Electrical characterization of Metal-Insulator-Metal contacts for superconducting qubits fabricated with CMOS production tools*

Metal-insulator-metal (MIM) contacts are crucial for understanding Josephson junctions employed in superconducting qubits. In this work, we focus on comprehensive studies of Al-AlOx-Al MIM contacts with different sizes from 0.04 µm² to 1600 µm² fabricated on 200 mm silicon wafers with CMOS production tools. A vacuum breaking approach between sputtered bottom and top Al electrodes is examined.

We observed direct as well as trap assisted tunneling. For 0.35x0.35 µm² devices a mean resistance of 13 kΩ at room as well as cryogenic temperatures is achieved, which is the target value for qubits. It will be shown that besides SEM, TEM and EELS room temperature electrical characterization with current, capacitance, and voltage breakthrough measurements are useful.

Etching and different surface treatments (Ar ion etch) before oxidation strongly influence the overall device performance. For MIM stacks of 1 µm² size, depending on ion cleaning and corresponding surface roughness, mean resistance changes from 12 GΩ to 10 kΩ were observed. Furthermore, a significant current proportion is transported through the junction sidewalls, and not through the main overlapping area. Depending on the etching process the ratio varied between 1:25 and 1:1.25.