ZnO Piezoelectric Film Research Articles

Piezoelectricity of ZnO Films Prepared by Sol-Gel Method

ZnO piezoelectric thin films were prepared on crystal substrate Si(111) by sol-gel technology, then characterized by scanning electron microscopy, X-ray diffraction and atomic force microscopy (AFM). The ZnO films characterized by X-ray diffraction are highly oriented in (002) direction with the growing of the film thickness. The morphologies, roughness and grain size of ZnO film investigated by AFM show that roughness and grain size of ZnO piezoelectric films decrease with the increase of the film thickness. The roughness dimension is 2.1880.914 nm. The piezoelectric coefficient d33 was investigated with a piezo-response force microscope (PFM). The results show that the piezoelectric coefficient increases with the increase of thickness and (002) orientation. When the force reference is close to surface roughness of the films, the piezoelectric coefficient measured is inaccurate and fluctuates in a large range, but when the force reference is big, the piezoelectric coefficient d33 changes little and ultimately keeps constant at a low frequency.

Control of in-plane and out-of-plane texture in shear mode piezoelectric ZnO films by ion-beam irradiation

ZnO polycrystalline films have a strong tendency to grow their c-axis perpendicular to the film surface, even on an amorphous substrate. However, unusual (101¯0) preferred orientations in which the c-axis lies in the substrate plane are often observed when the film is exposed to ion irradiation during its growth. To investigate the effect of ion irradiation on the (101¯0) preferred orientation, ZnO films were fabricated using a 0–1 keV oxygen ion-beam-assisted electron-beam evaporation of zinc. The results clearly indicated that the tendency of (101¯0) preferred orientation was enhanced with increasing ion energy and amount of ion irradiation. This demonstrated that the ion bombardment induced the (0001) preferred orientation to change into a (101¯0) preferred orientation which corresponds to the ion channeling direction. An in-plane preferred orientation was also obtained, probably because of deviations in the incident ion-beam direction from 5° to the substrate surface normal. These in-plane textured (101¯0) ZnO films, fabricated under the ion-beam irradiation of 0.5–1 keV, excited a shear acoustic wave without any longitudinal wave. The highest shear mode electromechanical coupling coefficient was found to be k15=0.16 in the film with ion-beam irradiation of 1 keV. This k15 value corresponds to 60% of that in a ZnO single crystal (k15=0.26).

ZnO film based surface acoustic wave micro-pump

In this study, a micro-pump unit based on surface acoustic wave (SAW) on piezoelectric ZnO film is designed and fabricated as a micro-fluidic device. It employs a mechanical wave, which is generated electrically using an aluminum interdigital transducer (IDT), and propagates on the surface of the ZnO film. The ZnO film was used in this study because it has a high electromechanical coefficient and an excellent bonding with various substrate materials, in particular silicon. The sputtering parameters for ZnO film deposition have been optimized, and the ZnO films with different thickness from 1 micron to 5.5 microns were prepared. The film properties have been characterized using different methods, such as scanning electron microscopy, X-ray diffraction and atomic force microscopy. Aluminum IDT with a finger width and spacing of 8 microns was patterned on the ZnO film using a lift-off process. The frequency generated was measured using a network analyzer, and it varies from 130 MHz to 180 MHz as a function of film thickness. A signal generator was used to generate the frequency with a power amplifier to amplify the signal, which was then applied to aluminum IDT to generate the surface acoustic wave. If a liquid droplet exists on the surface carrying the acoustic wave, the energy and the momentum of the SAW will be coupled into the fluid, causing the liquid to vibrate and move on film surface. The strength of this movement is determined by the applied voltage and frequency. The volume of the liquid drop loaded on the SAW device in this study is of several hundreds of nanoliters. The movement of the liquid inside the droplet and also on the ZnO film surface can be demonstrated. The performance of ZnO SAW device was characterized as a function of film thickness.

Two-step sputtered ZnO piezoelectric films for film bulk acoustic resonators

This study investigates high-performance ZnO piezoelectric films used for thin film bulk acoustic resonators (TFBAR). The ZnO piezoelectric film was deposited on a Pt/Ti electrode using an RF magnetron sputter by a two-step method at room temperature. The Pt/Ti electrode was deposited by a DC sputtering system, on which, ZnO piezoelectric films were deposited in one step and in two steps to minimize roughness in the first step and produce the preferred orientation in the second. Both field-emission scanning electron microscopy (FESEM) and atom force microscopy (AFM) revealed that ZnO piezoelectric film deposited by two-step sputtering exhibited favorable characteristics, such as a rigidly precise surface structure with surface roughness of 7.37 nm, even better than in one-step sputtering. Examining the ZnO thin film by X-ray diffraction (XRD) showed a much higher c-axis-preferring orientation than in one-step sputtering. The reflection coefficient of the resonator device was measured using an HP8720 network analyzer. The frequency response of the FBAR device exhibited a return loss of -25 dB at a resonant frequency of 2212 MHz with a high coupling coefficient of 6.7%.

CHARACTERISTICS OF PIEZOELECTRIC MICRO-SPEAKER FABRICATED WITH ZNO THIN FILM

ABSTRACT A piezoelectric microspeaker was fabricated on the 5 × 5 mm2 diaphragm with highly c-axis oriented ZnO thin film on a heavily compressive-stressed silicon-nitride film. Four masks were used in the fabrication processes of the piezoelectric microspeaker. 0.5 μ m-thick piezoelectric ZnO film was deposited by RF-magnetron sputtering method on an Al bottom electrode. Then, 0.2 μ m-thick Al film was deposited for the top electrode. To enhance the ZnO film quality for improving the microspeaker performance, two-step deposition technique was adopted. The back-side silicon nitride was patterned, and silicon substrate was removed to make a diaphragm. The speaker output pressure level (SPL) of the fabricated microspeaker was measured with the variation of sinusoidal input frequency and compared to that of the commercial piezo-ceramic speaker. The largest SPL of the microspeaker was about 92 dB at the input voltage of 6Vpeak-to − peak, which was measured at the distance of 3 mm from the microspeaker in open field. The SPL of microspeaker was nearly same as that of a commercial piezo-ceramic speaker.

Electromechanical coupling coefficient k15 of polycrystalline ZnO films with the c-axes lie in the substrate plane

The (1120) textured polycrystalline ZnO films with a high shear mode electromechanical coupling coefficient k15 are obtained by sputter deposition. An over-moded resonator, a layered structure of metal electrode film/(1120) textured ZnO piezoelectric film/metal electrode film/silica glass substrate was used to characterize k15 by a resonant spectrum method. The (1120) textured ZnO piezoelectric films with excellent crystallite c-axis alignment showed an electromechanical coupling coefficient k15 of 0.24. This value was 92% of k15 value in single-crystal (k15 = 0.26).

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.

Properties analysis of Mn-doped ZnO piezoelectric films

The Mn-doped ZnO piezoelectric films were prepared by sol–gel method. The ZnO films with perfect c-axis orientation were obtained in the annealing temperature range of 470–700 °C when 1% Mn ion (molar percent) was doped into precursor sol. The resistivity of the ZnO films annealed at 600 °C increased from 800 Ω cm (undoped) to 2 × 10 7 Ω cm (1% Mn-doped). The XPS spectra of Mn-doped ZnO films were analysed.

1-05-05 Texture control of ZnO piezoelectric films by ion beam irradiation(Bulk wave devices)

1-05-05 Texture control of ZnO piezoelectric films by ion beam irradiation(Bulk wave devices)

Deposition of Piezoelectric ZnO Films by RF Magnetron Sputtering and its Application to SAW Devices

AbstractHighly c-axis oriented, dense, and fine-grained polycrystalline ZnO films with smooth surface and high resistivity were deposited on 4 inch silicon wafers by employing ZnO targets in a radio-frequency (RF) magnetron sputtering system. By changing applied RF power, substrate temperature and O2/Ar gas ratio, the optimum process parameters were found to be 150 W, 200 °C and 30/70, respectively. Applying the ZnO films deposited under these optimum conditions, surface acoustic wave (SAW) devices of ZnO/IDT/SiO2/Si structure were fabricated by conventional photolithography and etching processes. The interdigital transducers (IDT), made of the aluminum deposited by DC magnetron sputter, were patterned as 2.5/2.5 μm of finger width/spacing. Another type of SAW filter of IDT/ZnO/diamond/Si structure was fabricated. In this structure, high-quality nanocrystalline diamond (NCD) films were deposited on 4 inch silicon wafers by direct current (DC) plasma assisted chemical vapor deposition method using H2-CH4 mixture as precursor gas. On the top of the diamond films, ZnO films were deposited under the optimum conditions. The aluminum IDT pattern was fabricated on the ZnO/diamond layered films. The characteristics of the fabricated SAW devices were evaluated in terms of center frequency, insertion loss, and wave propagation velocity.

VHF/UHF range bioultrasonic spectroscopy system and method

We developed the bioultrasonic spectroscopy system and method for doing ultrasonic tissue characterization studies in the VHF and UHF ranges. The system can determine the frequency characteristics of bulk acoustic properties of the sound velocity, attenuation, acoustic impedance, and density simultaneously by an ultrasonic transmission line comparison method using pure water as the reference, and using rf tone burst pulse signals. A biological tissue/liquid specimen is sandwiched between the parallel surfaces of buffer rods having ZnO piezoelectric film transducers on their opposite ends. Measurements are conducted with a typical thickness of 0.1 to 1 mm in the frequency range up to 700 MHz, and in the frequencies higher than 500 MHz, with a thin layer of specimen, several micrometers, operating in the multiple interference method. The system was applied for demonstration to measure the acoustic properties of bovine tissues, egg yolk and albumen, and aqueous solutions of biomacromolecules such as bovine hemoglobin and sugars in the frequency range 10 MHz to 1.6 GHz. The system and methodology will enable us to provide basic data for understanding the interaction of high-frequency ultrasonic waves with biological media and to make contributions to the establishment of the scanning acoustic microscope in the fields of biological and medical sciences.

Optimization of SAW sensors with a structure ZnO–SiO 2–Si to detect volatile organic compounds

Surface acoustic waves (SAW) devices are fabricated on ZnO piezoelectric films by reactive sputtering on SiO 2–Si substrates with different O 2 concentrations (7.5%, 10%, 15%) in Ar plasma. Crystalline structures and roughness characteristics of the films were investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements, respectively. Preparation conditions were optimized to obtain films of good quality suitable for the fabrication of SAW devices. Different designs of these ZnO structures have been characterized through RF measurements in order to choose the best design with the purpose of realizing a SAW sensor with good characteristics to detect volatile organic compounds (VOCs). The best design of structure ( W = 80 λ, L = 100 λ, N = 72) has been covered with polyepichlorohydrine (PECH) obtaining the best responses for toluene and above all with the ZnO film prepared with a 10% O 2 concentration.

Preparation of free-standing diamond films for high frequency SAW devices

Free-standing diamond films were prepared by hot filament chemical vapor deposition (HFCVD) method under different conditions. Inter-digital transducers (IDTs) were formed on the nucleation sides of free-standing diamond films by photolithography technique. Then piezoelectric ZnO films were deposited by radio-frequency(RF) reactive magnetron sputtering to obtain the ZnO/diamond film structures. Surface morphologies of the nucleation sides and the IDTs were characterized by means of scanning electron microscopy (SEM), atomic force microscope (AFM) and optical microscopy. The results indicate that the surfaces of nucleation sides are very smooth and the IDTs are of high quality without discontinuity and short circuit phenomenon. Raman spectra show the sharp diamond feature peak at about 1 334 cm −1 and the small amount of non-diamond carbon in the nucleation side. X-ray diffraction (XRD) patterns of the structure of ZnO/diamond films show a strong diffraction peak of ZnO (002), which indicates that as-sputtered ZnO films are highly c-axis oriented.

The Preparation of Piezoelectric ZnO Films by RF Magnetron Sputtering for Layered Surface Acoustic Wave Device Applications

Crystalline structures, intrinsic stress, and surface roughness of piezoelectric ZnO films deposited on sapphire substrates by rf magnetron sputtering and sequential post-annealing treatments were investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements. The rf power used and O2/(Ar+O2) gas ratio greatly affected the crystallinity and surface roughness of the ZnO film. A c-axis preferred orientation with superior surface roughness was achieved at a specific rf power and O2/(Ar+O2) gas flow ratio. In addition, the original intrinsic stress was also markedly released (stress=0.325 ×1010 dyn/cm2) and a uniform surface (Ra=2.42 nm) was produced by a post-annealing treatment at a temperature of 400 °C under oxygen ambient. An acoustic wave at the center frequency of 240 MHz (hZnO/λ=0.1) was obtained from the ZnO/sapphire-layered surface acoustic wave (SAW) device using this optimized ZnO film.

Effects of H 2O partial pressure on the crystallinity of ZnO thin film and electrical characteristics of film bulk acoustic wave resonators

We have improved electrical characteristics of a film bulk acoustic wave (BAW) resonator that features the injection of H 2O gas into a process chamber. The preferred crystallinity of piezoelectric ZnO film was obtained by RF sputtering at a high H 2O partial pressure 1.5×10 −4 Pa. The effective electromechanical coupling coefficient ( k t eff 2 ) of the BAW resonator remarkably goes up from 1.8% to 4.7% for which the corresponding H 2O partial pressures are 2.7×10 −5 and 1.5×10 −4 Pa. Injection of H 2O during the deposition process contributes to the improvement of crystallinity of ZnO thin film and the electrical characteristics of the BAW resonator.

Fabrication of Piezoelectric ZnO and PZT Films for FPW Device

ABSTRACTPiezoelectric ZnO and PZT films for acoustic wave device have been investigated. The films used for acoustic wave devices require textured structure, high piezoelectric coefficients, and high electromechanical coupling coefficients, because the piezoelectric films launch and receive the acoustic wave. We fabricated ZnO films by RF magnetron sputtering, and PZT films by chemical solution deposition (CSD). Results showed that uniform, dense and highly textured films were obtained under optimal process parameters. The film texture was controlled by modifying process parameters, such as gas kinds, gas ratio, and substrate type for sputtered ZnO films, and the presence of chelating agent, temperature of heat treatment, and substrate type for solution derived PZT films. Flexural plate wave (FPW) device has been successfully integrated onto 4-inch silicon wafers with optimized piezoelectric films.

Characteristics of (101~0) and (112~0) textured ZnO piezofilms for a shear mode resonator in the VHF-UHF frequency ranges

This paper reports the fabrication and characterization of ZnO piezoelectric thin films in which the crystallite c-axis is unidirectionally aligned in the plane. The films were deposited by a conventional radio frequency (RF) magnetron sputtering apparatus without epitaxy. We have measured reflection coefficient S11 of the ZnO film/glass substrate composite shear mode resonator and confirmed that the resonator excites shear wave only in the very high frequency to ultra high frequency ranges (VHF-UHF). The crystallites c-axis orientation and alignment were determined by x-ray diffraction (XRD) patterns, phi-scan pole figure analysis, omega-scan rocking curves, and atomic force microscope (AFM) measurement. The transduction of the shear wave showed good agreement with properties of the crystallite alignment in the film.

Influence of Li+ dopants on the properties of the ZnO piezoelectric films

ZnO piezoelectric films with the preferred 002-orientation were prepared by sol-gel method. The annealing temperature was 600∘C and the resistivity of the ZnO film was 1 × 106 Ω ⋅ cm. Li2CO3 and LiCl were added respectively into ZnO precursor as source of Li+-ion. The molar ratio of [Li+]/[Zn2 +] was 0.05. It is observed that the annealing temperature for forming preferred 002-orientation of ZnO films decreases from 660 to 550∘ C after Li2CO3 being doped. When Li2CO3 and LiCl are doped, the resistivity of ZnO films increases to 108Ω ⋅ cm and 109Ω ⋅ cm, respectively, with an annealing temperature of 550∘ C. When annealing temperature is 600∘ C, the resistivity of the ZnO film with LiCl dopant increases to 107Ω cm. The influence mechanism of the two dopants on the properties of the ZnO films is analyzed.

Effect of preheating and annealing temperatures on quality characteristics of ZnO thin film prepared by sol–gel method

In order to fabricate qualified ZnO piezoelectric thin films by sol–gel method, the relationships between the heat treatment temperatures (preheating temperature and annealing temperature) and the quality characteristics of ZnO piezoelectric thin films ( c-axis orientation, residual stress, grain size, roughness and resistivity) were investigated. The chemical composition of the precursor sol and the intermediate produced in the films heating process were analyzed by TGA–SDTA and FT-IR. The c-axis orientation and the residual stress of the ZnO thin film were identified by XRD. The morphologies, roughness and grain size were observed and estimated by AFM. The I– V characteristics were measured by using semiconductor characterization system. Experimental results show that the c-axis orientation is determined by both preheating and annealing temperatures, and that the residual stress, grain size, roughness and resistance of the ZnO thin films are mainly influenced by the annealing temperature. A qualified ZnO piezoelectric thin film has been prepared by using sol–gel with preheating temperature 400 °C for 10 min and annealing temperature 700 °C for 30 min.

Characterization of ZnO/diamond SAW devices elaborated on the smooth nucleation side of MPACVD diamond

AbstractWe designed a SAW filter in this work by combining the piezoelectric ZnO film with a freestanding double layer diamond film deposited using CH4–H2 pulsed MPACVD process through two growth stages. The AFM measurements on the nucleation side of the diamond film has shown that this side is smooth enough to perform photolithography process in the SAW device elaboration, while the good quality of the diamond film produced on this side has been confirmed through Raman spectroscopy.Furthermore, the ZnO films having their c axis perpendicular to the substrate were deposited using reactive magnetron sputtering under the optimal experimental conditions found from XRD measurements. In the SAW device elaboration on this ZnO/bilayer diamond structure, aluminium interdigital transducers (IDTs) were developed by conventional contact UV photolithography on the nucleation side of the freestanding diamond. The frequency response of the device presents harmonics of the acoustic wave modes propagating in the smooth diamond nucleation side. The good agreement obtained between the experimental and theoretical frequency responses attests that the elastic properties of the ZnO and the diamond are similar to those of the single crystal assumed in the theoretical study. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)