A111D3 Dfi75Mfi

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

A1 11 03087548

Goodman, Alvin M/Suppresslon of prematur QC100 .U57 NO.400-37, 1977 C.2 NBS-PUB-C

CO Q

'USAM Of * NBS SPECIAL PUBLICATION 400-37

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Semiconductor Measurement Technology.

Suppression of Premature Dielectric Breakdown

for High-Voltage Capacitance Measurements

OF STANDARDS

UBRABI

AUG 1 1977

Alvin M. Goodman

RCA Laboratories

Princeton, New Jersey 08540

Jointly supported by: The Defense Advanced Research Projects Agency

and The National Bureau of Standards

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

V.^ NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued July 1977
Library of Congress Cataloging in Publication Data

Goodman, Alvin M

Suppression of premature dielectric breakdown for high-voltage

capacitance measurements.

(Semiconductor measurement technology) (NBS special publication

; 400-37)

Supt. of Docs. No.; C 13.10:400-37

1. Electric capacitors —Testing. 2. Metal insulator

semiconductors—Testing. 3. Breakdown (Electricity) 4. High

voltages. I. Title. II. Series. III. Series: United States. National Bureau of Standards. Special publication ; 400-37. QC100.U57 no. 400-37 [TK.7872.C65] 602Ms [621. 3r5]

77-608129

National Bureau of Standards Special Publication 400-37

Nat. Bur. Stand. (U.S.), Spec. Publ. 400-37, 27 pages (July 1977) CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1977

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

Price $1.30 Stock No. 003-003-01809-1
CONTENTS

PAGE

1. Introduction 1

1.1. Description of the Problem 1

1.2. Conventional Techniques for Suppression of Premature

Breakdown 2

2. A New Technique 3

2.1. Basic Description 3

2.2. Operation 5

3. Experimental Results 6

Acknowledgment 6

References 7

Appendix 8

FIGURE CAPTIONS

Figure 1. Sample configuration illustrating: (a) Critical

region where premature breakdown occurs; and (b) Use

of silicone grease to suppress premature breakdown . . 2

Figure 2. Oblique view of apparatus. The washer frame is

raised, the baseplate is moved to the side, and the

probe-carrier box is partially raised on its hinge

for visual clarity 4

Figure 3. Overhead view of apparatus with sample in place and

probe-carrier box in extreme upward position 4

Figure 4. Side view of apparatus with silicone washer pressed

firmly against sample and probe lowered to contact

sample electrode 5

LIST OF TABLES

Table 1. List of Major Mechanical Parts Used in Probe and

Breakdown Suppression Washer Apparatus 9

Table 2. List of Major Mechanical Parts Used in Test Chamber . . 10

iii

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.

Larger scale drawings of the mechanical parts are available on request

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

DC 20234.

DISCLAIMER

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.

^Through ARPA Order 2397, Program Code 6D10.

V

SEMICONDUCTOR MEASUREMENT TECHNOLOGY: Suppression of

Premature Dielectric Breakdown for High-Voltage

Capacitance Measurements

Alvin M. Goodman

RCA Laboratories

Princeton, N.J. 08540

Abstraot: Surface-initiated premature dielectric breakdown is

encountered in extended-range MIS C(V)* measurements at applied-bias

voltages above some sample-dependent threshold value, e.g., 3 to 5 kV

across a ISO-ym-thick wafer of sapphire. It is necessary to suppress

this premature breakdown in order that a much larger applied-bias voltage may be used without damaging the sample. This may be accomplished

by eliminating the air space adjacent to the sample surface at the junction of the dielectric and the electrode edge. A simple, easy-to-use

apparatus (sample holder and probe assembly) which allows this to be

done conveniently and quickly by using a silicone rubber washer to cover

the edge of the electrode and the adjacent area is described. Construction

details of the apparatus and a test chamber which have been tested to

30 kV are provided in an appendix.

Key Words: Capacitance-voltage measurements; dielectric breakdown

suppression; discharge suppression; electronics; extended-range MIS C(V)

measurements; high-voltage C(V) measurements; MIS capacitor; premature

dielectric breakdown; semiconductor devices.

1. INTRODUCTION

1.1. Description of the Problem

A recently developed modification of the MIS C(V)* technique [1] has

extended its useful range allowing it to be used for the measurement of

samples with insulating layers more than two orders of magnitude greater

in thickness than was previously possible. This use, however, requires

the application of large bias voltage to the capacitor sample to be

measured. When the bias voltage exceeds some threshold value (e.g., 3

to 5 kV across a 150-ym-thick wafer of sapphire [2]) there exists the

danger of breakdown in the critical region along the insulator surface

or in air at an electrode edge as shown schematically in figure 1(a).

The breakdown at the surface or in air usually leads to breakdown of the

dielectric under test (sapphire in the present work). This problem is

similar to the one of "premature breakdown" encountered in the measurement of "breakdown strength" in dielectric media [3]. The present problem is different, however, in three respects: for C(V) measurements

(a) the high-field region should have a well-defined area; (b) the field

within this region should be as uniform as possible; and (c) there is no need to raise the applied field to the breakdown level.

*MIS C(V) is a commonly used abbreviation for metal-insulator-semiconductor

capacitance as a function of voltage.

1
METAL (GATE)

1 b

Figure 1. Sample configuration illustrating:

(a) Critical region where premature breakdown

occurs; and (b) Use of silicone grease to

suppress premature breakdown.

1.2. Conventional Techniques for Suppression of

Premature Breakdown

The known methods of preventing premature breakdown [3] can be roughly

categorized as follows:

1. Immersion of the sample or its critical region in

(a) suitable gas at high pressure (e.g., N2 at 100 atmospheres)

(b) liquid, either insulating (e.g., oil) or partially conducting (e.g., aniline)

(c) solid (e.g., silicone grease or elastomer)

2. Shaping of sample geometry to reduce the electric field at the

electrode edges (e.g., "recessed specimen" or "McKeown specimen"

technique)

.

2
Each of these methods in its conventional embodiments is undesirable

in the present application for one or more reasons: 1(a) would be

difficult and time-consuming to implement; 1(b) and 1(c) are messy

and inconvenient to use; and 2 is inconsistent with our requirement

of a uniform field over a well-defined area. Method 1(c) was used [2]

with partial success up to about 8 kV. The solid used to immerse the

critical region was a silicone grease (Dow Corning "A-Compound") .

This is shown in schematic cross section in figure 1(b). The application of the silicone grease to the sample and its removal after the

measurement were difficult, messy, and time-consuming. In addition,

the technique did not consistently prevent breakdown above about 6 kV.

In one set of experiments half of the samples were destroyed by dielectric

breakdown when voltages up to 8 kV were applied across ISO-ym sapphire

wafers during measurements. These and other equally catastrophic results

made it clear that a better method for breakdown prevention was needed

to make the measurement technique a practical one for everyday use rather

than a research laboratory curiosity.

The new breakdown suppression technique and the associated apparatus

and test chamber described in the following sections of this report were developed as part of a measurement system, to be used for routinely

carrying out high-voltage extended-range MIS C(V) measurements. The

first part of that system (a capacitance meter bias-isolation unit)

has been previously described [4]

.

2. A NEW TECHNIQUE

2.1. Basic Description

It has been found that if an insulating silicone rubber toroidal washer

having appropriate properties is pressed over the sample surface including the electrode edge, premature breakdown can be effectively prevented.

The surface of the rubber in contact with the sample must be smooth and

the rubber itself must be very resilient. It is also important that the

rubber contain no voids in the vicinity of the critical region and that

it have both a very high resistivity and a low dissipation factor. The

technique includes a means for pressing the washer uniformly and con- trollably over the surface of the sample. Washers molded from two

silicone compounds have been used successfully; these are Dow Corning

Sylgard #182 and #186.* It has been found that a thin coating of silicone grease on the surface of the washer in contact with the sample

gives even more reliable protection against breakdown than using just

the washer alone. Photographs of one version of the apparatus used

for implementing the technique are shown in figures 2, 3, and 4.

*See Disclaimer on p.iv.
Figure 2. Oblique view of apparatus. The washer frame

Is raised, the baseplate Is moved to the side, and the

probe-carrier box Is partially raised on Its hinge for

visual clarity.

Figure 3. Overhead view of apparatus with sample

In place and probe-carrier box In extreme upward

position.

4
\

Figure 4. Side view of apparatus with silicone ' washer pressed firmly against sample and probe

lowered to contact sample electrode.

In figure 2 an oblique view of the apparatus is shown. The washer frame

has been raised and the baseplate has been moved to the side for visual

clarity. The silicone washer is held in a recessed hole which has been

counter-bored in the bottom of the washer frame. The probe carrier box

is shown unlatched and partially raised on its hinge. In figures 3 and

4, the sample is shown in place with the washer frame lowered and the

washer pressed firmly against the sample.

2.2. Operation

In operation, the baseplate with the sample mounted on it is slid under

the washer frame and the two horizontal-axis micromanipulators are adjusted to make the washer concentric with the edge of the electrode on

the sample. Both the washer and washer frame are transparent allowing

this adjustment to be made quickly and easily. During this adjustment,

the probe carrier box is swung up as far as it can go to allow the operator maximum visibility; this is illustrated in figure 3. The verticalaxis micromanipulator is then used to move the washer frame down, pressing

the washer against the sample.

5
It is important that the washer be pressed against the sample hard enough

to fill any air spaces or voids at the sample surface. This pressure

extrudes the washer slightly as can be seen in figures 3 and 4.

The probe carrier box is then swung down and latched in place. The

probe can then be lowered through the concentric holes in the washer

and frame to make contact with the electrode. This is accomplished by

turning either of the two cam-shaft knobs, thereby rotating the cam

against the probe carrier and forcing it down. The probe has a springloaded retractable tip to provide a reliable contact to the evaporated

electrode on the sample without damaging it. ^

3. EXPERIMENTAL RESULTS

The use of the technique described in the preceding section has virtually

eliminated dielectric breakdown during MIS C (V) measurements at appliedbias voltages up to 12 kV across 150-ym- thick sapphire wafers. In addition, further tests were carried out at higher voltages. A 125-ym-thick

sapphire wafer was tested to breakdown at 16 kV. One 350-ym-thick

sapphire wafer was tested without breakdown up to 30 kV which was the

approximate limit of our test capability.

In summary, a new, simple, easy-to-use technique for suppressing premature

dielectric breakdown during high voltage C(V) measurements has been described. The technique allows the use of a much larger applied-bias

voltage than was previously possible.

ACKNOWLEDGMENT

I am indebted to J. D. Morgan for his contribution to the design and

construction of some of the essential hardware, to N. Capone, Jr. for

the drafting in the appendix of this report, and to J. M, Breece for

his help with the sample preparation and measurements.

6
REFERENCES

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

Devices ED-21 , 153-151 (1974).

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

Using the MIS Capacitance Method, IEEE Trans. Electron Devices

ED-22 , 63-65 (1975).

3. Carton, C. G., Intrinsic and Related Forms of Breakdown in Solids,

Eigh Voltage Technology , L. L, Alston, Ed., pp. 144-157 (Oxford

University Press, London, 1968).

4. Goodman, A. M. , Semi-conductor Measurement Technology: Safe Operation of Capacitance Meters Using High Applied-Bias Voltage, NBS

Special Publication 400-34 (December 1976).

7
APPENDIX

A. 1.0. General Description

The apparatus described in the body of this report has been designed for

use inside a closed test chamber. This is necessary for electrical and

optical shielding of the sample as well as for human safety. This appendix describes a safety-interlock arrangement used in conjunction with

the test chamber and provides assembly and construction detail drawings

of both the apparatus and the test chamber.

A. 1.1. Safety Interlock

To prevent high (and potentially lethal) voltages from being applied to

the sample while the test chamber is open, a pair of microswitches is

provided as part of a safety- interlock system. Their location is shown

in the Test Chamber Assembly drawing D-1686997 (See section A. 1.3

below). The switches, which are closed only when the test chamber cover

is fully closed, are wired in series and connected to an insulated BNC

connector. A cable from this connector to the high-voltage power supply

allows a relay-control circuit in the power supply to sense whether the

switch circuit is open or closed. If the switch circuit is closed,

voltage may be applied to the sample. If the switch circuit is open, a pair of high-voltage relays inside the power supply is de-energized;

this opens the output lines and grounds the output terminals.

A. 1.2, Probe and Breakdown-Suppression Washer Apparatus

The assembly and construction details for this apparatus are shown in

drawings C-1687340, C-1687338, C-1687339, C-1687347, C-1687337; a list

of the major mechanical parts and drawing reference numbers is given in

table 1.

A. 1.3. Test Chamber

The assembly and construction details of the test chamber are shown in

drawings C-1687330, C-1687331, C-1687332, C-1687333, C-1687343,

D-1686997; a list of the major mechanical parts and drawing reference

numbers is given in table 2.

Two further explanatory notes:

1. Three high-voltage coaxial chassis connectors are provided in

the back of the test chamber. Normally, only two of these are

used. The third is provided for possible use with a sample

having a guard-ring structure.

8
2. The poppet valve motinted in the back of the test chamber is

connected to the line providing vacuum to the sample base.

When the cover is closed, this line is opened to atmospheric

pressure relieving the vacuum in the line to the base. This

prevents an electrical discharge which might otherwise occur

when high voltage is applied to the sample under test. The

vacuum hold-down is of course not needed when the sample is

held in place by the breakdown-suppression washer.

Table 1. List of Major Mechanical Parts Used in

Probe and Breakdown Suppression Washer

Apparatus

Part

No. Part Description Drawing No

1. Cam and camshaft L— loo

/

JHV

2. Modified knob Li— ±00 / JH\J

3. Standard ffiyiU Knob L» XOO/ OH\J

4. Latch-lever yoke C-1687340

5. Probe-connection rod C-1687340

6. Nylon screw C-1687340

7. Latch-lever spring C-1687340

8. Anti-corona ball C-1687340

9. Latch lever C-1687340

10. Washer frame C-1687338

11. Probe-carrier box C-1687339

12. Probe carrier C-1687339

13. Washer mold C-1687347

14. Probe-carrier spring C-1687347

15. Silicone washer C-1687347

16. Probe (sleeve jacket and

spring-loaded plunger) C-1687347

17. Micromanipulator C-1687337

9
Table 2. List of Major Mechanical

Parts Used in Test Chamber

Part

ULCiWJLLL^ Li\J

J-

.

Test chamber base Vj— XOo / J JU

9 iesu cnaniDer oactc f—l Li~±0OAR7 1

/ J JX

oJ • iesc cnamDer cover U~ J.OO / J A't

.

Retaining ways Li— XDO / J J J

c Thumb screw L.— XDo / J J J

o • noxQ Qown wasner u ±oo / J J

J

7 vacuum leeutnrougn U—XOO/ JJJ

o •

9. Baseplate C-1687333

10. Baseplate insulator C-1687333

11. Switch bracket C-1687343

12. Actuating-arm extension C-1687343

13. Thumb-screw cap C-1687343

10
11
12
13
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15
16
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NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

• BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. Special Publication 400-37

2. Gov't Accession

No.

3. Recipitni's Acces.sion No.

4. TITLE AND SUBTITLE

Semiconduotov Measurement Technology: Suppression of

Premature Dielectric Breakdown for High-Voltage Capacitance

Measurements

5. Publication Date

Jill V 1 Q77

6. Performing Organization Code

7. AUTHOR(S)

Alvin M. Goodman

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

RCA Laboratories

Princeton, New Jersey 08540

10. Project/Task/Work Unit No. 4258408

11. 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 & Period CoveredInterim

14, Sponsoring Agency Code

15. SUPPLEMENTARY NOTES This work was funded by the Defense Advanced Research Projects

Agency under ARPA Order 2397, Program Code 6D10.

Library of Congress Catalog; Card Number; 77-608129

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

bibliography or literature survey, mention it here.)

Surface- initiated premature dielectric breakdown is encountered in extendedrange MIS C(V) measurements at applied-bias voltages above some sample-dependent

threshold value, e.g., 3 to 5 kV across a 150-ym-thick wafer of sapphire. It is

necessary to suppress this premature breakdown in order that a much larger appliedbias voltage may be used without damaging the sample. This may be accomplished by

eliminating the air space adjacent to the sample surface at the junction of the

dielectric and the electrode edge. A simple, easy-to-use apparatus (sample holder

and probe assembly) which allows this to be done conveniently and quickly by using

a silicone rubber washer to cover the edge of the electrode and the adjacent area

is described. Construction details of the apparatus and a test chamber which have

been tested to 30 kV are provided in an appendix.

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) Capacitance-volt age measurements; dielectric breakdown

suppression; discharge suppression; electronics; extended-range MIS C (v) measurements;

high-voltage C (V) measurements; MIS capacitor; premature dielectric breakdown;

semiconductor devices.

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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

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, S25.00 ; Foreign, $30.00'. Liquified Natural Gas. A literature survey issued quarterly. Annual subscription: $20.00.

Superconducting Devices and Materials. A literature survey issued quarterly. Annual subscription: S.'^O.OO . Send subscription orders and remittances for the pre- ceding bibliographic services to National Bureau of Standards, Cryogenic Data Center (275.02) Boulder,

Colorado 80302.
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