Thin-film tunnel junction detector based on aluminum-hafnium technology

Abstract

This utility model relates to instrumentation, measurement technology, and superconducting microelectronics, specifically to devices with thin-film tunnel junctions of the superconductor-insulator-normal metal (SIN) type, which can be used in cryogenic detectors designed to detect massive particles, X-ray photons, and microwave electromagnetic waves, for example, for studying the cosmic microwave background or searching for dark matter. The developed device with thin-film tunnel junctions includes a first hafnium film, a second iron film, and a third aluminum film, deposited by electron-beam evaporation normal to the sample surface. An aluminum oxide layer formed on the third film, and a top layer of two aluminum films deposited at angles of 45 and -45 degrees, respectively, to the sample normal.The introduction of a hafnium sublayer into the superconductor-insulator-normal metal structure allows for more efficient reduction of electron-phonon coupling, increasing detector sensitivity and reducing heat leakage from the nanoabsorber to the substrate. The aluminum oxide tunnel junction also becomes more uniform and thinner due to the higher density and homogeneity of the hafnium sublayer, enhancing the advantages of cold electron bolometers. 1 inc. ph., 5 fig.

Description

[0001] The utility model relates to the field of instrument making, measuring technology and superconducting microelectronics, in particular to devices with thin-film tunnel junctions of the superconductor-insulator-normal metal (SIN) type, which can be used in cryogenic detectors designed to register massive particles, X-ray photons, and microwave electromagnetic waves, for example, to study the cosmic microwave background or search for dark matter.

[0002] A well-known analogue of the developed technical solution is a device with thin-film tunnel junctions (patent RU 2733330 "Method for manufacturing devices with thin-film tunnel junctions", IPC H01L39 / 24, published October 1, 2020). The tunnel junction manufacturing technology used is based on the use of two resists with different sensitivities for exposure in an electron lithograph and the deposition of two layers of a normal metal or superconductor with intermediate oxidation to form a tunnel barrier.

[0003] However, such detectors lack a hafnium sublayer in the absorber structure, which deprives them of the following advantages. First, hafnium reduces electron-phonon coupling by a factor of three compared to aluminum. This reduces heat leakage from the nanoabsorber to the substrate, thereby increasing detector sensitivity. Second, hafnium has a higher density and smaller crystallite sizes, so the aluminum deposited on top of the hafnium layer has higher homogeneity.

[0004] A well-known prototype of the developed technical solution is a cold electron bolometer with thin-film superconducting junctions (patent RU 2442246 "Method for manufacturing devices with thin-film superconducting junctions," IPC H01L39 / 24, published February 10, 2012). These junctions utilize standard aluminum technology for producing thin-film tunnel SYN junctions, in which the absorbing structure consists of a bilayer of iron and aluminum. These SYN junctions are manufactured using a shadow sputtering method, which requires two resists with different sensitivities to form a cavity into which the material is deposited at an angle.

[0005] A thin iron layer deposited on top of hafnium suppresses the superconductivity of the underlying aluminum layer and has virtually no effect on the aluminum crystallite size when its evaporation rate is appropriately selected. Further oxidation of this aluminum layer results in the formation of a more uniform, thin, and dense oxide layer, which, in turn, reduces leakage current at ultra-low temperatures in the range of 10 to 100 mK. This improvement over the prototype is also relevant at temperatures of 250 and 300 mK, typical for single- and two-stage sorption 3He cryostats used in space environments, as it increases the efficiency of electron cooling.

[0006] The objective of the claimed solution is to develop a device based on tunnel SIN junctions with an absorber with an increased ratio of tunnel resistance Rj to normal resistance Rn, characterized by an increase in sensitivity at temperatures of 300 mK and below.

[0007] The technical result is achieved in that the developed device with thin-film tunnel junctions includes a first film of hafnium, a second film of iron, a third film of aluminum, applied by electron beam evaporation along the normal to the surface of the sample, a layer of aluminum oxide formed on the third film, as well as a top layer of two aluminum films applied at angles of 45 and -45 degrees, respectively, to the normal of the sample.

[0008] In a particular case of implementation, the first hafnium film has a thickness of 7.2 nm, the second iron film has a thickness of 1.2 nm, the third aluminum film has a thickness of 8.3 nm, before applying the top layer, oxidation was performed in oxygen for 10 minutes at a pressure of 49 Torr, and the top layer itself consists of two aluminum films with a thickness of 60 nm and 70 nm.

[0009] The utility model is explained by the following figures.

[0010] Fig. 1 shows a general view of a detecting element with bolometers based on a SIN junction.

[0011] Fig. 2 shows a bright-field TEM micrograph of the cross-section of layers of two samples.

[0012] Figure 3 shows the current-voltage characteristics (CVC) and differential resistance of the structure.

[0013] Figure 4 shows a graph of the electron temperature obtained by approximating the I–V characteristics of a bolometric structure.

[0014] Fig. 5 shows the volt-watt sensitivity SV and the noise equivalent power (NEP) of the bolometric structure at a cryostat temperature of 300 mK.

[0015] In the proposed utility model, during the production of SIN junctions, a layer of hafnium was applied as a sublayer under the bilayer of aluminum and iron.

[0016] To fabricate SIN junctions using this modified technology, methyl methacrylate (MMA) is first applied as the bottom resist and AR 600 as the top resist. Next, a resist mask is formed in an electron microscope with a laser interference stage. N-amyl acetate and MIBK:IPA 1:1 developers are then used for the top and bottom resists, respectively. The following films are then applied using electron beam evaporation: first, a hafnium film, then a second iron film, and then a third aluminum film. All of these are applied normal to the sample surface. The aluminum is then oxidized in oxygen. After the oxide layer is formed, two aluminum films are applied at angles of 45 and -45 degrees, respectively, to the sample normal. Lift-off lithography is then used to remove the unexposed resist, along with excess material from unnecessary areas.

[0017] At a temperature of 300 mK, for a signal at 14.5 GHz, a MES of less than 6 aW / √Hz was obtained with the following parameters for fabricating the SIN junction: the first film is 7.2 nm hafnium; the second film is 1.2 nm iron; the third film is 8.3 nm aluminum; oxidation in oxygen for 10 min at a pressure of 49 Torr; the top layer is two films of 60 and 70 nm aluminum.

[0018] Figure 1 shows a general view of a detecting element with bolometers based on a SIN junction (silver), manufactured using aluminum-hafnium technology. Also shown are the power and readout electrodes (gold).

[0019] Figure 2 shows a bright-field TEM micrograph of a cross-section of layers of a sample fabricated using a standard technology for fabricating bolometers based on a SIN junction according to a prototype method (left) and a modification of this technology using a hafnium sublayer (right).

[0020] In Fig. 3, the solid curves show the current-voltage characteristics (CVC) of the bolometric structure, measured at temperatures from 20 to 300 mK. The dotted curves show the differential resistance of the structure.

[0021] Figure 4 shows the solid curve of the electron temperature plot obtained by approximating the I-V characteristic of a bolometric structure. The dots represent the experimentally obtained I-V characteristic at a cryostat temperature of 300 mK, while the dashed curve represents the I-V characteristic obtained from a theoretical model used to reconstruct the dependence of the electron temperature on voltage.

[0022] Figure 5 shows the volt-watt sensitivity of the SV, which reaches 6×108 V / W, and the MESH of the bolometric structure at a cryostat temperature of 300 mK, obtained by approximating the I-V characteristics.

[0023] The cold electron bolometer based on the SIN junction utilizes decoupling of the electron and phonon subsystems, resulting in electron cooling. This decoupling results in the electron cooling effect, whereby heated, high-energy electrons are extracted from the non-superconducting nanoabsorber through the potential barrier of the tunnel junction. Only low-energy electrons, and therefore low temperatures, remain in the absorber. The phonon subsystem maintains its temperature, and cooling occurs specifically for the electronic subsystem of the sensing element. This ensures a rapid response to external signals and reduces the detector's internal noise. The introduction of a hafnium sublayer into the superconductor-insulator-normal metal structure allows for more effective reduction of electron-phonon coupling, increasing detector sensitivity and reducing heat leakage from the nanoabsorber to the substrate.Furthermore, compared to superconducting structures fabricated using the prototype method, the aluminum oxide tunnel junction becomes more uniform and thinner due to the higher density and homogeneity of the hafnium sublayer. This, in turn, enhances the advantages of cold electron bolometers.

Claims

1. A detector with thin-film tunnel junctions based on aluminum-hafnium technology, comprising a first film of hafnium, a second film of iron, a third film of aluminum, deposited by electron beam evaporation along the normal to the surface of the sample, a layer of aluminum oxide formed on the third film, and a top layer of two aluminum films deposited at angles of 45 and -45 degrees, respectively, to the normal to the sample.

2. A detector with thin-film tunnel junctions based on aluminum-hafnium technology according to claim 1, characterized in that the first hafnium film has a thickness of 7.2 nm, the second iron film has a thickness of 1.2 nm, the third aluminum film has a thickness of 8.3 nm, the aluminum oxide layer is formed by oxidizing the third aluminum film in oxygen for 10 minutes at a pressure of 49 Torr, and the top layer itself consists of two aluminum films with a thickness of 60 nm and 70 nm.

Images