Characterization of EUV induced contamination on multilayer optics

Extreme ultraviolet lithography (EUVL) is a next generation photolithographic technique that uses 13.5 nm or Extreme UV radiation and multilayer coated reflective optics. The reflectance of these optical elements can be strongly reduced when, as a consequence of exposing the optics to EUV photons, a contamination layer is built up on the mirrors. Since this will reduce the throughput of EUV lithography machines, contamination monitoring is considered to be necessary. Direct observation of the EUV reflectance of the mirrors is hardly possible since the required accuracy can only be achieved in very sophisticated lab reflectometers. This thesis describes experimental research on the topic of EUV induced contamination and its monitoring using alternative, in situ techniques. Occasional techniques for such a task have been mentioned, but no real investigations were carried out. This thesis reviews the suggested and new techniques and describes experimental work on the three most promising: laser-generated surface acoustic waves (LG-SAWs), secondary electron yield (SEY), and spectroscopic ellipsometry (SE). The goal was not only to develop an appropriate monitoring method, but also to get insight in the material properties, both mechanical and optical, of the contamination layer in order to predict the EUV reflectance loss and the possibilities to clean the optics.