Sensitive Transfer-Free Wafer-Scale Graphene Microphones

Abstract

During the past decades micro-electromechanical microphones have largely taken over the market for portable devices, being produced in volumes of billions yearly. Because performance of current devices is near the physical limits, further miniaturization and improvement of microphones for mobile devices poses a major challenge that requires breakthrough device concepts, geometries, and materials. Graphene is an attractive material for enabling these breakthroughs due to its flexibility, strength, nanometer thinness, and high electrical conductivity. Here, we demonstrate that transfer-free 7 nm thick multilayer graphene (MLGr) membranes with diameters ranging from 85− 155 to 300 μm can be used to detect sound and show a mechanical compliance up to 92 nm Pa −1 , thus outperforming commercially available MEMS microphones of 950 μm with compliances around 3 nm Pa −1 . The feasibility of realizing larger membranes with diameters of 300 μm and even higher compliances is shown, although these have lower yields. We present a process for locally growing graphene on a silicon wafer and realizing suspended membranes of patterned graphene across through-silicon holes by bulk micromachining and sacrificial layer etching, such that no transfer is required. This transfer-free method results in a 100% yield for membranes with diameters up to 155 μm on 132 fabricated drums. The device-to-device variations in the mechanical compliance in the audible range (20−20000 Hz) are significantly smaller than those in transferred membranes. With this work, we demonstrate a transfer-free method for realizing wafer-scale multilayer graphene membranes that is compatible with high-volume manufacturing. Thus, limitations of transfer-based methods for graphene microphone fabrication such as polymer contamination, crack formation, wrinkling, folding, delamination, and low-tension reproducibility are largely circumvented, setting a significant step on the route toward high-volume production of graphene microphones.

Ultra-sensitive graphene membranes for microphone applications

“…For example, as shown in ref. 37 graphene membranes with diameters of 85–150 μm exhibit resonance frequencies in vacuum of 250–320 kHz and mechanical sensitivities still comparable to a MEMS membrane with a diameter of 950 μm. For a fairer comparison, one can correct the obtained compliances in Fig.…”

Ultra-sensitive graphene membranes for microphone applications

“…In addition to the poor uniformity and control of strain, the transfer process can degrade the quality of the graphene by introducing contamination, cracks and wrinkles, unwanted for practical application and large-scale production. 37 In a recent study, 36 wafer-scale fabrication of multilayer graphene membranes was achieved using a transfer-free method, by which the graphene is grown and released directly on the target substrate. This novel method could prove beneficial in terms of uniformity and scalability in fabrication of graphene-based microphones and sensors.…”

Nanomechanical absorption spectroscopy of 2D materials with femtowatt sensitivity

“…Along with this comes a massively reduced effective mass, increased resonance frequencies, easily accessible non-linearity, and the ability to tune resonance frequencies. [9] This technological boost allows using such resonators as sensors for light, [10] magnetic fields, [11,12] sound [6,[13][14][15], gases [6,16] or even to study live bacteria. [17] Recently, the use of 2D materials-based resonators as fast and broadband optical spectrometers has been demonstrated.…”