RESUMO
A graphene/poly(methyl methacrylate) (PMMA) closed cavity resonator with a resonant frequency at around 160 kHz has been fabricated. A six-layer graphene structure with a 450 nm PMMA laminated layer has been dry-transferred onto the closed cavity with an air gap of 105 µm. The resonator has been actuated in an atmosphere and at room temperature by mechanical, electrostatic and electro-thermal methods. The (1,1) mode has been observed to dominate the resonance, which suggests that the graphene/PMMA membrane has been perfectly clamped and seals the closed cavity. The degree of linearity of the membrane's displacement versus the actuation signal has been determined. The resonant frequency has been observed to be tuned to around 4% by applying an AC voltage through the membrane. The strain has been estimated to be around 0.08%. This research puts forward a graphene-based sensor design for acoustic sensing.
RESUMO
We report the realization of an acoustic capacitive microphone formed by graphene/poly(methyl methacrylate) (PMMA). It is the first time that the ultra-large graphene/PMMA membrane suspended fully over the cavity has been fabricated by releasing the silicon dioxide sacrificial layer underneath the membrane. The novelty in the fabrication method is that the silicon dioxide layer has been etched by hydrogen fluoride vapor from the back of the partly etched silicon substrate. Using the new process, the ultra-large graphene/PMMA membrane, with a diameter to thickness ratio of 7800, has been suspended over the cavity with a 2 µm air gap. The spacing of 2 µm is the minimum gap over the graphene-based acoustic capacitive microphones which have been reported so far. The static deformation of the suspended graphene/PMMA membrane after silicon dioxide has been etched is estimated to be 270 nm. The aspect ratio of the membrane's diameter over its static deformation is around 13,000, which shows that the graphene/PMMA membrane with a diameter of a few millimeters can be transferred and suspended over the substrate with relatively small deformation by releasing the sacrificial silicon dioxide layer. The dynamic behavior of the device under electrostatic actuation has been characterized. The acoustic response of the graphene/PMMA capacitive microphone has been measured, and the sensitivity has been observed to be -47.5 dB V (4.22 mV/Pa) ± 10%. The strain in the graphene/PMMA membrane is estimated to be 0.034%.
RESUMO
In the drive to miniaturise and integrate reference oscillator components, microelectromechanical systems (MEMS) resonators are excellent candidates to replace quartz crystals. Silicon is the most utilised resonator structural material due to its associated well-established fabrication processes. However, when operation in harsh environments is required, cubic silicon carbide (3C-SiC) is an excellent candidate for use as a structural material, due to its robustness, chemical inertness and high temperature stability. In order to actuate 3C-SiC resonators, electrostatic, electrothermal and piezoelectric methods have been explored. Both electrothermal and piezoelectric actuation can be accomplished with simpler fabrication and lower driving voltages, down to 0.5 V, compared to electrostatic actuation. The vibration amplitude at resonance can be maximised by optimising the design and location of the electrodes. Electrical read out of the resonator can be performed with electrostatic or piezoelectric transduction. Finally, a great deal of research has focused on tuning the resonant frequency of a 3C-SiC resonator by adjusting the DC bias applied to the electrodes, with a higher (up to 160-times) tuning range for electrothermal tuning compared to piezoelectric tuning. Electrothermal tuning lowers the frequency, while piezoelectric tuning can be used to raise the frequency.