Changlei Zhao

Fabrication and characterization of piezoelectric micromachined ultrasonic transducers with thick composite PZT films

Ferroelectric microelectromechanical systems (MEMS) has been a growing area of research in past decades, in which ferroelectric films are combined with silicon technology for a variety of applications, such as piezo-electric micromachined ultrasonic transducers (pMUTs), which represent a new approach to ultrasound detection and generation. For ultrasound-radiating applications, thicker PZT films are preferred because generative force and response speed of the diaphragm-type transducers increase with increasing film thickness. However, integration of 4- to 20-microm thick PZT films on silicon wafer, either the deposition or the patterning, is still a bottleneck in the micromachining process. This paper reports on a diaphragm-type pMUT. A composite coating technique based on chemical solution deposition and high-energy ball milled powder has been used to fabricate thick PZT films. Micromachining of the pMUTs using such thick films has been investigated. The fabricated pMUT with crack-free PZT films up to 7-microm thick was evaluated as an ultrasonic transmitter. The generated sound pressure level of up to 120 dB indicates that the fabricated pMUT has very good ultrasound-radiating performance and, therefore, can be used to compose pMUT arrays for generating ultrasound beam with high directivity in numerous applications. The pMUT arrays also have been demonstrated.

Low temperature processing of nanocrystalline lead zirconate titanate (PZT) thick films and ceramics by a modified sol-gel route

Dispersing fine particles into a sol-gel matrix is a promising process to get a thick 0–3 composite coating layer. In this paper, we have further improved this modified sol-gel process by nanocrystalline composite technique to realize the low temperature annealing. Dense Pb(Zr, Ti)O3 (PZT) thick films of 10 to 50 µm in thickness have been obtained on the platinum-coated silicon substrates by spin-coating at sintering temperature of 600–700°C and fully developed submicron-sized grains have been demonstrated in screen-printing piezoelectric films on alumina substrates at sintering temperature of 700–800°C. The dependence of various properties such as microstructure, crystallization, ferroelectric and dielectric properties of such made thick films on the processing parameters have been investigated. For a 10 µm-thick film spin-coated on silicon wafer, the dielectric loss and relative permittivity are 0.010 and 1024, respectively, at 1 kHz. The remanent polarization (Pr) and the coercive field (Ec) are 13.6 µC/cm2 and 34.5 kV/cm, respectively. Obviously, such made thick film has comparable properties with bulk PZT ceramic. This novel technique can be extensively used in sol-gel, screen-printing, tape-casting, even in traditional ceramic process to reduce the process temperature.

PZT THICK FILMS FABRICATION USING A SOL-GEL BASED 0-3 COMPOSITE PROCESSING

A sol-gel based 0-3 ceramic-ceramic composite processing has been applied to fabricate lead zirconate titanate thick films with a composition near the morphotropic phase boundary, PZT53/47. In order to eliminate the effects of humidity on the stability of the sols, solid PZT precursors are prepared from partially stabilized titanium isopropoxide, lead acetate trihydrate and zirconium acetylacetonate without any solvent. Then the solid precursors were dissolved in 2-methoxythanol to form sols. Certain quantum modified PZT ceramic powder was dispersed into this obtained solution. Crack-free thick films with series thickness of 5 ~ 20-μm were fabricated using spin-coating method. The surface morphology and structural properties of the films were studied with Field Emission-SEM and X-ray diffraction techniques respectively. Electrical properties were compared with those of thin films. Thick PZT films should have good perspective in the application of new piezoelectric devices that cannot be achieved using conventional thin film or bulk techniques.

Deflection characteristics of a trapezoidal multilayer in-plane bending piezoelectric actuator

A novel multilayer split-morph actuator has been designed and fabricated using the thick film screen-printing technology. Deflection characteristics of the split-morph actuator have been investigated by theoretical analysis and experimental measurement. The results indicate that the tip displacement is inversely proportional to the thickness of each piezoelectric layer, but is independent of the number of layers and the total thickness of the actuator. The displacement/voltage sensitivity of the trapezoidal actuator is larger than that of the rectangular design, assuming both have the same width of clamped end and the same thickness of the piezoelectric layers. The maximum displacement/voltage sensitivity of 0.157 micron/V was obtained with a split-morph actuator with 30-micron thick layers. The proposed actuator is a promising candidate for the secondary fine-tuning actuator of a dual stage head-positioning servo system in high density hard disk drives.

Sol-Gel Derived PZT Thick Films with Nano-Sized Microstructure

Lead zirconate titanate (PZT) films are promising for acoustic micro-devices applications because of their extremely high electromechanical coupling coefficients and excellent piezoelectric response. Thicker PZT films are crucial for these acoustic applications. A hybrid sol-gel technology has been developed as a new approach to realize simple and cost-effective fabrication of high quality PZT thick films. In this paper, PZT53/47 thick films with a thickness of 5–50 μ m are successfully deposited on Pt-coated silicon wafer by using the hybrid sol-gel technology. The obtained PZT thick films are dense, crack-free, and have a nano-sized microstructure. The processing parameters of this technology have been evaluated. The microstructure of the film has been observed using field-emission scanning electron microscopy and the crystallization process has been monitored by the X-ray diffraction. The thick films thus made are good candidates for fabrication of piezoelectric diaphragm which will be an essential element of microspeaker and microphone arrays.