Work package 2 – Thin film technology

Here we cover the development of CQPS devices from disordered superconductor thin films fabricated mostly by atomic layer deposition (ALD). This includes the refining of established ALD technologies for superconducting nitrides like Niobium nitride (NbN) as well as the developing of deposition technologies of new thin film materials for CQPS like silicides and amorphous carbides. ALD has in general the advantage of precise film thickness control within the sub-nanometer range which is one requirement for controlled nanowire fabrication. A second requirement is the control over the lateral dimensions of the CQPS basic structures in the order of some nanometers and the possibility for fine-tuning of the nanowire properties. Here etching techniques with nanoscale precision such as focused ion beam (FIB) play an important role.

The Voltage standard is based on Josephson junctions – quantum elements fabricated by a well-developed Niobium thin film technology.  The new Current standard would be based on CQPS elements realized by new state-of-the-art thin film and/or 2d material superconductor synthesis and patterning techniques. Thus, another challenging task of the work package is the development of a hybrid technology to combine both fabrication worlds.

Furthermore, the CQPS elements have to be embedded into a measurement environment protecting the coherent quantum behavior. Therefore, additional non-superconducting circuity must be developed in thin film technology.  The current standard application also requires long-term stability of all fabricated thin film layers. Thus, possible aging effects must be investigated and the complete thin film technology optimized.

The fabricated CQPS elements show their applicable physics very close to a point where the superconducting behavior transits to electrical isolation (superconductor-insulator-transition, SIT).  A detailed study of the fundamental properties of the disordered thin films and nanowires close to SIT is essential from both sides – theory and experiment. The foci of investigation are set on the impact of quasiparticles, the microwave losses and the decay of superfluid stiffness close to SIT.