Photocopying – Projection printing and copying cameras – Step and repeat
Reissue Patent
1998-07-09
2001-08-07
Mathews, Alan A. (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S067000, C355S071000, C353S122000, C359S589000, C359S622000
Reissue Patent
active
RE037309
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a scanning exposure apparatus, used in a photolithography process in the manufacture of, e.g., a semiconductor element, a liquid crystal display element, a thin-film magnetic head, or the like, for transferring a pattern on a mask onto a substrate by synchronously moving the mask (or reticle) and the substrate and, more particularly, to a scanning exposure apparatus suited for a case wherein light having a high spatial coherency (e.g., harmonics of a KrF or ArF excimer laser, YAG laser, or the like) is used.
2. Related Background Art
In the photolithography process for the manufacture of semiconductor elements, a reduction projection exposure apparatus (stepper) adopting a step-and-repeat method for transferring a pattern on a mask or reticle (to be generally referred to as a “reticle” hereinafter) onto a semiconductor wafer coated with a photosensitive material (photoresist) via a projection optical system is used. In a stepper of this type, in order to improve the resolution by decreasing the wavelength of exposure light, it has been proposed to use, as exposure light, laser light in a far (or deep) ultraviolet range, e.g., harmonics or the like of a KrF or ArF excimer laser, a YAG laser, or an argon laser. At present, a stepper using the KrF excimer laser has been put into practical use, and is operating in manufacturing lines.
Laser light generally has a high spatial coherency (coherence) and forms a speckle pattern (interference fringes) on a reticle. As a result evenness of the illuminance on the reticle and wafer is impaired. In view of this, problem as disclosed in, e.g., U.S. Pat. No. 4,619,508 and Japanese Laid-Open Patent Application No. 1-259533 (corresponding to U.S. Pat. No. 5,307,207 (Mar. 13, 1989)), a pivot mirror is arranged at the light source side of a fly-eye lens in an illumination optical system to change the incident angle of laser light onto the fly-eye lens for every one to several pulses. With this arrangement, the interference fringes sequentially move on the reticle during exposure. Therefore, the evenness of the illuminance on the reticle or wafer, i.e., the evenness of the exposure amount, can be improved.
Recently, it is required to widen the image field of the projection optical system and to improve its resolution in correspondence with an increase in size and a decrease in line width of semiconductor elements. However, it is very difficult in terms of design and manufacture to realize both the high resolution and wide field of the projection optical system. Under the circumstances, a scanning exposure apparatus as disclosed in, e.g., U.S. Pat. Nos. 4,747,678, 4,924,257, and 5,194,893 has been the subject of much attention. In such an apparatus, a pattern on a reticle is transferred onto a wafer by illuminating only a local area on the reticle with light and synchronously moving the reticle and wafer. The scanning exposure apparatus can transfer a large-area pattern image onto the wafer even if the image field of the projection optical system is small, and can relatively easily improve the resolution of the projection optical system.
In the scanning exposure apparatus, since the reticle and wafer are synchronously scanned, the relationship between the moving amount (pitch) of the reticle and wafer and the pitch (in the scanning direction) of interference fringes in the illumination area between pulse emissions changes depending upon the scanning speed of the stage (i.e., an optimal exposure amount of the wafer). Therefore, when the scanning exposure apparatus uses light having a high spatial coherency as exposure light, it is difficult to reduce exposure amount unevenness caused by interference fringes even when the above-mentioned pivot mirror is used.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a scanning exposure apparatus which can minimize exposure amount unevenness on a photosensitive substrate due to interference fringes, even when light having a high spatial coherency is used as exposure light.
A first scanning exposure apparatus according to the present invention comprises a light source for emitting a light beam having a predetermined spatial coherence, an illumination optical system for illuminating a local area on a mask with the light beam, and a device for synchronously moving the mask and a photosensitive substrate to transfer a pattern formed on the mask onto the photosensitive substrate, wherein a direction with higher spatial coherence of the light beam (i.e. a direction in which spatial coherence of the light beam is high) substantially coincides with the scanning direction of the mask with respect to the illumination area in the illumination area on the mask.
According to the first apparatus of the present invention, the direction in which the spatial coherence (degree of coherence) of the light beam is high is measured in advance in a plane perpendicular to the optical axis of the illumination optical system for guiding the light beam from the light source to the mask, and the direction with a higher spatial coherence is made to coincide with the scanning direction of the mask in the illumination area. Therefore, as shown in, e.g.,
FIG. 4
, the illuminance distribution, in the scanning direction (X direction), in the illumination area varies at a predetermined pitch and at a relatively large amplitude, as indicated by a curve
40
. On the other hand, the illuminance distribution, in the non-scanning direction (Y direction) perpendicular to the scanning direction, in the illumination area is relatively flat, as indicated by a curve
41
. In this case, even when the illuminance distribution (curve
40
) largely varies in the scanning direction, since the mask is scanned along the direction corresponding to a higher spatial coherence, exposure amount unevenness in the scanning direction on the photosensitive substrate after scanning exposure is remarkably reduced. Since the illumination distribution (curve
41
) in the non-scanning direction is originally flat, to begin with exposure amount unevenness in the non-scanning direction on the photosensitive substrate is also very small. Therefore, even when illuminance evenness is impaired by interference fringes in the illumination area, exposure amount unevenness on the entire surface of the shot area on the photosensitive substrate can be reduced, i.e., the evenness of the exposure amount can be improved.
A second scanning exposure apparatus according to the present invention comprises a light source for emitting pulse light having a predetermined spatial coherence, an illumination optical system for receiving the pulse light and forming a local illumination area on a mask with the pulse light, and a device for synchronously moving the mask and a photosensitive substrate to transfer a pattern formed on the mask onto the photosensitive substrate. The apparatus further comprises an interference fringe moving member for changing a position of interference fringes in the illumination area for every one to several pulses in accordance with a relative scanning speed between the illumination area and the mask and a pitch, in the relative scanning direction, of the interference fringes formed in the illumination area. The second apparatus may also comprise a detector for detecting a spatial coherence of the pulse light, and a controller for controlling the operation of the interference fringe moving member in accordance with the detected spatial coherence.
According to the second apparatus of the present invention, pulse light is used as exposure light. When the pulse light is far ultraviolet laser light, KrF excimer laser light having a wavelength of 248 nm, it is not easy to satisfactorily correct the chrominance aberration of a projection optical system. The pulse light source therefore preferably narrows the wavelength band of the pulse light using a diffraction grating (or etalon), a slit, and the like. For this reason, in
FIG. 1
, for example, pulse light (LB
Hamatani Masato
Nakashima Toshiharu
Ozawa Ken
Mathews Alan A.
Miles & Stockbridge P.C.
Nikon Corporation
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