Josephson junction device and method for manufacturing the same

Abstract

A Josephson junction (JJ) device is provided. The JJ device includes an operating JJ, a first hydrogen trap JJ having a first end coupled to a first end of the operating JJ and a second end coupled to a first superconducting wiring, and a second hydrogen trap JJ having a first end coupled to a second end of the operating JJ and a second end coupled to a second superconducting wiring. The first hydrogen trap JJ and the second hydrogen trap JJ reduce hydrogen diffusion into the operating JJ.

Description

Technical Field

[0001] The present invention generally relates to superconductors, and more specifically to Josephson junction devices and methods of manufacturing the same. This application claims the priority of U.S. Patent Application No. 17 / 746,724, filed on May 17, 2022, the entire disclosure of which is incorporated herein by reference.

Background Art

[0002] Superconducting circuits are one of the major technologies being developed for improving the integrity of communication signals or increasing computing processing power. Superconducting circuits operate at temperatures below 100 Kelvin. Most efforts related to the manufacture of superconducting devices are limited to universities or government research institutes, and little has been published regarding the mass production of superconducting devices. For this reason, many of the methods used to manufacture superconducting devices in these laboratories utilize processes or equipment that are not capable of rapid and consistent manufacturing. Furthermore, the need for cryogenic processing is currently one of the greater barriers to the mass production of superconducting devices.

[0003] One of the common devices used in superconducting circuits is the Josephson junction (JJ). JJs have important applications in quantum mechanical circuits such as superconducting quantum interference devices (SQUIDs), superconducting qubits, and rapid single flux quantum (RSFQ) digital electronics. A JJ is an element consisting of two superconductors weakly coupled by a region that can be a non-superconductor or a weak superconductor. For example, one type of JJ consists of two superconductors separated by a thin insulating barrier. JJs have a critical current. The critical current is the maximum supercurrent that the junction can sustain without dissipating. Currents below the critical current will cause a change in the phase of the junction. To operate properly in applications using JJs, it is important to maintain a consistent critical current for the JJ. The I C (critical current) is known to drift over time. This effect is known as the aging degradation of the junction and can have an adverse effect on the consistent operation of JJ applications.

Summary of the Invention

[0004] In one embodiment, a Josephson junction (JJ) device is provided. The JJ device includes an operating JJ, a first hydrogen trap JJ having a first end coupled to the first end of the operating JJ and a second end coupled to a first superconducting wiring, and a second hydrogen trap JJ having a first end coupled to the second end of the operating JJ and a second end coupled to a second superconducting wiring. The first hydrogen trap JJ and the second hydrogen trap JJ reduce hydrogen diffusion into the operating JJ.

[0005] In one embodiment, a method of forming a Josephson junction (JJ) device is provided. The method includes forming a photoresist material layer on a substrate to form a JJ structure, patterning the photoresist material layer to form an opening in the photoresist material layer and a photoresist bridge near a central region of the JJ structure, performing a first superconducting inclined deposition process on the JJ structure to form a plurality of superconducting lower electrodes on the substrate, and performing an oxidation process to provide an oxidized top surface on the plurality of superconducting lower electrodes. The method further includes performing a second superconducting inclined deposition process on the JJ structure to form a plurality of superconducting upper electrodes. An overlap portion of one of the plurality of superconducting upper electrodes overlaps with one of the plurality of superconducting lower electrodes with the oxidized top surface therebetween, thereby forming a JJ below the photoresist bridge near the central region of the JJ structure. The method further includes performing an oxidation process to form a protective oxide on the JJ and removing the photoresist material layer to provide a final JJ structure.

Brief Description of the Drawings

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DETAILED DESCRIPTION OF THE INVENTION

[0007] The present disclosure relates to a Josephson junction (JJ) device and a method of manufacturing the same. The JJ device has a critical current (I C) is manufactured using elements that enhance the operation JJ against aging degradation that can vary. The JJ device is configured to be part of a microwave circuit. The JJ device is two additional hydrogen trap JJs inserted in series with the operation JJ, including a first hydrogen trap JJ inserted on the first side of the operation JJ and a second hydrogen trap JJ inserted on the second side of the operation JJ. The hydrogen trap JJs are designed with a critical current much larger than that of the operation JJ and do not affect the function of the microwave circuit. These large junctions function as hydrogen traps to protect the device junctions from hydrogen diffusion in superconducting (e.g., niobium) wiring that couples the hydrogen trap JJs to other parts of the microwave circuit by "pinching" the operation JJ. Further, an additional process is used to enhance the operation JJ against aging degradation. This additional process includes a post-junction oxidation process that is performed after the operation JJ is formed.

[0008] In one example, the JJ device includes a shadow-evaporated aluminum (Al)-based superconducting JJ (e.g., Al / aluminum oxide (AlOx) / Al) as the operation JJ incorporated into the operation JJ structure. The operation JJ is enhanced against aging degradation by a combination of hydrogen trapping action through two additional hydrogen trap JJs and a robust oxide barrier that protects the operation JJ from multiple aging degradation mechanisms. The entire JJ device can be oxidized by the same process used to form the AlOx barrier that forms the insulating layer of the operation JJ. The shadow-evaporated Al / AlOx / Al Josephson junction has a low I C is an academic standard for constructing superconducting quantum bit circuits. However, due to its inherent temporal instability and I C drift, using this type of junction outside of academic experiments has been hindered. The enhancement techniques described above enable the operation of the device outside of an academic environment by reducing the instability of the I C drift over time.

[0009] FIG. 1 shows a schematic diagram of an exemplary JJ device 10. The JJ device 10 includes a first hydrogen trap JJ14 having a first end coupled to a first end of the operating JJ12, and a second hydrogen trap JJ16 having a first end coupled to a second end of the operating JJ12. The first hydrogen trap JJ14 is coupled at a second end to a first superconducting wiring 18 (e.g., niobium) configured to be coupled to one or more microwave circuits, and the second hydrogen trap JJ16 is coupled at a second end to a second superconducting wiring 20 configured to be coupled to one or more microwave circuits. In one example, the first and second hydrogen traps JJ14, 16 are sized 100 times or more the size of the operating JJ12. The first and second hydrogen traps JJ14, 16 are inserted as a hydrogen barrier between the operating JJ12 and the superconducting wirings 18, 20 in the microwave circuit. Hydrogen diffusing from the superconducting wiring typically accumulates at the operating junction barrier and decreases the height of the electron barrier, thereby increasing the junction I C over time. The larger hydrogen traps JJ14, 16 are intended to trap hydrogen and prevent accumulation in the more sensitive operating JJ12, thereby stabilizing the junction I C of the operating JJ12.

[0010] The operating JJ12 and the hydrogen traps JJ14, 16 are connected to each other via superconducting lines 22, 24 (e.g., aluminum conductive lines) and are configured to be or are connected to a microwave circuit via the superconducting wirings 18, 20 to form the JJ device 10. The elements of the JJ device 10 can be sequentially formed on a wafer under vacuum to form the JJ device 10. Then, in the same process chamber in which the elements of the JJ device 10 are formed, an additional oxidation process can be performed on the wafer to form a robust high-quality native oxide on the surface of the JJ device 10. When this wafer is removed from the vacuum chamber, its surface is immediately exposed to water vapor and organic contaminants in the atmosphere. The robust native oxide passivates its surface and prevents the diffusion of contaminants into the tunnel barrier region of the operating JJ12, thereby stabilizing the I C of the operating JJ12.

[0011] Next, with reference to FIGS. 2 to 13, the manufacturing related to the formation of the JJ structure including the operation JJ of FIG. 1 will be described. In this example, a process flow showing the formation of the JJ structure including the operation JJ will be described, but it can be understood that this example can also be used similarly to manufacture the JJ structure including the first hydrogen trap JJ or the second hydrogen trap JJ. Further, the first hydrogen trap JJ, the second hydrogen trap JJ, and the operation JJ use the first hydrogen trap JJ and the second hydrogen trap JJ having different dimensions from the operation JJ in order to provide a larger critical current for the first hydrogen trap JJ and the second hydrogen trap JJ compared to the operation JJ, and all three JJs can be manufactured by sequentially or simultaneously using the same process repeated for all three JJs.

[0012] FIG. 2 shows a cross-sectional view of the JJ structure at an initial stage of manufacturing. FIG. 3 shows a plan view of the JJ structure of FIG. 2. The JJ structure includes a first photoresist material layer 52 covering the substrate 50 and a second photoresist material layer 54 covering the first photoresist material layer 52. The substrate 50 may be, for example, a silicon or glass wafer that provides mechanical support for formation. The first photoresist material layer 52 is a lift-off resist material, and the second photoresist material layer 54 is a standard photoresist material layer. Next, the first photoresist material layer 52 and the second photoresist material layer 54 are patterned and developed in order to make the trench openings 56, 58 appear according to the trench pattern within the first photoresist material layer 52 and the second photoresist material layer 54. A central bridge 57 (for example, a Dolan bridge) is formed between the trench opening 56 and the trench opening 58. The cross-sectional view of FIG. 4 shows the resulting structure. The first photoresist material layer 52 is configured to provide undercut regions 60, 62 by being undercut by a photoresist material developer used during the patterning and development processes. FIG. 5 shows a plan view of the structure of FIG. 4.

[0013] The first photoresist material layer 52 and the second photoresist material layer 54 can have various thicknesses corresponding to the wavelength of the radiation used to pattern the first photoresist material layer 52 and the second photoresist material layer 54. The first photoresist material layer 52 is formed on the substrate 50, and the second photoresist material layer 54 is formed on the first photoresist material layer 52 using spin coating or spin casting deposition techniques, and then selectively irradiated (e.g., using deep ultraviolet (DUV) irradiation) and developed to form trench openings 56, 58, central bridge 57, and undercut regions 60, 62.

[0014] Thereafter, this JJ structure can be placed in a deposition process chamber under vacuum. Then, as shown in FIG. 6, a first superconductor inclined deposition process 200 is performed on the structure of FIG. 5. The angle of the superconductor inclined deposition process can be realized by rotating the JJ structure counterclockwise by a rotation angle of about 30° to about 60° (e.g., 45°) with respect to the superconductor deposition source. That is, the incident angle of the superconductor metal is an angle of about 30° to about 60° with respect to the upper surface of the JJ structure. The deposition process 200 can be realized by various deposition processes such as sputter deposition of aluminum to form the first superconductor lower electrode 66 and the second superconductor lower electrode 68. The deposition process 200 can also be a superconductor metal evaporation process such as evaporation of aluminum to form the first superconductor lower electrode 66 and the second superconductor lower electrode 68. FIG. 7 shows the resulting structure.

[0015] Next, as shown in FIG. 8, the upper surfaces of the first superconductor lower electrode 66 and the second superconductor lower electrode 68 are oxidized, and an oxidized upper surface 70 is formed on the first superconductor lower electrode 66 and the second superconductor lower electrode 68. By arranging the JJ structure in the oxidation chamber and performing an oxidation process on the JJ structure, the oxidized upper surface 70 is formed on the first superconductor lower electrode 66 and the second superconductor lower electrode 68. The oxidation chamber may be the same chamber as the deposition chamber or a different chamber as long as the JJ structure remains in a clean environment under vacuum. During this oxidation process, the oxidized upper surface 70 of the superconductor metal covering the second photoresist material layer 54 is also formed. In one example, the oxidized upper surface 70 is aluminum oxide formed by oxidizing the upper surfaces of the first superconductor lower electrode 66 and the second superconductor lower electrode 68 formed of aluminum.

[0016] Next, as shown in FIG. 9, a second superconductor inclined deposition process 210 is performed on the structure of FIG. 8. The angle of the superconductor inclined deposition process can be realized by rotating the JJ structure clockwise by a rotation angle of about 30° to about 60° with respect to the superconductor deposition source. That is, the incident angle of the superconductor metal is an angle of about 30° to about 60° with respect to the upper surface of the JJ structure. The deposition process can be realized by various deposition processes such as sputter deposition of aluminum, for example, to form the first superconductor upper electrode 73, the second superconductor upper electrode 75, and the third superconductor upper electrode 77 as shown in the resulting structure of FIG. 10. The second superconductor inclined deposition process 210 can also be a superconductor metal vapor deposition process such as vapor deposition of aluminum for forming the first superconductor upper electrode 73, the second superconductor upper electrode 75, and the third superconductor upper electrode 77.

[0017] As shown in FIG. 10, the operation JJ80 is formed between the overlapping portions of the first superconducting lower electrode 66 and the second superconducting upper electrode 75, and an oxidized portion 70 is disposed between the first superconducting lower electrode 66 and the second superconducting upper electrode 75 under the central bridge region. In the example of aluminum / aluminum oxide / aluminum, the thin aluminum oxide (oxidized portion 70) forms a tunnel barrier, and the aluminum layers on both sides (the first superconducting lower electrode 66 and the second superconducting upper electrode 75) set the gap voltage of the operation JJ80.

[0018] After the second superconductor deposition process shown in FIG. 9, the resulting JJ structure shown in FIG. 10 is placed in an oxidation chamber before exposure to the atmosphere. This oxidation chamber may be the same as or different from the deposition chamber and the oxidation chamber described above as long as the JJ structure remains in a clean environment under vacuum. Thereafter, the JJ structure is oxidized in the same manner as the method by which the oxide barrier is formed and is exposed to a high pressure of about 450 torr to about 850 torr (e.g., about 650 torr) for a period of about 1 hour to about 3 hours (e.g., about 2 hours). This process forms a robust and high-quality protective oxide layer 82 on the surfaces of the first superconducting upper electrode 73, the second superconducting upper electrode 75, and the third superconducting upper electrode 77 on the operation JJ80, and this protective oxide layer 82 suppresses the diffusion of oxygen or other impurities into the oxide barrier 71 of the operation JJ80. FIG. 11 shows the resulting structure.

[0019] In the example of aluminum / aluminum oxide / aluminum, the first superconducting lower electrode 66, the AlOx tunnel barrier 71, and the second superconducting upper electrode 75 are all sequentially formed under vacuum. Then, an additional oxidation step is performed on the JJ structure in the same process chamber to form a robust and high-quality native oxide on the Al surface. When the JJ structure is removed from the vacuum chamber, its surface is immediately exposed to water vapor and organic contaminants in the atmosphere. The robust native oxide passivates the Al surface and prevents the diffusion of contaminants into the tunnel barrier region, thereby affecting the I of the operation JJC Stabilize it.

[0020] Next, by removing the first photoresist material layer 52 and the second photoresist material layer 54 from the JJ structure, the final JJ structure 90 of FIG. 12 is obtained. FIG. 13 is a plan view of the cross-sectional view of FIG. 12. The first photoresist material layer 52 and the second photoresist material layer 54 can be removed by a lift-off process. Alternatively, the first photoresist material layer 52 and / or the second photoresist material layer 54 can be removed by a peeling process, for example, O 2 It can be removed by ashing in plasma.

[0021] As described above, the process flow showing the formation of the operating JJ shown in FIGS. 2 to 13 can be similarly used to manufacture the first hydrogen trap JJ or the second hydrogen trap JJ having larger dimensions. Also, the first hydrogen trap JJ, the second hydrogen trap JJ, and the operating JJ can be manufactured simultaneously using the same process repeated for all three JJs, using the first hydrogen trap JJ and the second hydrogen trap JJ having larger dimensions than the operating JJ to provide a larger critical current for the first hydrogen trap JJ and the second hydrogen trap JJ compared to the operating JJ.

[0022] FIG. 14 shows a schematic diagram of an exemplary JJ device 100. The JJ device 100 includes a first hydrogen trap JJ104 having a first end coupled to the first end of the operational JJ102, and a second hydrogen trap JJ106 having a first end coupled to the second end of the operational JJ102. The first hydrogen trap JJ104 is coupled at its second end to a first superconducting wiring 108 (e.g., niobium) configured to be coupled to one or more microwave circuits, and the second hydrogen trap JJ106 is coupled at its second end to a second superconducting wiring 110 configured to be coupled to one or more microwave circuits. As shown in FIG. 14, the first arrow A points to a plan view of a JJ structure 103 similar to the structure shown in FIG. 13 including the operational JJ102. Also, the second arrow B points to a plan view of a JJ structure 105 similar to the structure shown in FIG. 13 including the first hydrogen trap JJ104. Further, the third arrow C points to a plan view of a JJ structure 106 similar to the structure shown in FIG. 13 including the first hydrogen trap JJ106. This indicates that each of the JJ structures 103, 105, 107 can be formed sequentially or simultaneously from the processes shown in FIGS. 2 - 13. FIG. 14 shows that the JJ structures 105, 107 are larger than the JJ structure 103.

[0023] FIG. 15 shows a cross - sectional view of an exemplary JJ device 150. The JJ device 150 includes a first hydrogen trap JJ structure 154 having a first end coupled to the first end of the operational JJ structure 152 via a superconductor line 174, and a second hydrogen trap JJ structure 156 having a first end coupled to the second end of the operational JJ structure 152 via a superconductor line 176. The first hydrogen trap JJ structure 154 is coupled at its second end to a first superconducting wiring 168 via a superconductor line 172, and the second hydrogen trap JJ156 is coupled at its second end to a second superconducting wiring 170 via a superconductor line 178. The operational JJ structure 152, the first hydrogen trap JJ structure 154, and the second hydrogen trap JJ structure 156 are each present on a substrate together with the superconductor lines 172, 174, 176, 178 and the first superconducting wiring 168 and the second superconducting wiring 170.

[0024] The operation JJ structure 152 includes an operation JJ 158, the first hydrogen trap JJ structure 154 includes a first hydrogen trap JJ 160, and the second hydrogen trap JJ structure 156 includes a second hydrogen trap JJ 162. The first superconducting wiring 168 and the second superconducting wiring can be formed of, for example, niobium. The superconducting lines 172, 174, 176, 178 can be formed of, for example, aluminum. In one example, the first and second hydrogen trap JJs 160, 162 are sized 100 times or more the size of the operation JJ 158. The first and second hydrogen trap JJs 160, 162 are inserted as a hydrogen barrier between the operation JJ 158 and the superconducting wirings 168, 170 in a microwave circuit. Hydrogen diffusing from the superconducting wiring typically accumulates at the junction barrier during operation, reducing the height of the electron barrier, thereby increasing the junction I C over time. The larger hydrogen trap JJs 160, 162 are intended to trap hydrogen and prevent accumulation in the more sensitive operation JJ 158, thereby stabilizing the junction I C of the operation JJ 158. By forming a post-junction oxide layer (not shown) on the operation JJ 158, the drift instability of the operation JJ over time can be further reduced. C

[0025] The foregoing description is an example of the present invention. Of course, for the purpose of explaining the present invention, it is impossible to describe every conceivable combination of components or methods, but those skilled in the art will recognize that many further combinations and permutations of the present invention are possible. Accordingly, the present invention is intended to embrace all such changes, modifications, and variations that fall within the scope of this application, including the appended claims.

Claims

1. A Josephson junction (JJ) device, an operating JJ, a first hydrogen trap JJ having a first end coupled to a first end of the operating JJ and a second end coupled to a first superconducting wiring, a second hydrogen trap JJ having a first end coupled to a second end of the operating JJ and a second end coupled to a second superconducting wiring, comprising the first hydrogen trap JJ and the second hydrogen trap JJ reducing hydrogen diffusion into the operating JJ.

2. The JJ device according to claim 1, wherein the operating JJ is an aluminum / aluminum oxide / aluminum JJ.

3. The JJ device according to claim 1, wherein each of the first hydrogen trap JJ and the second hydrogen trap JJ has a size that is 100 times or more the size of the operating JJ.

4. The JJ device according to claim 1, wherein the first hydrogen trap JJ is formed within a first hydrogen trap JJ structure, the second hydrogen trap JJ is formed within a second hydrogen trap JJ structure, and the operating JJ is formed within an operating JJ structure.

5. Each of the first hydrogen trap JJ structure, the second hydrogen trap JJ structure, and the operating JJ structure is formed from a set of superconducting lower electrodes covered by an insulating layer and a set of superconducting upper electrodes, and an overlap portion between one lower electrode of the set of superconducting lower electrodes and one upper electrode of the set of superconducting upper electrodes, together with a relevant portion of the insulating layer, integrally forms the JJ of the corresponding JJ structure.

6. A part of the JJ structure of the operation JJ is covered by a post-joining oxide layer to reduce the instability of the I Ｃ drift over time, the JJ device according to claim 5.

7. The JJ device according to claim 4, wherein the first hydrogen trap JJ structure has a first end connected to a first end of the operating JJ structure via a first superconducting line and a second end connected to the first superconducting wiring via a second superconducting line, and the second hydrogen trap JJ structure has a first end connected to a second end of the operating JJ structure via a third superconducting line and a second end connected to the second superconducting wiring via a fourth superconducting line.

8. The JJ device according to claim 7, wherein the first superconducting line, the second superconducting line, the third superconducting line, and the fourth superconducting line are formed of aluminum.

9. The JJ device according to claim 7, wherein the first superconducting wiring and the second superconducting wiring are formed of niobium.

10. A method of forming a Josephson junction (JJ) device, comprising: forming a photoresist material layer on a substrate to form a JJ structure; forming an opening in the photoresist material layer and forming a photoresist bridge near a central region of the JJ structure by patterning the photoresist material layer; forming a plurality of superconducting lower electrodes on the substrate by performing a first superconducting inclined deposition process on the JJ structure; providing an oxidized upper surface on the plurality of superconducting lower electrodes by performing an oxidation process; forming a plurality of superconducting upper electrodes by performing a second superconducting inclined deposition process on the JJ structure, wherein an overlap portion of one of the plurality of superconducting upper electrodes overlaps with one of the plurality of superconducting lower electrodes with an oxidized upper surface therebetween, so that a JJ is formed below the photoresist bridge near the central region of the JJ structure; forming a protective oxide on the JJ by performing an oxidation process; and providing a final JJ structure by removing the photoresist material layer. A method comprising the above steps.

11. The method according to claim 10, wherein the JJ is an aluminum / aluminum oxide / aluminum JJ, and both the first superconducting inclined deposition process and the second superconducting inclined deposition process are aluminum evaporation processes.

12. The method according to claim 10, wherein forming the photoresist material layer includes forming a first photoresist material layer on the substrate and forming a second photoresist material layer on the first photoresist material layer, the first photoresist material layer being a lift-off resist material and the second photoresist material layer being a standard photoresist material layer.

13. The JJ structure is an operating JJ structure, the JJ is an operating JJ, and the method further comprises a first hydrogen trap JJ structure having a first end coupled to a first end of the operating JJ structure and a second end coupled to a first superconducting wiring, and a second hydrogen trap JJ structure having a first end coupled to a second end of the operating JJ structure and a second end coupled to a second superconducting wiring, the first hydrogen trap JJ structure including a first hydrogen trap JJ, the second hydrogen trap JJ structure including a second hydrogen trap JJ, and the first hydrogen trap JJ and the second hydrogen trap JJ reducing hydrogen diffusion into the operating JJ. The method according to claim 10.

14. The method according to claim 13, wherein the first hydrogen trap JJ structure and the second hydrogen trap JJ structure are formed from the same sequential process as the operating JJ structure.

15. The method according to claim 13, wherein the first hydrogen trap JJ and the second hydrogen trap JJ each have a size that is 100 times or more the size of the operating JJ.

16. The method according to claim 13, wherein the operating JJ structure, the first hydrogen trap JJ structure, and the second hydrogen trap JJ structure are simultaneously formed in the same sequential process as the operating JJ structure using the same photoresist material.

17. The method according to claim 13, comprising depositing a first superconducting line, a second superconducting line, a third superconducting line, and a fourth superconducting line, the first hydrogen trap JJ structure having a first end connected to a first end of the operating JJ structure via the first superconducting line and a second end connected to the first superconducting wiring via the second superconducting line, and the second hydrogen trap JJ structure having a first end connected to a second end of the operating JJ structure via the third superconducting line and a second end connected to the second superconducting wiring via the fourth superconducting line.

18. The method according to claim 17, wherein the first superconducting line, the second superconducting line, the third superconducting line, and the fourth superconducting line are formed of aluminum.

19. The method according to claim 18, wherein the first superconducting wiring and the second superconducting wiring are formed of niobium.

20. The method according to claim 10, wherein each of performing a first superconductor inclined deposition process on the JJ structure, performing an oxidation process for providing the oxidized upper surface, performing a second superconductor inclined deposition process on the JJ structure, and performing an oxidation process for forming a protective oxide on the JJ is performed under vacuum in the same processing chamber.

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