1. Field of the Invention
The present invention relates to exposure apparatus, exposure methods, and device manufacturing methods, and more particularly to an exposure apparatus and an exposure method used in a lithography process to manufacture microdevices such as a semiconductor device, and a device manufacturing method using the exposure method.
2. Description of the Background Art
Conventionally, in a lithography process for manufacturing electron devices (microdevices) such as semiconductor devices (such as integrated circuits) and liquid crystal display devices, a projection exposure apparatus by a step-and-repeat method (a so-called stepper), a projection exposure apparatus by a step-and-scan method (a so-called scanning stepper (which is also called a scanner) or the like are mainly used.
In this type of exposure apparatus, attempts have been made for a shorter wavelength of exposure light and an increase (a higher NA) in the numerical aperture of the projection optical system, in order to improve the resolution of the projection optical system. However, by the shorter wavelength of exposure light and a higher NA in the projection optical system, the depth of focus has become small. Therefore, as a method of substantially shortening the exposure wavelength and also increasing (widening) the depth of focus when compared with the depth of focus in the air, the exposure that uses the liquid immersion method has recently begun to gather attention.
Meanwhile, as well as high resolution, the exposure apparatus is also required to have a high throughput. As an approach for improving the throughput, various proposals are made also on a twin wafer stage type exposure apparatus which employs a method where a plurality of wafer stages holding a wafer, such as for example, two wafer stages, are arranged, and concurrent processing of different operations is performed on the two stages.
Furthermore, recently, a proposal has also been made on a twin wafer stage type exposure apparatus which employs a liquid immersion method (e.g. refer to U.S. Pat. No. 7,161,659).
Now, in the case of measuring a position (height) of the wafer stage in a Z-axis direction orthogonal to an XY plane, a Z interferometer is known (e.g. refer to U.S. Pat. No. 6,208,407) which measures the height of the wafer stage by arranging a reflection surface (a Z measurement reflection surface) tilted by a predetermined angle, such as for example, 45 degrees with respect to the XY plane on the side surface of the wafer stage, irradiating a measurement beam parallel to the XY plane on the reflection surface, and receiving the return beam.
However, in the case of employing the Z interferometer above in the exposure apparatus similar to the one described in, for example, the U.S. Pat. No. 7,161,659, it is desirable to configure both wafer stages so that the height measurement can be performed with the Z interferometer from both sides. However, in the case of such a configuration, when the two wafer stages are made to be in contact or in proximity and a liquid immersion area is passed between the two wafer stages as disclosed in the U.S. Pat. No. 6,208,407, there was the risk of the Z measurement reflection surfaces coming into contact and being damaged. Further, in order to prevent such a situation from occurring, when the two wafer stages try to pass the liquid immersion area in a state where the two wafer stages are spaced apart, the liquid may leak from between the stages, which is likely to make the passing of the liquid immersion area impractical. Further, the liquid that has leaked could wet the Z measurement reflection surface. Further, even in the case where the Z measurement reflection surface is not used, or in the case when at least one of the wafer stages has a mechanism section protruding more than the other sections, a problem similar to the description above could occur when the two wafer stages are made to be in contact or in proximity and the liquid immersion area (liquid) is passed between the two wafer stages.