Stress and Refractive Index Control of SiO(2) Thin Films for Suspended Waveguides

Film stress and refractive index play an important role in the fabrication of suspended waveguides. SiO(2) waveguides were successfully fabricated on multiple substrates including Si, Ge, and Al(2)O(3) wafers; the waveguides were deposited using inductively coupled plasma chemical vapor deposition at 100 °C. The precursor gases were SiH(4) and N(2)O at 1:3 and 1:9 ratios with variable flow rates. The occurrence of intrinsic stress was validated through the fabrication of suspended SiO(2) 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 SiO(2). 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 SiO(2) bridges. To test if this method can be used to produce multi-layer devices, three layers of SiO(2) 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 SiO(2) bridge fabrication that is substrate and refractive index independent, providing a framework for future tunable waveguide fabrication.