Patent Description:
Extremely sensitive magnetometers are in high demand in a wide range of technology fields, including medicine (e.g., magnetoencephalography), mining (e.g., detection of ore bodies) and biology (e.g., monitoring of biological processes). A popular type of sensitive magnetometers is a superconducting quantum interference device (SQUID) that are used to measure extremely subtle magnetic fields.

At the heart of a SQUID is a superconducting loop with two quantum barriers in opposite loop paths. These barriers are also referred to as Josephson junctions and allow the tunnelling of superconducting electrons, which result in a voltage that depends on the magnetic field. This disclosure relates to the fabrication of these Josephson junctions. In particular, this disclosure relates to the fabrication of a step edge that can be the basis of a Josephson junction.

One technique for fabricating a Josephson junction is to use the direction of a superconducting crystal. More particularly, it is possible to grow the crystal in a particular direction on one side of the junction and in a different direction on the other side of the junction. When both meet in the middle (the "grain boundary"), the difference in crystal directions forms the barrier that then acts as a Josephson junction.

Growing crystals in different directions can be achieved by changing the direction of the surface on which the crystal grows. <FIG> illustrates a profile <NUM> of a superconductor substrate. Importantly, the profile <NUM> comprises a step edge <NUM>, which is a sharp transition from a sloped surface <NUM> to a horizontal surface <NUM>. There is usually a flat horizontal region between <NUM> and a smooth return path <NUM>, with the return path <NUM> having a gradual gradient to the level of surface <NUM>, which is a relatively smooth part of the profile <NUM> and ensures that the profile height returns back to the height of the horizontal surface <NUM> without a further step edge. <CIT> discloses a method of fabricating a step edge.

<FIG> shows the result of growing crystal onto the profile <NUM> in <FIG>. Importantly, this forms a grain boundary <NUM> where the different directions of crystal growth can be seen as they meet at the boundary <NUM>. This results in a barrier that then constitutes the Josephson junction. A similar structure is fabricated on the other side of a superconducting loop (not shown) so that two junctions are in the loop. It is noted that the smooth return path <NUM> does not form a Josephson junction due to the smoothness of the return path <NUM>, as well as the relatively low angle. It is also noted that the angle at the bottom of the step where <NUM> meets <NUM> (or <NUM> meets the flat horizontal section between <NUM> and <NUM>) is also smooth so that no junction forms. This is important because, according to the understanding in the prior art, a second Josephson junction in this part of the loop would render the SQUID less sensitive or even unusable. Typically, the profile in <FIG> is repeated so that the resulting profile resembles a sawtooth profile with a step edge <NUM> and a smooth return path <NUM>.

It is further noted that the characteristic (i.e. critical current) of the Josephson junction can be tuned by changing the step angle <NUM> shown in <FIG>. Further, the sensitivity can be further increased by combining a large number of SQUIDs into an array. However, with an increasing number of SQUIDs in the array, it becomes a problem that the smooth return path <NUM>/<NUM> uses a significant amount of space that cannot be used for further SQUIDS. Further, the smooth return path <NUM>/<NUM> also introduces variations in the crystal which are difficult to control and as a result, the crystal is of poorer quality, may also consist of multiple randomly oriented crystals, and may be damaged easily.

<FIG> illustrates a Josephson junction in plan view again comprising a step edge <NUM> (the dashed line), a sloped surface <NUM> (dark shaded area), a horizontal surface <NUM> and smooth return path <NUM> (light shaded area). The solid outline <NUM> delineates areas of crystal growth. It can be seen that there are also sidewalls which are orthogonal to the step-edge <NUM> and use further area. The crystal is removed from the side walls <NUM> to prevent poor Josephson junctions forming. The step edge angle is defined by an ion beam that is directed at the substrate at a particular angle. Therefore, if the shown device is replicated many times on the chip, all step edges on the same chip have the same orientation, limiting the options available for device design. Further, step-edge morphology varies with distance from the ion beam, so it can differ between the top and bottom of the substrate. This contributes to junction device parameter variability, which reduces device efficiency (arrays) and reliability. Junction uniformity - superconducting junction parameters vary by as much as <NUM> % or more across chip. <NPL>) discloses submillimeter wave detectors and mixers based on YBCO step-edge Josephson junctions incorporated into planar antennas. A meander-line detector structure with a series array of step-edge Josephson junctions is formed by depositing and patterning an YBCO film over several parallel grooves with steep flanks etched in a substrate.

<CIT> discloses a magnetic sensing element with two SQUID loops, each having two step-edge Josephson junctions located at an edge of a recess formed in a substrate.

This shows that there is a need to improve the existing methods and address the problems described above.

A method of forming multiple step edges in a surface of a crystalline substrate according to independent claim <NUM> comprises:.

The two walls may be substantially parallel and opposing each other.

Each of the multiple step edges may define an upper level and a lower level and the superconducting material may be deposited to form a first path section that crosses one of the two step edges from the upper level to the lower level; a second path section that crosses the same one of the two step edges from the lower level to the upper level; and a first connection on the lower level that connects the first path section to the second path section without reaching the upper level.

The first connection may be parallel to the one of the two step edges.

The first path section, the second path section and the first connection may form a first loop and material may be deposited to form a second connection to connect the first loop to a second loop deposited on a different one of the two step edges.

According to the present invention, the superconducting material is deposited onto the substrate to form a loop comprising two Josephson junctions each time the path crosses one of the two step edges.

The superconducting material may be deposited onto the substrate to form an array comprising more than two Josephson junctions each time the path crosses one of the two step edges.

Gradually rotating the substrate may comprise continuously rotating the substrate. Continuously rotating the substrate may comprise rotating the substrate at a constant rotation rate. The rotation rate may be greater than one rotation during exposing the resist and the substrate to the ion beam. The rotation rate may be more than <NUM> rotation per minute. The rotation rate may be more than <NUM> rotations per minute.

The two walls may be opposing each other on either side of the exposed area and a distance between the two opposing walls may be less than <NUM>. Junction parameters of the Josephson junctions may be identical within manufacturing variations.

The selected area may comprise multiple shapes each having two walls forming two respective step edges and the path crosses the two step edges of each shape multiple times. The shapes may be rectangular and arranged side by side such that the path crosses the two walls of all of the shapes in a straight line.

A device according to independent claim <NUM> comprises:.

The device comprises at least eight Josephson junctions.

A distance between the two step edges may be less than <NUM>.

Optional features of one of the method and the device are also optional features of the other of the method and the device.

This disclosure provides a method for fabricating step edges which opens up new degrees of freedom for junction positioning and orientation on a chip, and thus introduces new potential for device designs. The disclosed method may provide four key advantages over the current state-of-the-art:.

The methods disclosed herein can be used in all step-edge Josephson's junction devices such as SQUIDs, gradiometers, high frequency mixers and superconducting quantum interference filters (SQIF). In the case of SQIF devices, more junctions could be fabricated in a smaller area which will increase device performance. This has applications involving magnetometry such as geo-exploration, non-destructive evaluation, health sciences (biomagnetism and medicine), RF sensing and detection and communications, including satellite communications.

<FIG> illustrates a method <NUM> of forming multiple step edges in a surface of a crystalline substrate. The method comprises forming <NUM> a layer of resist over the surface. The layer of resist comprises openings to expose selected areas of the surface. For example, this layer can be formed by deposition of a layer of photo-resist on the surface of an MgO substrate or another suitable substrate and removing areas of the resist to expose selected areas of the surface using known photolithography techniques. This involves preparing a mask that is open for the selected areas, placing the mask on the photo-resist, directing light at the mask and removing the mask. In some examples, the opening in the mask has a rectangular shape. The areas of the photo-resist that were subjected to the light can then be removed. In this way, two opposing parallel walls are formed in the layer of resist on the perimeter and as the side walls of a rectangular depression, which is referred to as a 'trench' herein. As a result, the two walls are opposing each other. A negative resist can also be used for the same effect (where exposed areas of resist remain, while unexposed areas are removed). It is noted that it is equally possible to use alternative material, such as a hard mask to photoresist to pattern steps during step edge formation, for example a metal deposited by sputtering or e-beam deposition, or a ceramic material or alloy deposited by sputtering. This material can be deposited using positive or negative photo-resist to make the pattern. For example it could be deposited using a lift-off process.

It is noted that in some examples, the walls define a square or a rectangle (i.e. a trench) on the substrate and there are four walls on the perimeter of the trench which differ in their orientations by <NUM> degrees between neighbouring sections. In other examples, however, the shape of the depression may be more complex, including hexagonal or other regular or irregular polygons. In yet further examples, the walls are not straight/planar but curved and may define a circular depression. In this sense, multiple wall sections flow continuously into each other along the perimeter of the area and at a first point along the wall the orientation is different to the orientation at a second point. In this case there may be an infinite number of wall sections that could be defined. A wall may have a varying surface orientation along the wall and not every point along the wall has the same orientation. In other words, the wall may not be a single planar surface (but may comprise multiple planar surfaces), noting that in the example of a rectangular trench, each of the opposing walls are single planar surfaces. It is noted that the walls are on the perimeter of the area that is selected for exposure and etching.

The next step of method <NUM> is exposing <NUM> the resist and the substrate to an ion beam. This etches the resist and the exposed areas of the surface. While exposing the resist and the substrate to the ion beam, the substrate is gradually rotated <NUM> about an axis normal to the surface to thereby form a step edge at each of the two opposing walls.

The gradual rotation may be a continuous rotation, such as at a rotation speed of <NUM> rotations per minute during an exposure of <NUM>-<NUM> minutes, or a rotation speed of <NUM>, or even <NUM> rotations per minute, for a similar exposure time of <NUM> - <NUM> (exposure time is dependent on angle <NUM> (<FIG>). Other rotation rates may equally apply and may be dependent on a range of manufacturing parameters, such as greater than one rotation during the entire exposure time of the resist to the ion beam or more than <NUM> rotation per minute or more than <NUM> rotations per minute. In other examples, the gradual rotation may be a rotation at varying speed or in small steps, such as less than <NUM> degrees in each step or less than <NUM> degrees or less than <NUM> degree. It is further noted that in most examples, the rotation comprises multiple full (X*<NUM>°) rotations. But in other examples, the rotation may comprise less than a full rotation, such as a half (<NUM> degrees) or a quarter (<NUM> degrees).

It is important to note that the rotation is performed while the resist is exposed to the ion beam. This means the ion beam is not turned-off to rotate the chip but instead, the ion beam remains turned on during the rotation so that the etching occurs at all angles of the rotation.

The method then continues by depositing <NUM> superconducting material onto the substrate in a meandering shape. This forms a path that crosses the two opposing step edges multiple times. Further, this step forms a Josephson junction each time the path crosses one of the two opposing step edges as will be described below with reference to <FIG>.

Returning back to the different shapes that the walls can define, it should be noted that there may be a wall that forms a step edge and an opposite wall such that a second step edge is formed in a return path of the first step edge. It is noted here that the term "return path" is used to indicate any path that returns from a lower level back to the upper level of the substrate. In <FIG>, the smooth return path <NUM>/<NUM> is smooth in the sense that the return path has a gradual gradient to the upper level, having no edge when it reaches the upper level and therefore, no junction is created. In contrast, with using the methods and systems disclosed herein, the return path does comprise an edge and a junction is formed as will be shown in <FIG> and <FIG>. It is further noted that in symmetrical structures, such as trenches, the "return path" may be considered to lie on either one of the opposing side walls. In another example where the concept of a return path does not directly apply, two walls are located perpendicular to each other such that two step edges are fabricated that are also perpendicular to each other. In this example, it will often be the case that both walls are adjacent to the same exposed area of the surface and may even bound the exposed area of the surface. For example, if the exposed area is a rectangle, there are four walls that bound/define the rectangle and a step edge is formed on each of the four sides of the rectangle.

<FIG> illustrates a superconductor substrate <NUM>, such as MgO or other suitable substrate, during fabrication. Substrate <NUM> has a layer of photoresist <NUM> disposed thereon which forms a wall <NUM> (further walls are not shown for clarity) as a result of the previous photolithography steps. Substrate <NUM> is mounted on a substrate holder <NUM> and an ion beam <NUM> is incident on the wall <NUM> at an inclination angle <NUM> between the plane of the substrate <NUM> and the ion beam <NUM>. The inclination angle <NUM> may preferably be <NUM> degrees in some examples but may also be any angle between <NUM> and <NUM> degrees or even between <NUM> and <NUM> degrees. In another example, the angle is between <NUM> and <NUM> degrees. While the ion beam <NUM> is incident on the wall <NUM>, the substrate holder <NUM> gradually rotates the substrate <NUM> (and photoresist <NUM>) about a rotation axis <NUM>. This gradually changes the azimuth angle <NUM>.

This substrate holder <NUM> may have the following features:.

The rotation of the substrate holder during ion etching opens up new degrees of freedom for step edge formation on a patterned chip. For example, for an MgO substrate patterned with a typical rectangular shaped photoresist step edge pattern of exposed MgO surface, a step edge will now be formed on all four sides of the resist, where previously the step edge would have been formed only on one side, with two side walls and one smooth return path making up the other three sides.

<FIG> shows an example device with four step edges <NUM>, <NUM>, <NUM>, <NUM> fabricated according to method <NUM> in <FIG>. A meandering YBCO path <NUM> was also deposited as shown. As a result, Josephson junctions <NUM> are formed on opposite sides of the patterned rectangular region. Due to the meandering path as shown in <FIG>, the superconducting material forms a path that crosses the two opposing step edges multiple times. In other examples, junctions may also be placed on the top and bottom horizontal edges, making use of all four sides of the etching window. Previously this was difficult using the state of the art technique of step edge formation.

With the method disclosed herein, it may be possible to fabricate a higher density of step edges on a single chip, as there will no longer be space used by the presence of a smooth return path or side walls. This may help with scaling up the number of the junctions on a single substrate and make the process more appealing to manufacturers.

In some cases the performance is directly linked to the number of junctions in an array. Therefore this technique also has the potential to improve device performance. This process may also improve the on-chip uniformity of step edge junctions, and thus improve reliability of junctions across a substrate.

<FIG> shows a cross sectional comparison between two step edges after the first etch and prior to the second etch. One is prepared by the original method of creating step edge (<NUM>, without substrate holder rotation) and one is prepared by the method involving use of the rotating substrate holder (<NUM>). For illustration purposes, <FIG> also shows the ion beam <NUM> and angle <NUM> from <FIG>, which is shown in-plane (parallel to the plane of the drawing) for clarity. For the case without rotation <NUM>, the ion beam <NUM> would remain in-plane but for the rotation case <NUM>, ion beam would gradually change and rotate out of the plane of the drawing and eventually hit the opposite wall (not shown) such that the shown step edge would essentially lie in the shade. It is noted that <FIG> shows the right hand portion of <FIG> discussed below. The surface morphology at the bottom of the step and the step angle Φ (see <FIG>) are slightly different, but this change in morphology should not affect device function and the step angle can be engineered by adjusting the substrate holder angle. In particular, the step edge <NUM> has a height profile that varies along a distance from the step edge as a result of exposing the resist and the substrate to an ion beam while gradually rotating the substrate about an axis normal to the surface to thereby form a step edge. In this example, the profile <NUM> is less abrupt and changes more gradually than profile <NUM>. It can be seen that profile <NUM> extends from <NUM> to <NUM>, which means an opposing side wall could be placed within about <NUM> from the step edge <NUM>. While the nonrotating step edge <NUM> is steeper and therefore appears to occupy less space, an opposite step edge is not formed due to the static configuration of the ion beam <NUM> and a smooth return path is formed instead over about <NUM>, which is not shown due to the limited space on the x-axis.

<FIG> shows the cross sections of two opposite step edges formed only <NUM> from each other within the same photolithography window. This can be reduced to as low as <NUM> or less in other example designs. This is in contrast to previous technologies that rely on smooth return paths, usually up to <NUM> wide, where the distance between step edges would be at least <NUM>. As a result, the method disclosed herein allows the manufacturing of arrays with an increased density of step edges. It is noted that the distance of <NUM> is not a theoretical limit and depending on the manufacturing technology, a distance of less than <NUM>, such as less than <NUM> or even less than <NUM> is possible. <FIG> shows the three dimensional AFM images of the same scanned areas shown in <FIG>.

An array of <NUM>,<NUM> step edge Josephson junctions has been prepared using the rotating substrate holder <NUM>. This array utilises the principles of SQIF design, whereby greater than a thousand SQUID loops of varying size are fabricated in series and parallel in an array, in order to get a large voltage response to small changes in magnetic field. The voltage response from individual SQUID loops adds together via constructive interference to form an anti-peak around zero magnetic field, and destructive interference of the SQUID loop voltage response at non-zero magnetic fields. The array design was not optimised for the new type of junctions, however, a device was prepared as a proof of concept. <FIG> shows an image of the array with step edges highlighted by arrows, and where the lack of the return path is noted.

The observations from this device are as follows:.

The device gave the expected I-V response for Josephson junction arrays as shown in <FIG>.

The device also gave the expected SQIF voltage response to magnetic field as shown in <FIG>.

These results show that the step edges formed by the rotating substrate holder perform as expected and have similar properties to those produced by the state-of-the-art technique described in.

<CIT>; <CIT>; <CIT>; <CIT>; <CIT>; <CIT>; <CIT>, Available: <Go to ISI>://DIIDW:<NUM>.

The proposed process has the following advantages over existing process:.

The proposed method also allows a large range of step edge angles to be selected (<NUM> - <NUM> °, or <NUM> - <NUM>°) with commensurate control over critical junction parameters such as the critical current and normal resistance, and with high quality junctions. It is further noted that the step edge angle can be identical for all step edges, such as opposing step edges described below. This also means that the critical junction parameters are identical. "Identical" in this context means that the design sets these parameters as identical but of course, they are subject to manufacturing variations, which can be significant. For example, the step angle may vary by about plus/minus <NUM> degree or plus/minus <NUM> degrees and further variations, such as film quality, may lead to variations in critical current of about <NUM>%, or <NUM>% or <NUM>%. Identical junction parameters are advantageous in a range of applications, such as SQIFs with a large number of loops, for example.

<FIG> illustrates an example device <NUM> having a substrate <NUM> with two opposing steps <NUM> and <NUM> delimiting a lower level <NUM> from an upper level <NUM>. When comparing <FIG>, it is noted that in <FIG> the photoresist <NUM> is still present and the substrate <NUM> has not yet been exposed to the ion beam <NUM> to manufacture the multiple step edges. <FIG> shows the device after those steps, which means the photoresist has been removed and the steps <NUM> and <NUM> have been created. The lower level <NUM> is where no photoresist had shielded the substrate from the ion beam <NUM> and corresponds to the "exposed area" mentioned above. It is further noted that the profiles in <FIG> are taken along dashed line <NUM> in <FIG>.

Device <NUM> further comprises a first loop <NUM> and a second loop <NUM> made of superconducting material, such as YBCO, that has been deposited onto substrate <NUM> as shown in <FIG> but without smooth return path <NUM>. Similar to step edge <NUM> in <FIG> that forms grain boundary <NUM> in <FIG> (i.e. Josephson junction), the first loop <NUM> forms two Josephson junctions <NUM> and <NUM> on the upper step edge of first step <NUM>. It is noted that there are no further junctions at the bottom of step <NUM> (indicated at <NUM> and <NUM>) because the bottom is rounded as shown in <FIG> and <FIG>, which is indicated by the dashed lines in <FIG>.

Comparing <FIG> again to <FIG>, instead of smooth return path <NUM>, there is the second step <NUM> on the return path on which second loop <NUM> is formed. In this sense, instead of the sawtooth profile of <FIG>, there is now a trench profile where the edges of the two opposing side walls <NUM> and <NUM> face each other. In this example, the edge between upper level <NUM> and side wall <NUM> is parallel to the edge between upper level <NUM> and side wall <NUM>. The same could be said about the lower level <NUM> but this is more difficult to define due to the rounding of those edges. As a result of the trench structure shown in <FIG>, there are two further junctions <NUM> and <NUM> that are not present in <FIG> by the smooth return path <NUM> but now used in the second loop <NUM>. It is noted that the further junctions <NUM> and <NUM> are collinear with each other and parallel to step edges <NUM> and <NUM>, which are also collinear to each other due to the trench structure. Again, there are no further junctions at the bottom of second step <NUM>. Each of the loops <NUM> and <NUM> now acts as a SQUID to detect a magnetic field and together they have a higher sensitivity than a single loop.

Importantly, the two loops <NUM> and <NUM> may or may not be connected to each other. When un-connected they form two separate SQUIDs and when connected they form a SQUID array. In all cases, within the lower level <NUM> they have a connection <NUM> and <NUM>, respectively, that is parallel to the respective steps <NUM> and <NUM> (but not necessarily parallel). In this sense the path taken by the deposited material forming loop <NUM>, leads down step <NUM> forming a first path section from upper level <NUM> to lower level <NUM> and returns back up the same step <NUM> in the opposite direction forming a second path section from lower level <NUM> to upper level <NUM> without crossing another step in between and without reaching the upper level <NUM> in-between to form a loop. As a result, there are now two loops <NUM>, <NUM> in an area that would have previously been taken up by the step and smooth return path of a single loop. Importantly, the lower area <NUM> would be larger for a smooth return path than for two steps <NUM> and <NUM>, which means that the increase in density is significantly more than twice the previous densities as two loops now fit into an area that is smaller than the area previously required for a single loop. In other words, <FIG> now defines a trench shape whereas <FIG> defines a sawtooth shape between two adjacent step edges.

<FIG> show how the loops <NUM> and <NUM> can be connected. This may be important and necessary for SQIF arrays. The dashed lines are the step edge and the solid lines define YBCO area. <FIG> shows unconnected and <FIG> show different ways the loops may be connected. In particular, in <FIG>, there is a contiguous area, such as a rectangle in this case, and the loops are formed by cut-outs (e.g. square or rectangular) from this area.

In a further example, a device comprises two opposing walls in the surface of the crystalline substrate in the sense that a trench (also referred to as trough) lies between the walls and the surfaces of the walls face each other. The trench is formed where openings are in the resist so that the area that later forms the trench is exposed to an ion beam to etch the exposed area to form the trench. The trench is rectangular in this case and the walls are the opposite sides of the trench. Superconducting material is then deposed on the substrate in a meandering shape. This means the superconducting material forms a path that crosses the trench multiple times from side to side. In other words, the path crosses the trench in one direction, returns in an opposite direction, crosses again in the first direction, returns in the opposite direction and so on. The turning point, that is a <NUM> degrees turn or U-turn may be formed by two <NUM> degrees turns or right angle corners in the sense that the path abruptly changes direction by <NUM> degrees. Alternatively, the corners may be rounded or have other shapes, including quarter circles or continuous curvature.

Preferably, the angle (i.e. steepness) of the walls of the trench in the substrate are identical (within manufacturing variations), which results in an identical junction parameters, such as critical current on either side of the rectangle.

By crossing the step edges multiple times, each time a Josephson junction is formed due to the grain boundaries at the step edge (with identical junction parameters). This means if the path crosses the trench and returns a single time, there are at least four Josephson junctions in the path. These include a first junction going down into the trench over the first step edge, a second junction going up out of the trench over the second step edge, a third junction after turning around and again going down into the trench over the second step edge and finally a fourth junction going up out of the trench over the first step edge. In this case, there are four junctions connected in series since the path of superconducting material provides a connection between the junctions. There may be additional junctions in parallel at each crossing as described below.

According to the present invention, the total number of junctions is at least eight. This is the case when there are two junctions in parallel at each crossing, which together form a respective SQUID loop. So there are at least four SQUID loops including a first loop going down into the trench over the first step edge, a second loop going up out of the trench over the second step edge, a third loop after turning around and again going down into the trench over the second step edge and finally a fourth loop going up out of the trench over the first step edge. In this case, the four loops are connected in series since the path of superconducting material provides a connection between the junctions.

It is noted that a single meander goes through the trench twice in opposite directions (there and back) and may be referred to as a building block, or simply 'block'. A block can be repeated many times along the trench to create a large number of junctions or loops. Each block comprises two 'arms' where the path goes through the trench and each arm comprises two step-edge crossings (down and up). As a result, each block comprises four crossings. The two arms may be parallel connected or series connected. Each crossing of a block may comprise a single junction or two junctions forming a loop, such that each block comprises either four junctions or four loops with eight junctions in total. Combinations of loops and junctions and parallel and series connected arms are also possible.

<FIG> shows a device <NUM> according to the present invention, particularly similar to <FIG> in the sense that there is also a meandering YBCO path <NUM> on substrate <NUM> with two opposite step edges <NUM> and <NUM> fabricated according to method <NUM> in <FIG>. Meandering means, throughout this disclosure, that the path forms a series of regular sinuous curves, bends, loops, turns, or windings across trough <NUM>. In other words, the path swings from side to side as it runs across trough <NUM>.

Again, Josephson junctions are formed on opposite sides of a trough <NUM> in substrate <NUM>. Due to the meandering path as shown in <FIG>, the superconducting material forms a path that crosses the two opposing step edges <NUM> and <NUM> multiple times. In contrast to <FIG>, where each crossing of a step edge constitutes a single Josephson junction, in <FIG> each crossing where the path crosses a step edge constitutes two Josephson junctions in parallel.

A step-edge crossing is indicated at <NUM>. This crossing <NUM> comprises a first junction <NUM> and a second junction <NUM>, which are connected in parallel and together form a SQUID loop. A connection <NUM> at the bottom of trough <NUM> connects the SQUID loop at crossing <NUM> to a further SQUID loop at the opposite step edge <NUM>.

At the same time, a further connection <NUM> at the top layer connects the SQUID loop at crossing <NUM> to a further SQUID loop on the same step edge <NUM>. This patterns continues in a meandering or zigzag form to create a number of SQUID loops that are connected in series. <FIG> also shows pads <NUM> and <NUM> to connect to the series connected SQUID loops.

As described above, the device <NUM> can be constructed from multiple instances or copies of basic building blocks as indicated at <NUM>. In this example, building block <NUM> comprises four loops in series and the device <NUM> includes <NUM> building blocks.

With the method disclosed herein, it may be possible to fabricate a higher density of SQUID loops on a single chip, as there will no longer be space used by the presence of a smooth return path or side walls. This may help with scaling up the number of the SQUID loops on a single substrate and make the process more appealing to manufacturers. In the embodiment of <FIG> there are ten SQUID loops in a relatively small area demonstrating the scalability of the proposed method.

In some cases the performance is directly linked to the number of loops in an array. Therefore this technique also has the potential to improve device performance. This process may also improve the on-chip uniformity of loops, and thus improve reliability of junctions across a substrate.

<FIG> illustrates another embodiment according to the present invention, where the top level connections <NUM> are modified compared to <FIG>, so as to connect two loops in parallel and then connect the parallel connected loops in series. As can be seen, the parallel-connected loops are adjacent to each other on the same step edge. In some embodiments, the adjacent loops may also be connected at the bottom level within the trough. Many different combinations are possible with potentially a large number of parallel connected loops. Again, a building block is indicated at <NUM> where block <NUM> now comprises two parallel connected arms with two series connected loops in each arm noting that each block may comprise a larger number of parallel connected arms, such as <NUM> parallel connected arms with <NUM> loops in total. These larger blocks are then series connected in the meander shape as described herein.

<FIG> illustrates yet another example similar to the aforementioned meander-line detector structure of KONOPKA J ET AL, where there are multiple areas that are exposed in the photo resist to create three trenches (i.e. depressions) <NUM>, <NUM> and <NUM>. The three trenches <NUM>, <NUM> and <NUM> form six step edges in total (two on either side of each trench). A meandering path <NUM> of superconducting material crosses the trenches <NUM>, <NUM> and <NUM> multiple times before it returns. Each time path <NUM> crosses a step edge, a Josephson junction is formed. In <FIG>, the junctions are indicated as arrows where the direction points upwardly from the bottom of the trench to the top layer. As can be seen, the path <NUM> in <FIG> forms <NUM> junctions in total, with a block consisting of <NUM> junctions. Accordingly, there can be multiple areas that are exposed in the photo resist to create more than one trench, with each trench, having a meandering path consisting of two Josephson junctions and a block consisting of an even number of Josephson junctions.

Claim 1:
A method of forming multiple step edges in a surface of a crystalline substrate (<NUM>, <NUM>), the method comprising:
forming a layer of resist (<NUM>) over the surface, the layer of resist comprising openings to expose a selected area of the surface, thereby forming a first wall and a second wall in the layer of resist on a perimeter of the selected area; and
exposing the resist and the substrate to an ion beam (<NUM>), thereby etching the resist and the exposed areas of the surface to thereby create an upper level (<NUM>) outside the selected area and a lower level (<NUM>) within the selected area of the surface;
while exposing the resist and the substrate to the ion beam, gradually rotating the substrate about an axis (<NUM>) normal to the surface to thereby form a first step edge (<NUM>) at the first wall and a second step edge (<NUM>) at the second wall; and depositing superconducting material onto the substrate to form a path that crosses the first and second step edges multiple times,
to form multiple Josephson junctions (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>) comprising at least one Josephson junction each time the path crosses one of the first and second step edges,
wherein the path comprises multiple connections (<NUM>, <NUM>, <NUM>) on the lower level (<NUM>), each connection connecting one of the multiple Josephson junctions on the first step edge to one of the multiple Josephson junction on the second step edge, and
the superconducting material is deposited onto the substrate to form a loop (<NUM>, <NUM>) comprising two Josephson junctions each time the path crosses one of the first and second step edges.