We describe a method for measuring the complex impedance of transition-edge-sensor ͑TES͒ calorimeters. Using this technique, we measured the impedance of a Mo/Au superconducting transition-edge-sensor calorimeter. The impedance data are in good agreement with our linear calorimeter model. From these measurements, we obtained measurements of unprecedented accuracy of the heat capacity and the gradient of resistance with respect to temperature and current of a TES calorimeter throughout the phase transition. The measurements probe the internal state of the superconductor in the phase transition and are useful for characterizing the calorimeter.

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We present measurements of high fill-factor arrays of superconducting transition-edge x-ray microcalorimeters designed to provide rapid thermalization of the x-ray energy. We designed an x-ray absorber that is cantilevered over the sensitive part of the thermometer itself, making contact only at normal metal-features. With absorbers made of electroplated gold, we have demonstrated an energy resolution between 2.4 and 3.1 eV at 5.9 keV on 13 separate pixels. We have determined the thermal and electrical parameters of the devices throughout the superconducting transition, and, using these parameters, have modeled all aspects of the detector performance. PACS numbers:The requirements of the X-ray Microcalorimeter Spectrometer on NASA's Constellation-X mission include a full-width-at-half-maximum (FWHM) energy resolution A E of 2.5 eV at count rates up t o 1000 s-' per pixel and at least 95% quantum efficiency (QE) at an x-ray energy E of 6 keV. In order to cover a 5-arcminute field of view with spatial sampling matched t o the performance of the optics, a 64x64 array of 0.25-mm pixels wit11 i3 high fill fBc-tor is required. We report significant progress in the development of superconducting transition-edge sensor (TES) microcalorimeterl arrays for missions such as Constellation-X.The TES thermometers in our arrays consist of bilayers of Mo and Au with a superconducting transition temperature T, of N 0.1 K. Each 0.13-mm square TES is centered on a 0.5-pm silicon-nitride membrane, which is the thermal link with thermal conductance G t o the solid silicon frame at Tb w 50 mK. The TES pixels are arrayed on a 0.25-mm pitch. To achieve high QE and fill factor requires thick-film x-ray absorbers that fill the space above and between each TES. Our original approach to this requirement was t o construct Bi/Cu absorbers that make contact to the entire area of the TES, but extend cantilevered over the surrounding area. However, we determined that the need for good thermal contact to the TES and fast thermal equilibration of the absorber ( t o a\.oitl variwtioil of pulst: sl~apt: with posilio~~ ill tlw i l l ) -5orl)rr) conflicts with the requirement that the absorber not alter the superconducting transition of the s e n s~r .~ We have developed absorbers that are cantilevered over the sensitive regions of the TES's themselves as well as the surrounding area, making contact to the TES only at ing the properties of the TES thermometer, even when a normal metal is used as the absorber.In order to avoid shunting the TES bias current through the absorber, the absorber and TES may make contact at only one region along the direction of current flow. This constraint limits the size of the contact area within the TES. To ensure mechanical stability of the cantilevered structure, we extend the contact region onto the membrane. In this letter we present results from electroplated Au absorbers of two basic designs, one limiting the absorber-TES contact to the device edges and one making contact to the interdigitated stripes, as shown in...

Absorber Materials for Transition-Edge Sensor X-ray MicrocalorimetersArrays of superconducting transition-edge sensors (TES) can provide high spatial and energy resolution necessary for x-ray astronomy. High quantum efficiency and uniformity of response can be achieved with a suitable absorber material, in which absorber x-ray stopping power, heat capacity, and thermal conductivity are relevant parameters. Here we compare these parameters for bismuth and gold. We have fabricated electroplated gold, electroplated gold/electroplated bismuth, and evaporated gold/evaporated bismuth 8x8 absorber arrays and find that a correlation exists between the residual resistance ratio (RRR) and thin film microstructure. This finding indicates that we can tailor absorber material conductivity via microstructure alteration, so as to permit absorber thermalization on timescales suitable for high energy resolution x-ray microcalorimetry. We show that by incorporating absorbers possessing large grain size, including electroplated gold and electroplated gold/electroplated bismuth, into our current MoIAu TES, devices with tunable heat capacity and energy resolution of 2.3 eV (gold) and 2.1 eV (goldhismuth) FWHM at 6 keV have been fabricated.

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