Optical film thickness monitor

Claim 1

An optical film thickness measuring device for measuring the thickness of a thin film deposited on a substrate surface comprising, in combination: means for providing two light beams of different wavelengths, each beam exhibiting a varying curve of light intensity vs. film thickness when passed through said film, each said curve having alternate maximum and minimum points and one said curve being displaced from the other so that crossover points occur at thicknesses of said film which vary by one-quarter of the average wavelength between said beams; means for alternately interrupting each light beam so that the light beams comprise a series of alternate pulses with respect to time; means for collimating said interrupted light beams; means for projecting said interrupted light beams upon said film and substrate; means for collecting the light after it has passed through or been reflected from said film and substrate and focusing the transmitted or reflected light beams, said light beams being passed through said substrate if it is light-transmissive and being reflected therefrom if it is opaque; means for detecting said transmitted or reflected light beams, and providing an electrical output in accordance with its intensity; means connected to said detection means for subtracting alternate pulses from each other and for averaging the differences; and means connected to said subtraction-and-averaging means for providing an indication of the averaged differences.

Claim 4

A method for measuring the thickness of a thin film deposited on a substrate surface comprising the steps of: alternately interrupting two light beams of different wavelengths so that the beams form two series of light pulsEs, those in one series occurring during the spaces in the other series and vice versa; projecting these pulsed beams in parallel disposition upon said film and substrate; focusing the transmitted beams or the reflected beams upon light-detecting means providing an electrical output proportional to light intensity; subtracting alternate pulses of said electrical output from each other and averaging the differences; and utilizing the null points in the averaged differences to indicate the thickness of the deposited film.

Claim 5

A method for measuring the thickness of a thin film deposited on a substrate surface comprising the steps of: alternately interrupting two light beams of different wavelengths so that the beams form two series of light pulsEs, those in one series occurring during the spaces in the other series and vice versa; projecting these pulsed beams in parallel disposition upon said film and substrate; focusing the transmitted beams or the reflected beams upon light-detecting means providing an electrical output proportional to light intensity; subtracting alternate pulses of said electrical output from each other and averaging the differences; and utilizing the null points in the averaged differences to indicate the thickness of the deposited film.