Partially developed speckle fields applied for photoresist characterization

Gray-scale lithography is an increasing technology field. It consists of numerous sophisticated fabrication steps making it necessary to characterize the process of substrate structuring. For photoresist characterization in general deterministic patterns are imaged onto the resist by a mask or interference lithography. Our intention is to replace these patterns by random structures i.e. partially developed speckle fields (PDSF). Such special fields arise from the interference of two coherent waves: a homogeneous plane wave and a speckle wave with a statistical phase distribution. While using the resulting constant background intensity for the adjustment of the operating point of the resist, the speckle field part is generating the microstructure during the exposure. After development the specimen is illuminated. The light intensity distribution results in a PDSF which is characteristic for the response of the resist. We present simulations for various characteristic cases of linear/nonlinear resist performance. A statistical analysis of the PDSF based on distribution parameter (probability density function, speckle contrast) reveals the response of the whole lithographic process.