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

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.Scanning superconducting quantum interference device microscopy (sSQUID) has become an important magnetic imaging technique to study superconductivity, magnetism and band topology in quantum materials and devices in reduced dimensions 1,2,3 . The direct flux sensitivity of sSQUID is a fundamental advantage over techniques relying on short-range magnetic interactions for magnetic sensitivity (e.g., MFM 4 , spin-polarized STM 5 , magnetic exchange-force microscopy 6 , spin-polarized low energy electron microscopy 7 , scanning NV microscopy 8 ). It liberates sSQUID from stringent requirements on the cleanness of sample surface and greatly facilitates its application to a broad range of material systems, interfaces of heterostructures 9 and even device prototypes made of new materials 10,11 .

The on-chip scanning probe with dual niobium nitride nanoscale superconducting quantum interference devices for magnetic imaging at the high temperature

“…The spatial resolution of current image was approximately 2 µm, which was determined by measuring the peak-to-peak distance along Y direction in figure 4(e) [40]. The spatial resolution is comparable with the reported numbers of SQUID probe in the [18], and is ready for application to investigate the distribution of vortices in YBCO film.…”

“…The fabricated NbN nano-SQUID on-chip probe is shown in figure 1(a). The probe was shaped by the deep silicon etching [18]. The probe includes two nano-SQUIDs.…”

“…Therefore, we can image the sample at high temperatures without degrading of the nano-SQUID performance. The Q-plus technique was used to achieve a stable tip-sample distance control [18,39]. Briefly, due to the van der Waals force, the resonance frequency of the tuning fork will shift when the probe approaches to the sample surface.…”

“…These nano-SQUIDs allow imaging at tens of nanometers above the sample surface. The other type of SQUID probe is designed with on-chip planar loop to integrate the field coil for achieving multi-functional magnetic imaging [17][18][19][20][21]. The SQUID loop of a few millimeters long was usually designed in a gradiometric structure to reduce the influence of the background magnetic field.…”

Flux focusing with a superconducting nanoneedle for scanning SQUID susceptometry

“…Due to the lack of distance feedback without the needle, we fixed the scanning height at 500 nm above the touch-down point to avoid damaging the SQUID and the sample. This distance was several times smaller than the size of the pickup loop, so the resolution was limited by the latter for a planar device 14 . The pickup loop on this nano-SQUID measured 2 μm in diameter instead of the 5 μm diameter used with the NoS.…”