Josephson Junctions Fabricated by Focussed Ion Beam
This thesis details recent work on an innovative new approach to Josephson junction fabrication. These junctions are created in low TC superconductor-normal metal bilayer tracks on a deep submicron scale using a Focused Ion Beam Microscope (FIB). The FIB is used to mill away a trench 50_nm wide in the upper layer of niobium superconductor (125 nm thick), weakening the superconducting coupling and resulting in a Josephson junction. With the aid of a newly developed in situ resistance measurement technique it is possible to determine the cut depth to a high degree of accuracy and hence gain insight into how this affects the resulting device parameters. Devices fabricated over a wide range of cut depths and copper normal metal layer thicknesses (0-175 nm) have been thoroughly characterized at 4.2 K in terms of current-voltage (I-V) characteristics, magnetic field- and microwave-response. In selected cases I-V characteristics have been studied over the full temperature range from TC down to 300 mK. Devices with resistively-shunted (RSJ) I-V characteristics and ICRN products above 50 µV at 4.2 K have been fabricated reproducibly. This work has been complemented by Transmission Electron Microscopy (TEM) studies that have allowed the microstructure of the individual devices to be inspected and confirm the validity of the in situ resistance measurement. The individual junction devices are promising candidates for use in the next generation of Josephson voltage standards. In collaboration with Dr. Sam Benz at the National Institute of Standards and Technology (NIST) in the U.S., series arrays of junctions have been fabricated and characterized. Phase-locking behaviour has been observed in arrays of 10 junctions of spacings 0.2 to 1.6 µm between 4.2 K and TC in spite of the relatively large spread in individual critical currents. Strategies for minimizing junction parameter spread and producing large-scale arrays are discussed. The opportunities offered by the FIB for the creation of novel device structures has not been overlooked. By milling a circular trench in the Nb Cu bilayer a Corbino geometry SNS junction is created. In this unique device the junction barrier is enclosed in a superconducting loop, implying that magnetic flux can only enter the barrier as quantized vorticies. This gives rise to a startling magnetic field response – with the entry of a vortex the critical current is suppressed from its maximum value to zero. Experimental results and theoretical modeling are reported. Possible future applications of this novel device geometry (which may be of relevance to Quantum Computing and to studies of Berry’s phase effects) are considered.