Optimization of optical properties of resist processes

Thin film interference plays a dominant role in CD control of single layer photoresist processes, causing large changes in the effective exposure dose due to a tiny change in optical phase. Such interference effects are evident in the sinusoidal undulations of a plot of dose to clear versus resist thickness, the 'swing curve'. To quantify the interference swing, we define the swing ratio S as the ratio of the peak to valley change to the average value in the swing curve. S is a fundamental figure of merit for photoresist processes, since linewidth variations with small changes in resist thickness are proportional to S. A simple optical model of photoresist (as a Fabry-Perot etalon) leads to the following analytical expression for the swing ratio S: S ≈4√R1R2e-αD where R1 is the reflectivity of the resist/air interface, R2 is the reflectivity of the resist/substrate interface, and α is the resist absorption coefficient. Efforts to improve process control have led to the invention of many clever resist processes including Top Surface Imaging(TSI), Anti-Reflection Coasts(ARC), dyed resists, etc.