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Cost and yield effective IC fabrication requires the use of a large variety of substrates with distinct reflectivity, which may cause problems during the lithographic process either due to reflective notching and standing wave formation on highly reflective substrates, or trapezoidal resist patterns and deterioration of resolution, when a substrate with low reflectivity is employed. Reflectivity problems become more evident, when i-line radiation is replaced by DUV illumination tools. In addition, the non-bleaching nature of state-of-the-art chemically amplified resists further aggravates the reflectivity issues. It is therefore generally accepted that substrate reflectivity and resist transparency have to be closely matched to gain maximized lithographic performance, i.e., dissolution characteristics, resolution, depth-of-focus and exposure latitude. We have reported previously that poly-N,O-acetals act as effective dissolution inhibitors/promoters for PHS-based chemically amplified DUV resist materials. Alkylsubstituted poly(benzaldehyde-N,O-acetal)s are basically transparent in the 248 nm wavelength region, and therefore do not contribute to resist absorption. On the other hand, poly(naphthaldehyde-N,O-acetal)s are quite strong absorbants in the deep UV region. It was found that certain benz- and naphthaldehyde poly-N,O-acetal derivatives exhibit essentially identical inhibition and dissolution properties combined with similar cleavage kinetics. By both, physical mixing or co-condensation, of these materials, it is possible to adjust the optical resist absorption to precalculated values between approx. 0.30 - 0.80 micrometer MIN1 solely by poly-N,O-acetal selection without deterioration of other important resist properties. Basic chemistry, physico-chemical and optical properties of the resists are discussed in detail. Lithographic results including SEMs prove the versatility and efficiency of this approach.
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