Enabling cryogenic technologies for superconducting quantum devices

Low-temperature refrigerators cool systems down to cryogenic temperatures near absolute zero, where thermal noise and decoherence are suppressed. This allows quantum phases, such as superconductivity, to emerge in certain materials and enables the harnessing of individual quantum states for scientific and high-performance applications. However, the refrigerators used for these purposes are large and rely on cryoliquids, such as scarce and expensive 3He, which can be a limiting factor depending on the technological application. To enable more scalable, costeffective cryogenic platforms, new refrigeration technologies must be developed.

To this end, chip-scale coolers based on superconducting tunnel junctions have been envisioned to provide a fully solid-state alternative. Proof-of-principle operation of these coolers has been demonstrated at temperatures below 1.5 K, but to link them with commercially available 4He pulse tube cryocoolers, stage operating above 2.0 K is required. Additionally, thermally isolating and electrically conducting methods are needed to cascade coolers operating at different temperature ranges.

In this thesis, the fundamental components of a multi-stage chip-scale cooler operating at temperatures compatible with 4He pulse tube cryocoolers are developed. A superconducting flipchip assembly fabricated with In-bumps was characterized in the sub-kelvin temperature range, and the inter-chip thermal resistance was found to be suitable for chip-scale cooling applications. A through-chip signal routing method utilizing ALD TiN-based TSVs was developed, and the demonstrated critical temperature of 2.0 K enables dissipationless DC transport for multi-chip assemblies, such as cascaded coolers. Additionally, ALD MoCx was shown to exhibit a superconducting transition temperature up to 4.4 K and high conformality, showing promise as a TSV-compatible material.

The key achievement of electronic cooling of Al thin film from a bath temperature of 2.4 K down to 1.6 K was demonstrated using Nb-based superconducting tunnel junctions, probed by an onchip junction thermometer. Thermal model calculations highlighted the emergence of superconductivity in the Al beneath the cooler junctions, persisting up to a bath temperature of 2.4 K: one kelvin higher than the nominal critical temperature of the Al thin film. The single-stage cooler operating above 2.0 K enables solid-state on-chip cooling from 4He pulse-tube compatible temperature without the use of magnetic fields. Additionally, Al- and V-based tunnel junctions were fabricated at the wafer scale using degenerately doped Si as the normal electrode. The junctions exhibited suitable low-temperature electrical characteristics for cooling applications.

From superconducting interconnects to tunnel-junction components supporting high cooling power density above 1 K, the achievements presented in this thesis enable modular design of chip-scale cascade coolers. This technology is envisioned to support the scaling of several superconducting quantum devices from proof-of-principle to multi-component systems beyond experimental lab environments.