Scanning SQUID susceptometers with sub-micron spatial resolution

TL;DR

This paper presents the development of scanning SQUID susceptometers with sub-micron spatial resolution, combining advanced fabrication techniques with integrated features for enhanced magnetic imaging capabilities.

Contribution

The paper introduces a novel design and fabrication process for high-resolution SQUID susceptometers with integrated modulation and field coils, enabling superior magnetic imaging.

Findings

Achieved sub-micron spatial resolution in SQUID sensors.

Maintained a flux sensitivity of approximately 2μΦ₀/Hz^{1/2}.

Enabled imaging of magnetization, currents, and susceptibilities with high precision.

Abstract

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 $\approx 2\mu\Phi_0/Hz^{1/2}$. 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.