Design, fabrication, and applications of multi-mode nanoelectromechanical systems
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Miniaturization of systems allowed wide spread consumer use of microelectronics, integrated circuits and MEMS based sensors. Thanks to the advancement in microfabrication methods, it is possible to build structures with submicron dimensions. The integration of electronic control to these submicron structures started the NEMS eld. Due to their minuscule dimensions and very high frequency response, NEMS can sense external perturbations with unprecedented sensitivity. This made NEMS excellent candidates for sensor applications. NEMS are starting to evolve from academic research tools to become mass produced and large scale integrated sensing devices. Information extracted from the higher order modes further increase the capabilities of NEMS. In order to attain this extra information, we fabricated NEMS that can reach higher order mechanical modes. Every step of fabrication was done at Bilkent University research facilities such as UNAM and ARL. To pattern the submicron feature sizes, we relied on electron beam lithography. Thermal and electron beam evaporators were deployed for metallization of contacts and etch mask. In order to suspend the doubly clamped beams, we developed anisotropic silicon nitride and isotropic silicon dry etch recipes. At each step of the fabrication, tools such as SEM and stylus pro lometer was utilized for characterization. Fabricated NEMS were wirebonded to printed circuit boards for detection. Electrothermal actuation, an integrated method, was chosen to drive the nanomechanical resonator to its higher order modes. Piezoresistive down-mixing, another integrated method to complement the actuation, was used to detect the resulting nanomechanical motion. We used high frequency electronic equipment to detect RF range responses of our NEMS. Using these NEMS, we studied two novel applications on intermodal and mechanical coupling. First, we investigated intermodal coupling e ect of doubly clamped beams in order utilize this coupling e ect in higher order mode detection. When a doubly clamped beam is excited at its resonance frequency, every other mode of the device gets tuned. This occurs due to the clamping on both sides preventing longitudinal elongation and causing a stress on the beam. Using intermodal coupling method, we probed higher order modes of a nanomechanical resonator while tracking the fundamental frequency at the same time. We were able to detect mechanical modes up to 840 MHz, well out of the detection limit of our setup. We propose intermodal coupling as a novel detection method to acquire frequency response of NEMS at higher order modes which can not be detected with conventional methods. Finally, we studied nano scale energy sinks that absorb energy from a another structure. Energy sinks are linear oscillators that can trap the energy of a nearby structure within their phase space. When the natural frequency of these sinks are distributed optimally, nite number of sinks can mimic absorption of in nite sinks. We envisioned a real time dissipation controlled NEMS platform by deploying energy sinks. In order to test energy sink performance at nano scale, we devised an experimental setup, comparing identical nanomechanical resonators with and without energy sinks. We have shown that energy sinks successfully absorb energy of a resonator at nanoscale.
Electron Beam Lithography
Reactive Ion Etching
Quality Factor Modulation