Mask optimization in proximity lithography of thick resist

The thickness increase of photoresist can cause serious distortion of the resist patterns, which greatly affects the performance and application of the device. In this paper, the distortion characteristics of the two-dimensional slices of a high-aspect-ratio microstructure were theoretically investigated in the thickness direction. Meanwhile, an effective correction strategy that includes a doubleside exposure method and a combination of bright serif and gray-scale technique was proposed. The parameters of the mask shape and transmittance were optimized to correct the pattern distortions with the genetic algorithms. The diffracted light field modulation of various layers in the thick resist was employed as a merit function. Simulation results show that the pattern quality of internal layers of resist can be significantly improved by the proposed optimization method. The characteristic parameters such as feature size and sidewall angle match the design goal. Due to its flexibility, the optimization method proposed in this paper can be applied to more complex pattern by using appropriate extrapolation technique.