Development of cryogenic X-ray detectors based on Mo/Au TES

Development of cryogenic X-ray detectors based on Mo/Au TES | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Development of cryogenic X-ray detectors based on Mo/Au TES A. Camón, L. Fàbrega, C. Pobes, P. Strichovanec, J. Bolea, J. I. Rico-Camacho, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7700799/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 07 Jan, 2026 Read the published version in Journal of Low Temperature Physics → Version 1 posted 10 You are reading this latest preprint version Abstract Transition Edge Sensors (TES) have emerged as highly sensitive radiation detectors with applications ranging from astrophysics to materials science. In the framework of the Athena X-ray mission, we have developed Mo/Au-based TES devices designed to meet the specifications defined by ESA. The detectors were fabricated using a Mo/Au/Au trilayer process and characterized in a dilution refrigerator using a SQUID readout. From I–V curves and complex impedance measurements, we extracted the electrothermal parameters of the TES, which are consistent with theoretical models. Noise spectra were analyzed to estimate the energy resolution, and an excess noise factor was obtained from the comparison of theoretical and experimental data. A spectral resolution of 3.5 eV was achieved for 5.9 keV X-rays from a ⁵⁵Fe source. Cryogenic Detectors Superconductivity Transition Edge Sensors Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 1. Introduction Since K. D. Irwin proposed their use under voltage bias in 1995 [ 1 ], microcalorimeters based on Transition Edge Sensors (TES) have become one of the best radiation detectors available. Depending on the design and the absorber material used, they are able to detect individual photons across a wide range of frequencies, from microwaves to Gamma-rays, with exceptional spectral resolution [ 2 ]. Moreover, they exhibit extremely low dark count rates. TES devices are nowadays employed in a variety of fields, including astrophysics and materials science [ 3 ]. One notable upcoming application is the Athena mission [ 4 ], an X-ray space telescope to be launched by the European Space Agency (ESA) in the coming decade. Our group has been developing TES devices with the demanding Athena specifications in mind for more than a decade. This has also opened the possibility for new applications in the same energy range (synchrotron facilities and axion searches). In this work, we summarize the main milestones obtained up to now. Section 2 describes the detector fabrication process, Section 3 outlines the characterization setup, and Section 4 presents some results for a representative device. 2. Fabrication Four-inch silicon wafers with 500 nm thick low stress Si 3 N 4 layer in both sides are used as substrate. To avoid problems associated with Mo oxidation when the sample is transferred between the sputtering system and the e-beam during the fabrication process, our group developed the trilayer solution [ 5 ], combining magnetron sputtering and e-beam deposition. We use a magnetron AJA Orion 5 UHV sputtering system and an E-Beam PVD system Edwards 500 UHV. First, the 45nm of Molybdenum layer is deposited using Radio Frequency (RF) power supply from a 2-inch diameter Mo target of 99.95% purity. Then, a 15 nm protective Au layer is deposited by direct current (DC) power supply from a 2-inch diameter Au target of 99.99% purity. The argon working pressure is fixed to 3 mTorr. The deposition rates are 9 nm/min for Mo and 10 nm/min for Au. Afterwards, the wafer is taken out from the sputtering and loaded into the e-beam evaporation system, where 250 nm of high purity (99.99%) gold is evaporated (rate of 10 nm/min). These Mo and Au thicknesses determine a critical temperature close to 100 mK via the proximity effect [ 6 ]. After finishing the trilayer deposition, the Si 3 N 4 membranes on the back side of wafer are defined by using optical lithography (K.Suss MA6 aligner) with positive photoresist AZ ECI 3027. The size of the aperture in the Si 3 N 4 backside layer is calculated for anisotropic wet etching of (100) silicon in KOH solution to reach the correct size of membrane on the top side of the wafer. Then, Si 3 N 4 is etched away by reactive ion etching (RIE) with SF 6 plasma. Positive resin lithography (AZ ECI 3027) is used to define the area of the thermometer (Mo/Au). The combination of Ion Milling (IM) and RIE allows etching away the gold and partially the molybdenum layer, where approximately 10nm of Mo layer is left. After resin cleaning, 150 nm of Nb is deposited by DC sputtering with the rate 7.5 nm/min at a pressure 3 mTorr of Ar. Before Nb deposition the surface is cleaned by RF bias Ar plasma in the same chamber. This step allows to remove all surface contamination and produces a clean interface between Mo and Nb. Next, the Nb paths are defined with positive photoresist (AZ ECI 3027). The RIE (CF 6 plasma) is used to etch away the Nb and the rest of the Mo layer. After removal of the remaining photoresist, a two-level lithography process is carried out for the fabrication of a cantilevered X-ray absorber. First, the positive photoresist (AZ ECI 3027) is used to define 2.5µm stems that support the absorber on the Si 3 N 4 membrane, and produce a thermal contact point to the Mo/Au thermometer. Then, a Ti/Au (5/500 nm) seed layer is evaporated on, forming a bottom contact for plating. The area of absorber is defined by a second lithography using 7µm-thick positive photoresist (AZ XT10), being later exposed for plating. A homemade electrochemical cell (EC) supplied by an AUTOLAB PGSTAT12 potentiostat is used for gold plating. This method permits deposition of high purity gold (RRR = 35) of few micrometers thickness. Three-electrode system consists of a Platinum counter-electrode (CE), a Ag/AgCl reference electrode, and a working electrode (WE). The WE is the 4” wafer with the gold seed layer and chips distribution defined by lithography to open a total area of 10 cm 2 . A commercial solution, TECHNI GOLD 25 ES is used for Au plating. The 2.5 µm gold layer is deposited by applying a constant current mode with the solution heated to 55˚C. The current is set to 1.2 mA/cm 2 providing a deposition rate of 75 nm/min, that allows depositing a bright and flat Au gold layer. In some cases, to increase the quantum efficiency of the absorber, a Bi layer is deposited using another commercial plating solution, NB 100 (NB Technologies). A constant current of 2.5 mA/cm 2 at room temperature provides a deposition rate of approximately 80 nm/min. The typical Bi thickness achieved is between 3 to 5 µm. After cleaning the upper thick photoresist, the seed layer is etched off by IM. Finally, the bottom photoresist is dissolved and cleaned by acetone, isopropanol and ethanol bath. To release the Si 3 N 4 membranes, the Si substrate is wet etched selectively by 40% KOH solution at 80˚C. To protect the top side of the wafer, where the TES are, the wafer is hermetically closed into the PEEK wafer chuck, and only the backside of the wafer is exposed for wet etching. Up to 32 (10 x10 mm) chips are usually distributed in every 4” wafer. This allows for a wide variety of designs to be tested, from bilayer geometry or absorber coupling in single pixels, to arrays. Figure 1 shows a schematic cross-section of a typical single pixel. TES devices with different geometries, sizes, and absorber stem position have been designed and fabricated. An example of a representative TES, developed to achieve the best energy resolution in accordance with the specifications of the X-IFU instrument of the ATHENA mission, is presented in Fig. 2 . 3. Characterization setup Figure 3 shows a schematic of the circuit used for the TES. When a photon is absorbed, the TES heats up, and since it is biased within the superconducting transition, its resistance changes sharply. This leads to a change in the current flowing through the SQUID's pickup coil. The SQUID detects this variation and converts it into a voltage pulse. The SQUID used is a dual-channel model C6_X226LB provided by PTB (Berlin). It also includes the shunt resistor (R shunt ), which in our case is 2 mΩ. An encapsulated 55 Fe source, mounted directly above the TES, is eventually used as the X-ray source. The full setup is hosted in a Bluefors dilution refrigerator, achieving a base temperature below 10 mK. TESs are highly sensitive to magnetic fields. For this reason, the entire holder is magnetically shielded using a superconducting aluminum enclosure and a CRYOPERM shield. If any residual field remains, B res , it is compensated by using a superconducting Helmholtz coil placed at the sample holder, ensuring that the TES operates in a nearly zero-field environment. The residual field may vary between measurements, with typical values up to 10 µT. To estimate B res , we bias the TES within its transition region and sweep the applied magnetic field to maximize the SQUID output. To minimize external noise, the mains are filtered through an ultra-isolation transformer and an uninterruptible power supply (UPS). To avoid ground loops, optical fiber connections are used. A low-noise differential preamplifier (Stanford Research Systems SR560) is used to set the bandwidth and conditioning the signal from the SQUID. Finally, a 24-bit NI PXI-5922 digitizer is used to acquire voltage pulses generated by the arrival of X-ray photons at the detector. By using this setup, several measurements are performed to fully characterize the detector. I-V curves are acquired with a HP 3458A multimeter at different bath temperatures, T bath (Fig. 4 ), from which the TES thermal parameters are extracted (n, K, T 0 , and G) [ 7 ]. A Hewlett-Packard HP 3562A spectrum analyzer is used to perform complex impedance measurements [ 8 ] (Fig. 5 ). Based on a one thermal block model (1TB) [ 9 ], we estimate the electrical parameters: α, β (logarithmic derivatives of R(T)), the effective τ Ι , and heat capacity C. The noise performance parameters are also estimated [ 9 ]. 4. Results The characterization results of a representative TES are presented below. Assuming that in a 1TB model P bath = P joule , by fitting the set of I–V curves at different bath temperatures to the expression \(\:\) \(\:{P}_{bath}=K({T}^{n}-{\:T}_{bath}^{n})\) the thermal parameters of the TES (n, K, G, and T 0 ) are obtained, as summarized in Table 1 . The n value of 3.5, is in good agreement with the calculations reported by Anghel et at. [ 10 ] Table 1 Different TES parameters measured at 20% of Rn Lenght 80 um Width 40 um T 0 93 mK Rn 57 mΩ n 3,5 G 156 pW/K C 1.5 pJ/K α 300 β 4 L 57 nH τ Ι 145 us Electrical parameters of the TES (α, β, τ Ι and C) are derived by fitting the complex impedance data at different bias points to a 1TB model [ 9 ] (seeTable 1). The TES energy resolution can be estimated from the experimental noise spectrum (ΔE exp ) and also from the theoretical noise spectrum (ΔE th ), both based on the noise model (Fig. 6 – 7 ). In both cases, the definition follows Eq. (97) from [ 9 ] although the integral bandwidth is restricted between 10 Hz and 100 KHz. $$\:{\varDelta\:E}_{FWHM}=\:\frac{2\sqrt{2Ln\left(2\right)}}{\sqrt{{\int\:}_{0}^{\infty\:}\frac{4}{{NEP\left(\omega\:\right)}^{2}}d\omega\:}}$$ From the difference between the theoretical and experimental noise, the excess noise parameter M 2 is estimated (Fig. 8 ). The figure shows the relationship between M 2 and α, which remains approximately constant [ 11 ] for this device. A detailed analysis of the relationship between M 2 and the logarithmic derivatives for different TESs is presented elsewhere. This relation implies that TES with a larger α do not necessarily achieve better energy resolution, as it is often associated with higher M 2 values. Therefore, a trade-off between both parameters is required. Finally, Fig. 9 shows a histogram from the acquisition of 1730 pulses, yielding a final spectral resolution (ΔE final ) of 3.5 eV for the Ka complex of the 55 Fe source (5.9 keV). The discrepancy between ΔE final and ΔE exp suggests that probably, the intrinsic resolution of the devices is already at the state of the art and that the measured resolution is being limited by a set-up contribution. We are working on improving temperature stability and vibration isolation to reduce this contribution. A detailed analysis of the effect of T bath on the resolution at low operating point will be presented elsewhere. 5. Conclusions and Outlook We are developing Mo/Au-based TES X-ray detectors, focusing on the specifications defined by ESA for the X-IFU instrument of the Athena mission. We have developed fabrication and full characterization setups, and have designed, fabricated and characterized many devices. In this paper we presented the results achieved so far. Energy resolutions of 3.5 eV have been obtained, possibly limited by experimental setup contributions. Further improvements are currently being implemented, including temperature stability better than 1 µK and a new anti-vibration system. These upgrades are expected to enable further enhancement of the achieved energy resolution. Although this manuscript presents only the results of a representative TES for X-IFU, different TES have been developed in order to perform a broad range of studies. For instance, in [ 12 ] the physical origin of the superconducting transition was investigated; in [ 13 ] the temperature and current sensitivities of bare Mo/Au TES were analyzed; and in [ 14 ] the proximity effect in Mo/Au TES was explored. Ongoing work is currently focused on the optimal choice of the operating point and on the influence of TES design on fundamental parameters and excess noise as well as on the development of TES for other applications. Declarations The authors have no relevant financial or non-financial interests to disclose The data that support these findings are available from the corresponding author upon reasonable request. Author Contribution A.C. wrote the main manuscript text and prepared figures 4-8. P.S. prepared figures 1-2. J.I.R. prepared figure 3. C. P. prepared figure 9. All authors reviewed the manuscript. Acknowledgments The Spanish ”Ministerio de Ciencia, Innovación y Universidades” supports and funds this works, MICIU/AEI/ 10.13039/501100011033 under Grants No. PID2021-122955OB-C42 (also funded by FEDER, UE), No. PID2024-155779OB-C32, No. CEX2023-001286-S (Severo Ochoa Centers of Excellence). We also acknowledge the Aragón Regional Government through project QMAD (E09 23R). LF acknowledges also Spanish MICINN for support through the Severo Ochoa Programme for Centres of Excellence (project MATRANS42, CEX2023-001263-S). We also acknowledge PTB (Berlin) for providing us with excellent SQUIDs, and Jörn Beyer for his valuable help and advice. Data Availability The data that support these findings are available from the corresponding author upon reasonable request. References K.D. Irwin, Appl. Phys. Lett. 66 (15), 1998 (1995) J.N. Ullom, D.A. Bennett, Supercond Sci. Technol. 28 no 8, 084003, (2015) Appl, Sci, (2021) https://doi.org/10.3390/app11093793 D. Barret, V. Albouys, J.-W. den Herder, L. Piro et al., Exp. Astron. (2023). https://doi.org/10.1007/s10686-022-09880-7 L. Fàbrega, I. Fernandez, O. Gil, M. Parra, A. Camón, R. Gonzalez, J. Sese, F. Briones, IEEE Trans. Appl. Supercon. 19 (3), 460 (2009) J.M. Martinis, G.C. Hilton, K.D. Irwin, Nucl. Instr Meth A, (444), 23 (2000) T. Saab, E. Apodaca, S. Bandler, K. Boyce, J.A. Chevernak, E. Figueroa, F.M. Finkbeiner, M.J. Li, C.A. Kibourne, Nucl. Instr Meth A, (520), 281 (2004) M.A. Lindeman, S. Bandler, R.G. Brekosly, J.A. Chevernak, E. Figueroa, F.M. Finkbeiner, M.J. Li, C.A. Kibourne, Rev. Sci. Instrum. 75 , 1283 (2004) K.D.Irwin and G.C.Hilton, in Enss C. (eds.) Cryogenic Particle Detection, Topics in Appl. Phys. 99, Springer, Berlin Heidelberg, pp 63–150 (2005) D.-V. Anghel, C. Caraiani, Y.M. Galperin, Phys. Scr. 94 (10), 105704 (2019) N.A. Wakeham, J.S. Adams, S.R. Bandler, S. Beaumont, J.A. Chervenak et al., J. Appl. Phys. (2019). https://doi.org/10.1063/1.5086045 L. Fàbrega, A. Camón, C. Pobes, P. Strichovanec, J. Appl. Phys. (2024). https://doi.org/10.1063/5.0225871 L. Fàbrega, A. Camón, C. Pobes, P. Strichovanec, J. Bolea, M. Mas, J. Low Tem Phys. (2022). https://doi.org/10.1007/s10909-022-02721-0 L. Fàbrega, A. Camón, C. Pobes, P. Strichovanec, IEEE Trans. Appl. Supercon. (2019). https://doi.org/10.1109/TASC.2019.2903994 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 07 Jan, 2026 Read the published version in Journal of Low Temperature Physics → Version 1 posted Editorial decision: Revision requested 18 Nov, 2025 Reviews received at journal 04 Nov, 2025 Reviews received at journal 09 Oct, 2025 Reviewers agreed at journal 06 Oct, 2025 Reviewers agreed at journal 02 Oct, 2025 Reviewers agreed at journal 01 Oct, 2025 Reviewers invited by journal 01 Oct, 2025 Editor assigned by journal 29 Sep, 2025 Submission checks completed at journal 26 Sep, 2025 First submitted to journal 24 Sep, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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16:09:34","extension":"html","order_by":22,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":52791,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7700799/v1/373f5982b95b5d965985273b.html"},{"id":93514620,"identity":"c1adab71-8f42-4bce-a792-f09f05edbb66","added_by":"auto","created_at":"2025-10-14 16:09:34","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":84922,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eSchematic cross-section of a TES with absorber (SP: sputtering, EV: e-beam evaporation, PL: plating)\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7700799/v1/db467bfe9f9b12e8d72db5e8.jpeg"},{"id":93514625,"identity":"e43ebeb5-99f6-46ed-a09c-0051430e45bc","added_by":"auto","created_at":"2025-10-14 16:09:34","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":51185,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eOptical micrographs of TES. (a) Top-view image of a TES showing the Au absorber (240 × 240 µm), two central stems tangent to the edge of the trilayer, and four outer stems laying on the membrane. Mo/Nb paths with a width of 15 µm can also be seen. (b) Bottom-view image of the same detector through the membrane, showing the TES sensor in addition to the absorber stems and the paths.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7700799/v1/39f098ea196d04467afb0d54.jpeg"},{"id":93515659,"identity":"c944c0bf-5c76-4531-8715-41e479030690","added_by":"auto","created_at":"2025-10-14 16:25:34","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":57375,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eScheme of the circuit used for the TES characterization.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-7700799/v1/25efbe565d89e5613ab68a09.png"},{"id":93514897,"identity":"85ccb137-64a2-4965-8f82-84d16f156021","added_by":"auto","created_at":"2025-10-14 16:17:34","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":97333,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eA set of I-V curves at different T\u003c/em\u003e\u003csub\u003e\u003cem\u003ebath\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e, from 40 mK to 150 mK (right to left)\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7700799/v1/9f8001546798349c850e1b9b.jpeg"},{"id":93514627,"identity":"c6ef73b8-dc44-4f55-a4a3-587c2d1d5286","added_by":"auto","created_at":"2025-10-14 16:09:34","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":195519,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cem\u003eA set of Z(\u003c/em\u003eω\u003cem\u003e) curves at different operation points at T\u003c/em\u003e\u003csub\u003e\u003cem\u003ebath\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e = 50 mK.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7700799/v1/22ccb64ecfff58c34096bb39.jpeg"},{"id":93514904,"identity":"2d344ebd-7cb5-4da0-87b9-a0492572bfac","added_by":"auto","created_at":"2025-10-14 16:17:34","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":176621,"visible":true,"origin":"","legend":"\u003cp\u003eExperimental noise spectrum of the TES, along with the different noise contributions calculated using the TES parameters obtained from I-V curves and complex impedance using a 1TB model. Their sum defines the theoretical noise model (cyan line).\u003c/p\u003e","description":"","filename":"floatimage6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7700799/v1/bae0d5204ca75abaf02b5f86.jpeg"},{"id":93514908,"identity":"a7a85d73-591b-4216-9af8-5d15a1cc72af","added_by":"auto","created_at":"2025-10-14 16:17:34","extension":"jpeg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":104315,"visible":true,"origin":"","legend":"\u003cp\u003eΔE\u003csub\u003eTh\u003c/sub\u003e and ΔE\u003csub\u003eexp\u003c/sub\u003e as a function of R/Rn. For a typical operating point of 0.15, the ΔE\u003csub\u003eTh\u003c/sub\u003e is approximately 1.5 eV, while the ΔE\u003csub\u003eexp\u003c/sub\u003e is around 2.7 eV and ΔE\u003csub\u003efinal\u003c/sub\u003e is 3.5 eV.\u003c/p\u003e","description":"","filename":"floatimage7.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7700799/v1/059a19175323679c2cffb2fe.jpeg"},{"id":93514633,"identity":"9efaac59-4b54-4e14-938b-b878c90acb71","added_by":"auto","created_at":"2025-10-14 16:09:34","extension":"jpeg","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":89741,"visible":true,"origin":"","legend":"\u003cp\u003eM\u003csup\u003e2\u003c/sup\u003e and α as a function of R/Rn\u003c/p\u003e","description":"","filename":"floatimage8.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7700799/v1/522c93e18dbc336276d882b9.jpeg"},{"id":93515956,"identity":"bf989839-f794-4616-ab85-7c7fc4f8ca66","added_by":"auto","created_at":"2025-10-14 16:33:34","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":56444,"visible":true,"origin":"","legend":"\u003cp\u003eSpectrum of \u003csup\u003e55\u003c/sup\u003eFe Ka lines obtained with the pixel in Fig. 2. at 15 % of Rn\u003c/p\u003e","description":"","filename":"floatimage9.png","url":"https://assets-eu.researchsquare.com/files/rs-7700799/v1/863f33f4379e3c131233de15.png"},{"id":100069373,"identity":"854584fb-a89f-4fde-a6b5-d82dce541d78","added_by":"auto","created_at":"2026-01-12 16:13:36","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1335482,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7700799/v1/a6761613-7acd-4af3-8896-b18e41575e14.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Development of cryogenic X-ray detectors based on Mo/Au TES","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eSince K. D. Irwin proposed their use under voltage bias in 1995 [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e], microcalorimeters based on Transition Edge Sensors (TES) have become one of the best radiation detectors available. Depending on the design and the absorber material used, they are able to detect individual photons across a wide range of frequencies, from microwaves to Gamma-rays, with exceptional spectral resolution [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. Moreover, they exhibit extremely low dark count rates. TES devices are nowadays employed in a variety of fields, including astrophysics and materials science [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. One notable upcoming application is the Athena mission [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e], an X-ray space telescope to be launched by the European Space Agency (ESA) in the coming decade.\u003c/p\u003e\u003cp\u003eOur group has been developing TES devices with the demanding Athena specifications in mind for more than a decade. This has also opened the possibility for new applications in the same energy range (synchrotron facilities and axion searches). In this work, we summarize the main milestones obtained up to now. Section \u003cspan refid=\"Sec2\" class=\"InternalRef\"\u003e2\u003c/span\u003e describes the detector fabrication process, Section \u003cspan refid=\"Sec3\" class=\"InternalRef\"\u003e3\u003c/span\u003e outlines the characterization setup, and Section \u003cspan refid=\"Sec4\" class=\"InternalRef\"\u003e4\u003c/span\u003e presents some results for a representative device.\u003c/p\u003e"},{"header":"2. Fabrication","content":"\u003cp\u003eFour-inch silicon wafers with 500 nm thick low stress Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e layer in both sides are used as substrate. To avoid problems associated with Mo oxidation when the sample is transferred between the sputtering system and the e-beam during the fabrication process, our group developed the trilayer solution [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], combining magnetron sputtering and e-beam deposition. We use a magnetron AJA Orion 5 UHV sputtering system and an E-Beam PVD system Edwards 500 UHV. First, the 45nm of Molybdenum layer is deposited using Radio Frequency (RF) power supply from a 2-inch diameter Mo target of 99.95% purity. Then, a 15 nm protective Au layer is deposited by direct current (DC) power supply from a 2-inch diameter Au target of 99.99% purity. The argon working pressure is fixed to 3 mTorr. The deposition rates are 9 nm/min for Mo and 10 nm/min for Au. Afterwards, the wafer is taken out from the sputtering and loaded into the e-beam evaporation system, where 250 nm of high purity (99.99%) gold is evaporated (rate of 10 nm/min). These Mo and Au thicknesses determine a critical temperature close to 100 mK via the proximity effect [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eAfter finishing the trilayer deposition, the Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e membranes on the back side of wafer are defined by using optical lithography (K.Suss MA6 aligner) with positive photoresist AZ ECI 3027. The size of the aperture in the Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e backside layer is calculated for anisotropic wet etching of (100) silicon in KOH solution to reach the correct size of membrane on the top side of the wafer. Then, Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e is etched away by reactive ion etching (RIE) with SF\u003csub\u003e6\u003c/sub\u003e plasma.\u003c/p\u003e\u003cp\u003ePositive resin lithography (AZ ECI 3027) is used to define the area of the thermometer (Mo/Au). The combination of Ion Milling (IM) and RIE allows etching away the gold and partially the molybdenum layer, where approximately 10nm of Mo layer is left. After resin cleaning, 150 nm of Nb is deposited by DC sputtering with the rate 7.5 nm/min at a pressure 3 mTorr of Ar. Before Nb deposition the surface is cleaned by RF bias Ar plasma in the same chamber. This step allows to remove all surface contamination and produces a clean interface between Mo and Nb. Next, the Nb paths are defined with positive photoresist (AZ ECI 3027). The RIE (CF\u003csub\u003e6\u003c/sub\u003e plasma) is used to etch away the Nb and the rest of the Mo layer.\u003c/p\u003e\u003cp\u003eAfter removal of the remaining photoresist, a two-level lithography process is carried out for the fabrication of a cantilevered X-ray absorber. First, the positive photoresist (AZ ECI 3027) is used to define 2.5\u0026micro;m stems that support the absorber on the Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e membrane, and produce a thermal contact point to the Mo/Au thermometer. Then, a Ti/Au (5/500 nm) seed layer is evaporated on, forming a bottom contact for plating. The area of absorber is defined by a second lithography using 7\u0026micro;m-thick positive photoresist (AZ XT10), being later exposed for plating.\u003c/p\u003e\u003cp\u003eA homemade electrochemical cell (EC) supplied by an AUTOLAB PGSTAT12 potentiostat is used for gold plating. This method permits deposition of high purity gold (RRR\u0026thinsp;=\u0026thinsp;35) of few micrometers thickness. Three-electrode system consists of a Platinum counter-electrode (CE), a Ag/AgCl reference electrode, and a working electrode (WE). The WE is the 4\u0026rdquo; wafer with the gold seed layer and chips distribution defined by lithography to open a total area of 10 cm\u003csup\u003e2\u003c/sup\u003e. A commercial solution, TECHNI GOLD 25 ES is used for Au plating. The 2.5 \u0026micro;m gold layer is deposited by applying a constant current mode with the solution heated to 55˚C. The current is set to 1.2 mA/cm\u003csup\u003e2\u003c/sup\u003e providing a deposition rate of 75 nm/min, that allows depositing a bright and flat Au gold layer. In some cases, to increase the quantum efficiency of the absorber, a Bi layer is deposited using another commercial plating solution, NB 100 (NB Technologies). A constant current of 2.5 mA/cm\u003csup\u003e2\u003c/sup\u003e at room temperature provides a deposition rate of approximately 80 nm/min. The typical Bi thickness achieved is between 3 to 5 \u0026micro;m.\u003c/p\u003e\u003cp\u003eAfter cleaning the upper thick photoresist, the seed layer is etched off by IM. Finally, the bottom photoresist is dissolved and cleaned by acetone, isopropanol and ethanol bath. To release the Si\u003csub\u003e3\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e membranes, the Si substrate is wet etched selectively by 40% KOH solution at 80˚C. To protect the top side of the wafer, where the TES are, the wafer is hermetically closed into the PEEK wafer chuck, and only the backside of the wafer is exposed for wet etching.\u003c/p\u003e\u003cp\u003eUp to 32 (10 x10 mm) chips are usually distributed in every 4\u0026rdquo; wafer. This allows for a wide variety of designs to be tested, from bilayer geometry or absorber coupling in single pixels, to arrays. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows a schematic cross-section of a typical single pixel.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eTES devices with different geometries, sizes, and absorber stem position have been designed and fabricated. An example of a representative TES, developed to achieve the best energy resolution in accordance with the specifications of the X-IFU instrument of the ATHENA mission, is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"3. Characterization setup","content":"\u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows a schematic of the circuit used for the TES. When a photon is absorbed, the TES heats up, and since it is biased within the superconducting transition, its resistance changes sharply. This leads to a change in the current flowing through the SQUID's pickup coil. The SQUID detects this variation and converts it into a voltage pulse. The SQUID used is a dual-channel model C6_X226LB provided by PTB (Berlin). It also includes the shunt resistor (R\u003csub\u003eshunt\u003c/sub\u003e), which in our case is 2 mΩ. An encapsulated \u003csup\u003e55\u003c/sup\u003eFe source, mounted directly above the TES, is eventually used as the X-ray source. The full setup is hosted in a Bluefors dilution refrigerator, achieving a base temperature below 10 mK.\u003c/p\u003e\u003cp\u003eTESs are highly sensitive to magnetic fields. For this reason, the entire holder is magnetically shielded using a superconducting aluminum enclosure and a CRYOPERM shield. If any residual field remains, B\u003csub\u003eres\u003c/sub\u003e, it is compensated by using a superconducting Helmholtz coil placed at the sample holder, ensuring that the TES operates in a nearly zero-field environment. The residual field may vary between measurements, with typical values up to 10 \u0026micro;T. To estimate B\u003csub\u003eres\u003c/sub\u003e, we bias the TES within its transition region and sweep the applied magnetic field to maximize the SQUID output.\u003c/p\u003e\u003cp\u003eTo minimize external noise, the mains are filtered through an ultra-isolation transformer and an uninterruptible power supply (UPS). To avoid ground loops, optical fiber connections are used. A low-noise differential preamplifier (Stanford Research Systems SR560) is used to set the bandwidth and conditioning the signal from the SQUID. Finally, a 24-bit NI PXI-5922 digitizer is used to acquire voltage pulses generated by the arrival of X-ray photons at the detector.\u003c/p\u003e\u003cp\u003eBy using this setup, several measurements are performed to fully characterize the detector. I-V curves are acquired with a HP 3458A multimeter at different bath temperatures, T\u003csub\u003ebath\u003c/sub\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), from which the TES thermal parameters are extracted (n, K, T\u003csub\u003e0\u003c/sub\u003e, and G) [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. A Hewlett-Packard HP 3562A spectrum analyzer is used to perform complex impedance measurements [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e] (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). Based on a one thermal block model (1TB) [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e], we estimate the electrical parameters: α, β (logarithmic derivatives of R(T)), the effective τ\u003csub\u003eΙ\u003c/sub\u003e, and heat capacity C. The noise performance parameters are also estimated [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"4. Results","content":"\u003cp\u003eThe characterization results of a representative TES are presented below. Assuming that in a 1TB model P\u003csub\u003ebath\u003c/sub\u003e = P\u003csub\u003ejoule\u003c/sub\u003e, by fitting the set of I\u0026ndash;V curves at different bath temperatures to the expression\u003c/p\u003e\u003cp\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\)\u003c/span\u003e\u003c/span\u003e\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{P}_{bath}=K({T}^{n}-{\\:T}_{bath}^{n})\\)\u003c/span\u003e\u003c/span\u003e\u003c/p\u003e\u003cp\u003ethe thermal parameters of the TES (n, K, G, and T\u003csub\u003e0\u003c/sub\u003e) are obtained, as summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The n value of 3.5, is in good agreement with the calculations reported by Anghel et at. [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eDifferent TES parameters measured at 20% of Rn\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"2\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLenght\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003e80 um\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eWidth\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e40 um\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eT\u003csub\u003e0\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e93 mK\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eRn\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e57 mΩ\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003en\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e3,5\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eG\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e156 pW/K\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eC\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e1.5 pJ/K\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eα\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e300\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eβ\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e4\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eL\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e57 nH\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eτ\u003csub\u003eΙ\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u003cp\u003e145 us\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eElectrical parameters of the TES (α, β, τ\u003csub\u003eΙ\u003c/sub\u003e and C) are derived by fitting the complex impedance data at different bias points to a 1TB model [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e] (seeTable 1).\u003c/p\u003e\u003cp\u003eThe TES energy resolution can be estimated from the experimental noise spectrum (ΔE\u003csub\u003eexp\u003c/sub\u003e) and also from the theoretical noise spectrum (ΔE\u003csub\u003eth\u003c/sub\u003e), both based on the noise model (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e\u0026ndash;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eIn both cases, the definition follows Eq.\u0026nbsp;(97) from [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e] although the integral bandwidth is restricted between 10 Hz and 100 KHz.\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:{\\varDelta\\:E}_{FWHM}=\\:\\frac{2\\sqrt{2Ln\\left(2\\right)}}{\\sqrt{{\\int\\:}_{0}^{\\infty\\:}\\frac{4}{{NEP\\left(\\omega\\:\\right)}^{2}}d\\omega\\:}}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eFrom the difference between the theoretical and experimental noise, the excess noise parameter M\u003csup\u003e2\u003c/sup\u003e is estimated (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e). The figure shows the relationship between M\u003csup\u003e2\u003c/sup\u003e and α, which remains approximately constant [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e] for this device. A detailed analysis of the relationship between M\u003csup\u003e2\u003c/sup\u003e and the logarithmic derivatives for different TESs is presented elsewhere. This relation implies that TES with a larger α do not necessarily achieve better energy resolution, as it is often associated with higher M\u003csup\u003e2\u003c/sup\u003e values. Therefore, a trade-off between both parameters is required.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFinally, Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e shows a histogram from the acquisition of 1730 pulses, yielding a final spectral resolution (ΔE\u003csub\u003efinal\u003c/sub\u003e) of 3.5 eV for the Ka complex of the \u003csup\u003e55\u003c/sup\u003eFe source (5.9 keV). The discrepancy between ΔE\u003csub\u003efinal\u003c/sub\u003e and ΔE\u003csub\u003eexp\u003c/sub\u003e suggests that probably, the intrinsic resolution of the devices is already at the state of the art and that the measured resolution is being limited by a set-up contribution. We are working on improving temperature stability and vibration isolation to reduce this contribution. A detailed analysis of the effect of T\u003csub\u003ebath\u003c/sub\u003e on the resolution at low operating point will be presented elsewhere.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"5. Conclusions and Outlook","content":"\u003cp\u003eWe are developing Mo/Au-based TES X-ray detectors, focusing on the specifications defined by ESA for the X-IFU instrument of the Athena mission. We have developed fabrication and full characterization setups, and have designed, fabricated and characterized many devices. In this paper we presented the results achieved so far. Energy resolutions of 3.5 eV have been obtained, possibly limited by experimental setup contributions. Further improvements are currently being implemented, including temperature stability better than 1 \u0026micro;K and a new anti-vibration system. These upgrades are expected to enable further enhancement of the achieved energy resolution.\u003c/p\u003e\u003cp\u003eAlthough this manuscript presents only the results of a representative TES for X-IFU, different TES have been developed in order to perform a broad range of studies. For instance, in [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] the physical origin of the superconducting transition was investigated; in [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] the temperature and current sensitivities of bare Mo/Au TES were analyzed; and in [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] the proximity effect in Mo/Au TES was explored.\u003c/p\u003e\u003cp\u003eOngoing work is currently focused on the optimal choice of the operating point and on the influence of TES design on fundamental parameters and excess noise as well as on the development of TES for other applications.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eThe authors have no relevant financial or non-financial interests to disclose\u003c/p\u003e\u003cp\u003eThe data that support these findings are available from the corresponding author upon reasonable request.\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eA.C. wrote the main manuscript text and prepared figures 4-8. P.S. prepared figures 1-2. J.I.R. prepared figure 3. C. P. prepared figure 9. All authors reviewed the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgments\u003c/h2\u003e\u003cp\u003eThe Spanish \u0026rdquo;Ministerio de Ciencia, Innovaci\u0026oacute;n y Universidades\u0026rdquo; supports and funds this works, MICIU/AEI/\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.13039/501100011033\u003c/span\u003e\u003cspan address=\"10.13039/501100011033\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e under Grants No. PID2021-122955OB-C42 (also funded by FEDER, UE), No. PID2024-155779OB-C32, No. CEX2023-001286-S (Severo Ochoa Centers of Excellence). We also acknowledge the Arag\u0026oacute;n Regional Government through project QMAD (E09 23R). LF acknowledges also Spanish MICINN for support through the Severo Ochoa Programme for Centres of Excellence (project MATRANS42, CEX2023-001263-S). We also acknowledge PTB (Berlin) for providing us with excellent SQUIDs, and J\u0026ouml;rn Beyer for his valuable help and advice.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe data that support these findings are available from the corresponding author upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eK.D. Irwin, Appl. Phys. Lett. \u003cb\u003e66\u003c/b\u003e(15), 1998 (1995)\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eJ.N. Ullom, D.A. Bennett, Supercond Sci. 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Scr. \u003cb\u003e94\u003c/b\u003e(10), 105704 (2019)\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eN.A. Wakeham, J.S. Adams, S.R. Bandler, S. Beaumont, J.A. Chervenak et al., J. Appl. Phys. (2019). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1063/1.5086045\u003c/span\u003e\u003cspan address=\"10.1063/1.5086045\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eL. F\u0026agrave;brega, A. Cam\u0026oacute;n, C. Pobes, P. Strichovanec, J. Appl. Phys. (2024). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1063/5.0225871\u003c/span\u003e\u003cspan address=\"10.1063/5.0225871\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eL. F\u0026agrave;brega, A. Cam\u0026oacute;n, C. Pobes, P. Strichovanec, J. Bolea, M. Mas, J. Low Tem Phys. (2022). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1007/s10909-022-02721-0\u003c/span\u003e\u003cspan address=\"10.1007/s10909-022-02721-0\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eL. F\u0026agrave;brega, A. Cam\u0026oacute;n, C. Pobes, P. Strichovanec, IEEE Trans. Appl. Supercon. (2019). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1109/TASC.2019.2903994\u003c/span\u003e\u003cspan address=\"10.1109/TASC.2019.2903994\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"journal-of-low-temperature-physics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jltp","sideBox":"Learn more about [Journal of Low Temperature Physics](http://link.springer.com/journal/10909)","snPcode":"10909","submissionUrl":"https://submission.nature.com/new-submission/10909/3","title":"Journal of Low Temperature Physics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Cryogenic Detectors, Superconductivity, Transition Edge Sensors","lastPublishedDoi":"10.21203/rs.3.rs-7700799/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7700799/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eTransition Edge Sensors (TES) have emerged as highly sensitive radiation detectors with applications ranging from astrophysics to materials science. In the framework of the Athena X-ray mission, we have developed Mo/Au-based TES devices designed to meet the specifications defined by ESA. The detectors were fabricated using a Mo/Au/Au trilayer process and characterized in a dilution refrigerator using a SQUID readout. From I\u0026ndash;V curves and complex impedance measurements, we extracted the electrothermal parameters of the TES, which are consistent with theoretical models. Noise spectra were analyzed to estimate the energy resolution, and an excess noise factor was obtained from the comparison of theoretical and experimental data. A spectral resolution of 3.5 eV was achieved for 5.9 keV X-rays from a ⁵⁵Fe source.\u003c/p\u003e","manuscriptTitle":"Development of cryogenic X-ray detectors based on Mo/Au TES","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-10-14 16:09:29","doi":"10.21203/rs.3.rs-7700799/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-11-18T08:42:45+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-11-04T18:22:44+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-10-09T11:04:54+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"76364194841523509577399462328895219764","date":"2025-10-06T14:28:11+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"294274617622323472487859167051456178513","date":"2025-10-02T15:38:22+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"175090709765117565896348509602004369111","date":"2025-10-01T13:37:33+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-10-01T13:05:03+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-09-29T13:22:43+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-09-26T04:20:29+00:00","index":"","fulltext":""},{"type":"submitted","content":"Journal of Low Temperature Physics","date":"2025-09-24T07:39:31+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"journal-of-low-temperature-physics","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"jltp","sideBox":"Learn more about [Journal of Low Temperature Physics](http://link.springer.com/journal/10909)","snPcode":"10909","submissionUrl":"https://submission.nature.com/new-submission/10909/3","title":"Journal of Low Temperature Physics","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"a128c1fb-b653-48a7-93df-fdc0125ba936","owner":[],"postedDate":"October 14th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-01-12T16:05:36+00:00","versionOfRecord":{"articleIdentity":"rs-7700799","link":"https://doi.org/10.1007/s10909-025-03361-9","journal":{"identity":"journal-of-low-temperature-physics","isVorOnly":false,"title":"Journal of Low Temperature Physics"},"publishedOn":"2026-01-07 15:58:57","publishedOnDateReadable":"January 7th, 2026"},"versionCreatedAt":"2025-10-14 16:09:29","video":"","vorDoi":"10.1007/s10909-025-03361-9","vorDoiUrl":"https://doi.org/10.1007/s10909-025-03361-9","workflowStages":[]},"version":"v1","identity":"rs-7700799","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7700799","identity":"rs-7700799","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}