Bulk nanomachining of cantilevers with Nb nanoSQUIDs based on nanobridge Josephson junctions

Nanometer-scale superconducting quantum interference devices (nanoSQUIDs) were fabricated within a distance of 1 µm from the corners of 2 × 2 × 0.05 mm Si cantilevers that are intended for use in a scanning nanoSQUID microscope. The nanoSQUIDs contained Josephson junctions (JJs) in the form of Nb-based nanobridges, which had widths down to 10 nm and were patterned using hydrogen silsesquioxane negative resist. Numerical simulations of the superconducting current and the spatial distribution of the order parameter in the nanobridge JJs and the nanoSQUID, as well as the current–phase relationship in the nanobridge JJs, were performed according to Ginzburg–Landau equations on one-dimensional and two-dimensional grids. Bulk micromachining of the Si cantilever was performed using reactive ion etching with SF₆ gas through masks of nLOF 2020 photoresist from the front side and a quartz shadow mask from the back side of the substrate. An etch rate of 6 µmmin⁻¹ for Si was achieved for a power of 300 W of the inductively coupled SF₆ plasma. The nanoSQUIDs exhibited non-hysteretic current–voltage characteristics on the cantilever. The estimated spin sensitivity of 48 µ _B (√Hz)⁻¹ is sufficient for use of such a nanoSQUID as a magnetic field sensor for studying nanoscale objects, with a projected total distance to the object of below 100 nm.