Gerald W. Gibson

Scanning SQUID susceptometers with sub-micron spatial resolution

Superconducting QUantum Interference Device (SQUID) microscopy has excellent magnetic field sensitivity, but suffers from modest spatial resolution when compared with other scanning probes. This spatial resolution is determined by both the size of the field sensitive area and the spacing between this area and the sample surface. In this paper we describe scanning SQUID susceptometers that achieve sub-micron spatial resolution while retaining a white noise floor flux sensitivity of ≈2μΦ0/Hz1/2. This high spatial resolution is accomplished by deep sub-micron feature sizes, well shielded pickup loops fabricated using a planarized process, and a deep etch step that minimizes the spacing between the sample surface and the SQUID pickup loop. We describe the design, modeling, fabrication, and testing of these sensors. Although sub-micron spatial resolution has been achieved previously in scanning SQUID sensors, our sensors not only achieve high spatial resolution but also have integrated modulation coils for flux feedback, integrated field coils for susceptibility measurements, and batch processing. They are therefore a generally applicable tool for imaging sample magnetization, currents, and susceptibilities with higher spatial resolution than previous susceptometers.

The response of small SQUID pickup loops to magnetic fields

In the past, magnetic images acquired using scanning superconducting quantum interference device (SQUID) microscopy have been interpreted using simple models for the sensor point spread function. However, more complicated modeling is needed when the characteristic dimensions of the field sensitive areas in these sensors become comparable to the London penetration depth. In this paper we calculate the response of SQUIDs with deep sub-micron pickup loops to different sources of magnetic fields by solving coupled Londons and Maxwells equations using the full sensor geometry. Tests of these calculations using various field sources are in reasonable agreement with experiments. These calculations allow us to more accurately interpret sub-micron spatial resolution data obtained using scanning SQUID microscopy.

Scanning SQUID sampler with 40-ps time resolution

Scanning Superconducting QUantum Interference Device (SQUID) microscopy provides valuable information about magnetic properties of materials and devices. The magnetic flux response of the SQUID is often linearized with a flux-locked feedback loop, which limits the response time to microseconds or longer. In this work, we present the design, fabrication, and characterization of a novel scanning SQUID sampler with a 40-ps time resolution and linearized response to periodically triggered signals. Other design features include a micron-scale pickup loop for the detection of local magnetic flux, a field coil to apply a local magnetic field to the sample, and a modulation coil to operate the SQUID sampler in a flux-locked loop to linearize the flux response. The entire sampler device is fabricated on a 2 mm × 2 mm chip and can be scanned over macroscopic planar samples. The flux noise at 4.2 K with 100 kHz repetition rate and 1 s of averaging is of order 1 mΦ0. This SQUID sampler will be useful for imaging dynamics in magnetic and superconducting materials and devices.

Advanced sensors for scanning SQUID microscopy

As part of a joint Stanford/IBM effort to build a scanning SQUID microscopy user facility at Stanford, we have designed and fabricated three types of scanning SQUID microscope sensors. The first is a SQUID susceptometer, with a symmetric, gradiometric design, pickup loops with 0.1 micrometer minimum feature size integrated into the SQUID body through coaxially shielded leads, integrated flux modulation coils, and counterwound one-turn field coils. The second is a SQUID sampler, in which a picosecond current pulse generated on chip is inductively coupled into a hysteric scanning SQUID sensor. The feedback flux to keep the average SQUID voltage at a constant value is proportional to the flux through the sensor pickup loop at a fixed time delay. JSPICE simulations indicate that time resolutions below 10 picosec can be obtained. The third type is a dispersive SQUID, in which the capacitance and Josephson inductance of a one-junction SQUID are chosen so it has an LC resonance in the GHz range. The Josephson inductance depends on the magnetic flux through the SQUID. The magnetic flux is sensed through phase shifts in the reflected microwave signals at resonance. Calculations indicate spin sensitivities better than 1 Bohr magneton per root Hz for a 0.3 micrometer pickup loop diameter, with bandwidths of about 100 MHz possible.

Direct Measurement of Jitter in a JTL

A direct technique for the measurement of jitter in a Josephson transmission line (JTL), which is similar to an approach used to measure complementary metal-oxide-semiconductor latch metastability, is presented. The experiment yields a one-sigma jitter value of 74 fs per JTL stage in 1 kA/cm2 rapid single flux quantum technology. The experimental configuration has been modeled with JSIM, and the result agrees with the experimental data. Additionally, an analytical model has been developed to assess the scaling of the jitter-to-stage delay ratio with critical current density of the Josephson junctions comprising the JTL. Finally, we compare the measured upper bound jitter per stage of a 65-nm complementary metal-oxide-semiconductor inverter chain with that of a 1 kA/cm2 JTL.