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Fraunhofer re-invents MEMS micro-speakers

Fraunhofer re-invents MEMS micro-speakers

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By eeNews Europe



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.

MEMS chip with three micro loudspeaker components
on a PCB for control. One can see the parallel pairs of
actuators. © Fraunhofer IPMS.

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.


To prove their concept, the researchers designed three individually addressable groups of actuators on a single chip, as 14 pairs of long actuators in a 2-serial-7-parallel configuration. To increase the fill factor of the design, the researchers filled the remaining space to the edge of the rectangular shaped device with actuators of half the length.

The assembled MEMS chip, with cover wafer on top,
shows its acoustic opening and three individually controllable
micro speakers. The MEMS chip is glued to a carrier board
enabling wire bonds for electrical contacting.

Here the whole MEMS chip is under 1mm thick, with 400μm thick top and bottom capping wafers sandwiching the 75μm SOI layer and its patterned actuators. The active area of the device had a footprint of 9.3mm2. What the authors highlight is that since the height of the actuators equals the SOI layer thickness, it specifies the displaced volume for a given lateral actuator displacement. One could tune this height to increase the sound pressure without affecting the footprint of the device. What’s more, the MEMS can easily be mass fabricated using current silicon bulk micro machining processes.

The researchers at Fraunhofer IPMS anticipate that such tiny MEMS speakers could find their way in internet-enabled mobile terminals worn directly in the ear. This would be for applications such as simultaneous translation, payment services and marketing solutions, in effect, porting today’s voice-based home assistants into wearables. Such hearables would be predestined for what imec describes as the Internet of Voice, to be worn permanently in the ear, replacing bluetooth headsets.

The researchers are already working on improving the MEMS speaker for better linearity and higher sound pressure levels while reducing power consumption. Next, they will want to integrate the MEMS with its control electronics.

Fraunhofer IPMS – www.ipms.fraunhofer.de

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