24 Nanoelectromechanical Systems – Experiments andModeling

Nanoelectromechanical systems (NEMS) aremade of electromechanical devices that have critical dimensions from hundreds down to a few nanometers. By exploring nanoscale effects, NEMS present interesting and unique characteristics, which deviate greatly from their predecessor microelectromechanical systems (MEMS). For instance, NEMS-based devices can have fundamental frequencies in the microwave range (approximately 100GHz) [1]; mechanical quality factors in the tens of thousands, meaning low energy dissipation; activemass in the femtogram range [2]; force sensitivity at the attonewton level [3]; mass sensitivity up to attogram [4] and subattogram [5] levels; heat capacities far below a “yoctocalorie” [6]; power consumption in the order of 10 aW [7]; and extremely high integration levels, approaching 1012 elements per square centimeter [1]. All these distinguished properties of NEMS devices pave the way for applications such as force sensors, chemical sensors, biological sensors, and ultrahigh frequency resonators. The interesting properties of the NEMS devices typically arise from the behavior of the active parts, which, in most cases, are in the form of cantilevers or doubly clamped beams with dimensions at the nanometer scale. The materials for these active components include silicon and silicon carbide, carbon nanotubes (CNTs), gold, and platinum, to name a few. Silicon has been the basic material for integrated circuit technology and MEMS during the past few decades, and is widely used to build NEMS as well. Figure 24.1 is a scanning electron microscopy (SEM) image of a double-clamped resonator fabricated from a bulk, single-crystal silicon substrate [8]. However, ultrasmall silicon-based NEMS fail to achieve desired high quality factors owing to the dominance of surface effects, such as surface oxidation