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The demand for enhancement of the resolution in optical lithography requires a shift to shorter exposure wavelengths. The achievable resolution in a lithographic process is usually determined by means of the Rayleigh criterion, minimum resolvable feature size = k1 A/NA in which k1 is an adjustable parameter dependent on resist type and processing parameters. While according to this criterion, resolution is inversely proportional to the numerical aperture, the depth of focus is decreased in an inverse square power dependence according to depth — of—focus = k2 A/(NA)2 For high NA steppers, depth of focus may quickly become the process —limiting factor. It is a distinct drawback of the Rayleigh criteria that the k —factors may only be determined a posteriori from the results of a given process, so that their predictive value is severely limited. In this contribution, computer modeling is used to investigate the depth of focusfresolution problem for different exposure wavelengths. An extension to the photolithography simulator SAMPLE has been made allowing of the simulation of bulk imaging effects which occur during formation of the latent image in the resist layer. The model implemented accounts for the depthwise defocus of the lateral irradiance distribution within the resist film and gives the actual resist profile as a final result. This extended lithography model explains the asymmetries in the photolithographic response observed in the focus offset dependence of submicron resist images when the depth of focus becomes comparable to the resist film thickness. It can be applied to deternzine the absolute position of the resist film suiface with respect to the Gaussian image plane. Furtherniore, the algorithm can be utilized to establish the usable f ocus latitude for a given combination of a projection lens/ resist system from the basic characteristics of lens system and resist material. The general approach closely follows the one suggested by Mack [1] using the PROLITH simulator. SAMPLE results are compared to experiment for g —line exposure, and the treatment is extended to DUV materials.
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