1. Field of the Invention
The present invention relates to an illumination optical system and a projection exposure apparatus using the same.
2. Related Background Art
In recent years, the micropatterning techniques of semiconductor wafers have been greatly improved to realize highly integrated semiconductor devices such as ICs, LSIs, and the like. As principal projection exposure apparatuses of such micropatterning techniques, known are an equal-magnification projection exposure apparatus (mirror projection aligner) that performs exposure while scanning a reticle and a photosensitive substrate with respect to an equal-magnification mirror optical system having an arcuate exposure range, a reduction projection exposure apparatus (stepper) that forms a pattern image of a reticle on a photosensitive substrate by a refractive optical system to expose the photosensitive substrate by a step-and-repeat scheme, and the like. Also known is a step-and-scan type scanning projection exposure apparatus that can obtain a high resolution and can increase the screen size.
In the stepper, a reticle on which an electronic circuit pattern is formed to have a size m times the required size is illuminated by an illumination optical system, and the pattern on the reticle is projected onto the wafer surface at a reduced scale of 1/mx by a projection optical system. The scanning projection exposure apparatus employs a projection optical system for transferring a portion of the pattern on the reticle onto the wafer, an illumination optical system for illuminating the portion of the pattern on the reticle by forming a slit-like light beam, and a scan mechanism for scanning the reticle and wafer at a predetermined speed ratio with respect to the slit-like light beam and the projection optical system so as to scan and expose a pattern on a reticle onto a wafer.
A reticle formed with an electronic circuit pattern normally has a rectangular shape, and its pattern region has various sizes. On the other hand, the effective illumination range of the illumination optical system that illuminates the reticle surface in the projection exposure apparatus normally has a circular shape. For this reason, upon projecting and exposing the reticle pattern onto the wafer surface, a portion of the effective illumination range is shielded by a masking mechanism or the like to obtain a rectangular light beam similar to the reticle pattern shape. As described above, in the conventional projection exposure apparatus, since the portion of the effective illumination range is shielded by the masking mechanism to illuminate the reticle surface, the illumination efficiency is low. In order to solve this problem, U.S. Pat. No. 4,682,885 discloses an optical integrator in which cylindrical lenses are arranged to cross each other so as to obtain an illumination range corresponding to an arcuate slit.
On the other hand, in recent years, in order to improve the resolution and focal depth, modified illumination stops having various shapes (e.g., a ring like shape) are arranged at the light exit side of the optical integrator. The optical integrator is used as a member for forming a plurality of secondary light sources in the vicinity of the exit sides of the individual elementary lenses that make up the integrator, and for forming a uniform illumination region by integrating light emanating from these secondary light sources. However, when the number of the elementary lenses (corresponding to the number of secondary light sources) used in the optical integrator is reduced using a modified illumination stop, the uniformity of illumination lowers. In order to improve the uniformity of illumination, the diameters of the elementary lenses are reduced, and the number of elementary lenses used is increased. However, since the size of an elementary lens that can be manufactured is limited, it is difficult to increase the number of elementary lenses as conventional refractive optical elements.