NEW APPROACH TO RESOLUTION LIMIT AND ADVANCED IMAGE-FORMATION TECHNIQUES IN OPTICAL LITHOGRAPHY

Practical resolution, the minimum feature size with a depth of focus (DOF) required for LSI fabrication process, is analyzed. This analysis is based on experimentally obtained image quality criterion for pattern delineation, on simulated optical characteristics, and on similarity between aerial images formed by various optical systems. Dependence of practical resolution on various factors, such as optical system parameters (exposure wavelength: λ and numerical aperture: NA), resist processes, and required DOF, is investigated. It is shown that practical resolution in the sub-halfmicrometer region is not improved, and may even be degraded, with increasing NA. Furthermore, resolution improvement by increasing NA becomes less effective as λ becomes shorter. This means that the high-resolution capability of high-NA/short-wavelength optics cannot be utilized to create fine-pattern LSI's. In order to overcome this limitation, the effectiveness of advanced image formation techniques, the phase-shifting method and the FLEX method, in practical resolution enhancement is investigated. It is experimentally verified, using a phase-shifting mask and the excimer laser stepper, that a pattern feature size less than 0.2 μm can be clearly delineated with sufficient focus latitude. These advanced techniques make it possible to overcome the resolution limitation of conventional optical lithography. © 1991 IEEE