A technique is described for monitoring the thickness of transparent thin films growing on heated polished substrates (as in chemical vapor deposition). Thermal energy, radiated by a substrate and transmitted by a growing film, undergoes optical interference as a result of multiple reflections between the substrate‐film and film‐ambient interfaces. A narrow bandpass detector is used to measure the transmitted intensity and film thickness is correlated with the maxima and minima of the detector output. Using a 1P21 photomultiplier tube and a 6000 Å interference filter (90 Å half‐width) this technique has been applied for substrate temperatures as low as 800°C and thickness in the range of 1000–6000 Å with an accuracy of ± 100 Å. A special comparison technique is described which operates by sampling, storing, and subtracting signal intensities obtained from two substrates. Using this technique it is possible to terminate deposition of the outer film of a duplex combination (500 Å of Al2O3 on SiO2, for example) within 25 Å of the chosen thickness.

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