Control in sub-100 nm lithography in SAL-601

This article focuses on e-beam nanolithography in SAL-601, a negative tone, chemically amplified resist from the Shipley Corporation. It examines the role of exposure conditions and the postexposure bake (FEB) on the ability to define features of critical dimensions of 100 nm and below. Lithography was performed with a 50 kV e beam and 140 nm thick resist, where forward scattering of the e beam is expected to be minimal. Common FEB conditions of 105 degrees C for 1, 3, and 10 min and 110 degrees C for 1 min were investigated. Although the different FEB conditions produced a factor of up to 2.4x increase in sensitivity, no change was observed in the contrast, 5.8+/-0.1, or the minimum linewidths, similar to 60 nm. The measured line spread function forward Gaussian widths were characterized by standard deviations, sigma=27-33 nm, depending on FEB conditions. These are compared to a of 19 nm for PMMA and the expected a of 11 nm for the incident probe (sigma=10 nm) convoluted with a Monte Carlo code generated point spread function. The measured backscatter coefficient was 0.51, in agreement with our Monte Carlo simulation results. In the analysis of dose latitude to form critical dimension gaps (+/-10% precision) between a series of pads of differing sizes, the different FEB conditions produced no observed difference. The feature size, however, had a large effect on the ability to form the critical dimension gap. Furthermore, the dose latitude was much smaller than calculated by integration of Monte Carlo derived line spread functions. A semiempirical model is introduced in which the forward Gaussian width of a measured line spread function (LSF) in developed resist is convoluted with the Monte Carlo code LSF. The calculated exposure profiles based on the semiempirical LSF show much better agreement with the experiment than those based on the expected, sigma=11 nm, forward Gaussian width.