Development of Cryogen-Free Scanning SQUID-on-Tip Fabrication Process with Sensitivity from Near-Zero to Multi-Tesla Fields

We present the cryogen-free nanoscale SQUID-on-Tip (SOT) fabrication process implemented at University of Colorado Denver. The substrate, a 1 mm O.D. quartz glass capillary, is drawn down to diameters ranging from 50 to 300 nm using a commercial pipette puller. A wide variety of pipette geometries and superconducting (SC) materials can be accommodated to achieve multi-junction SOTs and others with unique geometries, achieving sensitivity to fields from near-zero to multi-Tesla. Normal-metal electrical contacts are deposited on the 1 mm O.D. region of the pipette via self-aligned thermal vacuum deposition; the pipette is then rotated 90° and masked such that a 250 μm long shunt resistor is deposited to bridge the future SC leads. The unique feature of this process is the use of a cryogen-free UHV chamber cooled to 8 K to deposit the self-aligned SC features at the very tip of the pipette. Low-pressure He gas is introduced between depositions to facilitate additional thermal coupling of the pipette to the cooling system. This process yields a powerful tool for investigating a wide range of superconducting, magnetic, and quantum materials.