Stepper lens aberration measurement pattern and stepper lens...

Optics: measuring and testing – Lens or reflective image former testing

Reexamination Certificate

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C430S005000

Reexamination Certificate

active

06654107

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a pattern configuration to be employed to evaluate the aberration characteristics of a reducing projection lens of a stepper which is utilized to form a pattern on a semiconductor substrate during a photolithography step implemented in a semiconductor device manufacturing process.
During the photolithography step implemented in a semiconductor device manufacturing process, a reducing projection exposure apparatus called a stepper is employed to repeat a step for transferring a circuit pattern formed on a reticule mask onto a semiconductor substrate (hereafter referred to as a wafer) to manufacture a semiconductor device. In the prior art, the aberration characteristics components of the reducing projection lens of the stepper are evaluated in order to ensure that the shape of the circuit pattern is accurately transferred during the photolithography step. Various types of testing and measurement are performed in correspondence to the specific aberration components of the lens during the stepper lens aberration characteristics evaluation.
As an example, coma aberration measurement is explained. If a coma aberration is present, convergence spots become asymmetrical, which causes asymmetry in the transferred pattern.
FIG. 20
illustrates a pattern configuration employed in an evaluation in the prior art. It is constituted of a line-and-space pattern achieved by a repeating arrangement of several lines and spaces having a width corresponding to the resolving power of the stepper lens. The width in this context refers to the length of the shorter side of the lines and spaces in the figure. In FIG.
20
(
a
), the widthwise direction extends along the horizontal direction. In FIGS.
20
(
b
), (
c
) and (
d
) respectively, the line-and-space pattern is rotated by 90 degrees, 45 degrees in the clockwise direction and 45 degrees in the counterclockwise direction relative to the state illustrated in FIG.
20
(
a
).
In the prior art, this pattern configuration is set at various exposure positions of the lens, as illustrated in
FIG. 21
, the widths of the outermost patterns in each pattern configuration are measured and the difference in the measured widths is evaluated as the coma aberration at the corresponding exposure position. An outermost pattern refers to a pattern that does not have another pattern lying adjacent to it on its outside within the same pattern configuration. This measurement method is based upon the principle that in line-and-space patterns, the shapes of the edges of patterns having other patterns lying adjacent to them are less likely to be affected by aberrations compared to outermost patterns.
For instance, the pattern configuration shown in FIG.
20
(
a
) is used to ascertain the coma aberration component along the horizontal direction. The dimensions of XL and XR which are the outermost pattern widths among the plurality of lines and spaces transferred and formed on the wafer are measured by using an SEM length measuring machine and then the coma aberration is calculated through:
coma aberration=(
XL
)−(
XR
) or
coma aberration=((
XL
)−(
XR
))/((
XL
)+(
XR
)).
This measurement and calculation process is implemented at each exposure position of the lens shown in
FIG. 21
to ascertain the coma aberration at each position.
The upper outermost pattern width YU and the lower outermost pattern width YL are measured and their difference is calculated to ascertain the coma aberration component along the vertical direction by using the pattern configuration in FIG.
20
(
b
). Likewise, the pattern widths +45L and +45R are measured and their difference is calculated as the coma aberration component along the diagonal direction extending from the upper left to the lower right by using the pattern configuration in FIG.
20
(
c
). The pattern widths −45L and −45R are measured and their difference is calculated as the coma aberration component along the diagonal direction extending from the upper right to the lower left by using the pattern configuration in FIG.
20
(
d
). Thus, by using the four types of pattern configurations shown in
FIG. 20
set at the individual exposure positions and measuring the transferred patterns formed by them, the coma aberrations manifested in the various directions at each exposure position can be evaluated.
Next, as another example of aberration measurement, measurement of astigmatism is explained. If an astigmatism is present, the correct focus position changes in correspondence to the direction in which a pattern is formed. For instance, if the exposure surface is set at the correct focus position for a given transferred pattern, another transferred pattern extending along a direction perpendicular to the first transferred pattern becomes defocused. Thus, the widths of the pre-transfer patterns that are equal to each other become different when they are transferred along two different directions at the exposure surface. Based upon this concept, the dimensional difference between transferred patterns along different directions at a given exposure surface is measured and calculated and then evaluated as the astigmatism.
In more specific terms, the pattern width 0C at the center of the pattern structure in FIG.
20
(
a
) and the pattern width 90C at the center on the pattern structure in FIG.
20
(
b
) are measured and the 0° direction—90° direction astigmatism is calculated and evaluated as:
0°-90° direction astigmatism=(0C)-(90C).
Likewise, 45C in FIG.
20
(
c
) and 135C in FIG.
20
(
d
) are measured and the 45° direction—135° direction astigmatism is calculated and evaluated as:
45 degrees 135 degrees direction astigmatism=(45C)-(135C). As an overall astigmatism quantity, the dimensional difference is calculated through:
astigmatism=MAX ((0C), (90C), (45C)), (135C))−MIN ((0C), (90C), (45C)), (135C))
for each pattern configuration on the same exposure surface by comparing the pattern widths along the 0 degree direction, the 90 degrees direction, the 45 degrees direction and the 135 degrees direction, and the dimensional difference thus ascertained is evaluated as the astigmatism. As in the coma aberration measurement explained earlier, this process of dimensional measurement, comparison and calculation is performed for the pattern configuration at each of the exposure positions set within the lens exposure range, so that the astigmatism can be evaluated in correspondence to each exposure position. The central width in each pattern structure is used in the astigmatism measurement since the edge shapes of patterns adjacent to other patterns are assumed to be affected by coma aberration to a lesser degree compared to outermost patterns.
SUMMARY OF THE INVENTION
In the measuring method using the patterns described above, a coma aberration is calculated by measuring the dimensional difference between the outermost patterns positioned symmetrical to each other. However, even when a pattern is affected by a coma aberration, a dimensional difference does not always manifest itself between the widths of the outermost patterns. FIG.
22
(
a
) is a sectional view of the pattern formed by using the pattern configuration shown in FIG.
20
(
a
). Even when the pattern is affected by an aberration, as illustrated in FIG.
22
(
a
), LE and RI may be very close to each other (LE·RI) and, as a result, no significant dimensional difference may be manifested. In other words, the measuring method in the prior art has a problem in that an aberration component may not be measured accurately.
FIG.
22
(
b
) is a cross section of one of the line patterns that have been formed. The coma aberration manifests itself as a difference between the left and right edge shapes within a given pattern as shown in FIG.
22
(
b
) and should, therefore, be evaluated as the dimensional difference ER−EL. However, since the dimensional difference between the left and right edges of the same pattern is not ascertained in the measuring m

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