Flexible superconducting circuit

Abstract

The invention relates to a flexible superconducting circuit (1). The flexible superconducting circuit comprises a first foil of dielectric material (2), for example a polyimide; a first capping layer (3) comprising chromium (Cr); a first superconducting layer (4); and a second capping layer (5) comprising chromium (Cr). The superconducting layer can comprise niobium titanium or niobium titanium nitride. The first and second capping layer of chromium protects the superconducting layer against impurities dissolved in the polyimide and acts as an adhesion layer between the superconductor circuit and the polyimide due to chemical bonding.

Description

[0001] Flexible superconducting circuit

[0002] Field of the invention

[0003] The invention relates to a flexible superconducting circuit for connecting electronic devices.

[0004] Background

[0005] A known flexible superconducting circuit, for example a superconducting communication line, can be applied for connecting an external electronic control device to a cryogenic electronic system. The cryogenic electronic system may comprise, for example, qubit devices, quantum processors, sensing and detector systems, quantum internet apparatuses, medical devices, cryptographic devices, classical computing processors, and any other electronic devices. However, there are many other applications using cryogenic electronic circuits, such as multi-pixel superconducting photon detectors used in astronomy and quantum communication applications.

[0006] Cryogenic cooling equipment is provided for maintaining the cryogenic electronic circuits at the required operating temperature of near zero Kelvin. This cryogenic cooling equipment is often built up from a stack of separated temperature stages, wherein each lower stage is cooled down to a lower temperature. Due to the fundamentals of thermodynamics, the power required to progressively cool down to lower temperatures increases exponentially. For example, a typical cryogenic cooling equipment consumes 20-30 kW for managing a thermal load of 12-18 pW at 100 mK.

[0007] The electronic control device is typically placed outside the cryogenic equipment to prevent their power dissipation from heating up the cryogenic equipment as a whole and thus the cryogenic circuits as well. Therefore, a communication path is required for exchanging signals between cryogenic circuits at the final stage of the cryogenic equipment through the top of the cryogenic equipment to the external control electronics. Such a path is typically constructed from a cascade of semi rigid communication lines, usually coax cables, to bridge the distance and to intercept mechanical tension and vibrations during the cooling down procedure and operation.

[0008] Recent developments show a possible shift from a part of the control electronic to cryogenics control stages, for example, at 4 K, however this still requires control lines to control devices between cryogenic stages. Cryogenic circuits, such as the qubit devices, require communication with the external control device for controlling the qubits and signaling back an actual state of each qubit to be to the control device. This requires also high frequency, HF, analogue signals. Typically, this signal can be in the range from low frequencies or DC to ultrahigh frequencies up to the infrared or visible wavelength ranges.

[0009] Recent cryogenic qubit devices have an increasing number of qubits. Each qubit requires individual communication to the control device outside the cryogenic device. This individual communication requires an increasing number of transmission lines for the qubits. For example, the qubit device can comprise 96 qubits and requires at least 288 individual transmission lines that should be guided through subsequent thermal stages to the outside. The transmission lines may comprise several coax connectors, for example, for bridging the consecutive stages of the cryogenic equipment. So, when the number of transmission lines increases, the total number of coax connectors in the transmission lines to bridge each stage is also increasing and relatively more space in the subsequent stages is required to accommodate for this increased number of coax connectors and may become a limiting condition for a further increase in numbers of qubits. The coax cables can be partly replaced by the flexible superconducting communication lines to reduce the volume of the connector. For example, eight coax cables can be replaced by a flexible superconducting communication line comprising a dielectric layer provided with eight channels or signal lines and a superconducting layer or ground layer provided at one or opposite sides of the flexible superconducting communication line. The flexible circuit can be connected to the cryogenic device via known connectors.

[0010] Thin-Film Nb / Polyimide Superconducting Stripline Flexible Cables by Vaibhav Gupta et al; IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 29, NO. 5, AUGUST 2019 discloses thin aluminum capping layers on the top and bottom of niobium signal traces to protect the niobium (Nb) during subsequent fabrication steps. A problem is that the quality of niobium traces of the flexible superconducting circuit is not sufficient, for example, because of deterioration of the behavior of the niobium, reduced adhesion of the niobium and mechanical stability of the traces.

[0011] Summary of the invention

[0012] It is therefore an object of the invention to mitigate the above indicated problems.

[0013] According to a first aspect of the invention this and other objects are achieved by a flexible superconducting circuit comprising in this order: a first foil of dielectric material; a first capping layer comprising chromium directly coupled at a first flat side of the first foil, a first superconducting layer; and a second capping layer comprising chromium, directly coupled to the first superconducting layer. The superconducting conducting layer has a normal superconducting behavior at 4 K. The dielectric material can for example polyimide. The first foil of dielectric material can comprise impurities like water H2O, hydrogen H2, and oxygen O2 dissolved in the dielectric material. During the vacuum deposition process, when the superconductor is applied at the foil, the temperature rises and water molecules are released from the dielectric material and pollute the superconductor. It is known that water vapor residue is harmful for niobium. The capping layer protects the first superconducting layer against diffusion of impurities dissolved in the dielectric into the first superconducting layer. A further advantage is that this capping also protects against diffusion of the impurities over a long time. Even after several years there is no diffusion of impurities in the superconducting layer. Furthermore, the capping layer also improves the adhesion between the superconductor layer and the polyimide due to chemical bonding. The first and second capping layer can be chromium or nickel chromium.

[0014] “Preserving niobium Superconductivity in Thin-Film Superconducting Flexible Cables” by Vaibhav Gupta*, John A. Sellers*, Charles D. Ellis*, Simin Zou*, George A. Hernandez*, Raj on Bai*, Yang Cao*, David B. Tuckerman f and Michael C. Hamilton* * Auburn University, Auburn, AL fMicrosoft Research, Redmond, WA. discloses a stack of oxidized silicon wafer, a superconducting line of niobium, a capping layer and a polyimide layer. Furthermore, that document discloses several results of experiments on the effect on the transition temperature Tc of the superconductor for different types and thicknesses of different capping layers of aluminum, chromium or tantalum. Also, that document discloses that a capping layer of aluminum retained a relatively high Tc of the niobium after curing the polyimide layer at 350 °C and that a capping layer of chromium or tantalum on the polyimide carrier result in a deteriorated superconductor with a reduced critical temperature Tc.

[0015] In an advantageous embodiment of the flexible superconducting circuit the first superconductor layer comprises niobium titanium or niobium titanium nitride. Both compounds show improved characteristics with respect to flexibility and critical temperature Tc of the superconducting layer. The inventors found that the critical temperature of the superconductor is hardly affected, even after a processing step at 300° C in the manufacturing process of the flexible superconducting circuit.

[0016] In a further advantageous embodiment of the flexible superconducting circuit a thickness of the first superconducting layer is in a range between 50 nm - 2 pm, preferably between 150 - 400 nm and more preferably 250 nm. The thickness can be selected in these ranges depending on process time of deposition of the superconductor, maximum current through the superconducting circuit and mechanical flexibility. The thickness of 250 nm is a good compromise with respect to process time of the deposition, flexibility of the superconducting circuit and a desired maximum current through the first superconducting layer.

[0017] In a further advantageous embodiment of the flexible superconducting circuit a thickness of the first capping layer and the second capping layer respectively is in the range between 15 nm and 50 nm, preferably 20 nm. An advantage of the thickness of 20 nm is that it provides a homogeneous layer with a constant thickness and a closed surface without any gaps.

[0018] In a further advantageous embodiment of the flexible superconducting circuit the dielectric material comprises a polyimide. Layers of polyimide are suitable for forming flexible substrates with a desired thickness. An example of a polyimide is Kapton as delivered by Dupont.

[0019] In a further advantageous embodiment the flexible superconducting circuit further comprises a first non-superconducting metal layer at a side of the second capping layer facing away from the first superconducting layer. The non-superconducting metal is both electrically conductive and thermally conductive. The first non-superconducting metal layer can comprise silver (Ag), copper (Cu), or gold (Au). In this arrangement the first non-superconducting metal layer provides a current by-pass for the superconductor layer and also acts a thermal path for reducing cool down time in a cryogenic system. The advantages are that the first non- superconducting metal layer allows for microwave measurements at room temperature, the first non-superconducting metal layer provides an improved solderability, and the first non- superconducting metal layer prevents forming of an electrically isolating barrier in the first superconducting layer. Furthermore, the first non-superconducting metal layer provides electrical conductivity above the critical temperature of the superconductor and can be used for connection between the superconductor and an external electronic device.

[0020] In a further advantageous embodiment the flexible superconducting circuit further comprises a first contact pad connected to a first portion of the first superconducting layer; and a second contact pad at a second portion of the first superconducting layer, wherein the location of the second portion is different from that of that of the first portion. The contact pads may provide a connection between a quantum device at one end of the flexible superconducting circuit and an electronic device at the other end of the flexible superconducting circuit.

[0021] In a further advantageous embodiment, the superconductor circuit comprises a plurality of parallel lines patterned in the first superconducting layer wherein the first contact pad is arranged at one end of a line of the plurality of lines and the second contact pad is arranged at another end of the line. This arrangement of lines or traces provides a multiple channel communication line between a quantum device and an electronic device. The plurality of lines can be for example 8 lines.

[0022] In a further advantageous embodiment, the flexible superconducting circuit comprises a third capping layer comprising chromium at a second flat side of the first foil of dielectric material facing away from the first superconducting layer, a second superconducting layer directly coupled to the third capping layer and a fourth capping layer comprising chromium coupled to the second superconducting layer. The second superconductor layer can be niobium titanium (NbTi) of niobium titanium nitride (NbTiN). In this arrangement the flexible superconducting circuit may comprise a microstrip communication line. The third and fourth capping layers can be chromium or nickel chromium.

[0023] In a further advantageous embodiment, the flexible superconducting circuit comprises a second non-superconducting metal layer directly coupled to the fourth capping layer at a side facing away from the second superconducting layer. The second non-superconducting metal layer can comprise silver, copper or gold. In this arrangement the second non- superconducting metal layer provides current stabilization of the superconductor in the superconducting layer. Furthermore, the second non-superconducting metal layer contacts the second superconductor layer and prevents forming of an electrically isolating barrier in the first superconducting layer. Furthermore, the second non-superconducting metal layer provides electrical conductivity above the critical temperature of the superconductor and can be used for connection between the superconductor and an external electronic device. Furthermore, the second non-superconducting metal layer acts as a ground plane of the flexible superconducting circuit.

[0024] In a further advantageous embodiment, the flexible superconducting circuit comprises in this order a second foil of dielectric material at the first non-superconducting layer facing away from the first superconducting layer, a fifth capping layer comprising chromium, a third superconducting layer and a sixth capping layer comprising chromium. The third superconducting layer can be niobium titanium or niobium titanium nitride. In this arrangement the flexible superconducting circuit may comprise a stripline communication line. The fifth and sixth capping layers can be chromium or nickel chromium.

[0025] In a further advantageous embodiment, the flexible superconducting circuit comprises a third non-superconducting metal layer at the sixth capping layer at a side facing away from the third superconducting layer. The third non-superconducting metal layer can comprise silver, copper or gold. In this arrangement the third non-superconducting metal layer provides current stabilization of the superconductor in the third superconducting layer. Furthermore, the third non-superconducting metal layer contacts the third superconductor layer and prevents forming of an electrically isolating barrier in the sixth capping layer. Furthermore, the third non-superconducting metal layer provides electrical conductivity above the critical temperature of the superconductor and can be used for connection between the superconductor and an external electronic device. The third non-superconducting metal layer can comprise silver, copper or gold. The third non-superconducting metal layer can also act as a ground plane.

[0026] According to a second aspect of the invention this and other objects are achieved by a method of manufacturing a flexible superconducting circuit, the method comprising providing a foil of a dielectric material; deposition of, in this order, a first capping layer comprising chromium at a first flat side of the foil of dielectric material; a first superconductor layer at the first capping layer; and a second capping layer comprising chromium at the first superconductor layer. The first and second capping layer can be chromium or nickel chromium.

[0027] These and other features and effects of the present invention will be explained in more detail below with reference to drawings in which preferred and illustrative embodiments of the invention are shown. The person skilled in the art will realize that other alternatives and equivalent embodiments of the invention can be conceived and reduced to practice without departing from the scope of the present invention.

[0028] Brief description of the drawings

[0029] Fig. 1 shows a cross-section in a longitudinal direction of flexible superconducting circuit according to an embodiment of this disclosure;

[0030] Fig. 2 shows a cross-section in a longitudinal direction of flexible superconducting circuit according to an embodiment of this disclosure;

[0031] Fig. 3 shows a cross-section in a longitudinal direction of flexible superconducting circuit according to an embodiment of this disclosure; and Fig. 4 shows a cross-section in a second direction in the plane of the flexible superconducting circuit according to an embodiment of this disclosure.

[0032] Detailed description of embodiments

[0033] In the figures like numerals refer to similar components. The invention is explained with reference to Figs. 1-4.

[0034] Although illustrative embodiments of the present invention have been described with reference to the accompanying drawings, it is to be understood that the invention is not limited to these embodiments. Various changes or modifications may be affected by one skilled in the art without departing from the scope of the invention as defined in the claims.

[0035] Accordingly, reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, it is noted that the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0036] The flexible superconducting circuit, for example, a communication line can be used to connect a cryogenic device, for example, a cryogenic electronic circuit at a temperature of about 1 mK to an electronic device at room temperature or another electronic device at 4 K. The cryogenic electronic circuit can be a qubit device or an astronomic electronic circuit or other scientific instruments.

[0037] Fig. 1 shows diagrammatically a cross-section of an embodiment of a superconducting circuit in a longitudinal direction of the superconducting circuit. In this embodiment the flexible superconducting circuit 1, for example a communication line, comprises a first foil 2 of dielectric material. The first foil 2 has a rectangular shape and the cross-section is in a longitudinal direction of the flexible superconducting circuit. The dielectric material can be polyimide, for example, Kapton as delivered by DuPont. In an embodiment, the first foil 2 of polyimide may comprise a stack of layers of polyimide and adhesive, wherein the thickness of the polyimide layer is 50 pm. The adhesive can be epoxy-based, acrylic based, or polymer based. The stack can be adapted to obtain a predetermined thickness of the flexible superconducting circuit. The thickness of the flexible superconducting circuit is about 0,3 mm. The length of and width of the flexible superconducting circuit can be 1 m and 10 mm respectively. Furthermore, the flexible superconducting circuit 1 comprises a first capping layer 3 of chromium at one side of the first foil 2, a first superconducting layer 4 and a second capping layer 5 of chromium. The first superconducting layer 4 is arranged between the first capping layer 3 and the second capping layer 5. The first superconducting layer 4 is niobium titanium (NbTi) or niobium titanium nitride (NbTiN).

[0038] In embodiments the thickness of the first superconductor layer 4 can be in a range 50 nm- 2 pm, preferably between 150 - 400 nm and more preferably 250 nm. The skilled person can select the thickness of the first superconducting layer 4 depending on process time of deposition of the superconductor, flexibility and maximum current through the superconducting circuit layer. A thickness of 250 nm for the first superconductor layer 4 appears well suitable with respect to flexibility of the superconducting layer and a desired maximum current through the superconducting circuit. In embodiments the maximum current through the first superconducting layer is in a range between 2 and 5 mA.

[0039] The first capping layer 3 of chromium protects the first superconducting layer 4 against impurities dissolved the polyimide, for example water H2O, hydrogen H2 and oxygen O2. Furthermore, the first capping layer of chromium also acts as an adhesion layer between the first superconducting layer 4 and the first foil 2 due to chemical bonding to the polyimide. A further advantage is that application of the chromium in the capping layer does not substantially affect the characteristic temperature Tc of the superconducting layer 4 after processing at 300 °C. Experiments show that the critical temperature Tc of the first superconducting layer 4 is hardly affected by the polyimide and is about 9 K.

[0040] In embodiment the thickness of the first capping layer 3 and the second capping layer 5 respectively is in the range between 15 and 50 nm, preferably 20 nm. A thickness of 20 nm provides a homogenous layer with constant thickness and a closed surface, without any gaps.

[0041] A further advantage is that the adhesion between the chromium layer and the polyimide layer is improved and more suitable for subsequent manufacturing steps and patterning of the different layers.

[0042] The second capping layer 5 of chromium, like the first capping layer 3 protects the superconducting layer 4 against impurities of a further layer. Alternatively, the first and second capping layers 3,5 are of nickel chromium.

[0043] In an embodiment the flexible superconducting circuit 1 comprises a first non- superconducting metal layer 6 at a side of the second capping layer 5 facing away from the first foil 4. The first non-superconducting metal layer 6 can be silver, copper or gold. Advantages of the first non-superconducting metal layer 6 are that the first metaling layer provides current stabilization in the first superconducting layer 4, that the first non- superconducting metal layer 6 is in electrical contact with the first superconducting layer 4, and that the first non-superconducting metal layer 6 improves solderability, prevents forming of an electrically isolating barrier and improves mechanical stability. Furthermore, the first non-superconducting metal layer 6 provides electrical conductivity above the critical temperature of the first superconductor layer 4 and can be used for connection between the superconductor layer and an external electronic device.

[0044] Optionally, the superconducting circuit 1 comprises a second non-superconducting metal layer 9 arranged at a second side of the first foil 2 facing away from the first superconducting layer 4. The second non-superconducting metal layer 9 can be made of silver Ag, copper Cu, or gold Au. The second non-superconducting metal layer 9 can be applied as a ground plane. In this embodiment the flexible superconducting circuit 1 can be a microstrip.

[0045] In embodiments the flexible superconducting circuit also comprises at least one of a resistor, a capacitor and a coil.

[0046] Fig. 2 shows diagrammatically an embodiment of the flexible superconducting circuit in a longitudinal direction of a superconducting circuit 10. The flexible superconducting circuit 10 is like that described with respect to Fig. 1. The cross-section is in a longitudinal direction of the flexible superconducting circuit 10. Furthermore, in this embodiment the flexible superconducting circuit 10 comprises a third capping layer 11 of chromium, a second superconducting layer 12 and a fourth capping layer 13 of chromium between the first foil 2 and the second non-superconducting metal layer 9. The second superconducting layer 12 is arranged between the third capping layer 11 and the fourth capping layer 13. The third capping layer 11 and fourth capping layer 13 are like the first and second capping layers 3,5 and are functioning like the first and second capping layers 3,5. Alternatively the third and fourth capping layers 11,13 are of nickel chromium. The second superconducting layer 12 is niobium titanium or niobium titanium nitride. The second superconducting layer 12 is also acting as a ground plane. In this embodiment the flexible superconducting circuit 1 can be a microstrip.

[0047] Fig. 3 shows diagrammatically an embodiment of the flexible superconducting circuit in a longitudinal direction of a superconducting circuit 20. The superconducting circuit 20 is like the superconducting circuit 10 described with respect to fig. 2. In this embodiment the flexible superconducting circuit 30 further comprises a second foil 21 of dielectric material, at a side of the first non-superconducting metal layer 6 at the side facing away from the first superconducting layer 4. The dielectric material of the second foil 21 is a polyimide, for example, Kapton. The second foil 21 has a similar shape of the first foil 2. Furthermore, the superconducting circuit 20 comprises a fifth capping layer 22 of chromium at the side of the second foil 21 facing away from the first superconducting layer 4, a third superconducting layer 23 and a sixth capping layer 24 of chromium, wherein the third superconducting layer 23 is between the fifth capping layer 22 and the sixth capping layer 24. The third superconducting layer 23 is niobium or niobium titanium nitride. The fifth capping layer 22 and sixth capping layer 24 are like the first and second capping layers 3,5 and are functioning like the first and second capping layers 3,5. Alternatively the fifth and sixth capping layers 22,24 are of nickel chromium.

[0048] Furthermore, in an embodiment, the superconducting circuit 20 comprises a third non- superconducting metal layer 25 at the side of the sixth capping layer 34 facing away from the third superconducting layer 23. The third non-superconducting metal layer can comprise of silver, copper or gold. The third superconducting layer 23 acts as a ground plane. In this embodiment the flexible conducting circuit can be a stripline.

[0049] Fig. 4 shows diagrammatically a first superconducting layer of an embodiment of the flexible superconducting circuit in a second direction perpendicular to the longitudinal direction of the flexible superconducting circuit. In this embodiment the flexible superconducting circuit comprises a plurality of parallel lines or traces 26, for example, eight parallel lines patterned in the first superconductor layer 4. Furthermore, the flexible superconducting circuit 20 comprises the first and second capping layers 3,5 of chromium and the first metal layer. The first and second capping layers 3,5 and the first metal layer 6 are patterned corresponding to the plurality of parallel lines 26 in the first superconductor layer 4. Furthermore, the superconducting circuit 20 is provided with first and second contact pads 7,8 wherein the first contact pad 7 is arranged at one end of the respective lines 26 and the second contact pad 8 is arranged at another end of the respective lines 26. The lines 26 and contact pads 7,8 can be obtained by lift-off processes, laser ablation, chemical etching or reactive ion etching of the first non-superconducting metal layer 6. These processes are well-known to the person skilled in the art.

[0050] A process to manufacture the flexible superconducting circuit 1 like the embodiment described in fig 1 comprises providing a first foil 2 of a dielectric material, and deposition of subsequently the first capping layer 3 of chromium, the first superconducting layer 4, the second capping layer 5 of chromium, the first non-superconducting metal layer 6 and the second non-superconducting metal layer 9. The dielectric material is a polyimide, for example Kapton. The thickness of the polyimide layer is 50 pm. The deposition of the subsequent layers can be in-situ performed in a vacuum deposition apparatus. The thickness of the first and second capping layers 3,5 of chromium is respectively 20 nm. Alternatively, the first and second capping layers are of nickel chromium.

[0051] The first superconducting layer 4 is niobium titanium or niobium titanium nitride. The thickness of the first superconducting layer 4 is 250 nm. The first non-superconducting metal layer 6 can be silver, copper or gold. A thickness of the first non-superconducting metal layer 6 is 500nm. Optionally, a second non-superconducting metal layer 9 is deposited on the other side of the first foil 2 facing a from the first superconducting layer 4. The thickness of the second non-superconducting metal layer is 200 nm. After deposition of the first non- superconducting metal layer 6 the contact pads 7,8 can be obtained by lift-off processes, laser ablation, chemical etching or reactive ion etching the first non-superconducting metal layer 6.

[0052] A process to manufacture the flexible superconducting circuit like the embodiment described in Fig. 2 comprises the steps as described hereinbefore in the vacuum deposition apparatus, but before the deposition of the second non-superconducting metal layer 9 by deposition of subsequently a third capping layer 11 of chromium, a second superconducting layer 12 of niobium titanium or niobium titanium nitride, a fourth capping layer 13 of chromium and then deposition of the second non-superconducting metal layer 9. The second non-superconducting metal layer 9 can be silver, copper or gold. The thicknesses of the third and fourth capping layers 11, 13 are 20 nm. Thickness of the second superconducting layer 12 is 200 nm and the thickness of the second non-superconducting metal layer 9 is 200 nm. Alternatively, the third and fourth capping layers 11,13 are of nickel chromium.

[0053] A process to manufacture the flexible superconducting circuit 30 like that as described in Fig. 3 comprises separated manufacturing steps for respectively a first subpart comprising the first foil 2 provided with the subsequent layers as described hereinbefore and a second subpart comprising a second foil 21 provided with subsequent layers as described in the following paragraph and a step of assembly of the flexible superconducting circuit by stapling of the first subpart part and the second part. The second manufacturing step of the second subpart comprises providing a second foil 21 of a dielectric material, for example, polyimide (Kapton) and deposition of subsequently the fifth capping layer 22 of chromium, the third superconducting layer 23 of niobium titanium or niobium titanium nitride, the sixth capping layer 24 of chromium, and the third non-superconducting metal layer 25. The third non- superconducting metal layer 25 is silver, copper or gold. The deposition of the subsequent layers on the second foil 2 can be performed in-situ in the vacuum deposition apparatus and can be similar as that described hereinbefore for deposition of the layers at the first foil 2. In this embodiment the thickness of the second foil of polyimide can be 50 pm. The thickness of the fifth and sixth capping layers of chromium is respectively 20 nm. The thickness of the third non-superconducting metal layer 25 is 200 nm.

[0054] Alternatively, the fifth and sixth capping layer can be of nickel chromium.

[0055] In a further manufacturing step, after manufacturing of the first and second subparts, the second subpart is turned such that the second foil and subsequent layers are turned upside down and fixed at the first non-superconducting metal layer 6 of the first foil 2 of the first subpart. Alignment marks can be applied on the first and second foil in order for a precise arrangement of the first and second foils.

[0056] The invention relates to a flexible superconducting circuit. The flexible superconducting circuit comprises a first foil of dielectric material, for example a polyimide; a first capping layer comprising chromium; a first superconducting layer; and a second capping layer comprising chromium. The superconducting layer can comprise niobium titanium or niobium titanium nitride. The first and second capping layer of chromium protects the superconducting layer against impurities dissolved in the polyimide and acts as an adhesion layer between the superconductor circuit and the polyimide due to chemical bonding.

[0057] Although illustrative embodiments of the present invention have been described with reference to the accompanying drawings, it is to be understood that the invention is not limited to these embodiments. Various changes or modifications may be affected by one skilled in the art without departing from the scope of the invention as defined in the claims. Accordingly, reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, it is noted that the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

Claims

Claims1. Flexible superconducting circuit (1,10, 20) comprising in this order: a first foil of dielectric material (2); a first capping layer (3) comprising chromium, Cr, directly coupled at a first flat side of the first foil; a first superconducting layer (4); and a second capping layer (5) comprising chromium, Cr, directly coupled to the first superconducting layer.

2. The flexible superconducting circuit (1,10, 20) of claim 1, wherein the first superconducting layer comprises niobium titanium, NbTi, or niobium titanium nitride, NbTiN.

3. The flexible superconducting circuit (1, 10, 20) of claim 1 or 2, wherein a thickness of the first superconducting layer is in the range between 50 nm -2 pm, preferably between 150 - 400 nm and more preferably 250 nm.

4. The flexible superconducting circuit (1, 10, 20) of any of the claims 1-3, wherein a thickness of the first capping layer (3) and the second capping layer (5) respectively is in the range between 15 nm and 50 nm, preferably 20nm.

5. The flexible superconducting circuit (1, 10, 20) of any of the claims 1-4, wherein the first foil (2) of dielectric material comprises a polyimide.

6. The flexible superconducting circuit (1, 10, 20) of according to any of the claims 1-5, wherein the flexible superconducting circuit further comprises a first non-superconducting metal layer (6) at a side of the second capping layer (5) facing away from the first superconducting layer (4).

7. The flexible superconducting circuit (1, 10, 20) of according to claim 6, wherein the first non-superconducting metal layer (6) comprises one of silver, Ag, copper, Cu, and gold, Au.

8. The flexible superconducting circuit (20) of according to any of the claims 1-7, wherein the flexible superconducting circuit further comprises: a first contact pad (7) connected to a first portion of the first superconducting layer (4); and a second contact pad (8) at a second portion of the first superconducting layer, wherein the location of the second portion is different than that of the first portion.

9. The flexible superconducting circuit (1) of claim 8 wherein the superconducting circuit comprises a plurality of parallel lines (26) patterned in the first superconducting layer (4), wherein the first contact pad (7) is arranged at one end of a line of the plurality of lines and the second contact pad (8) is arranged at another end of the line.

10. The flexible superconducting circuit (20) of any of the claims 6-9 further comprising, in this order, a third capping layer (11) comprising chromium at a second flat side of the first foil (2) of dielectric material facing away from the first superconducting layer, a second superconducting layer (12) directly coupled to the third capping layer and a fourth capping layer (13) comprising chromium coupled to the second superconducting layer.

11. The flexible superconducting circuit of claim 10 comprising a second non- superconducting metal layer (9) directly coupled to the fourth capping layer at a side facing away from the second superconducting layer.

12. The flexible superconducting circuit of claim 11 comprising in this order a second foil (21) of dielectric material at the first non-superconducting layer facing away from the first superconducting layer, a fifth capping layer (22) comprising chromium, a third superconducting layer (23) and a sixth capping layer (24) comprising chromium.

13. The flexible superconducting circuit of claim 12 wherein the flexible superconducting circuit comprises a third non-superconducting metal layer (25) at the sixth capping layer at a side facing away from the third superconducting layer.

14. A method of manufacturing a flexible superconducting circuit, the method comprising providing a first foil of a dielectric material; and deposition of, in this order, a first capping layer comprising chromium (Cr) at a firstflat side of the first foil of dielectric material; a first superconductor layer at the first capping layer; and a second capping layer comprising chromium (Cr) at the first superconductor layer.

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