Audio and ultrasound MEMS built on PZT substrate for liquid and airborne applications

This thesis presents piezoelectric microelectromechanical systems (MEMS) for sound generation, liquid pumping, peptide synthesizing, and microparticle trapping. The common thread for the piezoelectric MEMS is the usage of piezoelectric lead zirconate titanate (PZT) sheet in generating sound waves for various applications in air and liquid. ❧ For air-borne audio sound generation, a microspeaker based on bending movement of PZT bimorph diaphragm was designed and fabricated. A PZT substrate was mechanically polished and glued to a micromachined silicon with low viscosity glue to minimize the glue thickness for minimum electric field drop in the glue. Top and bottom electrodes of the microspeaker were patterned according to the stress distribution of a diaphragm for uniform loading to optimize the piezoelectric actuation of bimorph structure. With an order of magnitude lower power consumption compared to a commercial cell-phone electromagnetic microspeaker, the newly fabricated microspeaker produced about 20 dB higher sound output in frequency range of 500 ~ 2,000 Hz. The power consumption increased as the frequency increased, but the sound output was still much higher than a commercial cell phone microspeaker. ❧ For liquid pumping, 260 μm-thick PZT sheet was actuated on its fundamental resonance frequency to generate 8.6 MHz acoustic waves. Acoustic Fresnel lenses were arranged along with U-shape and straight-line liquid channel to produce unidirectional liquid flow. The sector angle of pie-shape electrodes and Fresnel lens were optimized to generate larger in-plane direction acoustic body force, which directly affect the liquid flow rate in the channel. To improve the pumping rate and stabilize the operation condition, acrylic reflector was set up between Fresnel lenses. The micropump with U-shaped lens arrangement produced 7.3 cm/sec particle drift velocity on the liquid surface, and the micropump having straight-lined lens arranged and acrylic reflector showed pumping rate of 9.5 mL/min. ❧ For peptide synthesis using the Spot technique, acoustic droplet ejector with silicon lens structure was used to dispense pre-activated amino-acid on modified glass surface. Conventional acoustic ejector employing Fresnel lens with air-reflect uses parylene D polymer film as its structural material. The parylene acoustic Fresnel lens structure layer was replaced with bare silicon not only to simplify the fabrication steps but also to improve the durability of the device and convenience of washing out solution inside the ejector. With surface modified glass substrate and acoustic ejector as a liquid dispenser, 9-mers long peptide was synthesized with 70% of final synthesis yield that is equivalent with 96% stepwise synthesis yield. ❧ To expand the functionality of the acoustic droplet ejector as a liquid dispenser in peptide synthesizing system, an ejector array consisting of eight directional ejectors were designed and fabricated. The Fresnel lenses and electrodes of eight transducers were patterned into pie-shape with apex angle of 90° to reduce the distances between ejectors in an ejector array. The silicon wafer was etched with Deep Reactive Ion Etching (DRIE), and combined to the PZT transducer, with a mask aligner aiding on the alignment, to form ejecting chambers and liquid transfer channels. The chambers were designed to provide identical acoustic echo circumstances for each ejector in the array. The six ejectors out of the eight ejectors in an array were able to eject droplets onto a very small area of 238 x 380 μm² when actuated individually. ❧ For particle trapping, an acoustic tweezers was designed and fabricated on 128 μm-thick PZT sheet. A Fresnel lens with multiple focal lengths was designed to generate an acoustic Bessel beam, similar to what axicon lens does. A multi-foci Fresnel was designed to have focal lengths of 830 μm, 860 μm, and 890 μm for its two most inner rings, next two rings, and the remaining 3 rings, respectively, out of total 7 Fresnel lens rings. The acoustic tweezers with the designed multi-foci Fresnel lens showed very good trapping ability on lipids particles, polystyrene microparticles, and zebra fish eggs, and confirmed the generation of an acoustic Bessel beam.