Thermal aberration control in projection lens

In order to respond to the constant demand for more productivity in the manufacture of IC devices, higher throughput and higher resolution are fundamental requirements for each new generation of exposure tools. However, meeting both requirements lead to unwanted aberration we refer to as "thermal aberration". In our experience, the problem of the thermal aberrations does not correlated only to the duration of heavy use. It depends very strongly on both the optical settings and the mask patterns, also even on the specific interaction between the two. So, even if using the same illumination configurations, there is a possibility to observe different distribution of thermal aberrations. In this paper, we define and investigate various patterns to be used as targets for thermal aberrations compensation. These patterns are identified as the "weak patterns" of the thermal aberration. We assess several cases of thermal aberrations, and show how the optimized compensation for each is determined and then applied on the actual exposure tools.