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Despite thorough and careful surface cleaning, evaporated or sputtered metallic aluminum mirror coatings are well known to be plagued by a multitude of pinholes in the coatings. These pinholes contribute to light scattering and light reduction. Subsequent corrosion and penetration of contaminants in the pinholes and on the edges of the metal coating result in degradation and eventual coating destruction. At this point, mirrors are stripped, cleaned, and recoated with aluminum. In this work, we report the reduction of coating pinholes by up to a factor of 100x by utilizing First Contact Polymer Apply-Dry Peel technology as a final step in surface cleaning before coating.
We fabricated an intensity-controlled imaging system to image backside illuminated optics and quantify the pinholes in aluminum mirror films. Imagej analysis of the size-calibrated pinhole distribution and light intensity resulted in statistical distributions plots of intensity vs pinhole size for each sample. Samples were prepared by stripping existing coatings with Green River mirror strip solution, followed by a standard cleaning procedure that consisted of a light NaOH wash, hand polishing with calcium carbonate slurry, alconox solution wash, distilled water(2X), and 200 proof ethanol wash (2x). After drying, one substrate was placed in the vacuum chamber and sputter coated with aluminum. The other sample was coated with Red First Contact Polymer Solution, allowed to dry, and the polymer film peeled of after pacing the substrate in the chamber for coating. Results show that the conventionally treated cleaned and dragwiped surfaces had 758 and 435 pinholes and the polymer strip coating cleaned surfaces had 38 and 9 pinholes. We believe that further studies and modified surface pretreatments can pave the way to reliably make zero defect coatings, not just for aluminum mirrors, but for all optical coatings.
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