Complex impedance of TESs under AC bias using FDM readout system

E Taralli,P Khosropanah,M P Bruijn,K Nagayoshi,J R Gao,M L Ridder,L Gottardi

doi:10.1063/1.5089739

Abstract

The next generation of Far-infrared and X-ray space observatories will require detector arrays with thousands of transition edge sensor (TES) pixel. It is extremely important to have a tool that is able to characterize all the pixels and that can give a clear picture of the performance of the devices. In particular, we refer to those aspects that can affect the global energy resolution of the array: logarithmic resistance sensitivity with respect to temperature and current (α and β parameters, respectively), uniformity of the TESs and the correct understanding of the detector thermal model. Complex impedance measurement of a TES is the only technique that can give all this information at once, but it has been established only for a single pixel under DC bias. We have developed a complex impedance measurement method for TESs that are AC biased since we are using a MHz frequency domain multiplexing (FDM) system to readout an array. The FDM readout demands for some modifications to the complex-impedance technique and extra considerations, e.g. how to modulate a small fraction of the bias carrier frequencies in order to get a proper excitation current through the TESs and how to perform an accurate demodulation and recombination of the output signals. Also, it requires careful calibration to remove the presence of parasitic impedances in the entire readout system. We perform a complete set of AC impedance measurements for different X-ray TES microcalorimeters based on superconducting TiAu bilayers with or without normal metal Au bar structures. We discuss the statistical analysis of the residual between impedance data and fitting model to determine the proper calorimeter thermal model for our detectors. Extracted parameters are used to improve our understanding of the differences and capabilities among the detectors and additionally the quality of the array. Moreover, we use the results to compare the calculated noise spectra with the measured data.

Highlights

Transition edge sensor (TES) microcalorimeters1 are very versatile superconducting devices, which can be used to detect radiation in a wide energy range e.g. from γ-ray down to submillimeter.2–8 A transition edge sensor (TES) consists of a superconducting thin film, typically with a transition temperature Tc∼100 mK, which is strongly coupled to its absorber but weakly thermal coupled to a lower temperature heat bath via a thermal conductance G
In this work we describe the details of a method for measuring the complex impedance of TESs that are biased in AC using a Base Band Feed-Back (BBFB) frequency domain multiplexing (FDM) readout system
We have extended this technique to be performed in the ACbias configuration that is intrinsic to the MHz FDM readout system

Summary

Introduction

Transition edge sensor (TES) microcalorimeters are very versatile superconducting devices, which can be used to detect radiation in a wide energy range e.g. from γ-ray down to submillimeter. A TES consists of a superconducting thin film, typically with a transition temperature Tc∼100 mK, which is strongly coupled to its absorber but weakly thermal coupled to a lower temperature heat bath via a thermal conductance G. TESs operate as thermometers: the absorption of incident photons by means of the absorber heats the device, which is biased in the transition between the superconducting and the normal states, causing a change in the resistance that is proportional to the photon energy absorbed. This variation is read out using a superconducting quantum interference device (SQUID). Large TES arrays are under development by many groups worldwide for ground-based and space-based applications and different technologies to readout a large number of detectors are under development..

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Discussion

Conclusion