C Polyethylene, grade 430,

[73] in 90 ml toluene. These materials are readily available.^'* The carbon black has a mean particle diameter of 22.0 nm and a nitrogen absorption surface area of 130 m^/g. The

surface of the carbon black as supplied is

acidic by addition of volatiles [71]. Lubrizol 894 is an ashless dispersant additive for

heavy duty engine oils; the weight percent of

nitrogen ranges from 1.65 to 1.95 percent

[72]. A-C Polyethylene, grade 430, is a polyethylene-vinyl acetate copolymer (30 percent vinyl acetate) ; its softening point is

60°C [73]. The concentrate is agitated for

about 5 min with a 100-W ultrasonic probe to disperse the carbon black.

To make the dilute suspension, about 1 ml of

the concentrate is added to 400 ml of Freon

TF'''* and stirred. This results in a suspension which contains about 0.1 g of carbon

black. This amount has been found to be adequate for decorating at least ten 3-in. (76-

mm) diameter wafers. Both the concentrate

and the dilute suspension are stable for long

periods (months). In this suspension, the

carbon black is negatively charged so that

positive corona potentials are used for re- verse decoration.

Measurements were made to characterize the

electrical state of the carbon particles in

the dilute suspension. These measurements

involved depositing the carbon black on metal

foils while recording the current and then

weighing the deposited carbon. This was done

as a function of applied voltage and time.

The weight of carbon black deposited and the

charge transported varied approximately linearly with voltage and time. For electrode

spacings of 1.7 cm on each side of a metal

foil, an applied voltage of 10 V for 4 min re- sulted in a deposition of 3.2 x 10"*^ g/cm^ of

carbon black. The average charge per single

particle of carbon black, with the assumption

that all such particles were 22.0 nm in diameter and singly dispersed, was 0.05 times the

electronic charge. Since the minimum charge

per particle is one electronic charge, it is

clear that the carbon black is aggregated to at least a factor of 20 times. This aggregation is essential for obtaining dense black

on the charged insulator surface.

The detection of cracks and pinholes decorated in this way is quite convenient under microscopic observation. An example is shown

in figure 33. It was demonstrated that every

.defect detected by etching of the underlying

aluminum through the defects was detected by

the carbon black reverse decoration procedure.

This was done in two ways . Wafers and chips were decorated with carbon black and photomicrographs were taken of crack and pinhole

patterns. The carbon black was then removed

and the aluminum was etched (NBS Spec. Publ.

400-25, p. 33) at 50°C for 5, 7, 10 or 15 min.

The specimens were then photomicrographed

again and the results compared. Hundreds of

micrographs from many different specimen

types were compared, and, in every case, all

defects detected by etching were also clearly

outlined by the carbon black. In the second

method, specimens were decorated with carbon

black and then aluminum etched with the car- bon black in place. Again, the correlation

was excellent. An example of the latter test

is shown in figure 34.

The density of demarcated or decorated localized structural defects in a dielectric layer

can be determined by counting the number of

This suspension was originally formulated by

G. S. Lozier, RCA Laboratories.

See disclaimer, p. viii.

44
MATERIALS AND PROCEDURES FOR WAFER PROCESSING

Figure 33- Photomicrograph of reverse decorated device showing high contrast which renders microscopic evaluation fast and sensitive. (Magnification: 'vlOOx.)

defect sites per unit area, using one of several microscopic techniques discussed previously [74]. The choice depends on the uniformity and density level of defects, the

type of material used in decorating, the degree of accuracy required, and the specific

purpose of analysis. Incident white-light,

bright-field illumination is most suitable

for observing specimens decorated with car- bon black. The magnification used should be

sufficient to allow resolution of individual

demarcated defect sites, but not higher, so that the largest possible specimen area is included within the field of view. It is impossible to specify a fixed magnification

that would hold for all cases, but a magnification in the range from 50 to 500 diameters

has been found to be most useful.

I The greatest difficulty in determining the

j density of defects by any method concerns

j the distribution uniformity. If the uniforj mity of distribution is "good", counting with-

' in a few sites may provide an adequate meaj sure of the true statistical distribution.

' If it is "fair", which is the most common occurrence, examining a substantial number of areas (5 to 15) over the specimen may be necessary to arrive at a valid average density

' value. Specimens with "poor" uniformity are best treated by dividing it into "good",

"fair", and "poor" regions and computing separate density values for each, rather than

computing a meaningless average value.

j The sampling areas for counting should be

spread reasonably uniformly over the specimen

surface. If the specimen is a device wafer

Figure 3^- Photomicrograph of portion of reverse decorated device aluminum-etched at 50°C for 5 min without removing carbon black.

(Magnification: '^325x; etched defects appear

inside the regions without carbon black as denoted by the arrows.)

or is otherwise patterned, one may choose to examine every nth unit, and count the defects

within that defined area. If the specimen is

not patterned, then the microscope stage micrometer can be used to space the observation

sites uniformly over its area. The circular

area of the microscope field of view for the

magnification may be used as a convenient

unit. Photomicrography affords a useful record of the defect density and often facilitates counting of the defect sites, especially if the density is high; it also provides

an accurately defined area of the specimen

surface

A major advantage of the carbon black reverse decoration method is the high optical contrast available in ref lected-light microscopy.

This high contrast permits a fairly simple

form of automated quality assessment with a microscope-mounted photocell to measure the

integrated light reflected from the defect

areas. This technique is much simpler than

the use of computer-implemented image analysis. Although the latter would provide a defect count, which the integrated ref lectedlight method does not furnish, it is believed

that the simple method explored here could be

used in a production facility to rank wafer

quality and set a pass, no-pass criterion.

Initially, photomicrographs were made of car- bon black reverse decorated devices on integrated circuit wafers. Transparencies made

from these micrographs were then mounted in an optical system so that a collimated light
MATERIALS AND PROCEDURES FOR WAFER PROCESSING

beam was passed through the transparency and

then focused on the faceplate of a vacuum

photocell to form an image of the light

source. An image of the transparency is not

desired since the photocell response may not

be uniform across the faceplate. The current

output of the photocell varies linearly with

incident light. It was found that the cur- rent output varied inversely with subjective

judgments of device quality made by counting

pinholes and estimating crack lengths.

Based on these preliminary encouraging' results, a photocell-microscope system was as- sembled to take direct readings of reflected

light from decorated wafers. The arrangement

used is shown schematically in figure 35. By

placing a plano-convex lens with a focal

length of 57 mm at the end of the camera tube, an image of the light source field stop was obtained at the photocell faceplate about 6 cm from the end of the camera tube. The photocell employed an S-11 cathode and was operated at 100 V. Light intensity was adjusted

to give convenient current levels and the

PHOTOCELL

IMAGE OF SOURCE FIELD STOP AT PHOTOCELL PLANO CONVEX LENS AT END OF CAMERA TUBE

OBJECTIVE

SPECIMEN

CONDENSER 3 <^ ] •» APERTURE

| -« FIELD STOP — COLLECTOR LENS

fMiil

SOURCE

Figure 35. Schematic representation of microscope modified to obtain image of source

field Stop at the photoceil to measure the

integrated light reflected from defective

areas. (Note that in the actual (vertical i lumination) system the source and photocell are on the same side of the specimen.)

overall magnification was chosen so that a large portion of the circuit was visible, but

bond pads and grid lines were excluded.

Shielding prevented introduction of ambient

light. Photocell current readings were taken on 20 circuits of each of two wafers. For

the "good" wafer, the lowest individual cur- rent was 2.9 nA, the highest was 5.5 nA, and

the average was 3.5 nA. For the "bad" wafer

the lowest current was 6.7 nA, the highest

was 15 nA, and the average was 10 nA. Pinhole counts on these wafers resulted in a dif

ference of a factor of two in defect density.

Thus, the photocell current measurement can easily detect a difference of a factor of two in pinhole density and is probably more sen- sitive than this.

The only fundamental limitation encountered

thus far has been devices with regions of

metal not connected to the semiconductor.

This floating metal is brought to a high potential by the corona so that the metal is

outlined with carbon black, but no defects

can be detected over it. Thus, the process

cannot be used with floating gate memory devices, for example, unless the floating metal

can in some way be grounded or connected to the semiconductor body.

The carbon black can be removed by soaking

the specimens in toluene for 15 min, followed

by ultrasonic agitation in toluene and rinsing in Freon TF-^'"' When ultrasonic energy is

used, care must be taken to avoid specimen

damage. Contact of the wafer with hard surfaces, such as beaker sides or bottom, should

be avoided. It has been found that careless

use of the ultrasonic bath has actually

caused pinholes in previously good passivating layers.

Several experiments were conducted to establish that this method is nondestructive.

First, a set of device wafers was aluminum

etched to reveal pinholes. Then the wafers were exposed to the corona discharge at 10,000 V dc for various times ranging up to 100 s. After the corona discharge, aluminum

etching revealed no new pinholes.

In a second test, one set of five device wafers was reverse decorated with carbon black

and a second set of five was direct decorated

with phosphor (NBS Spec. Publ. 400-25, p. 34)

After cleaning, the decoration was repeated.

This process was repeated for a total of five

times. Only one wafer showed new defects

(pinholes). During cleaning, this wafer had

See disclaimer, p. viii,

46
MATERIALS AND PROCEDURES FOR WAFER PROCESSING

been placed face down in the bottom of the

ultrasonic tank and it is believed that the

mechanical contact caused these defects. Subsequent tests with the ultrasonic tank used

in this manner did induce defects.

A third test involved automatic probe testing

of C-MOS wafers. These were chosen since it

is expected that C-MOS devices are most sus- ceptible to static electrical damage. Two

3-in. (76-ram) diameter C-MOS device wafers

were probed , and a map was made of good and

bad devices. Then the wafers were charged,

decorated, and cleaned. The wafers were

probed again. One wafer showed a net loss of

four devices out of about 130 good devices,

initially. The other wafer showed a net gain

of five devices out of about 100 good devices,

initially. These variations are within the

normal tolerance of repeated wafer probe testing.

It was concluded that the process of corona charging and decoration is nondestructive.

However, since no life tests have been carried out on decorated and cleaned devices

,

the effect of this test on service life and

reliability has not yet been directly estab-

! lished. (W. Kern'^ and R. B. Comizzoli^)

I 5.5. Process Materials Characterization

Boron Nitride Diffusion Sources — Transport

I studies were made of known amounts of zinc

il from simulated boron nitride diffusion

'! sources. Two glasses, which were designed to

I simulate the oxide layers on activated boron

: nitride of two types (pure and mixed with

j silica) , were prepared with admixtures of

.': 0.75 mole percent zinc oxide [75]. One of

i the glasses was boron oxide (B2O3) and the

1 second was a borosilicate glass composed of

;! silica (3102) and boron oxide in a 3 to 7

[ weight ratio. The transfer experiments were

; conducted at 1000°C in a dry nitrogen atmo-

[: sphere for times up to 12 h. The glass which

formed on the surface of the silicon wafer

was analyzed for zinc by means of flameless

Table 10 - Zinc Content in Glass Transferred

from Doped Borosilicate Glass to Silicon at 1000°C

Time, h Zinc Concentration,

Trial 1

ppm (by weight)

Trial 2

21

295

1.0 65 140

2.0 40 100

3.0 70 120

4.0 45 100

5.0 440 145

12.0 615 135

atomic absorption. In the case of the B2O3

glass, no zinc transfer was observed within

the detectability limit of one part per million by weight. The zinc content of the borosilicate glass is shown in table 10. The con- fidence limits for these data have not been

established at this time, but it seems likely

that the variations are not statistically significant. (J. Stach'^,

T. A. Yager+, and R. E. Tressler'"')

Hydrogen Chloride Gas — The SOLGAS program

[76] was used to compare the equilibrium partial pressures at 1100°C of the chemical species in the hydrogen chloride (HCl) and trichloroethylene (TCE) oxidation systems. The

most significant difference is that the partial pressure of water vapor in the TCE system is lower by an order of magnitude when

compared with that in the HCl system [77].

The calculations also indicate that the partial pressure of CIO is nearly equal in both

systems whereas the partial pressures of HCl,

HO, and HOCl are approximately five times

greater in the HCl system. (R. Tressler"*", W. H. Grubbs+, M. B. Das+, and J. Stach''")

*^Work performed at RCA Laboratories under

NBS Contract No. 5-35913. NBS contact for

additional information: T. F. Leedy.

"'"Work conducted at the Pennsylvania State

University under NBS Contract No. 5-35717.

NBS contact for additional technical information: R. I. Scace.
6 . PHOTOLITHOGRAPHY

6.1. Line-Width Measurement with Spatially

Filtered Coherent Optical Radiation

Measurement of the widths of lines and spaces on the order of 1 ym wide or less is made difficult since this regime lies at the resolution limit of optical microscopes. In this

regime, imagery tends to be nonlinear (see

sec. 6.2.) because of the partial coherence

of the illumination. The nonlinearity is

generally evidenced by edge ringing which distorts the image of the line edges.

The common attachments used on the microscope

for line-width measurement are the filar and

image-shearing eyepieces. The problems in

applying these when measuring narrow lines

have been detailed previously (NBS Spec. Publ.

400-19, pp. 34-41) [78]. An analysis has demonstrated that these problems can be alleviated if the image is properly spatially filtered [79].

The analysis is based on a microscope which

employs a single-lens coherent optical system

as shown in figure 36. For illustration, an opaque line in a clear background is shown as the object, A. The objective lens, B, as- sumed to be thin, forms a real image in the

image plane, D, a fixed distance, Z^, away.

This distance, which is called the tube

length, has been standardized at about 160

mm. Although an ocular is added in a microscope to form a virtual image with apparent

magnification given by the product of the

magnifications of both elements, the significant imagery is carried out by the objective

and the ocular can be ignored in the analysis. The Fourier transform of the object

field, the complex object spectrum, is located at a plane, C, one focal length, f, behind the lens

.

The physical makeup of the microscope in the

a-plane determines the frequency content of

the image. The outer aperture of the system

determines the spatial frequency limit of

this plane; the origin of the a-plane is

equivalent to optical "dc", and spatial frequency increases linearly outward to the aperture limit. A clear aperture therefore acts

as a low-pass filter, passing without attenuation all frequencies below its upper cutoff,

but truncating all those above. If an opaque

line of width 2ai is inserted in the center

of the a-plane, frequencies lower than a^ will also be truncated. With such an occluding band-pass filter, it can be shown that

the image of an opaque line in a clear background is identical with that of a clear line

in an opaque background [79]. For an optimum

filter, the image has a zero intensity at the

line-edge image locations; examples of such

images are given in figure 37. The optimum

filter is defined such that

ai = {l/[(2^)(2a)]}0Si-l{Si[(2^)(2aa2)]} (9)

where is the lower cutoff frequency, 02 is

the upper cutoff frequency, 2a is the width

of the line, Si[x] = jT'^ [ (sin?) /5 ]dC (the sine

integral) , and the pre-superscript zero on

the inverse sine integral indicates that the

lowest value (between 0 and tt) must be taken.

The upper cutoff frequency is determined from

02 = (NA)/X (10)

where (NA) is the numerical aperture of the

objective lens and X is the wavelength of the

incident illumination; this is the diffraction limit. Thus, for any given microscope

system, the optimum values of a^ and 02 are fully defined for a specific line width, 2a.

Since lines of unknown test width cannot a priori be paired with their optimum filters,

measurements must be made on images formed by

non-optimum filters. To assess the ability

to correct routine measurements made with nonA Object plane, with an opaque line as the

object

B P 1 ane of 1 ens

C Plane of object spectrum, located one focal length from the lens, with an opaque

obstacle inserted at the center of the

field.

D Image plane, with an idealized filtered

image.

Figure 36. Sketch of the single-lens coherent optical system, showing the locations of

various planes and coordinate systems.

48
PHOTOLITHOGRAPHY

Width of line:

X (MICROMETERS)

LU 1,2 o

-2 0 2 X (MICROMETERS)

b . Wi dth of line: 5 ym

Figure 37- Calculated images of clear lines

of wavelength 500 nm, and a 0.95 numerical aperture objective lens. (A

image without filtering; C: image with optical filtering.)

in opaque backgrounds for coherent illumination

obj ect line; B

optimum filters, calculations of the measured

dimension of 4.6-, 5.0-, and 5.75-ijm test line widths were carried out for filters optimized for 4.0-, 4.75-, 5.5-, and 6.0-ym

lines; and of 1.7-, 2.0-, and 2.3-pm test

line widths for filters optimized for 1.6-,

1.8-, 2.1-, and 2.4-ym lines. All of these

calculations were made for objective lenses

with numerical apertures of 0.5, 0.65, and

0.95. These calculations [79] showed that

when the lines are well-resolved by the optics, as are the lines greater than 4 ym for

all the numerical apertures used, measurements can be corrected to less than 1 percent

of their actual dimension for all of the filters used. However, when the lines are marginally resolved as in the case of the nominal 2-Mm grouping, the range of filters with

which they can be imaged and the measurements

corrected is greatly reduced. An important

result of the calculations shows that when

the measurements can be corrected, the errors

are reduced to the order of hundredths of a percent for all numerical apertures.

The problems associated with misalignment of

the filters were also examined [79]. The effect of such misalignment is to change the

zero intensity at the line-edge image location to a minimum because a shift in the cutoff frequencies results in unequal portions

of positive and negative sides of the spectrum passing through to the image. The largest error is produced by physically misaligning the lower cutoff filter. However, fabrication of adequate filters should be well

within normal machine tolerances since the effects of misalignment of optimum filters cor- responding to these tolerances are vanishingly small, and no significant measurement error should result.

To implement this filtering-measurement system successfully in a microscope, it is clear

that the lower cutoff frequency must be adjustable. With a continuously adjustable fil

ter, once a test line is placed in the system

the lower cutoff is continuously increased

from zero until an image is obtained similar

to those in figure 37 with the line edges defined by zero intensity and approximately

equal luminance peaks on either side of the zeroes. If an appropriate correction algorithm [79] is then applied, the resulting

line width can be accurately determined.

One way to achieve a variable filtering which

appears to be feasible would be to prepare a high-precision adjustable slit so that its

surfaces are reflecting. The lower cutoff is

achieved by opening the slit to allow the un- wanted portions of the spectrum to pass

through and be trapped while the desired portion of the spectrum is reflected into the

system along a different optical path and is

imaged. A conceptual scheme of this is

sketched in figure 38. Implementation of

this scheme would necessitate a partial redesign of some microscope systems, but in some models presently in use, such a path is already incorporated. Since nonuniformly opening slit jaws can be compensated through call

bration, imperfect jaw motions need not be a bar to application.

49
PHOTOLITHOGRAPHY

A Gubstage condenser lens

B Specimen (object plane)

C Objective lens

D Front-surface mirror, with its center one

focal length from the lens, C, and an opening which can be varied as the jaws of a slit; the portion of the image spectrum

that is eliminated passes through this

open i ng

E Plane of intermediate image, one tube

length from the lens, C F Microscope eyepiece

Figure 38. Sketch of a possible configuration for a microscope system with spatial fil- ter i ng

.

Although this technique appears to be theoretically feasible, there are several practical limitations. Most present microscope objectives are designed so that the object spectrum imaged by the objective lens is located

inside the physical dimensions of the lens

system where a filter cannot be constructed.

Introduction of additional optics to re-image

this spectrum would degrade the performance

of the objective and compromise the system.

Reflecting objectives or catadioptric systems

could be used without major modifications,

but their range of numerical apertures is limited. Full and unrestricted application of

these ideas will require that microscope objectives be redesigned to place the object

spectrum outside the last glass surface.

It was assumed for this analysis that the

line shapes had a rectangular profile and possessed symmetry about their centerlines. Not

very many lines in practice have such a perfect profile, and the modeling of lines to accommodate variations in their description

is necessary before these results can be ex- tended to the prediction of measurement ac- curacy on real lines and spaces.

The large number of reflecting surfaces in mi

croscope systems precludes the use of laser

illumination with a large temporal coherence

because the resulting interference patterns

would seriously affect image clarity. Further, because the size of the intended objects is so small, lateral coherence intervals need not be extremely large. The mercury arc, filtered for the 546.1-nm line, appears to provide sufficient lateral coherence

and virtually no temporal coherence, and

would seem to be ideal because the green wave

length is near the peak of the eye response

and would be reasonably comfortable for viewing. On the other hand, since radiance falls

off as the square of the magnification, and

because the central portion of the spectrum

where most of the image energy is located is

occluded in the filtering process, high illumination power is required. Additional testing is necessary to choose the best light

source.

A great many simplifications were introduced

in the analysis. The simple scalar theory

employed is sufficient to illustrate the

technique and develop the basic concepts,

but at the high numerical apertures that must

eventually be used in a practical system, a more rigorous analysis is needed. In addition the assumption of diffraction-limited

optics ignored the spherical aberration,

flare light, and the difficulty of focusing

in coherent illumination that are commonly

present with such systems. Flare light alone

is sufficient to significantly reduce the con

trast of the imagery of the zero crossings at

the line edges . To develop this technique more fully it will be necessary to take these

practical aspects of the microscope into account. (R. E. Swing")

6.2, Optical Imaging for Photomask Metrology

Theoretical and experimental investigation of

the optical microscope, as used for linewidth measurement on photomasks, continued.

A research microscope system, shown schematically in figure 39 , was assembled to make

NBS Optics and Micrometrology Section, Me- chanics Division.

50
PHOTOLITHOGRAPHY

photometric scans of line image profiles for

comparison with theoretically computed profiles .

The system is based on a commercially available research microscope modified to include

a higher-wattage tungsten-halogen lamp, smaller scanning apertures to produce higher reso- lution of the scanned image, a special one- dimensional scanning stage controlled by a stepping motor with function generator drive,

and linear voltage transducer (LVTD) gauges

for positional readout of stage and focus settings. Both analog and digital outputs are available from the photometer and gauges.

The analog outputs of the photometer and position gauge are fed to an X-Y recorder; the

digital outputs are intended for use with a microprocessor-based data handling system now being constructed.

In addition, the prism beam splitter, which

appears to obscure part of the light path, was removed and the incident illumination was filtered by a combination of a Wratten No. 60

filter and a heat reflecting mirror to produce a limited passband peaked at about 560

nm. Use of a scanning slit instead of a circular aperture was found to improve overall

system performance by reducing photon noise

as a result of the increased flux reaching

the detector and by averaging over some of

the minute imperfections along the edges of

the lines being scanned

Although a variety of optics and scanning

apertures may be used, the system has been

found to give best optical performance for

viewing photomasks in transmitted light when

employing a 63x, 0.9 NA objective lens, designed for use without a cover glass, as the

objective and a 32x, 0.6 NA long-working distance objective lens as the condenser. This

combination of optics appeared to produce

minimal flare.

Measured line-image profiles of typical clear

lines on a chromium-on-glass photomask are shown in figure 40. Some edge ringing, characteristic of images formed with coherent or partially coherent light, is evident.

Calculated line-image profiles corresponding

to the measured line images are also shown in

figure 40. These calculations were made with

a computer program which takes into account

a wide range of operating parameters including partial coherence, lens aberrations, scan

ning slit width, objective numerical aperture

and variations in the object being scanned

including geometry, edge degradation, and con

trast. The computation involves numerical in

tegration of a three-dimensional integral

whose integrand is complex and oscillates

over the range of integration. The derivation is omitted here, but the integral in one dimensional form is
PHOTOLITHOGRAPHY

=JJJ

r(a)t(a-y2)t(ct-Mi) x

exp[ — (Pi-Pa)] f^Cp^FCpa) ^

S(i^f^) dadyidp^, (11)

where HC^) is the image profile seen by the

photomultiplier , r(a) is the Fourier transform of the mutual coherence function at the

object, t(y) is the Fourier transform of the

amplitude transmittance of the object, F(y)

is the pupil function of the imaging objective, S(v) is the Fourier transform of the

intensity transmittance of the scanning aperture at the photomultiplier, and Zj and I2 axe the object and image conjugates of the

objective, respectively,

A detailed study of the coherence properties

of the microscope, which is not reproduced

here, has shown that for typical modes of operation, rCa) is the modulus squared of the

pupil function of the condenser. Thus, as has long been known [80], the aberrations of

the condenser lens do not affect the imagery. However, it is noted that the conditions under which this formulation is applicable are that the condenser aperture be uniformly illuminated (making it the limiting

aperture in the illumination system regardless of the mode of illumination) and that

the illuminated area at the object be large

compared to the dimensions of the impulse re- sponse of the condenser. When the area of

illumination becomes very small the system

must be treated theoretically as a microdensitometer for which a different description

applies [81 ]

.

The amplitude transmittance spectrum t(y) of

the object is general and can be changed as desired. At present, single and multiple

lines with variable contrast and edge degradation have been considered. The pupil function, F(p), of the imaging objective, assumed

to be diffraction limited, includes the aberration function and the various terms corre- sponding to defocus and spherical aberration,

etc., may be varied. This formulation assumes quasi-monochromatic illumination of mean wavelength A and does not take into ac- count the variation of the aberration function or other parameters with wavelength.

Although the agreement between the measured

and calculated line-image profiles is quite

good, which suggests that the system can be

used to measure the width of transparent

lines nearly as narrow as the impulse re- sponse of the objective lens, differences do

remain. It is felt these are dufe principally

to 1) the unknown influence of edge quality

which would affect the 1-ym line imagery more

than the 10-ym line imagery, 2) non-zero

transmittance of the opaque background, and

3) the presence of undetermined amounts of optical aberrations in the system, principally

spherical aberration. (D. Nyyssonen"''

)

NBS Optical Physics Division.
PHOTOLITHOGRAPHY

6.3. Calibration Standards for Photomask

Metrology

Artifacts of the design previously described

(NBS Spec. Publ. 400-17, pp. 37-38) have been

successfully manufactured. These artifacts

consist of the hard-surface material, either

"black" chromium approximately 90 nm thick or iron oxide approximately 180 nm thick, deposited directly on a 6-mm thick polished

glass substrate, approximately 36 mm square.

They contain design patterns of from one to four cycles with both clear lines on opaque

backgrounds and opaque lines on clear backgrounds. Compatibility with the scanning

electron microscope is achieved by overcoating the artifact with approximately 50 nm of

carbon (NBS Spec. Publ. 400-19, p. 41).

Figure 41 is a photomicrograph of the pattern

on a chromium-on-glass artifact as observed

in an optical microscope with bright-field

transmitted light at about 85x magnification.

At this magnification, the multiple 1-ym wide

lines and spaces cannot be resolved. Dimensional measurements (see sec. 6.4.) show that

the line widths as measured with an optical

microscope generally equal the design values

to within the specified tolerance of ±0.25 ym.

The 180-nm thick iron oxide is partially

transmitting and equivalent to coatings used

A

n (III

II

1

mil

Figure 42. Photomicrograph of four 1 -ym wide

clear spaces each separated by 1 ym in an

i ron-oxl de-on-g 1 ass artifact viewed under

bright-field transmitted light at a magnification of about 2CC0x.

on some "see-through" integrated circuit photomasks. Viewing these specimens under trans mitted light in an optical microscope shows a dark border at the edge of the iron oxide as shown in the photomicrograph of four 1-ym

wide clear spaces in an iron oxide artifact

in figure 42. The cause of this optical effect has not been identified, but it results

in a large uncertainty in setting the imageshearing and filar eyepieces for optical line

width measurements. For this reason and also

because the edge profile of iron-oxide lines

is less sharp than that of chromium lines,

iron-oxide artifacts are not being considered

for primary line-width calibration with the

SEM interferometer. (J. M. Jerke^)

5.4. Line-Width Measurements on Chromium-onGlass Artifactsi'

Line-width measurements were made with an optical microscope on several chromium-on-glass

artifacts (see sec. 6.3.). Single opaque

lines on a clear background and single clear

Figure 41. Photomicrograph of chromium-onglass artifact, viewed under bright-field

transmitted light at a magnification of about

85x.

NBS Optics and Micrometrology Section, Me- chanics Division.

Funded through the NBS Dimensional Metrology

Program.

53
PHOTOLITHOGRAPHY

Table 11 — Apparent Widths of Lines and Spaces Measured in Bright-Field

Transmitted Illumination

Nominal

Width, ym

Image-Shearing Eyepiece

(Intensity Splitting)

Line Width, Space Width,

pm \im

Filar Eyepiece

Line Width,

ym Space Width,

ym

1 1 04 + 0.07 0 87 ± 0.03 0 75 ± 0 21 1 18 ± 0 08

3 3 03 + 0.06 3 01 ± 0.06 2 84 ± 0 13 3 25 ± 0 02

10 9 84 + 0.05 9 98 ± 0.04 9 73 ± 0 26 10 31 ± 0 20

lines (spaces) in an opaque background were measured with filar and image-shearing (intensity splitting) eyepieces using bright-field

transmitted illumination. Five measurements

were made of the width of each of three lines

and spaces using each eyepiece. The means

and three-sigma values (three sample standard

deviations) are listed in table 11. These

data further confirm the previously reported

result that with transmitted illumination the

filar eyepiece indicates a wider width than

the image-shearing eyepiece for clear spaces

and a narrower width for opaque lines (NBS

Spec. Publ. 400-19, pp. 38-41, 41-43).

An investigation into the effect of focal position on line-width measurement with an optical microscope was also made. Measurements

were made on a single clear space, nominally

3-ym wide, in bright-field transmitted illumination with the image-shearing eyepiece. The

Table 12 - Effect of Focal Position on the

Apparent Width of a Clear Space

Measured with an Image-Shearing

Eyepiece in Bright-Field Transmitted Illumination

Di stance

from Best

Focus, ym

Measured

Width, ym

-1.5 2.87 ± 0.14

-1.0 2,93 ± 0.19

-0.5 2.91 ± 0.06

0.0 3.00 ± 0.05

+0.5 3.00 ± 0.06

+1.0 2.96 ± 0.10

+1.5 2.98 ± 0.10

objective lens had a magnification of 63x, a numerical aperture of 0.90, and a depth of

field of 1.23 ym. The position of best focus was a subjective decision by the operator

based on the quality of the image. The results obtained at seven focal positions, ex- tending beyond the normal depth of field, are given in table 12. Within this range, the

clear line measures maximum width at or near best focus. This result is also consistent

with the previously reported model (NBS Spec.

Publ. 400-19, pp. 38-41). . (F. W. Rosberry'^)

An experimental comparison was made of linewidth measurements using optical microscopes

and operating conditions representative of

those employed by the microelectrics industry for photomask inspection. Most photomask

line-width measurements made in industry are done with conventional optical microscopes

fitted with visual measuring eyepieces of either the filar or image-shearing types; to a limited degree, automatic TV-microscope systems are in use

.

The majority of measurements reported here were made with a monocular research-quality

microscope with separate filar and imageshearing eyepieces . During the course of the

study, a binocular image-shearing microscope

and an automatic TV-microscope system V7ere made available on loan for short term use.

The monocular microscope was equipped with an objective lens, with a numerical aperture of

0.95 and a magnification of 80x, and either

bright-field or dark-field illumination; its

image-shearing eyepiece had an intensity beamsplitter as the shearing element. The binocular microscope was equipped with an objective

NBS Optics and Micrometrology Section, Me- chanics Division.

54
PHOTOLITHOGRAPHY

lens, with a numerical aperture of 0.90 and a magnification of 63x, and bright-field illumination only; its image-shearing eyepiece had

a polarization beamsplitter as the shearing

element. The TV-microscope system was equipped with an objective lens, with a numerical aperture of 0.95 and a magnification of

80x, and bright-field illumination only. All

three microscopes had both reflected and

transmitted light capabilities. The three

systems differed in their fundamental principles of operation and means of data output.

The filar and image-shearing eyepieces re- quire operator judgment for locating image

edges in a line-width measurement; the TV system uses automatic image scanning and electronic thresholding. The filar eyepiece had

a vernier scale readout which requires the

calculation of line widths from arbitrary

scale divisions; the binocular image-shearing

system had direct digital readout as did the

TV-microscope system.

Three filar eyepieces were available for use with the monocular microscope. One, with a

7x magnification and a movable, full-field

pair crosshair configuration, had the best

"repeatability of the three and was used

throughout the study. Preliminary repeatability tests for each of the optical microscope

measurement systems resulted in variations

from the mean of ten measurements of the

width of a 5-ijm transparent line of 0.03 to 0.09 ym (three sample standard deviations).

The linearity of each of the microscope systems was checked for gross misbehavior by

measuring four line spacings after calibration with the NBS line-standard interferometer [82]. Within the measurement repeatability and the calibration accuracy, no significant deviations from linearity were observed.

Cosine error results when the axis of measurement is not perpendicular to the line whose

width is to be measured. In a filar eyepiece,

cosine error is minimized when the crosshair

lies parallel to the line and travels perpendicular to it. In the shearing eyepiece, the

direction of shear must also be perpendicular;

similarly with a TV-microscope system. For filar and image-shearing eyepieces , the degree

to which the line to be measured can be made

to lie along the axis of measurement depends

on the fraction of the field of view the line

occupies and the least detectable increment

of field of view between some part of the

line and a reference mark on the viewer. A

series of experiments showed that, provided

the line fills at least 5 to 10 percent of

the field, misalignment need not be a significant source of measurement error.

Detailed line-width measurements were made

with each of the systems on transparent lines

of 2-, 5-, and 10-ym nom.inal width in an opaque field. Each microscope was calibrated

in magnification using a line spacing previously measured to be 34.927 ± 0.007 ym with

the NBS line-standard interferometer [82]. In addition, the TV-microscope system was adjusted to measure a line-to-space ratio of a value predetermined by an artifact supplied

by the manufacturer.

The results of the measurements appear in

table 13. Each of the entries in the table

represents the mean of ten measurements taken

one after another. To determine how much significance can be attributed to each entry,

measurements on each of three systems were

repeated ten times successively on nine occasions for measurements of the 5-ym line in

bright-field transmitted illumination. Typical data for a series of ten measurements on

a single occasion are summarized in table 14

which also shows the arithmetic means and sam- ple standard deviations of the means for nine

such series taken on different occasions.

The spread in the means of measurements taken

on different occasions cannot be explained by

the spread within measurements on one occasion. If the standard deviation associated

with n repetitions of a measurement made on a single occasion is a , the sample standard

deviation of the means of these n repetitions

taken on a number of occasions is a measure

of the total standard deviation [83]:

where a^y is the between-laboratory component

of the variance. The reproducibility can be

defined as the spread about the grand mean

which includes 95 percent of all test results.

The reproducibility associated with test results based on a single measurement on each

occasion is therefore [83]:

Reproducibility = ±2.77^ a^^ + a^^ .

The results of the study above suggest that

the reproducibility is about ±0.3 ym for the

filar eyepiece and about ±0.1 ym for both the

shearing eyepiece and TV-microscope system.

Therefore, it can be concluded that the discrepancies between measurements made on the same line with different microscopes and illuminations are statistically significant.

(F. W. Rosberry'^ and D-. A. Swyt*^)

55
PHOTOLITHOGRAPHY

Table 13 - Apparent Widths of Clear Lines in an Opaque Field

Microscope Apparent Line Width, ym

a. Bright-Field Transmitted Illumination:

Filar Eyepiece 2 25 5. 25 10.05

Shearing Eyepiece (Intensity) 1 95 4, 65 9.10

Shearing Eyepiece (Polarization) 2 00 4 85 9.40

TV Display 2 55 5 35 9.75

b. Bright-Field Reflected Illumination:

Filar Eyepiece 2 15 5 00 9.80

Shearing Eyepiece (Intensity) 2 45 5 35 9.75

Shearing Eyepiece (Polarization) 2 50 5 40 9.90

TV Display 2 55 5 25 9.75

c. Dark-Field Transmitted Illumination:

Filar Eyepiece 2 10 4.80 9.30

Shearing Eyepiece (Intensity) 2 15 5 15 9.65

d. Dark-Field Reflected Illumination:

Filar Eyepiece 2 .25 5 15 9.50

Shearing Eyepiece (Intensity) 2 .25 5 20 9.55

Table 14 — Representative Data for Line-Width Measurements Repeated on the Same Line

Microscope Filar

Eyepiece

Shearing

Eyepiece

(Intensity Type)

TV Display

Typical Results of Mean, ym 5.25 4.65 5.40

Ten Measurements

on Same Occasion Sample Std. Dev. , ym 0.077 0.014 0.009

Average Sample Std. Dev. , pm 0.078 0.015 0.010

Mean of Means of Mean, ym 5.22 4.65 5.40

Ten Measurements

on Nine Occasions Sample Std. Dev. , ym 0.072 0.034 0.028

Reproducibility, ym 0.28 0.10 0.08

56
PHOTOLITHOGRAPHY

6.5. Line-Width Measurement by Diffraction

Pattern Analysis

A study was undertaken to delineate the tolerances, design criteria, and limitations as- sociated with the measurement of line widths

by analysis of the diffraction pattern. The

instrument for performing this measurement is

shown schematically in figure 43 [84]. A

; laser beam is shaped into a line by means of two cylindrical lenses, and C2, and is focused on the plane, P, through an adjustable

aperture slit A. The specimen to be measured

is placed at the focus of the slit image, S, formed by a spherical 50x microscope objec-

\ tive lens with a numerical aperture of 0.85.

This lens also serves to focus the effective

source, P, on a diode array, D, located an

I appropriate distance behind the specimen.

In this arrangement, the diffraction pattern

] in the plane of the diode array is given by

' the convolution of the source image with the

i object transform. In the case of a gap of

j width w in an opaque background in the object

plane (fig. 44a) with ideal transmission charj acteristics (fig. 44b), the object transform

is given by wsin(Trfw) / (irfw) (fig. 44c). The

intensity distribution of this diffraction

I pattern (fig. 44d) is measured and digitized

(fig. 44e) , and a discrete Fourier transform

I is computed (fig. 44f ) . The intercept of

! this autocorrelation function on the horizontal axis occurs a distance w from the origin.

I When the instrument is used to measure opaque

' lines, it is necessary to consider the image

of the aperture slit which surrounds the line

in the object plane. In this case, provided

' that the line is reasonably well centered in

the image of the slit, the autocorrelation

I function has a region of positive slope to

the right of the vertical axis which, when

extrapolated back, intercepts the horizontal

axis a distance equal to the width of the

line from the vertical axis [85].

Experiments were carried out to determine the

sensitivity of the system to focus, aperture

size, source intensity, the nature of the object measured (line or gap) , and the width

of the object. The measurement process was

found to be sensitive to the centering of the

line in the aperture, the aperture width relative to the line width, and focus and location of the aperture. In addition, a theoretical analysis showed that if the optical density of the opaque regions is less than 3.0

the positions of the intercept can be significantly different from that expected on the

basis of ideal considerations. Variations in

the intensity of the illuminating laser do

not appear to affect the measurement.

Better linearity and repeatability were

achieved for measurement of opaque lines in

a clear field than for measurem^>.nt of clear

gaps (or slits) in an opaque field. This appears to be due in part to the different criteria used for centering the aperture on the

object plane; the criterion used in the case

of opaque lines is both more sensitive and more repeatable.

Analysis of the optical system shows that the

system response depends on the uniformity and

phase of the aperture illumination and on the

numerical aperture and quality of the objective lens as well as the characteristics of

the line or gap being measured. Anomalies

which have been noted in the autocorrelation

functions suggest that additional analysis

and instrumental improvements are required to

LASER —

BEAM ^

A Adjustable aperture slit

Cj, C2 Cylindrical lenses

D Diode array

P Focal plane of source

S Specimen, focal plane of slit

Figure ^3. Schematic representation of optical system for diffraction pattern analysis,

57
PHOTOLITHOGRAPHY

t(x)

X

a. Gap of width w. b. Transmission function of gap (t(x) = 1 for X between ±w/2, 0 elsewliere).

e. Discrete sampling of transform intensity. _f. Inverse transform of It (f) 1 ^ (A(x)a:

1-lx/wl for X between ±w, 0 elsewhere).

Figure . Schematic flow diagram for estimation of width of gaps.

interpret accurately widths of lines less

than 2.5 ym wide. Various instrumental modifications are being considered.

(H. L. Kasdan^)

6.6. Automated Photomas''. Inspection

Detailed analyses of the state-of-the-art automated technologies previously identified

(NBS Spec. Publ. 400-4, pp. 49-50) as applicable to inspection of photomasks for defects were completed. In this work the basic physical principles on which these technologies

are based were analyzed, limitations of each

of these technologies were identified, the

approximate times necessary for inspection of

defects on a 76- by 76-mm photomask with 3.8-

by 3.8-mm die patterns were calculated and

problem areas were identified. A summary of

the results of the analyses are given in ta- ble 15.

A basic criterion used in these analyses was

that 2-pm defects must be detected (NBS Spec.

Publ. 400-17, p. 32). The approach used in

the calculation of the time for 100-percent

inspection of a photomask was to calculate

the sampling rate which is the maximum number

of 2- by 2-ym areas that can be inspected per

second. Total inspection times were calcuWork performed at Recognition Systems, Inc. under NBS Contract No. 5-35890. NBS contact

for additional information: D. B. Novotny.

58
PHOTOLITHOGRAPHY

Table 15 — Characteristics of Automated Photomask Inspection Techniques

Technique

Inspection

Sampl i ng Rate, s"^

Total

Inspection

Time, h:min

Required

Al ignment

or Indexing

Accuracy, ym

Estimated

Cost, l<$

Optical Overlay 4.0 X 106 0: 5 ±1 150

Spatial Filtering NA ^0: 5 NA 50

Dual -Beam Flying Spot

Scanner 3.5 X 106 0: 7i ±0.2 150

Single-Beam Flying Spot

Scanner 2.0 X 105 '^.2:10 ±1 500

Microdensitometer 1.5 X 10'+ ^26: 4 NA 150

TV -Microscope 1.2 X 106 '\.0:40 ±0.4 130

lated from this fundamental sampling rate on

the basis of 1.44 x 10^ such areas on a 76-

by 76-mm mask, plus all the additional times

that are needed to completely scan the masks.

The total inspection times, listed in the ta- ble, do not include loading times. The estimated costs listed in the last column are primarily based on estimates given by persons

working with similar equipment and should be

interpreted only in an approximate comparative sense.

In the optical overlay method two die patterns are viewed simultaneously through two microscope objectives, typically of lOx magnification. This technique has been used f or

many years for visual observation of the registration of the different layers in a mask

set. In an automated version, pattern information from the same coordinate position on

two die patterns is read by two identical inline diode or CCD arrays [86-88]. The signal

from each device in one of the arrays is subtracted from the corresponding signal on the

other array. Resultant differences are interpreted as defects. For the analysis it was

assumed that each array consisted of 512 devices in an area 13 mm long by 25 pm wide and

that scanning and inspection occur simultaneously and continuously. For a typical array

scanning rate of 4 x lo6 devices per second,

the entire array can be scanned once while

the stage, traveling at 20 mm/s, advances 2.5

ym, the width of the area observed by the array. These conditions imply that one scanning sweep across the mask can be made every

3.8 s and that the entire mask can be scanned

in 59 sweeps. Under these conditions the total inspection, time is less than 5 min which

includes about 1 min for reversing the stage

motion and indexing the scan.

One serious problem with this method is the

critical alignment required between the two die patterns being inspected. Misregistra-

tion resulting from various sources, such as run-out errors between the dice in the mask,

lack of consistency in line widths, and yaws

in the stage motion of the inspecting device,

can cause differences in the outputs that are erroneously interpreted as defects. The 2-pm

defect detection criterion requires that the

two patterns be in register with deviations

nowhere greater than ±1 ym. In addition, the

method requires intense illumination, and, in common with all directly comparative technologies, it is insensitive to any defect which

appears on all die patterns.

The spatial filtering method is based on the

property that for coherent light the Fraunhofer diffraction pattern of the object,

which is represented by the optical Fourier

transform of the object, is produced by a lens at the focal plane of the radiation

source. Therefore, if a photomask is placed

in front of a lens and illuminated by a collimated laser beam, the Fourier pattern of

the object is produced behind the lens at

the focal plane of the source (cf. sees. 6.1.

and 6.5.). If a mask that blocks the light

radiation produced by the desired mask pattern geometry is placed at this focal plane,

only the diffraction pattern produced by defects passes through this filter. These diffraction patterns may be reconstructed by means of a second lens to form a real space

mapping of the defects.

59
PHOTOLITHOGRAPHY

While this technique appears straightforward,

it is not practical for the detection of 2-ijm

defects. Both variations in the refractive

index of the glass larger than 1.6 x lO"'* or thickness variations greater than 0.17 ym are sufficient to distort the diffracted wavefronts by a half wavelength and prevent the

expected interference effects. Although the

surface irregularity problems can be alleviated through the use of liquid gates, this

creates contamination and cleaning problems

which are more severe than the original defect problems. In addition, the filters must

also be made with tolerances tighter than

those with which the photomasks being inspected are made. This technique is further

hampered by focusing problems and difficulties encountered in registering the filter

with the transformed image.

With the dual-beam, flying-spot scanner, the

transmittances through equivalent portions of two die patterns illuminated by two simultaneously scanning laser beams focused to a spot size of about 3 ym are compared, A difference in the transmission is interpreted as

a defect.

This technology has been developed and a working prototype exists [89]. Although the time

required to actually inspect a 76- by 76-mm

mask is only slightly greater than 7 min, an additional period of about the same time is

required for loading the mask and registering the two beams. The beam registration is

held to 0.2 um; misregistrations as large as 0.4 ym, caused by step and repeat errors or edge raggedness , do not register spurious defects. As in the case of the optical overlay method, this technique is not sensitive

to defects repeated on each of the die patterns .

In each of the other three technologies analyzed, the information from each die pattern

is digitized and compared with information

representing the ideal or true mask pattern

which is stored in the memory of a computer.

These techniques are limited by the data

transfer rate and require a high speed computer as an integral part of the equipment.

Since the storage requirements for a 3.8- by

3.8-mm die are well within the capabilities

of a computer system with digital disk storage and an 8.7-MHz bit transfer rate, the

criteria of word transfer rate and needed

bits per word can be used to select viable

combinations

.

With a single-heam, flying-spot scanner, a 2-ym diameter light spot is scanned over the

pattern and the transmitted intensity is re- j corded with a photomultiplier tube. The spot

may be generated either by a laser or a cathode ray tube (CRT) . The accuracy and speed

of the optical density measurement depends on

the time response of the photomultiplier and

the number of detectable grey levels. A CRT

flying-spot detector utilizing a 5-ys dwell

time per inspection point for beam indexing

and stabilization and a four-level grey scale

was assumed for the analysis. These conditions can be satisfied by commercially available cathode ray tubes with a 76- by 76-mm screen and a square matrix of 1.6 x 10^ accessible points. If these points are serially

focused on a mask through a lOx reduction

lens, it takes 80 s to scan each 7.6- by

7.6-mm field of view which includes four com- plete 3.8- by 3.8-mm die patterns. Since

there are 100 such fields of view on the mask,

about 2h h are required for the inspection.

Microdensitometers are designed primarily to measure the optical density of a pattern as

a function of position rather than to inspect for mask defects. To inspect for 2-ym

defects, the die pattern must be scanned with C a 2-ym spot in 2-ym swaths. If the stage i- travels at the highest practical velocity of

30 mm/s, a data acquisition rate of 1.5 x 10** words per second is required. This in turn

requires a photomultiplier response time of

less than 70 ys for accurate optical density

mapping. If these conditions can be achieved,

the scan time per individual die pattern, not

including stage indexing time, is about 4 min.

The total time to inspect the 400 die patterns on the mask is about 26 h.

There exists the possibility of reducing this scan time by scanning with a rectangular aperture. If a 2- by 100-ym aperture is used the

inspection time could be reduced by a factor

of 50 to about 32 min. However with this approach, serious questions arise concerning

the response time of the photomultiplier and

its ability to make accurate density measurements under these conditions. Furthermore,

the integrated intensity over a 2-ym scan length is changed by approximately 1 percent

for every 0.02 ym of misregistration of any

transparent to opaque edge extending the

length of the aperture. This effect combined

with those from scanning table yaws, die runout, and other registration errors commonly

found in photomasks would lead to the detection of copious spurious defects.

The analysis of the TV-microscope method has

been reported in detail (NBS Spec. Publ. 400-

17, pp. 32-36). The results listed in table

60
PHOTOLITHOGRAPHY

15 are for the case of a vidicon tube which

scans one 0.4- by 0.4-nnn field of view in

1/15 s.

The only two inspection systems commercially

available at the present time are modifications of systems that were developed for other purposes. One, a TV-microscope system, was originally designed to scan films of rocket launches. It is a defect inspection system, but its slow speed is a major factor inhibiting use. The second, a microdensitometer, was originally designed to scan astronomy films. It is most widely used for dimensional analysis of photomasks. Other defect

inspection systems have been developed by

several large corporations but these are not

available on the commercial market.

Masks can also be inspected for dimensional

errors with the systems analyzed above. Two basic limitations common to all approaches

are the dependence on suitable line-width

standards and the need for a thorough understanding of the optical system employed. It was concluded from this work that the search

for dimensional errors and the search for defects differ significantly, and that these

functions should be performed independently.

Speed is the most important consideration in

the automated search for visual defects; the

analyses showed that few systems as presently

envisioned have economically attractive inspection times.

To provide a means for evaluation of various

automated mask inspection systems, work was

initiated on the design and fabrication of a test mask with intentionally added defects.

This can be used to determine the minimimi detectable defect size, inspection speed,

missed defects, false indications, and differences in observation of pinhole and spot defects. In addition to comparing the characteristics of different machines, such a mask

can also be used on a regular basis to verify the performance of an operating system

[89].

An MOS metallization pattern of moderate com- plexity was chosen as the basis for this test mask. The smallest line widths or spaces in

this pattern are 0.30 mils (7.6 ym) . The

width of the pattern is 10.5 mils (0.27 mm)

and the height is 7.9 mils (0.20 mm). Each

die consists of a 10 by 10 array of this basic pattern. The final mask consists of a 14 by 14 array of dice with a center-tocenter spacing of 150 mils (3.81 mm) in each

direction. Of the 196 dice in the array, 15 contain intentional defects.

These 15 defective dice are inserted into the

array by the use of three extra lOx reticles

each of which contains 36 intentionally introduced defects including pinholes, spots,

breaks, bridges, intrusions, and protrusions

located in different parts of the pattern.

Each of these reticles is stepped into the

array at five different places, more or less

randomly located, so that each mask contains

540 defects. Since the area covered by the

14 by 14 array is 28.5 cm^, the defect density is about 19 cm~^.

Initial versions of these test masks have

been fabricated and are being evaluated. Preliminary test results suggest that although

not all of the smallest pinholes and spots

printed, a range of defects down to about

2 ym is available on the mask. It is intended that these test masks be subjected

both to additional visual inspection and to inspection on various automated inspection

systems at cooperating laboratories in order

to characterize them more fully and evaluate

their utility. (D. R. Ciarlo''", P. A. Schultz"*", and D. B. Novotny)

61

Lawrence Livermore Laboratory , Livermore

,

California 94550. Work conducted under

NBS Order No. 503183.
7. TEST PATTERNS

7.1. Square Array Collector Resistor

The square array, four-probe, collector resistor structure (NBS Spec. Publ. 400-17, pp. 25-

26) [21] was further analyzed for the case where the probe spacing is comparable with the

lateral dimensions of the specimen. It was assumed in the analysis that the specimen is

square, has nonconducting boundaries, and is thin with respect to the probe spacing, and

that the probe contacts lie on specimen diagonals. This configuration is one which may be

encountered in measuring the sheet resistance of the collector region of an integrated circuit .

An early attempt to solve this problem [90] was incorrect because the solution does not approach the van der Pauw [91] solution for the case where the probes are on the periphery of

the specimen. Other early solutions based on conformal transformation [92] and the method

of images [93,94] are correct but the results were not presented in a convenient form for

engineering use. The present solution is based on the method of images in a complex plane.

The results are expressed in the form of algebraic equations which are easy to evaluate and

converge rapidly.

The geometry analyzed is shown in figure 45. The current was assumed to be two-dimensional

in the plane of the specimen. The current carrying probes were assumed to be a point source

and a point sink so that the boundary conditions can be satisfied by a system of images

placed sjoranetrically about the boundaries. Complex variables theory was used to solve for

the potential difference, - \/2. Omitting the details of the solution, the result is:

Vi - V2 = (pI/277W)C

where

00 _ (coshx+1 ) (coshx-cosx) i (coshx+cosha) ^

[ (coshx-coshacosx) ^+slnh^asin^x]

" 2(I-cosx) ^ ^ ^

(cosha+l)2(cosha-cosx)2

m=l

a = mir, and x = sir/d.

This result goes to the proper limits for the cases where the specimen is an infinite sheet

(d >> s) and where the probes are on the periphery of the specimen (d = s). In each of

these cases the series in the expression for C is identically equal to zero. In the former

case, C approaches ln2; this is the result found by Uhlir [95] for a square four-probe array

on an infinite sheet with a nonconducting back side (NBS Spec. Publ. 400-25, pp. 39-40). In

the latter case, C approaches 21n2; this is the symmetrical van der Pauw solution [91].

When neither limiting case applies, C can be computed to an accuracy of 0.1 percent with the use of only the first three terms in the series.

Correction factors can be applied to the limiting cases to determine the region over which

the limiting case expressions are valid. When d >> s, the resistivity is given by

For large, but finite, ratios of d to s , a correction factor C5 = C/ln2 can be calculated

such that

P = Pco/Cs . (14)

The percentage error introduced by using the uncorrected value poo, is

Eco = 100(poo - p)/p = 100(C5 - 1) . (15)

For d/s ^23, the uncorrected value exceeds the true value by less than 1 percent.

When d

~ s the resistivity is

62
TEST PATTERNS

P = Pi/Ce (16)

where pj = 2poo and Cg = C/21n2, In this case

the percentage error introduced by using the

uncorrected value, p^, is

ei = 100(pi - p)/p = lOOCCe - 1) . (17)

For d/s between 1.0 and 1.35 the uncorrected

value is less than 1 percent below the true value. The correction factors C5 and Cg and

the errors Eoo and ej are tabulated in table

16.

These results can also be extended to other

cases. The present results are approximately

correct for the case in which the probe locations are rotated away from the diagonals of

the square even for small values of d/s [92-

94]; the differences are such that Pi approximates the true resistivity for somewhat larger values of d/s. For example, for d/s = 1.5,

Table 16 — Correction Factors for a Square Four-Probe

Array on a Thin Square Specimen

d/s C5 £00 Ce El

1.0 2 .0000 100.0 1 0000 0.0

1.1 1 .9997 100.0 0 9999 0.0

1.2 1 .9967 99.7 0 9984 0.2

1.3 1 .9880 98.8 0 9940 0.6

1.4 1 .9721 97.2 0 9860 1 .4 1.5 1 .9491 94.9 0 9745 2.6

1.6 1 .9199 92.0 0 9600 4.0

1.7 1 .8862 88.6 0 9431 5.7

1.8 1 .8493 84.9 0 9247 7.5

1.9 1 .8107 81.1 0 9054 9.5

2.0 1 .7716 77.2 0 8858 11 .4 3.0 1 .4500 45.0 0 7250 27.5

4.0 1 .2773 27.7 0 6386 36.1

5.0 1 .1849 18.5 0 5924 40.8

7.0 1 .0977 9.8 0 5488 45.1

10.0 1 .0487 4.9 0 5244 47.6

15.0 1 .0219 2.2 0 5109 48.9

20.0 1 .0123 1.2 0 5062 49.4

25.0 1 .0079 0.8 0 5040 49.6

30.0 1 .0055 0.6 0 5027 49.7

00 1 .0000 0.0 0 5000 50.0

63
TEST PATTERNS

the value of Cg for the case of 45 deg rotation, so that the probes lie on the bisectors

of the sides of the specimen, is 2.2 percent

larger than the value for the probes on the

diagonals. In addition, similar techniques

can be used to solve the problem of a square

array on a rectangular specimen.

(M. G. Buehler and W. R. Thurber)

7.2. Sheet Resistor Test Structures

Additional analysis of cross-type sheet re- sistor test structures was completed. These

analyses were carried out with the use of the same numerical analysis used previously in

the analysis of orthogonal quadrate crosses

and offset quadrate crosses (pinwheels) (NBS

Spec. Publ. 400-19, pp. 44-45) [96]. Two limiting cases of the cross structure were studied. The relative geometrical errors for

these cases were determined by computing the true sheet resistance, R^, and comparing it

with the sheet resistance calculated from the

symmetrical van der Pauw formula [91]:

R (vdP)

s (^/ln2)(AV/I), (18)

where AV = " ^2 for a current I passed into li and out of 1 2 as shown in figures 46

and 47. The error, E, is defined as

E = [R - R (vdP)]/R

s s s (19)

Figure kS. Normalized sheet resistance error resulting from shorting contacts on the

periphery of a square test structure.

Figure 47. Normalized sheet resistance error resulting from shorting contacts at the

ends of the arms of a Greek cross test struc- ture.

One limiting case, where the arm length is zero, is illustrated in figure 46. Over most

of the range the error is found to fit the em- pirical relation

E = (O.595±O.O15)(D/S)l"82±0.03 (20)

When D/S = 0, this case reduces to the symmet-:

rical van der Pauw structure and the error

goes to zero. If D/S < 0.1, the geometrical

error associated with this structure is less

than 1 percent. The integral equations for

this limiting case had been formulated by

Wick [97] but he did not solve and evaluate

the equations.

Another limiting case, where D = S, is the

Greek cross.* This is illustrated in figure

47. Except for a very short arm length

(A/S < 0.1), the error is found to fit the

empirical relation

"The conclusion that the Greek cross is a van

der Pauw structure was reached intuitively

and independently by W. E. Ham, RCA Laboratoj

ries (private communication). |i 64
TEST PATTERNS

E = (0.590±0.006)exp[-(6.23±0.02)A/S] . (21)

When A/S > 0.65, the geometrical error as- sociated with this structure is less than 1 percent. If A/S > 1.02, which is easily

achieved in practice, the error is less than

0.1 percent.

The technological implication of this test structure is found in the realization that

the sheet resistance of a very small region

can be accurately measured because the width

of the arms of the Greek cross is limited only by the minimum line width achievable by

the photolithographic technology utilized in

device fabrication.

(M. G. Buehler and J. M. David)

7.3. Test Pattern for TTL Technology

One approach to assurance of built-in component reliability involves the use of a process validation wafer (PVW) . This type of wafer is intended to evaluate process and cir-

, cuit parameter uniformity, and to measure the

I occurrence of random faults. The PVW is an

entire wafer of test structures and is intended to be fabricated along with product wafers on a periodic basis. This approach is a

I supplement or alternative to a test pattern

j substituted for an IC chip on a product wafer.

As a process characterization tool the PVW

can be used to qualify vendors and can serve

I as a circuit acceptance criterion. The PVW

1 concept has the potential of greatly reducing

the user-imposed requirements that now accom-

, pany a high-rel component purchase. Various

forms of this approach are currently being

used by the British Post Office [98] and by a few U.S. Government agencies in the procureI ment of reliable, custom components.

I As an illustration of a microelectronic test

! pattern under development for use as a proj cess validation wafer, consider the TTL test

I pattern shown in figure 48. This pattern

! consists of various test structures which are

!' intended to be tested electrically with a

I high-speed dc wafer tester. The test struci'

• tures are arranged so that all structures can

j be probed by a 2 by 10 probe array (NBS Spec.

Publ. 400-25, pp. 41-43). This modular arI rangement allows the structures to be called

I from a computer library and arranged quickly

into a new test pattern to fit available

space. In addition, the modular concept alii; lows for standardization of each test structure in its entirety including its probe pads

which must be an integral part of the test structure

The TTL pattern is divided into quadrants,

each 1.7 mm (67 mil) by 1.3 mm (51 mil). The

lower left quadrant contains strings of contacts and metal runs of varying length over oxide steps. The lower right quadrant contains multi-emitter transistors of varying

size. These structures are intended to check

for random faults such as contact window problems, incomplete metal coverage, and emittercollector shorts (pipes). The two upper quadrants contain process control structures, circuit elements (both transistors and resistors), and NAND gates. Among the process control structures are several versions of the cross sheet resistor (see sec. 7.2.).

At the present time artwork for the mask set

for this test pattern is being prepared. On

completion of the mask set, wafers are to be

fabricated and correlations established between the properties of appropriate test structures and the circuit characteristics.

(D. E. Sawyer, D. A. Maxwell, J. M. David,

and M. G. Buehler)

7.4. Test Pattern Design and Analysis for

SOS/LSI

The mask set for the SOS/LSI test pattern

(NBS Spec. Publ. 400-25, pp. 44-45) was com- pleted and two initial lots of wafers were processed with the previously outlined singleepitaxial, p^-polysilicon gate process. Preliminary observations were made on some of

the optically accessible structures in the

type V pattern and on the p-channel transistor in the type II pattern (NBS Spec. Publ.

400-19, p. 47).

The two-level interaction test structure for

p'*"-polysilicon over epitaxial silicon is

shown in figure 49. The structure consists

of polysilicon lines of various widths extending from the sapphire up onto a silicon island. The design line widths (in mils; 1 mil

-25 ym) are indicated outside the pattern.

It can be seen that no polysilicon remains

for designed dimensions of 0.1 mil (2.5 pm) or less. This suggests that the polysilicon

lines are smaller than their designed dimensions by about 0.05 mil (1.2 pm) on a side.

It can also be seen directly that the dimensions on top of the epitaxial silicon and on, top of the sapphire are nearly identical,

that a severe distortion in the polysilicon

occurs at the edge of the epitaxial silicon,

and that a discontinuity in the polysilicon

at the edge of the epitaxial silicon exists

for lines with designed width of less than

about 0.3 mil (7.6 urn)
TEST PATTERNS

0.00 IS. 00 30->aO <S.O0 60.00 75.00 90.00 lOS.OO 120-00 13S.00 ISO. 00 16S.00 180.00 19S.0D 210.00 22S.OO 240.00 2SS.00 270.00 MICROHETER "lO'

Figure k8. Junct ion- i sol ated TIL test pattern. (Upper left quadrant: process control structures, circuit elements, and NAND gates; upper right quadrant: process

control structures; lower left quadrant: contact resistors and metal step coverage

string; lower right quadrant: mul ti -emi tter transistors.)

66
TEST PATTERNS

Figure 49. Two-level interaction test structure.

j| The two-level alignment test structure is

! shown in figure 50. This structure consists

I of a trapezoidal polysilicon region over a

' rectangular silicon island. It is clear that

no difficulty exists in determining the intersection point where the trapezoid crosses the

rectangle. The index markers are placed on

\ 1.0-mil (25-Mm) centers. The difference in

I intersection points on the two sides of the

.1 trapezoid in figure 50a is approximately 0.4

mil (10 ym) which corresponds to an actual

I misalignment of about 0.02 mil (0.5 ym)

i An orthogonal pair of structures can be used

II for complete alignment measurement as shown

j in figure 50b. Even when the edge crossing

I is not ideal, as shown in figure 50c, the

J crossing points can be determined quite precisely. The resolution of the structure appears from these results to be about 1 pin.

I (25 nm) when using an ordinary microscope.

i| This structure can also be used as a very aci curate ruler to measure the change in the latI eral size of an object (such as a wafer) after some processing step or with temperature,

i In this case, the upper level would not nec-

'] essarily be printed but could be simply ob-

] served.

I An example of some preliminary results from

ij the type II tests is shown in figure 51.

'i This plot shows the distribution of drain

II current for a p-channel transistor in saturaI tion across approximately 1.5 in. (38 mm) of

S a 2-in. (51-mm) diameter wafer. Several very

ji interesting features of these data are noted:

1. A significant number of the devices

have no output on the scale used;

a. Single structure.

J. Orthogonal pair.

c. Single structure with non-ideal edge

cross i ng

Figure 50. Two-level alignment structures.

there is a tendency for these zero- output devices to cluster together;

2. The distribution observed shows a very "fuzzy" nature;

3. Gradual trends in the data over relatively large distances are observed;

approximately ±20 percent total varition is seen across the wafer for

operating devices

The interpretation of these data in detail is

not clear at present, but the following com- ments are relevant.

67
TEST PATTERNS

« 1.5 INCH

EDGE (FLAT)'

«±5% UNCERTAINTY

APPROXIMATE

SIZE OF "local"

REGION

0-

POSITION (6.0 MIL INCREMENTS)

Figure 51. Distribution of p-channel saturation current in heavy inversion along the

diameter of an SOS wafer perpendicular to the edge flat.

The zero-output devices are probably not caused by probing prob-lems

since low currents were detected on

the same devices under different

bias conditions. Furthermore, nchannel devices in the same row ex- hibited many fewer instances of

zero-output devices.

The fuzziness is probably caused

primarily by differences in the geometry of the channel regions.

3. Within a local area, approximately

50 mil (125 ym) in extent, an uncertainty of about ±5 percent exists.

Obviously, the most interesting observation

here is the zero-output devices since these

would be fatal to virtually any circuit. The

personality of the system is readily apparent

from data of this type taken over an extended

region of the wafer which provides considerably more information than data on one or a small number of single devices. (W. E. Ham''')[

tWork conducted at RCA Laboratories under

NBS Contract No. 5-35916.. NBS contact for

additional information: M. G. Buehler.

68
8. ASSEMBLY AND PACKAGING

8.1. Nondestructive Test for Beam-Lead Bonds

Detailed experiments were carried out to evaluate the acoustic emission test (NBS Spec.

Pubis. 400-12, pp. 31-32; 400-19, pp. 48-50;

and 400-25, pp. 46-49) [99] as a nondestructive means for evaluating the quality of beamlead and other types of multiple bonds. The

measurement system is capable of detecting

acoustic emission signals barely above the

average noise level of the preamplifiers and

considerably below various system and line

transients. Most acoustic emission detector

output signals produced in the present experiments were in the range of about 10 to 100 yV

and were easily captured by the equipment.

Because of the variety of gain adjustments

possible (preamplifiers, pulse capturing

equipment, and oscilloscope), the vertical

scale of most acoustic emission oscillograms

i has no particular significance. The only imI portant consideration is the signal-to-noise

ratio, and this can be easily observed from

the photographs. Signal sizes given in the

I discussion below refer to the detector output.

1 Putl Tests — In order to demonstrate the senj sitivity of the method, several beam-lead deI vices were tested by cutting all but one of

the beams and pulling the uncut beam to dei struction with an electrolytically etched,

' 150-ym diameter tungsten hook inserted into a

1 dab of silicone rubber on top of the device.

! Acoustic emission signals picked up by a substrate detector are displayed in figure 52. In the case of a well-bonded beam (fig. 52a),

failure occurred by a break at the bond heel.

The clipped waveform peaks indicate that the

i substrate detector output was significantly

I greater than 1 mV (peak-to-peak) during the

' initial part of the break. The signal from

such breaks generally continues erratically

for several times the time interval shown in

the figure. In the case of a well-bonded

j beam with a weak anchor (fig. 52b), failure

j occurred by peel at the anchor at a force of

, about 30 mN (1 gf = 9.8 mN) . The peak-to-

' peak detector output in this case was about

i 0.3 mV.

if Pull tests were conducted on several well-

! bonded devices with weak anchors. A series

j of short bursts was observed starting at

I about 15 millinewtons per beam (about half

I the peel strength of the anchors) . Similar

j well-bonded devices with strong anchors produced no signal until a pull force of about

25 millinewtons per beam was applied; the

, bursts in this case were longer and higher in

I amplitude than the devices with weak anchors.

69

Several groups of commercially available devices from four different manufacturers were

tested. Devices from three of these sources produced no acoustic emission signal until

they were stressed to about 25 millinewtons

per beam as noted above. However, devices

from the fourth source produced large bursts

of acoustic emission, such as those shown in

figure 53a, when they were stressed to only

about 10 millinewtons per beam; these bursts

increased in number and amplitude with increasing stress. Examination of these devices in a scanning electron microscope following stressing to 10 millinewtons per beam

revealed no evidence of mechanical failure.

However, examination following stressing at about 25 millinewtons per beam revealed elongation of the beams, separation of the relatively thick titanivun layer, peeling of the

anchors, separation of the silicon nitride

from the beams or silicon, and broken chips

of silicon at the anchor as shown in figure

53b. Any of these observed degradations

could be responsible for the acoustic emission signals. These problems can be attributed to design deficiencies; they were observed on three different device types from

the same source on lots purchased 18 months

apart. It should be noted that a normal destructive pull-off [100] or push test (NBS

Tech. Note 806, pp. 34-35) would not have revealed any problems with the structures because the beams ultimately broke with forces

typical of mechanically strong devices. The

consequences of the poor mechanical integrity

on device performance or reliability were not

established

In a large series of pull tests on devices

bonded to substrates with chrome-inhibited

bonding pads (NBS Spec. Publ. 400-25, pp. 46-

47) , it was found that a force of 10 to 15

millinewtons per beam was required to produce

acoustic emission from one or two weaklybonded beams on an otherwise well-bonded device. On the other hand, a force of only 5 to 10 millinewtons per bond was often sufficient to produce acoustic emission when all

beams were poorly bonded; such devices would

pull off completely at forces of 10 to 15

millinewtons per beam.

Push-Down Test on Devices Encapsulated in Silicone Rubber — Both well-bonded and poorlybonded devices were encapsulated, except for

the top of the chip, with a silicone rubber

thinned with xylene [99] and allowed to cure. For testing, the device was mounted on a vacuum chuck and the top of the chip was uniformly pressed downward with an acoustic
ASSEMBLY AND PACKAGING

a. Wei 1 -bonded beam which failed by breaking b. Well -bonded beam which failed by peeling

at the heel at a force of about 35 mN. at the anchor at a force of about 30 mN. Horizontal scale: 20 ys/div. Horizontal scale: 16 ys/div.

Figure 52. Acoustic emission signals from single beams pulled to destruction.

a. Acoustic emission signal at a pulling

force of about 12 millinewtons per beam.

Horizontal scale: 20 ys/div.

A Gold beam

B Separated titanium layer

C S i 1 icon ni tride

D Broken piece of silicon

E Portion of silicon chip

b. Scanning electron micrograph following ap

plication of a pulling force of about 25

millinewtons per beam.

Figure 53- Beam-lead device with poor mechanical integrity.

emission probe coated with silicone rubber

and mounted in a force gauge as shown in fig- ure 54. Well-bonded devices with good mechanical integrity required forces greater than

4.4 N on a 16-beam chip before producing

acoustic emission bursts; well-bonded devices

with poor mechanical integrity required 1.9

to 2.4 N. Devices of both types which had

one or more weak bonds generally emitted

their first bursts at forces of 0.5 to 1.5 N.

Appliaation of Force to Beams Only — Two probes, tipped with silicone rubber and designed to avoid contact with the chip, were

used to apply a do^imward force on the horizontal portion of the beam which extends outward from the chip. Simple resolution-offorces analysis of a single beam indicates

that nearly all the force is applied as a torque which tends to peel the anchor. Thus,

probing a single beam or a single row of

beams along one side of the chip provides an anchor adherence test; if all beams are probed simultaneously, the torque cancels and

the force is applied only to the bond system.

Individual beams were probed with a tungsten

carbide probe, which had a 75-ym diameter

flat region tipped with silicone rubber and

a lead-zirconate-titanate type acoustic emission detector, in order to establish emission

patterns for both anchor and beam failures.

70
ASSEMBLY AND PACKAGING

Force gauge

Force angle control

Probe acoustic emission detector

Acoustic waveguide and force probe

Substrate holder with vacuum hold

down and substrate acoustic emission

detector

Figure 5^. Apparatus for applying upward or downward force on beam-lead devices and for

detecting any resulting acoustic emission.

Acoustic emission traces from the probe and

substrate detectors are shown for typical

cases in figure 55. The traces resulting

from application of a downward force of about

20 mN to a beam with a weak anchor are shown

in figure 55a. This anchor ruptured at an applied force of about 34 mN; a well-bonded

beam with a strong anchor would typically

curve downward until it touches the substrate

at an applied force of 60 to 100 mN. For anchor failures, the probe detector usually produces the larger signal as shown in the figure. When a very weak bond is probed with a force of 30 to 40 mN, the signal intensities

are usually reversed, as shown in figure 55b.

Although these patterns are typical, it

should be noted that these failure modes may

at times produce entirely different patterns.

Some experiments were performed using the single probe to search for acoustic emission

from silicon nitride fractures. In general,

signals from fracture of thin ('^200 nm) silicon nitride skirts were not detected because

the emission is thought to occur at higher

frequencies and because the coupling into the

chip and substrate is poor. This was verified by coating a single beam probe with viscous acoustic coupling compound and moving it

sideways into an extended silicon nitride

skirt. A small acoustic emission burst was recorded in the probe detector (operating at 1.1 MHz) but not in the substrate detector

(operating at 375 kHz).

Experiments were also carried out using as a probe a modified beam-lead bonding tool. The

Signals from a well-bonded beam with a weak anchor at a force of about 20 mN.

b. Signals from a very weakly-bonded beam at

a force of about 40 mN.

Figure 55. Acoustic emission signals from individual beams subjected to downward force on

the horizontal portion of the beam. (Upper trace: probe detector; lower trace: substrate de- tector; horizontal scale: 20 ys/div.)

71
ASSEMBLY AND PACKAGING

BEAM

Figure 56. Beam-lead bonding tool modified

to apply force to the beams without contacting the chip.

inner walls of the shortened sides of the

tool are about 25 ym larger than the silicon

chip on all sides. The rim of the probe was

coated with silicone rubber and molded with

a deep undercut pattern of the beams as shown

in figure 56. In use, the probe is pressed

down on top of the beams; the lower portion

of the silicone rubber is forced against the

substrate and bulges underneath the beam in

such a manner as to lift the downward curving

portion of the beam, A weak beam will be

lifted upward resulting in acoustic emission.

If the bugging height is uniform and if the

probe is properly aligned to the chip and is

perpendicular to the substrate, about 1.5 N can be applied before the device collapses.

Most of this force is dissipated by compressing the silicone rubber and only a small

amount is actually applied as a lift force to the beams. This probe concept appears to offer the best method of stressing weaklybonded beams. The larger signal appears in

the substrate detector, partly because the

bond is more closely related to the substrate

and partly because the coupling from the

beams through the silicone rubber to the

probe is relatively poor. However considerably more development is required to increase

the operating life of the probe, and to obtain reproducible stressing forces and better

coupling to the probe detector. This type of

probe is only effective on devices with relatively uniform bugging height and bonds which

extend at least 75 to 100 ym from the chip.

Unlike the other probes employed in these

tests, this probe cleanly strips off the free

silicon nitride skirt in all areas away from

the beam and anchor. The silicon nitride on

top of the beams is not disturbed; the effect

of the damage which does occur on device performance and reliability has not been determined [99].

Use of Acoustic Emission in Other Areas — Ex periments were conducted to determine the fea

sibility of applying this technique to the

evaluation of bond integrity on film carrier

and reel systems. Tests on three different

types of such systems indicated that the

acoustic emission technique can be effectively used for this purpose [99]. In one case,

a signal was obtained which could be correlated with the separation of a single solder

bump. In a second case, several weak bonds

in a gang-bonded device produced an emission

burst. In a third case, a bond which was mechanically solid despite the fact that it had

a poor visual appearance emitted no bursts.

Acoustic emission bursts were also observed

when a weakly-bonded chip capacitor in a hybrid circuit was subjected to a downward

force of about 2 N. Preliminary experiments

suggest that acoustic emission can also be

used to supplement the nondestructive wire

bond pull test, both to assist in determining

the maximum force to be applied and to assure

the nondestructive nature of the test.

(G. G. Harman)

8.2. In-Process Bond Monitor

A series of experiments was carried out to determine whether there is any correlation

between the vibration amplitude of an ultrasonic bonding tool during bonding and the

bond quality as measured in terms of failure

modes and the mean and standard deviation of

the pull test.

The experimental set-up used was similar to that employed previously [101] except that

since only the envelope of the nodal pattern

was monitored there was no need for the two phase-delay pulse generators used in the previous work. The tool used in these experiments was a long 0.828-in. (21.0-mm) tungsten

carbide tool with a 4-mil (100-ym) foot

length. Also, the capacitor microphone was

fitted with a stainless steel taper tip having a 25-mil (0.64-mm) diameter hole. This

larger than usual taper tip was chosen for

two reasons, both of which enhance its us- ability in an industrial environment. First,

it is quite rugged and less easily damaged.

Second, it gives an acceptable signal at low

gain and hence is less susceptible to picking

up any extraneous noise.

In all the experiments reported here, the

node of the unloaded tool was found as follows: the tool was brought into contact with

the wire to be bonded and the power dial set72
ASSEMBLY AND PACKAGING

Table 17 — Pull Test Results for Monitored Ultrasonic Wire Bonds

Power Setting Failure Mode Pull Strength, Sample Standard Deviation,

^ gf(mN) gf(niN)

2.0 lift-off 3.06 (30.0) 2 54 (24.9)

5.0 heel breakage 9.23 (90.5) 0 97 (9.5)

7.0 heel breakage 8.81 (86.3) 0 75 (7.4)

10.0 heel breakage 6.3 (62.0) 1 0 (9.8)

Control mostly heel breakage 9.98 (97.8) 0 58 (5.7)

ting was turned down to its lowest position

(1.0); the power was then turned on and the

microphone was raised and lowered along the

length of the tool until a minimum signal was

obtained. This point was checked occasionally and was found to be fairly stable.

The first series of experiments employed aluminum pads of good bondability. On a single

substrate, two different groups of aluminum

bond pairs were fabricated. The first group,

the control group, was made using the following machine settings for both the first and

second bonds: power setting, 5.0; bonding

time, 50 ms; and bonding force, 0.25 N. The

control group bonds were pulled to destruction in order to establish that the powertime-force combination used in its fabricai tion produced acceptable bonds. The predomi-

|j nant failure mode found in the pull test was

] heel breakage. The overall mean, x", and sample standard deviation, s, were 9.98 and 0.58

j gf (97.8 and 5.7 mN) , respectively; these val-

: ues are indicative of a satisfactory bonding

process which is under control.

j The second group, the test group, was made

j with the same settings except that the power

j! setting and bonding time for the second bond

I were set to zero. The second bond of each

j: bond pair in this group was then made while

't the envelope of the nodal pattern was moniI tored. The force and time employed were the

d same as were used in bonding the control

I group; groups of eight bonds each were made

I' .with power settings of 2.0, 5.0, 7.0, and

10.0.

, The results of the pull test are given in taI ble 17. The pull strengths of the bonds made

! with a power setting of 2.0 are indicative of

' a bonding process which is out of control.

Typical microphone outputs (signatures) are

,| shown in figure 57. The interesting feature

': of the signature of bonds made with the lowest power setting is the ringing characteristic at the beginning of the bonding cycle and

the gradual tapering off of the tool vibration amplitude. The bonds made with higher

power settings showed no evidence of ringing;

all these bonds failed at the heel. This suggested that the microphone signal may be more sensitive to weld formation rather than to heel weakening due to excess deformation. It appeared likely that the nodal pattern contains only information concerning the quality

of the weld. As the power increases, the interfacial weld strength also increases. However, at the same time the bond heel is further deformed which decreases its strength

under the application of the pull test (NBS

Spec. Publ. 400-19, p. 51).

Other experiments made with the same tooltransducer-power supply combination yielded

similar results . When the experiments were

repeated with a tool with a 2-mil (50-ym)

footlength similar results were also obtained. When the experiments were repeated

with different power supply-transducer com- binations, however, no ringing was observed, even for the cases where the bonds failed by

lift off.

Since the node rises during the loading of

the tool during bonding, the vibrational am- plitude at various positions below the node

of the unloaded tool was studied at various

power settings with the original transducerpower supply combination. If there were any

ringing in the unloaded tool below the node

it was expected to travel up to the position

of the node during bonding where it could be

detected by the microphone; in this case the

ringing phenomenon would be suspected of being an artifact of the particular equipment

being used. If ringing were not there, then

there would be some reason to believe that

ringing is a true signature of the lift-off

failure mode. Ringing was observed 12 and 52

mils (0.30 and 1.32 mm) below the mode for a
ASSEMBLY AND PACKAGING

Power setting: 2.0. Power setting: 5.0.

c. Power setting: 7.0. d. Power setting: 10.0.

Figure 57- Typical signatures of ultrasonic aluminum-aluminum wire bonds made with different power settings.

power setting of 2.0 but not for power settings of 5.0 and 10.0.

These investigations indicate that the patterns which seemed to be typical of lift-off

failure mode bonds were really artifacts of

the particular coupling in the power supplytransducer-tool system when power is first

applied to the tool. Consequently, it is

doubtful whether ringing is a true signature of the lift-off failure mode, and so further development of this technique has

been abandoned. (J. H. Albers)

8.3. Radioisotope Leak Test

The first exploratory round-robin evaluation

of the radioisotope method for testing hermeticity of semiconductor devices (NBS Spec.

Publ. 400-8, p. 40) [102] has been completed

and the results analyzed. In undertaking the test it was resolved that the first priority

was to determine the precision (interlaboratory agreement) inherent in the radioisotope

method when applied to production devices and

that a data base was required upon which a suitable draft standard could be developed.

The resultant experiment was principally an accomplishment of the industry itself; NBS

supplied measurement guidelines, calibration

services, and participation in the direction

and refereeing of the tests.

Ten industrial organizations participated in

the experiment which was conducted according

to strict metrological guidelines. To this

end one specific type of equipment was used, one specific test sequence followed, all

equipment serviced and calibrated prior to test, specimens neutralized between tests,

and all testing refereed. A detailed trial

test sequence was formulated and evaluated

experimentally with 100 production integrated

circuit packages; the final test sequence was- modified in accordance with the results of

this trial. Since a reference transfer vac- uum gauge was essential for the determination

of specific activity of the test gas at each

site, a gauge was procured by and calibrated

at NBS.

Test packages were culled from large lots of : the selected type (14-lead ceramic dual inline) to provide a selection of leaks which

ranged from 10"^ to '\^10~'* atm-cm^/s (10~^ to

'^'10~^ Pa-m^/s) as indicated by this method.

One hundred test packages were circulated to j each of the participating test sites; all

packages survived the entire experiment . Al] 74
ASSEMBLY AND PACKAGING

tests were refereed. Following resolution of

several discrepancies, the ten measurements

on each test package yielded leak rates which

agree to within a fraction of the decade in

which the mean value occurred [102]. This

contrasts with prior results where the best

agreement was one or more orders of magnitude.

This experiment has demonstrated conclusively

that an interlaboratory comparison of hermetic packages can be accomplished under appropriate metrological guidelines. The inherent precision of production lipe radioisotope hermetic test equipment was determined on a stock integrated circuit item, and

simultaneously it was shown that package leakage can remain stable. There is now an understanding of the variations that can occur

from installation to installation and of some

of the corrections that are necessary to provide a more uniform measurement capability.

(S. Ruthberg)

8.4. Loose Particle Detection*

Particle detection methods in current use

fall into three major areas: x-ray (visual)

detection, monitored random vibration (MRV)

testing, and particle impact noise detection

(FIND). Generally speaking, x-ray methods

are limited in their ability to resolve very

small particles and are time consuming. Vibration testing methods rely on the loose particle causing a detectable electrical malfunction of the energized microcircuit tested,

[ Farticle impact noise detection methods rely

j on the particle being shaken loose by vibraI tion, hitting the circuit enclosure, and gen1 erating a signal which can be detected by an

attached, sensitive transducer.

It is the last method which is currently used

by a number of manufacturers of microcircuits

to detect loose particles, primarily in circuits for military end use in high reliability devices. Despite the fact that these or- ganizations have apparently used 100-percent

screening, a number of loose-particle-induced

circuit failures have occurred, suggesting

that the screening methods used are not com- pletely reliable.

I A part of the problem appears due to the lack

I of uniformity among the particular test procedures used by the manufacturers. This was

brought out at a recent meeting [103] at which it was concluded that no two contractors use the same set of conditions for the

test parameters such as frequency, acceleration, axis of application, duration, oscilloscope sensitivity, and conditioning shocks.

Consequently there is disagreement as to the

effectiveness of this test method. The desire was expressed for a uniform test method,

including calibration standards and optimized

parameters. Particular areas in need of standardization cited include: the transducer,

seeded packages for calibration purposes, attachment media (tape, couplant) , preconditioning, on-the-shaker shock, parameters such as the vibration frequency and amplitude, the

package size, and electrical bias effects.

Currently a draft procedure is being considered for inclusion in MIL-STD-883, Test Methods for Microcircuits. This procedure, in

brief, requires a small vibrational shaker

with low inherent noise, a noise detection

system, an attachment medium, and a shock

tool to help detach loose particles which may

have become temporarily attached to the inside of the microcircuit enclosure. In the use of the procedure, the microcircuit under

test is attached to an ultrasonic transducer

which is mounted on the shaker . During vibration, particles present in the microcircuit are shaken loose and impact the walls

of the circuit enclosure producing an acoustic impact signal. This signal is picked up by the ultrasonic transducer, amplified by

a transducer amplifier, and made audible by

a speaker. The output can also be displayed

on an oscilloscope. If the particle cannot

readily be shaken loose to produce the impact signal, a shock tool is applied to the

circuit enclosure to facilitate this without,

however, damaging the circuit itself.

The proposed particle impact noise test specifies vibration frequencies and amplitudes,

suggests a type of vibration shaker and several attachment media, recommends a particular commercial acoustic noise detection system, and a commercial shock tool. It also

specifies the test procedure to be used.

Discussions with a variety of manufacturers

of semiconductor circuits using variations

of this test suggested that: test equipment

specifications and details of procedure are

not sufficiently explicit in some areas to lead to repeatable and replicable results

and, where they are explicit, there is little assurance that they describe optimum conditions .

Principally funded by SAMSO with additional

funding from the NBS Nondestructive Evaluation Program.

75
ASSEMBLY AND PACKAGING

Accordingly, a task was undertaken to investigate the FIND test by examining the equipment and procedure in detail to obtain experimental data leading to an optimized test. An additional objective is to develop standard seeded microcircuits which can serve as reference standards for in-house checking of

FIND test equipment and procedures by way of

round-robin experiments.

Since seven of the nine major manufacturers

as well as some users of these microcircuits

use a particular brand of commercial FIND

system and a particular shock tool, these were procured for investigation. In addition, a selection of seeded circuits was obtained from one of the manufacturers. Preliminary experimental work was conducted, using the seeded cans, to gain experience with

the equipment. Following this, a more structured investigation was started.

An experiment was conducted to determine the

effectiveness of the various attachment media

specified in the FIND test procedure to transmit vibrational accelerations to a simulated

microcircuit , as represented by a subminiature piezoelectric accelerometer mounted on a small flat aluminum base. The total weight

of both components was 0.4 g. The test accelerometer was first directly mounted on the ta- ble of an electromagnetic vibration exciter

and a frequency response was run against the

built-in reference accelerometer. The test accelerometer was then attached to the table

by means of one of the attachment media and

the response was again measured over a frequency range from 30 to 5000 Hz, at acceleration amplitudes of 10 and 20 g^. The data

show response ratios between direct mounted

and couplant-attached accelerometer outputs

with a maximum spread from 0.94 to 1.03 for

the three couplants tested. Over the frequency range from 30 to 1000 Hz, considered

Table 18 — Representative Response Frequencies of Metal Spheres Impacting a Steel Surface

Material Weight, ug Frequency, kHz

Gold 10 621 Steel 10 825

Aluminum 1 .4 1270

most likely to be the FIND operating range,

response ratios ranged from 0.97 to 1.03.

This suggests that none of the three cou- plants tested is likely to significantly re- duce the acceleration applied to the microcircuits attached to the FIND equipment.

It was felt desirable to attempt to determine

the theoretical frequencies generated when

loose particles impact the walls of the microcircuit enclosure. This was considered as the case of the impact of a solid sphere on

an elastic plate which is a classical problem

[104]. If the velocity of impact is small

and the diameter of the sphere is small com- pared with the thickness of the plate, the

response frequency is proportional to the ratio of the impact velocity to Poisson's ratio.

Assuming vibrational accelerations with an am- plitude of 10 (zero to peak) at 60 Hz, values were calculated for representative metal

spheres impacting a steel surface; the results are given in table 18. These frequencies are considerably higher than the frequency of maximum response of the FIND system,

which is in the range from 130 to 170 kHz.

Nevertheless, the former should be able to shock excite the latter, thus producing a measurable output. (P. S. Lederer+,

J. S. Hilten''', and C. F. Vezzetti"*")

76

NBS Components and Applications Section,

Electronic Technology Division.
9 . DEVICE INSPECTION AND TEST

9.1. Dual-Laser, Flying-Spot Scanner

Temperatupe Measurement — A method has been

developed to obtain a quantitative measure of

the localized temperatures within operating

bipolar silicon devices. The method employs

the temperature enhancement of the 1.15-ym

photoresponse discussed previously (NBS Spec.

Publ. 400-25, p. 58). One of the advantages

of the method is that the spatial temperature

resolution may be as fine as the size of the

spot used for scanning, which is five to ten

times finer than the resolution which can nor- mally be achieved with other methods for measuring localized temperatures.

In developing the method, a generalized bipolar transistor element first was analyzed

with the temperature-indicating photogeneration allowed to occur simultaneously within

the emitter, base, and collector regions

while at the same time the device was biased

for its normal service as an amplifier of

electrical signals. For the usual commonemitter configuration, the collector current,

j , for the transistor element was found to

c be

Jc

= ^b^F JhuEMIT^F

(22)

where 3^ is the forward current gain, j, is

r b the base current supplied from an external

electrical source, and j^^EMIT' J'hvBASE'

^"'^

•'"hvCOLL^^^ the photocurrents generated in the

emitter, base, and collector regions respectively. For most devices of interest, and in

particular for the 2N4431 UHF transistors

used to test the method, the first two photocurrents may be neglected with respect to the

third

Jc

~- V ^ jhuCOLL^^F^') (23)

A plot of

j|^yQQ[_[_ a function of temperature is readily obtained by measuring the

base-collector photodiode response with the

transistor in thermal contact with a heat

source of known and adjustable temperature.

The base-collector photoresponses to 1.15-

and 0,633-vim radiation, normalized to their

values at 30°C are shown for a representative

device in figure 58. The temperature enhancement of the 1.15-ym photoresponse is a bulk

effect; analysis shows that the photoresponse

can be fitted by theory when the effects of

both the increase in phonon population and de- crease in bandgap are included.

By scanning a device with the 1.15-Mm laser,

a map of the photoresponse is obtained. If the current gain is spatially uniform, this

photoresponse may be interpreted directly

with the aid of a calibration curve such as the upper one in figure 58 to yield a map of

local temperature. If the gain is not spatially uniform, which occurs when a device is

operating in the hot-spot regime, it is necessary to correct the 1.15-pm photoresponse

for gain variations . This can be done by

scanning the device with the 0.633-ym laser,

since the 0.633-pm photoresponse is independent of temperature

Several 2N4431 transistors operating in the

hot-spot regime were used to test this electronic thermal mapping method. Temperature

profiles through the center of the hot spot were made with both the laser scanner [105]

and a commercial infrared microradiometer on

two devices. The resulting temperature profiles are shown in figure 59. The spatial

resolution for the microradiometer was 35 pm, compared to 4 ym for the laser scanner. Im- plicit in the calibration of the microradiometer is the assumption that the object filling the field of view has a uniform temperac/j

t/J

10.0

1 1 1 1 1 1 1 o 1.15/im1 1

1

• 0.633/^m

~ 1 1 1 1 1 1 1 1 1 ~

20 60 100 140 180

DISTANCE (mils]

220

Figure 58. Photoresponse of a typical 2N^^31 UHF transistor to 0.633" and l.lS-ym radiation as a function of temperature.

77
DEVICE INSPECTION AND TEST

DISTANCE ImilsJ DISTANCE |mlls|

a. Device No. 7. b. Device Ho. 9.

Figure 59- Temperature profile of hot spots in typical UHF transistors as measured

with a laser scanner (o) and an infrared mi crorad iometer (•)

.

Figure 60. Photograph of the reflected light

plus photoresponse image obtained when the

0.633"Mm laser beam scans *the bacl< side of a silicon-on-sapphire C-MOS 4007 inverter.

Figure 61. Photograph of the reflected light

image obtained when the l.lS-ym laser beam

scans the back side of a flip-chip C-MOS 4007

i nverter

.

I nput vol tage : 0 V

,

Input voltage: -1.5 V.

Figure 62. Photographs of the reflected light plus photoresponse image obtained when the

1.15-ym laser beam scans the back side of a flip-chip 709 operational amplifier.

78
DEVICE INSPECTION AND TEST

ture. When the microradiometer is focused on

the center of a hot spot in which the peak

temperature occurs over a region much smaller

than 35 ym across, it interprets the radiant

energy it intercepts as coming from a uniform,

but lower, temperature. The discrepancy

should not be as great when the microradiometer is focused on a temperature slope since

'i it is receiving energy both from hotter and

' colder portions simultaneously. The results

I shown are consistent with both of these premises. (D. E. Sawyer and H. P. D. Lanyon"

)

Back side Inspection — The possibility of using the laser scanner to look through the

1 back side of devices was briefly explored.

Use of this technique permits inspection or failure analysis of flip-chip or beam-lead

devices which are mounted circuit side down.

I Another significant advantage in scanning a

{ device from the back side is that greater ac- cessibility to the active regions is obtained

, because metal does not block the optical raj diation. A disadvantage is that the surface

' of the back side is generally unpolished, and

the roughness degrades the image obtained.

This problem can be alleviated somewhat by

placing a drop of oil or other substance on

the device in an attempt to match indices of

1 refraction at the rough surface. The interij face between the air and this substance is a

j smooth one, and image distortions are generI ally not caused by it.

jj Three devices were examined from the back

side with the laser scanner [105]. A siliconon-sapphire C-MOS 4007 inverter was bonded into a special flat-pack-type holder that could

be examined from either the circuit side or the back side. The sapphire is transparent

to the 0.633-um laser light which was used to scan the device. A drop of oil was put on

the surface to improve the image. The re- flected light image mixed with the photoresponse signal is shown in figure 60. Of the

four transistors shown in the figure, only

the lower right hand one was electrically

connected. The five brightly lit stripes are

the gate area.

A bulk-silicon, flip-chip C-MOS 4007 was also

examined from the back side with the laser

scanner. Unlike sapphire, silicon is not

transparent to the visible light and the infrared (1.15-ym) laser had to be used. The

bulk device was mounted in the same manner as the sapphire device. Figure 61 shows a reflected light image obtained by scanning

through the back side which was polished, so that no oil was needed.

A flip-chip type 709 operational amplifier

was also studied. This bulk silicon device

had its part number scratched into the back

surface which was unpolished. The infrared

laser was used, and a drop of oil was put on

the surface. The operational amplifier was connected for normal circuit operation in an amplifier stage with a gain of 10. The re- flected light image superimposed on the photoresponse is shown in figure 62 for two operating conditions. In the display obtained with

the input held at 0 V (fig. 62a) different

transistors in the circuit yield larger photoresponse than in the display obtained with

the input at -1.5 V (fig. 62b), which is sufficiently large to saturate the device.

(D. W. Berning and D. C. Lewis"*")

Instrumental Improvements — A conventional

color TV set was modified to serve as a three-gun color display for the laser scanner. With this display the various information

channels can be displayed at the same time

and still maintain their identity. For example, the reflected light signal can be presented in one color and photoresponse in another. If the device under investigation has

two power supply connections, a third color

can be used for the second supply photoresponse. Another application of the colors

involves mixing the colors in proportions relating to specimen photoresponse voltage.

The eye can discriminate between various hues

very easily. With the monochromatic display,

areas on the specimen producing equal photoresponse can only be accurately compared by

careful measurement using the vertical deflection technique, but with the color display

it is easy to see at a glance the areas producing equal photoresponse. Isothermal contours of transistors in hot-spot operation

can also be identified easily.

(D. W. Berning)

9.2. Scanning Acoustic Microscopy

This task was undertaken with the aim of producing, if feasible, a scanning acoustic microscope suited to examination of solid-state

devices. Rather than sensing the dielectric

properties of surfaces as in optical microscopy, the acoustic microscope responds to the

Worcester Polytechnic Institute, Worcester,

Mass. 01609.

''"office of Naval Research, Washington, D.C. 22217.

79
DEVICE INSPECTION AND TEST

SAPPHIRE LEKS

ZnO TRANSDUCER

SCAN NECHANISM

R F SOURCE —» MODULATOR

DETECTOR

I ROSTER

DISPLAr

PULSE GENERATOR

SAPPHIRE LENS

ZnO TRANSDUCER

X T SCAN

ROSTER

DISPLAY

SCAN MECHANISM

a. Transmission Tno'de, cw only. b. Reflection mode, cw or pulse.

Figure 63. Comparison of operating modes of the scanning acoustic microscope.

mechanical characteristics of surfaces and near subsurface regions. It is this basic

difference in response which, it is hoped,

will reveal useful information that is not

evident in ordinary light microscopy.

The initial activity of the Stanford portion

of this task has been focused on the problem

of perfecting an instrument which is capable

of operating in a pulsed mode with a frequency near 1000 MHz and with a resolution that

is less than 1 ym. Consequently, a major

part of the work has been concerned with instrument development.

The Stanford instrument which existed at the

start of the contract effort was a transmission mode microscope developed mainly for ex- amination of biological specimens [106].

This design has been modified for reflection

mode operation [107]. Figure 63 shows a com- parison between the two modes. The -transmission mode microscope was designed for cw operation; cw operation is possible in the reflection mode if the pulse generator, modulator,

gate and delay blocks are omitted.

If the specimen surface is not precisely parallel to the scan plane, changes in the

length of the transmission path during a cw scan produce a distracting moire pattern as shown in figure 64a. Pulsed operation avoids

this difficulty as figure 64b illustrates,

since the transmitted and reflected signals

do not co-exist and thus cannot interfere.

In the reflection mode, one must choose the

lens length such that reflections other than

those from the specimen (for example, from

the outer annulus of the lens face) do not ob- scure the desired signal.

Pulsed operation has necessitated a redesign

of the matching networks between the coaxial

line and the zinc-oxide transducer to provide

a. Reflection mode, cw operation, b. Reflection mode, pulsed operation,

Figure 6k. Acoustic images showing differences between cw and pulsed operation,

80
DEVICE INSPECTION AND TEST

a. Acoustic image, reflection mode, pulsed

operation; magnification: 'vigOOx.

b. Acoustic image, reflection mode, pulsed

operation; magnification: '\^1300x.

c. Optical image, interference contrast;

magnification: '^'1^00x.

Figure 65. Comparison between 9^0-MHz acoustic images of a bipolar microwave transistor

with 2-ym wide metal fingers and a photomicrograph taken with interference contrast.

!

wider bandwidth in order to avoid pulse distortion. The current bandwidth of 200 MHz is

adequate for the 75-ns pulse lengths now being used. In addition to removing the moire

pattern, use of these short pulses has apparently increased the image clarity and improved the resolving power. The reasons for

this improvement are not understood; much more remains to be done in developing the theory of imaging using focused acoustic waves.

New microscope systems have been built in

which the operating frequency has been increased from 350 MHz to 600 MHz and, most recently, to 940 MHz. The wavelength in water

decreased from 4.3 ym to 2.5 ym and 1.6 ym, respectively, with correspondingly improved

resolution. The resolution achievable with

the 940-MHz instrument is illustrated in figure 65, The scale in those images can be realized from the width of the fingers which is

2 ym. These are clearly resolved in the two

acous,tic images of different magnification.

These and related studies suggest that the

limit of resolution is now less than 0.5 ym

which is approaching one-third of a wavelength. An optical photo taken with a Nomarski type of interference microscope is included in the figure for comparison; this image represents something approaching the

state-of-the-art in optical microscopy. This

comparison shows that the details available

from the optical image are accurately reproduced in the acoustic image. The raggedness

at the edge of the rectangular pad in the

acoustic images is a result of excessive

noise on the mechanical scanning system.

This can be eliminated with some improvement

in the design of the scanning circuits in

this system.

Attenuation in water decreases significantly

with increasing temperature, as shown in the curves of figure 66. With this in mind it

should, in principle, be possible to monitor

point by point the temperature profile of an operating device. With a given circuit under power, those regions which are hot

should appear brighter in the image since the

liquid absorption in the heated region is diminished as compared to the unheated region.

With scanning this temperature profile should

show up as variations in the brightness of

the image. Although a first demonstration of

this principle was successful, the minimum

detectable temperature change and the possible spatial resolution have not yet been es- tablished .

Initial work was undertaken to study the effectiveness of using reflection-type imaging

i
DEVICE INSPECTION AND TEST

0.5 1.0 1,5 2.0 FREQUENCY |GHz|

Figure 66. Frequency dependence of attenuation of acoustic waves in water at various

temperatures

.

a. Normal incidence. b. Angle of incidence: 20 deg from norma

Figure 67. Acoustic images, reflection mode at 9^0 MHz, of an integrated circuit showing

tine effect of non-normal incidence.

Figure 68. Spherical tool grinder for making Figure 69. Photograph of the tool grinder,

acoustic lenses.

82
DEVICE INSPECTION AND TEST

where the acoustic beam is not at normal incidence to the plane of the specimen. It is

a relatively easy matter to tilt the substrate and record the image in this off-axis

arrangement. A comparison of images obtained

with normal and non-normal incidence is shown

in figure 67. It can be seen that the quality of the off-axis image is at least as good

as the image at normal incidence, and some of

I the finer detail stands out with more crispness. This technique appears to be worthy of

detailed study. (C. F. Quate^

)

Since the acoustic losses in water, the transj mission medium between lens and specimen, increase as the square of the frequency to about 100 dB/mm at 1 GHz, the path length is

j severely limited at the higher frequencies.

For this reason, it has been necessary to de-

' vise means for making concave lens surfaces

j of very small radius. As the lens diameter

is reduced the acoustic path length in the

liquid cell between the two confocal lens surfaces is reduced. This reduction permits use

of the microscope at higher operating frequency with a fixed attenuation.

A new machine has been designed and built for

the purpose of grinding spherical tips on

I tiny rods. These spherical tips are the tool

' which is used to grind and polish the lenses

The efficiency of the machine is so good that

several tips can be fabricated in an hour.

This means that it is feasible to use a separate tip for each lens. A schematic of the

principle of operation of the machine is

shown in figure 68. A spherical surface is

produced by rotation about axis H and oscillation about axis W if H and VV intersect.

A photograph of the completed instrument with

a tip mounted in the rotating holder is shown

in figure 69. With these improvements, lenses with a radius of 50 pm have been successI fully ground and polished. This is a signifi-

; cant decrease since the lenses used to record

the images as reported here had a radius of

i, 100 pm. The factor of two in radius should

] permit the operating frequency to be inI creased by 40 percent. (W. A. Bond^

)

I The inital objective of the Hughes portion of

! the task was to construct a second-generation

I laboratory microscope with the most useful

I features for examining solid-state devices,

building on the knowledge and experience

available from Stanford. Such an instrument

i has been constructed. This microscope was designed to operate at 400 MHz in the cw reflection mode. Since being placed in operation

around mid-April, the microscope has demonstrated good day-to-day mechanical and elec250 300 360 400 450 FREQUENCY, MHz

Figure 70. Frequency response of return and

transmission loss of two sputtered glass

acoustic ant i -ref 1 ect ion coatings (AARC)

(The experimental arrangement is shown inset.

tronic stability. The resolution appears to be better than 2.5 ym. Pulsed operation has

been achieved in a separate bench-top apparatus to establish the correct behavior of the

additional electronic circuitry but has not

yet been incorporated in the microscope. Pre

liminary experiments show that the useful

depth of field at 400 MHz is about 20 pm, a figure which does not appear to impose serious limitations on the utility of the instrument .

An anti-reflection coating has been developed

to reduce signal losses at the lens-water interface [108]. The performance improvement

gained by use of the anti-reflection coating

is shown in figure 70. Of the maximum possible 18 dB-signal loss reduction, 17-dB loss

reduction is achieved. The geometry of the

experiment is shown in the inset of the figure. (R. G. Wilson^ and R. D. Weglein^)

Work conducted at Microwave Laboratory,

Stanford University, under NBS Contract No. 5-35899. NBS contact for additional information: R. I. Scace

'work conducted at Hughes Research Laboratories under NBS Contract No. 5-35898. NBS

contact for additional information: R. I. Scace

83
DEVICE INSPECTION AND TEST

9.3. Scanning Electron Microscopy

Eleotron-Beam-Induaed Damage — Illustrations

of typical electrical parameter changes en- countered in integrated circuits examined using the scanning electron microscope (SEM)

and the use of radiation dose for describing

electron beam ionization damage have been

presented earlier (NBS Spec. Publ. 400-17,

pp. 43-46) [109]. These considerations are especially important in developing an analysis strategy to minimize the destructive as- pects of device examination with an SEM.

The relevance of the energy deposited, rather

than simply exposure time or electron energy,

in understanding the effects of SEM exposure

can be illustrated through the threshold voltage shifts (AVyj^) of a p-channel MOS transistor. For this device, the gate oxide (the

critical oxide) was 100 nm of silicon dioxide

beneath approximately 1 ym of aluminum (the

gate) which in turn was beneath a silicon dioxide overcoat approximately 1 pm thick. SEM

exposures were made of four specimens, two at 17.5 keV and two at 30 keV, at an electron

flux of 5.2 X 10^0 cm"^-s"l. The device

leads were floating during exposure. The

shift in threshold voltage for the two beam

energies is shown in figure 71. When the

shifts are plotted as a function of exposure

time, two curves result (fig. 71a). These da- ta can also be replotted as a function of the

radiation dose delivered to the critical oxide; plotted in this manner, the shifts approximately fall along the same curve (fig.

71b) demonstrating that the relevant parameter for understanding oxide effects is the

dose

.

Even though the calculations of dose are not

always convenient or even necessary, the ex- pression for dose,

00 l„t Dose =

'B

Dox

(24)

yields certain insights which are useful in

understanding how to minimize beam damage.

The first factor relates strictly to parameters of the SEM exposure: the beam current,

Ig; time, t; and magnification, which is re- lated to the scanned area, . Obviously the

energy deposited can be reduced by reducing

I g and t . Reducing the area scanned (increas

ing magnification) locally increases the dose

The second factor, the energy of the electrons in the beam, is the quantity which

links the strictly SEM parameters to the device geometrical parameters of the third factor; Eg determines the depth to which the

electrons penetrate. In the last factor, f^^ is the fraction of the incident electron en- ergy deposited in the critical oxide region

of thickness, Tox This fraction depends on both the device structure and the beam energy

In particular, f^^ is zero if the energy in

the beam is absorbed in inactive layers , such

as the passivation overcoat and metallization

above the critical oxide. For the example

100 EXPOSURE TIME Is]

5

4

3

2

Ih

10

—I—I I M I

1

1

1 1—I I M 1 1

1 r

^ 30 keV

• 17.5 keV y

J i I I I I I I I I I

I I I I I I I L

10° DOSE |rads|Si02|

10

a. Plotted as a function of exposure time. Plotted as a function of absorbed dose,

Figure 71. Change in threshold voltage, AVj^, of a p-channel MOS transistor as the result

of exposure to an electron flux of 5-2 x lOlU cm'^-s"-^ at energies of 17.5 and 30 keV in a scanning electron microscope.

84
DEVICE INSPECTION AND TEST

discussed above, no energy is deposited in

the critical oxide for beam energies of 15 keV and below. If energy is not deposited in

oxide regions critical to the device operation, the electrical parameters of the device

will not be altered.

Any possible electrical parameter degradation

resulting from the electron beam must be carefully considered when developing a quality assurance program, performing construction analysis, or planning a failure analysis procedure involving device inspection in the SEM.

If a device is expected to function electrically after SEM examination, it is essential

to develop a careful analysis strategy based

on the parameters in eq (24) before the device is placed in the SEM specimen chamber.

(K. F. Galloway,

W. J. Keery, and K. 0. Leedy)

Electron-Becm-Induaed-Current Mode — A simple method of calculation for estimating the

magnitude of the electron-beam-induced current (EBIC) in simple device structures has

been described previously (NBS Spec. Publ.

400-19, pp. 56-59) [110]. Some of the limitations of this method were illustrated by com- paring the EBIC signals from a diode intentionally doped with a metallic impurity and

from a simple npn transistor with the currents calculated using the simple method.

Consider first a typical EBIC application in

the imaging of a subsurface defect. An EBIC

image of a diode intentionally doped with

copper is shown in figure 72a. There is apparent copper clustering at defect sites in

the silicon as expected from the work of

Schwuttke [111]. The EBIC signal as a 250

pA, 10 keV electron beam traverses one of

these clusters at high magnification is shown

in figure 72b. Away from the cluster, the

gain is 720 whereas the calculation based on

the simple method would predict approximately

2500. In the cluster region, the gain is reduced further by a factor of 3. The gain results given in this example indicate one of

the deficiencies of the calculation. With

the lifetime reduction caused by the copper

impurities, the assumption of 100-percent

collection efficiency is no longer valid.

As might be expected, electron-beam-induced

currents and the resulting images in transistors and integrated circuits are often complex. The existence of additional leakage

paths and interacting junctions are sources

of additional complications. An example of

the errors which can be introduced by these

factors is illustrated in figure 73 for an

EBIC image of diode showing copper clusters .

720 (2500)

240

X ~

b. EBIC signal as lO-keV electron beam tra- verses a cluster. Measured gains, G, are indicated; the number in parenthesis is the gain calculated from the simple model.

Figure 72. E 1 ect ron-beam- i nduced-cur rent re- sponse of a copper-doped silicon p-n diode.

npn transistor with oxide and aluminum thick- nesses and junction depths as indicated in

the figure. EBIC scans across the device,

made under various conditions, are also shown.

In each scan, the signal was taken from one region of the transistor and the other regions were grounded. The gain scales are not

linear; selected values are indicated and com- pared with values in parentheses calculated

using the simple method. The signal is a maximum where the active junction vertically

approaches the surface. This geometrical ar- rangement allows for maximum collection efficiency, and the best agreement with the calculation occurs at these points. The measured signal from the emitter at 10 keV is in

85
DEVICE INSPECTION AND TEST

600 nm SOOnrr -^M Ir^^

180

I

D nm

t 1200 nm

[ 1 "^J

n +

P

n

a. Cross section with vertical dimensions,

-G = 1029 (1000) ,

-G = 514 (1900)

b. Current gain for lO-keV electrons with

the signal taken from the emitter with

the base and collector grounded.

G = 1886 (4500) —ri

G = 1543 (2100)

G = 1286 ( 3400)-

c. Current gain for 20-ke\l electrons with

the signal taken from the emitter with

the base and collector grounded.

-G = 1000 (1000)

-G = 183 (1000)

d. Current gain for 10-keV electrons with

the signal taken from the collector with

the base and emitter grounded.

Figure 73. El ectron-beam- i nduced-current re- sponse of a silicon npn transistor. (Measured gains, G, are indicated; numbers in parentheses are gains calculated from the

s impl e model .

)

better agreement with the calculation than

that at 20 keV because of differences in electron penetration. At 10 keV, most of the energy is deposited at depths less than 1000 nm while at 20 keV a significant fraction is deposited at greater depths. At 10 keV, the

signal drops off rapidly away from the point

where the active junction intersects the surface; at this energy, the beam does not penetrate to the horizontal part of the junction.

(K. F. Galloway,

W. J. Keery, and K. 0. Leedy)

Accurate models are not available to account

for these various complicating factors. Nevertheless, the EBIC mode has been very suc- cessfully applied in a comparative mode [112].j In circuits of a repetitive nature such as I multiple-gate devices, multiple-bit shift reg-j

isters, flip-flops, or any circuit where a

' circuit function is duplicated, operating and

nonoperating regions can be compared. Where | a duplication of function within a single die ' is not available, the EBIC display from a good circuit can be compared with that from

a defective circuit. In all applications of | a comparative nature, the instrument conditions must be held constant from specimen to ji specimen. '

One particularly useful application of this

mode of examination is in locating in failed

devices the malfunctioning region which can

then be the focus of investigation by other

means. Two examples of this application are

given. The first is a quad 2-input NAND gate

which contained four identical gates. An optical photomicrograph of the upper portion of

the die showing two of the four gates is re- produced as figure 74a. The gate on the

right was not functional in either an ac or dc mode. However, visual inspection failed

to detect any pertinent anomalies. Examination of an EBIC image obtained from the

ground pin of the device (fig. 74b) reveals

that there is a resistor (arrow) visible in

the inoperative circuit which is not apparent

in the good one. A careful analysis of the

circuit indicated that this must be due to an unwanted leakage path from that resistor

or its metallization to ground. Subsequently, the metallization was removed and a pin

hole was found beneath the metallization

stripe outside the resistor tub. This effectively shorted the resistor to ground en- abling the electron-beam-induced current in

the resistor to be collected and displayed.

The second example is a group of eleven

failed low-power four-bit shift registers.

EBIC images of the bad circuits were checked

against those of a functional one. As in the

first example, the EBIC signal was collected

from the device ground. Figure 75a shows an EBIC image of a good device; the arrow indicates the presence of a junction which was

not visible in the EBIC images of the bad devices, one of which is shown in figure 75b.

In a more thorough inspection of the suspect

area, it was found to consist of an input

protection diode junction. With greatly increased beam currents an EBIC response of

this junction was detectable as is shown in

86
DEVICE INSPECTION AND TEST

Mm

M.

MMJ

fa. Optical photomicrograph; no difference be- b. EBIC image; the response from the resistween the gates is visible. tor (arrow) identifies the faulty element,

Figure Jh. Portion of a quad 2-input NAND gate including a functional gate on the left and

a nonfunctional gate on the right.

EBIC image of a good device; note June

tion marked by arrow which does, not ap

pear in the images of faulty devices.

b. EBIC image of a failed device; note ab- sence of junction visible in good devices.

c. Junction made visible in EBIC image by use of larger beam current; note current

anomal ies (arrow)

d. Scanning electron micrograph of the junction after preferential etching, showing

damage (arrow)

Figure 75. Portions of a low-power, four-bit shift register,

87
DEVICE INSPECTION AND TEST

figure 75c; current anomalies around the junction are also visible in the figure. A sec- ondary electron examination of this area showed nothing until the glass passivation

was removed and the surface etched preferentially. A scanning electron micrograph of

the etched surface (fig. 75d) revealed damage across the diode junction which could ac- count for the leakage and result in the low

gain across the junction. Further investigation confirmed that all eleven devices had

failed in the same manner at the same pin.

This was traced to the improper set up of an automatic test machine.

(J. R. Devaney^ and K. 0. Leedy)

9.4. Automated Scanning Low Energy Electron

Probe

Additional investigations of wafer defects were carried out with the automated scanning

low energy electron probe (ASLEEP) (NBS Spec.

Publ. 400-29, pp. 58-59). Six wafers were

oxidized to cause the formation of oxidation

stacking faults. It was found that the oxidation stacking faults were too small to be re- solved by the ASLEEP. On the portions of the

wafers which were preferentially etched [113]

to reveal the defects, the etch pits were

found to be 1 to 2 ym in diameter. This appears to be below the resolution limit for

the ASLEEP electron optics.

A special test structure with minimum exposed

oxide (to reduce the effects of surface charging) was constructed for voltage resolution

measurements. A gold metallized alumina substrate was divided into two large pads separated by an 80-ym gap. An adjustable voltage

was applied between the two pads and its effect on the video signal was studied. A single line was scanned across the pads and the

target current was displayed on the oscilloscope. The potential on each pad was adjusted so that the currents on each were

equal and less than 1 nA. Then the potential

of one pad was adjusted until the target current was displaced by the width of the noise

on the scope. Under the best achievable con- ditions, a 25-mV change in potential was re- quired to do this. In addition, the computer

was used to measure voltage resolution by applying the cathode ramp and threshold detector . Comparison of measurements made over a period of less than a minute showed standard

deviations of 1 mV but long term drifts degraded the accuracy of the measurement and

increased the standard deviation to 10 mV. These drifts are due to the wandering of the

beam on the target surface and to variations

in heater current.

Efforts were made to observe surface potential differences between regions of a speci- men implanted with different densities of

phosphorus. The results of the experiments

showed that charging of the native oxide ob- scures the change in surface potential

caused by the Fermi level shift due to doping and prevents ASLEEP from making doping

density measurements.

Efforts were also made to observe potential

variations of an unpassivated integrated circuit chip. The ASLEEP was barely able to re- solve portions of the circuit consisting of

5-ym lines with 7-)jm spaces. Observations

were impaired because of surface charging effects; attempts to steer the beam so that it

probed only the metal interconnects were un- successful because of drifts in the deflection circuit. It was concluded, that in its

present form, the ASLEEP is not suitable for

examining integrated circuit chips. No further work on this instrument will be con- ducted under this program. (W. C. Jenkins^)

9.5. Thermal Properties of Transistors'*"

A model to describe the initiation of thermal

instability, the formation of stable hot

spots, and forward biased second breakdown in

power transistors has been developed and

tested [114]. The model has been used to pre- dict the locus of thermal instability (hotspot formation) in the I^, - V^^ [ (collectorcurrent) - (collector-emitter voltage)] plane

to within about 5 percent. For the stable

hot-spot mode, the model describes the qualitative behavior of the peak junction temperature, the current density distribution, and

the area of the stable hot spot. The model

also qualitatively describes the 1^, - V^^ locus at which second breakdown occurs and preWork conducted at Hi-Rel Laboratories under

NBS Order No. 513840.

Work conducted at the Naval Research Laboratory under NBS Order No. 501718. NBS

contact for additional technical information: K. F. Galloway.

Work at NBS funded through the NBS Electronic Technology Program; Westinghouse portion

funded by Westinghouse.
DEVICE INSPECTION AND TEST

diets a cross-over point of the hot-spot and

second -breakdown loci, which was observed ex- perimentally [114], and provides a basis for

axplaining the experimentally observed thermal hysteresis of the transistor current gain

:[115].

iThe model is based upon the concept of a staIbility factor [116] which accounts for the

j thermal-electrical feedback mechanisms of

'transistors. The expression for the stabiljity factor which determines initiation of

thermal instability is derived assuming ' that

jthe current distribution throughout the de-

;jvice is uniform prior to instability and that

is related to the emitter-base voltage,

V^, by the Ebers-Moll equation [117]. Once

BE instability has occurred, the model accounts

' for two phenomena which occur as the current

j! constricts to the hot-spot area. First, the

current density becomes large enough that

1 current crowding (to the edges of the emit1 ter fingers) occurs within the hot spot, and

second, base widening occurs due to the

large current densities involved. The effect

1 of these two phenomena is to modify the ex-

! preasion relating I_ to V,,^ and thus also to

I modify the expression for the stability factor. The net result is that the device can

' restabilize in a stable hot-spot mode of operation and exhibit thermal hysteresis.

ij

" The three device parameters required to prediet the onset of thermal instability are all

easily measured. The thermal resistance,

Rgjj,, is determined using the recommended

standard [118]. The emitter resistance, R^,

j is determined using the floating-collector

J technique [119]. A pre-exponential current,

I I^, which relates the collector current to

j the base emitter voltage, V^^, at low values

] of collector current:

' Ij. = I, exp [q(Vg^ - Eg)/kT], (25)

I where is the forbidden energy gap (in eleci tron volts), q is the electronic charge, k is

the Boltzmann's constant, and T is the absolute temperature, is determined from curvetracer measurements of the I. - V characL DC teristic at collector currents below 10 mA.

The quantitative predictions of device behavior in the stable hot-spot mode and the onset

of second breakdown are not as accurate as those for the initiation of thermal instability primarily because additional simplifying

assumptions for current crowding and base widening are contained in the model. However,

while this portion of the model may require

some refinements to give a more exact description of device parameters and characteristics

in the stable hot spot, it does appear to provide a satisfactory picture of the physical

behavior of the device for these conditions

The newly automated infrared microradiometer

proved to be extremely valuable in this study.

In this automated facility, an interactive

data acquisition system controls the steppermotor-driven X-Y stage upon which the device

under test rests, acquires the output of the

microradiometer through a digital voltmeter,

and sends the results to a printer to be

printed out in hard copy and paper tape. The

thermal measurements were used to demonstrate

the surprising fact that current crowding occurs within the hot spot. This was accom- plished by noting that the temperatures along

the emitter finger edges were higher than at their centers which can only be accounted for

by current densities being higher at the

edges than at the centers.

The results of this work are important for unders'tanding device operation and in the design of devices where trade-offs between various device performance characteristics must

always be made. However, the primary purpose

for undertaking this work was to establish an improved basis for developing safe operating

area limits for power transistors. These limits define the operating conditions in which

overheating and thermal instabilities do not

occur. (D. L. Blackburn, F. F. Oettinger,

S. Rubin, G. J. Rogers, and P. L. Hower""'")

Westinghouse Research Laboratories, Pittsburgh, PA 15235.

89
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Strausser, Y. E. , An Auger Analysis of

the Si02-Si Interface, J. Appl. Phys.

47, 3028-3037 (1976).

58. Coburn, J. W. , and Kay, E. , Techniques

for Elemental Composition Profiling in

Thin Films, Crit. Rev. Solid State Sci.

4, 561-590 (1974).

59. Ahn, J., Perleberg, C. R. , Wilcox, D. L.,

Coburn, J. W. , and Winters, H. F., Electron-Beam Effects in Depth Profiling

Measurements with Auger Electron Spectroscopy, J. Appl. Phys. 46^, 4581-4583

(1975).

60. Standard Method of Test for Thickness

of Epitaxial or Diffused Layers in Silicon by the Angle Lapping and Staining

Technique, ASTM Designation F 110, Annual Book of ASTM Standards, Part 43

(November 1976) , (Available as a sepa- | rate reprint from American Society for

Testing and Materials, 1916 Race Street,

Philadelphia, PA 19103.)

61. Taylor, N. J., Johannessen, J. S. , and

Spicer, W. E. , Crater-Edge Profiling in

Interface Analysis Employing Ion-Beam

Etching and AES, Appl. Phys. Letters

29, 497-499 (1976).

62. Mayo, S., Keller, R. A. , Travis, J. C,

and Green, R. B. , Detection of Sodium

Trace Contamination in Furnace Atmospheres at 1000°C, J. Appl. Phys. 47,

4012-4016 (1976).

63. Hetherington, G, , Stephenson, G, W,,

and Winterburn, J. A., Vitreous Silica

in Electronics, Electrical Engineering

41, No. 495, 52-56 (May 1969) and 41,

No. 496, 44-47 (June 1969).

64. Mayo, S., and Evans, W. H. , Development | of Sodium Contamination in Semiconductor

92
REFERENCES

Oxidation Atmospheres at 1000° C, (to be

published)

65. Hughes, H. L. , A Survey of RadiationInduced Perturbations in Metal-InsulatorSemiconductor Structures, 9th Annual Proceedings, Reliability Physics 1971, Las Vegas, Nevada, March 31-April 2, 1971,

pp. 33-39.

66. Lindmayer, J., Radiation and Oxide-Metal

Interactions in MOS, IEEE Trans, Nucl.

Sai. NS-18 , No. 6, 91-98 (1971)'.

67. Phillips, D. H. , Radiation Hardening of

MOS Integrated Circuits on <111> Silicon,

IEEE Trans. Nucl. Sci. NS-22 , 2190-2192

(1975).

68. Kjar, R. A., and Nichols, D. K. , Radiation Induced Surface States in MOS Devices, IEEE Trans. Nucl. Sci. NS-22 , 2193-2196 (1975).

69. Aubuchon, K. G. , Harari, E., and Chang,

P., CMOS Radiation Hardening, Annual Report, Contract No. N00014-72-C-0424

(Hughes Research Laboratories, Malibu,

California, October, 1974).

70. Kern, W. , and Comizzoli, R. B., Semiconductor Measurement Technology : Development of Techniques for Measuring the In- tegrity of Passivation Overcoats on In- tegrated Circuits, NBS Special Publication 400-31 (to appear).

71. Data Sheet, "Industrial Carbon Blacks,"

Columbian Division, Cities Service Co.,

3200 W. Market Street, Akron, OH 44313,

October 1975.

72. Data Sheet, "Lubrizol 894," Lubrizol

Corp., Cleveland, OH 44117, November

1974.

73. Data Sheet, "A-C Polyethylene," Allied

Chemical Co., Specialty Chemicals Div.,

P.O. Box 1087R, Morristown, NJ 07960,

January 1975.

74. Kern, W. , Detection and Characterization of Localized Defects in Dielectric

Films, RCA Review 34, 655-690 (1973); see also Solid State Technology 17 , No. 3, 35-42 (1974) and No. 4, 78-84

(1974)

75. Yager, T. A., Tressler, R. E. , and Stach,

J., Impurity Transfer Considerations for

the Use of BN Sources for Doping Silicon,

Extended Abstracts for the Fall Meeting

of the Electrochemical Society, Las Vegas, Nevada, October 17-22, 1976, pp. 853-855.

76. Eriksson, G. , Thermodynamic Studies of

High Temperature Equilibria. III. SOLGAS, a Computer Program for Calculating

the Composition and Heat Condition of an Equilibrium Mixture, Acta Chem. Scan.

25, 2651-2658 (1971).

77. Grubbs, W. H. , Das, M. B. , Stach, J.,

and Tressler, R. E. , A Comparison of HCl

and Trichloroethylene (TCE) Grown Oxides

on Silicon, Extended Abstracts for the

Fall Meeting of the Electrochemical Society, Las Vegas, Nevada, October 17-22,

1976, pp. 836-838.

78. Sjwt, D. A., NBS Program in Photomask

Linewidth Measurements, Solid State Technology 19, No. 4, 51-57 (April 1976).

79. Swing, R. E. , The Theoretical Basis of a New Optical Method for the Accurate Measurement of Small Line-Widths, Proc. Soc. Photo-Optical Instrum. Engrs. 80^, Developments in Semiconductor Microlithography, 65-11 (1976).

80. Born, M. , and Wolf, E. , Principles of

Optics, 5th edition, p. 522 (Pergamon

Press, New York, 1965).

81. Swing, R. E., Conditions for Microdensitometer Linearity, J. Opt. Soc. Am. 62 , 199-207 (1972).

82. Cook, H. D. , and Marzetta, L. A., An Automatic Fringe Counting Interferometer

for use in the Calculation of Line

Scales, J. Res. Natl. Bur. Stand. 65C , 129-139 (1961).

83. Mandel, John, Repeatability and Reproducibility, Materials Research and Standards 11, No. 8, 8-15, 52 (August 1971).

84. Kasdan, H. L. , Electro-Optical Method

for Measuring Gaps and Lines, U.S. Patent 3,937,580, February 10, 1976.

85. Kasdan, H. L. , and George, N. , Linewidth

Measurement by Diffraction Pattern Analysis, Proa. Soc. Photo-Optical Instrum.

Engrs. 80_, Developments in Semiconductor

Miorolithography , 54-63 (1976).

93
REFERENCES

86. Helen, R. , The tradeoffs in monolithic

image sensors: MOS vs. CCD, Electronics

46, No. 11, 106-111 (May 24, 1973).

87. Tull, R. G., Choisser, J. P., and Snow,

E. H. , Self -Scanned Digicon: A Digital

Image Tube for Astronomical Spectroscopy,

Applied Optics 14, 1182-1189 (1975).

88. Rodgers, R. L. , III, 512x320 Element

Silicon Imaging Device, Proc. Internat.

Solid State Circuits Conference , Philadelphia, Pennsylvania, February 13-14,

1975, pp. 188-189.

89. Bruning, J. H. , Feldman, M., Kinsel, T. S., Sittig, E. K., and Townsend, R. L.

,

An Automated IC Photomask Inspection System - AMIS, IEEE Trans. Electron Devices

ED-22 , 487-495 (1975).

90. Keywell, F., and Dorosheski, G., Measurement of the Sheet Resistivity of a Square Wafer with a Square Four-Point

Probe, Rev. Sci. Instr. 31, 833-837

(1960).

91. van der Pauw, L. J., A Method of Measuring the Resistivity and Hall Coefficient

on Lamella of Arbitrary Shape, Phillips

Research Reports 13, 1-9 (1958)

.

92. Mircea, A., Semiconductor Sheet Resistivity Measurements on Square Samples, J. Sci. Instr. 41, 679-681 (1964).

93. Green, M. A., and Gunn, M. W. , The Evaluation of Geometrical Effects in Four

Point Probe Measurements, Solid-State

Electronics 14, 1167-1177 (1971)

.

94. Green, M. A., and Gunn, M. W. , Four

Point Probe Hall Effect and Resistivity

Measurements Upon Semiconductors, SolidState Electronics 15_, 577-585 (1972).

95. Uhlir, A., Jr., The Potentials of Infinite Systems of Sources and Numerical

Solutions of Problems in Semiconductor

Engineering, Bell System Tech. J. 34 , 105-128 (1955).

96. David, J. M. , and Buehler, M. G., A Numerical Analysis of Various Cross Sheet

Resistor Test Structures, Solid-State

Electronics (to appear)

.

97. Wick, R. F., Solution of the Field Problem for a Germanium Gyrator, J. Appl.

Rhys. 25, 741-755 (1954); see problem 3.

98. Reynolds, F. N., Lawson, R. W., and

Mellor, P. J. T., Specifications for In- tegrated Circuits in Telecommunications

Equipment, Proc. IEEE ^2^, 223-230 (1974).

y9. Harman, G. G., The Use of Acoustic Emission in a Test for Beam Lead Bond Integrity, 14th Annual Proceedings, Reliability Physics 1976, Las Vegas, Nevada,

April 20-22, 1976, pp. 86-97.

100. Adams, J. R. , and Bonham, H. B., Analysis and Development of a Thermocompression Bond Schedule for Beam Lead Devices,

IEEE Trans. Parts, Hybrids and Packaging ?EP^, No. 3, 22-26 (1972).

101. Harman, G. G., Ed., Semiconductor Measurement Technology: Microelectronic

Ultrasonic Bonding, NBS Special Publication 400-2, p. 6 (January 1974).

102. Neff, G. R., Martin, B. D., and Ruthberg,

S., The Precision of the Radioisotope

Method for Leak Testing Integrated Circuits (to be published)

.

103. Screening for Particulate Contamination

in Cavity Devices, a meeting at Aerospace Corporation, Los Angeles, California, August, 1975 (unpublished).

104. Raman, C. V., On Some Applications of

Hertz's Theory of Impact, Phys. Rev. 15 , lll-lSli (1920).

105. Sawyer, D. E., and Berning, D. W. , Semiconductor Measurement Technology. A Laser Scanner for Semiconductor Devices,

NBS Special Publication 400-24 (to appear) .

106. Lemons, R. A., and Quate, C. P., Acoustic Microscope - Scanning Version, Appl.

Phys. Letters 24_, 163-165 (1974).

107. Lemons, R. A., and Quate, C. F., Integrated Circuits as Viewed with an Acoustic Microscope, Appl. Phys. Letters 25 , 251-253 (1974).

108. Weglein, R. D., Acoustic Properties of

Sputtered Glass at Microwave Frequencies,

Appl. Phys. Letters 29, 277-279 (1976).

109. Keery, W. J., Leedy, K. 0., and Galloway,

K. F., Electron Beam Effects on Microelectronic Devices , Scanning Electron

Microscopy/1976 , 0. Johari and I. Corvin,

Eds., pp. 507-514 (IIT Research Institute, Chicago, Illinois, 1976).

94
REFERENCES

.10. Galloway, K. F. , Leedy, K. 0., and Keery,

W. J., Electron-Beam-Induced Currents in

j Simple Device Structures, IEEE Trans.

I PartSy Hybrids^ and Packaging PHP-12 ,

' 231-236 (1976).

.11. Schwuttke, G. H. , Study of Copper Precipitation Behavior in Silicon Crystals, J.

I Eleotroohem. Soo. 108 , 163-167 (1961)

,L12. Thornton, P. R. , Hughes, K. A., Kyaw, H.

,

Millward, C, and Sulway, D. V., Failure

j Analysis of Microcircuitry by Scanning

Electron Microscope, Miaroeteatronics

j and Reliability 9-16 (1967).

L13. Sirtl, E., and Adler, A., Chromic AcidHydrofluoric Acid as Specific Reagents

for the Development of Etch Pits in Silij con, Z Metallkunde 52_, 529-531 (1961).

Il14. Rower, P. L., Blackburn, D. L., Oettinj ger, F. F., and Rubin, S., Stable Hot

I Spots and Second Breakdown in Power Transistors, PESO '76 Reaord, IEEE Power

Electronics Specialists Conference 1976,

j Cleveland, Ohio, June 8-10, 1976, pp.

I 234-246.

|ll5. Oettinger, F. F., and Rubin, S., The Use

of Current Gain as an Indicator for the

Formation of Hot Spots Due to Current

Crowding in Power Transistors, Tenth Annual ProceedingsJ Reliability Physios

1972, Las Vegas, Nevada, pp. 12-18.

116. Hower, P. L., and Govil, P. K. , Comparison of One- and Two-Dimensional Models

of Transistor Thermal Instability, IEEE

Trans. Electron Devices ED-21 , 617-62 3 (1974)

117. Gray, P. E. , DeWitt, D. , Boothroyd, A.

R. , and Gibbon, J. F., Physical Electronics and Circuit Models of Transistors , p . 181 (John Wiley and Sons , New

York, 1964).

118. Thermal Resistance Measurements of Conduction Cooled Power Transistors, EIA Recommended Standard RS-313-B (Revision

of RS-313-A), October 1975. (Available

from Electronics Industries Association,

2001 Eye Street, N.W. , Washington, DC

20006.)

119. Giacoletto, L. J., Measurement of Emitter and Collector Series Resistance,

IEEE Trans. Electron Devices ED-19 , 692-

693; 1224 (1972).

95
APPENDIX A SEMICONDUCTOR TECfiNOLOGY PROGRAM STAFF

Electronic Technology Division

(301) 921-3357

I. 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

,

Y. Trager

Semiconductor Characterization Section

(301) 921-3625

F. H. Brewer

Dr. W. M. Bullis, Chief'

Miss F. C. Butler*

Mrs. C. A. Cannon

Mrs. E. C. Cohen+

Dr. J. R. Ehrstein

Dr. K. F. Galloway

Mrs. A. D. Glover

W. J. Keery

Mrs. K. 0. Leedy

Dr. A. G. Lieberman

Dr. S. Mayo

Miss D. R. Ricks

R. I. Scace

H. A. Schafft

E. C. Watts+

L. R. Williams

Semiconductor Processing Section

(301) 921-3541

Dr. M. G. Buehler, Chief

J. M. David

M. R. Doggett

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

Electron Devices Section

(301) 921-3621

D. A. Maxwell

Dr. W. E. Phillips

L. A. Robinson

Dr. D. E. Sawyer

L. M. Smith

W. R. Thurber

Dr. J. H. Albers

D. W. Berning

D. L. Blackburn

G. W. Burdette"*" W. A. Cullins

Mrs. K. E. Dodson

H. E. Dyson

G . G . Harman

H. K. Kessler

Dr. D. B. Novotny

F. F, Oettinger, Chief

Mrs. B. A. Oravec*^

G. J. Rogers

S. Rubin

S. Ruthberg

" Secretary

t Part Time

§ Summer

H Telephone: (301) 921-3621

# Telephone: (301) 921-3821

X Telephone: (301) 921-3786

96
APPENDIX B SEMICONDUCTOR TECHNOLOGY PROGRAM PUBLICATIONS

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 IN DO iecn • iNote Date Issued JNiib Accession no

September 30, 1968 472 December 1968 AD 681330

December 31, 1968 475 February 1969 AD 683808

March 31, 1969 488 July 1969 AD 692232

June 30, 1969 495 September 1969 AD 695820

September 30, 1969 520 March 1970 AD 702833

December 31, 1969 527 May 1970 AD 710906

March 31, 1970 555 September 1970 AD 718534

June 30, 1970 560 November 1970 AD 719976

September 30, 1970 571 April 1971 AD 723671

December 31, 1970 592 August 1971 AD 728611

March 31, 1971 598 October 1971 AD 732553

June 30, 1971 702 November 1971 AD 734427

September 30, 1971 717 April 1972 AD 740674

December 31, 1971 727 June 1972 AD 744946

March 31, 1972 733 September 1972 AD 748640

June 30, 1972 743 December 1972 AD 753642

September 30, 1972 754 March 1973 AD 757244

December 31, 1972 773 May 1973 COM 73-50534

March 31, 1973 788 August 1973 AD 766918

June 30, 1973 806 November 1973 AD 771018

i| After July 1, 1973, progress reports were issued in the NBS Special Publication 400 subseries with the title. Semiconductor Measurement Technology:

I Quarter Ending NBS Spec. Publ. Date Issued NTIS Accession No.

;| September 30, 1973 400-1 March 1974 AD 775919

B.l. Prior Reports

400-4 November 1974 COM 74-51222

June 30, 1974

September 30, 1974

December 31, 1974

March 31, 1975)

June 30, 1975 f September 30, 1975

December 31, 1975 }

400-8

400-12

400-17

February 1975

May 1975

November 1975

AD/A 005669

AD/A 011121

AD/A 017523

400-19 April 1976 PB 251844

400-25 October 1976 PB 258555

97
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

been issued recently are listed below:

Schafft, H. A., Ed., Semioonduotor Measurement Teahnotogy: ARPA/NBS Workshop III. Test Pat

terns for Integrated Circuits, NBS Spec. Publ. 400-15 (January 1976).

Albers, J. H. , Ed., Semiaonduator Measurement Technology: The Destructive Bond Pull Test,

NBS Spec. Publ. 400-18 (February 1976).

Sawyer, D. E., and Berning, D. W. , Semiconductor Measurement Technology: Laser Scanning of

Active Semiconductor Devices - Videotape Script, NBS Spec. Publ. 400-27 (February 1976).

Sawyer, D. E. , and Berning, D. W., Laser Scanning of MOS IC's Reveals Internal Logic States

Non-Destructively, Proc. IEEE 6^, 393-394 (March 1976).

Lieberman, A. G., Introductory Concepts for Silicon Surface Analysis, Semiconductor Measurement Technology: ARPA/NBS Workshop IV. Surface Analysis for Silicon Devices, NBS Spec.

Publ. 400-23 (March 1976) pp. 3-6.

Lieberman, A. G., Semiconductor Measurement Technology: ARPA/NBS Workshop IV, Surface

Analysis for Silicon Devices, NBS Spec. Publ. 400-23 (March 1976),

Buehler, M. G. , Semiconductor Measurement Technology: Defects in Ptl Junctions and MOS

Capacitors Observed Using Thermally Stimulated Current and Capacitance Measurements - Videotape Script, NBS Spec. Publ. 400-26 (April 1976).

Kenney, J. M. , Semiconductor Measurement Technology: Permanent Damage Effects of Nuclear

Radiation on the X-Band Performance of Silicon Schottky-Barrier Microwave Mixer Diodes,

NBS Spec. Publ. 400-7 (April 1976).

Keery, W. J., Leedy, K. 0., and Galloway, K. F., Electron Beam Effects on Microelectronic De vices. Scanning Electron Microscopy/1976 , Toronto, April 5-9, 1976, pp. 507-514.

Harman, G. G., The Use of Acoustic Emission in a Test for Beam Lead Bond Integrity, 14th Annual Proceedings, Reliability Physics 1976, Las Vegas, Nevada, April 20-22, 1976, pp. 86-97.

Galloway, K. F. , Leedy, K. 0., and Keery, W. J., Electron-Beam-Induced Currents in Simple

Device Structures, Proceedings of the 26th Electronic Components Conference, San Francisco,

California, April 26-28, 1976, pp. 257-262.

Swyt, D. A., NBS Program in Photomask Linewidth Measurements, Solid State Technology 19 , No. 4, 51-57 (April 1976).

Buehler, M. G. , Semiconductor Measurement Technology: Microelectronic Test Pattern NBS-3

for Evaluating the Resistivity-Dopant Density Relationship of Silicon, NBS Spec. Publ.

400-22 (May 1976)

.

Ruthberg, S., Hermetic Test Procedures and Standards for Semiconductor Electronics, Proa. Symposium on Nondestructive Testing Standards, Gaithersburg , Maryland, May 19-21, 1976 (to

appear)

.

Novotny, D. B., Photoresist Exposure — Measurement and Control, Proc. Soo. Photo-Optical

Instrum. Engrs. 80, Developments in Semiconductor Microlithography , 9-13 (1976).

Swing, R. E., The Theoretical Basis of a New Optical Method for the Accurate Measurement of ; Small Line-Widths, Proc. Soc. Photo-Optical Instrum. Engrs. 80_, Developments in Semiconductor

Microlithography, 65-77 (1976).

98
APPENDIX B

Hower, P. L. , Blackburn, D. L. , Oettinger, F. F., and Rubin, S., Stable Hot Spots and Second

Breakdown in Power Transistors, PESO '76 Record, IEEE Power Electronics Conference 1976,

Cleveland, Ohio, June 8-10, 1976, pp. 234-246 (IEEE Cat. No. 76 CH 1084-3 AES)

Schafft, H. A., Ed., Semiconductor Measurement Technology: NBS/FDA Workshop, Reliability

Technology for Cardiac Pacemakers, NBS Spec. Publ. 400-28 (June 1976).

Galloway, K. F. , and Buehler, M. G. , The Application of Test Structures and Test Patterns to the Development of Radiation Hardened Integrated Circuits: A Review, NBSIR 76-1093 (August

1976).

Oettinger, F. F. , Blackburn, D. L., and Rubin, S., Thermal Characterization of Power Transistors, IEEE Trans. Electron Devices ED-23 , 831-838 (1976).

Buehler, M. G. , and Thurber, W. R. , A Planar Four-Probe Test Structure for Measuring Bulk

Resistivity, IEEE Trans. Electron Devices ED-23 , 968-974 (1976).

Buehler, M. G. , and Phillips, W. E. , A Study of the Gold Acceptor in a Silicon p'^n Junction

and an n-type MOS Capacitor by Thermally Stimulated Current and Capacitance Measurements,

Solid-state Electronics 19, (1976).

Mayo, W., Keller, R. A., Travis, J. C. , and Green, R. B., Detection of Sodium Trace Contamination in Furnace Atmospheres at 1000°C, J. Appl. Phys. 4_7, 4012-4016 (1976).

Galloway, K. F., Leedy, K. 0., and Keery, W. J., Electron-Beam-Induced Currents in Simple Dej vice Structures, IEEE Trans. Parts, Hybrids, and Packaging PHP-12 , 231-236 (1976).

Novotny, D. B., Sensitometry of Photoresists for Microelectronics, Proc. SPSE Symposium on

Sensitometry — Modern Aspects and Future Trends, Binghampton, N. Y., September 16-17, 1976

(to appear)

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, Proc. Microelectronics Seminar — Interface '76, Monterey, California, October 4-5, 1976 (to appear).

Buehler, M. G., and Sawyer, D. E. , Microelectronic Test Patterns for Use in Procuring Reliable, Custom Integrated Circuit Chips, 1976 Government Microcircuit Applications Conference Digest of Papers, Orlando, Florida, November 9-11, 1976, pp. 62-65.

Sawyer, D. E., and Berning, D. W., Thermal Mapping of Transistors With a Laser Scanner, Proc. IEEE bh^, 1634-1635 (1976).

Sawyer, D. E. , and Berning, D. W., Mapping Nonlinearities Over the Active Regions of Semiconductor Devices, Proc. IEEE 64, 1635-1637 (1976).

Harman, G. G., and Albers, J. H. , The Ultrasonic Welding Mechanism as Applied to Aluminum

and Gold Wire-Bonding in Microelectronics, Proc. Ldten und Schweissen in der Electronik,

Munchen, p. 113, November 25-26, 1976.

Mayo, S., Galloway, K. F. , and Leedy, T, F., Radiation Dose Due to Electron-Gun Metallization Systems, Trans. Nucl. Sci. NS-23 , 1875-1880 (1976).

Blackburn, D. L. , Rubin, S., and Rogers, G. J,, Measurements of Power Transistor Thermal

Instabilities, Stable Hot Spots and Second Breakdown, Technical Digest, 1976 International

Electron Devices Meeting, Washington, D.C., December 6-8, 1976, pp. 151-154.

Bullis, W. M., and Galloway, K. F., Semiconductor Electronics at the National Bureau of

Standards: Programs and Opportunities, Proc. 2nd IEEE (G-PEP) /ISEM University/Industry/

Government Microelectronics Symposium, Albuquerque, N. M. , January 3-5, 1977, Paper IX-1.

99
APPENDIX B

B.3. 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 either of the videotapes.

The following videotapes are available for distribution:

1. Defects in PN Junctions and MOS Capacitors M. G. Buehler August 1974

Observed Using Thermally Stimulated Current and

Capacitance Measurements

2. Laser Scanning of Active Semiconductor Devices D. E. Sawyer and October 1975

D. W. Berning

B.4. 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. Publications and reports of this kind which

have appeared since the beginning of the contract efforts are listed below:

Steigerwalt, S. T., and Lagnado, I., The CCD as a Test Structure for Semiconductor Process

Control, Microeleatronics J. 1_, 23-29 (1975).

Rudenberg, H. G., Automated Integrated Circuit Processing and Assembly, NBS-GCR-76-64 (April

1975)

.

Duffy, M. T., Zanzucchi, P. J., and Cullen, G. W., Method to Determine Quality of Sapphire,

NBS-GCR-76-61 (September 1975).

Johannessen, J. S., Spicer, W. E. , and Strausser, Y. E. , Phase Separation in Silicon Oxides

as Seen by Auger Electron Spectroscopy, Aippl. Phys. Letters 2]_, 452-454 (1975).

Johannessen, J. S., Spicer, W. E. , and Strausser, Y. E., Use of Auger Electron Spectroscopy

to Determine the Structure of Silicon Oxide Films, Semiconductov Measurement Technology:

ARPA/NBS Workshop IV. Surface Analysis for Silicon Devices, NBS Special Publication 400-23,

pp. 119-123 (March 1976).

Strausser, Y. E., and Johannessen, J. S., An Auger Electron Spectroscopy Study of Silicon

Spectra from Silicon Monoxide, Silicon Dioxide, and Silicon Nitride, Semioonduotor Measurement Technology: ARPA/NBS Workshop IV. Surface Analysis for Silicon Devices, NBS Special

Publication 400-23, pp. 125-138 (March 1976).

Kasdan, H. L. , and George, N., Linewidth Measurement by Diffraction Pattern Analysis, Proa.

Soc. Photo-Optical Instrum. Engrs. 80_, Developments in Semiconductor Micro lithography , 54-

63 (1976).

Johannessen, J. S., Spicer, W. E., and Strausser, Y. E. , An Auger Analysis of the Si02-Si

Interface, J. Appl. Phys. 4_7, 3028-3037 (1976).

Weglein, R. D. , Acoustic Properties of Sputtered Glass at Microwave Frequencies, Appl. Phys.

Letters 29_, 277-279 (1976).

Taylor, N. J., Johannessen, J. S., and Spicer, W. E., Crater-Edge Profiling in Interface

Analysis Employing Ion-Beam Etching and AES, Appl. Phys. Letters 29_, 497-499 (1976).

100
APPENDIX B

Goodman, A. M. , Semioonductor Measurement Technology. The Safe Operation of Capacitance

Meters Using High Applied Bias Voltage, NBS Special Publication 400-34 (December 1976).

Wilson, R. G., Planar and Axial Channeling of 800-keV As in <110> Si, Appl. Phys. Letters

29, nO-ni (1976).

B.5. Availability of Publications

In most cases reprints of articles in technical journals may be obtained on request to the author. NBS Technical Notes and Special Publications are available from the Superindendent of Documents, U.S. Government Printing Office, Washington, D. C. 20402, or

the National Technical Information Service, Springfield, Virginia 22161, or both.

Current information regarding availability of all publications issued by the Program is p vided in the latest edition of NBS List of Publications No. 72 which can be obtained on r quest to Mrs. E. C. Cohen, Room A327, Technology Building, National Bureau of Standards,

Washington, D. C. 20234.

101
APPENDIX C WORKSHOP AND SYMPOSIUM SCHEDULE

C.l. Proceedings or Reports of Past Events :

Symposium on Silicon Device Processing, Gaithersburg, Maryland, June 2-3, 1970.

(Cosponsored by ASTM Committee F-1 and NBS) . Proceedings: NBS Spec. Publ.

337 (November 1970)

.

ARPA/NBS Workshop I. Measurement Problems in Integrated Circuit Processing

and Assembly, Palo Alto, California, September 7, 1973. Report: NBS

Spec. Publ. 4C0-3 (January 1974).

ARPA/NBS Workshop II. Hermeticity Testing for Integrated Circuits, Gaithersburg

Maryland, March 29, 1974. Report: NBS Spec. Publ. 400-9 (December 1974).

Spreading Resistance Symposium, Gaithersburg, Maryland, June 13-14, 1974. (Cosponsored by ASTM Committee F-1 and NBS). Proceedings: NBS Spec. Publ.

400-10 (December 1974)

.

ARPA/NBS Workshop III. Test Patterns, Scottsdale, Arizona, September 6, 1974.

Report: NBS Spec. Publ. 400-15 (January 1976).

ARPA/NBS Workshop IV. Surface Analysis for Silicon Devices, Gaithersburg,

Maryland, April 23-24, 1975. Proceedings: NBS Spec. Publ. 400-23

(March 1976)

.

NBS/FDA Workshop. Reliability Technology for Cardiac Pacemakers, Gaithersburg,

Maryland, July 28-29, 1975. Report: NBS Spec. Publ. 400-28 (June 1976).

Reliability Technology for Cardiac Pacemakers, II, Gaithersburg, Maryland,

July 19-20, 1976. Report: in preparation.

102
APPENDIX D STANDARDS COMMITTEE ACTIVITIES

ASTM Connnittee F-1 on Electronics

J. H. Albers, Microelectronic Materials and Packaging SubcommitteeM. G. Buehler, Semiconductor Physical Properties, Semiconductor Process Materials,

Electrical and Optical Measurements, Microelectronic Materials and Packaging,

Microelectronics Imaging, and Quality and Hardness Assurance Subcommittees

*W. M. Bullis, Secretary; Editor, Semiconductor Physical Properties Subcommittee

J. R. Ehrstein, Semiconductor Physical Properties, Electrical and Optical Measurements, and Quality and Hardness Assurance Subcommittees

*J. C. French, Chairman, Editorial Subcommittee

G. G. Harman, Microelectronic Materials and Packaging Subcommittee

K. 0. Leedy, Semiconductor Physical Properties, Electrical and Optical Measurements, Microelectronic Materials and Packaging, and Quality and Hardness

Assurance Subcommittees

*C. P. Marsden, Honorary Chairman

R. L. Mattis, Editor, Semiconductor Measurements Subcommittee; Semiconductor

Physical Properties Subcommittee

*J. F. Mayo-Wells, Secretary, Editorial Subcommittee

D. B. Novotny, Editor, Microelectronics Imaging Subcommittee; Semiconductor Physical Properties and Microelectronic Materials and Packaging Subcommittees

W. E. Phillips, Semiconductor Physical Properties and Electrical and Optical Measurements Subcommittees

G. J. Rogers, Lasers and Quality and Hardness Assurance Subcommittees

S. Ruthberg, Microelectronic Materials and Packaging Subcommittee

*R. I. Scace, Second Vice-Chairman

*H. A. Schafft, Publicity Officer

A. H. Sher, Semiconductor Physical Properties, Semiconductor Process Materials,

Electrical and Optical Measurements, Microelectronic Materials and Packaging,

Microelectronics Imaging, and Quality and Hardness Assurance Subcommittees

W, R. Thurber, Semiconductor Physical Properties and Electrical and Optical Measurements Subcommittees

ASTM Committee E-10 on Radioisotopes and Radiation Effects

W. M. Bullis, Subcommittee on Radiation Effects on Electronic Materials and Devices and

Pulsed Radiation Effects

J. C. French, Subcommittee on Radiation Effects on Electronic Materials and Devices and

Pulsed Radiation Effects

R. I. Scace, Subcommittee on Radiation Effects on Electronic Materials and Devices and

Pulsed Radiation Effects

)| All subcommittees

103
APPENDIX D

Electronic Industries Association: Solid State Products Division, Joint Electron Device

Engineering Council (JEDEC)

D. L. Blackburn, Task Group JC-25-5 on Thermal Characterization on Power Transistors

and Committee JC-25 on Power Transistors

F, F. Oettinger, Chairman, Task Group JC-25-5 on Thermal Characterization on Power

Transistors, Committee JC-25 on Power Transistors; Technical Advisor, Thermal

Properties of Devices, Committees JC-13.1 on Government Liaison for Discrete

Semiconductor Devices, and JC-22 on Rectifier Diodes and Thyristors

S. Rubin, Chairman, Council Task Group on Galvanomagnetic Devices

D. E. Sawyer, Task Group JC-2A-5 on Transistor Scattering Parameter Measurement

Standard, Committee JC-24 on Low Power Transistors

H. A. Schafft, Technical Advisor, Second Breakdown and Related Specifications

Committee JC-25 on Power Transistors

lEC TC47, Semiconductor Devices and Integrated Circuits

S. Rubin, Technical Expert, Galvanomagnetic Devices; U.S. Specialist for Working

Group 5 on Hall Devices and Magnetoresistive Devices

IEEE Electron Devices Society

J. C. French, Standards Committee

F. F. Oettinger, Standards Committee Task Force on Second Breakdown Measurement

Standards

H. A. Schafft, Standards Committee Task Force on Second Breakdown Measurement

Standards

IEEE Industry Applications Society

F. F. Oettinger, Vice-Chairman, Standards Subcommittee, Power Semiconductor Committee

IEEE Magnetics Group

S. Rubin, Chairman, Galvanomagnetic Standards Subcommittee

Semiconductor Equipment and Materials Institute

R. I. Scace, Standards Committee

Society of Automotive Engineers

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

Processes

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

F. F. Oettinger, Steering Committee, Electronic Systems Committee

104
APPENDIX E SOLID-STATE T E C H PI 0 L 0 G Y S F A B R I C A T I 0 fl SERVICES

Technical services in areas of competence are provided to other NBS activities and other

government agencies as they are requested. Usually these are short-term, specialized services that cannot be obtained through normal commercial channels. Such services provided

during this and the previous reporting period, which are listed below, indicate the kinds

of technology available to the program.

I E.l. Scanning Electron Microscopy (W. J. Keery)

Tips of surface profile instruments were examined for the NBS Optics and Micrometrology

1 Section for use in an ANSI Round Robin.

Samples of fire retardant gypsum wall board, both new and fire damaged, were examined

for the NBS Fire Safety Engineering Division.

E.2. Semiconductor Device Fabrication (T. F. Leedy and J. Krawczyk)

' MOS capacitors were prepared for the Harry Diamond Laboratories for use in radiation hardj ness and charge transport studies.

Several special silicon samples for use in a metal-vacuum-metal tunneling experiment were

I prepared for the NBS Mechanics Division. 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,

I evaporation of aluminum and copper films, and use of preferential plasma etching, scribing,

and mounting chips onto headers

E.3. Metal Evaporation (J. Krawczyk)

Several special emulsion reticles were prepared for the NBS Physical Chemistry Division.

' E.4. Test Pattern Applications (M. G. Buehler)

Work was initiated on development of process validation wafers to assist the Navy Special

Projects Office* in identification and solution of yield problems in obtaining complex

I beam-lead, dielectrically-isolated , Schottky- clamped TTL circuits.

I Assistance was provided to the National Security Agency^ in designing a test pattern to

i evaluate C-MOS on sapphire processes at various prospective vendors. Eighteen test strucI tures were designed for measuring process and device parameters.

"SP-27 MIPR No. N00154-76-MP-01+590 and N00164-6-04590 . (NBS Cost Center 4251547).

tNSA MIPR No. H 98230-R3-76-8132. (NBS Cost Center 4259562).

105
INDEX*

acoustic emission 69-72

acoustic microscopy 79-83

ASTM Method F 374 7-9

Auger electron spectroscopy 31-35

beam-lead bonding 69-72

bond monitor, in-process 72-74

boron nitride diffusion sources 47

capacitance-voltage method 26,28 corona discharge method 42-47

cross-type sheet resistor 64-65

depletion approximation Z dopant profiles 11-13; 31-35

electron beam evaporator 26,28; 36,38-40

electron mobility 17-19

electron-beam-induced-current mode 85-88

electron-beam-induced damage 84-85

filar eyepiece 53-58

film carrier bonding 72 flame emission spectrometry 19-20

flying-spot scanner 60; 77-79

four-probe array, square 39-40

four-probe method 7-9

gross leak tests 55-56

hermeticity 74-75

hot spots 88-89

hydrogen chloride oxidation 47

image-shearing eyepiece 53-58

incremental sheet resistance 11-13

infrared microradiometer 77-79

infrared reflectance method 29-30

in-process bond monitor 72-74

ion implantation 40-42

ion microprobe mass analysis 3 Irvin's curves 15-17

junction temperature 77-79

line-width measurement 48-50; 50-52; 53; 53-

56; 57-58

line-width standards 53

loose particle detection 75-76

microdensitometer 60

microscopy, optical (theory) 48-50; 50-52

modular test patterns 65-66

NBS-3 test pattern 15-17; 17-19

neutron activation analysis 30-31

optical overlay method 59 oxidation furnace atmosphere 36-37

oxide films 18-19; 26,28

particle impact noise detection (FIND) test 75-76

passivation overcoats 42-47

photomask inspection 58-61

photomask metrology 48-58

photovoltaic method 13-15

power-device silicon 13-15; 19-24

process validation wafer 65-66

radioisotope leak test 74-75

resistivity variations 13-15

safe operating area, transistor 80-89

sapphire, silicon on 26,28; 29-30; 30-31; 65,

67-68

scanning acoustic microscopy 79-83

scanning electron microscopy 84-88

scanning low energy electron probe 88

Schottky barrier diodes 40

second breakdown 88-89

sheet resistor test structures 64-65

sodium contamination 36-37

spatial filtering techniques 48-50; 57-58;

59-60

spreading resistance 9-11, 11-13

square array collector resistor 62-64

surface preparation 9-11; 29-30

surface roughness 29-30

test patterns 61; 65-66; 65,67-68

thermally stimulated current 19-27

transistor, power 88-89

transistor-transistor logic circuit 65-66

TV-microscope method 53-58; 60-61

ultrasonic wire bonding 72-74

ultraviolet reflectance 29-30

van der Pauw sheet resistor 64-65

water vapor contamination 47 wire bonds 72-74

x-ray damage 26,28; 36,38-40

Page numbers in italics refer to entries in the Highlights section for those topics for

which detailed discussions do not appear in the main body of the report.

106 <rU. S. GOVERNMENT PRINTING OFFICE: 1977--240-848/6
NBS.114A,(REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS Spec. Publ. 400-29

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE 5. Publication Date Semiaonduator Ueasuvement Technology:

January 1 to June 30, 1976

Progress Report April 1977

6. Performing Organization Code

7. AUTHOR(S)

W. Murray Bullis

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No. see item 15

12. 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 Coveredfj^j-g^im; j^^^

to June 30, 1976

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

ARPA Order 2397, Program Code 6D10; DNA lACRO 76-806, SSPO Order N0016475P070030

,

MIPR FY76 167600366, ERDA Contract E(49-l)-3300, Mod. 3.

16. ABSTRACT (A 200-word or less factual summary of most sigfxificant information. If document includes a significant

bibliography or literature survey, mention it here.) This progress report describes NBS activities directed toward the development of

methods of measurement for semiconductor materials, process control, and aevices.

Both in-house and contract efforts are included. The emphasis is on silicon device

technologies. Principal accomplishments during this reporting period included (1)

development of theoretical expressions for electron mobility in silicon based on combinations of scattering mechanisms; (2) successful low-temperature processing of MOS

capacitors to permit measurement of thermally stimulated current and capacitance

without subjecting the specimens to potentially degrading heat treatments; (3) com- pletion of a study of the thermodynamics of reactions in an oxidation furnace tube

which provides a basis for models of the effect of water vapor, chlorine, and tube

wall conditions on sodium contamination levels; (4) development of a rapid, nondestructive method for reverse decoration of defects in passivation overcoats; (5)

development of the theoretical basis for accurate measurement of small line widths

by analysis of a spatially filtered image of the line; (6) extension of the acoustic

emission technique to the nondestructive testing of tape-bonded chips and hybrid

components; and (7) analysis of the results of a first exploratory interlaboratory

evaluation of the radioisotope method for testing hermeticity of semiconductor devices. New tasks were undertaken to develop techniques for measuring resistivity

uniformity of and non-dopant impurities and defects in power-device grade silicon

and to investigate the particle impact noise detection method for screening (over)

17. 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

)

Acoustic emission; Auger electron spectroscopy; beam-lead bonds; capacitance-voltage

methods; carrier mobility; C-MOS circuits; dopant profiles; electrical properties;

electronics: four-probe method; hermeticity; ion implantation; Irvin's curves j> (over)

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16. ABSTRACT (continued)

devices for the presence of loose particles in the package; initial results were

obtained in the study of scanning acoustic microscopy which was begun during the

previous reporting period. Because of technical limitations in the methods, efforts to develop both the automated scanning low energy electron probe and inprocess ultrasonic bond monitor were terminated. Also reported is other on-going

work on materials characterization by electrical and physical analysis methods,

materials and procedures for wafer processing, photolithography, test patterns, and

device inspection and test procedures. Supplementary data concerning staff, publications, workshops and symposia, standards committee activities, and technical ser- vices are also included as appendices.

17. KEY WORDS (continued)

leak tests; line-width measurement; measurement methods; microelectronics; optical

flying-spot scanner; oxidation particle impact noise detection test; passivation overcoats; photovoltaic method; power-device grade silicon; radioisotope method; resistivity; resistivity variations; safe operating area, transistor; scanning acoustic microscope; scanning electron microscope; scanning low energy electron probe; semiconductor

devices; semiconductor materials; semiconductor process control; silicon; silicon dioxide; silicon on sapphire; sodium contamination; spreading resistance; surface roughnessi

test patterns; thermally stimulated current; transistors, power; TTL circuits; ultrasonic wire bonding; ultraviolet reflectance; x-ray damage.

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