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Displacement Detection using Quantum Mechanical Electron Tunneling in Micro and Nano-electro-mechanical Systems

D. Taylor and K. Ekinci
Boston University, US

Keywords: MEMS, NEMS, displacement, membrane, STM

Micro-electro-mechanical systems (MEMS) and, more recently, nanoelectromechanical systems (NEMS) are emerging as candidates for a number of important technological applications ? such as ultra-fast actuators, sensors, and high frequency signal processing components. Experimentally, they are expected to make possible investigations of new phonon mediated mechanical processes and of the quantum behavior of mesoscopic mechanical systems. Finding precise methods for characterizing the electromechanical behavior of MEMS and in particular NEMS, however, remains a challenge. It appears that quantum mechanical tunneling of electrons between a sharp metallic tip and a metallic surface ? with its strong dependence on the tunnel gap ? is a suitable method for studying displacements in micro and nanoscale devices. Here, we report a technique for measuring electromechanical resonances in MEMS and NEMS based upon quantum mechanical electron tunneling. Micron and submicron scale electromechanical resonators ? such as the membranes shown in Figures 1 and 2 and the doubly-clamped beam in Figure 3 ? were fabricated using lithography, and dry and wet etching techniques. The structures were subsequently metallized by thermal evaporation of Chromium. After wire bonding, they were placed inside an ultra-high vacuum (UHV) environment, where they could be probed by a scanning tunneling microscope (STM) tip. In our measurements of the mechanical response of MEMS and NEMS (a schematic of which is shown in Figure 4), we monitored the modulations of the tunnel current between the STM tip and the device surface, while the mechanical resonances of the devices were electrostatically excited. To monitor displacements at high frequencies (MHz), we used the inherent non-linearity in the tunneling current across the tunnel junction; the tunnel current was ''mixed down'' by modulating the position of the STM tip. Finally, spatially accurate contour plots of device displacements were obtained by scanning the STM tip across the surface of the device. These results provide information about the specifics of electromechanical resonances in MEMS and NEMS devices and form a basis for future investigations of the mechanical properties of microscale and nanoscale devices using electron tunneling.

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