Low-cost piezo-MEMS speaker technology

In this paper, we are reporting low-cost piezo-MEMS speakers based on an SOI-free process. The speaker membrane is made of a 1.5 μm thick polyimide passive layer, obviating the need for SOI wafers. The custom-made PZT solution for depositing 1 μm sol-gel PZT thin films has further reduced fabrication costs. The membranes are realized by a backside Deep Reactive Ion etching (DRIE) on the silicon wafer. Laser doppler vibrometer (LDV) characterization has shown the resonance frequency of a single speaker element with a diameter of 2.5 mm to be 30 kHz with a sensitivity of 100 mm/s/V, whereas sensitivity at 4 kHz is around 1.2 mm/S/V. The speaker element can produce around 70–90 dB in-ear Sound Pressure Level (SPL) response in audible frequencies (100 Hz – 10 kHz) at 5 V excitation as shown by a simulation in an ear-coupler which is in line with the LDV measurements. Moreover, on average, the fabricated 4 × 4 arrays have shown a 10 dB higher SPL than a single-speaker element as measured by a microphone.