1> NBS SPECIAL PUBLICATION 400

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

I

feurement Technology

lugicdd iu:|juiy

October 1, 1976 to March 31, 19771
NATIONAL BUREAU OF STANDARDS

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

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

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

the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

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

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

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

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

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

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

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

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities' — Radiation Research — Thermodynamics and

Molecular Science — Analytical Chemistry — Materials Science. THE NATIONAL ENGINEERING LABORATORY provides technology and technical ser- vices to the public and private sectors to address national needs aad to solve national problems; conducts research in engineering and applied science in support of these efforts; builds and maintains competence in the necessary disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities;

provides engineering measurement traceability services; develops test methods and proposes

engineering standards and code changes; develops and proposes new engineering practices;

and develops and improves mechanisms to transfer results of its research to the ultimate user. The Laboratory consists of the following centers:

Applied Mathematics — Electronics and Electrical Engineering- — Mechanical

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

Consumer Product Technology — Field Methods.

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

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

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

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

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

The Institute consists of the following centers:

Programming Science and Technology — Computer Systems Engineering.

'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address Washington, DC 20234. -Some divisions within the center are located at Boulder, CO 80303.
Semiconductor Measurement Technology

October 1, 1976 to March 31, 1977

W. Murray Bullis, Editor

Electron Devices Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Washington, D.C. 20234

Jointly supported by: The National Bureau of Standards The Navy Strategic Systems Projects Office The Defense Nuclear Agency The Defense Advanced Research Projects Agency The Energy Research and Development Administration, and The Naval Ocean Systems Center

U.S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Jr., Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued November 1979
National Bureau of Standards Special Publication 400-38

Nat. Bur. Stand. (U.S.), Spec. Publ. 400-38, 95 pages (Nov. 1979) CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1979

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

Stock No. 003-003-02139-3 Price $3.50

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

li- U.S. GOVERNMENT PRINTING OFFICE: 1979 O—311-065 (272)
TABLE OF C 0 E NT S

SEMICONDUCTOR MEASUREMENT TECHNOLOGY

PAGE

List of Figures iv

List of Tables vii

Preface viii

Abstract and Key Words 1

1. Introduction 1

2. Highlights 2

3. Materials Characterization by Electrical Methods 6 3.1. Photovoltaic Method 6 3.2. Thermally Stimulated Current and Capacitance Measurements .... 10

3.3. Indium-Doped Silicon 15

4. Materials Characterization by Physical Analysis Methods 17

4.1. Electron Spectroscopy Techniques 17

4.2. Techniques for Chemical Diagnostics in Semiconductor Processing 20

4.3. Optical Test for Surface Quality of Sapphire Substrates 22

5. Materials and Procedures for Wafer Processing 25

5.1. Sodium Contamination in Oxidation Furnaces 25

6. Photolithography 26

6.1. Calibration Standards for Optical Photomask Line-Width Measurements 26

6.2. Optical Line-Width Measurements on Silicon and Iron-Oxide Photomasks 28

6.3. Optical Line-Width Measurements in Reflected Light 32

6.4. Submicrometer Line-Width Measurements 35

6.5. Primary Line-Width Calibration 39

7. Test Patterns 43

7.1. Test Patterns for Resistivity-Dopant Density Evaluation 43

7.2. Sheet Resistor Test Structures 43

7.3. Overetch Structure for Visual Measurement 48

7.4. Test Pattern Design Concepts 49

7.5. Probe Pad Evaluation 52

7.6. Computer Aided Design and Layout of Test Pattern Photomasks 53

7.7. Advanced Planar Silicon Test Structures 54

7.8. Test Pattern Design and Analysis for SOS/LSI 56

8. Assembly and Packaging 61

8.1. Radioisotope Leak Test 61

9. Device Inspection and Test 65

9.1. Scanning Acoustic Microscopy 65

9.2. Integrated Circuit Die Attachment Evaluation 70 9.3. Transistor Collector-Emitter Saturation Voltage 72

10. References 75

Appendix A Semiconductor Technology Program Staff 78

Appendix B Semiconductor Technology Program Progress Reports 79

Appendix C Computation of Dark-Field Line Images 81

iii
LIST OF FIGURES

PAGE

1. Photograph of probe holder with point contacts for photovoltaic scanning

apparatus 6

2. Examples of radial resistivity profiles measured on 2-in. diameter, n-type,

float-zoned silicon slices with photovoltaic and four-probe methods 7

|

3. Results of repeatability study of photovoltaic method for measuring radial

resistivity profiles 8 |

4. Schematic diagram of specimen geometry used for analysis of photovoltaic

method for measuring radial resistivity variation 9

5. Typical thermally stimulated capacitance and current responses of a gold-doped i n+p diode 11 '

6. Wafer maps of gold donor and boron acceptor densities in a 2-in. diameter sili- ' con wafer 12

7. Mesa diode fabrication process 13

8. Photomicrograph of typical ultrasonically formed mesa diode 13

9. Photomicrograph of typical plasma-etched mesa diode 13 j'

10. Postulated mesa-diode fabrication process using five-level photomask set .... 14 '

11. Resistance of an indium-doped silicon specimen at 77 K as a function of added

donor concentration 16

12. Computed signal obtained from a step interface with an Auger electron spectrometer with a cylindrical mirror analyzer 18 •

13. Oxygen concentration profile in MNOS structure 18

14. Oxygen KLL and silicon LVV signals as a function of distance near the interface

between silicon dioxide and silicon 19

15. Model for silicon-silicon dioxide interface 19

16. Diagram of variable speed spinner for controlled wet chemical etching 21

17. The oxygen-to-silicon ratio in a silicon-dioxide film as a function of distance

from the oxide-silicon interface 22

18. Example of data enhancement techniques 22

19. Correlation of SOS MOSFET channel current with infrared reflectance data of | corresponding sapphire substrates 23

20. Correlation of gated Hall mobility of SOS structures with infrared reflectance

data of corresponding sapphire substrates 23

21. Correlation of SOS MOSFET channel current with ultraviolet reflectance data of

corresponding epitaxial silicon films 24

22. Correlation of gated Hall mobility of SOS structures with ultraviolet reflectance data of corresponding epitaxial silicon films 24

I

Iv
LIST OF FIGURES

PAGE

23. A view of a pattern group on the chromium-on-glass line-width measurement

, artifact 26

24. Computed image profiles of a 2-pm clear line for an objective lens with

i numerical aperture of 0.90 and various condenser lenses and phase shifts .... 30

25. Schematic illustration showing method of computing threshold at edge with phase

j shift present 31

26. Schematics of optical microscope systems 33

27. Theoretical dark-field image of clear lines for 0.60 objective N.A. and 0.80 to 0.95 condenser N.A 34

28. Theoretical calibration curve for dark-field illumination, line width measured

i between peaks versus true line width for 0.85 objective N.A., 0.85 to 1.00

I condenser N.A. , and diffraction-limited optics 35

j29. Interference image of an iron-oxide artifact as seen in a Watson-Barnett inter-

] ference attachment for the microscope 35

j30. Theoretical image profiles of submicrometer clear lines from 0.4 to 1.0 pm wide

I on an opaque background for the optical scanning photometric microscope 36

jj31. Theoretical calibration curves for line-width measurements on chromium/chromium

j oxide artifacts for 0.60 condenser and 0.90 N.A 36

I j'32. Theoretical calibration curves for line-width measurements on see-through photoI, masks or wafers with 31-percent background transmittance or reflectance 37

33. Theoretical image profiles as a function of focus position of adjacent 0.5-ijm

lines and spaces for T =0.31 and A = tt/2 38

,34. Photomicrographs of a portion of resist pattern on a wafer showing nominal 0.5-

\im lines and spaces 38

35. Experimental image profile of nominal 0.5-pm lines and spaces in resist on a wafer before carbon coating 39

f'136. Schematic of the SEM/interf erometer system 40

37. Photograph of scanning stage with flexure-pivot system 40

38. Direct output of the SEM and interferometer data channels for a nominally 2-Mm

; wide opaque line 41

1^9. Line-edge profile for the model and a comparison of an experimental SEM line-

:ji edge trace with the calculated trace based on the model 41

jiO. Photomicrograph of test pattern NBS-4 for characterizing the resistivity-dopant

density relation in silicon 44

41. Four of the base sheet resistors on test pattern NBS-4 46

1+2. Base sheet resistance across diameter of Wafer B59Ph-l fabricated with test pattern NBS-4 47

I>3. Base sheet resistance across diameter of Wafer B3.0B-3 fabricated with test pat-

;i tern NBS-4 48

i| v

)

I
LIST OF FIGURES

PAGE

44. Base sheet resistance values for Wafer B92B-2 fabricated with test pattern NBS-4 48

45. Test structure for etch control . 49

46. Computer layouts of two quadrants of test pattern NBS-9 for dielectrically isolated TTL technology showing test structures arrayed in both the 2 by 10 modular

array and around the periphery of the die 50

47. Photomicrograph of test pattern NBS-5 for probe pad and probe card evaluation . . 52

48. Number of failures per cell as a function of relative position when probing

40-pm probe pads using different probe cards 53

49. Modular structure concept for photomask design 54

50. Flow chart indicating steps required to take a structure design to a working

mask set 54

51. Advanced test structure, showing common bus lines and gated diode with

integral reset transistor and MOSFET electrometer 55

52. Computer layout of test pattern for evaluation of advanced test structures ... 55

53. Complete test sequence for ungated van der Pauw test structure 57

54. Flow chart for complete UVDPl test sequence 58

55. Performance of gated van der Pauw structures at room temperature 60

56. Computed sample standard deviations, Sj, of measured leak sizes for all test leaks as a function of average leak size, Xj 63

57. Computed sample standard deviations, Sj-, of residuals for each test specimen

in the fine leak range as a function of average leak size, Xj . 64

58. Computed sample standard deviations, s^-, of residuals for each test specimen

in the gross leak range as a function of average leak size, Xj 64

59. Stacking fault on unoxidized [100] epitaxial silicon layer 25 ym thick grown

on sapphire substrate 65

60. Acoustic micrograph of pinhole defects in a 1.5-)jm thick borosilicate glass

layer sputtered on a flat polished sapphire substrate 66

61. Computed strength of reflected acoustic signal as a function of axial position

of single surface of sapphire , 66

62. Contrast reversal of acoustic micrographs of oxidized silicon pattern resulting

from change in focal position 67

63. Aluminum gate, silicon-on-sapphire STDL-4 integrated circuit 68

64. Cross sections of acoustic gray scale test patterns for measurement of contrast

characteristics of a scanning acoustic microscope 70

65. Laser scanner photoresponse image of integrated circuit used in this study ... 70

66. Circuit for evaluating integrated-c ircuit die-attachment integrity 71

vi
LIST OF FIGURES

PAGE

Percent increase in junction temperature as a function of intentional void

area 71

'*68. Test circuit for measuring V(-.]7^gy^r|,^ 72

69. Voltage waveform as observed with an oscilloscope in the pulsed method 74

'^70. Measuring circuit with clamping circuit attached 74

LIST OF TABLES

PAGE

1. Summary of Analysis of Spreading Resistance Method for Measuring Radial Resistivity Variation 9

2. Donor Formation in Silicon With Various Oxygen Contamination Levels on Heat

Treatment at 450°C 15

'3. Sample Optical Line-Width Calibration Data 27

14. Comparison of Filar-Eyepiece Measurements With Scanning Photometric Microscope

! Measurements of the Same Lines When the Eyepiece is Calibrated Against the

I Photometric System 29

p. Comparison of Image-Shearing Eyepiece Measurements With Scanning Photometric

J Microscope Measurements of the Same Lines When the Eyepiece is Calibrated

Against the Photometric System 29

|6. Computed Thresholds Corresponding to Edge Location 31

i|7. Line-Width Errors Resulting from Locating the Edges at the Center of the Adja-

'! cent Dark Band 32

js. Average Line-Width Measurements As Determined With the Optical Scanning Photo1 metric Microscope and the SEM/Interf erometer for a Black-Chromium Artifact ... 42

'p. Planar Test Structures on Test Pattern NBS-4 45

jp-O. Initial Parameters and Contact Pad Identification for Testing Ungated van der

Pauw (Cross) Sheet Resistors 57

11. Regression Coefficients for Radioisotope Leak Test Results 63

vii

1!
PREFACE

The Semiconductor Technology Program serves

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

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

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

controlling device fabrication processes.

This research leads to carefully evaluated

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

placed on the dissemination of the results

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

cost, and higher reliability of semiconductor

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

procurement by providing a coirjnon basis for

the purchase specifications of government

agencies, and, in addition, provides a basis

for controlled improvements in fabrication

processes and in essential device characteristics.

During this reporting period, the Program received direct financial support principally

from two major sponsors: the Defense Advanced Research Projects Agency (ARPA) and

the National Bureau of Standards (NBS). "I" In addition, the Program received support from

the Defense Nuclear Agency (DNA) , § the Navy

Strategic Systems Project Office,^ the Energy Research and Development Administration,'''*

and the Naval Ocean Systems Center.^ The

ARPA-suppor ted 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 em- phasize aspects of the work which relate to the specific needs of the supporting agency.

Measurement-oriented research 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 scheduled

symposia and workshops. In addition to progress reports such as this one, more detailed

reports such as state-of-the-art reviews, lit

erature compilations, and summaries of technical efforts conducted within the Program

are issued as these activities are completed.

Reports of this type which are published by

NBS appear in the NBS Special Publication 400

subseries

.

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.

Discla-tmer

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 7D10

(NBS Cost Center ^257555). Unless otherwise noted, work was funded from this source

.

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

§ Through Inter-Agency Cost Reimbursement Or- der 77-809 (NBS Cost Center 4259522).

H Code SP-2 3, through project order

N0016476WR7G036 administered by Naval Weap- ons Support Center, Crane, Indiana (NBS

Cost Center 4251533)

.

# Through ERDA Task Order A021-EES (NBS Cost

Center 4259561)

,

X Through Order N0095377M009005 (NBS Cost

Center 4259563)

.

viii
SEMICONDUCTOR MEASUREMENT TECHNOLOGY

PROGRESS REPORT

October 1, 1976 to March 31, 1977

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) refinement and extension of the analysis of the nondestructive, photovoltaic method for measuring radial resistivity variation of silicon slices; (2) development of a donor-addition technique for testing for the presence of X-levels in indium-doped silicon;

(3) development of a precision, wet chemical etching procedure for use in conjunction with x-ray photo- electron spectroscopy for profiling silicon dioxide-silicon interfaces; (4) completion of thermodynamic calculations of equilibrium sodium density in oxidation atmospheres contained in polycrys t al 1 ine silicon

tubes; (5) initial extensions of the line-width measurement procedure to partially transparent and re- flecting specimens and to submicrometer dimensions; and (6) completion of the development of a test pat- tern for characterizing a large-scale- integrat ion, silicon-on-sapphire process. Also reported is other ongoing 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 and publications are included as appendices.

Key Words: Auger electron spectroscopy; die attachment; electrical properties; electronics; hermeticity; indium-doped silicon; infrared reflectance; integrated circuits; line-width measurements;

oxidation; photolithography; photovoltaic method; power-device grade silicon; radioisotope leak test; resistivity variations; resistors, sheet; scanning acoustic microscope; semiconductor materials; semi- conductor process control; silicon; silicon dioxide; silicon-on-sapphire; sodium contamination; test patterns; test structures; thermally stimulated current and capacitance; transistors; ultraviolet re- flectance; V . ,; x-ray pho t oe lec t ron spectroscopy.

1 . INTRODUCTION

'This is a report to the sponsors of the Semi-

' conductor Technology Program on work carried

, out during the thirty-fourth and thirty-fifth

quarters of the Program. The report summaIrizes research on a wide variety of measurement methods for semiconductor materials,

iiprocess control, and devices that are being

studied at the National Bureau of Standards

(NBS). The Program, which emphasizes siliconibased device technologies, is a continuing

one, and the results and conclusions reported

here are subject to modification and refineiment

.

"The Program is divided into a number of task

jjareas, each directed toward the study of a i|particular material or device property or mea-

!l surement technique. This report is subdiI vided according to these tasks. Highlights

of activity during the reporting period are

given in section 2. This section provides a 'imanagement-level overview of the entire efjfort. Subsequent sections deal with each

fispecific task area. References cited are

illisted in the final section of the report.

The report of each task includes a narrative

idescription of progress made during this re-

;porting period. Additional information con-

^icerning the material reported may be obtained directly from individual staff memibers identified with the task in the report.

Organizational locations for Program staff

members at the end of the reporting period,

listed in Appendix A, are given for record

purposes only; many changes have occurred

because of a reorganization of the NBS which

took place on April 9, 1978.

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. Because of the long delay

between the end of the reporting period and

the issuance of this report, reports of this

type are not listed herein. A complete listing of publications issued by the Program is

available in NBS List of Publications LP72,

Semiconductor Measurement Technology, which

can be obtained on request to Electron Devices Division, National Bureau of Standards,

Washington, D.C. 20234; telephone (301) 921-

3786.

This is the next to last of these comprehensive progress reports. Work carried out after October 1, 1977 is reported in condensed

form under the title. Semiconductor Technology Program, Progress Briefs, in the NBSIR

series. The work carried out between April 1 and September 30, 1977 will be reported in

NBS Special Publication 400-45.

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) refinement and extension of the analysis of the nondestructive, photovoltaic method for measuring radial resistivity variation

of silicon slices;

(2) development of a donor-addition

technique for testing for the presence of

X-levels in indium-doped silicon;

(3) development of a precision, wet chemical etching procedure for use in conjunction

with x-ray photoelectron spectroscopy for profiling silicon dioxide-silicon interfaces;

(4) completion of thermodynamic calculations of equilibrium sodium density in oxidation atmospheres contained in polycrystalline

silicon tubes;

(5) initial extensions of the line-width

measurement procedure to partially transparent and reflecting specimens and to submicrometer dimensions; and

(6) completion of the development of a test pattern for characterizing a large-scaleintegration, silicon-on-sapphire process.

Unless another organization is identified,

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

of Standards

.

Materials Chavactevization by Electrical Methods — Automation of the photovoltaic scanning

apparatus to measure radial resistivity variations of circular semiconductor slices was completed. It was found that the most re- peatable results and profiles most like those

determined by the four-probe method could be

obtained on surfaces lapped with 12-Mm alumi- num grit. A spread of less than 3 percent of

the average resistivity was obtained in a preliminary set of eight measurements made over

a period of 4 days along the diameter of a single slice. This reproducibility is ex- pected to be adequate for use in evaluating

incoming float-zoned slices. The previous

analysis of the photovoltaic and photoconductive signals was refined and extended. The

algebraic simplification achieved through the use of a modified transformation equation

made it possible to improve the treatment of

the nature of the light spot in the analysis;

it was found that the expression for the gradient of the resistivity along the scanning

diameter should be 4/3 times the previously

obtained expression. In addition, it was po' sible to derive expressions for cases where

the current and voltage probes are arbitrari.

ly placed around the edge of the slice instead of being restricted to the ends of thej

scanning diam.eter.

The newly developed hot/cold wafer probing a paratus was used to map the densities of the

electrically active gold donor and the boron

acceptor centers in silicon as a function of

position across a processed wafer. The gold'

donor map was made by making thermally stimu

lated capacitance measurements on n^p diodes

Prelim.inary experiments to study the feasibi!

ity of forming mesa diode structures by ultr;

sonic machining and plasma etching was com- pleted. Although neither technique is, at the present time, fully satisfactory, neitheii can be ruled out for eventual use. To facili

tate subsequent experiments of this kind, a , photomask set of a test pattern which includes mesa diodes of various sizes , with anc. without guard rings , together with other re- 1, lated structures was designed. :

An electrical method, based on the addition

of donor centers, has been devised to verify

^

the existence of the X-level which adversely

affects the operation of indium-doped silicon in infrared detector arrays. This method, was used to confirm that the X-level hypothesis is appropriate for describing a specimen

which was thought to contain these levels on | the basis of Hall coefficient data. Further

experiments to test both the effectiveness

of the method and the X-level hypothesis are underway

.

Materials Characterization by Physical Analysis Methods — Study of limitations on the res- olution of interface width in Auger-sputter

profiling continued at Stanford University.

The analysis of the effects of electron escape depth was extended to the geometry of

the real cylindrical mirror analyzer system.

Results of measurements on a group of silicon

i

dioxide-silicon interfaces prepared by dry

oxidation were analyzed with this technique.

These results suggest that the transition re- gion between the silicon and the oxide consists of an undulating connective region with

a period less than 100 pm, a width no greater,

than 0.8 nm, and excursions of about 2 nm. j The results further suggest that any of the

suboxides of silicon (SiOj^) occur only within

the 0.8-nm wide connective region. In re- lated studies, it is suggested that experimentally observed differences in implanted

profiles measured by ion microprobe mass anal-;

2
HIGHLIGHTS

^sis and Auger-sputter profiling could be exjlained by broadening due to ion-beam effects,

In the continuing study of x-ray photoelectron spectroscopy and other techniques for

chemical diagnostics in semiconductor processing at the Jet Propulsion Laboratory, a preision wet etching procedure was devised and

^jsuccessf ully employed for profiling silicon

jiioxide films thermally grown on single-

'crystal silicon substrates. Work also conIjtinued on the development of data reduction

and presentation procedures for use with xIray photoelectron spectroscopy.

ilnitial correlations were made at RCA Labora-

'jtories between electrical properties of dejvices fabricated on silicon films grown on

{sapphire substrates and the infrared and ul-

|traviolet reflectance ratios of the substrate

jsurfaces and silicon films, respectively. It

,|was observed that the drain current measured

ion MOS transistors and the Hall mobility meajsured on gated Hall effect devices reached a isaturation value at an intermediate value of

jinfrared reflectance ratio characteristic of

;an intermediate level of polishing. No furither improvement in these electrical characIteristics was noted on more highly polished

isubstrates . On the other hand no saturation

[trend was observed for these parameters as a 'function of ultraviolet reflectance ratios

'iOf the silicon films; improved device perfor-

'mance can therefore be expected from improved film crystallinity . These data sugligest that substrate surface finishing is not

the ultimate limiting factor in determining

[film quality and that it may be possible to

ireduce the cost of sapphire substrates by reiducing the amount of polishing.

^ Materials and Procedures for Wafer Proaessing — Thermodynamic calculations for equilibrium concentrations of sodium and sodium compounds in oxidation furnaces were extended to the case of furnaces with silicon tubes. Although these tubes begin with much lower con- centrations of sodium than do the previously

analyzed fused silica tubes, they can become

contaminated with sodium from external

sources. As in the case of fused silica

tubes , the presence of extremely small quantities of water vapor in the furnace atmosphere enhances the leaching of sodium from

the oxidation tube and increases the sodium

equilibrium density in the furnace.

Photolithography — Initial line-width calibration measurements were made on several

black-chromium, photomask-like artifacts using the NBS scanning photometric microscope.

The three-slgma standard deviation around

the mean of nine line-width measurements

(three measurements on each of three days)

on a single line was typically between 0.03

and 0.09 (jm. This relatively high variability is attributed to edge raggedness in the

measured lines and focus displacements during scanning. In addition, procedures for

making accurate line-width measurements using these calibrated artifacts have been developed. These procedures are intended primarily for use on an optical microscope

equipped with a filar or image-shearing micrometer eyepiece. Comparison of line-width

measurements made after an initial line-scale

calibration showed that the systematic differ- ences between measurements by the various eyepieces can be eliminated with the use of the

line-width calibration.

Line-width measurements were made with the

scanning photometric microscope on silicon

and iron-oxide photomasks. Line-image profiles for these see-through masks were calculated for the case in which the numerical

aperture of the condenser lens is sufficiently smaller than that of the objective lens.

For this case, an expression can be derived

for the transmittance threshold which corresponds to line-edge location. For see- through masks, the phase difference between

light transmitted through the clear and background regions must be considered in calculating the transmittance threshold.

Conventional bright-field and dark-field microscope systems were evaluated for their

suitability for making line-width measurements in reflected light. For a bright-field

ref lected-light system, there is no analytic

expression to relate line-edge location to a transmittance threshold. Analytic expressions can be obtained for a dark-field

ref lected-light system by utilizing a newly

developed computation technique. However, it was found that the calibration curve for actual line width as a function of measured

line width is nonlinear and dependent on the

numerical apertures of the condenser and objective lenses. Consequently, neither conven- tional system appears suitable for making ac- curate line-width measurements on silicon wafers.

Line-image profiles were calculated for lines

less than 1 pm wide scanned with the photometric microscope. The calculations showed

the system can measure line widths on a blackchromium photomask as small as 0.4 ym with an error of 0.05 pm. For masks with a higher

background transmittance, the measurement ca3
HIGHLIGHTS

pability extends only to 0.5 lam; for narrower

lines, the line-image profiles change so rapidly that the transmittance threshold corre- sponding to edge location can no longer be

utilized. Preliminary measurements were made

on an exposed resist pattern on a silicon wafer. For the multiple 0.5-pm wide lines and

spaces on this pattern, the results showed

that the pattern features could be well re- solved optically. Criteria for locating the test optics at the position of best focus were also established. For isolated lines,

best focus occurs when the edge profile has

its steepest slope; for multiple lines, the

best focus appears to occur when the transmittance at the center of the line or space

(whichever has the lower transmittance or re- flectance) reaches the same relative background transmittance level as the larger semiopaque or clear areas of the test object.

The width of 1- and B-ym wide clear and

opaque lines was measured with a primary linewidth measurement system based on a scanning

electron microscope (SEM) with an in situ polarization interferometer. A model was developed to describe the interactions of the

electron beam with the material line. The

line-edge trace calculated from the model

agrees closely with the experimental SEM

trace in the regions of the edge. The results of measurements on the opaque lines were almost identical with those obtained using the scanning photometric microscope. For

the clear lines, however, differences were encountered which are greater than those ex- pected from the measurement uncertainties as- sociated with the two measurement systems.

The cause of this anomaly is currently under

investigation

.

Test Patterns — A new test pattern was designed and fabricated for use in the reevaluation of the resistivity-dopant density relationship in silicon. The principal feature

of this pattern was the inclusion of squarearray four -probe resistor structures with

various pipe sizes in order to permit measure- ments on wafers covering a wide range of re- sistivity. In addition, two new mask levels were added to the four used in earlier test patterns of this type. One of these is to facilitate contacting n-type base regions

with aluminum metallizations for fabricating

p-n-p structures. The second mask permits

delineation of high-quality gate oxides for

MOS devices

.

Sheet resistance measurements were made with

the six van der Pauw and two bridge base

sheet resistors on the new test pattern. For

measurements on low resistivity wafers, the

results were as expected; all of the van der

Pauw structures gave essentially the same sheet resistance, and the bridge structures

gave lower sheet resistance because over-etch

and lateral-diffusion effects caused the

width of the latter structures to be greater

than the design value which was used to calculate the sheet resistance. However, the

presence of an inverted collector surface lay er on a high resistivity p-type wafer caused

the various van der Pauw structures to yield

markedly different values for sheet resistance and the bridge structures to yield low- er than expected values. This experiment re- affirmed the importance of controlling the

surface layer characteristics in making electrical measurements on high resistivity ptype wafers.

A new test structure was designed to provide

a rapid way to estimate the amount of over- etching of a processed wafer from microscopical observations. The structure contains a built-in distance scale which allows one to calculate the amount of over-etching without

knowing the magnification of the microscope

used to observe the structure on the wafer.

Development work was completed on the soft- ware package for transforming test structure

designs into working photomasks. Use of

these computer programs is expected to facili- tate development of a catalog of well characterized, modular test structures which can be

combined easily into test pattern arrays for

specific applications.

A specially designed test pattern containing

arrays of various size contact pads was used

to test the feasibility of probing the 2- by

10-pad array. It was found to be possible to probe 60-Mm square pads with 100-percent re- liability across a 2-in. (50-mm) diameter waf- er; this pad size is smaller than the SO-pm

square pads selected for the initial modular

test patterns. It was also shown to be possible to combine the 2 by N modular layout of test structures with structures connected to | contacts around the periphery of a die ar- 1 ranged in the same manner as those of the integrated circuit being fabricated. This com- bination is particularly useful when a test pattern die is inserted at selected points on

a product wafer.

Design and layout were completed at the Westinghouse Advanced Technology Laboratories of

an array of advanced test structures which include ancillary integral circuitry to permit

measurement of very low currents at speeds

4
HIGHLIGHTS

compatible with automatic dc testing equipment available to the industry. The test pattern is designed with a 2 by 10 modular cell

arrangement; five different classes of advanced test structures have been included

for evaluation. In addition, a variety of

conventional test structures is included to permit comparison measurements as well as detailed study of the cross-bridge structure.

jExperimental work on the development of a test pattern for characterizing and monitoring a large-scale-integration, silicon-onsapphire process was completed at RCA Laboratories. Complete testing routines were es- tablished for all of the structures located

in parts 1, 2, and 4 of the pattern which

are intended to be tested on automatic elecijtronic test equipment. An extensive report

on this work is being prepared.

' AssembZij and Packaging — Detailed analysis

was completed of the results of the interlaboratory evaluation of the radioisotope method for testing hermeticity of semiconductor

devices being developed by ASTM Committee

fF-1 on Electronics. The estimated standard

[deviation ranged from 5 x 10~^ atm'cm^/s for

jfor a leak size of 1 x 10~^ atm*cm^/s to 3.2 X 10~^ atm'cm^/s for a leak size of

15 X 10~^ atm-cm^/s. Significant individual

laboratory biases were observed; if these

jcould be eliminated by use of suitable calibration artifacts, it is estimated that the

standard deviation could be reduced by a fac-

|tor of about 2.

'Pevioe Inspection and Test — Studies of the

application of the scanning acoustic microl^cope to the investigation of integrated circuits continued at Stanford University and

fiughes Research Laboratories. The Stanford

group has been able to observe individual

stacking faults in silicon epitaxial layers

with a clarity similar to that obtained in

high quality interference contrast photomicrographs, as well as defects underneath metallization layers. Considerable understanding of the response of the scanning acoustic

microscope to various layer structures typical of those found in integrated circuits is

being obtained through mathematical analysis.

The Hughes group is investigating techniques

for preparing reflection gray-scale bar

charts for use in measuring the acoustic contrast characteristics to complement earlier

measurements of spatial resolution.

The experimental investigation of the suitability of using the temperature response of

the substrate diode in an integrated circuit

to evaluate the integrity of the die attachment was completed. Results of tests on integrated transistor arrays with voided re- gions intentionally introduced into the die

attachment showed correlation between voided

area and increase in temperature of the substrate diode junction. The sensitivity under

pulsed conditions appeared adequate to detect

void areas as small as 17 percent of the die

area. These results suggest that at least in some cases the temperature response of the

substrate diode can provide information re- garding the quality of the integrated circuit die attachment.

An improved test method for determining the

saturation voltage of bipolar transistors was

developed and initial laboratory tests were

conducted to validate the procedure. This

test is intended for use in hardness assurance applications where it is desired to compare measurements of saturation voltage made

before and after irradiation.

5
3 . MATERIALS C H A R A C T E R I Z A T I 0

BY ELECTRICAL METHODS

3.1 . Photovoltaic Method

The resistivity uniformity of the starting ma- terial plays a critical role in determining

the characteristics and the performance of

high power semiconductor devices. Variations

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

slice, but also contribute substantially to poor yield in the manufacturing process and

adversely affect the reliability of finished

devices. It is the purpose of this work to refine the photovoltaic scanning technique

for measuring radial resistivity variations

of circular semiconductor slices [1]. This

technique is nondestructive in that no contact is required with the wafer surface area

in which devices are to be fabricated; also

it has better spatial resolution than the

widely used four-probe method [2,3].

Instrumental Development* — An automated data

collection and analysis system has been designed and assembled (NBS Spec. Publ. 400-29,

pp. 13-15) [4]. This system enables a user

to measure the resistivity profile along a wa- fer diameter in about 2 min with some user- system interaction after the measurement has

begun. Additional equipment modifications

are being made to eliminate the user-system

interaction after initiation of the measurement and to automate the measurement of the

average resistivity of the slice by the van

der Pauw method [5].

Because a number of problems, mostly mechanical, had been encountered with the original

specimen holder with knife-edge contacts (NBS

Spec. Publ. 400-29, pp. 13-15), a second holder with point-probe contacts, shown in figure

1, was fabricated and installed. With this

holder, contacts are made to the top surface

of the slice, about 0.25 mm from the periphery. It is expected that this location is

close enough to the edge that no correction

for probe position is required. Since the

peripheral ring is usually removed during device fabrication, any damage which might be

introduced by the probes would not affect the

finished device.

(G. J. PsOgers and D. L. Blackburn)

System Characterisation* — Study of the

current-voltage characteristics of the pointprobe contacts showed that the contact resistance was less than that of knife-edge contacts to the same slice, that the linearity

Figure 1. Photograph of probe holder with

point contacts for photovoltaic scanning ap- paratus .

of the point-probe contacts was better than

that of the knife-edge contacts, and that the

point-probe contacts were more repeatable

from slice to slice.

During initial measurements of resistivity

profiles, several problems were encountered

in both the photovoltage and photoconductivity measurements. A "lateral" photovoltage

appeared between the two contacts at the same end of a measurement diameter; consequently,

different photovoltages were measured by the two sets of contacts and the correct photovoltage could not be established. The mea- surements of photoconductivity, which re- quire use of both sets of contacts, were

found not to be reproducible. It was found

that both problems were caused by large photovoltages generated by light scattered to the

metal-silicon contacts. By carefully shielding the contacts from external stray light

Principally funded by the Energy Research

and Development Administration, Division of

Electric Energy Systems.

6
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

\ i

DISTANCE FROM CENTER (mm) DISTANCE FROM CENTER (mm)

a. Mean resistivity: 30.8 fi-cm. b. Mean resistivity: 133-3 ^'cm.

I DISTANCE FROM CENTER (mm) DISTANCE FROM CENTER (mm)

i c. Mean resistivity: I63 fi-cm d. Mean resistivity: 129 fi'cm.

igure 2. Examples of ra(dial resistivity profiles measured on 2-in. diameter, ;z-type, float

pned silicon slices with photovoltaic and four-probe methods. (o: Photovoltaic method; x: pur-probe method)

i 7
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

and using a strongly absorbed exciting laser

beam (wavelength = 632.8 nm) in place of the

previously used very penetrating one (wavelength = 1.15 ym) which might be scattered

internally to the contact area, it was possible to effectively eliminate the lateral photovoltage in all but a few instances. In these cases, which are still under investigation, the photovoltage measured appears to be

related to the surface condition of the wafer

being measured.

With the elimination of the lateral photovoltage, it became possible to compare resistivity profiles measured with the photovoltaic

and four-probe methods. Numerous such comparisons were made using the specimen holder

with shielded point-probe contacts and the

632.8-nm laser source. Four examples are shown in figure 2. The results of the photovoltaic measurements were adjusted so that

the average resistivity of the measured profile equals the resistivity of the slice as measured by the van der Pauw method. This

procedure establishes the absolute resistivity at the initial point of the profile.

All of these measurements were made on speci- mens on which both surfaces had been lapped

with 12-ym alumina grit. It has been found

that the most repeatable results and profiles

most like those determined by the four-probe

method are obtained on this surface or on a lapped surface that has been lightly etched.

These are the types of surface finish usually present for resistivity profiling of thyristor grade silicon because an abraded surface is usually desired prior to and during

the initial diffusion process for these devices .

Preliminary measurements were made to evaluate the repeatability of the system. Eight

measurements of the resistivity gradient were

made over a period of four days on a single

slice. The results are plotted in figure 3 together with two four-probe measurements of

the profile. The spread in the photovoltaic

data in this figure is less than 3 percent of

the average resistivity. This is probably

sufficient repeatability for a rapid, nondestructive method for measuring incoming floatzoned slices.

(D. L. Blackburn and G. J. Rogers)

Mil'xi-.

DISTANCE FROM CENTER (mm)

Figure 3- Results of repeatability study of

photovoltaic method for measuring radial re- sistivity profiles. (Specimen: 2-in. diameter, n-type, float-zoned silicon slice with

mean resistivity of 133-3 fi'cm. o: Mean of

eight photovoltaic measurements, plotted individually at the measurement points; +: four-probe measurement made at start of test;

-k : four-probe measurement made at end of

test .

)

fy the boundary conditions at the periphery

of the circular slice and the edge of the cir

cular light spot. However, it was found that

by modifying the analytic form of the transformation equation, the amount of algebraic

manipulation could be significantly reduced.

In particular, the most useful function found

for mapping the complex variable z = x + iy

into the transformed variable w = u + iv was

:/r

1 + z/r (1)

where r is either the radius of the slice, b, or the radius of the light spot, a, depending

upon which boundary condition is being met.

Theoretical Analysis — The previous analysis

of the photovoltaic and photoconductive signals [1] was reviewed with the intent of re- moving as many simplifying assumptions and

restrictions as possible. Basically the same conformal mapping approach was used to satisThe algebraic simplification achieved through

the use of this transformation made it feasible to generalize the analysis to consider

cases where the current and voltage probes

are arbitrarily placed around the edge of the

slice instead of being restricted to the ends

8
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

of the scanning diameter and to include the

effects of nonradial gradients in resistivity.

The specimen geometry is depicted in figure

4 where the slice is aligned with its x-axis

along the scanning diameter and probes are placed every 45 deg around the slice circumference. Probes 1 and 5, at the ends of the

scanning diameter, are those used with the

present measuring technique. Probes 3 and 7 along the y-axis could be used with the present technique to measure nonradial gradients

in the y-direction. Probes 2, 4, 6, and 8 on diameters oriented at 45 deg with respect

to the scanning diameter are the ones used in

the 45-deg orientation geometry.

The resistivity gradient in an extrinsic silicon slice is given by

dp qCi V V(x)

_ ^ maj min

d? " ~

TTkT * f(x) ' bt * AR(x) '

where c is x or y, q is the charge of the majority carrier, k is Boltzmann's constant, T

is the absolute temperature, u . and y . maj mm

are the mobilities of the majority and minority carriers, respectively, p is the average

resistivity of the slice, b is the radius of

the slice, and t is the thickness of the

slice. The constant Cj and the quantities

f(x), V(x) , and AR(x) , which depend on the

position of the light spot x (which is assumed to have a radius a << b) , are listed

in table 1 for four cases of interest: the

X- and y-gradients of resistivity measured

with probes at the ends of the x- and y-axes

(x-y geometry) or at the ends of the diameters oriented at 45 deg with respect to the x axis (45-deg geometry) . In the table the sub

scripts indicate the probes between which the

voltage is measured or between which the current is applied, e.g., V51 is the potential

difference between probes 5 and 1 and I51 is

the current between probes 5 and 1.

The symmetry of these equations is significant, although it should be pointed out that

V and AR have somewhat different definitions

in the two geometries. Simple symmetry arguments show that nonradial (tangential) resistivity gradients do not influence the photo3

Figure 4. Schematic diagram of specimen geometry used for analysis of photovoltaic meth

od for measuring radial resistivity variation

Table 1 — Summary of Analysis of Spreading Resistance Method for Measuring

Radial Resistivity Variation

Parameter

in eq (2)

x-y Geometry 45-deg Geometry

x-gradient y-gradient x-gradient y-gradient

^1 1 1

1 2/2

1 2/2

f(x) 1 - (x2/b2)

[1 - (x2/b2)]2

1 - (x2/b2) 1 + (x2/b2)

[1 + (x2/b2)J

V(x) V5I ^62 ^ ^48 ^62 - ^48

AR(x) A(V5^/l5^)a A(V5^/l5T)a A(Vg4/l82) ^(W^82)

^Two probes are required at each end of the diameter to avoid measuring the

potential difference between current-carrying contacts.

9
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

voltage observed at the ends of the scanning

diameter. These same symmetry arguments can be used to show that the contributions from

the radial and nonradial components of resistivity gradient can be separated by taking

the sum and difference of the photovoltages

observed at the ends of the diameters at 45

deg to the scanning diameter. This allows

one to measure each component of resistivity

gradient separately and independently in the

45-deg geometry. Thus, in the x-y geometry,

V is the photovoltage observed across the

scanning diameter, or the diameter perpendicular to it, but in the 45-deg geometry it is

the sum (or difference) of photovoltages observed across two diameters . The change in

resistance induced by the light spot, AR, is

observed in the x-y geometry by current and

voltage measurements across the scanning diameter, but in the 45-deg geometry it is observed by current and voltage measurements

using widely separated probes as in a van der

Pauw measurement [5]

.

In analyzing the photovoltaic aspects in the

derivation, conformal mapping was used once again to satisfy the boundary conditions at the periphery of the slice. The transformation function, eq (1), was used with r equal

to the radius of the slice. The boundary val

ue problem at the edge of the light spot includes a photoinduced electric field within

the light spot area (due to resistivity gradients interacting with the incident light)

and the resulting current, both inside and

outside the light spot area. In the previous

analysis [1], an averaging over the light

spot was employed, whereas in the present

analysis an approximate solution to the actual boundary value problem was obtained using

a uniform electric field inside the light

spot and a dipole field outside the light

spot. In this way, it was shown that the pre

vious averaging technique underestimated the

equivalent dipole moment of the light spot,

and the resultant photovoltage, by a factor

of two-thirds.

It should be noted that the equation for the

x-gradient of the resistivity in the x-y geometry is 4/3 times the expression obtained

in the original analysis [1,4]. This difference arises from two sources related to the

treatment of the nature of the light spot in

analyzing the photoconductive and photovoltaic aspects of the problem. In analyzing the

photoconductive aspects, conformal mapping

was used twice in succession, once to handle

the circular boundary at the periphery of the

slice, and then again to handle the circular

boundary of the small light spot . The transformation function, eq (1), was used both

times, with the appropriate value for r each

time. An image of the light spot is generated in satisfying the zero normal-current

boundary conditions at the edge of the slice

in the first transform space. The effects of

this image light spot must be included in the

second transformation. This had been ignored

in the previous analysis [1] because no way

was found to evaluate its effects. This difficulty arose because the image light spot

has a nonclrcular shape after being transformed back to the original coordinates for

use in the second transformation. However,

in the present analysis, it was observed that

the symmetry of the transformed real and image light spots, coupled with their symmetrical position with respect to the transformed

probe positions, leads to the conclusion that

each will have the same net effect of the com- puted value of AR. Therefore, the neglect of

the effect of the image light spot in the previous analysis led to an expression for AR

that was too small by a factor of two.

The 45-deg geometry is intuitively more attractive than the present x-y geometry because the light spot can be moved over the

complete scanning diameter and not come close

to a contact to produce extraneous effects

due to photogenerated carriers reaching the

metal-semiconductor contact. It does, however, require that three scans be made along

the diameter mapped rather than the two needed for the x-y geometry. (R. D. Larrabee)

3.2. Thermally Stimulated Current and Capaci

tance Measurements^

Spatial Distribution of Gold in Silicon —

Gold is frequently used to control the lifetime or switching characteristics of highpower diodes and thyristors. It is known to produce two centers in the band gap of silicon: an acceptor center 0.59 eV below the

conduction band edge and a donor center 0.35

eV above the valence band edge [6]. The

gold centers are generally produced by diffusing the gold near the end of the device

wafer fabrication. The uniformity of the

electrical activity of gold centers on wafersized power devices is an important consideration in the ability of the device to switch

uniformly over its entire area.

Principally funded by the Energy Research

and Development Administration, Division of

Electric Energy Systems

.

10
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

The newly developed hot/cold wafer probing

apparatus (NBS Spec. Publ. 400-29, pp. 20-22)

[7] was used to map the gold donor concentration in a processed wafer. Thermally stimulated current and capacitance measurements

were made on the 432-ym diameter n'^p gated

base-collector diode (structure 3.10) of test pattern NBS-3 [8]. This test pattern is repeated at 5.08-mm intervals across the wafer;

about 70 identical dice appear on a standard

2-in. diameter wafer. A typical bipolar process was used to fabricate the wafer on ptype silicon with a room temperature resistivity of 5 to 10 Q'cm and (111) surfaces.

After growth of the field oxide, the n"*" base

regions were formed by diffusing phosphorus

to form a junction about 1.5 to 2.0 pm deep.

The test diodes considered here were not ex- posed to a subsequent emitter diffusion which

\<ias performed to complete other test structures . The gold center was introduced by the

diffusion of gold, which had been evaporated

on the back surface, for 24 h at 825°C.

Figure 5 shows the thermally stimulated re- sponse from a typical diode on the processed

wafer. The upper curve is the capacitance re- sponse and the lower is the current response.

In each case, the response was measured by

first cooling the device to near liquid nitrogen temperature. Zero bias was applied to the diode to charge all defects with majority

carriers (holes) ; a reverse bias of 15 V was

then applied to form a depletion region. The

current or capacitance was measured with the

depletion bias maintained while the device

was heated at about 7 K/s. At the appropriate temperature, the gold donor emits its

trapped hole resulting in both a measurable

current and a slight collapse of the depletion region; the latter causes a measurable

increase in capacitance. The system noise as reflected in the x-y recorder traces was approximately 2 fF and 0.1 pA for the capacitance and current, respectively.

The gold donor density, , was determined

from the measured thermally stimulated capaci- tance response [9]:

2(C, C.)

1 ^t = \ \. ^ '

where is the background boron acceptor

density and and are the capacitances in- dicated in figure 5. This expression is apI propriate for the present case since only

hole emission from the gold donor center need

be considered and since the initial capaci3.24 -

3.20

3.16 -

93 113 133 153 173 TEMPERATURE |K|

Figure 5. Typical thermally stimulated capacitance (a) and current (b) responses of a gold-doped n+p diode. (Heating rate: 7 K/s.)

tance, <Z^, is much less than the capacitance

of the diode at zero bias. This determination is independent of heating rate since it

is the total change in charge on the center

that is important. However, it is necessary

to obtain an independent measure of N^. The

average acceptor density in the depletion re- gion was calculated from:

2Ci2c22(V2-Vi)

"a = qKg.£^(Ci2-C22) ' (4)

where and C2 are the capacitances of the

diode at two values of reverse bias, V;^ and

V2 , respectively; q is the electronic charge;

Kg_j^ is the relative dielectric constant of

silicon; and is the permittivity of free

space. Wafer maps of and as a function

of position on the wafer are displayed in fig- ure 6. In each case the darker areas represent regions of greater density. As indicated on the maps, the gold donor density

varies from 2.34 to 3.61 x 10-^^ and the

acceptor density ranges from 1 to 1.5 x 10-^^ cm~^. It appears that there is no relationship between the donor defect density and the

acceptor background. The system noise (2 fF)

suggests that donor densities as low as

11
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

Figure 6. Wafer maps of gold donor and boron acceptor densities in a 2-in. diameter silicon

wafer. (Higher density occurs in darker regions on the map; every fourth point is an actual data point; others are interpolated.)

1 X 10^^ cm ^ could be detected in this wafer.

(R. Y. Koyama, W. E. Phillips, and Y. M. Liu)

Mesa Diode Fabrication — As discussed previously (NBS Spec. Publ. 400-36, pp. 23-24),

special test structures are required to define and isolate the p-n junction in a partially or fully fabricated thyristor. This

is most conveniently done by defining a pattern of mesa diodes over the front surface of

the thyristor wafer. Three techniques are being considered for forming the mesa diode

structures: ultrasonic machining, plasma

etching, and chemical etching.

Initial test devices were produced by ultrasonic machining as illustrated in figure 7. In this process the diffused silicon is impact ground by an ultrasonically driven stainless steel tool and an abrasive slurry (fig.

7c) . During the cutting operation, the front

surface of the wafer is covered by a thin

glass plate (fig. 7b) to prevent chipping. A

special tool head was fabricated to form 22 mesa regions each about 1.5 mm in diameter

and spaced on a square grid with 5.08 mm between centers. About 10 min is required to cut through the cover plate and approximately

75 ym into the silicon surface. The moat sur- rounding each mesa is briefly chemically

etched in to remove the damaged surface

layer created by the grinding operation.

The remaining sections of the glass microscope slide serve as a mask to protect the

mesa surfaces during etching. The back sur- face diffused layer is lapped off (fig. 7d)

,

and, after removal of the protecting glass

plates, a 200-nm thick layer of undoped silicon dioxide is then deposited on the front

surface of the wafer by conventional chemical

vapor deposition techniques (fig. 7e) to passivate the junction edges which were exposed

during the formation of the mesas. The CONTACT mask of test pattern NBS-2 [10] is used

to define the contact areas (structure 2.09)

by conventional photolithographic techniques.

A 600-nm thick aluminum film is deposited on

the front surface of the wafer, and ring contacts are defined with the METAL mask of test pattern NBS-2 (fig. 7f ) . A metal contact is

also deposited on the back surface of the

wafer. A photomicrograph of the front surface of a completed mesa diode is shown in

figure 8.

About 20 percent of the diodes fabricated on several wafers had leakage currents of about

10 nA at a reverse bias of 10 V at room temperature; diodes with this magnitude of leakage current are suitable for making thermally

stimulated current and capacitance measureCP-6 (HN03:HF:CH3COOH, 5:3:4) is available

commercially under the name "silicon etch"

from such suppliers as Corco Chemical Corp.,

Fairless Hills, PA 19030 and Transom Co.,

Rte. 1, Rowley, MA 01969.

12
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

p-TYPE DIFFUSION

n-TYPE SILICON

a. Diffused wafer,

-GLASS PLATE

b. Wafer protected by thin glass plate,

c. Ultrasonic cutting,

^CONTACT CUTS

DIFFUSION REMOVED—

-"^

d. After removal of back-surface diffusion,

OXIDE ALUMINUM - m PI

e. After deposition of oxide and definition

of contact rings.

f. After deposition of aluminum.

Figure 7- Mesa diode fabrication process.

13
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

ments . Most of the devices, however, had

leakage currents which were much larger; surface leakage is the probable source. The

procedure for the ultrasonic fabrication of mesa diodes needs to be refined in order to achieve greater reproducibility of device

quality

.

Mesa structures were also produced by plasma

etching, with the use of a simple three-level

dot-matrix mask set. Plasma etching [11] is

a "dry" chemical process which is used for re- moval of various materials in the semiconductor industry. In the present application, it

is used to remove regions of silicon from the

surface of the wafer not protected by the mesa mask. An rf plasma is used to excite

an atmosphere of carbon tetrachloride and

oxygen to produce various chemically reactive

atoms, ions, electrons, and free radicals.

These species chemically react with the surface to etch the silicon.

Whereas the ultrasonically formed mesa is defined by the mechanical dimension of the tool,

the plasma etch technique requires a mask to protect the mesa region while the moat is being etched. In a first attempt at using this

technique, an oxide film 300 nm thick was thermally grown on a diffused wafer. The oxide was patterned by a photolithographic operation to protect the mesa areas of the diode structure. After plasma etching the moat,

an oxide film about 50 nm thick was thermally

grown for the passivation. Finally, contact

cuts were made and the contact metal was applied and patterned. A photomicrograph of

typical mesas, 230 ym in diameter, is shown

in figure 9. This procedure was successful

in fabricating diodes of good leakage characteristics. Of the more than 20 devices

tested most had a leakage current in the

range of a few nanoamperes at room temperature with a 10-V reverse bias.

Although the plasma etched mesa is a definite

possibility with respect to mesa diode fabrication, it has, in its current form, some disadvantages with respect to characterization of in-process power device material.

One disadvantage is the relatively timeconsuming step of the mesa etching; with present equipment the etching rate is only a few

tenths of a micrometer per minute. Further,

thermal oxidation steps were employed in this

initial test for both masking and passivation.

These steps require a temperature of 1000°C

for about 20 min. Although this is both

cooler and shorter than the deep diffusion

temperature and time, it has not yet been established that thermal oxidation does not al- ter the characteristics of the wafer. j

1

I To avoid some of the complications encountered in these preliminary studies of mesa di

ode fabrication, a special test pattern, des-'

ignated as test pattern NBS-13, was designed.!

This test pattern includes mesa diodes of var

ious sizes, with and without guard rings, con tact resistors, and mesa etch rate monitors. , The simplest procedure for fabricating the mesa diode requires a three-level mask set.

Di f fused wafer.

PASSIVATION

b. After mesa definition (Mask 1) and passi'

vation

.

METAL

c. After n-contact cut (Mask 2), n-metalliza^

tion, and n-metal definition (Mask 3)-

MESA DIODE

d. After p-contact cut (Mask 4), p-metallization, p-metal definition (Mask 5), and mi- croa 1 loy

.

Figure 10. Postulated mesa-diode fabrication

process using five-level photomask set. 14
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

,-.fter cleanup of the diffused starting wafer,

the first step is to define the diode mesa

area with the first level mask. After the

moat area surrounding the mesa is removed, a passivation layer is applied to protect the

junction. Following this, two mask levels

are required for cutting the contact areas

and defining the contact metallizations. Although there should be no problem associated

with the metallization to the p"'"-region, it

is likely that metallization to the high re- sistivity n-region could result in a rectifying Schottky barrier contact. Because the

fabrication procedures and test structures

are in a development phase, it was appropriate to incorporate flexibility into the mask

set. Hence, the mask set was designed with

separate levels for the contacts to the nand p-type regions

.

A possible use of this mask set is illustrated in figure 10. As in the three-level

case, described above, the first mask defines

the mesa areas . This is followed by passivation. The second and third masks are used to cut and define the gold-0.6% antimony contacts to the n-type regions. The fourth and

fifth levels are used to cut and define the

aluminum metallization to the p-type region.

A microalloy step (400°C) is included to insure good adhesion and low resistance, nonrectifying contacts. (J. Krawczyk

,

R. Y. Koyama, T. F. Leedy, and Y. M. Liu)

3.3. Indium-Doped Silicon

This work was undertaken to identify parameters associated with extrinsic infrared detection in silicon that present problems of measurement and to develop appropriate measurement technologies to solve these problems

(NBS Spec. Publ. 400-36, pp. 17-20). Initial

efforts are directed toward methods for characterizing indium-doped silicon.

Some observers [12-14] have reported an extraneous acceptor level in this material, but

it has not been seen by others. If present,

this additional level can seriously degrade

detector performance and, if not adequately

controlled, could give rise to appreciable

variations in photoresponse over the active

area of an imaging array. Therefore, it becomes important to determine if this is a real level or just an artifact of the measurement techniques used. If it proves to be a real level, it then becomes necessary to determine if it is present to the same extent

in material from a variety of sources.

This problem is being approached by collecting material from various sources, confirming

their characterization measurements, and performing new experiments aimed directly at checking the validity of any extraneous level

interpretation of the behavior observed. For

example, the temperature dependence of the

Hall coefficient is the traditional parameter

measured to determine dopant energies and densities. Unfortunately, the interpretation of

Hall measurements in terms of dopant parameters is not necessarily unique; the same data

can frequently be fitted, within the experimental error, by assuming different sets of

energies and concentrations of impurities.

Therefore, some method must be devised to distinguish between the various possible interpretations. One way of accomplishing this is

by measuring the effects of introducing additional donors to the test specimen. Since

the different interpretations generally predict quite different behavior with donor addition, a second Hall measurement after donor

addition can usually identify incorrect interpretations. Donors can be added by several

techniques: by neutron transmutation of the

silicon to form phosphorus by the reaction

[12]

30si + n - 31 P +

Table 2 Donor Formation in Silicon With Var- ious Oxygen Contamination Levels on Heat Treatment at 450°C

9-ym

Absorption Donor Density

r ^4:- 4. After 10 Min, Coefficient, ^^.3

cm"i

Maximum

Donor Density,

cm"3a

4 67 1 2 X 1015 7 X 1015

4 15 6 8 X 101^ 6 X 1016

3 63 3 8 X IQi'* 3 5 X IOI6

3 11 2 1 X IQi'* 2 4 X 1016

2 6 9 5 X 1013 1 5 X 1016

2 08 3 8 X 1013 6 5 X 1015

1 04 2 6 X lOii 8 X lOi^

0 52 1 4 X IQii 7 X 1013

0 10 1 5 X 108 7 X loii

0 052 8 0 X 106 7 X lOlO

This is the donor density formed after heating at '+50°C for very long times.

15
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

ADDED DONORS |cni-^| Figure 11. Resistance of an indium-doped

silicon specimen at 77 K as a function of

added donor concentration. (A: computed

curve based on a model which includes the

extraneous level; B: computed curve based

on a model which does not include the ex- traneous level; C: experimental curve.)

by indiffusion, at 300° to 500°C, of lithium,

wnich is an interstitial donor [15]; or, if

sufficient oxygen is present in the crystal,

by forming oxygen donor complexes by annealing in an inert atmosphere at 450°C for a short time [16, 17]

.

A preliminary experiment was carried out on a silicon specimen known to exhibit the extraneous level behavior, from a crystal with an absorption of 4 cm"-^ in the 9-ijm oxygen

band [18] . Varying amounts of donors were introduced by heating the specimen at 450°C

for a series of short periods of time.

Based on data of Baker [19], reproduced in

table 2, and with the assumption that the

rate of donor formation is constant for the

total period of exposure to 450°C. In this

experiment, the rate of donor introduction

was taken as 5 x 10 '^^ cm~ ^ 'min" '

.

The effects of adding oxygen donors to this

specimen (determined in this case by measurement of the resistivity rather than the Hall

coefficient) closely followed the behavior

predicted by a model which included the ex- traneous level, as shown in figure 11. The

behavior was quite different from that predicted by an alternative model which satisfactorily described the same Hall data but

did not include extraneous levels. These

data argue strongly for the validity of the

extraneous level hypothesis.

Additional test specimens with various

amounts of oxygen and extraneous levels are being obtained in order to conduct additional, more definitive experiments in which the

Hall coefficient, as well as the resistivity,

is to be monitored as donors are added. In addition, facilities have been set up to measure the spectral photoresponse attributable

to the extraneous level as a function of

added donor density, as a further test of

the extraneous level hypothesis.

(R. D. Larrabee

)

16
I 4. MATERIALS CHARACTERIZATION BY

PHYSICAL ANALYSIS METHODS

4.1. Electron Spectroscopy Techniques

Limitations on Interface Width Resolution in

Auger-Sputter Profiling: Electron Escape

Depth — If the detected Auger electrons are emitted normal to the specimen surface, the

emission strength, A(x), of a transition can

be written as*

A(x) = - /f (x+x')

-x'/L

dx' (5)

where x is the depth of material sputtered

away or, equivalently , the position of the

surface after some amount of sputtering; L

is the mean free path of an electron of energy E in the solid; and f (x) is the concentration profile to be determined from measurements of A(x) as a function of the sputtered

depth. The variable x' is the distance along

the normal with an origin at the sputtered

depth

.

It is assumed that the upper layer of the

specimen is uniformly excited by the incident

electrons and that the specimen surface is

smooth. The derivative of the emission

strength formula.

dA(x)

dx

df(x+x') -x'/L

dx

^ dx'

can be integrated by parts (noting that

df(x+x')/dx = df (x+x')/dx') to obtain:

dA(x) _ 1 __ f . ^ , 1 w s _f(^) +-A(x)

which may be rewritten:

dA(x)

f(x) = A(x) - L dx (6)

to provide a formula with which the measured

profile can be deconvolved.

The cylindrical mirror analyzer (CMA) system

used in the present work has a ring aperture

with an axis 30 deg from the normal to the

specimen surface. Hence, the electrons which

are detected are not emitted normal to the

surface but rather over a broad range of angles. Electrons emitted at an angle 9 from

the normal travel a distance x'/cos 0 inside

the specimen. Electrons which pass through

various parts of the ring aperture are emitted at different angles. If the width of the

aperture is assumed to be very small and if

the azimuthal angle on the ring is denoted by

(}), the geometry of the present apparatus is

such that

cose = 0.6405 + 0.3365 coscj) .

The emission strength can then be written

A(x) //J 0

c / _i_ i\ -x'/Lcose /, f(x+x ) e dx d(j) . (7)

If the integral over ^ is approximated by a summation,"^ f (x) may be obtained from A(x) by

a numerical routine. To check the effect of

the broadening on a step interface under

these conditions, the numerical routine was

run in reverse to obtain the signal which

would be obtained from a step interface, as well as reproduce the step. The results are illustrated in figure 12. The measured width,

over which the signal decreases from 90 to 10

percent of its maximum value, is 1.64 L. For

the on-axls system, the measured width of a step interface was found to be 2,2 L (NBS

Spec. Publ. 400-36, p. 32). The curve in

figure 12 can be approximated by an exponential with a decay constant approximately 0.75

(1.64/2.2) times the decay constant for the case of the on-axis detection system; this

result suggests that the following deconvolution formula is a useful approximation for

the CMA system:

f(x) = A(x)

3 ^ dA(x)

4 ^ dx

(8)

The difference between results obtained from

the approximation of eq (8) and from the com- plete calculation is illustrated in figure 13

for an MNOS structure which was comprised of

This formulation is equivalent to that derived previously (NBS Spec. Publ. 400-36,

pp. 31-32), but the origin of the x-axis

has been shifted from the interface to the

surface of the specimen.

'when this is done, it is convenient and

straightforward to eliminate contributions

from those portions of the aperture which

are blocked by obstructions in the apparatus .

17
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

J3Ozo

<

UJo

z

o

o

3 2 I DISTANCE /ESCAPE DEPTH

Figure 12. Computed signal obtained from a step interface with an Auger electron spectrometer with a cylindrical mirror analyzer,

a thin silicon dioxide layer sandwiched between pure silicon and silicon nitride. In the figure, the solid curve represents the

measured signal, and the dashed curve represents the corrected concentration as calculated by the numerical routine with the as- sumption of a 1.2-nm escape depth. Note that

the peak height is increased by 30 percent.

Points calculated from eq (8) are plotted as solid circles on the figure. The curve through these points is shifted somewhat more

to the right and reduced slightly in height,

but does appear to be a good approximation.

(C. R. Helms

S. Schwarz§ , and W. E. Spicer§

)

Auger-Sputter Profiling of MOS Oxides — Augersputter profile measurements were made on a group of oxide-silicon interfaces prepared by

oxidation of (100) silicon wafers in a dry

oxygen atmosphere at 1000°C. The oxidations

were carried out as follows: Chemically

cleaned wafers were inserted into an oxidation furnace in flowing nitrogen, with an insertion time of 2 min. The wafers remained

in the nitrogen ambient for an additional 8 min before dry oxygen was let into the furnace. This time interval insured that the

wafers were at the oxidation temperature before oxidation began. After oxidation, the

wafers were annealed for 8 min in nitrogen before being withdrawn from the furnace; withdrawal time was 2 min. The thickness of the

oxide films, as determined by ellipsometry

,

2,0 4 0 6 0 80 10.0 12.0 140 16,0 18.0 20.0 DEPTH (nm)

Figure 13- Oxygen concentration profile in MNOS structure. (Solid curve: measured Augei

signal; dashed curve: signal corrected with

numerical routine; solid circles: signal cor- rected with eq (8) .

)

C-V measurements, or step-height measurements;*

ranged from 24 to 96 nm.

j! Sputtering was carried out with neon, xenon,

or argon at 1 keV or argon at 500 eV and both'

the oxygen KLL transition at 510 eV and the

silicon LW transition at 92 eV were monitored as a function of sputter depth; typical!

profiles are depicted in figure 14 which

shows the results on a 96-nm thick oxide. Noi significant differences in the measured width

(10% to 90%) of either the oxygen or silicon | line were observed for films of different

thickness or for the different sputtering con ditions. The measured oxygen and silicon

widths were 3.35 + 0.2 5 nm and 2.5 ± 0.15 nm, respectively, where the variability indicated

is one sample standard deviation. The major

contribution to the difference in values obtained is the escape depth. When the escape

depth is deconvolved from the profiles, values of 2.5 ± 0.5 nm and 2.25 ± 0.25 nm are obtained. If in addition a correction is

made for the ion range (NBS Spec. Publ . 400-

25, pp. 16,17) [20], it can be concluded that

the transition region between pure silicon

and pure silicon dioxide is about 2.0 nm [21].

The significance of these measurements is two- fold. First, any nonunif ormities in the sputtering process should give a broadening ef- .

Work performed at Stanford Electronics Laboratories of Stanford University under NBS

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

18
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

SiO

85.0 90.0 95.0 100.0 DEPTH (nm)

1050

Figure ]h. Oxygen KLL and silicon LVV signals as a function of distance near the interface between silicon dioxide and silicon.

feet that is proportional to thickness; from

these data, these effects are clearly undetectable. Second, all of the interfaces studied

had identical electrical characteristics; it

is therefore encouraging that the measured interface properties were also identical.

Neither in this study nor in previous work

(NBS Spec. Publ. 400-25, pp. 18-19) [21] has

evidence been observed for a different chemical state of silicon, which might be expected

to occur in the transition region. This result, however, may be due to the ion knockons which have a tendency to "scramble" the

chemistry of the region of interest. There

is considerable uncertainty in the effect of

the ion beam on SiO type materials; their

X presence may only be detectable in the Auger

data taken during sputtering before the interface is reached. For 1-keV argon ions, the

depth for significant damage is estimated to be about 3.0 nm. With an escape depth of

3.0 nm (Si KLL transition), and assuming

a minimum sensitivity of 10 percent, it is

estimated that a layer of SiO , 0.8 nm thick

X

or thinner, could not be detected.

These results lead to the model shown in fig- ure 15 for the interface [22]. The width of

the transiton region is probably due to an un- dulating interface, the period of which is

certainly less than 100 pm, the distance over which the electron beam is rastered, and any

19

SIO,

Figure 15. Model for silicon-silicon dioxide

i nterf ace

.

SiO^ present is in a connective region no greater than 0.8 nm wide. The value for the

width of the transition region is consistent

with the results of recently reported ion

thinning, transmission electron microscopy

experiments [23] in which an undulating interface with about 2-nm excursions and a periodicity of about 1 ym was found. The value for

the upper limit of the width of the connective region is consistent with the results of

recent internal photoemission studies [24] in

which no evidence was found for an SiO rex gion or significant excess charge extending

further than 0.4 \im from the interface. On

the other hand, the results of experiments

[25] using x-ray photoelectron spectroscopy

suggest that the thickness of the connective

region is somewhat greater than 0.8 nm. The

experiments were, however, performed on very

thin oxides, and it is not unreasonable to expect the interface region of a thin oxide,

present during the initial stages of oxidation, to differ from that obtained after 25

to 100 nm of oxide had been grown.

(C. R. Helms §,

S. Schwarz§ , W. E. Spicer§ , and N. Johnson^)

A Comparison of Ion Miaroprobe and AugerSputter Profiling — Auger-sputter profiling

(ASP) measurements were made on a group of

silicon specimens implanted with phosphorus

or antimony that had also been profiled by

ion microprobe mass analysis (IMMA) . Tn general, the peak concentrations measured by ASP were about 30 percent higher than those measured by IMMA on the same profile. This difference appears to be due to ion-beam effects

which are present in both experiments but to

a lesser extent in the ASP work.

Work performed at Stanford Electronics Laboratories of Stanford University under NBS

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

^Xerox Palo Alto Research Center, Palo Alto,

California 94304.
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

The broadening due to the 1-keV ions used in

ASP has been found to be about 1.5 nm. For

the 20-keV ions used in IMMA the broadening

is estimated to be about 15 nm, a value which

is about the same order of magnitude as the

width of the implanted profile. Hence, significant broadening of the measured profile

is possible with an accompanying reduction

in the. peak height as compared with the effects due to the lower energy ion beam used

in ASP. (C. R. Helms§

,

S. Schwarz§, and W. E. Spicer^ )

4.2. Techniques for Chemical Diagnostics in

Semiconductor Processing

The objective of this task is the development

of a standardized measurement technology for

chemical diagnostics for implementation in

the semiconductor industry. This measurement

technology is based on the use of x-ray photoelectron spectroscopy (XPS or ESCA) as the

primary chemical probe. XPS and other com- plementary surface analytical spectroscopies

are used to follow the surface chemistry of

material at critical stages of semiconductor

device processing. This effort is currently

emphasizing two of the most important aspects

of that technology: depth profiling and data

reduction.

Depth Profiling — Since surface analytical

techniques can sample only the volume within

about 10 nm of the surface, it is necessary

to combine material removal with these methods in order to measure concentration profiles as a function of depth down through material. There are two basic approaches to this problem: 1) ion milling (sputter etching) and 2) wet chemical etching. Currently,

ion milling is in great favor since it can be

combined with simultaneous Auger spectroscopy

for elemental depth profiles. However, con- siderable chemical and physical damage to the

specimen is introduced by the ion beam. This

damage is related to ion energy. Although

damage is minimized at energises below 500 eV,

considerable chemical damage has been observed when even 100-eV argon ions are used

to mill silicon or silicon dioxide.

Wet chemical methods based on immersion in dilute solutions of hydrofluoric acid in water

or various nonaqueous solvents have not been

fully successful. The porosity of silicon

dioxide and the reactive solubility of the

water in the oxide leads one to expect chemical modification of the structure after long

term immersion in such solutions. In the course of the previously described study of

20

carbon at the silicon-silicon dioxide interface (NBS Spec. Publ. 400-36, pp. 34-35), it was observed that square specimens, about 1 cm on a side, when spun at 12,000 rpm could

be uniformly etched by drop-wise addition of

hydrofluoric acid solutions to the spinning

surface. Preliminary experiments were initiated to investigate the reproducibility of

this system.

Pipetting devices which dispense 5 pL with

an accuracy of ±0.1 percent or better were

used to dispense the etching solution in 50-

yL drops. Several oxide films, 10 or 30 nm

thick, were grown on silicon specimens and

etched back by this technique. The thickness

of the remaining oxide was measured ellipsometrically, and no thickness variations were observed over the surface. No topographical

detail was observed on several specimens

which were examined by electron microscopy

after the oxide thickness had been reduced to about 3 or 4 nm. These experiments suggested

that a depth profile can be taken in which

the depth coordinate is measured in terms of

chemical equivalents of solvation. Therefore

additional work was undertaken to refine the

etching apparatus and to optimize the etching chemistry.

The current technique for controlled wet

chemical etching is designed to apply welldefined amounts of chemical etchant to the

specimen surface for a controlled period of

time. The choice of a solution-phase etchant

permits control of the process on an atomic

scale. Subtly different chemical bonds show

different reactivities to the same etchant.

The various forms of silicon dioxide show a wide variation in etching rate when exposed

to hydrofluoric acid. Precise control of

the amount of etchant and the time of exposure should therefore enable the experimenter to discern small variations in the

silicon-oxygen chemistry.

As noted above, however, immersion of the

specimen in a solution, would expose the in- ner layers of the oxide to the solvent

through rapid diffusion. The presence of

solvent molecules within the lattice could

lead to reaction with the oxide lattice, re- distribution of impurities, and modification

of defects. This technique is therefore

based on the exposure of the specimen surface

Work performed at Stanford Electronics Laboratories of Stanford University under NBS

Contract No. 5-35944. NBS contact for additional information: K. F. Galloway.
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

to a liquid film of controlled composition

for time periods of 1 to 1000 ns. This solidliquid analog of a stopped-flow system is ac- complished by placing the specimen on a platform spinning at a constant velocity between

10,000 and 100,000 rpm and dispensing etchant

onto the specimen surface as 50-pL drops.

Each drop is permitted to evaporate to dryness before the next drop is dispensed. The

experiment is performed within the environmental chamber attached to the XPS spectrometer

introduction lock. This chamber is dynamically flushed with dry nitrogen gas and the residual water concentration is limited to less

than one part per million. All solutions are deoxygenated

.

Nonaqueous solvents have been used because

of the affinity of silicon dioxide for water

and because residual nonaqueous solvents

can readily be detected through observation

of the carbon Is photoelectron line in the

XPS spectrum. After examination of a number

of different solvent systems, anhydrous ethanol was chosen for these experiments by reason of its viscosity, ability to dissolve

surface fluoride species, and high volatility.

Saturated ethanalic hydrogen fluoride solutions are prepared by bubbling anhydrous hydrogen fluoride gas through deoxygenated ethanol. These saturated solutions are then dij luted with additional ethanol to a volume ratio of 10:1. These solutions are observed to give an etch rate of approximately 2 nanometers per milliliter of solution at 15,000 rpm. Following the drop-wise addition of the appropriate amount of etchant, the specimen is

rinsed with 10 mL of ethanol and then introduced into the spectrometer introduction lock.

After XPS analysis, which includes silicon 2p,

oxygen Is, carbon Is, nitrogen Is, and wide

scan spectra, oxide film thickness is measured by ellipsometry if the oxidized to ele- - mental silicon intensity ratio in the silicon

2p spectrum is greater than 3 to 1. Significant hydrocarbon and water adsorption occurs

j during the mounting of the specimen for the

ellipsometric measurement, and consequently

this method was not used when the spectral

intensity ratio could be used to establish

the required depth scale.

A diagram of the variable speed spinner is

given in figure 16. The system consists of

a dry-nitrogen driven turbine which rotates

at 10,000 to 100,000 rpm as the gas pressure

is varied from 12 to 95 psi (83 to 660 kPa)

.

The turbine chamber is isolated from the

spinning head which is enclosed within a TFE- fluorocarbon receptacle to capture etchant

and solvent thrown off the spinner surface.

21

PIPETTE CENTER

Figure 16. Diagram of variable speed spinner for controlled wet chemical etching.

The spinner is fabricated from polypropylene.

The test specimen is held in position by suction created by nozzles drilled into the

spinner block.

This wet etching procedure was used to profile a number of silicon dioxide films thermally grown on single-crystal silicon substrates. The etching conditions used in

these experiments were designed to remove 1.0

to 2.0 nanometers of oxide per step. The

oxygen-to-silicon ratio in the oxide was directly determined through measurement of the

oxygen Is and oxidized silicon 2p intensities

in the x-ray photoelectron spectra. A nor- malized plot of this ratio for a representative film prepared by dry oxidation at 900°C

is shown in figure 17. The integrated intensity ratio is effectively 2:1 over most of

the film. About 1.5 nm from the siliconsilicon dioxide interface, the ratio appears

to change rapidly. In this same region large

shifts in the position of the centroid of the

oxide peak are also observed which suggests
MATERIALS CHARACTERIZATION BY

T—1

1 I

I

I I I 1

I

I I I I

I

I . .

•

o

' ' '

I

' ' I' ' ' I I I I I I 0 5 10 15 20 nm

Figure 17. The oxygen-to-silicon ratio in a silicon-dioxide film as a function of distance from the oxide-silicon interface.

that although bulk oxide stoichiometry is

maintained throughout most of the film, lower

oxides are present in a narrov? region next to the interface.

Data Reduction — Work is continuing on the development of data reduction and presentation

procedures. The current state of the noise

removal and deconvolution routines is indicated in figure 18. The upper curve is a direct trace of the silicon 2p spectrum of a 6-nm thick film of silicon dioxide on silicon.

The middle curve shows the resolution enhancement that can be obtained by using a simple

filter. The lower curve results from the additional use of smoothing and line-shape con- straints .

An algorithm has been developed to facilitate

the problem of peak assignment and spectral

interpretation in XPS spectra. The algorithm

comprises five basic steps: 1) smooth spectrum with cross-correlation filters; 2) locate peak maximum and integrate above background to find intensity; 3) tabulate intensity and binding energy for all peaks with

intensity 50 percent greater than the standard deviation of the noise; 4) scan dictionary which contains peak positions and relative intensities of secondary elemental lines;

22

PHYSICAL ANALYSIS METHODS

l' 1

' ' ' '

1 •

l\ /A ORIGINAL

S All RES ENHANCE.

1

1 1

'U. .^Nl deconvolution

, , , , 1 , , , , 1 , , , , BINDING energy (eV)

Figure 18. Example of data enhancement techniques. (Upper curve: direct trace of silicon 2p spectrum of a 6-nm thick film of silicon dioxide on silicon; middle curve: resolution enhancement by simple filtering; lower curve: trace enhanced by application of deconvolution computer routines.

5) perform intensity and position fitting pro

cedure using a SIMPLEX driver and leastsquares criteria.

This algorithm reduces the spectral problem

to a sum of delta functions with particular

position and intensity. The interrelation

of additional lines for a single element,

such as 4f, 5s, 4d, 3d, etc., is included.

The implementation of the last step includes

a procedure for plasmon search. Peaks are assigned in order of decreasing intensity.

Following successful assignment, all peaks

related to the element are stripped from the

table which is then iterated once again.

(F. J. Grunthaner*)

4.3. Optical Test for Surface Quality of

Sapphire Substrates

The results of optical and x-ray analyses on silicon films have been shown to correlate

Work performed at Jet Propulsion Laboratory

under NBS Order No. 611377. NBS contact

for additional information: K. F. Galloway.
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

n 1 1 r ~i 1 r- n- CHANNEL

p- CHANNEL

I

I , 1/

00 Oi Q2 03 04 05 51 07 0.8 09 10

IR REFLECTANCE RATIO dgooCM-'-'UsoCM-ll

Figure 19. Correlation of SOS MOSFET channel

current with infrared reflectance data of

corresponding sapphire substrates.

well with the results of optical reflection

measurements on the underlying sapphire substrate (NBS Spec. Publ. 400-36, pp. 37-39).

The silicon films were subsequently ion1 implanted and fabricated into device structures using the test pattern generated under

a companion contractural effort (NBS Spec.

Pubis. 400-19, p. 47, and 400-25, pp. 44-45).

ro-m Electrical measurements were performed on nand p-channel transistors and on gated Hall

bars on each of the several variously polished substrates. Drain current corresponding to a given set of bias conditions on both

transistor types is plotted against the infrared reflectance ratio for each substrate

in figure 19. The bias conditions were the same in all cases; the source drain voltage

was 5 V, and the gate voltage was 10 V, with

appropriate polarity applied for each transistor type. Each data point in figure 19 is

representative of device performance over a given substrate. It was determined that the

threshold voltage was approximately the same

for all wafers except for the specimen with

the lowest infrared reflectance ratio. Therefore, the drain current is directly proportional to the field effect mobility. It is

interesting to note that the current, and

therefore the field effect mobility, satujj rates at a reflectance ratio of approximately

0.5 and that for higher ratios, corresponding

to more highly polished substrates, these devices, as operated, do not show a dependence

on substrate surface quality. A similar saturation also occurs in the gated Hall mobility,

shown in figure 20. 23

X 400

_jmg 300

_i_jI 200 o

IxJ g 100

Q

/

/ • /

•

/

/•

/

/

/

/

/ J

I 1 1

1 1 1 01 02 0,3 04 05 06 07

IR REFLECTANCE (IsooCM-'^UsoCM-'

>

08 09

Figure 20. Correlation of gated Hall mobil

i ty of SOS structures with infrared reflectance data of corresponding sapphire substrates .

The transistor current and Hall mobility data

from figures 19 and 20 are plotted against

the ultraviolet reflectance ratio of the sili- con films in figures 21 and 22, respectively.

The lower the ultraviolet reflectance ratio

is, the better the silicon film quality. No saturation trend is observed in these curves

and improved device performance, as determined by the parameters measured here, can

be expected from improved film crystallinity

.

Sowever , the data plotted in figures 19 and

20 suggest that substrate surface finishing

is not the ultimate limiting factor in determining film quality.

Some comments regarding the data presented

here are necessary. The infrared reflectance

range shown in figures 19 and 20 corresponds

approximately to the range observed (from the

worst to the best) among purchased substrates.

The substrates used here were deliberately

not annealed before silicon epitaxy, except

for one which had a good surface initially

represented by a reflectance ratio of 0.95.

The latter served as a reference with which

other specimens could be compared. Annealing

the other substrates can be expected to improve device performance, but would obscure

a decision on the level of damage tolerable

in the substrate polishing process.

The level of damage in the substrates used

was not always uniform across the substrate.

Consequently, some scatter in the data can

be expected. In addition, gated Hall mea- surements at the silicon film thickness used

here (0.6 pm) are complex because of anomalous behavior near zero gate bias [26] and

data must be obtained at a bias level suitable for the device being evaluated. This
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

08

07

I

06 2^ 05

UV REFLECTANCE RATIO, log (Ro, /Rv)

Figure 21. Correlation of SOS MOSFET channel

current with ultraviolet reflectance data of

corresponding epitaxial silicon films.

means that the devices measured here were

not all measured under the same gate bias

conditions. Despite these factors, there is

a good correlation between the infrared re02 03

UV REFLECTANCE RATIO. log(Ro^/R)^)

Figure 22. Correlation of gated Hall mobility of SOS structures with ultraviolet reflec

tance data of corresponding epitaxial silicon

f i 1 ms .

flectance measurements on the substrates,

the ultraviolet reflectance measurements on the epitaxial silicon films, and the electrical properties of the final devices.

(M. T. Duffy"", P. J. Zanzucchi^, W. E. Ham^ , and G. W. Cullen^)

Work performed at RCA Laboratories under

NBS Contract No. 5-35915. NBS contact for

additional technical information: K. F. Galloway

.
5. MATERIALS AND PROCEDURES FOR

WAFER PROCESSING

5,1. Sodium Contamination in Oxidation Fur- naces*

Thermodynamic calculations aimed at investigating the sodium equilibrium density established at 1000°C in oxidation atmospheres contained in polycrystalline silicon tubes were

made [27] as an extension of similar calculations (NBS Spec. Publ. 400-29, pp. 36-37)

made in oxidation atmospheres contained in

transparent fused silica tubes [28].

Silicon oxidation tubes are prepared by chemical vapor deposition through hydrogen reduction of pure trichlorosilane ; this fabrication process made under carefully controlled

conditions reduces the inclusion of external

contamination. The resulting polycrystalline

silicon has a sodium content of about 10 ppb

by weight [29] or about 1000 times less sodium than occurs in transparent fused silica

oxidation tubes . The low sodium content of

polysilicon makes it an excellent wall material to contain oxidation atmospheres.

The equilibrium density of sodium in the oxidation atmosphere in a new polycrystalline

silicon tube is about 32 times lower than

that of a fused silica tube operated under

the same conditions (temperature and water

content in the atmosphere) because of the initially lower sodium content in the bulk. However, a silicon oxidation tube tends to develop increased alkali contamination during normal operation unless special care is taken to isolate it from external contamination. At

oxidation temperatures, sodium diffuses easily through the wall into the oxidation atmosphere. The sodium may originate from the

furnace refractory impurities, the room ambient, or other sources outside the tube. Such

contamination is hard to avoid even in clean

production-oriented facilities. Further, because oxygen and trace water impurity contained in the oxidation atmosphere react with

the tube to form silicon dioxide deposited on

the wall, the situation qualitatively approaches the fused silica oxidation chamber

condition.

The sodium contamination build-up imposes a need for periodic in situ cleaning of silicon oxidation tubes if devices are to be fabricated with oxides which have a low sodium

content. The introduction of a dilute hydrogen chloride mixture causes the formation of

silicon chloride from the interaction with

sodium glasses contained in the silica film

25

formed on the silicon tube wall. At equilibrium, for the case where the cleaning mixture contains 10-percent hydrogen chloride

and 0.01-percent water in an inert carrier

gas, the equilibrium sodium number density

in the atmosphere is 3.66 x 10^^ cm~^. This

is 10'' times larger than the equilibrium so- dium density normally present in the oxidation ambient. This results in a leaching of

sodium from the inner wall of the tube and a reduction in the bulk sodium content below

the original level.

Chlorine gas is sometimes used to clean silica tubes since it is somewhat more efficient

than hydrogen chloride. However, the use of

dry chlorine as a cleaning agent for silicon

tubes should be avoided because of the risk

involved due to its high reactivity with sili

con. The calculations [27] show that the

tube could be severely damaged by chlorine

if the protective silica film deposited on

the inner tube wall has pinholes through

which chlorine may come in contact with silicon to produce quantities of gaseous silicon

chlorides. If moisture is present in the sys

tem, hydrogen chloride is generated, moderating the aggressive behavior of dry chlorine.

The silica film formed on the tube wall can also be removed by a dilute hydrogen atmosphere at oxidation temperature. The formation of water generated through the hydrogensilica interaction contributes to the leaching of sodium from the tube bulk. If such an atmosphere is maintained in silicon oxidation

tubes when they are not being used for semiconductor processing, the sodium impurity

from the tube wall bulk and the silica film

on the tube are constantly removed from the

hot zone. This tube conditioning reduces the

need for periodic hydrogen chloride cleaning

and consequently minimizes the incidence of

other species (such as chlorine) in the oxida

tion chamber. Experimental evidence shows

that chlorine is incorporated into oxide

films thermally grown on silicon wafers processed in oxidation tubes previously cleaned

with hydrogen chloride [30]. This has proved

to result in electrical instabilities under

exposure to ionizing radiation [31].

(S. Mayo and W. H. Evans+)

Principally funded by the Defense Nuclear

Agency

.

' NBS Chemical Process Data Evaluation Section

Physical Chemistry Division.
6. PHOTOLITHOGRAPHY

6.1. Calibration Standards for Optical Photomask Line-Width Measurements

A primary objective of the photomask metrology task is to develop line-width artifacts

in the 1- to 10-pm region which can be used

by the integrated circuit industry to calibrate optical systems used for measuring line

widths on photomasks with transmitted illumination. The first step toward issuance of

these artifacts is a collaborative test involving NBS and a limited number of participants from the industry.* This initial collaborative test has as its goals: 1) reduction of known systematic errors resulting

from instrument-reading scale offsets; 2)

evaluation of residual errors which remain

after reducing known systematic errors; and

3) evaluation of the proposed NBS recommended

line-width measurement procedure to determine

if the procedure is sufficiently well defined

to achieve the optimal transfer of measurements from NBS to the user.

In this initial experiment, each participant

is to make measurements on lines in two pattern groups of the initial chromium-on-glass

artifacts (NBS Spec. Pubis. 400-29, p. 53,

and 400-17, pp. 37-38). A view of one of the

pattern groups is shown in figure 23. The

four-bar pattern of 1-ym lines is used for

focusing the microscope. The widths of the

spaces and lines numbered 1 through 6 are measured at the locations marked by the arrows; measurements of these six patterns plus

the adjacent 2-Mm line are also made on a second pattern group. Line spacing measurements

(both left edge to left edge and right edge

to right edge) are made between lines A and B in the two line groups shown in the insets.

The measurement procedure for use in the collaborative test is based on a preliminary set

of measurements performed at NBS. These measurements, described below, established that

the systematic error in line-width measurements resulting from instrument scale offsets

could be reduced by an order of magnitude using line-width calibration rather than a con- ventional line-spacing calibration.

Calibration Measurements — Optical line-width

calibrations on these artifacts were made on the NBS scanning photometric microscope (NBS

Spec. Publ. 400-29, pp. 50-52) [32]. The important parameters of this image-scanning,

transmitted-light optical microscope are: 1)

spectrally filtered illumination peaked at 530 nm for optimum optical-image performance;

2) 0.90 objective N.A.; 3) 0.60 condenser N.A. (underfilled) ; and 4) an effective scanning

Figure 23. A view of a pattern group on the

chromium-on-glass line-width measurement art i fact

.

slit of 0,13 ym by 1.3 ym at the position of

the test specimen. The scanning slit re- mains fixed while the stage moves, and the

stage position is monitored with a linear

voltage transducer (LVTD) gauge. The digital outputs from the photomultiplier and

LVTD gauge are recorded by a microprocessor

at position increments of 0.01 ym. The

length scale of the LVTD gauge is calibrated

using a known center-to-center line-spacing

measurement

.

The optical line-width calibrations on the ar tifacts involve measurement of the widths of seven clear or opaque lines in each pattern

group measured. Each line is measured nine

times, three times a day for three days. For

the three measurements made within a single

day, the artifact remained in the same place,

but the optical system was refocused prior

to each measurement. For the day-to-day

measurements, the artifact was removed from

the system and reinserted, realigned, and

repositioned. Table 3 shows a sample of the

During the interval between the end of the

reporting period and the issuance of this re port, the initial collaborative test has

been completed and the results analyzed. A report of the results is given in Swyt , D. A., Rosberry , F. W. , and Nyyssonen, D., Cali

bration of Optical Microscopes for Photomask

Linewldth Measurements, Proa. Mioroeleatronias Seminar — Interface '77, Monterey, California, October 5-7, 1977, pp. 131-141+.

26
PHOTOLITHOGRAPHY

Table 3 - Sample Optical Line-Width Calibration Data

M^,^-;^,! Measured Line Width, um c + ^^^a.^a Nominal Standard

Line Width, Day 1 Day 2 Day 3 Deviation

Run 1 Run 2 Run 3 Run 1 Run 2 Run 3 Run 1 Run 2 Run 3

Opaque Lines

Clea

1 0 82 0 82 0 83 0 82 0 83 0 82 0 80 0 83 0 81 0 82 0 03

2 1 76 1 74 1 75 1 77 1 78 1 81 1 79 1 80 1 79 1 78 0 07

3 2 86 2 84 2 81 2 79 2 75 2 83 2 80 2 79 2 82 2 81 0 10

10 9 27 9 20 9 18 9 18 9 21 0 13

r Lines

1 0 88 0 86 0 89 0 88 0 90 0 89 0 92 0 93 0 93 0 90 0 07

3 3 06 3 05 3 01 3 06 3 08 3 03 3 03 3 01 2 99 3 04 0 09

10 9 54 9 58 9 52 9 52 9 50 9 53 0 09

measurement data on one particular pattern

group. Earlier measurement data established

that on a high quality, nominally 1-ym wide

line, the system was capable of a 3a variability of 0.025 ym with five measurements on any

given day; the 3a variability could be re- duced below 0.01 ym with a total of 30 measurements .

The present data reflect two primary sources

of measurement variation. Line-edge quality

is reflected in the day-to-day variations as shown, particularly in the measurement of the

nominally 2-ym wide opaque line. When replacing the artifact in the measurement system,

it is difficult to assure that the measurement is made in exactly the same place be- cause the serrations originally intended for

this purpose (NBS Spec. Publ. 400-17, pp. 37-

38) do not lie within the field of view.

Hence, the visible raggedness in the edge

j affects the day-to-day measurements. The

; other source of variation in measurements involves focus. Although initial focusing can

be performed extremely accurately, the height

variation of the artifact with linear displacement of the stage introduces focus displacements of 0.1 ym or more while scanning

over a distance of 10 ym. These displacements may be due to imperfections or dirt in

I the stage ball bearings. Care was taken to minimize this effect by leveling the stage.

Currently, the remaining focus error is cor-

' rected in mid-scan (for the 10-ym lines only)

.

This is possible since for a given position

of the artifact, the focus error introduced

over the length of scan may be determined using the piezoelectric focusing mechanism. As

long as the artifact is not moved with re1 spect to the stage, this focus error is re- peatable and therefore may be corrected for

by application of the correct voltage to the

piezoelectric elements. This technique is

not altogether satisfactory since the 3a values are still larger than is desirable. As

a long-term solution to this problem, the

flexure-pivot stage used in the SEM/interferometer system (see sec. 6.5) [33] is being

redesigned with a hole in the middle to ac- commodate the condenser for the transmittedlight system. It has been tested in re- flected light and shows no focus problem over distances of 10 ym or more when properly leveled. (D. Nyyssonen''"

)

Line-Width Measurements^ — While the underlying premise that proper calibration of microscopes for measuring line widths demands linewidth, not line-spacing, standards has not

been fully validated, the fundamental inadequacy of line-spacing calibrations has been

demonstrated (NBS Spec. Pubis. 400-19, pp. 41-

42, and 400-29, pp. 53-56) [34]. These results clearly showed a systematic dependence

of measured line widths on object, illumination, and measuring-eyepiece types.

The study reported here was undertaken as an initial demonstration of the positive value

of using the calibrated line-width as opposed

to the line-spaciiig standard. The standard

was a prototype artifact calibrated as described above. The study consisted of: 1) visual-eyepiece measurements (in arbitrary

scale units) of the widths of a number of

lines; 2) computation of the widths of the

lines subsequently designated "unknowns" in

NBS Optical Physics Division.

§ Partially funded through the NBS Dimensional

Metrology Program.

27
PHOTOLITHOGRAPHY

terms of the widths of those designated "calibration lines;" and finally, 3) comparison of

the widths of the unknowns so computed

against the relative widths as measured on

the scanning photometric microscope. The

study was performed partially blind in that

the operator made the visual measurements

knowing only the nominal widths of the lines,

independent of their designation. Only after

completion of the measurements were raw data

in scale units converted to dimensional units

relative to the photometrically measured line

widths

.

Measurements were made on both filar and

image-shearing eyepiece microscopes. Filtered green light and an objective/condenser

numerical aperture ratio of less than one were used by design. With all readings taken

in arbitrary scale units, the widths of the

five clear spaces were measured, then the

widths of the first two spaces remeasured.

Similarly, the widths of six opaque lines were measured, with repeat measurements on

the first two. Each of the line-width measurements consisted of three individual readings which were averaged to obtain the mean

value for the measurement. The entire set of

measurements for each group was repeated in

three "runs" for each eyepiece.

After completion of the measurements, computations and analysis of the results were performed by a different person. One pair of

lines in each of the clear and opaque groups,

the pair consisting of one 3-pm wide line and

one 10-pm wide line, served as calibration

lines. The remaining lines were treated as unknowns. For each individual run of lines

of like polarity (clear or opaque) , readings

in arbitrary units for each eyepiece were

converted to dimensional units by linear calibration against the photometrically measured

widths of the calibration pair.

For r^ and r2, respectively the means of

three readings on lines of photometrically

measured widths wj and W2 , the unknown width w corresponding to reading r is: X X

w = ar + b , X X

'

where

W1-W2

a = ri-r2

and

28

W2r i-wir2

b = . ri-r2

By this calibration relationship, the numerical results of this exercise (shown in tables

4 and 5 for filar and image-shearing eyepieces, respectively) were generated. In- cluded are specific results for each of three runs on clear spaces and opaque lines made

with each eyepiece.

The agreement is generally satisfactory. For

the filar eyepiece the RMS average difference

for all runs is 0.058 pm; because of the

large deviations on run 3, the RMS average

difference for the measurements on the clear

spaces is larger than that for the measurements on the opaque lines. For the imageshearing eyepiece, the RMS average difference

for the measurements on the clear spaces is

0.033 pm, and that for the measurements on

the opaque lines is 0.099 ym. Despite the

relatively large differences obtained in some

cases, it is apparent that systematic differences between the two types of eyepieces have

been eliminated.

(D. A, Swyt^ and F. W. Rosberry^)

{

6.2. Optical Line-Width Measurements on Silicon and Iron-Oxide Photomasks'^

Optical measurements of line widths on seethrough photomasks such as silicon and ironoxide have posed a problem to both manufacturers and users of photomasks because of the un- 1 certainty involved in locating edges in the

complex image structure. The difficulty lies I in determining the edge locations in relationship to the dark and bright bands that occur

near the edges (NBS Spec. Publ. 400-29, p. 53) . Clearly, measurements of line widths

made by locating the edge in the center of

the dark band yield significantly different

values than measurements made using the transition region between the clear area and the

dark band or even between the dark band and

the adjacent bright band. In order to determine the true edge location, an accurate description of the optical system, illumination, . and photomask is required, and the phenomena

which give rise to the banding must be understood .

NBS Optics and Micrometrology Section, Mechanics Division.

Funded through the NBS Optical Physics Division .

^.1
PHOTOLITHOGRAPHY

Table 4 - Comparison of Filar-Eyepiece Measurements With

Scanning Photometric Microscope Measurements of

the ^ame Lines When the Eyepiece is Calibrated

Against the Photometric System

Line Type

Line

Number^

Photometric

Line Width,

jjm

Visual Line Width, ym Run I Run 2 Run 3

Clear Spaces

Unknowns 3 0 78 0 78 0 78 0 69

4 2 98 2 99 3 01 2 85

5 10 10 10 08 10 15 10 19

Ca libration 1 2 98 3 02 2 98 2 97

(Check) 2 10 17 10 13 10 17 10 33

Opaque Lines:

Unknowns 8 0 96 0 88 0 88 0 93

9 1 86 1 83 1 90 1 87

10 2 96 3 01 2 99 3 00

11 10 03 9 96 9 92 10 02

Cal i brat ion 6 3 00 2 99 2 96 2 96

(Check) 7 9 95 9 99 9 94 9 95

The line numbering system in this experiment is different

from that shown in figure 2 3 which was used in the initial

collaborative test.

Table 5 — Comparison of Image-Shearing hyepiece Measurements

With Scanning Photometric Microscope Measurements

of the Same Lines When the Eyepiece is Calibrated

Against the Photometric System

I . Photometric . . , , ,. , Line Type

^ine

^ ^ine Width, Visual Line Width, ym

'^"'"^^'^ pm Run 1 Run 2 Run 3

Clear Spaces:

Unknowns 3 0 78 0 77 0 79 U 77

4 2 98 2. 97 2 97 2 96

5 10 10 10 17 10 16 10 16

Cal i bration 1 2 98 2 96 2 98 2 98

(Check) 2 10 17 10 16 10 21 10 18

Opaque Lines:

Unknowns 8 0 96 1 09 1 08 0 97

9 1 86 2 05 2 05 1 95

10 2 96 3 05 3 11 2 99

n 10 03 10 03 9 95 9 94

Cal i bration 6 3 00 3 05 3 06 2 94

(CheckJ 7 9 95 10 03 9 97 9 92

^The line numbering system in this experiment is differ^ent

from that shown in figure 23 which was used in the initial

collaborative test.

29
PHOTOLITHOGRAPHY

For see-through masks the image structure is

quite different from that of the previouslyanalyzed opaque masks (NBS Spec. Pubis. 400-

29, pp. 50-52, and 400-36, pp. 42-44) [32],

although the optical system parameters have

remained the same. The change which must be

made in the mathematical model of the system

is in the complex amplitude transmittance

function t(x). For chromium-on-glass photomasks, which have a transmittance of a few

percent or less, the complex amplitude transmittance was taken as real and the phase re- tardation of the material ignored. However,

for see-through masks, there is an optical

path difference between light transmitted

through the clear and the semi-opaque portions of the photomask. The resulting phase

difference in transmitted light is given by

= k(n - l)d , o

(9)

where k is the wave number, n is the index

o of refraction of the mask material, and d is

the mask thickness. The complex amplitude

transmittance must, in the general case, include this phase as well so that

t(x) = t (x)e

o

i(i.(x)

DISTANCE ( .m)

a. 0.60 condenser N.A., = 0.31

(b) tt/2, and (c) tt .

(a) 0, 0.22 condenser N.A.

t:/2, (b) TT. To = 0.31, (a)

0.90 condenser N.A. and (b) tt. To = 0.21 (a) 0

Figure 2k. Computed image profiles of a 2-ym

clear line for an objective lens with numerical aperture of 0.90 and various condenser

lenses and phase shifts.

30
PHOTOLITHOGRAPHY

Image profiles and associated parameters were

computed for several cases of interest [35]

.

Figure 24 illustrates the computed image profiles for an objective numerical aperture of

0.90 and varying condenser aperture for the

background transmittances and phases indicated. In the course of generating these image

profiles, Kintner introduced a new, faster

computation method based upon a Fourierseries expansion of the object and computation of the transmission cross-coefficient

which characterizes the combined condenser

and objective system [36,37]. This method,

which is discussed in connection with darkfield imaging (see sec. 6.3.), is outlined

in Appendix C. It enables a reduction in computer time of up to an order of magnitude in

some cases along with a slight improvement in

accuracy with phase included in the complex

amplitude transmittance of the object.

Because the transmittance threshold, T , c which corresponds to the edge location (denoted by the dashed lines in fig. 24) varies

with the amount of phase present as well as

the background transmittance of the photomask

material, a new expression must be derived

for the proper transmittance threshold to be

used to establish the edges of the line being

measured. As in the case of nearly opaque

masks (NBS Spec. Publ. 400-36, pp. 42-44)

[32], when a symmetrical impulse response is

centered at the edge between two materials,

as shown in figure 25, in coherent illumination the threshold at the edge is the ratio

of the transmittance computed at this position with respect to the transmittance of the

clear area. With a phase change present, T becomes, in coherent limit,

Figure 25. Schematic iilustration showing

method of computing threshold at edge with

phase shift present.

T = 0.25 (1 + T + 2/t~ cosd)) , (10)

c o o

where T is the relative transmittance of the

o semi-opaque areas of the mask with respect to the transmittance of the clear area and <p is

given in eq (9) for transmitted light. Partially coherent imaging systems are extremely sensitive to phase changes; for eq (10) to apply, the condenser aperture must be less

than it was for the case of nearly opaque

masks. Table 6 gives values of T^ for a variety of cases. The values for the case of

coherent light were computed from eq (10)

;

Table 6 — Computed Thresholds Corresponding to Edge Location

'condenser ' 'objective

1 0

9

00 (incoherent)

1 00 0 66 0 44 0 24

0 (coherent)

0 0 0.50 u 27 0 24 0 23 U.25

O.Z} 0 0.61 0 56 0 53 0,53

0.21 1^/2 0,61 0 41 0 31 0,30

0.21 TT 0.61 0 26 0 07 0.07

0.31 0 0.66 0 61 0.61

0.31 7t/2 0.66 0 36 0 35 0 33 0.33

0.31 TT 0.66 0 10 0 08 0 06 0.05

31

]
PHOTOLITHOGRAPHY

Table 7 - Line-Width Errors Resulting from Lo- cating the Edges at the Center of

the Adjacent Dark Band

Objective N.A.

0.90 0.60 0.45

0.21 0.20 ym 0.31 ym 0.41

0.21 IT 0.11 0.17 0.22

0.31 tt/2 0.15 0.24 0.32

0.31 0.08 0.13 0.17

1 .00 any 0 0 0

values for the other cases were found from

the computed image profiles.

In microscopes which do not produce a photometric image profile and cannot, therefore,

determine the proper threshold, it is of interest to know the magnitude of the error which results from locating the edge at the

center of the dark band which is the most re- peatable setting for a filar eyepiece. Table

7 shows the magnitude of this error computed

for various values of T^, <}> , and objective and

condenser numerical apertures [35].

(D. Nyyssonen'^)

6.3. Optical Line-Width Measurements in Re- flected Light^

Another major goal of the photomask metrology

task is the extension of the capability for

measuring line widths in the 1- to 10-ym re- gion to include measurements in reflected

light. Early in the effort it was recognized

that measurements made in reflected light are more difficult to characterize than those

made in transmitted light on black-chromium

photomasks. Several factors contribute to these difficulties. The reflectance of the

materials is much lower than the transmittance resulting in lower signal-to-noise ratio in the photomultiplier output of the scan- ning photometric microscope. The differences

in reflectances are also lower, thereby producing lower contrast imagery. Therefore,

the optical-path differences between light re- flected from the various parts of the test specimen become important in determining edge

locations. The study of transmittance measurements of see-through photomasks (see sec. 6.2.) indicates that the edge image profiles

change drastically along with the threshold

corresponding to edge location. In addition,

it is expected that the measurements would

be affected both by the surface characteristics of the materials and by the thickness

of the line-carrying layer in relation to the depth of focus of the microscope.

Evaluation of Bright-Field and Dark-Field

Refleated-Light Systems — One of the two illumination systems, bright field or dark

field, is generally used for viewing objects

in reflected light with a microscope. The

bright-field system, illustrated schematically in figure 26a, uses a beam-splitter to direct the illumination onto the objective

which, in turn, focuses the illuminating aper- ture onto the object. When unfolded, this

system corresponds to a transmitted-light

system with matched numerical apertures for

condenser and objective. Such a system is

partially coherent and sensitive to phase

(optical-path) differences in the object.

Since there is no analytic expression to relate edge location to transmittance threshold, this system is a poor choice for linewidth measurements of the desired accuracy.

In the dark-field system, shown schematically

in figure 26b, the illuminating condenser

aperture is annular, with an inner radius

corresponding to, or greater than, the numerical aperture of the objective. The outer

radius is usually as large as practicable in

order to gain the maximum flux at the detector. This system, when unfolded, may be ana- lyzed using the theory of partial coherence

as was done in the case of transmitted-light

systems [32]. However, because the thin, an- nular condenser aperture has a numerical aper

ture greater than that of the objective, a two-dimensional analysis must be made. The

computation method used in earlier calculations is prohibitively time-consuming for two dimensional computations, but fortunately

the new method of computation introduced by

Kintner [36,37], discussed below, reduces

the computation time by nearly an order of

magnitude; this reduction makes accurate anal

ysis of dark-field imagery feasible.

(D. Nyyssonen'')

Computation of Dark-Field Line Images — The

calculation of the image intensity distribution in a partially coherent imaging system

is, in general, a difficult task. It can be

NBS Optical Physics Division.

Principally funded by the Naval Ocean Systems Center.

32
PHOTOLITHOGRAPHY

hJI SCANNING SLIT SCANNING SlIT

APERTURE DIAPHRAGM

OBJECT (VIEWED IN REFIECTIDN)

APERTURE DIAPHRAGM

Bright-field, ref 1 ected- 1

i ght system. Dark-field, ref 1 ected- 1

i ght system.

Figure 26. Schematics of optical microscope systems

simplified by describing tlie system in terms

of a dimensionless abstract model which takes

advantage of the properties of the Fourier

transform and by making certain particular as- sumptions about the form of the object being

imaged. In particular, if the object function varies in one dimension only, if the object function is assumed to be periodic, and

if the image plane is sampled by a scanning

slit, the intensity spectrum observed is given by

sin(TTwg)

TTW^ •J(0 (11)

where E, is the geometrical coordinate in the

pupil plane divided by the numerical aperture of the objective lens, w is the full

width of the scanning slit in dimensionless

coordinates (actual width multiplied by the

numerical aperture of the objective lens and

divided by the wavelength of the light), and

1(5) is the image spatial frequency spectrum

in the pupil plane. It should be noted that

a non-periodic object may be expressed in

terms of a periodic model if a large period

is chosen so that the object is effectively

isolated and does not interfere with its

neighbors. The model and the derivation of

eq (11) are outlined in Appendix C. (E. C. Kintner"^)

Comparison of Calculated and Experimental

Dark-Field Image Profiles — Line images were

calculated from eq (11) for comparison with

image profiles experimentally measured with

dark-field illumination. One of the difficulties in comparing theoretical and experimentally measured image profiles is determination of the radii of the annular condenser

illuminator. For the available transmittedlight, dark-field systems, the numerical apertures of the annuli were available in the literature. However, for ref lected-light systems with a reflecting annular illuminator

built into the objective, the radii of the

annuli could only be estimated from the physical dimensions.

Images of 1-, 2-, and 5-ym wide clear lines were calculated for the case of an objective

lens with a numerical aperture of 0.60 and

an annular condenser aperture with inner and

NBS-NRC Postdoctoral Research Associate, NBS

Optics and Micrometrology Section, Mechanics

Division.

33
PHOTOLITHOGRAPHY

DISTANCE, ,um

a . 1 -ym c 1 ea r 1 i ne

,

S 0.5 -

DISTANCE, nm

b . 2-ym cl ea r 1 i ne

.

DISTANCE, idm

5-um c 1 ear 1 I ne

.

Figure 27. Theoretical dark-field image of clear

lines for 0.60 objective N.A. and 0.80 to 0.95

condenser N.A.

outer radii corresponding to numerical apertures of 0.80 and 0.95, respectively. The

calculated profiles, shown in figure 27, have

the observed bright band at the edge of the

line which is frequently used to make linewidth measurements. However, the peak of

this bright band does not always coincide

with the edge as is generally assumed. Other

calculations show that the peak displacement

is independent of phase difference and background transmittance of the photomask material. It is therefore possible to plot a calibration curve for a given condenser and objective combination; this curve relates the

line width measured peak-to-peak to the true

line width as shown in figure 28. This plot

varies with the numerical apertures of the

condenser and objective and is dependent on the optical aberrations of the system (a diffraction limited optical system is assumed)

.

Experimentally determined image profiles in

the transmitted-light , dark-field case were

found to be similar to the calculated profiles. However, ref lected-light , dark-field

imagery is much less satisfactory principally because of the low light level in the image plane and the resulting poor signal-tonoise ratio. This limitation, combined with

the nonlinear calibration relation (fig. 28)

led to the decision not to use reflectedlight, dark-field illumination as a method

for calibrating lines on wafers.

Optimum Line-Width Measuring System — Since

neither conventional reflection system seemed

to be a desirable optical calibration system,

other possible systems were examined. Based

on the large amount of information and re- search available on the transmitted-light sys

tem, an optimum choice would be the reflection analog of that system. The major modifi

cation which must be made to conventional re- flection systems is the restriction of the il

luminating aperture to a diameter less than

that of the imaging system. This can be

34
PHOTOLITHOGRAPHY

achieved by using a conventional bright-field

system but closing down the aperture diaphragm to yield a suitable value of the ratio

R of the numerical aperture of the condenser

lens to that of the objective lens. Investigations on semi-opaque photomasks (see sec.

3.5 <

J" 3.0

Q.

<

tuQ.a

111 oc

tn<

tuS

zH

O

UJ

2.0

1.0

1 1

1 1 1

i

1 1 1 1 1 y /

/

f

//

// // //

//

/

/ // - / / / / / / 1 1 1 1

1 1 0.5 1.0 1.5 2.0 2.5 3.0 3.5 LINEWIDTH,/im

Figure 28. Theoretical calibration curve for

dark-field illumination, line width measured

between peaks versus true line width for 0.85

objective N.A., 0.85 to 1.00 condenser N.A.,

and d i f f ract i on- 1 imi ted optics.

6.2) have shown that the coherent limit for

the threshold corresponding to the edge location, given in eq (10) , is appropriate when

R is less than 0.45 provided that the phase

change is 77/2 rad or less. It is desirable

to use this aperture for most measurements

to maintain reasonable signal-to-noise levels in the photomultiplier output. However,

for phase changes approaching it rad the value of R must be reduced to 0.25 or less for

the coherent limit to apply. Note that in

reflection, the phase difference is not in

general the same as that for transmitted

light, as given in eq (9), but is given by

= 2kd + (12)

where k is the wave number, d is the thickness of the defining film on the substrate,

and d) is the difference in the phase change

o on reflection at the film surface and at the

substrate which is dependent upon the materials. To determine the proper threshold, it

is necessary to know the phase change ts) ^ or

(J) . In reflected light, <)) may be determined

R o by using a microscope interef erence objective

of the Watson-Barnett type and measuring the

fringe displacement . Figure 29 shows a photo

graph of such a fringe pattern for an ironoxide photomask. The white-light, zero-order

fringe must be used to determine the direction of fringe displacement. (D. Nyyssonen^)

Figure 29. Interference image of an ironoxide artifact as seen in a Watson-Barnett

interference attachment for the microscope.

(Optical path differences on reflection from

iron-oxide as compared to glass substrate in- troduce a phase change of approximately 0.7

X.)

6.4. Submicrometer Line-Width Measurements^

Although primary interest until recently has

been on line widths in the 1- to 10-ym region, there is now strong interest in exploit

ing optical line-width-measurement capability

below 1 ym. Edge profiles of submicrometer

clear lines on an opaque background were com- puted [38J. For relatively wide clear lines

on an opaque background, the 25-percent relative intensity transmittance point corresponds with the true edge location. However,

as the width of the line becomes smaller,

this threshold point starts to deviate

slightly from the true edge location as illus

trated in figure 30. This deviation results

in a small error if the 25-percent threshold

is employed. Similar deviations occur for m.easurements on partially transparent backtNBS Optical Physics Division.

< Principally funded by the Naval Ocean Systems Center.

35
PHOTOLITHOGRAPHY

|1

I

h IS

E

1.0

t-Q

^

liJ Z 0.5

-JQ

UJ

OC3

(A

<

UJ 0

\ \ .6/nm-\-\-

' \ \ \ -V V-- 25 percent

.1 .2 .3 .4 5 6 7 .8 DISTANCE (micrometers)

Figure 30. Theoretical image profiles of submicrometer clear lines from 0.^ to 1.0 ym wide

on an opaque background for the optical scanning

photometric microscope. (Vertical dashed lines

indicate respective edge locations. Intersection of 25-percent threshold line with respective curves indicates where system would locate

edge .

)

1.0

1 1 _

_^*>

•-—*^ /

/

/

/

/

/

/

1

1

i"

I-9

ui

0.5 Q

UJ

QC

Z)

en<

UJ

1 1

y/ ~ .^'^

/

/

/

/

/

/

/

/

1 1 0.5 1.0 TRUE LINEWIDTH, yu. m

a . I sola ted c 1 ear 1 i nes

.

0.5 1.0 TRUE LINEWIDTH, /xm

b. Adjacent lines and spaces of equal width.

Figure 31. Theoretical calibration curves for line-width measurements on chromium/chromium

oxide artifacts for O.6O condenser and 0.90 objective N.A. (33.5-percent threshold used for

measurement based on 2.5-percent background transmi ttance , no phase change.)

36
PHOTOLITHOGRAPHY

E

10

UJ - -; 0.5

Q

lij

0)

<

UJ

0.5 1.0 TRUE LINEWIDTH,^tm

a . I so 1 ated 1 i nes .

1

h Xv

///

• //

/

/

/

/

/

/

/

/

/

1

1 0.5 1.0 TRUE LINEWIDTH,/xm

b. Adjacent lines and spaces of equal width,

Figure 32. Theoretical calibration curves for line-width measurements on see-through photomasks or wafers with 31"percent background t ransm i ttance or reflectance. (Curve a: 7t/2 phase

change with line width measured at 32.8-percent threshold; curve b: it phase change with line

width measured at ^.8-percent threshold. Condenser N.A.: O.AO; objective N.A.: 0.90.)

grounds or reflecting surfaces, in which

cases the threshold is given by eq (10) (see

sec. 6.2.). Calibration curves to relate measured line width employing the 25-percent

threshold criterion to actual line width were

computed for several cases of interest as shown in figures 31 and 32.

For isolated lines on black-chromium photomasks, measurements can be made on lines as narrow as 0.4 ym; as shown in figure 31a, the error in line width is 0.02 ym for a 0.5-ym

and increases to 0.05 ym for a 0.4-ym line.

There is no significant difference between

the edge location for multiple lines and that

for isolated lines; the calibration curves differ by less than 0.01 ym for a line 0.4-ym

wide. For specimens with a higher transmittance (or reflectance) background and phase

differences present, measurement capability

extends to 0.5 ym. and abruptly dies out. There is still image detail present, but the curves are changing so rapidly that the

threshold corresponding to edge location for

larger line widths can no longer be used.

The curves in figure 32 are plotted for a con- denser numerical aperture of 0.40 and an objective numerical aperture of 0.90. This

choice of condenser aperture is poor for specimens with phase differences of tt rad. When

the condenser numerical aperture is reduced

to 0.22, the curves for a phase difference of

37

IT rad become the same as the curves for a phase difference of •n'/2 rad. Again, there

are no significant differences between the

edge location for multiple and isolated lines.

Although these theoretical images agree with

the imagery seen experimentally, they do not

include the effect of edge slope; an ideal

edge of infinite slope has been assumed. The

effect of edge slope needs to be explored,

and a comparison of optical measurements on submicrometer lines with SEM measurements on

the same lines must be performed to verify

the calculated calibration curves.

In the above discussion, it was assumed that

the test object was located at the position

of best focus; no aberration term for defocus

was included in the computations . Examination of the optical imagery in the microscope

showed that with phase differences present,

the imagery going through focus is not symmetric with respect to best focus. A com- plete reversal of image polarity can occur

going through focus. For isolated lines,

best focus occurs when the edge profile has

the steepest slope, but for multiple lines

and spaces, this criterion cannot be used.

In the latter case, best focus appears to occur when the transmittance at the center of

the line or space (whichever is lower transmittance or reflectance) reaches the same
PHOTOLITHOGRAPHY

DISTANCE (Mm) Figure 33. Theoretical image profiles as a function of focus position of adjacent 0.5"

pm lines and spaces for Tq = 0.3' and ^ =

tt/2. (Dark dashed line (C) is best focus;

(A): -A/2; (B): -X/k; (D): (E): +X/2.)

a. Viewed with optical scanning photometric

mi croscope

.

Viewed with scanning electron microscope

used for line-width measurement.

Figure 3^- Photomicrographs of a portion of resist pattern on a wafer showing nominal 0.5- ym lines and spaces (smallest pattern).

relative background transmittance level as the larger semi-opaque or clear areas of the test object. This case is illustrated in

figure 33. This criterion for best focus re- quires scanning over longer distances which,

in turn, puts severe requirements on maintenance of focus. The accuracy and repeatability of this method remains to be verified ex- perimentally .

Preliminary measurements were made on an ex- posed resist pattern on a silicon wafer which

contained 0.5-ym wide lines and spaces.

This wafer was examined in both the scanning

electron microscope (SEM)'''"^' and optical linewidth-measuring systems. Figure 34 shows

samples of optical and SEM imagery of the

best pattern on the wafer. In the optical

image, a dark line occurs along the edges of

the pattern, but in the SEM image a bright

line occurs in the same place. The pictures

are mirror images of the same portion of the same line groups and show that the 0.5-ym

t+

This specimen was supplied by the Naval

Ocean Systems Center.

Scanning electron micrograph made by W. J, Keery

.

38
PHOTOLITHOGRAPHY

lUo

z

<

o

UJ

-J

u.

Ul

oc

lU>

liJ oc

Figure 35- Experimental image profile of nominal 0.5-)jm lines and spaces

in resist on a wafer before carbon coating. (Reflection optical microscope

is analog of t ransm i ttance line-widtin measuring system witli illuminating

numerical aperture determined by leaf diapliragm set to R 0.5.)

wide lines are well resolved optically. Before the scanning electron micrographs were

taken, this pattern was scanned in the scanning photometric microscope system. For the

image scan shown in figure 35, the system was operated in the reflection mode with the leaf

aperture diaphragm shut down to between 0.6

and 0.7 times the aperture of the objective.

At this time, the problem of determining best

focus was identified; before an objective criterion for determination of best focus was developed, the resist pattern was overcoated

for examination in the SEM. Subsequent measurements revealed that the carbon coating

had affected the reflectance properties of

the wafer in such a way that determination of

best focus was virtually impossible and no measurements could be made. This result es- tablished the necessity of making all optical

measurements on test wafers before carbon

coating. (D. Nyyssonen'1 )

6.5. Primary Line-Width Calibrations

To relate the generally unaccessible defined

unit of length (wavelength of radiation from

krypton 86) to an accessible artifact for the user, several calibration measurements must

generally be successively linked. In the

first stage of the line-width calibration pro39

cess, the physical standard for the defined

unit of length is related to the width of a material line on a photomask-like artifact by means of a primary measurement system which

includes a polarization interferometer located in a scanning electron microscope (SEM)

(NBS Spec. Publ. 400-19, p. 43). The wavelength of the interferometer laser is monitored by comparison with the radiation wavelength from a stabilized laser which, in turn

is compared with the krypton radiation.

This primary measurement system is too timeconsuming and costly for the routine calibration of user artifacts. Therefore, in the

second stage of the process a secondary measurement system is used to relate the line

widths calibrated by the SEM/interf erometer

on the first artifact to line widths on the user artifact. The secondary measurement sys tem is a scanning photometric optical microscope with modifications for optimal performance (NBS Spec. Publ. 400-29, pp. 50-53)

[32]. In the third stage, the line widths

calibrated by the secondary measurement system are related to the readouts of a lineNBS Optical Physics Division.

^Partially funded through the NBS Dimensional

Metrology Program

.
PHOTOLITHOGRAPHY

ELECTRON COLLECTOR

ELECTRONS

INTERFEROMETER

MEASUREMENT

SAMPLE

F i gu re 36

.

ter system.

Schematic of the SEM/ i nterf eromewidth measurement system, such an an optical

microscope, used in the field. Finally, the user makes measurements of unknown photomask

line widths with this calibrated instrument.

In the primary measurement system [33], shown

schematically in figure 36, a very small beam

of electrons, nominally a few tens of nanometers in diameter, is focused at normal incidence on the test specimen and remains relatively stationary as the test specimen is

moved. A double-pass polarization interferometer records the motion of the test specimen

as the latter is moved under the beam; the

locations of the line edges are detected by

the change in SEM signal which arises from

differences in the secondary-electron emission coefficients of the various materials.

In this system, the SEM is used as a very

fine fiducial mark or cross hair for locating

the line edges. Since conventional SEMs are

not designed to operate in this mode, a number of compromises have been made in the operation of the SEM. These compromises include:

1) replacing the normally used stage with a specially designed stage assembly consisting

of two substages for moving the test specimen,

2) operating the electron beam in a linesweep mode with approximately l-ym sweep

length on the test specimen, and 3) operating

the SEM at 10 keV to reduce contamination

buildup on the test specimen (even though a higher operating voltage would give a smaller

effective electron-beam diameter)

.

Figure 37. Photograph of scanning stage with

flexure-pivot system. (Stage is fixed at points A, while other stage sections move; F

- flexure pivot, P - stack of piezoelectric

elements, and S - specimen mounting area.)

which moves the line from one edge to the

other; this stage has a maximum travel of

about 60 pm and a smoothness of better than

0.001 \im over a few micrometers. A photograph of a flexure-pivot stage is shown in

figure 37. This stage uses a stack of piezoelectric elements to drive the stage at one point and, by means of flexure-pivot lever

arms, the original movement is magnified

about 20 times.

A sample plot of the output from the two data

channels of the SEM-interferometer is shown

in figure 38. The SEM and interferometer data are shown with respect to the same linear

time base; the interval between successive

data points is 1 ms . The SEM output signal

is used to determine the line-edge locations

so that the line width, or the separation of

the line edges, can be related to a number of

whole fringes plus fringe fractions from the

interferometer output signal. Therefore, the

electron interaction with the material line

must be modeled in order to specify accurately the true line-edge locations from the SEM

signal or edge trace.

One of the two substages uses two piezoelectric drivers with orthogonal motions to position the test specimen over a 10-mm by 10-mm area in the SEM field of view. The other substage is a mechanical flexure-pivot stage

An illustration of the line edge which is as- sumed in the model is shown schematically in

figure 39a. The line edge is a linear ramp

rising at an angle <j) from the glass substrate

to the top of the chromium-oxide layer on a
PHOTOLITHOGRAPHY

.AAAAAAAAAAAAAAAAAAA

Figure 38. Direct output of the SEM and in- terferometer data channels for a nominally

2-ym wide opaque line.

black-chromium artifact. The incident electron beam is assumed to have a Gaussian distribution of electrons. For the model, it is

necessary to specify a value of <i> , the diameter of the electron beam, the thickness of

the chromium/ chromium-oxide layer, and the

secondary-electron emission coefficients of

the glass substrate, chromium, and chromium

oxide. In order to get a match between optical and SEM measurements , the line-edge location is defined as the point on the line

slope corresponding to a 25-percent intensity

transmittance (50-percent amplitude transmittance) . A line-edge trace calculated from

this model is compared with an experimental

SEM trace in figure 39b. The following parameters were used in the calculation: electron

beam diameter, 70 nm; thickness of chromium/

chromium dioxide layer, 110 nm; (j) , 55 deg

;

secondary-electron emission coefficients for

the glass substrate, chromium, and chromium

oxide, 1.0, 1.026, and 1.265, respectively.

The model fits the data well in the region of

the edge, but in the region past the edge on

the chromium-oxide side , the SEM output does

not fall off as rapidly as the model predicts.

Measurements of the widths of clear and

opaque lines on a black-chromium artifact

were made with both the SEM/interf erometer

system and the scanning photometric optical

microscope. Typical results are given in ta- ble 8 which lists the mean values of a series

of measurements. Based on other experimental

data, the 3a limits associated with repeated

measurements on a single occasion are 0.016

ym for the SEM/interf erometer and 0.025 ym

for the optical microscope.

There is essentially no difference in the mean values for the opaque lines as measured

on the SEM/interf erometer and optical microscope. For the clear lines, however, the

differences are 0.13 ym for the nominally 1-

ym wide line and 0.34 ym for the nominally

3-ym wide line. Part of these differences

O

X

Chromium/chromium oxide

Glass substrate

DISTANCE

a. Model of line edge.

DISTANCE

b. Comparison of an experimental SEM lineedge profile with the profile calculated

from the model

.

Figure 39. Line-edge profile for the model

and a comparison of an experimental SEM

line-edge trace with the calculated trace

based on the mode 1

.

is probably due to the measurement uncertainties associated with each measurement system.

For the optical microscope, the measurement

uncertainty is estimated to be ±0.10 ym. For

the SEM/interf erometer , the uncertainty is not so well characterized, but is estimated

to be smaller than ±0.10 ym. The SEM/interferometer system has the potential for a significantly smaller uncertainty than the optical microscope as a result of the better reso- lution of the SEM. The relatively large difference of 0.34 ym appears to be an anomaly

which is currently under investigation.

(A. W. Hartman^

,

D. Nyyssonen"'' , R. E. Swing^ , and W. J. Keery)

t

NBS Optical and Micrometrology Section, Me- chanics Division.

NBS Optical Physics Division.

41
PHOTOLITHOGRAPHY

Table 8 - Average Line-Width Measurements As Determined With the Optical

Scanning Photometric Microscope and the SEM/ Interferometer for

a Black-Chromium Artifact

Nominal Line Width

Measured Line-Widths, ym

Optical Microscope SEM/ Interferometer Di fference

1 ym Opaque 0.77 0.77 0

1 pm Clear 0.94 1 .07 0.13

3 ym Opaque 2.90 2.91 0.01

3 ym Clear 3.12 3.46 0.34

42
7 . TEST PATTERNS

I 7.1. Test Patterns for Resistivity-Dopant

Density Evaluation

The test pattern for developing data for re- evaluation of the resistivity-dopant density

relationship in silicon has been redesigned

to provide square-array four-probe resistor

structures [39] of varying pipe size. Of the

i various structures for measuring resistivity

included on test pattern NBS-3 (NBS Spec.

Publ. 400-12, pp. 19-22) [8], the squarearray four-probe resistor has proven to be

the best. However, because of lateral difI fusion and depletion width effects (NBS Spec.

I Publ. 400-17, pp. 25-26), a single pipe size

proved inadequate for making measurements on specimens which covered a resistivity range

from less than 10"^ Q-cm to over 100 S7«cm. The square-array four-probe resistors on the

original mask set for test pattern NBS-3 had

0.25-mil (6.3-ym) square pipes. A modified

base mask was made which permitted fabrication of 0.50-mil (12.7-ym) square pipes [39];

however, both sizes could not be fabricated

on the same wafer. The new test pattern,

NBS-4, contains five square-array four-probe

resistors with pipe sizes of 0.20, 0.25,

0.30, 0.40, and 0.50 mils (5.1, 6.3, 7.6,

10.2 , and 12 .7 ym)

.

In addition two MOS capacitors with diffused

guard rings for making steady-state, deepdepletion, capacitance-voltage measurements

[40] of dopant density and four new base

sheet resistors (see sec. 7.2.) were included on the new pattern. Two new mask levels were added to the original four levels

[8]. One of these, the BASE-CONTACT mask, is

designed to delineate heavily doped regions

j for making electrical contact to base diffusions . These regions are generally used only

I in processing of p-type wafers to reduce the

resistance of the aluminum contacts to the n- type base regions; the aluminum metallization

makes satisfactory contact to typical p-type

base regions. The other mask, the GATE mask,

is used to delineate regions of thin, highquality oxide which are needed for the MOS

, devices; only one type of oxide can be fabricated with test pattern NBS-3 [8].

; A photomicrograph of the NBS-4 test pattern

is shown in figure 40. The overall pattern

is 200 mils (5.08 mm) on a side and is divided into four quadrants. The new structures

on the pattern were designed using the same

i design rules as for NBS-3 [8]. The 38 test structures on the NBS-4 pattern are listed in

j table 9. Note that the Hall effect device

(4.31) is functional only after it has been

scribed from the wafer. The devices specifically designed for the resistivity-dopant

density evaluation are indicated by an asterisk in the table. Bulk dopant density values

can be obtained from the following structures

the three MOS capacitors (4.2, 4.3, and 4.8),

the base-collector diode (4.10), the MOSFETs

(4.15 and 4.16), and the Hall effect device

(4.31). Bulk resistivity values can be found

from the five square-array four-probe collector resistors (4.1, 4.5, 4.7, 4.9, and 4.17)

and the Hall effect device (4.31).

A variety of other structures for various pur

poses was also included on the new test pattern. It contains eight base sheet resistors

four of which were not included on test pattern NBS-3 [8], in order to provide comparisons of the various geometries (see sec. 7.2.). The base-under-emitter sheet resistor

(4.20) is a new structure on test pattern NBS

4; it is intended to determine the sheet re- sistance of the active region of the base.

The metal sheet resistor (4.19) is constructed as a Greek cross (NBS Spec. Publ. 400-29,

pp. 64-65) [41] rather than as an offset

quadrate cross (pinwheel) (NBS Spec. Publ.

400-19, pp. 44-45) to avoid the problem of

burn-out due to high current density in nar- row connecting arms. The incremental base

sheet resistor (4.35) was redesigned with

wider contact arms than the equivalent structure (3.30) to reduce problems which arose from the thinning of the contact arms during

anodic oxidation and etching.

(W. R. Thurber and M. G. Buehler)

7.2. Sheet Resistor Test Structures

Measurements were made on the base sheet resistors in both n- and p-type wafers fabricated with test pattern NBS-4 (see sec. 7.1.)

There are six van der Pauw and two bridge

base sheet resistors on this test pattern.

Four of these structures are new and four

also were included on test pattern NBS-3 [8]

.

The four new structures are illustrated in

figure 41. The repeated structures are the

offset quadrate cross van der Pauw resistor

(4.26), the large circular van der Pauw re- sistor with a field plate and channel stop

(4.11), a still larger circular van der Pauw

resistor designed for incremental sheet re- sistance studies (4.35), and a bridge-type

sheet resistor with the probe pads in a linear configuration (4.33).

Results of measurements of all eight sheet

resistors along the diameter of a 60-fi-cm n-
TEST PATTERNS

Figure 40 . Photomicrograph of test pattern NBS-^ for characterizing the resistivity-dopant

density relation in silicon. (The 38 test structures are identified in table 9. The overall pattern is 200 mils (5-08 mm) on a side.)

44
TEST PATTERNS

Table 9 - Planar Test Structures on Test Pattern NBS-4

Number Test Structure^

Important Mask

Dimension, mil (mm)

1 ^fnllprtnr rp^i*^fnr fninp^ 0 ?n mil on Fi *^irip) s _

2 25 fO 057)

2 *MnS raoaritor ovpr rol lector fbasp DfiR) D 15 (0 38)

34 *MOS capacitor over collector (emitter DGR)

Alignment markers

D

_ 15 (0 38)

5 *Collector resistor (pipes 0.30 mil on a side) s

_ 2.25 (0.057)

6 LOQO

7 *Col lector resistor (pipes 0.40 mil on a side) s

_ 2.25 (0.057)

8 *M0S capacitor over collector (FP, CS) D

_ 15 (0.38)

9 *Collector resistor (pipes 0.50 mil on a side) S _ 2.25 (0.057)

10 *Base-col lector diode (FP, CS) D 17 (0.43)

11 Base sheet resistor (VDP, FP, CS) D 17 (0.43)

12 MOS capacitor over base D

- 15 (0.38)

13 Bipolar transistor s

_ 5.5 (0.14)

14 Base-collector diode (FP, CS) D

_

5 (0.13

15 MOSFET (circular, W/L = 18.2) L _ 3.5 (0.089)

16 MOSFET (rectangular, W/L = 4)

*Collector resistor (pipes 0.25 mil on a side)

L

_ 0.5 (0.013)

17 S _ 2.25 (0.057)

18 Collector spreading resistor D

_ 15 (0.38)

19 Mpffll t^hppt ^P"^i^t^^ f Rrppk rrfT^^ 1 s 1 5 (G 018)

20 Base-under-emi tter sheet resistor (Greek cross) s

_

1 .5 (0.038)

21 Racp rnntart shppt rpsistor (Grppk cross) s

_

1 5 (0 038)

22 Base sheet resistor (Greek cross) s

_ 0.25 (0.0064)

23 Base sheet resistor (B, W/L = 0.25) L _

5 (0.15)

24 Racp chppt rpsistnr f Grppk rrnssl s

_

1 5 fO 038)

25 Rasp shept resistor (Greek cross, tab rontactsl s

_

1 5 (0 038)

26 Racp chppf resistor foffset Quadrate rrnssl s _

1 5 (0 038)

27 Mpta 1 -to-pmi tter contact rpsistor s _

1 0 (0 025)

28 Fmiffpr ^hppt rp*^i^fnr f nf "f ^pf nuridrrifp ^^n<^*^l s 1 5 (0 038)

29 Mpf a 1 _(-(-) -ha sp rontart resistor s

_

1 0 fO 025)

30 MOS capacitor over collector S = 4 (0.10)

31 *Hall effect device (VDP) s 100 (2.54)

32 Emitter sheet resistor (B, W/L = 0.25) L 6 (0.15)

33 Base sheet resistor (B, W/L = 0.25) L 6 (0.15)

34 Surface profilometer structure

35 Incremental base sheet resistor (VDP) D 30 (0.76)

36 Etch-control structures^

37 Resolution structures'^

38 Metal step-coverage resistor

" = structures designed for resistivity-dopant density evaluation.

= bridge; CS = channel stop; DGR = diffused guard ring; FP = field plate;

L = length along current path; VDP = van der Pauw; W = width of current

path

.

"'d = diameter; L = length along current path; S = side of a square.

'B = BASE mask; BS = BASE CONTACT mask; C = CONTACT mask; E = EMITTER

mask; GO - GATE OXIDE mask; M - METAL mask.

45
TEST PATTERNS

Greek cross (4.22) b. Bridge (4.23) •

c. Greek cross {h.2h). d . Greek cross (4.25)

Figure 41. Four of the base sheet resistors on test pattern NBS-4.

type silicon wafer are shown in figure 42. All the van der Pauw configurations at a given location give essentially the same sheet resistance. Since the large incremental sheet resistor (4.35) is located at a considerable distance from the cluster of oth- er base sheet resistors (see fig. 40), the

sheet resistance was measured on devices

above and below the quadrant where most of

the other sheet resistors are clustered and

averaged to obtain the values plotted on fig- ure 42. This procedure reduces the effects

of gradients in the sheet resistance on the

inter comparison

.

The good agreement of the new Greek-cross

structures with the circular and offset quadrate cross structure supports the numerical

analysis of these configurations by David and

Buehler [41]. In particular, they showed

that the Greek-cross sheet resistor is a valid van der Pauw test structure if the length

of the arms is greater than the width of the arms. Thus, accurate measurements of the

sheet resistance of a very small region can

be made. Structure 4.22 has arms which are only 0.25 mil (6.4 ym) wide which is the mini mum line width on the pattern. Simple calculations of the effects of lateral diffusion

on sheet resistance predict that structure

4.22 could show a resistance up to 6 percent higher than the structures with 1.5-mil

(38.1-ijm) wide arms such as structures 4.24

and 4.25. However, the experimental data

suggest that any such effect, if present at all, is at most about 0.5 percent.

46
TEST PATTERNS

•d -

POSITION ACROSS WAFER

Figure 42. Base sheet resistance across di- ameter of Wafer B59Ph-l (60-f]'cm n-typej fabricated with test pattern NBS-4. (a: Circuj lar van der Pauw with field plate and channel

I stop (4.11); O: Greek cross (4.22); : Bridge

(4.23); •: Greek cross (4.24); : Greek cross

(4.25); : Offset quadrate cross (4.26); O: Bridge (4.33); ^- Large circular van der Pauw

' (4.35).)

As expected, the bridge structures show lower

sheet resistance values than the van der Pauw

structures because of the effects of overetching and lateral diffusion on the bridge

structures (NBS Spec. Publ . 400-17, pp. 22-

24) . It is interesting to note that the two lines in figure 42, through the van der Pauw

and through the bridge data points, are not

parallel. This means that the amount of over- etching varied across the wafer.

The results of measurements on two p-type

silicon wafers revealed problems due to the

presence of an inverted surface layer which

shunts the current in the van der Pauw structures. Measurements on a p-type wafer

yielded results similar to those obtained on

the n-type wafer discussed above. As shown

in figure 43, the base sheet resistance had

a steep U-shaped profile, but, within the un- certainty due to the large variation in sheet

resistance across the wafer, the van der Pauw

structures all gave about the same value at

a given location. The two bridge structures

had lower measured sheet resistance which

could be generally accounted for by overetching and lateral diffusion effects. These

two structures had about the same sheet re- sistance at the center of the wafer; one is

higher to the left of center and the other

higher to the right of center, as would be

expected from their positions on the test pat

tern

.

The results of measurements on a 92-fi*cm ptype wafer, which was processed at the same

time as the S-n-cm wafer except that following the emitter diffusion all oxide was re- moved and a new, thinner oxide was regrown

to reduce surface inversion caused by charge

in the oxide, were significantly different as shown in figure 44. On this wafer, the sheet

resistance as measured on some of the van der

Pauw structures was much higher than the nomi

nal 200 to 300 ^/a , and the sheet resistance

of the bridge structures was much lower than

would be predicted by overetching considerations. The results on both kinds of structures can be qualitatively explained by the

presence of an inverted surface layer on the

p-type collector region which occurred in

spite of the special oxidation procedure.

For the van der Pauw structures, some of the

current passes through this surface layer

which surrounds the body of the resistor;

this results in a higher than normal value of

measured sheet resistance. The fraction of

the current which passes through the surface

layer is strongly dependent on the geometry

of the structure; the arm resistance and the

spacing between arms are important factors.

Structure 4.22, which has narrow arms close

together, shows the highest sheet resistance.

Structure 4.11 which has a channel stop and

field plate appears to give the truest value

of sheet resistance. The basic design of

this structure is good because the arms, even

though, they are fairly narrow, are widely sep

arated from each other. Consequently, even when the plate was biased to accumulate the

collector surface surrounding the base diffusion, the measured sheet resistance was not

changed significantly from the value measured

with an inverted surface. That structure

4.25 is much less influenced by the surface

layer than structure 4.24 can be attributed

to the fact that the diffusion for structure

4.24 extends under the probe pads and con- sequently the amount of shunted current is

much greater than that for structure (4.25).

For the bridge structures, the inverted surface layer provides a parallel current path

which reduces the measured sheet resistance.

(W. R. Thurber, M. G. Buehler, and Y. M. Liu)

47
TEST PATTERNS

i i i 1 POSITION ACROSS WAFER

Figure h3 . Base sheet resistance across diameter of Wafer B3.0B-3 (3-fi'cm p-type) fabricated with test pattern NBS-^. (Symbols

are the same as in fig. ^2.)

7.3. Overetch Structure for Visual Measurement

An important parameter to be measured in integrated circuit processing is the amount

of overetching which has occurred whenever a pattern is delineated by etching selected areas such as the windows in the silicon dioxide layer prior to a diffusion or when patterning the metallization interconnect layer.

A test structure was designed to provide a rapid way to "estimate the amount of overetching from microscopical observations. This

structure contains a built-in distance scale

which allows one to calculate the amount of

overetching without knowing the magnification

of the microscope that was used to observe

the structure on the wafer. However, because

the scale is a line-spacing scale and the determination of the amount of overetching is a line-width measurement (see sec. 6.1.), systematic errors may be introduced; these errors

are the order of 0.25 to 0.5 ym [34].

The test structure consists of six 25-Mm by

25-pm squares arranged as shown in figure

45a, which illustrates the pattern as it

would appear on the photomask. For a metallization pattern, the numbered squares represent areas which will remain unetched, while

for oxide windows they are the areas in which

etching will occur. In the photomask pattern,

the squares are positioned such that the cor- ners of adjacent squares touch each other.

48

>^--t>-^—^0—0—<>—

1 2 3 4 5 6 ) 8 9 10

POSITION ACROSS WAFER Figure . Base sheet resistance values for

Wafer B92B-2 {32-Q'cm p-type) fabricated with

test pattern NBS-4. (Symbols are the same as

in fig. 42.

)

The appearance of the structure after etching

a metallization layer is shown in figure 45b,

while figure 45c shows the structure as it

would appear following the etching of an oxide layer. The effect of overetching (somewhat exaggerated in these figures) is the

shrinking of each square in the metallization

pattern and the swelling of each square in

the oxide pattern. Each side of the square

is displaced by one half the amount of the

overetching, X

oe Although etching causes the size of each

square to change from the value it had on

the photomask, certain distances which are conserved may be used as a convenient scale.

It can be seen by comparing the patterns in

figure 45 that the distance from any top,

bottom, right, or left edge of one square to the corresponding top, bottom, right, or left edge of any other square is conserved

in the etching process. For rough determinations, the measurements may be made on a photomicrograph of the structure and the amount

of overetching computed from

W

where X is the scale distance on the mask

e

^1
TEST PATTERNS

\\\W

\\\\\^

Overetch structure, com- posed of 25-ym by 25-Mrn squares, as it appears

on the photomask. The

numbered squares represent areas which remain

unetched in a metallization layer and areas which are etched in an oxide layer.

OR

Overetch structure after being etched in a meta 1 1 i za t i on I ayer

.

The effect of overetching is shown by the

shrini<ing of each square

by on each side.

OR

Appearance of the overetch

structure after being

etched in an oxide layer.

The effect of overetching

is shown by the swelling

of each square by Xqq/2

on each side.

Figure k5 • Test structure for etch control. (The dimensions X^g and Xg are the actual overetch and "ruler" lengths, while Wgg and Wg denote these quantities as they appear on an enlarged photomicrograph of the structure.)

pattern in micrometers, W is the scale dise tance on the photomicrograph in arbitrary

units, and W is the overetch distance on

oe

the photomicrograph in the same arbitrary

units. For purposes of illustration in figure 45, the scale chosen is the distance between the line drawn to connect the right

edges of squares 2 and 5 and the line drawn

to connect the right edges of squares 3 and

6.

For measurements on photomicrographs, additional sources of error include variations

in the locations of the edges on the photomicrograph due to aberrations in the camera optical system and to variations in exposure

and development of the film. For these measurements it may be sufficient to assume that

the scale length, X , has the value intended

e by the photomask design.

For more accurate determinations a line-width

measuring system (such as a microscope

equipped with a filar or image-shearing eyepiece or an automatic TV-microscope system)

calibrated with a line-width standard should

be used to make measurements of X directly

oe

on the pattern. In this case, it is not necessary to use the built-in-scale feature of

the pattern. (G. C. Taylor and M. G. Buehler)

7.4. Test Pattern Design Concepts

Layout of test patterns in modular cells (NBS

Spec. Publ. 400-25, pp. 41-43) has proven to be useful in structure design (see sec. 7.2.),

mask generation (see sec. 7.6.), and testing.

A comprehensive test pattern for junctionisolated TTL technology has been designed

(NBS Spec. Publ. 400-29, pp. 65-66) [42] using

modular cells with a 2 by 10 pad array.

Frequently, however, it is desirable to in- sert a test pattern die at selected points on

a product wafer. In such circumstances, testing is facilitated if the contact pad arrange49
TEST PATTERNS

j

Figure kS . Computer layouts of two quadrants of test pattern NBS-9 for d i el ectr i cal 1 y isolated

periplnery of the die.

50
TEST PATTERNS !

TTL technology showing test structures arrayed in both the 2 by 10 modular array and around the

51
TEST PATTERNS

merit is the same as that on the product die

itself. It is possible to combine this ar- rangement with the 2 by N modular layout of test structures.

As an illustration consider the two quadrants

of test pattern NBS-9 which are illustrated

in figure 46. This test pattern was designed

for a dielectrically isolated TTL integrated

circuit (IC) process. The quadrants illustrated contain process control structures for

measuring sheet and contact resistances of

various diffused layers and metallizations

and advanced test structures (NBS Spec. Publ.

400-36, pp. 46-48) for measuring very small

device leakage currents arranged in a 2 by

10 array. Around the periphery, connected to the IC contacts, are located selected circuit

elements and individual circuit devices.

Figure hj . Photomicrograph of test pattern

NBS-5 for probe pad and probe card evaluation

(The pads in the leftmost column are kO pm on

Testing of the circuit elements can be accom- plished by suitable software control flagged

by the results of the initial test of the entire IC as determined on the test pattern

die. Failure of any of the circuit elements

to operate according to designed specifications could trigger tests of the individual

circuit devices. Additional tests on the re- maining test structures (using a different

probe card) might also be necessary to identify the process step at fault.

The complete pattern also contains a number

of test structures intended to provide information from visual (microscopical) observation and, in the remaining two quadrants, ran- dom fault test structures designed to test

for oxide pinholes, metal bridges, metal step

coverage, metal step to resistor, number of

emitter openings without fault, and perimeter

or area emitter faults. (T. J. Russell, G. C. Taylor, D. A. Maxwell, and M. G. Buehler)

7.5. Probe Pad Evaluation

Studies were undertaken to define the limits

of probe pad size suitable for use with test patterns arranged in 2 by 10 cell arrays. A single-level test pattern, a cell of which is

shown in figure 47, was designed for these

studies. Each cell of the pattern contains

four 2 by 10 arrays of probe pads shorted to- gether. The four arrays have pads which are either 100, 80, 60, or 40 ]im on a side depending on the array. Each cell also contains

structures for electrically determining the

metal sheet resistance and metal overetch

and an array with hollow pads designed to test the centering of the probe pins in the

pad pattern. There is also an all-metal area which is intended to be used for visual inspection of the pattern of the probe marks.

The experiments were carried out on patterns

etched in aluminum deposited on an oxidized

silicon wafer.

Three probe cards, each from a different vendor, were used in the experiments. Each card was used to probe a portion of the pattern

which contained 20 rows by 12 columns of

cells. Within this sampling area, continuity

was checked between the 2400 pairs of adjacent pads in each of the four arrays. A sepa- rate patterned wafer was used for tests on each of the probe cards. In each case, the

first run was made on the 100-um square pads

with succeeding runs made on successively

sm.aller pads. In each case the probes were aligned under a microscope so that the vertical and horizontal displacements of the

probes from the centers of the pads in the

probe pad arrays in each of the four corners

of the sampling area were, on the average,

minimized. Probing proceeded from left to right and from top to bottom of the sampling

area

.

For each of the three probe cards, the 100-,

80-, and 60-pm probe pad arrays were all

probed without a single failure over all 2400

probe pad pairs. This indicated that pads as small as 60 ym on a side can be probed using

automatic probing equipment and that probe

cards satisfactory for probing pads of this

size are generally available from vendors.

In one instance it was possible to probe the

40-iJm square pad arrays within the sampling

area without a single failure. In the other

52
TEST PATTERNS

0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

0 0 1 0 0 3 6 3 3 3 4

b 0 0 0 0 0 0 0 0 0 0 0 1

3 5 2 2 3 4 4 3 5 5 5 0 0 0 0 0 0 0 0 1 1 0 1

3

I) 5 3 2 3 5 4 3 3 5 4 0 0 0 0 0 0 1 1 1 1 1 1

3 3 5 3 2 3 5 4 3 3 5 5 0 0 0 1 1 1 1 1 1 1 1 1

3 4 4 3 2 2 5 3 2 3 5 3 0 1 1 1 1 1 1 1 1 1 1 1

2 3 5 3 2 3 5 4 2 4 5 3 1 1 1 1 1 1 1 1 1 1 1 1

3 4 5 2 2 3 4 4 2 2 4 3 1 1 1 1 1 1 1 1 1 1 1 1

2 3 4 3 2 2 4 4 2 2 4 3 1 1 1 1 1 1 1 1 1 1 1 1

2 4 3 2 2 2 4 3 2 2 4 4 1 1 1 1 1 1 1 1 1 1 1 1

2 3 4 2 2 2 5 3 2 2 3 3 1 1 1 1 1 1 1 1 1 1 1 2

2 2 4 2 2 2 4 3 2 2 4 2 1 1 1 1 1 1 1 2 2 2 2 2

2 3 4 8 8 g 9 8 7

0 X 8 1 1 1 1 2 2 2 2 2 2 3 3

8 8 X 8 7 8 X

o 8 8 X 8 1 1 1 2 2 2 2 2 2 2 3 3

8 8 X 8

0o 8 X 8 8 8 X 9 1 1 1 2 2 2 2 2 2 2 2 3

8 9 X 8 o 7 X 8 8 8 X X 1 1 2 2 2 2 2 2 2 3 3 3

7 3 X 8 8 X 8 8 0 X 8 1 1 2 2 2 2 2 3 3 3 3 3

a Probe ca rd p . b. Probe ca rd B

Figure 48. Number of failures per cell as a ^function of relative position when probing

I^O-ym probe pads using different probe cards.

two cases the fault patterns appear to indicate the occasional occurrence of minor mechanical problems in the probing equipment

used in the experiment. Figure 48 shows

the number of probe pad pairs at which continuity was not detected as a function of

position within the sampling area in these

two experiments. The letter X indicates

j that all ten probe pad pairs were open withj:in that cell.

i

j In one case, shown in figure 48a, probing was

jsuccessful (with one exception) until the

'sixth site in the fifth row of the sampling

'area. From that point on probe pad pairs 3 and 7 (where the top pair in each 2 by 10 array is pair 1) failed in nearly every cell,

i,and pad pairs 2, 4, 6, and 8 failed occasionjally, to give a total of from 2 to 6 failures

'iwithin each cell. This behavior continued uni'til the fourth site in the sixteenth row. At

I that point other pad pairs began to fail also

'so that between 7 and 10 pairs failed in each

I cell. Such quantum jumps in the number of

jfailures per cell suggest that the prober

[Stepping motor lost or gained one or more

steps before stopping at the sites where the

quantum jumps occurred. Visual inspection of

the probe marks at and near these two sites

showed that there was an abrupt change in the

positions of the probe marks relative to the

probe pad array at these two sites.

In the second case, shown in figure 48b, no quantum jumps appear, but there is a gradual

increase in failure rate in moving from one corner of the sampling area to another. It has been speculated that this could be caused

by wafer slippage on the chuck or rotation of

the chuck as the test proceeded.

The test pattern can also be used to evaluate

the suitability of probe cards for probing

patterns with the 2 by 10 pad cell array.

M-easurements with malfunctioning probe cards

would yield a regular pattern of open pairs

which is easily distinguishable from the patterns observed above.

(R. L. Mattis and M. R. Doggett)

7.6. Computer Aided Design and Layout of

Test Pattern Photomasks

A software package, designated Computer Aided

Photomask Programs (CAPP) , was developed to provide a means of transforming test structure designs into working photomasks. The

program architecture is based on a modular

concept. The building-block levels are illustrated in figure 49. The basic element is

a rectangle which can be any size from a square 0.002 in. (51 ym) on a side to a square 0.060 in. (1.52 mm) on a side. Rectangles can be rotated between 0 and 89.9 deg.

A structure is composed of either a single

rectangle or a combination of rectangles.

Structures are combined to form modular cells

which can, in turn, be combined into complete

test patterns.

The flow chart in figure 50 indicates the

steps required to transform structure designs

to a working mask set. After design in ac- cordance with established design rules, the

test structures selected for inclusion in a pattern are digitized. In this phase, the

hand-drawn rectangles which constitute a structure are reduced to digital form and

stored in a computer file. Each structure re- quires one or more mask levels; each mask level of the structure is stored along with a label indicating both structure and mask level information. This step is carried out on an independent digitizing board.

The results of the digitization are processed

using CAPP to create a library index file

with entries for each structure and mask lev53
TEST PATTERNS

PATTERN

Figure kS . Modular structure concept for photomask design.

Figure 50. Flow chart indicating steps re- quired to take a structure design to a working mask set.

CAPP. The location of each structure within

the cell is determined by specifying the co- ordinates of the lower left corner of the

structure. It is possible, therefore, that more than one structure may occupy the same location or the same structure may occupy different locations in the cell with the restriction that each structure assignment uses one I of the 10 structure positions. Although the

cell is multi-leveled, it is not necessary

that the cell level and the structure level

be the same; since each structure level is a basic unit, it may be included in any cell

level. For example, a new structure could

be formed by using a structure emitter level

as a cell base level when the cell is generated. At completion of this phase, the cell

is the new basic unit. Each cell level contains the collection of structure-level data

for all structures contained in the cell.

There is no restriction on the shape of the

cell; however, modular design with a 2 by 10

array of contact pads (NBS Spec. Publ. 400-

25, pp. 41-43) is encouraged in order to maximize the pattern density and reduce parasitic effects due to design differences (c.f. sec .7.2.).

Next the cells are positioned into the pattern by specifying the coordinates of the

lower left corner of each cell. At this

stage additional computer-generated plots

may be obtained and checked to ensure that

the pattern layout is complete and correct.

Finally, a file which contains all rectangle

information for each level used in the pattern is written. This file is then reformatted and written on magnetic tape for use as the input source of the mask-making machine.

(R. C. Frisch and D. A. Maxwell)

7.7. Advanced Planar Silicon Test Structures

el combination. To do this, the input data

base, which is read in freefield format, is

written into a random access file with fixed

record length. At the same time, a second

file, the library file, keeps track of the

structure number, mask level, and record position in the random access file. The library

file can then be used to interrogate the ran- dom access file, record by record, and create

computer-generated plots of each structuremask level. These plots are used to verify

the design and check for digitization errors.

To reduce the data handling in the final pattern, cells, which consist of a group of

structures, are constructed with the use of

Design of the test pattern for studying the

limits and application of advanced test struc- tures containing integral electrometer amplifiers (NBS Spec. Publ. 400-36, pp. 46-48) was completed. The pattern consists of both advanced gated diode and capacitor test structures and conventional test structures.

The advanced test structures are arranged

modular ly with a 2 by 10 contact pad array.

The center-to-center pad spacing is 160 ym

and the standard pads are squares 4 mils (100

ym) on a side. The test structures are spatially interchangeable within the module and common bus lines are utilized to eliminate re-^ dundant pads. With the common bus approach.

54
TEST PATTERNS

SOURCE/DRAIN

O POLY SILICON M SUBSTRATE CONTACT

METAL

Figure 51. Advanced test structure, showing

common bus lines and gated diode with integral reset transistor and MOSFET electrometer.

Figure 52. Computer layout of test pattern for evaluation of advanced test structures.

55
TEST PATTERNS

each 20-pad module contains 14 to 16 test circuits and measures approximately 20 mils (0.5

mm) by 62 mils (1.57 mm).

To illustrate the modularity concept of the

advanced test structure array, a circuit featuring all the connections necessary to construct a 2 by N test module is shown in figure 51. Four pads located on the bottom of

the module supply all the signals necessary

to operate any of the capacitor or gated diode test structures through the four bus

lines marked V , V ,

(}> , and V . The innerB R K <j most of the four vertical lines (V ) is con- nected to the gates of all the diodes in the

module. Although this line is not connected

to capacitor structures, it runs the length

of the module to allow contact to be made to diode circuits located above the capacitor

structures. The second vertical line ((}>„) is

connected to the gates of all the reset transistors located in the module. The third

line (V ) is a supply bus for the drain of R

the reset transistor switch. The outermost

line (V ) is a supply bus for the drain of

D the electrometer. A fifth common pad is con- nected to a long vertical contact stripe (V^)

which runs the length of the module and assures a uniform ground contact for all circuits regardless of their location. The pad

(Vq) is a typical circuit output pad. The

circuit shown in the figure is that of the

standard gated diode structure. The module

layout incorporates built-in crossunders and

allows for all bus lines to run uninterrupted through the length of the module. With

the use of this design approach, the construction of any 2 by N test module requires only

one output pad per test structure, plus the

five common bus pads described above.

The complete pattern, designated NBS/(w) ATP,

is shown in figure 52. The overall die (chip)

size is 197 mils (5.00 mm) by 217 mils (5.51

ram) . There are six advanced test structure

modules, labeled A through F, located on the

bottom of the test pattern. Module A contains 15 diode circuits each featuring an onchip 1-kfi load resistor. The post-processing

value of the resistance of this resistor can be obtained directly by measuring the resistance between the circuit output and substrate pads. Module B consists of 15 diode

circuits each featuring an on-chip load transistor. The size of this active load device

is identical to that of the output electrometer. Complete access to the source, gate,

and drain of this device is available via the

56

output and substrate pads. Module C contains ' 14 diode circuits, each requiring an external

load resistor. One of the 14 circuits also

allows direct access to the diode diffusion

and electrometer gate. Module D consists of

14 capacitor circuits, each requiring an ex- ternal load resistor for circuit operation.

As with diode module C, one of the 14 circuits allows direct access to the capacitor

and electrometer gate. Module E is identical

to module C with the exception that the probe

pads are 3.2 mils (80 ym) square instead of

the standard 4-mil (100-pm) square pads found

on all other modules. Module F is a composite module which includes all the circuits

found in modules A through D. In addition,

three new versions of the diode circuit were

added which are not found in any other module.

These circuits are similar to the three diode

circuits in modules A, B, and C except that

the diode area was greatly increased to provide a means for studying the effect of diode

area on the circuit characteristics.

The conventional test structures are grouped

into five modules, labeled TPl through TP5 in

figure 52. Module TPl contains six

polysilicon-metal resolution patterns and pinhole testers. Module TP2 contains two contact strings, five step-continuity testers,

two bridge resistors, and two substrate contact windows. Module TPS contains three

cross-bridge resistors (NBS Spec. Publ. 400-

36, pp. 45-46) and two capacitors for conventional capacitance-voltage measurements. Modules TP4 and TP5 each contain three cross- bridge resistors; various sizes of lines and

contact arms are utilized in the nine cross- bridge structures in order to permit study of

the effects of size on results obtained with

this structure.

(D. McCarthy", B. Godwin", M. L. Lonky"'^, D. S. Herman", J. Victorey", and J. Acevedo")

^

7.8. Test Pattern Design and Analysis for

SOS/LSI

This task was undertaken to develop a comprehensive test pattern for characterizing and

monitoring a large scale integration (LSI),

silicon-on-sapphire (SOS) process and other

mask-compatible processes (NBS Spec. Publ.

400-25, pp. 44-45). During this final period

of experimental work, the software for testWork performed at Westinghouse Defense and

Electronic Systems Center under NBS Contract

No. 6-35766. NBS contact for additional information: M. G. Buehler.
TEST PATTERNS

Determine current level

for good voltage measurements

Test for spurious current

Test for residual voltage

Test for resistance between

opposite and adjacent arms

Test for linearity

Test for van der Pauw resistance between card

set of arms

Compute average sheet

res isCance

Compute asymmetry factor

Compute reciprocity factor

(I)

(ISPUR)

(VRES (I, J) - 4 tests

(R (I, J)) - 6 tests

(BILIN) , (Tl>fLIN)

(R (1, J)) - 4 tests

(RHOS)

(ASSFAC)

(RECFAC)

Figure 53- Complete test sequence for un- gated van der Pauw test structure.

ing the automatically testable structures in

J quadrants 1, 2, and 4 of the pattern was generated and used.

All the software necessary for creating and

using the test programs has been completed so that any collection of the structures contained in the pattern may be tested with only

very small programming effort. It is only

necessary to know the key parameters indicating the tests desired to obtain a working version of a test program. These Include the

pin connections and the desired limits for

the tests. The user does not have to be familiar with the details of the testing hardjware or the test programming syntax to obtain

a working program. An extensive report on

this work is being prepared.

The following relatively complete example of

[the testing cycle through preliminary data re- duction serves to illustrate the use of the

programming concepts. Since it is reasonably straightforward and concise, the procedure for testing the ungated van der Pauw

Table 10 — Initial Parameters and Contact Pad

Identification for Testing Ungated

van der Pauw (Cross) Sheet Resistors

A. Initial Parameters

Specified applied voltage (VAPP)

Minimum tolerable test current (IMIN)

Maximum tolerable test current (IMAX)

Maximum tolerable spurious current (ISPURM)

Maximum acceptable residual voltage

with no current

Maximum acceptable resistance between

arms

(VI(f>)

(RAMX)

B. Contact Pad Identification

Location Number Symbol

Upper left pad 1 PUL Lower left pad 2 PLL Upper right pad 3 PUR

Lower right pad 4 PLR Isolated pad 5 PXX

(cross) sheet resistor structures (see sec.

7.2.) in quadrant 1 is described.

The heart of the entire procedure is the test

block. Each structure has several types of test blocks which may be suitable for the par- ticular application of interest. The test block for the ungated van der Pauw structure

of quadrant 1 is mnemonically named UVDPl.

The block diagram of the complete test se- quence, which is appropriate for use when the

sheet resistance of the material is unknown,

is shown in figure 53. Other test blocks for

this structure, which are less complete, may

be used if less information is desired.

In performing the test, the parameters listed

in table 10 are specified together with the

appropriate pin numbers used in contacting

the structure. The actual tests performed by

the tester are shown in the block diagram of

figure 54. The current to be used for the

tests is first determined by applying the

specified voltage (VAPP) to each set of arms

and measuring the resulting current. If an average current between (IMIN) and (IMAX) can- not be supplied by the tester, then testing

is halted and the next structure is started.

Since the digital voltmeter employed has a resolution and accuracy of 100 pV, the

sensed voltage must be at least 100 mV in order to achieve 0.1-percent accuracy. This

requires an applied voltage of 10 V. 57
TEST PATTERNS

UVDP1

Apply VAPP to PUL

with PUR grounded

Measure I; I

yes

Print

"I TOO LOW

Quit

Apply VAPP to PUL, PUR, PLR, PLL and

ground PXX

Measure I (ISPUR)

Print "SPURIOUS CURRENT

Quit

Print "OPEN ARM"

Quit <-

Calculate asymmetry factor

1 [rZI + R34 R42 + RI3I (RAVE) L 2 2 J lASSFAC)

Calculate reciprocity factor

I R21 - R43I t I R42 - R13l (RAVEI (RECFAC)

Ground PUL, PUR

Measure voltage across PLL, PLR

(VRES)

Force (I) between PUL, PLL

Measure voltage between PUL, PLL; V

V/(l) = (R12)

Similarly measure

(R14), (R24),

(R31), (R43I, (R231

yes

Force (I) through PUL- PLL measure voltage

across PUR-PLR; VI

Force -(I) Measure V; V2

Force 0.5(1) Measure V, V3

Calculate VI - V2

2V1 2V3

VI

(BILIN)

(TWLIN)

Force (I) between PUL, PLL

Measure voltage between PUR, PLR; (V34)

Similarly measure (V13), (V21) and (V42)

Calculate average van der Pauw resistance IR34+ R21 + R13 + R421 14 = (RAVE)

Calculate average sheet

resistance (RAVE) in)

Figure 5^- Flow chart for complete UVDPl test sequence.

58
TEST PATTERNS

The structure is tested next for spurious current. Such currents are generally relatively

Ismail for SOS structures, but can be very sigInificant for bulk silicon structures. In this test VAPP is applied to the four pads of

the structure (PUL, PLL, PUR, and PLR) and a fifth, nominally isolated, pad (PXX) is

grounded. The current between the structure

and the nominally isolated pad is measured;

if this spurious current exceeds a specified

value (ISPURM), testing is halted.

Next, a test is done to ensure that the re- sistances of the contacts to the arms are sufficiently low that they can force the

voltage sensing system to ground in the time

allowed for the measurement. This is done by

grounding two of the arms and measuring the

voltage between the remaining two. If the

contact resistance is excessive, the digital

voltmeter is effectively open circuited and measures a large voltage. If the magnitude

of this voltage, VRES, exceeds the specified

maximum (VI(f)) , a bad contact is assumed, and

testing is halted.

I The two-point resistances between each set of

[arms are measured next. These data are valu-

! able for determining the structural integrity

I of the arms and also provide a direct interI nal check on the van der Pauw results if

I there is no appreciable contact resistance.

I If the resistance of any arm, RIJ, is greater

j than the specified maximum (RAMX) , testing is

[halted. This maximum resistance is provided

Ijmainly to ensure that the voltage sensing sysI tem is not loading the structure during the

I measurement

.

If the testing reaches this point, a test for

linearity and bilaterality is done. This

test consists of measuring the van der Pauw

voltage at the current level found for this

structure, at the negative of this value, and

!at half of this value. Two parameters, TWLIN

and BILIN, are calculated to reflect the linearity and bilaterality of the measurements,

irespectively

.

iFinally, the actual van der Pauw measurements

ijare performed. These consist of measuring

jjthe van der Pauw voltages between four differlent sets of arms. The sheet resistance, RHOS

,

land the asymmetry and reciprocity factors,

which represent the quality of the structure

land of the material, are calculated.

jlest blocks are combined to obtain a final

'modular test program. For the particular exlample described here, four different van der

Pauw sheet resistor structures were to be

59

tested in the same program. From a test program point of view, these structures differ

primarily by the pins used. The final test

program consisted of the UVDPl test block re- peated four times with only the pin numbers

and the structure identifiers changed. It is,

of course, necessary to have the appropriate

software to effect this integration.

Diagnostic messages are printed on the console during the test execution. These messages describe the faults, structure, and

the test block used for the test. If many

messages are obtained, something is seriously

wrong. Following test execution, the results

are presented in computer-generated wafer

maps, histograms, or tables, as appropriate.

Measurements were made on a variety of the

type I test structures. The results of measurements on the two gated van der Pauw sheet

resistors were particularly interesting be- cause they show examples of strong failure of

reciprocity. They also clearly show the limitations of the quasi-conventional methods of

characterizing SOS films.

These measurements are all performed under

constant current conditions. Several of the

voltages appearing across the arms are plotted as a function of gate voltage in figure

55. Results from both structures are plotted

on the same graph. These structures differ

from each other only in the type of contact

used; the undoped silicon under the gate is

the same in both cases. The undoped silicon

is of too high a resistance to be examined by

this technique unless the gate voltage is

such as to make the silicon conductive by the

induced surface charge. For gate voltages

near zero, no results are obtained; however,

good results are obtained in the range of

gate voltage where the surface is made rea- sonably conducting.

Three sets of curves are plotted for each device. The variation of the applied voltage

as a function of gate voltage and the variation of the potential of the intersection are plotted on the same scale (10 mV/div), while

the van der Pauw voltages for four different

combinations of arms are plotted on the same gate voltage scale but on a 0.1-mV/div scale.

The contacts used on these structures have a very small contact resistance. This is seen

in the fact that the voltage at the intersection is equal to one half the applied voltage

(within 1 mV) at each gate voltage. This means that the field across the oxide at the

intersection can be readily calculated from

the applied voltage and the gate voltage. It
Figure 55- Performance of gated van der Pauw structures at room temperature. (See text for discussion.)

also means that over distances of the order

of the 10-mil (0.25-rnm) size of the structure

that the voltage-current characteristic of

the epitaxial silicon in the resistor structure is nonlinear.

On the other hand, serious deviations from

the reciprocity demanded by linear systems

are found in the van der Pauw voltage, particularly for the structure with n"*" contacts.

Considering the highly defected structure of

the silicon, it seems to be reasonable to assume the epitaxial silicon exhibits some an- isotropy in its conductivity on a local scale

These results demonstrate that the diagnostic

power of these test structures is considerable, although accurate testing is somewhat

cumbersome and automatic testing of these

structures requires special equipment.

(V/. E. Ham+)

^Work performed at RCA Laboratories under

NBS Contract No. 5-35916. NBS contact for

additional information: M. G. Buehler.

60
8 . ASSEMBLY AND PACKAG I NG

8.1. Radioisotope Leak Test

Detailed analysis was completed of the results of the interlaboratory evaluation of

the radioisotope method for testing hermeticity of semiconductor devices being developed

by ASTM Committee F-1 on Electronics [43J

.

The purposes of this experiment were both to determine the precision (between-laboratory

agreement) inherent in the radioisotope method when applied to production devices with

commercially available test equipment and to generate a data base for further refinement

of the draft method. To reduce the number of

variables involved, one specific model of

equipment was used so that all installations

could follow the identical operational se- quence in using their equipment to carry out

the prescribed test procedure. The test procedure itself, however, can be performed with

other suitable commercially available test equipment, although actual controls may vary.

All equipment was serviced and calibrated

prior to carrying out the test procedure.

The test specimens were 100, 14-lead, ceramicglass, dual-in-line integrated circuit packages with a nominal internal free volume of

0.02 cm^ . The packages were selected to provide a range of leak sizes from less than 1 x 10"^ atm«cm^/s* to about 5 x 10"^ atm'cm^/s.

Although the packages were selected from norjmal production lots, they were not subjected

to fluid immersion at any time.

.Even though it was not clear at the onset

'that real package leaks were sufficiently

stable for such an exercise, the test specimens were circulated through 10 laboratories^

with a final repeated test made by the first

laboratory to close the loop. All specimens

survived

.

The radioactive gas used in the test procedure, krypton-85, is a radioisotope with a

ijhalf life of approximately 10 years. Over

'99 percent of the disintegrations involve

the emission of beta particles with a maximum energy of about 0.7 MeV, and about 0.5

(percent of the disintegrations leads to gamma

[emission with an energy of approximately 0.5

jiMeV. In use, the krypton-85 is diluted with

Initrogen in the ratio of about 1 to lO'* to obtain an increased quantity of test gas with

la viscosity equivalent to that of air. For

I the test, the packages are first placed in

Ian activation tank, which is then sealed and

evacuated. The krypton-nitrogen gas mixture

'is pumped into the tank to a prescribed actijlvation pressure. During a prescribed "soak61

ing" time, the gas passes through any package leaks and accumulates in the interior.

At the end of the soaking time, the gas mixture is pumped from the activation tank and

stored for reuse, the tank is vented, and

the packages are examined with a gamma radiation counter to measure the amount of krypton

85 that remains in the interior.

The equation traditionally used to relate the

gamma count rate to the leak rate is [44,45]:

R P

2

^ " SKT(P 2 _ p_2) ' (13)

e 1

where Q is the leak rate (atm-cmVs), R is

the count rate above ambient background

(counts/min) , S is the specific activity

(pCi/atm'cm^) of the gas mixture, K is the

overall counting efficiency of the detector

for the package type (counts/min-yCi) , T is

the soaking time, is 1 standard atmosphere pressure, P is the activation pressure (atm absolute) , and ?^ is the initial

internal package pressure (atm absolute)

.

This formula assumes that the gas flow

through the leak is laminar, that the gas accumulates within the package linearly in

time, and that little pressure change oc- curs in the interior. No provision is included for gas escape after pressurization

.

Since the emphasis of this experiment was on measurements agreement rather than on absolute value, all but the last of these assumptions were not of consequence. To minimize

the effect of gas escape from the package interior, count rate determinations were to be

completed within a specified short (dwell)

time

.

In practice, the hermetic screening process

is not a measurement procedure but is a deAlthough the SI system of units is now preferred, present engineering practice uses

the mixed units of atm-cm^/s for leak rate,

torr for vacuum, and psi for pressures near

or greater than 1 atmosphere . Conversion

factors are 1 atm-cm^/s = 0.10133 Pa-m^/s,

1 torr = 133.33 Pa, and 1 psi = 6892 Pa.

+'The companies which participated in the ex- periment were the Burroughs Corp . ; Fairchild

Semiconductor; Hughes Aircraft; IBM Corp.

(2 installations); IsoVac Engineering, Inc.; Lockheed Missiles and Space Co.; Motorola,

Inc. (2 installations); and TRW Systems.
ASSEMBLY AND PACKAGING

tection operation for indicated leak rates

greater than some set value. Normally the

radioisotope method is used as a fine leak

test procedure with the rejection limit set

at such a value as to allow the aetection of some of the larger leak sizes [45J. However,

the dynamic range for quantitative measurements with this procedure depends on the

range of the rate meter. This range is generally about two orders of magnitude. To meet the objectives of the interlaboratory

evaluation, two successive cycles were em- ployed in order to obtain accurate count

rates and to prevent overfill with the radioactive gas. The first cycle was a test for

the range of leak sizes greater than 1 x 10~^

atm'cm^/s, and the second cycle was a fine

leak test for the indicated range from 1 x 10"^ to about 1 X 10"^ atm*cm^/s. Packages

which unambiguously evidenced a leak greater

than 1 X 10~° atm«cm^/s in the first cycle were set aside and assigned values as measured. Those packages which evidenced borderline values of about 1 x 10~^ atm-cm^/s

were retested in the second cycle along with

the remaining packages which had passed the

first cycle.

The count rate for each device was measured

initially at each installation to establish

the background level for each device. For

the first cycle, the activation pressure was nominally 3 atm(abs) and the soaking time was

36 s with a final pump-down to 2 torr. In- dividual packages were measured in the counting station; all measurements were scheduled

for completion within 15 min after removal

from the activation tank. Actually, the time

taken for completion ranged from 15 to 40

min. Leak rate values were calculated from

eq (13).

The fine leak test was performed in the same manner as the first-cycle leak test. Pressurization was nominally 5 atm(abs) ; soaking

time was determined from eq (13) based upon

the particular value of indicated specific

activity at each test site so that the count

rate for a leak size of 1 x 10~^ atm-cm^/s

was about 1000 counts/min higher than the

background count rate. The calculated soaking time was generally 12 to 18 min. Final

venting pressure was 0.5 torr. Although not

generally considered necessary for fine leakers, a 30-min time limit for completion of

counting is good practice. Actual times

ranged from 17 to 80 min.

After testing at each site, the specimens

were deactivated of krypton-85 by storing

them in a vacuum chamber for about 48 h. At

each of the succeeding installations, background count rates before test were nominally

2000 counts/min or less for each device.

Of those borderline packages which evidenced

leak rates of about 1 x 10 ^ atm*cm^/s in the

first cycle, most exhibited a well-defined

test value in the second cycle which was

about one decade smaller than the value measured in the first cycle. In such cases, the

fine leak values were assigned to them. The

remaining borderline leakers exhibited large

count rates of the order of 10^ counts/min in

the second cycle, in which cases the value

measured in the first cycle was assigned.

With this distinction, the between-laboratory

agreement of whether leaks were greater than

1 X 10~^ atm'cm^/s or not was 100 percent.

This agreement would not have resulted had

the borderline specimens not been retested in

the fine leak cycle.

In the initial selection, 25 packages were identified as non-leakers (leak size less

than 1 X 10~® atm*cm^/s). Of the 275 measure

ments on these packages, 261 (95 percent)

confirmed the initial assignment. The 14 measurements which indicated leak sizes great

er than 1 x 10"^ atm'cm^/s were made on 10

packages; no more than two such measurements

(of a total of 11) occurred on any package.

The average leak size and the sample standard

deviation were determined for each test package from the 11 measured values of leak size; an indication of no leak (less than 1 x 10"^

atm*cm^/s) was assigned a value of zero. The

sample standard deviation is plotted in fig- ure 56 as a function of average leak size;

from this plot it is apparent that the imprecision increases systematically with leak

size for both leak ranges. The data also

show that no packages had leak sizes between

3 X 10"'' and 1 x 10~6 atm-cm^/s. It is not clear whether this result occurred because of

the selection process or whether it is reflec

tive of the nature of the leaks in this type

of package. The large dispersion for the

range of leak sizes greater than 1 x 10 ^ atm-cm^/s is certainly affected by the variation in dwell times that occurred at some of

the installations.

Regression calculations show that for both

the fine and larger leak ranges the variation

in between-laboratory reproducibility is well

described by equations of the form

S . X. log -1 = aQ + aj log , (14

X ^ X

o o 62
ASSEMBLY AND PACKAGING

10"^ 10''' 10"^ 10"^

AVERAGE LEAK SIZE, x, |atm cm3/sl Figure 56. Computed sample standard devia-

! tions, Sj, of measured leak sizes for all test le_aKS as a function of average leak

' size, Xj . (The solid lines are calculated

from eq ( 1 ^) .

)

I

I where s. is the samule standard deviation and

! — ^

)j x_. is the average leak size of the j th test

| package , Values of the regression coeffiI cients ag and a^ and the normalization leak

i size X are listed in table 11, together with

' the correlation coefficient of the fitted

' lines. The regression lines are plotted in

I figure 56; note that they are not continuous

' over the entire leak size range covered.

, More detailed examination of the data indicates that part of the overall systematic de-

: pendence of dispersion on leak size is the rei. suit of individual laboratory measurement biases. It is of interest to speculate on the

precision that could possibly be obtained

were laboratory biases eliminated. This possible precision was anticipated by taking the

apparent precision at each laboratory and

pooling the results for each test specimen.

Specifically, a linear regression calculation

of the form x . = a + bx , where x . is the

iJ j iJ leak size measured by laboratory i on test specimen j, was applied to each laboratory's

results. The residuals were calculated for

each test specimen from each laboratory's re- gression line as X . . - x.., where x.. is the

expected value as calculated from the regression line for each laboratory at the corre- sponding value of Xj . A standard deviation

s was then calculated from all the residuals

r for each specimen and, again, a regression

curve was fitted. Results are represented by curve a in figure 57 for the fine leak range

and by curve a in figure 58 for the larger

leak range. Actual precisions that were obtained in the test, when laboratory biases

are included, as computed from eq (14), are given as curves b in the figures. From these

results it would appear that the precision

could be improved by a factor of about 2 if

laboratory biases could be eliminated. Identification of the factors which cause this

bias remains for further effort.

The actual precisions determined in this ex- periment are dependent on the guidelines imposed and on the type of test specimen used.

Values would probably be affected by change

in package materials and internal free volumes. However, these results demonstrate

that an interlaboratory comparison of hermetic packages can be accomplished with this ra- dioisotope method and that one can achieve a measurement precision which is greatly improved over the one or more orders of magnitude differences that have heretofore been

the rule. Although the precision achieved

Table 11 — Regression Coefficients for Radioisotope Leak Test

Results

ao

Xq , atm-cm^/s

Ccrrel ation

Coefficient

Fine Leak Range -0.280 0.839 10-8 0.980

Gross Leak Range -o.ice 0.947 10-6 0.989

63

1
ASSEMBLY AND PACKAGING

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 X

10'

AVERAGE LEAK SIZE, xj latin- crnVs |

Figure 57. Computed sample standard deviations, Sr, of residuals for each test speci- men in the fine leak range as a function of

average leak size. (Computed points are represented by open circles; the solid line

is obtained from an equation of the form of

eq (H). For comparison, the regression

line for the measured sample standard deviations, S[, is shown as the dashed line.)

here is probably better than that to be ex- pected in the normal production environment,

the guidelines imposed can be transferred

readily and should lead to improvements there

also. (S. Ruthberg§

)

15 20 25 30 35

AVERAGE LEAK SIZE, x, |atm cmysl

45

Figure 58. Computed sample standard deviations, Sp. of residuals for each test specimen in the gross leak range as a function of

average leak size, xj . (Computed points are represented by open circles; the solid line

is obtained from an equation of the form of

eq (1^). For comparison, the regression

line for the measured sample standard deviations. Si, is shown as the dashed line.)

G. R. Neff, IsoVac Engineering, Inc., and

B. P. Martin, IBM Corp., were co-leaders of

the interlaboratory experiment

.

64
9. DEVICE INSPECTION AND TEST

9.1. Scanning Acoustic Microscopy

Additional studies of resolution and contrast

tj were conducted on the developmental scanning

acoustic microscopes [46] at both Stanford

j University and Hughes Research Laboratories.

In addition, modifications have been made to improve the instruments at both locations.

Stanford Projeot — Investigation of a selecI tion of silicon and sapphire surfaces con-

, tinued in order to improve the understanding

of acoustic microscopy as applied to silicon

l' devices. Stacking faults have been observed

' in epitaxial layers of silicon. Figure 59

shows a typical example, a stacking fault on

the surface of a 25-pm thick, (100) epitaxial

j silicon layer, as viewed both by interference

j contrast (Nomarski) optical microscopy and by

i| scanning acoustic microscopy. Although the

ii acoustic micrographs are somewhat more coarse

I due to the lower resolving power and the pres-

' ence of the scan lines, they seem to contain

! all of the important features contained in

! the optical image.

It has been found that small pinholes in a

. film, approximately one-quarter wavelength

ij thick sputtered on a smooth surface, show up

las very bright high contrast points in the im-

:!age. This is to be expected on the basis of

'j the work at Hughes on anti-reflection coat-

! ings on the sapphire lens surfaces (NBS Spec.

jjlPubl. 400-29, p. 83) [47]. An example of pinj holes in a 1.5-ym thick borosilicate glass

! layer sputtered on flat polished sapphire is

a. Optical (Nomarski) photomicrograph.

(Magnification: 120X.)

Figure 59- Stacking fault on unoxidlzed [lOO]

sapphi re substrate

.

shown in figure 60. In this image the beam

was focused at the surface of the glass layer;

in addition to the pinholes, two scratches

can be seen. Similar effects were also observed on silicon wafers covered with a 1-pm

thick silicon dioxide layer.

These studies also revealed a dependence of

the image on axial position of the focal

point of the acoustic beam. Previous work at Hughes demonstrated that there were variations in contrast and contrast reversals as a function of the axial position (NBS Spec.

Publ. 400-36, pp. 55-56). These differences were attributed to differences in levels at the surface of the test specimens and to the

properties of the transducer-lens combination.

The present work showed that the strength of

the returning signal can be strongly affected

by the acoustic energy reflected from the various interfaces beneath the surface which, in

turn, is strongly dependent on the angle of

convergence of the incident beam. As the test specimen is translated along the axis toward

the lens, the beam convergence at the liquidspecimen interface increases.

The focus of the beam can be moved from the

surface at the liquid interface into the interior of the specimen when the specimen is

accurately translated toward the lens. If

this focal point coincides with an interface

between two different materials, such as silicon dioxide and aluminum, the return signal

changes in strength. For this focal condi-

, b. Acoustic (800-MHz) micrograph. (Magnification: 200X.)

epitaxial silicon layer 25 yrn thick grown on

65
DEVICE INSPECTION AND TEST

Figure 60. Acoustic micrograph of pinhole de- fects in a 1.5-ym thick borosilicate glass

layer sputtered on a flat polished sapphire

substrate. The acoustic beam is focused on

the glass surface. (Magnification: ^OGX.)

tion, the image reveals structural information that is beneath the surface. Information of this kind is not accessible to other

forms of microscopy.

Consider first the case of a single layer. A

clean surface of sapphire, which has an impedance that is much larger than water, serves

as an excellent reflector. The variation of

the reflected signal as the surface is moved

along the beam axis is shown in figure 61. At the point marked 0 on the figure the re- flected signal is a maximum; at this point

the beam is focused on the surface. As the

specimen is moved from this point, either to- ward the lens or away from the lens, the return signal decreases in amplitude. This is

a result of the defocusing of the beam at the

reflecting surface. The width of the curve

gives an effective "depth of focus" of the

acoustic beam.

Figure 62 is a series of images of a silicon

wafer covered with silicon dioxide. The islands (which show up as bright regions in fig.

62a and dark regions in fig. 62b) are com- posed of three layers: a 28-nm thick film of

silicon dioxide covered by a 350-nm thick deposited layer of silicon nitride covered by a 200-nm thick deposited layer of silicon dioxide. The remainder of the slice is covered

with a 1.56-ym thick silicon dioxide film

thermally grown in a wet ambient at 1100°C

for 5 h. In this process the level of the

silicon is lowered and as a result the oxidesilicon interface for the regions outside the

islands is 450 nm below that of the islands.

Figure 61 . Computed strength of reflected

acoustic signal as a function of axial position of single surface of sapphire. (At 0,

acoustic beam is focused on the sapphire

surface; negative values indicate motion of

surface toward acoustic lens.)

The surfaces of the islands are about 530 nm below the surface of the oxide layer. The

image in figure 62a was made with the beam

focused at the surface of the islands and

the image in figure 62b was made with the

focal point of the beam 10 pm below this sur- face .

The reversal of contrast in these images may

be understood by referring to the measured

curves of reflected signal as a function of

axial distance shown in figure 62c. When the

acoustic beam is directed on the thermal oxide, the response is given by the curve marked A, and when the beam is directed on the islands, the response is given by the curve marked B, The origin is the position

of the specimen when the beam is focused on

the top of the islands; negative distances

represent translation of the specimen toward

the lens. At the origin, the signal from

the island areas is greater than that from

the areas covered with thermal oxide so the

island areas appear lighter as in figure 62a

When the specimen is moved 10 pm toward the

lens, the signal levels are reversed and the

islands appear darker as in figure 62b.

The reflected signal curves (fig. 62c) also

show another interesting feature. To the

left of the primary maximum, near the origin,

the response curve has a steep slope. The

signal changes by 1 dB for a translation of

150 nm. Since it should be possible to monitor changes in signal strength that are a fraction of a decibel, thickness changes of

66
DEVICE INSPECTION AND TEST

a. Beam focused on island surface, b. Beam focused 10 pm below island surface,

c. Reflected acoustic signal as a function of

axial position. Origin represents focus of

beam on island surface.

Curve B: On-island.

Curve A: Off-island

Figure 62. Contrast reversal of acoustic micrographs of oxidized silicon pattern resulting

from change in focal position.

few tens of nanometers should be detectable.

jPreliminary calculations indicate that sensi-

(jtivity to thickness changes are greatest

liwhen, as in this case, the nominal layer

thickness is about one-quarter of a wave-

,|i length

.

It is not yet possible to define with precision the type of configuration that gives

rise to the second peak, but it appears to be related to the value of the acoustic im-

_pedance of the layer as compared to the substrate. When the layer impedance is smaller

than the impedance of the substrate, the low

impedance layer acts to couple energy more efficiently into the substrate, and the second peak occurs when the focal point is

moved in depth until it coincides with the

interface. Aluminum layers on sapphire and

silicon are examples of this case. When the

impedance of the layer is higher than the impedance of the substrate, the presence of

the layer increases the reflected signal.

For that situation, a second peak is not always obtained. There is no evidence for a second peak with a gold layer on silicon,

but there is for a layer of silicon nitride

on silicon.

67
DEVICE INSPECTION AND TEST

(IFIELD OXIDE - 300 nm THICK,

I GATE OXIDE - 100 nm THICK)

400 nm OF LOW DENSfTY OXIDE OVER TOP SURFACE (Not Slravml

*****

MM^mm

a. Depth profile of a transistor section of

c i rcu i t

.

b. Acoustic micrograpin wi tin beam focused on sapphire surface. (Magnification: 225X.

c

.

e

.

Acoustic micrograph with beam focused 2 ym below sapphire surface. Arrows indicate typical silicon islands. (Magnification: 225X.)

Acoustic micrograph with beam focused 6 ym below sapphire surface. C: Aluminum

over 100 nm of oxide over silicon. D: Aluminum over silicon. (Magnification:

225X.)

Acoustic micrograph with beam focused h pm below sapphire surface. A: Aluminum

over 300 nm of oxide over silicon. B: Aluminum over silicon. (Magnification:

225X.)

B C

Acoustic micrograph of different portion

of circuit showing different crossover characteristics at crossover C-6. (Magnification: 325X .

)

Figure 63. Aluminum-gate, silicon-on-sapphire STDL-4 integrated circuit,

68
DEVICE INSPECTION AND TEST

These results are illustrated by the images

of a portion of an aluminum-gate, silicon-onsapphire STDL-4 circuit [48] shown in figure

63. The depth profile of a transistor section of this circuit is shown in figure 63a.

The image in figure 63b was made with the

beam focused on the sapphire surface. The

image there compares favorably with an optical photomicrograph. The aluminum layers

overlaying the silicon appear dark since this

combination effectively couples the energy into the sapphire substrate and reduces the reflected signal. Translation of the device

2 ym toward the lens results in an image (fig.

63c) that does not differ significantly from

the previous image except that the silicon islands (arrows) show greater contrast to the

sapphire. Translation an additional 2 ym to- ward the lens results in an image (fig. 63d)

with contrast reversals in the regions where

aluminum passes over silicon bars covered

with 300 nm of silicon dioxide (such as A)

and aluminum-silicon contacts (such as B)

.

In the image obtained after an additional 2 ym translation toward the lens (fig. 63e)

,

the gate electrodes of the transistors appear

very bright. The gate electrode (such as C)

is separated from the silicon by 100 nm of

oxide while the source and drain electrodes

(such as D) are applied directly to the silicon.

An image of a different part of this circuit

(fig. 63f) illustrates the ability of the

scanning acoustic microscope to observe features under the surface. The vertical line

of metallization A exhibits regions of high

contrast at the points where it crosses horizontal silicon bars such as those marked 1,

||2, and 3. An illustration of why this is poItentially important is seen where vertical

jline C crosses horizontal line 6. There is

jia dark square where the vertical and horizontal lines cross except for the crossing C-6.

lAt this crossing, the dark square is missing,

Ijprobably because of a defect in the structure

jtat this point. Other features on this circuit which stand out clearly in the acoustic

(micrograph are: 1) the apparent bubble or

ijball in the center of the horizontal line

p and 2) the apparent narrowing of vertical

jline B as it crosses horizontal line 1. Neijither of these features was readily apparent

|pn an optical photomicrograph of this portion

|0f the circuit.

Ibhe principal improvement in the instruraentailtion is the addition of a microprocessor-

'based signal processing capability. In one

'jinstance the signal is stored one line at a

jjtime and the second derivative computed with

69

the microprocessor. A portion of this second

derivative is added to the original signal to form the final image. This provides significant edge enhancement and emphasizes those

features that are dim if not unobservable in

the unprocessed image. (C. F. Quate")

Hughes Project — High resolution test targets

have been used to measure the spatial frequency response of the scanning acoustic microscope [49,50]. The measurements of the

acoustic contrast characteristics and gray

scale present greater difficulties. First,

image contrast may be obtained using either

phase or amplitude information and it is con- ceivable that a combination of the two (not

necessarily related) may be used to obtain

greater contrast and possibly greater enhancement of detail than can be realized by using

either type of information separately. Second, and fundamentally different from the

light image system, the available material

properties for producing layered structures

do not permit the fabrication of a continuum

of reflective properties through which contrast is achieved.

One way to realize a reflection gray-scale

bar chart in a water immersion system is to fabricate a series of identical bars with increasing thickness up to one-quarter wavelength of a material with a large reflection

coefficient, such as gold, on a highly absorbent substrate. In the absence of an easily

obtainable substrate with this desirable

property, the gold can be deposited on a substrate such as sapphire on which an acoustic

anti-reflection coating (AARC) [47] has been

deposited. This type of structure, illustrated in figure 64a, can suffice for a pulsed reflection experiment if all but the

desired image signal is gated out and if the

measurement is carried out at the characteristic frequency of the AARC. An alternative

version of an acoustic gray-scale bar chart

can be realized by depositing (sputtering)

layers of light crown glass on sapphire in

thickness increments from zero to one-quarter

wavelength to achieve equal increments of am- plitude or power reflection coefficient as illustrated in figure 64b. Both types of

gray scale may be useful in examining the

validity of the analytical predictions of

the two systems, using metal-dielectric or solely dielectric materials. A gray-scale

Work performed at Microwave Laboratory,

Stanford University, under NBS Contract No. 5-35899. NBS contact for additional information: R. I. Scace.
DEVICE INSPECTION AND TEST

a. Gold bars on sapphire substrate with antireflection coating.

b. Glass layers on sapphire substrate

Figure 6k. Cross sections of acoustic grayscale test patterns for measurement of contrast

characteristics of a scanning acoustic microscope .

pattern of the sputtered-glass type has been

fabricated and is being tested. It should

be noted that the dimensions given In figure

64b are appropriate for a frequency of about

375 MHz and scaling would be required if a different frequency were used.

A variety of electronic and mechanical improvements to the scanning acoustic microscope are being made. In particular, the hydraulic scan drive was converted to a direct

mechanical drive (vertical scan direction)

,

a simple adjustable specimen mount was built,

and sample-and-hold circuitry to provide improved display capabilities was assembled,

(R. G. Wilson+ and R. D. Weglein^)

9.2. Integrated Circuit Die Attachment Evaluation

The experimental investigation of the suitability of using the temperature response of

the substrate diode in an integrated circuit

to evaluate the integrity of the die attachment was completed. This method is similar

70

Figure 65. Laser scanner pho to response image of integrated circuit used in this study.

(Electrically active devices are light.)

to that previously developed for evaluating .: the integrity of the die attachment of tran- -j sistors and diodes [51] . The circuit is | pulsed for a time short compared with the

thermal time constant of the case but longer . than the thermal time constant of the device

die. The temperature rise of the die is de- , termlned by the use of an appropriate tem- perature sensitive parameter. Since defects ; in the bond between the die and the header ; result in higher die temperatures, the

amount of temperature increase above that , which would be expected for a good bond gives

an indication of the quality of the die attachment. In the present study, the voltage

drop across the substrate diode with a fixed

forward current was evaluated as the temperature sensitive parameter.

In the test devices, a voided region was introduced between the header and the die by j making a circular depression on the header - and bonding the die over the depression. The^ devices used for the investigation were integrated circuit transistor arrays. Each inte-a

grated circuit die consisted of seven common , collector transistors on a common substrate.

Four of the seven transistors on the die were connected after the die was bonded to the

header as illustrated by the laser scanner ; photoresponse image in figure 65. The cir- ;j cuit dice were squares 50 mils (1.27 mm) on ^ a side and were bonded with an eutectic al- [

Work performed at Hughes Research Laborato- ' ries under NBS Contract No. 5-35898. NBS ; contact for additional information: R. 1. ' Scace

.
DEVICE INSPECTION AND TEST

Figure 66. Circuit for evaluating integratedcircuit die-attachment integrity.

loy over voids with diameters ranging from

23,5 to 35 mils (0.60 to 0.89 mm), thus representing bond deficient areas ranging from

about 17 to 38 percent of the die area. In addition, 33 dice were successfully bonded

to headers that had no voids to serve as control devices.

The temperature rises in both the voided and

ijcontrol devices were measured to determine

jthe increase in die temperature as a function

)of void size. The schematic diagram of the

:|circuit used to measure the temperature rise

'is shown in figure 66. Prior to heating, the

ijvoltage of the substrate diode was measured

'with a forward current of about 0.5 mA. To heat the die, a 12.8-ms wide pulse was apiplied to the bases of the four transistors

^'which were connected in a common collector

configuration and operated as emitter followers. During the heating pulse, the substrate

diode was reverse biased to insure proper

transistor action. After the heating pulse

had been applied, the transistors in the integrated circuit were turned off and the

voltage of the substrate diode was again

measured with a forward current of 0.5 mA. A small delay was needed to allow transients

to die out of the system. The temperature

coefficient of the diode voltage was found to be -2.3 mV/°C by heating the entire device up

to known temperatures with no heating pulses

applied to the integrated circuit, and measuring the substrate diode voltage as a function

of temperature. During the heating pulse,

the circuit dissipated 8 W, but since the duty

cycle was 0.8 percent, the average heat dissipation was 64 mW; because of this low average dissipation, special heat sinking was not

required

.

Co 35 -

<

tro

z

UJ

cc3

<

CC

UJ

a.Z

UJ

g

o

10 15 20 25 VOID AREA (%)

30 40

Figure 67. Percent increase in junction tem- perature as a function of intentional void

area. (Solid dots: measured increase.

Dashed line: estimated best fit to the data.)

The results of this study are given in figure

67 where the percent increase in temperature

is plotted as a function of the nominal void

size. The spread in the data is fairly large.

This is due mostly to variations in the quality of the die attach in those areas where

there was no intentional void. The zero point on the graph was based on the average

temperature rise for the control group. It can be seen that with the transient technique

used in this study a void covering roughly 35 percent of the die area produced a 27.5 percent increase in the temperature rise over what it would be had there been no void. As had been noted in the previous study [51]

,

steady-state thermal resistance measurements

on these devices were much less sensitive;

only a 10-percent increase in steady-state

temperature rise was produced by a void cover- ing 35 percent of the die area.

Substantial variations in the quality of the

die attach were observed on some of these devices with the use of radiographic x-ray

techniques. Several dice which had very

large temperature rises (not included in fig.

67) appeared to be bonded to the header only

along one side of the die. Unfortunately,

the resolution of the radiographs was insuf71
DEVICE INSPECTION AND TEST

ficient to permit quantitative determination

of an actual void area.

These results suggest that, at least in some

cases, the temperature response of the substrate diode can provide information regarding the quality of integrated circuit die attachment. (D. W. Berning)

9.3. Transistor Collector-Emitter Saturation

Voltage^

Investigation of the measurement of transistor collector-emitter saturation voltage was undertaken to develop a standard method for

determining this parameter which is important

because it can be used to predict the performance of a transistor used in switching apparatus such as in dc to ac power inverters,

chopping circuits, or logic circuits. It represents the "zero" logic level in many types

of digital circuits. It is important to know

how low the collector-emitter voltage becomes

in saturation because during saturation the

transistor passes the most current for the

longest periods of time, and power dissipation at the collector can become excessive if

the voltage drop is too high. When a system

design is to be used in a radiation environment, it is important to know the change in

saturation voltage as a result of radiation

dose so that the design can accommodate the

changed value. The experimental study was completed and a standard procedure has been

proposed [ 52 ]

.

By definition [53], a transistor is operating

in the saturation region when both the

emitter-base and the collector-base junctions

are forward biased. The collector-emitter

voltage in this condition is usually represented in specification literature by the

symbol V , This voltage is the sum of

Ch ( bAi ) the junction saturation voltage and the re- sistive drops in the neutral regions of the

transistor. The junction satmration voltage

is the difference between the forward-biased

junction voltages of the collector-base and

the emitter-base junctions. This voltage is

normally small and rather insensitive to variations in collector or base current because

of its logarithmic dependence on these currents. There are three elements to the re- sistive drops but the most significant, particularly for power transistors, is usually

that across the collector region. The collector region of many power transistors is

lightly doped to obtain high breakdown voltages and the transistor is usually operated

at high currents in saturation.

Pulsed method.

Figure 68. Test circuit for measuring

^CE(SAT)

•

In the application of a transistor in a circuit, it is not always necessary to know the

actual value of V , , . Rather, it is fre

quently only important to know that for the

bias conditions of interest, the collectoremitter voltage is less than a specified val

ue. To determine this, it is necessary only

to control the test conditions so that the

measured value of V , , is low enough to Ct V oAL

)

meet the specification. To measure a change

in V . however, it is necessary to makf

Lb (, oAi ) TM

the measurement twice with all of the important independent variables specified and

carefully controlled. The purpose of the

present work was to provide a method with

test conditions defined thoroughly enough

that accurate measurements of changes in

VCE(SAT)can be made.

Circuits for making the measurement under dci or pulsed conditions are given in figure 6£ The dc method is applicable for currents low{'

enough to produce negligible heating in the

junction, while the pulsed method is applica^

ble for currents which would cause significant heating in the junction if not pulsed,

In carrying out the measurement, the base anc^ collector currents of the transistor under

test are each in turn adjusted to be at a

Principally funded by the Defense Nuclear

Agency

.

72
DEVICE INSPECTION AND TEST

specified value. Then the collector-emitter

voltage is measured.

Experiments were performed to determine the

sensitivity of the measured ^^^(g^rj,^ to the

jambient and junction temperatures, the base

land collector currents and their ratio, and

iseries resistance and other properties of

[the measuring circuit. Three transistor

Itypes were chosen for these experiments: a [small-signal transistor (2N2222) , a single-

[diffused power transistor (2N3055) , and a [triple-diffused power transistor (2N5840)

.

These device types were selected to provide a

iwide variation in measured results for variaijtions in the measurement conditions. Three

ildevices of each type were used for each exIperiment. All were off-the-shelf commercial

'[transistors. Though the number of devices

l|used is small, the data indicate the trends

llin the effects of the variables and, as such,

indicate which parameters are necessary to ontrol carefully and which are not. r

To simulate a rise in the ambient temperature,

ithe transistors were placed in a heated oil

(bath whose temperature was controlled at aptproximately 10°C intervals from room tempera-

£ure to 100°C during measurement of V , Lt, \ oAi ) ufficient time was allowed for the transis-

|tor to reach a steady-state temperature be-

'itween measurements. The results showed that

'for ordinary measurements on most devices norjpial laboratory temperature conditions are adeiquate as deviations of less than 10°C result

!in changes in V of less than 5 percent.

CE(SAT)

'However, when very precise measurements are

itrequired on certain transistors, especially

transistors with a very low doping level in

the collector region, greater control of the

lent temperature is required.

ased on the manufacturer's specified value

of thermal resistance for these devices, the

prise in junction temperature would not exceed

:i2 or 3°C at the low power levels of the satu-

.ration region if the transistor is properly

^mounted with a heat sink. Such a temperature

rise would have a very small effect on the

I'measured value of V, However, should

CE(SAT)

the measurement be made in a condition where

the transistor was not properly mounted on a heat sink, careful attention must be paid to the power dissipation during the measurement.

Measurements of the collector-emitter voltage

were made for the three transistor types as

.a function of the base and collector currents.

Variations in these currents affect the measurement in two ways. First, when the resistive drop across the collector region is the

most significant contributor to ^Qg^g^'p)' ^ change in collector current produces a change

in the measured value of V , . which is

approximately proportional to the change in

collector current. Second, for some values

of base and collector current, the transistor may operate in the active region instead

of the saturation region. This will increase

the measured value of V by several hundred

Cil percent for changes in of only tens of

percent. Obviously, this value of collectoremitter voltage is not V , , . Assuring

L Jl (. bAi ) that the transistor is indeed operating in

the saturation region during the measurement

of V , , is of utmost importance. In

CE ( oAl ) practice both the base current, I , and the

B collector current, I , to be used in performC

^ ing the measurement are specified. The dc common emitter current gain, 6, is the ratio

of I to I . When the transistor is operating in the saturation region, this ratio is

referred to as the forced gain, since both

I and I are set in order to force the tranC B sistor to operate in saturation. It is con- venient to consider a transistor to be in

saturation if the forced 3 is much lower than

the normal 3 of the transistor operated in

the active region. When the forced 3 approaches the normal 3 of the transistor,

small variations in I^ give increasingly

larger variations in V . Under these condiCil tions, precise measurement of V is diffiCh

cult and it is doubtful whether the results

represent V^^^^^^^.

There are other practical considerations for

measuring V , . particularly in power tran

Lb (. oAi ) sistors where the collector currents used can

be many amperes so that even a small series

resistance in the voltage measuring circuit

can be a serious source of error. The use of

Kelvin connections, in which the voltage measuring circuit is separated from the current

driving circuit, can eliminate most of these

series resistance problems. The voltage measurement is made with a very high impedance

instrument so that there is a negligible

amount of current in that portion of the circuit. There is a separate set of leads to which the current driving circuit is attached

If there is an extraneous resistance in the

voltage measuring circuit, the voltage drop

73
DEVICE INSPECTION AND TEST r

BASE CURRENT PULSE TURNED ON

- BASE CURRENT PULSE TURNED OFF

'CE ISMl ZERO VOLTS

TIME

Figure 69. Voltage waveform as observed with

an oscilloscope in the pulsed method.

across it is negligible. If there is an ex- traneous resistance in the current driving

circuit, the voltage drop across it is not

involved in the voltage measurement and therefore will not lead to an error in measuring

V_,„, Kelvin connections need be made

CE (, SAi ) only to the emitter and collector leads; no voltage measurement is made in the base circuit so that Kelvin connections are not re- quired .

The effect of the series resistances in the

circuit between the transistor and the point

where the voltage sensing connection is made

is not correctable by the use of Kelvin con- nections. The current provided by the current driving circuit causes a voltage drop across such a resistance which is measured by

the voltage measuring circuit as part of

V„„,„,^,. An example of such a resistance is

CE(SAT) ^ the resistance of the leads of the transistor.

For this reason, it is important that contact

be made reproducibly to the transistor within

3 mm of the transistor case. Another source

of such a resistance is poor or dirty contacts. This source can be avoided by using a transistor holder which has good quality contacts which are kept clean.

Another source of error in the measurement of

V - . is variation in the measurement cirCti (. bAi j cuit itself. The current levels used, particularly for power transistors, require considerable power dissipation in the resistors in

the collector circuit. Care should be taken

to select resistors that are capable of handling this power without changing resistance.

If the currents I and I„ are determined by

B C

'

PULSED

VOLTteC

SOUBCE ±

Figure 70. Measuring circuit with clamping

c i rcu it at tached .

measuring the voltage drops across resistors

in the base and collector circuits, it is

particularly important to assure that these

resistors do not heat up because of excessivi

power dissipation. A temperature drift in \ these resistors could cause serious error in

the measurement of V^^^g^^^ because the measurement would be made at the wrong values o: I^ and I^.

When the pulsed method is used, the voltage

measurement of V^„,„.^s must be made with an

Lb ( bAi ) instrument capable of measuring the voltage

of one portion of a pulse with respect to thi ground potential. A typical pulse is shown

in figure 69. This measurement can be made

with an oscilloscope if it is capable of ac- curately following the pulse from the high

level which occurs when the current pulse is

off to the low level of V , , when the

CL (, bAl ) current pulse is on. If, however, the amplifiers in the oscilloscope saturate, an errons ous measurement of ^^g^g^'p) will be made.

One way to avoid this is by application of a clamping circuit, such as that shown in fig- ure 70, at the input of the oscilloscope.

The effect of this circuit is to lower the

voltage during the time the current pulse is

off so that it is similar in magnitude to the value of ^{^g^g^rp) when the current pulse is on. An oscilloscope can easily follow the new pulse shape without saturating.

(K. 0. Leedy!

74
10. REFERENCES

Blackburn, D. L., Schafft, H. A., and

Swartzendruber , L. J., Nondestructive

Photovoltaic Technique for the Measurement of Resistivity Gradients in Circular Semiconductor Wafers, J. Electvoahem. Soa. 119 , 1773-1778 (1972).

Swartzendruber, L. J., Four -Point Probe

Measurement of Non-Uniformities in Semiconductor Sheet Resistivity, Solid-State

Electronics 2^, 413-422 (1964).

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

Spitzen-Gleichstron-Messverfahrens an Silicium-Proben, Tiel I u. II, Archiv.

techn. Messen, No. 369, 237-240 (October

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

1966)

.

Blackburn, D. L., Koyama, R. Y. , Gettinger, F. F., and Rogers, G. J., Measurement Techniques for High Power Semiconductor Materials and Devices: Annual

Report for the Period January 1, 1976 - December 31, 1976, ERDA Report CONS/ 3800-

2, pp. 4-21 (May 1977)

.

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)

.

Sah, C. T., Forbes, L., Rosier, L. L.,

Tasch, A. F., Jr., and Tole, A. B.,

Thermal Emission Rates of Carriers at Gold Centers in Silicon, Appl. Phys

.

Letters 15, 145-148 (1969).

See reference 4, pp. 26-42.

Buehler, M. G., Semiconductor Measurement Technology: Microelectronic Test

Pattern NBS-3 for Evaluating the Resistivity-Dopant Density Relationship of

Silicon, NBS Special Publication 400-22

(June 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, 111-19>Q (1978).

Buehler, M. G., Semiconductor Measurement Technology: Microelectronic Test

Patterns: An Overview, NBS Special Publication 400-6 (August 1974), pp. 4-6,

12-19.

11. Kumar, R., Ladas, C, and Hudson, G.,

Characterization of Plasma Etching for

Semiconductor Applications, Solid State

Technology 19 (10), 54-59 (October 1976)

12. Thomas, R. N., Braggins, T. T., Hobgood,

H. M., Takei, W. J., and Nathanson, H. C, Compensation of Extrinsic Si: In Detector Material by Neutron Transmutation

of Silicon, IRIS Specialty Group Meeting

on Infrared Detectors, U.S. Air Force

Academy, Colorado, March 22-23, 1977.

(Unpublished)

13. Baron, R. , Young, M. H., Baukus , J. P.,

and Marsh, 0. J., Properties of Si:In as an Extrinsic Detector Material, IRIS Spe

cialty Group Meeting on Infrared Detectors, U.S. Air Force Academy, Colorado,

March 22-23, 1977. (Unpublished)

14. Scott, M. W., Spectroscopic Investigation of Si: In, IRIS Specialty Group Meet

ing on Infrared Detectors , U.S. Air

Force Academy, Colorado, March 22-23,

1977. (Unpublished)

15. Dearnaley, G., and Northrop, D. C, Semi

conductor Counters for Nuclear Radiations, 2nd edition, pp. 150, 173 (E. & F. N. Spon Limited, London, 1966).

16. Kaiser, W., Electrical and Optical Properties of Heat-Treated Silicon, Phys. Rev. 105, 1751-1756 (1957).

17. Kaiser, W., Frisch, H. L., and Reiss, H. Mechanism of the Formation of Donor

States in Heat-Treated Silicon, Phys.

Rev. 112, 1546-1554 (1958).

18. Standard Test Method for Interstitial

Atomic Oxygen Content of Silicon by In- frared Absorption, ASTM Designation F 121, Annual Book of ASTM Standards, Part

43 (ASTM, Philadelphia, 1978).

19. Baker, J. A., Oxygen in Silicon, Paper

82, The Electrochemical Society, Chicago

May 13-18, 1973. (Unpublished)

20. Johannessen, J. S., Spicer, W. E., and

Strausser, Y. E., An Auger Analysis of

the Si02-Si Interface, J. Appl. Phys.

47, 3028-3037 (1976).

21. Helms, C. R., Garner, C. N., Miller, J.,

Lindau, I., Schwarz, S., and Spicer, W. E., Studies of Si/Si02 Interfaces by

Auger Sputter Profiling and Photoelec75
REFERENCES

tron Spectroscopy Using Synchrotron Radiation, Proa. 7th Intern. Vao. Congr. and

Srd Intern. Conf. Solid Surfaces, Vienna,

September 12-16, 1977, pp. 2241-2243.

22. Helms, C. R., Spicer, W. E., and Johnson,

N. M., New Studies of the Si-Si02 Interface Using Auger Sputter Profiling, Solid State Communiaations 25, bTi-61(:)

(1978) .

23. Blanc, J., Buiocchi, C. J., Abrams, M.

S., and Ham, W. E., The Si/Si02 Interface Examined by Cross-Sectional Transmission Electron Microscopy, Appl. Phys.

Letters 30_, 120-122 (1977).

24. DiStefano, T. H., Field Dependent Internal Photoemission Probe of the Electronic Structure of the Si-Si02 Interface,

J. Vac. Sci. Tech. 13, 856-859 (1976).

25. Raider, S. I., and Flitsch, R., Abstract:

Stoichiometry of Si02/Si Interfacial Regions. I. Ultrathin Oxide Films, J. Vao. Sci. Tech. 13, 58 (1976) .

26. Ham, W. E., The Measurement and Interpretation of the Electrical Properties

of SOS, Extended Abstracts for the Fall

Meeting of the Electrochemical Society,

Las Vegas, Nevada, October 17-22, 1976,

pp. 462-464.

27. Mayo, S., and Evans, W. H. , Thermodynamic Considerations in the Use of Polysilicon Oxidation Tubes for Clean Si02-Film

Preparation, J. Electrochem. Sac. 125 , 106-110 (1978).

28. Mayo, S., and Evans, W. H., Development

of Sodium Contamination in Semiconductor

Oxidation Atmospheres at 1000°C, J. Electrochem. Sac. 124 , 780-785 (1977)

.

29. Dietze, W., Hunt, L. P., and Sawyer, D.

H. , The Preparation ai\d Properties of

CVD-Silicon Tubes and Boats for Semiconductor Device Technology, J. Electrochem.

Soa. 121 , 1112-1115 (1974).

30. Chou, N. J., Osburn, C. M. , and van der

Meulen, Y. J., Auger Analysis of Chlorine in HCl- or Cl2-grown Si02 Films,

Appl. Phys. Letters 22, 380-381 (1973).

31. Bunghard, R. A., Gregory, B. L., Gwyn, C.

W. , and Derbinwich, G. F., Process Dependence of Radiation Effects in CMOS Integrated Circuits, Final Report, Sandia 75-

0136, May 1975.

32. Nyyssonen, D., Linewidth Measurement

with an Optical Microscope: The Effect

of Operating Conditions on the Image Pro

file, Appl. Optics 16, 2223-2230 (1977).'

33. Jerke, J. M., Hartman, A. W., Nyyssonen,

D., Swing, R. E., Young, R. D., and

Keery, W. J., Comparison of Linewidth

Measurements on an SEM/Interferoraeter System and an Optical Linewidth-Measur- | ing Microscope, Proc. Soc, Photo-Optical'

Instrum. Engrs. 100 , Developments in

Semiconductor Microlithography II, 37-45

(1977)

.

34. Swyt, D. A., and Rosberry, F. W., A Comparison of Some Optical Microscope Measurements of Photomask Linewidths, Solid

State Technology 20 (8), 70-75 (1977).

35. Nyyssonen, D., Optical Linewidth Measure

ments on Silicon and Iron-Oxide Photomasks, Proc. Soc. Photo-Optical Instrum.

Engrs. 100 , Developments in Semiconductor

Microlithography II, 127-134 (1977) .

j,

36. Kintner, E. C, Investigations Relating

to Optical Imaging in Partially Coherent

Light, Ph.D. dissertation, Univ. of Edinj

burgh, 1975.

j; 37. Kintner, E. C, and Sillitto, R. M. , j Edge Ringing in Partially Coherent Imag-j

ing. Optica Acta 2A, 591-605 (May 1977).

38. 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 Bu- reau of Standards, Proc. Microelectronics Seminar - Interface '76, Monterey,

California, October 3-5, 1976, pp. 51-

59 (Kodak Publication G-47)

.

39. 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).

40. Brown, D. M., Connery, R. J., and Gray,

P. v.. Doping Profiles by MOSFET Deep

Depletion C(V), J. Electrochem. Soc.

122 , 121-127 (1975).

41. David, J. M., and Buehler, M. G., A Numerical Analysis of Various Cross Sheet

Resistor Test Structures, Solid-State

Electronics 20, 535-543 (1977)

.

42. Buehler, M. G., and Sawyer, D. E., Microelectronic Test Patterns Aid Produc76
REFERENCES

tion of Custom ICs , Circuits Manufaaturing 17_ (2), 46-56 (February 1977).

3. Ruthberg, S., Neff, G. R., and Martin,

B. D., Radioisotope Hermetic Test Precision, Proa. 1977 Intemat. Mioroeleatronias Symposium, Baltimore, Maryland, October 24-26, 1977, pp. 131-137.

.&4. Cassen, B., and Burnham, D., A Method of

( Leak Testing Hermetically Sealed Components Utilizing Radioactive Gas, Int. J.

l Appl. Radiation & Isotopes 9_, 54-59

\ (1960).

f5. MIL-STD-883B, Test Methods and Procedures for Microcircuits , Method 1014.2,

Seal, Test Condition B; MIL-STD-750B

,

Test Methods for Semiconductor Devices,

Method 1071.1, Hermetic Seal, Test Condition B.

6. Quate, C. F., A Scanning Acoustic Microscope, Semioonduator Silicon/197 7 , H. R. Huff and E. Sirtl, Eds., pp. 422-430

(Electrochemical Society, Princeton,

N.J., 1977).

I7. Weglein, R. D., Acoustic Properties of

Sputtered Glass at Microwave Frequencies

Appl. Phys. Letters 29_, 277-279 (1976).

ij^8. Hagon, P. J., Milakim, K. , and Seymour,

R. H., Programmable Acoustic Signal Processing Devices, R&D Technical Report

ECOM-05011-1 (February 1974) . (AD 775-

840)

49. Wilson, R. G., Weglein, R. D., and Bonnel, D. M., Scanning Acoustic Microscope

for Integrated Circuit Diagnostics,

Semiconductor Silicon/1977 , H. R. Huff

and E. Sirtl, Eds., pp. 431-440 (Electrochemical Society, Princeton, N.J.,

1977) .

50. Weglein, R. D., Wilson, R. G., and Bonnell, D. M., Scanning Acoustic Microscopy — Application to Fault Detection,

15th. Annual Proceedings , Reliability

Physics 1977, Las Vegas, Nevada, April

12-14, 1977, pp. 37-43.

51. Oettinger, F. F., and Gladhill, R. L.,

Thermal Response Measurements for Semiconductor Device Die Attachment Evaluation, Technical Digest, 1973 Int. Electron Devices Meeting, Washington, D.C.,

December 3-5, 1973, pp. 47-50.

52. Leedy, K. 0., Measurement of Transistor

Collector-Emitter Saturation Voltage,

NBSIR 77-1231 (June 1977).

53. Low Frequency Power Transistors, JEDEC

Suggested Standard No. 10, January 1976.

(Available from Engineering Department,

Electronic Industries Association, 2001

Eye Street, N.W., Washington, D.C.

20006.

)

77
APPENDIX A

SEFIICONDUCTOR TECHNOLOGY PROGRAM STAFF

(on March 31, 1977)

Significant changes in assigned organizational units and telephone numbers have

occurred between the end of the reporting period and the issue date of this report. Call the Electron Devices Division, (301) 921-3786, for current information.

Electronic Technology Division

(301) 921-3357

J. C. French, Chief

Miss B. S. Hope*

J. F. Mayo-Wells

Dr. W. M. Bullis, Asst. Chief for

Semiconductor Technology

Mrs. M. L. Stream"

Mrs. E. J. Walters*+

Dr. A. H. Sher, Asst. Chief for Operations

R. L. Raybold

Mrs. T. J. Talbott

Mrs. E. Y. Trager*

Semiconductor Characterization Section

Dr. J. H. Albers

F. H. Brewer

Dr. W. M. Bullis, Chief

Mrs. C. A. Cannon

Mrs. E. C. Cohen"*" Dr. J. R. Ehrstein

(301) 921-3625

Dr. K. F. Galloway

Mrs. A. D. Glover'' Mrs. F. C. Hoewing*

W. J. Keery

Dr. R. D. Larrabee

Mrs. K. 0. Leedy

Dr. S. Mayo

Miss D. R. Ricks

R. I. Scace

H. A. Schafft

L. R. Williams

Semiconductor Processing Section

(301) 921-3541

Dr. M. G. Buehler, Chief J. Krawczyk L. A. Robinson

M. R. Doggett T. F. Leedy Dr . T. J. Russell

R. L. Frisch Y. M. Liu Dr D. E. Sawyer

Miss D. L. Hines* R. L. Mattis Dr G. C. Taylor

Dr. R. Y. Koyama D. A. Maxwell W. R. Thurber

Dr . W. E. Phillips

Electron Devices Section

(301) 921-3621

D. W. Berning

D. L. Blackburn

W. A. Cullins

Mrs. K. E. Dodson*

H. E. Dyson

G. G. Harman

H. K. Kessler

M. D. Nelligan"^

Dr. D. B. Novotny

F. F. Oettinger, Chief

G. J. Rogers

S. Rubin

S. Ruthberg

* Secretary + Part Time

78
APPENDIX B SEMICONDUCTOR TECH^JOLOGY PROGRAM

PROGRESS REPORTS

'A review of the early work leading to this Program is given in Bullis, W. M., Measurement

Methods for the Semiconductor Device Industry — A Review of NBS Activity, NBS Tech. Note

511 (December 1969)

.

Progress reports covering the period July 1, 1968, through June 30, 1973, were published

as NBS Technical Notes with the title. Methods of Measurement for Semiconductor Materials,

Process Control, and Devices:

Quarter Ending

tSeptember 30, 1968

December 31, 1968

March 31, 1969

June 30, 1969

September 30, 1969

December 31, 1969

March 31, 1970

June 30, 1970

September 30, 1970

December 31, 1970

March 31, 1971

June 30, 1971

September 30, 1971

December 31, 1971

March 31, 1972

June 30, 1972

September 30, 1972

December 31, 1972

March 31, 1973

June 30, 1973

Quarter Ending

Iseptember 30, 1973

December 31, 1973 \ IMarch 31, 1974 |

June 30, 1974

September 30, 1974

December 31, 1974

March 31, 1975 \ June 30, 1975 |

September 30, 1975

i

(December 31, 1975 ( jIMarch 31, 1976 \ [june 30, 1976 ) September 30, 1976

December 31, 1976 \ iMarch 31, 1977 | June 30, 1977 i September 30, 1977/

NBS Tech. Note Date Issued NTIS Accession No.

472 December 1968 AD 681330

/i 7"^ t / J

1? ,^ t-v >- 11 >- T T 1 O/^Q r eoruary j.yoy 00 JoUo

488 July 1969 AD 692232

495 September 1969 AD 695820

520 March 1970 AD 702833

527 May 1970 AD 710906

555 September 1970 AD 718534

560 November 1970 AD 719976

571 April 1971 AD 723671

592 August 1971 AD 728611

598 October 1971 AD 732553

702 November 1971 AD 734427

717 April 1972 AD 740674

727 June 1972 AD 744946

733 September 1972 AD 748640

743 December 1972 AD 753642

754 March 1973 AD 757244

773 May 1973 AD 762840

788 August 1973 AD 766918

806 November 1973 AD 771018

reports were issued in the NBS Special Publication 400- subonductor Measurement Technology:

NBS Spec Publ Date Issued NTIS Accession No.

400-1 March 1974 AD 775919

400-4 November 1974 COM 74-51222

400-8 February 1975 AD/A 005669

400-12 May 1975 AD/A 011121

400-17 November 1975 AD/A 017523

400-19 April 1976 PB 251844

400-25 October 1976 PB 258555

400-29 April 1977 AD/A 040011

400-36 July 1978 PB 283482

400-38 This report Not yet available

400-45 To be issued Not yet available

79
APPENDIX B

After October 1, 1977, progress reports were issued as NBSIRs with the title, Semiconductor Technology Program, Progress Briefs:

Quarter Ending

December 31, 1977

March 31, 1978

June 30, 1978

September 30, 1978

December 31, 1978

March 31, 1979

June 30, 1979

NBSIR

78-1444

78-1444-2

78- 1444-3

79-1591

79-1591-2

79-1591-3

79-1591-4

Date Issued

March 1978

July 1978

October 1978

January 1979

March 1979

June 1979

August 1979

NTIS Accession No.

PB 284809

PB 284478

PB 292681

PB 294846

PB 298063

PB 298064

Not yet available

80
APPENDIX C COMPUTATION OF DARK-FIELD LINE IMAGES

E. C. Kintner*

The calculation of the image intensity distribution in a partially coherent imaging system

is, in general, a difficult task. It can be simplified by describing the system in terms of

a dimensionless abstract model which takes advantage of the properties of the Fourier transform and by making certain particular assumptions about the form of the object being imaged.

The method outlined here has been described in some detail by Kintner [C-1] and discussed

elsewhere by Hopkins [C-2] and by Ichioka and Suzuki [C-3]

.

Let positions in the object plane of a microscope be described by the dimensionless cartesian coordinates

:

P _

X = — X (C-la)

A and

P _

y = ^ y , (C-lb)

where the geometrical coordinates are given by x and y, p is the numerical aperture of the

objective, and A is the mean wavelength of the (quasi-monochromatic) illumination. Points

in the image plane conjugate to (x,y) in the object plane are described by the same coordi-

!j nates (x,y). Similarly, points in the pupil plane can be described by coordinates

' ? = Jh^ (C-2a)

I and

n = ^/p , (c-2b)

o

j| where E, and n are the geometrical coordinates and again is the numerical aperture. Where

the Fraunhofer diffraction theory is valid, the coordinate system (Cjl) corresponds to the

Fourier-transform plane of the coordinate system (x,y)

.

']! The image spatial frequency spectrum J(?,ri) is defined to be the Fourier transform of the image intensity distribution I(x,y) through the equations:

-Z"(5,n) :,n, /=// I(x,y) exp |2TTi(xC + yn)} dx dy (C-3a)

and

J(C,n) exp|-2TTi(xC + yn)} d? dn . (C-3b)

Similarly, if the complex amplitude transmittance (or reflectance) of the object is given by

the function F(x,y), its spatial-frequency spectrum is given by the Fourier transform

i| NBS-NRC Postdoctoral Research Associate, NBS Optics and Micrometrology Section, Mechanics

' Division.

81
APPENDIX C

F(C,n)

=JJ

F(x,y) exp |2ui(x5 + yn)[ dx dy . (C-4)

—oo

The imaging properties of the objective lens system may be described by the amplitude pointspread function K(x,y), whose Fourier transform is given by

K{^,r\) =jj K(x,y) exp |2^i (x? + yn) [ dx dy . (C-5)

The function has a special physical significance, because in Fourier optics it may be

identified with the pupil function of the optical system. In the dimensionless coordinates \ used here, the radius p = (C^ + n^)^^^ = 1 corresponds to the edge of the optical pupil and :! simultaneously corresponds to the limiting spatial frequency detected by the objective lens j system under coherent illumination. ;

Finally, in the presence of stationary partially coherent illumination, the mutual intensity

of the illumination in the object plane may be described by the function J(x,y). Its Fourier

transform

^(C,n)

=JJ

J(x,y) exp|2Tri(xC + yn)[ dx dy (C-6)

—oo

is called the "effective source," and, where certain conditions are met, it corresponds to the intensity distribution in the aperture of the condensing-lens system. Thus, for example,

in a microscope system with matched numerical apertures between the objective and the con- denser, the effective source is a uniform distribution which cuts off at the limiting radius

P = 1.

With these definitions, it is possible to show that the image spatial-frequency spectrum produced by a specified optical system is given by the equation

J(?,n) =// y(?+5',n+n';C',n')^(C+C',n+n')^^^(C',n')d5'dn' , (c-7)

where

r(5i,ni;C2.n2) =// ^(C",n")Z(Si+5",ni+n")x*(C2+C",n2+n") dc" dn" . (c-8)

In eqs (C-7) and (C-8) the asterisk denotes complex conjugation. The function ^ (? i , 1 1 5 C2 > '^2^ called the transmission cross-coefficient [C-4], characterizes the complete optical system

including condenser and objective. When the illumination is incoherent, it can be shown that

the function T becomes the optical transfer function; similarly, for coherent illumination,

the function becomes the product of the two pupil functions K and K . Therefore, eq (C-7)

combines the attributes of the optical system with the attributes of the object in a manner

which is both intuitively meaningful and practically useful.

If the object function varies in one dimension only (e.g., a straight edge), eqs (C-7) and

(C-8) may be simplified into a one-dimensional form:

82
APPENDIX C /

(C-9)

where

7(^1, ?2) J(S",n")if(S+5",n")^^(C2+C",n") dC dn" (C-10)

Equation (C-10) still allows for a two-dimensional optical system.

Equation (C-9) is further simplified if the one-dimensional object is periodic (e.g., a grating). Then the object may be represented by a Fourier series, whose transform is a distribution of delta functions. If the periodic function is even, then the Fourier series consists

of cosine terms only. Thus, an even periodic function with period P = 1/C may be represented by the Fourier series ^

F(x) = a cos(2irn5 x) , n p (C-11)

n=-oo

with a = a and whose Fourier transform is simply

n -n

FiO = a 6(n5 -?)

n p (C-12)

n=-oo

The coefficients {a } are complex when the object function includes phase variations. (The

;! ^ infinite limits on the series simplify the presentation of the next step.)

Equation (C-12) may be inserted into eq (C-9) , which after simplification becomes

no = a a ^T(nC ;0) + > [a ^^,a* ,r((n+n')5 ;n'? ) n'=l

+ a ,a* ,T((n-n')? ;-n'? )] n-n -n p p

n+n ' n

'

(n?p-0

CO c^6(nCp-0 ,

(C-13)

with c = -c

n -n Therefore, the intensity in the image plane is given by

I(x) = c cos(2Tm£ x) n p n=-oo

(C-14)

Since the intensity distribution in the image is necessarily real, the coefficients c are

I n also real. When the series in eq (C-14) is evaluated, only two cosines must be calculated

83
APPENDIX C

directly; the remainder of the harmonics may be calculated by using a simple recurrence re- lation for the cosine function.

The use of a periodic object greatly simplifies the calculation of the intensity distribution in partially coherent imaging. In general, the calculation of the image intensity re- quires three separate integrations: (1) the integration over the source as given by eq

(C-8) to determine the performance of the optical system; (2) the integration over the object spectrum as given by eq (C-7) to determine the intensity spectrum; and (3) the integration over the intensity spectrum as given by eq (C-3b) (the inverse Fourier transform) to obtain the intensity distribution itself. In practical calculations, these integrations can

be intolerably tedious. The above analysis shows that with a one-dimensional periodic object, two of the three integrations, eqs (C-3b) and (C-7), reduce to discrete summations,

eqs (C-13) and (C-14) , thus eliminating the problems of convergence associated with numerical integration. Although infinite limits are specified for the summations in eqs (C-13)

and (C-14), the function T(Ci;52) vanishes for large values of the arguments, due to the finite extent of the pupil function and the source function. Therefore, the summations in eqs

(C-13) and (C-14) may be truncated at predetermined limits without loss of accuracy.

So great are the advantages of using a periodic object that it is worthwhile to express even

a non-periodic object in terms of a periodic model. This is readily accomplished if a large

period is chosen so that the object is effectively isolated and does not interfere with its

periodic neighbors. The periodic model may be compared directly with the non-periodic object to verify the accuracy of the approximation. It should be emphasized that this periodic approximation, .which is so easily checked, is, in fact, the only approximation in the imaging calculation; all the other steps of the calculation are exact.

In practice, the image plane is sampled by a scanning slit, which must be convolved with the

intensity distribution to obtain the distribution recorded by the scanning system. Since

convolution in the image plane corresponds to multiplication in the pupil plane, it is con- venient to multiply the intensity spectrum in eq (C-13) by the Fourier transform of the slit

Thus, the intensity spectrum actually observed is

I a) = s(o • no = • i(o , (c-15;

S TTWt

where w is the full width of the scanning slit in dimensionless coordinates.

Therefore, the complete calculation for the partially coherent imaging process is characterized by the equation

l(x)=c+2 / cs cos(2TTn5 x) , (C-16)

s o X ^ n n p

' n=l

where

^n = Vo^^^^S'^^ ^ S Vn'\''^((^^"')S'"'S^^Vn'\''^^^"-"')S'~"'S^ ^'"''^ n'=l

and

sin (rnrw^ ) s = —^ . (C-18) n nirw^ P

A periodic rectangular waveform as shown in figure C-1 , which characterizes a periodic line

object, is given by a Fourier series, eq (C-11) , with coefficients

84
APPENDIX C

Figure C-1. A periodic rectangular waveform.

nd

a = n

sin(2Tmw^)

(C-19a)

(C-19b)

hen w < 0.1, each line is effectively isolated and behaves as a single line object. The

|i t ituantities t and t, may, of course, be complex to model a line object with phase variation,

b llthough the procedures described above simplify the calculation of the image intensity disitibution in partially coherent imaging, the integration for the function T describing the

Iptical system, eq (C-8) or eq (C-10) , remains a formidable probl em. It will be noted that

.jhis integration represents a generalized convolution of the source function J with the objective pupil function K. Where the source and objective apertures are circular and the obiiective pupil includes aberrations, this convolution has no simple solution and implies an

bctremely tedious numerical integration. To avoid this, two alternative compromises are mailable

.

iirst, where the source aperture is small (less than the objective aperture), it is possible

p reduce the integration to one dimension rather than two, in which case numerical integraion becomes feasible. It is thus possible to test the effects of aberrations such as depcusing and spherical aberration. As the source size approaches the size of the objective

perture, this model becomes less reliable.

^cond, if all variations in the source aperture and objective pupil are suppressed (i.e.,

b aberrations) , then the two-dimensional integration corresponds to the generalized convoluiion of three uniform circles; that is, the problem reduces to finding the area of the mutu-

|1 intersection of the source aperture and the shifted objective pupils (see fig. C-2 and

pf. [C-4]). This is a fairly strenuous exercise in logic and geometry, but it yields an ef-

||icient algorithm for the optical performance function T.

he above method may be extended to systems with annular sources (see fig. C-3) . First,

he function Tq is computed for a system including a source with radius equal to the outer

^^dius of the annulus. Second, the function Tj is computed for a source with radius equal

jjo the inner radius of the annulus. Then the difference between Tq and yields the value

85
APPENDIX C

Figure C-2. The convolution for the transmission cross-coefficient.

AMMIUIS OUTER aiClE INNER CHtCLE

J (£,v) Jo (f-*?) J

I

m [To ] [Ti]

Figure C-3- An annular source obtained using two circular sources.

of T corresponding to the annular source. This method is entirely analogous to the method

used to calculate the diffraction pattern of an annular aperture [C-5].

References

:

C-1. Kintner, E. C, Investigations Relating to Optical Imaging in Partially Coherent Light",

Doctoral dissertation, University of Edinburgh, 1975.

C-2. Hopkins, H. H., On the Diffraction Theory of Optical Images, Proc. Roy. Soc. A217 , 408: 432 (1953).

C-3. Ichioka, Y., and Suzuki, T., Image of a Periodic Complex Object in an Optical System - under Partially Coherent Illumination, J. Opt. Soa. Am. 66_, 921-932 (1976).

C-4 . Born, W. , and Wolf, E., Principles of Optics^ fifth edition, pp. 530-532 (Pergamon

Press Inc., Elmsford, N.Y., 1975).

C-5. Ibid. , pp. 416-417.

86
NBS>1UA (REV. 8-761

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. Spec. Publ. 400-382. Gov't Accession No, 3. ft^cifjient's AcCASsiort >jo.

4. TITLE AND SUBTITLE

Semiconductor Measurement Technology: Progress Report,

October 1, 1976 to March 31, 1977

5. Publication Date November 1979

Performing Organization Code

7. AUTHOR! S) W. M. Bullis, Editor

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

19. Pro{ect/rasl</Wort< Unit No.

11. Contract/Grant No. See item 15,

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS fS(ree(, city, state. ziP)

NBS - Washington, DC 20234; SSPO - Washington, DC 20376;

DNA - Washington, DC 20305; ARPA - 1400 Wilson Boulevard,

Arlington, VA 22209; ERDA - Washington, DC 20545; Naval

Ocean Systems Center,

13. Type of Report & Period Covered Interim: Oct. 1, 197(

to March 31, 1977

15. SUPPLEMENTARY NOTES

ARPA Order 2397, Program Code 7D10; DNA lACRO 77-809; SSPO Order NOO16476WR70036 , Code

SP-23; ERDA Task Order A021-EES; Naval Ocean Systems Center Order N0095377M009005

.

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or leas tactual summary of moat aigniticant 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 devices. Both in-house and contract efforts are included. The emphasis is on silicon device technologies. Principal accomplishments during this reporting period included (l) refinement and extension of the analysis of the nondestructive, photovoltaic

method for measuring radial resistivity variation of silicon slices; (2) development of a donor-addition

technique for testing for the presence of X-levels in indium-doped silicon; (3) development of a precision, wet chemical etching procedure for use in conjunction with x-ray photoelectron spectroscopy for profiling silicon dioxide-silicon interfaces; (4) completion of thermodynamic calculations of equilibrium sodium density in oxidation atmospheres contained in polycrystalline silicon tubes; (5) initial ex- tensions of the line-width measurement procedure to partially transparent and reflecting specimens and

I to submicrometer dimensions; and (6) completion of the development of a test pattern for characterizing

a large-scale-integration, silicon-on-sapphire process. Also reported is other ongoing 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 and publications are included as appendices.

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) Auger electron spectroscopy; die attachment; electrical properties; electronics; hermeticity; irdium-

'1 doped silicon; infrared reflectance; integrated circuits; line-width measurements; oxidation; photolithography; photovoltaic method; power-device grade silicon; radioisotope leak test; resistivity variations; resistors, sheet; scanning acoustic microscope; semiconductor materials; semiconductor process

llicontrol; silicon; silicon dioxide; silicon-on-sapphire; sodium contamination; test patterns; test struc-

'tures; thermally stimulated current and capacitance; transistors; ultraviolet reflectance; VcE(SAT)'

jx-ray photoelectron spectroscopy.

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

21. NO. OF

PRINTED PAGES

95

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price

$3.50

18. AVAILABILITY QUnlimited

I I For Official Distribution. Do Not Release to NTIS

[x] Order From Sup. of Doc, U.S. Government Printing Office, Washington, DC

20402, SD Stock No. SN003-003-02 139-3

Order From National Technical Information Service (NTIS), Springfield, VA. 22161 USCOMM-DC

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