TiAu-based micro-calorimeters for space applications B.P.F. Dirks a,Ã , M. Popescu a , M. Bruijn a , L. Gottardi a , H.F.C. Hoevers a , P.A.J. de Korte a , J. van der Kuur a , M. Ridder a , Y. Takei b a SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands b Institute of Space and Astronautical Science (ISAS/JAXA), 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan article info Available online 27 May 2009 Keywords: IXO XEUS EURECA Transition edge sensors Micro-calorimeters Superconductors Cross-talk abstract We present the latest results of the performance of micro-calorimeters based on transition edge sensors (TESs) for space applications. Sensors based on TiAu superconductive layers with Cu/Bi absorbers are discussed and have been characterized. Different coupling schemes between absorber and TES have been tested leading to an optimal (preferred) design for a new batch of arrays. We discuss the progress on array development for the International X-ray Observatory (IXO) in terms of pixel uniformity and filling factor. Inter-pixel cross-talk is discussed as well. & 2009 Elsevier B.V. All rights reserved. 1. Introduction SRON, Netherlands Institute for Space Research, works on the development of the cryogenic narrow field imager for the International X-ray Observatory (IXO), a space mission dedicated to the study of the hard X-ray Universe (0.1–40 keV). The project is led by a large collaboration of ESA/NASA/JAXA and several international institutes and is in fact a merging of two large space projects known as XEUS (ESA) and Constellation-X (NASA). The satellite is planned for a launch around 2020. A detailed description of IXO is still in progress but as its scientific targets will be close to that of XEUS, Ref. [1] gives a good overview of the mission’s requirements. The narrow field imager will operate in the energy range from 0.1 to 20 keV aiming at an energy resolution of 2 eV at 2 keV and 5 eV at 5.9 keV. The imager will have 32-by-32 pixels of 240  240 mm 2 . The filling factor of the focal plane is 95%. SRON and collaborators started the EURECA (EURopean- JapanEse Calorimeter Array) project [2] to show the technological readiness of a 5-by-5 pixel array of micro-calorimeters based on transition edge sensors (TESs) and SQUID read-out using frequency domain multiplexing (FDM). The overall performance demonstration is planned for mid 2009. In this paper we focus on recent developments in the single pixel and array development; details about FDM read-out can be found in Ref. [3]. 2. Pixel development 2.1. Detection principle The TES X-ray detection principle is as follows: a photon is absorbed and heats up the absorber material (in our case Cu/Bi). The temperature change is sensed by a very sensitive thermo- meter, the TES, which is directly connected to the absorber by means of a specific coupling scheme. The SRON TESs are based on TiAu bilayers with a superconducting to normal transition at 100 mK. In the transition, a small change in temperature (10 4 K), due to the absorption of a photon, will lead to a large change in resistance (100 mO). If a constant voltage bias is applied, the change in current, directly related to the photon energy, can be measured with high precision by a low-impedance SQUID read-out system. 2.2. Sensor variations Fig. 1(a) shows a standard chip with 13 sensors, each having its specific absorber–TES coupling and geometry. These geometries include bar-coupled, dot-coupled (not shown in the figure) and central square-coupled absorbers. All of these variations have been tested extensively. The devices in which a central square provides the absorber–TES coupling have shown a good performance and this geometry will be pursued in the remainder of the study. An example of such a sensor is depicted in Fig. 1(b). It consists of a 1-mm-thick Cu absorber layer (100  100 mm 2 ) centrally coupled to a 146  150 mm 2 TiAuTi (20/50/5 nm) bilayer resting on a 1-mm-thick SiN membrane connected to the heat bath ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/nima Nuclear Instruments and Methods in Physics Research A 0168-9002/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2009.05.114 Ã Corresponding author. Tel.: +3130 2535558. E-mail address: b.p.f.dirks@sron.nl (B.P.F. Dirks). Nuclear Instruments and Methods in Physics Research A 610 (2009) 83–86