Reliability of different fabrication processes for window- and cross-type Nb/Al-AlO${}_x$/Nb Josephson junctions

Josephson tunnel junctions (JJs) are the basic element of many superconducting electronic devices such as SQUIDs or qubits. To allow for a successful and predictable operation of these devices, JJs having high quality and well-defined parameters are needed. The fabrication of window-type JJs is an often used approach to reliably produce such junctions. However, restrictions in alignment accuracy practically limits the junction size if not using advanced technologies such as e-beam lithography. Additionally, the overlap between the base electrode and the top wiring as well as the minimum junction size leads to constraints with respect to the junction capacitance and therefore to practical design parameters of SQUIDs and qubits. Both issues can be addressed by introducing a cross-type JJ fabrication process. Within this context, we present different fabrication processes for Nb/Al-AlO${}_x$/Nb based JJs. This includes not only our well-established, anodization-free fabrication process for window-type JJs but also different types of fabrication processes for cross-type JJs. We will compare all processes with each other by means of different figures of merit such as the ratio of subgap to normal resistance R${}_{sg}$/R${}_n$ or the characteristic voltage R${}_{sg}$I${}_c$ and show that our JJs exhibit a very high quality.