Bi-directional electrostatic microspeaker with two large-deflection flexible membranes actuated by single/dual electrodes

This paper reports a bi-directional electrostatic microspeaker that has three main features: (1) a thin and flexible Parylene membrane for large deflection (>7.5 mum), thus, large acoustic pressure generation, (2) dual electrodes on both sides of the membrane to pull it in two opposite directions and overcome the weak restoration force of the polymer membrane, thus allowing the use of flexible membranes as a high-speed vibration element, and (3) bi-directional (forward/backward) packaged structures for symmetric and loud acoustic sounds of 113.4 dB (SPL: sound pressure level) at 7.68 kHz when actuated using a single electrode, and 98.8 dB (SPL) at 13.81 kHz when actuated using two electrodes at a distance of 1 cm from the microspeaker. By actuating two membranes, the microspeaker also produced symmetric sound pressures at both the front and back sides of the device. The generated sound frequency range is from 1 kHz (measurement cut-off frequency) to 25 kHz, and the sound level was reasonably uniform over a wide frequency range. The fabricated microspeaker has an active area footprint of 3 times 5 mm², while the total packaged device (including electrical pads area of 3.5 times3.5 mm²) has a volume of 13 times 16 times 1 mm³. The micropump is built on two bonded silicon wafers and incorporates a flexible polymer, Parylene, membrane as the actuation membrane