A. W. Kleinsasser

Reliable single‐target sputtering process for high‐temperature superconducting films and devices

We report a simple, single‐target magnetron sputtering process for films of high‐temperature superconductors involving an off‐axis sputtering geometry. The process lends itself both to film growth with high‐temperature post‐anneals and to low‐temperature in situ film growth. The post‐anneal process routinely yields YBa2Cu3O7−x films on SrTiO3 substrates that are fully superconducting at 86–89 K. Current densities at 77 K range from 104 to 8×105 A/cm2. A single‐level superconducting quantum interference device (dc SQUID), made by photolithographically patterning a low current density film, has a flux noise level at 77 K of 3×10−4 Φ0/(Hz)1/2 at 20 Hz, dominated by low‐frequency noise associated with flux motion in the film.

Sub‐μm, planarized, Nb‐AlOx‐Nb Josephson process for 125 mm wafers developed in partnership with Si technology

We have demonstrated a new planarized all‐refractory technology for low Tc superconductivity (PARTS). With the exception of the Nb‐AlOx‐Nb trilayer preparation, the processing is done almost exclusively within an advanced Si technology fabrication facility. This approach has allowed us to leverage highly off of existing state‐of‐the‐art lithography, metal etching, materials deposition, and planarization capabilities. Using chemical‐mechanical polish as the planarization technique we have fabricated Josephson junctions ranging in size from 0.5–100 μm2. Junction quality is excellent with the figure of merit Vm typically exceeding 70 mV. PARTS has yielded fully functional integrated Josephson devices including magnetometers, gradiometers, and soliton oscillators.

Niobium trilayer Josephson tunnel junctions with ultrahigh critical current densities

We describe the fabrication and properties of Nb‐AlOx‐Nb Josephson tunnel junctions having critical current densities as high as 400 kA/cm2, roughly an order of magnitude larger than any previously reported for this materials system. The quality of the junction characteristics, as indicated by the current level at subgap voltages, is considerably lower in high‐Jc than in low‐Jc junctions. However, over the entire high‐Jc range of 20–400 kA/cm2, the quality of our junctions remains the same (i.e., subgap current scales with critical current).

Design, fabrication, and performance of integrated miniature SQUID susceptometers

The design, construction, and performance of miniature SQUID (superconducting quantum interference device) susceptometers is discussed. Spins (in units of mu /sub beta /) per square root Hz has been identified as an important figure of merit. Simple expressions for S/sub n/ (spin sensitivity) in miniature SQUID susceptometers are developed and the implications of dimensional scaling explored. The details of several existing and proposed designs are reviewed, including versions that utilize commercial SQUIDs. With thin-film DC SQUIDs, S/sub n/ values of a few thousand spins/ square root Hz have already been obtained, and it is projected that values of a few hundred will be achieved soon. >

Low‐frequency noise in low 1/f noise dc SQUID’s

We demonstrate that the low‐frequency noise in our edge junction dc superconducting quantum interference devices, with a basic 1/f flux noise of 2×10−12 Φ20/Hz at 1 Hz, can all be accounted for in terms of junction critical current fluctuations. A novel modulation readout scheme is able to cancel the effect of junction critical current fluctuations and reduce our total noise to 1×10−12 Φ20/Hz at 0.1 Hz, a level that is three times lower than the lowest flux noise ever previously reported at this frequency.

Subpicosecond optoelectronic study of resistive and superconductive transmission lines

We have studed the propagation of subpicosecond electrical pulses on coplanar resistive and superconductive Nb transmission lines. Pulses with 0.9 ps full width at half maximum were generated and detected by shorting fast photoconductive switches with 80 fs laser pulses. Dramatic improvements in propagation characteristics were achieved when the Nb was superconductive. We observed the strong dispersion and attenuation predicted to occur for frequency components near the superconducting enery gap frequency.

Low-noise modular microsusceptometer using nearly quantum limited dc SQUIDs

A flexible combination of superconducting integrated circuits was used to construct a low‐temperature magneto‐optic microsusceptometer utilizing a dc superconducting quantum inteference device (SQUID) detector operating near the quantum limit (coupled energy sensitivity of 1.7ℏ). Miniature pick‐up loop assemblies on transparent substrates were joined by superconducting interchip connections to a thin‐film dc SQUID, which is in turn read out by a second dc SQUID connected to room‐temperature electronics. Measurements on an 8.5‐μm‐diam titanium dot evaporated directly into the pick‐up loop demonstrate a spin sensitivity of ∼103 spins/(Hz)1/2 at T=290 mK.

Practical DC SQUIDs with extremely low 1/f noise

A large number of highly robust and reliable thin film DC SQUIDs have been designed and fabricated which have excellent low frequency noise properties. Measurements performed on isolated devices have yielded a limit on the low frequency (1/f) flux noise component which is at least a factor of 60 below the average value reported for devices of this kind. The corresponding energy factor in the white noise region is 770 h at 0.1 Hz. The input coil inductance is 0.7 μH and the coupling efficiency α = 0.9. The substantial reduction of the low frequency noise in these SQUIDs demonstrates that improvements in the performance of these devices in the white noise region can be obtained without sacrificing the low frequency resolution.

Superconducting InGaAs junction field-effect transistors with Nb electrodes

We describe the design, fabrication, and characterization of superconducting In0.47Ga0.53As junction field‐effect transistors (JFETs) with Nb source and drain electrodes. In0.47Ga0.53As has the advantage of combining large coherence length and high Schottky barrier transmission, making it a very attractive material on which to base superconducting FETs. At large voltages these devices behave as normal FETs in either enhancement or depletion modes, while at small voltages they act as Josephson junctions or super‐Schottky diodes. Both normal and super‐ currents are controlled by the gate.

Josephson integrated circuit process for scientific applications

We have developed and are regularly practicing a seven mask-level Josephson integrated circuit fabrication process tailored to dc SQUID requirements and intended for SQUID studies and other scientific applications of Josephson technology. The process incorporates low capacitance Nb/Nb 2 O 5 /PbAuIn edge junctions, PdAu shunt resistors, and a wiring pitch of 5 μm for the SQUID input coil level (which is PbAuIn). The junctions can be made as small as 2μm by 0.3μm, with a capacitance (including parasitics) of ∼0.14 pF. This process yields stable and reliable junctions and integrated circuits.