Performance and qualification of CdTe pixel detectors for the Spectrometer/Telescope for Imaging X-rays

Abstract

The Spectrometer/Telescope for Imaging X-rays (STIX) is a remote sensing instrument on-board the ESA Solar Orbiter spacecraft. STIX is designated to the study of energetic phenomena in solar flares. A Fourier-imaging technique using tungsten grid collimators in front of CdTe pixel detectors is employed, covering the 4 to 150 keV energy range with a full-width-half maximum resolution around 1 keV at low energies.Acrorad CdTe detectors of 1 mm thickness with a planar aluminum Schottky contact are used as basis for a subsequent patterning process into eight large pixels, four small pixels, and a guard ring. The patterning is done by means of microfabrication technologies. The area of the patterned sensor is 10×10 mm 2 .Test equipment has been developed for selecting the detectors with best performance prior to integration with the read-out system, and for qualification purposes. The set-up allows pixel-based dark current measurements at low temperatures. Pixel dark currents below 60 pA are needed to avoid excess noise in the read-out ASIC. The best pixels show dark currents below 10 pA at 300 V bias and −20°C. Spectroscopic measurements with 133 Ba sources confirm the good performance.This paper briefly explains the mission context of the CdTe detectors and then gives details of the production and testing procedures. Typical results are shown, with emphasis on performance degradation studies from displacement damage by proton irradiation. This is expected to be the dominant degradation mechanism for this application.

Signal generation in CdTe X-ray sensors

“…This makes it difficult to maintain the correct stoichiometric proportions of the two components for intrinsic material during crystal fabrication. 4 The band gap is about of 1.6 eV at 4 K and decreases to approximately 1.5 eV at room temperature. large pixel in STIX, then has a resistance of 2 GΩ, or a leakage current of 50 nA at 100 V. At -20°C this will be reduced to a few nanoamperes due to the exponential dependence of the resistivity on temperature.…”

“…A blocking Schottky contact on the anode limits the leakage current. The sensors are produced by Acrorad, Japan, and the pixel structure on the anode is applied at the Paul Scherrer Institute, Switzerland [4]. The crystals are integrated into the Caliste-SO read-out hybrids by CEA Saclay, France.…”

“…Moderate cooling of the material is sufficient to achieve low leakage current due to the large band gap. 4 The bulk resistivity of the best obtainable CdTe crystals is about 2 × 10 9 Ω cm at room temperature. A 1 mm thick piece of 10 mm 2 area, corresponding to the dimensions of one 1 The mass attenuation coefficient is taken from a National Institute of Standards and Technology database, accessible at www.nist.gov/pml/data/xraycoef/.…”

“…The band edges in an energy band diagram indicate the electrostatic potential energy of an electron at a given location. 4 To describe quantitatively how the bands deform when materials are in contact, (1.1) has to be solved, thus the charge density has to be known.…”

The Spectrometer/Telescope for Imaging X-rays (STIX)

“…Raw CdTe crystals of size 14×14×1 mm 3 with planar electrodes were procured from Acrorad (Japan) with a platinum cathode and gold-titanium-aluminum Schottky multi-layer anode. Patterning was done at the Paul Scherrer Institute using photolithography and a plasma etching process (Grimm et al 2015). The final step was dicing to the required size of 10 × 10 mm 2 .…”