Real-time control of the deposition of optical coatings by multiwavelength ellipsometry

Real-time control. by multiwavelength phase modulated ellipsometry (PME), of the growth of optical coatings is reviewed. The structures consist of plasma deposited SiO2 and SiNx slacks, and a SiOxNy gradient-index. A first feedback method is used for the growth control of a standard quarterwave filter on c-Si, and on glass by incorporating incoherent modelling of the transparent substrate. This feedback control method is based on the comparison between the real-time PME measurements and pre-computed target trajectories. The resulting optical coatings characterized by spectroscopic PME and transmission measurements show a reproducible accuracy, with less than 1% deviation between target and measured spectral responses. A different feedback method is used to control of the deposition of gradient-index coatings. An efficient method for estimating the optical parameters based on a least square fitting of the most recent recorded measurements is used in the latter case. Furthermore, an integral expansion of the reflection coefficients, giving a continuous optical model describing inhomogeneous films is used in the real-time modelling of the deposited structure. As an example of application, a successful real-time control of the growth of a linear gradient index is demonstrated. (C) 1998 Elsevier Science S.A.