Detection of contamination in EUV systems

摘要

A sensor is described for sensing contamination in an application system. The sensor (100) comprises a capping layer (108). The sensor (100) is adapted so as to cause a first reflectivity change upon initial formation of a first contamination layer (110) on the capping layer (108) when the sensor (100) is provided in the system. The first reflectivity change thereby is larger than an average reflectivity change upon formation of a thicker contamination layer (110) on the capping layer (108) and larger than an average reflectivity change upon formation of an equal contamination on the actual mirrors of the optics of the system.