Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
2000-11-17
2003-04-15
Nguyen, Henry Hung (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S067000, C355S072000, C356S401000
Reexamination Certificate
active
06549269
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an exposure apparatus and an exposure method for exposing a sensitive substrate with a laser beam, an electron beam and other charged particle beams. In particular, the present invention relates to an exposure apparatus and an exposure method; which is used for producing semiconductor elements or liquid crystal display elements by means of the photolithography process, and which exposes the sensitive substrate by projecting a pattern formed on a mask via a projection optical system onto the sensitive substrate. Especially, the present invention relates to an exposure apparatus and an exposure method suitable for performing exposure and alignment of two substrates in parallel using two substrate stages.
BACKGROUND OF THE INVENTION
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 “sensitive substrate”, if necessary) such as a wafer or a glass blade applied with a photosensitive material such as phtoresist 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 sensitive substrate is placed on a substrate stage which is movable two-dimensionally, and the sensitive 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 sensitive 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 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 thereby 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 time.
With-this type of projection exposure apparatus, alignment between the reticle and the wafer needs to be performed highly precisely prior to exposure. To carry out this alignment, the wafer is provided with a position detecting mark (alignment mark) formed (or exposure transferred) by a previous photolithographic process. By detecting the position of this alignment mark, the exact position of the wafer (or a circuit pattern on the wafer) can be detected.
Alignment microscopes for detecting the alignment mark are roughly classified into the on-axis type for detecting the mark through a projection lens, and the off-axis type for detecting the mark without allowing the detecting light pass through a projection lens. With regard to a projection exposure apparatus with an excimer laser light source, which would be predominent in this field, an alignment microscope of the off-axis type is optimal. This is because the projection lens has been corrected for chromatic aberration due to exposure light, so that the on-axis type cannot condense alignment light, or if it could, an error due to chromatic aberration would be marked. An alignment microscope of the off-axis type, on the other hand, is provided separately from the projection lens; therefore, free optical design is possible without regard for such chromatic aberration, and various alignment systems can be used. For example, a phase contrast microscope or a differential interference microscope may also be used.
When the sensitive substrate is subjected to exposure by using the scanning type projection exposure apparatus, the so-called complete pre-measurement control method has been carried out as follows as described, for example, in Japanese Laid-Open Patent Publication No. 6-283403 corresponding to U.S. Pat. No. 5,448,332. That is, all detecting points included in one array provided on a front side in the scanning direction with respect to an exposure field are used as sample points. All values of focus positions at the sample points are previously measured before exposure, followed by the averaging process and the filtering process. The autofocus and the autoleveling mechanisms are controlled in an open manner during the exposure in consideration of phase delay. Concurrently with the foregoing operation, an inclination in the non-scanning direction is determined by means of the least square approximation method from the measured values of the focus positions at the respective sample points in the one array described above to perform the leveling control in the non-scanning direction in accordance with the open control.
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 described above, 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. As a result, 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 bcforehand, 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 basi
Nishi Kenji
Ota Kazuya
Nguyen Henry Hung
Nikon Corporation
Oliff & Berridg,e PLC
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