Fabrication of robust electrothermal MEMS devices using aluminum–tungsten bimorphs and polyimide thermal isolation

We report a novel fabrication process for bimorph based electrothermal devices that involves surface and bulk micromachining on an SOI (silicon-on-insulator) wafer. The bimorph transducers consist of aluminum (Al)-–tungsten (W) beams with a thin layer of SiO₂ encapsulating the W-layer and electrically isolating it from the Al-layer. The widely different coefficients of thermal expansion (CTE) of Al and W ensure high bimorph sensitivity. The W-layer acts as an active layer of the bimorph and also as a resistive heater for actuation by Joule heating. Proof mass and rigid beams can be incorporated into the design by utilizing the device layer of the wafer. Thermal isolation between the bimorphs and the substrate; and between bimorphs and the proof mass or rigid beams is achieved by a spin-coated polyimide layer. The process ensures improved robustness compared to comparable designs that typically utilize SiO₂ thin-film for thermal isolation. The process can be adapted to a wide range of electrothermal devices, and enables design and layout engineers to trade off speed for achieving lower power consumption and vice versa. We report fabrication and test results on 1D (one-dimensional) electrothermal scanning micromirrors. The fabricated devices have significantly better robustness compared to previously reported mirrors with Al-SiO₂ bimorphs and SiO₂ thermal isolation. (paper)