Wang, H; (2014) Focused-Ion-Beam Growth of Nanomechanical Resonators. Doctoral thesis , UCL (University College London).

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Abstract

Nanoscale mechanical resonators exhibit excellent sensitivity and therefore potential advantages for application as ultrasensitive mass sensors by comparison with micromachined cantilevers. We fabricated three dimensional vertical C-W-nanorods on silicon substrates by focussed ion beam induced deposition (FIB-CVD) and investigated the factors which affected the growth rate and smoothness of the nanorod sidewall, including the heating temperature of precursor gas and the ion beam current. We also discussed the effects on reducing the thickness of the nanorod with FIB milling, including the ion beam current, ion beam energy and ion incident angle. We fabricated a doubly-clamped beam and a singly-clamped beam by felling a vertical nanorod over a trench with FIB milling. We investigated the static mechanical properties (i.e. Young’s modulus) of doubly-clamped and singly-clamped nanorods by atomic force microscopy (AFM) with force displacement measurement. Since the optical signal reflected from a cantilever whose dimensions are sub-wavelength is very weak, it is difficult to measure the absolute nanoscale displacement of such cantilevers with an optical technique. We describe an electron microscope technique for measuring the absolute oscillation amplitude and resonance of nanomechanical resonators with a model-independent method. A piezo-actuator mounted in a field-emission scanning-electron microscope (SEM) is used to excite the nanomechanical resonator to vibrate. The secondary electron signal is recorded as the primary electron beam is scanned linearly over the resonator. An absolute oscillation amplitude as low as 5 nm can be resolved, this being comparable to the size (~1.5 nm) of the primary electron beam. The Q-factor of nanomechanical resonators was measured ranging 300 to 600. The mass resolution of the resonators was also estimated to the level of 1E-15 g.

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