Characterisation of nanographite for MEMS resonators

Abstract

Thin-film graphite and graphene are promising materials for nanoelectomechanical systems (NEMS) resonators, for sensors and signal processing applications. The high in-plane stiffness, low mass density and electrical conductivity of graphene are key properties to obtain NEMS resonators with high natural frequencies, sensitivities and tunability. Chemical vapor deposition (CVD) onto a copper catalyst is the most widely-used method to obtain large-scale graphene. However this requires transfer to a desired substrate which adds complexity and can cause wrinkling and polymer contamination. As an alternative, plasma-enhanced CVD (PECVD) has been used to deposit nanographene and nanographite films directly onto insulating substrates, such as SiO₂. Such films have graphitic domains ~10 nm in diameter. In this work, we fabricate electrostatically actuated MEMS resonators from nanographite, establishing this as a route towards integration of nanographene/graphite using CMOS-compatible fabrication. To fabricate our devices, 300 nm thick nanographite is deposited by PECVD onto 6-inch silicon wafers with 200 nm SiO₂ layer. Methane is the carbon precursor with hydrogen diluent in ratio 60:75 sccm and material characterisation is performed using Raman spectroscopy and atomic force microscopy. The film is patterned via optical lithography into 10 µm wide doubly-clamped and cantilever beams and etched using O₂ based reactive ion etching. E-beam evaporated nickel pads are used as contacts, then the device is released by isotropically etching the underlying SiO₂ using HF vapour. The nanographite is under a relatively high compressive stress which causes buckling of the doubly-clamped beam. However, we over-etch the SiO₂ to achieve a ~30 µm undercut of the beam anchors. The stress gradient in the film creates an upward deflection of the anchors and imparts an effective tension to the suspended beam. Finite element simulation has been undertaken to take account of the added ‘length’ which is added to the beam. We then model the fundamental mode of vibration as a beam under tension. To measure the resonant frequency of the resonators, we apply DC bias plus a time varying AC voltage, between the beam and substrate, causing a varying force at the frequency of the AC voltage. The velocity of the beam is measured using laser Doppler vibrometry and becomes large at mechanical resonance. Natural frequency of vibration has been measured for a large number of devices: 257 kHz for 150 µm beams, 420 kHz for 100 µm, 595 kHz 75 µm beams and 15 kHz for 100 µm cantilevers. Quality factors have been calculated from a fitted Lorentzian curve and at ambient pressure are 20 and 1300 at 30 mTorr. Application of increasing DC Bias (up to 50 V maximum) enables tuning of the natural frequency by electrostatic spring softening, with an average tunability of 1.19 kHz per volt across this range.

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Identifiers

ORCID for J.W. McBride: orcid.org/0000-0002-3024-0326

ORCID for H.M.H. Chong: orcid.org/0000-0002-7110-5761

ORCID for Suan-Hui Pu: orcid.org/0000-0002-3335-8880

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