Study on optimization of annular off-axis illumination using Taguchi method for 0.35 mu m dense line/space

Abstract

The simulation and experiment study on optimizations of process factors under annular off-axis illumination (AOAI) using Taguchi method for design of experiment (DOE) for 0.35 mu m dense line/space in i-line was reported in this paper. The optimized simulation results for both resolution and DOF are as follows: inner ring sigma(i) = 0.35, outer ring sigma(o) = 0.70, NA = 0.5, negative bias on mask for line in dense line/space similar to 0.03 mu m (L/S = 0.320/0.380 mu m), positive bias on mask for isolated line similar to 0.03 mu m (iso. L = 0.38 mu m), overlapped DOF (dense + isolated) similar to 1.86 mu m, exposure dose similar to 278 mJ/cm(2). The experimental results are in good agreement with simulations. The optimized results by experiments with a NA of 0.57 and with AOAI are as follows: inner ring sigma(i) = 0.35, outer ring sigma(o)= 0.70, negative bias on mask for line in dense line/space similar to 0.015 mu m (L/S=0.335/0.365 mu m), positive bias on mask for isolated line similar to 0.03 mu m (iso. L = 0.38 mu m), overlapped DOF (dense + isolated) similar to 1.7 mu m, exposure time similar to 520 ms (exact dose uncertain). Without AOAI but with the same mask bias, the overlapped DOF is similar to 1.2 mu m by experiment. The overlapped DOF from mask patterns without any bias (L/S = 0.35/0.35 mu m; iso. L = 0.35 mu m) with 1/2 AOAI is similar to 0.93 mu m; and from conventional illumination without mask bias is up to similar to 0.72 mu m at most. The DOF, proximity effect and resolution for dense L/S are obviously improved from 1/2 AOAI exposure after optimization if compared with conventional exposure. The Taguchi method is also proved valuable in this study.