Development of optical monitor for control of thin-film deposition

The design of a multichannel optical monitor for transmittance measurements of thin-film coatings during deposition is described. The system comprises a light source and one or more spectrum analyzers each incorporating a prism monochromator and a 256-element photodiode array detector. This multiple-channel design, in conjunction with an HP 1000 computer and data acquisition period of 100 ms, enables the coating uniformity to be precisely monitored and controlled. High-system throughput has been achieved with a large numerical aperture (f/1.5), while retaining excellent spectral resolution over the 350–1100-nm wavelength range. Experimental measurements indicate a practical resolution equal to the detector-limited resolution, a wavelength reproducibility of 0.1 nm at 400 nm and 0.6 nm at 700 nm, and a photometric accuracy and precision of ~1 and ±0.3%, respectively. The problem of unequal energy distribution across the spectrum is handled by optical rather than the usual electronic compensation.