Wafer scale fabrication of silicon nitride MEMS phase shifters with XeF2 dry vapor release etch process
Integrated photonic systems require reliable, low power consumption phase shifters in order to scale circuit complexity. Micro-electromechanical systems (MEMS) are a viable candidate to provide low-power phase shifting, without the significant drawbacks of thermo-optic effects (high power usage and cross-talk). Previous works have demonstrated MEMS phase tuning through vertically displaced microbridges. However, these require > 40 V for a π phase shift  using gradient electric fields, though lower voltage requirements have been demonstrated for direct electrostatic actuation . Ultra-low voltage (∼ 1 V) designs using horizontal slot waveguides have recently been demonstrated [3-5] although these methods require complex mechanical structures that must be released using vapor-HF or critical point drying post-processing. Here we present low-voltage phase tuning of vertically actuated beams that are released at wafer level with a vapor XeF2 etch completely in-house. Our process is carried out on an i-line photolithography stepper to define the waveguiding, metal, and MEMS structures. We use a sacrificial polysilicon layer between the SiN waveguide and the SiN beam. The XeF2 undercuts the beam, enabling a simple MEMS release process that does not undercut the waveguide. The movable SiN beam on top of the waveguide utilizes a single-sided anchor so that it resembles a wide and short single-clamped cantilever. This enables a phase shifter that is capable of a π phase shift with < 10 V and length < 100 μm. We measure the optical transmission versus applied voltage for multiple voltage sweeps and extract the phase shift per voltage at various wavelengths. We demonstrate reliable tuning over multiple sweeps with an average voltage of 7 V ± 0.5 V for a π phase shift. This phase shifter is central to the scalability of programmable photonic circuits, including quantum photonics.