Superconducting flip-chip devices using indium microspheres on Au-passivated Nb or NbN as under-bump metallization layer

Superconducting flip-chip interconnects are crucial for the three-dimensional integration of superconducting circuits in sensing and quantum technology applications. We demonstrate a simplified approach for a superconducting flip chip using commercially available indium microspheres and an in-house-built transfer stage for bonding two chips patterned with superconducting thin films. We use a gold-passivated niobium or niobium nitride layer as an under-bump metallization (UBM) layer between an aluminum-based superconducting layer and the indium interconnect. At millikelvin temperatures, our flip-chip assembly can transport a supercurrent with tens of milliamperes, limited by the smallest geometric feature size and critical current density of the UBM layer. We also show that the indium interconnect itself can carry a supercurrent exceeding 1\,A. Our flip-chip assembly does not require patterning and electroplating of indium, nor does it need a flip-chip bonder. Thus, the presented flip-chip devices can be utilized in applications that require few superconducting interconnects carrying large currents at millikelvin temperatures.