NAT L INST. OF STAND & TECH R.LC.

' "III llll llll Illl III

A111DM 7S723D

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PUBLICATIONS

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*. NIST SPECIAL PUBLICATION 400-97

U.S. DEPARTMENT OF COMMERCE/Technology Administration

National Institute of Standards and Technology

Semiconductor Measurement Technology:

Test Structure Implementation Document:

DC Parametric Test Structures and Test Methods

for Monolithic Microwave Integrated Circuits (MMICs)

C. E. Schuster

0.400-97

995
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2 Some elements at Boulder, CO 80303.
Semiconductor Measurement Technology:

Test Structure Implementation Document:

DC Parametric Test Structures and Test Methods

for Monolithic Microwave Integrated Circuits (MMICs)

C. E. Schuster

Semiconductor Electronics Division

Electronics and Electrical Engineering Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899-0001

Sponsored by:

Defense Advanced Research Projects Agency

Arlington, VA 22209-2308

and

U.S. Air Force Wright Laboratory

Wright-Patterson AFB, OH 45433-6543

September 1995

U.S. DEPARTMENT OF COMMERCE, Ronald H. Brown, Secretary

TECHNOLOGY ADMINISTRATION, Mary L. Good, Under Secretary for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Arati Prabhakar, Director
National Institute of Standards and Technology Special Publication 400-97

Natl. Inst. Stand. Technol. Spec. Publ. 400-97, 85 pages (September 1995)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1995

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
TABLE OF CONTENTS

Page

ABSTRACT 1 Key Words l EXECUTIVE SUMMARY 2

1. INTRODUCTION 5

1.1. Purpose 5

1.2. Scope 5

1.2.1. NISTGAAS Form and Function 5

1.2.2. Documentation 5

2. IMPLEMENTATION CONSIDERATIONS 6

2.1. Selecting Test Structures 6

2.2. Developing a Test Vehicle 8

2.3. Assuring the Testing Environment 9

2.4. Developing Data Analysis Capabilities 9

3. COMPUTER-AIDED DESIGN (CAD) CELL LIBRARY 9

3.1. Overview 9

3.2. Process Layers 10

33. Plot Colors and Patterns 10

3.4. Cell Naming Conventions 10

3.4.1. Test Structure Names 12

3.4.2. Layer Names 12

3.4.3. Dimensions 13

3.5. CAD Drawing Hierarchy 13

3.6. Obtaining the NISTGAAS Cell Library 13

4. ACKNOWLEDGMENTS 14

5. REFERENCES 14

APPENDICES

Appendix A - NISTGAAS Cell Library Contents 15

A.l List of Test Structures 17

A. 2 Library Hierarchy 17 A.3 Process Layer Designators 17

A4 Cell Contents 17

Appendix B - NISTGAAS Test Structure Specification 23

B. l Kelvin-Cross Interfacial Contact Resistor (Four-Pad) 27

B.2 Kelvin-Cross Interfacial Contact Resistor (Shared-Pad) 33

B.3 Nanometer-Resolution Electrical Alignment Structure 41

B.4 Mesa /Channel van der Pauw Sheet Resistor 51

B.5 Step Coverage/Interconnect Meander 57

B.6 Interconnect Resistor 61

B.7 Cross-Bridge Resistor 65

B.8 MIMIC-Standard 200 urn FET 71

Appendix C - NISTGAAS Test Structure Sideviews 79

iii
LIST OF FIGURES

Page

Figure 1. Selecting Test Structures and Test Vehicles 7

Figure 2. Fabrication Process Steps 11 Figure A-l. NISTGAAS Top-Level CAD Drawing 20

Figure A-2. NISTGAAS 2nd-Level CAD Drawing 21

Figure C-l. Sideview: Kelvin-Cross Interfacial Contact Resistor (Four-Pad) .81

Figure C-2. Sideview: Kelvin-Cross Interfacial Contact Resistor (Shared-Pad) 82

Figure C-3. Sideview: Nanometer-Resolution Alignment Structure 83

Figure C-4. Sideview: Mesa /Channel van der Pauw Sheet Resistor 84

Figure C-5. Sideview: Step Coverage/Interconnect Meander 85

Figure C-6. Sideview: Interconnect Resistor 86

Figure C-7. Sideview: Cross-Bridge Resistor .87

Figure C-8. Sideview: MIMIC-Standard 200 urn FET 88

LIST OF TABLES

Page

Table 1. NISTGAAS Cell Types and Their Application 3

Table 2. NISTGAAS Design Rules 10

Table A-l. NISTGAAS Test Structures and CAD File Locations (Page 1 of 2) 18

Table A-2. Mapping of Color/Pattern to Process Layer 22

LIST OF SCHEMATICS

Page

Figure B-l. Kelvin-Cross Interfacial Contact Resistor (Four-Pad) 29

Figure B-2. Kelvin-Cross Interfacial Contact Resistor (Shared-Pad) 34

Figure B-3. Nanometer-Resolution Electrical Alignment Structure (Horizontal) 43

Figure B-4. Nanometer-Resolution Electrical Alignment Structure (Vertical) 44

Figure B-5. Mesa/Channel van der Pauw Sheet Resistor 52

Figure B-6. Step Coverage/Interconnect Meander 58

Figure B-7. Interconnect Resistor 62

Figure B-8. Cross-Bridge Resistor 66

Figure B-9. MIMIC-Standard 200 urn FET (Horizontal) 72

Figure B-10. MIMIC-Standard 200 urn FET (Vertical) 73

iv
Semiconductor Measurement Technology:

Test Structure Implementation Document:

DC Parametric Test Structures and Test Methods for

Monolithic Microwave Integrated Circuits (MMICs)

C. E. Schuster

Semiconductor Electronics Division

National Institute of Standards and Technology

Gaithersburg, MD 20899

ABSTRACT

This document describes a set of microelectronic test structure designs for manufacturers of

GaAs MMIC devices. These designs enable the dc measurement of process and device

parameters that can be used to diagnose, monitor, compare, and predict the performance of the

fabrication process or the devices produced. The test structure designs are embodied in a computer-aided design library known as NISTGAAS, which contains 8 types of test structures,

implemented in 125 combinations of process layer and size, and based on a 2 x 6 probe-pad

array. Any design, once fabricated on a wafer, can be probed using commonly available

commercial parametric test system equipment. This document specifies how to implement and

test each type of test structure and how to analyze the results. It also provides guidance on how

to apply the set of test structures at the wafer level. Although NISTGAAS was designed for the

process described in this document, it was also designed and demonstrated to be adaptable for

other MMIC processes. Since NISTGAAS contains cell designs rather than a chip design, it provides a flexible test structure methodology that also provides the MMIC community with a common reference point for assessing process and device performance.

Key Words: CAD cell library; GaAs; integrated circuit; MMIC; parametric test method; process

control; test structure

DISCLAIMERS

Certain commercial and public-domain products are identified to specify the procedures

described in this document. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the

products identified are necessarily the best available for the purpose.

The NISTGAAS cell library was produced by the National Institute of Standards and

Technology, an agency of the U.S. Government, and by statute is not subject to copyright in the

United States. Recipients of this software assume all responsibility associated with its operation, modification, maintenance, and subsequent re-distribution.

1
EXECUTIVE SUMMARY

The NISTGAAS computer-aided design (CAD) cell library was initially developed as part of the

Defense Advanced Research Projects Agency (DARPA)/Tri-Service effort known as the

Microwave/Millimeter Wave Monolithic Integrated Circuits (MIMIC) Program. The

NISTGAAS library includes designs developed by NIST and the U.S. Air Force, Wright

Laboratory (WL). The NISTGAAS development and validation were funded in part by WL

MIPR Nos. FY1123-90-N9514, FY1175-91-N9518, FY1123-92-N9520, and FY1175-93-N9519.

Under the MIMIC Program, DARPA sponsored contracts for processes, products, and services

to develop high-performance, affordable, and available microwave and millimeter-wave

technology for manufacturing electronic systems. One aspect of this program was the

Materials/Devices Correlation task (MIMIC Phase 1, Task 4.E), to investigate and quantify

GaAs process performance through the use of test structures.

Six contractor foundries participated in Task 4.E, using test structures provided by WL. A

common test chip, replicated about 200 times on each 3-inch wafer, was used by each contractor

to produce high-density test structure data for 6 lots of 4 wafers each. Test structure data from

the contractors and WL were analyzed by WL, with NIST assistance. Most test structures

performed adequately for most process lots and lines. Some observations about performance

variations provided guidance for improved test structure designs, test procedures, and test procedure specifications [1]. A second-generation wafer-level test vehicle, known as the High-Density Test Reticle (HDTR),

was designed (partly) to investigate new test structure designs intended to improve and extend

the performance of particular Task 4.E test structures and of the general test structure

methodology. The HDTR test structures were designed by WL and NIST, fabricated by WL,

and tested by WL and NIST [2, 3]. The HDTR test structures and other NIST test structure

designs are contained in the NISTGAAS library, which includes the eight test structure types

shown in Table 1. These types are implemented in 125 layer and size combinations, where each

combination is a cell in the NISTGAAS library.

The NISTGAAS library and this implementation document provide the GaAs-based microwave

community with a readily available, thoroughly documented, and flexible implementation

method to use as a common reference point for assessing process and device performance.

2
Table 1. NISTGAAS Cell Types and Their Application

TEST STRUCTURE TYPES APPLICATION

Kelvin -Cross Interfacial

Contact Resistor (Four-Pad)

Determine interfacial contact resistances to assess quality of a contact type

Kelvin -Cross Interfacial

Contact Resistor (Shared-Pad)

Determine interfacial contact resistances to assess quality of a contact type

Nanometer-Resolution

Electrical Alignment Structure

Determine placement of a feature relative to two reference features to assess the

performance of lithography processes/tools

Mesa/Channel van der Pauw

Sheet Resistor

Determine sheet resistance of mesa and

channel, thickness of channel to assess

potential FET performance

Step Coverage/Interconnect

Meander

Determine current continuity to assess step

coverage for two conducting layers

Interconnect Resistor Determine load resistance of a contact type to provide circuit design parameter

Cross-Bridge Resistor Determine sheet resistance and linewidth to assess the quality of a conducting layer

MIMIC-Standard 200 am FET Determine dc FET parameters to assess FET

performance

3

1. INTRODUCTION

1.1. Purpose

This document describes how to implement the test structures included in the NISTGAAS cell library. It specifies each test structure design, how to test it, and how to analyze the results. It also addresses how to apply the set of NISTGAAS test structures at the wafer level.

1.2. Scope

1.2.1. NISTGAAS Form and Function

The NISTGAAS cell library contains dc parametric test structures designed for a MMIC process

that has design rules typical for fabricating 1 (im, low-noise, depletion-mode devices. The

library can be used with other MMIC processes, as it is designed so the test structures can be

easily customized for processes with similar but different design rules and mask layers. The

CAD software used to create NISTGAAS is the (public-domain) Magic graphic layout editor [4], so the mask-level layout information for the test structure designs is transferable in either

Caltech Intermediate Form (CIF) or Calma Stream format. Each test structure design can be

tested using a dc parametric test system with a probe card that can access a 2 x 6 probe-pad

array, where the probe pads are 75 \im x 75 um each and have a 125 (xm pitch.

Since the NISTGAAS library provides individual test structure designs (cells), rather than a test chip, only the designs useful for a particular application need be implemented. This enables the user to develop a flexible monitoring capability without sacrificing area (to non-applicable parts

of a test chip), when such area might be better used for product or for design development.

1.2.2. Documentation

Each test structure type is specified in terms of its purpose, application, design variations, CAD,

layout constraints, measurement method, computation and interpretation of results, and

historical references. These specifications are found in Appendix B, while the remaining

document provides information needed to understand and effectively use Appendix B.

This document is organized to provide adequate information for various types of users but to minimize the effort each needs to find details of interest or reproduce working-level aids.

Potential users include program managers, process engineers, CAD personnel, tester code

developers, and data analysts. Generally applicable background information is in section 2

(general perspective on test structure implementation) and in section 3 (specific rationale,

assumptions, and conventions used in designing the NISTGAAS cell library). Appendix A

(library configuration) will be the most useful to CAD personnel. Appendix B (the test structure

specifications) is intended mainly for coders and analysts, but can also be useful to program

managers and process engineers in assessing the process-related aspects of test structure design

and implementation. Process engineers may also find Appendix C (process sideviews) a useful

extension to the Appendix B checkplots.

5
2. IMPLEMENTATION CONSIDERATIONS

To gain the maximum benefit from using test structures, users need to be conscientious not only

in applying the specifications in Appendix B, but also in several other areas: selecting test structures, developing an appropriate test vehicle, assuring the testing environment, and

developing data analysis capabilities. These areas are discussed in detail elsewhere [5] and here

within the context of MMIC manufacturing environments and the NISTGAAS cell library. The

sections on selecting test structures and test vehicles are summarized in figure 1.

2.1 . Selecting Test Structures

As the first step in the test structure implementation process, the user should determine why

test structures are needed - both broadly and specifically. In doing this, the expertise of process

and design engineers is valuable. The reasons for using test structures affect the data needed

and the test structures and test vehicle design to be used. The major consideration involves

whether the test structures will be used to monitor a well-controlled production environment,

to diagnose a new process, to develop process models, or to provide data for comparison to data from other process lines.

In a well-controlled process, the physics and the yield goals of the process and products are understood, so the critical parameters needed to monitor the process are well known and

limited. In the other process environments and applications, many more parameters may be

needed to develop correlations between process parameters or between process parameters and

device parameters or to compare the capabilities of different process lines. These considerations

affect not only which test structure types are needed, but also what layer/size combinations are needed.

In comparing data from different process lines, a common set of test structures, appropriate for

all processes, is needed. If one of the common test structures has not previously been used on a process line, a line-specific test structure providing similar parameters should be retained to correlate process conditions based on previous history.

The layer/size combinations selected should be those actually used in the device or process step

for which the test structure parameter is needed. If devices require features larger than the

minimum size, designs with the same size features should be included. For example, to assess a FET with a 10 um channel width, the mesa /channel van der Pauw is implemented with a 10 (i.m vs 4 \im linewidth. In determining a load resistance for the source or drain of a FET, a

corresponding-size, ohmic-to-mesa interconnect resistor is used.

Further, particular process sensitivities may affect the choice of the NISTGAAS cell to be used.

For example, if ohmic sheet resistance is adequately controlled but does vary, test structures

with ohmic runs long enough to be susceptible to such variations should not be used. For the

alignment structure, such a sensitivity could be minimized by using the OG vs GO cell.

Another consideration in selecting test structures involves how a mask set will be developed

and used. By including designs with linewidths smaller than the process currently supports,

6
c

To select a set of test structures and a test vehicle

well-controlled process

select test structures

to monitor critical process parameters

use full set of test structures

to monitor as many process parameters as possible

assure appropriate

layer/size combinations

to diagnose or model

place test structures on product chip or

in kerf areas

Y

replicate test chip on

full wafer or at high density

assure enough sites for each

test structure for statistically significant data analysis

Fabricate

test structures/test vehicle on wafer

Figure 1. Selecting Test Structures and Test Vehicles.

7
the design limits of the process may be assessed and a new mask set is not needed for the next

generation of the process. Such decisions are a trade-off with other factors such as test vehicle

size and the cost, time, and needs associated with a new mask set.

2.2. Developing a Test Vehicle

Once the necessary test structure types and layer/size combinations are identified, the test vehicle implementation and layout can be developed. As in choosing test structures, a major

consideration in test vehicle design is its application, e.g., to monitor, diagnose, model, or compare. Again, process and product design expertise is needed.

For process diagnosis and modeling, a full-wafer or other high-density test vehicle is needed to provide robust statistics for correlation and wafer-mapping analyses. Frequently, all of the test structures are included on a test chip that is replicated on an entire wafer or at a high density

and uniformly distributed with product-related chips on a wafer. For monitoring a wellcontrolled and well-characterized process, reduced implementations that fit on the product chip

or wafer or in the kerf area are usually adequate. Possible implementations are usually striplike and known by such terms as test strip, coupon, pellet, or plug-bar.

Regardless of the test vehicle type, several layout factors are important. Most important is that

the test structures should be located near the MMIC they are to characterize. Thus, a test chip

implementation should either contain on-chip MMIC devices or have the salient test structures near the edge of the test chip that will be adjacent to the edge of a product chip with a near-by

device. Similarly, test strips should have their test structures arranged to be near the related

device. For example, if gate-to-ohmic alignment is critical for a FET, the gate-to-ohmic

alignment structure should be located near a sample of the FET.

Another layout consideration is whether correlation between the test structure parameters is needed. The user must then decide which test structures need to be placed near each other and

assure that their relation to near-by device needs are maintained.

Each test structure design should be replicated at sites that are uniformly distributed on the

wafer. This provides some indication of parameter uniformity, the sites needed to perform the

preliminary validation procedures included in some of the test structure specifications, and the

sites needed to demonstrate data reproducibility. Although five sites are often used, this is about half the sites usually needed to provide a robust statistical basis for data analysis [6].

If comparisons between process lines are needed, a standard test vehicle with moderate to high

density is usually needed. In addition to the above factors, this test vehicle must be designed to be accommodated by all the process lines. Process lines should consider retaining previously

used process-specific test vehicles to correlate new parameters to previous history.

If adequate space is not available for the initially identified set of test structure designs, the set must be prioritized according to which designs provided the most useful information. This

exercise should consider not only test structure type but also layer and size choices. For

example, in assessing the quality of the different contacts in the process, having 3 vs 5

linewidths for all types of contacts may be preferable to having 5 linewidths for fewer types.

8
2.3. Assuring the Testing Environment

Before collecting data that will be analyzed for significant purposes, the hardware and software

in the testing environment must be thoroughly validated. Instruments should be calibrated, the

switching matrix and probe card connections verified, and other tester diagnostics performed.

Software function and correct data logging should be demonstrated using known artifacts or benchmarks. This should verify that correct currents or voltages are forced or measured and

that sufficient settling times are allowed to produce realistic results that are accurately stored in

the intended locations in a database.

These activities are simply good engineering practice. Other assurances that relate more to specific measurement procedures for MMIC environments and NISTGAAS test structures are discussed in the introductory notes in Appendix B.

2.4. Developing Data Analysis Capabilities

Automated techniques with analyst-friendly interfaces should be developed to assure

consistency in analyzing test structure data of more than minimal volume. The first step of such

automation should include an algorithm to assure that measurements are reproducible. If the

measurements are not reproducible, the cause needs to be determined, the problem resolved,

and the measurements repeated. If measurements are reproducible, the next step should

include a robust outlier exclusion algorithm, such as in reference [7], to remove data points that

will skew further statistical analysis. Once reproducibility is demonstrated and outliers are excluded, the integrity of the data is sufficient to support further analysis. Some mechanism

should also exist so that analysts can easily distinguish the original data from data that have

successfully completed these evaluations.

3. COMPUTER-AIDED DESIGN (CAD) CELL LIBRARY

3.1. Overview

The NISTGAAS cell library was designed using the Magic CAD layout editor (version 6.3 on a Sun SPARCsystem 300 with SunOS 4.1.2 and Open Windows 2.0). The library layout and its associated technology file are designed in a hierarchical manner. The test structures are designed with constraints intended to assure portability and immunity to yield-limiting defects.

These features enable the library to be customized easily for different mask layers and design

rules. Because the library is hierarchical by test structure type and a cell naming convention is used, test structure designs are easily located and the framework for adding customized designs

is available and obvious.

For most test structure types, a number of design variations are provided. The variations for a given test structure type result from different combinations of process layers and dimensions,

producing a set of unique designs for that test structure type. Each design variation is a "cell" in

the NISTGAAS library and is stored as a separate file within the library file hierarchy.

9
All the design variations for each test structure type are listed in Appendix A and are referenced in the specification for each test structure type in Appendix B. Although each

specification shows the drawing for only one of the available cells, all cells referenced in the

specification are found in the electronic version of NISTGAAS. To understand the NISTGAAS

cell designs, some background process- and library-related information is needed.

3.2. Process Layers

The NISTGAAS technology file represents a common Metal-Semiconductor Field Effect

Transistor (MESFET) process, where the design rules and processing steps support a planar

process fabricating 1 ^m, low-noise, depletion-mode devices. The design rules are summarized

in table 2, and the processing steps are summarized in figure 2.

Table 2. NISTGAAS Design Rules

M © R 5 Ml DB AIR T

Minimum

width (urn)

4 4 4 1 1 4 4 4

Minimum

space (^im)

5 4 4 2 2 4 4 4

where:

M = mesa

0 = ohmic

R = resistor

G = gate

M1 = first metal

Dl = dielectric (nitride)

AIR = air bridge

T = thick metal

3.3. Plot Colors and Patterns

Each process layer or construct in a cell is represented as a color, in the electronic format of

NISTGAAS, and as a black and white pattern in the drawings in Appendix B. In the electronic

version, a legend shows each layer name and the associated color. When the NISTGAAS cells are viewed on a Sun system or in black and white, the colors and patterns equate to the process

layers and constructs as indicated in Appendix A.

3.4. Cell Naming Conventions

Cell names consist of two or three fields concatenated with the character "_" between each. The

three fields include test structure name, layer name, and dimension. A description of each field

follows, along with its valid entries.

10
Substrate

GaAs

TOO^m

Mesa

2x1

1

cm-3

0.3 /jm

Ohmic

Ni/GeAu/Au

0.3 fjm

Resistor

NiCr

0.0150^

Recess

0.1 /jm

., ... ...

,

, ,

, ^>^^^^!^<jj^^7>j .

jj . i .i .

. jffiyysfr , , . , . , . , . , . , . , . , . , . , . , . , .

, . , . gate

Ti/Pt/Au

0.9

11
3.4.1. Test Structure Names

The test structure name field is an abbreviation of the full test structure name:

1. CONRES - Kelvin-Cross Interfacial Contact Resistor (Four-Pad)

2. CONRESSH - Kelvin-Cross Interfacial Contact Resistor (Shared-Pad)

3. ALIGNH and ALIGNV - Nanometer-Resolution Electrical Alignment Structure;

names denote different designs for horizontal and vertical orientations

4. VPAUW - Mesa/Channel van der Pauw Sheet Resistor

5. MEANDER - Step Coverage/Interconnect Meander

6. INTRES - Interconnect Resistor

7. CROSSBR - Cross-Bridge Resistor

8. RFFETH and RFFETV - MIMIC-Standard 200 urn FET; names denote different

horizontal and vertical designs based on the Task 4.E MESFET designs with the same names.

3.4.2. Layer Names

The layer name field identifies the process layer(s) in the test structure design. In most cases, a

layer is indicated by its first letter, and multi-character layer names indicate two layers, with the

upper layer occurring first in the layer name. The exceptions are: Ml; AIR; and names noted as

"in CONRESSH," where the name indicates which layer in the two sets of layers in the test structure is shared at pads used by both designs. The NISTGAAS layer names are:

1. M - mesa

2. O - ohmic; in CONRESSH: layer shared by OM and MIO designs

3. R - resistor; in CONRESSH: layer shared by RO and MIR designs

4. G-gate

5. Ml - first metal; in CONRESSH: layer shared by M1G and TM1 designs

6. T - thick metal; in CONRESSH: layer shared by TO and TG designs

7. AIR - air bridge (thick metal-dielectric-first metal)

8. OM - ohmic-to-mesa

9. RO - resistor-to-ohmic

10. GO - gate-to-ohmic

11. MIO - first metal-to-ohmic

12. TO - thick metal-to-ohmic

13. MIR - first metal-to-resistor

14. M1G - first metal-to-gate

15. TG - thick metal-gate

16. TM1 - thick metal-to-first metal

12
3.4.3. Dimensions

These represent dimensions in the test structure design, in micrometers: the width of a square

contact or a bridge, or the dimensions of a FET gate. The NISTGAAS dimensions include:

1. 1

2. 2

3. 3

4. 4

5. 6

6. 8

7. 10

8. 12

9. 1x200

Thus, some typical cell names are VPAUW_M_4, CONRES_MlR_12, MEANDER_AIR, and

RFFETH_lx200. For a complete list of the cell names for all of the NISTGAAS designs, see Appendix A.

3.5. CAD Drawing Hierarchy

The NISTGAAS cell library is hierarchical, with like cells grouped together. The library

configuration is shown in Appendix A, which also describes how to navigate through the

hierarchy to any test structure design.

3.6. Obtaining the NISTGAAS Cell Library

To obtain the NISTGAAS cell library and its associated technology file, or for more information,

please contact:

C. E. Schuster

NIST

Bldg 225/Rm B360

Gaithersburg, MD USA 20899-0001

Phone: 1-301-975-2241

e-mail: schuster@sed.eeel.nist.gov

13
4. ACKNOWLEDGMENTS

The efforts of these individuals are notable in developing the NISTGAAS cell library and are appreciated: P. Roitman for guidance on designing the CAD library; R. A. Allen, for guidance

on designing and testing the electrical alignment test structure; J. C. Marshall for developing the

Magic technology file; C. H. Ellenwood for digitizing and managing the CAD library; and L. W.

Linholm for many helpful comments. The NIST team also thanks WL personnel for their

contributions: M. Mah for providing process information; D. Via for digitizing the MIMICStandard 200 [im FET; J. K. Gillespie and S. E. Cummins for providing test information for the

MIMIC-Standard 200 Jim FET, as well as many other helpful technical discussions; and,

especially, R. T. Kemerley for encouraging and supporting the concept of a CAD cell library for

MMIC test structures.

5. REFERENCES

[1] Schuster, C. E., Linholm, L. W., and Gillespie, J. K., High-Density Test Structures for

Assessing Microwave/Millimeter Wave Monolithic Integrated Circuit (MIMIC)

Performance, Digest of Papers, 1991 Government Microcircuit Applications Conference,

Orlando, FL, November 5-7, 1991, Vol. XVII, pp. 335-338 (1991).

[2] Gillespie, J., Bozada, C, Kehias, L., Quach, T., Mah, M., Worley, R., Cummins, S., and

Eppers, C, High Density Test Reticle, U.S. Air Force Wright Laboratory Interim Report

WL-TR-93-5011 (February 1993).

[3] Schuster, C. E., and Linholm, L. W., Semiconductor Measurement Technology: Test Structure

Assessment Document: DC Parametric Test Structures and Test Methods for Monolithic

Microwave Integrated Circuits (MMICs), NIST Special Publication (to be published).

[4] Mayo, R. N., Arnold, M. H., Scott, W. S., Stark, D., and Hamachi, G., 1990

DECWRL/Livermore Magic Release, Regents of the University of California, Lawrence

Livermore National Labs, Stanford University, and Digital Equipment Corp. (September

1990).

[5] Kopanski, J. J., and Schuster, C. E., Review of Semiconductor Microelectronic Test

Structures with Applications to Infrared Detector Materials and Processes, Semicond. Sci. Technol. 8, 892-894 (July 1993).

[6] Suehle, J. S., Linholm, L. W., and Kafadar, K., A Method for Selecting a Minimum Test Chip

Sample Size to Characterize Microelectronic Process Parameters, Proc. 1983 Custom

Integrated Circuits Conf., Rochester, NY, May 23-25, 1983, pp. 308-312 (1983).

[7] Rosner, B., Percentage Points for a Generalized ESD Many-Outlier Procedure, Technometrics

25 (2), 165-172 (1983).

14
Appendix A - NISTGAAS Cell Library Contents

15

Appendix A - NISTGAAS Cell Library Contents

This section describes how to navigate through the CAD drawing hierarchy to a particular test structure design and how to understand the checkplots found in Appendix B. In the following

text, references to process layers or constructs are made in terms of color and pattern. The

colors pertain to what is seen when viewing the CIF format of NISTGAAS, while the patterns

(in black and white) pertain to the Appendix B checkplots.

A.1 List of Test Structures

For each test structure type, a number of design variations related to process layers and size are included. Table A-l shows the available design variations and associated cell names. The cell name is a concatenation of the test structure type, process layer, and size. In the cell name, the

notation "{2,3,4,6,8,10,12}" indicates that seven separate cells actually exist, with the last character of their names being "2", "3", ... "12", respectively. The information in the Layout Label

column relates to the library cell hierarchy as follows.

A.2 Library Hierarchy

When the NISTGAAS file is viewed, the top-level drawing, shown in figure A-l, has blocks

whose labels show only partially. The numbers to the left of each set of blocks are keyed to table A-l, to indicate the content of the layout label blocks at the next lower level of the drawing

hierarchy. When a block at the lower level is viewed, as shown in figure A-2, the layout label in

table A-l is clearly seen in the large full-length block on the left. Each block to the right contains one test structure design variation (i.e., one cell), whose cell name corresponds to an entry in

table A-l.

A.3 Process Layer Designators

Upon viewing a cell, a checkplot of the test structure design is seen. Checkplots for each test structure type are included in the specifications in Appendix B. The meaning of the colors and

patterns is defined in the top-level drawing (figure A-l), in the legend found in the lower left corner, where the process layer names are partially spelled out in their corresponding color or

pattern. For convenience when viewing a design on the screen and in Appendix B, the

mapping of process layer to color and pattern is shown in table A-2. As an aid in identifying

the design when the fabricated test structure is viewed under a microscope or on a CAD

monitor, most designs include a layer designator, formed in thick metal and placed adjacent to a run of the layer.

A.4 Cell Contents

Most cells include two independent sets of probe pads to accommodate either a horizontalvertical or a 0°-90° combination of orientation variations. The convention used is: the upper set of pads is the horizontal or 0° orientation and the lower set of pads is the vertical or 90°

orientation. Both sets of pads can be probed with one touchdown of a 2 x 6 probe card.

17
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21
Table A-2. Mapping of Color/Pattern to Process Layer

PROCESS COLOR PATTERN

LAYER fAnnpnHiy R\

mesa green black

ohmic gray "x" in clear box black "x" in clear box

rpcictnr brown rhprkprhoard nrav rhprkprhoarri

gate rust dark gray

first metal brown 7" gray 7"

dielectric gray 7" in clear box

air bridge yellow "x"

thick metal blue gray

22
Appendix B - NISTGAAS Test Structure Specification

B.l - Kelvin-Cross Interfacial Contact Resistor (Four-Pad)

B.2 - Kelvin-Cross Interfacial Contact Resistor (Shared-Pad)

B.3 - Nanometer-Resolution Alignment Structure

B.4 - Mesa /Channel van der Pauw Sheet Resistor

B.5 - Step Coverage /Interconnect Meander

B.6 - Interconnect Resistor

B.7 - Cross-Bridge Resistor

B.8 - MIMIC-Standard 200 urn FET

23
Appendix B - NISTGAAS Test Structure Specification

These notes apply to all the test structure specifications in this Appendix.

Users are responsible for selecting the exact values needed for forcing currents, tolerances,

limits, or other comparative purposes cited in these procedures. Values should be processspecific and based on good engineering practice or previously demonstrated performance.

Scope

Each test structure type is specified in terms of its purpose, application, design variations, CAD

checkplot, layout constraints, measurement and computation methods, analysis of results, and

historical references. Some noteworthy aspects of these elements follow.

Since this document is implementation-oriented, the theoretical details of each test structure are primarily in the references. Some theoretical design considerations are reflected in the layout

constraints section, although its main purpose is to help the user develop wafer-level layouts or adapt test structure designs for a different process or layer/size combination.

All design variations listed for each test structure type are available in the electronic version, as indicated in Appendix A. However, each specification includes a checkplot of only one design

variation, for convenient use with this document. For a more detailed understanding of the

implementation of this representative design, see the corresponding process sideview in

Appendix C.

The measurement and computation methods are described independently of any particular

computer-based system.

The analysis of results section is intended to help an analyst interpret what the measurements

mean and how the results might be significant to process engineers and device designers.

Format

To the extent possible, the measurement procedures are described in tabular format, so they are easy to use when creating tester code. The preliminary measurement procedures, however, are

in expository format to adequately explain intent, criteria for successful completion, and

possible problems and solutions.

The tabular format includes columns specifying quantities to force, measure, and compute. In the Force and Measure columns, these conventions apply:

"1-3" means "current into probe pad 1 and out of probe pad 3" or "voltage at probe pad 1 with respect to probe pad 3."

"upper" and "lower" refer to the location of an interconnected set of probe pads, as cited

in the layout constraints section.

24
Instrumentation

Some test procedures require specific, but commonly available, measurement system

capabilities, as noted. Instruments with nanovolt vs microvolt resolution and teraohm vs gigaohm input impedance may be needed if the maximum current that can safely be forced will cause only a small voltage for measurement. If such instruments are not available, making such

measurements is likely to produce invalid data.

All measurements should be made with dark-kits installed and microscope lights off. As MMIC

measurements are affected by light, total darkness provides a reliable standard that enables

meaningful comparisons of any future data from the same test structure design.

Validation of Method

Most measurement methods include a preliminary procedure to be run at "five sites" (see next

paragraph) prior to the procedure intended for full-data collection. Preliminary procedures are

often needed to assure that a valid measurement can be made or to determine a value to be

used in the full-data procedure.

In this Appendix, references to "five sites" indicate something less than a full-data set. The exact

number of sites and their location depend on the factors discussed in section 2.2. If the test structures are distributed over a whole wafer, "five sites" means "a minimum of one site near

the center of the wafer and one site near the center of each quadrant, with any additional sites

distributed as uniformly as possible over the rest of the wafer." If test structures are distributed

some other way, use a minimum of five sites that are as evenly distributed as possible. If less

than five sites are on the wafer, use all sites.

Most of the validation procedures refer to "normalized current density," which is the product of

current density and layer thickness. The normalized values cited are based on a typical current

density for metals of 10

6 A/cm2 (10 mA/fim2 ) and the layer thicknesses for the representative

fabrication process described in section 3.2. Users should follow the methodology of the

examples provided, substituting their layer characteristics to find appropriate forcing currents.

Validation of Data

Sometimes the Compute column computation does not reflect the most efficient computer code

implementation. However, it does provide intermediate values needed to fully understand the

data collected, as explained in the analysis of results section. Such analyses can be valuable in

resolving problems in tester code, equipment set-up, or on the fabricated sample.

After all data are collected for all the test structure types, retest the "five sites" on the wafer to provide an assessment of the ability to consistently measure each test structure. Compare each

retest data value to the full-data value for the corresponding site. If the values are not within

the predetermined limits established by the user, the cause needs to be determined before

meaningful analysis can be performed on the full-data set.

Before analyzing or comparing any data collected, remove outliers (that will skew the results of

such investigations) by applying a robust outlier exclusion method [7].

25

Kelvin-Cross biterfacial Contact Resistor (Four-Pad)

PURPOSE To determine the interfacial contact resistance and the uniformity of the

interfacial contact layer.

APPLICATION To assess the quality of a contact type.

The interfacial contact resistance is a more suitable quantity for assessing

contact quality than those provided by other frequently used "contact

resistance" methods. Other methods, such as the transmission line model

(TLM), measure end contact resistance or indirectly measure front contact

resistance. The Kelvin-cross is more accurate than these because: its Kelvin measurement eliminates parasitic resistances from each probe-toprobe pad, tap, and contact layer; its layout considerations minimize

other parasitic resistances; and it is a direct measurement rather than an

extrapolated value with an inherently large error.

The uniformity of the interfacial contact resistance over a range of contact

sizes has been shown to be a good indicator of the quality of contacts, and

hence, the performance and reliability of subsequently fabricated circuits.

This uniformity is assumed, rather than measured, in the TLM method,

making it unreliable for predicting contact resistance for contact sizes not

measured. Such characterization can be accomplished accurately with

the Kelvin-cross once it has been used to demonstrate the uniformity of

the interfacial layer.

DESIGN See cell names (for contact type):

VARIATIONS

CONRES. OM_L (ohmic-to-mesa)

CONRES. GO_L (gate-to-ohmic)

CONRES. RO_L (resistor-to-ohmic)

CONRES. MIO.L (metal 1-to-ohmic)

CONRES. M1R_L (metal 1-to-resistor)

CONRES._M1G_L (metal 1-to-gate)

CONRES. TOJL (thick metal-to-ohmic)

CONRES. TG_L (thick metal-to-gate)

CONRES. TM1_L (thick metal-to-metal 1)

where L = {2, 3, 4, 6, 8, 10, 12} indicates the design linewidth in |im.

27
LAYOUT These constraints apply to the design shown in figure B-l.

CONSTRAINTS

1) Each cell includes two sets of 2 x 2 probe pads, with a Kelvin-cross

between each set. Each Kelvin-cross contains one contact composed of

the same two layers. The upper set of pads contains the 0° orientation

and the lower the 90° orientation, which has its current and voltage taps

rotated counterclockwise from those in the upper set. (Measuring two

such orientations enables the analysis of orientation effects; it also has

long use as a measurement technique to eliminate the effects of any

offsets in the measurement system.)

2) Each contact is square, is of minimum-design width, and has taps the

same width as the contact edge (to eliminate parasitic resistances due to current crowding as current passes from the taps to the contact).

3) If a contact uniformity assessment is not needed, include only the

minimum-design width test structure for the contact types of interest.

However, if a small current will not produce a sufficient voltage (see the

"Note:" in the measurement validation procedure), several larger size

designs should be used to demonstrate uniformity, in order to enable

extrapolation of the contact resistance for the (smaller) size of interest.

4) If determining the uniformity of a contact type, include the minimumdesign width test structure and those with different size square contacts

(to the extent that sufficient voltage is not a problem), and place them

adjacent to each other on the wafer. At least three different sizes are needed, but five (or more) provide a better basis for the curve fit used in

analyzing the data.

METHOD For some layer combinations, the largest current that can be forced safely

produces a small voltage that is measured accurately only if using a voltmeter with nanovolt resolution and teraohm input impedance.

For each design variation, perform the validation procedure in 1) below

and then use the measurement procedure in 2) below to collect the

intended test structure data. The validation procedure determines the

forcing current to be used in the measurement procedure and assures that a valid test structure measurement can be made.

1) Determine the appropriate forcing current, I

f

, needed to measure the

interfacial contact resistance, R, . This procedure requires considering the

current densities of the process layers and the resolution of the voltmeter,

making some preliminary measurements, and assuring that certain

conditions are met. If such a determination has never been made, or the

fabrication process has changed since the previous determination, then

perform the following procedure; otherwise, proceed to the measurement

procedure in 2).

28
conres_m1r_10

Figure B-1. Kelvin-Cross Interfacial Contact Resistor (Four-Pad)*

29
a. Determine the (theoretical) maximum I

{ that can be applied

without Joule heating by identifying the lesser of the maximum

normalized currents that can be sustained by the layers of the given

design variation. For example, in a gate-to-ohmic contact, the maximum

normalized current is limited by the ohmic layer if it is 3 mA/|jm for

ohmic metal and 9 mA/(im for gate metal. For a 4 (im contact edge, this

means an Z

f = 6 mA gives an effective normalized current of 1.5 mA/}im,

or 50 % of the 3 mA/jim constraint, while an 7

f = 12 mA gives 100 %.

To assure that Joule heating is avoided, limit an initial test value for If to a value significantly less than the constraint. Also consider any process

history and knowledge about the minimum measurable and maximum

sustainable currents for the particular layer types. An automated

sequence of sweeps (e.g., from 50 % to 80 % of the constraint value) may

be helpful in determining the I

f that best satisfies the criteria below, as

several tests may be needed to assure these conditions exist.

Note: the l

( must also create a sufficient voltage across the contact for an

accurate voltage measurement. The minimum voltage needed is in the

decade that is 100 times the resolution of the meter. If a differential

voltmeter with a resolution of 40 |iV is used, the minimum voltage

needed is 1 mV. If the target value for R; for a gate-to-ohmic contact is V

0.01 SI, the minimum 7f that can be used, /fmin > -mL, is 100 mA. This

Ri creates a normalized current of 25 mA/(im, which exceeds the 3 mA/pm

constraint. However, if a voltmeter with a resolution of 10 nV is used, a

smaller minimum voltage of 1 p.V is needed; the safe Zf = 6 mA cited

above creates 60 across the contact, which can be accurately measured

with this voltmeter.

in small contacts with a small R{ and a low normalized current limit, such

measurements can be difficult. If using the maximum safe current and

the best resolution meter available, and the voltage is still insufficient, use

a larger contact to enable forcing a larger current. Then, demonstrate

uniformity for the contact type, and use the extrapolation technique

suggested in layout constraints 3 and 4 to find the Rj for the smaller

contact. Alternatively, the best measurement possible may be one that

only indicates a low level of noise or shows that Rj is less than 1 SI.

b. Collect and evaluate data from both orientations of the test structure. For each orientation, collect data from at least five sites that are uniformly distributed on the wafer.

For each site, sweep and plot the I-V curve from -I{ to +I(

. To perform a sweep, force the current from pad 1 to pad 3, and measure the voltage at pad 2 with respect to pad 4 on each orientation of the test structure. In

the plots for each orientation, observe whether linear operation exists and

the curves are approximately centered near 0 V.

30
If the curves are linear and approximately centered, proceed to 2) and use

the I

f just evaluated to perform the measurement procedure for the same

design layer and size variation. A separate validation for other design

sizes of a given layer combination is usually not necessary, as the I{ should scale linearly.

If the curves are not linear and approximately centered, the cause needs

to be determined and resolved before proceeding .to 2). Some potential causes may be light or heat from the measurement process, offsets in the

measurement system, or problems in the fabrication process. Also,

determine if both orientations and all sites provide approximately

equivalent results. If not, the cause may need to be resolved before

proceeding to 2). Some problems may be resolved by performing

additional sweeps at progressively higher, but constrained, currents.

2) Determine the interfacial contact resistance, JR

S , for a contact type:

Force Measure Compute

1-3, upper: +I{ 2-4, upper: Vn

1-3, upper: -I( 2-4, upper: Vu Vn +

2/f

1-3, lower: +If 2-4, lower: V21 1-3, lower: -7

f 2-4, lower: V22 *2 =

Vn + v

2 If

2

3) Determine the uniformity of the interfacial layer for a contact type by

making the measurements in 2) above on at least three, but preferably

five or more, different size contacts of that type.

ANALYSIS OF 1) For each site, validate the data as follows. Compare the pairs of V

RESULTS values for a given orientation of a given contact type: Vn and Vn(0°

orientation); and V2l and (90° orientation). If both magnitudes in a

pair are not approximately equal, the cause should be determined. Also,

Vn and Vn should be positive, and Vn and V22 should be negative.

Compare the R1 and R2 values. If they are not approximately equal, this

indicates possible orientation effects. Such possibilities should be

investigated by analyzing equivalent-orientation data from other test structures and devices on the wafer.

2) For the valid data for each design variation, find the mean of Rt and

compare it to the target value. An important point to remember is: determining and comparing Rt values to more than two or three

31
significant digits should not be as important as simply determining that

the measured is small, where small is considered to be something less

thanlQ.

3) To demonstrate the uniformity of the interfacial layer for a given

contact type, plot the log of the mean of Rj vs the log of its associated area, A, and perform a linear curve fit. The better the points fit a straight line,

the better the uniformity of the interfacial layer and the better the quality

of that contact type.

REFERENCES 1) Proctor, S. J., and Linholm, L. W., A Direct Measurement of Interfacial

Contact Resistance, IEEE Electron Device Letters EDL-3, 294-296 (1982).

2) Proctor, S. J., Linholm, L. W., and Mazer, J. A., Direct Measurements of

Interfacial Contact Resistance, End Contact Resistance, and Interfacial

Contact Layer Uniformity, IEEE Trans. Electron Devices ED-30 (11), 1535-

1542 (1983).

32
B.2 Kelvin-Cross Interfacial Contact Resistor (Shared-Pad)

PURPOSE To determine the interfacial contact resistance and the uniformity of the

interfacial contact layer.

APPLICATION To assess the quality of a contact type.

The interfacial contact resistance is a more suitable quantity for assessing

contact quality than those provided by other frequently used "contact

resistance" methods. Other methods, such as the transmission line model

(TLM), measure end contact resistance or indirectly measure front contact

resistance. The Kelvin-cross is more accurate than these because: its Kelvin measurement eliminates parasitic resistances from each probe-toprobe pad, tap, and contact layer; its layout considerations minimize

other parasitic resistances; and it is a direct measurement rather than an

extrapolated value with an inherently large error.

The uniformity of the interfacial contact resistance over a range of contact

sizes has been shown to be a good indicator of the quality of contacts, and

hence, the performance and reliability of subsequently fabricated circuits.

This uniformity is assumed, rather than measured, in the TLM method,

making it unreliable for predicting contact resistance for contact sizes not

measured. Such characterization can be accomplished accurately with

the Kelvin-cross once it has been used to demonstrate the uniformity of

the interfacial layer.

This shared-pad implementation is a space-saving alternative to the

classic four-pad Kelvin-cross contact resistor, requiring 12 vs 16 pads to implement the test structure for two different contact types.

DESIGN

VARIATIONS

LAYOUT

CONSTRAINTS

See cell names (for contact type):

CONRESSH.O.L

CONRESSH_R_L

CONRESSH_Ml_L

CONRESSH T L

(ohmic-to-mesa and metal 1-to-ohmic)

(resistor-to-ohmic and metal 1-to-resistor)

(metal 1-to-gate and thick metal-to-metal 1)

(thick metal-to-ohmic and thick metal-to-gate)

where L = {2, 3, 4, 6, 8, 10, 12} indicates the design linewidth in Jim.

These constraints apply to the design shown in figure B-2.

1) Each cell includes a set of 2 x 6 probe pads and a total of four Kelvincrosses, providing 0° and 90° orientations for two different contact types.

33
conressh_o_10
2) The upper two crosses implement contact type a: the top cross (at pads

2 to 11) represents the 0° orientation; the bottom cross (at pads 3 to 10)

represents the 90° orientation, which has its current and voltage taps

rotated counterclockwise from those in the top cross. (Measuring two

such orientations enables the analysis of orientation effects; it also has

long use as a measurement technique to eliminate the effects of any

offsets in the measurement system.)

3) The lower two crosses, implementing contact type b, contain one of the

process layers used in the upper two crosses: the layer between pads 1 and 2 is also used from pads 3 to 4 and pads 5 to 6. The top cross (at pads

4 to 9) represents the 0° orientation, and the bottom cross (at pads 5 to 8)

represents the 90° orientation.

4) The NISTGAAS convention is to use the layer "shared" by the upper

and lower sets of crosses as the layer name field in the test structure name.

5) Each contact is square, is of minimum-design width, and has taps the

same width as the contact edge (to eliminate parasitic resistances due to current crowding as current passes from the taps to the contact).

6) If a contact uniformity assessment is not needed, include only the

minimum-design width test structure for the contact types of interest.

However, if a small current will not produce a sufficient voltage (see the

"Note:" in the measurement validation procedure), several larger size

designs should be used to demonstrate uniformity, in order to enable

extrapolation of the contact resistance for the (smaller) size of interest

7) If determining the uniformity of a contact type, include the minimumdesign width test structure and those with different size square contacts

(to the extent that sufficient voltage is not a problem), and place them

adjacent to each other on the wafer. At least three different sizes are needed, but five (or more) provide a better basis for the curve fit used in

analyzing the data.

METHOD For some layer combinations, the largest current that can be forced safely

produces a small voltage that is measured accurately only if using a voltmeter with nanovolt resolution and teraohm input impedance.

For each design variation, perform the validation procedure in 1) below

and then use the measurement procedure in 2) below to collect the

intended test structure data. The validation procedure determines the

forcing current to be used in the measurement procedure and assures that a valid test structure measurement can be made.

35
1) Determine the appropriate forcing current, 1

{

, needed to measure the

interfacial contact resistance, . This procedure requires considering the

current densities of the process layers and the resolution of the voltmeter,

making some preliminary measurements, and assuring that certain

conditions are met. If such a determination has never been made, or the

fabrication process has changed since the previous determination, then

perform the following procedure; otherwise, proceed to the measurement

procedure in 2).

a. Determine the (theoretical) maximum If that can be applied

without Joule heating by identifying the lesser of the maximum

normalized currents that can be sustained by the layers of the given

design variation. For example, in a gate-to-ohmic contact, the maximum

normalized current is limited by the ohmic layer if it is 3 mA/^im for

ohmic metal and 9 mA/|jm for gate metal. For a 4 \im contact edge, this

means an 7

f = 6 mA gives an effective normalized current of 1.5 mA/fim.

or 50 % of the 3 mA/(im constraint, while an I

{ = 12 mA gives 100 %.

To assure that Joule heating is avoided, limit an initial test value for l

{ to a value significantly less than the constraint. Also consider any process

history and knowledge about the minimum measurable and maximum

sustainable currents for the particular layer types. An automated

sequence of sweeps (e.g., from 50 % to 80 % of the constraint value) may

be helpful in determining the If that best satisfies the criteria below, as

several tests may be needed to assure these conditions exist.

Note: the I

f must also create a sufficient voltage across the contact for an

accurate voltage measurement. The minimum voltage needed is in the

decade that is 100 times the resolution of the meter. If a differential

voltmeter with a resolution of 40 \lV is used, the minimum voltage

needed is 1 mV. If the target value for R| for a gate-to-ohmic contact is V

0.01 n , the minimum Jf that can be used, /fmjD > -!m!L

, is 100 mA. This

creates a normalized current of 25 mA/jim, which exceeds the 3 mA/^im

constraint. However, if a voltmeter with a resolution of 10 nV is used, a

smaller minimum voltage of 1 ^iV is needed; the safe I{ = 6 mA cited

above creates 60 \iV across the contact, which can be accurately measured

with this voltmeter.

In small contacts with a small Rt and a low normalized current limit, such

measurements can be difficult. If using the maximum safe current and

the best resolution meter available, and the voltage is still insufficient, use

a larger contact to enable forcing a larger current. Then, demonstrate

uniformity for the contact type, and use the extrapolation technique

suggested in layout constraints 3 and 4 to find the Rt for the smaller

contact. Alternatively, the best measurement possible may be one that

only indicates a low level of noise or shows that R{ is less than 1 CI.

36
b. Collect and evaluate data from both orientations and contact

types of the test structure. For each orientation and contact type, collect

data from at least five sites that are uniformly distributed on the wafer.

For each site, sweep and plot the I-V curve from -I( to +Zf

. To perform a sweep for contact type a, force the current from pad 1 to pad 3 on the 0°

orientation (pads 2 and 4 on the 90° orientation), and measure the voltage

at pad 2 with respect to pad 11 on the 0° orientation (pads 3 and 10 on the

90° orientation). For contact type b, force the current from pad 3 to pad 5

on the 0° orientation (pads 4 and 6 on the 90° orientation), and measure

the voltage at pad 4 with respect to pad 9 on the 0° orientation (pads 5

and 8 on the 90° orientation). In the plots for each orientation for a given

contact type, observe whether linear operation exists and the curves are approximately centered near 0 V.

If the curves are linear and approximately centered, proceed to 2) and use

the If just evaluated to perform the measurement procedure for the same

design layer and size variation. A separate validation for other design

sizes of a given layer combination is usually not necessary, as the I

{ should scale linearly.

If the curves are not linear and approximately centered, the cause needs

to be determined and resolved before proceeding to 2). Some potential causes may be light or heat from the measurement process, offsets in the

measurement system, or problems in the fabrication process. Also,

determine if both orientations and all sites provide approximately

equivalent results. If not, the cause may need to be resolved before

proceeding to 2). Some problems may be resolved by performing

additional sweeps at progressively higher, but constrained, currents.

37
2) Determine the interfacial contact resistance, R{ , for two contact types:

type a at pads 2, 3, 10, and 11 and type b at pads 4, 5, 8, and 9:

Force Measure Compute

1-3: +I{ 1-3: -If 11-2: Va2 +

2

2-4: +If 10-3: Va3 2-4: -If 10-3: Va4 + v»

2 It

3-5: +If 9-4: Vbl 3-5: -If 9-4: Vb2 +

4-6: +I{ 8-5: Vb3 4-6: -I{ 8-5: VM

^b2 - K3 | +K

-

2

_Rbl + Rb2 ^

2

3) Determine the uniformity of the interfacial layer for a contact type by

making the measurements in 2) above on at least three, but preferably

five or more, different size contacts of the same type.

ANALYSIS OF 1) For each site, validate the data as follows. Compare the pairs of

RESULTS V values for a given orientation of a given contact type: Val and (0°,

type a); V* and Va4 (90°, type a); Vbl and Vb2 (0°, type b); and VM and VM

(90°, type b). If both magnitudes in a pair are not approximately equal,

the cause should be determined. Also, the Vxl and Vx3 values should be

positive, and the Vx2 and Vxi values should be negative. Compare the

pairs of R values from the 0° and 90° orientations for the given contact

type: JRal and Ra2 (type a); and Kbl and Kb2 (type b). If both magnitudes in

each pair are not approximately equal, this indicates possible orientation

effects. Such possibilities should be investigated by analyzing equivalentorientation data from other test structures and devices on the wafer.

2) For the valid data for each design variation and contact type, find the

mean of and compare it to the target value. An important point to remember is: determining and comparing R{ values to more than two or

38
three significant digits should not be as important as simply determining

that Rj is small, where small is considered to be something less than 1 Q.

3) To demonstrate the uniformity of the interfacial layer for a given

contact type, plot the log of the mean of R{ vs the log of its associated area, A, and perform a linear curve fit. The better the points fit a straight line,

the better the uniformity of the interfacial layer and the better the quality

of that contact type.

REFERE' iCES 1) Proctor, S. J., and Linholm, L. W., A Direct Measurement of Interfacial

Contact Resistance, IEEE Electron Device Letters EDL-3, 294-296 (1982).

2) Proctor, S. J., Linholm, L. W., and Mazer, J. A., Direct Measurements of

Interfacial Contact Resistance, End Contact Resistance, and Interfacial

Contact Layer Uniformity, IEEE Trans. Electron Devices ED-30 (11), 1535-

1542 (1983).

39

B.3 Nanometer-Resolution Electrical Alignment Structure

PURPOSE To determine placement of a feature relative to two reference features.

APPLICATION To determine the misregistration between two mask layers, in order to assess the performance of the lithography alignment process and tools.

This design enables measurements with a total uncertainty of 10 nm or

less and repeatability within 1.5 nm (6 sigma), when compared to NIST

length standards which are based on an optical interferometer.

Measurement results from this design are more accurate than those from

the late 1970s "NBS Alignment Structure," which has 0.1 |im resolution

This design is more sensitive to slight misalignments and less sensitive to sheet resistance variations. This improvement is achieved by shortening

the potentiometer bridge (to minimize material- and process-induced

errors), and then compensating for the resultant systematic error of each

voltage tap (due to a tap no longer being equivalent to a point contact to the bridge), which effectively shortens the length of the bridge.

Other applications are possible with this test structure. By implementing

the design in a single mask layer, it can be used to evaluate the precision

and accuracy of primary pattern generation or printing tools. Then, the

measurements determine the placement of a feature obtained by single

exposure or the registration of the printed features.

DESIGN

VARIATIONS

See cell names (for mask layer types):

ALIGNH_OM_4 and ALIGNV_OM_4

ALIGNH_GO_4 and ALIGNV_GO_4

ALIGNH_M10_4 and ALIGNV_M10_4

ALIGNH_TO_4 and ALIGNV_TO_4

ALIGNH_TG_4 and ALIGNV_TG_4

ALIGNH OG 4 and ALIGNV OG_4

(ohmic-to-mesa)

(gate-to-ohmic)

(metal 1-to-ohmic)

(thick metal-to-ohmic)

(thick metal-to-gate)

(ohmic-to-gate)

The "H" or "V" following "ALIGN" denotes the different designs used to measure horizontal and vertical misregistrations. The two orientations are functionally equivalent but have different topologies which require

different probing procedures. Functionally, the pads of the two

orientations map as shown:

horizontal 1 2 3 4 5 6 7 8 9 10 11 12

vertical 11 1 2 6 3 12 9 10 7 4 5 8

41
LAYOUT These constraints apply to the designs shown in figures B-3 and B-4.

CONSTRAINTS

1) Each cell includes a set of 2 x 6 probe pads and a potentiometer-type

test structure. The voltage taps at horizontal pads 5, 6, 7, 8, and 11

(vertical pads 3, 12, 9, 10, and 5) are used to determine the lengthshortening effects and to assess line uniformity.

2) The other voltage taps implement three fixed offsets. The first offset is used to determine the misregistration and the other two provide known

values which are used for measurement verification. Three taps define

each offset, which is the distance from the center of the "middle" tap with

respect to the center between the centers of the "end" taps. The first offset

is 0 um (horizontal pads 2, 4, and 9; vertical pads 1, 6, and 7). The other

offsets are +1 |im (horizontal pads 5, 3, and 2; vertical pads 3, 2, and 1)

and -1 Jim (horizontal pads 9, 10, and 11; vertical pads 7, 4, and 5).

3) The spacings of the voltage taps relate to the design length of a bridge

segment, L, whose length is 20 um. The following spacings cannot be

changed without making other design considerations and changing the

documented methods for verification and measurement.

between centers of taps from

horizontal vertical

pads pads spacing (um)

5 to 2 3tol

2 to 9 1 to 7

9 to 11 7 to 5

11 to 8 5 to 10 L

6 to 5 12 to 3

8 to 7 10 to 9 3L

2 to 4 1 to 6

4 to 9 6 to 7 L/2

5 to 3 3 to 2

10 to 11 4 to 5 (L/2) + l 3 to 2 2tol

9 to 10 7 to 4 (L/2)-l

4) The taps for measuring voltages (all horizontal pads except 1 and 12;

all vertical pads except 11 and 8) must all cross the bridge and extend

from the same side of the bridge. The extension of each tap must be > 2

times the width of the tap. (These constraints minimize the effects of

inside corner rounding.)

5) All taps and the bridge are implemented in the minimum-design

linewidth. To evaluate the limits of the current process or to design a

42
align h_go_4
alignv_go_4

Figure B-4. Nanometer-Resolution Electrical Alignment Structure (Vertical)

44
mask set that is also usable on future, smaller-geometry processes,

include smaller-design linewidth test structures as well. This design can be scaled down to 0.7 |im; smaller geometries cannot be reliably

fabricated due to imaging nonlinearities in current lithography tools.

6) The NISTGAAS convention is to implement the bridge in the lower

mask layer and the center tap of the measurement and fixed offset taps in

the upper mask layer. Thus, in the gate-to-ohmic design, the bridge is ohmic metal, and the measurement and fixed offset center taps are gate

metal. To minimize the effect of ohmic sheet resistance variations in

some processes, this implementation can be inverted to obtain a measurement least affected by these variations: the longer length bridge

uses gate metal and the shorter length taps use the more variable ohmic

metal. This inverse implementation is the "OG" design, provided in

addition to the conventional "GO" design.

7) A designator in thick metal is provided for easy identification when

viewing these structures. For the horizontal (vertical) cells, the letter below (above) pad 2 indicates the layer of the bridge, while the letter

below (above) pad 11 indicates the layer of the center taps.

8) Each cell includes metal runs that are outside the 2x6 probe-pad area. These runs extend from the pad 2 and pad 11 sides: by 24 \tm and 22 |im,

respectively, for the ALIGNH cell, and by 16 \im and 16 um, respectively,

for the ALIGNV cell. While any other NISTGAAS cell can be placed

adjacent to an ALIGNH or ALIGNV cell without violating any design

rule, the user needs to assure that this is also true if placing non- NISTGAAS constructs adjacent to these cells.

METHOD For each design variation, perform the validation procedure in 1) below

and then use the measurement procedure in 2) below to collect the

intended test structure data. The validation procedure determines the

forcing current to be used in the measurement procedure and assures that a valid test structure measurement can be made.

1) Determine the appropriate forcing current, I

{

, needed to determine the

mask layer misregistration. This procedure requires considering the

current densities of the process layers and the resolution of the voltmeter,

making some preliminary measurements, and assuring that certain

conditions are met. If such a determination has never been made, or the

fabrication process has changed since the previous determination, then

perform the following procedure; otherwise, proceed to the measurement

procedure in 2).

a. Determine the (theoretical) maximum J

f that can be applied

without Joule heating by identifying the lesser of the maximum

normalized currents that can be sustained by the layers of the given

design variation. For example, in a gate-to-ohmic design, the maximum

45
normalized current is limited by the ohmic layer if it is 3 mA/(im for

ohmic metal and 9 mA/|im for gate metal. For a 4 fim bridge, this means

an I

f = 10 mA gives an effective normalized current of 2.5 mA/fim, or 83 % of the 3 mA/jim constraint, while an If = 12 mA gives 100 %.

To assure that Joule heating is avoided, limit an initial test value for I

f to a value significantly less than the constraint. Also consider any process

history and knowledge about the minimum measurable and maximum

sustainable currents for the particular layer types. An automated

sequence of sweeps (e.g., from 50 % to 80 % of the constraint value) may

be helpful in determining the If that best satisfies the criteria below, as

several tests may be needed to assure these conditions exist.

Note: the If must also create a sufficient voltage in the bridge, Vb/ for an

accurate voltage measurement. The minimum voltage needed is in the

decade that is 100 times the resolution of the meter. If a differential

voltmeter with a resolution of 40 uV is used, the minimum voltage

needed is 1 mV. The niinimum Vb to be measured will occur for the

minimum bridge length of 9 |im. For a 4 um ohmic bridge design (with

target sheet resistance, RA , of 0.5 Q.), the minimum If that can be used,

K W

Amin - — — ' *s 0-89 mA. This creates a normalized current of

0.22 mA/um, which is well below the 3 mA/um constraint. More than a minimum I{ should be used when possible. If the sufficient and safe

I{ = 10 mA cited above is used, the minimum Vb to be measured will be

11.2 mV, which can be accurately measured with this voltmeter.

b. Collect and evaluate data from both orientations of the test structure. For each orientation, collect data from at least five sites that are uniformly distributed on the wafer.

For each site, sweep and plot the I-V curve from -If to +If

. To perform a sweep, force the current from pad 1 to pad 12 on the horizontal structure

(pads 11 and 8 on the vertical structure), and measure the voltage at pad 2

with respect to pad 3 on the horizontal structure (pads 1 and 2 on the

vertical structure). In the plots for each orientation, observe if linear

operation exists and the curves are approximately centered near 0 V.

If the curves are linear and approximately centered, proceed to 2) and use

the If just evaluated to perform the measurement procedure for the same

design layer variation.

If the curves are not linear and approximately centered, the cause needs

to be determined and resolved before proceeding to 2). Some potential causes may be light or heat from the measurement process, offsets in the

measurement system, or problems in the fabrication process. Also,

determine if both orientations and all sites provide approximately

equivalent results. If not, the cause may need to be resolved before

46
proceeding to 2). Some problems may be resolved by performing

additional sweeps at progressively higher, but constrained, currents.

2) Determine the misregistration between two mask layers by applying

the following procedure and the analysis of results procedures to both

orientations of the test structure.

a. Determine and test the a values and b values as indicated in

Steps 1 and 2 in the table below, to assure that the voltage taps are functional and the bridge is uniform. If these values are not within T, a predetermined tolerance (see next paragraph), the computations in Step 3

are not meaningful for this site. In this case, log all of the data values for

this site as invalid and proceed to the next site. (Once data for all sites are

collected, analyze the invalid data sites as indicated in the analysis of

results section.) If all the voltages measured in Steps 1 and 2 are valid,

continue to Step 3.

b. In Steps 1 and 2, the value of T is a function of the length and

resistivity of the material, as well as an uncertainty value, k, supplied by

the user. The value for k is less than the uncertainty desired for the

measurement of misregistration. The value for T is then determined by

k

T - — ( V4 + V5 ) . The V4 and V5 values are found as indicated in Step 1 in

L

the table. Thus, if the desired measurement uncertainty is 20 nm, and the

V4 + V5 sum is 2 mV, the value for T is 2 \iV.

c. In Step 3, average the Vs measured in Steps 1 and 2, as shown

in the table. Use the indicated averages to find the length-shortening of

the bridge due to the voltage taps: 8LL for one tap in the lower metal

layer, and SL^ for one tap in the upper metal layer. Use the 8Ls and Vs

indicated to determine xp and xm , verification measurements of the fixed

offsets of +1 |im and -1 \im, and to determine x, the measured value of

the mask misregistration at the site.

See next page for table.

47
Force Measure

H

6-5

5-3

3-2

2-4

4-9

9-10

10-11

11-8

8-7

5=11

6-7

12-3: 3-2

2-1 1-6

6-7

7-4

4-5

5-10

10-9

3-5

12-9

Vu

Vu

Vu

V4a V5. V*

V7> V^

V9a v10> Via

Compute

(and test tolerances)

Stepl = vm -<yu + v3a +

^ + v5a +

^

+ v7a ) «2 = Ku-(Ka + v10a + v8a + v9a )

if {(a1 >r)u(a2 >r)u(a3 >r)}

then {log "invalid" xp , xm/ and x for site; go to next site}

else {continue to next Step)

1-12 11-8 -If as in

Step 1

as in

Stepl

Vlbto Vnb

Step 2

repeat Step 1, substituting "b" for "a" and

"b"for "a":

- measure values Vlb to Vnb

- compute and test values h to b3 Step 3

Vn = V"a ± v»*

t Where n = {1, 2, ... 10}

v =

Vi + V9 2

SL= Vc ~ 3Vs L

ve -vt = _2L V10 -3V& 3 Vc -V8

V2 + V3 2

_v6 -y7 l-^l

V6 + V7 2

_V4 -V5 L-SLlv4 +vs

48
ANALYSIS OF 1) If invalid measurements are found in Step 1, further analysis is needed

RESULTS to determine whether this is due to measurement system problems or processing problems. To begin such an investigation, remeasure each

invalid data site to see if the problem is reproducible, assuring that good

contact exists between the probes and probe pads. If valid data cannot be

obtained with careful, manual measurement, log the V values and the x

values (in an area separate from the database for the valid data).

Analyze the Vx to Vu values to see if a particular bridge segment is a frequent problem. Check that the components of V1 , V10 , and Vn are

consistent with their measured values, and check that V4 = V5 , V2 = and V3 = V6

. If measurement system problems cannot be identified and a

significant amount of data remains invalid, other process problems may

exist. Analyze and correlate other test structure data to investigate this

possibility.

2) Perform this analysis on the valid data collected for each orientation of

the test structure. Compare the verification measurements, xp and xm , to their corresponding design values of +1 ^m and -1 \im, as follows. Find

the mean of all the valid measurements for each quantity: xp ,xm , and x. Plot each mean value vs its corresponding design value; perform a linear curve fit for the three points. Then, subtract the value of the y-intercept

from the mean of the xp values and from the mean of the xm values. If either result is significantly different from its corresponding design value

(i.e., not within the desired range given the k value specified by the user),

further evaluation is needed as described in the next two paragraphs.

If stepper-based lithography was used to implement the test structures,

distinguish between a patterning error and either a measurement system

error or a processing problem as follows. For each site, plot the xp ,xm , and x values. If all the plots consistently show three points not in an

acceptably straight line, then a patterning error is likely. If, in the set of

plots with the three points not in an acceptably straight line, the random

(non-repeated) errant points can be treated as outliers, then a measurement system or process error is likely.

If stepper-based lithography was not used, data from more than one

wafer is needed to perform further analysis. If data exist for a statistically

significant number of wafers, for each site on each wafer, plot the xp ,xm , and x values. For a given site, if the plots for all the wafers show the

same relationship between the three points, then a systematic error exists.

3) For each orientation, the misregistration between the two mask layers

is the extracted value of the y-intercept, as determined in 2) above. A

smaller value means lesser misregistration. Positive values from the

horizontal (vertical) structure indicate that features fabricated on the

wafer near the site are more to the right (bottom) than they would be if

49
they were perfectly aligned. The difference between the mean of the

measured x value and the extracted misregistration value is the residual

error (due mainly to imperfections in the material or patterning of the

mask layer), as described in Reference 2.

4) Examine the individual and overall effects of both the horizontal and

vertical misregistration by creating wafer maps of full-wafer data for xp

, xm , and x. Translational (run-out) or rotational effects, can be extracted, as discussed in Reference 4.

REFERENCES 1) Linholm, L. W., Allen, R. A., and Cresswell, M. W., Microelectronic

Test Structures for Feature Placement and Electrical Linewidth

Metrology, in the Handbook of Critical Dimension Metrology and Process

Control, Vol. CR52, K. M. Monahan, Ed. (SPIE, Bellingham, WA, 1994),

pp. 91-118.

2) Allen, R. A., Cresswell, M. W., Linholm, L. W., Owen, J.C., III, Ellenwood, C.H., Hill, T. A., Benecke, J. D., Volk, S. R., and Stewart, H. D.,

Application of the Modified Voltage-Dividing Potentiometer to Overlay

Metrology in a CMOS/Bulk Process, Proceedings of the 1994 IEEE

International Conference on Microelectronic Test Structures, San Diego,

CA, March 22-24, 1994, Vol. 7, pp. 51-56.

3) Cresswell, M. W., Penzes, W. B., Allen, R. A., Linholm, L. W.,

Ellenwood, C. H., and Teague, E. C, Electrical Test Structure for Overlay

Metrology Referenced to Absolute Length Standards, Proceedings of

SPIE, International Society of Optical Engineering, Integrated Circuit

Metrology, Inspection, and Process Control VIII 2196, 512-521 (1994).

4) 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, Tech. Digest, Intl Electron Devices Meeting, Washington, DC, December 5-7, 1977,

pp. 7A-7F (1977).

50
B.4 Mesa/Channel van der Pauw Sheet Resistor

...

PURPOSE To determine active layer sheet resistance and channel thickness.

APPLICATION To assess the quality of the active layer and the channel and to assess the

potential performance of a FET formed using the active layer and

channel.

If channel thickness for a transistor is needed as a monitor or predictor of

FET performance, this test structure can replace the conventional van der

Pauw test structure fabricated in the active layer and provide the

measurement needed to determine channel thickness. A van der Pauw

structure fabricated in the active (mesa) layer enables direct measurement

of the sheet resistance of the mesa. A gated van der Pauw structure

fabricated in the channel (recess) enables direct measurement of the sheet

resistance of the channel, which can be used with doping profile

information to determine channel thickness.

This method of determining channel thickness is inherently more

accurate than methods using the transmission line model (TLM) test structure because it is more direct and accurate. The TLM-based methods

require multiple measurements on each test structure and multi-step

graphical and extrapolation techniques to obtain channel thickness. They

also assume that the TLM can provide an accurate contact resistance

value (see sections B.l and B.2) and that a uniform contact resistance does

exist over the several hundred micrometers from which TLM

measurements are made.

DESIGN See cell names:

VARIATIONS

VPAUW_M_L

where L = {2, 3, 4, 6, 8, 10, 12} indicates the design linewidth in \im.

LAYOUT These constraints apply to the design shown in figure B-5.

CONSTRAINTS

1) Each cell includes two van der Pauw sheet resistors. The lower 4 pads

of each cell contain the mesa sheet resistor. The upper 6 pads contain the

channel sheet resistor and include a gate connection to bias the channel,

as explained in the measurement method section.

2) The arms of the van der Pauw cross must have a length £ 2 times their

width.

3) The linewidth of the test structure implemented on the wafer should

be the same as the width of the recess (bottom) in the device of interest.

51
vpauw_m_1

2
METHOD 1) Perform this validation procedure and then use the measurement

procedure starting at 2) below to collect full-wafer data. This validation

procedure assures that a valid test structure measurement can be made.

a. Select a forcing current, I{

, based on process history or other

knowledge (0.5 mA is typical for a MESFET process).

b. Collect and evaluate data from the 4-pad and 6-pad structures. For each structure, collect data from at least five sites that are uniformly

distributed on the wafer.

For each site, sweep and plot the I-V curve from -I{ to +/f. To perform a sweep, make the conventional 0° and 90° van der Pauw measurements as

follows. Force the current from pad 2 to pad 1, and measure the voltage

at pad 3 with respect to pad 4; this represents the 0° measurement. For

the 90° measurement, force the current from pad 3 to pad 2, and measure

the voltage at pad 4 with respect to pad 1. In the plots for each

measurement orientation, observe whether linear operation exists and the curves are approximately centered near 0 V.

If the curves are linear and approximately centered, proceed to 2) and use

the I

{ just evaluated to perform the measurement procedure for the same

design size variation. A separate validation for other design sizes is usually not necessary, as the I

f should scale linearly.

If the curves are not linear and approximately centered, the cause needs

to be determined and resolved before proceeding to 2). Some potential causes may be light or heat from the measurement process, offsets in the

measurement system, or problems in the fabrication process. Also,

determine if both orientations at a site and for all sites provide

approximately equivalent results. If not, the cause may need to be

resolved before proceeding to 2). Some problems may be resolved by

performing additional sweeps at other currents, based on process history.

53
2) Determine the sheet resistance of the mesa, Rsh(m)/ using the 4-pad van

der Pauw cross sheet resistor (lower 4 pads):

Force Measure Compute

2-1: +7f 3-4: V1 2-1: -Jf 3-4: V2

2|/f |

3-2: +If 4-1: V3 3-2: -If 4-1: V4

3) Determine the sheet resistance of the channel, Rsh(ch), using the 6-pad

van der Pauw cross sheet resistor (upper 6 pads):

Force Measure Compute

2-1: +Jf 5-6: V,

2-1: -If 5-6: V2 ^chl -

v; + v2 2

5-2: +Jf 6-1: V3 5-2: -Jf 6-1: V4 ^ch2 -

+

2 h

^h(ch) 2

4) To determine how a change in gate bias affects the channel sheet

resistance, perform the procedure in 3) above while also forcing the

desired Vg at pad 3. The resultant channel thickness and sheet resistance

values can then be used as desired in conjunction with doping profile

information (see analysis of results section).

ANALYSIS OF 1) For each site, validate the data as follows. For the mesa data, compare

RESULTS the pairs of V values for each measurement orientation: Vx and V2 (0°

measurement); and V3 and V4 (90° measurement). If the compared

magnitudes are not approximately equal, the cause should be

determined. Also, Vj and V3 should be positive, and V2 and V4 should be

negative. Compare the R^ and values. If they are not approximately

equal, this indicates possible orientation effects. Such possibilities should

54
be investigated by analyzing equivalent-orientation data from other test structures and devices on the wafer. Also perform this analysis for the

corresponding V and R values for the channel.

2) For the valid data, find the means of Rsh(m) and RMch) and compare them

to the target values.

3) Using the value, read the channel thickness from a graph of sheet

resistance and thickness, derived from doping profile data obtained

either from measurement or simulation.

REFERENCES 1) Buehler, M. G., and Thurber, W. R., An Experimental Study of Various

Cross Sheet Resistor Test Structures, /. Electrochem. Soc. 125 (4), 645-650

(1978).

2) 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, /. Electrochem. Soc. 125 (4), 650-654 (1978).

3) Buehler, M. G., and Thurber, W. R., Measurement of the Resistivity of a Thin Square Sample with a Square Four-Probe Array, Solid-State

Electronics 20, 403-406 (1977).

4) David, J. M., and Buehler, M. G., A Numerical Analysis of Various

Cross Sheet Resistor Test Structures, Solid-State Electronics 20, 539-543

(1977).

5) Buehler, M. G., and Thurber, W. R., A Planar Four-Probe Test

Structure for Measuring Bulk Resistivity, IEEE Trans. Electron Devices

ED-23 (8), 968-974 (1976).

6) Williams, R., Modern GaAs Processing Methods (Artech House,

Norwood, MA, 1990), p. 303.

55

PURPOSE To detect step coverage problems between two conducting layers.

APPLICATION To assess qualitatively if catastrophic step coverage problems exist.

Step coverage is most often a problem in a planar process when the

thickness of the upper layer is not more than several times as thick as the

lower layer. If step coverage problems exist, they are most frequently

catastrophic and therefore yield-limiting. Such problems are observable

as loss of continuity in a series chain of contacts.

This design is intended to detect, but not locate or rigorously quantify,

catastrophic step coverage failures. For this application, a large number

of contacts are not needed for robust statistics or detectability, so the

design is compact and probe-pad compatible with the rest of the cell library. Because this design has a small number of contacts, it should not

be used to determine a failure rate, as some similar but larger designs

could be used.

DESIGN

VARIATIONS

See cell names (for mask layer types):

LAYOUT

CONSTRAINTS

MEANDER_RO_4

MEANDER_M1R_4

MEANDER_M1G_4

MEANDER AIR 4

(resistor-to-ohmic)

(metal 1-to-resistor)

(metal 1-to-gate)

(air bridge)

These constraints apply to the design shown in figure B-6.

1) Each cell includes two sets of 2 x 2 probe pads, with a meander

between each set. Each meander contains 170 contacts, all composed of

the same layers, with taps after the 4th and 12th contacts. The upper set of pads contains horizontal contacts and the lower vertical contacts, with

the current and voltage taps rotated counterclockwise from those in the

upper set. (Measuring two such orientations enables the analysis of

orientation effects; it also has long use as a measurement technique to eliminate the effects of any offsets in the measurement system.)

2) Each contact is square, is of minimum-design width, and has taps the

same width as the contact edge (to eliminate parasitic resistances due to current crowding as current passes from the taps to the contact). The

adjacent contacts are separated by the minimum-design spacing. Only

minimum-design widths and spacings are implemented.

57
meander_m1g_4

Figure B-6. Step Coverage/Interconnect Meander

58
METHOD 1) Determine if an open circuit exists for a contact type by measuring

continuity for different length contact chains. Select the forcing current,

If/ based on process history or other knowledge (1 mA is typical for a MESFET process).

Force Measure

1-3, upper: I

f 1-3, upper: Vu

2-3, upper: I

f 2-3, upper: Vn

1-4, upper: I

( 1-4, upper: V13 4-2, lower: I

f 4-2, lower: V21 1-2, lower: I

( 1-2, lower: V22 4-3, lower: I

( 4-3, lower: V23

ANALYSIS OF 1) If the voltmeter does not indicate a compliance limit on Vu

or V21 RESULTS for a given step type, then step coverage should not be a problem.

Otherwise, apply 2) below to estimate the extent of the apparent problem,

assuming that only a single failure is the cause of the compliance

indication as follows.

2) An estimate of the range of the relative failure rate can be found, based

on the number of contacts through which current is observed to flow and

not flow. The relative failure rate is simply the inverse of the number of

contacts through which current should flow. If compliance is not

indicated for Vu or V2l , current flows through all 170 contacts without

failure. If compliance is indicated for a Vxl value, the failure rate is at least 1/170, or 0.00588. Compliance readings for the V values indicate the

failure rates shown:

Compliance limit found for Number of contacts

in failed chain

Minimum

Failure Rate

no Vs 0 0.00

170 0.00588

12 0.085

4 0.250

Note that, while it is possible to make measurements for chains of 158

and 166 contacts, the failure rates that would result are the same order of

magnitude as for the 170 contact chain. Making such additional

measurements would provide little added information, especially since

this design is not intended to locate failures. The computed failure rates should be interpreted qualitatively to indicate that, at best: step coverage

is not a problem, may be a problem, or is definitely a problem.

59
3) Compare the Vlx values to the V2x values to assess horizontal vs

vertical orientation effects.

REFERENCES 1) Buehler, M. G., The Use of Electrical Test Structure Arrays for

Integrated Circuit Process Evaluation, /. Electrochem. Soc. 127 (10),

2284-2290 (1980).

60
B.6 Interconnect Resistor

PURPOSE To determine the load resistance associated with a contact.

APPLICATION

DESIGN

VARIATIONS

LAYOUT

CONSTRAINTS

METHOD

To provide the load resistance of a contact type to a circuit designer.

See cell names (for contact type):

INTRES_OM_4

INTRES_M10_4

INTRES MIR 4

(ohmic-to-mesa)

(metal 1-to-ohmic)

(metal 1-to-resistor)

These constraints apply to the design shown in figure B-7.

1) Each cell includes two sets of 2 x 2 probe pads, with two contacts

between each set. Each contact contains the same two layers. The upper

set of pads contains horizontal contacts and the lower vertical contacts,

with the current and voltage taps rotated counterclockwise from those in

the upper set. (Measuring two such orientations enables the analysis of

orientation effects; it also has long use as a measurement technique to eliminate the effects of any offsets in the measurement system.)

2) Each contact is square, is of the minimum-design width, and has taps

the same width as the contact edge (to eliminate parasitic resistances due

to current crowding as current passes from the taps to the contact). Use

minimum-design spacing between contacts. The contacts are separated

by the minimum-design spacing. Only the minimum-design widths and

spacings are implemented.

1) Perform this validation procedure and then use the measurement

procedure in 2) below to collect full-wafer data. This validation

procedure assures that a valid test structure measurement can be made.

a. Select a forcing current, If , based on process history or other

knowledge (1 mA is typical for a MESFET process).

b. Collect and evaluate data from both orientations of the test structure. For each orientation, collect data from at least five sites that are uniformly distributed on the wafer.

For each site, sweep and plot the I-V curve from -If to +If. To perform a sweep, force the current from pad 1 to pad 4 on the horizontal orientation

(pads 1 and 2 on the vertical orientation), and measure the voltage at pad

2 with respect to pad 1 on the horizontal orientation (pads 4 and 3 on the

vertical orientation). In the plots for each orientation, observe whether

61
intres_m1r_4

Figure B-7. Interconnect Resistor

62
linear operation exists and the curves are approximately centered near OV.

If the curves are linear and approximately centered, proceed to 2) and use

the I{ just evaluated to perform the measurement procedure for the same

design layer variation.

If the curves are not linear and approximately centered, the cause needs

to be determined and resolved before proceeding to 2). Some potential causes may be light or heat from the measurement process, offsets in the

measurement system, or problems in the fabrication process. Also,

determine if both orientations and all sites provide approximately

equivalent results. If not, the cause may need to be resolved before

proceeding to 2). Some problems may be resolved by performing

additional sweeps at progressively higher, but constrained, currents.

2) Determine the load resistance, KL

, for a contact type:

Force Measure Compute

1-4, upper: +If 2-3, upper: Vn

1-4, upper: -I{ 2-3, upper: V12 *L1 = Vn

2 h

1-2, lower: +Jf 4-3, lower: V21 1-2, lower: -If 4-3, lower: V22 RL2 ~

+ V22

lit R

_ RL\ + RL2

L " 2

ANALYSIS OF 1) For each site, validate the data as follows. Compare the pairs of V

RESULTS values for a given orientation: Vn and V12 (horizontal orientation); and

V21 and V22 (vertical orientation). If the compared magnitudes are not

approximately equal, the cause should be determined. Compare the Ru

and Ru values. If they are not approximately equal, this indicates

possible orientation effects. Such possibilities should be investigated by

analyzing equivalent-orientation data from other test structures and

devices on the wafer.

REFERENCES 1) None.

63

Cross-Bridge Resistor

PURPOSE To determine sheet resistance and linewidth of a conducting layer.

APPLICATION To assess the quality of a conducting layer.

The cross-bridge resistor provides two critical parameters which can be

used in process control or evaluation of lithography tool performance. In

either application, the variations in sheet resistance and linewidth across

the wafer are compared with predetermined tolerances to assess whether

adequate control is maintained.

The cross-bridge resistor has been used extensively to provide precise,

fast, and easily-made measurements. Many design variations with

respect to cross, bridge, and tap dimensions are possible. The designs

provided minimize design-induced errors for linewidth results to less

than 20 ran for the 4 |im design and less than 2 ran for the 1 \im design.

For further detail on dimensions, uncertainties, and design requirements

for a test structure to measure sub-micrometer linewidths, see Reference 1 at the end of this chapter.

DESIGN See cell names (for mask layer types):

VARIATIONS

CROSSBR. M 4 (mesa)

CROSSBR. 0_4 (ohmic)

CROSSBR. R 4 (resistor)

CROSSBR. G_l (gate)

CROSSBR. Ml_4 (metal 1)

CROSSBR. T_4 (thick metal)

LAYOUT These constraints apply to the design shown in figure B-8.

CONSTRAINTS

1) Each cell includes a set of 2 x 3 probe pads and a cross-bridge resistor

that meets the minimum-design geometries specified in cross-bridge

Reference 2. Each cross-bridge design includes a van der Pauw cross (pads 1, 2, 5, and 6) for measuring sheet resistance and a bridge resistor

(pads 1, 3, 4, and 5) for measuring linewidth.

2) The bridge and all taps are the minimum-design width for the layer of

interest. The length of the bridge (between pads 4 and 5) is L = 125 \im.

3) The arms of the van der Pauw cross must have a length > 2 times their

width. The van der Pauw width, Wc (dimension of one edge of the square

"boxed" area of the cross), must be at least 10 ^m. The Wc value is 10 pm

in each design except the gate design, where it is 11 |im.

65
crossbr_t_4

Figure B-8. Cross-Bridge Resistor.

66
METHOD For some layers, the largest current that can be forced safely produces a small voltage that is measured accurately only if using a voltmeter with

nanovolt resolution and teraohm input impedance.

For each design variation, perform the validation procedure in 1) below

and then use the measurement procedure starting at 2) below to collect

the intended test structure data. The validation procedure determines the

forcing current to be used in the measurement procedure and assures that a valid test structure measurement can be made.

1) Determine the appropriate forcing current, Jf , needed to measure the

sheet resistance, Ksh . This procedure requires considering the current

density of the process layer and the resolution of the voltmeter, making

some preliminary measurements, and assuring that certain conditions are met. If such a determination has never been made, or the fabrication

process has changed since the previous determination, perform the

following procedure; otherwise, proceed to the measurement procedure

in 2).

a. Determine the (theoretical) maximum lf that can be applied

without Joule heating by considering the maximum normalized current

that can be sustained by the layer in the given design variation. For

example, consider a gate layer where the maximum normalized current is 9 mA/|im. For a gate cross with Wc = 11 |im, this means an I

{ = 50 mA

gives an effective normalized current of 4.5 mA/Jim, or 50 % of the

9 mA/|im constraint, while an 7

f = 99 mA gives 100 %.

To assure that Joule heating is avoided, limit an initial test value for I

f to a value significantly less than the constraint. Also consider any process

history and knowledge about the minimum measurable and maximum

sustainable currents for the particular layer types. An automated

sequence of sweeps (e.g., from 50 % to 80 % of the constraint value) may

be helpful in determining the I

f that best satisfies the criteria below, as several tests may be needed to assure these conditions exist.

Note: the 7

f must also create sufficient voltage in the cross for an accurate

voltage measurement. The minimum voltage needed is in the decade that

is 100 times the resolution of the meter. If a differential voltmeter with a resolution of 40 (iV is used, the minimum voltage needed is 1 mV. If the

target value for Rsh for gate metal is 0.03, the minimum 2, that can be used,

V n

I = , is 151 mA. This creates a normalized current of fmjn ln2'

13.7 mA/|im, which exceeds the 9 mA/|im constraint. However, if a voltmeter with a resolution of 10 nV is used, a smaller minimum voltage

of 1 |iV is needed; the safe If = 50 mA cited above creates 330 |iV in the

cross, which can be accurately measured with this voltmeter.

67
b. Collect and evaluate data from the cross area of at least five

sites that are uniformly distributed on the wafer.

For each site, sweep and plot the I-V curve from -7

f to +7f

. To perform a sweep, force the current from pad 6 to pad 1, and measure the voltage at pad 5 with respect to pad 2; this represents the 0° measurement. For the

90° measurement, force the current from pad 1 to pad 2, and measure the

voltage at pad 6 with respect to pad 5. In the plots for each measurement

orientation, observe whether linear operation exists and the curves are approximately centered near 0 V.

If the curves are linear and approximately centered, proceed to 2) and use

the I

( just evaluated to perform the measurement procedure for the same

design layer and size variation.

If the curves are not linear and approximately centered, the cause needs

to be determined and resolved before proceeding to 2). Some potential causes may be light or heat from the measurement process, offsets in the

measurement system, or problems in the fabrication process. Also, check

if both orientations at a site and for all sites provide approximately

equivalent results. If not, the cause may need to be resolved before

proceeding to 2). Some problems may be resolved by performing

additional sweeps at progressively higher, but constrained, currents.

2) Determine the sheet resistance, Rsh , of a given layer:

Force Measure Compute

6-1: +7f 5-2: Vx *i = Y\

It

6-1: -7f 5-2: V2 R2 = h

1-2: +7f 6-5: V3 R3 = h

1-2: -7f 6-5: Vi *4 =

**i - 2

*sh2"

2 ^h In 2 2

68
3) Determine the linewidth, W, of a given layer, using the Rsh value

determined in 2) above:

Force Measure Compute

1-3: +Zf 5-4: Vx M

1-3: -7, 5-4: V2 ^2b =

D _ ^lb + ^2b

b "

Ah\

w- ^hL

*b

ANALYSIS OF 1) Compare the Rx and R2 values (0° measurement) and compare the

RESULTS R3 and R4 values (90° measurement). If the compared magnitudes are not

approximately equal, the cause should be determined. Here, Vx and V3 should be positive, while V2 and V4 should be negative. Compare the Rshl and R^2 values, and if they are not approximately equal, this indicates

possible orientation effects. Such possibilities should be investigated by

analyzing equivalent-orientation data from other test structures and

devices on the wafer.

2) Compare the Klb and R2b values. If the compared magnitudes are not

approximately equal, the cause should be determined. Here, Vr should

be positive, while V2 should be negative.

3) Compare the Rsh value to its target value and the W value to the design

linewidth.

REFERENCES 1) Linholm, L. W., Allen, R. A., and Cresswell, M. W., Microelectronic

Test Structures for Feature Placement and Electrical Linewidth

Metrology, in the Handbook of Critical Dimension Metrology and Process

Control Vol. CR52, K. M. Monahan, Ed. (SPIE, Bellingham, WA, 1994),

pp. 91-118.

2) Buehler, M. G., and Hershey, C. W., The Split-Cross-Bridge Resistor

for Measuring the Sheet Resistance, Linewidth, and Line Spacing of

Conducting Layers, IEEE Trans. Electron Devices ED-33 (10), 1572-1579

(1986).

3) Carver, G. P., Mattis, R. L., and Buehler, M. G., Design Considerations

for the Cross-Bridge Sheet Resistor, NBSIR 82-2548 (1982).

69
4) 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,/. Electrochem. Soc. 125 (4), 650-654 (1978).

5) Buehler, M. G., and Thurber, W. R., An Experimental Study of Various

Cross Sheet Resistor Test Structures, /. Electrochem. Soc. 125 (4), 645-650

(1978).

6) David, J. M, and Buehler, M. G., A Numerical Analysis of Various

Cross Sheet Resistor Test Structures, Solid-State Electronics 20, 539-543

(1977).

7) Troccolo, P., Mantalas, L., Allen, R. A., and Linholm L. W., Extending

Electrical Measurements to the 0.5 um Regime, Proceedings of the SPIE,

International Society for Optical Engineering, Integrated Circuit Metrology,

Inspection, and Process Control V 1464, 90-103 (1991).

70
B.8 MIMIC-Standard 200 *im FET

PURPOSE To determine dc FET parameters.

APPLICATION To enable device and circuit designers to assess FET performance.

DESIGN See cell names:

VARIATIONS

LAYOUT

CONSTRAINTS

RFFETH_1X200 and RFFETV_1X200

The "H" or "V" following "RFFET" denotes the different designs that

enable the horizontal and vertical FET orientations to be probed with the

same probing procedure and without rotating the wafer.

These constraints apply to the designs shown in figures B-9 and B-10.

1) Each cell includes a set of 2 x 3 probe pads and a FET. Each FET has a

1 ^im gate length and a 200 pm gate width, as indicated by the "1X200" in

its cell name. The gate is centered in a channel in the 5 ^m space between

the source and drain. The left three pads are source-gate-source terminals

and the right three pads are source-drain-source terminals, to allow on- wafer probing by rf network analyzer test equipment. (Note: this

document includes only the dc testing specifications.)

2) The RFFETH cell includes gate metal that is outside the 2x6 probepad area by 8 Jim and 6 nm from the pad 2 and 5 sides, respectively.

While any other NISTGAAS cell can be placed adjacent to an RFFETH cell without violating any design rule, the user needs to assure that this is also

true if placing non-NISTGAAS constructs adjacent to this cell.

METHOD Determine these dc FET parameters for each orientation:

dc FET Parameter

Parameter

Symbol

Test Number

Saturated source-drain current ^dss 1 Saturation voltage vr sat 2

Pinch-off voltage 3

Source-drain resistance 4

Drain resistance 5

Source resistance K 6

J Transconductance 8m 7

J Source-gate breakdown voltage Vrgs 8

| Drain-gate breakdown voltage vv Idg 9

71
rffeth_1x200

Figure B-9. MIMIC-Standard 200 \im FET (Horizontal).

72
rffetv_1x200

Figure B-10. MIMIC-Standard 200 \im FET (Vertical).

73

1) For some of the tests specified on the next sheet, a preliminary

characterization procedure is needed before full-wafer data is collected to assure that valid measurements can be made. Such tests include a

parenthetical note referencing a circled number that is found on the

subsequent page.

If the procedure following the note has never been performed, or the

fabrication process has changed since the previous characterization,

perform the "Preliminary characterization" portion before using the "Post- characterization" portion.

In general, the preliminary characterization portion involves performing

a sweep to demonstrate appropriate operation (i.e., pinch-off or linearity)

and to aid in choosing 1 or 2 appropriate forcing values for use in the full- wafer data collection procedure. This approach increases the efficiency of

the full-wafer data collection process without ignoring critical validation

measurements.

Characterization measurements are made at five sites that are uniformly

distributed on the wafer.

In applying these procedures, the user should also consider the following.

The procedures reference typical values, which may need to be refined

for the user's process. Also, for some parameters, other procedures may

provide adequate data. If the user has such procedures which are associated with significant process history, and wants to retain them, the user should demonstrate the two procedures provide equivalent or acceptably accurate data.

2) Execute tests 1-3 on the next sheet. If Vpo

is not within the

predetermined limits, further testing is not meaningful; otherwise,

continue with tests 4-9.

75
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77
ANALYSIS OF 1) If an unacceptable standard deviation exists for a full-wafer

RESULTS data set, the preliminary characterization procedure may need to be

performed at all sites to investigate this observation.

2) Measured/computed values should be compared to target values and

limits obtained from process simulation or history.

3) Check for possible orientation effects by creating wafer maps of full- wafer data for each dc FET parameter. Orientation effects may be further

analyzed by examining possible correlations between the various dc FET

parameters and equivalent-orientation data from other test structures and

devices on the wafer.

REFERENCES Williams, R., Modern GaAs Processing Methods (Artech House, Norwood,

MA, 1990), pp. 345-355.

78
Appendix C - NISTGAAS Test Structure Sideviews

79
Appendix C - NISTGAAS Test Structure Sideviews

The following process sideviews correspond to the checkplots provided in Appendix B. Each

sideview is taken from a horizontal slice through the contact(s) or construct(s) where

measurements are made. Since this slice is the same regardless of the orientation of a given test structure type, sideviews are shown for only a single orientation of the test structure type. The

layer thicknesses depicted in each sideview are not precisely to scale (per the process shown in

figure 2) because of the range of magnitudes represented. However, the thicknesses shown do

reflect realistic relative differences between all layers except the resistor layer, which is much

thinner than is (relatively) shown.

80
conres_m1r_10

Figure C-1. Sideview: Kelvin-Cross Interfacial Contact Resistor (Four-Pad)

81
conressh_o_1

0

Figure C-2. Sideview: Kelvin-Cross Interfacial Contact Resistor (Shared-Pad)

82
alignv_go_4

\

M

Figure C-3. Sideview: Nanometer-Resolution Alignment Structure •

83
vpauw_m_1

2

Figure C-4. Sideview: Mesa/Channel van der Pauw Sheet Resistor

84
meander_m1g_4

Figure C-5. Sideview: Step Coverage/Interconnect Meander

85
intres_m1 r_4
crossbr_t_4

Figure C-7. Sideview: Cross-Bridge Resistor.

87
rffeth_1x200

Figure C-8. Sideview: MIMIC-Standard 200 urn FET

88
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20056.

Building Science Series—Disseminates technical information developed at the Institute 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—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 NIST under the sponsorship of

other government agencies.

Voluntary Product Standards—Developed under procedures published by the Department of Commerce

in Part 10, Title 15, pf the Code of Federal Regulations. The standards establish nationally recognized

requirements for products, and provide all concerned interests with a basis for common understanding of

the characteristics of the products. NIST administers this program in support of the efforts of private-sector

standardizing organizations.

Order the following NIST publications—FIPS and NISTIRs—from the National Technical Information

Service, Springfield, VA 22161.

Federal Information Processing Standards Publications (FIPS PUB)—Publications in this series

collectively constitute the Federal Information Processing Standards Register. The Register serves as the

official source of information in the Federal Government regarding standards issued by NIST pursuant to the Federal Property and Administrative Services Act of 1949 as amended, Public Law 89-306 (79 Stat.

1 127), and as implemented by Executive Order 1 1717 (38 FR 12315, dated May 11, 1973) and Part 6 of

Title 15 CFR (Code of Federal Regulations).

NIST Interagency Reports (NISTIR)—A special series of interim or final reports on work performed by

NIST for outside sponsors (both government and nongovernment). In general, initial distribution is handled

by the sponsor; public distribution is by the National Technical Information Service, Springfield, VA 22161,

in paper copy or microfiche form.