Simulation of proximity printing

A simple and easy to apply model is presented for contact and proximity lithographic steps. A first order approximation to the wave propagation inside a resist layer during exposure is provided, including diffraction and standing wave interference effects. This model takes refraction at the air-resist interface into consideration, as well as the depthwise change of the distance from the diffracting mask feature. The algorithm is easy to implement since the data structures of simulators such as SAMPLE are preserved, thus requiring only a modest increase in computational effort. A comparison of simulated resist profiles with those obtained from experiments shows that the model qualitatively explains the shape of profiles which arise under various lithographic conditions. The question of profile degradation, caused by exceeding certain limits for the proximity distance, is discussed. Computed irradiance distributions inside the resist layer at the beginning of the exposure, when the optic al properties of the resist material are still isotropic, are used.