Analysis of the effective reflectance of digital micromirror devices and process parameters for maskless photolithography

The evaluation of the effective reflectance of digital micromirror devices (DMD) is used to predict exposure doses in optical maskless photolithography applications. The effective reflectance is determined by multiplying the diffraction efficiency by the reflectance of the DMD in a dynamic mode. The diffraction efficiency is calculated using multiple slit diffraction theory and the two-dimensional grating equation for the wavelengths of the lithography sources, G-line (436 nm), H-line (405 nm), and I-line (365 nm). The diffraction efficiency is also used to determine the optimized dimensions of a DMD's pitch size. The reflectance of the DMD in a dynamic mode linearly decreases as the frame rate increases, dropping by ∼8% at a frame rate of 16,000 frames per second. The process parameters are analyzed in the terms of spot overlap and blur, which can have an effect on pattern quality. The spot overlap tends to increase with the frame rate of DMD, and it decreases with the stage speed. The spot blur, analyzed as a function of the illumination time and stage speed, also shows proportional behavior with those parameters.