Electrostatic transfer of sub-micron magnetic particles onto cantilevers using a focused ion beam system

Abstract

In this paper, we present a focused-ion-beam-assisted method for preparing magnet tips for magnetic resonance force microscopy measurements. The method electrostatically transfers prefabricated magnetic nanoparticles to microcantilevers, achieving precise control over the magnet overhang past the cantilever leading edge while minimizing the fabrication damage to the leading edge of the tip magnet. We demonstrate successful fabrication of magnets ranging in size from 460 nm to 2.8 um. These magnets were affixed to two types of cantilevers: silicon cantilevers with a spring constant of 800 uN/m, and single-crystal silicon cantilevers with a spring constant of 30 uN/m. We show that the electrostatic transfer method enables a wide variety of tip shapes, sizes, and materials that were previously not possible with conventional fabrication methods. The transfer procedure allows us to prefabricate the desired particle geometry with minimal ion-beam damage, as confirmed by Monte Carlo simulations. We show that the technique is versatile and can be used to fabricate custom-tipped cantilevers for a broader range of scanning probe techniques.

Figures (6)

• Figure 1: Electrostatic transfer of a 460nm Ni nanosphere onto a Type A MRFM cantilever. Left column: schematic. Right column: SEM images. (a) Spin-coated Ni particles on a silicon substrate. (b) Milled a cantilever notch at the leading edge. (c) Picked up the particle with the tungsten probe tip. (d) Attached the particle electrostatically to the cantilever. (e) Deposited Pt for adhesion, with a probe tip stabilizing the cantilever. Scale bars: 1µm.
• Figure 2: Electrostatic transfer of an FIB-milled NdFeB cylinder onto a Type A MRFM cantilever. Left column: schematic. Right column: SEM images. (a) Spin-coated NdFeB particles on a silicon substrate. (b) Milled a spherical NdFeB particle (2.8µm diameter) into a sub-1µm-diameter cylinder. (c) Milled a cantilever groove at the leading edge. (d) Picked up the particle with the tungsten probe tip. (e) Slotted the cylinder into the groove with the unexposed bottom facing away from the cantilever. (f) Deposited Pt by EBID to secure the magnet. The SEM image (b) was taken from a different transfer run from the rest of the SEM panels. Scale bars: 1µm.
• Figure 3: Energy dispersive X-ray spectroscopy (EDS) measurements of the magnetic nanoparticles before and after the transfer procedure. (a) Ni particle after spin coating, before transfer. (b) NdFeB magnet before and after milling and transfer. The inserted figures show the region of the EDS spectra. The Ni particle EDS post-transfer was not obtained because we do not expect significant changes in element contents. Scale bars for inserted images: (a) 200nm; (b) top: 10µm, bottom: 1µm. Both spectra were acquired at an accelerating voltage of 20keV, and only the 010 energy range is shown.
• Figure 4: SEM images of cylindrical NdFeB magnet attached to Type A cantilever using the cylinder transfer protocol. The 45nm layer at the leading edge of the magnet is hypothesized to result from redeposition of NdFeB and silicon during milling. Scale bars: 200nm.
• Figure 5: SEM image of spherical NdFeB magnet placed onto a Type B MRFM cantilever using the electrostatic transfer protocol. The NdFeB magnet diameter was 2.2µm. Scale bar: 5µm.