Quasicontinuous refractive index tailoring of SiN_x and SiO _x N_y for broadband antireflective coatings

A broadband antireflective coating for silicon was fabricated by tailoring the compositions of SiN_x and SiO_x N_y during conventional plasma enhanced chemical vapor deposition. The coating's refractive index was quasicontinuously graded, e.g., from 3.22 to 1.44 at 1550 nm. Over the 280-3300 nm wavelength range, the reflectance was below 8% peak and 4.3% average. The deposited stack was composed of dense dielectric materials. This enables patterning and processing into robust devices after coating deposition. Using single layer ellipsometry data, the transfer matrix method was applied to predict the multilayer coating's reflectance spectra. The results showed good agreement with experimental data.