Micromachined Acoustic Transducers With Embedded Vertical Capacitive Arrays

Acoustic transducers are the crucial interface between acoustic signals and electrical signals, playing a pivotal role in converting and manipulating sound waves for a wide range of applications across industries and healthcare, such as non-destructive evaluation, range finding, proximity sensing, ultrasonic actuation and sensors, medical imaging probes, therapeutic ultrasound, microphones, and micro speakers. Such applications require transducers operating at frequencies spanning from tens of hertz to hundreds of megahertz. For most of the applications, generating strong acoustical signal is the most important design parameter. Achieving strong acoustic signals and heightened sensitivity demands a high output pressure per transducer unit area. To generate high output pressure per transducer unit area, higher vibration amplitude is required. When a transducer vibrates with a large vibrational amplitude, it can generate high output pressure per surface area even at a lower frequency. For example, when an acoustic membrane generates high output pressure per surface area, it would enable the membrane to produce enough audible sound at low frequency and works as a low frequency speakers or hearing aid instruments. Over the past century, acoustic transducer technology has evolved from piezoelectric crystals to contemporary capacitive micromachined ultrasonic transducer (CMUT) or piezoelectric micromachined ultrasonic transducer (PMUT). However, current piezoelectric or electrostatic micromachined transducer face design and fabrication limitations for generating substantial vibration amplitudes. The main objective of this work is to demonstrate a novel approach that transforms the electrostatic transduction that is conventionally performed by a closely spaced electrode next to the vibrating membrane to an array of electrostatic cells embedded within the membrane. The air gap between the fixed electrode and moveable membrane of the conventional electrostatic acoustic transducers limits the vibration amplitude in the range of tens of nm to few microns. Expanding this gap further is restricted by concerns related to reliability, difficulties in fabrication, and the need for higher operating voltages. The array structures of this research can bypass all the above-mentioned issues and enable the realization of ultrasonic transducers and microspeakers with large out-of-plane displacement, resulting in high sound pressure output per unit area at moderate operating voltage. Extremely narrow air gaps can be made in the vertical electrostatic cells which allows the devices to be operated at low operating voltage while generating high electrostatic force and energy per unit area. Electrostatic cells embedded within the membrane also facilitate the membrane to vibrate with much larger vibration amplitude compared to the conventional devices. Using this novel approach, an acoustic membrane operating in the audible range has shown almost 5 times higher output pressure per surface area per volt compared to the state-of-the-art. Smaller membrane with a resonance frequency in the ultrasonic range would have much higher output pressure per surface area compared to the conventional CMUT and PMUT. This approach can also be used to design a much stronger MEMS micropump for drug delivery, and other MEMS devices where vibrating membrane is the crucial part.