We fabricate a MEMS microphone that incorporates a graphene-based membrane that vibrates in response to acoustic forcing. We employ a novel fabrication process, where a graphene/PMMA bilayer membrane is transferred over a cavity on a separate chip before being affixed to the surface of another chip containing an electrode, resulting in the fabrication of a moveable capacitor with a membrane-to-electrode gap of 8 µm. The gap, which is less than half the size of other reported graphene membrane-based audio transducers, allows for the device to operate with low DC bias voltages of about 1 V and, when integrated with a custom-designed readout circuit, demonstrates a sensitivity to sound pressure between 0.1 mV/Pa and 10 mV/Pa across the range 100 Hz to 20 kHz. As well as a sensitivity that is comparable to previous work, the flat frequency response is stable when the sound pressure is varied between 70 dB SPL and 80 dB SPL , with the sensitivity value not varying by more than 0.2 mV/Pa.
This letter reports an experimental study of an electrothermal actuator made from an ultra-large graphene-based bilayer thin film with a diameter to thickness aspect ratio of ∼10 000. Suspended thin films consisting of multilayer graphene and 350–500 nm-thick Poly(methyl methacrylate) have been transferred over circular cavities with a diameter of 3.5 mm. The use of bilayer materials with different mechanical and thermal properties results in thin film structures that can be induced to vibrate mechanically under the electrothermal transduction mechanism. The dynamic response of the bilayer has been investigated electrothermally by driving the structures with a combination of alternating current and direct current actuation voltages (Vac and Vdc) and characterizing their resonant frequencies. It has been found that the bilayer thin film structure behaves as a membrane. In addition, the actuation configurations affect not only the amplitude of vibration but also the tuning of the resonant frequency of the vibrating membranes. The existence of Joule heating-induced tension lowers the mechanical stiffness of the membrane and hence shifts the resonant frequency downwards by −108187 ppm. A resonant frequency of 3.26 kHz with a vibration amplitude of 4.34 nm has been achieved for 350 nm-thick membranes under actuation voltages of 1 V of Vac and 8 V of Vdc.
Large area graphene-poly (methyl methacrylate) (PMMA) closed cavity resonator has been fabricated. The resonator has been formed by transferring an ultra-large graphene-PMMA membrane over 3.5 mm diameter circular closed cavity with 220 µm depth. The graphene-PMMA membrane includes 6-layer graphene and 450 nm PMMA film. A modified graphene-PMMA dry transfer method has been developed in this work. Using the Kapton tape supporting frame, the graphene-PMMA membrane has been dry transferred onto the substrate with a small membrane's static deformation of around 180 nm. The membrane's static deformation aspect ratio (suspended membrane's diameter over the membrane's deformation) is around 19,500. The graphene-PMMA closed cavity resonator has been actuated mechanically, acoustically and electro-thermally. The dynamic behaviour of the membrane suspended over the closed cavity shows that the (1, 1) mode dominates the graphene-PMMA membrane's resonance with a resonant frequency of around 10 kHz and suggests the device is under good gas encapsulation. Acoustic vibration amplitude sensitivity of graphene-PMMA membrane over the closed cavity is measured to be around 6 µm/Pa. The membrane's dynamic behaviour, simulated under similar mechanical and electro-thermal actuation conditions, has been shown to be consistent with the trend of the device's experimental results. The strain in the suspended graphene-PMMA membrane is estimated to be 0.04 ± ± ± 0.01 % % %.
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