Modular apparatus for electrostatic actuation of common atomic force microscope cantilevers

Abstract

Piezoelectric actuation of atomic force microscope (AFM) cantilevers often suffers from spurious mechanical resonances in the loop between the signal driving the cantilever and the actual tip motion. These spurious resonances can reduce the accuracy of AFM measurements and in some cases completely obscure the cantilever response. To address these limitations, we developed a specialized AFM cantilever holder for electrostatic actuation of AFM cantilevers. The holder contains electrical contacts for the AFM cantilever chip, as well as an electrode (or electrodes) that may be precisely positioned with respect to the back of the cantilever. By controlling the voltages on the AFM cantilever and the actuation electrode(s), an electrostatic force is applied directly to the cantilever, providing a near-ideal transfer function from drive signal to tip motion. We demonstrate both static and dynamic actuations, achieved through the application of direct current and alternating current voltage schemes, respectively. As an example application, we explore contact resonance atomic force microscopy, which is a technique for measuring the mechanical properties of surfaces on the sub-micron length scale. Using multiple electrodes, we also show that the torsional resonances of the AFM cantilever may be excited electrostatically, opening the door for advanced dynamic lateral force measurements with improved accuracy and precision.

Contact Resonance Force Microscopy for Viscoelastic Property Measurements: From Fundamentals to State-of-the-Art Applications

“…Not surprisingly, the actuation methods that have been successful in liquids (i.e., Brownian motion, 71 magnetic, 119 and photothermal 120 ) represent best practice for measurements in air because they retain a clean, artifact-free response. Other options that provide best-practice viscoelastic CRFM actuation include, but are not limited to, electrostatic 55 and Lorentz force excitation. 39 The benefits of direct excitation are especially evident for heavily damped materials (large tan δ) and when operating in heavily damped environments (e.g., in liquids).…”

“…On point 1, most early CRFM measurements employed acoustic excitation at either the sample or cantilever base, but these methods suffer from spurious vibrations, resulting in a "forest of peaks" in heavily damped materials and environments. 48,49 Recent developments in magnetic, 50,51 thermal, 52,53 Brownian motion, 54 and electrostatic 55,56 excitation schemes have largely addressed this issue via artifact-free responses in ambient and nonambient conditions. On point 2, most CRFM users require not only viscoelastic property measurements but also property maps, which required development of an understanding of the trade-offs between speed and accuracy for various tracking methods.…”

“…However, accurate Q n c and thus Q n s measurements have proven problematic due to three main shortcomings: (1) cantilever actuation, (2) resonance acquisition and tracking, and (3) model design. On point 1, most early CRFM measurements employed acoustic excitation at either the sample or cantilever base, but these methods suffer from spurious vibrations, resulting in a “forest of peaks” in heavily damped materials and environments. , Recent developments in magnetic, , thermal, , Brownian motion, and electrostatic , excitation schemes have largely addressed this issue via artifact-free responses in ambient and nonambient conditions. On point 2, most CRFM users require not only viscoelastic property measurements but also property maps, which required development of an understanding of the trade-offs between speed and accuracy for various tracking methods. − Recent work has focused on these trade-offs as they pertain to viscoelastic CRFM. , On point 3, early CRFM models omitted damping in the contact or failed to accurately account for damping in the environment.…”

Electrostatically actuated encased cantilevers

“…Reliable implementation remains difficult because alignment of an electrode is generally cumbersome and electrostatic forces frequently convoluted with the tip–sample interaction where changes in capacitance gradient due to topographical features influence cantilever excitation. An optically transparent electrode [ 20 ] or conductive sample [ 21 – 22 ] has been used as driving electrodes and Long et al presented a designated cantilever holder to position an excitation electrode within few tens of micrometers on top of a regular cantilever [ 23 ]. However, accurate positioning in the confined space without perturbing the laser beam path remains very challenging.…”