Vacuum ultraviolet directed design, synthesis and development of 157nm photoresist materials
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The design of 157 nm materials for photolithography presented many challenges, stemming from the inherently strong absorbance of the majority of organic compounds in the vacuum ultraviolet (V-UV). A spectrophotometer designed to operate in the vacuum was utilized to screen a multitude of materials for high transparency at 157 nm. These hydrogenated monomers served as model compounds for the repeat units in polymers. A variety of fluorinated, hydrogenated norbornanes were found to be transparent for use at 157 nm, and empirical evidence showed that the position and amount of fluorine incorporation mattered significantly in norbornene systems. The best of these are geminally substituted but, unfortunately, geminally disubstituted norbornenes do not polymerize to any significant degree using transition metal addition catalysts such as palladium or nickel. A variety of different methods were used to incorporate fluorinated and transparent monomers into polymers. Dinorbornene monomers were used in conjunction with the Grubbs second-generation catalyst to produce polymers using ring-opening metathesis polymerization (ROMP), but these compounds were found to be too strongly absorbing for use at 157 nm. Tricyclononene analogues of the norbornene materials were synthesized, found to be transparent in the V-UV and can be polymerized via addition polymerization. The absorbance of these materials matched the absorbance trends first seen in the gas phase spectra of the monomers, which bolstered the case for screening materials in the vacuum UV first. Vicinal cis-exo-norbornane diols were also synthesized of these fluorinated, geminally disubstituted norbornenes for use as condensation polymer precursors. Attempts to form polycarbonates and polyethers from these diols proved unsuccessful, because of the propensity for the cis-exo-norbornane diols to form cyclic carbonates. The final, optimized photoresist that gave the best results in imaging experiments at 157 nm employed poly(2-(3,3,3-trifluoro-2-trifuoromethyl-2- hydroxypropyl)bicyclo[2.2.1]heptane-5-ene) (PNBHFA) or the Asahi Glass RS001 fluoropolymer in conjunction with a variety of oligomeric dissolution inhibitors. These formulations allowed us to print high resolution, high aspect ratio images at 157 nm. The vacuum ultraviolet spectroscopy, polymer synthesis and imaging of 157 nm materials are presented.