Meander-shaped piezoelectric MEMS loudspeaker with maximized area efficiency for in-ear applications

State-of-the-art MEMS loudspeakers use innovative actuator concepts to improve the acoustic performance from minimal chip volumes. Lateral vibration of multiple vertical actuators is a promising lever to increase the active emitting area while keeping the chip dimensions constant. Recent publications on this topic require a multiple wafer fabrication process to realize such actuator principles. This work demonstrates a new concept fabricating a closed folded membrane with vertical actuators in a single wafer process. Conformal deposition of piezoelectric aluminum nitride (AlN) thin films allow the lateral actuation of the transducer. Laser-doppler-vibrometer (LDV) measurements show that no actuator resonances are located below 40 kHz. Acoustic characterization of a manufactured device in an IEC ear simulator demonstrates a linear behavior of this loudspeaker with THD values below 1.5 % over the audible frequency range. At 1 kHz, a SPL of 64 dB and a THD of 0.3 % are measured with a driving voltage of 16 V_p. A comparison of the acquired frequency response with simulated results shows a very good fit with deviations of less than 3.2 dB, successfully proving the concept and the potential of the folded MEMS loudspeaker.