A111D3 057525

(O NBS SPECIAL PUBLICATION 400-34

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NATIONAL BUREAU OF STANDARDS

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»nal Burenu of Standards

? e '977

- Semiconductor Measurement Technology:

^ ,^^Safe Operation of Capacitance Meters

Using High Applied-Bias Voltage

Alvin M. Goodman

RCA Laboratories

Princeton, New Jersey 08540

This acti\ily was supported by The Defense A(l\anced Researeh Projects Agency

and

The National Bureau of Standards

1

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

Edward O. Vetter, Under Secretary

Dr. Betsy Ancker-Johnson, Assisfanf Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued December 1976
Library of Congress Cataloging in Publication Data Goodman, Alvin M

Safe operation of capacitance meters using high applied-bias

voltage. (Semiconductor measurement technology) (NBS special

publication ; 400-34)

Supt. of Docs, no.: C 13,10:400-34

1. Capacitance meters. 2. Electric measurements. 3. High

voltages. I. Title. II. Series. III. Series; United States. National Bureau of Standards.

Special publication : 400-34. QC100.U57 no. 400-34 [TK335] 602Ms [621.37'42]

76- 608338

National Bureau of Standards Special Publication 400-34

Nat. Bur. Stand. (U.S.), Spec Publ. 400-34, 57 pages (Dec. 1976) CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1976

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

(Order by SD Catalog No. C13.l0:400-.34). Stock No. 003-003-017-07-8 Price $1.45

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

PAGE

1. Introduction ^

2. Simplified Circuit and Principles of Operation 4

3. Actual Circuit 6

3.1. Circuit and Functional Description 6

a. Bias-Protection Circuit , 6

b. Zero Adjustment and Zero Suppression 6

c. Peaker Circuit ..... , 9

3.2, Measurement Accuracy . «... 10

a. Basic Considerations 10

b. Equivalent Circuit 10

(1) = 0 . . . « . , 11

(2) ^ 0 13

4. Experimental Results 15

4.1. General 15

4.2. Transient Suppression 15

4.3. Effect of BIU on Measurement Accuracy ..... I7

5. Discussion and Conclusions , 26

Acknowledgment , 27

Appendix „ 28

References «....,* 49

FIGURE CAPTIONS

Figure 1, Conventional high-frequency C(V) measurement. The inset

is a schematic illustration of a typical plot of normalized capacitance C^^ versus voltage 2

iii
PAGE

Figure 2, Isolation box arrangement for measuring C(V) with an applied bias voltage larger than the C-meter limit:

(a) block diagram, and (b) isolation box circuit ... 3

Figure 3, Simplified version of C-meter bias-protection circuits:

(a) normal operating mode, and (b) after a capacitor

sample breaks down with a large bias voltage applied

to it 3

Figure 4. Schematic circuit diagram of C-meter bias- isolation

Figure 5, Plots of relative sensitivity factor S(Cs) and relative incremental sensitivity factor S'(Cg) versus

Cs with series resistance R as a parameter 12

Figure 6, (a) Schematic representation of the experimental

arrangement for testing the transient suppression capability of the BIUo (b) Photograph of a typical

observed transient (HI terminal to ground) 16

Figure 7. Calibration curves for BIU operating with BEC Model

71A C-meter 18

Figure 8. Calibration curves for BIU operating with BEC Model

72AD C-met

Figure 9. Calibration curves for BIU operating with PAR Model

410 C-meter 20

Figure 10, Relative incremental sensitivity S' (Cs) for BIU

operating with BEC Model 71A C-meter 22

Figure 11, Relative incremental sensitivity S'(Cs) for BIU

operating with BEC Model 72 AD C-meter 23

Figure 12, Relative incremental sensitivity S'(Cs) for BIU

operating with PAR Model 410 C-meter , , . 24

Figure 13. Schematic circuit diagram of 3-MHz filter 25

Figure ML Front panel Capacitance-Meter Bias-Isolation Unit ... 29

Figure M2 Side panels 30

Figure M3 Rear panel 31

Figure M4 Chassis top and bottom covers 32

Figure M5 Clear acrylic (polymerized methyl methacrylate) base . 33

iv
PAGE

Figure Mb Peaker/zero suppression shield 34

Figure M7 Drawing containing: A. Shield cover 35

B. Shield cover bracket ..... 35

C. Mounting bracket for L6, L7 . 35

D. PIN diode heat sink 35

Figure M8 Physical layout of bridge 36

Figure M9 Drawing containing: A. Acrylic bias-divider mounting

plate 37

B. Acrylic front and rear .... 37

C. Acrylic sides 37

Figure MIO Peaker /zero-suppression acrylic mounting bracket ... 38

Figure Mil Zero-suppression fine capacitor acrylic mounting

bracket 39

Figure M12 Support 40

Figure M13 Rectifier mounting plate 41

Figure M14 Zener heat sink 42

Figure M15 Peaker zero-suppression shield rear insulator 43

Figure M16 Acrylic cover insulator 44

Figure Al Front view of Capacitance-Meter Bias-Isolation Unit . , 45

Figure A2 Top view with top cover removed 46

Figure A3 Top view of Peaker/Zero Suppression circuit with top cover and shield cover removed 47

Figure A4 Bottom view with bottom cover removed 48

LIST OF TABLES

Table 1. Electrical Parts List for Capacitance-Meter Bias-Isolation

Unit 8

Table 2. Test-Signal Spectral Analysis for Three C-Meters .... 26

Table 3. List of Mechanical Parts 28

V
PREFACE

This study was carried out at the RCA Laboratories as a part of the

Semiconductor Technology Program in the Electronic Technology Division

at the National Bureau of Standards, The Semiconductor Technology Program serves to focus NBS efforts to enhance the performance, interchangeability, and reliability of discrete semiconductor devices and

integrated circuits through improvements in measurement technology for use in specifying materials and devices in national and international

commerce and for use by industry in controlling device fabrication processes. The work was supported by the Defense Advanced Research Projects Agency* through the National Bureau of Standards' Semiconductor

Technology Program, Contract 5-35912. The contract was monitored by

R, L. Raybold as the Contracting Officer's Technical Representative

(COTR) and R. Y. Koyama as Assistant COTR.

Certain commercial equipment, instruments, or materials are identified

in this report in order to adequately specify the experimental procedure.

In no case does such identification imply recommendation or endorsement

by the National Bureau of Standards, nor does it imply that the material or equipment identified is necessarily the best available for the

purpose.

Larger scale drawings, of the mechanical parts are available on request

from the COTR, TECH-A-361, National Bureau of Standards, Washington,

DC 20234.

) r

^Through ARPA Order 2397, Program Code 6D10.

vi

I
SEMICONDUCTOR MEASUREMENT TECHNOLOGY: Safe Operation of Capacitance

Meters Using High Applied-Bias Voltage

by

Alvin M. Goodman

Abstract: The use of capacitance meters (C-meters) to

determine small-signal (differential) capacitance at 1 MHz

as a function of applied-bias voltage is widespread. The

maximum value of the bias voltage which may be applied to a sample under test with any commercially available C-meter is

600 V or less. A larger bias-voltage capability is required

for certain applications.

This report describes a technique for using a commercial

C-meter with a Bias-Isolation Unit (BIU) for capacitance measurements at bias-voltage magnitudes up to 10 kV without

damage to the measurement equipment. The basic principles

of operation and the details of the electrical design of a BIU are discussed.

The use of the BIU imposes certain limitations on the range

of sample capacitance which may be measured without introducing excessive error. The theory of these limitations is

presented and compared with experimental results obtained

from the use of the BIU with each of three commercially

available C-meters, The measurement capability demonstrated

by these results appears to be adequate for all current and

future applications. For less than +1% error in the indicated (measured) capacitance, the measurable range of the

sample capacitance is found to be from 0 to at least 400 pF,

In some applications, it is important to be able to accurately

measure small changes in the sample capacitance; for less than

+1% error in the indicated (measured) value of a small change

in the sample capacitance, the measurable range of the sample

capacitance is found to be from 0 to at least 130 pF,

Construction details of the BIU are appended.

Key Words: Bias-Isolation Unit; capacitance measurements at high applied-bias voltage; capacitance-meter; ex- tended-range capacitance measurement; high-voltage C(V)

measurements; modified MIS C(V) measurements.

1 . INTRODUCTION

The use of capacitance meters (hereinafter abbreviated as C-meters) to

determine small-signal (differential) capacitance as a function of

applied-bias voltage is commonplace today in many research, development,

and manufacturing applications. Many, if not most, of these applications

1
are connected with the semiconductor industry. A variety of commercial

instruments* is available to meet most existing measurement requirements.

Typically, an instrument of this type uses a crystal-controlled 1-MHz

test signal whose amplitude is "^15 mV, The test signal is applied to

the unknown capacitance; the resulting current is amplified, and its

quadrature component (with respect to the applied voltage) is determined

using some form of phase-locked synchronous detector. The quadrature

component of the current is directly proportional to the measured ca- pacitance, and the C-meter is usually calibrated to read directly in

picofarads. In addition, an analog output voltage is generally made

available to enable the plotting of capacitance on a recorder. Means

are usually provided for applying a quasi-dc bias voltage to the un- known capacitance through the C-meter. This allows the capacitance to

be recorded as a function of the applied-bias voltage using the arrangement shown in figure 1, The magnitude of the bias voltage that may be

applied in this way is limited. Although the limit differs for different

instruments, it is in no case greater than 600 V.

BIAS

VOLTAGE

LO"

N

X - Y

RECORDER

C - METER

I

SAMPLE

Figure 1, Conventional high-frequency C(V) measurement. The inset is a schematic illustration

of a typical plot of normalized capacitance Cfj versus voltage.

It is sometimes necessary to apply a bias voltage larger than 600 V to a capacitor sample [1,2], This can be accomplished by using an arrangement which applies the bias directly to the sample but not to the

C-meter. An example (based on a circuit described in reference 3) is

shown in figure 2, The isolation box allows the bias voltage to be

*Some examples of commercial C-meters (or instruments containing

C-meters) are as follows: BEC (Boonton Electronics Corporation) Models

71A, 71AR, 72A, 72AD, 72B, 72BD; and PAR (Princeton. Applied Research

Corporation) Model 410.

2
BIAS ISOLATION

C-METER VOLTAGE BOX

TEST

SAMPLE

(a) BLOCK DIAGRAM

0.5-lmH

(COARSE TUNING)

TO

BIAS

VOLTAGE

SUPPLY

CURRENT

LIMITING

RESISTOR

.OOl/iF

lOkV

T

36pF

l.5-3.lpF

(FINE TUNING)

l.5-3.lpF

(FINE TUNING)

.02/tF

lOkV

SAMPLE TO

C-METER

36pF

0.5-lmH

(COARSE TUNING)

(b) ISOLATION BOX CIRCUIT

Figure 2. Isolation box arrangement for measuring C(V) with an applied

bias voltage larger than the C-meter limit: (a) block diagram, and (b)

isolation box circuit.

applied to the sample while keeping it out of the C-meter; at the same

time, it isolates the bias-voltage supply from the high-frequency test

signal. It is assumed that there is a dc path through the C-meter to

allow the high-voltage-blocking capacitor to charge and discharge as the

bias voltage is (slowly) varied. The parallel resonant circuits are tuned to the test frequency (1 MHz in this case) to provide the necessary ac isolation. This isolation can also be obtained by using sufficiently high-value resistors instead of the tuned circuits. The

3
blocking capacitor between the sample and the C-meter must be sufficiently large that it does not introduce unacceptable error into the

measurement. For the value shown (0.02 yF) , the maximum error for

samples with C < 100 pF would be < 0.5%. The voltage rating of the

blocking capacitor must, of course, be at least as large as the greatest anticipated bias voltage.

The arrangement shown in figure 2 does not, however, provide complete

protection for the C-meter in the case of sample failure. If the sample

develops a short-circuit while a large bias voltage is applied to it,

the blocking capacitor (which is also charged up to the bias voltage)

must discharge through the C-meter. If the voltage across the blocking

capacitor just before the sample "shorts" is sufficiently large, the

C-meter will be damaged.

In order to prevent this type of damage to the C-meter, a "bias-protection circuit" has been developed. The basic principles of this circuit

and some of the design considerations are discussed in section 2. The

actual circuit and operation of a C-meter "Bias-Isolation Unit" (BIU)

,

which allows safe capacitance measurements at bias voltage up to + 10 kV, are described in section 3. In section 4, some of the experimental results obtained using the BIU with commercial C-meters are presented. Finally, in section 5, present and possible future applications

of capacitance measurements at high applied-bias voltage are discussed.

2. SIMPLIFIED CIRCUIT AND PRINCIPLES OF OPERATION

The operation of the C-meter bias-protection circuit can best be described by considering a simplified version: first, in the normal

operating mode, as shown in figure 3(a), and second, the equivalent

circuit after a sample "breaks down" with a large bias voltage applied

to it, as shown in figure 3(b). The sample capacitance being measured

is represented by Cs. The series combination of L and Cb is tuned to

resonance (at the measurement frequency of the C-meter) so that there

is no reactance in series with Cs. The diodes I) exhibit very small

capacitance and conductance at the measurement signal level ('^^ millivolts) and thus do not interfere with the measurement of Cs. The shunt

capacitance and conductance of the diodes can be reduced still further

by applying a small reverse bias to each of the diodes. The value of

R is much less than Xcsj and in a first approximation its effect on the

measurement of Cs may be ignored. A detailed consideration of its effect will be presented later. The bias voltage is supplied to Cs

through high-value resistances (r >> Xcs)j they serve two functions:

. (i) to effectively isolate the bias supply from the measurement circuit,

and (ii) to limit the current in case of a sample breakdown (short

circuit)

.

The C-meter is represented by a parallel RLC circuit at the measurement

frequency. There is a low-resistance dc path between the terminals of

the C-meter. This allows the dc voltage at the C-meter terminal to re- main effectively zero during slow variations of the applied-bias voltage.

4
HIGH VOLTAGE

POWER —

SUPPLY

PROTECTION CIRCUIT C-METER

(a)

(b)

Figure 3. Simplified version of C-meter bias-protection circuits:

(a) normal operating mode, and (b) after a capacitor sample breaks

down with a large bias voltage applied to it.

During large rapid variations of the sample bias-voltage, the voltage

at the C-meter terminals would start to become significantly different

from zero; one or the other of the diodes would then start to conduct

heavily when biased in the forward direction. One rather important

example of rapid variation of sample bias-voltage is the case in which

the sample develops a short-circuit during application of a large bias

voltage; the voltage across the sample drops (almost) instantaneously

to zero and Cfi must discharge through R, L, and the parallel combination

of the diodes and the C-meter. This is illustrated schematically in

figure 3(b). The forward resistance of the conducting diode rf must be

sufficiently low that the voltage at the input terminals of the C-meter

never exceeds the maximum allowable value; i.e., it remains "clamped"

within some allowable range.

5
3. ACTUAL CIRCUIT

3.1. Circuit and Functional Description

The schematic circuit diagram of the C-meter bias- isolation unit

(hereinafter abbreviated as BIU) is shown in figure 4 and the individual

circuit elements are described in table 1. The circuit performs not

only the bias-protection function but a number of others as well; these

will now be described.

The high and low terminals of the output to the C-meter are individually

clamped by diodes (Dl and D2 on the high side, D3 and D4 on the low

side) to remain within some allowable range with respect to ground. The

diodes Dl through D4 are PIN diodes with a low forward resistance, high reverse resistance, and fast turn-on. To reduce their loading effect on

the C-meter, they are normally maintained at a reverse bias of 6,8 V. The bias voltages (+6.8 and -6.8 V) are provided with a very low source impedance by the Zener diodes, VRl and VR2.

The high-voltage blocking capacitors C3 and C4 are at (or nearly at)

series resonance at 1 MHz with LI + L6 and L2 + L7, respectively. Adjustment of L6 and L7 can be used to compensate for series inductance

of the connecting leads both inside and outside the BIU.

The voltage dividers formed by Rl and R8 on the HIGH side and R2 and R9

on the LOW side serve to attenuate the bias input by a factor of 1000:1

so that it may be fed directly to the X-axis of an X-Y recorder.

High-frequency noise from the bias supply is attenuated by the low-pass

filter formed by R5, R6, and CI. The resistors R3 and R4 and the capacitor CI form a filter isolating the bias supply from the 1-MHz measurement signal.

A bias voltage equal to that at the high side of the sample is available

at J5 for application to a guard ring electrode. The 1-MHz series res- onant circuit formed by L3 and C2 assures that the guard ring is effectively grounded at the measurement signal frequency.

In order to facilitate the nulling ("canceling out") of capacitance between the high side and low side test terminals, a circuit (labeled ZERO

SUPPRESSION in figure 4) has been provided to allow a wide range of zero adjustment. This is useful not only for the usual nulling of stray ca- pacitance due to sample holder and/or connecting leads but also for

operation of the C-meter in a suppressed-zero mode as is required for

"modified MIS C(V) measurements" [1,2].

a. Bias-Protection Circuit

b. Zero Adjustment and Zero Suppression

6
7
Table 1. Electrical Parts List for Capacitance-Meter

Bias-Isolation Unit

Schematic ^ Reference Description (Commercial designation. If appropriate, shovm In parentheses )

Rl, R2 48.8 X lO^-fJ nominal value, "high-voltage stable"; series string of 17 resistors: 2.8/ x 10^ fi, 1%, T2.

R3, R4 5 X 10^ a, 1%, 12.5 W, "high-voltage stable."

R5, R6 4.7 X 10^ Q, 10%, 2 W. R7 4.7 X 10^ a, 10%, 2 W. R8, R9 51,350-fi nominal value. Actual values are obtained by selecting resistor

combinations that will result in 1000:1 bias voltage attenuation at JIO and Jll when those terminals are shunted by the input impedance of the recorder (10^ Q) . RIO, Rll 25-n, "high-voltage stable"; series string of 25 resistors: 1-^, 3-W non inductive.

R12, R13 820 fi, 10%, 2 W. R14 51 n, 1%, 1/2 W, noninductive.

CI 0.01 yF, 14 kVDCW (Plastic Capacitors, Type HG 140-103).

C2, C3, C4 ' 0.002 uF, 14 kVDCW (Plastic Capacitors, Type HG 140-202).

C5, C6, C7 36 pF silvered mica capacitor.

C8, C13 5.2 - 75 pF variable capacitor (E. F. Johnson, Type "L" , No. 167-0004-001).

C9, CIO, Cll, c, r ji J ^ J. 51 pF silvered mica capacitor.

C14 2.2 - 10 pF variable capacitor with speed-reducing planetary drive.

(E. F. Johnson Type "L", No. 16 7-0001-001) (Jackson Brothers, Type

4511/DAF planetary ball drive with 6:1 ratio).

Dl, D2, D3, D4 PIN diodes (Unitrode, Type 7201D diodes)

LI, L2 Special choke, 33 turns wound on 1 in. dia. x 4 in . length plastic form. L3 Special choke, ~50 turns wound on 1 in. dia. x 4 in. length plastic form

to resonate with C2 at 1 MHz. L4 145 yH, high Q choke (Boonton Electronics Corp., Type 400124).

L5 1000 uH, L6, L7 1.2 to 2.5 uH (North Hills, Type 120A with 3 turns removed leaving

12 turns remaining)

.

Jl, J2, J3,

J4, J5 High voltage coaxial chassis connector (Amphenol, Type 97-3102A-18-420S)

,

J6, Jl , J8, Insulated BNC coaxial chassis connector (Amphenol, Type 31-010)

.

JIO, Jll, J12 Insulated banana jack.

SI SPST toggle switch.

S2, S3 Ceramic insulated continuous shorting type switch. VRl Type 1N3305B Zener diode. VR2 Type 1N3305RB Zener diode. Fuse Type 3AG-1A. Power Supply Regulated with + and - 15 V outputs (Semiconductor Circuits, Type

2,15.100)

.

*The coimnercial designations listed above are for components whose

dimensions are consistent with those of the mechanical parts described

in the appendix. The use of other components may require modification

of the mechanical parts on which they are mounted. This commercial

identification of components does not imply recommendation or endorsement by the National Bureau of Standards nor does it imply that these

conqjonents are necessarily the best available for the purpose. 8
The circuit consists of a variable amount of capacitive susceptance

that is applied between the high side test terminal and a source of

voltage* which is approximately equal to the test signal in magnitude

but is 180 deg out of phase with it. A range of greater than 250 pF of zero suppression is available. In order that this range include

"zero" (i.e., no zero suppression) the minimum value of C13 + C14 plus

the stray capacitance of the wiring must be "canceled out" by a small

inductive susceptance (L5). The resistor R14 is approximately equal to

the sum of RIO + Rll to minimize the phase imbalance between the signals

applied to the high-side input terminal of the C-meter during operation

in a suppressed-zero mode.

c. Peaker Circuit

A C-meter with a high- impedance input is sensitive to loading due to

stray capacitance from the (high side) input terminal to ground. There

is in the BEC Models 71A/71AR an adjustable tuned circuit that provides

inductive susceptance from the high side input terminal to ground; this

circuit may be used to null (or compensate) up to about 100 pF of stray

capacitance. The BIU, sample holder, and connecting cables may provide

more stray capacitance than can be nulled by this circuit. Therefore,

an additional circuit providing inductive susceptance has been built

into the BIU for use if needed; it is capable of nulling up to an additional 150 pF. This portion of the circuit is labeled PEAKER in

figure 4. In operation the PEAKER is adjusted as follows:

(i) With the cables and sample connected, the C-meter is set

to its highest sensitivity (highest input impedance),

(ii) The ZERO SUPPRESSION control is set to produce a reading

in the upper half of the scale,

(iii) The PEAKER is then adjusted to obtain a maximum reading.

If the meter goes off-scale, repeat steps (ii) and (iii).

The peaker circuit adjustment is important only when the C-meter used

has a high input impedance. If the C-meter has a low- impedance input

like the PAR Model 410, its performance will be completely indifferent

to adjustment of the peaker circuit.

The capacitor to be measured is connected to the terminals marked

SAMPLE and the bias-voltage power supply is connected to the terminals

marked BIAS INPUT. If a guard-ring electrode system is to be used on

the sample, the guard ring is connected to the terminal marked GUARD and

the guarded electrode must be connected to the HIGH side; the unguarded

electrode is connected to the LOW side.

If the BIU is used with a BEC C-meter, the TEST and DIFF terminals of

the C-meter are connected to the correspondingly marked terminals of

of the BIU. The external bias terminals of the C-meter should be con- nected together (short-circuited).

*This voltage is available at the LO side of the terminals marked DIFF

on BEC Models and at the terminal marked NULL on the PAR Model 410.

9
If the BIU is used with a PAR Model 410 C-meter, the HI side of the

TEST and DIFF terminals of the BIU should be connected together and to

the C-meter terminal marked INPUT. The LO side of the TEST and DIFF

terminals of the BIU should be connected, respectively, to the C-meter

terminals marked DRIVE and NULL,

3.2, Measurement Accuracy

a, Basic Considerations

The use of the BIU places certain constraints upon the range of test

capacitance values which can be measured and the accuracy with which

these measurements can be made. In what follows we shall first consider

a simple equivalent circuit for the combination of the test capacitance

and the BIU, and second, derive expressions describing the deviation of

the apparent or "measured" capacitance from the actual value as a function of the equivalent circuit parameters.

b. Equivalent Circuit

The C-meter measures the capacitive susceptance of a test capacitance Cg

connected between its terminals and displays this value on a suitably

calibrated linear scale reading directly in units of capacitance

(usually picofarads). Let us define this value of susceptance as:

(Cg) E CO Cg (1)

When the C-meter is used with the BIU to measure the same test capacitance, the equivalent circuit is one in which the test capacitance

appears to be in series with a resistance R and (possibly) a reactance X,

The resistance is due principally to the sum of RIO + Rll in figure 4,

There may also be a small contribution due to the resistance in the windings of Ll, L2, L6, and L7 and in the blocking capacitors C3 and C4,

The reactance X is equal to the difference between the inductive reactance

of the sum of the circuit inductances (Ll + L2 + L6 + L7 + lead inductance) and the capacitive reactance of the series combination of C3 and

C4. We shall see shortly that there is no advantage to X being capacitive (negative); in practice, we would like it to be either zero or slightly inductive. We shall therefore treat it as if it were due to an "excess" inductance Lx*

The susceptance B(Cs) of the series combination of Cs, R, and Lx measured

by the C-meter is

(.VCg^ + [l-a)^L^Cg]^

We may now define a relative sensitivity factor, S(C„).

10
s(c,)

= t^-^Vs^ (3)

- B (C_) 2-,2_ 2 , 2_ ,2 o S 03 R Cg + [1-0) LjjCgJ

This gives the ratio of the apparent value of the Cs that would be indicated by the C-meter when used with the BIU to the actual value of Cg

that would be indicated by the C-meter alone. In some measurement applications [1,2] it is necessary to determine small changes in a test

capacitance. It is, therefore, of interest to derive an expression for

the relative inaremental sensitivity factor which we shall call S'CCg).

s'(c,)

2 2 2 2 2

[1-0, 4Cg] -.RCg

^ [(1-A^Cg)^ + 0)VCg2]^

This gives the ratio of the apparent value of a small change in Cg that

would be indicated by the C-meter when used with the BIU to the actual

value of the small change in Cg that would be indicated by the C-meter

alone. Ideally, of course, both S and S' should be equal to 1.0 for all

values of Cs» but this could be true only for both Lx and R equal to

zero; i.e., without the protection circuit to which R is essential.

It is helpful to consider two separate regimes (a) L = 0 and (b) L ?^ 0.

X X

(1) Lx = 0

In this regime, eqs (3) and (4b) reduce to

S(c ) = 1

i>\S-o) 2 2 2

and

1 2^2^ 2 - s

^ ^^S^ ~ 2 2 2 2

^ [1 + oiVCg'^]^

In figure 5, S and S' are plotted as a function of Cg with R as a parameter. For the value R = 50, it can be seen that the added error in

measurement due to the BIU should be less than 1% in S for Cg < 300 pF

and less than 1% in S' for Cg < 180 pF, These ranges of Cg measurable

with low added error far exceed any application requirements encountered

to date.

11
4-1 •H>

•H

4-) •Hd

0)

CO

4-1

0)

cuo

0)>

•H V-i 0)

4-1 B

5-l

enm ^—

o

4-)

Cfi •H

4J>

•H

4J •H

COC

0) CO m

4-1 'A

fS >

•H

4-)

0)U COd

IW CD O U

(U CO > O /-N rH C/) ^ O

IT)

m

0)

u

u o

bO

U

•ri

12
(2) ^ 0

It is helpful to write eqs (3) and (4b) in the form

and

S (5a) S(C ) = —

(1-aCg)^ + 3Cg^

- ^^-"V'-^^s' (5b)

[(1-aCg)^ + ^C^^r

2 2 2 where a = oo L and 3 = w R . A

We are particularly interested in the region in which S(Cs) and S' (Cg) are close to 1; i.e., the region in which aCs<<l and BCs^<<l. Equations

(5a) and (5b) may be simplified by carrying out the indicated operations

and retaining only first-order terms in aCs and 3Cs^» This gives

S(Cg) ^ 1 + aCg - 3Cg^ (7a)

and

S'(Cg) = 1 + 2aCg - 36Cg^ (7b)

For non-zero a, both S(Cs) and S'(Cs) are increasing functions of Cs at

low values of CSj go through maxima and become decreasing functions of

Cg. These maxima may be found by differentiating eqs (7a) and (7b) and

setting them equal to zero. This gives

S(C_) = 1 + 3c/ (8a)

S max S at Cg = a/2B (8b)

and

S'(C ) = 1 + 33C„^ (8c)

b max o at Cg = a/33 (8d)

It was previously stated that there is no advantage to the reactance X

being capacitive (negative); that this is true can be seen from the

following argument. If the reactance were negative, the terms in eqs

(7a) and (7b) which are linear in Cg would become negative and the

13
departure of S(Cs) and S' (Cg) from their ideal values of 1.00 would

occur more rapidly with increasing Cs. This is clearly undesirable

It is easily demonstrated that for a given allowable error magn'itude in

either S(Cs) or S'CCg), a wider range of measurable Cs can be obtained

by using an "appropriate" value of a(Lx) than would be possible for

a = 0.

Let us consider two examples. First, we consider the case in which the

maximum allowable error in S(Cs) is + e. At its maximum value,

S(Cs) Txisix - 1 + It follows from eqs (8a) and (8b) that the appropriate value of a is

a (e) = 2/17 (9)

The maximum value and, therefore, the maximum range of Cs which can be

measured without exceeding the allowable error e, is found from eqs (7a)

and (9) to be

ACg(a,£) = [1 + v'2~] (10)

This may be compared with the maximum range of Cs which can be measured

without exceeding the allowable error in S(Cs) when a = 0, i.e., when

there is no excess inductance; in this case the range is

ACg(0,£) = (11)

Thus, the appropriate value of a can provide an increase in measurable

range of Cs by a factor of

AC„ (a, e) ^'^^ ''''

As a second example, we consider the case in which the maximum allowable

error in S' (Cs) is + e'. At its maximum value, then, S' (Cs) = 1 + e'.

It follows from eqs (8c) and (8d) that the appropriate value of a for

this case is

a(£') = /Tal' (13)

The maximum value and, therefore, the maximum range of Cs which can be

measured without exceeding the allowable error e* is found from eqs

(7b) and (13) to be

ACg(a,e') = [1 + /~2 ] [e/3&]^^^ (14)

This may be compared with the maximum range of Cs which can be measured

without exceeding the allowable error in S' (Cs) when a = 0, i.e., when

there is no excess inductance; in this case the value is

14
ACg(0,e') = [e/33] (15)

Here again, there is an increase in the measurable range of Cg by a factor of

AC^(a,e')

Note, however, that the "appropriate" values of a are not the same for

equal error magnitudes e and e

*

In summary then, excess inductance Lx can be used to increase the measurable range of Cg without exceeding a set of preassigned error limits

on either S(Cs) or S'(Cs).

4. EXPERIMENTAL RESULTS

4.1. General

The BIU described in section 3 was tested to ensure that it would properly perform the desired bias-protection (transient-suppression) function

and that the accuracy of measurements made with it would not be excessively degraded. These tests and their results will now be described.

4.2. Transient Suppression

The experimental arrangement for testing the transient-suppression

capability of the BIU is shown in figure 6. A test capacitor periodically short-circuiting under high bias (10 kV) is simulated by a motordriven spark gap. The resulting transient voltage at the C-meter

terminals is picked up with a probe and displayed on a fast oscilloscope.

The voltage from eaoh C-meter terminal to ground was checked individually

for each polarity of applied bias and found to have a peak value less

than + 200 V. A typical example is shown in figure 6(b).

In addition, repeated sample breakdowns were simulated under actual test

conditions; i.e., with a C-meter connected. These tests were conducted

with three different types of commercially available C-meters: (i) a BEC Model 71A, (ii) a BEC Model 72AD, and (iii) a PAR Model 410. The

C-meters were tested before and after the simulated breakdowns; no damage or change in calibration was found.

Furthermore, many actual sample breakdowns have occurred during C (V)

measurements using the BIU at high voltage levels (approaching 10 kV)

In none of these breakdowns was a C-meter damaged.

15
OSCiLLO

SCOPE

CE

L

iO:|

VOLTAGE

DIVIDER

PROBE

CAPACITANCE METER

BIAS-ISOLATION UNIT

HI

o

TEST

o

LO 0 Q

MOTOR-DRIVEN

SPARK GAP

lOkV

POWER

SUPPLY (a)

ii

>>

o

o

(b)

lOOnS/div.

Figure 6. (a) Schematic representation of the experimental arrangement

for testing the transient suppression capability of the BIU. (b) Photograph of a typical observed transient (HI terminal to ground)

,

16
4.3. Effect of the BIU on Measurement Accuracy

In section 3.2 the effect of the BIU on the accuracy of C-meter measurement was considered from a theoretical point of view. The actual

effect was determined experimentally by measuring S(Cs) and S' (Cg) for

each of the three C-meters used in the transient suppression tests

(section 4.2). Prior to these measurements L6 and L7 were adjusted to

produce minimum reactance at 1 MHz between the terminals J3 and J6 and

between J4 and J7 of figure 4. That is, there was no intentional use

of excess Inductance to "stretch" the measurable range of Cs. Some

slight additional inductance was, of course, present due to the leads

connecting the BIU to the C-meter and the test capacitor to the BIU.

In order to measure S(Cs) and S'(Cg), a precision decade capacitor was

used as a standard. This 3-terminal capacitance standard has an accuracy of 0.25% for each of its component capacitors. In each case, the

output of the C-meter was read on the 10- inch scale of an X-Y recorder

with an accuracy of 0.2% of full scale. Each C-meter was calibrated

on the appropriate scale (s) by connecting the standard capacitor

d-ireotly to the C-meter (i.e., with BIU out of the circuit), and following the manufacturer's calibration Instructions.

After the completion of the calibration procedure, the standard capacitor was connected to the C-meter through the BIU, and the apparent capacitance (as measured through the BIU) was determined as a function of the

standard capacitance.

From section 3.2 S(Cs) = apparent value of Cg/standard value of Cs.

It was not possible to obtain S'(Cs) directly. Therefore, the following

approximation was used: S' (Cg) = (measured increase in the apparent

value of Cg due to an actual increase of 6Cs)/6Cs. For all of the

measurements discussed in this report, 6Cs = 1 pF. The actual measurement procedure was as follows: (1) a value of Cg was set on the standard

capacitor, (11) the output was reduced to zero using the ZERO SUPPRESSION

controls, (ill) the sensitivity of the recorder was increased by a factor

such that an additional 1 pF should cause full-scale deflection on the

recorder, (Iv) the output was again adjusted to zero, (v) the standard

capacitor was Increased by 1 pF, and (vi) the value of S'(Cg) was read

directly from the recorder with full scale corresponding to S' = 1.00;

if the deflection exceeded full scale by more than 1% (the available

recorder over-range) the sensitivity was reduced by a factor of 1/2,

in which case half-scale deflection corresponded to S' = 1.00.

Measurements of apparent capacitance versus standard capacitance were carried out using each C-meter, and the value of S(Cg) was computed for

each data point. The results are shown in figures 7, 8, and 9. For

comparison, the theoretical expression, eq (5) for S(Cg) based on the

simplified equivalent circuit with Lx = 0, is also shown in each figure.

Qualitatively, the results are similar for the three C-meters, viz.,

the falloff of S(Cg) with increasing Cg is smaller than would be expected

from eq (5) over most of the measured range of Cg. This occurs un- doubtedly because the connecting leads provide a small excess inductance

17
(jd) lN3ilVddVQ

= S 'A1IAI1ISN3S 3AllVn3a

u

00

•H

18
(jd)lN3UVddVQ

QdVONViSQ

lN3dVddVQ

«S 'AilAlllSN3S 3AllV13a

19
20
Lx, having the effect discussed In section 3.2. This is most evident

in the data for the Boonton Model 72AD C-meter (see fig. 8), where

there is a detectable peak in S(Cs).

There is one other deviation from SCCg) = 1.00 using the BEC Model 71A.

This occurs on the lower capacitance ranges where the input resistance

is high. On the 1, 3, and 10 pF scales, S(Cs) was 0.892, 0.968, and

0.995, respectively, independent of Cg. This falloff in sensitivity is

due to the loading of the input circuit by the BIU. The effect is not

detectable on the 30 pF and higher capacitance ranges. The effect is

not a serious one in any case since it is independent of Cg and may

easily be compensated by an appropriate internal adjustment of the Cmeter sensitivity on the lower scales (if the C-meter is dedicated to

operation with the BIU) or, alternatively, by appropriately adjusting

the recorder sensitivity. The input impedance of each of the other

two C-meters tested is sufficiently low that they are not loaded by the

BIU on any scale.

The value of S'(Cs) was then determined for each of the C-meters. The

results are plotted as open circles in figures 10, 11, and 12. The

other data in figures 11 and 12 will be discussed shortly. For com- parison, each figure also shows the theoretical expression, eq (6),

for S' (Cg) based on the simplified equivalent circuit with = 0.

The results for the three C-meters are distinctly different in this case.

For the BEC Model 71A C-meter (see fig. 10) the experimentally determined values of S' (Cg) rise above 1.0 at low Cg and fall below 1.0 at

higher Cg. The initial rise is thought to be due to the excess inductance provided by the connecting leads. Above Cg % 120 pF, the experimental values of S'(Cg) fall off more rapidly with increasing Cg than

would be expected from eq (6). There are probably two reasons for this:

(i) The measurements were made using the 100 pF scale corre- sponding to an input resistance of approximately 60 Q; as

Cg and the zero-suppression capacitance are increased,

the effective capacitance from the HI terminals to ground

increases, shunting the 60-fi input resistance and lowering

the apparent sensitivity of the C-meter.

(ii) The effective series resistance of the BIU may be larger

than 50 fi due to the series resistance contribution of the

coils and blocking capacitors.

Nevertheless, there is a measurement range of 140 pF in Cg in which S'

does not deviate from its ideal value of 1.0 by more than 1%.

For the BEC Model 72AD C-meter (see fig. 11), the initial experimental

results (unfilled circles) were rather bizarre. Similar results were

obtained from the initial experimental measurements (unfilled circles

in fig. 12) of S'(Cg) using the PAR Model 410 C-meter. The undulation

of S' (Cg) suggested the possibility of resonances occurring at frequencies other than the 1-MHz test signal. An analysis of the equivalent

circuit at harmonics of 1 MHz, taking into account the value of Cs at

21


(®0),S

24
which the largest undulation takes place, indicated that the major

culprit was probably a third-harmonic component in the test signal.

To test this hypothesis, a crude filter was assembled for use between

the BIU and the C-meter (fig. 13). The purpose of the filter was to

attenuate the third-harmonic component of the current flowing from

the test capacitor Cs into the C-meter HI terminal. The measurements

of S' (Cs) were repeated for the BEC Model 72AD and PAR Model 410 Cmeters using the filter. The results are shown as the squares in

figures 11 and 12. It was clear that the filter provided a substantial

improvement in the 0 to 100 pF range of Cs; it was equally clear that

further improvement was desirable. For this reason, a spectral analysis of the test signal of each of the three C-meters was carried

out. The results (shown in table 2) indicate that the BEC Model 71A

has the "cleanest" test signal, while the test signals of the other two are relatively rich in harmonics. These results confirmed that the

imexpected behavior of S' (Cs) was indeed due to the presence of undesirable harmonics in the test signal.

TO HI TERMINAL

OF C-METER

V

La

La= 36-6VH

Lg =3- 5.5/iH

C= 620pF

La IS ADJUSTED TO

RESONATE WITH

C AT 1MHz

IS ADJUSTED TO

PROVIDE MINIMUM

IMPEDANCE AT 3MHz

FROM BIU TERMINAL

TO GROUND

Figure 13. Schematic circuit diagram of 3-MHz filter.

25
It was felt that the best place to eliminate the harmonics was in the

C-meter, either in the test signal generator or in the amplifier chain

preceding the phase-sensitive detector. Accordingly, arrangements were

made with the PAR Corp. to obtain a modified version of the PAR Model

410 C-meter with extra filtering to eliminate the unwanted harmonics.

The results of a measurement of S' (Cg) using the BIU with this C-meter

are shown as the filled circles in figure 12. The deviation of S' (Cg)

from 1.0 is less than 1% over the range 0 to 200 pF. The experimental

points do not fall below 1.0 as rapidly with increasing Cg as would be

expected from eq (6) ; this is consistent with the behavior expected ' from a small excess inductance provided by the connecting leads. It is

reasonable to expect that similar results could be obtained using the

BIU with a EEC Model 72AD C-meter suitably modified to filter out the

test signal harmonics.

Table 2. Test-Signal Spectral Analysis for Three C-Meters

F (MHz)

Test Signal Component (mV ) rms

BEC Mod. 71A BEC Mod. 72 AD PAR Mod. 410

1 13.0 12.7 10

2 0.112 0.590 0.35

3 0.050 0.063 0.400

4 0.010 0.044 0.100

5 <0.007 0.050 0.112

6 <0.007 0.014 0.050

7 <0.007 0.009 0.045

8 <0.007 0.009 0.040

9 <0.007 0.009 0.032

10 <0.007 0.010 0.028

These results may be summarized as follows: It has been demonstrated

that the BIU can be used with any of three commercially available C-meters

at applied voltages up to 10 kV. For less than + 1% error in relative

sensitivity S(Cs), the measurable range of Cg is from 0 to greater than

400 pF; for less than + 1% error in relative incremental sensitivity

S'(Cg), the measurable range of Cs is from 0 to greater than 130 pF.

5. DISCUSSION AND CONCLUSIONS

It is useful to consider the results which have been achieved thus far

with the BIU and to place them in the broader perspective of the requirements of present and possible future applications.

The two applications of high-voltage capacitance measurements which have

already been investigated in some detail are:

(i) measurements of C(V) of metal-glass-silicon capacitors for

the purpose of characterizing the interface between silicon

and a passivating glass layer (tg^^ss ym) , and

26
(ii) measurements of C(V) of metal-sapphire-silicon capacitors

for the purpose of characterizing SOS and the siliconsapphire interface (tg^ppj^-j^j-g ^ 100 to 150 ym) .

In these applications the range of values of Cs encountered was 5 to

55 pF. Clearly, the equipment described herein is entirely adequate

for these measurements. It is anticipated, however, that future measurement applications (using thicker sapphire wafers) will require biasvoltage capability up to about + 25 kV, and current efforts are directed

toward the development of equipment that will be capable of operating

at these higher voltage levels.

Two other applications have been investigated only in sufficient detail

to show that the measurement technique is a useful one. These applications are: (i) characterization of the interface between heavily

doped silicon and a thick (> 1 ym) overlying insulator, (ii) measurement

of the base-collector capacitance of a very high voltage transistor

(max Vc ^ 1500 V) in order to nondestructively characterize the doping

profile of its base-collector region. Other possible future applications

include the characterization of new passivating and encapsulating layers

for semiconductor devices (e,g,, plastics, resins, and epoxies) , studies

of the electric field dependence of dielectric polarization under high

fields, and studies of dielectric-electrolyte interfaces. In each

of these applications, the equipment described appears to be adequate

for all current and future applications.

In conclusion:

(1) A technique has been described which allows the safe operation of

commercially available C-meters for capacitance measurements with

applied-bias voltage up to + 10 l^V,

(2) The technique requires a bias- isolation unit for which the circuit,

theory, and practical results have been presented,

(3) Details of construction of the bias- isolation unit are provided in

the appendix,

ACKNOWLEDGMENT

I am indebted to Chester J, Halgas for the mechanical design and the

construction of the bias- isolation unit described in this report, to

Paul Kuczer for construction of an earlier developmental version, and

to James M, Breece for assistance with some of the measurements.

27
APPENDIX

This appendix contains information which is intended for use by anyone

who wants to duplicate the BIU described in the body of the report.

It consists of:

(i) A list of the mechanical parts which are not commercially

available,

(ii) detailed drawings of those parts, and

(iii) a drawing and photographs showing the placement of all

parts (the circled numbers refer to the mechanical parts

and electrical components identified in tables 1 and 3).

Table 3. List of Mechanical Parts

(not commercially available)

Part Dtawi n Q

No. Part Description No.

1.-J- • Front panel Ml

2. Left side panel M2

3, Right side panel M2

A Rear panel M"^ iij

5. Top cover M4

6. Bottom cover M4

7. Clear acrylic base M5

8. Peaker /zero-suppression shield M6

9. Shield cover M7

10. Shield cover bracket M7

11. Mounting bracket for L6, L7 M7

12. Acrylic bias-divider mounting plate M9

13. Acrylic front M9

14. Acrylic rear M9

15. Acrylic left side M9

16. Acrylic right side M9

17. Peaker/zero-suppression acrylic mounting bracket MIO

18. Zero-suppression fine-capacitor acrylic mounting bracket Mil

19. Support M12

20. Rectifier mounting plate M13

21. Zener diode heat sink M14

22. Peaker/Zero-suppression shield rear insulator M15

23. Acrylic cover insulator M16

24. PIN diode heat sink M7

28
29
30
< <

- rM 9^

< 3

C J §5

r-I

CJ

0)ca

u

CO

OJ Pi

0)u

3

•H

® —

to

31
It «l

-450-

6H0LP5 .l&7oiA

450

'8 2'A5(e^ liCDi^ 1/

i_

—

®

1000-

Z8I

4 50

4,5o

'c' HOL.es

.

CHASSIS TOP 4 SoTTDWS

L

Figure M4 Chassis top and bottom covers.

32

i

a fSi

5 <

>

r

Li*

®—

1

I/)

cvN

6

2

3<

\8li

4

"5" Tf

(U •H,G

CO

CIo

•rlCO

CO<U

)-l (Xa

COo

u

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CO(U PMS

(U

00

•H

fa

34
^1

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a! 5

a

0

2

< a

< a /a

1^CD V•vr

O 2 O

< 0

..^<

T

"

5

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FT

n

r-JC o

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a o

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(1) c o

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oo

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35
36
-I.e«- >-)*— <»« 'I*

12.15"- — 15<>-

-3.4*(-

in

100 ©'•— I

Ji5

0

3.35-

-4 01)-

ACRYUC SibES

-H — '25

2 ttOoieED

.12'ibiA (S) J4^i}l^

=-l—

^ 7t I 7&

t _ *. U f

00

[«- .15 -»-U-.7{ .>j

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.,.|.TT

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-----

j

-> 3.53 —• 3 36 —J i

i

2 eccjitSO

7t l53- 'a—1^ 4 © .20/ .'"!

(5) .MTCXA

Figure M9 Drawing containing: A. Acrylic bias-divider mounting plate

B. Acrylic front and rear

C. Acrylic sides.

37
38
39
o

>•

<5;

J

• *

o

<

6

Io

CL

a

o

u

u

oa

3

40
41
0

J

<

J

J

I

5

-J

-4- 0

IP

r

<7\

O

X

42
43

45
Figure A2 Top view with top cover removed.

46
47
i
REFERENCES

1. Goodman, A. M. , A Useful Modification of the Technique for Measuring Capacitance as a Function of Voltage, IEEE Trans, Eleatvon

Devices ED-21 , 753-757 (1974).

2. Goodman, A. M. , An Investigation of the Silicon-Sapphire Interface

Using the MIS Capacitance Method, IEEE Trans, Electron Devices

ED-22 , 63-65 (1975).

3. Instruction Manual for BEC Models 71A and 71AR Capacitance/Inductance

Meter, Boonton Electronics Corporation, Whippany, N.J.

49

NBS.114A (REV. 7-73)

U.S. DEPT. OF COMM. 1. PUBLICATION OR REPORT NO. 2. Gov't Accession BIBLIOGRAPHIC DATA No. SHEET Special Publication 400-34

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

SemioonduotoT Measva>ement Teahnology : Safe Operation of

Capacitance Meters Using High Applied-Bias Voltage

5. Publication Date

6. Performing Organization Code

7. AUTHOR(S)

Alvin M. Goodman

8. Performing Organ. Report No. PRRL-76-CR-27

9. PERFORMING ORGANIZATION NAME AND ADDRESS

RCA Laboratories

Princeton, New Jersey 08540

10. Project/Task/Work Unit No. 4258408

n. Contract/Grant No. 5-35912

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

NBS - Washington, D.C. 20234

ARPA - Arlington, Virginia 22209

13. Type of Report 8c Period CoveredInterim

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES 'j>Y]±s work was funded by the Defense Advanced Research Projects

Agency under ARPA Order 2397, Program Code 6D10,

Library of Congress Catalog Card Number: 76-6O8338

16. ABSTRACT (A 200-word or less tactual summary of most si0iiticant information. If document includes a significant bibliography or literature survey, mention it here.) iiie use or capacitance meters mecersy to ae ue.'. uixne omaxi oj-gncii v.*ini"-*-*=nLXcij.y

capacitance at 1 MHz as a function of applied-bias voltage is widespread. The maximum

value of the bias voltage which may be applied to a sample under test with any commercially available C-meter is 600 V or less. A larger bias-voltage capability is required

for certain applications.

This report describes a technique for using a commercial C-meter with a Bias-Isolation

Unit (BIU) for capacitance measurements at bias-voltage magnitudes up to 10 kV without

damage to the measurement equipment. The basic principles of operation and the details

of the electrical design of a BIU are discussed.

The use of the BIU imposes certain limitations on the range of sample capacitance

which may be measured without introducing excessive error. The theory of these limitations is presented and compared with experimental results obtained from the use of the

BIU with each of three commercially available C-meters. The measurement capability

demonstrated by these results appears to be adequate for all current and future applications. For less than + 1% error in the indicated (measured) capacitance, the measurable

range of the sample capacitance is found to be from 0 to at least 400 pF. In some

applications, it is important to be able to accurately measure small changes in the

sample capacitance; for less than + 1% error in the indicated (measured) value of a small change in the sample capacitance, the measurable range of the sample capacitance

is found to be from 0 to at least 130 pF.

Construction details of the BIU are appended.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Blas-Isolation Unit; capacitance measurements at high appliedbias voltage; capacitance-meter; extended-range capacitance measurement; high-voltage

C(V) measurements; modified MIS C(V) measurements.

18. AVAILABILITY Q Unlimited

1 For Official Distribution. Do Not Release to NTIS

1 x' Order From Sup. of Doc, U.S. Government Printine Office Washington. D.C. 20402. SD Cat. No. C13 .lOti+UO-S^

1 1 Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

21. NO. OF PAGES

57

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price

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Superintendent of Documents,

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Please add my name to the announcement list of new publications to be issued in the series: National Bureau of Standards Special Publication 400-,

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ftU.S. GOVERNMENT PRINTING OFFICE: 1976 240-848/69 1-3

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NBS TECHNICAL PUBLICATIONS

PERIODICALS JOURNAL OF RESEARCH reports National Bureau

of Standards research and development in physics,

mathematics, and chemistry. It is published in two

sections, available separately:

• Physics and Chemistry (Section A)

Papers of interest primarily to scientists working in these fields. This section covers a broad range of physical and chemical research, with major emphasis on

standards of physical measurement, fundamental constants, and properties of matter. Issued six times a year. Annual subscription: Domestic, $17.00; Foreign, $21.25. • Mathematical Sciences (Section B)

Studies and compilations designed mainly for the mathematician and theoretical physicist. Topics in mathematical statistics, theory of experiment design, numerical

analysis, theoretical physics and chemistry, logical design and programming of computers and computer sys- tems. Short numerical tables. Issued quarterly. Annual

subscription: Domestic, $9.00; Foreign, $11.25.

I DIMENSIONS/NBS (formerly Technical News BulleI tin)—This monthly magazine is published to inform

I scientists, engineers, businessmen, industry, teachers,

I students, and consumers of the latest advances in science and technology, with primary emphasis on the

j work at NBS. The magazine highlights and reviews

j such issues as energy research, fire protection, building

technology, metric conversion, pollution abatement,

! health and safety, and consumer product performance.

In addition, it reports the results of Bureau programs

. in measurement standards and techniques, properties of

; matter and materials, engineering standards and services, instrumentation, and automatic data processing.

Annual subscription: Domestic, $9.45; Foreign, $11.85.

NONPERIODICALS

Monographs—Major contributions to the technical liter- ature on various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and

! industrial practice (including safety codes) developed

' in cooperation with interested industries, professional

organizations, and regulatory bodies.

Special Publications—Include proceedings of conferences

sponsored by NBS, NBS annual reports, and other

special publications appropriate to this grouping such

as wall charts, pocket cards, and bibliographies.

Applied Mathematics Series—Mathematical tables, manuals, and studies of special interest to physicists, engineers, chemists, biologists, mathematicians, comI puter programmers, and others engaged in scientific

ji and technical work.

National Standard Reference Data Series—Provides

quantitative data on the physical and chemical properties of materials, compiled from the world's literature

j and critically evaluated. Developed under a world-wide

1 program coordinated by NBS. Program under authority

of National Standard Data Act (Public Law 90-396).

NOTE: At present the principal publication outlet for these data is the Journal of Physical and Chemical

Reference Data (JPCRD) published quarterly for NBS

by the American Chemical Society (ACS) and the American Institute of Physics (AIP). Subscriptions, reprints,

and supplements available from ACS, 1155 Sixteenth

St. N.W., Wash. D. C. 20056. Building Science Series—Disseminates technical infor- mation developed at the Bureau on building materials,

components, systems, and whole structures. The series presents research results, test methods, and performance criteria related to the structural and environmental

functions and the durability and safety characteristics of building elements and systems.

Technical Notes-r-Studies or reports which are complete

in themselves but restrictive in their treatment of a

subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under proce- dures published by the Department of Commerce in Part

10, Title 15, of the Code of Federal Regulations. The

purpose of the standards is to establish nationally rec- ognized requirements for products, and to provide all concerned interests with a basis for common understanding of the characteristics of the products. NBS

administers this program as a supplement to the activities of the private sector standardizing organizations.

Consumer Information Series—Practical information,

based on NBS research and experience, covering areas

of interest to the consumer. Easily understandable language and illustrations provide useful background knowledge for shopping in today's technological marketplace.

Order above NBS publications from: Superintendent

o f Documents, Government Printing Office, Washington,

D.C. 20402. Order following NBS publications—NBSIR's and FIPS

from the National Technical Information Services,

Springfield, Va. 22161. Federal Information Processing Standards Publications

(FIPS PUBS)—Publications in this series collectively constitute the Federal Information Processing Standards Register. Register serves as the official source of information in the Federal Government regarding stand- ards issued by NBS pursuant to the Federal Property

and Administrative Services Act of 1949 as amended.

Public Law 89-306 (79 Stat. 1127), and as implemented

by Executive Order 11717 (38 FR 12315, dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal

Regulations).

NBS Interagency Reports (NBSIR)—A special series of interim or final reports on work performed by NBS for outside sponsors (both government and non-government). In general, initial distribution is handled by the sponsor; public distribution is by the National Technical Information Services (Springfield, Va. 22161) in paper copy or microfiche form.

BIBLIOGRAPHIC SUBSCRIPTION SERVICES

I The following current-awareness and literature-survey

bibliographies are issued periodically by the Bureau:

Cryogenic Data Center Current Awareness Service. A

literature survey issued biweekly. Annual subscription: Domestic, $20.00; Foreign, $25.00.

Liquified Natural Gas. A literature survey issued quarterly. Annual subscription: $20.00.

Superconducting Devices and Materials. A literature survey issued quarterly. Annual subscription: $20.00.

Send subscription orders and remittances for the pre- ceding bibliographic services to National Bureau of Standards, Cryogenic Data Center (275.02) Boulder,

Colorado 80302.
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington. D.C. 20234

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