High Performance Optical Waveguides based on Boron and Phosphorous doped Silicon Oxynitride

Silicon oxynitride (SiON) is a highly attractive material for integrated optics, due to its excellent properties such as high transparency, adjustable refractive index and good stability. In general, the growth of SiON layers by plasma enhanced chemical vapor deposition (PECVD) is followed by a high temperature annealing step in order to remove hydrogen and to achieve low propagation losses in the 1.5-μm wavelength window. The high annealing temperature (>1100 ̊C) required for sufficient hydrogen removal induces, however, side effects like significant inter-layer diffusion and microcracks resulting in deterioration of the device performance. In this paper compositional and optical properties of as-deposited and annealed boron (B) and phosphorous (P) doped SiON layers were investigated. The doped layers have been fabricated by introducing PH3 and B2H6 gaseous precursors into the PECVD process. Hydrogen contents of the samples have been studied by Fourier transform infrared (FTIR) spectroscopy. Compared to undoped film, a 50% reduction of the hydrogen content was measured in as-deposited Pdoped SiON layers. Further reduction down to the FTIR detection limit was achieved upon annealing at temperatures as low as 700 ̊C. Besides hydrogen reduction the reflow properties of B and P doped SiON are also highly relevant for the realization of low-loss integrated optical circuits. Reactively ion etched channel waveguides have been reflown applying a temperature of 900 ̊C. Significant reduction of the sidewall roughness has been confirmed by scanning electron microscopy.