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A UNITED STATES DEPARTMENT OF COMMERCE

PUBLICATION

national Bureau ot Standaras

Library, E-Ol Admin. Bldg.

DEC \

1969

NBS TECHNICAL NOTE 511

Measurement Methods for the

Semiconductor Device Industry

—A Summary of NBS Activity

j -I. I mm
NATIONAL BUREAU OF STANDARDS

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

in addition to serving as the Nation's central measurement laboratory, the Bureau is a principal

focal point in the Federal Government for assuring maximum application of the physical and

engineering sciences to the advancement of technology in industry and commerce. To this end

the Bureau conducts research and provides central national services in four broad program

areas. These are: (1) basic measurements and standards, (2) materials measurements and

standards, (3) technological measurements and standards, and (4) transfer of technology.

The Bureau comprises the Institute for Basic Standards, the Institute for Materials Research, the

Institute for Applied Technology, the Center for Radiation Research, the Center for Computer

Sciences and Technology, and the Office for Information Programs.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United

States of a complete and consistent system of physical measurement; coordinates that system with measurement systems of other nations; and furnishes essential services leading to accurate and

uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of an Office of Measurement Services and the following technical

divisions:Applied Mathematics—Electricity—Metrology—Mechanics—Heat—Atomic and Molecular Physics—Radio Physics -—Radio Engineering -'—Time and Frequency -—Astrophysics -—Cryogenics.2 THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to im- proved methods of measurement standards, and data on the properties of well-characterized

materials needed by industry, commerce, educational institutions, and Government; develops,

produces, and distributes standard reference materials; relates the physical and chemical properties of materials to their behavior and their interaction with their environments; and provides

advisory and research services to other Government agencies. The Institute consists of an Office of Standard Reference Materials and the following divisions: Analytical Chemistry—Polymers—Metallurgy—Inorganic Materials—Physical Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to promote

the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations in the development of technological standards, and test methodologies; and provides advisory and research services for Federal, state, and local government agencies. The Institute consists of the following technical divisions and

offices: Engineering Standards—Weights and Measures — Invention and Innovation — Vehicle Systems Research—Product Evaluation—Building Research—Instrument Shops—Measurement Engineering—Electronic Technology—Technical Analysis.

THE CENTER FOR RADIATION RESEARCH engages in research, measurement, and application of radiation tlo the solution of Bureau mission problems and the problems of other agencies and institutions. The Center consists of the following divisions: Reactor Radiation—Linac Radiation—Nuclear Radiation—Applied Radiation. THE CENTER FOR COMPUTER SCIENCES AND TECHNOLOGY conducts research and

provides technical services designed to aid Government agencies in the selection, acquisition,

and effective use of automatic data processing equipment; and serves as the principal focus

for the development of Federal standards for automatic data processing equipment, techniques,

and computer languages. The Center consists of the following offices and divisions: Information Processing Standards—Computer Information — Computer Services — Sys- tems Development—Information Processing Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and

accessibility of scientific information generated within NBS and other agencies of the Federal government; promotes the development of the National Standard Reference Data System and a system of information analysis centers dealing with the broader aspects of the National Measure- ment System, and provides appropriate services to ensure that the NBS staff has optimum accessibility to the scientific information of the world. The Office consists of the following

organizational units:

Office of Standard Reference Data—Clearinghouse for Federal Scientific and Technical Information ' —Office of Technical Information and Publications—Library—Office of Public Information—Office of International Relations.

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

- Located at Boulder. Colorado 80302.

3 Located at 5285 Port Royal Road, Springfield, Virginia 22151.
UNITED STATES DEPARTMENT OF COMMERCE

Maurice H. Stans, Secretary

NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

NBS TECHNICAL NOTE 511

ISSUED DECEMBER 1969

Nat. Bur. Stand. (U.S.), Tech. Note 511, 24 pages (Dec. 1969)

CODEN: NBTNA

Measurement Methods for the Semiconductor

Device Industry—A Summary of NBS Activity

W. Murray Bullis

Electronic Technology Division

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

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

(Order by SD Catalog No. CI 3.46:511 ), Price 30 cents
Foreword

The work reported in this technical note resulted from the efforts

of many NBS staff members. J. C. French has been instrumental in all

phases of the activity since its inception. F. H. Brewer and Dr. L. J.

Swartzendruber carried out much of the work on the resistivity project.

H. A. Schafft conducted the projects on second breakdown and resistivity

inhomogeneity. The germanium program was begun in cooperation with Dr.

J. A. Coleman; significant contributions have been made by W. K. Croll,

Dr. A. H. Sher, and W. R. Thurber. R. L. Mattis and Dr. W. E. Phillips

carried out much of the work on the lifetime project. Present project

responsibilities are identified in the Quarterly Progress Reports for the

present program on Methods of Measurement for Semiconductor Materials,

Process Control, and Devices (see reference 33 ).

Acknowledgement is made to various other agencies which sponsored

portions of the work: Advanced Research Projects Agency (resistivity),

Air Force Materials Laboratory (lifetime),' Arms Control and Disarmament

Agency (germanium), Atomic Energy Commission (germanium), National

Aeronautics and Space Agency (resistivity), and Rome Air Development

Center (inhomogeneities , second breakdown).

In addition it must be emphasized that success of both past and

present projects depends on the cooperation which has been obtained from

industrial members of the various standards committees in which project

staff are participating.

11
Measurement Methods for the Semiconductor Device Industry - A Summary of NBS Activity

W. Murray Bull is

Work at NBS which led to the development of a broad program on Methods of Measurement for Semiconductor Materials, Process Control, and Devices is

described. Initial work was concentrated on resistivity of silicon wafers and second breakdown in

transistors. In the first case, the basis for a significant improvement in the method for measuring

resistivity of silicon wafers was established, and

in the second, concepts were developed which formed

the basis of a new type of specification for operating conditions free from second breakdown. Work was extended to include other projects, including

studies of germanium for gamma-ray detectors , car- rier lifetime, and resistivity inhomogeneities which

are still in progress. Formation of the broad program in response to increased interest in improved

measurement methods is described.

Key Words: carrier lifetime; germanium; lithiumdrifted gamma-ray detectors; resistivity; resistivity inhomogeneities; second breakdown; silicon.

1. Introduction

The semiconductor device industry is one of the most technologically

advanced industries in the world. The properties of these devices depend

on the presence of exceedingly small but controlled amounts of impurities

in some of the purest and most perfect crystalline material known to man.

Not only the amount of impurity but also its spatial location must be

controlled to exceedingly fine dimensions . Although much sophisticated

technology is employed, the manufacture of semiconductor devices is still

done on a batch basis. The basic unit in the process is a slice or wafer

of semiconductor material. From each wafer one or many thousand devices

may be fabricated depending on the use and size of the device. Many

steps in the processing, particularly the assembly and packaging operations, are still done by hand.

The manufacturing process is exceedingly complex; many chemical,

metallurgical, and mechanical steps are involved. The measurement re- quirements in such processing can be grouped into three broad areas. First is acceptance of incoming materials which include the semiconductor

wafer itself; processing materials such as water, acids and solvents,

carrier and doping gases, and photoresists; and the materials used in

assembly and packaging of finished devices. Next is the actual fabrication of the device. The widely used epitaxial, planar process, for

example, involves vapor deposition of single crystal material; oxidation;

selective etching of the oxide by means of photolithographic techniques;

1
selective high-temperature, solid-state diffusion of impurities to con- trolled depths; evaporation of metals to form interconnections; assembly;

and packaging. Each individual step must be closely controlled in order

to obtain the desired device at the end of the line. Finally, it is nec- essary to test the finished device in order to determine that the desired

characteristics have been achieved and also to establish some measure of

the expected life of the device.

The Electronic Technology Division has been concerned with the first

and last of these areas for some time. Recently, increased demands for

improved device performance in terms of operation at higher power levels

,

at higher frequencies , and with greater reliability under both normal and

abnormal conditions have led to increasingly severe control requirements

at all stages in device processing. This in turn has led to a need for

improved measurement methods throughout the manufacturing process.

This note describes the two projects with which the division's involvement in this measurement area began. Growth of the program in order

to meet present requirements and the new activities now in progress are also discussed.

2. Resistivity

In 1960, NBS was asked by Committee F-l on Materials for Electron

Devices and Microelectronics of the American Society for Testing and

Materials (ASTM) to investigate the problems associated with the measurement of silicon wafers. Resistivity of wafers is the most important

parameter in the exchange of silicon and germanium across the interface

between crystal grower and device manufacturer. The importance of this

parameter arises from the fact that for device-quality silicon and germanium there is a well-documented relationship between resistivity and

impurity concentration in the region of room temperature. 1 ' 2 The free

carrier concentration which is the important consideration in device

design is controlled by the concentration of impurities in the semiconductor.

Although the measurement of resistivity is relatively simple and

adaptable to production processes , problems and disagreements had plagued

the industry for some time. Two methods of measuring the resistivity

were in common use. The four-probe method 3 was the preferred one since

it was not necessary to cut a particularly shaped sample in order to make

the measurement. In this method, four equally spaced point contacts are placed on a flat surface of the material as shown in Fig. la. A current,

I , is passed through the outer pair of contacts and the potential difference , V, across the inner pair is measured with a sensitive, highimpedance voltmeter. The resistivity may be calculated from:

p = —- 2tts, (1)

where s is the spacing between adjacent probes, if the specimen is a semi-infinite sample. If measurements are made on wafers of finite
dimensions, additional correction factors must be included in order to

account for thickness and diameter. An additional factor to allow for

unequal probe spacings may also be included.

In the two-probe method4 a specimen is cut from the crystal in the

form of a rectangular parallelepiped as shown in Fig. lb. The contacts

through which the current, I, passes are plated with a metal so that each

end of the specimen is an equipotential plane. The potential difference,

V, between a pair of point contacts placed a distance, I, apart is measured and the resistivity calculated from:

V Wt / O \ P =— — , (2)

where w and t are the width and thickness of the specimen.

Despite the preference for the four-probe method, it was generally

believed in the industry that the two-probe method was the more precise.

It was proposed therefore that NBS should certify standards of semiconductor materials of various resistivities as determined by the two-probe

method and furnish them for use in calibrating four-probe systems in the

field. Calibration standards with 1 per cent accuracy were expected to

be satisfactory.

Because two-probe and four-probe measurements were to be compared,

it was first necessary to solve the potential distribution problem en- countered in making four-probe measurements on thin, rectangular wafers.

These correction factors could not be obtained in closed form, so computer programs for the computation of correction factors for a variety of

circumstances were developed and published. 5 Differences in the results

obtained from the two-probe and four-probe measurements led to a study of

the homogeneity of silicon wafers.

In order to carry out this study without having to cut rectangular

samples in each case, correction factors for off-center measurements in

circular wafers were calculated in detail. Again these correction factors could not be obtained in closed form, but detailed tables of their

value for the cases of interest were calculated and published. 6 In

addition to their application in the study at hand, these tables proved

to be extremely useful to the industry as a whole; more than 1500 copies

of the technical note containing them have been distributed, and they

have been widely reprinted and distributed by material manufacturers

themselves.

The identification of the homogeneity problem led to an analysis of

the effect of inhomogeneities on four-probe measurements

.

7 Calculations

were carried out for several idealized models which consisted of a circular region with resistivity p + Ap embedded in a thin sheet of infinite

extent with resistivity p

,

It was concluded that the specimen uniformity available at the time

was inadequate to provide certified standards of the precision that the

industry desired. It was also found during these experiments that the

3
reproducibility of measurements by the four-probe method appeared to be

as good as that which could be obtained with the two-probe method.

Therefore the goals of the project were reoriented toward the development

of a four-probe method with procedures and conditions-of-measurement

which were sufficiently well defined that the reproducibility desired by

the industry could be obtained.

Detailed experiments on various aspects of the four-probe method

were carried out. The effects of variations in probe force, surfaae

roughness, and point material were investigated. From the results of

these experiments it was possible to specify optimum surface preparation

procedures and point contact characteristics . It turned out that tungsten

carbide points applied with a load of 175 grams on a surface lapped with

5 ym abrasive gave satisfactory results without undue point wear or probe

wander

.

The probe wander which results from several circumstances is worthy

of special consideration. A photograph of a typical probe is shown in

Fig. 2. The construction is such that four independently sprung pointed

wires are guided in hard bearings . Typically the spacing between the

centers of adjacent probe points is 1.6 mm. Therefore a variation in

position of only 16 ym represents a 1 per cent change in probe spacing.

Some freedom must be allowed the probes in order for them to glide up and

down within the bearings as the probe assembly is applied to the specimen

surface. If this freedom is too great, excessive wander will take place.

Furthermore, if the direction of travel of the probe as it is applied to

the specimen is not exactly perpendicular to the surface, some gliding

along the surface will take place. The probe wander associated with a good probe is less than 0.3 per cent of the mean probe spacing. Additional wander may be expected if the specimen surface is exceedingly

rough. The points may then slide down hills which they encounter on being applied to the surface of the specimen. Hence, the need for lapping with fine abrasive.

A second criterion for probe quality is equality of the probe spacing. A test in which the imprint of the probes on a polished silicon wafer is measured was developed in cooperation with ASTM Committee F-l to

enable the operator to verify the quality of his own probe both as to

probe wander and probe spacing. To allow for unequal probe spacing, a correction factor, which could be calculated by the operator for his

probe and applied to the results of his measurement, was developed for

the case of measurements on thin wafers.

In addition, a test for the quality of the electrical measuring circuit both as to accuracy and input impedance was developed. This test

involved the use of large resistors in series with a standard precision

resistor in order to simulate both probe contact and specimen bulk resistances. Test circuits with the standard resistor equal to 0.1, 0.01, and

0.001 ft were designed and constructed as part of the project. Larger

standards were assembled from commercially available components.

Another factor to be considered was the temperature dependence of

the resistivity. When measurements are made at room temperature it is

4
often naively assumed that the temperature of the room is sufficientlywell controlled that no further control is necessary. Unfortunately,

this is not usually the case. Imposition of a requirement that the measurements be made at some specified, fixed temperature would considerably

reduce the flexibility and ease with which the measurement could be made.

Since it was found that a thin wafer of silicon or germanium placed on a massive copper block assumes the temperature of the block in a very short

time the order of half a minute or so even if it is electrically isolated from the block by a thin sheet of mica, it was concluded that if

such a heat sink were employed and if its temperature were measured at

the time of the resistivity measurement one would know the temperature of

the wafer.

Detailed studies of the temperature variation of resistivity of silicon and germanium showed that over the temperature range between 18 and

28°C it was possible to fit the resistivity-temperature curve for extrinsic specimens with a straight line. In such cases, a linear temperature

coefficient was obtained which the operator could use to correct resistivity measured at a temperature t in this range to a reference temperature which was selected as 23°C. 8 It was found that the linear temperature coefficients for both silicon and germanium were 1 per cent per degree or less . Linear temperature coefficients could not be obtained for

germanium with resistivity greater than a few ohm-centimeters because of

the influence of carrier pairs thermally generated across the forbidden

gap (intrinsic conduction).

An improved test method based on these findings was prepared in co- operation with ASTM Committee F-l. 9 During the preparation of the method

several round-robin tests were conducted by the Committee to establish

the precision which could be achieved. 10 In the first of these, a few

silicon wafers with resistivity in the range of 10 fi-cm at room temperature and of as good a uniformity as could be obtained were selected and

circulated among five groups experienced in precision measurements of resistivity. In order to reduce the number of variables as much as possible and to establish the validity of the test for the quality of the

probe, a probe was included with the samples so that everyone in the test

used the same mechanical equipment. In each case a standard deviation

less than 1 per cent was achieved. In 1967, a second round-robin test

was conducted. This time wafers with resistivities between 0.01 and 1000

fi-cm were included. All of the experienced laboratories obtained values

which gave a standard deviation less than 1 per cent on all wafers except

an inhomogeneous 1000 fi-cm specimen. Inexperienced laboratories participating in a parallel round-robin test encountered numerous difficulties

principally centered about sensitivity and accuracy of their electrical

measuring equipment.

The results of several round-robin tests made in the last decade are summarized in Fig. 3. The data included are for samples with resistivity

of 100 fi-cm or less. The 1967 data are based on the results of the round

robin among experienced laboratories. The beneficial effects of including controls for environmental conditions and tests for both probe and

electrical measuring systems are evident since these were included only

on the 1967 round robin.

5
While the method was being improved, a direct-reading apparatus,

which contained circuits to compensate for the diameter, thickness, and

off-center correction factors was designed and built. This instrument,

shown in Fig. 4, permits measurements to be made with a precision of 5 per cent or better over a wide range of geometrical conditions

.

1 l Corrections for both temperature and unequal probe spacing could easily be

added to the system. Another output of the project was a comprehensive

bibliography of methods of measurement of resistivity which was compiled

in 1964. 12

At the present time additional work is being carried on in cooperation with ASTM Committee F-l to extend further the range of applicability

of this method. In addition, studies are being made of the effect of

relaxation of certain of the test conditions so that the method may be

applied in circumstances other than the testing of bulk silicon wafers.

For example , in the measurement of diffused layers one requires that the

measurement be made on whatever surface one obtains after the diffusion

process takes place. Similarly, measurements on epitaxially deposited

layers require that the measurement be made on the shiny surface which

results after the epitaxial growth process. Application of this method

in these important areas of process control is extremely important to the

industry.

3. Second Breakdown

At about the same time that the Resistivity Project was undertaken

NBS was asked by Committee JS-6 on Power Transistors, of the Joint Electron Device Engineering Councils , to investigate problems associated with

the phenomenon later to be known as second breakdown. In transistors,

where second breakdown was first identified, its initiation is characterized by a sharp decrease in the voltage between the collector and emitter

together with a constriction of the current in the transistor.

The three regions of a transistor emitter, base, and collector

are doped in such a way that rectifying p-n junctions occur between each

pair of regions. In normal operation, the emitter-base junction is biased in the forward or conducting direction and the collector-base junction in the reverse or blocking direction. Most of the minority carriers,

which are injected from the emitter to the base, diffuse through the base

and are collected in the collector to form a current in the collector circuit. A small fraction of these carriers recombines in the base with

majority carriers to form a very small current in the base circuit. This

is the normal or amplifying mode of operation of the transistor. In this

mode the collector current is controlled by the much smaller base current

and, above a rather small collector-emitter voltage, is essentially independent of voltage for a given value of base current. If the emitterbase junction is reverse biased, then the current in the collector circuit is usually very small until a voltage is applied across the collector-base junction which is high enough to cause a sufficiently large

avalanche multiplication current. The transistor is then said to be

operating in the avalanche breakdown mode. If the voltage in the collector circuit is increased further, the collector-base voltage will

6
initially decrease and then approach a constant value as the collector

current continues to increase. When the collector current reaches a value, usually called Im , the collector-base voltage decreases sharply.

Further increases in the collector current occur as a result of this

decrease and the finite resistance in the collector circuit. Since this

'decrease in collector-base voltage occurs after the first or avalanche

breakdown, the phenomenon associated with the second change of voltage

was called second breakdown. It was later observed that second breakdown

could also occur in a transistor with zero or forward base-current drive,

as shown in Fig. 5, and in diodes. 13 It was also found that rather than

being unique 4 Im depended on the operating conditions of the transistor.

Although a transistor is not necessarily harmed by an excursion into

second breakdown, irreversible damage can occur if the current is sufficiently large. The characteristic catastrophic failure mechanism is a collector-to-emitter short circuit which arises because the current tends

to become localized in a small region of the transistor which is then

melted by the large localized dissipation of power. At the time NBS was asked to investigate the problem a number of conflicting models for

second breakdown had been proposed and it was impossible to specify operating conditions which would avoid the phenomenon.

As with the resistivity project, work began with visits to various

companies interested in the problem in order to establish its extent and

the scope of effort which would be required. Initial experimental work

demonstrated that all the existing models were inadequate to^ explain the

effect and showed that second breakdown was a characteristic of all transistors. 13 Pulse measurements of second breakdown revealed the existence

of a delay time between the start of the pulse and the initiation of

second breakdown as shown in Fig. 6. This observation led to the concept

of a triggering energy for second breakdown dependent on the operating

condition of the transistor which formed the basis of a new type of specification for maximum operating conditions free of second breakdown. 14 ' 15

This type of specification is now almost universally used in the industry. In Fig. 7, which shows the essential features, the maximum safe

dwell times are given for the amplifying mode of operation of a transistor. The operating limits governed by the lines with a slope of -1 show

where the transistor is limited by straightforward thermal considerations.

Regions where the slope is steeper are those where the transistor is

second breakdown limited. If most high power and high frequency transistors are to be utilized at all realistically, they must be operated under

conditions where second breakdown could occur if certain precautions were

not observed. Thus, as capabilities of transistors have been extended in

power and frequency these specifications have increased in importance.

Some of the work which was carried out during this project was directed toward better defining the conditions under which second breakdown can occur. Observations of the current distributions in transistors

under various conditions of operation were made with the use of temperature-sensitive phosphors. 16 The temperature of the device will increase

in areas of high current density. The current constriction associated
with second breakdown was demonstrated by these experiments. It was also

shown that the site of the constriction occurs in a region of high cur- rent density which can result from various types of structural irregularities in the transistor. Some of the structural irregularities that can significantly alter current distributions are localized regions of reduced base width, 17 regions of reduced collector avalanche breakdown voltage, poor or nonuniform heat sinking of the transistor wafer, diffusioninduced dislocations, 18 and precipitates.

Despite the better characterization of the phenomenon, improved

specifications for avoiding it , and the development by the industry of

device designs and circuit conditions which reduce the incidence of sec- ond breakdown, the problem is still not under complete control. Further,

because of the complexity of the interacting mechanisms believed to be

important, the phenomenon itself is not yet fully understood. Several

invited papers have been prepared at various times in the last several

years in order to summarize and review the status of the second breakdown

field. 1

5

» 19 » 20 In 1967, a comprehensive review21 and a comprehensive

bibliography22 of work in the field of second breakdown were compiled and

published. In addition, tutorial lectures have been presented throughout

this period at a number of meetings and liaison has been maintained with

Committee JS-6 regarding various aspects of specification and data formats related to the second breakdown problem.

4. New Activities

Specification of Germanium Several years ago the scope of the overall program was expanded in several directions. For some time a group

had been engaged in the fabrication of lithium-drifted germanium and sili- con nuclear radiation detectors. These detectors are p-n junctions which

have extremely wide high-resistance region between thin p-type and n-type

layers. This region is formed by the compensation of residual impurities

in this volume with lithium atoms. By the end of 1965 serious difficulties in attempting to fabricate lithium-compensated germanium detectors

were encountered at many laboratories. The origin of the difficulties

was traced to the starting materials used in the fabrication of the devices, but the exact cause was not identified. Although suitable material, primarily from foreign suppliers, has subsequently become available,

neither the cause of the past trouble nor a satisfactory means of identifying "poor" quality material prior to the drifting of lithium has been

discovered. The problem, then, was one of developing specifications for

germanium material which could be used in selecting this material for

application to lithium-drifted detectors.

Only a few properties were customarily specified at the time of the

start of this program. These included room temperature resistivity, dislocation density, and minority carrier lifetime as determined from meas- urement of photoconductive decay. It was apparent that knowledge of

these few properties was not sufficient to discriminate between good

quality and poor quality crystals. A series of isolated experiments at

various laboratories had suggested that some other parameters such as lithium precipitation rate2 ^ or low-temperature electron drift mobility2
might be correlated with material quality. The approach25 adopted was to

characterize a sizable group of germanium crystals as fully as possible

with respect to the standard parameters such as resistivity, etch pit

density, and lifetime as well as other parameters such as Hall mobility

at various temperatures , electron drift mobility at liquid nitrogen temperature, infrared absorption due to interstitial oxygen, and lithium

precipitation and drift rates. After a detector was fabricated from the

characterized crystal by means of a standardized procedure, its performance was determined by measuring a number of device characteristics such

as leakage currents, noise, capacitance, energy resolution, and charge

collection efficiency, in order to determine whether correlations exist

between detector quality and initial material properties.

In addition to the experimental work now going on, ASTM Committee

F-l was assisted in setting up a task force to study germanium properties,

This task force is now developing a standard method for measuring the

drift rate of lithium in germanium. In its original version, this procedure required a drift time of 100 hours or more. A new version of the

method is being prepared to incorporate findings from experimental work

at NBS 26 which enable one to predict the mobility from the measurement of

junction capacitance during the first few hours of the drift. It was also shown that characteristics previously attributed to low drift mobility of the lithium ion may also be caused by precipitation of lithium

atoms during the drifting process . The questions related to the specification of germanium for this application are not completely resolved at

the present time and it is expected that considerable additional work

will need to be done.

Cai'vier Lifetime In 1966, a second project was undertaken in order

to study the relevance of the existing standard method for measuring car- rier lifetimes in semiconductors—the method of photoconductive decay. 2 In this method, carriers in excess of the equilibrium density are generated in a block of semiconductor material by means of a pulse of light of

sufficient energy to excite electron-hole pairs across the forbidden

energy gap. After the pulse is abruptly terminated, the decay of con- ductivity is measured. The time constant of this decay is associated

with the time constant of the decay of the excess carriers. In cases where each electron recombines directly with a hole , the determination of

lifetime is unambiguous. On the other hand, if the recombination takes

place through impurity centers and if the charge density of the impurity

center population changes during the recombination process , then the

electron and hole lifetimes need not be the same. Under these circumstances the decay time associated with the photoconductivity may be different from the lifetimes associated with either carrier population.

Since this mode of recombination—through impurity centers--is the prevalent one in both germanium and silicon, the possibility that the photoconductive decay technique is not measuring the true lifetime of the

minority carrier, is a real one. Since it is the minority carrier lifetime that is important in the operation of junction devices it is desirable to establish convenient ways of measuring this quantity more directly.
In the project attempts are being made both to define better the

conditions appropriate to the measurement of the photoconductive decaytime and to establish other means for measuring lifetime. Experimental

work on several aspects of the diode recovery technique, on the spectral

dependence of the surface photovoltage , and on steady state photoconductivity and photomagnetoelectric effects has been initiated. The output

of the first phase of this project has been a preliminary delineation of

the conditions which affect photoconductive decay measurements28 together

with a comprehensive bibliography on methods of measuring minority car- rier lifetime. 29 The second phase of the project, which is now in progress, involves further delineation of the characteristics of the photoconductive decay method together with measurements which enable the results obtained by the various techniques to be compared.

Resistivity Irihomogeneity A third project was designed to investigate the homogeneity of silicon wafers. As larger and larger areas of a single wafer are utilized for high power devices, nuclear radiation detectors , or large area integrated circuits , homogeneity of the material

becomes more and more important. Since the most important single characteristic is the resistivity, work was undertaken to establish ways of

determining resistivity homogeneity on semiconductor wafers. This had

been done previously with the four-probe method but it had been shown

that this method is not sensitive to variations which occur over regions

with linear dimensions smaller than about one-fourth the probe spacing.'7 A number of years ago a photovoltaic method was introduced for use on thin germanium filaments. 30 The initial objective of the project was to

verify the use of this method on germanium filaments and extend it to

silicon filaments. The feasibility of applying this technique to silicon

at least on a laboratory scale was demonstrated quite quickly, and it was decided to extend this study to include uncut wafers during the first

phase. Detailed analysis of measurements on whole wafers are complicated

by the fact that the illuminated portion of the wafer is shunted by un- illuminated portions. Since the theoretical analysis of this problem had

not yet been carried out, the preliminary experimental results were ana- lyzed by ignoring the shunting effects. Both heavily-doped p-type re- gions diffused into a p-type wafer in specific pattern, and the core which occurs in most high resistivity n-type silicon were observed in

qualitative fashions. The results obtained in these preliminary experiments were sufficiently encouraging that it was decided to continue the

project along these lines. Also as a part of the first phase, a bibliography of methods of measurement of various kinds of inhomogeneities in

semiconductor wafers was compiled and published.

Other Areas In the process technology area, facilities are being

expanded to include diffusion, sputtering, and photomasking operations in

addition to the alloying, evaporation, and contacting facilities which

had been required for earlier projects. Studies of various types of

measurement techniques for in-process control are being considered.

A major effort has been mounted in the study of methods of evaluating wire bonds to semiconductor wafers. Failure of the bond structure

is the largest single cause of the failure of low-power semiconductor

10
devices in high reliability applications. At the request of several

other government agencies , detailed studies of measurement methods appropriate to the evaluation of wire bonds were undertaken.

5. Present Program

If high device reliability is required in a system, as for example

in military and space applications, some sort of assurance that a particular device will perform adequately must be obtained before the system

is assembled. At the present time the most common way of achieving this

is with the use of screening tests at the end of the device line. In

addition to being expensive, this approach is frequently unsatisfactory

since it is virtually impossible to develop tests which will simulate all

of the conditions subsequently encountered by the circuit in the field.

When one adds to this the possibility of operator error in performing the

tests or improper design of test conditions the failure rate observed may

be abnormally large.

A broader approach is to develop techniques which will help to build

reliability into devices . This approach covers the entire spectrum of

device fabrication procedures rather than just a testing sequence on the

finished device and forms the basis of the Program on Methods of Measurement for Semiconductor Materials, Process Control, and Devices which is now being undertaken in the Division. The three major tasks of this program are (1) to evaluate the technical adequacy of relevant measurement

methods developed in industrial or agency organizations and make recom- mendations for necessary modifications, (2) to develop specific methods

in accordance with the guidance received from participating groups , and

(3) to provide to standardizing groups in industry and government agencies such technical or other assistance as is appropriate to encourage

rapid development, publication, and implementation of improved measurement method standards.

The concepts and methodology of this program have evolved from the

earlier projects described above. One of the key elements is cooperation

and liaison with both manufacturers and users. This is essential in the

development of realistic, useful standards and is accomplished through

participation in technical committees of organizations such as the ASTM

and the Electronic Industries Association, through consultations with and

sponsorship from various government agencies , and through personal con- tacts in industrial plants and government laboratories. These contacts

also serve to provide project staff members with a broader view of the

problems faced by industry and help in insuring that the output of the

projects will be usable in the field.

It is possible to select only a few properties for detailed study.

The primary objective of such study is the development of a technical

basis for a standard method for measuring the property to an established

precision. Since existing methods form the basis of these studies, an important step taken soon after a study begins is a thorough review of

the state-of-the-art . Based on this review particular methods are

11
selected for detailed investigation. In addition bibliographies on methods of measurement of a given property and summary review papers are prepared as may be appropriate. Because the program is rather small when

considered relative to the total R S D capability of the semiconductor

device industry, the areas which must be emphasized are those where con- siderable leverage can be applied and the value of the work extended

beyond direct application of the results of specific experimental investigations.

In the case of the resistivity project, the usefulness of the method

to the industry has been documented in a benefit-cost analysis conducted

in 1967 as part of a Bureau-wide study. Improvements in other measurement techniques are expected to yield corresponding value to manufacturers

and users alike and will assist in solving the problem of securing high

reliability devices for specialized applications.

12
6. References

1. J. C. Irvin, "Resistivity of Bulk Silicon and of Diffused Layers in

Silicon," Bell System Tech. J. 41, 387-410 (1962).

2. D. B. Cuttriss , "Relation Between Surface Concentration and Conductivity in Diffused Layers in Germanium," Bell System Tech. J. 40,

509-521 (1961).

3. F. Wenner, "A Method for Measuring Earth Resistivity," Bull. Bur.

Std. 12, 469 (1915); L. B. Valdes , "Resistivity Measurements on Germanium for Transistors," Proa. IRE 42, 420-427 (1954); F. M. Smits

,

"Measurement of Sheet Resistivities with the Four-Point Probe," Bell

System Tech. J. 37_, 711-718 (1958).

4. See, for example, Method A of "Methods of Test for Resistivity of

Semiconductor Materials" (ASTM Designation: F43-67T) 1968 Book of

ASTM Standards, Part 8.

5. L. J. Swartzendruber , "Calculations for Comparing Two-Point and FourPoint Probe Resistivity Measurements on Rectangular Bar- Shaped Semiconductor Samples," NBS Tech. Note 241, June 1, 1964; J. M. Reber,

"Potential Distribution in a Rectangular Semiconductor Bar for Use

with Four-Point Probe Measurements", Solid-State Electron. 7,

525-529, 945 (1964).

6. L. J. Swartzendruber, "Correction Factor Tables for Four-Point Probe

Resistivity Measurements on Thin, Circular Semiconductor Samples,"

NBS Tech. Note 199, April 15, 1964.

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

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

"Temperature Coefficient of Resistivity of Silicon and Germanium

near Room Temperature," Solid-State Electron. 11, 639-646 (1968).

9. "Method of Test for Resistivity of Silicon Slices Using Four Pointed

Probes" (ASTM. Designation: F84-68T) 1968 Book of ASTM Standards3 Part 8.

10. W. M. Bullis, "Standard Measurements of the Resistivity of Silicon

by the Four-Probe Method," NASA-CR-86032, Dec. 29, 1967. (Available

from the Clearinghouse for Scientific and Technical Information

(CFSTI), Springfield, Va. 22151, Accession No. N68-18067).

11. L. J. Swartzendruber, F. H. Ulmer, and J. A. Coleman, "Direct-Reading Instrument for Silicon and Germanium Resistivity Measurement,"

Rev. Sci. Instrum. 39, 1858-1863 (1968).

13
12. J. C. French, "Bibliography on the Measurement of Bulk Resistivity

of Semiconductor Materials for Electron Devices," NBS Tech. Note

232, October 21, 1964.

13. H. A. Schafft and J. C. French, " 'Second Breakdown' in Transistors,"

IRE Trans. Electron Devices ED-9 , 129-136 (1962).

14. H. A. Schafft and J. C. French, "Breakdown Characteristics of Semiconductor Materials," Electro-Technology 75_, No. 6, 77-82, June 1965.

15. H. A. Schafft, "Avoiding Second Breakdown," Proc. XIII Inter. Sci.

Congress on Electronics3 Rome, June 1966, pp. 119-130.

16. H. A. Schafft and J. C. French, "Second Breakdown and Current Distributions in Transistors," Solid-State Electron. 9_, 681-688 (1966).

17. H. A. Schafft and J. C. French, "Characteristics of Second Breakdown

and Transistor Failure," Physics of Failure in Electronics, M. F.

Goldbert and J. Vaccaro , Eds. (Baltimore, Spartan Books, 1963) pp.

104-107.

18. H. A. Schafft, G. H. Schwuttke, and R. L. Ruggles , Jr., "Second

Breakdown and Crystallographic Defects in Transistors," IEEE Trans.

Electron Devices ED-13 , 738-742 (1966).

19. H. A. Schafft and J. C. French, "A Survey of Second Breakdown," IEEE

Trans. Electron Devices ED-13 , 613-618 (1966).

20. H. A. Schafft, "Second Breakdown in Transistors," Proc. Second

Symposium on Reliability in Electronics3 Budapest, October 1968,

pp. 225-1 to 225-14.

21. H. A. Schafft, "Second Breakdown—A Comprehensive Review," Proc.

IEEE 5_5, 1272-1288 (1967). .

22. H. A. Schafft, "Second Breakdown in Semiconductor Devices—A Bibliography," NBS Tech. Note 431, October 1967.

23. R. J. Fox, "Lithium Drift Rates and Oxygen Contamination in Germanium," IEEE Trans. Nucl. Sci. NS-13 , No. 3, 367-369 (1966).

24. G. A. Armantrout, "Correlation Between Lithium Drift Mobility and

Minority-Carrier Drift Mobility in Germanium," IEEE Trans. Nucl.

Sci. NS-13 , No. 3, 370-372 (1966).

25. W. M. Bullis and J. A. Coleman, "Characterization of Germanium and

Silicon for Nuclear Radiation Detectors," Nucleonics in Aerospace,

P. Polishuk, Ed. (New York, Plenum Press, 1968) pp. 166-175.

26. A. H. Sher, "Lithium Ion Drift-Mobility in Germanium," J. Appl. Phys.

40, 2600-2607 (1969).

14
27. "Standard Method for Measuring the Minority Carrier Lifetime in Bulk

Germanium and Silicon" (ASTM Designation: F28-66) 1968 Book of ASTM

Standards> Part 8.

28. R. L. Mattis, W. E. Phillips, and W. M. Bullis , "Measurement and Interpretation of Carrier Lifetime in Silicon and Germanium" AFML-TR68-81, July 1968. (Available from CFSTI, Springfield, Va. 22151,

Accession No. AD671552).

29. W. M. Bullis, "Measurement of Carrier Lifetime in Semiconductors—An

Annotated Bibliography Covering the Period 1949-1967," NBS Tech. Note

465, November 1968.

30. J. Oroshnik and A. Many, "Quantitative Photovoltaic Evaluation of

the Resistivity Homogeneity of Germanium Single Crystals," SolidState Electron. 1, 46-53 (1960).

31. H. A. Schafft, "Photovoltaic Method for the Measurement of Resistivity (Inhomogeneities) in Germanium and Silicon," RADC-TR-68-64,

June 1968. (Available to qualified users from Defense Documentation

Center, Alexandria, Va. 22314, Accession No. AD835778.)

32. H. A. Schafft and S. G. Needham, "A Bibliography on Methods for the

Measurement of Inhomogeneities in Semiconductors," NBS Tech. Note

445, May 1968.

33. Jointly supported by NBS, Defense Atomic Support Agency, National

Aeronautics and Space Administration (Electronics Research Center),

U. S. Navy Strategic Systems Project Office. Progress reports are issued quarterly. The first four (NBS Tech. Notes 472, December

1968, 475, February 1969, 488, July 1969, and 495, September 1969) cover the period between July 1, 1968 and June 30, 1969.

15
( Q ) FOUR- PROBE METHOD

(b) TWO - PROBE METHOD

r

v_i

w

F

I i

Where:

P- Resistivity

V = Potential

Difference

I = Current

S = Probe Spacing

w = Width

t = Thickness

I = Length

Figure 1 - Geometrical Arrangements for Four-Probe and TWo -Probe Methods

for Measuring Resistivity of Semiconductor Crystals.

16
Figure 2 - Typical Four-Point Probe with 1.6 -ram Spacing.

17
100

o

<

t

CO

0.1 y

-INDUSTRIAL ROUND ROBIN PRIOR TO

AN A.S.T.M STANDARD

ROUND ROBIN TEST

OF A.S.T.M. STANDARD

ADOPTED 1964

VALUE DESIRED FOR MATERIALS

EXCHANGE - 1%

A.S.T.M. ROUND ROBIN \ i TEST OF N.B.S. METHOD -U

1

58 60 62 64

YEAR

66 68

Figure 3 - History of the Interlaboratory Precision of Measurement of

Silicon Resistivity in the Range 0.01 to 100 fi-cm, by the

Four-Probe Method, Showing the Spread of the Sample-Dependent Standard Deviations by the Vertical Bars.

18
-P

ftft <O

ftI &o

fthO •H

T)

CT5 <D ftI +JO

0)&

•HQI

CD

bO •H

ft

19
I.

SECOND

BREAKDOWN

w

^~V°)

^VR)

F o

vCE

Figure 5 - Sketch of the swept V - I characteristics of a transistor

with constant forward IF), zero (0), and reverse (R) base

current drive. The characteristics are drawn for only the

first half of the sweep cycle. Second breakdown is initiated

at currents I„(F)„ MMI (0), and I (R).

M

20
-k LJ

I-

'30 I A c u o m o H a; 3 CO bO Po

a)

p

k

v

IT)

0) oo

•H

,c a) P

a)e o

•H P P

Mh co o -HH O ft • •H (tf H Pown) c -h e

Mh E-h -P

ft! a

re! o ) -H ) p

rd Ok) 3

T3 > H o

£1op

a) CO P

U3

CD -Pm

•HH

•HC

•H

CO •Hc

<u o

rd

M P rd i—I

a)

O

U > A 3

ft

<D

bO •H

U-,

21
Figure 7 - A presentation of safe areas of operation for forward-base

drives indicating the regions safe for dc operation and for

pulsed operations with dwell times t , t , and t„. Logarithmic

scales are used to show more clearly the voltage range over which the transistor is thermally limited in the usual sense

(where the slope of the line has a vlue of -1) and the range

over which the transistor is second-breakdown limited.

22
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