Weiguang Zhu

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.

Design and fabrication of a high performance multilayer piezoelectric actuator with bending deformation

A new bending mode multimorph actuator was designed and fabricated successfully by a multiple screen printing process. Unlike the conventional bimorph actuator in which the bend occurs in the thickness direction, the bend in the multimorph actuator occurs in the widthwise direction because of synchronistical deformation of each single monolithic layer in the multilayer structure. The theoretical analysis and experimental measurements were conducted to study the performance of this type of actuator, and a comparison was made with the conventional bimorph actuator. Larger displacement, higher resonance frequency, and much larger blocking force could be achieved with the multimorph actuator than with a bimorph actuator of similar dimensions. The multimorph actuator presented in this paper provides a valuable alternative for actuator applications beyond those available with the popular bimorph and longitudinal multilayer actuators.

Improved preparation procedure and properties for a multilayer piezoelectric thick-film actuator

Abstract Low-voltage multilayer piezoelectric lead zirconate titanate (PZT) based thick-film displacement actuators were fabricated on Al 2 O 3 ceramic substrate using a multiple screen-printing process. High isostatic pressure was applied on the multilayer green film before firing to increase the material density and the bonding strength between electrode layer and piezoelectric layer. It was demonstrated that the feasibility for producing multilayer actuators on the substrate was significantly improved with the compression process introduced and the ferroelectric and piezoelectric properties of the final actuator samples were greatly improved as well.

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.

A SYSTEMATIC STUDY ON STRUCTURAL AND DIELECTRIC PROPERTIES OF LEAD ZIRCONATE TITANATE/(PB,LA)(ZR(1-X)TI(X))O3 THIN FILMS DEPOSITED BY METALLO-ORGANIC DECOMPOSITION TECHNOLOGY

Pin‐hole free ferroelectric (Pb,La)(Zr1−xTix)O3 thin films with uniform composition have been fabricated using the metallo‐organic precursor compounds, which were carefully home synthesized. The structural development, spectroscopic, and dielectric properties of these films have been systematically investigated using atomic force microscopy (AFM), x‐ray diffraction, Fourier transform infrared spectroscopy, Raman scattering, and dielectric measurements. It has been found from our experimental results of PZT 40/60 thin films that the overlapping of (h00) and (00l) peaks of these films in x‐ray diffraction patterns, mainly due to the small grain sizes in films, makes it very difficult to distinguish individual diffraction peaks and to identify the phases. However, Raman measurements undoubtedly reveal the Raman spectra of these films in the tetragonal phase field, demonstrating that Raman spectroscopy is an effective tool to identify structures, especially in the case of thin films having very small grains. ...

Ultrasound radiating performances of piezoelectric micromachined ultrasonic transmitter

This letter reports on a membrane-type piezoelectric micromachined ultrasonic transducer (pMUT). It consists of 3.5 or 7μm thick composite lead zirconate titanate (PZT) film on a multilayered membrane and works as an ultrasonic transmitter. The ultrasound-radiating performance of the transmitter, in response to a continuous ac driving voltage, has been tested systematically. The generated sound pressure level at resonance frequency of the transmitter varies with the applied dc bias and driving voltage and can exceed 110dB at a measuring distance of 12mm. For a given square membrane with the same multilayer structure and thickness, the resonance frequency of the transmitter is inversely proportional to the area of the membrane, and can be further tuned by control of the poling conditions of the PZT film or the applied dc bias.

Lead zirconate titanate ceramics derived from oxide mixture treated by a high-energy ball milling process

Abstract Lead zirconate titanate (Pb(Zr 0.52 Ti 0.48 )O 3 or PZT) ceramics were directly prepared from their oxide mixture, which was treated by a high-energy ball milling process. X-ray diffraction (XRD) result indicated that there was no reaction between the component oxides during the milling process. The particle size of the mixture was greatly reduced to nanometer scale by the milling process. Sintering behavior of the mixture demonstrated a distinguished volumetric expansion before densification, clearly showing that a reaction sintering occurred at about 770°C. PZT ceramics sintered at 1000°C for 4 h exhibited a dielectric constant of 1156 and dielectric loss of 0.03, a remnant polarization of 29 μC/cm 2 and a coercive field of 18.4 kV/cm. Piezoelectric parameters of the 1000°C-sintered PZT ceramics were k 33 of 40.4%, k 31 of 40.9%, d 33 of 175 pC/N and d 31 of −148 pC/N, respectively. These results have shown that preparation of PZT ceramics in this way is practical.

Structural and thermal analyses on phase evolution of sol-gel (Ba, Sr)TiO3 thin films

Abstract The thermal decomposition process of barium strontium titanate (BST) xerogel precursors has been analyzed using thermogravimetric analysis, Differential thermal analysis and X-ray diffraction. The results indicate that the precursors are amorphous in nature when annealed below 520 °C, and perovskite BST grains crystallize at 620 °C via a solid-state reaction between (Ba,Sr)CO 3 and TiO 2 . It is revealed that the intermediate phase suppresses the crystallization of perovskite BST grains in films, and results in higher anneal temperature. To minimize the intermediate phase, the first three layers are crystallized prior to the deposition of subsequent layers, and 340-nm-thick perovskite BST films have been fabricated at a process temperature of 625 °C. The low process temperature results in a fine-grained structure in films, producing films that exhibit low dielectric constant and low tunability accordingly.

Self-biased dielectric bolometer from epitaxially grown Pb(Zr,Ti)O3 and lanthanum-doped Pb(Zr,Ti)O3 multilayered thin films

A self-biased dielectric bolometer has been realized by epitaxially grown lead zirconate titanate Pb1.1(Zr0.3Ti0.7)O3 (PZT 30/70) and lanthanum-doped lead zirconate titanate Pb0.83La0.17(Zr0.3Ti0.7)0.9575O3 (PLZT 17/30/70) multilayered thin films using the sol-gel technique. The high-resolution transmission electron microscopy images and electron diffraction pattern showed that the multilayered film had its preferred (111) orientation and the epitaxial growth between the PZT 30/70 and PLZT 17/30/70 layers. The self-biased phenomenon and a large pyroelectric current were observed in the prepoled sample without any external bias at the PLZT 17/30/70 phase transition temperature of 120 °C. Pyroelectric coefficient as high as 990 μC/m2K is obtained in the multilayer thin films around 120 °C. It is obvious that there exists an induced polarization in the PLZT 17/30/70 layer. We attribute this self-biased effect to the strong remnant polarization in the immediately contacted tetragonal PZT 30/70 layers.

Highly enhanced sinterability of commercial PZT powders by high-energy ball milling

Abstract High-energy ball milling technique was successfully applied to a commercial lead zirconate titanate (PZT) powder to decrease its sintering temperature by 100°C. The sinterability enhancement of commercial PZT powder through this technique was evidenced by experimental results. The commercial PZT powder is made up of ball-shaped particles of tens of micrometers in size, consisting of