Coding gray-tone mask for refractive microlens fabrication

A new method has been developed to design gray-tone masks for microlens fabrication. Unlike previous methods which modulate the light intensity by changing the cell size or cell pitch only, the method relays on adjusting both the shape and position of a cell which gives an extra freedom to control the design accuracy. Based on the theory of partial coherent light and the resist development model, the intensity distribution through the gray-tone mask and exposure of photoresist have been simulated. Nonlinear effects in aerial image and resist development have been take into account to correct the mask design. A grey tone mask for microlens array has been fabricated and printed by a g-line stepper. Hemispherical photoresist structures have been achieved.