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5 Terminology 5.1 Acro nyms related to wafer flatness param eters as defined in SEMI M 1 , Appendi x 1, Flatness Decision Tree, are summarized in Table 1. NOTE 1: The most comm only specified flatnes s measure- ments for…

SEMI MF1530-1104

TEST METHOD FOR MEASURING FLATNESS, THICKNESS, AND

TOTAL THICKNESS VARIATION ON SILICON WAFERS BY

AUTOMATED NON-CONTACT SCANNING

This test method was technically approved by the Global Silicon Wafer Committee and is the direct

responsibility of the North American Silicon Wafer Committee. Current edition approved for publication by

the North American Regional Standards Committee on July 11, 2004. Initially available at www.semi.org

September 2004; to be published November 2004. Original edition published by ASTM International as

ASTM F 1530-94. Last previous edition SEMI MF1530-0704.

1 Purpose

1.1 Flatness, thickness and thickness variation are vital

factors affecting the yield of semiconductor device

processing.

1.2 Knowledge of these characteristics can help the

producer and consumer determine if the dimensional

characteristics of a specimen wafer satisfy given

geometrical requirements.

1.3 This test method is suitable for measuring the

flatness and thickness of wafers used in semiconductor

device processing in the as-sliced, lapped, etched,

polished, epitaxial or other layer condition.

2 Scope

2.1 This test method covers a noncontacting,

nondestructive procedure to determine the thickness

and flatness of clean, dry, semiconductor wafers in such

a way that no physical reference is required.

2.2 This test method is applicable to wafers 50 mm or

larger in diameter, and 100 m (0.004 in.)

approximately and larger in thickness, independent of

thickness variation and surface finish, and of wafer

shape.

2.3 This test method measures the flatness of the front

wafer surface as it would appear relative to a specified

reference plane when the back surface of the water is

ideally flat, as when pulled down onto an ideally clean,

flat chuck. It does not measure the free-form shape of

the wafer.

2.4 Because no chuck is used as a measurement

reference, this test method is relatively insensitive to

microscopic particles on the back surface of the wafer.

2.5 The values stated in SI units are to be regarded as

the standard. The values given in parentheses are for

information only.

NOTICE: This standard does not purport to address

safety issues, if any, associated with its use. It is the

responsibility of the user of this standard to establish

appropriate safety and health practices and determine

the applicability of regulatory or other limitations prior

to use.

3 Limitations

3.1 Any relative motion between the probes and along

the probe measuring axis during scanning produces

error in the lateral position equivalent-measurement

data.

3.2 Most equipment systems capable of this

measurement have a definite range of wafer thickness

combined with sori/warp (dynamic range) that can be

accommodated without readjustment. If the sample

moves outside this dynamic range during either

calibration or measurement, results may be in error. An

overrange signal can be used to alert the operator and

measurement data examiners to this event.

3.3 The quantity of data points and their spacing may

affect the measurement results (see Section 7.1.5.2).

3.4 Site flatness measurements may be affected if the

site boundaries and corners do not contain data array

elements. This effect may be reduced through

interpolation techniques.

4 Referenced Standards

4.1 SEMI Standards

SEMI M1 — Specifications for Polished

Monocrystalline Silicon Wafers

SEMI M20 — Practice for Establishing a Wafer

Coordinate System

SEMI MF1241 — Terminology of Silicon Technology

4.2 ASTM Standard

E 691 — Practice for Conducting an Interlaboratory

Study to Determine the Precision of a Test Method

NOTICE: Unless otherwise indicated, all documents

cited shall be the latest published versions.

1 Available from ASTM International, 100 Barr Harbor Drive, West

Conshohocken, PA 19428, Tel: 610-832-9500, Fax: 610-832-9555,

Website

http://www.astm.org. Appears in Volume 14.02 of Annual

Book of ASTM Standards.

SEMI MF1530-1104 © SEMI 2003, 2004 1

5 Terminology

5.1 Acronyms related to wafer flatness parameters as

defined in SEMI M1, Appendix 1, Flatness Decision

Tree, are summarized in Table 1.

NOTE 1: The most commonly specified flatness measure-

ments for advanced IC production in 2004, the time of the last

revision of this standard, are SFQR with a site size of either

26 mm by 8 mm or 25 mm by 8 mm and GBIR (TTV).

5.2 Definitions related to wafer flatness can be found

in SEMI M1.

5.3 Definitions of other terms related to silicon

material technology can be found in SEMI MF1241.

6 Summary of Test Method

6.1 A calibration procedure is performed. This sets the

instrument's scale factor and other constants.

6.2 The wafer is supported by a small-area chuck and

is scanned along a prescribed pattern by both members

of an opposed pair of probes.

6.3 The paired displacement values are used to

construct a thickness data array (t[x,y]). This array

represents the front surface of the wafer when the back

surface of the wafer is ideally flat, as when pulled down

onto and ideally clean, flat chuck.

6.4 The data array is used to produce one or more of

the parameters required by the application.

6.4.1 If flatness measurements are required, a reference

plane and a focal plane suitable to the application are

constructed on the back or front surface.

6.5 Thickness or flatness, or both, values are calculated

and reported, as required.

7 Apparatus

7.1 Measuring Equipment — consisting of wafer-

holding device, multiple-axis transport mechanism,

probe assembly with indicator, and system controller/

computer, including data processor and suitable

software.

7.1.1 The measuring equipment shall have means to

input the choices listed in the Procedure (see Section 12

7.1.2 The measuring equipment shall be direct reading

with all necessary calculations performed internally and

automatically as outlined in Section 13 .

7.1.3 The system shall be equipped with an overrange

signal.

7.1.4 Instrument data reporting resolution shall be 10

nm or smaller.

7.1.5 The measuring equipment contains the following

subsystems:

7.1.5.1 Wafer-holding device, for example a chuck

whose face is perpendicular to the measurement axis,

and on which the wafer is placed for the measurement

scan. The nature and size of the wafer holding device

shall be agreed upon between the parties to the test.

7.1.5.2 Multiple-axis transport mechanism, which

provides a means for moving the wafer-holding device,

or the probe assembly, perpendicularly to the

measurement axis in a controlled fashion in several

directions. This motion must permit data gathering

over a prescribed scan pattern within the entire fixed

quality area. Data point spacing to be used shall be

agreed upon between the parties to the test.

7.1.5.3 Probe assembly with paired noncontacting

displacement-sensing probes, probe supports, and

indicator unit (see Figure 1).

Probe A

Probe B

Wafer

Fixture

Probe A

Probe B

Wafer

Fixture

Figure 1

Schematic View of Wafer, Probes, and Fixture

7.1.5.3.1 The probes shall be capable of independent

measurement of the distances a and b between the

probed site on each surface of the sample wafer and the

nearest probe surface.

7.1.5.3.2 The probes shall be mounted above and

below the wafer in a manner so that the probed site on

one surface of the wafer is opposite the probed site on

the other.

7.1.5.3.3 The common axis of these probes is the

measurement axis.

7.1.5.3.4 The probe separation D shall be kept constant

during calibration and measurement.

7.1.5.3.5 Displacement resolution shall be 10 nm or

better.

7.1.5.3.6 The probe sensor size shall be 4 × 4 mm, or

other value to be agreed upon between the parties to the

test.

SEMI MF1530-1104 © SEMI 2003, 2004 2

Table 1 Acronyms for Wafer Flatness Parameters

Reference Plane

Acronym

Measurement

Method

Reference Surface

Construction Area

Measurement

Parameter

GBIR Global Back Ideal back surface Entire FQA Range (TIR)

GF3R Global Front Three-point Range (TIR)

GF3D Global Front Three-point Deviation (FPD)

GFLR Global Front Least squares Entire FQA Range (TIR)

GFLD Global Front Least squares Entire FQA Deviation (FPD)

SBIR Site Back Ideal back surface Entire FQA Range (TIR)

SBID Site Back Ideal back surface Entire FQA Deviation (FPD)

SF3R Site Front Three-point Range (TIR)

SF3D Site Front Three-point Deviation (FPD)

SFLR Site Front Least squares Entire FQA Range (TIR)

SFLD Site Front Least squares Entire FQA Deviation (FPD)

SFQR Site Front Least squares Site Range (TIR)

SFQD Site Front Least squares Site Deviation (FPD)

SFSR Site Front Least squares Sub-site Range (TIR)

SFSD Site Front Least squares Sub-site Deviation (FPD)

8 Materials

8.1 Set-up Masters — suitable to accomplish

calibration and standardization as recommended by the

equipment manufacturer.

8.2 Reference Wafer — with total thickness variation

(TTV) value and flatness value similar to the product or

process to be monitored and with a data set that is used

to determine the level of agreement between the data

set obtained by the measuring equipment under test and

the reference wafer data set (see Section 9 ).

9 Suitability of Measuring Equipment

9.1 Determine the suitability of the measuring

equipment with the use of a reference wafer and its

associated data set in accordance with the procedures of

Sub-section 9.2 , or by performance of a statistically-

based instrument repeatability study to ascertain

whether the equipment is operating within the

manufacturer's stated specification for repeatability.

9.1.1 The reference wafer data set is a set of thickness

or flatness parameters based on corrected data in that all

possible interferences have been removed and the data

has been replanarized in accordance with this method.

9.1.2 Each reference wafer (artifact) is accompanied by

its own data set, the reference data set (RDS), in which

each parameter value is the average of a number of

values obtained for that parameter over a number of

“passes” (repeat measurements). The artifact is

measured on a measurement equipment under test and

its RDS is compared against the resultant-measured

sample data set. Delta-parameter and other values are

computed from the differences. The parameter used to

determine agreement between the artifact and the

measuring equipment under test and the acceptable

level of this agreement is to be agreed upon between the

parties to the test.

9.2 Procedure

9.2.1 Select a reference wafer of appropriate criteria

together with its associated reference data set (RDS).

9.2.2 Measure the reference wafer on the measurement

equipment under test to obtain a sample data set (SDS).

9.2.3 Subtract the two data sets on a parameter basis to

obtain a difference data set (DDS):

DDSSDSRDS 



(1)

9.2.3.1 The DDS represents the differences between

the measurements made on the measuring equipment

under test and the reference data set. The DDS contains

many values.

9.2.4 The simplest metric that can be used to determine

acceptability is maximum difference, the largest

absolute value in the DDS. This represents the worst-

case disagreement between the results obtained from

the measurement equipment under test and the

reference data. Accept the measurement equipment as

suitable for measurement if the maximum difference is

less than a value that is agreed upon between the parties

to the test.