Piezoresistor device and method of operating piezoresistor device

Abstract

The invention relates to a varistor device and a method of operating the varistor device. A varistor device for voltage surge protection of an electronic circuit at low temperatures includes an electrical lead comprised of a superainsulator material and an electrical contact element. The electrical contact elements are used to connect different locations along the electrical lead to the electronic circuit. The electrical contact elements are in electrical contact with the electrical lead at different locations along the electrical lead. The electrical leads are adapted to provide a super-insulating state or a Cuppel-to-Perot insulating state at cryogenic temperatures, and to provide a non-linear resistance between different locations at cryogenic temperatures.

Description

Technical Field

[0001] The present disclosure relates to varistors operable at cryogenic temperatures, such as for use in superconducting coils or qubits, employing superinsulators whose electrical conductivity substantially vanishes at cryogenic temperatures. Background Art

[0002] An important use of varistors is to protect electronic devices from voltage fluctuations (such as voltage fluctuations from power supplies or data inputs) and from the resulting undesirably high power intake. To this end, the varistor is connected in parallel with the device to be protected. The varistor has a nonlinear resistance. Ideally, as long as the voltage at the transistor is lower than a predefined threshold voltage, the varistor will be substantially non-conductive. When the voltage exceeds the threshold voltage, the resistance of the ideal varistor will decrease sharply (i.e., nonlinearly), and the current through the varistor will increase accordingly with the voltage. In this ideal case, the parallel current path provided by the varistor reduces the voltage at the electronic device and the current through the electronic device. Therefore, the nonlinear voltage-dependent resistance of the varistor enables it to be effectively protected from voltage fluctuations.

[0003] Existing varistors, such as those based on semiconductors (e.g., forming diodes) or on metal oxides forming internal diodes, can only be operated at relatively high temperatures. Varistors according to the prior art are not suitable for use at cryogenic temperatures, at which devices such as superconducting coils or qubits operate. This is due to high heating, which occurs due to the passage of current through the varistor at voltages above the threshold voltage. Another disadvantage of conventional varistors is that the value of the on-state resistance is poorly controlled. The on-state resistance directly determines the current passing through the varistor at a voltage greater than the threshold voltage, and thus determines the amount by which the varistor is heated. Therefore, poor control of the on-state resistance leads to poor control of the heating of the varistor. Summary of the invention

[0004] In view of the above technical problems, there is a need for an improved varistor which can be operated at cryogenic temperatures. This problem is solved by a varistor device according to claim 1. Claim 15 provides a method of operating a varistor device.

[0005] Varistor devices employ superinsulator materials. Superinsulator materials are dual superconductors, i.e., materials that exhibit a superinsulating state at low but finite temperatures, with essentially infinite resistance, so that no current can pass through them. The transition from the superinsulating state to a state with greater resistance can be triggered by a sufficient temperature increase or by applying a sufficiently high voltage. The superinsulating state has been observed in titanium nitride films, niobium titanium nitride films, and indium oxide films.

[0006] According to a first aspect, a varistor device for voltage surge protection of an electronic circuit at cryogenic temperatures comprises an electrical lead made of a superinsulator material and an electrical contact element. The electrical contact element is used to connect different positions along the electrical lead to the electronic circuit. The electrical contact element is in electrical contact with the electrical lead at different positions along the electrical lead. The electrical lead is suitable for providing a superinsulating state or a Cooper pair insulating state at cryogenic temperatures, and is suitable for providing a nonlinear resistance between different positions at cryogenic temperatures.

[0007] Varistor devices with superinsulator materials provide strong nonlinear resistance at cryogenic temperatures, where the absolute value is substantially zero. Therefore, varistor devices based on superinsulator materials are very suitable for voltage surge protection of electronic devices operating at cryogenic temperatures. The varistor device itself shows low power consumption and low voltage noise levels at cryogenic temperatures, where the leakage current is substantially zero. These are the main advantages of varistor devices over varistors according to the prior art.

[0008] In addition, the varistor device with superinsulator material provides a voltage-current characteristic that increases steeply (in other words, steps or jumps, or has a large nonlinearity) at the threshold voltage. The steepness of the increase or the magnitude of the nonlinearity at the threshold voltage determines the extent to which excess voltage (and thus current, and thus heating) is taken away from the device to be protected. The varistor device according to the present specification takes away more excess voltage from the device to be protected than a conventional varistor, and thus provides improved voltage surge protection.

[0009] When the voltage applied to the varistor device exceeds the threshold voltage, the resistance of the varistor device can exhibit ohmic behavior. The resistance in this case is also referred to as the on-state resistance. This is an advantage over the prior art, in which the on-state resistance exhibits non-ohmic behavior but increases more strongly with the voltage applied to the varistor (e.g., exponentially as in a diode), resulting in poor control of the on-state resistance. The on-state resistance of the varistor device according to the present specification can be easily controlled by forming electrical leads having a size preselected according to the desired on-state resistance.

[0010] By forming its dimensions (ie, in terms of width and thickness of the electrical leads) in accordance with a pre-selected threshold voltage, a varistor device having a pre-selected threshold voltage can be fabricated.

[0011] Furthermore, the varistor device may be sized (ie, in terms of the width and thickness of the electrical leads) to exhibit a preselected response time.

[0012] The electrical contact element may comprise or consist of a superconductor material.The superconductor material may be adapted to provide a superconducting state at cryogenic temperatures.

[0013] The corresponding electrical contact elements may provide a minimum resistance at cryogenic temperatures for connecting the electronic circuit to, for example, a varistor device and / or other electrical or electronic components, such as a voltage source.

[0014] Electronic circuits may include superconductor components adapted to exhibit superconductivity at cryogenic temperatures, such as superconductor coils, superconductor qubits, SQUIDs, or superconductor single electron transistors.

[0015] The corresponding electronic circuits are typically operated at cryogenic temperatures. For such devices, there is a particular need for voltage surge protection, which can be cooled to cryogenic temperatures together with the electronic circuits and still function / operate. Conventional voltage surge protection devices typically operate at much higher temperatures, such as at room temperature. The use of such conventional voltage surge protection devices requires a connection between it and the electronic circuit to be protected, i.e., a connection between the electronic circuit operating at cryogenic temperatures and a device at a higher temperature (e.g., room temperature). Such a connection is undesirable, for example because it causes heat transfer, and such a connection can be avoided using a varistor device according to the present specification.

[0016] The electrical leads may be connected to the electronic circuit via the electrical contact elements.

[0017] The electrical leads may be electrically connected in parallel to the electronic circuit.

[0018] The superinsulator material may include or be a material from the group consisting of titanium nitride, titanium niobium nitride, and indium oxide.

[0019] The superinsulator material may include or be a material from the group consisting of titanium nitride and niobium titanium nitride.

[0020] 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.

[0021] The superinsulator material may be titanium nitride and the thickness of the electrical leads may not exceed 5 nm.

[0022] The small thickness can help ensure that the electrical leads exhibit super-insulation properties at cryogenic temperatures.

[0023] Cryogenic temperature may refer to a temperature of at most 1.5K, or at most 0.5K, or at most 0.15K, or at most 0.05K.

[0024] The electronic circuitry and the electrical leads may be arranged in thermal contact with the same cooling element.The cooling element may be adapted to provide a cryogenic temperature.

[0025] The electrical lead may be electrically connected to a reference potential at one of a variety of locations.

[0026] Alternatively, the electrical leads may be electrically connected to different reference potentials at different locations.

[0027] The length of the electrical leads between different locations may be at least 1 mm.

[0028] The corresponding embodiments of the varistor device can provide maximum resistance, thereby minimizing energy loss and energy dissipation. The leakage current through the metal black can be significantly less than -1 pA.

[0029] In some embodiments, the length of the electrical leads between different locations can be at least 0.01 mm or at least 0.1 mm.

[0030] The length of the electrical leads between different locations can be less than 1 mm.

[0031] The corresponding embodiment of the varistor device can minimize its reaction time to voltage surge and thus provide a high-speed varistor. However, a leakage current of about 1 pA through the metal black may occur.

[0032] Superinsulator materials may be obtained using etching (eg plasma etching) of a layer of titanium nitride or niobium titanium nitride.

[0033] The superinsulator material of the electrical leads may be in physical contact with the dielectric layer.

[0034] Dielectric layers may be used to electrically insulate a superinsulator material, for example, from nearby semiconductor or conducting materials.

[0035] The superinsulator material of the electrical lead may be in physical contact with the growth promoting layer. The growth promoting layer may be adapted to promote highly crystalline growth of the superinsulator material. In particular, the growth promoting layer may be a dielectric layer, such as the dielectric layer described above.

[0036] The dielectric layer may be adapted to provide a growth promoting layer.

[0037] The growth promoting layer may include or be a nitride, such as aluminum nitride.

[0038] In particular, the inventors have experimentally verified that the use of a growth promoting layer of aluminum nitride can increase the critical temperature of the metal lead, and thus the maximum temperature at which the varistor device can operate, by approximately 10%.

[0039] Alternatively, the growth promoting layer or the dielectric layer may comprise or be an oxide, such as silicon dioxide.

[0040] The dielectric layer and / or the growth promoting layer may be disposed on a substrate (eg, a silicon substrate).

[0041] A dielectric layer and / or a growth promoting layer may be arranged between the substrate and the superinsulator material.

[0042] According to an 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 one millimeter.

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

[0044] According to another embodiment, the superinsulator material is composed of niobium titanium nitride, the substrate is a silicon substrate, and the varistor device includes a dielectric layer composed of silicon dioxide. In this embodiment, the length of the electrical leads may exceed one millimeter.

[0045] According to another embodiment, the superinsulator material consists of niobium titanium nitride, the substrate is a silicon substrate, and the varistor device includes a growth promoting layer consisting of aluminum nitride. In this embodiment, the length of the electrical leads may exceed one millimeter.

[0046] According to another embodiment, the superinsulator material consists of titanium nitride and the varistor device comprises a dielectric layer consisting of silicon dioxide. In this embodiment, the length of the electrical leads may be less than one millimeter.

[0047] According to another embodiment, the superinsulator material consists of titanium nitride and the varistor device comprises a growth promoting layer consisting of aluminum nitride. In this embodiment, the length of the electrical leads may be less than one millimeter.

[0048] According to another embodiment, the superinsulator material consists of niobium titanium nitride and the varistor device comprises a dielectric layer consisting of silicon dioxide. In this embodiment, the length of the electrical leads may be less than one millimeter.

[0049] According to another embodiment, the superinsulator material consists of niobium titanium nitride and the varistor device comprises a growth promoting layer consisting of aluminum nitride. In this embodiment, the length of the electrical leads may be less than one millimeter.

[0050] Niobium titanium nitride may refer to a material having the chemical composition Nb x Ti y N material, wherein x is in the range of 0.3 to 0.33, and y is in the range of 0.7 to 0.67.

[0051] 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 that connects 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 method includes cooling the electrical lead to cryogenic temperatures so that the electrical lead is in a superinsulating state or a Cooper pair insulating state and provides a nonlinear resistance between different locations at cryogenic temperatures.

[0052] A third aspect may relate to a method of forming a varistor device for voltage surge protection of an electronic circuit at cryogenic temperatures. The method may include forming an electrical lead composed of a superinsulator material. The method may further include forming an electrical contact element electrically contacting the electrical lead at different positions along the electrical lead to connect different positions along the electrical lead to the electronic circuit. The electrical lead may be formed to provide a superinsulating state or a Cooper pair insulating state at cryogenic temperatures and to provide a nonlinear resistance between different positions at cryogenic temperatures.

[0053] The method may further comprise 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.

[0054] The method may further include providing a substrate. A dielectric layer and / or a growth promoting layer may be disposed on the substrate.

[0055] The electrical lead may comprise a layer of titanium nitride or niobium titanium nitride, in particular provided by atomic layer deposition.

[0056] Providing a layer of titanium nitride or titanium niobium nitride may include depositing titanium nitride or titanium niobium nitride onto the dielectric layer and / or the growth promoting layer while maintaining the dielectric layer and / or the growth promoting layer at a temperature exceeding 300 K, or exceeding 310 K, or exceeding 320 K, or exceeding 330 K, or exceeding 340 K. In some embodiments, the temperature does not exceed 400 K, or does not exceed 390 K, or does not exceed 380 K, or does not exceed 370 K, or does not exceed 360 K.

[0057] The method may include performing an etching process on the layer of titanium nitride or titanium niobium nitride.

[0058] The etching process may be performed by plasma etching.

[0059] In some embodiments, the etching process is performed for not less than one second, or not less than three seconds.

[0060] In some embodiments, the method includes photolithographically patterning the metal lead to define its width and / or its length.

[0061] Forming the electrical contact elements may include depositing a superconductor material, for example using patterned deposition. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] The techniques of the present disclosure and advantages associated with these techniques will become most apparent from the description of exemplary embodiments according to the accompanying drawings, in which:

[0063] Figure 1 A perspective view of a varistor device according to a first embodiment is given;

[0064] Figure 2 The current-voltage characteristics of the varistor device at three different temperatures are given;

[0065] Figure 3a The current-voltage characteristics of the varistor device for magnetic fields of zero and 1 T on a log-log scale are given;

[0066] Figure 3b Given the Figure 3a The differential conductance corresponding to the measurement of

[0067] Figure 4a The current-voltage characteristics of varistor devices with different lengths are given;

[0068] Figure 4b The basis for different lengths of varistor devices is given Figure 4a The threshold voltage is determined by the current-voltage characteristic;

[0069] Figure 5 shows the time evolution of the current through the varistor device when different voltages (as indicated by the reference numerals in mV) are applied to the varistor device at t≥0, wherein for t<0 the applied voltage is zero;

[0070] Figure 6 A top view of a varistor device according to another embodiment is provided;

[0071] Figure 7 A perspective view of a varistor device according to another embodiment is provided;

[0072] Figure 8a The process steps for manufacturing a varistor device are illustrated;

[0073] Figure 8b illustrates another process step for manufacturing a varistor device; and

[0074] Fig. 9 A method of operating a varistor device is illustrated. DETAILED DESCRIPTION

[0075] Figure 1 A schematic diagram of a varistor device 10 according to an embodiment is given.

[0076] The varistor component 10 comprises an electrical lead 2 consisting of a superinsulator material.

[0077] Superinsulators are the dual materials of superconductors, that is, materials that provide a superinsulating state at low but finite temperatures. In this superinsulating state, the superinsulator has essentially infinite electrical resistance, so that no current can pass through it. The superinsulating state has been observed in titanium nitride films, niobium titanium nitride films, and indium oxide films.

[0078] Figure 1 The electrical leads 2 of the varistor device 10 are made of titanium nitride, but another known superinsulator, such as niobium titanium nitride or indium oxide or an aluminum layer, as described in US2021 / 068320 A1, may be used instead.

[0079] 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.

[0080] The electrical contact element 4 allows the electrical lead 2 to be connected to the electronic circuit at different locations 6 , 8 .

[0081] The electrical contact element 4 is electrically conductive or metallic, respectively. According to some embodiments, the electrical contact element consists of a superconductor material, such as aluminum or niobium.

[0082] The varistor device 10 is typically connected in parallel to an electronic circuit for operation at cryogenic temperatures using the electrical contact elements 4. This is typically an electronic circuit comprising superconductor components such as superconductor coils, superconductor qubits, SQUIDs or superconductor single electron transistors.

[0083] Figure 2 Shows something like Figure 1 The current-voltage characteristics of the varistor device 10 of the varistor device have been performed. More specifically, Figure 2 The measured varistor device 10 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 the different locations 6, 8. Figure 2 During the measurement, a magnetic field B=0.2 T has been applied to the electrical lead 2.

[0084] Figure 2 The current-voltage characteristics between the electrical contact elements 4 are given for temperatures of the metal lead 2 of 20 mK, 40 mK and 120 mK. The horizontal axis V indicates the voltage in mV. The vertical axis I indicates the current in A.

[0085] In addition to the above-mentioned superinsulating state and the classical state (e.g., Ohmic state) at a much higher temperature (e.g., room temperature), most superinsulator materials exhibit a Cooper pair insulating state at an intermediate temperature. In other words, when a superinsulator material is heated from a low temperature at which it exhibits a superinsulating state, it undergoes a first phase transition from the superinsulating state to the Cooper pair insulating state at a first critical temperature. When the superinsulator material is further heated, it undergoes a second phase transition from the Cooper pair insulating state to a classical state (e.g., Ohmic state) at a second critical temperature.

[0086] At a temperature of the metal leads 2 below 50 mK and when a voltage less than the threshold voltage of the varistor device 10 is applied between the electrical contact elements 4, the superinsulator material is in its superinsulating state. Figure 2 The dependence of the current I on the applied voltage V in the superinsulating state can be described by the power law I∝V α The power α increases with decreasing temperature. At the first phase transition out of the superinsulating state, a jump from α=1 to α=3 is observed.

[0087] In principle, a non-linear dependence I(V) of the current I on the applied voltage V can be exploited in the varistor device 10 to provide a non-linear resistance of the varistor device 10 .

[0088] When the applied voltage increases to a threshold voltage of about 0.02mV, the current I increases sharply. This sharp increase, or jump, or step, or nonlinear increase reflects the first phase transition leaving the superinsulating state, respectively. This sharp increase is preferably used to provide the nonlinear resistance of the varistor device 10. It is particularly suitable for protecting electronic circuits from voltage surges at cryogenic temperatures. This steep jump is a unique feature of the varistor device described and cannot be realized in any prior art varistor. It forms the basis of the unprecedented fast operating time described below.

[0089] At temperatures above 50 mK and below 0.5 K, superinsulator materials exhibit a Cooper pair insulating state that is different from both the superinsulating state at lower temperatures and the classical state (e.g., Ohmic state) at higher temperatures. Figure 2It is observed for a temperature of 120mK. The dependence of the current I on the applied voltage V, I(V), is linear within the Cooper pair insulation state. However, the current-voltage characteristic of the varistor device 10 exhibits nonlinearity, and increases significantly when the applied voltage increases, and drives the metal lead 2 from the Cooper pair insulation state through the second phase transition into the classical state (e.g., ohmic state). Although this nonlinearity is not as obvious as the nonlinearity of the first phase transition, it can also be used to provide a nonlinear resistance of the varistor device 10. In other words, it can constitute a varistor operating regime.

[0090] Under zero magnetic field and for the metal lead 2 composed of titanium nitride, the phase transition (i.e., the first phase transition) into or out of the superinsulating state occurs at a first critical temperature in the range of 10mK to 50mK, which depends on the thickness of the metal lead 2. Films with smaller thicknesses exhibit a larger first critical temperature. In order to ensure the emergence of the superinsulating state, the thickness T of the metal lead 2 composed of titanium nitride should be no greater than 5nm. The metal lead 2 composed of titanium nitride that exhibits a superinsulating state exhibits a Cooper pair insulating state at a temperature exceeding the first critical temperature. In other words, the first critical temperature corresponds to a phase transition between a superinsulating state and a Cooper pair insulating state. At a higher temperature (second critical temperature) up to 0.5K, the metal lead 2 undergoes a second phase transition between a Cooper pair insulating state and a classical state (such as an Ohmic state).

[0091] The metal lead 2 composed of niobium titanium nitride undergoes a corresponding phase transition, wherein the first critical temperature is in the range of 0.05K to 0.15K and the second critical temperature is up to 1.5K. In order to ensure the emergence of the superinsulating state, the thickness T of the metal lead 2 composed of niobium titanium nitride should not be greater than 10nm. x Ti y The occurrence of the superinsulating state in niobium titanium nitride has been experimentally verified with x in the range of 0.3 to 0.33 and y in the range of 0.7 to 0.67. However, the superinsulating state is likely to occur over a wider range of x and y.

[0092] Figure 3a and Figure 3b Shows something like Figure 1 The current-voltage characteristics and differential conductance of the varistor device 10 of the varistor device have been performed. More specifically, Figure 3a , Figure 3b The measured varistor device 10 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 of 2.3 mm between different locations 6, 8. Figure 3a , Figure 3bIn , the measured data for the applied magnetic field of 0T and 1TC are given. The experiment was performed at a temperature of 0.1K for the metal lead 2. Figure 3a In FIG. 1 , the horizontal axis V indicates the voltage in V, and the vertical axis I indicates the current in A. Figure 3b In the figure, the horizontal axis V indicates the voltage in V, and the vertical axis I indicates the voltage in Ω. -1 The differential conductance is in units of .

[0093] exist Figure 3a In the example, a sharp increase, jump, 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. The corresponding voltage is called the threshold voltage.

[0094] Correspondingly, when the metal lead 2 is composed of titanium nitride (not shown), the threshold voltage also increases with the applied magnetic field.

[0095] exist Figure 3a , Figure 3b The current flowing between the electrical contact elements during the measurement is about 1 fA in the super-insulating state and about 100 nA in the Cooper pair insulating state. These incredibly low currents are not achievable in prior art varistors, resulting in low leakage and low noise that were previously unachievable in varistors.

[0096] Figure 3b Presentation Figure 3a The differential resistance of the varistor device 10 is shown in FIG. 1 . The step-like characteristic at the threshold voltage indicates the high quality of the varistor device 10 .

[0097] Figure 4a Shows something like Figure 1 The current-voltage characteristics of the varistor device 10 of the varistor device have been performed. More specifically, Figure 4a The measured varistor device 10 comprises an electrical lead 2 made of niobium titanium nitride with a thickness T of 10 nm, a width of 50 μm and different distances L between different locations 6, 8. Figure 4a During the measurement of , no magnetic field is applied to the electrical lead 2. The temperature of the metal lead 2 is 20 mK. The horizontal axis V indicates the voltage in V. The vertical axis I indicates the current in nA.

[0098] For all distances L between the different positions 6, 8, the current-voltage characteristic exhibits a sharp increase, or a jump, or a step, or a nonlinear increase in the current as a function of the applied voltage, respectively, similar to that described above for the varistor device 10 according to the previous embodiment. However, the threshold voltage at which the sharp increase begins depends on the distance L between the different positions 6, 8 along the metal lead 2.

[0099] Figure 4b Summarizes from Figure 4a The threshold voltage is obtained from the current-voltage characteristics. For each current-voltage characteristic, the threshold voltage is determined as the voltage V at which the current I exceeds 1nA. Figure 4b In FIG. 1 , the horizontal axis indicates the distance L in mm between different positions 6 , 8 of the electrical lead 2 . The vertical axis Vth indicates the threshold voltage in V.

[0100] like Figure 4a , Figure 4b As shown, by changing the distance L between different positions 6 , 8 of the electrical lead 2 , the threshold voltage can be easily adjusted.

[0101] Figure 5 Shown by a method similar to Figure 1 More specifically, a time evolution of the current of the varistor device 10 has been performed. Figure 5 The measured varistor device 10 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 locations 6, 8. Figure 5 No magnetic field is applied to the electrical lead 2 during the measurement of . The temperature of the metal lead 2 is 20 mK. The threshold voltage of the device is 0.058 V. The horizontal axis t indicates the time in μs. The vertical axis I indicates the current in nA. For t<0, zero voltage is applied to the electrical contact element 4. At t≥0, a voltage (in mV) indicated by the reference numeral (i.e., 175 mV, 185 mV, or 200 mV) is applied to the electrical contact element 4.

[0102] For all applied voltages, the current is Figure 5 The initial value is essentially zero, ie it is zero within the measurement accuracy.

[0103] The details of the temporal evolution and in particular the saturation value depend on the difference between the applied voltage and the threshold voltage. The response time can be defined as the time difference between the time t at which the current I reaches 20% of the saturation value and the time t at which the current I reaches 80% of the saturation value. The response time corresponds to the steepness of the temporal evolution of the current I during its increase. The response time of the varistor device 10 is unprecedentedly short, i.e. less than 2 μs, which is not achieved in any prior art device known to the inventors.

[0104] When the applied voltage is close to the threshold voltage, the response time is about 2 μs. As the voltage increases, the reaction time of the varistor device 10 can be reduced to a few ns. Another way to achieve a reaction time of less than 1 μs, less than 0.1 μs, or less than 0.01 μs is to reduce the varistor length, for example, to less than 1 mm. However, a side effect of this length reduction is that the threshold behavior becomes blurred.

[0105] Figure 6 A varistor device 10 according to another embodiment is presented. The varistor device 10 and Figure 1 Similar components are indicated by the same reference numerals and will not be described again for the sake of brevity.

[0106] exist Figure 6 In the embodiment of the invention, the varistor device 10 is arranged on a cooling element 14 and in thermal contact with it. The electronic circuit 12 to be protected from voltage surges by the varistor device 10 is also arranged on the cooling element 14 and in thermal contact with it. The varistor device 10 is electrically connected in parallel to the electronic circuit 12 via the electrical contact element 4. Figure 6 In the embodiment of the present invention, the electronic circuit 12 and the varistor device 10 form an integrated circuit, or in other words, a single chip.

[0107] Reference potentials 16 , 18 are provided to the electronic circuit 12 and the varistor device 10 via electrodes 16 , 18 connected to an external power supply.

[0108] The electronic circuit 12 includes at least one superconductor component adapted to exhibit superconductivity at cryogenic temperatures. For example, the electronic circuit 12 may include a superconductor coil, a superconductor qubit, a SQUID, or a superconductor single electron transistor.

[0109] Figure 7 A varistor device 10 according to another embodiment is presented. The varistor device 10 and Figure 1 Similar components are indicated by the same reference numerals and will not be described again for the sake of brevity.

[0110] Figure 7 The electrical leads 2 are formed on the substrate 22. In some embodiments, the substrate 22 is formed as Figure 6 part of the cooling element 14 .

[0111] In the depicted embodiment, substrate 22 is a silicon wafer.

[0112] The substrate 22 serves as a mechanical support for the electrical leads 2 .

[0113] Layer 20 is formed between electrical leads 2 and substrate 22 .

[0114] According to some embodiments, layer 20 serves as an electrical insulator between electrical leads 2 and substrate 22. In a corresponding embodiment, layer 20 consists of a dielectric material.

[0115] Alternatively or additionally, the layer 20 serves as a growth promoting layer of the superinsulator material of the electrical lead 2. Suitable materials for the growth promoting layer include aluminum nitride and silicon dioxide.

[0116] Figure 8a , Figure 8b A method 30 of manufacturing a varistor device 10 is illustrated.

[0117] At step 32, a layer 20 is provided. In the depicted embodiment, the layer 20 is disposed over a substrate 22 that provides mechanical support.

[0118] In the depicted embodiment, layer 20 is composed of aluminum nitride or silicon oxide, more specifically silicon dioxide. Substrate 22 is a silicon substrate 22 .

[0119] At step 34, a layer 2' of titanium nitride or niobium titanium nitride is provided.

[0120] Providing 34 the layer 2' of titanium nitride or niobium titanium nitride is achieved using atomic layer deposition, resulting in a stepwise film growth. Compared to chemical vapor deposition (the standard technique for growing TiN or NbTiN films), this highly controllable process provides excellent thickness and stoichiometric uniformity and an atomically smooth surface. However, in principle chemical vapor deposition could also be used.

[0121] For the ALD deposition of the TiN film, TiCl4 and NH3 were used as gaseous reactants. The temperature of the layer 20 during deposition was 350° C. The deposited thickness of the titanium nitride film was 5 nm.

[0122] For ALD deposition of NbTiN films, NbCl5, TiCl4 and NH3 were used as gaseous reactants. The stoichiometry was adjusted by varying the ratio of TiCl4 / NbCl5 in the gas used during the growth cycle. The temperature of layer 20 during deposition was 350°C. The deposited thickness of the niobium titanium nitride film was 10 nm. The chemical composition of the deposited niobium titanium nitride film was 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.

[0123] At step 36, an etching process 26 is performed on the layer 2' of titanium nitride or titanium niobium nitride.

[0124] The etching process 26 ensures that the metal lead 2 comprising the superinsulator material is produced from the layer 2 ′ of titanium nitride or niobium titanium nitride.

[0125] In the case of the layer 2 ′ of titanium nitride, the etching process 26 is performed for one second by plasma etching.

[0126] In the case of the layer 2 ′ of niobium titanium nitride, the etching process 26 is performed for three seconds by plasma etching.

[0127] In a subsequent optional step (not shown), the metal wire 2 is photolithographically patterned to define its width and its length. For example, the metal wire 2 is photolithographically patterned into stripes with a width of 50 μm.

[0128] exist Figure 8b At step 38 of the depicted method 30 , the electrical contact elements 4 are formed to make electrical contact with the electrical leads 2 at different locations 6 , 8 along the electrical leads 2 to connect the different locations 6 , 8 along the electrical leads 2 to the electronic circuit 12 .

[0129] In the depicted 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 at a distance of 2.5 mm (equal to the distance between the different locations 6, 8). For experiments measuring the current through the electrical leads 2, additional electrodes are deposited between the electrodes 4 for current measurement. The distance between the additional electrodes is 0.45 mm.

[0130] Fig. 9 A method 40 of operating a varistor device to provide voltage surge protection to an electronic circuit at cryogenic temperatures is illustrated.

[0131] Any of the varistor devices 10 described above may be used as the varistor device of method 40 .

[0132] The method 40 includes, at step 42 , cooling 42 the electrical lead 2 to a cryogenic temperature such that the electrical lead 2 is in a superinsulation state or a Cooper pair insulation state and provides a non-linear resistance between the different locations 6 , 8 at the cryogenic temperature.

[0133] List of Reference Numerals

[0134] 2 Electrical leads

[0135] 4 Electrical contact elements

[0136] 6, 8 Different positions along the electrical leads

[0137] 10 Varistor Devices

[0138] 12 Electronic Circuits

[0139] T Thickness of electrical leads

[0140] L Length of electrical leads

[0141] 14 Cooling element

[0142] 16, 18 Reference potential

[0143] 20 Growth promotion layer

[0144] 22 Substrate

[0145] 24 Deposition of superconducting materials

[0146] 26 Etching / Plasma Etching of Superconducting Materials

[0147] 28 Deposition of conductive material

[0148] 30 Method for forming a varistor device

[0149] 32 Providing a growth promoting layer on the substrate

[0150] 34, 36 Forming electrical leads

[0151] 38 Forming electrical contact elements

[0152] 40 Methods of operating varistor devices

[0153] 42 Cool the electrical leads to cryogenic temperature

Claims

1. A varistor device (10) for protecting an electronic circuit (12) from voltage surges at cryogenic temperatures, the varistor device (10) comprising: an electrical lead (2) composed of a superinsulator material, and an electrical contact element (4) for connecting different locations (6, 8) along the electrical lead (2) to the electronic circuit (12), wherein the electrical contact element (4) is in electrical contact with the electrical lead (2) at the different locations (6, 8) along the electrical lead (2), The electrical lead (2) is adapted to provide a superinsulating state or a Cooper pair insulating state at the cryogenic temperature, and is adapted to provide a nonlinear resistance between the different positions (6, 8) at the cryogenic temperature.

2. The varistor device (10) according to claim 1, wherein: The electronic circuit (12) comprises a superconductor component adapted to exhibit superconductivity at the cryogenic temperature, such as a superconductor coil, a superconductor qubit, a SQUID or a superconductor single electron transistor.

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

4. The varistor device (10) according to any one of the preceding claims, wherein The superinsulator material includes or is a material from the group consisting of titanium nitride, titanium niobium nitride, and indium oxide.

5. The varistor device (10) according to any one of the preceding claims, wherein The thickness (T) of the electrical lead (2) does not exceed 10 nm.

6. The varistor device (10) according to any one of the preceding claims, wherein The superinsulator material is titanium nitride, and the thickness (T) of the electrical lead (2) does not exceed 5 nm.

7. The varistor device (10) according to any one of the preceding claims, wherein The cryogenic temperature refers to a temperature of at most 1.5K, or at most 0.5K, or at most 0.15K, or at most 0.05K.

8. The varistor device (10) according to any one of the preceding claims, wherein The electronic circuit (12) and the electrical leads (2) are arranged in thermal contact with a same cooling element (14), and wherein the cooling element (14) is adapted to provide the cryogenic temperature.

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

10. The varistor component (10) according to any one of the preceding claims, wherein The length (L) of the electrical lead between the different locations (6, 8) is at least 1 mm.

11. The varistor component (10) according to any one of the preceding claims, wherein The superinsulator material can be obtained by etching (26), such as plasma etching (26), a layer (2') of titanium nitride or niobium titanium nitride.

12. The varistor component (10) according to any one of the preceding claims, wherein The superinsulator material of the electrical lead (2) is in physical contact with a growth promoting layer (20), in particular a dielectric growth promoting layer (20), which is suitable for promoting high crystalline growth of the superinsulator material.

13. The varistor device (10) according to claim 12, wherein: The growth promoting layer (20) includes or is: Nitrides, such as aluminum nitride, or Oxides, such as silicon dioxide.

14. The varistor device (10) according to claim 12 or 13, wherein: The growth promoting layer (20) is arranged on a substrate (22), such as a silicon substrate (22).

15. A method (40) of operating a varistor device (10) to provide voltage surge protection to an electronic circuit (12) at cryogenic temperatures, the varistor device (10) comprising: an electrical lead (2) composed of a superinsulator material, and an electrical contact element (4) for connecting to the electronic circuit (12) at different locations (6, 8) along the electrical lead (2), wherein the electrical contact element (4) makes electrical contact with the electrical lead (2) at the different locations (6, 8) along the electrical lead, Wherein, the method (40) comprises: The electrical lead (2) is cooled (42) to the cryogenic temperature so that the electrical lead (2) is in a superinsulating state or a Cooper pair insulating state and provides a nonlinear resistance between the different locations (6, 8) at the cryogenic temperature.

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