The membranes may exhibit different sizes and shapes, but they vibrate out of plane, meaning that they are area-dependant. Now researchers from Fraunhofer IPMS have demonstrated a novel type of MEMS micro-speakers consisting of all-silicon electrostatic, in-plane bending micro actuators working in air chambers. By design, the new CMOS-compatible MEMS concept uses the chip’s bulk volume for air displacement and sound pressure generation, meaning chip area can be substituted for volume.
The researchers reported their results in the Microsystems & Nanoengineering journal in a paper titled “Concept and proof for an all-silicon MEMS micro speaker utilizing air chambers”. It describes what they call a nanoscopic electrostatic drive (NED) featuring paired electrostatic bending actuators in rows and columns within the device layer of a bonded silicon on insulator (SOI) wafer and covered by another wafer bonded to the SOI wafer.
A tiny separation allows the actuators to move without friction against the top and bottom layers and because the device’s top and bottom covers integrate acoustic openings between each neighbouring row of actuators, the MEMS can output sound when actuated at relevant frequencies. The double-S shaped NEDs can be densely packed across the surface. Driving a voltage across the electrodes of the individual actuators bends them laterally in-plane according to their orientation in an odd mode relative to each other, altering the volume inside the air chambers between the actuators. Effectively, the air chambers push out air through the acoustic openings in one mode and suck air back in as the actuators deflect in the opposite direction. Driving this actuation at any frequency in the audible range generates sound.