High-Precision Stress Measurement in Thin Films for X-Ray Mirrors

Abstract

Future X-ray observatories aim to achieve sub-arcsecond angular resolution with unprecedented sensitivity. Silicon meta-shell optics technology will enable the X-ray astronomy instrumentation community to create such an observatory. The light-weighted silicon mirrors used in meta-shell optics have a low stiffness which makes them susceptible to deformations caused by stress in their reflective coatings. Much research has been dedicated to figuring coated mirrors that have been deformed by their coatings and in creating low stress coatings. These coatings need to be stable over decades for the length of the observatory's mission. However, the stress stability of candidate X-ray reflective coatings has not been measured or proven to be small enough as to not re-deform the mirrors after they have been corrected.

Membrane resonance techniques have been used to study thin film stress evolution during deposition. It has a superior sensitivity as compared to other techniques, such as substrate curvature methods. A novel device that uses the membrane resonance technique to repeatably measure stress in thin films is described. Sources of non-repeatability are discussed and repeatability studies are performed. The results presented in this thesis suggest that the membrane resonance technique is suitable for use in measuring X-ray reflective coating stress stability to the minute levels required for future X-ray observatories.