Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
1999-07-06
2004-03-02
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
Reexamination Certificate
active
06700667
ABSTRACT:
This Application claims the benefit of Japanese Patent Application Nos. HEI 08-277913 filed Oct. 21, 1996 and HEI 08-298756 filed Nov. 11, 1996, which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an exposure apparatus and method for transferring a mask pattern onto a photosensitive substrate, in a photolithography process, used in the manufacture of semiconductor devices, image pickup devices (such as CCD), liquid crystal display devices, thin-film magnetic heads. The invention relates more particularly to a scanning exposure type exposure apparatus and method that operates in a step-and-scan mode.
2. Discussion of the Related Art
In the manufacture of semiconductor devices, a step-and-repeat type (one-time exposure type) reduction projection exposure apparatus (“stepper”) has been widely used as an exposure apparatus for transferring a pattern of a reticle as a mask onto each of shot areas of a wafer (or a glass plate) coated with a photo-resist. The industry desires the ability for transference of a large area of a circuit pattern with high accuracy, without increasing the burden on the optical projection system. To accomplish this goal a step-and-scan type projection exposure apparatus has been developed wherein the reticle and the wafer are scanned in synchronization with each other, wherein a part of the pattern on the reticle is projected on the wafer through the optical projection system, so that an image of the pattern on the reticle is sequentially transferred onto each shot area on the wafer.
In a conventional aligner, as a prototype of the scanning exposure apparatus, a pattern of the entire area of the reticle is transferred onto the entire area of the wafer. This provides a non-reverse image at a magnification of 1.0 in a one-time scanning exposure operation using a unit-type stage system.
However, in the step-and-scan type exposure apparatus the optical projection system normally projects an image at a certain reduction ratio or magnification that is smaller than 1.0. Therefore a reticle stage and a wafer stage must be driven independent of each other and at a speed ratio that is dependent upon the reduction ratio of the optical system. Since a stepping mode is employed to position one shot area after another in an exposure region, the driving mechanism of the stage system tends to be complicated and to require highly sophisticated control (refer to Japanese laid-open patent publication No. 7-176468, U.S. Pat. No. 5,646,414 for example).
In the conventional step-and-scan type projection exposure apparatus, therefore, the speed and position of each stage are controlled based on measurement values of laser interferometers, as shown in FIG.
10
. Referring to FIG.
10
(
a
1
) and (
a
2
), a mirror
52
X for measuring displacement along the X-axis and a mirror
52
Y for measuring displacement along the Y-axis are fixed on a wafer stage
51
on which a wafer W is mounted. A mirror
55
X for measuring displacement along the X-axis and a mirror
55
Y for measuring displacement along the Y-axis are fixed on a reticle stage
54
on which a reticle R is mounted. If the rectangular coordinate system of the plane on which the wafer W is moved is defined by the X-axis and Y-axis, and the reticle and wafer are scanned during scanning exposure in a direction (Y-direction) parallel with the Y-axis, two measuring laser beams
53
Y
1
,
53
Y
2
or
56
Y
1
,
56
Y
2
are incident upon a corresponding one of the Y-axis mirrors
52
Y,
55
Y of the scanning direction. Also, one measuring laser beam
53
X or
56
X is incident upon the corresponding X-axis mirror
52
X,
55
X for the non-scanning direction, so that the position (Y-coordinate) of the stage in the scanning direction is measured by the corresponding two-axis laser interferometric system. The position (X-coordinate) of the stage in the non-scanning direction is measured by the corresponding one-axis laser interferometric system.
In this case, the two-axis laser interferometric system for the scanning direction includes a laser interferometer for measuring yawing. The Y-coordinate in the scanning direction is represented by the average of measurement values of the two-axis laser interferometric system. The angle of rotation of each of the wafer stages
51
, on which wafer W rests and on the reticle stage
54
reticle R rests, is measured based on a difference between the Y-coordinates at which the two laser beams are incident upon the corresponding Y-axis mirror. During a scanning exposure operation, the wafer stage
51
and reticle stage
54
are moved in synchronization with each other so that the relationship between the X-coordinate and Y-coordinate of the wafer stage
51
and those of the reticle stage
54
match the projection scale (reduction ratio) of the optical projection system, so that the relative rotation angle of these stages is maintained to a fixed value. Although the optical projection system normally used in the conventional apparatus is adapted to project a reverse image of the reticle pattern on the wafer and thus the wafer stage
51
and reticle stage
54
are scanned in opposite directions, it is assumed, for the sake of simplicity, that a non-reverse image of the reticle pattern is projected and the wafer and reticle stages are both scanned in the Y-direction.
If the reflecting surfaces of the mirrors extend parallel with the X-axis and Y-axis with high degree of accuracy, a scanning exposure operation is performed such that the wafer W on the wafer stage
51
is moved in the Y-direction relative to a slit-like exposure region
58
. The reticle R on the reticle stage
54
is then moved in the Y-direction relative to a slit-like illuminated region
57
in synchronization with the movement of the wafer W so that an image of a pattern of the reticle R is transferred onto one of shot areas on the wafer W. The thus exposed shot area SAa has an accurate rectangular shape, as shown in FIG.
10
(
a
3
). The shot array formed on the wafer W is in the form of a grid in which the shot areas are arranged along the X-axis and the Y-axis, as shown in FIG.
10
(
a
4
).
However, if the mirrors
52
X,
52
Y are rotated clockwise by an angle &thgr; due to yawing of the wafer stage
51
, as shown in FIG.
10
(
b
1
), the wafer W is scanned in a direction parallel with the reflecting surface of the mirror
52
X (direction that is inclined by the angle &thgr; with respect to the original Y-axis), as indicated by the arrow
60
b
. The stepping of the wafer W in the non-scanning direction is conducted in a direction parallel with the reflecting surface of the mirror
52
Y, as indicated by the arrow
61
b
. In this case, the rotation of the wafer stage
51
is detected based on the inclination of the mirror
52
Y. The reticle stage
54
is also rotated by the angle &thgr; in accordance with the rotation of the wafer stage
51
, whereby the reticle R is scanned in the rotated direction, as indicated by the arrow
59
b
, as shown in FIG.
10
(
b
2
), while it is being rotated through angle &thgr;. Accordingly, the shot area (on which the pattern image of the reticle R is transferred) exposed on the wafer W in the scanning exposure operation is rotated, but has an accurate rectangular shape, as represented by shot area Sab, as shown in FIG.
10
(
b
3
). The shot array (shown in FIG.
10
(
b
4
)) formed on the wafer W is also in the form of a grid (which will be called “rectangular grid”) in which the shot areas are arranged in orthogonal directions.
In the conventional step-and-scan type projection exposure apparatus, as described above, the coordinate positions of the wafer stage and reticle stage are measured by the respective laser interferometric systems. Where the X-axis and Y-axis mirrors of the laser interferometric systems have a good orthogonal relationship with each other the exposed shot area has a rectangular shape, even if the wafer stage is rotated due to yawing and the obtained shot array is in the form of a rectangular grid.
However
Font Frank G.
Lee Andrew H
Morgan & Lewis & Bockius, LLP
Nikon Corporation
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