Contamination Resistant Sol-Gel Antireflective Coatings

Silica-based sol-gel antireflective (AR) optical coatings are critical components of high peak power laser systems. Water vapor and volatile organic compounds can contaminate these coatings, reducing both their antireflective efficiency and laser-damage resistance. Previous work has shown that alkylation of free hydroxyl groups on the silica sol-gel with organosilicon compounds in solution can improve the contamination resistance of silica sol-gels. In this work, alkylation with organosilanes in the vapor state was studied as an alternative to solution-based methods. Sol-gel AR coatings were vaporphase treated with hexamethyldisilazane (HMDS), tetramethyldisilazane (TMDS) and bis-(trifluoropropyl)-tetramethyldisilazane (FTMDS), at both room temperature and 50 ̊C. Contact angle measurements were taken to discern the degree of silanization as a function of exposure time to the reactive silane vapor. The contamination resistance of these vapor-phase treated sol-gels was determined by exposing them to a saturated vapor environment of a common organic contaminant (vacuum pump oil) at 60°C and 80 mtorr for 24 hrs. Results suggest that elevated temperatures enable higher degrees of silanization and therefore increase contamination resistance. Contamination testing suggests that HMDS treatment is more effective than TMDS and FTMDS treatments. However, none of the vapor phase techniques have provided as good protection against vacuum pump oil contamination as solution-based methods.