Sensitive Transfer-Free Wafer-Scale Graphene Microphones

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.