MEMS Self-focused Piezoelectrical Acoustic Transducers

Micro-Electrical-Mechanical Systems (MEMS) are the integration of mechanical elements, sensors, actuators and electronics on a wafer, which are relied on micro-fabrications. People use well-established IC fabrication techniques with chemical and mechanical processes to construct micro structures and devices. At the microscopic level, MEMS bridge the gap between the "electrical/computer world" and the real physical world, which is a great-leap-forward development. Since the first electrostatic micro-motor demonstrated in 1988, MEMS have been increasingly developed in the worldwide.The study on MEMS is divided into two parts:micro-structures and micro-transducers. Micro-structures include micro-lens, micro-nozzles, micro-probes and micro-fluidic systems, while micro-transducers always refer to mirco-sensors and micro-actuators. As an important MEMS transducer, self-focused piezoelectrical acoustic transducers with Fresnel lens have been developed by Dr. Kim's group at University of Southern Califonia. In this paper, we extensively explored two kinds of these transducers and applied them to two important applications alternatively. Moreover, two novel micro-facbrcation techniques were developed as well.First, most of the techniques for cell detachments either randomly affect a large area of cells or require much human labor. We used focused acoustic beams to generate cavitation bubbles for the localized cell detachment. A self-focused piezoelectrical acoustic transducer was used to focus the acoustic waves at a spot of 60μm in radius, where the peak intensity of the pressure reached 3 atmospheres, strong enough to activate cavitation bubbles at a high frequency (12 MHz). Due to the high pressure and temperature produced by the cavitation bubbles, around 200 cells in the localized area were detached from the substrate while leaving adjacent cells intact. This technique demonstrated the potential applications of localized cell detachment for small size sample preparations and localized bio-medical therapies. Second, high frequency ultrasond imaging has become an important medical diagnosis tool. We developed high-overtone self-focused acoustic transducers for high frequency ultrasonic imaging and Doppler. By using harmonic frequencies of a thick bulk Lead Zirconate Titanate (PZT) transducer with a novel air-reflector Fresnel lens, we obtained strong ultrasound signals at 60 MHz (3rd harmonic) and 100 MHz (5th harmonic). Both experimental and theoretical analysis demonstrated that the transducers could be applied to Doppler systems with frequencies up to 100 MHz.Third, we explored a new microfabrication technique to form controllable MEMS curvertural structures on Silicon wafers. By using Silicon-dioxide sputter, photolithography, DRIE (Deep Reactive Iron Etching) and polyimide coating, we achieved the first cost-effective fabrication on common Silicon wafers without using gray masks which is very expensive. Arbitrary structures can be applied to, but are not limited to, optical elements, needle arrays, and any other MEMS devices which require controlled profiles. The invention and development of this method showed great promise to form microstructures with controllable 3D structures. Meanwhile, it could be a potential to fabricate the self-focused acoustic transducers on a curvatural structure with great focal effects.Finally, we demonstrated a novel technique for covering microfluidic systems using Parylene-C. Microfluidic systems consisting of micro channels and reservoirs need to be covered to protect or isolate liquid samples from the environment. Thick photoresist and wax were employed as the sacrificial layers in the enclosed micro channels and reservoirs before Parylene-C sealing. And both spinning and depositing Teflon-like amorphous fluorocarbon polymer methods were applied. The results showed that the melted wax improved adherence on a flat and neat Parylene-C film cover and could greatly benefit the mass production. After removing the sacrificial layers, Parylene-C was heated to 120℃to change the residual stress of Parylene-C film to strongly tensile for a flatter surface.