A SIMPLE IN-SITU OPTICAL FILM THICKNESS MONITOR FOR INFRARED COATINGS

A unique in-situ film thickness monitor for infrared (IR) coatings has been designed and fabricated by using a visible laser (HeNe laser). T he accuracy of the monitor has been verified by comparing the results of the optical monitoring using the HeNe laser with those obtained by the optical monitoring using an IR laser (CO2 Laser). The two lasers a re aligned to couple the laser beams so that they are incident on the same spot of the sample bring coated and, after transmission through i t, they are separated and directed to the respective detectors for the HeNe laser and CO2 laser. The coupling of the two beams helps to alig n the in-situ monitor, The percentage transmission of the CO2 laser be am is used for the monitoring of film thickness at 10.6 mu m Simultane ously, the transmission of the HeNe laser beam is also used for monito ring the film thickness as multiples of HeNe laser wavelength. The res ults have been compared using the in-situ film thickness monitor based on the IR laser specially designed for this purpose. The system has b een incorporated in the Balzers Vacuum Coating Unit and has been used for the fabrication of an output laser mirror at 10.6 mu m Eight lambd a/4 layers of the ThF4 and ZnSe have been coated on a ZnSe substrate b y electron beam evaporation and the reflectivity R similar to 97% has been achieved. (C) 1997 Elsevier Science S.A.