Fabrication and characterization of integrated silicon nanowire electromechanical systems

Lin Yu, Purdue University

Abstract

This research addresses the design and implementation of nanoelectromechanical systems (NEMS) based on silicon nanowires. As the counterpart of MEMS in the nanosale regime, NEMS have the potential to reach higher frequencies, larger quality factors, higher mass sensitivities and lower powers. Two fabrication approaches, both compatible with CMOS processing, have been introduced: bottom-up and top-down. In the bottom-up approach, silicon nanowires are synthesized using a Vapor-Liquid-Solid (VLS) process. The silicon nanowires synthesized by VLS process with large diameters tend to grow along a <111> orientation with a hexagonal cross section. Due to nanofabrication process variance and defects, however, the VLS silicon nanowires often show irregular hexagonal cross sections. This phenomenon leads to resonant mode splitting near the natural frequency of the nanowire observed by laser vibrometery. Due to high resistance of VLS silicon nanowire resonators, however, electrostatic measurement of resonance above the thermoelectrical noise floor is not possible. In light of such limitation, silicon nanowire resonators based on SOI wafers are prepared using the top-down technique. In the top-down approach, silicon nanowires are prepared based on silicon-on-insulator (SOI) with dual gate structure using e-beam lithography. The mechanical resonance is electrostatically excited and detected by the use of the mixing technique. Both in-plane and out-of-plane vibration modes are observed. The peak frequency of the silicon nanowire resonators can be tuned both upward and downward due to the combination of tension hardening effects and capacitive softening effects by the dual gate design. Nonlinear tuning has also been investigated on the top-down silicon nanowire resonators. Hysteresis in the resonant response has been observed by sweeping the excitation frequency up and down, which shows the onset of nonlinearity. The ability of simultaneously tuning the peak frequency both downward (softening) using the d.c voltage on the gate and upward (hardening) using the a.c. voltage on the drain has been demonstrated. To the author's knowledge, the research is the first demonstration of electrostatically transduced silicon nanoresonators based on SOI technology. This technique could be a promising solution to the integration of future on-chip NEMS resonators and paves the way for future nanoelectromechanical circuits.

Degree

Ph.D.

Advisors

Hu, Purdue University.

Subject Area

Condensed matter physics|Nanotechnology

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