Eun Sok Kim

Micromachined acoustic resonant mass sensor

This paper describes a highly sensitive, film bulk acoustic resonator (FBAR) mass sensor (built on a micromachined silicon-nitride diaphragm with a piezoelectric thin film and Al electrodes) that can operate in vapor and liquid. The sensitivity of the device to mass change on its surface has been investigated by having various thicknesses of silicon-nitride support layer and also of Al layer. The sensor is measured to have a mass sensitivity of 726 cm/sup 2//g, which is about 50 times that of a typical quartz crystal microbalance (QCM). In vapor, the sensor (operating at around 1 GHz and having a relatively high quality (Q) factor of 200-300) shows a minimum detectable frequency shift of about 400 Hz, which corresponds to a mass change of 10/sup -9/ g/cm/sup 2/ on the sensor surface, comparable with that detectable by a QCM. In liquid, though the Q usually drops more than an order of magnitude, we obtain a Q of 40 at 2 GHz by using a second harmonic resonance of the resonator. And with the Q, a minimum 5 ppm resonant frequency shift can be detected, which corresponds to 10/sup -8/ g/cm/sup 2/ change on the sensor surface.

IC-Processed piezoelectric microphone

A miniature diaphragm pressure transducer having sensitivity to acoustic signals at the level of conversational speech has been fabricated by combining micromachining procedures (to produce a thin silicon-nitride diaphragm) with ZnO thin-film processing. The sensor consists of a patterned ZnO layer (which acts as a piezoelectric transducer) deposited on a thin square micromachined diaphragm made of LPCVD silicon nitride. The diaphragm, 2 µm in thickness, is the thinnest yet reported for a piezoelectric readout structure of relatively large area (3 × 3 mm2). The transducer shows an unamplified response of roughly 50 µV/µbar when excited by sound waves at 1 kHz with the variation of the sensitivity from 20 Hz to 4 kHz being approximately 9 dB. These results are obtained using a 0.1-mm-wide annular pattern that measures 3.6 mm in circumference.

Piezoelectric microphone with on-chip CMOS circuits

An IC-processed piezoelectric microphone with on-chip, large-scale integrated (LSI) CMOS circuits has been designed, fabricated, and tested in a joint, interactive process between a commercial CMOS foundry and a university micromachining facility. The 2500*2500*3.5 mu m/sup 3/ microphone has a piezoelectric ZnO layer on a supporting low-pressure chemical-vapor-deposited (LPCVD), silicon-rich, silicon nitride layer. The optimum residual-stress-compensation scheme for maximizing microphone sensitivity produces a slightly buckled microphone diaphragm. A model for the sensitivity dependence of device operation to residual stress is confirmed by applying external strain. The packaged microphone has a resonant frequency of 18 kHz, a quality factor Q approximately=40, and an unamplified sensitivity of 0.92 mV/Pa. Differential amplifiers provide 49 dB gain with 13 mu V A-weighted noise at the input. >

Micropump based on PZT unimorph and one-way parylene valves

This paper describes a micropump composed of a piezoelectric PZT unimorph and one-way parylene valves. Two different designs of the valves (cantilever- and bridge-type) are studied, fabricated and tested. The micropump (13 × 13 × 1.2 mm3 in size) is capable of pumping liquid up to 700 μL min−1 with its pumping speed being insensitive to a backpressure up to 2.5 kPa. The flow rate of 700 μL min−1 is obtained when the micropump is driven with square pulses at 6 kHz with 50% duty cycle and 100 V peak-to-peak. The maximum static pumping pressure is measured to be 4 kPa.

Micromachined acoustic-wave liquid ejector

This paper describes the design and performance of micromachined, self-focusing acoustic-wave liquid ejector (AWLE) that requires no heat, nozzle, nor acoustic lens. The AWLE has a very simple device structure and is easy to fabricate. Three versions of AWLE have been designed, fabricated, and tested for an ink-jet printing application. Also developed are computer simulation and design aids that take into account the acoustic loss in water and the two-time wave reflections at the water-air and water-transducer interfaces. The AWLE has been observed to eject water droplets of about 5 /spl mu/m in diameter with radio frequency (RF) pulses of 5 /spl mu/s pulsewidth. Overall, the AWLE has been shown to be capable of improving the printing resolution and speed of ink-jet printing significantly.

Microfluidic motion generation with acoustic waves

Abstract This paper presents a novel approach for producing microfluidic motion by loosely focused acoustic waves (generated by piezoelectric zinc oxide thin film). Results show that the acoustic waves generated by radio frequency (r.f.) sources with frequencies corresponding to the thickness-mode resonances of the piezoelectric film are very effective in moving liquid around when the waves are loosely focused. The device operates without any significant temperature increase in the liquid, and will be very attractive for mixing or transporting temperature-sensitive fluids.

A film bulk acoustic resonator in liquid environments

A highly sensitive film bulk acoustic resonator (FBAR) mass sensor in liquid environments is described in this paper. A transmission line model is used to theoretically predict the dependence of the FBARs resonant frequency on added mass. FBAR performance in a liquid environment is experimentally characterized for the first time and the effects of the liquid nature and conductivity on the FBAR series and parallel resonant frequencies are investigated. A TiO2-coated FBAR is developed for sensitive mass sensing of metal ions in a liquid environment.

Single- and Triaxis Piezoelectric-Bimorph Accelerometers

This paper describes the novel single- and triaxis piezoelectric-bimorph accelerometers that are built on parylene beams with ZnO films. The unamplified sensitivity and the minimum detectable signal of the fabricated single-axis accelerometer are measured to be 7.0 mV/g and 0.01 g, respectively, over a frequency range from 60 Hz to subhertz. The linearity of the sensitivity as a function of acceleration is measured to be 0.9% in the full scale. A highly symmetric quad-beam bimorph structure with a single proof mass is used for triaxis acceleration sensing and is demonstrated to produce high sensitivity, low cross-axial sensitivity, and good linearity, all in a compact size. The unamplified sensitivities of the X-, Y-, and Z-axis electrodes (of the triaxis accelerometer) in response to the accelerations in X-, Y-, and Z-axes are 0.93, 1.13, and 0.88 mV/g, respectively. The worst-case minimum detectable signal of the triaxis accelerometer is measured to be 0.04 g over a bandwidth ranging from subhertz to 100 Hz. The cross-axial sensitivity among the X-, Y-, and Z-axis electrodes is less than 15% in the triaxis accelerometer. The theoretical analyses of the charge sensitivities and resonant frequencies along with the effects of residual stress on the charge sensitivities are presented for both the single- and triaxis accelerometers.

Self-focused high frequency ultrasonic transducers based on ZnO piezoelectric films

A micromachined self-focusing high frequency ultrasonic transducer was fabricated with a 13μm thick ZnO film deposited on a silicon substrate by sputtering. X-ray diffraction shows that the film has a high (002) orientation. The element aperture size of the transducer was 2.5mm, and the fundamental resonant frequency was designed to be over 200MHz with approximately 28% bandwidth through only one matching layer. Experimental results show that this type of focused high frequency ultrasound device may have potential for cellular microstructure imaging and skin cancer detection.

Piezoelectrically actuated dome-shaped diaphragm micropump

This paper describes a piezoelectric micropump built on a dome-shaped diaphragm with one-way parylene valves. The micropump uses piezoelectric ZnO film (less than 10 /spl mu/m thick) to actuate a parylene dome diaphragm, which is fabricated with an innovative, IC-compatible process on a silicon substrate. Piezoelectric ZnO film is sputter-deposited on a parylene dome diaphragm with its C-axis oriented perpendicular to the dome surface. Two one-way check valves (made of parylene) are integrated with a piezoelectrically actuated dome diaphragm to form a multi-chip micropump. The fabricated micropump (10/spl times/10/spl times/1.6 mm/sup 3/) consumes extremely low power (i.e., 3 mW to pump 3.2 /spl mu/L/min) and shows negligible leak up to 700 Pa static differential pressure.