Bistable silicon photonic MEMS switches

We present the design of a non-volatile, bistable silicon photonic MEMS switch. The switch architecture builds on our previously demonstrated silicon photonic MEMS switch unit cell, using vertically movable adiabatic couplers. We here propose to exploit compressive stress in the movable polysilicon waveguides in a controlled manner, to intentionally displace the movable waveguides out of plane upon release. We design the waveguide suspensions to achieve close alignment with the fixed bus waveguide in the ON state, and positioning of the movable waveguide far from the fixed waveguide in the OFF state. Both ON and OFF positions are stable mechanically, without the need for maintaining an actuation voltage. In order to actuate the movable waveguide, we design vertical comb drive actuators that allow to commutate between both stable ON and OFF positions. Finite Element simulations predict electrostatic switch actuation with less than 30 V for compressive stress typically accessible in deposited polysilicon thin films. We validate the bistability mechanism by comparison with a representative experimental demonstrator. The demonstrator consists of a structured 100 nm poly-Si layer, deposited by chemical vapor deposition onto a thermally oxidized (1 μm) silicon wafer, exhibiting a compressive intrinsic stress of 275 MPa. Upon direct writing laser based photolithography, etching and final HF vapor release, the suspended structures bend into either stable position, and we measure a total buckling amplitude of 800 nm, sufficient to entirely de-couple the waveguides optically in the OFF state.