# Microstructured magnetic calorimeter with meander-shaped pickup coil

A. Burck, Sebastian Kempf, S. Schäfer, Hannes Rotzinger, Matias Rodrigues, Thomas Wolf, Loredana Gastaldo, Andreas Fleischmann, Christian Enss

► To cite this version:

A. Burck, Sebastian Kempf, S. Schäfer, Hannes Rotzinger, Matias Rodrigues, et al., Microstructured magnetic calorimeter with meander-shaped pickup coil. Journal of Low Temperature Physics, 2008, 151 (1-2), pp.337-344. 10.1007/s10909-007-9659-4. cea-05472177v3

HAL Id: cea-05472177

https://cea.hal.science/cea-05472177v3

Submitted on 16 Feb 2026

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
# Microstructured Magnetic Calorimeter with Meander-Shaped Pickup Coil

A. Burck · S. Kempf · S. Schäfer · H. Rotzinger · M. Rodrigues · T. Wolf · L. Gastaldo · A. Fleischmann · C. Enss

## Abstract

In the last years metallic magnetic calorimeters (MMC) showed an energy resolution of a few eV for x-rays up to 10 keV. This makes MMCs a promising and powerful tool for many applications where photons or energetic massive particles have to be detected—like absolute activity measurements of radioactive isotopes, high resolution x-ray spectroscopy and x-ray fluorescence material analysis. However, in order to fulfill all requirements of these applications and to allow to reach the maximum resolving power a consequent micro-fabrication of the MMC detectors is needed. The micro-fabrication of metallic magnetic calorimeters requires reliable deposition and patterning processes for niobium structures with high critical currents and for paramagnetic sensors.

As one result of our advances in microstructuring a fully microfabricated MMC which consists of a meander shaped niobium thin film pickup coil and a 3 µm thick sputter deposited paramagnetic Au:Er temperature sensor will be presented. Deposition of energy in the paramagnetic sensor causes a rise in temperature and results in a change of magnetization, which is measured by a low noise high bandwidth dc-SQUID. The sputter deposited Au:Er films we report on are working well and show thermodynamic properties close to the ones known from bulk material down to temperatures of 45 mK.

**Keywords:** MMC · Metallic magnetic calorimeter · Low temperature detectors · Rare earth alloys · Gold-erbium alloy · X-ray spectroscopy

## 1 Introduction

It was shown [1] that metallic magnetic calorimeters with meander-shaped pickup coil have, depending on the application, many advantages compared to magnetic calorimeter which make use of a circular pickup loop. This gave rise to us to develop a metallic magnetic calorimeter with a meander shaped pickup coil. In order to allow to reach the maximum resolving power and to forward the development of MMC detector arrays a consequent micro-fabrication of the MMC detectors is needed. In the following we report on our first fully micro-fabricated detector. An overview of the micro-fabrication process and the process parameters will be given. Furthermore the results which were achieved with this detector will be discussed.

Figure 1 shows the design and coupling scheme of the detector. The detector consists of two meander-shaped Nb thin film pickup coils which are connected in parallel to the input coil of a current-sensor SQUID to form a gradiometric superconducting flux transformer. A Au:Er sensor is placed on top of the meander shaped pickup coil(s). In order to produce a field around the meandering wire of the pickup loop within the volume of the sensor a large persistent current is injected into the superconducting loop formed by the two meanders. The deposition of energy in one of the sensors causes a small temperature increase in the sensor. Consequently the magnetization of the sensor material decreases. The conservation of flux in the superconducting circuits of the flux transformer—consisting of the two meander shaped pickup coils and the input coil of the SQUID—leads to a small current change $dI$ in the input coil which induces a flux change in the SQUID.

## 2 Experimental Details

Figure 2 shows a photograph and the corresponding schematic drawing of the meander-shaped pickup coils. In the picture the meander-shaped pickup coil is partially covered by a toaster like structure. This structure consists of gold and is used for a better thermalization. The meandering pickup loop which is placed below each of the thermalization structures covers an area of 1 mm × 1 mm and is formed by 4.5 µm-wide thin film Nb wires with a 10 µm pitch. Because the scale of the picture prevents a clear visibility of these structures Fig. 3 shows a magnification of the area marked with white dashed lines. The critical current of the 4.5 µm wide and 400 nm thick Nb structures of the

<figure><img src="image_2.png" /><figcaption>**Fig. 1 (Color online) Readout scheme of the detector.** The increase of temperature upon a deposition of energy in the detector results in a magnetization change of the sensor material. Flux conservation causes a small current change $dI$ in the input coil which induces a flux change in the SQUID</figcaption></figure>
meander shaped pickup coil was measured to be 125 mA. This corresponds to a current density of 6.9 × 10⁶ A/cm² which agrees well with the data found in Huebener et al. [2]. The residual resistivity ratio of the films was about RRR = 5.8. The bond pads labeled by VS are needed to connect the pickup coil electrically to the input coil of the SQUID. In order to generate a magnetic field within the sensor the superconducting circuit is equipped with a persistent current switch that allows for large currents to be frozen in these meanders. The bond pads for the electrical connection of the heater and the field lines with the corresponding leads are labeled by IH and IF, respectively.

<figure><img src="image_3.png" /><figcaption>**Fig. 2** (Color online) Photograph and the corresponding schematic drawing of the meander shaped pickup coil. A thermalization structure is placed on top of the pickup coil (only shown in the photograph). The magnetic field is generated by a persistent current. Therefore the meander is equipped with a persistent current switch ($I_H$: heater, $I_F$: field lines)</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Fig. 3** Zoom in the marked region in Fig. 2. A pad, the thermalization structure and the meandering 4.5 µm wide Nb stripes of the pickup loop are shown. A 3 µm thick Au:Er film is placed on top of the pickup loop which serves as sensor (not present in Fig. 2)</figcaption></figure>

To produce the Niobium structures firstly a 400 nm thick Niobium layer is sputtered on a Saphir wafer. The sputtered films are made by magnetron dc-sputtering of a pure Nb target. Typically deposition rates are 47 nm/min in 3 × 10⁻⁴ mbar of Ar (6.0 N) after pumping to a base pressure of 5 × 10⁻⁸ mbar. The Nb structures are then patterned by a lithographical wet etching process where a solution of 1 part HF, 6 parts HNO₃ and 10 parts H₂O is used to etch the niobium. Hereby the photoresist AZ 5214E [3] protects the structures which in the end should remain.

In order to electrically isolate the Nb structures an anodization process is used to oxidize the Niobium. The used fluid consists of Amonium Pentaborat (39 g), H₂O (190 ml) and Ethylen Glycol (280 ml). By applying a voltage between the meander and a platin electrode in the fluid the anode (meander) is oxidized. Thereby the surface of the anode is transformed into Nb₂O₅. The resulting oxide layer is about 30–50 nm thick and has a resistance of about 30 kΩ. Additionally a layer of SiOx is RF-sputtered and structured by a lift-off process on top of the anodized Niobium.

A 250 nm thick Au structure which later serves as heater and thermalization layer is produced by a standard lift-off process. Below the gold a 50 nm thick sticking layer of Cu is used. Both films are DC-magnetron-sputtered.

Figure 3 shows a REM picture of a MMC chip at a later stage of fabrication. The displayed area corresponds to the area of Fig. 2 marked by white dashed lines. In addition to the structures of Fig. 2 one can see a part of a 3 µm thick sputterdeposited Au:Er sensor. The sensor is structured by a standard lift-off process using the positive image reversal resist Ti xLift [4], which can be processed several µm thick and allows for edges with reasonable undercut. After patterning the photoresist the Au:Er films are dc-magnetron-sputtered. The used target
consists of Au doped with 820 ppm of natural, i.e. isotopically non-enriched, erbium. Typical deposition rates are 90 nm/min in 4 × 10⁻³ mbar of Ar (6.0 N) after pumping to a base pressure of 5 × 10⁻⁸ mbar.

## 3 Results

For the characterization of the sputtered Au:Er-films the magnetization M of both, the film and a small bulk piece of the target material, was measured in the temperature range from 2 K to 300 K with a commercial SQUID magnetometer system.

<figure><img src="image_5.png" /><figcaption>**Fig. 4** (Color online) Magnetization M vs. 1/T for bulk Au:Er (≈ 820 ppm) and magnetron dc sputtered Au:Er film (≈ 780 ppm) in the range of 2–300 K. In both cases the Er is isotopically not enriched. The measurement of the two samples was done with a commercial SQUID magnetometer system</figcaption></figure>

Figure 4 shows the zero-field cooled magnetization M of both samples in a magnetic field of 1 T as a function of the inverse temperature (symbols). The solid lines in both plots are fits to the data consisting of a temperature independent offset to represent the diamagnetic behaviour of the host material and a temperature dependent contribution, being proportional to the erbium concentration, to represent the paramagnetic behavior of Er in Au, as it can be derived from the crystal field parameters of Hahn et al. [5] following the formalism discussed in [6–8] including an appropriate powder average to take care of the poly-crystalline state of the material. Both, the dia magnetic offset $\chi^{\text{exp}}$ and the erbium concentration are free parameters of the fit. For both samples only data at temperatures above 60 K was used for the fit, as indicated in the plots, and still we can find very good agreement down to 2 K. We interpret this result as a convincing proof, that the majority of the Er ions are indeed located at regular lattice sites of the gold host material, as expected for this type of dilute alloy. Our second conclusion from this measurement is, that the Er concentration of sputter deposited Au:Er thin films is only slightly smaller than the concentration of the target material.

<figure><img src="image_6.png" /><figcaption>**Fig. 5** (Color online) (a) Magnetization M vs. 1/T of a magnetron dc sputtered Au:Er film for two different field currents between 30 mK and a few hundred mK. The 3 µm thick film covers one of the pickup loops of our first fully microstructured meander shaped detectors. The magnetization was measured by the MMC itself. The data (solid lines) show an excellent agreement with the numerical calculation (dashed lines) down to temperatures of 45 mK. The deviation at lower temperatures is due to the weak coupling to the heat bath combined with the high rate of the used X-ray source. (b) Corresponding temperature of the chip vs. the temperature of the bath</figcaption></figure>

Figure 5a shows the temperature dependence of the magnetic flux in the primary SQUID of the detector setup discussed above, that is caused by the magnetization of the 3 µm thick Au:Er film sputtered on the meander-shaped pickup coil. By means of the persistent current switch two different currents, 40 mA and 55 mA, were frozen in the meander in order to generate a magnetic field in the sensor volume.
Beside the measured data (solid lines) the expected flux change for a 775 ppm Au:Er sen- sor (dashed lines) are shown. Both, the film discussed here and the film discussed above, were produced in the same sputtering process and should have the same Erbium concentration. In the calculation of the theoretical curves we took the hyperfine splitting of the naturally abundant isotope 167Er into account. Down to temperatures of about 45 mK the data for both currents agree well with the numerical calculation. For lower temperatures the measured magnetization becomes more flat than the numerical curves.

We assume that this is a sign for the detector chip temperature TChip not following the temperature Tbath of the cold stage of the ADR
cryostat at lowest temperatures due to insufficient heat sinking of the connected wires. There might also be an additional direct eddy
current heat load on the sensor due to the backaction of the SQUID. However, we can not exclude at the moment, that the observed
flattening of the magnetization indeed represents a degraded paramagnetic behavior of the sputter-deposited Au:Er. In the case of the
latter interpretation being correct, further improvement of the sputter-deposition process will be required. For further characterization we
exposed the detector to x-ray fluorescence photons of manganese from an 55Fe-source. The detector was operated at 30 mK and with a
field current of 55 mA corresponding to an average field of 3 mT and a heat capacity of 0.4 nJ/K. The rate of detected x-rays was about 15
events/s. The thermal decay time of the signals was approximately 80 ms, resulting from the poor phononic thermal link to the bath,
through the sapphire substrate of the detector chip.

<figure><img src="image_7.png" /><figcaption>**Fig. 6** (Color online) (a) X-ray fluorescence spectrum of <sup>55</sup>Mn. The solid line represents a fit to the data with a FWHM linewidth of 162 eV. The corresponding baseline distribution has a FWHM linewidth of 102 eV. (b) Measured (symbols) and the numerically calculated (lines) detector response δΦ<sub>S</sub>/δE vs. temperature for two different field currents of 40 mA and 55 mA</figcaption></figure>

Figure 6a shows the pulse height spectrum of about 18000 events. The energy scale was calibrated based on the pulse height of the events of the Kα-line. The shape of the measured Kα-line can be described by a FWHM linewidth of 162 eV. This value is 60% larger than the linewidth ΔE<sub>FWHM</sub> = 102 eV that we derived from acquired baseline traces. The difference between the two values can be due to a number of reasons, among them: (i) The area of the sensor being somewhat larger than the one of the meander-shaped pickup coil. This might lead to a position sensitivity of the detector, because events that are stopped along the edge of the sensor would lead to a reduced signal size. This effect might also explain the tails of the lines in the spectrum towards lower energy. (ii) Varying loss of high frequency phonons that are generated in an early phase of the downconversion of the deposited energy. (iii) A variation of the temperature of the coldstage of the ADR, which was not stabilized during this measurement.

The measured baseline width itself is about a factor of 4 larger than the expected ΔE<sub>FWHM</sub> ≈ 25 eV for the MMC discussed here. This discrepancy is almost entirely due to the enormous pile-up problem from which the experiment suffered.

Figure 6b shows the measured (symbols) and the numerically calculated (lines) detector response, i.e. the magnetic flux change in the SQUID per unit of deposited energy, as a function of sensor temperature for two different field currents of 40 mA and 55 mA, respectively. The temperature of the data below 45 mK was corrected according to the measured dc-magnetization discussed above. At high temperatures the measured detector response is about 30% smaller than the numerically calculated one. This discrepancy increases to almost 50% towards the lowest temperatures. In parts this difference is expected, because it is not trivial to include the hyperfine splitting of the Er isotope 167Er in the calculation in a proper way, and we believe that our calculation corresponds rather to a lower bound of the true heat capacity in this temperature range. A reasonable fraction of this discrepancy also results from the fact that the sensor area is somewhat larger than the meander, a detail not taken into account in the calculation. A third contribution to the observed discrepancy might be an increased specific heat of the sputter-deposited Au:Er material as compared to bulk Au:Er. If future experiments will support this latter interpretation, further improvement of the sputter-deposition process will be needed to increase the quality of the paramagnetic thin films.

4 Conclusions

We presented our process for the micro-fabrication of magnetic calorimeters based on meander-shaped pick-up coils and sputter-
deposited paramagnetic Au:Er sensors. We described the geometry of our first fully micro-fabricated detector and discussed the measured
thermodynamic properties of the sputter-deposited Au:Er material. In the temperature range from 300 K down to 45 mK the magnetization
of the sputtered film agrees well with the known behavior of bulk Au:Er. Concerning the heat capacity of the sputtered film, the result is
not as conclusive as in the case of the magnetization, due to the geometry of the film under investigation and uncertainties in the
numerical calculation of the heat capacity of interacting Er ions including the hyperfine splitting of 167Er. Within the corresponding
uncertainty we can conclude that the observed heat capacity is less than 30% larger than the one expected. Both results are a clear
indication that MMCs can be successfully micro-fabricated by standard micro-lithography techniques, a necessary requirement on the way
towards medium and large scale arrays.

References

1. A. Fleischmann, C. Enss, G.M. Seidel, in Cryogenic Particle Detection, ed. by C. Enss. Topics in Applied Physics, vol. 99 (Springer, Berlin, 2005), pp. 151–216

2. R.P. Huebener, R.T. Kampwirth, R.L. Martin, J. Low Temp. Phys. 19(3/4) (1975)

3. The Clariant photoresist AZ 5214E is distributed by Microchemicals GmbH, www.microchemicals.eu (Ulm in Germany)

4. The photoresist TI xLift is distributed by Microchemicals GmbH, www.microchemicals.eu (Ulm in Germany)

5. W. Hahn, M. Loewenhaupt, B. Frick, Physica B 180/181, 176–178 (1992)

6. K.R. Lea, M.J.M. Leask, W.P. Wolf, J. Phys. Chem. Solids 23, 1381–1405 (1962)

7. Y. Ebina, Rep. Res. Inst. Electr. Commun. Tohoku Univ. 15, 47 (1963)

8. A. Fleischmann, J. Schönefeld, J. Sollner, C. Enss, J.S. Adams, S.R. Bandler, Y.H. Kim, G.M. Seidel, J. Low Temp. Phys. 118(1/2) (2000)