Formation mechanism and compensation methods of profile error in focused ion beam milling of three-dimensional optical microstructures

The formation mechanism and compensation methods of the profile error in focused ion beam (FIB) bitmap milling of three-dimensional (3D) optical microstructure are studied in this paper. The processing parameters such as ion beam currents, dwell times, and beam overlap ratios on the profile error have an optimized parameters combination of 21 nA, 10 mu s, and 50%, and the shape accuracy processed under the conditions is 0.44 mu m of P-V and 0.142 mu m of RMS, respectively. Inappropriate processing parameters lower either shape accuracy or processing efficiency. By means of an accurate simulation model of the FIB machined 3D profiles, the intrinsic formation mechanism of the profile error can be attributed to ion beam diameters, redeposition effect, and sputter yields, and these factors are inevitable in FIB milling of 3D microstructures. In situ modification and iterative optimization are proposed to compensate the FIB milling error, which both can improve the shape accuracy more than 50%. The in situ modification method needs to load the original bitmap and modified bitmap separately, which is less convenient than the iterative optimization method that only need loading one bitmap file. However, the in situ modification is better than the iterative optimization at decreasing the RMS value of the profile error which indicates the global profiles error of the processed microstructures.