Post-release deformation and motion control of photonic waveguide beams by tuneable electrothermal actuators in thick SiO₂

Photonic packaging, which includes high-precision assembly of photonic sub-systems, is currently a bottleneck in the development of commercially-available integrated photonic products. In the pursuit of a fully-automated, high-precision, and cost-effective photonic alignment scheme for two multi-channel photonic chips, this paper explores different designs of the on-chip electrothermal actuators for positioning mechanically-flexible waveguide structures. The final alignment goal is ~100nm waveguide to waveguide. The on-chip actuators, particularly for out-of-plane actuation, are built in a 16μm-thick SiO2 photonic-material stack with 5μm-thick poly-Si as an electrothermal element. A major challenge of out-of-plane positioning is a 6μm height difference of the waveguides to be aligned, due to different built-up material stacks, together with a misalignment tolerance of 1 μm-2μm from the pre-assembly (flip-chip) process. Therefore, the bimorph-actuator design needs to compensate this height difference, and provide sufficient motion to align the waveguides. We propose to exploit the post-release deformation of so-called short-loop bimorph actuator designs to meet these joint demands. We explore different design variants based on the heater location and the integration of actuator beams with waveguide beams. The actuator design (with 30μm poly-Si and 900μm SiO2 in length) has ~8μm out-of-plane deflection and is able to generate ~4μm motion, which meets the design goal.