Interferometric Scheme for High-Sensitivity Coaxial Focusing in Projection Lithography

Focusing of wafer plane is an essential factor to determine the ultimate feature size of the stepper such as projection lithographic system. Based on Michelson interferometeric system, this paper demonstrates an interferometric focusing scheme for projection lithography to coaxially locate the ideal focal plane of the projective objective. The collimated incident laser beam is divided into the reference arm and object arm. The latter propagates through the objective lens and then interferes with the slightly deflected reference beam that reflected back by a fixed mirror, giving rise to an interferential pattern on the CCD. Any amounts of defocusing can be directly indicated from the demodulated phase of the interferential pattern. In this manner, the focusing sensitivity at nanometer scale is experimentally attainable, which shows great superiority over traditional methods, particularly the limited focal length of current projective objective lens.