Material characterization of silicon dioxide cladding for photonic devices

The top cladding is an important factor when designing photonic devices. It has implications on waveguide properties such as mode guidance and optical loss. Silicon dioxide (SiO₂) is a commonly used cladding material for photonic devices as it has low optical loss and also enables a large refractive index contrast between the waveguide core and cladding. We performed an investigation into the material properties of SiO₂ thin films deposited by different methods, namely, plasma-enhanced chemical vapor deposition (PECVD) undoped silicate glass (USG), high density plasma (HDP), PECVD tetraethyl orthosilicate (TEOS), and low pressure chemical vapor deposition (LPCVD) TEOS. Material properties including refractive index, surface roughness, absorption, and film stress were characterized for the different SiO₂ cladding samples. Our results show that the surface roughness of HDP films was around 55 times better than that of USG films. In terms of film stress however, USG films had the lowest, whereas PECVD TEOS films were found to have the highest film stress, more than twice of that of the USG films. These findings indicate that when deciding on a particular deposition method for the SiO₂ cladding, besides considerations of thermal budget and device application, an equally important compromise needs to be made to give a balance among the various material properties.