Vertical integration of superconducting solid state coolers

Solid state coolers based on superconducting tunnel junctions are promising low temperature cooling devices which are routinely fabricated using modern microfabrication methods. Arising applications from quantum technologies often depend on subKelvin temperature to operate, requiring the use of bulky and expensive dilution refrigerators. Solid state coolers can potentially replace dilution refrigerators by cascading several microcoolers operating in different temperature ranges. In order to achieve this, coolers have to be packed efficiently, thus 3D integration is essential. In this thesis, vertical integration of superconducting solid state coolers is studied and developed. Flip-chip bonding is utilized to interconnect two microchips together. Samples are studied before and after bonding, and the thermal resistance of 3D integrated device is determined. Quality assurance of the process focuses on indium bumps and isolation etching of adjacent islands in the cooler device. Cooling is not
observed in the device, as the flaws in the isolation etching limit the full capability of the device. However, thermal response of a device is studied, finding that the thermal resistance is significantly improved compared to previous work.