Fabrication of Coaxial and Triaxial Atomic Force Microscope Imaging Probes
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Fabrication of Coaxial and Triaxial Atomic Force Microscope Imaging Probes Keith A. Brown1 and Robert M. Westervelt2,* 1 International Institute for Nanotechnology, Northwestern University, Evanston, IL 60208, U.S.A. 2 School of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138, U.S.A. ABSTRACT Herein, we detail the fabrication of atomic force microscope (AFM) probes that have two and three coaxial electrodes at their tips. This fabrication strategy leverages the availability of conductive AFM probes and encompasses a general method for processing their complex and delicate structure through the deposition of insulating and conductive layers by shadow masked chemical and physical vapor deposition, respectively. Focused ion beam milling is used to expose the two electrode (coaxial) or three electrode (triaxial) structures at the tip of the AFM probe. Finally, we discuss new imaging modalities enabled by these probes including electrically-driven contact resonance imaging for nanoscale mechanical characterization, imaging the local dielectric constant by quantifying the dielectrophoretic force, and trapping functional particles at the tip of a probe using dielectrophoresis. These imaging techniques illustrate the generality and utility of this fabrication approach and suggest that such probes could be widely applied to image many nanoscale materials. *Corresponding author: [email protected] INTRODUCTION Our ability to learn about nanoscale materials is defined by the techniques available for imaging their properties. Atomic force microscopy (AFM) has become a standard technique for measuring surface properties with nanoscale resolution [1]. Of particular interest is the application of electric fields to an AFM probe, which represents a controllable way to image electrical properties such as work function, dielectric constant, and impedance [2, 3]. However, two physical principles limit the use of electric fields: (1) the long range of the Coulomb interaction and (2) the “sticky finger” problem that nanoscale materials can become irreversibly stuck to surfaces on account of the relative importance of surface forces to body forces. Recently, we have explored one path to circumvent these effects, namely, the use of AFM probes that have multiple coaxial electrodes at their tip [4-8]. By independently controlling the voltage applied to these electrodes, we have found that such probes are useful for imaging and manipulating nanoscale materials. In this manuscript, we detail the fabrication of coaxial and triaxial AFM probes and outline several new imaging approaches that they enable. EXPERIMENT The principle experimental challenge associated with the use of coaxial and triaxial AFM probes is the deposition and processing of high quality films on the delicate three dimensional structure that makes up AFM probes. A number of fabrication-intensive methods have been
explored toward this end including a 20-step method for fabricating the cantilever and tip for near-field microwave microscopy [
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