Fabricating a device using a multilayer stack

1. A method of fabricating a device, comprising:

providing a substrate, wherein the substrate comprises a metal layer on a principal surface of the substrate;

forming a multilayer stack on the metal layer, the multilayer stack comprising:

a first polymer layer;

a second polymer layer on the first polymer layer, wherein the second polymer layer is in contact with a surface of the first polymer layer; and

a third polymer layer on the second polymer layer, wherein the third polymer layer is in contact with a surface of the second polymer layer;

patterning the multilayer stack to form opening through the multilayer stack such that, within the opening, the third polymer layer is undercut by the second polymer layer and the second polymer layer is overcut by the first polymer layer,

wherein patterning the multilayer stack to form the opening comprises

exposing the third polymer layer and the second polymer layer,

developing the third polymer layer and the second polymer layer, wherein during development of the second polymer layer, the second polymer layer recedes relative to the third polymer layer to form an undercut profile with respect to the third polymer layer,

dry etching the first polymer layer;

dry etching the metal layer to form a patterned metal layer of the device; and

removing the multilayer stack,

wherein the device comprises a quantum computing circuit element.

2. The method of claim 1 , wherein the third polymer layer and the second polymer layer are exposed simultaneously.

3. The method of claim 1 , wherein the third polymer layer is exposed and developed before the second polymer layer is exposed and developed.

4. The method of claim 1 , wherein exposing the third polymer layer and the second polymer layer comprises exposing the third polymer layer and the second polymer layer with electron beams.

5. The method of claim 1 , wherein the first polymer layer is poly (methyl methacrylate) (PMMA), and the second polymer layer comprises polydimethylglutarimide (PMGI).

6. The method of claim 1 , wherein the second polymer layer and the third polymer layer are different materials.

7. The method of claim 6 , wherein the second polymer layer and the third polymer layer are developed simultaneously using a developer solution, wherein exposed portions of the third polymer layer are less soluble in the developer solution than exposed portions of the second polymer layer.

8. The method of claim 6 , wherein the entire second polymer layer is flood exposed with deep ultraviolet light before the third polymer layer is formed on the second polymer layer.

9. The method of claim 6 , wherein the third polymer layer and the second polymer layer are exposed simultaneously.

10. The method of claim 9 , wherein, after developing the third polymer layer, the second polymer layer is exposed a second time.

11. The method of claim 1 , wherein dry etching the first polymer layer comprises performing reactive ion etching of the first polymer layer.

12. The method of claim 1 , wherein the first polymer layer recedes laterally as a result of the dry etching.

13. The method of claim 12 , comprising controlling a degree of recession of the first polymer layer during the dry etching.

14. The method of claim 1 , wherein dry etching the metal layer comprises ion milling the metal layer.

15. The method of claim 1 , wherein dry etching the metal layer comprises performing reactive ion etching of the metal layer.

16. The method of claim 1 , wherein a thickness of the first polymer layer is between 10 nm and 100 nm.

17. The method of claim 1 , wherein the metal layer comprises a superconductor.

18. The method of claim 1 , wherein the device comprises a Josephson junction, a superconducting quantum interference device (SQUID) or a qubit.