Conventionally, in an exposure apparatus which is used to produce semiconductors, etc., a circuit pattern formed on a mask (reticle) is projected and transferred onto a photosensitive substrate (for example, a wafer) via a projection optical system. Resist is coated on the photosensitive substrate, is exposed by being subjected to the projection exposure via the projection optical system, and thus a pattern of the resist (resist pattern) corresponding to a pattern of the mask (mask pattern) is obtained. The resolving power (resolution) of the exposure apparatus depends on the wavelength of an exposure light (exposure light beam) and the numerical aperture of the projection optical system. Therefore, in order to improve the resolving power of the exposure apparatus, it is required to shorten the wavelength of the exposure light and to increase the numerical aperture of the projection optical system.
Increasing the numerical aperture of the projection optical system to be not less than a predetermined value is generally difficult in view of the optical design. Therefore, it is necessary to shorten the wavelength of the exposure light. In view of this, attention is directed to an EUVL (Extreme UltraViolet Lithography) technique as a next-generation exposure technique (exposure apparatus) to be used for patterning semiconductor elements. The EUVL exposure apparatus uses an EUV light (Extreme UltraViolet light or light beam) having a wavelength of, for example, about 5 nm to about 40 nm. In a case that the EUV light is used as the exposure light, any usable light-transmissive optical material is absent. Therefore, in the EUVL exposure apparatus, a reflection type mask is used, and a reflective optical system is used as an illumination optical system and a projection optical system (see, for example, U.S. Pat. No. 6,452,661).
USPTO Backgrounds
USPTO Backgrounds