Capping layer for reducing ion mill damage

1. A method of fabricating an electrical contact junction that allows a current to flow, the method comprising:

providing a substrate comprising a first layer of superconductor material;

removing a native oxide of the superconductor material of the first layer from a first region of the first layer, wherein removing the native oxide from the first region of the first layer of superconductor material comprises

applying a first photoresist layer to the first layer of superconductor material,

patterning the first photoresist layer to expose the first region of the first layer of superconductor material, and

ion milling the exposed first region of the first layer of superconductor material;

forming a capping layer in contact with the first region of the first layer, wherein the capping layer prevents reformation of the native oxide of the superconductor material in the first region, wherein forming the capping layer comprises

forming the capping layer on the patterned first photoresist layer such that part of the capping layer is in direct contact with the ion milled exposed first region of the first layer of superconductor material, and

removing a section of the capping layer that is not in direct contact with the ion milled exposed first region of the first layer of superconductor material;

forming, after forming the capping layer, a second layer of superconductor material that electrically connects to the first region of the first layer of superconductor material to provide the electrical contact junction that allows current to flow.

2. The method of claim 1 , wherein forming the second layer of superconductor material comprises:

applying a second photoresist layer;

patterning the second photoresist layer to expose the capping layer and a portion of a substrate surface; and

forming the second layer of superconductor material on the capping layer and an exposed portion of the substrate surface.

3. The method of claim 1 , wherein forming the second layer of superconductor material comprises:

applying a second photoresist layer;

patterning the second photoresist layer to expose the capping layer and a portion of a substrate surface;

removing the capping layer to expose the first region of the first layer without the native oxide; and

forming the second layer of superconductor material on the exposed first region of the first layer without the native oxide and on the exposed portion of the substrate surface.

4. The method of claim 1 , wherein the capping layer comprises a material that, when subject to ion milling at a predetermined set of ion beam parameters, has an etch rate that is higher than an etch rate of the native oxide subject to the same predetermined set of ion beam parameters.

5. The method of claim 4 , wherein the predetermined set of ion beam parameters comprises a beam voltage, a beam current, and a beam width.

6. The method of claim 4 , wherein the etch rate of the capping layer material is at least five times higher than the etch rate of the native oxide.

7. The method of claim 4 , wherein the etch rate of the capping layer material is at least fifteen times higher than the etch rate of the native oxide.

8. The method of claim 4 , wherein the capping layer material comprises silver or gold.

9. The method of claim 4 , wherein the capping layer material comprises a metal having a thickness such that the metal behaves as a superconductor material due to the superconductivity proximity effect when the electrical contact junction is cooled below a critical temperature of the superconductor material of the first layer.

10. The method of claim 9 , wherein the thickness of the metal is between about 5 nm and about 10 nm.

11. The method of claim 4 , wherein the capping layer material comprises a superconductor material.

12. The method of claim 11 , wherein the superconductor material of the capping layer comprises titanium nitride, rhenium, or ruthenium.

13. The method of claim 1 , wherein the superconductor material of the first layer comprises aluminum.

14. The method of claim 1 , wherein the superconductor material of the second layer comprises aluminum.

15. A device comprising:

a substrate;

a first layer of superconductor material on the substrate, the first layer of superconductor material having first and second opposing surfaces, wherein the second surface faces away from the substrate;

a capping layer in contact with the second surface of the first layer of superconductor material, wherein a region of the second surface of the first layer of superconductor material in contact with the capping layer is free of a native oxide of the superconductor material,

wherein the capping layer comprises a material that is associated with an ion milling etch rate that, at a set of predetermined ion beam parameters, is higher than an ion milling etch rate associated with the native oxide of the superconductor material of the first layer at the same set of predetermined ion beam parameters,

wherein a thickness of the capping layer is between about 5 nm and about 10 nm; and

a second layer of superconductor material in contact with the capping layer.

16. The device of claim 15 wherein the first layer of superconductor material, the capping layer, and the second layer of superconductor material form an electrical contact junction that allows DC current to flow unimpeded.

17. The device of claim 16 , wherein the capping layer material comprises a metal having a thickness such that the metal behaves as a superconductor material due to the superconductivity proximity effect when the electrical contact junction is cooled below a critical temperature of the superconductor material of the first layer.

18. The device of claim 15 , wherein the etch rate associated with the capping layer material is at least five times higher than the etch rate of the native oxide of the superconductor material of the first layer.

19. The device of claim 15 , wherein the ion milling etch rate associated with the capping layer material is at least fifteen times higher than the ion milling etch rate of the native oxide of the superconductor material of the first layer.

20. The device of claim 15 , wherein the ion milling etch rate associated with the capping layer material is at least fifteen times higher than the ion milling etch rate of the native oxide of the superconductor material of the first layer.

21. The device of claim 15 , wherein the capping layer material comprises a metal having a thickness such that the metal behaves as a superconductor material due to the superconductivity proximity effect when the electrical contact junction is cooled below a critical temperature of the superconductor material of the first layer.

22. The device of claim 15 , wherein the capping layer material comprises a superconductor material.

23. The device of claim 22 , wherein the capping layer material comprises titanium nitride, rhenium, or ruthenium.

24. The device of claim 15 , wherein the superconductor material of the first layer is aluminum.

25. The device of claim 15 , wherein the superconductor material of the second layer is aluminum.

26. The device of claim 15 , wherein the device is a qubit.

27. The device of claim 15 , wherein the device is a capacitor.

28. The device of claim 15 , wherein the device is a cross-over bridge.

29. A method of fabricating an electrical contact junction that allows a current to flow, the method comprising:

providing a substrate comprising a first layer of superconductor material;

removing a native oxide of the superconductor material of the first layer from a first region of the first layer;

forming a capping layer in contact with the first region of the first layer, wherein the capping layer prevents reformation of the native oxide of the superconductor material in the first region,

wherein the capping layer comprises a material that, when subject to ion milling at a predetermined set of ion beam parameters, has an etch rate that is higher than an etch rate of the native oxide subject to the same predetermined set of ion beam parameters;

forming, after forming the capping layer, a second layer of superconductor material that electrically connects to the first region of the first layer of superconductor material to provide the electrical contact junction that allows current to flow.

30. The method of claim 29 , wherein a thickness of the capping layer is between about 5 nm and about 10 nm.