Mark A. Lindeman

Impedance measurements and modeling of a transition-edge-sensor calorimeter

We describe a method for measuring the complex impedance of transition-edge-sensor (TES) 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.

Performance of Mo/Au TES microcalorimeters

We are developing X-ray calorimeters to meet the specifications of the Constellation-X mission. Each calorimeter consists of a transition-edge-sensor (TES) thermometer, which is suspended on a silicon-nitride membrane. Our TES thermometers are Mo/Au bilayer films that are biased in the sharp phase transition between the superconducting and normal-metal states. These calorimeters have demonstrated very good energy resolutions: 2.4 eV at 1.5 keV and 3.7 eV at 3.3 keV. The energy resolutions are limited by thermal noise and Johnson noise (which are intrinsic to any resistive calorimeter) plus excess noise. The excess noise, which is several times larger than the Johnson noise, is consistent with frequency-independent voltage noise in the TES. Detailed measurements of one Mo/Au TES demonstrate that the excess noise is independent of the voltage applied to the TES over a range of biases at the same TES resistance. The magnitude of the excess noise is smallest at the high-resistance end of the phase transition. We also compared noise in square Mo/Au TES’s ranging in size from 300 microns to 600 microns to learn how the excess noise is affected by the geometry of the TES.

Fabrication of Mo/Au transition-edge sensors for X-ray spectrometry

We present fabrication details of our Mo/Au X-ray microcalorimeters, which are being developed as one of the candidate high resolution spectrometers for the Constellation-X mission. We have reproducibly fabricated Mo/Au transition-edge sensors with Tcs of /spl sim/100 mK on etched silicon nitride membranes and connected via superconducting Nb leads. Our single pixel devices have, so far, attained resolution of 3.7 eV at 3.3 keV. We also discuss our plans for fabrication and testing of fully functional multi-pixel array of X-ray microcalorimeters.

Optimization of X-ray Absorbers for TES Microcalorimeters

We have investigated the thermal, electrical, and structural properties of Bi and BiCu films that are being developed as X-ray absorbers for transition-edge sensor (TES) microcalorimeter arrays for imaging X-ray spectroscopy. Bi could be an ideal material for an X-ray absorber due to its high X-ray stopping power and low specific heat capacity, but it has a low thermal conductivity, which can result in position dependence of the pulses in the absorber. In order to improve the thermal conductivity, we added Cu layers in between the Bi layers. We measured electrical and thermal conductivities of the films around 0.1 K, the operating temperature of the TES calorimeter, to examine the films and to determine the optimal thickness of the Cu layer. From the electrical conductivity measurements, we found that the Cu is more resistive on the Bi than on a Si substrate. Together with a SEM picture of the Bi surface, we concluded that the rough surface of the Bi film makes the Cu layer resistive when the Cu layer is not thick enough to fill in the roughness. From the thermal conductivity measurements, we determined the thermal diffusion constant to be 2 x 103 μm2μs-1 in a film that consists of 2.25 μm of Bi and 0.1 μm of Cu. We measured the position dependence in the film and found that its thermal diffusion constant is too low to get good energy resolution, because of the resistive Cu layer and/or possibly a very high heat capacity of our Bi films. We show plans to improve the thermal diffusion constant in our BiCu absorber.

Detailed characterization of Mo/Au TES microcalorimeters

We are optimizing Mo/Au transition-edge-sensor (TES) calorimeters to meet the specifications of NASA’s Constellation-X mission. Our calorimeters have already demonstrated very good energy resolution of X rays (2.4 eV at 1.5 keV). We wish to further improve the energy resolution by reducing excess noise in the calorimeters. Development of a detailed model and understanding of the noise is instrumental to reaching this goal. Towards that end, we employ a linear model that describes the response of a calorimeter to signal and various sources of noise. The model is based on detailed measurements of the parameters that affect the calorimeter’s performance, such as current-voltage characteristics of the TES, thermal conductance of our silicon-nitride membranes, and inductance in the electronic circuit used to bias the TES. We determine the sharpness of the superconducting phase transition by fitting the model to the measured responsivity of the calorimeter. The model relates sources of noise, such as phonon noi...

First results from Position-Sensitive quantum calorimeters using Mo/Au Transition-Edge Sensors

We report the first results from a high-energy-resolution imaging spectrometer called a Position-Sensitive Transition-Edge Sensor (PoST). A PoST is a quantum calorimeter consisting of two Transition Edge Sensors (TESs) on the ends of a long absorber to do one dimensional imaging spectroscopy. Comparing rise time and energy information, the position of the event in the PoST is determined. Energy is inferred from the sum of the two pulses. We have fabricated 7- and 15-pixel PoSTs using Mo-Au TESs and Au absorbers. We have achieved 32 eV FWHM energy resolution at 1.5 keV with a 7-pixel PoST calorimeter.

Fabrication of close-packed TES microcalorimeter arrays using superconducting molybdenum/gold transition-edge sensors

We present an overview of our efforts in fabricating Transition-Edge Sensor (TES) microcalorimeter arrays for use in astronomical x-ray spectroscopy. Two distinct types of array schemes are currently pursued: 5×5 single pixel TES array where each pixel is a TES microcalorimeter, and Position-Sensing TES (PoST) array. In the latter, a row of 7 or 15 thermally-linked absorber pixels is read out by two TES at its ends. Both schemes employ superconducting Mo/Au bilayers as the TES. The TES are placed on silicon nitride membranes for thermal isolation from the structural frame. The silicon nitride membranes are prepared by a Deep Reactive Ion Etch (DRIE) process into a silicon wafer. In order to achieve the concept of closely packed arrays without decreasing its structural and functional integrity, we have already developed the technology to fabricate arrays of cantilevered pixel-sized absorbers and slit membranes in silicon nitride films. Furthermore, we have started to investigate ultra-low resistance throug...

Position Sensitive Microcalorimeters

We present our latest results from our development of Position-Sensitive Transition-Edge Sensors (PoSTs). Our devices work as one-dimensional imaging spectrometers. They consist of a long absorber (segmented or solid) with a transition-edge sensor (TES) on each end. When X-rays hit the absorber, the comparison of the signals sensed in the two TESs determine the position of the TES, while the addition of the signals gives the energy of the X-ray. We obtained impedance curves for three different devices and obtained reasonable fits with our theoretical PoST model.

Probing the phase transition of Mo/Au TES microcalorimeters

We present recent measurements obtained using a new method for characterizing transition edge sensor (TES) calorimeters: We measured the electrical impedance of a TES calorimeter throughout the superconducting to normal metal phase transition. The impedance method enables us to previously measure how the resistance and heat capacity of the TES varied throughout the phase transition. These measurements probe the internal state of oru Mo/Au TES. We also present recent results from measurements of noise in our TESs. Our measurements are instrumental toward understanding and optimizing our TES calorimeters.

A constant temperature TES microcalorimeter with an external electronic feedback system

One major problem with TES micro-calorimeters is the very narrow temperature range in which they work. To detect high energy events the heat capacity of the detector must be sufficiently large to keep the sensor running in its sensitive region. The large heat capacity limits the best energy resolution that can be obtained. One way to avoid a saturation effect while keeping the heat capacity small is to run the detector at constant temperature, reducing the power dissipated into it by an amount equal to the power conducted into the thermometer from the energy deposition. We studied the possibility of such a detector using an external electronic feedback system (EEF) [1,2] that reduces the bias voltage on the sensor. Using traditional electro-thermal feedback to run a constant temperature detector, the signal amplitude can be reduced, but the noise of the readout electronics does not change and this may worsen the energy resolution. Moreover the gain of the feedback, determined by the thermometer sensitivity alpha, may not be large enough to keep the temperature constant. With external feedback, the noise of the readout electronics is reduced proportionally to the signal and the feedback can be made larger. In this paper we outline the characteristics of such a system, investigate its performances and discuss our optimization efforts.