Polysilane lithography

Contrast enhancement lithography is a clever procedure which uses a bleachable contrast enhancing layer to restore the distorted aerial image of the mask which has been blurred by diffraction effects into a sharp image at the underlying photoresist surface. The process is too complicated to explain in detail here and the interested reader is referred to the cited literature (60,61). Suffice it to say that the large extinction coefficients of most polysilane derivatives coupled with their ready bleachability make them ideally suited for such purposes and we have demonstrated this application at 313 nm (16,18). 

MILLER ETAL. Polysilanes Solution Photochemistry Lithography... 

It was found that 5-nm-thick resist-mask polysilane films worked well in a direct lithography process on silicon substrates, resulting into a line width of 40 nm prepared by scanning probe microscope lithography, using a carbon nanotube tip.57 Thin PMPS films of 6—8 nm, with a molecular weight of 30,000 were prepared by spin casting and cured at 150°C to obtain a smooth surface. It has been interpreted that moisture was essential for the oxidation of the polysilane. The proposed mechanism involved dissociation of Si-Si bonds in polysilane by the electron injection from the carbon nanotube tip catalyzed by moisture. 

Combining the lithographic and etch mask functions into a single polymer can be a major challenge, especially for deep-UV lithography. The latitude in resist design is limited, because at least 10 and preferably 15 wt % of the polymer structure must be reserved for silicon. A few materials, like silicon-substituted poly(methyl methacrylates) (6) and polysilanes (7, 8), have been used as positive two-layer resists for deep-UV lithography, but these materials suffer from either poor to moderate sensitivities to deep-UV radiation or an excessive absorption in the UV that limits exposure depth in the resist layer. 

Typical resists include cyclized polyisoprene with a photosensitive crosslinking agent (ex bisazide) used in many negative photoresists, novolac resins with diazoquinone sensitizers and imidazole catalysts for positive photoresists, poly(oxystyrenes) with photosensitizers for UV resists, polysilanes for UV and X-ray resists, and polymethacrylates and methacrylate-styrenes for electron-beam resists (Clegg and Collyer, 1991). Also note the more recent use of novolac/diazonaphthoquinone photoresists for mid-UV resists for DRAM memory chips and chemically amplified photoacid-catalysed hydroxystyrene and acrylic resists for deep-UV lithography (Choudhury, 1997). 

The conventional bilayer resist systems in which the top imaging layer (typically organosilicon polymer) also serves as an etch mask was first proposed by Hatzakis et al. in 1981, ostensibly for electron-beam lithography. Since then, a number of organosilicon resists for bilayer resist systems have been reported for use in near-UV, DUV, mid-UV, electron-beam, and x-ray applications, a good review of which has been provided by Ohnishi et al. In recent times, negative-tone resist systems and processes based on silicon-backbone polymers such as polysilanes,polysilynes, and plasma-deposited polymers have been developed for 193-nm lithography. 

Strong absorption of polysilanes in the UV region makes these materials suitable for application to photo (81) and X-ray lithography (82). Photo-bleaching by UV irradiation indicates scission as the major process for most polysilanes scission proceeds via silyl radical and/or silylene (R2Si ) extrusion (Fig. 9) (83). 

Miller RD, Willson CG, WaUroff GM, Clecak N, Sooriyakumaran R, Michl J, Karatsu T, McKinley AJ, Klingensmith KA, Downing J (1989) Polysilanes photochemistry and deep-UV lithography. Polym Eng Sci 29(13) 882-886... 

Polysilanes have been investigated as the raw materials of new ceramics, photoresist polymers for high resolution lithography, semiconductors, electro-conductive polymers, etc. [46]. Polydimethylsilane is prepared by adding di-chlorosilane to the dispersed minute particles of sodium in toluene solution. 

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