AlllOD TfiSaSD

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PUBLICATIONS

NATL INST OF STANDARDS & TECH R.I.C.

A1 11 00985250

Blackburn, David L/An automated photovol QC100 .U57 V400-52;1979 C.I NBS-PUB-C 19

00 Q NBS SPECIAL PUBLICATION 400-52

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards
NATIONAL BUREAU OF STANDARDS

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Semiconductor Measurement Technology:

An Automated Photovoltaic System

for the Measurement of Resistivity Variations

in High-Resistivity Circular Silicon Slices

David L. Blackburn

Electron Devices Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Washington, D.C. 20234

This activity was supported by

The Department of Energy

Division of Electric Energy Systems

Washington, D.C. 20545

U.S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Jr., Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued November 1979
Library of Congress Catalog Card Number: 79-600178

National Bureau of Standards Special Publication 400-52

Nat. Bur. Stand. (U.S.), Spec. Publ. 400-52, 41 pages (Nov. 1979) CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1979

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

Stock No. 003-003-02138-5 Price $2

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

ir U.S. GOVERNMENT PRINTING OFFICE: 1979 O—311-065 (266)
TABLE OF CONTENTS

Page

Abstract 1

1. Introduction i

2. System Construction 2

3. System Operation 14

4. References 17

Appendix A 19

Appendix B 26

LIST OF FIGURES

1. Automated photovoltaic system for the measurement of resistivity variations in high-resistivity circular silicon slices 3

2. Electrical switching circuitry 6

3. Motor-drive circuitry 7

4. Laser support, light chopper, x-y stage, and mirror assembly 8

5. A pneumatic probe-lowering assembly 9

6a. Composite drawing of specimen holder lU

6b. Details of parts 1, 2, and 3 of the specimen holder 11

6c. Details of parts 4 through 11 of the specimen holder .... 11

6d. Details of part 12 of the specimen holder 12

6e. Further details of probe assembly 12

6f. Details of apparatus for centering slice 13

6g. Details for air distribution system for controlling pneumatic

probe-lowering apparatus 13

7. Slice geometry used for van der Pauw average-resistivity measurement 16

iii
Disclaimer

Certain commercial equipment, instruments, or materials are identified in

this report for the purpose of providing a complete description of the

work performed. The experiments reported do not constitute a complete

evaluation of the performance characteristics of the products so identified. In no case does such identification imply recommendation or en- dorsement by the National Bureau of Standards, nor does it imply that the

material or equipment identified is necessarily the best available for

the purpose.

iv
SemiaonduGtov Measurement Teohnology:

An Automated Photovoltaic System for the Measurement of

Resistivity Variations in High-Resistivity

Circular Silicon Slices

ABSTRACT

This report describes an automated photovoltaic system for non- destructive measurement of resistivity variations of highresistivity circular silicon slices. The computer-based system

for making the measurements is described, detailed construction

diagrams are given to facilitate reproduction of the system, and

a listing of the computer program for controlling the system is

given. Comparisons between resistivity profiles determined using the automated photovoltaic system and the four-probe technique indicate that the photovoltaic system is adequate for production screening and incoming inspection of high-resistivity

float-zoned silicon slices.

Key Words: Automation; bulk, photovoltaic effect; computer control; homogeneity; measurement method; nondestructive testing;

photoconductivity; power devices; resistivity variation.

1 . INTRODUCTION

The overall objective of this task was to evaluate the feasibility of the

photovoltaic method [1,2 J as a rapid, nondestructive technique for characterizing the resistivity uniformity of high-resistivity, large-diameter

silicon slices intended for fabrication of high-power devices. In the

photovoltaic technique, a light spot is scanned along a slice diameter.

The photovoltage and photoinduced change in specimen resistance are measured as a function of position along the slice diameter. From these two

measurements, the variation in resistivity along the diameter can be computed.

Most of the results of this work have already been published [3-6 J. A

copy of reference [3] which contains a brief description of the theory of

the bulk photovoltaic effect, a description of the measurement procedure

and the apparatus employed, and a discussion of the results obtained for

float-zoned (FZ) silicon is included as Appendix A to this report. Discussion of the characterization of the photovoltaic system, the illumination wavelength, surface finish, measurement time, theoretical analysis,

experimental results, and extension to neutron transmutation doped (NTD)

silicon is given elsewhere [6]. A more detailed description of the

mathematical analysis has been prepared for separate publication [7j.

This report provides the details necessary to replicate and use the mea- surement system. In the next section, the computer-based system for making the measurement is described, and detailed construction diagrams are given to facilitate reproduction of the system. Procedures for using the

1
system are outlined in section 3. The complete listing of the computer

program for controlling the system is given in Appendix B.

2. SYSTEM CONSTRUCTION

A picture and diagram of the completed photovoltaic system are shown in

figure 1. A simplified block diagram can be found in Appendix A, figure

3.

In table 1 are listed the major components of the system which were com- mercially purchased. The multiprogrammer is an integral part of the

computer/controller system.

A diagram of the electrical switching circuitry employed for the measurements is shown in figure 2. The digital input boards (Hewlett Packard

Model 69431A) and relay boards (Hewlett Packard Model 69433A) are physically located within the multiprogrammer. The digital panel meter is used

to measure the current in the specimen during the measurement of the photoinduced change in specimen resistance, AR, as well as to measure the

current and voltages for the measurement of the van der Pauw average slice

resistivity

.

A diagram of the circuitry to control the stepping motor is shown in fig- ure 3. Both the relay board and motor control card (Hewlett Packard Model 6y335A) are physically located in the multiprogrammer.

The mechanical layout of the laser support (constructed at NBS), light

chopper (commercial), x-y stage (commercial), and mirror assembly (constructed at NBS) is shown in figure 4. The support metal is 1/2-in.

(12.7-mm) thick aluminum. The light chopper frequency was adjusted to a nominally 4U0-Hz chopping rate and the lock-in amplifier tuned to that

frequency

.

A drawing of a pneumatic probe-lowering assembly (constructed at NBS) is

shown in figure 5. The system has four such assemblies. Each probe assembly contains two side-by-side spring-loaded probes (one is to carry

the current for the photoinduced change in resistance measurement, the

other to measure voltage). The pneumatic system assures that all eight

probes are lowered simultaneously onto the surface of the slice. The entire pedestal assembly can move left or right in the figure. All four

pedestals are equidistant from the center of the slice support and move in

unison via a worm gear controlled by the system user. This permits automatic centering of the slice for measurement.

Drawings of the specimen holder (constructed at NBS) and its constituent

parts including the pneumatic probe-lowering assembly are shown in figure

6. Except where noted, all parts are constructed of brass and the dimensions are given in inches.

Figure 6a is a composite drawing top view and side view of the specimen

holder. The front probe assembly has been removed for clarity in the

side view. The circled numbers in the side view refer to the various
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RETURN SPRING

PROBE ASSEMBLY -

SLICE

I

NEOPRENE DIAPHRAGM

SLICE SUPPORT

STAGE

REGULATED AIR

DOVETAIL SLIDE

-PEDESTAL

Figure 5. A pneumatic probe-lowering assembly.

9
Figure 6a. Composite drawing

10

of specimen holder.
Figure 6b. Details of parts 1, 2, and 3 of the specimen holder.

4-40 TAP h-75-*| 7

MAKE 4 BRASS

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Figure 6c. Details of parts 4 through 11 of the specimen holder.

11
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Figure 6e. Further details of probe assembly (part 5) .

12
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SECTION B-B

WORM, PINION a RACK ARRANGEMENT

SECTION A-A Figure 6f . Details of apparatus for centering slice,

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DRILL a TAP 12-24

MAKE ONE

BRASS Figure 6g. Details for air distribution system for controlling pneumatic probe-lowering apparatus.

13
components for which detailed drawings follow. Figure 6b shows the details of parts 1, 2, and 3. Figure 6c shows details of parts 4 through

11. A drawing of part 12 is shown in figure 6d. The circled numbers 1 through 5 on this drawing refer to the various components of the assembly

in this figure. In figure 6e, a further detailing of part 5 is shown.

The circled numbers on this drawing also refer to the items of the assembly on the figure. The details of the worm, pinon, and rack arrangement

for centering the slice are shown in figure 6f. Finally, the details of

the air distribution valve which controls the air supplied to the pneumatic probe- lowering system (part 12) are shown in figure 6g.

3. SYSTEM OPERATION

The sequence of events for making a measurement of the resistivity gradient using the photovoltaic system is as follows:

1. The operator places the slice onto the specimen holder, turns

the screw thread to bring in the probe assembly in unison to

center the slice, and lowers the probes pneumatically onto the

slice

.

2. The operator loads the program to control the measurement (see

Appendix B) into the memory of the computer.

3. The operator instructs the computer to start the measurement by

depressing the start key on the computer.

4. The computer displays a series of messages asking for the following information:

- specimen identification

- date of measurement

- time of measurement

- specimen type {n or p)

- slice diameter (cm)

- slice thickness (cm)

- slice average resistivity* (f^.*cm) - number of measurement points

- distance between measurement points (cm)

- lock-in amplifier time constant (s) - request if user desires to make a series of measurements on the slice center (where the light probe is located at the

beginning) to assure that the proper time constant and

sensitivity of the lock-amplifier is being used.

The operator supplies all of the above information by typing the

appropriate responses on the calculator keyboard. For produc-

*See below for a procedure to obtain this quantity.

14
tion measurements, most of these responses could be stored in

the computer memory before the measurement begins.

5. After the last response, the computer instructs the motor translator to move the specimen stage so that the light probe is at

the end of the specimen diameter where the measurement is to begin.

6. The stage moves to the beginning point and the printer-plotter

prints out the information on specimen identification, date,

etc.

7. At the first measurement point, the computer instructs the stage

to wait lU lock-in amplifier time constants. After this time

has passed, the lock-in amplifier is instructed to make a mea- surement and display the photovoltage measured at that point and

to relay the value for storage in the computer.

y. The motors are instructed to move the stage to the next measurement point and wait 10 time constants. The lock-in amplifier is

again instructed to display and relay its measurement. This se- quence is repeated until all the specified number of measurements has been made.

9. When the light probe has reached the other end of the specimen

diameter from which the measurement begins, the photovoltage

scan is completed. The user is instructed by a display to turn

on the constant current supply and to adjust it to the desired

value (typically 1.0 mA for 10 <^ P <^ bO ^'cm and 0.1 raA for 60 <_ p <_ 200 9.' cm),

10. The operator is asked by a displayed message if a one-time voltage sampling is again desired to assure the proper lock-in am- plifier sensitivity is being used. Usually, the sensitivity is

reduced by one to two orders of magnitude from that used for the

photovoltage measurement to make the measurement of photoinduced

change in specimen resistance.

11. After the proper sensitivity is selected, the measurement can begin. By depressing the continue key, the operator instructs

the computer to begin the measurement. The measurements of the

photoinduced change in specimen resistance are made at the same locations as were the photovoltage measurements, but the stage

is traveling in the opposite direction. The motors are instructed to wait 10 time constants at each measurement position

before a measurement is made.

12. When the light probe reaches the last point of the measurement,

the computer begins calculating the resistivity variations at

each point and performing the summation as described in section

3 of Appendix A. These results are listed by the printerplotter. The resistivity is computed by adding a constant to

15
Table 2. Steps for the Measurement of Average Resistivity.

Step Number

Current

Between Probes Quantity Measured Symbol

1 'J 0 _4- L.urrenc ^12

2

1 1 i+, / — Voltage between V34

terminals 3 and

if 9-1- Current I2I

4 . 1-, 2+ Voltage between ^43

terminals 3 and 4

5 2+, 3- Current ^23

0 2+, 3- Voltage between ^41

terminals 4 and 1 7 2-, 3+ Current ^23

QO 2-, 3+ Voltage between Vl4

terminals 4 and 1

p = 1.1331 • t

• V + V

34 43

[ '21

V + V

' 41 14

I 23

»

where t is the wafer thickness in cm.

Figure 7. Slice geometry used for van der

Pauw average-resistivity measurement.

16
each computer value so their average equals the van der Pauw

average resistivity,

13. The computer then asks the operator via the display if a plot of

resistivity versus position is desired. If so, the user changes

paper and the plot is generated by the printer-plotter.

The system has also been programmed to measure the average specimen re- sistivity using the van der Pauw technique. If required, this measurement is performed between steps 1 and 2 above. The measurement sequence

is

1. The operator loads the van der Pauw measurement program into the

memory of the computer.

2. The computer asks, via the display, what the specimen thickness

(in cm) is. The answer is typed in by the user on the computer

keyboard.

3. The computer tells the user, via the display, to turn on the

constant current source and adjust it to the desiied value,

which is typically 0.1 mA.

4. The user instructs the computer to begin the measurement by

pressing the start button. The measurement sequence is listed

in table 2; the probes are identified by number in figure 7.

5. The computer calculates the average resistivity using the equation at the bottom of table 2.

Acknowledgments

The author thanks Mr. G. J. Rogers for his excellent efforts in assembling the automated system and in developing the required software. Dr. R. D. Larrabee for sharing his intuitive insights of the bulk photovoltaic

effect and for his precise and in-depth analysis of the effect for circular slices, Mr. F. F. Oettinger for his many helpful discussions. Dr. W. M. Bullis for his critical reading of this manuscript, and Mr. W. A.

Culllns for his craftsmanship in helping to design and in supervising the

construction of the slice holder and mechanical supports.

4. REFERENCES

1. Oroshnik, J., and Many, A., Quantitative Photovoltaic Evaluation of

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

2. Blackburn, D. L., Schafft, H. A., and Swartzendruber , L. J., Nondestructive Photovoltaic Techniques for the Measurement of Resistivity Gradients in Circular Semiconductor Wafers, J. Eleatvochem, Soe.

119, 1773-1778 (1972).

17
3. Blackburn, D. L. , Photovoltaic Technique for Measuring Resistivity

Variations of High Resistivity Silicon Slices, J. Res, Nat. Bur,

Stand,, vol. 83, No. 3, 265-271 (May-June 1978).

4. Koyama, R. Y. , Blackburn, D. L., and Oettinger, F. F., Measurement

Techniques for High Power Semiconductor Materials and Devices,

Proc. : Light-Fived Thyristor Workshop, epri Report WS-77-049, unpaged (March 1978).

5. . Blackburn, D. L. , and Larrabee, R. D., Automated Photovoltaic Technique for Nondestructively Measuring Resistivity Variations of

High-Resistivity Silicon Wafers, Semioonductor Chavaatevization

Techniques, Proceedings Volume 78-3, P. A. Barnes and G. A.

Rozgonyi, Eds, pp. 168-179 (The Electrochemical Society, Princeton,

1978).

6. Oettinger, F. F. , Ed, Measurement Techniques for High Power Semiconductor Materials and Devices: Annual Report, January 1 to December

31, 1977, NBSIR 78-1474 (May 1978).

7. Larrabee, R. D. , and Blackburn, D. L., Theory and Application of a Nondestructive Photovoltaic Technique for the Measurement of Resistivity Variation in Circular Semiconductor Slices (to be published)

.

18
APPENDIX A

JOURNAL OF RESEARCH of the National Bureau of Standards Volume 83, No. 3, May-June 1978

Photovoltaic Technique for Measuring Resistivity Variations of

High Resistivity Silicon Slices'^

D. L. Blackburn

Institute for Applied Technology, National Bureau of Standards, Washington, D.C. 20234

January 13, 1978 A description of an automated, photovoltaic system for measuring the resistivity variation of high resistivity, large diameter sihcon wafers is given. The photovohaic technique utilizes a scanning light spot to induce a bulk

photovoltage and a change in resistance from which is calculated the local variation in resistivity. This

nondestructive technique requires no contacts to the useful fabrication area of the wafer, and measured resuhs have good correlation with the results of the four-probe technique. Specific examples of measured resistivity gradients are presented along with a discussion of the tlieory, measurement conditions and limitations, and

description of a calculator-based automated system to perform the measurements.

Key Words: Automation; computer control; homogeneity; measurement method; photovoltage, bulk; power

semiconductor materials; resistivity; semiconductors; silicon; ihyristors; transistors, power.

1 . Introduction

The radial variation of resistivity is one of the principal

characteristics specified in the procurement of silicon for the manufacture of high power semiconductor devices. Resistivity variations result from an inhomogeneous doping density

in the starting silicon and both the solid state diffusion and

alloying steps used in the fabrication of high power devices

are affected by such inhomogeneities. Gross variations in resistivity can cause wide variations in device characteristics and contribute to poor device yields. Poor junction geometry and nonunifortn current distributions are frequent problems in devices fabricated froin inhoinoge- neous material. Failures due to hot spots or thermal runaway

may result. Also, localized low resistivity regions of a wafer

limit operating voltage of a device fabricated from that wafer

to values lower than those expected from the remainder of the wafer. The method commonly used for determining the resistivity variations of starting silicon wafers is the four-probe tech- nique. There are a number of difficulties and limitations with

this method. A basic limitation in the application of this method to the measurement of resistivity variations is the

relatively large sampling region of the probe [1].' Because of

this, it has been recommended that to obtain better spatial

resolution, a two-probe measurement be made along a narrow

bar cut from along the measurement diameter of interest [2]. There can also be large errors introduced into the four-probe measurement due to slight mislocations of the probes,

particularly near the wafer edge [1, 3]. The spreading

resistance technique has none of these difficulties, but it can

be very slow and time consuming. Perhaps the greatest

limitation of any probing method such as spreading resistance

or four-probe is the placement of probes onto the wafer

surface in precisely those areas where diffusions and other

* This work was conducted as part of the NBS program on Semiconductor Measurement

Technology and was funded principally by the Energy Research and Development Administration.

Divisions of Administration and Electrical Energy Systems, trhough ERDA Task Order A021-EES.

' Figures in brackets indicate literature references at the end of this paper.

processing steps are to occur. The damage caused bv the probes may itself be detrimental to the reliability and yield of devices fabricated in these slices. The photovoltaic- technique offers an alternative for measuring resistivity variations along the diameter of circular

wafers. The technique, which is based upon the theory of Tauc [4], requires no contact with the wafer surface in the area where a finished device or devices are to be fabricated. The validity of a modified photovoltaic theory for circular wafers was demonstrated several years ago [5]. The present

work has established the feasibility of automating the photovoltaic technique for the rapid measurement of radial resistivity variations in high-resistivity, large-diaineter, power

grade silicon.

2. Theory of the Photovoltaic Effect

The underlying theory of the bulk photovoltaic effect was

derived and its physical aspects discussed by Tauc [4]. The

bulk photovoltaic effect is a phenomenon which occurs when

electron-hole pairs are photogenerated in a region of an

impurity-density gradient (or, equivalently, a resistivity gradient). In such a region, an internal electric field exists in the semiconductor. This is much like the situation at a pn

junction, except that the field is much smaller than for a

junction. When excess electron-hole pairs are generated in

this region, they are separated by the internal electric field, and the steady-state distribution of the separated carriers is such that the magnitude of the net internal field is reduced.

It is the reduction in magitude of the internal field which

results in the photovoltage which can be measured at contacts made to the specimen. It is possible to relate this measured

photovoltage to the resistivity gradient in the region where

the excess electron-hole pairs are distributed. The physics of the photovoltaic effect in one dimension is described graphically in figure 1. Figure la is a representation of a typical radial resisitivity profile found in highresistivity Czochralski-grown silicon. Figure lb shows the band structure for such a resistivity variation and illustrates 19
ILLUMINATED REGION

POSITION

Figure 1. ,4 representation of (a) a typical resistivity profile, (b) the associated energy hand structure, (c) photovoltage, and (d) integrated photovoltage for an n- type specimen.

For circular specimens, where a small light spot falls on a

wafer diameter and the photovoltage is measured at the ends

of that diameter, the measured voltage also depends upon

the position of the light spot on the diameter. The equation

relating the resistivity variation to the photovoltage and

photoinduced change in specimen resistance for a circular semiconductor wafer is:

1 + 1^

df^ V{x) ^ _9

dx TvhT 1 - {x/bf bt AR{x)

where b and x are shown in figure 2 and — = resistivity gradient (11 • cm/cm)

(1)

dx

</kT

Ma/

p

V{.x)

Mi

majority carrier charge (C) Boltzmann's Constant (J/K)

temperature (K)

majority carrier mobility (cm^/V • s) minority carrier mobility (cm^/V • s) average resistivity (H-cm)

thickness (cm)

photovoltage (V)

photoinduced change in resistance (fl)

Equation (1) differs from the equation for dp/dx derived

previously [5] by a multiplicative factor of 4/3. In the course

of this work, a more exact analysis of the effect of the circular geometry upon the measured photovoltage and photoinduced

change in specimen resistance has shown that eq (1) is a

better approximation for dp/dx than that presented previously.

how the internal field (as manifested by the variation with

position of the conduction and valence band edges) causes

the photogenerated electron-hole pairs to be physically

separated. Thus, a local voltage dipole is created. This

results in a voltage measurable at the ends of the specimen.

If the light is scanned along the specimen, a voltage profile can be determined, as shown in figure Ic. If this voltage is integrated over position, assuming that the steady-state excess density of photogenerated carriers is independent of

position, and then multiplied by a constant dependent only

upon known material parameters, one obtains the profile in figure Id. This profile is identical to the variation in

resistivity, as shown in figure la. Because the entire profile

in figure Id is shifted by an unknown integration constant,

this method determines the variation in resistivity and not

the absolute resistivity at each position. It will be shown later

that a measurement of a weighted average specimen resistivity using the van der Pauw technique [6] can be employed to estimate the absolute resistivity at each position from the photovoltaic resistivity variation profile. For most specimens, the excess density of steady-state

photogenerated carriers is not constant with position. This is principally because the effective lifetime of the material is not constant but varies with position. This effect is accounted

for in this measurement technique by also measuring the photoinduced change in specimen resistance as a function of

position.

Figure 2. The wafer geometry upon which

eq ( 1 ) is based.

3. Measurement Procedure

To measure the resistivity gradient using the photovoltaic

technique, the circuit shown in simplified form in figure 3 is used. First, the photovoltage, V{x), is measured as a function 20
[ SPECIMEN

j

LOCK-IN AMPLIFIER

, COMPUTER/^

CONTROLLER

AUTOMATIC

PRINT-OUT

Figure 3. Simplified version of the measurement system.

BCD INPUT STEPPINB MOTOR CONTROLLER PROGRAMMABLE RELAYS WAFER HOLDER MOTOR DRIVEN LINEAR STASE

H MIRROR

P PROBES

R MOTOR DRIVEN ROTART STA6E W WAFER UNDER TEST

CALCULATOR/CONTROLLER

NOTE 1: THE PROBES ARE ACTUALLY PERPENDICULAR TO THE SURFACE OF THE WAFER UNDER TEST.

Figure 4. Schematic of automated measurement system.

of position. This is done by having the switch S open,

stepping the wafer beneath the Hght probe in constant increments. Ax, and measuring V(x) at each position. Next, the photoinduced change in specimen resistance,

A/? (a:), is measured. This is accomphshed by closing switch S and again stepping the wafer beneath the hght probes in increments of Ax such that each A/?(x) is measured at the

same positions as V (x). Because a constant current. /. exists

in the specimen with the switch closed, a decrease in the voltage across the specimen occurs as the light sweeps along

the diameter. This is because the total specimen resistance

is decreased due to the localized decrease in resistivitv caused by the excess electron-hole pairs generated by the

light. If the change in voltage is AV(a:), then:

21
A«W = ^. (2,

The quantities V(x) and Mt(x) are then inserted in eq (1) to compute the gradient, dp/dx, at each position, x. To deter- mine the variation in p, the plot of dp/dx is numerically

summed (Pm(x) = 2 i^i {dp/dx)i^). Thus, a plot of the

variation in resistivity is obtained. As mentioned, an absolute determination of resistivity is not obtained, but each Pm{x)

above differs from the absolute value by a constant, po, the value of the resistivity at the position the summation was

begun. To estimate the value of po, the average of all the Pmixfs (PmAVG = ^/^ Sm=i Pm(^)) is subtracted from the measured van der Pauw average resistivity, p [6]. This value

for Po is then added to each Pm{x) to determine the absolute

resistivity profile. The van der Pauw average resistivity measurement has been automated and is an intrinsic part of the measurement system.

4. Apparatus

A schematic of the automated photovoltaic measurement

system is shown in figure 4. The entire measurement system

is under the control of a computer/controller. The results of the measurements are furnished in hard copy by the printer/

plotter. He Ne Laser — A. laser, operating at 0.6328-p,m wavelength, is used as the source for generating the electron-hole

pairs in the silicon. The laser used in this work has about a 15-mW power output at 0.6328 pm. The laser can also be

operated at 1.152 pm with >2 mW of power output. It has been found, however, that the internal reflections within the

silicon at this longer, very penetrating, wavelength can

induce a barrier photovoltage at the measurement contacts which masks the bulk photovoltage generated at the light

spot position. The 0.638-/xm radiation is all absorbed very near the silicon surface and so is not reflected internally to the contact region. The laser is used because of the high energy density easily obtained within a small spot about 1 mm in diameter.

Lock-in Amplifier — The magnitude of the photovoltage is usually very small. In this work, for specimens with resistivity between 50 and 200 fl- cm, the voltage is of the order of ± 1 p\. For specimens with resistivity between 10 and 50

fl'cm, the photovoltage is of the order of ±0.01 pV. To

allow measurement of these small signals, the laser radiation

is chopped mechanically with a light chopper and a lock-in amplifier used to measure the voltage. The chopping fre- quency is nominally 400 Hz. The frequency should be as high as possible while assuring the radiation stays on the specimen long enough during each cycle for the photovoltage

to reach a steady-state value. Motor Driven Stage — The specimen is held in a specimen

holder (see next paragraph) which rests on a stepper-motor

driven stage. The stepper motors are under control of the computer and are typically stepped so that measurements are made at 1-mm intervals. Specimen Holder — A picture of the specimen holder is shown in figure 5. The holder is constructed so that measure- ment contact is made to the specimen on the top surface within about 1 mm of the rim. Contact is made at four

locations at 90-deg increments around the wafer (on perpendicular diameters) in a van der Pauw arrangement [6]. At

each contact location, two probes (similar to those used for spreading resistance or four-probe measurements) make

contact with the wafer and are separated by about 1.5 mm.

The loading of each of the probes has been approximately

1.0 to 1.5 N in this work. During measurement of the bulk

photovoltage, one of the probes at each end of the diameter

being scanned is used to measure the voltage. During the measurement of the photoinduced change in specimen resist- ance, the other two probes (one at each end) are used to carry the constant current, and the change in potential across

the specimen is measured with the same two probes as were

used to measure the bulk photovoltage. Contact is made on

perpendicular diameters to permit the measurement of the average specimen resistivity (p) by the van der Pauw tech- nique.

Figure 5. Future of specimen holder.

5. Results

Two comparisons between the resistivity profiles as deter- mined by the photovoltaic technique and by the four-probe

method will be given to show the qualitative agreement that can be obtained. The comparisons are for high resistivity

(~150 CI- cm) and lower resistivity (~10 O- cm) specimens.

The surfaces of the wafers were lapped with 12-p,m alumina. Measurements have been made on polished surfaces, but

these measurements were not as reproducible as those made

on lapped surfaces.

In figure 6 is shown a comparison between the respective

resistivity profiles as determined for a nominal 150 fl-cm ,

22
ohm-cm

r175

MEAN RESISTIVITY

165

155

151.2 ohm-cm '3 .

135/

o PHOTOVOLTAIC MEASUREMENT

t- FOUR POINT PROBE DATA

125

-25 -20 -15 -10 -5 0 5 10 15 20 25 DISTANCE FROM CENTER OF WAFER-mm

RESISTIVITY VERSUS POSITION

Figure 6. Comparison of resistivity profiles as determined by the four-probe technique and

the photovoltaic technique for a nominal ISOCl-cm n-type silicon wafer.

rt-type silicon wafer. The total time required to determine the photovoltaic resistivity profde on the 5-cm diameter wafer was 2 to 3 min. The total time required for measurement

depends upon the magnitude of the photovoltage, which was

between — 1 /xV to 1 /xY in this instance. The lock-in amplifier time constant was set to 100 ms and the light

remained at each measurement point for about 20 time

constants (i.e., ~2 s). Generally, for p > 50 fl-cm, the 100-ms time constant has been found satisfactory. For lower resistivity specimens, the signal level is usually smaller because of a decrease in dp/dx, and also perhaps

because of a decrease in minority carrier life-time. Thus, the

lock-in amplifier time constant must be increased and the measurement time increased. A comparison of photovoltaic

and four-probe resistivity profiles for a nominal 10 CI - cm n- type silicon wafer is shown in figure 7. The photovoltage for

this specimen was usually in the range —0.01 /LtV to +0.01

/LtV. A 300-ms lock-in amplifier time constant was used and

the total measurement time was about 8 min.

An indication of the reproducibility of the photovoltaic

technique is given in figure 8. In this figure are shown the data for six resistivity profiles as determined by the photovoltaic technique along the same diameter of an n-type 135

O'cm silicon wafer. The measurements were made six

different times over a period of several days. The wafer was removed and then reinserted into 'the specimen holder between measurements.

Detailed measurements to determine the spatial resolution

of the photovoltaic technique have yet to be made. This will be done by comparing photovoltaic and spreading resistance

resistivity profiles along the same wafer diameter. It is expected that the resolution will be one or two carrier

diffusion lengths. In most high resistivity material, the

diffusion length will be on the order of 0.5 to 1 mm. Thus, a

spatial resolution of no better than 1 to 2 mm may be

expected, regardless of light probe size. The minimum resistivity gradient which can be measured

depends upon a number of parameters. The more electron- hole pairs that are generated, the larger the signal for a given

gradient and thus the smaller the minimum gradient measurable. Thus, the more intense the radiation source, the better the resistivity gradient resolution. Also, the longer the carrier

lifetime, the better the resistivity gradient resolution (but the poorer the spatial resolution). In this work, the minimum

gradient measurable has usually been of the order of 1.0

fl- cm/cm (a 0.1 fi-cm gradient over the 1-mm diameter

light probe).

23
ohm-cm

T14.0

MEAN RESISTIVITY K,

10.0 ohm-cm

Oti +_

-13.0

o PHOTOVOLTAIC MEASUREMENT

+ FOUR POINT PROBE DATA

12.0

i i

4 .

.

11.0

• * : t

10.0'

* f

• w

^ 9.0

-25 -20 -15 -10 -5 0 5 10 15 20 25 DISTANCE FROM CENTER OF WAFER-mm

RESISTIVITY VERSUS POSITION Figure 7. Comparison of resistivity profiles as determined by the four-probe technique and

the photovoltaic technique for a nominal lOCl-cm n-type silicon wafer.

Several photovoltaic resistivity measurements have been

made on neutron transmutation doped (NTD) silicon. Be- cause of the low resistivity gradients and the low lifetime usually present in this material, the bulk photovoltage has been extremely small; less than 0.01 /xV. In addition, the presence of a front-to-back surface photovoltage interferes with the bulk photovoltage to such an extent that meaningful measurements on NTD material have not usually been

possible. The front-to-back surface photovoltage is present on all specimens, but is only a problem when it is of the same order of magnitude as the bulk-photovoltage. This has been observed to be the case only in NTD silicon.

6. Conclusion

An automated system based upon the bulk photovoltaic

effect for measuring the resistivity variations of high-resistivity power grade silicon has been described. A brief description of the theory of the photovoltaic effect was given.

Comparisons between the resistivity profiles as determined

using the new system and as determined by the four-probe

technique have been given as well as an indication of the

reproducibility of the photovoltaic technique. In the work

described, the measurement of variations of about 0. 1 O • cm

for a 1-mm light probe diameter was possible. The spatial resolution of the technique is expected to be about 1 to 2

minority carrier diffusion lengths. The time of measurements

is about 2 to .3 min for p > .50 H- cm and 8 to 10 min for 10

< p < 50 n-cm.

The author gratefully acknowledges the contributions of R.D. Larrabee, G. J. Rogers and F. F. Oettinger to the

results of this paper.

24
ohm-cm

[-145

• •

' s

"

• "v. [

140

:^ +

t+

+.S

t •135 r 1 MEAN RESISTIVITY * 4 . - 133.3 ohm-cnfi +

+

. + , -130 /

+++

+

+

+

+

++

+

\ 125

+

+

++

* i

i

+

4-

-120

4+4 ;

-25 -20 -15 -10 -5 0 5 10 15 20 25 DISTANCE FROM CENTER OF WAFER mm

RESISTIVITY VERSUS POSITION

Figure 8. Repetitive photovoltaic resistivity profiles made along the same diameter of an n- type silicon wafer.

7. References

[1] Swartzendruber, L. J., Four-point probe measurement of non-uniformities in semiconductor sheet resistivity, Solid-State Electronics 7, 413-422 (1964).

[2] Vieweg-Gutberlet, F., and Schiinhofer, F. X., Grensen der anwendbarkeit des 4-spitzen-gleichstrom-messverfahrens an silicium-proben.

Part I, Arch. Tech. Messen. 369, 237-240 and Part II 370, 259- 262 (1966).

[3] Swartzendruber, L. J., Correction factor tables for four-point probe

resistivity measurements on thin circular semiconductor samples,

Natl. Bur. Std. (U.S.), Tech. Note 199. 34 pages (April 1964).

[4] Tauc, J., The Theory of a Bulk Photo- Voltaic Phenomenon in Semiconductors, Czech. J. Phys. 5, 178-179 (19.55).

[5] Blackburn, D. L., Schafft, H. A., and Swartzendruber, L. J.. Nondestructive Photovoltaic Techniques for the Measurement of Resistivity Gradients in Circular Semiconductor Wafers, J. Electrochem. Soc. 119, 1773-1778(1972).

[6] van der Pauw. L. U., A Method of Measuring Specific Resistivity and

Hall Effect of Discs of Arbitrary Shape, Philips Res. Repl. 13, 1-9

(1958).

25
APPENDIX B

Typical Calculator/ Controller Program for Measuring, Recording, and

Plotting the Data Needed to Determine the Resistivity Variation of a Silicon Slice by the Photovoltaic Technique.

0: dim A$ [ 1 ] ,B$ [ 1] , C$ [ 8 ] ,D$ [ 4 ] ,E $ [ 12 ] ,F$ [ 1] ,G$ [ 3] , H$ [ 32

]

1: ent "Date ", C$ , "Time ", D$

2: ent "Wafer number", E$

3: ent "Wafer type [P or N]",F$

4: ent "Wafer diameter [cm]",D

5: ent "VJafer thickness [mm] " ,rl

6: ent "Vvafer mean resistivity [ohm cra]",r2

7: ent "Label diameter ( 1-2 , 2-1 , 3-4 , 4- 3

) " ,G$

8: ent "Length of step [mm]",L

9: ent "Number of steps [even]",S

10: ent "How many readings to be aver aged?" , r

3

11: ent "Amplifier time constant [msec]",T

12: ent "Store data in file number?", rl2

13: 6*r0 ; 0-»I->-M->-Q*X-<-Y-«-Z-*rB->-r9 ; 1-*N;1000* J ; 10 0*L-A

14: if G$="l-2" ;4402*rl0; 3000*rll

15: if G$ = "2-l" ; 3002-rlO; 3000-«-rll

16: if G$="3-4" ;222-rl0;4400*rll

17: if G$="4-3" ;142H.rlO; 4400*rll

18: ent "Plot 4-point probe data? (Y/N)",A$

19: if A$="N";jmp 14

20: dsp "INSEPT DATA CARTRI DGE" ; beep ; s tp

21: ent "Number of data points", r8

22 : dim G[r 8+1] ,L [r8 + l]

23: ent "Data file number", r9

24: if r8=17;l*rl3

25: if r 8=33; 4 0-«.rl3 26: if r8=35;62*rl3

27: if r8=37;50*rl3

28: if r8=41;63*rl3

29: Idf rl3,L[*];ldf r9,G[*];l*N

30: if G[N1] >M;G[N]*M

31: if G[N] <J ;G [N]*J

32: if (N+1--N) <r8 + l; jmp -2

33: dsp "WAFEF CENTERED? LINEAR STAGE? "; beep ; s tp

34: dim A [S + 2] ,B [S + 2] ,C [S + 2] ,D [0 :S + 2] ,E [S + 2] ,F [0 :S + 2]

35: dim J [ S + 2 ] ,K [ S + 2 ] , M [ r 3+1 ] , P [0 : S + 2

]

36: fmt 1 , c, f 4 . 0 , c, z ;wr t 7 2 3 . 1 , " 01 6 OTI?" , r 11 + 31 , " T04 OT"

37: ent "Calibrate? (Y/N)",A$

38: if A$="Y";gsb "calibrate"

39: if A$="N";jmp 3 40: ent "Continue calibration? (Y/N)",B$

41: if B$="Y" ; jmp -3

42: 1-N; (N-l-S/2) *L-»-P [N] ;dsp "BEAM MOVING TO 'START' POSITION"

43: 100*L* (S/2+l)*B; if B<4096;jmo 2 44: B-4095-«-B;7777-Y

45: dtoB+Z;fmt 1 , c , z ; wr t 7 2 3. 1

, "O160TG0002TO40T" ;g sb "motor"

46: fmt I,c5,cl0;wrt 6 . 1

, " Date : " ,C$

47: fmt 1 , c5 , c7 , c27 ; wr t 6. 1 ," Time :", D$ , "WAFER"

48: fmt 2 , c25 , cll , cl 3 , c3 ; wr t 6.2, "Number :",E$,"Type :",F$

49: fmt 3 , c25 , f 6 . 2 , c3 , cl5 , f 7 . 3 ,c3

*31690

26
0: reserves memory space for identifying data (date, slice number, etc.)

1-12: asks operator to enter slice identification and characteristics and conditions of test

13: assigns initial values to variables

14-17: selects current and voltage relays to correspond with diameter chosen in 7

18-32: provides for copying four-probe data from tape cassette

33: reminds the operator to center the slice and energize linear stage

34-35: reserves memory space for data

36: connects lock-in amplifier to diameter to be scanned

37-41: if the response is YES (Y in 37), triggers the meter at one-half second intervals

for 15 seconds to permit selection of optimum meter range

42-45: moves stage to starting position

46-58: prints slice identification and characteristics and conditions of test

27
50: wrt 6 . 3

, " Diarae ter : " , D

, "cm ", " Th ickness : " , r 1

, "mm"

51: fmt 4 ,c40, f 8,3 ,c7 ;wr t 6. 4, "Average resistivity :", r2 , "ohm cm"

52: wrt 6 53: fmt 4 , c41 , c5 ; wr t 6 . 4 ," Diameter :",G$

54: fmt 5 ,c41, f 6 . 2 ,c3 ; wr t 6. 5, "Length of step :",L,"mm"

55: fmt 6 , c41 , f 6 . 0 ; wr t 6. 6, "Number of steps:",

S

56: fmt 6 ,c41 , f 6 . 0 , c3 ; wr t 6. 6, "Time constant :",T,"ms"

57: fmt 7 , c58 , f 2 , 0 ; wr t 6 . 7

, " Locat ion of data file: Track 1, file, ",rl2

58: wrt 6;wrt 6 59: fmt 7 , cll , c9 , c21 , clO , c6 , cl4 , cll

60: fmt 8,cl4 ,cll,cl5,cl0,c9,cll,cl0

61: wrt 6.7," Locat ion "

, " I =0 "

, "d R (x ) ", " Sum ( Rd x) ", " r ho "

, "d R

'

( x

) ", " sum ( R

' dx )

62: wrt 6.8," [mm fm center 1" , "Volts" , "Ohms" , "Ohm-cm" , "Ohm-cm" , "Ohms" , "Ohm-cm"

63: wrt 6 64: if F$="N" ; 12. 2427*3. 6*r2*r2/( . 05*D*rl) *r4

65: if F$="P" ;-12. 2427*1. 36*r2*r2/( .05*D*rl) *r4

56: fmt 1 , c, z ; wr t 7 2 3. 1

, "016 OTGOOO 1TO40T" ;gsb "step"

67: fxd l;dsp "BEAM IS",P[N],"mm FROM CENTER" ;gsb "meter"

68: if J[N]=99;fmt 1 , 4 x , f 6 . 2 ,4 x , c8 ; wr t 6 . 1 , P [N] ," OVERLOAD" ; jmp 2 69: fmt 2 ,4x , f 6 . 2 , 5x , f 6 . 3 ; wr t 6. 2 ,P [N] ,A[N]

70: P [N] +L-«-P [N + 1] ; if (N+1* N ) <S + 2 ; jmp -4

71: dsp "TURN CURRENT SUPPLY ON";beep;stp

72: fmt 1 , c, f 4 . 0 , c , z ; wr t 7 2 3. 1

, "016 OTG" , r 10 , " T04 OT"

73: wrt 7 2 3. 1

, " Ol60T@ 002 1T04 OT"

74: dsp "ADJUST CURRENT" ; beep ; s tp

75: wtb 6 ,27 , 10; J [S/2] *r5;P [N] -L-P [N] ;N-l>N;ent "Current [mA]",Q

76: fmt 1 , c, f 4 . 0 , c , z ; wr t 7 2 3. 1

, "016 0T@" , r 11+2 1, "T04 OT"

77: ent "Calibrate? (Y/N)",A$

78: if A$="Y";gsb "calibrate"

79: if A$="N";jmp 5 80: ent "Continue calibration? (Y/N)",B$

81: if B$="Y";jmp -3

82: jmp 2 83: gsb "step"

84: fxd l;dsp "BEAM IS",P[N],"mm FROM CENTER" ;gsb "meter"

85: if J[N]=99;fmt 1 , 2 2x , c8 ; wr t 6. 1 , "OVERLOAD" ; jmp 2 86: fmt 2 ,2 3x, f 6 . 3 ; wr t 6.2,B[N]

87: wtb 6 ,27 , 10,27 , 10; P [N] -L-P [N-1] ; if ( N- 1-N) >=1 ; jmp -

4

88: fmt 3 , 16x , "xlO" , f 2. 0 , 3x , " xlO " , f 2 .

0

89: J [S/2]+r6;wr t 6.3,r5,r6

90: if r6<-5;wtb 6,27,10;fmt 3 , c4 1 , c29 ; wr t 6. 3

, "xlO-3" ,"xl0-3"

91: wtb 6,27,10,27,10,27,10;fmt 4 ,2 2x , " I=" , f 5 . 3

, "mA" ; wr t 6 . 4 ,Q

92: wtb 6,27 ,77 ,10,10;l-N;0->-r7

93: fmt 1 , c, z ; wr t 7 2 3 . 1 , "016 0T(a002lTO40T"

94: dsp "CHECK CURRENT" ; beep ; beep ; beep ; wa i t 5000

95: -l*abs( ( B [ S/2 + 1 ] *10'^ r 6-A [S/2+1 ] *10'^r5)/( .001*Q) )*C[S/2;i-l]

96: -l*abs((B[N] *10'r6-A(N] *1

0

" r 5 ) / ( . 00 1*Q ) )*C[N]

97: 1/(1-(P[N]/(5*D) )^2)*J[N]

98: C[N1/J [N]~2*K[N]

99: L/10*r4*J [N] *A[N] * 10" r 5/C [N ] [N

]

*11314

28
59-63: prints heading for data

64,65: calculates constants for n- and p-type material,

66-70: steps the stage through the number of positions prescribed in steps 8 and 9 of

section 3 of this report, reads the meter at each position, and prints the position

of the stage and photovoltage in two columns. If the meter overloads, prints the

position of the stage and the word OVERLOAD.

71: instructs the operator to turn on the current supply

72-76: connects current supply to the diameter to be scanned, reads the current with the

digital panel meter, instructs operator to adjust the current and enter the value,

and connects the lock-in amplifier to the diameter to be scanned

77-82: if the response is YES (Y in 77), triggers the meter at one-half second intervals

for 15 seconds to permit selection of optimum meter range

83-87: steps the stage through the same positions covered in 66-70 but in the reverse direction (the motor reversal was included in the value of rll in 76), reads the

meter at each position, and prints the photoinduced change in voltage in a third

column, starting at the bottom of the column and moving up. If the meter overloads,

prints the word OVERLOAD.

88-91: prints the multipliers for the photovoltage and photoinduced change in voltage

columns and the value of the current

92: moves the printer to the first row of data and sets counter

93,94: disconnects the lock-in amplifier and directs the operator to read the current

again

95-112: calculates the photoinduced change in specimen resistance and the change in

resistivity for both the actual and idealized cases and prints these values in the

next five columns

29
100: D [N] +D [N-1] *D [N]

101: if P [rJ] > = -16 ; if P [N] < = 16 ; D [N] +r 7-* r7 ; R+l-«-R 10 2 : L/10*r4*l/J [N ] *A[lsl] * 10 r 5/C [ S/2 + 1] -^F [N]

103: F [N] +F [N-ll-F [N]

104: if {N+1*N) <S+2;gto 96

105: r 2-r7/R*r7;D[S/2+l] +r7-E [S/2+1] ; l-N

10 6: D[N]+r7*E[N]

10 7: K[N] *(E [S/2 + l]/E [N]

) ^2-K[N]

108: if r6<-5;1000*C[N]-C[N] ;1000*K[N]-K[N]

109: fmt I,32x,3f9.3,3x,f9.4,f9.3;wrt 6. 1 ,C [N ] , D [N ] ,E [N] ,K [N] , F [N]

110: if E [N] >M; E [N]->-M

111: if E [N] <J;E [N]*

J

112: if {N+1*N) <S+2; jmp -6

113: wrt 723.1,"O160T@003lTO40T"

114: fmt l,c,z;wrt 7 2 3. 1

, "O160TG0001TO40T"

115: dsp "BEAM RETUPNING TO WAFER CEiMTER"

116: 100*L*S/2->C; 0*X*Y-*Z; if C<4096;jmD 2 117: C-4095-C; 7777*Y

118: dtoC-«-Z ;f mt l,c,z;wrt 7 2 3 . 1 , " 016 OTG 000 1T04 OT" ; g sb "motor"

119: dsp "To plot, change paper in pr inter beep ; s tp

120: if r8>0;ent "PLOT FOUR-POINT PROBE DATA? (Y/N)",F$

121: fxd 2;dsp " Rmin = "

, J , "Rjna x=" ,M , " Ym in=? "

; ent "",J

122: fxd 2;dsp " Rm in=" , J

, " Rma x=" , M, " Yma x=? " ; ent "",M

123: oil 'form' (9.5 ,13 ,13)

124: ell 'psiz '

( 9 . 7 5 , 8 , 3 , . 5

)

125: cll 'scl' (-26,25, J-(M-J)/5,M+(M-J)/10)

126: cll 'move' (-25, J- (M-J)/10) ;wait 1000

127: cll 'xaxis' (J-(M-J)/10,5,-25,25)

128: cll 'move' (0 , J) ;wait 1000

129: cll 'yaxis' (0, (M-J)/10, J,M)

130: -25*I;fmt f3.0,z

131: cll 'move' (I ,J- (M-J)/10) ;cll ' space '(- 1) ; cll 'skip'(l)

132: wrt 6,I;if ( I +5+ I ) <-5 ; jmp -1

133: cll 'move' (I ,J-(M-J)/10) ;cll ' space '(- 1) ; cll 'skip'(l)

134: fmt f2.0,z;wrt 6,I;if (I +5*1 ) < = 25 ; jmp -1

135: cll 'space' (-32) ;cll 'sk ip '( 1) ; wr t 6, "DISTANCE FROM CENTER OF WAFER-mm"

136: J-«-I;if r2<10;fmt l,f5.2,z;jmp 3 137: if r2<100;fmt l,f6.1,z;jmp 2 138: if r2>=100;fmt l,f4.0,z

139: cll 'move' (0 , 1 ) ; wr t 6.1,I;jmp (I + ( M-J ) /5->-I ) >M

140: cll 'move' (-1 . 5 ,M) ;cll 'skip '

( -1) ; wr t 6, "OHM-CM"

141: cll 'move' (P [1] ,E [1] ) ; l->-N;cll ' char '

( 46 , 5 , 3 ) 142: cll 'fplt' (P [N] ,E [N] , 111) ; jmp (N+l-«-N ) >S + 1 143: 25-N

144: cll 'pit' (N, r2 ,45) ; jmp (N-l*N)=-25

145: cll 'move' (-25, r2) ,-cll ' sk ip '

( -1) ; wr t 6, "MEAN RESISTIVITY"

146: cll ' skip' (1) ;fmt 1, 7x , f 6 . 2 ,c7 , z ; wr t 6 . 1 , r2 , "ohm-cm

147: 1-^N 148: if F$="Y";cll ' pi t ' ( L [N ] ,G (N ] , 4 2 ) ; jmp (N+l*N)>r8

149: cll 'move' (-26 , J) ; wr t 6

, "o PHOTOVOLTAIC MEASUREMENT"

*31005

30
113-114:

115-118:

119-122:

123-129:

130-135:

136-140:

141,142:

143-146:

147-148:

149,150:

shorts current supply and reverses direction of stepping motor

returns laser beam to center of slice

determines if plots of resistivity and four-probe data are desired and requests

operator to set abscissa limits

locates plot on page and draws x- and y-axes

scales and labels x-axis

scales and labels y-axis

plots resistivity, E(N) , versus position, P(N) , using the symbol o to locate the

data points

plots the mean resistivity specified in 6 as a dashed line

plots four-probe data, G(N), versus position, L(N), using the symbol * to locate

the data points

prints legend for plots

31
150: if F$="Y";cll 'move '

( -26 , J )

; cll ' sk ip ' ( 1 ) ; wr t 6,"* FOUR POINT PPOBE DATA'

151: cll 'move' (0 , J) ;cll ' sk ip '

( 8 ); cll 'space'(-13)

152: wrt 6

, "RESISTIVITY VERSUS POSITION"

153: cll 'move' (0 , J) ;cll ' skip '

( 9) ;cll ' space '

( -1 0 ) ; f ra t c 8 , cl2 , 14 x , c8 , c8

154: wrt 6, "WAFER: ",E$,C"$,D$

155: cll 'move' (0 , J) ;cll ' sk ip '

( 10 ) ; cll ' space ' (-6 ) ; f mt c 10 , c3

156: wrt 6, "DIAMETER " ,G$

157: cll 'move' (0 , J) ;cll ' sk ip '( 11 ) ; cll 'space' (-16)

15b: ent "Comment? (32 characters) " ,H$ ; fmt c32;wr t 6 ,H$

159: ent "Record data on cassette? (Y/N)",A$

160: if A$="Y";dsp "INSERT DATA CARTRIDGE" ; beep ; s tp

161: trk l;rcf rl2,E[*]

162: wtb 6,13;trk 0;dsp "END" ; beep;gto 119

163: "calibrate":

164: 0*1

165: wrt 7 2 3

, "O260TAT 240T" ; wait 500;jmp (I+500-I ) =15000

16 6: 0->-I;ret

16 7: "meter":

168: 1*I;0*P

169: fmt ;wait 10*T

170: wait 1*T

171: wrt 723, "O260TAT240TAT" ;red 723,

E

172: wrt 72 3

, "O240TCT" ; red 723,

F

173: if E> = 10000 ; E-IOOOO-E; otdE+E; E-6int (E/16) -60int (E/256)

174: if F>=100 ;F-100*int (F/100)-F

175: if F>=40;99-^J [N] ;gto 191

176: if F>=20 ;F-20*F;1*G; jmp 2 177: -1*G

178: if F> = 10;F-10->-F;l*K; jmp 2 179: 0*K

180: F-7->-J[N]

181: G* (K+E/1000)-«-M[I ] 182: M[I] +P-«-P;if ( I +1* I ) < =r 3 ; g to 170

183: if r 3=l;r3*r22;gto 189

184: P/r3*r20 ; l+I; 0-P

185: (M [I ] -r 20 ) (M [I ] -r20) +P-P; jmp (I + l-<-I)>r3

186: V(P/(r3-l) )*r21;l-I;0-P*r22

187: if M [ I ] > = r20-2r21; if M [ I ] < = r 20 + r 21 ; M [ I ] +P-»-P ; r 22 + l*r 22

188: if ( I+l-I) <=r 3; jmp -1

189: if Q=0;P/r22+A[N] ; jmp 2 190: P/r22*B[N]

191: ret

192: "motor":

193: fmt 3 , c, f 4 . 0 , c, z ; wr t 7 2 3. 3 , "016 OTK" , X

, " T04 OT"

194: wrt 7 2 3 . 3

, "O160TK" , Y

, " TO40T"

195: wrt 7 2 3 . 3 , "O160TK" , Z

, " T04 OT"

196: ret

197: "step":

198: 0-«>X-«-Y;dtoA*Z;gsb "motor"

199: ret

*20643

32
151-158: prints title of graph and allows one additional line to enter additional

information as a commeni:

159-161: provides for recording resistivity values on cassette if desired

162: end statement

163-166: "calibrate" subroutine triggers meter at one-half second intervals

166-191: "meter" subroutine

168-172: pauses and reads meter

173-181: converts BCD to decimal and separates overload and polarity information

182-186: takes r3 measurements, averages them, and determines the standard deviation

187,188: selects r22 measurements which are within two standard deviations of the mean

189,190: averages the measurements that are within two standard deviations of the mean and

records photovoltage (A[N]) or photoinduced change in specimen voltage (.B[NJ)

192-196: "motor" subroutine moves the linear stage X+Y+Z steps in the direction, and using

the values for X, Y, and Z, determined by the main program

197-199: "Step" subroutine converts the length of step selected by the operator to an octal number and calls the "motor" subroutine

33
200: "torm":

201: wtb r0,27,77

202: wtb r0,27,84

203: wtb r0,27 ,87 , int (120*pl/64) ,120*pl

204: wtb r0,27 ,76 , int (96*p2/64) ,96*p2

205: wtb r0,27 ,70, int(96*p3/64) ,96*p3

206: ret

207: "psiz":

20 8: pl-H;p2*W

209: wtb r0,27 ,79 ,int(p4*120/64) ,p4*120, int(p3*96/64) ,p3*96

210: ret

211: "scl":

212: 120W/(p2-pl) -^U 213: 96H/ (o4-p3) *V

214: pl-X; p3*Y

215: ret

216: "xaxis":

217: wtb 6,27,46,95,0,5,9

218: wtb 6,27,6 5, in t( (p3-X)U/64) , int ( (p3-X)U) ,int( (pl-Y)V/64) , int ( (pl-Y) V)

219: p3-p5;wtb 6,43;wtb 6,8

220: wtb 6,27,114 ,int(p2U/64) ,int{p2U) ,0,0;wtb 6,43,8;jmp (p5+p2+p5 ) >=p4

221: ret

222: "yaxis":

223: wtb 6,27,46,124,0,3,0

224: wtb 6 ,27 ,6 5, int { (pl-X)U/64) , int ( (pl-X)U) ,int( (p3-Y)V/64) ,int( (p3-Y)V)

225: p3-«-p5;wtb 6,43;wtb 6,8

226 : wtb 6 , 27 , 114 ,0 , 0 , int (p2V/64 ) , int (p2V) ; wtb 6,43,8;jmp (p5+p2->-p5 ) >=p4

227: ret

228 : "move" : 22 9: wtb 6 , 27 , 6 5 , in t( {pl-X)U/64) ,int( (pl-X)U) ,int( {p2-Y)V/64) , int ( (p2-Y)V)

230: ret

2 31: "space":

232: if pl<0;gto +2

233: wtb r0,32;jmp 2 ( ( pl-l-pl) =0

)

234: wtb r0,8;jmp (pl+l+pl) =0

235: ret

2 36: "skip":

237: if pl<0;gto +2

238: wtb rO,10;jmp 2 ( ( pl-l-pl) =0)

239: wtb r 0 , 27 , 10 ; jinp (pl+l-pl) =0

240: ret

241: "pit":

242: wtb r 0,27 ,6 5 , int ( (pl-X)U/64) , int ( (pl-X)U) , int ( (p2-Y)V/64) , int ( (p2-Y)V)

243: wtb r0,p3;wtb r0,8

244: ret

245: "fplt":

246: wtb 6 ,27 ,97 , int ( (pl-X)U/64) , int ( (pl-X)U) , int ( (p2-Y)V/6 4) , int ( (p2-Y)V)

247: wtb 6,p3;wtb 6,8

248: ret

249: "char":

250: if p2=0; 5*p2;0-^p3

251: wtb 6 ,27, 46 ,pl , int (p2/64) ,p2 ,p3

252: ret

*1722

34
200-206: "form" subroutine sets dimensions of plotter page

206-210: "psiz" subroutine locates the graph on the page

211-215: "scl" subroutine scales the graph

216-221: "xaxis" subroutine draws the x-axis

222-227: "yaxis" subroutine draws the y-axis

228-230: "move" subroutine moves the plotter to position pl,p2

231-235: "space" subroutine moves the plotter horizontally

236-240: "skip" subroutine moves the plotter vertically

241-244: "pit" subroutine plots data points with no connecting lines

245-248: "fplt" subroutine plots data points and connects them by a line

249-252: "char" selects the character to be used to connect the data points

35

NBS.114A (REV. 0-78)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

SP 400-52

SI.<3ov*L AcoBsa^on No. 3. ie<jipten$'$ Acctisjoft Ho.

4. TITLE AND SUBTITLE Semioonduotor Measurement Technology: An Automated Photovoltaic System for the Measurement of Resistivity Variations

in High-Resistivity Circular Silicon Slices

5. Publication Date November 1979

$. Performing Organization Code

7. AUTHOR(S)

David L . Blackburn

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON DC Onj'M

10, Project/Task/Work Unit No.

11. Contract/Grant No. ERDA Task Order

A021-EES

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS rsfreer, City, state, ZIP) Department of Energy

Division of Electric Energy Systems

Washington, DC 20545

13. Type of Report & Period Covered Final

12/75 - 10/77

fliK%diSi«^^^»8»cy Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 79-600178

Document descrit)es a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or leaa tactual summary ol most significant inlormation, II document includes a significant bibliography or

literature survey, mention it here.)

This report describes an automated photovoltaic system for nondestructive measurement

of resistivity variations of high-resistivity circular silicon slices. The

computer-based system for making the measurements is described, detailed construction

diagrams are given to facilitate reproduction of the system, and a listing of the

computer program for controlling the system is given. Comparisons between

resistivity profiles determined using the automated photovoltaic system and the

four-probe technique indicate that the photovoltaic system is adequate for production

screening and incoming inspection of high-resistivity float-zoned silicon slices.

17. KEY WORDS Cs/x to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name;

separated by semicolons) Automation; bulk photovoltaic effect; computer control; homogeneity; measurement

method; nondestructive testing; photoconductivity; power devices; resistivity

variation.

18. AVAILABILITY Unlimited

1 1 For Official Distribution. Do Not Release to NTIS

[5cl Order From Sup. of Doc, U.S. Government Printing Office, Washington, DC

20402, SD Stock No. SN003-003-02138-5

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

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

21. NO. OF

PRINTED PAGES

41

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price

$2.00 USCOMM-DC

FEDERAL INFORMATION PROCESSING STANDARD SOFTWARE SUMMARY

01. Summary date

Yr. Mo. Day

04. Software date

Yr. Mo. Day

02. Summary prepared by (Name and Phone) David L. Biackburn

05. Software title Automated Photovoltaic System

06. Short title 08. Software type

Automated Data System

I I Computer Program

I I Subroutine/Module

09. Processing mode

Interactive Batch

I I Combination

03. Surrimary action New Replacement Deletion

Previous HQInternal Software ID

07. Internal Software ID PVI

10. General

Application area

Specific Computer Systems Q Support/Utility

pr| Scientific/Engineering

I I Bibliographic/Textual

Management/

Business

I I Process Control Other

11. Submitting organization and address Electron Devices Division

Center for Electronics & Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Washington, DC 2023^

12. Technical contact(s) and phone D. L. Blackburn

Bldg. 225, Rm. B310

National Bureau of Standards

Washington, DC 2023^

13. Narrative This software controls an automated photovoltaic system for measuring resistivity

variations of circular semiconductor slices. The software is used with an HP 9825A

cal culator/control ler , controls a system using the IEEE Standard 488-1975 interface

bus, and gives hardcopy output via a printer. The software requires about 9000 bytes

of storage and uses Hewlett Packard's version of BASIC.

14. Keywords

Automated measurement; nondestructive; resistivity variation; silicon slices.

15. Computer manuf'r and model Hewlett Packard

9825A

16. Computer operating system Photovol ta i c17. Programing language(s) Hewlett Packard

BASIC

18. Number of source program state- ments252

19. Computer memory requirements 9000 bytes

8 bits/byte

20. Tape drives One - integral to

HP 9825A

21. Disk/Drum units 022. Terminals One - integral to HP 9825A

23. Other operational requirements Printer/plotter (HP 9871A) , Mul ti programmer (HP SSkOB)

24. Software availability

Available Limited In-house only m .

25. Documentation availability

Available ODDInadequate In-house only

26. FOR SUBMITTING ORGANIZATION USE

185-101 Standard Form 185

1974 July

U.S. Dept. of Commerce—NBS

(FIRS. Pub. 30)

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characteristics of the products. NBS administers this program as a supplement to the activities of the private sector standardizing

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Superconducting Devices and Materials. A literature survey issued

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