Stress and Refractive Index Control of SiO2 Thin Films for Suspended Waveguides

Film stress and refractive index play an important role in the fabrication of suspended waveguides. SiO2 waveguides were successfully fabricated on multiple substrates including Si, Ge, and Al2O3 wafers; the waveguides were deposited using inductively coupled plasma chemical vapor deposition at 100 °C. The precursor gases were SiH4 and N2O at 1:3 and 1:9 ratios with variable flow rates. The occurrence of intrinsic stress was validated through the fabrication of suspended SiO2 bridges, where the curvature of the bridge corresponded to measured intrinsic stress, which measured less than 1 µm thick and up to 50 µm in length. The flow rates allow film stress tunability between 50 and −65 MPa, where a negative number indicates a compressive state of the SiO2. We also found that the gas ratios have a slight influence on the refractive index in the UV and visible range but do not affect the stress in the SiO2 bridges. To test if this method can be used to produce multi-layer devices, three layers of SiO2 bridges with air cladding between each bridge were fabricated on a silicon substrate. We concluded that a combination of low temperature deposition (100 °C) and photoresist as the sacrificial layer allows for versatile SiO2 bridge fabrication that is substrate and refractive index independent, providing a framework for future tunable waveguide fabrication.