Cryogenic positioning and alignment with micrometer precision

TL;DR

This paper presents rapid, reproducible protocols for cryogenic microcantilever alignment with micrometer precision, utilizing resonance frequency shifts and optical imaging to improve scanning probe microscopy accuracy under challenging conditions.

Contribution

It introduces novel methods combining resonance frequency monitoring and optical fiber imaging for precise alignment of microcantilevers at cryogenic temperatures and high vacuum.

Findings

Resonance frequency shifts effectively map sample features near cantilever edges.

Polymer coatings and metallic ground planes do not hinder edge detection.

Optical fiber imaging simplifies and accelerates the alignment process.

Abstract

Aligning a microcantilever to an area of interest on a sample is a critical step in many scanning probe microscopy experiments, particularly those carried out on devices and rare, precious samples. We report a series of protocols that rapidly and reproducibly align a high-compliance microcantilever to a $< 10 \: \mu\mathrm{m}$ sample feature under high vacuum and at cryogenic temperatures. The first set of protocols, applicable to a cantilever oscillating parallel to the sample surface, involve monitoring the cantilever resonance frequency while laterally scanning the tip to map the sample substrate through electrostatic interactions of the substrate with the cantilever. We demonstrate that when operating a cantilever a few micrometers from the sample surface, large shifts in the cantilever resonance frequency are present near the edges of a voltage-biased sample electrode. Surprisingly, these "edge-finder" frequency shifts are retained when the electrode is coated with a polymer film and a $\sim 10 \: \mathrm{nm}$ thick metallic ground plane. The second series of methods, applicable to any scanning probe microscopy experiment, integrate a single-optical fiber to image line scans of the sample surface. The microscope modifications required for these methods are straightforward to implement, provide reliable micrometer-scale positioning, and decrease the experimental setup time from days to hours in a vacuum, cryogenic magnetic resonance force microscope.