ECR CVD of silicon oxynitride films for antireflection coating of 10 Gbit/s, 1.56 μm multi-quantum well InP/InGaAsP Mach-Zehnder modulators

This paper describes the development of electron cyclotron resonance (ECR) plasma-deposited SiO_xN_y single layer anti-reflecting (AR) optical coatings for III-V 10 Gbit/s, 1.56 μm Mach-Zehnder (MZ) modulators. To minimize the effect of reflections from the cleaved facets, MZ modulators require low reflectivity (1% for TE and TM polarizations) facet coatings, which remain stable over operating life (25 years). Several reports in the literature have addressed the problem of achieving a low reflectance facet coating by monitoring; the reflectance of, for example, one laser at a time. However, for production quantities, the uniformity, stability, run-to-run reproducibility and device performance across several bars held in a coating fixture must be optimized. This places constraints on the deposition method and fixturing. E-beam evaporated coatings tend to produce films which have poor stability (i.e. drift in reflectance); deposition at high substrate temperatures reduces this effect but can also result in incongruent loss of the group V element and give rise to absorption and/or scattering at the facet. In this paper we report of the fabrication and characterization of SiO_x N_y films deposited by ECR CVD which provides low ion energy (~<25 eV) bombardment, low plasma damage and low deposition temperature to produce high quality, stable AR facet coatings