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Interest has steadily grown during the past few years in materials that are resistant to dry etching techniques that can be used in multilevel resist processes. This is perhaps a result of the greater demands placed on resist technology by increased device complexity and resolution requirements. The most well known multilevel resist methods are the two-level PCM (portable conformable mask, first described by B. J. Lin ) and the BTL trilevel processes. In the latter, the pattern defined in the top level is transfered first to a thin silicon dioxide layer and subsequently to the substrate by dry etching. A two-level photoresist process employing 200-300 nm irradiation of the upper layer followed by transfer of the resultant image into the lower layer by dry etching, combines the attractive features of both deep UV lithography and multilevel resist 3processing. Deep UV lithography affords improved resolution due to decreased diffraction, while multilevel processing2alleviates the problems associated with substrate topography and sur-face reflectivity. An outline of this resist processing scheme is shown in Figure 1. The substrate is coated first with a thick layer of an organic polymer that effectively planarizes the wafer surface. A layer of photoresist is then applied. Deep UV exposure and routine development of the top layer generates the desired pattern which is transferred to the substrate by oxygen RIE. One means of iiproving the resistance of a material to RIE is incorporation of silicon. Taylor and Wolf have reported that the addition of silicon containing compounds to organic polymers improves their resistance to erosion by an oxygen discharge, presumably by the formation of a protective layer of SiO2. Dialkyl siloxane polymers have been Eeported to be usable as negative deep UV resists in the two-level process described above. Anotheg negative, oxygen RIE resistant, deep UV resist has been reported by MacDonald et al. This material, a copolymer of trimethylsilylstyrene and chlorostyrene, shows excellent resistance to oxygen RIE. Similar trimethylsilylstyrene-chloromethylstyrene copolymers have been described by Suzuki et al. Negative resists, however, may swell during development and thus limit the degree of resolution. Poly(methyl methacrylate) (PMMA)3 and various substituted methacrylates 8,9 are well known, high resolution, positive deep UV resists. However they are not sufficiently resistant to an oxygen plasma to be usable in a two-level process employing RIE pattern transfer. Ideally, the etching rate ratios for the planarizing material vs. resist should be >10:1 and preferably >20:1 in order to minimize the effects of line width erosion and allow optimization of the thickness of the lower layer. This ratio may also be maximized through appropriate selection of the planarizing layer material. Our initial efforts have concentrated on effecting an improvement in the oxygen RIE resistance of PMMA, without destroying its high resolution characteristics. Improvements by factors of over 30 have been accomplished through the incorporation of siloxane substituted esters in the poly-mers; these effects are described as a function of silicon content. A preliminary litho-graphic evaluation of the materials is presented and while the etching rate ratio of HPR-204 vs. the P(DS-A-M) polymer employed is only ≈7 it is demonstrated that these new deep UV resists show promise as oxygen RIE masks for a two-level photoresist system employing dry etching pattern transfer techniques.
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