Superconductor junctions for solid-state coolers.

Abstract

The superconductor junction includes a normal metal layer having a first side and a second side, an insulating layer overlying the second side of the normal metal layer, and a first superconductor layer formed of a first superconductor material overlying a side of the insulating layer opposite the side overlying the normal metal layer. The superconductor junction further includes a second superconductor layer formed of a second superconductor material, the first side overlying a side of the first superconductor material opposite the side overlying the insulating layer. The second superconductor material has a higher diffusion coefficient than the first superconductor material and / or the second superconductor material has a lower recombination coefficient than the first superconductor metal layer. A normal metal layer quasiparticle trap is coupled to the second side of the second superconductor layer.

Description

[Technical field]

[0001] The present invention relates generally to superconductors, and more particularly to superconductor joints for solid-state coolers. [Background technology]

[0002] Solid-state electronic cooling by tunneling of "hot" electrons across normal metal-insulator-superconductor (NIS) junctions with a bias voltage has been demonstrated to operate below 1 K, essentially like the well-known and expensive room-temperature Peltier thermoelectric coolers. These NIS cryocoolers are built from the same materials and with the same lithographic fabrication foundry tools as the Josephson junctions used in superconducting circuits, and are essentially fully compatible with the Josephson junction components. They can be integrated with and fabricated simultaneously with the Josephson junction itself. However, the temperature throw of current NIS coolers is very limited, with a maximum temperature difference between the hot and cold sides of about 150 mK.

[0003] One of the main limitations to the full performance of NIS coolers is the presence of non-equilibrium quasiparticles in the superconducting leads resulting from the large currents flowing through the device. The low quasiparticle relaxation rate and thermal conductivity in superconductors bind these hot particles in the vicinity of the junction resulting in severe overheating in the superconducting electrodes. There are several methods to reduce the accumulation of quasiparticles in superconductors. The most common method is to use a normal metal bound to the superconductor called a quasiparticle trap, which allows the quasiparticles to transfer to the normal metal where they relax their energy through electron-electron and electron-phonon interactions. This device is called a normal metal-insulator-superconductor-normal metal (NISN) junction. However, the phonon heat generated in the quasiparticle trap can be transferred back to the superconducting electrodes, thus limiting the temperature difference between the hot and cold sides of a NISN junction solid-state cooler. Summary of the Invention

[0004] In one example, the superconductor junction includes a normal metal layer having a first side and a second side, an insulating layer overlying the second side of the normal metal layer, and a first superconductor layer formed of a first superconductor material overlying a side of the insulating layer opposite the side overlying the normal metal layer. The superconductor junction further includes a second superconductor layer formed of a second superconductor material, the first side overlying a side of the first superconductor material opposite the side overlying the insulating layer. The second superconductor material has a higher quasiparticle diffusion coefficient than the first superconductor material and / or the second superconductor material has a lower quasiparticle recombination coefficient than the first superconductor layer, the first superconductor layer having a thickness of about 1 coherence length to about 3 coherence lengths associated with the first superconductor material, and the second superconductor layer having a thickness of at least 4 coherence lengths associated with the second superconductor material. A normal metal layer quasiparticle trap is coupled to the second surface of the second superconductor layer.

[0005] In another example, a refrigerator device is disclosed that includes a pair of normal metal-insulator-superconductor-superconductor-normal metal (NIS1S2N) junctions, each of which includes a normal metal layer having a first side and a second side, an insulating layer overlying the second side of the normal metal layer, a first superconductor layer formed of a first superconductor material, the first superconductor layer having a first side overlying a side of the insulating layer opposite the side overlying the normal metal layer, and a second superconductor layer formed of a second superconductor material, the second superconductor layer having a first side overlying a side of the first superconductor material opposite the side overlying the insulating layer. The second superconductor material has a higher diffusion coefficient than the first superconductor material and / or the second superconductor material has a lower recombination coefficient than the first superconductor layer, the first superconductor layer having a thickness of about 1 coherence length to about 3 coherence lengths associated with the first superconductor material, and the second superconductor layer having a thickness of at least 4 coherence lengths associated with the second superconductor material. Each NIS1S2N junction further includes a normal metal layer quasiparticle trap coupled to the second surface of the second superconductor layer via a normal metal bump bond, and a bias voltage applied between the normal metal layer quasiparticle trap of a first NIS1S2N junction of the pair of NIS1S2N junctions and the normal metal layer quasiparticle trap of a second NIS1S2N junction of the pair of NIS1S2N junctions causes hot electrons to be removed from the normal metal layer, through the insulating layer of the second NIS1S2N junction, the first superconductor layer of the second NIS1S2N junction, the second superconductor layer of the second NIS1S2N junction, and to the normal metal layer quasiparticle trap of the second NIS1S2N junction.

[0006] In yet another example, a method of forming a superconductor junction is provided. The method includes providing a normal metal layer having a first side and a second side, disposing an insulating layer on the second side of the normal metal layer, disposing a first superconductor layer formed of a first superconductor material with its first side on a side of the insulating layer opposite the side overlying the normal metal layer, and disposing a second superconductor layer formed of a second superconductor material with its first side on a side of the first superconductor material opposite the side overlying the insulating layer. The second superconductor material has a higher diffusion coefficient than the first superconductor material and / or the second superconductor material has a lower recombination coefficient than the first superconductor material, the first superconductor layer having a thickness of about 1 coherence length to about 3 coherence lengths associated with the first superconductor material, and the second superconductor layer having a thickness of at least 4 coherence lengths associated with the second superconductor material. The method further includes bonding a normal metal layer quasiparticle trap to a second surface of the second superconductor layer with or without a normal metal bump bond. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is an exemplary schematic diagram of a cross-sectional side view of a superconductor junction. [Diagram 2] FIG. 2 is a graph illustrating the operating principle and net heat flow of the superconductor junction of FIG. [Diagram 3] FIG. 3 is a table of parameters for different superconductor pair combinations with a first superconductor layer acting as an energy filter and a second superconductor layer acting as a transport layer. [Figure 4] FIG. 4 is an exemplary schematic cross-sectional view of a solid-state cooler device. [Diagram 5] FIG. 5 is a schematic cross-sectional view of an example of a superconductor junction with a bump bond coupling a second superconductor layer to a normal metal layer quasiparticle trap. [Figure 6] FIG. 6 is a block diagram of a refrigeration system using solid state devices such as those of FIGS. [Figure 7]FIG. 7 illustrates a method for forming a superconductor junction. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] The present disclosure relates to a superconductor junction comprised of a normal metal-insulator-superconductor metal structure (NIS), where the superconductor metal structure is formed of a first superconductor layer adjacent to an insulator and a second superconductor layer adjacent to the first superconductor layer. A normal metal is a metal that does not superconduct at cryogenic operating temperatures. A normal metal trapping layer can be bonded to the second superconductor layer by a normal metal bump bond or placed on the second superconductor layer opposite the first superconductor layer to form a NIS1S2N junction. The normal metal trapping layer acts as a quasiparticle trap. A voltage can be applied to the NIS1S2N junction to raise some electrons above the Fermi level of the normal metal. These hot electrons (quasiparticles) tunnel across the insulator layer to the first superconductor layer and remove heat from the normal metal layer. Since the second superconductor layer has a smaller energy band gap than the first superconductor layer, the hot electrons easily transfer to the second superconductor layer. Finally, the hot electrons transfer to the quasiparticle trap in the normal metal layer on the hot side of the NIS1S2N junction. The reduction in the superconducting energy gap reduces the backflow of quasiparticles, while the use of the quasiparticle trap inhibits the quasiparticles from reintegrating into phonons while in the superconducting material.

[0009] However, as the quasiparticles pass through the superconductor layer, they can recombine into Cooper pairs, which generates phonons (heat) depending on the time it takes to pass through the superconductor material. Phonons are quantized lattice vibrations that carry heat in superconductors and partially in normal metals, which also carry heat by electrons. Whenever heat encounters a superconductor layer, it is carried as vibrations primarily by phonons. The vibrations have a dominant wavelength based on the particular material and its current temperature. Each superconductor material has a specific diffusion coefficient (cm) that defines the speed at which quasiparticles diffuse across the particular superconductor material. 2 / s) and the recombination coefficient (μm 3 / s) It is desirable to have a superconductor material with a high diffusion coefficient and a low recombination coefficient to minimize the recombination of quasiparticles into Cooper pairs.

[0010] However, another limitation is the cryogenic temperature (required for the junction to operate properly). That is, the superconductor acts as an energy filter for the proper operation of the NISN junction at half its transition temperature (the temperature required for superconductivity). The lower the cryogenic temperature, the more costly the refrigeration requirements to maintain the superconducting properties of the superconductor layer of the NISN junction. Unfortunately, certain superconductors with higher transition temperatures have higher recombination coefficients and / or lower diffusion coefficients. These issues are addressed by selecting a superconductor material for the second superconductor layer that has a relatively high diffusion coefficient and / or low recombination coefficient compared to the superconductor material of the first superconductor layer. That is, the diffusion coefficient of the second superconductor layer is higher than that of the first superconductor layer and / or the recombination coefficient of the second superconductor layer is lower than that of the first superconductor layer. This reduces the recombination of quasiparticles into Cooper pairs, which leads to the generation of heat in the superconductor material. The first superconductor layer acts as an energy filter and the second superconductor layer acts as a transport layer for the superconductor junction.

[0011] For example, niobium can be used to form NIS junctions with large thermal lift due to its large energy gap. However, niobium has a thermal lift of 8.7 cm. 2 / s and a diffusion constant of 311 μm 3 / s. This leads to unacceptably large quasiparticle losses when attempting to build junctions with diffusion paths longer than 0.1 μm. As an example, to maintain low quasiparticle losses, a second thick aluminum layer is deposited on top of the superconductor layer of a NIS junction, providing a layer with a relatively high diffusion rate and low recombination rate. Aluminum has a recombination constant of 122 cm 2 / s and 22 μm 3 / s recombination constant, allowing the construction of large junctions.

[0012] The superconductors can be close to each other; therefore, a thickness of more than four coherence lengths is usually desired to ensure that the layers function properly. There is also a thermal boundary resistance mismatch between the superconductors; this is tuned to block the dominant phonon wavelength, preventing phonons from the "hot" electron trap from flowing into the NIS junction. The dominant phonon wavelength is the wavelength at which, at any temperature, heat can modulate a single frequency, ν, of phonon vibrations. max The approximation is that the dominant phonon wavelength is then λ dom =ν / ν max where v is the speed of sound. Similar to optical fibers, by choosing a quarter-wavelength (or multiple) film thickness, one can set up a phonon filter where constructive interference in each layer minimizes the phonon heat flow.

[0013] In one example, superconductor junctions can be used to separate circuits that reside on different superconducting structures in a multi-chip device. The superconductor junctions can be a bump bond structure or part of a bump bond structure that joins two chips in a multi-chip device. In another example, the superconductor junctions can be used in solid-state cooler devices that can be used in refrigeration systems. The solid-state cooler devices can form the final cooling stage of multiple cooling stages to provide cooling to millikelvin temperatures.

[0014] FIG. 1 is an exemplary schematic diagram of a cross-sectional side view of a superconductor junction 10. The superconductor junction 10 can be configured as a refrigeration stage used in cryogenic cooling applications where the superconductor heat cooler is configured as a solid-state device that is one of multiple solid-state cooler devices arranged around a cryocontainer that resides in a vacuum and holds a superconducting circuit. The multiple solid-state devices can provide a final stage in a cryogenic refrigeration system, allowing efficient cooling by removing heat from the cold side of the refrigeration stage and preventing heat return from the hot side of the final refrigeration stage in the multiple refrigeration stages. Alternatively, the superconductor junction 10 can be used as a connection structure between two devices, where the connection structure is a multi-layer connector or bump bond structure.

[0015] The superconductor junction 10 is a NISN device with a superconductor portion that is a multi-layer superconductor structure 20 forming a NIS1S2N device. The superconductor junction 10 includes a normal metal layer 12 having a first surface disposed on a cold side of the superconductor junction 10. The normal metal layer 12 has a second surface separated from the multi-layer superconductor structure 20 by an insulator layer 14. A normal metal layer quasiparticle trap 22 has a first surface disposed on a second or hot side of the superconductor junction 10. The normal metal layer quasiparticle trap 22 has a second surface disposed on a surface of the multi-layer superconductor structure 20 opposite the surface disposed on the insulator layer 14. The multi-layer superconductor structure 20 includes a first superconductor layer 16 (S1) having a first energy band gap and a second superconductor layer 18 (S2) having a second energy band gap. The first energy band gap is larger than the second energy band gap.

[0016] The first superconductor layer 16 (S1) is formed of a first superconductor material and the second superconductor layer 18 (S2) is formed of a second superconductor material. The diffusion coefficient of the second superconductor material is selected to be larger than the diffusion coefficient of the first superconductor material (S1) and / or the recombination coefficient of the second superconductor material is selected to be smaller than the recombination coefficient of the first superconductor material. This allows for rapid diffusion of quasiparticles and / or reduces the recombination of quasiparticles into Cooper pairs, which leads to the generation of heat in the superconductor material as the quasiparticles pass through the multilayer superconductor structure 20.

[0017] In one example, the first superconductor layer 16 (S1) is 8.7 cm 2 / s and 311 μm 3 The second superconductor layer 18 (S2) is made of niobium (Nb) having a recombination coefficient of 122 cm 2 / s and 22 μm 3 In another example, the first superconductor layer 16 (S1) is made of aluminum (Al) having a recombination coefficient of 8.7 cm 2 / s and 311 μm 3 The second superconductor layer 18 (S2) is made of niobium (Nb) having a recombination coefficient of 371 cm 2 / s and 306 μm 3 In yet another example, the first superconductor layer 16 (S1) is made of tin (Sn) having a recombination coefficient of 371 cm 2 / s and 306 μm 3 The second superconductor layer 18 (S2) is made of tin (Sn) having a recombination coefficient of 122 cm 2 / s and 22 μm 3 The diffusion and recombination coefficients are formed of aluminum (Al) having a recombination coefficient of 1 / s. The diffusion and recombination coefficients mentioned above are merely examples and may vary based on the particular quality and process variations of the formation of the superconductor material.

[0018] A bias voltage (+V / -V) is applied between the normal metal layer 12 located on the cold side of the superconductor junction 10 and the normal metal layer quasiparticle trap 22 located on the hot side of the superconductor junction 10. The bias voltage (V+ / -V) raises the energy level of the hot electrons on the normal metal layer 12. FIG. 2 is a graph 30 illustrating the operating principle and net heat flow of the superconductor junction 10 of FIG. 1, where hot electrons in the normal metal layer 12 above the Fermi level tunnel across the insulator layer 14 into the first superconductor layer 16 (S1), thus removing heat from the normal metal layer 12. Since the second superconductor layer 18 (S2) has a smaller energy band gap than the first superconductor layer 16 (S1), the hot electrons easily move to the second superconductor layer 18 (S2). Finally, the hot electrons transfer to a normal metal layer quasiparticle trap 22 on the hot side of the superconductor junction 10. The reduction of the superconducting energy gap reduces the backflow of quasiparticles, while the use of a quasiparticle trap inhibits the quasiparticles from reintegrating into the superconducting material.

[0019] In one example, the first superconductor layer 16 (S1) has a capacitance of 2Δ=2.73×10 -3 The second superconductor layer 18 (S2) is made of niobium (Nb) having a superconducting energy band gap of 2Δ=3.4×10 eV. -4 In another example, the first superconductor layer 16 (S1) is made of aluminum (Al) having a superconducting energy band gap of 2Δ=2.72×10 eV. -3 The second superconductor layer 18 (S2) is made of niobium (Nb) having a superconducting energy band gap of 2Δ=11.5×10 eV. -4 In yet another example, the first superconductor layer 16 (S1) is made of tin (Sn) having a superconducting energy band gap of 2Δ=11.5×10 eV. -4 The second superconductor layer 18 (S2) is made of tin (Sn) having a superconducting energy band gap of 2Δ=3.4×10 eV. -4The superconducting material is made of aluminum (Al) having a superconducting energy bandgap of 100 eV. It should be understood that a variety of different superconducting material layers and different numbers of superconducting material layers can be used, so long as they are selected to have a superconducting energy bandgap that gradually decreases from the normal metal layer 12 to the normal metal layer quasiparticle trap 22.

[0020] The thickness (S1) of the first superconductor layer 16 is selected to be about 1 coherence length to about 3 coherence lengths to maximize diffusion and minimize recombination. This minimizes the travel distance of the quasiparticles, so a superconductor material with a higher transition temperature can be selected that can have a lower diffusion coefficient and / or a higher recombination rate. The second superconductor layer is selected to have a higher diffusion coefficient and / or a lower recombination coefficient. Additionally, the second superconductor layer 18 (S2) acts as a transport layer that does not affect the function of the NIS junction as the first superconductor layer does. Thus, the second superconductor layer 18 (S2) is not limited to operating at 1 / 2 its transition temperature, but can operate at or near its transition temperature. The thickness of the second superconductor layer 18 (S2) should be at least 4 coherence lengths so as not to inverse-proximitize the normal metal layer quasiparticle trap.

[0021] 3 is a table of parameters for different superconductor pair combinations with a first superconductor layer acting as an energy filter and a second superconductor layer acting as a transport layer. The first combination 42 includes niobium (Nb) as the first superconductor layer and aluminum (Al) as the second superconductor layer. Niobium (Nb) has a superconducting transition temperature of 9.2 Kelvin, a superconducting energy gap of 1.36 meV, and a lattice constant of 8.7 cm. 2 / s, and a diffusion coefficient of 311 μm 3 Aluminum has a superconducting transition temperature of 1.2 Kelvin, a superconducting energy gap of 0.172 meV, and a recombination coefficient of 122 cm 2 / s, and a diffusion coefficient of 22 μm 3The cryogenic temperature of the energy filter needs to be half the superconducting transition temperature for proper operation of the NISN junction. For niobium, this temperature is about 4.6 Kelvin. The cryogenic temperature of the transport layer can be its transition temperature. For aluminum, this temperature is about 1.2 Kelvin. Thus, for this superconductor pair, the cryogenic temperature needs to be 1.2 Kelvin to allow proper operation of the first and second superconductor layers.

[0022] The second combination includes niobium (Nb) as the first superconductor layer and tin (Sn) as the second superconductor layer. Tin has a superconducting transition temperature of 3.75 Kelvin, a superconducting energy gap of 0.57 meV, and a lattice constant of 371 cm. 2 / s, and 306 μm 3 The third combination includes tin (Sn) as the first superconductor layer and aluminum (Al) as the second superconductor layer. In this superconductor pair, the cryogenic temperature is 1 / 2 the transition temperature of tin (Sn) (Tc=1.75 Kelvin) and therefore needs to be about 0.875 Kelvin or less to allow proper operation of the first and second superconductor layers. As can be seen from all of these combinations, there is a trade-off between the required cryogenic temperature, the diffusion coefficient, and the recombination coefficient. Again, the diffusion coefficients and recombination coefficients mentioned above are merely examples and may vary based on the specific quality and process variations of the formation of the superconductor material.

[0023] FIG. 4 is a schematic cross-sectional view of an example of a solid-state refrigerator device 60 using a pair of superconductor junctions. The solid-state refrigerator device 60 can be configured as a cooling stage used in a cryogenic cooling application where the solid-state structure is one of multiple solid-state refrigerator devices arranged around a cooling vessel that resides in a vacuum and holds a superconducting circuit. The multiple solid-state structures can provide a final stage in a cryogenic refrigeration system and enable efficient cooling by removing heat from the cold side of the refrigeration stage and prevent heat return from the hot side of the final refrigeration stage in the multiple refrigeration stages. The solid-state refrigerator device can include a NIS1S2 junction or a NIS1S2N junction. The return of heat extracted from the cold side of the solid-state refrigerator device is mitigated by using a two-layer superconductor structure having superconductor layers with decreasing band gaps from the cold side to the hot side.

[0024] As shown in FIG. 4, the solid-state refrigerator device 60 includes a substrate 62 disposed on the hot side of the cooling stage. A first conductive pad 64 is disposed on the substrate 62, and a second conductive pad 66 is disposed on the substrate 62. The first conductive pad 64 is separated from the second conductive pad 66 by a gap 68. A first multilayer superconductor structure 75 includes a first surface (or cold side) configured to face the cold side of the cooling stage and a second surface (or hot side) configured to face the hot side of the cooling stage. The second surface of the first multilayer superconductor structure 75 is bonded to the first conductive pad 64 by a normal metal bump bond 65. A second multilayer superconductor structure 85 includes a first surface facing the cold side of the cooling stage and a second surface facing the hot side of the cooling stage. The second surface of the second multilayer superconductor structure 85 is bonded to the second conductive pad 66 by a normal metal bump bond 67.

[0025] The first multi-layer superconductor structure 75 includes a first superconductor layer 74 and a second superconductor layer 72. The second multi-layer superconductor structure 85 includes a third superconductor layer 84 and a fourth superconductor layer 82. The first superconductor layer 74 and the third superconductor layer 84 are formed of a first superconductor material having a first energy band gap, and the second superconductor layer 72 and the fourth superconductor layer 82 are formed of a second superconductor material having a second energy band gap. The first energy band gap is larger than the second energy band gap. The diffusion coefficient of the second superconductor material is selected to be greater than the diffusion coefficient of the first superconductor material and / or the recombination coefficient of the second superconductor material is selected to be less than the recombination coefficient of the first superconductor material to allow rapid diffusion of quasiparticles and / or reduce recombination of quasiparticles into Cooper pairs, which results in the generation of heat in the superconductor material.

[0026] The first insulating layer 76 is disposed between the surface of the first side of the first multi-layer superconductor structure 75 and the first end of the normal metal layer 78, and the second insulating layer 88 is disposed between the surface of the first side of the second multi-layer superconductor structure 85 and the second end of the normal metal layer 78. The normal metal layer 78 is a shared normal metal layer. A bias voltage is applied between the first conductive pad 64 and the second conductive pad 66 to remove hot electrons from the normal metal layer 78. The bias voltage increases the energy level of the hot electrons on the normal metal layer 78. The first conductive pad 64 and the second conductive pad 66 can be formed from a normal metal or a superconducting metal. When formed from a normal metal, the first conductive pad 64 and the second conductive pad 66 function as quasiparticle traps.

[0027] The thicknesses of the first superconductor layer 74 and the third superconductor layer 84 are selected to be between about one coherence length and about three coherence lengths to maximize diffusion and minimize recombination. The thicknesses of the second superconductor layer 72 and the fourth superconductor layer 82 should be at least four coherence lengths so as not to inverse-proximitize the normal metal layer quasiparticle traps. The normal metal layers and normal metal bump bonds can be formed of normal metals such as one or a combination of gold, titanium, chromium, platinum, or another metal alloy above its superconducting transition temperature. The superconductor material layers can be formed of niobium, tin, aluminum, or some other superconducting metal.

[0028] Quasi-particles (hot electrons) can be lowered in energy by inelastic collisions in the normal metal layer quasiparticle traps which generate phonons on the hot side of the NIS1S2N junction when necessary. These phonons tend to flow back from the normal metal layer quasiparticle traps to the superconductor structure of the NIS1S2N junction, resulting in the generation of heat in the superconductor material. Normal metal bump bonds, which bond the superconductor structure to the normal metal layer quasiparticle traps, alleviate this problem. The normal metal bump bonds form a point cooler that allows electrons to easily move into the normal metal layer quasiparticle traps, while the smaller cross-sectional area of ​​the normal metal bump bonds compared to the direct contact between the quasiparticle traps and the superconductor structure inhibits phonons from returning from the normal metal layer quasiparticle traps to the superconductor structure.

[0029] FIG. 5 is a cross-sectional view of an exemplary NIS1S2N junction 100 including a normal metal bump coupling a superconductor structure to a normal quasiparticle trap that can be used in the cryocooler of FIG. 4. The NIS1S2N junction 100 includes a normal metal layer 102 disposed on the cold side of a cryogenic stage. In this example, the normal metal layer 102 is formed of a titanium tungsten (TiW) alloy. A very thin and inverse proximity effect-subject aluminum layer 104 including an aluminum oxide portion 106 is disposed on the normal metal layer 102. The aluminum oxide portion 106 forms an insulator for the NIS1S2N junction 100. A first superconductor layer 108 is disposed on the aluminum oxide portion 106. In this example, the first superconductor layer 108 is formed of niobium (Nb). A second superconductor layer 110 is disposed on the first superconductor layer 108. In this example, the second superconductor layer 110 is formed of aluminum (Al). A normal metal layer quasiparticle trap 114 is disposed on top of the second superconductor layer 110 and is coupled to the second superconductor layer 110 via a normal metal bump bond 112. In this example, both the normal metal layer quasiparticle trap 114 and the normal metal bump bond 112 are formed of gold.

[0030] The normal metal bump bond 112 allows the electrons to easily move into the normal metal layer quasiparticle trap 114 while inhibiting the phonons from returning from the normal metal layer quasiparticle trap 114 to the second superconductor layer 110. As shown in FIG. 5, the hot electrons move as quasiparticles from the normal metal layer 102 through the aluminum layer 104, the aluminum oxide portion 106 (i.e., the aluminum oxide layer), through the first superconductor layer 108 and the second superconductor layer 110, through the normal metal bump bond 112 to the normal metal layer quasiparticle trap 114, and to the hot side of the cryogenic stage where the hot electrons can dissipate heat. Electrons that bind to phonons are inhibited from returning to the second superconductor layer 110 due to the small cross-sectional area of ​​the normal metal bump bond 112. The thickness of the first superconductor layer can be selected to be about one coherence length to about three coherence lengths to maximize diffusion and minimize recombination. The thickness of the second superconductor layer should be at least four coherence lengths so as not to proximitize quasiparticle traps in the normal metal layer.

[0031] FIG. 6 is a block diagram of a refrigeration system 140 using solid-state devices such as those of FIGS. 1, 4, and 5. The refrigeration system 140 includes multiple stages labeled stage #1 through stage #N, where N is an integer equal to or greater than 2. Each refrigeration stage provides an additional temperature drop from the previous stage, with the Nth stage being the final stage and providing the final temperature drop and lowest temperature of the refrigeration system 140. In other examples, the Nth stage is not the last stage, but is the first stage or an intermediate stage. Stage #N of the refrigeration system 140 includes a cryocontainer 150 with multiple solid-state devices 152 disposed therearound, similar to those shown in FIGS. 1, 4, or 5, that cooperate to provide a final lowest temperature of the refrigeration system 140 within the cryocontainer 150. The cryocontainer 150 is in a vacuum environment and can be configured to house a superconducting circuit. In another example, one or more of the other stages use solid-state devices similar to those of stage #N to provide a gradual temperature drop throughout the refrigeration system 140. In another example, the cryocontainer 150 may be formed of a normal metal that provides the final normal metal layer of each solid state device 152 .

[0032] FIG. 7 illustrates an example of a method 200 for providing a superconductor junction. At 210, a normal metal layer having a first side and a second side is provided. At 220, an insulating layer is disposed on the second side of the normal metal layer. The insulating layer can be the result of a deposition of an insulating material or a deposition of a metal layer and oxidation of the metal layer. At 230, a multi-layer superconductor structure is disposed on the insulating layer. The multi-layer superconductor structure has a first side overlying a side of the insulating layer opposite the side overlying the normal metal layer. The multi-layer superconductor structure includes a first superconductor layer formed of a first superconductor material and a second superconductor layer formed of a second superconductor material. The first superconductor layer has a first side disposed over the insulating layer and the second superconductor layer has a first side disposed over the second side of the first superconductor layer.

[0033] The first superconductor material is selected to have an energy band gap larger than that of the first superconductor material. The diffusion coefficient of the second superconductor material is selected to be larger than that of the first superconductor material, and / or the recombination coefficient of the second superconductor material is selected to be smaller than that of the first superconductor material to allow rapid diffusion of quasiparticles and / or reduce recombination of quasiparticles into Cooper pairs resulting in heat generation in the superconductor material. The thickness of the first superconductor layer can be selected to be about 1 coherence length to about 3 coherence lengths to maximize diffusion and minimize recombination. The thickness of the second superconductor layer should be at least 4 coherence lengths to not inverse-proximitize with the normal metal layer quasiparticle trap.

[0034] At 240, the normal metal layer quasiparticle trap is coupled to a second side of the multilayer superconductor structure, and thus to a second side of the second superconductor layer, either via a normal metal bump bond or by placing the normal metal layer quasiparticle trap directly on top of the second superconductor layer to form a superconductor junction. At 250, the superconductor junction is placed around a cryocontainer of a cryogenic refrigerator, for example, along with several other similar superconductor junctions. A bias voltage is then coupled between the normal metal layer and the normal metal layer quasiparticle trap to remove hot electrons from the normal metal layer through the insulating layer and the multilayer superconductor structure to the normal metal layer quasiparticle trap at 260.

[0035] For ease of explanation, the terms "overlay", "overlaying", "underlay" and "underlying" (and derivatives) are used throughout this disclosure to indicate the relative position of two adjacent surfaces in a selected orientation. Additionally, the terms "top" and "bottom" used throughout this disclosure indicate opposite surfaces in a selected orientation. Similarly, the terms "upper" and "lower" indicate relative positions in a selected orientation for purposes of explanation. Indeed, the examples used throughout this disclosure show one selected orientation. However, in the examples described, the selected orientation is arbitrary and other orientations (e.g., upside down, rotated 90 degrees, etc.) are possible within the scope of this disclosure.

[0036] The above are examples of the present disclosure. Of course, it is not possible to describe every conceivable combination of components or methods for purposes of describing the present disclosure, but one skilled in the art will recognize that many further combinations and permutations of the present disclosure are possible. Accordingly, the present disclosure is intended to embrace all such changes, modifications, and variations that fall within the spirit and scope of the appended claims. In addition, when the present disclosure or claims recite "a," "an," "a first," or "another" element, or the equivalent thereof, it should be construed as including one or more such elements, and does not require or exclude two or more such elements. Furthermore, to the extent that the term "includes" is used in either the detailed description or the claims, such term is intended to be inclusive, in the same manner as the term "comprising" is construed when employed as a transitional term in the claims. Finally, the term "based on" is construed to mean based at least in part on.

Claims

1. a normal metal layer having a first surface and a second surface; an insulating layer overlying the second surface of the normal metal layer; a first superconductor layer formed of a first superconductor material, the first superconductor layer having a first surface overlying a surface of the insulating layer opposite a surface overlying the normal metal layer; a second superconductor layer formed of a second superconductor material, the second superconductor layer having a first surface overlying a surface of the first superconductor material opposite a surface overlying the insulating layer, the second superconductor material having a higher diffusion coefficient than the first superconductor material and / or the second superconductor material having a lower recombination coefficient than the first superconductor layer, the first superconductor layer having a thickness of about 1 coherence length to about 3 coherence lengths associated with the first superconductor material, and the second superconductor layer having a thickness of at least 4 coherence lengths associated with the second superconductor material; a normal metal layer quasiparticle trap coupled to a second surface of the second superconductor layer; A superconductor junction comprising:

2. 2. The superconductor junction of claim 1, wherein the first superconductor layer has an energy bandgap greater than an energy bandgap of the second superconductor layer.

3. 2. The superconductor junction of claim 1, wherein a bias voltage applied between said normal metal layer and said normal metal layer quasiparticle trap removes hot electrons from said normal metal layer, through said insulating layer, said first superconductor layer, and said second superconductor layer, and into said normal metal layer quasiparticle trap.

4. 2. The superconductor junction of claim 1, further comprising a normal metal bump bond coupling the normal metal layer quasiparticle trap to the second surface of the second superconductor layer, the normal metal bump bond configured to allow quasiparticles to flow into the normal metal layer quasiparticle trap while blocking phonons from returning from the normal metal layer quasiparticle trap to the second superconductor layer.

5. 2. The superconductor junction of claim 1, wherein the first superconductor layer is formed of niobium (Nb) and the second superconductor layer is formed of aluminum (Al).

6. 2. The superconductor junction of claim 1, wherein the first superconductor layer is formed of niobium (Nb) and the second superconductor layer is formed of tin (Sn).

7. 2. The superconductor junction of claim 1, wherein the first superconductor layer is formed of tin (Sn) and the second superconductor layer is formed of aluminum (Al).

8. 10. The superconductor junction of claim 1, wherein the second superconductor layer has a film layer thickness that is a quarter wavelength or a multiple of a quarter wavelength of a dominant phonon associated with an operating temperature of the second superconductor material.

9. A solid-state cooler comprising the superconductor junction of claim 1.

10. 13. A refrigeration system comprising a plurality of refrigeration stages, at least one stage of the refrigeration system comprising a cryocontainer and a plurality of the superconductor junctions of claim 1 disposed about the cryocontainer.

11. 1. A cooler device comprising: Normal metals - insulators - superconductors - superconductors - normal metals (NIS １ S ２ N) pairs of junctions, each NIS １ S ２ The N junction is a normal metal layer having a first surface and a second surface; an insulating layer overlying the second surface of the normal metal layer; a first superconductor layer formed of a first superconductor material, the first superconductor layer having a first surface overlying a surface of the insulating layer opposite a surface overlying the normal metal layer; a second superconductor layer formed of a second superconductor material, the second superconductor layer having a first surface overlying a surface of the first superconductor material opposite a surface overlying the insulating layer, the second superconductor material having a higher diffusion coefficient than the first superconductor material and / or the second superconductor material having a lower recombination coefficient than the first superconductor layer, the first superconductor layer having a thickness of about 1 coherence length to about 3 coherence lengths associated with the first superconductor material, and the second superconductor layer having a thickness of at least 4 coherence lengths associated with the second superconductor material; a normal metal layer quasiparticle trap coupled to a second surface of the second superconductor layer via a normal metal bump bond; Including, The NIS １ S ２ The first NIS of the pair of N junctions １ S ２ The normal metal layer quasiparticle trap of the N junction and the NIS １ S ２ The second NIS of the N junctions １ S ２ A bias voltage is applied between the normal metal layer and the quasiparticle trap of the N junction, and hot electrons are transported from the normal metal layer to the second NIS １ S ２ The insulating layer of the N-junction, the second NIS １ S ２ The first superconductor layer of the N-junction, the second NIS １ S ２ Through the second superconductor layer of the N junction, the second NIS １ S ２ The normal metal layer of the N-junction is removed to trap quasiparticles.

12. The first superconductor layer is １ S ２ The first NIS of the pair of N junctions １ S ２ N-junction and the second NIS １ S ２ 12. The cooling device of claim 11, wherein both N-junctions have an energy band gap greater than the energy band gap of the second superconductor layer.

13. The first NIS １ S ２ The normal metal layer of the N-junction and the second NIS １ S ２ The cooler device of claim 11 , wherein the normal metal layer of an N-junction is a single shared normal metal layer.

14. The NIS １ S ２ The first NIS of the pair of N junctions １ S ２ N-junction and the second NIS １ S ２ 12. The cooling device of claim 11, wherein for at least one of the N-junctions, the first superconductor layer is formed of niobium (Nb) and the second superconductor layer is formed of aluminum (Al).

15. The NIS １ S ２ The first NIS of the pair of N junctions １ S ２ N-junction and the second NIS １ S ２ 12. The cooling device of claim 11, wherein for at least one of the N-junctions, the first superconductor layer is formed of niobium (Nb) and the second superconductor layer is formed of tin (Sn).

16. The NIS １ S ２ The first NIS of the pair of N junctions １ S ２ N-junction and the second NIS １ S ２ 12. The superconductor junction of claim 11, wherein for at least one of the N junctions, the first superconductor layer is formed of tin (Sn) and the second superconductor layer is formed of aluminum (Al).

17. 12. A refrigeration system comprising a plurality of refrigeration stages, at least one stage of the refrigeration system comprising a cryocontainer and a plurality of cooler devices according to claim 11 disposed about the cryocontainer.

18. 1. A method of forming a superconductor junction, comprising the steps of: providing a normal metal layer having a first surface and a second surface; disposing an insulating layer over the second surface of the normal metal layer; disposing a first superconductor layer formed of a first superconductor material having a first surface on a surface of the insulating layer opposite a surface overlying the normal metal layer; disposing a second superconductor layer formed of a second superconductor material having a first surface on a surface of the first superconductor layer opposite the surface overlying the insulating layer, the second superconductor material having a higher diffusion coefficient than the first superconductor material and / or the second superconductor material having a lower recombination coefficient than the first superconductor layer, the first superconductor layer having a thickness of about 1 coherence length to about 3 coherence lengths associated with the first superconductor material, and the second superconductor layer having a thickness of at least 4 coherence lengths associated with the second superconductor material; bonding the normal metal layer quasiparticle trap to a second surface of the second superconductor layer with or without a normal metal bump bond. A method comprising:

19. 20. The method of claim 18, further comprising coupling a bias voltage between the normal metal layer and the normal metal layer quasiparticle trap to remove hot electrons from the normal metal layer, through the insulating layer, the first superconductor layer, and the second superconductor layer, and into the normal metal layer quasiparticle trap.

20. 20. The method of claim 18, further comprising arranging the superconductor junction with a plurality of additional superconductor junctions about a cooling vessel to form a final stage of a cooling system.

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