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Polymethacrylamide (PMAAm) and copolymers of MAAm with methyl methacrylate were synthesized and evaluated for their applicability to electron beam lithography. The sensitivity of PMAAm has previously been reported as less than 1 μC/cm2, with thermal stability at temperatures up to 330°C.' Despite these claims, further lithographic evaluation of this resist system is apparently absent from the literature. This research was conducted to further investigate the lithographic performance of these resists and to determine their sensitivity using current definitions. Using PMAAm homopolymer (Mw = 8.1 x 105), with a 15 minute prebake at 200°C, the lithographic results were much poorer than expected. Patterns exposed to doses of 10 μC/cm2 or lower could not be developed using water as the developing solvent. Forced developing with Na2SiO3 solution (pH=10) developed lower doses than water, but much greater thinning was observed. An unexposed thinning of 10% occurred when developing exposures of 15 μC/cm2 with water, and 40 μC/cm2 with Na2SiO3 solution (20 KV). Swelling of the unexposed polymer and some adhesion problems were observed. The high sensitivity previously reported for PMAAm' can not be attributed solely to chain scission efficiency (Gs), which has been reported to be only 1.5 times that of PMMA (Gs determined by Y-irradiation). An induction period in the dissolution of unexposed polymer has also been sugggested as contributing to the sensitivity of this resist. In the present work, dissolution induction periods were observed with laser interferometry for the unexposed films, but the magnitude of these induction periods could not account for a large enhancement of sensitivity. Imide crosslink formation may have been responsible for the previously reported sensitivity of PMAAm.' In the present work, imid formation was not observed, either after prebaking coated wafers at 180 to 240°C or heating of polymer solutions for 7 hours at 80°C. Apparently, the conditions for useful formation of imide are difficult to reproduce. Because of the problems encountered with the PMAAm homopolymer during lithography, a series of MMA MAAm copolymers were synthesized and evaluated as potential resists. These copolymers eliminated the problems of swelling and poor solubility of the homopolymer, but sensitivity enhancement over that of PMMA was minimal.
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