Determination of resist exposure parameters in helium ion beam lithography: Absorbed energy gradient, contrast, and critical dose

We study the relationship between the true resist contrast (gamma (true)) a nd the apparent contrast (gamma (HD)), as determined by the slope of the Hu rter-Driffield (HD) curve, in He+ ion beam lithography. These parameters ca n differ significantly because the absorbed energy density is a function of depth in the resist. We formulate an analytical model, using a linear appr oximation to the deposited energy distribution, that permits the extraction of the absorbed energy gradient, the true resist contrast, and the critica l dose from experimental HD curves. The model accurately describes both exp erimental and simulation results. We show that the contrast for poly(methyl methacrylate) resist is 2.9 for various He+ ion energies even though the ap parent contrast can be as low as unity at 50 keV. (C) 2000 American Vacuum Society. [S0734-211X(00)08406-7].