Depth Profiling of Polishing-Induced Contamination on Fused Silica Surfaces

Laser-induced damage on optical surfaces is often associated with absorbing contaminants introduced by the polishing process. This is particularly the case for W optics. In the present study, secondmy ion mass spectroscopy (SIMS) was used to measure depth profiles of finishing-process contamination on fused silica surfaces. Contaminants detected include the major polishing compound components (Ce or Zr from CeOz or Z@z), Al present hugely because of the use of AlzO~in the final cleaning process, and other metals (Fe, Cu, Cr) incorporated during the polishing step or earlier grinding steps. Depth profile data typically showed an exponential decay of contaminant concentration to a depth of 100-200 nm. This depth is consistent with a polishing redeposition layers formed during the chemo-mechanical polishing of fused silica. Peak contaminant levels are typically in the 10-100 ppm range, except for Al which often exceeds 1000 ppm. A strong correlation has been shown between the presenee of a “gray haze” damage morphology and the use of Ce02 polishing compound. It has not been proven, however, that linear absorption by CeOz, or any other contaminant, is the relevant damage mechanism. Simple thermomechanical calculations show that for the contaminant levels present, temperatures high enough to cause damage m only likely if the contaminant was present as particles with diameters of 10-30 nm. We are not able to prove or disprove the presenee of such particles. No strong correlation between high levels of Ce, or any other contaminant, ad low damage threshold is observed. In fact one of the strongest indications of a correlation is between increased damage thresholds and inaeasd Zr contamination. This suggests that the connection between redeposition layer contamination and laser damage threshold is not simply an absorbing contaminant issue.