A DIRECT ROBUST FEEDBACK METHOD FOR GROWTH-CONTROL OF OPTICAL COATINGS BY MULTIWAVELENGTH ELLIPSOMETRY

Real time control by multiwavelength phase modulated ellipsometry (PME ), as applied to the growth of optical coatings deposited on various s ubstrates, is reviewed. The structures consist of plasma deposited SiO 2, and SiNx, stacks and SiOxNy gradient-index coatings. A feedback met hod is used for growth control of a standard quarterwave filter on c-S i and on glass, the latter achieved by incorporating incoherent modell ing of the transparent substrate. The feedback control method is based on a comparison between the real time PME measurements and pre-comput ed target trajectories. The resulting optical coatings characterised b y spectroscopic PME and transmission measurements show reproducible sp ectral characteristics, with less than 1% deviation between the target and measured spectral responses. The feedback control method is furth er generalised to the control of gradient-index coatings. (C) 1998 Els evier Science S.A.