Fabrication of Capacitive Micromachined Ultrasonic Transducer (CMUT) With PMMA/Graphene Membrane

An experimental study of a capacitive micromachined ultrasonic transducer (CMUT) made from a polymethyl methacrylate (PMMA)/graphene composite membrane is presented. Graphene has superior electrical and mechanical properties that make it desirable for sensing and micro-electro-mechanical systems (MEMS) applications. The novel aspect of this technique is that it fabricates CMUTs from the PMMA/graphene membrane rather than the more common silicon-based membrane. The PMMA/graphene membrane comprised bilayer graphene and 320 nm of PMMA layer. The CMUT was formed by transferring a PMMA/graphene membrane over SU-8 patterned circular cavities with diameters of
$60~\mu \text{m}$
, and this dry transfer method helped to eliminate problems such as stiction and membrane damage that can occur during the bonding process. After bonding, the Raman spectrum of the PMMA/graphene membrane red-shifts, which indicates tensile strain. The surface resistance of the composite membrane measured by the four-wire probe technique was
$724.6~\Omega /\square $
. Additionally, Young’s modulus of the composite membrane was determined to be 2.2 GPa through a single-axis stretching test. The resonant frequency, measured in air, was 1.69 MHz, and the quality factor was estimated to be around 34 at the resonance frequency. Based on resonant frequency measurements of five CMUT chips, it was observed that the fabricated devices demonstrated a high level of uniformity.