* ^ z NBS SPECIAL PUBLICATION 400-45

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

Semiconductor

Measurement Technology

M^rti^i'jf¥rtn>^iirtiiiittlb'ili'SiiTiiih^

JUN i % 1981

NEW BOOK SHELF

Progress Report

April 1, 1977 to September 30, 1977
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 and 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:

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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:

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

April 1, 1977 to September 30, 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. Philip M. Klutznick, Secretary

Luther H. Hodges, Jr., Deputy Secretary

Jordan J. Baruch, Assistant Secretary for Productivity, Technology and Innovation

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued August 1980
National Bureau of Standards Special Publication 400-45

Nat. Bur. Stand. (U.S.), Spec. Publ. 400-45, 43 pages (Aug, 1980) CODEN: XNBSAV

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TABLE OF CONTENTS

SEMICONDUCTOR MEASUREMENT TECHNOLOGY

Page

List of Figures iv

List of Tables v

Preface vi

Abstract and Key Words 1

1. Introduction 1

2. Highlights 2

3. Materials Characterization by Electrical Methods 4 3.1. Reevaluation of Irvin's Curves 4 3.2. Hole Mobility in p-Type Silicon 4 3.3. Capacitance-Voltage Methods 5 3.4. Thermally Stimulated Current and Capacitance Measurements 5

4. Materials Characterization by Physical Analysis Methods 10 4.1. Calibration Standards for Ion Microprobe Mass Analysis 10 4.2. Electron Spectroscopy Techniques 10 4.3. Use of the Scanning Electron Microscope for Total Dose Testing 12

5. Photolithography 13 5.1. Calibration Standards for Optical Linewidth Measurements on Photomasks 13 5.2. Spatial-Filtering Microscope 13 5.3. Primary Linewidth Calibration 14 5.4. Submicrometer Linewidth Measurement 17

6. Test Patterns 20

6.1. Random Fault Test Structures 20

7. Assembly and Packaging 26 7.1. Wire-Bond Pull Test 26

8. Device Inspection and Test 30 8.1. Scanning Acoustic Microscopy 30

9. References 33

Appendix 36

iii
LIST OF FIGURES

Page

1. Schematic presentation of the pulsed applied voltage and the resulting capacitance transient 6

2. Block diagram for the pulsed isothermal transient capacitance measurement .... 7

3. Oscilloscope traces of the gate pulse, the ungated capacitance transient, and the

gated capacitance transient 7

4. Oscilloscope traces of a single-gated capacitance transient and the gated capacitance transient averaged 2000 times 7

5. Oscilloscope trace of the calculated emission rate 7

6. Representative isothermal transient capacitance response of a reverse-biased

p'^n junction 8

7. Calculated luminance distributions for filtered and unfiltered images 14

8. Illustration of the procedure for locating the edge position for a given SEM

output 16

9. Effects of "bloom" on the SEM output of an edge 16

10. Model profile and calculated SEM output trace for a line of width w = 300 nm . . . 17

11. Computed and measured image profiles of nominally O.S-pm wide lines and spaces in

a chromium-on-glass photomask 18

12. Computed image profiles of multiple line object with 0.5-ym wide lines and 0.6-yra

wide spaces 18

13. Confidence limits for the defect density D for a confidence level of 95 percent . 24

14. Scanning electron micrograph of typical bonds on a substrate used in the bond

pull test interlaboratory comparison 27

15. Graphical presentation of the wire-bond pull test interlaboratory comparison data 27

16. Resolution pattern 30

17. The amplitude of the SAM output voltage versus lens-to-specimen spacing for sili- con surfaces as recorded by translating the specimen axially through the focal

plane 31

18. The amplitude of the SAM output voltage versus lens-to-spacing distances for

various crystal surfaces as recorded by translating the specimen axially through

the focal plane 32

iv
LIST OF TABLES

Page

1. Results of Study of Effect of Phosphorus on Measured Width of Silicon-SiliconDioxide Interfaces 11

2. Example of Results Obtained in Pilot Linewidth Measurement Study 14

3. Typical Set of Values of the Ratio, R, of SecondaryHilectron Current at Edge

Position to Average Secondary Electron Current as a Function of Beam Diameter,

d, and Edge Angle, 0 16

4. Comparisons of Defect Densities as Estimated by Six Methods 22

5. Multiple Comparisons of Means of Wire-Bond Pull Test Results 29

V
PREFACE

The Semiconductor Technology Program serves

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

discrete semiconductor devices and integrated

circuits through improvements in measurement

technology for use in specifying materials

and devices In national and international

commerce and for use by industry in controlling device fabrication processes. This re- search leads to carefully evaluated and welldocumented test procedures and associated

technology. Special emphasis is placed on the dissemination of the results of the re- search to the electronics community. Application of these results by industry will con- tribute to higher yields, lower cost, and

higher reliability of semiconductor devices.

Improved measurement technology also leads to greater economy in government procurement by

providing a common basis for the purchase

specifications of government agencies, and,

in addition, provides a basis for controlled

improvements in fabrication processes and in

essential device characteristics.

of technical efforts conducted within the

Program are issued as these activities are completed. Major reports of this type which

are published by NBS appear in the 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.

During this reporting period, the Program re- ceived direct financial support principally

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

the National Bureau of Standar ds (NBS)+ and,

in addition, 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-supported portion of

the Program, Advancement of Reliability, Processing, and Automation for Integrated Circuits with the National Bureau of Standards

(ARPA/IC/NBS) , addresses critical Defense Department problems in the yield, reliability,

and availability of digital monolithic integrated circuits. Other portions of the Program emphasize aspects of the work which re- late 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,

detailed reports such as state-of-the-art re- views, literature compilations, and summaries

vi

^Through ARPA Order 2397, Program 7D10 (NBS

Cost Center 4257555). Unless otherwise

noted, work was funded from this source.

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

^Through Inter-Agency Cost Reimbursement Order 77-809 (NBS Cost Center 4259522).

^Code SP-23, through project order

N0016476WR70036 administered by Naval Weapons Support Center, Crane, Indiana (NBS Cost

Center 4251533).

^^hrough ERDA Task Order A021-EES (NBS Cost

Center 4259561).

"through Order N0095377MP09005 (NBS Cost Center 4259563).
f SEMICONDUCTOR MEASUREMENT TECHNOLOGY

PROGRESS REPORT

April 1, 1977 to September 30, 1977

Abstraot: This progress report describes NBS activities directed toward the development of methods of measurement for semiconductor materials, process control, and devices.

Both in-house and contract efforts are included. The emphasis is on silicon device technologies. Principal accomplishments during this reporting period included (1) development of theoretical expressions for computing resistivity and liole mobility for boron-doped silicon;

(2) completion of a study of problems associated with use of a scanning electron microscope

for total dose testing of semiconductor devices; (3) completion of a pilot study to evaluate procedures for measuring 1- to 10-pm wide clear and opaque lines viewed with transmitted il- lumination; (4) completion of a preliminary study of test structures for estimating densities of process-induced random faults in device wafers; and (5) completion of an interlabo- ratory evaluation of the destructive wire-bond pull test. 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.

Key Words: Capacitance-voltage methods; deep level transient spectroscopy; electrical properties; electron irradiation; electronics; hole mobility; integrated circuits; ion knock-on effect; ion microprobe mass analysis; Irvin's curves; isothermal transient capaci- tance method; linewidth measurements; measurement technology; microelectronics; phosphorus

pileup; photolithography; pull test; random fault test structures; resistivity; scanning acoustic microscope; scanning electron microscope; semiconductor devices; semiconductor materials; semiconductor process control; silicon; silicon dioxide; spatial filtering microscope; sputter-Auger method; test structures; transistors; wire bonds. 1. INTRODUCTION

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

out during the thirty-sixth and thirtyseventh quarters of the Program. The report

summarizes research on a wide variety of measurement methods for semiconductor materials,

process control, and devices that are being

studied at the National Bureau of Standards

(NBS). The Program, which emphasizes

silicon-based device technologies, is a con- tinuing one, and the results and conclusions

reported here are subject to modification and

refinement

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

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

Subsequent sections deal with each specific

task area. The report of each task includes

a narrative description of progress made during this reporting period. References cited

are listed in the final section of the re- port. Affiliations of staff members identified with each task are those during the re- porting period; unless otherwise identified,

staff were affiliated with the Electronic

Technology Division. These affiliations are given for record purposes only; many changes

have occurred because of a reorganization of the NBS which took place on April 19, 1978.

Background material on the Program and individual tasks may be found in earlier progress

reports as listed in the Appendix. 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 publication 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.

This is the 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. These

Progress Briefs also contain listings of more detailed publications printed during the re- porting period covered or scheduled for publication in the near future.

Additional information concerning the projects described in this report, information on current projects, and copies of List of Publications LP72 or of current issues of Progress Briefs can be obtained on request to Electron Devices Division, National Bureau of Standards, Washington, D.C. 20234, telephone

(301) 921-3786.

i
2. HIGHLIGHTS

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

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

included:

(1) development of theoretical expressions for computing resistivity and hole mo- bility for boron-doped silicon;

(2) completion of a study of problems

associated with use of a scanning electron

microscope for total dose testing of semiconductor devices;

(3) completion of a pilot study to evaluate procedures for measuring 1- to lO-ym

wide clear and opaque lines viewed with

transmitted illumination;

(4) completion of a preliminary study of

test structures for estimating densities of

process-induced random faults in device

wafers; and

(5) completion of an interlaboratory

evaluation of the destructive wire-bond pull

test

Unless another organization is identified,

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

of Standards.

Materials Charaatevization by Eleatvieal

Methods — Work continued on the experimental

redetermination of the resistivity-dopant

density (p-N) relations for silicon (Irvin's

curves). Because the impurities in silicon

are not fully ionized at room temperature in

the intermediate dopant density range, the

dopant density (N) is not always equal to the

carrier density (n). It was found to be con- venient to display the results of this work

in the form of the product qpN (or its reciprocal) against N or p as well as in the form

of mobility (the reciprocal of q pn) against n or p.

Theoretical expressions were derived for the

resistivity and hole mobility as functions of

dopant density and temperature in boron-doped

silicon. The model upon which the expressions are based is applicable for dopant densities from 1013 to 3 x 10^^ cm"^ and for

temperatures from 100 to 400 K. This project

was carried out at the University of Florida.

An analytical study of the validity of the

models used in analyzing differential capacitance impurity profiling measurements was un- dertaken with the identification of several

critical problems.

The isothermal transient capacitance (ITCAP)

technique was extended to combine the use of

a commercial 1-MHz capacitance bridge with

the repetitively pulsed charging technique

used in deep level transient spectroscopy.

The extension allows more rapid data analysis

and the measurement of shorter emission decay

rates than can be achieved with the conventional ITCAP method.

In addition, a detailed analysis of the isothermal initial and final capacitance method

demonstrated that this method, in conjunction

with the conventional ITCAP method, can be

used for distinguishing between the majority

and minority carrier emission rates of midgap defect centers.

Materials Chavaatevization by Physical Analysis Methods — As part of an ongoing task at Texas Instruments to develop methods for preparation of standards for calibrating an ion

microprobe mass analyzer (IMMA), an experiment was carried out to estimate the repeatability of the IMMA. Measurements were made over periods ranging from about 15 min to 1 week. The average relative sample standard

deviations for all measurement series were

less than 7.5 percent.

Several thermally oxidized phosphorus-doped

silicon wafers were examined to determine if the width of the oxide-silicon interface as measured by the sputter-Auger method is affected by the presence of the large concen- trations of phosphorus which pile up on the

silicon side of the interface during oxidation. The measured interface width was found

to increase significantly with increased

phosphorus density.

In a related project, a simple model, based

on an analogy to thermal diffusion theory, was developed to account for ion knock-on

broadening during sputter-Auger profiling.

These activities were carried out at Stanford

University.

A study of the problems associated with the use of a scanning electron microscope (SEM)

to simulate a gamma source such as cobalt-60

for total dose testing of semiconductor devices was completed. It was concluded that

the SEM can serve as a convenient tool for

this purpose if attention is given to several

critical factors.

Photolithography — The initial threelaboratory pilot study of procedures for mea2
HIGHLIGHTS

suring 1- to lO-ym wide clear and opaque

lines viewed with transmitted light was com- pleted. The results showed wide variations

between various participating groups and indicated that some of the systematic errors Introduced into the measurement cannot be

eliminated by a simple two-point calibration

technique. An artifact with a new pattern

has been designed so that appropriate calibration data can be determined.

The analysis of the spatial filtering microscope was extended to the case of a single

edge. It was shown that any band-pass filter

provides accurate edge location (in the form

of a sharp minimum at the edge position) provided there is sufficient energy left in the

image for observation.

An improved model was developed to describe

the interaction of an electron beam with a line edge. Although this model does not con- sider either "bloom" of the electron beam or fairing and rounding at the top and bottom of

the real edge, the model appears to provide a reasonably satisfactory basis for locating

the edge.

Satisfactory measurements of a chroraium-onglass pattern of 0.5-ym wide lines and spaces were made with the scanning photometric microscope operating in transmitted light. Operation in reflected light was hampered by

lack of adequate source intensity; a krypton

laser source is being incorporated into the

system to overcome this limitation.

Test Patterns — Preliminary evaluations were conducted of random fault test structures for

determining defect densities related to metal

step coverage, contacts to resistors, and

multi-emitter transistors. Various methods

for estimating defect densities were compared; it was found that good estimates of

defect density could be obtained from limited

data when the test structure has a number of

elements such that one (but not more than

one) defect occurs in a reasonable fraction

of the structures. Measurements on five test sites on a wafer were found to be inadequate

to provide a reasonable estimate.

Assembly and Packaging — Analysis of an interlaboratory evaluation of the destructive

wire bond pull test was completed. Problems

with calibration of a widely used pull tester were identified; these were subsequently cor- rected by the manufacturer. Satisfactory

agreement was obtained except for one group

of bonds which had been intentionally poorly

made and overly deformed. The test results

support the conclusion that the permitted

bond deformation should not exceed two wire

diameters for aluminum wire bonds.

Device Inspection and Test — It was observed

that when a polished surface of a single

crystal is examined with a converging acoustic beam, the reflected signal has a characteristic response that is dependent upon the

elastic properties of the reflecting surface.

This property can be used in the scanning

acoustic microscope to monitor the thickness

of films deposited on these surfaces.

Preliminary results of experiments to study

the resolution of the scanning acoustic microscope showed that a periodic pattern of

2-ym wide lines and spaces could be resolved

at an operating frequency of 750 MHz and that

both this pattern and a periodic pattern of

1-ym wide lines and spaces could be resolved

at an operating frequency of 1100 MHz.

The feasibility of a reflective acoustic ten- step gray scale for use with a scanning

acoustic microscope operated at 375 MHz was demonstrated although the dynamic range is quite limited. The gray scale employs a single contoured layer of light borate crown

glass on a sapphire substrate.

The gray-scale project was carried out at Hughes Research Laboratories, and the other

scanning acoustic microscope projects were carried out at Stanford University.

3
3. MATERIALS CHARACTERIZATION BY

ELECTRICAL METHODS

3.1. Reevaluation of Irvin's Curves

Carrier mobility is frequently calculated

from measurements of resistivity and dopant

density. However, such calculations are correct only when the carrier density, n, is

equal to the dopant density, N. At room tem- perature, the two are generally not equal in

the dopant density range from about 10'^'' cm~^

to about 10^^ cm~^ because of incomplete

ionization of the dopant atoms (NBS Spec

Publ. 400-19, pp. 13-16). For example, for

silicon doped with phosphorus to a density of

about 10^^ cm"^, it is estimated that only 85

to 90 percent of the phosphorus atoms are ionized [1,2]. For silicon doped with boron

to about the same density, it is estimated

that 70 to 80 percent of the boron atoms are ionized [3,4]. This difference occurs because, even though boron and phosphorus have

essentially the same ionization energy, the

effective masses of the majority carriers and

the degeneracy factors for the impurity levels are different in the two cases. Other

?7-type dopants, such as arsenic and antimony

have nearly the same ionizaton fraction as phosphorus, but for other p-type dopants,

such as gallium and indium, the percent ionization at a given dopant density is smaller

because of the larger ionization energies of

these dopants [5].

Thus, the quantity 1/qpN, where q is the

electron charge and p is the resistivity,

yields the mobility only when N = n. This

assumption is frequently made. The CaugheyThomas [6] mobility expressions, which are fits to the Irvin [7] curves, are based both

on data from Hall effect measurements, which

give carrier density within the uncertainty

of the Hall scattering factor, and on data

from chemical techniques which give the total

dopant density; no distinction between the

two was made. Likewise, Mousty et at. [8]

did not include deionization effects in their

work on phosphorus-doped silicon. Both Hall

effect and neutron activation analysis data were plotted as phosphorus density. Differences between the two were attributed solely

to the Hall scattering factor. Wagner [9]

obtained an expression for hole mobility

based on measurements on silicon implanted

with boron. The amount of boron retained in

the silicon was determined by integration of

data obtained from incremental sheet resistance and Hall effect measurements on layers

which were sectioned by anodic oxidation. A

mobility value as a function of boron density

was derived which made the integrated dose

equal to the implanted dose. This is equivalent to the assumption that all of the boron

was electrically active and ionized.

There is a need for curves and analytical ex- pressions to relate resistivity to both car- rier density and dopant density. It is con- venient to use the mobility ( y = 1/qpn) and

the quantity 1/qpN (or its reciprocal) to re- late resistivity to carrier density and dopant density, respectively, since these quantities exhibit details and structure not easily displayed on a p-N or p -n plot which

must generally cover many decades. Equilibrium cases involve the use of mobility as a function of carrier density. For example,

consider the determination of a dopant density profile of a diffused layer from incremental sheet resistance measurements [10].

The resistivity values are converted to car- rier density using knowledge of the mobility

as a function of carrier density. Then dopant density is calculated from carrier density using information on percent ionization.

Depletion conditions, under which the dopant

atoms are fully ionized, involve the use of curves of 1/qpN as a function of dopant density. Examples include the determination of the resistivity of the specimen from measurements of voltage breakdown or of capacitance

as a function of applied voltage.

(W. R. Thurber)

3.2. Hole Mobility in p-Type Silicon

Theoretical expressions for computing resistivity and conductivity of holes as functions

of dopant density and temperature were derived for boron-doped silicon. The model Is applicable for dopant densities from 1013 to

3 X 10^^ cm~^ and temperatures, T, between

100 and 400 K [3,4].

Using a 3-band [I.e., heavy-hole, light-hole,

and spln-orblt split-off (SOS) bands] model,

the hole mobility was calculated by properly

combining the contributions from scattering

by lattice phonons , Ionized impurities, and

neutral Impurities. In addition, the effect

of hole-hole (h-h) scattering and the nonparabollclty of the valence bands were taken

Into account In the mobility formulation.

To verify the theoretical calculations, re- sistivity measurements on nine boron-doped

silicon slices with dopant densities from 4.5

4
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

X 10^^ to 6 X 10-^° cm~^ were performed for

100 <^ T _< 400 K, using a planar square-array

test structure [11]. Agreement between the

calculated and measured resistivity values

was within 6 percent over the entire range of

dopant density and temperature studied here.

Excellent agreement (within +5 percent) between the calculated hole mobility values and

those of Wagner [9] was obtained for boron

17 — 3 densities less than 10 cm . Differences

found for boron densities greater than 10-^^ cm~^ are attributed to Wagner's neglect of

the effect of the deionization of boron atoms

in this dopant range (see sec 3.1.).

(S. S. Li*)

3.3. Capacitance-Voltage Methods

An analytical study of the theoretical validity of the differential capacitance impurity

profiling method was undertaken. In the initial phase, the following problems and questions related to successful application of

the method were identified:

1. The Schottky equations, which form

the foundation of the classical differential

capacitance method, are strictly only valid

for one-sided junctions [12,13,14]. What

kind of errors does this cause in practical

applications?

2. Are the claims that the Schottky

equations profile the equilibrium majority

carrier density near a junction rather than

the impurity distribution [15,16] correct?

How can the seemingly conflicting answers

[17,18] to these claims be resolved?

3. A method has been suggested for

theoretically testing the validity of the

Schottky equations [15,16]. This method does

not depend on the depletion layer approximation but does formally depend on the theoretical equations one uses in carrying through

the calculations. This raises the question

of whether one should use the electron-hole

transport equations with recombination-

*Work performed at the University of Florida

under NBS Contract No. 7-35741. NBS contact

for additional information: W. R. Thurber.

Work also supported in part by National

Science Foundation Grant No. ENG 76-81828.

+NBS Applied Mathematics Division.

^Principally funded by the Department of Energy, Division of Electric Energy Systems.

generation terms or the Poisson-Boltzmann

equation. Also, the results of applying the

testing method depend on the choice used in

defining capacitance and depletion layer

width. What are the proper definitions

[19,20,21] for these quantities in more exact

theories than the depletion layer approximation? (S. R. Kraft+)

3.4. Thermally Stimulated Current and Capacitance Measurements^

Transient Capacitanae Technique [22] — Several methods for the detection of deep level

defects in semiconductors by the measurement

of depletion capacitance transients have been

utilized by a number of investigators [23-

28]. In the traditional thermally stimulated

capacitance (TSCAP) measurement [23,24], defects charged at low temperatures are induced

to discharge by heating the device. In the

deep level transient spectroscopy (DLTS)

technique [25], the device is repeatedly

pulsed to charge the defect, and the discharge during a fixed time interval is monitored as the device temperature is changed

slowly. In a third technique, the isothermal

transient capacitance (ITCAP) technique (NBS

Spec. Publ. 400-36, pp. 20-22) [26], the discharge transient is observed directly after

the device has been switched from charging

bias to depletion bias at a fixed temperature. Whereas DLTS typically utilizes a 20-

MHz bridge [27] for measurement of capacitance, both TSCAP and ITCAP are typically

performed with a commercial 1-MHz capacitance

bridge. All three methods are capable of determining the density, energy level, and

emission rate of the defect.

The ITCAP method was extended to combine the use of the commercial 1-MHz capacitance

bridge with the repetitively pulsed charging

technique. A digital processing oscilloscope

(DPO) is used in conjunction with a minicomputer to capture the capacitance transient

and perform computations to characterize it.

This extension allows rapid data analysis and

the measurement of shorter emission decay

rates than can be achieved with the conventional ITCAP method. In addition, it permits

rapid measurements to facilitate wafer mapping of defect densities (NBS Spec. Publ.

400-38, pp. 10-12).

Figure 1 shows a typical series of bias

pulses and the resulting capacitance transients. A p^n junction structure is as- sumed for specificity and only majority carrier charging [25] is assumed for simplicity.

5
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

L«_l CD

S 0

1_LJ i toti t2 TIME

Figure 1. Schematic presentation of (a) the

puLsed applied voltage and (b) the resulting

capacitance transient.

v

. - The large capacitance pulse accompanying the

zero-bias voltage pulse is due to the momentary collapse of the depletion region which

allows charging of the traps. After re- establishing depletion, the charged traps

emit carriers at a rate governed by the temperature, and the initial capacitance, Cj^, decays exponentially to a final value of ca- pacitance, C^:

C(t,T) = - (C^ - cp exp(-et) (1)

where C(t,T) is the capacitance at time t and

temperature T and e is the emission rate. In the DLTS mode of operation, the difference in

capacitance at fixed times t^ and t2 after

depletion is reestablished is monitored as a function of temperature. This is typically

done with a dual-channel box car integrator.

In the usual ITCAP measurement , a single ca- pacitance transient is captured on a recording medium such as an x-y recorder. The re- sponse time of the recorder and the background noise level generally limit this mea- surement to transient times in excess of 100

ms. In the modified ITCAP technique, the device under test is maintained at a fixed tem- perature, but it is repetitively pulsed as in

the DLTS mode. The dc depletion bias is applied to the device via the "LO BIAS" input

of the commercial 1-MHz capacitance bridge,

as shown in the block diagram in figure 2. The charging pulses are applied in series

with the device via a pulse transformer on the "LO" side of the input. In order to

utilize the useful dynamic range of the

digital processing oscilloscope fully, it is necessary and desirable to suppress the large

zero-bias capacitance pulse which precedes

the transient capacitance signal. This is

simply accomplished by suitably gating the

analog output of the capacitance bridge.

Figure 3 shows three typical signals obtained

while studying a gold-doped p'^n device

which was held at a -15 V reverse bias and

pulsed to zero bias with a 1-ps pulse. The

signal applied to the analog gate (a) appears

at the point marked (l) in figure 2. The

gate width is adjusted to blank out the input

of the DPO during the large zero-bias capacitance pulse, ^ which dominates the ungated

analog signal (b) which appears at point (2)

in figure 2. The width of this capacitance

pulse is indicative of the response time of

the capacitance bridge, generally less than 1 ms. Note that the negative-going capacitance

transient is barely observable in this trace. The gated capacitance transient (c) appears

at point (3) in figure 2. The signal-tonoise ratio for each of the three traces in

figure 3 was enhanced by averaging the signal over 100 transients by use of the capabilities of the DPO and its associated minicomputer. Note that in adjusting the repetition

period of the applied pulses, it is necessary

to insure that equilibrium to has, in

fact, been established.

The signal-to-noise ratio can be improved by

averaging over more transients. Figure 4 shows the full gated capacitance transient,

both a single trace and the average over some 2000 transients. The latter trace can be

analyzed to obtain the emission rate, shown

in figure 5. The relative "flatness" of the

calculated emission rate is indicative of the

true exponential behavior of the curve. The

increase in noise as a function of time is

due to the statistical increase in the uncertainty of the signal as the transient gets

smaller with time. The absence of the differential discontinuity at the beginning of

the transient and the abrupt zero near the

end of the transient are due to software ma- nipulations to avoid overdriving the dynamic

range of the processor. A value for the

emission rate is obtained simply by taking an average over the relevant range. In this case, the emission rate is calculated to be

The adjustable gate delay is convenient for

setting of the gate position when a pulse

generator which has an adjustable delay time

between the sync and gate pulses is employed.
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

PUISE

TRANS.

PULSE

OUT

0:

BUS „, GtTE

DieiTtL PROCESSOII

PULSE

OUT

eiTE DEL«(

74121

SATE WIDTH

10 2 ms|

Figure 2. BLock diagram for the pulsed isothermal transient capacitance measurement.

GATE DELAY , , GATE WIDTH -J

0.5pF

5IF 17niV

|a| 1

T 5irV *

ijhj

I4f

|b|

too 200 300 TIME Imsl

400 500

Figure 4. Oscilloscope traces of (a) a single-gated capacitance transient and (b)

the gated capacitance transient averaged 2000

times

TIME Imsl Figure 3. Oscilloscope traces of (a) the

gate pulse, (b) the ungated capacitance

transient, and (c) the gated capacitance

transient. (Each trace is the average of 100

scans.)

100 200 300 400

TIME (ms)

500

Figure 5. Oscilloscope trace of the calculated emission rate.

3.3 s~l at a temperature of 238 K; this value

is in good agreement with the data of Sah et

at. [28] for electron emission from the gold

acceptor. (R. Y. Koyama)

Analysis of Mid-Gap Levels — Characterization

of some mid-gap defect centers requires distinguishing between majority and minority

carrier emission rates. Analysis of isothermal capacitance transient response measurements (NBS Spec. Publ. 400-36, pp. 20-22)

yields the sum of the majority and minority

carrier emission rates of the defect centers :

e = e + e n p (2)

where e_, is the hole emission rate and e. F is the electron emission rate. For those defect centers whose electron and hole emission

rates are nearly equal, additional measurements are required to completely characterize

the defect center.

7
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

The Isothermal initial and final capacitance

method [26] was analyzed to determine its applicability for this purpose [29]. Under

conditions for which the defect center energy

level is several kT from the Fermi level

(where k is the Boltzmann constant), the defect centers of a reverse-biased junction can

be fully charged initially by removing the reverse bias. Then the initial net spacecharge density on the n-side of a p'^n junction is [26]

- Nj. = (2V/qeA^)/(C^^ - C^^) (3a)

and on the p-side, where the defect centers

are not charged, it is

N, = (2V/q£A^)/(C.^ - C,^)

d lb (3b)

where N^j is the dopant (net acceptor or do- nor density), Nj. is the defect density, V

is the change in bias voltage from the applied bias value to the charging (zero) bias,

q is the electronic charge, e is the dielectric constant of silicon, A is the junction

area, C^^ is the initial junction capacitance (with defect center charged), and C^^ is the unbiased junction capacitance. The

capacitance decays in accordance with eq (1);

a representative capacitance transient is

shown in figure 6. After discharge, the defect centers return to their steady-state

charge density, and the net space-charge density on the n-side of a p n junction be- comes

^d

" fVp/^^n'^^p^^ " (2V/q£A^)/(C^^-C^^)(4a)

and on the p-side

N. + [N e /(e +e )] = (2V/qe:A^)/(c"^-c'^)(4b)

d t n n p t b where is the final (steady-state) junction capacitance. Subtraction of eq (4a)

from eq (3a) or eq (4b) from eq (3b) gives

the same result [26]:

N e

t n

2V

2 2 2 i^'f - ^i)

e+e 22 2 2 2'

n ^p qeA^ (C^ - Cp (C^ - Cp

(5)

This can be arranged to give the ratio of

hole to electron emission rates in the p'^n

junction:

% _ (^b - (^b

-

2V 4 2 2 C (C - C ) b f i

- 1 (6)

106 0 -

TIME |s|

Figure 6. Representative isothermaL transient capacitance response of a reverse- biased p'*'n junction. (Device 87AN9.1 at 226.66 K with bias of -5 V.)

Hence, e^^ and ep are separable by combining eq (6) and eq (2) (with e determined from

isothermal capacitance transient measurements) .

An independent, accurate measurement of Nj. is required to evaluate e^/ e^^- The ratio

N^/Nj. can be obtained from a dynathermal

thermally stimulated capacitance measurement

[30], and N^j can be determined from the re- sistivity of the lightly doped side of the

junction. It is the fractional uncertainty

of the Nj. determination which places a lower limit on the value of the e^/e^

ratio which can be reliably determined.

The emission rates can be expressed in terms

of the empirical equation of Arrhenius as

= B T

X exp(-AE^/kT) (7)

where x is either n or p, AEjj is the energy

difference between the defect center and the

conduction band and AEp is the energy difference between the defect center and the

valence band. The pref actor ^n^^ '^^ BpT^ is equal to the product of the electron or hole thermal velocity, the electron

or hole capture cross section, and the density of states in the conduction or valence

bands, respectively. When AE^ » kT, the

exponential term dominates the variation with

temperature. Present experimental precision

is inadequate to distinguish between various

small values of the temperature exponent in
MATERIALS CHARACTERIZATION BY ELECTRICAL METHODS

the prefactor; the frequently made choice of

a = 2 results from the assumption of a temperature-independent capture cross section.

With and known at several temperatures, Arrhenius plots of the natural logarithm of en/T"^ and e„/T2 against reciprocal temperature, T , can be made. Each

plot has a slope of -AE^^/k. and an intercept

of UnBjj. It is expected that the isothermal capacitance transient, initial and final

capacitance, and dynathermal capacitance mea- surements will be adequate to characterize

the majority and minority carrier emission

rates of a defect center. (W. E. Phillips)

9
M. MATERIALS CHARACTERIZATION BY

PHYSICAL ANALYSIS METHODS

4.1. Calibration Standards for Ion Microprobe Mass Analysis

As part of a task to develop methods for

preparation of standard specimens for empirically calibrating an ion microprobe mass ana- lyzer (IMMA) (NBS Spec. Publ. 400-19, pp. 20-22), an experiment to estimate the repeatability of the IMMA was carried out. A bulk

sample of phosphorus-doped silicon with a nominal room temperature resistivity of 0.004

0"cm (equivalent phosphorus density, 1.6 x 10^^ cm~^) served as the test specimen.

A 22 -nA, 19.4-keV TD ion beam was used to analyze the test specimen. The beam was ras- tered over an area nominally 125 tJm by 100

ym, and a 42-Mm by 33-pm aperture was ^gni"

ployed. Counts were made of the ^ P and Si

secondary ions. The beam was electrostatically switched each 100 ms until a total of

10 s of counting time had been accumulated

for each isotope, and a ^^P/^'^'si count ratio was computed. This took 22 s of elapsed time

(10 s for 3^P counting, 10 s for ^^'Si counting, and 2 s for switching). Generally, 25 to 50 count ratios were taken in each run to obtain a statistically meaningful sample.

To estimate the short-term ('^15-min) repeatability, 7 runs were made and each was ana- lyzed to obtain the mean value of the

31p^30g^ count ratio and the relative sample

standard deviation (RSSD) for the run. The

average RSSD was 7.4 percent with a sample

standard deviation of 1.1 percent.

To estimate the intermediate-term ('^2- to 4-h) repeatability, 3 sets of data, comprising 3 to 8 runs each, were analyzed. In this case, each datum was the average of 25 to 50

count ratios. The mean and RSSD of these

averages were computed for each set of data.

The average RSSD was 3.2 percent with a sam- ple standard deviation of 1.0 percent.

To estimate the longer term (7-day) repeatability, 10 runs were made during the period

July 11 to July 18, 1977. Runs were made

during the afternoon of the first day, the

morning and afternoon of the next four days,

and the afternoon of the last day. The count

ratios were analyzed to obtain the mean value

(1.113 X 10~3) and RSSD (6.1%). An analysis

of this same specimen about 18 months earlier

yielded a count ratio of 1.249 x 10~^, a value only about 12 percent higher than was obtained in the present series.

It should be noted that differences in count

ratios obtained in different runs may reflect

microinhomogeneities in the silicon slice be- cause a new 125-Mm by 125-iJm area was used

for each run. The doping uniformity on this

scale is not known for this particular slice,

but silicon slices with dopant densities

suitable for measurement with the IMMA cannot

be obtained with microresisti vity variations

smaller than 5 to 10 percent.

(R. Dobrott* and G. B. Larrabee*)

4.2. Electron Spectroscopy Techniques

Studies of Dopant Redistribution During Oxide

Growth: Phosphorus Pileup — When a thermal

oxide is grown on a silicon substrate doped

with phosphorus, the phosphorus segregates

and piles up on the silicon side of the interface. The presence of phosphorus at the

interface increases the oxide growth rate. In addition, the silicon is more heavily

doped near the interface than in the bulk,

thus affecting device parameters. Study of

the pileup phenomenon yields insights into

both the oxidation kinetics of silicon and

the nature of the silicon-silicon dioxide interface .

The phosphorus density near the interface was measured by means of the crater-edge profiling technique [32]. The test specimens were uniformly doped <111> silicon wafers on which

oxide films had been thermally grown. ^ The

results of this work [33] showed that the

peak density of phosphorus was much higher

than the value predicted from diffusionredistribution theory. The phosphorus peak was also found to be quite narrow, on the or- der of 4 nm. The phosphorus density in the

oxide was below the detectability limit.

Work performed at the Central Research Laboratories of Texas Instruments Incorporated

under NBS Contract No. 5-35917. This task was completed between the end of this re- porting period and the publication date of

this report. A complete report of the work

and the results obtained has been prepared

[31].

The test wafers were prepared by J. Plummer

and C. Ho of the Stanford Integrated Circuits Laboratory.

10
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

Table 1 - Results of Study of Effect of Phosphorus on Measured Width of Silicon-SiliconDioxide Interfaces.

Wafer ^^^^ Phosphorus 10- to 90-Percent Width, nm 50-Percent Width, Peak Phosphorus

Density, cm-3

Q^^g^^ ^Ll STTi^o-n-TW Density, cm-3

u 1.0 X 10

D 5.1 X 10

E 7.2 X 10

F 1.8 X 10

G 2.8 X 10

H 3.2 X 10

J 3.2 X 10

15 4.1 2.95 1.55 undetectable

19'^ 21 4.7 3.55 1.4 1.3 x 10^^

4.5 3.4 1.35 2.1 x 10^^

4.45 3.05 1.35 2.6 x 10^^

20 21 4.5 3.25 1.45 2.6 x 10^^

20 21 4.8 3.7 1.6 3.2 x

10"^^ 20 21 5.1 3.75 1.7 2.7 x 10^^

These wafers were recently reexamined to determine if the apparent width of the interface is affected by the presence of phosphorus. The experiments were performed using a 1-yA, 4.5-k.eV electron beam, rastered over a lOO-ym by 100-ym square area. Sputtering was performed with 1-k.eV argon ions. Oxide

thicknesses measured on an interferometer

were used to establish the sputtering rates. Profiles of the silicon LVV and oxygen KLL

signals were taken. The 10- to 90-percent

widths, uncorrected for escape depth, are presented in table 1. The widths between the

50-percent points of these profiles are also

tabulated. The measured interface width is seen to increase somewhat with increased

phosphorus doping.

Crater-edge profiles were also performed.

The approximate phosphorus peak density was determined for each profile by comparison

with the bulk silicon peak-to-peak heightThese densities are also given in table 1. Interestingly, the bulk silicon peak-to-peak

height was also observed to increase with increased doping. (S. Schwarz,^

C. R. Helms, ^ and W. E. Spicer§)

Limitations an Intevfaae Width. Resolution in

Augev-Sputtev Profiling: Icn Knock-On Effects — A simple model of ion knock-on broadening has been developed. This model is

based on an analogy to thermal diffusion theory [34]. Broadening is found to occur in rms fashion:

W = /w ^ - , (8)

o -y m

where Wq is the actual width of the

interface, is the measured width, and W

is the broadening parameter.

At low energies (E <^ 1 keV), it was found

that

where E is the energy transferred to the solid by the incident ion; is the energy

required to displace an atom from its lattice

position; S is the sputter yield (the number

of atoms sputtered per incident ion), a directly measurable quantity; and d is the

nearest-neighbor-distance of the atoms in the

solid. The energy dependence of W is rather

weak because S increases as E increases . The

sputtering yield dependence indicates that,

for two different bombarding ions of the same energy

(10)

At higher energies (1 keV < E < 10 keV), the

following semi-empirical formula was found to be appropriate for cases where the electronic

energy losses of both the incident ion and

the lattice (or dopant) atoms are small:

W = (U)

where Y = 4M-|^M2(M-|^ +1^2)"^, M-j^ is the mass of

the incident ion, M2 is the mass of the lattice (or dopant) atom, and R is the ion

range.

Work performed at Stanford Electronics Laboratories of Stanford University under NBS

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

11
MATERIALS CHARACTERIZATION BY PHYSICAL ANALYSIS METHODS

This model was compared [34] with reasonable

success against Monte Carlo calculations and

SIMS data reported in the literature and with

Auger sputter profile measurements made with

several primary ions on the interface between

silicon and a thermally grown silicon-dioxide

film. The most important attribute was the

variation of the broadening parameter, W,

with the sputtering yield, S. Profiles were observed to broaden in rms fashion.

(S. R. Schwarz ^ and C. R. Helms ^

4.3. Use of the Scanning Electron Microscope for Total Dose Testing^

Reliable prediction techniques for the radiation survivability of semiconductor devices

in an ionizing radiation environment based on preirradiation measurements have not been es- tablished. For this reason, total dose

screening of sample devices is frequently em- ployed as part of a hardness assurance program. The kilovolt electron beam in a scan- ning electron microscope (SEM) has been used

in many experiments to explore the effects of

ionizing radiation on semiconductor devices

and to examine its applicability to hardness

screening. This technique has been of interest for hardness assurance because of its

convenience and because devices can be selectively irradiated directly at the wafer level. A study of the problems associated with

using an SEM in a hardness assurance program

to simulate total dose testing with a gamma

source such as Co has been completed [35].

At least three factors must be considered in

order to simulate a '^'^Co radiation exposure

in an SEM. First, for a kilovolt electron

beam, the depth-dose distribution through the

oxide may be quite different from the assumed

constant depth-dose distribution for Co ex- posure. A method for estimating the total

absorbed dose in critical oxides has been

developed [36]. This method is based on an empirical expression for electron energy dissipation as a function of electron energy and

penetration depth [37].

quately specifying SEM total dose testing

procedures

It was concluded that the SEM can serve as a convenient tool for total dose testing of

semiconductor devices if attention is given

to: (1) the depth-dose distribution of kilovolt electrons and how it relates to the device structure and (2) achieving a uniform

electron exposure. The ability to adjust the

dose rate delivered in the SEM offers the advantage of being able to deposit a significant dose in a short time period. This technique has unique features in that devices can

be selectively irradiated directly at the

wafer level, thus saving packaging and possibly bookkeeping costs usually associated

with a total dose screen. Also, the radiation sensitivity of different regions of an integrated circuit can be separately investigated. Possible problems, such as application of bias to the device under test, are certainly surmountable.

If the procedures for utilizing the SEM for

total dose testing are carefully planned in

view of the characteristics of the instrument

and the kilovolt electron beams employed, the

SEM provides a powerful tool for hardness as- surance screening. Programs requiring wafer

level traceability should particularly benefit from the application of this technique.

(K. F. Galloway and P. Roitman*)

Second, the dose rate during a typical SEM

exposure can be considerably higher than typical ^'^Co dose rates. Third, an SEM properly

adjusted for imaging using secondary electrons does not deliver a laterally uniform

electron flux to the specimen. In addition,

there are practical problems associated with

device positioning, device biasing, and possible damage to adjacent devices. All of

these considerations are important for ade-

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

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

Principally funded by the Defense Nuclear

Agency.

%BS-NRC Postdoctoral Research Associate.

12
5. PHOTOLITHOGRAPHY

5.1. Calibration Standards for Optical

Linewidth Measurements on Photomasks

The initial collaborative (pilot) study (NBS

Spec. Publ. 400-38, p. 27) to evaluate procedures for measuring 1- to 10-pm wide clear

and opaque lines viewed with transmitted illumination was completed [38]. In a blind

study, each of three major manufacturing

firms in the microelectronics industry re- ceived one NBS photomask-like target from the same production batch. Without knowledge of

the NBS-measured linewidth values, each of seven groups among the three firms measured

selected lines with their own optical microscope using their own non-NBS calibration

standards, but following (to widely varying

degrees of exactness) NBS directions for performing the measurements. The values which

each company group measured for nominally

3-pm wide lines, expressed relative to the

NBS measured value (normalized to a value of

3.00 tJm), are given in table 2. The differences between company and NBS values are ac- tual observed differences for measurements on the same line and target. Different companies measured different targets; different

groups within a company measured the same

line on the same target.

Although six of the seven groups used automatic TV-microscope systems (which can give

highly reproducible results), the discrepancies between companies, between divisions of

the same company, even between groups of the same division of the same company, can be seen from the table to be very large. The

appearance of these discrepancies in measurements by technologically advanced and metrologically sophisticated companies points up the serious nature of the linewidth measurement problem.

The pilot study also yielded data for 1- and

lO-pm wide lines. Analysis of the results

suggests that: (1) the "discrepancies" in

linewidth measurements reported to NBS are recurrent and systematic; (2) the discrepancies appear in the results of the most so- phisticated microelectronics companies; (3)

automatic TV-type systems are as susceptible

to systematic errors as visual eyepiece systems; and (4) the discrepancies are not simple constant "offset" types; although complex, they are describable in terms of the

phenomena of optical imaging near the resolution limit of the microscope.

Although there is ample evidence that the use

of appropriate measurement procedures and

calibration artifacts will substantially re- duce systematic errors in the measurement

system [39] (NBS Spec Publ. 400-38, pp. 28-29), the results of the present study demonstrate the prevalence of a type of systematic error which cannot be eliminated by the

expediency of a "two-point" calibration technique. To overcome the limitations of the

present artifact, a new pattern [40] has been

designed for use in the next phase of the

evaluation which involves NBS and ten indus^

trial laboratories. , (D. A. Swyt,

F. W. Rosberry, and D. Nyyssonen )

5.2. Spatial -Fi 1 teri ng Microscope

In the previous development of the concept of

a spatial-filtering microscope (NBS Spec.

Publ. 400-29, pp. 48-50) [41], emphasis was placed on the accurate measurement of small

lines and spaces. This was a natural result

of the thrust of the current studies which

are concerned with improving the accuracy and

precision of linewidth measurement. However,

in a great many measurement applications,

particularly in locating objects within a field that is well-outlined, it is often useful to measure from the edge of the field to somewhere in the inner region. As in mea- surement of linewidth, this is an asymmetric

measurement , the accuracy of which may be improved by use of the spatial-filtering method. It is, therefore, useful to study the

effects of band-pass filtering on the images

of edges.

The general analysis closely follows the previous study; namely, calculate the spectrum

of the complex object, multiply the spectrum

by the function that describes the spatial

filter, Fourier transform the product, and

complex square the result. The resulting ex- pression is the filtered image located in the

image plane of the microscope objective.

Figure 7 contains plots of calculated image

distributions for a diffraction-limited, 0.50

numerical-aperture (N.A.) objective with filtering (10 cycles per millimeter lower cutoff

frequency) and without filtering. The original edge object is shown for comparison. The

upper cutoff frequency is determined from the

diffraction-limited properties of the lens at

NBS Optics and Micromet rology Section,

Mechanics Division.

NBS Optical Physics Division.

13
Table 2 -

PHOTOLITHOGRAPHY

Example of Results Obtained in Pilot Linewidth Measurement Study.

Type of

Measurement

Company A

Div. 1 Group 1 Group 2 Div. 2 Group 3

Company B Company C Div. 1 Group 1 Group 2 Div. 2 Group 3

NBS

Width of Clear Space, m 2.98 2.95 2.75 2.82 3.00 2. 76 3.36 3.00

Width of Opaque Line, ym 2.97 3.50 3.31 3.19 3.10 3.12 3.19 3.00

Spacing of Two Lines, pm 5.98 5.99 5.99 5.96 6.00

^Different companies measured different targets; different groups within a company measured

the same line on the same target. Measured linewidths were normalized to NBS measurements

on the same line and target.

a wavelength of 500 nm- This plot clearly

illustrates the efficacy of the technique for

locating edges.

In general, the results of this study show

that: (1) for a given lower cutoff frequency, the width of the dip, or zero, be- comes smaller as the numerical aperture of

the objective increases and thereby defines

the edge location more precisely; and (2) for

a given numerical aperture, the larger the

lower cutoff frequency, the less energy is

_j 1 I : -3-2-1 ] ; ^

0 I Z 3 X -

Figure 7. CaLcuLated Luminance distributions

for filtered and unfiLtered images. (A:

original edge of object; B: coherent image of

edge; C: filtered image of edge. The optics

used consist of a 0.50 N.A. objective with a calculated upper cutoff frequency of 1000

cycles/mm at an illumination wavelength of

500 nm. The lower cutoff frequency for the

filtered image (curve C) is 10 cycles/mm.

The abscissa units are micrometers. Unit

magnification is assumed.)

available in the image, but the smaller is the width of the dip. For a 0.50 N.A. objective with a lower cutoff frequency of 80

cycles per millimeter, the image energy in

the region about the edge is less by an order

of magnitude than that obtained with a cutoff

frequency of 10 cycles per millimeter. Since

any band-pass filter provides accurate edge

location, provided there is sufficient energy

left in the image for observation, it is not necessary to employ an optimum filter [45] as

it is in measuring narrow lines.

The remaining problem is that of determining

the width of a line beyond which it is possible to treat it as a set of two independent

edges. The solution of this problem has a bearing on the determination of the upper

limit on the use of a spatial-filtering microscope and will also help to delineate the

general measurement precision associated with

asymmetric optical measurements.

(R. E. Swing''')

5.3. Primary Linewidth Calibration

In making linewidth determinations in the

scanning electron microscope (SEM), the actual measurement of width is based on interferometry in which the location of each edge of

the line is obtained from the SEM output. In the model currently being used [42] (NBS

Spec. Publ. 400-38, pp. 40-43), the line edge

is taken to be a linear ramp, rising at an angle 'i> from the substrate to a thickness t, and the incident electron beam is assumed to

"NBS Optics and Micrometrology Section,

Mechanics Division.

14
PHOTOLITHOGRAPHY

have a Gaussian distribution of electrons.

The model also reflects the material characteristics of the edges to be studied; for

black-chromium edges, there are three areas

of concern: the substrate, the sloped face,

and the bulk of the edge. These three re- gions have different secondary-electron emission coefficients (SEEC) which must be taken

into account.

In the previous work which was concerned with

a chromium film (covered with a very thin

layer of chromium oxide) on a glass substrate, the mid-point on the steep portion of

the output was designated as the edge location, since early calculations showed that

the mid-point was related to the origin in

the edge model. Because this relationship is

only approximately true for most cases, it is appropriate to extend the analysis. For the

ideal model, the minimum value of the

secondary-electron current, Iinin> is equal to 2oi, where oj^ is the SEEC of the glass substrate. The maximum current, Iniax> occurs at a value of x given by

_ "1 InA

^ 2 2x^ '

where x-j^ = t cot

(12)

, A = [(n^ sec (|) ) Oi]/[(o3 sec 4)) -

02], 03 is the SEEC of the

chromium (in the transition region of the

edge), and 02 is the SEEC of the chromium ox- ide (at the other side of the edge). The

edge location is taken as the beginning of

the slope of the edge (x = 0); the current at this position is given by

1(0) = (Oj^+a^) + [(o^ sec <i')-o^] erf x^^ .(13)

The ratio, R, of 1(0) to I^^g [= ''2(In,ax +

^min)] '^^^ calculated. It is a function

of the three SEECs , <}), and t. Values of R as

a function of beam diameter, d, and <t> are listed in table 3 for the case where the beam

diameter is assumed to be that within which

80 percent of the beam energy is contained,

oi = 1.000, 02 = 1.265, CT3 = 1.030, and t = 120 nm. Use of this model is illustrated in

figure 8. ^avg found by averaging the

maximum and minimum values of current. 1(0)

is calculated as the product of Ig^g arid R (found in this case from table 3 for d = 65 nm and 4* = 75 deg); and the position of the

edge is located by dropping a perpendicular

line from the 1(0) point on the curve which

fixes the origin of the distance scale.

This model is deficient on two known counts.

It fails to account for the "bloom" (which

has been shown to occur at high SEM voltages)

and it does not consider the fairing and

rounding at the top and bottom of the real

edge. A mathematical function that describes

the fairing and rounding at the edges could

be developed; this function would require additional experimental evidence from scanning

electron micrographs for quantification.

This approach does not appear to be difficult. However, consideration of bloom is more Important.

The effect of bloom on SEM output is twofold:

(1) the current in the region just before the

steep rise Is somewhat depressed and (2) the

region following the maximum is filled in.

These effects are illustrated in figure 9. Similar effects have been observed in experimental secondary-electron current curves [42]

(NBS Spec. Publ. 400-38, figure 39b). It has

been shown that the shape of the curve in the

region from which the determination of edge

location is made is unchanged by these effects, but the suggested use of maxima and

minima for locating the edge may need revision; the mean value cannot be expected cl priori to remain constant.

The physical phenomena associated with bloom seem clear enough. When the scanning beam is located in the plane regions of constant

thickness (before and after the edge discontinuity), a stable system is established so that the flow of secondary electrons is con- stant for these regions. As the beam approaches the edge from the lower end, the

bloom generates electrons that are scattered

sldewise under the edge and are prevented

from reaching the detector; that is, some of the secondary-electron energy is trapped,

thus depressing the current in that region.

As the scanning beam moves along the face of

the edge, the system again reaches a stable

condition until it approaches the top of the

edge. There the bloom generates electrons

that exit from the top of the edge (in the

region of constant thickness, just to the rear of the edge) and cause an apparent in- crease in secondary-electron energy. As the

beam crosses the top of the face and proceeds

into the plane region of constant thickness,

electrons can now similarly exit from the

face near the top, and this behavior results

in a slower fall-off in energy than that predicted by the ideal system without bloom.

In the above analysis, it is assumed that the

two edges of a line are sufficiently separated so that their interactions with the

scanning beam are Independent of each other.

This is a safe assumption for llnewldths of

1 )jm and larger. However, since the ongoing

15
PHOTOLITHOGRAPHY

Table 3 — Typical Set of Values of the Ratio,

R, of Secondary-Electron Current at Edge Position to Average Secondary

Electron Current as a Function of

Beam Diameter, d, and Edge Angle,

d, nm deg R

50 50 1.0017

50 55 1.0073

50 60 1.0231

50 65 1.0591

50 70 1. 1274

50 - 75 1. 2347

50 80 1.3695

55 50 1.0033

55 55 1.0118

55 60 1.0326

55 65 1.0748

55 70 1. 1471

55 75 1.2508

55 80 1.3722

60 50 1.0055

60 55 1.0171

60 60 1.0425

60 65 1.0895

60 70 1.1635

60 75 1.2620

60 Bo 1.3714

65 50 1.0082

65 55 1.0229

65 60 1.0525

65 65 1.1028

65 70 1.1766

65 75 1.2694

65 80 1.3682

70 50 1.0113

70 55 1.0291

70 60 1.0621

70 65 1.1145

70 70 1.1869

70 75 1.2738

70 80 1.3635

study of submicrometer linewidths must subsequently be considered, it is necessary to formulate expressions for closely spaced

edges analogous to those for an isolated

edge. The procedure for locating the position of the edge (cf. fig. 8) needs no reviFigure 8. ILlustration of the procedure for

Locating the edge position for a given (calculated) SEM output.

o

Figure 9. Effects of "bloom" on the (calculated) SEM output of an edge. (The crosses

(+) show the modification of the ideal output

[shown by the asterisks (*)] for a given

amount of "bloom.")

sion for application to linewidth, provided

there are no perturbing effects between the

edges. Typical results are shown in figure

10. Linewidth formulae can also be modified

to account for an asymmetric line in which

the angles on each edge differ or the materi16
PHOTOLITHOGRAPHY

a. Model of a Line with width w and showing

the thickness parameter t, angles on each

face, and '2/ and the five regions

with secondary-electron emission coefficients as noted.

b. Calculated SEM output for a line with w = 300 nm, d = 40 nm, t = 110 nm, •t'l = (t>2 = 55°, = 1.000, 02 = 1-265, and 03 =

1 .026.

Figure 10. Model profile and calculated SEM

output trace for a line of width w = 300 nm.

tern is a partially coherent system imaging

with a 0.90 numerical aperture (N.A.) objective. A theoretical model of the system was developed, and the image profiles computed

from this model were shown to be in good

agreement with experimental results [43,44].

Although the system was originally developed

for measurements in the 1- to 10-,.m regime,

it was shown theoretically that the method of

edge detection could produce accurate mea- surements for 0.5-Him wide lines and spaces

(NBS Spec. Publ. 400-38, pp. 36-40).

A chromium-on-glass photomask^ was used to test the system capability at these submicrometer dimensions. The computed and mea- sured image profiles of nominally O.S-ym wide

lines and spaces are shown in figure 11. Because of the unequal widths of the lines and

spaces in the experimental traces, the theoretical and experimental data cannot be su- perimposed. The experimental traces show the

period of the multiple O.S-ym lines to be

within a few percent of 1.0 um, while the

chromium lines appear narrower than the

spaces with a line-to-space ratio of approximately 0.8. The measuring system has been

shown to have a three-sigma precision of

+0.025 ym when measuring good quality blackchromium masks with nominally 1.0-ym wide

lines and spaces. Sufficient data are not currently available to establish the precision of the measurements on the specimen used

in this present study. It appears, however,

that the raggedness in the edges determines

the repeatability of measurements.

Further theoretical calculations were also

made to test the sensitivity of the measuring

system to small changes in line and space

widths at these dimensions. Figure 12 shows

typical calculated image profiles. In all cases, the equation used for the threshold

for edge detection [45]:

T = 0.25 (1 + T + 2/t~ cosv') , c 00

where Tg is the relative transmittance of

the nearly opaque region and Ct is the phase

difference between the transparent and nearly

opaque regions, yields the correct line or

al in the center section of the line is of nonuniform thickness. (R- E. Swing*)

5.4. Submicrometer Linewidth Measurement'

NBS Optics and Micrometrology Section,

Mechanics Division.

^Principally funded by the Naval Ocean

Systems Center.

Measurements in Transmitted Light — The NBS

transmitted-light linewidth-measurement sysThis specimen was supplied by the Naval

Ocean Systems Center.

17
PHOTOLITHOGRAPHY

LU3

0.5 1.0 1.5 2.0 DISTANCE l/iinl

2.5

a. Image profile on a bright chromium photomask as seen by NBS Linewidth measuring

system.

0.5 1.0 1.5 2.0 DISTANCE \^xm]

2.5

b. Computed image profile based on model of

the NBS transmitted- light Linewidth mea- suring system (background transmittance

of 2.5% [no phase difference]).

Figure 11. Computed and measured image profiles of nominally O.S-pm wide lines and spaces in

a chromium-on-glass photomask.

0.5 1.0 DISTANCE [^JLm]

1.5

a. Tq = 0.025; * = 0 rad.

2.0

5 0.5

0.5 1.0 DISTANCE i/xm|

1.5

b. Tq = 0.31; 4) = irrad.

2.0

Figure 12. Computed image profiles of multiple line object with 0.5-pm wide lines and 0.6-ijm

wide spaces.

18
PHOTOLITHOGRAPHY

space width to within 1 percent.

(D. Nyyssonen ' and E. C. Kintner )

Measuvements in Reflected Light — The

ref lected-light system (NBS Spec. Publ.

400-38, pp. 33-34) was modified by insertion

of a fixed circular aperture in place of the

leaf aperture diaphragm which controlled the

illuminating numerical aperture. When used

with the bright-field reflector, this modification produces the reflection analog of the

transmitted-light linewidth-measurement system with the ratio of condenser N.A. to objective N.A. equal to 0.25; this ratio is re- quired for artifacts which produce phase differences of up to TT rad on reflection.

Image profiles produced with this reflectedlight system have very poor signal-to-noise

ratio (S/N). In addition, all of the specimens examined, including resist on wafers,

showed differences in spectral reflectance

between the substrate and the patterned layer; for example, the substrate may appear

yellow-green and the patterned layer, bluegreen. An analysis of this effect shows that

a thin-film phenomenon effectively makes the

specimen appear to differ in contrast with

illumination wavelength. Accurate linewldth

measurements in reflected light require a

much narrower spectral bandwidth than was needed for the transralCted-light system.

A brief analysis of the reduction of S/N for

the ref lected-light system as compared with

the transraitted-light system shows: (1)

there is a reduction factor of 20 or more in

S/N because of the lower reflectance of the

wafer (typically 4-percent reflectance for

the wafer as compared with 90-percent or more transmittance for the photomask); (2) there

is an additional reduction factor of 7 in

going from 0.66 to 0.25 for the ratio of con- denser N.A. to objective N.A.; (3) the beam

splitter in the bright-field illumination

system introduces another reduction factor of

4; and (4) reducing the spectral bandwidth

introduces at least another reduction factor

of 6. The transmitted-light system has an S/N ratio of better than 400/1. The losses

in the ref lected-light system yield an S/N

ratio of about 1/10. Since 100/1 is probably

the lowest reasonable value required for a good measurement system, the enormous In- crease in brightness required can only be

achieved by use of a laser. Since optimum

optical performance of the system requires a wavelength of 530 nm, a krypton laser operating at this wavelength is currently being incorporated in the system."^ (D. Nyyssonen^ )

'NBS Optical Physics Division.

%BS-NRC Postdoctoral Research Associate, NBS

Optics and Micrometrology Section, Mechanics

Division.

^The system has since been satisfactorily

operated with the laser source; see reference [46].

19
6. TEST PATTERNS

6.1. Random Fault Test Structures

This study was undertaken to evaluate experimentally various ways of determining processinduced wafer defect densities from measurements on the random fault test structures

which are included on test pattern NBS-7 [47]

(NBS Spec. Publ. 400-29, pp. 65-66). This

pattern was designed to be fabricated by a junction-isolated bipolar process, typical of

that employed for making transistortransistor logic (TTL) integrated circuits.

The pattern contains three random fault test structures: metal-step-coverage, contact-toresistor, and multi-emitter-transistor structures .

Since the wafers measured in this study were especially prepared to exhibit a wide range

of defect densities, it is possible to ob- serve the effect of the magnitude of the mean defect density on the results obtained by

several different methods. This permits an evaluation of various methods of analysis

used to estimate the defect density, the degree of success of using reduced testing

schemes, and the suitability of the test structures as they exist on test pattern

NBS-7 for evaluating a wide range of defect

densities

The metal-step-coverage test structure con- sists of four segments of 6-ym wide metal

runs crossing over cuts in the oxide layer.

The segments cross over 46, 92, 253, and 713

oxide cuts, respectively. The presence of a fault was indicated by an open circuit in a particular segment when a current source attempted to drive a 100-mA current through the

metal run. The probe pads were large enough

to allow two probes to make contact with each

pad. This enabled Kelvin-type measurements

to be made in order to prevent resistance at the current carrying probe contact from interfering with the potential measurement and

also enabled a check to be made to ensure

that both probes were contacting the pad,

thus avoiding a fault indication due to poor

probe contact.

The contact-to-resistor test structure consists of four segments containing a total of

180 diffused p-type resistors of minimum dimensions connected in series with IS-pm wide

metallization. All the resistors of the entire structure are located in a conunon isolation region. Metal probe pads connected to the structure divide the resistor string into

four segments containing 12, 24, 72, and 252

resistor contacts, respectively. The presence of a fault was indicated by an open circuit, rectification greater than 20 percent,

or nonlinearity indicating a difference of

greater than 20 percent between the resistances measured at applied voltages of 5 and

10 V across the resistor string. Moderate

variation in the absolute value of the resistance was not used as a fault Indicator since

in most instances this was caused by variations in the sheet resistance of the diffused

layer across the wafer. As in the case of

the raetal-step-coverage structures, the probe

pads were large enough to permit two probes

to make contact with each pad.

The multi-emitter-transistor test structure

consists of nine transistors grouped in a common collector region. The base region of

each transistor contains either 1, 2, 4, 9,

16, 36, 72, 140, or 286 emitter diffusions.

The emitters (24-pm by 24-ym mask dimensions)

are arranged in regular rows and columns with

a 24-pm spacing between emitters. For each

transistor, a probe pad is connected to the

base regions and another probe pad is con- nected to a large metallized area connecting

together all the emitters of that transistor.

A single probe pad provides contact with the common collector region of the structure. A

fault in this test structure results in either an emitter-collector pipe (a low resistance path between the emitter and the collector) or a leaky junction (a short between

the base and emitter or base and collector).

In practice, a fault was indicated by a collector-emitter breakdown voltage with the

emitter shorted to the base (BV^^ ) of less

than 10 V at a current of 10 pA.

For a random distribution of defects on the

wafer such that equal areas have an equal

probability of intercepting a defect, the

probability of a given test structure intercepting a defect is given by the binomial

distribution [48]. Let D equal the mean defect density, A equal the area of the test structure susceptible to the defect, and N equal the total number of test structures on the wafer. For N » 1, the binomial distribution can be approximated by the Poisson

distribution to give the following equation

for the yield Y [49]:

Therefore, for a large enough sample of test structures N, the defect density D can be

found from the slope of the plot of the natural log of yield against area A. For the case of the metal-step-coverage test structures, it was convenient to let A equal the

20
TEST PATTERNS

number of oxide steps crossed by the metal

run. For the contact-to-resistor and multiemitter transistors, A is taken to be the

number of contacts in a test segment or the

number of emitters in the test transistor,

respectively. The four segments of the

contact-resistor test structure allow the

following combinations of resistor contacts:

12, 24, 36, 72, 84, 96, 252, 264, 296, and

360. By adding together the test results of

segments from adjacent test sites in blocks

of 2, 4, and 8 test sites, the following com- binations of resistor contacts can be added

to this list: 48, 144, 192, 288, 504, 576,

720, 1008, 1440, 2016, and 2880. Similar

lists of combinations of the number of steps

and number of emitters can be generated from

the test segments present in the raetal-stepcoverage and multi-emitter-transmitter test structures

.

For a random distribution of defects, the

most rigorous method for estimating the defect density involves the measurement of Y for all the possible values of A generated by

combining results from segments of individual

test sites and blocks of adjacent test sites,

followed by determination of the slope of the In Y-A plot. This procedure is designated

method I in the discussion which follows.

This procedure tends to be rather timeconsuming, so several simplifications of this

method were also studied.

An obvious simplification is to reduce the

number of combinations of A's by limiting

them to the combinations found at each test site. This approach is referred to as method

II. A possible problem with method II occurs when the defect density D is very small in

relation to the maximum number of test entities (contacts, steps, or emitters, etc.)

present in each test site. Under such conditions, it is possible that defects will only

be encountered by the largest test segment,

and it would be impossible to plot the natural log of yield as a function of A.

A third method works well under the condition

where the product of the defect density and

the total number of entities at the test site

is small, the condition where method II might

fail. Method III involves the use of an approximation to eq (14) in order to obtain the

defect density estimate directly from the

failure rate F [50] which is the complement

of the yield:

F = 1 - Y . (15)

Using a power series expansion of the exponential function, and retaining only the

first two terms of this series, one obtains a simple expression for the defect density:

D = F/A . (16)

Since the power series expansion Is an alternating series, the error in terminating it at the second term is no greater than the third

term in the series. The error in this method

should be small if the failure rate is small;

for F less than 0.3, the expected error is

less than 10 percent. For simplicity, A was taken to be the total number of test entities

in the test structure. Then, the failure

rate F is that fraction of the test sites of

a particular size which contain at least one defect

.

These three methods involve testing all of

the segments of each test structure with the

test pattern stepped and repeated to cover

the entire wafer. Three other methods involved data from only a portion of the wafer

to see how much the estimates of defect density would change if the number of measurements was substantially reduced. In methods

IV and V, only the results obtained from mea- surements on the largest segment of each test structure were used. In method IV, the defect density was obtained from the slope of

the plot of InY versus A (the areas employed

are nA , where n = 1, 2, 3, or 4 and A max ' * ' max

is the number of entities in the largest segment of each single test structure), and in

method V it was calculated from eq (16). In method VI, D was calculated from eq (16) using only the data gathered from five selected

test sites on the wafer. The five sites

chosen consisted of one at the center and

four located synmietrically about the center

approximately two-thirds of the distance from

the center to the outer edge of the wafer.

Since an average wafer contained 200 usable

test sites, method VI represents a very substantial reduction in testing time. This ar- rangement is intended to simulate the placement of a limited number of test pattern

sites on a product wafer.

These six methods of estimating the defect

density are compared in table 4 for 10 wafers

covering a wide range of defect densities.

Wafers IX, 2Y, 3 6, and 228 were produced us- ing normal processing, whereas the rest of

the wafers were produced with processing intentionally altered to induce defects. This was done to be able to evaluate the performance of the test structures over a considerable range of defect densities. The first

column of the table contains the wafer identification numbers, arranged in order of increasing defect density as determined by

21
TEST PATTERNS

Table 4 — Comparisons of Defect Densities as Estimated by Six Methods.

Wafer Method I Percent Difference from Result Obtained by Method I Number D, defects/unit

Method II Method III Method IV Method V Method VI

a. Metal -Step-Coverage Test Structure

228 7.47 X _5

10 I -15.8 -17.4 + 1.74 -13.7 -142

36 8.21 X - 0.5 +17.8 -55.9 + 9.26

IX 8.6 X 10:4 -33.7 +14.5 0.0 -14.5

2Y 1.03 X 10.4 - 8.7 - 2.9 - 6.7 - 8.7

425 S-11 4.70 X 1^-4 -12.3 -14.7 -14.9 - 0.85 + 51.4

425 S-12 5.93 X 10.4 -24.0 +12.9 + 5.2 + 0.5 -145

425 S-10 6.15 X 10 z - 5.8 +28.9 +59.8 +10.4

425 S-3 1.26 X 10":: - 3.8 +42.0 + 2.3 +28.1 - 15.0

425 S-2 1.60 X 10":: - 0.6 +47.5 -53.0 +49.4 - 69.0

425 S-4 1.67 X

10"-^ +14.9 +54.0 - 1.2 +38.9 + 45.8

b. Contact-to-Resistor Test Structure

IX 8.24 X 10_4 +48.3 -28.6 +13.6 + 21.8

3e 1.30 X 10.4 -43.0 -59.0 - 8.45 -111.0

228 1.67 X 10-4 +44.5 +19.2 -41.4 - 15.0

2Y 1.68 X 10 I +10.1 -40.5 +32.0 - 22.7 -288.0

425 S-11 1.30 X 10":: + 7.8 +19.2 +22.7 - 8.45 + 14.6

425 S-12 1.32 X 10":: - 6.81 + 4.55 +25.0 - 22.0 -195.0

425 S-10 1.57 X 10"^ +23.6 +22.9 - 0.63 + 7.0

425 S-5 2. 14 X 10"^ +12.6 +33.2 + 6.1 + 7.48 -137.0

425 S-4 2.35 X 10":: - 0.42 +29.8 + 8.90 + 8.52 - 25.5

425 S-3 3.13 X

10"-^ -24.9 +32.2 + 2.7 + 6.6 + 1.5

c. Multi -Emitter Transistor Test Structure

IX 1.66 X +36.1 +36.8 -49.4 - 1.81

36 2.51 X 10.4 +26.3 +13.9 +56.5 + 4.0

228 2.90 X 10_4 + 1.0 +11.7 +10.0 -24.1 -143.0

2Y 3.67 X 10 ^ 0.0 + 9.6 - 5.18 +12.5 - 92.2

425 S-3 9.98 X 10"^ - 6.2 +77.0 +65.9 + 29.1

425 S-5 1.1 X 10"^ - 2.7 +60.0 +59.0 + 18.8

425 S-11 1.21 X 10"^ + 3.3 +45.5 +71.4 + 24.0

425 S-12 1.27 X 10"^ -11.8 +86.5 +72.0 + 33.0

425 S-4 1.62 X 10"^ -15.4 +89.0 +78.5 + 56.0

425 S-10 1.71 X

10""^ + 0.6 +89.5 +79.5 + 52.5

Percent difference = 100 " ^^^^^"^ p^Meth^d'tf

22
TEST PATTERNS

method I, listed in the second column. Columns three through seven contain values of

the percent difference between the defect

density obtained from method I and that obtained by method II, III, IV, V, or VI, re- spectively. A dash is used to indicate that

the method of analysis failed to give any es- timate of the defect density.

It is most instructive to look at the results

of each method of estimating the defect density (a given column) for all three test structures. In method II (column 3), yield

data for those values of A obtained by com- bining segments of adjacent test sites is not considered. This eliminates the largest A

values, so one would expect to see a significant difference from method I when the defect

density is small and the total number of en- tities per test site (number of entities in

the test structure) is small. This problem

is evident for the contact-to-resistor test structure. This structure has only 360 contacts per test site compared to 566 emitters

for the multi-emitter structure and 1104

steps for the metal-step-coverage test structure.

Method III (column 4) which just uses the

failure rate F of the entire test structure

agrees only moderately well with method I for

wafers with low defect densities. An increasing discrepancy between method I and

method III with increasing defect density is clearly evident for the two test structures

with the largest number of entities.

Except in the case of the multi-emitter transistor test structure, the results of methods

IV and V, which use the test results from

only the largest segment of the test structures, agree quite well with the defect density estimate of method I. These methods

suffer large errors when the defect densities

are large, especially when the number of en- tities in the largest segment of the test structure is also large. For method IV, once

the defect density is large enough that adjacent test sites always have at least one defect in the largest segments, it is not possible to plot the yield versus A using information from only the largest segment of

the test structure, so no estimate is possible under these conditions. These results

indicate that it is not necessary to have a test structure containing segments with different numbers of entities to obtain good es- timates of the wafer defect density, provided

that the defect density is small enough for

the above approximations made to the yield

formula to be valid.

In most cases, method VI, where only five

sites on the wafer are tested, gives rather

unsatisfactory results. When the defect density was low, often none of the five test structures contained a defect, so that no es- timate of the defect density could be obtained. In those cases where defects were detected, the density varied considerably

from that given by method I.

All of the above six methods of analysis rely on the assumption that the defects causing

the test structures to fail are randomly distributed point defects. To investigate the

validity of this assumption, the spatial

variation of defects was investigated. To estimate the angular dependence, each wafer

was divided into nine wedges, each with an angle of 40 deg. The total number of defects

within each wedge was counted, and this num- ber was normalized with respect to the largest number of defects found in any wedge on the wafer. Although there was considerable

variation in defect density, especially for

those wafers which had the smallest defect

densities, there appears to be no clear evidence for any overall angular dependence.

The radial dependence of defect density was investigated by dividing the wafer into two regions, separated by a circle with a radius

equal to two-thirds of the radius of the

wafer. The number of defects was counted for

each area and divided by the number of test pattern sites within that area. In the case

of the metal-step-coverage test, there seems

to be no evidence for a radial dependence of

defect density. The results are markedly

different for the contact-to-resistor and

multi-emitter transistor test results. For

these structures, the number of defects per

test site for the outer portion of the wafer

is consistently greater than the number of

defects per test site for the inner portion.

The assumption of a random distribution of

defects does not seem to be valid for the defects causing the faults in the contact-toresistor and multi-emitter test structures.

It appears that more realistic values of the

defect densities could be found by analyzing

the two regions of the wafer separately, as- suming a random distribution of defects within each region.

In the preceding discussion, nothing has been

said about estimating the degree of confidence one can have that the defect density,

obtained by even the most rigorous of the six

methods used. Is very close to the actual

value of the wafer defect density. In general, when the number of failures in M defect

23
TEST PATTERNS

D IDEFECTS/STEPI

Figure 13. Confidence Limits for the defect density D for a confidence Level of 95 percent. (X is the number of faulty entities

found in a sample of M entities.)

monitors (M resistor contacts, for example)

Is measured, one can estimate the defect density as the quotient of the number of defective monitors found and the total number of

monitors tested. This is exactly what one Is doing when using method 111, provided that

each failure of a test site Is due to only one defect. The quality of this estimate depends on the magnitude of the defect density

and the size of the sample M. If a random

distribution of defects can be assumed, the

number of faults encountered by a sampling of N monitors follows a binomial distribution.

The statistical charts of the confidence limits for the binomial distribution can be used

to answer questions such as: (1) If x defective entities were found in a sample of M en- tities, what are the 95-percent or 99-percent

confidence Intervals for the wafer defect

density? or (2) For a given range of defect

densities, what sample size is required to provide a desired degree of confidence In the

estimation of wafer defect density? The statistical charts for the confidence limits of

the binomial distribution as derived by Clopper and Pearson [51] do not cover the range

of values encountered in this study. Fortunately, under the condition of a large sample

size (total number of entitles on the wafer)

and a small defect density, the binomial distribution may be approximated by the Poisson

distribution. Therefore, the statistical

tables of Molina [52] may be used to extend

the statistical charts for the confidence

limits to range of defect densities desired.

Figure 13 presents a version of the extended

statistical charts for the 95-percent confidence intervals. The abscissa represents the

defect density value multiplied by 10"

where n is a positive whole number greater

than one. (For n less than one, the statistical charts of Clopper and Pearson [51] may

be used. ) The ordinate represents the fraction of defective entities found in the total

number of entities examined, multiplied by

10". The quantity M is the total number of

entities examined; for example, it would

equal the total number of contacts per test structure times the total number of test structures measured for the contact-toresistor test structure. The quantity X is

the total number of defects found on the wafer. The two lines labeled with the same M

numbers represent for a given X/M ratio the

upper and lower bounds of the corresponding

defect density values at the 95-percent con- fidence level. In drawing M items at random

from a larger population of mean defect density D, the number of defective items X in

the sample will, in general, be different

24
TEST PATTERNS

each time a sample is taken. However, the

X/M ratio will be inside the interval between

the 95-percent confidence limits, 95 times

out a hundred, in the long run of sampling.

To illustrate the use of this chart, consider

the metal-step-coverage test results in table

4 for wafer number 228. The fraction of bad

steps in the total number tested is found

from method III results. Using the percent

difference formula and method I defect density, this number is given by D = X/M = 8. 7 x 10 . To use figure 13, let n equal 5. Since there are 1104 steps per test site and

approximately 200 test sites per wafer, the

sample size M is approximately 2 x 10^. With

X/M equal to 8.7 x 10"-, the 95-percent con- fidence limits on the defect density are read

from figure 14 as 5.1 x 10"'' to 1.23 x 10 or the defect density can be expressed as 8.7 + 3.6 X 10"-' at the 95-percent confidence

level.

Figure 13 can also be used to see why method

VI which used the test results from only five

sites yields unsatisfactory results. For five test sites, the sample size is M 5000.

Although the 95-percent confidence interval

lines are not shown in the figure , they would

lie outside the 5

x lo ^ lines, so for a given

X/M ratio, there is a very wide range in the

possible values of the defect density.

It should be mentioned that in using figure

13 in conjunction with the results of method

III it must be assumed that each faulty test segment found contains no more than one fault. The figure is based on a separate

examination of each test entity. However, if the defect density is low and the number of entities per test segment is small enough

that the occurrence of multiple faults per

test segment is rare, then figure 13 may be

used. (G. C. Taylor)

25
7. ASSEMBLY AND PACKAGING

7.1. Wire-Bond Pull Test

The wire-bond pull test is the most widely

accepted method for controlling the quality

of the wire bonding operation and thereby offering added assurance that semiconductor devices will not fail in the field due to weak

wire bonds. Specific test procedures and

pull-force values are called out in military

and other device purchase specifications and

yet, until the present study, there has never been a carefully controlled interlaboratory

comparison of this important test.

The variables known to affect the bond pull

test including ones that cannot be treated

theoretically, such as bond peeling and tearing, have been summarized elsewhere [53,54],

and ASTM standard procedures for both the

destructive [55] and nondestructive [56] bond

pull tests have been adopted by the American

National Standards Institute.

An interlaboratory comparison (ILC) was car- ried out under the auspices of ASTM Committee

F-1" to verify the destructive bond pull

method [55] and to develop data on its reproducibility. For this comparison, all bonds were made with the same bonding machine by

the same operator in order to minimize the

effect of uncontrolled variables. One substrate was prepared for each of seven participants in the experiment other than NBS

The bonding substrates were 1.5-in. (38-mm)

diameter silicon wafers on which a 0.8- to

1 . 0-pm thick film of silicon dioxide was thermally grown. A film of aluminum 0.7 to 0.8 um thick was evaporated onto the oxide

and subsequently sintered. The aluminum was patterned in square bonding pads 0.005 in.

(0.13 mm) on a side spaced 0.005 in. apart over the entire surface to yield approximately 15,000 pads on each wafer. Each wafer was mounted on a steel base for rigidity and ease

of handling.

An ultrasonic bonding machine was set up using previously developed methods [57]. The

machine parameters were optimized for bonding

in each desired bond-quality group on samples

from the same wafer lot and then verified on each individual test wafer. Wire bonds were made with 0.0010-in. (25-pm) diameter,

stress-relieved, aluminum 1-percent silicon

wire from the same production lot, having a breaking load of 12 to 14 gf.- A bond-tobond spacing of 0.04 in. (1.0 mm) was chosen

so that the center of the loop would be over an unused bonding pad, making it easy for the

bond pull operator to center the pulling hook

in the bond loop. The center-position loop

height was chosen to be 0.011 to 0.012 in.

(0.28 to 0.30 mm) in order to minimize the

effect that slight variations in loop height

or bond-to-bond spacing would have on the re- sulting pull force. This geometry also

yields pull-force values that are the same as the force in the wire at the break point so that no calculation is required to satisfy

the referee method [55]. A scanning electron

micrograph of a part of a substrate including

typical bonds used in the interlaboratory

comparison is shown in figure 14.

In order to determine whether there could be

any dependence of the precision of the pullforce data on bond quality (e.g., weak or cracked bond heels could be damaged by bond

loop movement during accidental misplacement

of the pulling hook), the test substrates

contained bonds in three quality groups. The

first group was prepared to represent highquality bonds and contained approximately 100

bonds designed to have a mean pull force of

approximately 10 gf with a standard deviation, s^, less than 1 gf. The second group

consisted of approximately 100 averagequality bonds designed to have a mean pull

force of approximately 7.5 gf with s^^ % 1 gf. The third group consisted of poorly

made, overly deformed bonds designed to have

a mean pull force of approximately 4 gf with

Sjj ^ 1'5 gf. Because of the large variability, this group contained approximately

200 bonds so that the data obtained would be

statistically meaningful.

Because test samples were made on different

days and the characteristics of the wire used

and the production techniques may have varied

somewhat from day to day, these samples may

"Committee F-1 on Electronics of the American

Society for Testing and Materials, 1916 Race

Street, Philadelphia, PA 19103.

^The participating organizations were Aerospace Corporation, Bell Telephone Laboratories, Cominco American, Fairchild Semiconductor Corporation, Hewlett-Packard Corporation, Johns Hopkins Applied Physics Laboratory, National Bureau of Standards, and

Rockwell International Corporation.

^Nearly all measuring gages are calibrated in

grams-force. The preferred SI unit for

force in this range is the millinewton (mN);

1 gf = 9.8 mN.

26
ASSEnrsLY AND PACKAGING

12

Figure 14. Scanning electron micrograph of

typical bonds on a substrate used in the bond

pull test inter laboratory comparison. (The

bonding pads are square; the rectangular

appearance is an artifact resulting from the

angle of incidence of the electron beam.)

not represent the same statistical population. Also, being performed by different or- ganizations in differeat locations, test re- sults from different organizations do not necessarily represent data from the same statistical population. Because of these possible differences, the results of test samples

in this interlaboratory test could be directly compared only against the results obtained

by the originating laboratory on the same sample. Therefore, approximately one half of

the bonds in each group on each wafer were

pull tested by the originating laboratory,

NBS.

The bond pulling apparatus used in the NBS

test [57] employed a graphic readout. The

pull mechanism is based on a spring deflection type system similar to that used in a gram gage; the readout is accomplished by the use of a transducer whose output is amplified

and displayed as a vertical line on an x-y recorder. Ten calibration points, produced

by hanging weights on the hook [55], were fitted by a computer-generated polynomial to obtain a best fit calibration curve. Subsequently, all graphical pull data were entered

into the same program, automatically con- verted into grams-force, and analyzed statistically.

Eleven test samples were produced in March

and April 1975. The first distribution to

E 6

-J 3 3

H

GROUP

10 qt

5i

GROUP

7 5 gf PULL FORCE GROUPS

Figure 15. Graphical presentation of the

wire-bond pull test interlaboratory comparison data. (The numbers are used to represent

the various participating laboratories, ex- cept 8 which is an NBS retest. Error bars

represent +_1 standard deviation of the lab

data. Horizontal lines not at an end of a vertical line represent +^ standard deviation

of the original NBS data on that laboratory

specimen. Group I data are normalized to 10-gf, II to 7.5-gf, and III to 4-gf pull

force.

the participating laboratories took place at the June 1975 ASTM Committee F-1 meeting held

at NBS. Six samples were given to volunteers

among the attendees at the interconnection

bonding section meeting. One sample was

lost, and pull test data from two other sam- ples had to be disqualified as discussed below; two additional samples were distributed

at a later date. Before performing the test, some laboratories held their samples for sev- eral months; one laboratory held its sample

for two years. Thus, some change in the wire

properties as well as annealing of the ultrasonically work hardened bond heel could have

occurred before the pull tests were performed.

The analysis of the interlaboratory data took

place in April and May 1977 after the results

from all tests were received. In most cases,

the mean value of the original NBS pull data

27
ASSEMBLY AND PACKAGING

from each group was within 0.5 gf of the intended value. However, to better compare the

laboratory-to-laboratory data, these mean values were normalized to the intended values

of 10, 7.5, and 4 gf, and the normalization

factors were then used to normalize each appropriate test group from the participating

laboratories

Figure 15 is a plot of the doubly normalized

data from all of the laboratories. The wide

spread in results indicated that there were serious problems with the ILC. Because of

the time lag from production to pulling by

the participating organizations, one of the

remaining original samples was tested at NBS on June 3, 1977, to determine if aging effects on the pull strength could be responsible. These samples had remained in mechanically protected, nonhermetic containers at room temperature since their production. An

extremely careful recalibration of the equipment was carried out before this test, and

the results were Incorporated in the computer

data analysis calibration program. These

data are included in figure 16 as the eighth

set. It can be seen that there was no significant change in bond pull force due to the

26-month room temperature storage of the wire

bonds. ^ Thus, other reasons had to be found

for the sometimes large variations in pull

force from the NBS control values.

An examination of data from Laboratory 6 indicated that all values were given in even grams-force. A discussion with the personnel

involved revealed that the operator had not received the calibration procedure and in addition had rounded all values up to the next

highest gram mark on the scale of the pulling

gage. The gram gage was an old one and old

gages tend to read higher as the return

spring relaxes or is damaged. Thus, these

data were disqualified from the ILC.

Data from Laboratory 7 ranged from 30 to 45

percent lower than the reference values.

Lengthy discussions with the laboratory personnel as well as examination of the pulling

equipment revealed that the static calibration, specified in the ILC instructions and

recommended by the pull tester manufacturer,

did not correspond with the dynamic readout.

Also, the pulling hook, occasionally lurched

during pulling in a way that was operator dependent, resulting in its displacement from

the bond midpoint. These faults were re- ported to and were subsequently observed by

the manufacturer who has since made available

a retrofit kit and has redesigned the tester

to eliminate these problems. The data from

Laboratory 7 were also disqualified from the ILC.

Examination of the data from the qualified

participants (Laboratories 1 through 5), presented in figure 15, shows that the normalized pull-force values were, with only one exception, lower than those of the NBS reference. The test data from the qualified participants were run through a battery of statistical tests. Of these, only the means of

Group II were not significantly different in

the analysis of variance test. This test as- sumes that all data came from the same population, which may not have been the case even after the double normalization if unknown

pull test variables had been introduced.

Perhaps the most useful of the tests are various multiple comparisons of means [58] as shown in table 5. In this table, the lab

numbers are grouped according to the results

from the Newman-Keuls procedure; the means

are grouped according to results from the

Scheffe technique. Lines under a group indicate that the means are not significantly

different. Of the two tests, the NewmanKeuls Is normally the stricter, but in this case must be considered only approximate

since its rigorous application requires that

each group be made up of the same number of

points. The Scheffe technique corresponds

with the f-test and indicates that only the means of Group II are not significantly different, which is evidence that unknown pull

test variables were encountered in the other

two groups. The data from Group III, which

had the highest bond deformation (>2.5 wire

diameters), show the poorest correlation in

this comparison of means.

The precision of the test was estimated on the basis of relative sample standard deviations which were 7.9, 8.1, and 17.2 percent

for Groups I, II, and III, respectively.

One conclusion that may be drawn from the

above data is that poor, overly deformed

bonds give the widest percentage variation in

pull test results. When the test procedure

and its precision are sources of contention

This conclusion applies to the wire that was used in the ILC, which was stress-relieved

(partially annealed) aluminum 1-percent

silicon wire. A slight decrease in pull

force of hard (unannealed) aluminum wire

bonds of similar composition is not ruled

out by these results. Very pure wire (e.g.,

99.999-percent aluminum) is known to change

its hardness with time.

28
ASSEMBLY AND PACKAGING

Table 5— Multiple Comparisons of Means of between buyer and seller, pull test coraparlWi re-Bond Pull Test Results.^ sons should not be made using standard production devices since these usually contain a ^2435 mix of good, average, and mediocre bonds,

often having a variety of bond geometries.

n <ro n oo n i\ i n yi /i c\ cc The test should be prepared on a single-level

8.62 9.38 9.41 9.44 9.65 , , , s , ^ substrate, rather than in packages, and conGrou II sist of high quality bonds. If a low pull-

"^^Lab" 1 3 2 4 5 force range is required, then a relatively

low loop-height-to-bond-spacing ratio should

Group I Lab: Mean:

Mean: 6.51 6.61 6.74 7.12 7.23 be used rather than overdeformed bonds. The

test results support the conclusion that the

Group III permitted bond deformation should not exceed

Lab: _2 5 3 4 1 two wire diameters for aluminum wire bonds.

(G. G. Harman and C. A. Cannon)

Mean: 2.86 3.11 3.63 3.89 4.17

Lab numbers are grouped according to the re- sults from the Newman-Keuls procedure; the means are grouped according to results from

the Scheffe technique. Lines under a group

indicate that the means are not significantly different.

29
8. DEVICE INSPECTION AND TEST

8.1. Scanning Acoustic Microscopy

Spatial Resolution — Experiments were carried

out to study the resolving power of a scan- ning acoustic microscope (SAM). A resolution

pattern, " formed in a 150-nm thick aluminum

film on glass, was used. It contains groups

of equal lines and spaces, 1 and 2 ym wide.

Preliminary results are shown in figure 16. In the amplitude trace for the SAM operating

at 750 MHz (fig. 16b), the 2-pm wide lines

and spaces are clearly resolved, but the 1-pm

wide lines and spaces are not. In the amplitude trace for an SAM operating at 1100 MHz,

the 1-pm wide lines and spaces are clearly

resolved with an amplitude nearly equal to that of the 2-ym wide lines and spaces.

(C. F. Quate+)

Phase Imaging in Reflection — When a polished

surface of a single crystal is examined with

a converging acoustic beam, the reflected

signal has a characteristic response that is

dependent upon the elastic properties of the

reflecting surface [59-61] (NBS Spec. Publ.

400-38, pp. 65-69). For example, consider

the scanning acoustic microscope amplitude

traces shown in figure 17. These long exposure photographs display the variation in the

height of the returning pulses as the test specimen is translated axially through the

focal point. The triggering of the trace is

such that the horizontal scale on the scope

is a direct measure of the lens-to-specimen

spacing. It is only the upper envelope of

these curves that is significant and the

structure beneath the envelope should be ignored. These curves could be important for

they indicate that the striking differences

in the acoustic response could be used to monitor the thickness of metallization layers

on silicon. Similar curves are shown in fig- ure 18 for bare pure surfaces of sapphire,

silicon, and gallium arsenide. These curves emphasize the striking differences in the re- sponse of different crystals. (C. F. Quate,^

A. Atalar,"^ and H. K. Wickramasinghe''')

Scanning acoustic microscope image. A: 1-ym wide lines and spaces. B: Z-ym wide

Lines and spaces, f = 1100 MHz.

Amplitude trace, f = 750 MHz.

"Supplied through the courtesy of Bell La bo- ^^^^^^^H^|H|H^^HHhH^^^^^^^^^^|

^Work performed at Microwave Laboratory,

Stanford University, under NBS Contract No. c. Amplitude trace, f = 1100 MHz. 5-35899. NBS contact for additional information: R. I. Scace. Figure 16. Resolution pattern.

30
DEVICE INSPECTION AND TEST

a. Bare silicon surface, f = 750 MHz. b. Bare silicon surface, f = 1100 MHz.

c. A 1-ym thick film of aluminum on silicon,

f = 750 MHz.

d. A 1-pm thick film of aluminum on silicon,

f = 1100 MHz.

.

I

e. A 2-Mm thick film of aluminum on silicon,

f = 750 MHz.

Figure 17. The amplitude of the SAM output voltage versus Lens-to-specimen spacing for

silicon surfaces as recorded by translating the specimen axially through the focal plane,

(The horizontal scale is 3.75 pm/div.)

31
DEVICE INSPECTION AND TEST

c. GaLLium arsenide.

Figure 18. The amplitude of the SAM output voltage versus lens-to-spacing distances for

various crystal surfaces as recorded by translating the specimen axially through the focal

plane. (The horizontal scale is 3.75 ym/div and the signal frequency is 750 MHz.)

Gray Scale — The feasibility of a reflective

acoustic gray scale (NBS Spec. Publ. 400-38,

pp. 69-70) for operation at 375 MHz was demonstrated [62]. The principle of the technique, useful in calibrating the object-image

brightness transfer characteristics of a scanning acoustic microscope, is based on the

inherent impedance-matching properties of

layers between the immersion liquid (water in

tills case) and a sapphire crystal substrate.

In the demonstration vehicle, a single contoured layer of light borate crown glass was used. The stair-step pattern in the layers

was designed to obtain ten equal increments

of power reflection coefficient that cover

the available dynamic range of the material

system. The ten-step glass gray scale, fabricated by ion-beam etching, yielded close to a predicted 6-dB bright-to-dark intensity

range, with 8 dB representing the upper limit

for the materials involved here. Although

the performance was predicted from a planewave model, not generally valid in the case

of the spherical wavefronts encountered in

the highly convergent acoustic beam, the ex- perimentally observed transfer curve was close to predictions. It was found that the

axial position relative to the focal distance, at which agreement was realized, is a function of the material in which the gray

scale layer is fabricated. (R. D. Weglein ^)

Work performed at Hughes Research Laboratories under NBS Contract No. 5-35898. NBS

contact for additional information: R. I. Scace

32
9. REFERENCES

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ASTM Standards, Part 43 (November 1979).

57. Harman, G. G. , Ed., Semiconductor Measurement Technology: Microelectronic

Ultrasonic Bonding, NBS Special Publication 400-2 (January 1974).

58. Snedecor, G. W. , and Cochran, W. G.,

Statistical Methods, Chapter 10 (Iowa

State University Press, Iowa City, Iowa,

1967).

59. Atalar, A., Quate, C. F. , and Wickramasinghe, H. K. , Phase Imaging in Reflection with the Acoustic Microscope, Appl.

Phys. Letters 21, 791-793 (1977).

60. Weglein, R. D. , and Wilson, R. G. , Characteristic Material Signatures by Acoustic Microscopy, Electron. Lett. 14 , 352-

354 (1978).

61. Atalar, A., An Angular Spectrum Approach

to Contrast in Reflection Acoustic Microscope, J. Appl. Phys. 49, 5130-5139

(1978).

62. Weglein, R. D. , Gray Scale for Scanning

Acoustic Microscopy, Electron. Lett. 14 , 656-657 (1978).

35
APPENDIX

SEMICONDUCTOR TECHNOLOGY 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 NBS Tech. Note Date Issued NTIS Accession No

September 30, 1968 472 December 1968 AD 681330

December 31, 1968 475 February 1969 AD 683808

March 31, 1969 488 July 1969 AD 692232

June 30, 1969 495 September 1969 AD 695820

September 30, 1969 520 March 1970 AD 702833

December 31, 1969 527 May 1970 AD 710906

March 31, 1970 555 September 1970 AD 718534

June 30, 1970 560 November 1970 AD 719976

September 30, 1970 571 April 1971 AD 723671

December 31, 1970 592 August 1971 AD 728611

March 31, 1971 598 October 1971 AD 732553

June 30, 1971 702 November 1971 AD 734427

September 30, 1971 717 April 1972 AD 740674

December 31, 1971 727 June 1972 AD 744946

March 31, 1972 733 September 1972 AD 748640

June 30, 1972 743 December 1972 AD 753642

September 30, 1972 754 March 1973 AD 757244

December 31, 1972 773 June 1973 AD 762840

March 31, 1973 788 August 1973 AD 766918

June 30, 1973 806 November 1973 AD 771018

After July 1, 1973, progress reports were issued in the NBS Special Publication 400- subseries with the title. Semiconductor Measurement Technology

Quarter Ending NBS Spec. Publ. Date Issued NTIS Accession No

September 30, 1973 400-1 March 1974 AD 775919

December 31, 1973 1 March 31, 1974

400-4 November 1974 COM 74-51222

June 30, 1974 400-8 February 1974 AD/A 005669

September 30, 1974 400-12 May 1975 AD/A 011121

December 31, 1974 400-17 November 1975 AD/A 017523

March 31, 1975 ) June 30, 1975 J 400-19 April 1976 PB 251844

September 30, 1975)

December 31, 1975

400-25 October 1976 PB 258555

March 31^ 1976

June 30, 1976 J 400-29 April 1977 AD /A 040011

September 30, 1976 400-36 July 1978 PB 283482

December 31, 1976 ] March 31, 1977 ) 400-38 November 1979 PB 80-120231

36

t, U. S. GOVERNMENT PRINTING OFFICE : 1980 311-046/167
HBS-lUA (REV. 9-781

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

Spec. Publ. 400-45

2.007*1. Accession No. ifecijiifefit's Access ior^ Nof -^^^^

r.mmmYY/'Mirir-'^-r^^

4. TITLE AND SUBTITLE

Semiconductor Measurement Technology: Progress Report,

April 1. 1977 to September 30, 1977

5. Publication Date August 1980

6. Performing Organization Code

7. AUTHOR(S) W. Murray Bullis, Editor

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No. See item 15.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS fsrreef, city, sute. ziP)

NBS, Washington, DC 20234; SSPO, Washington, DC 20376; DNA,

Washington, DC 20305; ARPA, 1400 Wilson Blvd, Arlington, VA

22209; ERDA, Washington, DC 20545; Naval Oceans Systems

Center, 271 Cataline Blvd., San Diego, CA 92152

13. Type of Report & Period Covered Interim: Apr. 1, 1977

to Sept. 30, 1977

14. ^nsoring Agency Code

15. SUPPLEMENTARY NOTES

ARPA Order 2397, Program Code 7D10 ; DNA lACRO 77-809; SSPO Order N00016476WR70036 , Code

SP-23; ERDA TAsk Order A021-EES; Naval Ocean Systems Center Order N0095 377MPO9005

1 Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or less factual summary ol moat significant information. If document includes a signi ficant bibliography or

literature survey, mention it here.) This progress report describes NBS activities directed toward the development of methods

of ftieasareraent for semiconductor materials, process control, and devices. Both in-house

and coatract efforts are iacluded. The emphasis is on silicon device technologies.

Principal accomplishments during this rerjorting period iacluded (1) developraent of theoretical expressions for computing resistivity and hole mobility for boron-doped silicon;

(2) completion of a study of problems associated with use of a scanning electron microscope for total dose testing of semiconductor devices; (3) completion of a pilot study

to evaluate procedures for measuring 1- to 1 O-ym wide clear and opaque lines viewed with

transmitted illumination; (4) completion of a preliminary study of test structures for

estimating densities of process-induced random faults in device wafers; and (5) completion of an interlaboratory evaluation of the destructive wire bond pull test. Also re- ported is other ongoing work on materials characterization by electrical and physical

analysis methods, materials and procedures for wafer processing, photolitiiography , test

patterns, and device inspection and test procedures.

17. KEY WORDS ("six to twelve entries; alphabetical order: capitalize only the first letter ol the first key word unless a proper name;

separated bv semicolons) Capacitance-voltage methods; deep level transient spectroscopy; electrical properties; electron irradiation; electronics; hole mobility; integrated circuits; ion

knock-on effect; ion microprobe mass analysis; Irvin's curves; isothermal transient

capacitance method; linewidth measurements; measurement technology; microelectronics;

phosphorus pileup; photolithography; pull test; random fault test structures; resistivity; scanning acoustic microscope; scanning electron microscope; semiconductor devices; semiconductor materials; semiconductor process control; silicon; silicon dioxide; spatial filtering microscope; sputter-Auger method; test structures; transistors; wire bonds.

18. AVAILABILITY QUnlimited

1 1 For Official Distribution. Do Not Release to NTIS

[x] Order From Sup. of Doc, U.S. Government Printing Office, Washington, DC

20402

1

1 Order From National Technical Information Service (NTIS), Springfield, VA. 22161

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(THIS REPORT)

UNCLASSIFIED

21. NO. OF

PRINTED PAGES

43

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