Circuit, varistor device, and method of operating the circuit

Abstract

To provide an improved varistor device capable of operating at cryogenic temperatures.SOLUTION: A varistor device 10 for protecting an electronic circuit from voltage surges at cryogenic temperatures includes an electrical lead 2 constructed of a superinsulator material and an electrical contact element 4. The electrical contact element is for connection to the electronic circuit at different locations 6, 8 along the electrical lead and is in electrical contact with the electrical lead at different locations along the electrical lead. The electrical lead is adapted to provide a superinsulating or Cooper pair insulating state at cryogenic temperatures and to provide a non-linear resistance between the different locations at cryogenic temperatures.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to varistors capable of operating at cryogenic temperatures, such as superconducting coils or qubits, that employ superinsulators whose electrical conductivity becomes essentially zero at cryogenic temperatures. [Background technology]

[0002] An important application of varistors is to protect electronic devices from voltage fluctuations and the resulting undesirable high power consumption, e.g., due to power supply or data input. For this purpose, varistors are connected in parallel with the device to be protected. A varistor has a nonlinear electrical resistance. Ideally, it is essentially nonconductive as long as the transistor voltage is below a certain threshold voltage. Once the voltage exceeds the threshold voltage, the electrical resistance of an ideal varistor decreases significantly (i.e., nonlinearly), and the current flowing through the varistor increases with voltage. In this ideal situation, the parallel current path provided by the varistor reduces the voltage of the electronic device and the current flowing through it. Therefore, the nonlinear voltage-dependent resistance of the varistor allows for effective protection against voltage fluctuations.

[0003] Existing varistors, such as those based on semiconductors (e.g., diode-forming) or metal oxides that form internal diodes, can only operate at relatively high temperatures. Modern varistors are not suitable for use at the cryogenic temperatures at which devices such as superconducting coils or quantum bits operate. This is because current flowing through the varistor at voltages above the threshold voltage generates high heat. Another drawback of conventional varistors is the poor control of the value of their on-state electrical resistance. This on-state electrical resistance directly determines the current flowing through the varistor at voltages above the threshold voltage, and thus the amount of heating of the varistor. Therefore, poor control of the on-state electrical resistance leads to poor control of the heating of the varistor device. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] US Patent Application Publication No. 2021 / 068320 Summary of the Invention

[0005] In view of the above technical problems, there is a need for an improved varistor that can operate at cryogenic temperatures. This problem is solved by the varistor device as set forth in claim 1. Claim 15 provides a method for operating the varistor device.

[0006] Varistor devices employ superinsulating materials. Superinsulating materials are the flip side of superconductors; that is, materials that exhibit a superinsulating state with essentially infinite resistance at low, but finite, temperatures, and thus cannot pass electrical current. The transition from the superinsulating state to a state with greater resistance can be induced by a sufficient increase in temperature or the application of a sufficiently high voltage. The superinsulating state has been observed in titanium nitride, niobium nitride, and indium oxide films.

[0007] According to a first aspect, a varistor device for voltage surge protection of an electronic circuit at cryogenic temperatures comprises an electrical lead constructed of a super-insulating material and an electrical contact element for connecting different locations along the electrical lead to the electronic circuit, the electrical contact element being in electrical contact with the electrical lead at different locations along the electrical lead, the electrical lead adapted to provide a super-insulating or Cooper pair insulating state at cryogenic temperatures and to provide nonlinear resistance between the different locations at cryogenic temperatures.

[0008] Varistor devices using super-insulating materials offer strong nonlinear resistance at cryogenic temperatures, the absolute value of which is virtually zero. Therefore, varistor devices based on super-insulating materials are ideal for voltage surge protection electronic devices operating at cryogenic temperatures. At cryogenic temperatures, the varistor device itself has low power consumption and voltage noise levels, and current leakage is virtually zero. These are the main advantages of this varistor device over state-of-the-art varistors.

[0009] Furthermore, varistor devices using superinsulator materials provide a voltage-current characteristic with a sharp increase in threshold voltage (in other words, a step or jump, or a large nonlinearity, respectively). The steepness of the increase in threshold voltage or the magnitude of the nonlinearity determines how much excess voltage (and resulting current, and therefore resulting heating) is removed from the device being protected. The varistor device of the present invention removes more excess voltage than conventional varistor devices, providing improved voltage surge protection.

[0010] When the voltage applied to the varistor device exceeds a threshold voltage, the electrical resistance of the varistor device may exhibit ohmic behavior. The resistance in this state is also referred to as the on-state resistance. This is an advantage over prior art techniques in which the on-state resistance exhibits non-ohmic behavior and increases more strongly (e.g., exponentially, like a diode) with the voltage applied to the varistor, resulting in poor control of the on-state resistance. The on-state resistance of a varistor device according to the present invention can be easily controlled by forming the electrical leads with preselected dimensions according to the desired on-state electrical term.

[0011] The varistor device can be manufactured with a preselected threshold voltage by forming it with dimensions (ie, width and thickness of the electrical leads) that correspond to the preselected threshold voltage.

[0012] Additionally, the varistor device can be dimensioned (ie, width and thickness of the electrical leads) to exhibit a preselected response time.

[0013] The electrical contact element may include or consist of a superconducting material, which may be adapted to provide a superconducting state at cryogenic temperatures.

[0014] The corresponding electrical contact elements can provide minimal resistance at cryogenic temperatures for connecting electronic circuits to other electrical or electronic components, such as, for example, varistor devices and / or voltage supply devices.

[0015] The electronic circuit may include a superconducting component adapted to exhibit superconductivity at cryogenic temperatures, such as a superconducting coil, a superconducting qubit, a SQUID, or a superconducting single-electron transistor.

[0016] The corresponding electronic circuits typically operate at cryogenic temperatures. Such devices particularly require voltage surge protection that can function / operate even when cooled to cryogenic temperatures along with the electronic circuits. Conventional voltage surge protection devices typically operate at much higher temperatures, such as room temperature. The use of such conventional voltage surge protection devices requires a connection between the device and the electronic circuit to be protected, i.e., a connection between the electronic circuit operating at cryogenic temperatures and the device at high temperature (e.g., room temperature). Such a connection is undesirable, for example because it causes heat conduction, which can be avoided with the varistor device according to the present disclosure.

[0017] The electrical leads may be connected to an electronic circuit via electrical contact elements.

[0018] The electrical leads may be electrically connected in parallel to the electronic circuit.

[0019] The superinsulator material may be comprised of or may be a material from the group including titanium nitride, niobium titanium nitride, and indium oxide.

[0020] The superinsulator material may be comprised of or may be a material from the group including titanium nitride and niobium titanium nitride.

[0021] According to an embodiment, the thickness of the electrical leads does not exceed 10 nm. In such an embodiment, the superinsulator material may include or be titanium nitride or niobium titanium nitride or a combination thereof.

[0022] The superinsulator material may be titanium nitride and the thickness of the electrical leads may not exceed 5 nm.

[0023] The small thickness may help ensure that the electrical leads exhibit a super-insulating state at cryogenic temperatures.

[0024] Cryogenic temperatures may refer to temperatures below 1.5K, or below 0.5K, or below 0.15K, or below 0.05K.

[0025] The electronic circuitry and the electrical leads may be placed in thermal contact with the same cooling element, which may be adapted to provide cryogenic temperatures.

[0026] The electrical leads may be electrically connected to a reference potential at one of the different locations.

[0027] Alternatively, the electrical leads may be electrically connected to different reference potentials at different locations.

[0028] The length of the electrical leads between the different locations may be at least 1 mm.

[0029] Corresponding embodiments of the varistor device can provide maximum electrical resistance and minimize energy loss and dissipation. Leakage current through metallic black can be significantly less than 1 pA.

[0030] In some embodiments, the length of the electrical leads between different locations may be at least 0.01 mm, or may be 0.1 mm.

[0031] The length of the electrical leads between the different locations may be less than 1 mm.

[0032] A corresponding embodiment of the varistor device can minimize response time to voltage surges, providing a fast varistor, although leakage currents of approximately 1 pA can occur through the metallic black.

[0033] The superinsulator material may be obtainable by using etching, such as plasma etching, on a titanium nitride layer or a niobium titanium nitride layer.

[0034] The superinsulator material of the electrical leads may be in physical contact with the dielectric layer.

[0035] The dielectric layer may serve, for example, to electrically insulate the superinsulator material from nearby semiconducting or conductive materials.

[0036] The superinsulator material of the electrical lead may be in physical contact with a coolant-promoting layer. The growth-promoting layer may be adapted to promote high crystal growth of the superinsulator material. More specifically, the growth-promoting layer may be a dielectric layer, such as the dielectric layers described above.

[0037] The dielectric layer may be adapted to provide a growth promoting layer.

[0038] The growth enhancing layer may include or be a nitride, such as aluminum nitride.

[0039] More specifically, the inventors have experimentally verified that the use of an aluminum nitride growth-promoting layer can increase the critical temperature of the metal lead wire, thereby increasing the maximum temperature at which the varistor device can operate by approximately 10%.

[0040] Alternatively, the growth-enhancing layer or dielectric layer may comprise or be an oxide, such as silicon dioxide.

[0041] The dielectric layer and / or growth-enhancing layer may be disposed on a substrate, such as a silicon substrate.

[0042] A dielectric layer and / or a growth promoting layer may be disposed between the substrate and the superinsulator material.

[0043] According to one embodiment, the superinsulator material is titanium nitride, the substrate is a silicon substrate, and the varistor device comprises a dielectric layer composed of silicon dioxide. In this embodiment, the length of the electrical leads may exceed 1 millimeter.

[0044] According to another embodiment, the superinsulator material is titanium nitride, the substrate is a silicon substrate, and the varistor device comprises a growth promotion layer composed of aluminum nitride. In this embodiment, the length of the electrical leads may exceed 1 millimeter.

[0045] According to another embodiment, the superinsulator material is comprised of niobium titanium nitride, the substrate is a silicon substrate, and the varistor device includes a dielectric layer comprised of silicon dioxide. In this embodiment, the electrical leads may be greater than 1 millimeter in length.

[0046] According to another embodiment, the superinsulator material is comprised of niobium titanium nitride, the substrate is a silicon substrate, and the varistor device includes a growth promotion layer comprised of aluminum nitride. In this embodiment, the length of the electrical leads may exceed 1 millimeter.

[0047] According to another embodiment, the superinsulator material is comprised of titanium nitride and the varistor device includes a dielectric layer comprised of silicon dioxide. In this embodiment, the electrical leads may be less than 1 millimeter in length.

[0048] According to another embodiment, the superinsulator material is comprised of titanium nitride and the varistor device includes a growth promoting layer comprised of aluminum nitride. In this embodiment, the electrical leads may be less than 1 millimeter in length.

[0049] According to another embodiment, the superinsulator material is comprised of niobium titanium nitride and the varistor device includes a dielectric layer comprised of silicon dioxide. In this embodiment, the electrical leads may be less than 1 millimeter in length.

[0050] According to another embodiment, the superinsulator material is comprised of niobium titanium nitride and the varistor device includes a growth promotion layer comprised of aluminum nitride. In this embodiment, the electrical leads may be less than 1 millimeter in length.

[0051] Niobium titanium nitride has the chemical composition Nb x Ti y N, where x is in the range of 0.3 to 0.33 and y is in the range of 0.7 to 0.67.

[0052] A second aspect relates to a method of operating a varistor device to protect an electronic circuit from voltage surges at cryogenic temperatures, the varistor device including an electrical lead made of a superinsulator material and electrical contact elements connecting different locations along the electrical lead to the electronic circuit. The electrical contact elements electrically contact the electrical lead at different locations along the electrical lead. The method includes cooling the electrical lead to a cryogenic temperature such that the electrical lead becomes a superinsulating or Cooper pair insulating state, and providing a nonlinear resistance between the different locations at the cryogenic temperature.

[0053] A third aspect relates to a method of forming a varistor device for protecting an electronic circuit from voltage surges at cryogenic temperatures. The method may include forming an electrical lead from a superinsulator material. The method may further include forming electrical contact elements in electrical contact with the electrical lead at different locations along the electrical lead to connect the different locations along the electrical lead to the electronic circuit. The electrical lead may be formed to provide a superinsulating or Cooper pair insulating state at cryogenic temperatures and to provide nonlinear resistance between the different locations at cryogenic temperatures.

[0054] The method may further include providing a dielectric layer and / or a growth-promoting layer. Electrical leads may be formed on the dielectric layer and / or the growth-promoting layer.

[0055] The method may further include providing a substrate. A dielectric layer and / or a growth promoting layer may be provided on the substrate.

[0056] The electrical leads may more specifically include providing a layer of titanium nitride or niobium titanium nitride by atomic layer deposition.

[0057] Providing the titanium nitride or niobium titanium nitride layer may include depositing the titanium nitride or niobium titanium nitride on the dielectric layer and / or the growth promotion layer while maintaining the dielectric layer and / or the growth promotion layer at a temperature greater than 300 K, or greater than 310 K, or greater than 320 K, or greater than 330 K, or greater than 340 K. In some embodiments, the temperature is not greater than 400 K, or not greater than 390 K, or not greater than 380 K, or not greater than 360 K.

[0058] The method may include performing an etching process on the layer of titanium nitride or niobium titanium nitride.

[0059] The etching process may be carried out by plasma etching.

[0060] In some embodiments, the etching process is carried out for 1 second or more, or 3 seconds or more.

[0061] In some embodiments, the method includes lithographically patterning the metal leads to determine their width and / or length.

[0062] Forming the electrical contact element may include depositing a superconducting material, for example, using patterned deposition. [Brief explanation of the drawings]

[0063] The techniques of the present disclosure and their associated advantages will be best apparent from a description of exemplary embodiments taken in conjunction with the accompanying drawings. [Figure 1]FIG. 1 is a perspective view of a varistor device according to a first embodiment. [Figure 2] FIG. 2 shows the current-voltage characteristics of the varistor device at three different temperatures. [Figure 3a] FIG. 3a shows the current-voltage characteristics of the varistor device on a log-log scale when the magnetic field of the varistor device is zero or 1 T. [Figure 3b] FIG. 3b shows the differential conductance corresponding to the measurement of FIG. 3a. [Figure 4a] FIG. 4a shows the current-voltage characteristics of varistor devices of different lengths. [Figure 4b] FIG. 4b shows the threshold voltage determined from the current-voltage characteristics of FIG. 4a for varistor devices of different lengths. [Figure 5] FIG. 5 shows the time evolution of the current through the varistor device when different voltages (indicated by reference signs in mV) are applied to the varistor device at t≧0, where the applied voltage is zero for t<0. [Figure 6] FIG. 6 is a top view of a varistor device according to another embodiment. [Figure 7] FIG. 7 is a perspective view of a varistor device according to another embodiment. [Figure 8a] FIG. 8a shows the processing steps for manufacturing the varistor device. [Figure 8b] FIG. 8b illustrates another processing step for fabricating a varistor device. [Figure 9] FIG. 9 is a diagram illustrating the method of operation of the varistor device. DETAILED DESCRIPTION OF THE INVENTION

[0064] FIG. 1 is a schematic diagram of a varistor device 10 according to an embodiment.

[0065] The varistor device 10 comprises electrical leads 2 made of a super-insulating material.

[0066] A superinsulator is the flip side of a superconductor, a material that exhibits a super-insulating state at a low but finite temperature. In this super-insulating state, the superinsulator has essentially infinite resistance and therefore no electrical current can pass through it. Super-insulating films include titanium nitride, niobium titanium nitride, and indium oxide.

[0067] The electrical leads 2 of the varistor device 10 of FIG. 1 are constructed of titanium nitride, although other known superinsulators such as niobium titanium nitride, indium oxide, or an aluminum layer as described in US Pat. No. 5,649,292 may be used instead.

[0068] The varistor device 10 further comprises electrical contact elements 4 that make electrical contact with the electrical leads at different locations 6, 8 along the electrical leads.

[0069] The electrical contact elements 4 allow the electrical leads 2 to be connected to an electronic circuit at different locations 6,8.

[0070] The electrical contact elements 4 are each made of a conductive or metallic material, and according to some embodiments are made of a superconducting material such as aluminum or niobium.

[0071] The varistor device 10 is typically connected in parallel to an electronic circuit for cryogenic operation using electrical contact elements 4. This is typically an electronic circuit containing superconducting components such as superconducting coils, superconducting qubits, SQUIDs, or superconducting single-electron transistors.

[0072] Figure 2 shows the current-voltage characteristics of a varistor device 10 similar to that of Figure 1. More specifically, the varistor device 10 on which the measurements of Figure 2 were taken is equipped with an electrical lead 2 made of titanium nitride having a thickness T of 5 nm, a width of 50 μm, and 6 and 8 turns at different positions with a distance L of 2.3 mm. During the measurements of Figure 2, a magnetic field B = 0.2 T is applied to the electrical lead.

[0073] 2 shows the current-voltage characteristics between the electrical contact element 4 when the temperature of the metal lead wire 2 is 20 mK, 40 mK, and 120 mK. The horizontal axis V represents voltage (mV), and the vertical axis I represents current (A).

[0074] In addition to the super-insulating state described above and a classical state, e.g., an ohmic state, at much higher temperatures, such as room temperature, most super-insulating materials exhibit a Cooper pair insulating state at intermediate temperatures. In other words, when a super-insulating material is heated from a low temperature at which it exhibits a super-insulating state, a first phase transition occurs from this super-insulating state to a Cooper pair insulating state at a first critical temperature. When the super-insulating material is further heated, a second phase transition occurs from this Cooper pair insulating state to a classical state, e.g., an ohmic state, at a second critical temperature.

[0075] The super-insulating material enters a super-insulating state when the temperature of the metal lead wire 2 is below 50 mK and a voltage less than the threshold voltage of the varistor device 10 is applied across the electrical contact elements 4. This can be seen in Figure 2 for a temperature of 20 mK and a voltage less than ≈0.02 mV. In the super-insulating state, the dependence of the current I on the applied voltage V, I(V), follows a power law function: I∝V α where the exponent α increases with decreasing temperature. The first phase transition from the superinsulating state sees a jump from α=1 to α=3.

[0076] In principle, the non-linear dependence I(V) of the current I on the applied voltage V can be applied to the varistor device 10 to realize a non-linear resistance of the varistor device 10 .

[0077] As the applied voltage increases to a threshold voltage of approximately 0.02 mV, the current I increases abruptly. This abrupt increase, or jump, or step, or nonlinear increase, respectively, reflects the first phase transition from the super-insulating state. This abrupt increase is preferably used to provide the nonlinear resistance of the varistor device 10, which is particularly suitable for voltage surge protection of electronic circuits at cryogenic temperatures. This abrupt jump is a unique feature of the varistor device of the present invention and is not achievable with prior art varistors. This is the basis for the unprecedentedly fast operating times described below.

[0078] At temperatures above 50 mK and below 0.5 K, superinsulator materials exhibit a Cooper pair insulating state that is distinct from the superinsulating state at low temperatures and the classical (e.g., ohmic) state at high temperatures. This can be seen in Figure 2 at a temperature of 120 mK. The dependence of current I on applied voltage V, I(V), is linear in the Cooper pair insulating state. However, the current-voltage characteristic of varistor device 10 exhibits a nonlinearity that increases significantly as the applied voltage increases, causing metal lead wire 2 to transition from the Cooper pair insulating state through a second phase transition to a classical state, e.g., an ohmic state. This nonlinearity, while less pronounced than the nonlinearity of the first phase transition, can also be used to provide the nonlinear resistance of varistor device 10. In other words, a varistor region can be configured.

[0079] In a zero magnetic field, for a titanium nitride metal lead 2, a phase transition to or from the super-insulating state (i.e., the first phase transition) occurs at a first critical temperature, which ranges from 10 mK to 50 mK depending on the thickness of the metal lead 2. The thinner the film, the higher the first critical temperature. To ensure the super-insulating state, the thickness T of the titanium nitride metal lead 2 must be 5 nm or less. An electrical lead 2 made of titanium nitride that exhibits a super-insulating state exhibits a Cooper pair insulating state at temperatures above the first critical temperature. In other words, the first critical temperature corresponds to a phase transition between the super-insulating state and the Cooper pair insulating state. At higher temperatures up to 0.5 K (the second critical temperature), the metal lead 2 undergoes a second phase transition between the Cooper pair insulating state and a classical state, such as an ohmic state.

[0080] The metal lead 2 made of niobium titanium nitride undergoes a corresponding phase transition at a first critical temperature in the range of 0.05 K to 0.15 K and a second critical temperature up to 1.5 K. To ensure the super-insulating state, the thickness T of the metal lead 2 made of niobium titanium nitride must be less than 10 nm. The occurrence of the super-insulating state is due to the chemical composition Nb x Ti yN (where x is in the range of 0.3 to 0.33 and y is in the range of 0.7 to 0.67), although the superinsulating state may occur over a wider range of x and y.

[0081] Figures 3a and 3b show the current-voltage characteristics and differential conductance of a varistor device 10 similar to that of Figure 1. More specifically, the varistor device 10 on which the measurements of Figures 3a and 3b were taken comprises an electrical lead 2 made of niobium titanium nitride, having a thickness T of 10 nm, a width of 50 μm, and a distance L between different positions 6 and 8 of 2.3 mm. Figures 3a and 3b show measurement data for applied magnetic fields of 0 T and 1 T. The experiment was carried out with the temperature of the metal lead 2 at 0.1 K. In Figure 3a, the horizontal axis V represents voltage (V) and the vertical axis I represents current (A). In Figure 3b, the horizontal axis V represents voltage (V) and the vertical axis I represents differential conductance (Ω). -1 ) is shown.

[0082] In Figure 3a, the sudden increase, or jump, or step, or nonlinear increase in current occurs at a voltage of 0.006 V for B = 0 and 0.2 V for B = 1 T, respectively. The corresponding voltages are called threshold voltages.

[0083] Similarly, when the metal lead wire 2 is made of titanium nitride (not shown), the threshold voltage also increases in response to the applied magnetic field.

[0084] The current flowing between the electrical contact elements during the measurements in Figures 3a and 3b is approximately 1 fA in the super-insulating state and approximately 100 nA in the Cooper pair insulating state. These extremely low currents are not achievable with prior art varistors, resulting in low leakage and low noise never before achieved with varistors.

[0085] Figure 3b shows the differential resistance of the varistor device 20 of Figure 3a. The step-like characteristic at the threshold voltage indicates that the varistor device 10 is of high quality.

[0086] Figure 4a shows the current-voltage characteristics of a varistor device 10 similar to that of Figure 1. More specifically, the varistor device 10 on which the measurement of Figure 4a was taken comprises electrical leads 2 having a thickness T of 10 nm, a width of 50 µm, and different distances L between different positions 6, 8. During the measurement of Figure 4a, no magnetic field was applied to the electrical leads 2. The temperature of the metal leads 2 was 20 mK. The horizontal axis V represents voltage (V). The vertical axis I represents current (nA).

[0087] Regardless of the distance L between the different positions 6, 8, the current-voltage characteristic will exhibit an abrupt increase, or a jump, or a step, or a non-linear increase, respectively, of the current as a function of the applied voltage, similar to that described above for the varistor device 10 according to the previous embodiment, except that the threshold voltage at which the abrupt increase begins depends on the distance L between the different positions 6, 8 along the metal lead 2.

[0088] Figure 4b summarizes the threshold voltages obtained from the current-voltage characteristics of Figure 4a. For each current-voltage characteristic, the threshold voltage was determined as the voltage V at which the current I exceeded 1 nA. In Figure 4b, the horizontal axis represents the distance L between the different positions 6 and 8 of the electrical lead 2 in mm. The vertical axis, Vth, represents the threshold voltage in volts.

[0089] As shown in Figures 4a and 4b, the threshold voltage can be easily adjusted by varying the distance L between the different positions 6, 8 of the electrical lead 2.

[0090] FIG. 5 shows the time evolution of the current through a varistor device 10 similar to that of FIG. 1. More specifically, the varistor device 10 on which the measurements of FIG. 5 were taken comprises an electrical lead 2 made of niobium titanium nitride, with a thickness T of 9 nm, a width of 50 μm, and a distance of 2.3 mm between different positions 6 and 8. During the measurements of FIG. 5, no magnetic field was applied to the electrical lead 2. The temperature of the metal lead 2 was 20 mK. The threshold voltage of this device is 0.058 V. The horizontal axis t represents time (μs). The vertical axis I represents current (nA). For t<0, zero voltage is applied to the electrical contact element 4. For t≧0, a voltage (mV) indicated by the reference sign (i.e., 175 mV, 185 mV, or 200 mV) is applied to the electrical contact element 4.

[0091] Regardless of the applied voltage, the current increases to a saturated value after a sufficiently long time (e.g., 6-10 μs in Figure 5) from the initial force t = 0. The initial value is essentially zero, i.e., zero within the accuracy of the measurement.

[0092] The details of the time change, specifically the saturation value, depend on the difference between the applied voltage and the threshold voltage. The response time can be defined as the difference in time between the time t when the current I reaches 20% of its saturation value and the time t when the current I reaches 80% of its saturation value. The response time corresponds to the steepness of the time change of the current I during the period when the current I increases. The response time of the varistor device 10 is unprecedentedly short, i.e., less than 2 μs, and, to the inventors' knowledge, has not been achieved by prior art devices.

[0093] A response time of approximately 2 μs occurs when the applied voltage is close to the threshold voltage. As the voltage increases, the response time of the varistor device 10 can be reduced to a few nanoseconds. Another way to achieve response times of less than 1 μs, less than 0.1 μs, or even less than 0.01 μs is to reduce the length of the varistor, for example to less than 1 mm. However, this reduction in length has the side effect of blurring the threshold behavior.

[0094] Figure 6 shows a varistor device 10 according to another embodiment. The varistor device 10 is similar to the varistor device of Figure 1. Similar components are given the same reference numerals and will not be described again for the sake of brevity.

[0095] In the embodiment of Figure 6, the varistor device 10 is placed on top of, and in thermal contact with, the cooling element 14. The electronic circuit 12 that is to be protected against voltage surges by the varistor device 10 is also placed on top of, and in thermal contact with, the cooling element 14. The varistor device 10 is electrically connected in parallel to the electronic circuit 12 via the electrical contact elements 4. In the embodiment of Figure 6, the electronic circuit 12 and the varistor device 10 form an integrated circuit, in other words a single chip.

[0096] Reference potentials 16, 18 are provided to the electronic circuit 12 and the varistor device 10 via electrodes 16, 18 which are connected to an external power source.

[0097] The electronic circuit 12 includes at least one superconducting component adapted to exhibit superconductivity at cryogenic temperatures. For example, the electronic circuit 12 may include a superconducting coil, a superconducting qubit, a SQUID, or a superconducting single-electron transistor.

[0098] Figure 7 shows a varistor device 10 according to another embodiment. The varistor device 10 is similar to the varistor device of Figure 1. Similar components are given the same reference numerals and will not be described again for the sake of brevity.

[0099] 7 is formed on a substrate 22. In some embodiments, the substrate 22 is formed as part of the cooling element 14 of FIG.

[0100] In the illustrated embodiment, the substrate 22 is a silicon wafer.

[0101] The substrate 22 serves as a mechanical support for the electrical leads 2 .

[0102] A layer 20 is formed between the electrical lead 2 and the substrate 22 .

[0103] According to some embodiments, layer 20 acts as an electrical insulator between electrical leads 2 and substrate 22. In a corresponding embodiment, layer 20 is made of a dielectric material.

[0104] Alternatively or additionally, layer 20 serves as a growth promoting layer for the superinsulating material of electrical lead 2. Suitable materials for growth promoting layers include aluminum nitride and silicon dioxide.

[0105] 8a and 8b show a method 30 for manufacturing the varistor device 10. FIG.

[0106] In step 32, layer 20 is provided. In the illustrated embodiment, layer 20 is provided on a substrate 22 that provides mechanical support.

[0107] In the illustrated embodiment, layer 20 is composed of aluminum nitride or silicon oxide, more specifically silicon dioxide. Substrate 22 is a silicon substrate 22.

[0108] In step 34, a layer 2' of titanium nitride or niobium titanium nitride is provided.

[0109] The titanium nitride or niobium titanium nitride layer 2' of 34 is achieved using atomic layer deposition, resulting in stepwise film growth. This highly controllable process provides superior thickness and stoichiometric uniformity and an atomically smooth surface compared to chemical vapor deposition, the usual technique used to grow TiN or NbTiN films, although in principle chemical vapor deposition could also be employed.

[0110] For the deposition of TiN films by ALD, TiCl4 and NH3 are used as gas reactants. The temperature of layer 20 during deposition is 350° C. The titanium nitride film is deposited to a thickness of 5 nm.

[0111] For the deposition of NbTiN films by ALD, NbCl, TiCl, and NH are used as gas reactants. The stoichiometry is 4 / The temperature of layer 20 during deposition is 350°C. The niobium titanium nitride film is deposited to a thickness of 10 nm. The chemical composition of the deposited niobium titanium nitride film is Nb x Ti y N, where x is 0.3 and y is 0.7, and x is 0.33 and y is 0.67.

[0112] In step 36, an etching process 26 is performed on the titanium nitride or niobium titanium nitride layer 2'.

[0113] The etching process 26 ensures that the metal lead 2 comprising the superinsulator material is produced from the titanium nitride or niobium titanium nitride layer 2'.

[0114] In the case of a layer 2' of titanium nitride, the etching process 26 is carried out by plasma etching for 1 second.

[0115] In the case of a layer 2' of niobium titanium nitride, the etching process 26 is carried out by plasma etching for 3 seconds.

[0116] In a subsequent additional step (not shown), the metal leads 2 are lithographically patterned to define their width and length, for example, the metal leads 2 are lithographically patterned into 50 μm wide stripes.

[0117] In step 38 of method 30 shown in FIG. 8b, electrical contact elements 4 are formed that electrically contact electrical lead 2 at different locations 6, 8 along electrical lead 2 to connect the different locations 6, 8 along electrical lead 2 to electronic circuit 12.

[0118] In the illustrated embodiment, a patterned deposition 28 of aluminum or niobium is performed to deposit electrodes 4 as electrical contact elements 4. The electrodes 4 are deposited with a spacing of 2.5 mm (equal to the distance between the different positions 6, 8). In experiments to measure the current flowing through the electrical leads 2, additional electrodes were added between the electrodes 4 for current measurement. The distance between the additional electrodes was 0.45 mm.

[0119] FIG. 9 illustrates a method 40 of operating a varistor device to protect electronic circuits from voltage surges at cryogenic temperatures.

[0120] Any of the varistor devices 10 described above can be used as the varistor device in method 40 .

[0121] The method 40 includes step 42. The cooling step 42 is to cool the electrical lead 2 to a cryogenic temperature such that the electrical lead 2 becomes super-insulating or Cooper pair insulating, resulting in a non-linear resistance between the distinct locations 6, 8 at the cryogenic temperature. [Explanation of symbols]

[0122] 2 Electrical Leads 4 Electrical Contact Elements 6, 8 Different positions along the electrical lead 10. Barista equipment 12 Electronic circuit T Electrical lead wire thickness L length of electrical lead 14 Cooling element 16, 18 Reference potential 20 Growth promotion layer 22 PCB 24 Deposition of superconducting materials 26 Etching of superconducting materials / Plasma etching 28 Deposition of conductive materials 30 Method for forming a varistor device 32. Providing a growth promotion layer on the substrate 34, 36 Forming electrical leads 38 Forming electrical contact elements 40 How to operate the barista device 42 Cooling electrical leads to cryogenic temperatures

Claims

1. A circuit comprising an electronic circuit (12) and a varistor device (10) for providing voltage surge protection for the electronic circuit (12) at cryogenic temperatures, which refers to temperatures below 1.5 K, The varistor device (10) comprises: an electrical lead (2) made of a superinsulator material; a plurality of electrical contact elements (4) for connecting different locations (6, 8) along the electrical lead (2) to an electronic circuit (12); the plurality of electrical contact elements (4) electrically contact the electrical lead (2) at the different positions (6, 8) along the electrical lead (2); the electrical leads (2) are connected to the electronic circuit (12) via the plurality of electrical contact elements (4) at the different positions (6, 8); the electrical leads (2) are adapted to provide a super-insulating or Cooper pair insulating state at the cryogenic temperature and to provide a non-linear resistance between the different locations (6, 8) at the cryogenic temperature; the electronic circuit (12) comprises a superconducting component adapted to exhibit superconductivity at the cryogenic temperatures from the group consisting of a superconducting coil, a superconducting qubit, a SQUID, and a superconducting single-electron transistor; circuit.

2. The electrical leads (2) are electrically connected in parallel to the electronic circuit (12). The circuit of claim 1 .

3. the superinsulator material comprises or is a material from the group including titanium nitride, niobium titanium nitride, and indium oxide; 3. A circuit according to claim 1 or 2.

4. the thickness (T) of the electrical lead (2) does not exceed 10 nm; 3. A circuit according to claim 1 or 2.

5. the superinsulator material is titanium nitride and the thickness (T) of the electrical lead (2) does not exceed 5 nm; 3. A circuit according to claim 1 or 2.

6. The cryogenic temperature refers to a temperature of 0.5K or less, 0.15K or less, or 0.05K or less.

3. A circuit according to claim 1 or 2.

7. the electronic circuit (12) and the electrical leads (2) are arranged in thermal contact with the same cooling element (14), the cooling element (14) being adapted to provide the cryogenic temperature; 3. A circuit according to claim 1 or 2.

8. the electrical leads (2) at one of the different locations (6, 8) are electrically connected to a reference potential (16, 18) provided to the electronic circuit (12) and the varistor device (10); 3. A circuit according to claim 1 or 2.

9. the length (L) of the electrical lead between the different locations (6, 8) is at least 1 mm; 3. A circuit according to claim 1 or 2.

10. The superinsulator material is obtained by etching (26) a layer (2') of titanium nitride or niobium titanium nitride, 3. A circuit according to claim 1 or 2.

11. The superinsulator material of the electrical lead (2) is in physical contact with a growth promoting layer (20), specifically a dielectric growth promoting layer (20), adapted to promote high crystalline growth of the superinsulator material.

3. A circuit according to claim 1 or 2.

12. The growth promotion layer (20) comprises or is a nitride. A varistor device (10) according to claim 11.

13. The growth promotion layer (20) comprises or is an oxide. A varistor device (10) according to claim 11.

14. The growth promotion layer (20) is provided on a substrate (22). A varistor device (10) according to claim 11.

15. A method (40) for operating a varistor device (10) to protect an electronic circuit (12) from voltage surges at cryogenic temperatures, referring to temperatures below 1.5 K, wherein the electronic circuit (12) comprises a superconducting component adapted to exhibit superconductivity at said cryogenic temperatures from the group consisting of superconducting coils, superconducting qubits, SQUIDs, and superconducting single-electron transistors; The varistor device (10) comprises: an electrical lead (2) constructed of a super-insulating material; and a plurality of electrical contact elements (4) connected to the electronic circuit (12) at different locations (6, 8) along the electrical lead (2), the plurality of electrical contact elements (4) being in electrical contact with the electrical lead (2) at the different locations (6, 8) along the electrical lead (2), and the electrical lead (2) being connected to the electronic circuit (12) via each of the plurality of electrical contact elements (4) at the different locations (6, 8); The method (40) comprises: cooling (42) the electrical leads (2) to the cryogenic temperature such that the electrical leads (2) are in a super-insulating or Cooper pair insulating state and provide a non-linear resistance between the different locations (6, 8) at the cryogenic temperature. Method (40).

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