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SEMI P25-94 © SEMI 1994, 2004 5 b. Alternatively, if point- like objects are available in the image for evaluation, the focal surface may be obtained by findi ng the z displacement for the best processed point image at e…

SEMI P25-94 © SEMI 1994, 2004 4

permissible deviations. The depth of focus maps may be

dependant, among other things, on the process, line width,

and image geometry, especially the choice of point-like

patterns or line-like patterns along the saggital, tangential, or

other directions. Several maps may be produced as part of a

test, covering these variations.

6 Procedure

6.1 Pattern — See SEMI P19. A pattern and pattern

dimensions are to be selected in accordance with the

most reasonable correspondence to the intended

application of the equipment. Typically, the pattern will

consist of two sets of evaluative lines. A sequence of

patterns of varying line widths may also be employed,

in which case the depth of focus may be reported as a

function of the line width. The pattern selection and

dimensions must be included as part of the report.

6.2 Test Sites — The pattern is to be repeated at least

nine times over the image field. The minimum of nine

sites will include the four corners of the image, the four

edge midpoints of the image field and the center of the

image field. Whichever many sites are used, the sites

must be distributed symmetrical and uniformly over the

image field, giving no special weight to any portion of

the image field. For examples, see Figure 2.

6.3 Process — A test image will be printed under the

same conditions and processes to be used in the

practical application (see Section 2) of the instrument

(stepper, scanner, etc.). The process description will be

included in the depth of focus and best focus report.

6.4 Focus Steps — The image will be printed several

times, varying the defocus by a fixed amount between

each print. The amount of variation of defocus between

steps and the limits of the defocus will be selected so as

to bracket the expected best focus position and to reach

beyond the depth of focus at the extremes of the image

series, as best as can be determined. The number and

size of the defocus steps must be specified in the report.

6.5 Image Array — These multiple images can be a

row of separate exposure fields on one substrate,

multiple stepped displaced exposures of the test

geometry within one image field region, or even

exposures on entirely separate substrates. The particular

strategy is usually constrained by the functions of the

instrument under test.

6.6 Test Measurements — For each image site for both

test pattern orientations and at each value of defocus,

the processed image geometries will be examined and

either accepted or rejected on the basis of image

sharpness, image size or some related image

characteristic. Usually this examination will use the line

width as the acceptance criteria. The total number of the

measurements will be twice the number of sites times

the number of defocus steps.

6.7 Quantitative Image Evaluation — The photoresist

image patterns will be examined or measured for

deviation from the target, in a manner specified by the

vendor or user of the instrument. An acceptable image

is where the variation of the pattern is not outside the

permissible deviation under the conditions of the

intended use of the equipment. The defocus values for

which the image is usable will be determined at each

image site within the image field.

6.8 Depth of Focus and Best Focus Determination —

The defocus variation over which there are usable

images at simultaneously every test site within the field,

by the above criteria, will be reported as the practical

focal range. In accordance with the definition, the best

focus will be the z axis position where some target

specification is met or the possible defocus is

maximized or the line deviation minimized, etc. If there

is no common range of usable defocus across the field,

the depth of focus is zero and the best focus is

undetermined. This can occur due to, among other

things, a large plane tilt of substrate to the optical axis,

high aerial image focus surface curvature, high

astigmatism, and auto focus system instabilities.

6.9 Focal Astigmatism Determination — The test

measurements are separated into two groups, one group

for each of the two different orthogonal orientations of

the test pattern. These groups are referred to as the “S”

and “T” groups, however the saggital and tangential

orientations have not necessarily been used in

accordance with the definition of “rotated astigmatism.”

6.9.1 For each group separately, the focal surface is

determined by using for each site the midpoint of the

defocus values where the processed image is found to

be acceptable, in accordance with Section 6.7.

6.9.2 The difference in z position between these

surfaces for each image site is calculated. These

differences are plotted as a function of image position.

This map of values is the astigmatism.

6.9.3 The maximum of the absolute values is the

maximum astigmatism.

6.9.4 The average of the absolute values is the mean

astigmatism.

6.10 Field Flatness Determination — Two alternative

but essentially equivalent methods of determining the

focal surface are allowed:

a. The focal surface which is the midpoint of the S and

T focal surfaces is calculated.

SEMI P25-94 © SEMI 1994, 2004 5

b. Alternatively, if point-like objects are available in

the image for evaluation, the focal surface may be

obtained by finding the z displacement for the best

processed point image at each image site.

6.10.1 This focal surface is reported as a map of z

displacements as a function of image position. This

map is the curvature of field.

6.10.2 The difference between the minimum z

displacement in the focal surface and the maximum z

displacement in the focal surface is the maximum

curvature of field.

NOTE 21: This includes optical image curvature, wafer

curvature, fixture curvature, etc. This procedure for the sake

of simplicity does not separate these effects. It is an error to

consider this measure of field flatness equivalent to a measure

of the optical image focal surface.

7 Report

7.1 The depth of focus, best focus, etc. reports will

include a description of the practical use, per definition

5.1.5. In addition, the report should include mention of

any special mechanical considerations — in particular,

among others, mention should be made if a chip

leveling scheme is used and whether this scheme is site

by site or global.

7.2 List of report contents:

 Process: substrate and layer types and processing,

exposure conditions

 Pattern: description or diagram of the test

structures, including the choice of S and T or X and

Y orientations

 Wafer: the wafer quality and specifications.

 Leveling: leveling mechanism.

 Sites: list or map of the image sites.

 Criteria: the acceptance criteria used in the

evaluation of the test structures.

 Depth of Focus or Focal Range (single overall

values)

 Best Focus (single overall value)

 Astigmatism map

 Maximum Astigmatism

 Mean Astigmatism

 Field Flatness map

 Maximum Curvature of Field

Figure 1

The Coordinate System and the Sagittal and

Tangential Lines, Shown for the Optical Image

Cylindrical Symmetry and for the Actual

Processed Images of Steppers and Scanners

Figure 2

Typical Test Site Placements in

Various Types of Image Fields

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