Creating Nanostructures with Controllable Sidewall Profiles for Mechanical Sensor Applications
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Creating Nanostructures with Controllable Sidewall Profiles for Mechanical Sensor Applications Yi Zhao and Xin Zhang Laboratory for Microsystems Technology, Department of Manufacturing Engineering, Boston University, 15 Saint Mary’s Street, Boston, MA 02215, USA ABSTRACT This paper presents a novel approach to create silicon nanostructures with controlled sidewall profiles. The nanostructures are fabricated by alternating the reactive ion etching process and the exposure process to the atmosphere with moisture. The air exposure is believed attributed to the sidewall passivation by facilitating the fast formation of a thin SiO2/SiOxFy layer. Using this approach, three types of representative nanostructures are demonstrated, namely: nanopillars with vertical sidewall, nanopillars with narrowed necks, and suspending nanocantilevers. Without requiring expensive facilities and extensive expertise, this work is expected to provide an alternative for developing nanostructures with a variety of geometric profiles for mechanical sensing applications. INTRODUCTION A variety of mechanical sensors have been developed and applied at micrometer scale to measure the minute forces that can not be detected using conventional approaches, especially the mechanical responses from biological objects [1-3]. Many of these mechanical sensors are in cantilever-like profiles with a high aspect ratio (noted as pillars hereafter). According to the measuring principle [4], these structures have a high probing sensitivity in the lateral direction. It is therefore sensible to shrink these sensors to nanoscale, in order to enhance the probing sensitivity and the spatial resolution. In prior works, the nanostructures with high aspect ratios were formed using deep reactive ion etching (DRIE). The vertical sidewall is achieved by alternating cycles of etching and passivation. In the etching cycle, fluoride based etchant (SF6 is often used) is ionized and driven by RF voltage source applied between a pair of parallel plate electrodes. In the passivation cycle, the Teflon like polymer (C4F8) is deposited on the sample. The polymer deposited on the bottom is stripped off in the following etching cycle, while that on the sidewall remains due to the nature of directional etching. There are some inherent drawbacks for building nanostructures with DRIE. First, the etching rate of DRIE is relatively high so it is difficult to control the etching depth which is usually on the order of or less than a few µm. Second, the lateral penetration is usually comparable with the width of the nanostructures. Third, the expensive facilities and extensive expertise required in DRIE hinder a wide adaptation. In addition, deep etching can not make lateral suspending nanostructures for measuring out-of-plane forces without pre-depositing multi-layered materials. In this paper, we demonstrate a novel approach based on reactive ion etching to overcome the above problems. The etching is performed by tuning the operation pressures in reactive ion etching (RIE) and
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