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SEMI P25-94 © SEMI 1994, 2004 1 SEMI P25-94 (Reapproved 1104) SPECIFICATION FOR MEASURING DEPTH OF FOCUS AND BEST FOCUS This specification was technically approved by the Global Mi cropatterning Com mittee and is the dir…

SEMI P24-94 © SEMI 1994, 2004 5

RELATED INFORMATION 1

NOTICE: The material contained in this related information is not an official part of SEMI P24 and is not meant to

modify or supersede the standard in any way. This information is provided as a source of information to aid in the

application of the standard. As such, it is to be considered as reference material only. The standard should be

referred to in all cases. This related information was approved for publication by full balloted procedures.

R1-1 Short Term Test Duration

R1-1.1 It is recommended the short term test duration

be 30 measurements done in the shortest possible time

under the most limited of conditions to eliminate

extraneous sources of variation.

R1-2 Long Term Duration

R1-2.1 Common practice requires about 30 degrees of

freedom in the highest level of sources of variation.

For example, one should continue a long term study

over 30 working days, if days is the highest level of

variation.

R1-3 Confidence Interval

R1-3.1 This is one way in which confidence interval is

calculated.

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where

v = df in the extimated standard deviation,

/2 – the alpha risk accepted for the estimate (1-) is

the “confidence level”, and

refers to the “chi-square” distribution which fits

variances. The equation is arranged such that the area

under the chi-square distribution is to the right of the

designated value.

R1-3.2 The reference distribution looks something like

Figure R1-1 — its shape depends on the degrees of

freedom in the estimate.

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Figure R1-1

Example of a Chi-Square Distribution

NOTICE: SEMI makes no warranties or

representations as to the suitability of the standards set

forth herein for any particular application. The

determination of the suitability of the standard is solely

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refer to manufacturer's instructions, product labels,

product data sheets, and other relevant literature,

respecting any materials or equipment mentioned

herein. These standards are subject to change without

notice.

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Equipment and Materials International (SEMI) takes no

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mentioned in this standard. Users of this standard are

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such rights are entirely their own responsibility.

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SEMI P25-94 © SEMI 1994, 2004 1

SEMI P25-94 (Reapproved 1104)

SPECIFICATION FOR MEASURING DEPTH OF FOCUS AND BEST

FOCUS

This specification was technically approved by the Global Micropatterning Committee and is the direct

responsibility of the North American Microlithography Committee. Current edition approved by the North

American Regional Standards Committee on August 16, 2004. Initially available at www.semi.org

September 2004; to be published November 2004. Originally published in 1994.

1 Purpose

1.1 This document provides a common descriptive

vocabulary and outline of basic technique for use by

photolithographers in the IC industry to gauge and

report the depth of focus, astigmatism, and field

curvature of IC photolithographic instruments (e.g.,

scanners, steppers). [Hereafter referred to as

“instrument” or “instruments.”]

2 Scope

2.1 This specification is limited to the measurement of

focus and depth of focus for photolithography as used

in the manufacture of integrated circuits and closely

allied technologies. Because of the wide variation in

equipment techniques, it is not possible to provide a

definitive measurement procedure for these parameters.

Rather, in this document, a basic guideline is offered.

NOTE 1: This technique has value in determining the best

focus setting for a given application, however the main

concern will be the determination for the depth of focus,

astigmatism and field curvature for the purpose of comparing

different instruments and processes.

NOTICE: This standard does not purport to address

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

responsibility of the users 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 It must be emphasized that the values of depth of

focus, astigmatism, and field curvature cannot be

determined for a given instrument independent of the

effects of the image geometries and the image transfer

process. The values will have to be determined under

the constraints of a practical application process,

suitable for the instrument, illumination, process, object

pattern, and environment. Thus, the process to be used

for a fair measure of instrument performance must be

one that is appropriate and has been optimized for the

given instrument and application. Comparison of the

performance of two different instruments will

inherently be a comparison of the the total application,

instrument specific processes included. A description of

the process is a necessary part of the report of a depth

of focus, astigmatism or field curvature measurement.

Relying on values that were obtained under

significantly different application conditions from the

desired application will result in error and potential

unexpected process failure.

4 Referenced Standards

4.1 SEMI Standards

SEMI P19 — Metrology Pattern Cells for Integrated

Circuit Manufacture

NOTICE: Unless otherwise indicated, all documents

cited shall be the latest published versions.

5 Terminology

5.1 Definitions

NOTE 2: The scope of the following definitions does not

necessarily cover application outside of the procedures

discussed herein. For more universal definitions and

discussion, the user is referred to any of the standard optical

texts.

5.1.1 image (micropatterning) — any single geometric

form appearing in a layout: (1) drafting — as a part of a

master drawing or layout; (2) optical — as projected on

a screen or viewed, usually at some magnification or

reduction; (3) oxide — as etched in the silicon dioxide

layer on an oxidized silicon wafer; (4) photographic —

as in a photomask or in the emulsion of a photographic

film or plate; (5) as a photoresist, an exposed and

developed coating on a substrate.

5.1.2 processed image (micropatterning) — any single

geometric form appearing in the realized pattern or

topographical variation in a material surface or material

constitution, obtained by a physical process of pattern

transference from an optical image.

NOTE 3: This definition is intended to extend the discussion

of depth of focus and focus to include both cases where the

images are realized in photoresist films, per the image

definition given above, and cases where there may not in fact

be a photoresist film involved in the optical pattern

transference process. Examples include photoactive chemical

vapor deposition and photo ablation.

SEMI P25-94 © SEMI 1994, 2004 2

5.1.3 standard coordinates — a system of Cartesian

coordinates with the z axis along the optical axis of the

system and with the x and y axes in the flat plane

perpendicular to the optical axis. The system user or the

supplier will specify the x and y directions in this plane

for any particular equipment studies.

NOTE 4: Upon occasion, cylindrical coordinates may be

used for special discussions, for example, mapping

astigmatism. The standard transform is used between the

Cartesian (x,y) and the cylindrical (r,ø) is used.

NOTE 5: The x and y coordinates are taken as displacements

from the optical axis along a flat plane. Thus, if the image

plane is curved, the x and y coordinates are taken from the

projection of the curved image plane onto a flat plane.

Consistently, r is taken as the distance along a perpendicular

to the optical axis.

NOTE 6: A displacement along the z axis (and hence the

optical axis) from a reference position will be negative for

displacements toward midpoint of the optical system and

positive for displacements away from the midpoint of the

optical system, regardless of whether the reference position is

on the object side or on the image side or coincident with

some portion of the optical system.

NOTE 7: For folded optical systems, the coordinate system is

allowed to rotate with position along the optical axis so that

the above definition holds at any given position along the

axis. Alternately, it may be assumed that for purposes of using

this coordinate system, all optical paths are represented as

unfolded.

5.1.4 image field — the extent of the image along the x

and y axes. It may be defined by the limits of image

quality, as a practical matter, for the intended

application.

NOTE 8: Definitions 5.1.1 and 5.1..2 in combination with

5.1.4 define the drafting image field, the optical image field,

the processed image field, etc. The term “image” refers to a

single geometrical form. The term “field” refers to the

complete set of forms.

5.1.5 practical use — the conditions of photoprocess,

image geometries, etc. that are required for the intended

use of the instrument. The practical use is specified by

the following:

 process — photoresist type and chemical

processing, exposure conditions, and

environmental conditions (temperature and

humidty setpoints and stability).

 pattern — description or diagram of the test

structures, including the choice geometrical

dimensions, orientations, positions.

 substrate — the substrate quality and

specifications, including all pre-existing structures

and layers on the substrate.

 leveling — the leveling mechanism used to hold

the substrate at the desired position and angle with

respect to the optical image plane.

 site — list or map of the image sites in the

processed image field used for the tests.

 criteria — the acceptance criteria used in the

evaluation of the processed images.

NOTE 9: Typically, the instrument vendor will indicate a

range of practical use parameters and the instrument user will

select from this range the actual conditions, as is appropriate

to the user’s needs.

5.1.6 resolution, practical — the minimum line width

that reproduces the mask (or drafting) dimensions

faithfully. [SEMI Micropatterning — Terms and

Definitions Relating to the Microlithography Industry]

NOTE 10: The practical resolution is a number specified by

the equipment vendor.

5.1.7 point-like object — A circular or square form in

the image where the diameter or width is equal to the

practical resolution.

5.1.8 evaluative line pattern — A pattern in the image

constructed of 3 to 5 straight parallel lines where the

lines are oriented at some specified angle with respect

to the standard coordinates and where the width of the

lines is equal to the practical resolution and the pitch of

the lines is twice the practical resolution. (See SEMI

P19.)

5.1.9 saggital lines — An evaluative line pattern where

the lines lie along a radius to the optical axis. (See

Figure 1.)

NOTE 11: Saggital lines are primarily formed from light rays

that do not lie in planes containing the optical axis. For optical

systems having cylindrical symmetry, such as some reduction

steppers, the saggital lines will be radial lines from some

particular point of the image field. For other systems the lines

may not be radial. For example, the saggital direction for

scanners is perpendicular to the direction of the scan

throughout the image field.

5.1.10 tangential lines — An evaluative line pattern

where the lines lie perpendicular to a radius to the

optical axis. (See Figure 1.)

NOTE 12: Tangential lines are primarily formed from light

rays that do lie in planes containing the optical axis. For

systems having cylindrical symmetry, tangential lines lie upon

circles about some particular point of the image field. For

other systems, the lines may not be so arranged. For example,

the tangential direction for scanners is parallel to the direction

of the scan throughout the image field.

5.1.11 focus — a condition of geometric adjustment of

the lens’s object, the optical system and the image plane

such that the optical image rays originating from a

given point in the object converge to the smallest