REFERENCE

NBS

PUBLICATIONS

NBSIR 86-3495

A 11 IDE

NAT’L INST OF "tructive Evaluation

/Nondestructive Ill'll fva'uatl°n activities In ss in the Semiconductor

QC100 .U56 NO.86-3495 1986 V19 C.1 NBa r Materials and Processes Division

R. D. Larrabee and ML I. Bell, Editors

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Electronics and Electrical Engineering

Semiconductor Electronics Division

Gaithersburg, Maryland 20899

December 1986

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

-QC

100

.056

# 86-3495

1986

NBS

RESEARCH

INFORMATION CENTER

NBSiR 86-3495

NONDESTRUCTIVE EVALUATION

ACTIVITIES IN THE SEMICONDUCTOR

MATERIALS AND PROCESSES DIVISION

R. D. Larrabee and M. I. Bell, Editors

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Electronics and Electrical Engineering

Semiconductor Electronics Division

Gaithersburg, Maryland 20899

December 1986

U.S. DEPARTMENT OF COIV1IV1ERCE, Malcolm Baldrig®, S&cretasy

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Director
/.
Table of Contents

Page

Abstract 1

Introduction 1

Electrical Resistivity of Extrinsic Silicon (J. R. Ehrstein) 3

Electrical Resistivity and Lifetime of Near-Intrinsic Silicon (R. D. Larrabee) .... 6

Deep-Level Transient Spectroscopy of Semiconductor Devices (J, R. Lowney) .... 10

Ellipsometric Techniques of Materials Characterization

(G. A. Candela and D. Chandler-Horowitz) 13

Optical Linewidth Measurements (D. Nyyssonen) 15

Characterization of Semiconductors by Infrared Spectroscopy (A. Baghdadi) .... 18

Characterization of Compound Semiconductor Wafers by X-Ray Topography

(R. A. Forman, S. Mayo, and M. I. Bell) 23

Characterization of Semiconductors by Raman and Photoluminescence Spectroscopy

(M. I. Bell and R. A. Forman) 26

iii

NONDESTRUCTIVE EVALUATION ACTIVITIES IN

THE SEMICONDUCTOR MATERIALS AND PROCESSES DIVISION

R. D. Larrabee and M. I. Bell, Editors

Semiconductor Materials and Processes Division

National Bureau of Standards

Gaithersburg, MD 20899

ABSTRACT

This is the first in a planned series of annual reports describing the nondestructive evaluation and measurement development activities of the National Bureau of

Standards in the area of semiconductor materials and devices. Present activities

include production and certification of standard reference materials, development

of new measurement techniques, and coordination of interlaboratory experiments

and other activities of voluntary standards organizations. Standard reference

materials are produced for the determination of the electrical resistivity of semiconductors and for the optical measurement of linewidths on transparent photomasks. New techniques have been developed for optical deep-level spectroscopy,

resistivity and recombination lifetime profiling in ingots of very high resistivity

silicon, measurement of oxygen in silicon by infrared absorption, characterization

of compound semiconductor wafers by optical and x-ray techniques, and extension of photomask linewidth measurements to thicker lines on silicon substrates.

Interlaboratory studies are in progress for electrical deep-level characterization,

spreading resistance, and measurement of layer thickness by ellipsometry.

Key Words: deep-level spectroscopy; Fourier-transform infrared spectroscopy;

gallium arsenide; linewidth resistivity; nondestructive evaluation; silicon; x-ray

topography.

INTRODUCTION

The Semiconductor Materials and Processes Division and the Semiconductor Devices and

Circuits Division comprise the Semiconductor Technology Program at the National Bureau

of Standards (NBS). 1 This report summarizes the status of selected projects relating to

the nondestructive evaluation or characterization of semiconductor materials and the fabrication processes required to make them into commercial devices and integrated circuits. 2 The reports were drafted by those actually performing the work, and the reader is referred

to these individual authors for specific additional information about their projects.

1 In October 1985, the Semiconductor Materials and Processes Division and the Semiconductor Devices and Circuits Division were combined into the Semiconductor Electronics

Division.

2 The Linewidth Metrology activity was transferred to the Center for Manufacturing

Engineering early in FY-1986.

1
The Semiconductor Technology Program (STP) at NBS deals with measurement-related

problems of importance to the semiconductor industry, its suppliers, and its customers

in other industries and in the government. As part of the Department of Commerce,

NBS has a responsibility for objective technical work in developing measurements and

standards. Other U.S Government agencies frequently utilise and sponsor the work of

the program. All of the work is related in some way to the measurement of a physical

quantity such as electrical resistivity, dopant distribution, linewidth, or oxide thickness.

The program includes the development and certification of standard reference materials,

but does not presently offer any calibration services. Other projects are concerned with the

development of new or improved test methods for characterizing semiconductor materials

and processes. Program activities include the publication of technical papers and reports,

organization of workshops and seminars, supervision of interlaboratory studies (round

robins) , and assistance to standards organizations such as the American Society for Testing

and Materials (ASTM), the Semiconductor Equipment and Materials Institute (SEMI), the

Joint Electron Device Engineering Council (JEDEC), and Deutsches Institut fur Normung

(DIN).

This report has been prepared for the NBS Office of Nondestructive Evaluation as a means

of disseminating information about our current work in the broad area of nondestructive

evaluation. We welcome the reader’s comments and suggestions, not only about the specific

projects discussed in this report, but also on how we might do a more effective or complete

job in serving the semiconductor industry and the users of semiconductor products.

Descriptions of nondestructive evaluation projects in many other divisions and laboratories

of NBS are available in NBSIR 85-3187, “Nondestructive Evaluation, Technical Activities,

1985.” For further information, contact the NBS Office of Nondestructive Evaluation,

Materials Building, Rm. B344, National Bureau of Standards, Gaithersburg, MD 20899.

2
ELECTRICAL RESISTIVITY OF EXTRINSIC SILICON

James R. Ehrstein

I. Objective

To develop the ability to measure the resistivity of silicon slices or ingot sections in which

the resistivity lies in the range 5 x IQ-4

to 100 fl-cm, and is dominated by a single intentional dopant. Precision of about 5% is required for process control applications, while 1%

precision is desirable for materials acceptance testing.

II. Background

Resistivity is one of the primary parameters specified in the sale of silicon for semiconductor

devices. (Other primary parameters are physical dimensions and conductivity type, i.e., p- or n-type or the specific atomic species to be used for doping.) Early measurements of

resistivity required sectioning of a portion of the silicon ingot to obtain a rectangular barshaped specimen onto which contacts were plated or pressed. For this geometrical shape,

voltage and current measurements can be readily related to specimen resistivity, but the

procedure is clearly destructive. Early attempts to use a mechanical fixture to apply

four pointed probes under pressure to the specimen surface in order to do a four-terminal

(Kelvin-type) measurement of resistivity suffered from inaccuracy and imprecision. Such

measurements were nondestructive, however, in that only four small specimen areas each

about 50 /im in diameter are damaged by the probes.

III. Approach

NBS involvement began about 1962, at a time when interlaboratory precision of 10% to

30% was typically achieved for resistivity measurements in the range 0.01 to 100 O-cm.

Geometric correction factors for the finite thickness of thin circular specimens had been

developed, as had an empirical knowledge of proper measurement current levels (as a function of resistivity) which would minimize the effects of Joule heating and minority

carrier injection.

Early NBS contributions included the evaluation of the temperature coefficient of resistivity

for measurement temperatures in the vicinity of room temperature [l], the development

of geometric scaling factors for circular slices with diameters only a small multiple of the

probe separation [2], a geometric correction for probes with nonuniform spacing [3], and a

calculation to estimate the effects of lateral resistivity nonuniformity [4]. In addition, the

use of analog test boxes was introduced to verify instrument performance, and extensive

analytical and experimental tests were run to establish the errors that were introduced by

various components of the measurement [5,6].

IV. Results

Ensuing interlaboratory round robins, conducted under the auspices of ASTM Committee

F-l on Electronics, showed that, with proper control of experimental conditions and appli3
cation of the full set of geometric corrections, interlaboratory precision of 0,75%, or better, can be attained over the resistivity range 0.001 to 100 H-cm. Using the same procedures,

subsequent round robins demonstrated interlaboratory precision of 1.7% in the resistivity

range 100 to 500 and 5% in the resistivity range 500 to 2000 fbcm.

V. Output

NBS Report 9666 was originally published to summarize the NBS contribution to this

measurement development as well as the analyses of the round robins to test the multilaboratory precision up to 120 H-cm; this report was subsequently reprinted with the addition

of the temperature coefficient of resistivity data as NBSIR 74-496 [5]. The tests required to verify proper instrument performance and to evaluate probe spacing

are laborious. Further, they do not suffice to predict the quality of measurement data as influenced by the nature of the metal-to-semiconductor contacts between the probe tips

and the silicon specimen. As a result, ASTM Committee F-l petitioned NBS in the early

1970s to make available silicon resistivity standards which could be used to check the

calibration of a complete four-probe measurement station.

Such a set of standards, SRM 1520, consisting of two slices of silicon with resistivity levels

appropriate to the integrated circuit industry, was first made available in 1973. A second

set of standards, consisting of three slices of silicon having higher resistivity values, as appropriate to the manufacture of silicon power control devices, was first made available

in 1978. With the advent of eddy-current-based instruments for the rapid nondestructive measurement of resistivity, the availability of resistivity standards became even more

important since eddy current instrument-gain must be calibrated against specimens of

known resistivity. As a result, SRM 1523, consisting of two specimens with resistivity

values specifically chosen to suit the operating characteristics of commercial eddy current

instrumentation, was made available in 1980, and SRM 1520 was made available with thin- ner slices, more appropriate to eddy current instruments, and was renamed SRM 1521. A

total of more than 500 sets of these silicon resistivity standards have been sold worldwide

for calibrating four-probe or eddy current instruments.

VI. Impact

Procedures have been developed and tested to allow highly repeatable resistivity measure- ments to be made by four-point probe. A series of calibrated reference specimens allow

rapid verification of four-point probe instrument performance, calibration of eddy current

testers, and a common base for resistivity scale throughout the semiconductor industry.

VII. List of Publications

1. Bullis, W. M., Brewer, F. H., Kolstad, C. D., and Swartzendruber, L. J., Temperature

Coefficient of Resistivity of Silicon and Germanium near Room Temperature SolidState Electronics 11, 639-646 (1968).

4
2. Swartzendruber, L. J., Correction Factor Tables for Four-Point Probe Resistivity Measurements on Thin, Circular Semiconductor Samples, NBS Tech. Note 199 (April 15,

1964).

3. Swartzendruber, L. J., private communication.

4. Swartzendruber, L. J., Four-Point Probe Measurement of Non-Uniformities in Semiconductor Sheet Resistivity Solid-State Electronics 7, 413-422 (1964).

5. Bullis, W. M., Standard Measurements of the Resistivity of Silicon by the Four-Probe

Method, NBSIR 74-496 (August 1974).

6. Bullis, W. M., Editor, Methods of Measurement for Semiconductor Materials, Process

Control, and Devices, Quarterly Report, October 1 to December 31, 1972, NBS Tech.

Note 773 (June 1973).

5
ELECTRICAL RLJISTiWi/Y ElJuviE c:F NEAR-INTRINSIC SILICON

Robert D. Larrabee

I. Objective

To measure the average bulk resistivity and minority-carrier recombination lifetime of short

ingot sections or slices of high-resistivity semiconductor silicon (e.g., resistivities of 10,000

to 50,000 0*cm and lifetimes in the millisecond range).

II. Background

There are a wide variety of techniques that have been developed for measuring the resistivity and lifetime of extrinsic semiconductor silicon in the range of resistivities commonly

used in the integrated-circuit or power-device industries (i.e., below several hundred fl*cm).

Some of these techniques may be destructive (e.g., techniques depending on ohmic contacts or electrical probing). Others are nondestructive, but have inadequate sensitivity at

the low free-carrier densities in higher resistivity silicon (e.g., techniques based on eddy

currents or free-carrier infrared absorption). In addition, as the resistivity increases, the

relative importance of surface effects increases and gives rise to nonreproducible systematic errors. As applications for near-intrinsic high-resistivity silicon have arisen, a need has

developed for rapid, practical, nondestructive, and fairly accurate methods of measuring

resistivity and lifetime in this regime.

III. Approach

A. Average Resistivity of Thin Slices

A new approach to resistivity measurement is being developed that takes into account

the possibility of relatively large depletion or significant accumulation layers at the specimen surfaces. The nondestructive nature of the measurement is achieved by providing

removable capacitive contacts to the end surfaces of thin slices or ingot sections (front and

back surfaces) as shown in figure 1A, and by measuring the complex impedance of the

resulting lossy capacitor els a function of frequency. The charge-neutral internal (bulk)

and the charged surface regions of the specimen are modeled by the equivalent circuit of

figure IB, and the parameters of that circuit found by fitting the model to the measured

impedance data. The resistivity of the interior (bulk) region is then computed from the

resulting values of the circuit parameters.

B. Profiling Lifetime and Resistivity of Ingot Sections

An electro-optic approach to nondestructive profiling is being developed that uses a 1.15-

jum He-Ne laser normally incident on the end surface of the ingot to generate electron-hole

pairs in the interior of a silicon ingot and senses the resulting perturbation of resistivity

by radio-frequency electrical measurements using externally applied capacitive contacts.

This perturbation has been analyzed and solved analytically for the case of a cylindrical

6
specimen with four linear contacts on the ends of two mutually perpendicular diameters

and a small highly penetrating laser beam on a diameter 45° to a line joining the contacts

[l], as shown in figure 2. Radial profiling is achieved by scanning the laser beam along this

diameter, while azimuthal profiling is achieved by rotating the cylindrical specimen with

respect to the four capacitive contacts. Axial profiling is achieved by breaking these four

linear contacts into segments and measuring each segment separately. The recombination

lifetime is determined by measuring the time constant of decay of the perturbation when

the laser beam is turned off, and the relative resistivity is measured by analyzing the

magnitude of the electrical response to turning on the laser for a fixed time interval that

is short compared to the lifetime.

IV. Results

A. Average Resistivity of Thin Slices

The impedance method has been tested on slices of single-crystal semiconductor silicon in

the resistivity range from about 15,000 to 30,000 fhcm. The results of the measurement

have been shown to be relatively independent (i.e., to within about 10%) of the physical and

chemical nature of the contacted surface and of the formulation of the conductive material

painted on that surface to form the capacitive contacts. Experiments are currently in

progress to compare the results of this impedance technique with the results of other

techniques applicable in this resistivity range (which are generally destructive or otherwise

impractical).

B. Profiling Lifetime and Resistivity of Ingot Sections

A facility for performing the laser-scan profiling has been built and shown to function

as predicted with acceptable signal-to-noise levels for high-resistivity silicon ingots, using

a 1-mW He-Ne laser at 1.15 /im. The absorption length of this infrared light is several

inches, and therefore, the maximum length of the ingot sections is limited to about this

value. Measured lifetimes (in the millisecond regime) are consistent with an average value

determined by the conventional technique of photoconductive decay applied to the wholeingot section. The measured resistivity profiles will be checked by cutting a profiled ingot

section into smaller test specimens that can be measured by the above slice-impedance

method.

V. Output

A publication describing the impedance technique and the initial results summarized above

is in preparation. The lifetime and resistivity profiling technique is summarized in NBSIR

83-2792 [l]. A more definitive report will be written after the profiling technique is checked

by applying the slice-impedance technique to small specimens cut out of a previously

profiled ingot.

7
VI. Impact

Several companies are participating with NBS to determine the value of these techniques

as a way of monitoring the resistivity of near-intrinsic semiconductor silicon to assure compliance with purchase specifications and for in-process quality control. In addition,

the Department of Defense is interested in establishing better domestic sources of highresistivity silicon and has indicated an interest in having NBS participate in their programs

to assist in the characterization of high-resistivity silicon, particularly with respect to the

measurement of resistivity and minority-carrier lifetime.

VII. List of Publications

1. Larrabee, R. D., and Lowney, J. R., Measurement Techniques for High-Resistivity

Detector-Grade Silicon: Progress Report, July 1, 1982 to June 30, 1983, NBSIR 83-

2792 (December 1983).

Figure 1A. Semiconductor slice or ingot section provided with capacitive contacts to form

a two-terminal test device.

8
Cb C S —1

1—

Rb R S ——-aa/v——

J

•wv

Bulk

Semiconductor

Effects of

Near-surface Regions

Figure IB. Equivalent circuit of the test device of figure 1. Rb and Cb represent the bulk

resistance and capacitance. Rs and Cs characterize the effects of the near-surface regions

(e.g., a surface depletion region).

Chopping

Wheel

Figure 2. Schematic diagram of the laser-scan lifetime and resistivity profiling technique.

The capacitive linear electrical contacts are labeled 1, 2, 3, and 4.

9
DEEP-LEVEL TRANSIENT SPECTROSCOPY OF SEMICONDUCTOR DEVICES

Jeremiah R. Lowney

I. Objective

To determine the densities, energies, and capture cross sections of deep levels in semiconductor devices.

II. Background

Deep levels are electronic states deep within the energy gap of a semiconductor which are caused by impurities and defects and which can alter or degrade device performance. Some

are present in the starting material while others are either accidentally or intentionally

added during device fabrication. Since these levels can affect the performance of a device,

it is imperative that techniques be available to characterize them. An important technique

for doing so is deep-level transient spectroscopy (DLTS). This technique generally measures

the liberation of carriers trapped in deep levels when the ambient carrier densities are modulated with either an optical or an electrical pulse. The liberated carriers are measured

either by a collected current or by a change in diode capacitance if a diode test device

is used. The most common method is to change the reverse bias of a diode abruptly

and repetitively and measure the change in capacitance as a function of time as carriers are thermally emitted from deep levels in the space-charge region during the period of

greater reverse bias. The measurements may be made at a constant temperature or as the

temperature is slowly swept. In conventional DLTS, the temperature is swept, and the

capacitance is measured at two times during the transient. The difference in capacitance

at these two times is then plotted as a function of the temperature. A peak in the signal

is observed at a temperature determined by the so-called rate window given by a known

relation between the two measurement times. The rate window is equal to the emission

rate of carriers from the trap at that temperature. It is possible to determine the position

of a level in the energy gap from the emission rate, and the density of the level from the

amplitude of the signal. Trap concentrations can be determined to about one part in a thousand. Subsequent measurements of the filling of the levels with majority carriers (and,

if possible, minority carriers) are needed to determine the majority and minority capture

cross sections. These measurements are more difficult and involve pulsing the diode rapidly

either electrically or optically. From all these data, one can predict the effect of the deep

levels on the net carrier density and recombination lifetime of the semiconductor material

used to make the device.

III. Approach

A difficulty in the general acceptance of deep-level transient spectroscopy has been that

inconsistencies in the results of measurements made at different laboratories have led to

distrust of the method. The goal of the NBS effort has been to make the method reliable

by improving the control inherent in the instrumentation and the rigor obtained in the

10
theoretical interpretation. To achieve this goal, a special cryostat was built which can measure and control the temperature of the sample to within 0.1 K. A digital oscilloscope

is used to collect the capacitance transients associated with the emission of trapped carriers.

These data are taken isothermally, because this method is more accurate than one based on temperature sweeping. The data are then transferred to a computer for analysis. The

theory of data analysis is based on a solution of Poisson’s equation which takes into account

that traps are initially filled with carriers in only part of the space-charge region and that

traps do not emit in another part of the space-charge region because they lie below the

quasi-Fermi energy. From this theory, simple relations have been derived which are useful

in reducing the data and providing accurate values of the deep-level parameters.

A second aspect of this w'ork has been to investigate deep levels in semi-insulating GaAs

with a technique called photoresistance deep-level transient spectroscopy (PR-DLTS). An

optical pulse is used to generate excess carriers, and the ac resistance of the sample is measured as the traps return to thermal equilibrium. This technique is truly nondestructive

because the contacts may be removed after the measurement.

IV. Results

The most recent work centers on the interpretation of nonexponential transients which

occur when the trap density is larger than 10% of the dopant density on the lightly doped

side of the junction. A method was devised for determining if nonexponentiality of the

transient is a problem [1]. A shift in the temperature corresponding to the peak of the

DLTS signal is taken as a sign of nonexponentiality when the two times at which the

capacitance is measured are varied while maintaining a constant rate window. A theoretical

formula was deduced to correct for nonexponentiality so that it is possible to determine the

correct emission rate in this case [2]. This formula involves the steady-state capacitances

when the diode is biased to fill the traps and when fully reverse biased, as well as that at the

beginning of the transient just after the larger reverse bias is restored. Formulas applicable

to both isothermal measurements as well as conventional DLTS were derived. This work

enables the proper determination of trap energy and resolves a source of disagreement in

the literature. Further work is presently being done to obtain improved analysis of trap

density.

The PR-DLTS method [3] has yielded good agreement with other techniques such as the dc current-transient method known as photoinduced transient spectroscopy (PITS).

Measurements were made on a wide range of samples grown by both horizontal Bridgman

and liquid encapsulated Czochralski techniques. Many deep levels were identified.

11
V. Output

Results of this work have been published [1-6], both as reports to sponsors and in archival

journals. Platinum-doped silicon diodes are being produced and, once fully characterized,

will serve as an aid for researchers and technicians so that more reliable measurements can

be made. Through participation in round robins and interactions with the ASTM, the staff

have helped engineers in the field to appreciate better the utility of the method. An ASTM

test method is being devised and interactions with various companies who manufacture

DLTS and related equipment are continuing.

VII. List of Publications

1. Thurber, W. R., Forman, R. A., and Phillips, W. E., A Novel Method to Detect

Nonexponential Transients in Deep Level Transient Spectroscopy, J. Appl. Phys. 53,

7397-7400 (1982).

2. Thurber, W. R., Lowney, J. R., and Phillips, W. E., Measurement Techniques for

High Power Semiconductor Materials and Devices: Annual Report, January 1, 1982

to March 31, 1983, NBSIR 84-2838 (U.S.G.P.O., Washington, DC, 1984), p. 4.

3. Seabaugh, A. C., Bell, M. I., Larrabee, R. D., and Oliver, J. D., Jr., High-Frequency

Transient-Resistance Spectroscopy of Deep Levels in SI GaAs, in Semi-Insulating III-V

Materials: Kah-nee-ta 1984, D. C. Look and J. S. Blakemore, Eds. (Shiva Publishing,

Ltd., Cheshire, England, 1984), pp. 437-445.

4. Lowney, J. R., Larrabee, R. D., and Thurber, W. R., The Relationship Between DeepLevel Measurements and Lifetime in Devices, IEEE Proc. Custom Integrated Circuits

Conference, Rochester, New York, May 23-25, 1983, pp. 152-156.

5. Phillips, W. E., and Lowney, J. R., Analysis of Nonexponential Transient Capacitance

in Silicon Diodes Heavily Doped with Platinum, J. Appl. Phys. 54, 2786-2791 (1983).

6. Phillips, W. E., Thurber, W. R., and Lowney, J. R., Improved Analysis Procedures for

Deep-Level Measurements by Transient Capacitance, Defects in Silicon 83-9, W. M.

Bullis and L. C. Kimerling, Eds., pp. 485-490 (Electrochemical Society, Pennington,

New Jersey, 1983).

12
ELLIPSOMETRIC TECHNIQUES OF MATERIALS CHARACTERIZATION

George A. Candela and Deane Chandler-Horowitz

I. Objective

To develop supporting theory and models and to determine optimum measurement conditions for variable angle/wavelength ellipsometry of single and multiple layers of materials

used for photomasks and integrated circuits.

To develop, certify, and issue step-height and layer-thickness standard reference materials

(SRMs) for silicon dioxide films on single-crystal semiconductor silicon.

II. Background

Thin films of electrically insulating silicon dioxide on semiconductor silicon are used extensively in integrated circuits. In particular, the characteristics of the thin silicon dioxide gate

of metal-oxide-semiconductor (MOS) devices are of particular interest because this oxide

forms part of the active region of these devices. The need for improved characterization

techniques and for tighter standards in thin-film metrology is increasing as the demands

on these films increase (e.g., smaller thickness, increased radiation resistance, ability to

withstand higher voltages, etc.). Ellipsometry provides a practical and accurate way to

measure the thickness and index of refraction of such films. The thickness is important

as a critical device parameter that affects performance (e.g., threshold voltage), and the

index of refraction is of interest because it provides a measure of the physical condition

of the film (e.g., density, composition, microstructure, etc.). Ellipsometry is inherently

nondestructive as the probe is a low-intensity beam of light.

The applications of ellipsometry for the characterization of integrated circuit structures

extend beyond single films of silicon dioxide on silicon. Other types of films of interest

include: photoresist films used for patterning; silicon nitride films used for passivation; and

polysilicon films used for conductive lines. These films are usually not deposited on the

semiconductor silicon substrate, but on top of some other film (e.g., a film of photoresist

on top of a silicon dioxide layer about to be patterned). Multilayer structures such as these

have more parameters of interest (e.g., thickness and index of refraction of each layer), and

the resulting ellipsometric characterization must provide a correspondingly greater number

of independent measurements. This is usually accomplished by making the ellipsometric

measurements at several selected angles of incidence (multi-angle ellipsometry) or over a range of wavelengths (spectroscopic ellipsometry).

HI. Approach

The approach being pursued for improving ellipsometric metrology involves three basic

steps. First, a fully automated state-of-the-art research-grade ellipsometer has been constructed. Second, a complete error analysis of the various ellipsometric methods is in

13
progress in order to determine how best to use the instrument to minimize the errors of

measurement of selected material parameters. Third, a set of SRMs (several thicknesses of

silicon dioxide films on silicon) is being developed for the calibration of ellipsometric and

other optical and mechanical thickness measurement techniques.

IV. Results

The basic research ellipsometer has been built and is nearing its final stages of check-out

and optimization. A complete analytic error analysis for single transparent films on an

opaque substrate has been developed and implemented in a computer (FORTRAN) program. This program allows one to design the details of a test specimen (e.g., film thickness)

and the ellipsometric parameters (e.g., angle of incidence) to optimize the measurement

for any given material property (e.g., real part of the index of refraction of the film).

Future plans include extending this approach to multilayer structures. A developmental

thin-film SRM has been designed; prototypes are being evaluated, and the procedures for

its certification are being developed.

V. List of Publications

The following publications describe some of the recent ellipsometric work:

1. Chandler-Horowitz, D., and Candela, G. A., Principal Angle Spectroscopic Ellipsometry Utilizing a Rotating Analyzer, Applied Optics 21, 2972 (1982).

2. Chandler-Horowitz, D., Ellipsometric Accuracy and the Principal Angle of Incidence,

Proc. Soc. Photo-Optical and Instrumentation Engineers 342, Integrated Circuit

Metrology, 121 (1982).

3. Chandler-Horowitz, D. and Candela, G. A., On the Accuracy of Ellipsometric Thickness Determinations of Very Thin Films, J. de Physique 44, CIO-23 (December 1983).

4. Candela, G. A. and Chandler-Horowitz, D., An Ellipsometry System for High Accuracy Metrology of Thin Films, Proc. Soc. Photo-Optical and Instrumentation Engineers 480, Integrated Circuit Metrology II, 2 (1984).

5. Chandler-Horowitz, D., Ellipsometric Metrology of Ultrathin Films: Dual Angle Measurements, Proc. Soc. Photo-Optical and Instrumentation Engineers (to be published).

6. Chandler-Horowitz, D., Semiconductor Measurement Technology: Analytic Analysis

of Ellipsometric Errors, NBS Special Publication 400-79 (to be published).

14
OPTICAL LINEWIBTH MEASUREMENTS

Diana Nyyssonen

I. Objective

To develop optical measurement and calibration techniques for accurate linewidth measurements of 0.5- to 10-/^m features patterned on integrated-circuit photomasks, reticles,

wafers, and in photoresist.

To develop, certify, and issue photomask, reticle, and silicon-wafer standard reference

materials (SRMs).

II. Background

Accurate linewidth measurements are needed by the integrated circuit (IC) industry to ensure that optical, electron-beam, and x-ray lithographic systems, as well as critical dimension measuring systems, perform to specifications and to ensure that the corresponding

lithographic patterning and etching processes are under control and produce feature sizes

within the required tolerances. Optical linewidth measurements are inherently nondestructive as the probe is a low-intensity beam of light. In recent years, the minimum feature

size of commercial integrated circuits has decreased steadily and is presently approaching

the submicrometer regime. As feature sizes approach the wavelength of the light used for

their measurement, diffraction effects and the three-dimensional nature of the features and

edge geometry become dominant and begin to seriously affect the measurement. Therefore, new and specialized techniques are required for accurate measurement at these small

dimensions.

III. Approach

Two basic approaches are being pursued depending upon the nature of the specimen.

For features patterned in thin layers, i.e., less than approximately one-quarter of the il- luminating wavelength, accurate measurements can be made using coherent, bright-field

microscopy in either: (1) transmitted light using a tungsten-halogen source (opaque photomasks on a transparent glass substrate), or (2) reflected (incident) light using an ion-laser source (integrated circuit structures). Algorithms have been developed for accurate edge

detection and linewidth measurement using the scalar theory of partially coherent imaging of thin layers in a microscope. This approach is practical for linewidths greater than

approximately 0.5 fim (for a wavelength of 530 nm). For lines of smaller dimensions, the

diffraction effects from the optical system dominate, and shorter wavelengths must be used.

The second approach was developed to deal with the more complicated measurements of

lines patterned in thicker layers (greater than one-quarter of the illuminating wavelength)

including resist, thick metal, and dielectric layers on wafers. For these thick layers, scalar

imaging theory cannot be used to predict or analyze the image structure. A waveguide

15
model has been developed to predict the scattering and resulting image structure for such

three-dimensional features patterned in thick layers with the complex edge geometries

found on integrated circuit wafers and in developed resist.

IV. Results

The first approach was applied to the measurement of antireflecting-chromium photomasks

and has been used routinely for more than five years for calibration of an NBS-developed

SRM covering the linewidth range from 0.5 to 10 pm.

For thin layers of oxide and metal on semiconductor wafers (which must be viewed in

reflected laser illumination), calibration techniques have been developed, and extensive

work has been done on the development of a suitable SRM for which prototypes are currently being fabricated. An interlaboratory study involving the prototype SRM and

the accompanying calibration and measurement procedures is planned before this SRM is issued.

Measurement and calibration techniques for lines patterned in thicker layers are also being

developed. For this case, a waveguide model has been developed, and it is now possible to

predict the scattered field and resulting image structure for any combination of different

component materials and arbitrary edge geometry. The task of implementing practical

measurement and calibration techniques based on this model has not been completed, and

fabrication of suitable SRMs with controlled edge geometry will require better process

control than is currently available.

V. List of Publications

The following key publications describe the progress of these two approaches:

1. Nyyssonen, D., Theory of Optical Edge Detection and Imaging of Thick Layers,

J. Opt. Soc. 72, 1425 (1982).

2. Kirk, C. P., and Nyyssonen, D., Modeling of the Optical Microscope Images of Thick

Layers for the Purpose of Linewidth Measurement, Proc. Soc. Photo-Optical and

Instrumentation Engineers 538 Optical Microlithography IV, 179 (1985).

3. Nyyssonen, D., Design of an Optical Linewidth Standard Reference Material for

Wafers, Proc. Soc. Photo-Optical and Instrumentation Engineers 342, Integrated Circuit Metrology, 27 (1982).

Numerous other publications relating to the various aspects of linewidth measurement on

IC photomasks and wafers are available. Photomask SRMs 474/475 have been available

since 1979 and more than 100 have been sold at costs of $4689 (SRM 474) and $3092 (SRM

475). Two other closely related photomask SRMs are nearing completion and are expected

to be available in the near future: SRM 476, a bright chromium photomask, and SRM 473,

16
an antireflecting-chromium reticle SRM with 3X dimensions. For current price listings and

availability, consult the NBS Office of Standard Reference Materials, Chemistry Building,

Rm. B311, National Bureau of Standards, Gaithersburg, MD 20899.

NBS has conducted and sponsored (in cooperation with SPIE) over a dozen state-of-the-art

seminars on linewidth measurement and calibration for the IC industry.

VI. Impact

The NBS photomask SRMs 474 and 475 are the smallest, most accurate dimensional standards in the world today. They provide world-wide traceability to the national standard

of length at micrometer dimensions. NBS is seen as the world leader in the development

of accurate linewidth measurement and calibration techniques and thousands of copies of

its publications in this area have been requested and distributed.

This project has had a long and continuing history of collaboration with major IC producers, equipment manufacturers, standards laboratories, and academia including: research

associates (currently with HP and Vickers Instruments), co-op and graduate students

(Rochester Institute of Technology, University of Rochester, etc.), collaborations with the

IC industry (IBM, Hewlett-Packard, RCA, Honeywell, etc.), collaborations with instrument manufacturers (Vickers Instruments, Nikon, Leitz, etc.), interactions with academia

(University of California at Berkeley, University of Arizona, Stanford, etc.), and visits and

interactions with visitors to NBS from all these groups. Virtually all major U.S. semiconductor houses use SRM 474 as the basis for linewidth measurement, and the demand for

NBS-sponsored seminars on linewidth measurement continues.

17
CHARACTERIZATION OF SEMICONDUCTORS BY INFRARED SPECTROSCOPY

Aslan Baghdadi

I. Objective

To determine the concentration of impurities in semiconductor substrates.

II. Background

The processing of semiconductor substrates into integrated circuits (ICs) can be described

as the introduction of very low levels (in the ppm range) of impurities into a nearly perfect

host lattice under carefully controlled conditions. Thus, the presence of even very small

concentrations of undesirable impurities in a substrate can have dramatic effects on the

performance of IC devices manufactured on that substrate. For example, impurities can

result in emitter-to-collector leakage in a bipolar transistor or can reduce the time an NMOS dynamic memory cell can hold a charge. Consequently, careful characterization of

the substrate prior to fabrication can significantly increase the yield of the manufacturing

process.

Impurities in semiconductors can be classified by their electrical activity: they can be

neutral, shallow levels (i.e., acceptors or donors), or they can be deep levels. Shallow levels are ionized at the operating temperature of the device, and thus they determine whether

the semiconductor is n-type or p-type. Deep levels are levels closer to the middle of the

band gap of the semiconductor. These levels can either release or trap an electron or a

hole and thus act as generation/recombination centers.

Infrared spectroscopy can be used as a tool to detect and quantify of all three kinds of

impurities.

III. Approach

A. Neutral Impurities

Neutral impurity atoms which are lighter than the atoms of the host lattice, such as oxygen

or carbon in silicon, can be detected by infrared absorption spectroscopy because they have

local vibrational modes which absorb infrared radiation with a characteristic wavelength.

These measurements can often be carried out at room temperature. Such room temperature measurements are quick, nondestructive, and are easily amenable to automation.

They are sufficiently inexpensive so that 100% of the material can be evaluated by this

method, if necessary. The primary difficulty in accomplishing this goal lies in adapting

the test method to as-received substrates, so that no special preparation of the material

is needed to make the measurement. In other words, the measurement method must be

adapted to the material, rather than the material to the measurement.

18
B. Shallow Impurities

These impurities can be detected by infrared absorption spectroscopy because the shallow

electronic levels can absorb infrared radiation. These measurements must be carried out

at low temperatures, e.g., 10 K. In principle, this would be a nondestructive measurement,

since the measurement itself does not alter the sample in any way. However, most lowtemperature equipment cannot accommodate samples larger than about a centimeter, so that in many cases the semiconductor wafer (which may be 15 cm or more in diameter)

must be cut up in order to fit into the low-temperature cryostat. Moreover, since some

time is needed for the sample to equilibrate at these low temperatures, this would not be

a very fast measurement. Given these limitations, the measurement of shallow impurities

would not be considered nondestructive. These limitations are not fundamental, however,

and equipment could possibly be designed to make this a nondestructive technique.

C. Beep Impurities

These impurities may be detected by infrared spectroscopy using their electronic levels or from their local vibrational modes if their atomic masses are lighter than the atomic mass

of the host lattice. However, this is not anticipated to become a test method used for

the routine evaluation of semiconductor substrates, because most deep-level impurities of

interest can be detected better by electrical measurements.

IV. Application: Impurities in Silicon

A. Neutral Impurities

The most important neutral impurities present in semiconductor silicon are oxygen and

carbon. Oxygen is present at roughly the 10 to 20 ppma level in all silicon grown by

the conventional Czochralski method. Carbon is present at levels ranging from the limit

of detectability (about 0.1 ppma) to 2 ppma in both Czochralski and float-zone silicon.

Although both these impurities, being neutral, do not directly influence the device performance, they are present in such high concentrations (actually often greater than the concentrations of the intentionally introduced impurities) that they can have serious secondorder effects. Either too high or too low an oxygen concentration can result in reduced

structural strength in Czochralski silicon. Small oxygen complexes can pin dislocations,

thus preventing their propagation. At much higher concentrations, however, large oxide

precipitates actually become the source of undesirable dislocations.

Carbon has been shown to be a nucleation center for the formation of either structural

defects, such as dislocation loops, or precipitates. Since the carbon distribution in the

silicon is generally nonuniform, the presence of carbon can result in a variation in the

physical properties of the material across the substrate. This variation in the material

properties implies a variation in the diffusion rates of the dopant impurities - thus, it can

result in an undesired distribution of the dopant after processing.

19
B. Shallow Impurities

Shallow impurities such as boron, phosphorus, and arsenic are occasionally present in

the silicon feedstock prior to crystal growth. Since all of these impurities have moderate

segregation coefficients (0.3 to 0.8), they cannot be eliminated easily during crystal growth.

The presence of any of these impurities in material intended to isolate two independent

devices could lead to an unwanted channel between these devices.

C. Deep Impurities

Most metallic impurities can produce deep levels in silicon. These impurities have very low

segregation coefficients, so that they are removed from the silicon during growth. However,

some metallic impurities, such as copper, iron, or gold can be introduced into the silicon

during processing. These impurities are fast diffusers in silicon and are not easily removed

by surface cleaning. One method has been developed for controlling metallic impurities

in silicon by “gettering” the impurities. Oxygen precipitates of controlled size and distribution are formed beneath the surface during high-temperature anneals. The metallic

impurities are trapped in the bulk of the silicon by these precipitates, and are thus prevented from affecting the active regions of the devices fabricated on the substrate surface.

The oxygen content of these substrates must be known within narrow limits (typically

±10%) in order for oxide precipitation to occur with the desired size and distribution.

Thus, the oxygen content measurement, discussed in section IV A, is the first step in the

control of deep impurities in silicon substrates during processing.

V. Results and Output

A. Measurements on As-Received Substrates

Typical semiconductor substrates are not ideally suited for infrared measurements for two

reasons: they are too thin, and they are not polished on both the front and the back

surfaces. Methods have been developed for working with thin samples [1-3] and for dealing

with the effects of unpolished surfaces [2-5].

B. Automated Data Reduction

An algorithm has been developed, and a computer program written implementing that

algorithm, for calculating the oxygen content of as-received silicon wafers [3,6].

C. FT-IR Metrology

The success of these test methods also depends upon the basic accuracy and precision of

the infrared measurement. Commercial Fourier transform infrared (FT-IR) spectrometers

have only recently become a standard instrument in analytical laboratories. These instruments are vulnerable to a different set of systematic errors than the dispersive infrared

spectrometers they are replacing. Thus, the best accuracy possible can only be achieved

after examining all possible sources of systematic errors in an FT-IR instrument, and after

20
understanding how they affect the spectrum after Fourier transformation. We have worked

on the effects of different instrument design [7], compositional striations [8,9] instrumental

artifacts [1,5,7,10] and errors in the analog-to-digital converter [11,12] of Fourier transform

instruments.

VI. Impact

This work, by increasing the accuracy and interlaboratory reproducibility of infrared absorption measurements, will ultimately result in an improved yield for microelectronics

fabrication processes. A test method incorporating some of this work has been proposed

for consideration by the American Society for Testing and Materials [3],

VII. List of Publications

1. Baghdadi, A., and Forman, R.A., Tertiary Interferograms in Fourier Transform Spectroscopy, Appl. Spectrosc. 35, 473 (Sept-Oct. 1981).

2. Baghdadi, A., Multiple Reflection Corrections in Fourier Transform IR Spectroscopy

of Back Surface Damaged Wafers, Proc. Symposium on Defects in Silicon, May 1983,

San Francisco, CA The Electrochemical Society, p. 293.

3. F-l PI 12. Proposed Test Method for Interstitial Oxygen Content of Silicon Slices by

Computer-Assisted Infrared Spectrophotometry, 1985 Annual Book of ASTM Standards, Volume 10.05, American Society for Testing and Materials, Philadelphia, Pennsylvania (1985).

4. Baghdadi, A., Comment on ‘Precise Determination of Oxygen in Silicon,’ J. Electrochem. Soc. 132, 510 (1985).

5. Baghdadi, A., Measurement of the Oxygen and Carbon Content of Silicon Wafers

by Fourier Transform Spectrophotometry, Microelectronic Processing, Inorganic Materials Characterization, ACS Symposium Series 295, L.G. Casper, Ed. (American

Chemical Society, 1985), 208.

6. Slaughter, S., Gladden, W. K., Duncan, W,, and Baghdadi, A., Computer Algorithm and Programs for the Automatic Determination of Interstitial Oxygen in Silicon

(manuscript in preparation).

7. Baghdadi, A., The Effect of Instrumental Artifacts on the Quantitative Determination

of Oxygen in Silicon, Semiconductor Processing, ASTM STP 850, D.C. Gupta, Ed.

(American Society for Testing and Materials, Philadelphia, 1984), p. 343.

8. Forman, R. A., Bell, M. L, Baghdadi, A., and Mayo, S., The Effect of Striations on the

Compositional Analysis of Silicon Crystals, Proc. Symposium on Defects in Silicon , May 1983, San Francisco, CA (The Electrochemical Society), 303.

9. Forman, R. A., Bell, M. L, Mayo, S., and Kahn, A., Effect of Spatial Averaging on

the Compositional Analysis of Crystals by Absorption Spectroscopy, J. Appl. Phys.

21
55, 547 (1984).

10. Baghdadi, A., Implicit Apodization of Interferograms in Fourier Transform Spectroscopy, Appl. Spectrosc. 37, 520 (1983).

11. Baghdadi, A., and Gladden, W. K., ADC Errors in Quantitative FT-IR Spectroscopy,

Proc. Soc. Photo-Optical and Instrumentation Engineers 553 Fourier and Computerized Infrared Spectroscopy, 207 (1986).

12. Baghdadi, A., Gladden, W. K., and Flach, D. R., Nonlinear Effects of Digitizer Errors

in FT-IR Spectroscopy, submitted for publication to Applied Spectroscopy.

22
CHARACTERIZATION OF COMPOUND SEMICONDUCTOR WAFERS

BY X-RAY TOPOGRAPHY

Richard A. Forman, Santos Mayo, and Michael I. Bell

I. Objective

To develop rapid, nondestructive methods for imaging the distribution of strain, dislocations, and other defects in semiconductor wafers.

II. Background

The production of III-V compound semiconductor crystals by the liquid-encapsulated

Czochralski (LEG) process is of growing commercial importance. Wafers of this material are used as substrates in the fabrication of transistors and integrated circuits which

offer significant advantages in speed, power consumption, and radiation hardness over their

counterparts made of silicon. A major drawback of this technology is the variability of the

starting material and the lack of effective techniques to screen out poor quality wafers

before they enter the lengthy and expensive device fabrication process.

At present, the only technique in common use for wafer quality control is etching. This

is destructive to the wafer surface and has meant that only small samples of production

lots can be tested. Experience indicates, however, that examination of every wafer to be

processed would be desirable; defects induced during crystal growth tend to be localized,

causing wafer characteristics to vary significantly within a boule as well as from boule to

bouie. Further, wafer-finishing steps such as lapping and polishing can introduce strain

and microcracks which can be expected to vary from wafer to wafer in an unpredictable

way.

III. Approach

Strain and defect patterns are easily observed in reflection x-ray topographs of these materials. A rapid, low-cost, nondestructive method has recently been developed for qualitative

x-ray reflection topography [l]. Results demonstrate the feasibility of 100% inspection of

incoming wafers by fabrication facilities of the size contemplated In plans recently an- nounced by government agencies and industrial firms. The asymmetric double-crystal

topography (DCT) system developed achieves high speed and ease of operation at the

expense of reduced sensitivity to individual microdefects such as dislocations. This is not

a major sacrifice in studying materials such as LEG GaAs which have a very high defect

density. Three features of this system contribute significantly to its low cost and high

throughput:

1. A commercial silicon wafer is used as a low-magnification monochromator for a conventional laboratory x-ray source. Both the low magnification and the exceptionally

good surface finish of the monochromator aid in achieving high intensity in the x-ray

beam incident on the specimen.

23
2. An efficient method for precise specimen alignment was devised [2] which greatly

simplifies the process of alignment for both reflection and transmission topographs.

3. Topographs are recorded on high-speed instant film, using x-ray fluorescent screens. In the present application, the loss of resolution is more than compensated by the

reduction in measurement time.

IV. Applications

A. Dislocations in LEC GaAs

The new methods have made it possible to obtain both reflection and transmission to- pographs of LEC GaAs wafers in reasonably short exposure times. The transmission to- pographs exhibit the characteristic cellular patterns and four-fold symmetric distributions

of dislocations. The reflection topographs reveal regions of slip and sub-grain boundaries

which result from the same conditions of temperature and stress during crystal growth

which produce the dislocation patterns.

B. Growth Striations in LEC GaAs

Growth rate fluctuations can lead to nonuniform incorporation of impurities. If the pres- ence of an impurity produces a sufficiently large change in the lattice constant, variations

in its concentration will produce contrast in an x-ray topograph. These inhomogeneities

(known as growth striations) are important not only because they lead to point-to-point

variations in the electrical and mechanical properties of the material, but also because

they can be used to study important aspects of the crystal growth process. The surfaces of

constant impurity concentration represent approximate “snapshots” of the instantaneous

shape of the liquid-solid interface as the crystal is pulled from the melt, and valuable information about the shape and stability of the interface at various stages of growth can

be obtained by examination of the topographs. Recent work on the incorporation of indium into GaAs in order to reduce the generation of dislocations has had the side effect

of producing crystals with pronounced striations in the indium concentration, and the opportunity provided by this material is being exploited in order to examine the influence

of crystal diameter and growth conditions on the behavior of the interface between the

crystal and the melt.

C. Strain and Breakage in GaAs Wafers

GaAs wafers, whether cut from ingots grown by the LEC or horizontal Bridgman (HB)

method, often exhibit considerable internal strain and a tendency to fracture “spontaneously.” Large, slightly strained regions generally result from the growth process, while

small regions of high strain can be produced by lapping, polishing, and handling during

wafer fabrication. Double-crystal topographs reveal these strains, as well as occasional

buried or incipient cracks which are not visible optically, but which can lead to fracture.

Unfortunately, wafers with considerable strain or distortion cannot be imaged completely

in a single topograph, especially when relatively high resolution is required as in examining

24
low-disloeation-density HB substrates for optoelectronic applications. In suck cases, it is necessary to generate composite images from a series of topographs. This, together with

the demands of 100% incoming inspection, creates a need for continued improvement in

techniques and apparatus in order to achieve further reductions in measurement time.

V. Results and Output

The design and operation of the DOT system have been reported in two publications and

a talk at a major international conference. The results have attracted the attention of a number of industrial crystal growth and device fabrication groups which now collaborate

with NBS in projects designed to correlate the results of x-ray topography with the conditions under which the crystal was grown and with the outcome of attempts to use the

material in devices.

VI. Impact

The information which can be obtained from x-ray topographs is of potential value to

crystal growers wishing to optimize and control the growth process and to device manufacturers needing to screen out poor quality wafers prior to processing. The basic advances in

methods and instrumentation made in this work can be translated into significant reductions in cost in an industrial setting. This will lead to increased use of double-crystal x-ray

topography as a tool in process development and control and as a method for incoming

inspection.

VII. List of Publications

1. Forman, R. A., and Mayo, S., In Situ Alignment Procedure for X-ray Topography,

J. Appl. Crystallog. 18, 106 (1985).

2. Forman, R. A., Bell, M. I., and Mayo, S., Rapid X-Ray Topographic Examination

of GaAs Crystals, Proc. Symp. Defect Recognition and Image Processing in III-V

Compounds, to be published.

25
CHARACTERIZATION OF SEMICONDUCTORS

BY RAMAN AND PHOTOLUMINESCENCE SPECTROSCOPY

Michael I. Bell and Richard A. Forman

I. Objective

To develop and refine laser spectroscopic methods to identify, characterize, and quantify

impurities and structural defects in semiconductors.

II. Background

The introduction of small, carefully controlled quantities of electrically active impurities

into semiconductor crystals is used to control the electrical properties of the material, permitting the fabrication of electronic devices. Sensitive measurement methods, preferably

noninvasive and nondestructive, are required to verify that sufficiently precise control of

impurity concentration and distribution has been obtained. The structural perfection of

the crystal lattice also influences the device fabrication process.

III. Approach

A. Raman Spectroscopy

Raman scattering can be used to detect and quantify impurities (both electrically active

and inactive) and to assess the structural perfection of a crystal. The technique requires

essentially no sample preparation, can be used to examine regions as small a few cubic

micrometers, and can often be performed at room temperature.

B. Photoluminesence

Laser-excited photoluminescence shares many of the advantages of Raman spectroscopy as a method of nondestructive evaluation. The experimental apparatus and procedures have

many similarities, with photoluminescence generally offering greater sensitivity but often

requiring cryogenic temperatures.

IV. Application

A. Raman Spectroscopy of Substitutional Impurities

A substitutional impurity with smaller mass than the host ion it replaces can be observed

by Raman scattering because it gives rise to a local vibrational mode at a frequency which

does not overlap those of the host lattice. Since the local mode frequency depends on the

ionic mass, characteristic isotope shifts can be used to confirm the identification of these

spectral features [l] . With appropriate care, measurements of local mode intensity can be

used to determine the concentration and distribution of impurity ions [2].

26
In addition, electrically active impurities give rise to electronic Raman scattering in the

form of a broad continuum [2] which often interacts with vibrational modes to produce

characteristic interference lineshapes. These features can be analyzed to yield estimates

of the free carrier concentration. In polar semiconductors such as GaAs, there is an

additional interaction between collective electronic excitations (plasmons) and the lattice

phonons which give rise to coupled transverse plasmon-phonon modes. Measurement of

the frequencies of these modes yields accurate values of the free carrier concentration.

This procedure is used routinely in our laboratory to verify the results of Hall effect and

resistivity measurements.

B. Raman Spectroscopy of Strain and Lattice Damage

Homogeneous deformation of the crystal lattice (strain) causes frequency shifts and splittings of the vibrational modes, and lattice damage alters the selection rules that govern

which vibrations (phonons) give rise to Raman scattering. The calculation of the quantitative effects of stress [3] permits the interpretation of Raman scattering data to yield

the magnitude and distribution of strain. The authors have also observed the effects of

residual ion implantation damage [2] and of the disorder created by a very high density of

impurity ions [l].

C. Photoluminescence of Hot Carriers

Photoexcited carriers with energies greater than the minimum of the band they occupy

can recombine before reaching thermal equilibrium. This produces emission (hot photoluminescence) at energies greater than the bandgap. Careful examination and interpretation

of the hot photoluminescence of acceptor-doped GaAs have led to more precise knowledge

of the structure of the conduction band of this material and to improved understanding of

its transport properties [4].

V. Results and Output

The results of this work have been reported in four publications and in talks at major

conferences and numerous government, industrial, and university laboratories.

VI. Impact

A. Raman Spectroscopy

The procedures developed for quantitative determination of impurity concentration from

local mode intensity [2] have been adopted by many laboratories and used or cited in all subsequent publications of which the authors are aware.

B. Photoluminescence

The improved values for the energy difference between conduction band minima obtained

in this study of hot photoluminesence [4] have been used by several groups involved in

27
the modeling of GaAs transport properties and device behavior. Informal reports indicate

that calculations which incorporate these results yield better agreement with experiment

than had been obtained previously.

VII. List of Publications

1. Forman, R. A., Bell, M. I., Myers, D. R., and Chandler-Horowitz, D., The Raman

Spectrum of Carbon in Silicon, Jpn. J. Appl. Phys 24, L848 (1985).

2. Forman, R. A., Bell, M. I., and Myers, D. R., Comments on “Raman Scattering from

B-Implanted Laser Annealed Silicon,” J. Appl. Phys. 52, 4337 (1981).

3. Bell, M. I., Effects of Stress on the Raman-Active Modes in Semiconductors, Proc.

Vllth Int. Conf. on Raman Spectroscopy, W. F. Murphy, Ed. (North Holland,

Amsterdam, 1980), 76.

4. Imhoff, E. A., Bell, M. I., and Forman, R. A., Hot Photoluminescence in BerylliumDoped Gallium Arsenide, Solid State Comm. 54, 845 (1985).

28
Hi

5

5G14A (rev. 2 -e v

)

U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DAsA

REPORT NO.

$Hs£T (See instructions) NBSIR 86-3495 December 1986

4. TITLE AND SUBTITLE

Nondestructive Evaluation Activities in the Semiconductor Materials and

Processes Division

5. AUTHOR(S)

Editors: Robert D. Larrabee and Michael I. Bell

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State. ZIP)

10.

SUPPLEMENTARY NOTES

Q3 Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here) This is the first in a planned series of annual reports describing the nondestructive

evaluation and measurement development activities of the National Bureau of Standards

in the area of semiconductor materials and devices. Present activities include

production and certification of standard reference materials, development of new measurement techniques, and coordination of inter laboratory experiments and other

activities of voluntary standards organizations. Standard reference materials are produced for the determination of the electrical resistivity of semiconductors and for

the optical measurement of linewidths on transparent photomasks. New techniques have

been developed for optical deep-level spectroscopy, resistivity and recombination lifetime profiling in ingots of very high resistivity silicon, measurement of oxygen in

silicon by infrared absorption, characterization of compound semiconductor wafers by

optical and x-ray techniques, and extension of photomasks linewidth measurements to

thicker lines on silicon substrates. Interlaboratory studies are in progress for

electrical deep-level characterization, spreading resistance, and measurement of layer

thickness by ellipsometry

.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) deep-level spectroscopy; Fourier-transform infrared spectroscopy; gallium arsenide;

linewidth resistivity; nondestructive evaluation; silicon; x-ray topography

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