Direct-write Helium Ion Lithography of YBa2Cu3O7 Nano Josephson junctions and SQUIDs
The 1986 discovery of high transition temperature (high-TC) superconductivity in copper-oxide materials set in motion an intense research effort to develop superconducting electronics functioning in the range of liquid nitrogen temperatures (77 K). Scientists and engineers soon after discovered that these materials were much more difficult than initially imagined. Anisotropic electrical properties and a very short superconducting coherence length seriously narrowed or eliminated the possibility of using existing fabrication techniques of conventional superconductors. These new materials demanded novel device architectures that proved very difficult to realize. Nearly three decades have passed and progress in high-TC superconducting devices has been very slow because process control at the sub ten nanometer scale is required to make high-quality, reproducible Josephson junctions: the basic building block of superconducting electronics. Recent advances in gas field focused helium ion beams provide a new and promising approach for direct-write lithography of high-TC materials for the realization of predictable and scalable high-TC electronics.
I will describe how my laboratory uses a 0.5 nm diameter focused helium beam to directly write Josephson barriers into the a-b plane of a YBa2Cu3O7−δ (YBCO) superconducting thin film. The key to this method is that YBCO is sensitive to point defects in the crystal lattice caused by ion irradiation. Increasing irradiation levels has the effects of increasing resistivity and reducing the superconducting transition temperature. At very high irradiation levels YBCO becomes insulating and no longer superconducts. The very small size of the barrier created using a focused helium beam allows us to create junctions with higher barriers than previously possible which allows for insulating barriers and higher resistance devices. I will present data showing how the Josephson barrier goes through the metal to insulator transition with increasing irradiation dose, and discuss a modified restively shunted junction model that describes our data. I will conclude with a discussion about how the helium ion direct-write method enables for novel experiments in nanoscale Josephson junctions and studies of the in-plane anisotropy of the superconducting order parameter symmetry.