Experimental optimization of the electron-beam proximity effect forward scattering parameter

The electron-beam forward scattering parameter a characterizes the width of the incident beam plus an additional radius due to scattering of primary electrons in the resist. These forward scattering effects can be included in proximity-effect correction algorithms by using the point-spread energy function generated by a Monte Carlo simulation. Alternatively, correction algorithms may use a superposition of Gaussian functions to fit Monte Carlo simulations or to fit experimental data. Long-range (beta > 10 mu m) effects due to electrons backscattered from the substrate are well characterized by simulations; however, forward scattering effects are difficult to model or measure. Experimental methods of measuring a include the exposure of dot arrays, line arrays, or so-called doughnut patterns over a large range of doses. Proximity parameters are then inferred indirectly through fitting line and dot widths. We have instead used a simpler and faster technique based on the method of [Dubonos et al., Microelectron. Eng. 21, 293 (1993)] which uses a liftoff technique to choose the optimal value of the forward scattering parameter a. We have applied this technique to determine a for 100 kV exposure of poly(methyl methacrylate) and KRS resists. Negative resist presents more of a challenge, and so a method of optimizing alpha for negative resists is presented. This work includes the variation of a as a function of resist thickness, and compares these values to those inferred from Monte Carlo simulations. We also discuss the dependence of a on resist developer and the effects of distortions from the developer meniscus. (c) 2005 American Vacuum Society.