Simulation of focus effects in photolithography

A simple physical model is proposed for bulk imaging effects during latent image formation in a resist layer. The model considers refraction at the air-resist interface, as well as depthwise defocus of the lateral intensity distribution within the resist layer. The approach represents a first-order correction to the vertical propagation model used in conventional photolithography simulation, yet preserves the data structures of simulators such as SAMPLE and SPESA, and requires only a modest increase in computational effort. Comparison of simulated resist profiles with published experimental data shows that this model qualitatively explains the asymmetries in photolithographic response observed as a function of focus offset position in a single layer resist process. The question of the optimum focal position within the resist layer is discussed using simulated focus-exposure diagrams and the concept of effective defocus.>