Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
2001-09-27
2003-07-08
Nguyen, Henry Hung (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S075000, C355S077000
Reexamination Certificate
active
06590636
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a projection exposure method and a projection exposure apparatus. In particular, the present invention relates to a projection exposure method and a projection exposure apparatus for exposing a photosensitive substrate by projecting an image of a pattern formed on a mask onto the photosensitive substrate by the aid of a projection optical system. Especially, the present invention relates to a projection exposure method and a projection exposure apparatus for exposing a photosensitive substrate by overlaying and transferring images of patterns formed on a plurality of masks onto a predetermined area on the photosensitive substrate.
2. Description of the Related Art
Various exposure apparatuses have been hitherto used, for example, when semiconductor elements or liquid crystal display elements are produced by means of the photolithography step. At present, a projection exposure apparatus is generally used, in which an image of a pattern formed on a photomask or reticle (hereinafter generally referred to as “reticle”) is transferred via a projection optical system onto a substrate (hereinafter referred to as “photosensitive substrate”, if necessary) such as a wafer or a glass blade applied with a photosensitive material such as photoresist on its surface. In recent years, a reduction projection exposure apparatus (so-called stepper) based on the so-called step-and-repeat system is predominantly used as the projection exposure apparatus, in which a photosensitive substrate is placed on a substrate stage which is movable two-dimensionally, and the photosensitive substrate is moved in a stepwise manner (subjected to stepping) by using the substrate stage to repeat the operation for successively exposing respective shot areas on the photosensitive substrate with the image of the pattern formed on the reticle.
Recently, a projection exposure apparatus based on the step-and-scan system (scanning type exposure apparatus as described, for example, in Japanese Laid-Open Patent Publication No. 7-176468 corresponding to U.S. Pat. No. 5,646,413), which is obtained by applying modification to the stationary type exposure apparatus such as the stepper, is also used relatively frequently. The projection exposure apparatus based on the step-and-scan system has, for example, the following merits. That is, (1) the projection optical system is easily produced because a large field can be exposed by using a smaller optical system as compared with the stepper, and a high throughput can be expected owing to the decrease in number of shots because a large field is exposed. Further, (2) an averaging effect is obtained owing to relative scanning for the reticle and the wafer with respect to the projection optical system, and it is possible to expect improvement in distortion and depth of focus. Moreover, it is considered that the scanning type projection exposure apparatus will be predominantly used in place of the stepper, because a large field will become essential in accordance with the increase in the degree of integration of the semiconductor element, which is 16 M (mega) at present and will become 64 M for DRAM, 256 M, and 1 G (giga) in future as the progress proceeds along with times.
When the photosensitive substrate is subjected to exposure by using the projection exposure apparatus as described above, it has been contemplated to improve the resolution and the depth of focus for a pattern to be formed, by using the modified illumination method, for example, the SHRINC (Super High Resolution by Illumination Control) method as described in Japanese Laid-Open Patent Publication No. 4-273245.
Such a projection exposure apparatus is principally used as a mass-production machine for semiconductor elements or the like. Therefore, the projection exposure apparatus necessarily required to have a processing ability that how many sheets of wafers can be subjected to the exposure process for a certain period of time. That is, it is necessarily required for the projection exposure apparatus to improve the throughput.
In this context, in the case of the projection exposure apparatus based on the step-and-scan system, when a large field is exposed, the improvement in throughput is expected because the number of shots to be exposed on the wafer is decreased as described above. However, since the exposure is performed during movement at a constant velocity in accordance with synchronized scanning for the reticle and the wafer, it is necessary to provide acceleration and deceleration areas before and after the constant velocity movement area. If a shot having a size equivalent to a shot size of the stepper is exposed, there is a possibility that the throughput is rather decreased as compared with the stepper.
The outline of the flow of the process in such a projection exposure apparatus is as follows.
(1) At first, a wafer load step is performed, in which a wafer is loaded on a wafer table by using a wafer loader.
(2) Next, a search alignment step is performed, in which the position of the wafer is roughly detected by using a search alignment mechanism. Specifically, the search alignment step is performed, for example, on the basis of the contour of the wafer, or by detecting a search alignment mark on the wafer.
(3) Next, a fine alignment step is performed, in which the position of each of the shot areas on the wafer is accurately determined. In general, the EGA (enhanced global alignment) system is used for the fine alignment step. In this system, a plurality of sample shots included in the wafer are selected beforehand, and positions of alignment marks (wafer marks) affixed to the sample shots are successively measured. Statistical calculation based on, for example, the so-called least square method is performed on the basis of results of the measurement and designed values of the shot array to determine all shot array data on the wafer (see, for example, Japanese Laid-Open Patent Publication No. 61-44429 corresponding to U.S. Pat. No. 4,780,617). In this system, it is. possible to relatively accurately determine the coordinate positions of the respective shot areas at a high throughput.
(4) Next, an exposure step is performed, in which the image of the pattern on the reticle is transferred onto the wafer via the projection optical system while successively positioning the respective shot areas on the wafer to be located at exposure positions on the basis of the coordinate positions of the respective shot areas having been determined in accordance with the EGA system or the like described above and the previously measured baseline amount.
(5) Next, a wafer unload step is performed, in which the wafer on the wafer table having been subjected to the exposure process is wafer-unloaded by using a wafer unloader. The wafer unload step is performed simultaneously with the wafer load step (1) described above in which the exposure process is performed. That is, a wafer exchange step is constructed by the steps (1) and (5).
As described above, in the conventional projection exposure apparatus, the roughly classified four operations are repeatedly performed by using one wafer stage, i.e., wafer exchange→search alignment→fine alignment→exposure→wafer exchange.
The throughput THOR [sheets/hour] of such a projection exposure apparatus can be represented by the following expression (1) assuming that the wafer exchange time is T
1
, the search alignment time is T
2
, the fine alignment time is T
3
, and the exposure time is T
4
.
THOR
=3600/(
T
1
+T
2
+T
3
+T
4
) (1)
The operations of T
1
to T
4
are executed repeatedly and successively (sequentially) as in T
1
→T
2
→T
3
→T
4
→T
1
. . . . Accordingly, if the individual elements ranging from T
1
to T
4
involve high speeds, then the denominator is decreased, and the throughput THOR can be improved. However, as for T
1
(wafer exchange time) and T
2
(search alignment time), the effect of improvement is rel
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