Assessment of thermal loading-induced distortions in optical photomasks due to e-beam multipass patterning

Thermal loading-induced distortion in the photomask during e-beam patterning has recently received special attention due to its significant contribution to overlay errors. Multipass e-beam writing, a strategy proposed to reduce the heating effects and associated distortions, was simulated using three-dimensional finite element models. Thermal responses of the photomask during multipass patterning were determined and global in-plane distortions were calculated. For the given system exposure conditions of 40 mu C/cm(2) at 50 keV, the average value of the 3 sigma pattern placement error due to the bulk heating of the photomask obtained from multipass writing was found to be approximate to 3.5 nm which is 28% lower than that of single pass writing. Parametric studies showed that thermal radiation has a large influence on the mask cooling. (C) 1998 American Vacuum Society. [S0734-211X(98)06506-8].