Varistor device and method for operating varistor device

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 the protection of electronic devices against voltage fluctuations and the resulting undesirable high power consumption, for example due to power supply or data input. For this purpose, varistors are connected in parallel to the device to be protected. The varistor has a non-linear electrical resistance. Ideally, it is essentially non-conductive as long as the voltage of the transistor is below a certain threshold voltage. When the voltage exceeds the threshold voltage, the electrical resistance of an ideal varistor decreases significantly (i.e. non-linearly) and the current through the varistor increases with the voltage. In this ideal situation, the parallel current path provided by the varistor reduces the voltage of the electronic device and the current through it. The non-linear voltage-dependent resistance of the varistor therefore allows for effective protection against voltage fluctuations.

[0003] Existing varistors, for example those based on semiconductors (e.g. forming diodes) or metal oxides forming 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 qubits operate. This is because the 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 the 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 by which the varistor is heated. Thus, 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 capable of operating at cryogenic temperatures. This problem is solved by the varistor device as claimed in claim 1. Claim 15 provides a method of operating the varistor device.

[0006] Varistor devices employ superinsulator materials. Superinsulator 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 through which no electrical current can pass. A transition from the superinsulating state to a state with greater resistance can be induced by a sufficient increase in temperature or application of a sufficiently high voltage. The superinsulating state is found 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 is in electrical contact with the electrical lead at different locations along the electrical lead. The electrical lead is adapted to provide a super-insulating or Cooper pair insulating state at cryogenic temperatures and to provide a non-linear resistance between the different locations at the cryogenic temperatures.

[0008] Varistor devices using super-insulator materials provide strong non-linear resistance at cryogenic temperatures, the absolute value of which is practically zero. Therefore, varistor devices based on super-insulator materials are ideal for voltage surge protection electronic devices operating at cryogenic temperatures. The varistor device itself at cryogenic temperatures has low power consumption, low voltage noise levels and practically zero current leakage. 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 steep increase in threshold voltage (in other words, a step or jump, or 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 electrical resistance. This is an advantage over the prior art, where the on-state electrical 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 electrical resistance. The on-state electrical resistance of the varistor device according to the 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) according 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 may 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, SQUID, or a superconducting single-electron transistor.

[0016] The corresponding electronic circuitry usually operates at cryogenic temperatures. Such devices are especially required to have voltage surge protection that is capable of functioning / operating even when cooled together with the electronic circuitry to cryogenic temperatures. Conventional voltage surge protection devices usually 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 circuitry to be protected, i.e. a connection between the cryogenically operating electronic circuitry and a hot (e.g. room temperature) device. Such a connection is undesirable, for example as it leads to 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 an electronic circuit.

[0019] The superinsulator material may be comprised of or may be a material from the group including titanium nitride, titanium niobium 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] Cryogenics 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 disposed 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 the 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 provide a fast varistor with minimal response time to voltage surges, although leakage currents of approximately 1 pA can occur through the metallic black.

[0033] The superinsulator material may be obtainable by using an etch, such as a plasma etch, 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 enhancement 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 promotion 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 include or be an oxide, such as silicon dioxide.

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

[0042] A dielectric layer and / or a growth promotion 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 comprised of silicon dioxide. In this embodiment, the length of the electrical leads may be greater than 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 made of aluminum nitride. In this embodiment, the length of the electrical leads may be greater than 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 length of the electrical leads may be greater than 1 millimeter.

[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 be greater than 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 promotion 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 includes an electrical lead made of a superinsulator material and an electrical contact element connecting different locations along the electrical lead to the electronic circuit. The electrical contact element electrically contacts 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 is in 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 of 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 a 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 on the growth-promoting layer.

[0055] The method may further include providing a substrate. A dielectric layer and / or a growth enhancing 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 a layer of titanium nitride or niobium titanium nitride may include depositing 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 of 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 description of the drawings]

[0063] The techniques of the present disclosure and their associated advantages will be best understood from a description of exemplary embodiments taken in conjunction with the accompanying drawings. [Figure 1]FIG. 1 is a perspective view of the varistor device according to the first embodiment. [Diagram 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 for zero and 1 T magnetic fields of the varistor device. [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. [Diagram 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 for t<0 the applied voltage is zero. [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 illustrates the process steps for manufacturing a varistor device. [Figure 8b] FIG. 8b illustrates another processing step for manufacturing a varistor device. [Figure 9] FIG. 9 is a diagram illustrating the method of operation of the varistor device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

[0065] The varistor device 10 comprises an electrical lead 2 constructed from a superinsulator material.

[0066] A superinsulator is the flip side of a superconductor, a material that provides a superinsulating state at a low but finite temperature. In this superinsulating state, the superinsulator has essentially infinite resistance and therefore no electric current can pass through it. Superinsulating films include titanium nitride, titanium niobium 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. 5,399,323 may alternatively be used.

[0068] The varistor device 10 further comprises electrical contact elements 4 which 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, according to some embodiments they 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 a superconducting coil, a superconducting qubit, SQUID, or a superconducting single-electron transistor.

[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 comprises an electrical lead 2 made of titanium nitride with a thickness T of 5 nm, a width of 50 μm and a distance L of 2.3 mm between 6 and 8 turns at different positions. 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 temperatures of the metal lead wire 2 are 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 mentioned 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 superinsulator material enters the superinsulating state when the temperature of the metal leads 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 superinsulating state, the dependence I(V) of the current I on the applied voltage V follows a power law: 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] The current I increases abruptly when the applied voltage is increased to the threshold voltage ≈0.02 mV. This abrupt increase, or jump, or step, or non-linear increase, respectively, reflects the first phase transition from the super-insulating state. This abrupt increase is preferably used to provide a non-linear resistance of the varistor device 10, which is particularly suitable for voltage surge protection of electronic circuits at cryogenic temperatures. Such an abrupt jump is a unique feature of the varistor device of the present invention, which is not achievable with prior art varistors. This is the basis for the unprecedented fast operating times described below.

[0078] At temperatures above 50 mK and below 0.5 K, the superinsulator material exhibits 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 is seen in FIG. 2 at a temperature of 120 mK. The dependence I(V) of the current I on the applied voltage V is linear in the Cooper pair insulating state. However, the current-voltage characteristic of the varistor device 10 exhibits a nonlinearity that increases significantly as the applied voltage increases to phase-transition the metal lead 2 from the Cooper pair insulating state through a second phase transition to a classical state, e.g. ohmic. This nonlinearity, although not as pronounced as the nonlinearity of the first phase transition, can also be used to provide a nonlinear resistance of the varistor device 10. In other words, a varistor region can be configured.

[0079] At zero magnetic field, for a metal lead 2 made of titanium nitride, the phase transition to or from the super-insulating state (i.e., the first phase transition) occurs at a first critical temperature, which is in the range of 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 occurrence of the super-insulating state, the thickness T of the metal lead 2 made of titanium nitride 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 (second critical temperature), the metal lead 2 undergoes a second phase transition between the Cooper pair insulating state and a classical state, such as the 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 occurrence of 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 achieved by 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] 3a and 3b show the current-voltage characteristics and differential conductance of a varistor device 10 similar to that of FIG. 1. More specifically, the varistor device 10 on which the measurements of FIGS. 3a and 3b were taken comprises an electrical lead 2 made of niobium titanium nitride with a thickness T of 10 nm, a width of 50 μm and a distance L between the different positions 6, 8 of 2.3 mm. In FIGS. 3a and 3b, measurement data are shown for applied magnetic fields of 0 T and 1 T. The experiment was carried out with a temperature of the metal lead 2 of 0.1 K. In FIG. 3a, the horizontal axis V represents voltage (V) and the vertical axis I represents current (A). In FIG. 3b, the horizontal axis V represents voltage (V) and the vertical axis I represents differential conductance (Ω). -1 ) is shown.

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

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

[0084] The current flowing between the electrical contact elements during the measurements of Figures 3a and 3b is approximately 1 fA in the super-insulating state and approximately 100 nA in the Cooper pair insulating state. These very 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 performed comprises electrical leads 2 with 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 the voltage (V). The vertical axis I represents the 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 exceeds 1 nA. In Figure 4b, the horizontal axis shows the distance L between the different positions 6, 8 of the electrical lead 2 in mm. The vertical axis Vth shows the threshold voltage in V.

[0089] As Figures 4a, 4b show, the threshold voltage can be easily adjusted by varying the distance L between 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 measurement of FIG. 5 was performed 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 between the different positions 6, 8 of 2.3 mm. During the measurement of FIG. 5, no magnetic field is applied to the electrical lead 2. The temperature of the metal lead 2 was 20 mK. The threshold voltage of the 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) as 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 at a time long enough (e.g., 6-10 μs in Figure 5) from the initial force of 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 may be defined as the difference in time between the time t at which the current I reaches 20% of its saturation value and the time t at which 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 has not been achieved, to the inventors' knowledge, by prior art devices.

[0093] A response time of about 2 μs occurs when the applied voltage is close to the threshold voltage. As the voltage is increased, the reaction time of the varistor device 10 can be reduced to a few ns. 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 making the threshold behavior unclear.

[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 numbers and will not be described again for the sake of brevity.

[0095] In the embodiment of Figure 6, the varistor device 10 is disposed on top of, and in thermal contact with, the cooling element 14. An electronic circuit 12, which is to be protected against voltage surges by the varistor device 10, is also disposed on, 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 numbers and will not be described again for the sake of brevity.

[0099] The electrical leads 2 of Figure 7 are formed on a substrate 22. In some embodiments, the substrate 22 is formed as part of the cooling element 14 of Figure 6.

[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 a substrate 22 .

[0103] According to some embodiments, layer 20 serves 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 promotion layer for the superinsulator material of electrical lead 2. Suitable materials for growth promotion layers include aluminum nitride and silicon dioxide.

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

[0106] In step 32, layer 20 is provided. In the embodiment shown, layer 20 is provided on a substrate 22 which 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' at 34 is achieved using atomic layer deposition, resulting in stepwise film growth. This highly controllable process offers 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, TiCl 4 and N.H. 3 is used as the gas reactant. The temperature of layer 20 during deposition is 350° C. A titanium nitride film is deposited with a thickness of 5 nm.

[0111] For the deposition of NbTiN films by ALD, NbCl 5 , TiCl 4 and N.H. 3 is used as the gas reactant. The stoichiometry is the TiCl 4 / NbCl 5 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, where x is 0.33 and y is 0.67.

[0112] In step 36, an etching process 26 is performed on the titanium nitride or titanium niobium 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 determine their width and length. For example, the metal leads 2 are lithographically patterned into stripes 50 μm wide.

[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 circuitry 12.

[0118] In the embodiment shown, 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 through the electrical leads 2, additional electrodes were added between the electrodes 4 for the measurement of the current. The distance between the added 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 may be used as the varistor device in method 40 .

[0121] The method 40 includes a step 42, which 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. Varistor device 12 Electronic circuit T Electrical lead wire size L Length of electrical lead 14 Cooling element 16, 18 Reference potential 20 Growth promotion layer 22 Substrate 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 a varistor device 42 Cooling Electrical Leads to Cryogenic Temperatures

Claims

1. A varistor device (10) for voltage surge protection of an electronic circuit (12) at cryogenic temperatures, comprising: The varistor device (10) comprises: an electrical lead (2) made of a superinsulator material; and electrical contact elements (4) for connecting different positions (6, 8) along said electrical leads (2) to an electronic circuit (12); the electrical contact elements (4) electrically contact the electrical lead (2) at the different locations (6, 8) along the electrical lead (2); the electrical leads (2) being 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; A varistor device (10).

2. the electronic circuit (12) comprises 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; A varistor device (10) as claimed in claim 1.

3. The electrical leads (2) are connected to the electronic circuit (12) via the electrical contact elements (4) and / or in electrical parallel. A varistor device (10) according to claim 1 or 2.

4. The superinsulator material comprises or is a material from the group including titanium nitride, niobium titanium nitride, and indium oxide. A varistor device (10) according to claim 1 or 2.

5. The thickness (T) of the electrical lead (2) does not exceed 10 nm; A varistor device (10) according to claim 1 or 2.

6. the superinsulator material is titanium nitride and the thickness (T) of the electrical lead (2) does not exceed 5 nm; A varistor device (10) according to claim 1 or 2.

7. The cryogenic temperature refers to a temperature of 1.5K or less, 0.5K or less, 0.15K or less, or 0.05K or less. A varistor device (10) according to claim 1 or 2.

8. the electronic circuit (12) and the electrical leads (2) are arranged in thermal contact with a same cooling element (14), the cooling element (14) being adapted to provide the cryogenic temperature; A varistor device (10) according to claim 1 or 2.

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

10. the length (L) of the electrical lead between the different locations (6, 8) is at least 1 mm; A varistor device (10) according to claim 1 or 2.

11. The superinsulator material is obtained by using an etching (26), such as a plasma etching (26), on a layer (2') of titanium nitride or niobium titanium nitride, A varistor device (10) according to claim 1 or 2.

12. the superinsulator material of the electrical lead (2) is in physical contact with a growth promotion layer (20) adapted to promote high crystal growth of the superinsulator material, in particular a dielectric growth promotion layer (20); A varistor device (10) according to claim 1 or 2.

13. The growth promotion layer (20) includes or is a nitride such as aluminum nitride or an oxide such as silicon dioxide. A varistor device (10) according to claim 12.

14. The growth promotion layer (20) is provided on a substrate (22), such as a silicon substrate (22). A varistor device (10) according to claim 12.

15. A method (40) of operating a varistor device (10) to protect an electronic circuit (12) from voltage surges at cryogenic temperatures, comprising the steps of: The varistor device (10) comprises: an electrical lead (2) constructed from a superinsulator material and electrical contact elements (4) for connecting to the electronic circuit (12) at different locations (6, 8) along the electrical lead (2), the electrical contact elements (4) being in electrical contact with the electrical lead (2) at the different locations (6, 8) along the electrical lead (2); The method (40) comprises: cooling (42) the electrical leads (2) to the cryogenic temperature such that the electrical leads (2) are in a superinsulating or Cooper pair insulating state and provide a non-linear resistance between the distinct locations (6, 8) at the cryogenic temperature. Method (40).

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