Compensator Selection In The Tolerancing Of A Microlithographic Lens

A microlithographic projection lens is expected to give constant results from one lens to another, anywhere within the field, over the full Rayleigh depth of focus, and with a finite variation of exposure dose. In particular, the requirements for image dimension and position control are an order of magnitude smaller than the features being printed. These demand that the paper design have wavefront correction significantly better than the Marechal criterion, and almost zero distortion. Furthermore, impressive theoretical performance is of no use if the manufactured lens cannot always approach the ideal as closely as possible. Lens designers traditionally bridge the gap between customer requirements and production realities. Compensators -- parameters that are adjusted to compensate for construction imperfections -- are a well-known way of relaxing tolerances. For a lens on the edge of possibility, they come to the aid of the designer caught between two vociferous groups of people with contradictory needs -- the users and the manufacturers. A lens will be shown that is used in a Wafer Stepper capable of printing features in the 0.7- to 0.6-micron range, and the rationale given for an extensive scheme of compensation, both before and after assembly. A selected number of elements are adjustable after assembly, while the lens is tested both interferometrically and lithographically, after which they are fixed in place.