Etching characteristics of silica-based optical waveguide for planar lightwave circuit

We have successfully synthesized high quality silica by using plasma enhanced chemical vapor deposition (PECVD) of tetraethyl orthosilicate (TEOS) precursors. Optical transmission spectroscopy and Fourier transform infrared (FTIR) spectroscopy indicated that, the TEOS/O-2 ratio had pronounced influence on the deposition rate and optical properties for SiO2 films. Modification on refractive index (n) for the synthesis of single mode optical waveguide can be easily achieved in PECVD process, by addition of TMP in the precursors. Based on this, a Si/SiO2/P-SiO2/SiO2 ridge-type waveguide structure could be derived following a photolithography, lift-off and reactive ion etching (RIE) process sequence. In this study, silica etch characteristics of different etching parameters were investigated using CHF3/O-2 as the etch gases. Metal mask could offer higher selectivity and yielded smaller deposition of polymer, so that an optical quality ridge structure with high vertical sidewall angle and dimension controllability, and smooth sidewall surface became possible. The results came out that the etching parameters such as RF power and CHF3/O-2 ratio had significant influence on the etch rates, etch selectivities and etch profiles. Under optimal etching conditions and post-treatment, a single mode waveguide with vertical etch profile and smooth sidewall could be fabricated successfully.