An optical system and technique for monitoring a monotonic change in the thickness of a transparent film by means of optical interference, and for eliminating ambiguity in the identification of absolute film thickness. The system is particularly adapted for monitoring the etching of a dielectric film...http://www.google.de/patents/US4293224?utm_source=gb-gplus-sharePatent US4293224 - Optical system and technique for unambiguous film thickness monitoring
