Calibration and Testing of Small High-Resolution Transition Edge Sensor Microcalorimeters with Optical Photons.

Pulses of narrow line-width optical photons can be used to calibrate and test sub-2 eV full-width at halfmaximum (FWHM) energy resolution transition-edge sensor (TES) microcalorimeters at low energies (< 1 keV), where it is very challenging to obtain X-ray calibration lines comparable to (or narrower than) the detector resolution. This scheme depends on the ability to resolve the number of 3 eV photons in each pulse, which we have recently demonstrated up to photon numbers of about 300. At LTD-18 we showed preliminary results obtained with this technique on a 0.25 eV baseline resolution TES microcalorimeter designed for the ultra-high-resolution subarray of the Lynx mission. The line-shape was well described by a simple Gaussian. However, the difficulty of delivering photons to the small 46 µm square absorbers resulted in a large thermal crosstalk signal, whose random nature is expected to rapidly degrade the observed energy resolution towards higher photon numbers/energies. We have since improved the coupling between the optical fiber and the TES absorber and report here our current results.

Effects of uniaxial stress on Mo and Mo/Cu bilayer superconducting transitions.

Transition-edge sensors (TES) are widely used as sensing elements in X-ray microcalorimeters. Further improvement of their energy resolution hinges on a thorough understanding of the transition surface (as function of temperature, current, and magnetic field) to achieve high sensitivity (α) and low noise (small ß), as well as the capability to repeatably fabricate the proximity superconducting/normal metal bilayers with a predictable transition surface. One aspect that is poorly understood is the impact of film stress on the transition. Data from Mo films deposited using e-beam evaporation onto heated substrates, as well as sputtered films, show a strong correlation between film stress and superconducting transition temperature (≈-0.2K/GPa, corresponding to shift of about -0.1 K for a 0.1% change in biaxial strain). However, this correlation is of opposite sign and much larger than one would expect from the pressure dependence of bulk Mo. Furthermore, modifications in fabrication details of the devices, such as membrane perforations and absorber attachment, have been observed to result in large qualitative differences in the transition surface for otherwise identical TES geometry. It seems reasonable to ask whether associated changes in film stress distribution can cause these differences. To shed some light on this issue, we have subjected a bare Mo film as well as Mo/Cu bilayers to in-situ tunable uniaxial stress produced by a piezo-electric stack. Our results indicate that the direct strain induced changes to the transition temperature are rather small (about +0.3 mK for a 0.1% strain change on a Mo film) and consistent in sign and order of magnitude with that derived from the bulk.

Mapping of the resistance of a superconducting transition edge sensor as a function of temperature, current, and applied magnetic field.

We have measured the resistance R(T, I, B ext) of a superconducting transition edge sensor over the entire transition region on a fine scale, producing a 4-dimensional map of the resistance surface. The dimensionless temperature and current sensitivities ( α ≡ ∂ log R / ∂ log T | I and ß ≡ ∂ log R / ∂ log I | T ) of the TES resistance have been determined at each point. α and ß are closely related to the sensor performance, but show a great deal of complex, large amplitude fine structure over large portions of the surface that is sensitive to the applied magnetic field. We discuss the relation of this structure to the presence of Josephson "weak link" fringes.

Fabrication of superconducting Mo/Cu bilayers using ion-beam-assisted e-beam evaporation.

Superconducting/normal metal bilayers with tunable transition temperature are a critical ingredient to the fabrication of high performance transition edge-sensors (TES). Popular material choices include Mo/Au and Mo/Cu, which exhibit good environmental stability and provide low resistivity films to achieve adequate thermal conductivity. The deposition of high quality Mo films requires sufficient adatom mobility, which can be provided by energetic ions in sputter deposition, or by heating the substrate in an e-beam evaporation process. The bilayer T c depends sensitively on the exact deposition conditions of the Mo layer and the superconducting/normal metal interface. Because the individual contributions (strain, crystalline structure, contamination) are difficult to disentangle and control, reproducibility remains a challenge. Recently, we have demonstrated that low energy ion beam assist during e-beam evaporation offers an alternative route to reliably produce high quality Mo films without the use of substrate heating. The energy and momentum delivered by the ion beam provides an additional control knob to tune film properties such as resistivity and stress. In this report we describe modifications made to the commercial end-Hall ion-source to avoid iron contamination allowing us to produce superconducting Mo films. We show that the ion beam is effective at enhancing the bilayer interface transparency and that bilayers can be further tuned to reduced T c and higher conductivity by vacuum annealing.