Microstructured magnetic calorimeter with meander-shaped pickup coil

Transcription

1 Journal of Low Temperature Physics manuscript No. (will be inserted by the editor) A. Burck S. Kempf S. Schäfer H. Rotzinger M. Rodrigues T. Wolf L. Gastaldo A. Fleischmann C. Enss Microstructured magnetic calorimeter with meander-shaped pickup coil Received July 23, 27, Accpted October 5, 27 Keywords MMC, metallic magnetic calorimeter, low temperature detectors, rare earth alloys, gold-erbium alloy, x-ray spectroscopy Abstract In the last years metallic magnetic calorimeters (MMC) showed an energy resolution of a few ev for x-rays up to 1keV. 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 45mk. PACS numbers: 7.2.Mc, 7.85.Fv, 7.85.Nc, 29.3.Kv, Dq, Oj, g, 75.2.En A. Burck S. Kempf S. Schäfer T. Wolf L. Gastaldo A. Fleischmann C. Enss Kirchhoff Institute for Physics, University of Heidelberg, INF 227, 6912 Heidelberg, Germany H. Rotzinger present address: Code 662, NASA/GSFC and Brown University, Greenbelt, MD 2771, USA M. Rodrigues Laboratoire National Henri Becquerel, DRT/DETECS, Gif-sur-Yvette, CEA-Saclay, France Andreas.Burck@kip.uni-heidelberg.de

2 2 L s L i δi SQUID sensor L I + δi' L substrate 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 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.

3 3 Heater Tests Heater Tests Process Tests Persistent Current Switch Process Tests Bond Wire 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) 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 1mm 1mm and is formed by 4.5 µm-wide thin film Nb wires with a 1 µm pitch. Because the scale of the picture prevents a clear visibility of these structures figure 3 shows a magnification of the area marked with white dashed lines. The critical current of the 4.5 µm wide and 4nm thick Nb structures of the meander shaped pickup coil was measured to be 125 ma. This corresponds to a current density of A/cm 2 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 V S 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 theses meanders. The bond pads for the electrical connection of the heater and the field lines with the corresponding leads are labeled by I H and I F, respectively. To produce the Niobium structures firstly a 4nm 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 47nm/min in mbar of Ar (6.N) after pumping to a base pressure of mbar. The Nb structures are then patterned by a lithographical wet etching process where a solution of 1 part HF, 6 parts HNO 3 and 1 parts H 2 O is used to etch the niobium. Hereby the

4 4 thermalisation structure (Au) thermalisation structure (Au) (imprint) meander shaped pickup loop (Nb) bonding pad (Nb) sensor (Au:Er) Fig. 3 Zoom in the marked region in figure 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 figure 2). 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 (39g), H 2 O (19ml) and Ethylen Glycol (28ml). 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 Nb 2 O 5. The resulting oxide layer is about 3 5nm thick and has a resistance of about 3kΩ. Additionally a layer of SiO x is RF-sputtered and structured by a lift-off process on top of the anodized Niobium. A 25nm thick Au structure which later serves as heater and thermalization layer is produced by a standard lift-off process. Below the gold a 5nm 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 figure 2 marked by white dashed lines. In addition to the structures of figure 2 one can see a part of a 3 µm thick sputter-deposited 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 82 ppm of natural, i.e. isotopically non-enriched, erbium. Typical deposition rates are 9 nm/min in mbar of Ar (6.N) after pumping to a base pressure of 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 2K to 3K with a commercial SQUID magnetometer system.

5 5 15 Bulk Au:Er 15 Sputtered Au:Er Magnetization M [A/m] 1 5 H = 1. T/µ Conc. Er 3+ = 817. ppm Magnetization M [A/m] 1 5 H = 1. T/µ exp χ dia = lit χ dia = Conc. Er 3+ = ppm Fit range: 6 K - 33 K Fit range: 1 K - 33 K Inv. Temperature 1/T [1/K] Inv. Temperature 1/T [1/K] Fig. 4 (Color online) Magnetization M vs. 1/T for bulk Au:Er ( 82ppm) and magnetron dc sputtered Au:Er film ( 78ppm) in the range of 2 3K. In both cases the Er is isotopically not enriched. The measurement of the two samples was done with a commercial SQUID magnetometer system. 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 6,7 including an appropriate powder average to take care of the poly-crystalline state of the material. Both, the diamagnetic offset χ exp and the erbium concentration are free parameters of the fit. For both samples only data at temperatures above 6 K was used for the fit, as indicated in the plots, and still we can find very good agreement down to 2K. 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 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, 4 ma and 55mA, 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 775ppm Au:Er sensor (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 167 Er into account. Down to temperatures of about 45mK the data for both currents

6 6 Magnetic Flux Φ S [Φ ] a) 4 ma 55 ma T Chip [mk] b) I F =4mA Inv. Temperature 1/T [K -1 ] T bath [mk] Fig. 5 (Color online) a) Magnetization M vs. 1/T of a magnetron dc sputtered Au:Er film for two different field currents between 3mK 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 nummerical calculation (dashed lines) down to temperatures of 45mK. 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. 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 T Chip not following the temperature T bath 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 55 Fe-source. The detector was operated at 3mK and with a field current of 55 ma corresponding to an average field of 3 mt and a heat capacity of.4nj/k. The rate of detected x-rays was about 15 events/s. The thermal decay time of the signals was approximately 8ms, resulting from the poor phononic thermal link to the bath, through the sapphire substrate of the detector chip. Figure 6a shows the pulse height spectrum of about 18 events. The energy scale was calibrated based on the pulse height of the events of the K α -line. The shape of the measured K α -line can be described by a FWHM linewidth of 162eV. This value is 6% larger than the linewidth E FWHM = 12eV 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

7 Events / 9.4 ev Mn T=3mK I F =55mA E FWHM =162 ev K α δφ S /δe [Φ /kev] ma 55 ma 5 a) K β Energy [kev].5 b) Temperature T [mk] Fig. 6 (Color online) a) X-ray fluorescence spectrum of 55 Mn. The solid line represents a fit to the data with a FWHM linewidth of 162eV. The corresponding baseline distribution has a FWHM linewidth of 12eV. b) Measured (symbols) and the numerically calculated (lines) detector response δ φ S /δ E vs. temperature for two different field currents of 4mA and 55mA. 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 E FWHM 25eV 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 4 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 3% smaller than the numerically calculated one. This discrepancy increases to almost 5% towards the lowest temperatures. In parts this difference is expected, because it is not trivial to include the hyperfine spitting of the Er isotope 167 Er 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 sputterdeposited 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.

8 8 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 3 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 167 Er. Within the corresponding uncertainty we can conclude that the observed heat capacity is less than 3% 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, and G.M. Seidel, Cryogenic Particle Detection, Topics Appl. Phys. 99, , (25) 2. R.P. Huebener, R.T. Kampwirth, and R.L. Martin, J. Low Temp. Phys. 19, Nos. 3/4, (1975) 3. The Clariant photoresist AZ 5214E is distributed by Microchemicals GmbH ( Ulm in Germany) 4. The photoresist TI xlift is distributed by Microchemicals GmbH ( Ulm in Germany) 5. W. Hahn, M. Loewenhaupt, B. Frick, Physica B 18/181, (1992) 6. K.R. Lea, M.J.M. Leask, W.P. Wolf, J. Phys. Chem. Solids 23, (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, and G.M. Seidel, J. Low Temp. Phys. 118, Nos. 1/2, (2)