Improving spatial resolution of scanning SQUID microscopy with an on-chip design

TL;DR

This paper presents a novel on-chip SQUID design with nano-scale pick-up coils and precise fabrication techniques, achieving sub-micron spatial resolution for magnetic imaging of quantum materials.

Contribution

It introduces a new fabrication method combining electron beam lithography and deep silicon etching to significantly improve SQUID-on-chip spatial resolution.

Findings

Achieved pick-up coils with diameters down to 150 nm.

Demonstrated sub-micron spatial resolution in magnetometry.

Enhanced magnetic imaging capabilities for quantum materials.

Abstract

Scanning superconducting quantum interference device microscopy (sSQUID) is currently one of the most effective methods for direct and sensitive magnetic flux imaging on the mesoscopic scale. A SQUID-on-chip design allows integration of field coils for susceptometry in a gradiometer setup which is very desirable for measuring magnetic responses of quantum matter. However, the spatial resolution of such a design has largely been limited to micrometers due to the difficulty in approaching the sample. Here, we used electron beam lithography technology in the fabrication of the 3D nano-bridge-based SQUID devices to prepare pick-up coils with diameters down to 150 nm. Furthermore, we integrated the deep silicon etching process in order to minimize the distance between the pick-up coil and the wafer edge. Combined with a tuning-fork-based scanning head, the sharpness of the etched chip edge enables a precision of 5 nm in height control. By scanning measurements on niobium chessboard samples using these improved SQUID devices, we demonstrate sub-micron spatial resolutions in both magnetometry and susceptometry, significantly better than our previous generations of nano-SQUIDs. Such improvement in spatial resolution of SQUID-on-chip is a valuable progress for magnetic imaging of quantum materials and devices in various modes.