Collection efficiency of EUV sources

摘要

The collection efficiency is one of the most important characteristics of a source in an EUV lithography system. It is defined as the fraction of the total in-band EUV radiation power which can actually be picked up and used in the subsequent total optical path of an EUV projection system down to the wafer. It is not a property of the source alone, but is determined both by the details of the optical system with its etendue limitations and by the geometrical intensity distribution of source plasma. Until now, the source performance had to be evaluated by ray-tracing calculations of the complete optical lithographic system including the source, which were quite time-consuming. Therefore, a simple and fast, but still reliable method for evaluation of collection efficiency has been developed which is based on taking pinhole images of the source plasma with a CCD camera. From these, a source intensity distribution is constructed, which is used as input to a subsequent simple projection algorithm. By taking into account radiation from an "allowed" source volume only, which is constructed beforehand by rigorous ray tracing, the actual collected power and the collection efficiency is calculated. Comparison with detailed raytracing model calculations of the complete optical system indicate the good accuracy of the method. It can be used in a simple way for source optimization and enables reliable evaluation and specification of source performance in EUV projection systems.