Airbridge for making connections on superconducting chip, and method for producing superconducting chips with airbridges

Abstract

A bridge (113) is implemented on a superconducting wafer. It comprises a strip of superconducting material between a first superconducting region (104) and a second superconducting region (105). A first end of the strip includes a first planar end (301) attached to and parallel to the first superconducting region (104), and a second end of the strip includes a corresponding second planar end (302). A middle portion (303) is located between the first planar end (301) and the second planar end (302), forming a curve away from the plane defined by the surfaces of the first and second superconducting regions (104 and 105). A first separation line (304) and a second separation line (305) separate the ends (301, 302) from the middle portion (303). At least one of the first separation line (304) and the second separation line (305) does not point laterally across the strip.

Description

Technical Field

[0001] In general, this invention relates to the technology of superconducting wafer hardware. In particular, this invention relates to forming conductive (superconducting) contacts between conductive regions on a wafer. Prior Technology

[0002] Superconducting wafers are used in a variety of applications, such as quantum computing. These typically involve very high frequencies of electrical signals, such as those on the order of several GHz, making a well-defined and extended ground plane important. A ground plane is a relatively large, substantially continuous sheet of conductive material on one or more surfaces of a substrate. Adjacent ground planes can be separated from each other by signal transmission lines or other discontinuities, making it uncertain whether the potentials of the ground planes on either side of a discontinuity are truly the same. These discontinuities can cause undesirable phenomena, such as the so-called slot line mode in the ground plane.

[0003] Besides the ground plane, there may be other locations on the chip where a good electrical connection must be ensured between two conductive regions separated from each other by a discontinuity. Examples of such locations include places where signal transmission lines must pass through resonators or other signal transmission lines, or places where a point on a resonator or signal transmission line must be locally connected to ground.

[0004] The traditional method of forming an electrical connection at a discontinuity is wire bonding. Wire bonding machines use thermal and / or ultrasonic energy to form a metallurgical weld between one end of a thin metal wire and a first conductive region. The wire extends through the surrounding free space into a second conductive region, where a similar attachment is made before cutting the loose end of the wire.

[0005] While wire interconnects are a well-known and widely used technique, they have drawbacks, especially when aiming for miniaturization and high resistance to noise and interference. It is not uncommon for a quantum computing chip with six individual qubits to have more than 150 interconnects. Quantum computing chip designers are currently aiming to increase the number of qubits on the chip, which is expected to further increase the number of connections required between conductive regions. Although some interconnects can extend between conductive regions at the edge of the chip and adjacent conductive regions on the circuit board to which the chip is attached, there are still numerous locations on the chip where the drawbacks of wire interconnects become problematic. Summary of the Invention

[0006] The objective is to provide a structural design for forming conductive connections between adjacent conductive regions on a superconducting wafer in a scalable architecture. Another objective is that this structural design requires very little space on the wafer. Yet another objective is to make the fabrication of this conductive connection easy and reliable. Still another objective is to make this conductive connection as non-destructive as possible and as resistant as possible to noise and interference.

[0007] The objective of this invention is achieved by utilizing a thin-film deposition method to create microscopically small, strip-shaped airbridges that span discontinuities. The ends of the suspended portions of the airbridge can be shaped differently from those defined by straight lines, thereby improving structural stability.

[0008] According to a first configuration, a bridge configuration is used to form a connection on a superconducting wafer. The bridge comprises a strip of superconducting material between a first superconducting region and a second superconducting region, the second superconducting region being separated from the first superconducting region by a discontinuity. A first end of the strip includes a first planar end attached to and parallel to the first superconducting region, and a second end of the strip includes a second planar end attached to and parallel to the second superconducting region. The strip includes an intermediate portion between the first and second planar ends. The intermediate portion is curved away from the plane defined by the first and second superconducting regions, leaving a gap between the intermediate portion and the plane. A first separation line separates the first planar end from the intermediate portion, and a second separation line separates the second planar end from the intermediate portion. At least one of the first and second separation lines does not point laterally across the strip.

[0009] According to one embodiment, the strip is constrained by two longitudinal edges between the first end and the second end, and the length of the middle portion along the midline between the two longitudinal edges from the first separation line to the second separation line is different from the length of the middle portion along either of the two longitudinal edges. This has the advantage that, by using a relatively simple manufacturing method, the middle portion can exhibit curvature in at least two different directions.

[0010] According to one embodiment, the two ends of the first separation line are closer to the first end of the strip than the midpoint of the first separation line; and the two ends of the second separation line are closer to the second end of the strip than the midpoint of the second separation line. The length of the middle portion along the middle line between the two longitudinal edges from the first separation line to the second separation line is shorter than the length of the middle portion along either of the two longitudinal edges. This has the advantage that, by using a relatively simple manufacturing method, the middle portion can exhibit curvature in at least two different directions.

[0011] According to one embodiment, the strips of superconducting material comprise at least two material layers on top of each other, wherein the material layers have been deposited using different thin-film deposition methods. This has the advantage of allowing the air bridge to exhibit high structural strength and other favorable structural properties.

[0012] According to one embodiment, the intermediate portion has a saddle-shaped surface. This has the advantage of utilizing the inherent structural stability properties of the saddle-shaped surface.

[0013] According to one embodiment, the thickness of the strip in the direction perpendicular to the plane is at least 1 micrometer. This has the advantage of providing significantly better structural stability than a thinner air bridge.

[0014] According to one embodiment, the length of the strip from the first end to the second end includes a limit value between 80 and 120 micrometers. This has the advantage of allowing the air-bridge structure to fit various parts of a superconducting microelectronic wafer.

[0015] According to one embodiment, the width of the strip in the lateral direction includes a limit value between 40 and 80 micrometers. This has the advantage of allowing the air-bridge structure to fit into various parts of a superconducting microelectronic wafer.

[0016] According to the second configuration, a superconducting wafer is provided, comprising a substrate, a first superconducting region, and a second superconducting region. The first and second superconducting regions are located on the surface of the substrate, and the second superconducting region is separated from the first superconducting region by a discontinuity. The superconducting wafer includes an air bridge of the aforementioned type located between the first and second superconducting regions.

[0017] According to one embodiment, the superconducting wafer includes circuit elements located in the discontinuity between the first and second superconducting regions. These circuit elements can be, for example, transmission lines between other circuit elements, resonators, DC flux lines, DC voltage bias lines, RF lines such as control buses, or capacitive coupler lines. This has the advantage of providing relatively free possibilities for placing the circuit elements on the superconducting wafer without worrying about how to form conductive connections between the separated superconducting regions using these circuit elements.

[0018] According to a third embodiment, a method for generating a superconducting connection is provided, the superconducting connection spanning a discontinuity between a first superconducting region and a second superconducting region of a superconducting wafer. The method includes using one or more thin-film deposition methods to form a strip of superconducting material on top of the first and second superconducting regions, spanning the discontinuity. A first end of the strip includes a first planar end attached to and parallel to the first superconducting region; and a second end of the strip includes a second planar end attached to and parallel to the second superconducting region. The strip includes a middle portion between the first and second planar ends. The middle portion is formed to be curved away from a plane defined by the surfaces of the first and second superconducting regions, leaving a gap between the middle portion and the plane. At least one of the first and second planar ends is formed such that a corresponding separation line separating the planar end from the middle portion does not point laterally across the strip.

[0019] According to one embodiment, one or more thin film deposition methods is sputtering. This has the advantage of allowing for the relatively rapid preparation of material layers with considerable thickness.

[0020] According to one embodiment, the method includes: removing oxides from the surfaces of the first and second superconducting regions at locations where the strips will be respectively attached to the first and second superconducting regions; and using a first thin-film deposition method to deposit a first material layer at the locations. The removal of the oxides can be performed in situ in a vacuum environment, and the first material layer can be deposited at the locations using the first thin-film deposition method without compromising the vacuum environment. This has the advantage of producing very good conductive connections without concern about any intermediate oxidation that might cause undesirable conductivity loss.

[0021] According to one embodiment, ion milling is used to remove the oxide, and electron beam evaporation is used to deposit a first material layer at the location. This has the advantage that these method steps can be combined and performed in the same apparatus, provided it includes the capabilities required for both processes.

[0022] According to one embodiment, the method includes, after depositing the material at the location using a first thin-film deposition method, depositing a second material layer on top of the first layer using a second thin-film deposition method (a different thin-film deposition method). This has the advantage that a large number of material layers can be prepared relatively quickly, even if a faster deposition method is suboptimal for depositing the earliest first material layer.

[0023] According to one embodiment, at least one of the first planar end and the second planar end is formed such that the length of the intermediate portion along the longitudinal edges of the strip from the first separation line to the second separation line is different from the length of the intermediate portion along either of the two longitudinal edges. This has the advantage that, by using a relatively simple manufacturing method, the intermediate portion can exhibit curvature in at least two different directions. Simple Explanation of the Diagram

[0024] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this specification, illustrate embodiments of the invention and, together with the specification, help explain the principles of the invention. In the drawings: Figure 1 shows an example of an air bridge between conductive regions on the surface of a superconducting wafer; Figure 2 shows an example embodiment of an air bridge; Figure 3 shows an example embodiment of an air bridge; Figure 4 shows an example embodiment of an air bridge; Figure 5 illustrates a method; Figure 6 shows an example embodiment of an air bridge; Figure 7 shows an example embodiment of an air bridge. Implementation

[0025] Figure 1 is a top view of a portion of a superconducting wafer. The superconducting regions, indicated by oblique cross-sectional lines in the figure, are relatively extended and denoted by element symbols 101 to 106. These regions can be, for example, part of a ground plane within the superconducting wafer or other extended structure with a fixed potential. The superconducting region is a substantially two-dimensional surface region made of, covered by, and / or having an intermediate material layer capable of becoming superconducting at the temperature at which the wafer is intended to be used. Assuming the wafer is intended to be used as part of a quantum computing device, it may be intended to be used at temperatures below 1 K. Materials capable of being used to form superconducting regions on the wafer include, but are not limited to, aluminum, niobium, tantalum, titanium, and their superconducting alloys.

[0026] In a portion of the wafer shown in Figure 1, adjacent superconducting regions are separated from each other at discontinuities in the superconducting material. Circuit elements may be located at these discontinuities; examples of such circuit elements in Figure 1 are transmission lines 107 between other circuit elements (not shown in Figure 1) and four resonators 108 to 111. Other examples of such circuit elements are DC flux lines, DC voltage bias lines, RF lines such as control buses carrying microwave signals, and capacitive coupler lines.

[0027] At the lower end of the resonator is a coupling device used to provide signal coupling between transmission line 107 and resonators 108 to 111. In this example, the coupling device has the shape of two opposing, T-shaped material portions, from which the transmission line and resonators are made.

[0028] Air bridges are used to form connections between adjacent superconducting regions, spanning discontinuities and circuit elements located within those discontinuities. A total of 16 such air bridges are shown in Figure 1, with air bridges 112 and 113 illustrated as examples.

[0029] Figure 2 shows an enlarged isometric view of the air bridge 112, which forms a connection between a first superconducting region 104 and a second superconducting region 105, separated from each other by a discontinuity. As shown in Figure 2, the superconducting regions on the wafer do not need to be completely continuous: in the example of Figure 2, each of the first superconducting region 104 and the second superconducting region 105 is shown as comprising a regular array of openings, commonly referred to as flux traps. These prevent the generation of any unwanted magnetic fields that might otherwise be generated due to current flowing through a relatively large, continuous, conductive ground plane. The size of the openings is small enough that, at the relevant frequencies, the superconducting region in question has a grounding effect substantially similar to that of a continuous superconducting region. A portion of the circuit element 111 located at the discontinuity is also shown in Figure 2; in this example, the circuit element 111 is a resonator.

[0030] The air bridge 112 comprises a strip of superconducting material situated between the first superconducting region 104 and the second superconducting region 105. The term "strip" refers to the overall profile of the material constituting the air bridge 112: it is somewhat elongated and has a substantially constant thickness, which is relatively small compared to its length and width. Specifically, the strip comprises superconducting material deposited on top of the first superconducting region 104 and the second superconducting region 105 using a thin-film deposition method. Examples of suitable thin-film deposition methods are described later herein. The use of thin-film deposition methods has inherent advantages over wire bonding, such as allowing the air bridge 112 to be relatively short and rigid, and enabling its very precise placement on the superconducting wafer.

[0031] The first end of the strip includes a first planar end 201, which is attached to and parallel to the first superconducting region 104. Similarly, the second end of the strip includes a second planar end 202, which is attached to and parallel to the second superconducting region 105. The strip includes a middle portion 203 between the first planar end 201 and the second planar end 202. The middle portion 203 is curved away from the plane defined by the surfaces of the first and second superconducting regions 104 and 105, leaving a gap between the middle portion 203 and the plane. This gap isolates the air bridge 112 from the circuit element 111 located at the discontinuity separating the first and second superconducting regions 104 and 105. The axes of the Cartesian coordinate system are selected in the manner shown in FIG2, and the plane defined by the surfaces of the first and second superconducting regions 104 and 105 is the xy plane or at least parallel to the xy plane.

[0032] It has been found that the air bridge formed as shown in Figure 2 exhibits many advantageous characteristics. Sufficient thickness can be achieved through thin-film deposition, resulting in a robust structure; the thickness d of the strip in the direction perpendicular to the plane defined by the surfaces of the first superconducting region 104 and the second superconducting region 105 is preferably at least 1 micrometer. In the coordinate system shown in Figure 2, this perpendicular direction is the z-direction. The strip-like shape has a bend in the middle portion 203, which reasonably resists any forces that tend to cause the air bridge to collapse. If desired, the mechanical strength of the strip can be enhanced by composing it from a cluster of different materials or heterogeneous structures. If necessary, the empty space below the middle portion 203 allows the air bridge 112 to be disconnected later without damaging the underlying circuitry 111.

[0033] However, measurements using an air bridge like the one in Figure 2 have shown that some suboptimal performance may be encountered in certain situations. At least part of this suboptimal performance can be attributed to the mechanical vibrations of the air bridge itself.

[0034] Figure 3 illustrates another embodiment of a void bridge 113 for forming a connection on a superconducting wafer. Many parts in Figure 3 are similar to their counterparts in Figure 2 and are labeled with the same element symbols, so they need not be described again here. The first end of the strip forming the void bridge 113 includes a first planar end 301 attached to and parallel to a first superconducting region 104, and the second end includes a second planar end 302 attached to and parallel to a second superconducting region 105. The strip includes a middle portion 303 between the first planar end 301 and the second planar end 302, the middle portion 303 being curved away from the plane defined by the surfaces of the first and second superconducting regions 104 and 105, leaving a gap between the middle portion 303 and the plane.

[0035] The difference from the embodiment of FIG. 2 lies in the manner in which the portions 301, 302, and 303 of the strip are formed. A first separating line 304 separates the first planar end 301 from the middle portion 303. A second separating line 305 separates the second planar end 302 from the middle portion 303. At least one of the first separating line 304 and the second separating line 305 does not point to traverse the strip exactly laterally. In the embodiment of FIG. 3, this applies to both separating lines 304 and 305: they both have a curved shape, which differs from the straight, lateral shape seen in the embodiment of FIG. 2.

[0036] Here, a line pointing laterally across the strip means a straight line perpendicular to the longitudinal direction of the strip. A separating line not pointing exactly laterally across the strip can be a curve or a zigzag line, with its ends equidistant from the ends of the strip. Alternatively, a separating line not pointing exactly laterally across the strip can be a directional line that is not perpendicular to the longitudinal direction of the strip.

[0037] It has been found that forming the separation lines 304 and 305 in this way causes the middle portion 303 to have a slightly curved shape in both directions, as shown in Figure 3. This is used to provide additional structural stability, making it possible to largely avoid the suboptimal performance mentioned above with reference to Figure 2.

[0038] Figure 4 illustrates some possible shapes of the separation line, which separates the ends from the middle section. In Figure 4, each individual rectangle represents a top view of the strip-shaped air bridge. In each case, the strip is limited by two longitudinal edges between its first and second ends. In most cases, the length of the middle section along the median line between the two longitudinal edges (see the dashed line in Figure 4) from the first separation line to the second separation line differs from the length of the middle section along either of the two longitudinal edges. In case f) shown in Figure 4, the length of the middle section along the median line 401 from the first separation line to the second separation line is the same as the length of the middle section along the two longitudinal edges, but there is a significant variation in length along any other line between the two longitudinal edges.

[0039] A common feature of the embodiments in Figure 3 and the cases a), b), c), and h) in Figure 4 is that the middle of each separation line extends towards the other end. In other words, the two ends of the first separation line are closer to the first end of the strip than the midpoint of the first separation line, and the two ends of the second separation line are closer to the second end of the strip than the midpoint of the second separation line. It has been found that this method of forming the separation lines allows the intermediate portion to exhibit a saddle-shaped surface shape, as shown in Figure 3. A saddle-shaped surface is a surface that exhibits curvature in two opposite directions along different intersecting planes. For example, in Figure 3, the intermediate portion 303 shows the following curved shape in cross-section of the xz plane: the midpoint of the curved shape is closest to the plane defined by the surfaces of the first superconducting region 104 and the second superconducting region 105. The intermediate portion 303 shows the following curved shape in cross-section of the yz plane: the midpoint of the curved shape is farthest from the plane.

[0040] In Case g), the middle of the first separation line is closer to the first end of the strip than either end of the first separation line, and the middle of the second separation line is closer to the second end of the strip than either end of the second separation line. This way of forming the separation lines will tend to make the middle part appear bubble-shaped, again exhibiting curvature in both directions, although in this case, the vertical cross-section is shown as curving in a similar direction in both directions, not in the opposite direction as in the saddle-shaped surface.

[0041] Case d) in Figure 4 is an example where one of the separating lines can simply point laterally across the strip, while the other separating line does not. This principle can be generalized to cover all possible shapes of other separating lines, not just those similar to Case b) as in Case d) of Figure 4.

[0042] As illustrated in Case h) of Figure 4, the ends of the strip do not need to be straight and / or perpendicular to the longitudinal direction of the strip. This principle can be extended to strips with all types of separation lines, not just those similar to Case c) of Figure 4.

[0043] Similar to Figure 2, in the embodiment of Figure 3, the thickness d of the strip in the z-direction is preferably at least 1 micrometer. This allows for a thinner air bridge, but at the cost of reduced structural stability. Other advantageous dimensions of the strip are found to be that the length of the strip from its first end to its second end includes these limits between 80 and 120 micrometers; and the width of the strip in the transverse direction includes these limits between 40 and 80 micrometers.

[0044] Regarding the strips of superconducting material constituting the air bridge, it has been found advantageous that the strips comprise at least two material layers on top of each other. According to one embodiment, these material layers are deposited using different thin-film deposition methods. Next, referring to Figure 5, we will consider this or other patterns for preparing air bridges of the aforementioned type.

[0045] Figure 5 illustrates five main steps of the method. For each step, an isometric view is shown on the left and a cross-section is shown on the right. In the top step of Figure 5, the imaginary plane drawn with dashed lines and intersecting the structure on the left shows the plane of the cross-section taken on the right.

[0046] The first step, shown in Figure 5, includes providing a substrate 501 and covering its surface 502 with a patterned photoresist. Here, as in all other embodiments, the substrate 501 can be any suitable material that serves as a structural support layer for the superconducting wafer. Examples of substrate materials include, but are not limited to, silicon, gallium arsenide, and sapphire.

[0047] The surface covered by photoresist 502 includes a first superconducting region and a second superconducting region, which are connected to the air bridge, but are not shown separately in Figure 5. The pattern in the photoresist includes two openings 503 and 504, located at the first and second planar ends of the strip where they will be placed in the completed air bridge. A piece of photoresist 505 remains intact between openings 503 and 504: this piece of photoresist will ultimately define the blank space below the middle portion of the air bridge.

[0048] The first step in Figure 5 shows that the edges of openings 503 and 504 that face each other are not straight, or at least do not follow a direct lateral path across an imaginary straight line connecting the center points of the two openings. In this embodiment, the edges all have a wide V-shape, where the mid-angles of the V face each other. This method step has a significant impact on the shape of the separation line defined between the planar ends and the middle portion of the strip, as the separation line will follow the path of these edges of the openings.

[0049] The accompanying diagram is simplified to include all material layers constrained by strict planar cross-sections: in practice, the openings 503 and 504 in the photoresist are formed such that the midpoints of their facing edges are closer together, which tends to make the entire sheet of photoresist 505 between the openings thinner at its narrowest point. This helps to form a hollow bridge with a saddle-shaped surface in the middle.

[0050] The second step, shown in Figure 5, involves depositing a material layer 511 using one or more thin-film deposition methods. A portion of the material layer 511 ultimately forms strips of superconducting material on the patterned photoresist. Many thin-film deposition methods inherently possess the ability to fabricate conformal material layers of substantially uniform thickness across all geometries of the underlying substrate. This is schematically shown in the right-hand cross-section of the second step in Figure 5. Also noteworthy in the isometric drawing is the presence of grooves 513 and 514 on the visible surface of the material layer 511 at the locations of openings 503 and 504 in the underlying patterned photoresist.

[0051] The entire material layer 511 can be prepared using a single thin film deposition method. However, it has been found advantageous, at least in some cases, to utilize several different thin film deposition methods. Sputtering is advantageous as one of these methods because it can advantageously involve a rapid increase in the thickness of the deposited material layer.

[0052] In an advantageous embodiment, the second step shown in FIG5 includes removing oxides from the surfaces of the first and second superconducting regions at locations where strips of superconducting material will be attached to the first and second superconducting regions. These locations are visible and accessible via openings 503 and 504 in the photoresist 502. Subsequently, a first material layer is deposited at these locations using a first thin film deposition method. The removal of oxides can be performed in situ in a vacuum environment. To prevent the formation of any additional oxides, it is advantageous to deposit the first material layer at these locations using the first thin film deposition method without compromising the vacuum environment therebetween. Alternatively, with the substrate rapidly loaded into a protective environment such as a vacuum, non-in-situ methods such as chemical etching with hydrofluoric acid can be used to remove the oxides.

[0053] An advantageous method for removing oxides is ion milling, and an advantageous method for depositing a first material layer at the location is electron beam evaporation. The first material layer can be, for example, composed of aluminum, and its thickness can be relatively small compared to the final thickness of the completed strip. Using electron beam evaporation, an aluminum layer, for example, 20 nanometers thick, can be deposited as the first layer.

[0054] A second, different thin-film deposition method can be used to deposit a second material layer on top of the first layer. The choice of method and material depends on the available thin-film deposition systems and their characteristics. One material may have, for example, good superconductivity, while the other may have good structural strength, or be deposited using methods that rapidly increase the layer thickness to a desired range of approximately 1 micrometer. As previously explained, depending on, for example, the properties of the materials involved and their interactions in the composite structure, thinner (or thicker) material layers can be used to form air bridges.

[0055] The third step shown in Figure 5 involves fabricating another layer of patterned photoresist 521. This time, only portions of the material layer 511 deposited in the previous step are covered by the photoresist 521 forming strip-shaped air bridges.

[0056] In the fourth step shown in Figure 5, the portion of material layer 511 not covered by the latest photoresist 521 is removed. The latest photoresist 521 protects the strip-shaped portion of material layer 511 that constitutes the air bridge. Some cavities may be formed at the ends and sides of photoresist 521, shown as 531 and 532, but photoresist 521 mostly protects the portion of material layer 511 beneath it.

[0057] Finally, all remaining photoresist is removed. This final step removes not only the protective photoresist 521 at the top of the strip, but also the portion 505 of the first photoresist that remains below the middle section, leaving the structure shown in the final step of Figure 5.

[0058] Figures 6 and 7 illustrate further embodiments of using air bridges in superconducting wafers. In the example of Figure 6, the superconducting wafer includes a substrate and a first linear superconducting region 601 on its surface, one end of which is visible in Figure 6. The linear superconducting region 601 can be, for example, part of a resonator or a signal transmission line. The end of the first superconducting region 601 is surrounded on the substrate surface by a ground plane 602 in the form of a second superconducting region. It is separated from the first superconducting region by a discontinuity 603. An air bridge 604 is disposed between the end of the first superconducting region 601 and the ground plane 602, bridging the discontinuity 603. This is an example illustrating that, in any case, an air bridge is not necessarily only used to connect two superconducting regions separated from each other by some actual circuit elements located at the discontinuity. The air bridge 604 can be of any kind already described above.

[0059] Figure 7 shows a portion of a superconducting wafer, which includes a substrate and a first linear superconducting region 701 on its surface. Similar to Figure 6, the first linear superconducting region 701 can form part of, for example, a resonator or a signal transmission line. A second linear superconducting region 702 exists, which can also be part of, for example, a resonator or a signal transmission line. Each of these two superconducting regions is surrounded by a U-shaped portion of a corresponding ground plane 703 or 704. Discontinuities 705 and 706 separate the first superconducting region 701 from the first U-shaped ground plane portion 703 and the second superconducting region 702 from the second U-shaped ground plane portion 704, respectively. An additional linear superconducting region 707 extends at a linear discontinuity that separates the U-shaped ground plane portions 703 and 704 from each other. The additional linear superconducting region 707 can be part of, for example, a resonator or a signal transmission line.

[0060] A first air bridge 708 connects the first superconducting region 701 and the second superconducting region 702 by bridging a discontinuity between them. This discontinuity is the connection between the U-shaped ground plane portions 703 and 704 and another linear superconducting region 707. Two other air bridges 709 and 710 connect the U-shaped ground plane portions 703 and 704 by bridging this discontinuity, with another linear superconducting region 707 located within the discontinuity.

[0061] It will be apparent to those skilled in the art that, with advancements in technology, the basic principles of this invention can be implemented in various ways. Therefore, this invention and its embodiments are not limited to the examples described above, but rather can vary within the scope of the claims.

[0062] 101: Superconducting region

[0063] 102: Superconducting region

[0064] 103: Superconducting region

[0065] 104: First superconducting region

[0066] 105: Second superconducting region

[0067] 106: Superconducting region

[0068] 107: Transmission Line

[0069] 108: Resonator

[0070] 109: Resonator

[0071] 110: Resonator

[0072] 111: Resonator

[0073] 112: Sky Bridge

[0074] 113: Sky Bridge

[0075] 201: End of the first plane

[0076] 202: End of the second plane

[0077] 203: Middle Section

[0078] 301: End of the first plane

[0079] 302: End of the second plane

[0080] 303: Middle Section

[0081] 304: First separation line

[0082] 305: Second separation line

[0083] 401: Centerline

[0084] 501:Substrate

[0085] 502: Optical Resist

[0086] 503: Opening

[0087] 504: Opening

[0088] 505: Optical Resist

[0089] 511: Material layer

[0090] 513: Groove

[0091] 514: Groove

[0092] 521: Optical Obscuration

[0093] 531: Cavity

[0094] 532: Cavity

[0095] 601: Superconducting region

[0096] 602: Grounding plane

[0097] 603: Discontinuity

[0098] 604: Sky Bridge

[0099] 701: Superconducting region

[0100] 702: Superconducting region

[0101] 703: Grounding Plane Section

[0102] 704: Grounding plane portion

[0103] 705: Discontinuity

[0104] 706: Discontinuity

[0105] 707: Linear superconducting region

[0106] 708: Sky Bridge

[0107] 709: Sky Bridge

[0108] 710: Sky Bridge

Claims

1. A hollow bridge (113) for forming a connection on a superconducting wafer, wherein: - The air bridge comprises a strip of superconducting material between a first superconducting region (104) and a second superconducting region (105), the first superconducting region (104) and the second superconducting region (105) being separated by a discontinuity; - The first end of the strip includes a first planar end portion (301) attached to and parallel to the first superconducting region (104); - The second end of the strip includes a second planar end portion (302) attached to and parallel to the second superconducting region (105); - The strip includes a middle portion (303) at the first planar end portion (301) and the second planar end portion. (302) between; and - the intermediate portion (303) forms a bend away from the plane defined by the surfaces of the first superconducting region (104) and the second superconducting region (105), leaving a blank space between the intermediate portion (303) and the plane; - a first separation line (304) separating the end of the first plane (301) from the intermediate portion (303); - a second separation line (305) separating the end of the second plane (302) from the intermediate portion (303); and - at least one of the first separation line (304) and the second separation line (305) does not point laterally across the strip.

2. The air bridge as described in claim 1, wherein: - The strip is limited by two longitudinal edges between the first end and the second end; - The length of the middle portion from the first separation line to the second separation line along the midline between the two longitudinal edges is different from the length of the middle portion along either of the two longitudinal edges.

3. The air bridge as described in claim 2, wherein: - The two ends of the first separation line (304) are closer to the first end of the strip than the midpoint of the first separation line (304); and - The two ends of the second separation line (305) are closer to the second end of the strip than the midpoint of the second separation line (305); such that the length of the middle portion (303) along the middle line between the two longitudinal edges from the first separation line (304) to the second separation line (305) is shorter than the length of the middle portion (303) along either of the two longitudinal edges.

4. An air bridge as described in any one of claims 1 to 3, wherein, The superconducting strip comprises at least two material layers on top of each other, wherein the material layers are deposited using different thin film deposition methods.

5. An air bridge as described in any one of claims 1 to 3, wherein, The middle part (303) has a saddle-shaped surface.

6. An air bridge as described in any one of claims 1 to 3, wherein, The thickness of the strip in the direction perpendicular to the plane is at least 1 micrometer.

7. An air bridge as described in any one of claims 1 to 3, wherein, The length of the strip from the first end to the second end includes limits between 80 and 120 micrometers.

8. An air bridge as described in any one of claims 1 to 3, wherein, The width of the strip in the lateral direction includes a limit value between 40 and 80 micrometers.

9. A superconducting wafer, comprising: - Substrate (501); - First superconducting region (104) and second superconducting region (105) on the surface of the substrate, the first superconducting region (104) and the second superconducting region (105) being separated by a discontinuity; and - an air bridge (113) as claimed in any one of claims 1 to 8, located between the first superconducting region (104) and the second superconducting region (105).

10. The superconducting wafer as claimed in claim 9, comprising circuit elements (107, 108, 109, 110, 111) located at a discontinuity between the first superconducting region (104) and the second superconducting region (105), wherein, The circuit element (107, 108, 109, 110, 111) is one of the following: a transmission line (107) between other circuit elements, a resonator (108, 109, 110, 111), a DC flux line, a DC voltage bias line, an RF line such as a control bus, or a capacitive coupler line.

11. A method for generating a superconducting connection spanning a discontinuity between a first superconducting region and a second superconducting region of a superconducting wafer, the method comprising using one or more thin-film deposition methods to form a strip of superconducting material on top of the first and second superconducting regions and spanning the discontinuity, such that: - a first end of the strip includes a first planar end attached to and parallel to the first superconducting region; - a second end of the strip includes a second planar end attached to and parallel to the second superconducting region; - the strip includes a middle portion between the first and second planar ends; - the middle portion is formed with a bend away from a plane defined by the surfaces of the first and second superconducting regions, leaving a gap between the middle portion and the plane; and - at least one of the first and second planar ends is formed such that a corresponding separation line separating the planar end from the middle portion does not point laterally across the strip.

12. The method as described in claim 11, wherein, One of these one or more thin film deposition methods is sputtering.

13. The method of any one of claims 11 or 12, the method comprising the steps of: - removing oxide from the surfaces of the first superconducting region and the second superconducting region at the location where the strip is respectively attached to the first superconducting region and the second superconducting region; and - depositing a first material layer at the location using a first thin film deposition method; wherein, The oxide is removed in situ in a vacuum environment, and the first material layer is deposited at the location using a first thin film deposition method without compromising the vacuum environment.

14. The method as described in claim 13, wherein, The oxide is removed using ion milling, and the first material layer is deposited at that location using electron beam evaporation.

15. The method as described in claim 13, the method comprising: - After depositing the material at the location using the first thin film deposition method, a second material layer is deposited on top of the first layer using a second, different thin film deposition method.

16. The method as described in either claim 11 or 12, wherein, At least one of the first planar end and the second planar end is formed such that the length of the middle portion of the midline between the longitudinal edges of the strip from the first separation line to the second separation line is different from the length of the middle portion along either of the two longitudinal edges.

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