Surface micromachined transition edge sensor microcalorimeters on polyimide membranes

A surface micromachining process is described for the fabrication of transition edge sensor (TES) microcalorimeters on polyimide membranes. The TES consists of a low-stress superconductor/normal metal bilayer of Mo/Cu. The deposition parameters for the sputter-deposited Mo and Cu films are optimized using a design of experiment and statistical models to obtain a T_c around 100 mK, bilayer sheet resistance of around 15 to 20 mΩ/sq, and low bilayer film stress of <100 MPa. To fabricate the membranes for the microcalorimeters, a sacrificial poly-Si film is sputter-deposited on a Si wafer with thermal oxide, after which poly-Si mesas are patterned where the membranes are to be formed. Then, a planarizing polyimide film is spin-coated on the wafer. After curing the polyimide, the TES bilayers and wiring layers are deposited and patterned using standard dry etch processes. To release the membranes, windows in the polyimide film above the poly-Si mesas are opened using a dry etch, and the sacrificial poly-Si is removed using a XeF₂ etch process. Using a polyimide film with low cure temperature (e.g., <200°C), wiring and first-stage SQUID readouts for the TES may be deposited underneath the microcalorimeter prior to membrane fabrication, thereby simplifying the design of large, multi-pixel TES microcalorimeter arrays with integrated first-stage SQUID readouts.