Surface micromachining for transition-edge detectors

We are developing arrays of high-performance detectors based on superconducting transition-edge sensors (TES) for application in x-ray materials analysis as well as x-ray and sub-millimeter astronomy. In order to obtain the desired thermal time constants, as well as to provide thermal isolation from adjacent pixels, these arrays utilize micromachined thermal-isolation structures. Until recently, we have achieved thermal isolation of single-pixel devices by anisotropic wet etching of the entire Si wafer behind the pixel, leaving the detector supported by a thin Si₃N₄ membrane. Limitations of this technique make it undesirable for the fabrication of close-packed arrays. One possible means to achieve thermal isolation of close-packed arrays is surface micromachining. Here, a TES is fabricated on top of a Si₃N₄ membrane that is held above the substrate by a small number of support legs. Because the underlying wafer is not thinned or removed, the resulting detector chip is strong and requires no special handling. In this paper we describe the fabrication processes and present preliminary data on the properties of 64-pixel arrays of surface-micromachined TES x-ray detectors.