Fabrication Process for Deep Submicron SQUID Circuits with Three Independent Niobium Layers.

We present a fabrication technology for nanoscale superconducting quantum interference devices (SQUIDs) with overdamped superconductor-normal metal-superconductor (SNS) trilayer Nb/HfTi/Nb Josephson junctions. A combination of electron-beam lithography with chemical-mechanical polishing and magnetron sputtering on thermally oxidized Si wafers is used to produce direct current SQUIDs with 100-nm-lateral dimensions for Nb lines and junctions. We extended the process from originally two to three independent Nb layers. This extension offers the possibility to realize superconducting vias to all Nb layers without the HfTi barrier, and hence to increase the density and complexity of circuit structures. We present results on the yield of this process and measurements of SQUID characteristics.

NanoSQUIDs from YBa2Cu3O7/SrTiO3 superlattices with bicrystal grain boundary Josephson junctions.

We report on the fabrication and characterization of nanopatterned dc SQUIDs with grain boundary Josephson junctions based on heteroepitaxially grown YBa2Cu3O7 (YBCO)/SiTrO3 (STO) superlattices on STO bicrystal substrates. Nanopatterning is performed by Ga focused-ion-beam milling. The electric transport properties and thermal white flux noise of superlattice nanoSQUIDs are comparable to single layer YBCO devices on STO bicrystals. However, we find that the superlattice nanoSQUIDs have more than an order of magnitude smaller low-frequency excess flux noise, with root-mean-square spectral density at 1 Hz (Φ0 is the magnetic flux quantum). We attribute this improvement to an improved microstructure at the grain boundaries forming the Josephson junctions in our YBCO nanoSQUDs.

YBa2Cu3O7 nano superconducting quantum interference devices on MgO bicrystal substrates.

We report on nanopatterned YBa2Cu3O7-δ (YBCO) direct current superconducting quantum interference devices (SQUIDs) based on grain boundary Josephson junctions. The nanoSQUIDs are fabricated by epitaxial growth of 120 nm-thick films of the high-transition temperature cuprate superconductor YBCO via pulsed laser deposition on MgO bicrystal substrates with 24° misorientation angle, followed by sputtering of dAu = 65 nm thick Au. Nanopatterning is performed by Ga focused ion beam (FIB) milling. The SQUID performance is comparable to devices on SrTiO3 (STO), as demonstrated by electric transport and noise measurements at 4.2 K. MgO has orders of magnitude smaller dielectric permittivity than STO; i.e., one may avoid Au as a resistively shunting layer to reduce the intrinsic thermal flux noise of the nanoSQUIDs. However, we find that the Au layer is important for avoiding degradation during FIB milling. Hence, we compare devices with different dAu produced by thinning the Au layer via Ar ion milling after FIB patterning. We find that the reduction of dAu yields an increase in junction resistance, however at the expense of a reduction of the critical current and increase in SQUID inductance. This results in an estimated thermal flux noise that is almost independent of dAu. However, for two devices on MgO with 65 nm-thick Au, we find an order of magnitude lower low-frequency excess noise as compared to nanoSQUIDs on STO or those on MgO with reduced dAu. For one of those devices we obtain with bias-reversal readout ultra-low flux noise of ∼175 nΦ0 Hz-1/2 down to ∼10 Hz.