Plasma treatment of porous SiN_x: H films for the fabrication of porous-dense multilayer optical filters with tailored interfaces

Porous and dense silicon nitride films with low (1.58) and high (1.88) refractive indices were prepared by using successively microwave and radio frequency (rf) plasma-enhanced chemical vapor deposition. Surface treatments were performed on porous layers using argon and nitrogen rf plasmas in order to densify and flatten their surface, and hence to obtain an abrupt transition between porous and dense films. The processes during deposition and interface treatment were studied by in situ real-time spectroscopic ellipsometry as well as by other characterization techniques. We show that besides the densification effect, preferential sputtering and annealing phenomena occur during plasma treatments at high bias (∫ VBtreat ∫ 400 V), leading to silicon enrichment at the film surface and chemical stabilization of the film bulk. Using atomic force microscopy, we observed a significant reduction of the thickness of the surface roughness layer after treatment for single layers (≈70% reduction) and multilayer stacks (≈60% reduction). Porous-dense Fabry-Ṕrot filters were fabricated using this approach that led to both enhanced optical performance due to minimized interface thickness and improved environmental stability.