Functional scanning probes for sensing, actuation and deposition

Abstract

The imaging of objects by standard bright field microscopy is limited by the wavelength of light. An advanced high resolution microscopy technique, based on a relatively simple principle, is atomic force microscopy (AFM). In this technique the topography of a surface is mapped by measuring the force acting on an extremely sharp tip when it is scanned over a surface of interest. Nanoscale resolution can be obtained with AFM, which makes it an indispensable tool for nanotechnology. Next to imaging, even more exciting possibilities become available by the integration of additional functions into the probe. In this thesis, this functionalization of AFM probes is explored.Regarding actuation, the integration of an electrostatic microactuator in the probe was investigated, to obtain a mechanically active AFM probe. The motivation for this work comes from the drive towards high-speed (video-rate) AFM, which requires tips that are sharp and made of a durable material (such as silicon nitride). One of the limiting factors in high-speed AFM is the bulk piezo, which suffers from hysteresis and creep and has limited displacement at higher frequencies. By replacing the piezoelectric actuator by an integrated capacitive micro electro mechanical system (MEMS) actuator, higher scan rates can be obtained. To make the actuator integration more straightforward, a silicon-on-insulator (SOI) compatible fabrication process for the fabrication of in-plane tips (tips directed in the plane of the fabrication wafer) was developed and tested. With this process, sharp silicon nitride tips (best measured radius 8 nm) can be batch fabricated on monocrystalline silicon cantilevers. The in-plane aspect of the fabrication allows for arbitrarily shaped cantilevers.Electrochemical sensing functionality in AFM was obtained by filling a nanofountain pen probe with mercury, resulting in an in-situ renewable mercury microelectrode. Both dropping mercury electrode and hanging mercury droplet configurations were obtained by the control of the pressure on the mercury. In the static droplet configuration, chronoamperometric measurements and cyclic and square-wave voltammograms were obtained with the potential of microscale spatial resolution.Liquid deposition in AFM was studied by using fountain pen probes with various tip geometries. Spotting experiments in contact mode showed a t 0.21±0.01 dependence of the spotsize on the contact time. A significant increase in droplet deposition speed was obtained by writing lines, which subsequently break up into droplets. Regular arrays of mono and bimodal dispersed droplets were obtained by varying the tip-substrate speed. By using electrohydrodynamic deposition, scaled down to micrometer sized gaps, contactless deposition of solids dissolved in liquid was achieved. This technique allows for the deposition of polar liquids on nonwetting surfaces, which is challenging for contact mode techniques. Furthermore, the precise z-control in the AFM setup allows for significantly smaller deposits than can be achieved by, fo...