Fabrication of Superconducting Qubits With Al Trilayer Josephson Junctions

To implement a large-scale integration of superconducting quantum bits (qubits), we have investigated a process of fabricating submicrometer Al/AlOx/Al trilayer Josephson junctions. Moreover, on the basis of this process, we developed a means of fabricating different kinds of superconducting qubits. Superconducting qubits require not only small junctions, but also peripheral circuit elements, such as a large resonator, on the same chip. As a result, we chose to use planarization in the self-aligned junction contact formation, which suppressed a pattern-area-dependence of the chemical-mechanical polishing rate. In addition, Al fine wires with a width of 0.2 μm were required in some parts of the qubits to increase the coupling strength between the SQUIDs and qubits. We developed a two-step etching process to form such fine wires using i-line photoresist, which had a high etching resistance. The operation of three-junction flux qubits was confirmed in a dilution refrigerator.