Contaminants under a Dielectrical Coating on a Laser Mirror

The performance of a laser mirror with a dielectrical coating deteriorated faster than the specifications allowed. Since no contaminants could be detected on the surface of the mirror, it was suspected that the substrate was contaminated before the deposition of the coating. It was not known, which substances (remnants of polishing material, organic contaminants, residues of cleaning agents, etc.) were potentially causing the problem. Therefore, ToF-SIMS depth profiling was performed, which is well suited for a survey analysis, because all elements can be detected in parallel with good sensitivity. The depth profile shows a higher intensity of Mg and Al at the interface between the substrate and the dielectrical multi-layers. This contamination was due to remnants of the polishing material left on the substrate after insufficient cleaning.