1. Field of the Invention
The present invention relates to an exposure apparatus and a method for producing a device in which a substrate is exposed with a pattern via a projection optical system and a liquid.
2. Description of the Related Art
Semiconductor devices and liquid crystal display devices are produced by the so-called photolithography technique in which a pattern formed on a mask is transferred onto a photosensitive substrate. The exposure apparatus, which is used in the photolithography step, includes a mask stage for supporting the mask and a substrate stage for supporting the substrate. The pattern on the mask is transferred onto the substrate via a projection optical system while successively moving the mask stage and the substrate stage. In recent years, it is demanded to realize the higher resolution of the projection optical system in order to respond to the further advance of the higher integration of the device pattern. As the exposure wavelength to be used is shorter, the resolution of the projection optical system becomes higher. As the numerical aperture of the projection optical system is larger, the resolution of the projection optical system becomes higher. Therefore, the exposure wavelength, which is used for the exposure apparatus, is shortened year by year, and the numerical aperture of the projection optical system is increased as well. The exposure wavelength, which is dominantly used at present, is 248 nm of the KrF excimer laser. However, the exposure wavelength of 193 nm of the ArF excimer laser, which is shorter than the above, is also practically used in some situations. When the exposure is performed, the depth of focus (DOF) is also important in the same manner as the resolution. The resolution R and the depth of focus δare represented by the following expressions respectively.R=k1·λ/NA  (1)δ=±k2·λ/NA2  (2)
In the expressions, λ represents the exposure wavelength, NA represents the numerical aperture of the projection optical system, and k1 and k2 represent the process coefficients. According to the expressions (1) and (2), the following fact is appreciated. That is, when the exposure wavelength λ is shortened and the numerical aperture NA is increased in order to enhance the resolution R, then the depth of focus δ is narrowed.
If the depth of focus δ is too narrowed, it is difficult to match the substrate surface with respect to the image plane of the projection optical system. It is feared that the margin is insufficient during the exposure operation. Accordingly, the liquid immersion method has been suggested, which is disclosed, for example, in International Publication No. 99/49504 as a method for substantially shortening the exposure wavelength and widening the depth of focus. In this liquid immersion method, the space between the lower surface of the projection optical system and the substrate surface is filled with a liquid such as water or any organic solvent so that the resolution is improved and the depth of focus is magnified about n times by utilizing the fact that the wavelength of the exposure light beam in the liquid is 1/n as compared with that in the air (n represents the refractive index of the liquid, which is about 1.2 to 1.6 in ordinary cases).
However, the conventional technique as described above involves the following problem. The exposure apparatus, which is disclosed in International Publication No. 99/49504, is constructed such that the liquid is supplied and recovered to form the liquid immersion area on a part of the substrate. In the case of this exposure apparatus, for example, when the substrate stage is moved to the load/unload position in order to unload the substrate having been placed on the substrate stage and load a new substrate in a state in which the liquid in the liquid immersion area is not recovered sufficiently after the completion of the liquid immersion exposure, there is such a possibility that the liquid, which remains on (adheres to) the end portion of the projection optical system, the liquid supply nozzle, and/or the liquid recovery nozzle, may fall onto surrounding units and members including, for example, the guide surface of the stage and the reflecting surface for the interferometer for the stage.
Further, when the liquid remains on the optical element disposed at the end portion of the projection optical system, the remaining liquid leaves any adhesion trace (so-called water mark) on the optical element disposed at the end portion of the projection optical system after the evaporation of the remaining liquid. There is such a possibility that any harmful influence may be exerted on the pattern to be formed on the substrate during the exposure process to be subsequently performed. It is also assumed that the liquid immersion area is formed during any process other than the exposure process, i.e., when the reference mark member and/or the reference plane member arranged around the substrate on the substrate stage is used. In such a situation, there is such a possibility that the liquid in the liquid immersion area cannot be recovered sufficiently, the adhesion trace may remain on the member as described above, and the liquid remaining on the member as described above may be scattered.