Conformal high aspect ratio fill using RF sputtering

RF sputtering is a common deposition technique used in fabrication of optical waveguide-based devices. This technique is predominantly used to deposit thin film of high index dielectric oxide/nitride for waveguides. RF sputtering provides uniform deposition of film on a planar substrate. Challenges arise while depositing film on a high aspect ratio structure, e.g. depositing top cladding on a strip waveguide. Uniform and conformal deposition on a high aspect ratio structure requires higher substrate temperature and annealing. High temperature processing inhibits lift-off processing. It also affects the material property, e.g., amorphous silicon starts converting into polysilicon at 300 C. We present here an RF sputtering process, which can be used to deposit a conformal layer on a high aspect ratio structure without introducing heating. We introduce a DC bias to the substrate in addition to RF, which allows deposition and etching to happen simultaneously. DC bias to wafer/substrate results in re-deposition.