Nonspecular Scattering in EUV Lithography: Determining Specifications for Surface Finish

Abstract

As EUV lithography progresses from laboratory research to prototype development the realistic performance of manufacturable components becomes a primary concern. Nowhere is this more evident than in the fabrication and implementation of the EUV imaging optics. It is now well understood that the structure of the optical surfaces and the multilayer coatings (ML) that make the surfaces reflective at soft x-ray wavelengths must be specified and fabricated with unprecedented accuracy. Errors in the structure, which include deviations in the surface profile of the substrate and unintentional variations in the multilayer period, cause aberations in the imaging process. When these errors are at very long spatial wavelengths they are treated deterministically, and can be evaluated (and hence corrected) using interferometric methods now under development. However, there are errors in the surface profile at all spatial frequencies. Describing the exact structure of the surfaces of the optics at all spatial scalelengths is an intractable problem. Instead the surface figure errors at mid- and high-spatial frequency (called “roughness” or “finish”) are treated statistically. Within this statistical description, the effect of surface (and multilayer) roughness is to remove intensity from the image (the specular field) and scatter it throughout the image field. This nonspecular scattering is problematic for two reasons: (1) it decreases the useful throughput of the optical system and, (2) it produces a backround halo which reduces the contrast of the image. In this paper we describe a method of relating the nonspecular scattering to the roughness of the optical surfaces in a distributed EUVL imaging system. Our ultimate goal is to develop a robust specification of surface finish that can be used as the guideline for manufacturing EUV optics.