PECVD SiNx Thin Films for Protecting Highly Reflective Silver Mirrors: Are They Better Than ALD AlOx Films?

Protection of silver surface from corrosion is an important topic, as this metal is highly susceptible to damage by atomic oxygen, halogenated, acidic and sulfur-containing molecules. Protective coatings need to be efficient at relatively small thicknesses, transparent and must not affect the surface in any detrimental way, during the deposition or over its lifetime. We compare PECVD-deposited SiNx films to efficiency of ALD-deposited AlOx films as protectors of front surface silver mirrors against damage by oxygen plasma. Films of different thickness were deposited at room temperature and exposed to O2 ECR-plasma for various durations. Results were analyzed with optical and SEM microscopy, pulsed GD-OES, spectroscopic ellipsometry and spectrophotometry on reflection. Studies indicate that both films provide protection after certain minimal thickness. While this critical thickness seems to be smaller for SiNx films during short plasma exposures, longer plasma treatment reveals that the local defects in PECVD-deposited films (most likely due to erosion of some regions of the film and pinholes) steadily multiply with time of treatment and lead to slow drop of reflectance of SiNx-protected mirrors, whereas we showed before that ALD-deposited AlOx films reliably protect silver surface during long plasma exposures.