ABSTRACT
The process of imaging a patterned reticle by steppers and scanners onto photoresist-coated silicon wafers has been the backbone of the mass production of integrated circuits for over four decades. Like photography, imaging optics allows the parallel transfer of tremendous amounts of information from the object to image, or the reticle to wafer, in a fraction of a second. The specialized projection lenses of microlithography must accomplish this image transfer while maintaining uniformity of imagery and image placement on the nanometer level over centimeter sized image fields. Although demands on the imaging lens are severe, performance cannot be obtained without a properly designed illuminator. The illuminator controls or sculpts the diffraction pattern in the pupil that accomplishes the imaging on the wafer and maintains the radiometric uniformity across the image field. The illuminator does not require the same fabrication precision as the projection lens but has its own set of difficult design problems. Nonetheless, the imaging lens and illuminator must be designed together as a single system to achieve the desired overall performance. This chapter examines the design considerations, difficulties, solutions, and techniques of designing these optical systems from the source to reticle.
