Modern sophisticated optical film systems require materials with a structure controlled on the nanometer scale ("nano-engineering"). This aspect is particularly important when non-quarterwave films, inhomogeneous index depth profiles, nano-porous, nanocomposite and doped films with specific interfacial effects are considered for the fabrication of optical filters and passive and active optical waveguides. At the same time, the mechanical performance and wear and scratch resistance of such structures can also be optimized to meet the device requirements. Plasma-enhanced chemical vapor deposition (PECVD) is a very attractive technique to fabricate such "nano-structured" filters since it allows one to control the refractive index, n, at every moment of the film deposition. In the present work, two PECVD approaches were applied to control the refractive index depth profile n(z) using a single silicon nitride based material: a) variation of film packing density (porosity) in Si3N4 with n ranging from 1.60 to 2.00 (@ 500 nm) controlled by the instantaneous ion bombardment energy; and b) variation of x in SiNx:H alloys controlled solely by the duty cycle in the pulsed discharge. We show (using both experimental and dynamic Monte Carlo simulation evidence) that the process monitoring and the control of the film microstructure of such coatings require detailed knowledge of the growth mechanism in terms of plasma-surface interactions.
Using the above approaches, we fabricated different optical film systems, such as rugate filters, single cavity Fabry-Perot filters, as well as a superlattice structure displaying interesting photoluminescence properties. These examples of optical applications point out the main advantages of the surface-controlled film growth in high frequency PECVD, in particular low deposition temperature, reproducibility, versatility, fast process response and ease of use. In addition, we demonstrate the use of in-situ and ex-situ broad range spectroscopic ellipsometry (UV-VIS-NIR, 246–1650 nm, and FIR, 2–30 micrometers) as a powerful technique for the characterization of homogeneous and inhomogeneous optical films, of interfaces, as well as for real-time process control and monitoring.
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