ZnO Piezoelectric Film Research Articles

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.

Fabrication of MEMS ZnO dome-shaped-diaphragm transducers for high-frequency ultrasonic imaging

This paper presents the microfabrication technique for a dome-shaped-diaphragm transducer (DSDT) for 200 MHz cellular microstructure imaging. The DSDT uses piezoelectric ZnO film to generate acoustic waves, and is fabricated on a silicon substrate. The fabricated DSDTs have been tested with a pulse-echo method using a quartz target, and shown to produce an echo signal at 210 MHz with 20% bandwidth. The DSDT fabrication uses spherical balls to precisely shape wax molds, onto which parylene is deposited as a support layer for the DSDT. The wax molds are removed by toluene to release the parylene dome diaphragms. Piezoelectric ZnO film is sputter deposited on the parylene dome diaphragm. E-Solder silver epoxy is placed and cured on the back surface to function both as an acoustic backing and as structural support. Quarter wavelength thick parylene is deposited on the front side of the wafer for acoustic matching. The fabrication technique for the DSDTs is meant for low-cost mass production of the devices for high-frequency biomedical imaging. Moreover, the technique allows the precise control of the radius and curvature of the dome-shaped diaphragm through adjusting the size of the front-to-backside thru holes using different radii of the spherical balls.

Freestanding CVD diamond elaborated by pulsed-microwave-plasma for ZnO/diamond SAW devices

In previous work, the feasibility of developing high performance AlN/diamond surface acoustic wave (SAW) devices has been demonstrated using the unpolished nucleation side of freestanding CVD diamond. This process shows the advantage of avoiding the diamond polishing, which is a tedious and a time-consuming technological step. As alternative, we propose the use of a diamond substrate obtained by two stages of growth in pulsed MPACVD to constitute two superposed layers. The first one was deposited at optimal conditions with 2% CH 4 in H 2 gas mixture, leading to a high diamond quality. The second diamond layer is used as a support in order to get a freestanding diamond. It has a lower quality because it's obtained by increasing the CH 4 content, thus the growth rate. The first diamond layer combined with piezoelectric film defines the SAW properties. Its thickness is chosen as a function of the acoustic wave penetration depth in order to limit their propagation only in the first diamond layer. Aluminium inter-digital transducers (IDTs) were deposited by conventional contact UV photolithography on the nucleation side of the freestanding diamond. Piezoelectric ZnO film was then deposited by reactive magnetron sputtering to obtain the ZnO/IDT /diamond structure. Electrical characterization of the SAW device exhibits high filtering properties. The following parameters are obtained with 32 μm wavelength, 3 μm ZnO and 25 μm diamond first layer thickness: acoustic phase velocity: v ϕ=9696 m/s, electromechanical coupling coefficient: K 2=0.75%, temperature coefficient of frequency: TCF=29 ppm/°C.

Applications of piezoelectric ZnO film deposited on diamond-like carbon coated onto Si substrate under fabricated diamond SAW filter

The applications of surface acoustic wave (SAW) devices aim directly at piezoelectric thin films deposited onto amorphous diamond-like carbon (DLC) coated on silicon substrates from target ZnO in this study. Our high-velocity SAW filters are characterized using multi-layer structures of thin films by DLC depositing. However, if propagating velocity of diamond is much higher, then the SAW phase velocity of piezoelectric thin film ZnO deposited on DLC coated onto Si substrate is raised from 2800 to11430 m/s. Consequently, the GHz-band SAW devices could be fabricated using thin films of DLC without sub-micron photolithography technology. We have been also successfully fabricated and demonstrated the diamond-like SAW filters by evaporating interdigital transducers on ZnO thin films with ULSI technology.

Fabrication and characterization of high frequency SAW device with IDT/ZnO/AlN/Si configuration: role of AlN buffer

Surface acoustic wave (SAW) devices with a ZnO/buffer/Si configuration have been fabricated, employing poly-AlN (deposited by RF magnetron sputtering) and epi-AlN (deposited by plasma-assisted MBE) as the buffer layer. In particular, the effects of the AlN buffers on the orientation of ZnO films as well as the performance of SAW devices are studied. Piezoelectric ZnO films are deposited using RF sputtering at a nominal condition of RF power of 300 W, O 2/(Ar+O 2) ratio of 10%, and substrate temperature of 300 °C. For all the deposited AlN and ZnO films, the X-ray diffraction (XRD) spectra and full width at half maximum ( ω FWHM) of rocking curves are measured in terms of the deposition conditions, to characterize the c-axis preferred orientation and crystal quality. The frequency response characteristics (including S 21) of the fabricated SAW devices are also measured. The experimental results indicate that the c-axis orientation and crystal quality of ZnO films are determined mainly by the properties of AlN buffer. Furthermore, the insertion loss of ZnO-SAW devices is found to be dependent on the crystal quality and surface morphology of AlN buffer. The correlations between the properties of AlN buffers and the characteristics of ZnO-SAW devices are also discussed in detail.

Polyimide membrane with ZnO piezoelectric thin film pressure transducers as a differential pressure liquid flow sensor

Fabrication and characterization of ZnO thin film piezoelectric sensors on a polyimide membrane is presented in this paper. As a test device a differential pressure liquid flow sensor has been fabricated. We discuss new material combinations for the fabrication of the test devices. The pressure sensor membrane is a thin polyimide sheet bonded to a silicon wafer and the sensing material is sputtered ZnO piezoelectric thin film. The fabricated liquid flow sensor has been tested with a piezoelectric micropump for flow rates from 30 µl h−1 to 300 µl h−1. Stroke volumes of 1 to 10 nl have been measured. The strain in the sensing layer has been modeled and a transverse piezoelectric coefficient of e31,f = −0.294 C m−2 has been extracted.

Investigation of piezoelectric ZnO film deposited on diamond like carbon coated onto Si substrate under different sputtering conditions

The applications of surface acoustic wave (SAW) devices aim directly at piezoelectric thin-films deposited onto amorphous diamond like carbon-coated on silicon substrates from target ZnO in this study. This study discusses the physics and electrics of the thin-films with the sputtering conditions of RF power, ambient pressure and mass flow ratios oxygen/argon (O 2/Ar) under constant substrate temperature. This investigation analyses the thin-films of microstructure, grain size, micro-morphology, and O 2/Ar ratio by XRD, SEM, TEM, AFM, and AES. The analytical results show increasing the deposition ratio of the thin-films increases power and decreases ambient pressure and O 2 flow ratio. However, the RF power has a threshold limit value (TLV), and the growth ratio of thin-films decreases, if the RF power exceeds the TLV. The O 2/Ar flow ratio not only affects the depositing ratio, but also the surface of the crystal type. A suitable O 2/Ar flow ratio in the cavity, obtains a thin-film with a high-density structure and smooth surface. The optimal parameters for sputtering are RF power 200 W, ambient pressure 3×10 −3 Torr, O 2/Ar flow ratio of 1/8 and the distance between target and substrate of 35 mm. Finally, interdigital transducers were fabricated on the films to measure the characteristics of the SAW device. Consequently, the characteristics of SAW devices can be significantly enhanced by using diamond like carbon thin film as the inter-layer.

The compatibility of ZnO piezoelectric film with micromachining process

This paper presents the compatibility of ZnO piezoelectric films with the process of micromachining. As a reactive material, ZnO films are sensitive to temperature, acids, bases, and even water. These films may easily be damaged in the subsequent process steps. Therefore, different from most of the present investigations that focus on how to fabricate ZnO films with good performance, this paper discusses how to protect ZnO films against degradation in subsequent micromachining processes. The main micromachining processes related to ZnO films, including cleaning, photolithography, etching, photoresist stripping, metal layer patterning, passivation and dielectric layer deposition, and reaction ion etching (RIE), are investigated. How these subsequent micromachining processes change ZnO films performance and how to minimize or avoid such performance changes is presented. Some suggestions about the subsequent micromachining process steps are given.

Piezoelectric ZnO films by r.f. sputtering

Piezoelectric zinc oxide films are used in microelectromechanical systems (MEMS) applications, where they can be used in sensors to detect, e.g., pressure or acceleration. Beside sensors, ZnO films are applied in activation devices, where force is needed. Conductive-doped zinc oxide (most often with aluminum) is also used in optoelectronics. Piezoelectric films including AlN and ZnO are more difficult to produce than the corresponding conductive materials. In order to achieve good piezoelectricity in ZnO films, they have to possess high purity, a (0 0 1) orientation (ZnO has hexagonal crystal structure), high resistivity, and fine columnar microstructure perpendicular to the substrate. We have used r.f. magnetron (13.56 MHz) sputtering from a ZnO target in an oxygen atmosphere to achieve the piezoelectric ZnO. The aim of this work has been to develop an r.f. sputtering process for ZnO to achieve highly piezoelectric thin films. As a test vehicle to measure the piezoelectricity of the ZnO films we have fabricated resonators and passband filters in the 1–2 GHz range using standard microelectronics photolithography, deposition, and etching techniques on 100-mm diameter Corning glass or silicon wafers. The influence of the sputtering-process parameters on the film properties has been studied by X-ray diffraction, scanning electron microscopy, atomic force microscopy, and electrical measurements. In this study, the effects of the process parameters on the final material properties of the ZnO film are discussed in detail.

Shear Bulk Wave Transducer Made of (112̄0)-Plane Epitaxial ZnO Film on R-Sapphire

A (112̄0)-plane epitaxial ZnO film is deposited on an R-plane ((011̄2)-plane) sapphire substrate. In order to generate a shear bulk wave using the ZnO film, metal electrodes are required at both planes of the ZnO film; namely, the top plane on the top surface of the ZnO film and the bottom plane at the boundary between the ZnO film and the R-sapphire substrate. A c-plane ((0001)-plane) polycrystal ZnO film is conventionally deposited on the metal film on the R-sapphire substrate. Thus, the (112̄0)-plane epitaxial ZnO film for the shear bulk transducer has not been realized until now. The authors realized a (112̄0)-plane epitaxial ZnO film with low electrical resistivity (ρ of the order of 10-4 Ω·cm) on an R-sapphire substrate by doping with an impurity such as Al, Ga, or V as a substitute for the metal electrode, and a (112̄0)-plane piezoelectric epitaxial ZnO film with high electrical resistivity (ρ>10+10 Ω·cm) on this epitaxial ZnO film with low electrical resistivity for fabricating a shear wave transducer. An Al electrode was deposited on the piezoelectric ZnO film. By using the Al electrode as the top-plane electrode and a (112̄0)-plane epitaxial ZnO film with low electrical resistivity as the bottom-plane electrode substituting for the metal film, strong shear bulk waves have been generated.

Fabrication of Piezoelectric Thin Film Resonators with Acoustic Quarter-Wave Multilayers

Expanding electrical communication systems demand new acoustic wave devices that can be used in high frequency ranges. The solidly mounted piezoelectric thin film resonator is a candidate for such devices. In this study, first, acoustic properties of a number of thin films are evaluated and a combination of SiO2 and ZnO is shown to be suitable for such devices as quarter-wave multilayer materials. Then, piezoelectric thin film resonators consisting of a ZnO piezoelectric film on alternately deposited SiO2 and ZnO quarter-wave multilayers are designed and fabricated. Their measured characteristics are presented and discussed.

Characteristics of surface acoustic wave convolver in the monolithic metal–zinc oxide–silicon nitride–silicon dioxide–silicon structure

The nonlinear analysis of the metal–insulator semiconductor shows that the ac currents charging the interface traps lead to large dc operating voltage and an inefficient operation of the monolithic convolvers. These interface traps are annihilated during a low temperature anneal, which utilizes hydrogen atoms implanted underneath the SiO2–Si interface. The overlay piezoelectric ZnO film in the metal–ZnO–Si3N4–SiO2–Sistructure is protected from the influx of hydrogen atoms by an interposed silicon nitride layer. Hydrogen implantation and rapid thermal annealing steps are integrated in the process sequence of realizing an efficient metal–ZnO–Si3N4–SiO2–Si monolithic surface acoustic wave convolver.

Spectroscopy of Pure and Eu 3+ Doped ZnO

Pure and Eu 3+ doped ZnO piezoelectric ceramics and thin films were prepared via solid state chemical reaction and by sol-gel method, respectively. Strong green emission band were observed from all samples. Both broad band emission from ZnO and line emission from Eu 3+ were observed in the sample ZnO doped with Eu 2 O 3 . In contrast to the report in the literature, excitation spectrum showed that energy transfer from ZnO host to Eu ions in these samples was very weak and the transfer was through Eu 3+ absorption of the green emission of ZnO.

Investigation of Li-doped ferroelectric and piezoelectric ZnO films by electric force microscopy and Raman spectroscopy

We have grown Li-doped ZnO films on silicon (100) using the rf planar magnetron sputtering method. The surface charges induced piezoelectrically by defect and by polarization can be observed by electric force microscopy. The Li-doped ZnO films have been proven to be ferroelectric. The Raman spectra of ZnO and Li-doped ZnO films have been measured.

Control of morphology and orientation of ZnO thin films grown on SiO2/Si substrates

ZnO is a wide bandgap semiconductor possessing unique electrical, mechanical, and optical properties. Piezoelectric ZnO film has a high electro-mechanical coupling coefficient, which makes it a promising material for high frequency and low loss surface acoustic wave (SAW) devices in RF/microwave applications. High quality piezoelectric ZnO films grown on Si substrates also pave the way for integration of SAW devices with Si IC technology. In this work ZnO films are grown on SiO2/Si substrates by metal–organic chemical vapor deposition. The growth process is optimized to obtain highly oriented ZnO films with a smooth surface morphology. The structural properties of the films are investigated using X-ray diffraction, electron microscopy, and scanning probe microscopy. To obtain ZnO films with both good crystallinity and smooth surfaces, we have developed a two-step growth technique. A high temperature (450–500°C) buffer layer is initially deposited, which provides a highly crystalline template for the subsequent growth of a low temperature (300–330°C) layer. High quality ZnO thin films have been achieved, which are needed for fabrication of low-loss SAW devices.

Surface acoustic wave propagation properties of nitrogenated diamond-like carbon films

Nitrogenated diamond-like carbon (a-C:N) thin films on Si(100) were deposited by radio frequency magnetron reactive sputtering. Using ZnO piezoelectric thin films, the surface acoustic wave (SAW) propagation properties of a-C:N thin films on Si(100) have been experimentally investigated using bidirectional interdigital transducers between ZnO and a-C:N/Si(100). A generalized SAW mode with velocities between 5000 and 7000 m/s were observed. A high velocity pseudo-SAW mode was also observed and identified with simulated data, of which the velocity was 13 850 m/s. We calculated the film elastic constants from the phase velocities in the layered structure. The calculated constants for a-C:N thin films on Si were c11=392.1 and c44=185.7 GPa.

Film Bulk Acoustic Resonator Fabrication for Radio Frequency Filter Applications

In this paper, a two-step radio frequency (RF) sputtering deposition technique for piezoelectric ZnO film formation and its successful application for film bulk acoustic resonator (FBAR) devices are presented. Several critical parameters of the RF sputtering process such as deposition pressure, RF power and O2 concentration were studied to clarify their effects on the material characteristics of the ZnO films. The ZnO films deposited by the proposed two-step deposition are shown to have the growth characteristic of strongly preferred orientation toward the c-axis. The FBAR devices with the ZnO films showed a large return loss of ∼50 dB at the center frequency of 1.49 GHz. It was also found that the impedance matching of the FBAR could be easily achieved simply by controlling the resonance area of the resonator.

Optimization of ZnO Piezoelectric Thin Films and Bottom Electrodes in Thin Film BulkAcoustic Wave Resonator

A thin film bulk acoustic wave resonator (TBAR) has been fabricated using a ZnO piezoelectric thin film sandwiched by top and bottom electrodes on a SiO2 diaphragm by MEMS techniques. The elctrode/ZnO/electrode/SiO2 structure can be designed to cancel a temperature coefficient of frequency constant (TCF) by optimizing the ZnO/SiO2 thickness ratio, because the TCF of ZnO is negative, and that of SiO2 is positive. The ZnO thin film on the SiO2 shows a c-axis orientation by RF sputtering. The crystallinity of the ZnO thin film is influenced by surface conditions of under layer. ZnO thin films have been deposited on bottom electrode, that is Au/NiCr, Au/Ti and Au/Cr, respectively. The Au/Ti/ZnO/Au/Ti/SiO2 structure thin film bulk acoustic resonator shows the best resonant characteristics in this experiment. The resonant characteristics depend on the crystallinity of the ZnO thin film. The resonant resistance at 205MHz using a Au/Ti bottom electrode is about 8% less than that using a Au/Cr electrode. The X-ray diffraction result shows that the crystallinity of ZnO is greatly influenced by the crystallinity of the bottom electrode. The buffer electrode between an Au electrode and substrate give an influence on both the crystallinity of the ZnO thin film and the resonant characteristics through the Au electrode.

Piezoelectric properties and micro structures of lead titanate thin films for GHz-band bulk acoustic wave filters

The bulk acoustic wave filters at GHz bands were fabricated using lead titanate, PbTiO3 (PT) piezoelectric films which were deposited by RF magnetron sputtering and were oriented along <111> crystal direction. To improve the quality of PT films, the bottom electrodes which had the Ti-diffusion suppressible layers were designed and prepared. X-ray diffraction intensity of the PT(111) became 5 times larger and degree of crystal orientation became 2 times stronger in comparison with those of the former PT films. The filters using the improved PT films had the insertion loss of -14dB. The PT film had effective electromechanical coupling coefficient (k) of 31% and electrical Q of 50. This k value was superior to those of well-known ZnO and AlN piezoelectric films.

The use of sputtered ZnO piezoelectric thin films as broad-band microactuators

An actuation system is described which utilizes a single active ZnO piezoelectric thin film deposited onto a silicon cantilever. The method for obtaining films for effective actuation by r.f. magnetron sputtering is discussed. The actuator has been incorporated into a system which enables unwanted vibrations of the cantilever to be cancelled, whilst maintaining the ability simultaneously to deflect the cantilever as required.