NBS

PUBLICATIONS

' 1

«;|&«3-. BS SPECIAL PUBLICATION 400-51

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards

100

.U57

NO.TO-Sll

1979

C.2

Semiconductor Measurement Technology:

A Production-Compatible

Microelectronic Test Pattern

for Evaluating Photomask

Misalignment
NATIONAL BUREAU OF STANDARDS

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APR 2 7 19^

Semiconductor Measurement Technology:

A Production-Compatible IMicroelectronic 1

Test Pattern for Evaluating

Photomask Misalignment

T. J. Russell

D. A. Maxwell

Electron Devices Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Washington, D.C. 20234

Sponsored by the

National Bureau of Standards Washington, D.C. 20234 and

Advanced Research Projects Agency

Arlington, Virginia 22209

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

Jordan J. Baruch, Assistant Secretary for Science and Technology

il NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued April 1979
Library of Congress Cataloging in Publication Data

Russell, Thomas James, 1943- A production-compatible microelectronic test pattern for evaluating

photomask misalignment.

(Semiconductor measurement technology) (NBS special publication ; 400-5 1

)

1. Integrated circuits— Masks—Testing. I. Maxwell, Dwight A., joint author. II. United States. National Bureau of Standards. III. United

States. Advanced Research Projects Agency. IV. Title. V. Series. VI. Series: United States. National Bureau of Standards. Special

publication ; 400-51. QC100.U57 no. 400-51 [TK7874] 602Ms [621 .38 r73'028] 79-230

National Bureau of Standards Special Publication 400-51

Nat. Bur. Stand. (U.S.), Spec. Publ. 400-51, 32 pages (Apr. 1979) CODEN: XNBSAV

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WASHINGTON: 1979

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

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U.S. GOVERNMENT PRINTING OFFICE: 1979 O-281-067 (29)
TABLE OF CONTENTS

Page

Abstract 1

1. Introduction 1

2. Description of the Test Structures 2

3. Description of the Test Pattern 4

4. Use of the Test Pattern 5

Acknowledgments 5

References 5

Appendix A 23

Appendix B , 28

LIST OF FIGURES

1. Contact-to-doped region electrical alignment resistor .... 9

2. Metal-to-doped region electrical alignment resistor 10

3. Metal-to-doped region visual alignment indicator 11

4. Metal-to-doped region visual alignment indicator 12

5. Metal-to-contact visual alignment indicator 13

6. Metal-to-contact visual alignment indicator 14

7. Doped region cross brige sheet resistor 15

8. Metal cross bridge sheet resistor 15

9. Metal-to-doped region contact resistor 17

10. PNP isolation diode 18

11. Computer drawing of NBS-15 test pattern with structures

numbered 19

12. Photomicrograph of NBS-15 test pattern 20

13. Square mask alignment markers 21

14. Plus sign mask alignment markers 22

ill

Semioonduatov Measurement Technotogy

A Production-Compatible Microelectronic Test Pattern

for Evaluating Photomask Misalignment*

by

T. J. Russell and D. A. Maxwell

Abstract: Microelectronic test pattern NBS-15 is

composed of several potentiometric , productioncompatible electrical alignment resistor test structures,

visual alignment indicator test structures, cross bridge

sheet resistors, and contact resistor test structures. The

pattern was originally designed as a study vehicle for the

electrical alignment resistor, but it was also demonstrated

that, when stepped over an entire wafer, the pattern is

suitable for use in evaluating misalignment which may result

from photomask generation, photomask exposure, or other fabrication processes. This report summarizes the test structures that are included in the pattern and contains explanation of how each of the structures is measured.

Key Words: Contact resistor, cross bridge sheet resistor; electrical alignment resistor; semiconductor; silicon;

test pattern; test structures; visual alignment.

1 . INTRODUCTION

Microelectronic test pattern NBS-15 was designed to study several

designs of a potentiometric, production-compatible electrical alignment resistor test structure and to compare these to a sensitive

visual alignment indicator test structure. Results of our studies

show that any of the electrical alignment resistors can be used to

quantitively measure the misalignment between two photomask levels

with a resolution of about 0.1 ym [1] . The purpose of this report

is to describe the test structures included in the NBS-15 test pattern

and to indicate how they and the test pattern can be used.

The electrical alignment resistor is measured using dc techniques.

Since it can be tested on an automated integrated circuit wafer prober,

data can be collected much more quickly using the electrical alignment

test structure than they can be using visual alignment test structures.

Because of the additional potential for computer-controlled measurement,

This work was conducted as part of the Semiconductor Technology

Program at the National Bureau of Standards. This work was supported

by the Defense Advanced Research Projects Agency (Order No. 2397,

Program Code 7D10) . Not subject to copyright.

1
data can be rapidly obtained and analyzed for assessing photomask

alignment and determining causes for misalignment when the structure

is stepped across an entire wafer.

Test pattern NBS-15 was designed for a three-photomask-level

process to produce doped regions, contact windows, and metal probe

pads and interconnects. The doping can be done either by diffusion

or by ion implantation.

,

, 2. DESCRIPTION OF THE TEST STRUCTURES

For the three-level process used here, two types of potentiometric electrical alignment test structures [1] were included. Both

types were designed with different combinations of contact window size,

doped region width, and voltage sensing arm spacing. One type, structures 1 through 5 and structure 8, is used to measure the misalignment

between the doped region and the contact window photomask levels.

This structure is illustrated in figure 1; in this and subsequent

figures each level is identified as follows: doped regions are en- closed by heavy solid lines, contact areas are denoted by cross- hatched rectangles, and the metal areas are enclosed by light solid

lines, and the critical dimensions are labeled. The critical dimensions of the various forms of the structure are given in table 1. The

second type, structures 6 and 7, is used to measure the misalignment

between the doped region and the metal photomask levels. This structure is illustrated in figure 2, and the critical dimensions are also

given in table 1. Both types of structures are measured in the same

way; the structure is analogous to a linear potentiometer where the

mid-point of the resistor is sensed by measuring the voltage imbalance

between the sliding contact and the ends of the potentiometer when a constant current is passed through the resistor. The structure is designed so that when there is perfect alignment between the two mask

levels, the distances L2 between the center of the tap and the center

of the contact window opening (type 1) or the center of the metal

contact arm (type 2) are equal. Then, the resistance R29 between pads

2 and 9 is equal to the resistance Rg 3 between pads 3 and 9, and the

resistance R55 between pads 5 and 6 is equal to the resistance R75

between pads 5 and 7. When the contact window opening (type 1) or the

metal contact arm (type 2) is displaced from the design position, the

resistances are no longer equal. The displacements in the x- and

y-directions , AL and AL , are calculated from [2] X y

AL

and

\ ^65 ^7 6 (1)

X 2 R^2

h ^93 ^29 . (2) % 2 R

12

2
Two types of visual alignment indicator test structures [1,3,4,] were also included. Structures 15 and 23, are used to measure the

misalignment between the metal and doped region photomask levels wliile

structures 17 and 24 are used to measure the metal and contact mask

levels misalignment. In each of these structures, a "Vee" from one mask level is superimposed on a rectangle from a previous mask level,

as shown in figures 3 through 6. The displacement of the crossing

points of the Vee and the rectangle is proportional to the misalignment

of the two levels and is given by

£tan0 (3)

where Zi is the displacement of the crossing points and Iz is the separation of the leading edges of the small rectangles. Both Zi and £2 are measured by a visual technique and have arbitrary units. To relate

ll and £2 to an absolute scale, the designed separation, £, between

the leading edges of the small rectangles is used. For the structure

in this test pattern, £ was chosen to be 20 ym and 0 was chosen to be

0,1 rad (5.7 deg) so that i£tan0 = 1 ym, and eq (3) becomes

£1

AL = —

£2

(4)

where AL is in micrometers.. In any visual measurement, there is a cer- tain amount of subjectivity. It was expected that when the structure

is measured one would prefer to see the Vee part of the structure in

the metal with rectangles in the oxide as in structures 16 and 17. This

order was reversed in structures 23 and 24; here the metal is seen as a rectangle and the oxide contains the Vee. In experiments to compare

the electrical and visual structures [1] , no differences were observed

for these two locations of the Vee.

Since the potentiometric electrical alignment resistor may be

sensitive to several process nonuniformities, it was necessary to include several other test structures in the test pattern in order to

test for these. Variations in either the sheet resistance or the

width of the doped region within the test structure lead to resistance

differences which will interfere with the measurement. To determine

the magnitude of these variations, cross bridge sheet resistors [5] were included for the doped region, structures 9 and 10, shown in

figure 7, and for the metal, structure 21, shown in figure 8. Procedures for measuring these structures and calculating sheet resistance

and line width for the measurements are given in [5] which is reproduced as Appendix A.

Incomplete etching of the contact window or other metalsemiconductor contact faults lead to erroneous results. The contact

resistance test structure [6] used in several previous test patterns

was included to provide information on contact quality. This structure

is shown in figure 9. Because the structure is affected by contact

window geometry, five versions (structures 11 through 15) which repeat

3
the various geometries of the contact window in the electric alignment structure were included in the test pattern. Use of this structure is described in [6] which is reproduced as Appendix B. Initial

experiments have indicated that large variations in the measured contact resistance are due to aluminum and silicon incompletely alloying

over the entire contact window.

The isolation diode, structure 22, shown in figure 10, is composed

of interdigitated doped regions of opposite conductivity type to the

substrate which therefore form pn junctions with the substrate. If a potential is applied between the two contact pads, the presence of a significant current implies either surface leakage or poor isolation of

the doped region both of which may interfere with measurements.

3. DESCRIPTION OF THE TEST PATTERN

The various structures were assembled into a rectangle 2.44 mm

wide and 1.60 mm high. The pattern is arranged modularly with six 2 by

10 columns of probe pads which contain the various electrically measured test structures. The visual alignment indicators are placed in

two skipped columns as shown in the computer drawing of the complete

test pattern in figure 11. As in the drawings of the individual test

structure, each level of the test pattern is identified as follows:

doped regions enclosed by heavy solid lines, contact window areas denoted by cross hatched rectangles, and the metal areas enclosed by

light solid lines. This drawing contains the digitized rectangles for

computer-controlled pattern generation. Two types of fabrication alignment marks are included in the pattern: square (structure 19) and plus

sign (structure 20) . A photomicrograph of a fabricated die is shown in

figure 12. The doped regions appear as black regions, the contact opens

as gray regions enclosed by black lines, and the metallizations are white

.

The test pattern was implemented in a three-level mask set intended for use with positive photoresist. Therefore, the doped region

and contact window masks are dark field and the metal mask is light

field. The pattern was laid out in metric units with a computer-aided

design system which results in an Electromask* compatible magnetic

tape which contains the information necessary for a computer con- trolled pattern generator to fabricate the three lOX reticles. On this

*This instrument is identified in this publication for the purpose

of providing a complete description of the work performed. In no case does such identification imply recommendation or endorsement

by the National Bureau of Standards nor does it imply the item identified is necessarily the best available for the purpose.

4
tape, level 1, for the doped regions, is composed of 490 rec tangles,

level 2, for contact windows, 380; and level 3, for the metal level,

575.

4. USE OF THE TEST PATTERN

The individual electrical alignment test structures can be incorporated into any test pattern to provide local information about mask

registration. Working masks have been made in which the entire pattern

has been stepped and repeated on 2.54-mm (100-mil) centers to com- pletely cover a wafer."'" Vector maps from measurements can be used to

evaluate [1,7,8,9] the misalignment which results from photomask generation, photomask exposure, and other fabrication processes. Preliminary

results of measurements of mask registration using this test structure

have been reported elsewhere [1]

.

ACKNOWLEDGMENTS

The authors wish to thank several members of the Electron Devices

Division for their contributions to this project. Y. M. Liu for fabricating the wafers which contain the test pattern, W. M. Bullis, M. G.

Buehler, and G", P. Carver for their helpful comments, and J. S. Halapatz

and E. J, Walters for preparing this manuscript.

Contact the Electron Devices Division, National Bureau of Standards,

Washington, D.C. 202 34 for information on availability of this tape.

To be able to test dice on a wafer using a certain metric or an English automatic prober, the dice must be centered on multiples of

1.27 mm (50 mil). Since the minimum step on the metric prober is 10

ym and on the English prober 2.54 ym (0.1 mil), the minimum step com- mon to both is 1.27 mm or 50 mil.

5
REFERENCES

1. Russell, T. J., Leedy, T. F., and Mattis, R. L., A Comparison of

Electrical and Visual Alignment Test Structures for Evaluating

Photomask Alignment in Integrated Circuit Manufacturing, Teahnical

Digest, International Electron Devices Meeting, Washington, D.C.

Dec. 5-7, 1977, pp. 7A-7F.

2. Thomas, D. R. , and Presson, R. D. , An Electrical Photolithographic

Alignment Resistor, Proa. Government M'torocivcuit Applicat-ions

Conf., Boulder, Colorado, June 25-27, 1974, pp. 196-197.

3. Ham, W. E., Test Pattern Design and Analysis for SOS/LSI, Semiconductor Measurement Technology, Progress Report, January 1 to

June 30, 1976, NBS Special Publication 400-29, pp. 65-68

(April 1977)

.

4. Ham, W. E., Semiaonduotov Measurement Technology: A Comprehensive

Test Pattern and Approach for Character izating SOS Technology, NBS

Special Publication 400-56, pp. 300-307 (submitted for publication)

.

5. Buehler, M. G. , Grant, S. D., and Thurber , W. R. , Bridge and van der Pauw. Sheet Resistors for Characterizing the Line Width of

Conducting Layers, J. Eteotrochem. Soc . 125,, 550-654 (1978).

6. Buehler, M. G., Semioonduotor Measurement Teohnology : Microelectronic Test Pattern NBS-3 for Evaluating the Resistivity-Dopant

Density Relationship of Silicon, NBS Special Publication 400-22,

p. 13 (June 1976)

.

7. Matsuzawa, T. , Sunami, H., and Hashimoto, N., Analysis of Superposition Error on Semiconductor Slices, MicroeZectronias and

Reliability 15, 173-177 (1977) .

8. Leebrick, D. H. and Kisker, D. W. , An Electrical Alignment Test

Device and Its Use in Investigating Processing Parameters Proc

.

Kodak Microelectronics Seminar, Monterey, California, October 5-7,

1977, pp. 55-83.

9. Perloff, D. D., A Four Point Electrical Measurement Technique for

Characterizing Mask Superposition Errors on Semiconductor Wafers,

IEEE J. Solid-State Circuits, SC-13, 435-444 (1978)

.

6
Table 1. Test Structure List

Structure Test Structure Specification/Notes

Number Description W, Cj, C21 L^, I/2, M(ijm) Page

1 Contact-to-doped-region 18, 6, 6, 122, 61, - 9

electrical alignment

resistor

2 Contact-to-doped-region 24, 6, 6, 122, 61, - 9

electrical alignment

resistor

3 Contact-to-doped-region 24, 12, 6, 122, 61, - 9

electrical alignment

resistor

4 Contact-to-doped-region 24, 6, 12, 122, 61, - 9

electrical alignment

resistor

5 Contact-to-doped-region 24, 12, 12, 122, 61, - 9

electrical alignment

resistor

6 Metal-to-doped-region 24, 12, 16, 122, 61, 8 10

electrical alignment

resistor

7 Metal-to-doped-region 24, 12, 24, 122, 61, 8 10

electrical alignment

resistor

8 Contact-to-doped-region 18, 6, 6, 80, 80, - 9

electrical alignment

resistor

9 Doped-region cross- 18, -, 200, 40, - 15

bridge sheet resistor

10 Doped-region cross- 24, -, -, 190, 44, - 15

bridge sheet resistor

11 Metal-to-doped-region 18, 5, 6, -, -, - 17

contact resistor

12 Metal-to-doped-region 24, 5, 6, -, -, - 17

contact resistor

7
structure

Number

Test Structure

Description

Specification/Notes

W, Ci C2, Li, L2, M(ym) Page

13 Metal-to-doped-region 24, 6, 12, -, -, - 17

contact resistor

14 Metal-to-doped-region 24, 12, 5, -, -, - 17

contact resistor

.15 Metal-to-doped-region 24 , 12 , 12 , - , - , - 17

contact resistor

16 Metal-to-doped-region Vee in metal level 11

visual alignment £ = 20 ym, 0 = 5.7 deg

indicator

17 Metal-to-doped-region Vee in metal level 13

visual alignment Z = 20 ym, 0 = 5.7 deg

indicator

18 NBS-15 logo

19 Square mask alignment 21

markers

20 Plus sign mask align- 22

ment markers

• 21 Metal cross bridge 8, -, -, 200, 40, - 16

sheet resistor

22 PNP isolation diode 18

23 Metal-to-doped-region Vee in doped region level 12

visual alignment £ = 20 ym, 0 = 5.7 deg

indicator

24 Metal-to-contact Vee in contact level 14

visual alignment I = 20 ym, 0 = 5.7 deg

indicator

8
W

C,

22

10

V 9

La

1'

8

<- L2

5

I

223

I

6

I

160 )im

Figure 1. Contact- to-doped region electrical alignment resistor

(structures 1,2,3,4,5 and 8).

9
"I

c,

C2

10

L,

V 9

8

5

I

160 |im

Figure 2, Metal-to-doped region electrical alignment resistor

(structures 6 and 7)

.

10
11
Figure 4. Metal-to-doped region visual alignment indicator

{structure 23) .

12
Figure 5. Metal-to-contact visual alignment indicator (structure 17)

.

13
14
^1

0

w

0

0

160 Mm

Figure 7. Doped region cross bridge sheet resistor (structures 9 and 10)

15
160

L,

gure 8. Metal cross bridge sheet resistor (structure 21)

16
160 |im

Figure 9. Metal-to-doped region contact resistor (structures

13, 14 and 15)

.

17

19
-H

OC

0) 0) > M

-P 03 O +J

03 (D -p e O !^ U 0

4:;

m

-p- 4-1 a; u U fO

03

X!u

03

0} 4-J C -H o 4:: H 5

0) 0)

cuCMO

T3

0)

03 +J

0)

0) +J

+J

03

4J

0)

4-1 0) Ln -H

rH fH I COm o

03

O

cu

03 X!

J4CnOu

•Heo

J-> o

cnH

i-f >

+JO

>i 03

03 (U

CPU

(U

•H 20
21
22
APPENDIX A

Reprinted from Journal of the Electrochemical Society

Vol. 125, No. 4, April 1978

Printed in the U.S.A.

Bridge and van der Pauw Sheet Resistors for Characterizing

the Line Width of Conducting Layers

M. G. Buehler,* S. D. Grant, and W. R. Thurber

National Bureau of Standards, Electronic Technology Division, Washington, D.C. 20234

ABSTRACT

It is shown that the line width of conducting layers can be computed from

simple d-c electrical measurements made on bridge and van der Pauw shaped

test structures. A compact six-contact, cross-bridge sheet resistor test structure was developed to make this measurement directly. Line widths measured on

boron nitride diffused layers indicate that the method is sensitive to width

variations of the order of ±0.1 /jm (±4 iiin..)

The measurement of line widths in the microelectronic industry is needed for the process control and

design of integrated circuit components. For process

control, line-width measurements are useful in moni-

• Electrochemical Society Active Member. Key words: cross-bridge structure, diffused layer, line width,

process control, sheel resistance.

toring the many etching processes which define the width of oxide windows and conducting layers. In cir- cuit design, the value of diffused or implanted resistors

is determined in part by the line width of the conducting layer. Conventional methods for measuring the line width

include both optical (1) and electrical techniques.

23
Vol. 125, No. 4 LINE WIDTH OF CONDUCTING LAYERS 651 The electrical techniques commonly employ the double-bridge method (2) which requires the fabrication

of two bridge structures of different width. Since this measurement technique is a differential-type method,

it suffers from errors in taking the difference of large numbers. An alternative to these methods is the determination of line widths using a bridge structure (3) in combination with a van der Pauw test structure (4). The

width of the bridge structure is calculated from measurements of its resistance along with knowledge of the length between the voltage taps and the sheet re- sistance value obtained from the van der Pauw structure. The advantage of these vehicles is that they can be combined into one compact structure with the same

line width, which means that the conductivity of the composite is more uniform than would generally be

true for separate structures. In addition, the computation of the line width does not depend on the differ- ence of large numbers; however, the analysis of the van der Pauw structure requires the measurement of low voltages, usually in the range from 1 to 10 mV.

This is not a problem for laboratory-based instrumentation but precludes the use of those integrated circuit testers which do not measure these low voltages. Test Structure Design and Fabrication Line drawings of the test structures used in this study are shown in Fig. 1-4. Metallized contact to dif- fused regions is shown by the crosshatched areas. Structures 3.22 and 3.28 were fabricated as a part of

test pattern NBS-3 (5) and the structures 7.18 and

7.23 were fabricated as a part of test pattern NBS-7

(6). The dimensions for these structures are listed in Table I. The bridge test structure shown in Fig. 1 was designed in accordance with the ASTM standard (3) which requires that Wm ^ 3Dm and Bm — 2Wm. In addition the lateral diffusion and overetch effects were

minimized by requiring that Am s= 5Xj and Cm — 5Xj, where Xj is the junction depth. The subscript m indicates a parameter referred to photomask dimensions. The offset quadrate-cross test structure shown in

Fig. 2 was designed to eliminate the contact shorting

1. Bridge sheet resistor test structure (3.28). The edges

^1/ ^2/ and ^3 ore used to determine the width from optical photo- microscopic measurements.

Am

m

// /

Dm

Sm

i

t

V2

Fig. 3. Greek-cross sheet resistor test structure (7.18)

SYMMETRY TAB

Fig. 4. Cross-bridge sheet resistor test structure (7.23)

correction factor (7) associated with the van der Pauw equation. For this structure where A/S — 0.33 and D/S = 0.17, the correction factor is much less than

0.1% and may be neglected. Likewise, the Greek-cross

test structure shown in Fig. 3 was designed so that the arm length is greater than the arm width which means

that the contact shorting correction factor associated with this structure also can be neglected (7)- The cross-bridge structure shown in Fig. 4 is a combination of the bridge structure shown in Fig. 1 and

Table I. Test structures

Number Figure Type

Pliotomask dimensions

/tm (mil)

3.28

7.18

7.22

Bridge

Offset quadrate cross

Greek cross Cross bridge

Fig. 2. Offset quadrate-cross test structure (3.22)

Am 25.4 (1.00) Bm 127.0 ( 5.00) Cm 12.7 (0.50) Dm 12.7 (0.50) Lm 139.7 (5.50) Wm 38.1 (1.50) Am 25.4 (1.00) Dm 6.4 (0.25) Sm 38.1 (1.50) Sm 14.0 (0.55) Dm 8 0 (0.31) Lm 200.0 (7.87) Wm 16.0 (0.63)

24
652 J. Electrochem. Soc: SOLID-STATE SCIENCE AND TECHNOLOGY April 1978

the Greek-cross structure shown in Fig. 3. Hie bridge

part was designed in accordance with the ASTM

standard (3) except that Wm = 2Dm- The cross part was designed to eliminate the contact shorting correction factor (7). In order to preserve the symmetry of the cross, symmetry tabs were introduced at the ends

of the arms denoted Ii, I2, and Vi. These tabs are in- tended to compensate for the extension of the struc- ture along the arm denoted V2. The test structures shown in Fig. 1-4 were fabricated by photolithographic processes using contact printing and an n-p-n transistor process. Both emitter and base

sheet resistors were fabricated. The base sheet resistors, which were used to obtain the results reported

in this study, were fabricated on n-type, (111)-

oriented silicon substrates on which a 300 nm thick oxide was grown in steam at 1100°C. Diffusion windows

were etched in the oxide. The boron nitride predeposition diffusion was at 965° C in dry nitrogen for about

30 min. After a deglaze in 10% hydrofluoric acid solution, the base was diffused at 1100°C for 18 min in wet

oxygen, followed by 40 min in dry oxygen, followed by 15 min in dry nitrogen. This creates over the base

regions a 350 nm thick oxide that masks the subsequent

emitter diffusion. The final high temperature steps oc- curred at 1025°C for 20 min and at 925'C for 45 min. The structures were complete after opening the contact windows and patterning the aluminum metallization. The base sheet resistance was nominally 200 !!/.

Test structures 3.22 and 3.28 were fabricated in wafers numbered 28, 29, and 30; the junction depth was 2.1

iiim. Structures 7.18 and 7.23 were fabricated in a 9 n- cm wafer, D8.8 Ph-1; the junction depth was 1.6 ^m.

Electrical Measurements The electrical measurements utilized the d-c four- probe method where current was forced between two

contacts to the resistor and the voltage was measured between two additional contacts. The current from a constant current source was determined either from

the voltage drop across a standard resistor or was as- sumed to be within the stated accuracy. Current re- versal was achieved by switching the current through

a mechanical switch or relay to ensure that the magnitude of the reverse current equals that of the forward

current. The magnitude of the current was chosen so that the measured voltage was between 0.5 and 10 mV.

The substrate was allowed to float electrically during

these measurements. The line width is determined from measurements on

the bridge structure in combination with measure- ments on the van der Pauw structure. The sheet resistance is determined from the van der Pauw struc- ture by utilizing probe pads labeled Ij, I2, Vi, and V2 shown in Fig. 2, 3, or 4 and is calculated from the van der Pauw formula (4) Rs = JiR(±I)/ln2 [1] where R(±/) is the incremental resistance determined from the current and voltage measurements for both

directions of current and both contact orientations; see preceding paper (8). The parameter f which appears in the complete form of the van der Pauw formula is as- sumed to be unity in this study since the asymmetry

factor, Fa, was much less than 10% (8). The line (or window) width is determined by utiliz- ing probe pads labeled Ii*, I2*, V:*, and V2* shown in

Fig. 1 and 4. The effective width. We, of the currentcarrying channel is calculated from We = RsLm/R'(±I) [2] where Rs is determined from Eq [1] and Lm is the

distance between voltage taps as determined from the photomask. The incremental resistance determined from measurement for both forward (+1) and re- verse (—/) currents is given by

For measurements made in the forward current mode,

the current !*( + !) is passed into h* and out of h'

where both and V*{+I) - Vi*(+I) -

V2''( + l) are positive quantities. For measurements made in the reverse current mode, both /*(—/) and V*(— 7) are negative quantities. The above expressions assume that the voltage taps do not significantly per- turb the current flow in the current-carrying channel.

If the test structure is a diffused layer then the ef- fective width is We = + aXj + Woe [4] where Wm is the width determined from the photomask, Woo is the incremental change in the width due

to photolithographic effects and overetching, and oXj

is the effective incremental change in the width due to lateral diffusion where Xj is the junction depth and a

is the lateral diffusion factor (9). Since the lateral dif- fusion cannot be observed from the top surface of the

structure, the optically observable line width is W = We - aXj = Wn, + Woe [5] These quantities are shown in detail in Fig. 5.'

Line-Width Measurements

Line widths for the bridge structure shown in Fig. 1 were measured as a function of position across a wafer by both electrical and optical techniques. In order to compare the two techniques, the factor aXj, with a assumed to have the value U.3 (9) and Xj — 2.1 nm

for wafers 28, 29, and 30, was subtracted from the ef- fective line width. We, which was evaluated from the

electrical measurements with the use of Eq. [2]. In

I ig. 6 the sensitivity of the method to etch time is illustrated by an experiment where the base diffusion

' If the test structure is a deposited layer, the effective width, which is the same as the optically observable width, W, is We = Wm - Woe.

Fig. 5. Cross-sectional view of a diffused region showing various dimensions. To account for lateral diffusion effects, the width of the conducting channel is increased by on amount aXj where the conductivity in this region is the same as in the main channel.

1.75

B 1,70

—I

1 1 r ADO OPIIC«L AU • ElECIRICAL

7 5 min. Hunt 30

i *

1 55

1.50

S 0 min, WAFER 29 _0 a Jj^ g B2 5 mm, WAFER 28

-| 5,08 mm 1200 mil]

I I I I L

R*(±l) = [V*( + I) - V*(-

[3]

2 3 4 5 6 7 8 9 10

POSITION ACROSS WAFER

Fig. 6. Width of the base diffusion window as measured electri- cally and from photomicrographs along the diameter of wafers etched for different times.

25
Vol. 125, No. 4 LINE WIDTH OF CONDUCTING LAYERS 653 window was etched for a different time on each of

tliree wafers. The measurements demonstrate the effect of increased etch time on the window width. The optical measurements were made directly on

photomicrographs of various bridge test structures. An

optical microscope with a 40x objective was operated

in the dark field mode to obtain photomicrographs.

The magnification as measured on the photomicrograph was 585 X. The method used to determine the

line width is illustrated by the points labeled Ei, Ea, and En shown in Fig, 1 The distance between edges

El and E,3 is assumed to be unaffected by the photolithographic processes or the etch times. This distance was

taken from the nominal photomask dimension and for structure 3.28 is 63.5 ± 1.3 nm (2.50 ± 0.05 mils). The

distance between edges Ei and Eo corresponds to the

optically observable width. In measuring this width a scale was placed on the photomicrograph at a convenient angle such that the distance between Ei and E.-j was a convenient multiple of the actual dimension. The

distance between edges Ei was read directly from the

scale. The estimated repeatability of both the electri- cal and optical line-width measurements was ±0.05 nm

(±2 ^in.). The results obtained from the photomicrographs and shown in Fig. 6 compare favorably with the values obtained from the electrical measurements and in all cases the values are larger than the measured photomask width, Wm = 39.2 ± 0.3 ij.m (1.545 ± 0.01 mils). Sheet resistance values are typically more uniform in the center of a wafer than at the periphery. The fact that the bridge and van der Pauw resistors are located about 50 mils apart on the test pattern would affect the agreement between the electrical and optical measurements more at the edge of the wafer. Tlie results from the center of the wafer illustrate that the electri- cal width measurements are sensitive to width changes

of the order of ±0.1 ij.m (±4 /j-in.). Line widths were also measured with the use of the cross-bridge test structure shown in Fig. 4. Before computing the line width it is necessary to verify that the sheet resistance is correctly determined from the cross-bridge structure. This demonstration is shown in

Fig. 7 where the sheet resistance from adjacent Greekcross (7.18) and cross-bridge structures are compared.

The results shown in Fig. 7 were analyzed by the least squares method where the slope of the line is 0.98 and

the intercept is 2.9 a/D. The standard deviation for these measurements is 0.7% which means that sheet

BASE SHEET RESISTANCE |0/a), 7.23

Fig. 7. Comparison of sheet resistance results from the Greekcross (7.18) and cross-bridge (7.23) test structures.

Table II. The mean and standard deviation of various sheet

resistor factors (in percent)

Structure number Fo Fa

7.18 0.133 ± 0 050 -O.OU ± 0031 -0.55 ± 0.65 7.18* 1.39 ± 0.39 -0.005 ± 0 007 + 0.13± 1 38

7.23 0.141 ± 0.050 -0.011 ± 0 010 -1.48 ± 0.72 7.23* 0.53 ± 0.11 -0.019 ± 0.006 -1.00 ± 1.21

* Data on structures designated by an • were obtained with an automated data acquisition system.

resistance values can be adequately determined from

the cross-bridge test structure. Other comparisons between the Greek-cross (7.18) and the cross-bridge (7.23) test structures are listed in Table 11. The factors Fo, Fl, and Fa listed in Table II are defined in the preceding paper (8). The offset factors, Fo, measured with an automated data acquisition

system are significantly larger, because of thermal

voltages in the relay scanners, than those m.easured on

a system where the digital voltmeter was connected

directly to the voltage contacts. The linearity factors, Fl, are quite small being less than 0.02% which indi- cates that the conduction mechanism in these test structures is ohmic. As expected, the asymmetry factors. Fa, indicate that the Greek cross (7.18) is more symmetrical than the cross bridge (7.23); however, since Fa << 10%, the van der Pauw formula can be used without correction to compute the sheet resistance. As an example of how electrical line width measure- ments can be utilized in process control, wafer maps of sheet resistance and effective line width We from Eq.

[2] as determined from measurements on the cross bridge (7.23) are shown in Fig. 8. Each wafer map is divided into five increments indicated by symbols

given in the key. The numbers listed in the left-hand column are measured parametric values. The numbers

in the right-hand column are the number of devices

RESISTANCE NO. P-BASE SHEET RESISTANCE | n/n

)

EFFECTIVE P BASE WIDTH |pm|

WIDTH NO.

Fig. 8. Wafer mops of a p-type base diffusion into a 9 fl-cm

n-type substrate as analyzed by the cross-bridge test structure

(7.23).

26
654 J. Electrochem. Soc: SOLID-STATE SCIENCE AND TECHNOLOGY April 1978

measured in each increment. Every other symbol

shown in the wafer map represents a measured point: from these values the remaining symbols were obtained by interpolation. The sheet resistance shows a rather

large top to bottom variation which is indicative of a diffusion problem; however, the effective line width shows a reasonably flat profile which is indicative of good etch control. Similar wafer maps have been re- ported elsewhere (6) for both the base and emitter

sheet resistors using separate cross and bridge structures. The use of the cross-bridge method in evaluating

the width of very narrow diffused (or im-planted)

layers should be viewed with caution. This situation occurs when the incremental lateral diffusion width

aXj is comparable with the over-all width. In fact, the caution applies to any layer in which strong lateral

resistivity gradients exist. In such cases the effective sheet resistance determined at the cross may not be

the correct value for use in the computation of the

effective width from Eq. [2]. The method described here has also been applied to the measurement of the

line width of phosphorus emitter diffusions with a sheet

resistance of about 1 n/Q (5, 6). In addition, the width

of aluminum metallization with a sheet resistance of about 0.03 n/n has been measured.

Conclusion

It was demonstrated that effective line width can be determined from electrical measurements on dif- fused bridge and van der Pauw sheet resistors and that these electrical measurements agree with those deter- mined from optical photomicrographs of the sheet resistors. The method is sensitive to width changes of the order of ±0.1 ^m (±4 ^in.). A new cross-bridge

sheet resistor was developed; wafer maps of the sheet

resistance and line width illustrate the utility of this structure for process control. The advantage of electrical line-width measurements

is that they can be made quicltly and at low cost using automatic data acquisition equipment with sufficient voltage resolution. Thus these measurements are important because of their potential use as process control and design vehicles in the manufacture of semiconductor devices.

Acknowledgment

The authors are indebted to Y. M. Liu for developing

the various diffusion processes used to fabricate the NBS-3 and NBS-7 test pattern wafers, to R. L. Mattis

for the wafer maps shown in Fig. 8, and to C. A. Cannon for making the photomicrographs. This work was

conducted as part of the Semiconductor Technology

Program at NBS. Portions of this work were supported

by the Defense Advanced Research Projects Agency

(Order No. 2397, Program Code 7D10) and the Defense

Nuclear Agency (lACRO 77-809). Manuscript submitted Aug. 3, 1977; revised manuscript received Nov. 28, 1977. Any discussion of this paper will appear in a Dis- cussion Section to be published in the December 1978 Journal. All discussions for the December 1978 Discussion Section should be submitted by Aug. 1, 1978.

Publication costs of this article were assisted by the National Bureau of Standards. REFERENCES

1. D. A. Swyt, Solid State Technol, 19, 55 (April 1976).

2. W. E. Ham, Semiconductor Measurement Technol. NBS Special Publication 400-15, p. 30 (January

1976).

3. ASTM Designation F 76, "Annual Book of ASTM

Standards," Part 43 (November 1977).

4. L. J. van der Pauw, Philips Tech. Rev., 20, 220

(1958); L. J. van der Pauw Philips Res. Rep., 13,

1 (1958).

5. M. G. . Buehler, Semiconductor Measurement Technol, NBS Special Publication 400-22 (May 1976). Requests for releasing the NBS-4 mask set (or the earlier NBS-3 set) from a commercial maskmaking company should be made directly to M. G. Buehler, National Bureau of Standards, Washington, D.C. 20234. The cost of a set is about $200.

6. M. G. Buehler and D. E. Sawyer, Circuits Manufacturing, 17, 46 (February 1977).

7. J. M. David and M. G. Buehler, Solid-State Electron., ZO, 539 (1977).

8. M. G. Buehler and W. R. Thurber, This Journal,

125, 645 (1978).

9. W. M. Penney and L. Lau, Editors, "MOS Integrated

Circuits," p. 80, van Nostrand Co., New York

(1972).

27
APPENDIX B

B-1. Metal-to-Base Contact Resistor, Structure 3.24

Metal-to-base contact resistor, structure 3.24, is a four-terminal resistor

which consists of a base diffused into the collector and metal which touches

the base at a square window 1.00 mil (25.4 ym) on a side; see figure 1. The

effective contact resistance for a unit area, R^-. , as given by this structure

is calculated from

R^ = AV/I (1)

where the potential, V, is V]_-V2 for a current, I, passed into I]_ and out of

I2 and A is the area of the contact. For a square contact window of 1.00 mil

(25.4 ym) on a side, A = 6.45 x 10~^ cm"^ . This structure is intended to serve as a process control monitor. It is not intended to yield absolute

values for the specific contact resistance, for the specific contact resistance determined by this structure is influenced by over-etch of the contact

window and by current crowding effects since only a fraction of the contact

carries a significant current [B-1] . Over-etch of a contact window occurs because the contact window is etched at the same time as the scribe lines.

Etching the scribe lines requires removal of the collector oxide which is

thicker than the base oxide.

Figure 1. Photomicrograph of the

metal-to-base contact resistor,

structure 3.24.

Reference:

B-1. Kelley, F. R. , Mattis, R. L.,

Resistor Study, Semioonduator

NBS Special Publication 400-1

and Buehler , M. G. , Metal-to-Base Contact

Measurement Teahnology , W. M. Bullis, Ed.,

(March 1974), pp. 22-25.

28
NBS-114A IREV. 9-76)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. Special Publication 400-51

2. Gov't. Accession No. 3. R«cip)ent's Accession No.

4. TITLE AND SUBTITLE

Semiconductor Measurement Technology

:

A Production-Compatible Microelectronic Test Pattern

for Evaluating Photomask Misalignment

5. Publication Date April 1979

6. Performing Organization Code

7. AUTHOR(S) T. J. Russell and D. A. Maxwell

8. Perforrr,ing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

10. Pfoject/Task/Work Unit No. Order No. 2397, Code 7D1C

11. Contract/Grant No.

12 <;pnN<;nRI NH ORH AN 1 7ATinN NAMF ANH rOMPi FTP ADORFSS i'Sfr<!>i-( Citv Si^tp 7IP) NBS - Washington, DC 2023^

ARPA - 1400 Wilson Blvd., Arlington, VA 22209

13. Tvnp nf Rpnnrt %j PprinH C^r\\}Q'Xpi(\ Interim

Jan. 1977 to Dec. 1978

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 79-230

I

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

16. ABSTRACT {A 200-word or less tactual surnmary of most significant information. If document includes a significant bibliography or

literature survey, mention it here.) Microelectronic test pattern NBS-I5 is composed of several potent i omet r i c

,

production-compatible electrical alignment resistor test structures, visual alignment indicator test structures, cross bridge sheet resistors, and contact resistor

test structures. The pattern was originally designed as a study vehicle for the

electrical alignment resistor, but it was also demonstrated that, when stepped over

an entire wafer, the pattern is suitable for use in evaluating misalignment which

may result from photomasl< generation, photomask exposure, or other fabrication

processes. This report summarizes the test structures that are included in the

pattern and contains explanation of how each of the structures Is measured.

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) Contact resistor; cross bridge sheet resistor; electrical alignment resistor;

semiconductor; silicon; test pattern; test structures; visual alignment.

18. AVAILABILITY [^Unlimited

1 1 For Official Distribution. Do fJot Release to NTIS

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