Beamsplitter type lens elements with pupil-plane stops for lithographic

Abstract

An object of the present invention is to provide an optical system for lithography wherein the zero order component of the main lithographic image is unobstructed and the zero order component in the ghost image is removed. The optical system includes a beamsplitter component and a condensor lens structure for illuminating a lithographic mask from a range of directions which excludes a range of directions that are sufficiently close to the axis to add undesired background to the exposure field after multiple reflections with the lens. The optical system further includes an excimer laser and a lens system having an array of stops for intercepting multiple images of the excimer laser after they reflect from the primary wafer image or other surfaces such as the mask or lens surfaces. The illuminating zero-order beams reflect obliquely off the wafer after contributing to the image, where they are then refocussed to the opposite side of the pupil (the primary mirror). The beam then exits the lens towards the ghost image in an oblique fashion, and it will miss the wafer expose field entirely if the angles involved are sufficiently large. This redirection of the zero-orders is accomplished in the system with a molded fused silica fly's-eye-lens. A still further element of the system for stray light control is the use of a small stop in the pupil which blocks each zero-order beam after reflection from the wafer.