Piezoelectric Zinc Oxide Film Research Articles

Thermally stable piezoelectric sensors for quantitative pressure sensing based on linear piezoelectric zinc oxide thin films

Linear piezoelectric materials offer new opportunities for next-generation multifunctional micro/nano systems on chip aside from ferroelectrics. However, the advantage of quantitative pressure sensing of linear piezoelectric materials over ferroelectrics has not been demonstrated, while the thermal stability of linear piezoelectric zinc oxide (ZnO) is yet to be evaluated. In this work, pressure sensors based on linear piezoelectric undoped ZnO and ferroelectric vanadium doped ZnO (V-ZnO) films are studied side by side. Both films possess similar fiber textures oriented along the polar ZnO [002] direction, yet each of the V-ZnO grains consists of strip-like domains observed by SEM. Applying triangular-wave pressures, the linear piezoelectric ZnO sensor generates triangular-wave voltage signals that are one-to-one linearly correlated to the applied pressure, showing decisive advantages for quantitative pressure sensing compared to the ‘sail-like’ voltage signals of the ferroelectric V-ZnO sensor that is many-to-one correlated to the applied pressure. Superior thermal stability is demonstrated in the linear piezoelectric ZnO sensor showing stable pressure sensing properties up to 450 °C compared to the degraded sensing properties of the ferroelectric V-ZnO sensor at 150 °C. The thermally stable piezoelectric sensors for quantitative pressure sensing based on linear piezoelectric ZnO films serve as candidates for high-performance multifunctional micro/nano systems.

Creation and research of the SAW transducer with a single-phase grid and a piezoelectric zinc oxide film

A method for obtaining a new type of surface acoustic wave (SAW) transducer operating at double frequency with a single-phase closed-loop lattice and a piezoelectric zinc oxide film is developed and experimentally investigated. A method for calculating such a transducer has been developed, its equivalent circuit has been compiled, taking into account propagation losses, losses in the metal film and the inductance of the connecting wires. When the frequency is doubled, the SAW attenuation per unit length increases.

Influence of thickness nonuniformity of piezoelectric zinc oxide layer on parameters of microelectronic BAW solidly mounted resonator

The paper presents the experimental results of a study of the influence of the thickness nonuniformity of a piezoelectric zinc oxide films on electrical equivalent parameters of microelectronic BAW resonators: the static and dynamic capacitances, dynamic inductance, and dynamic resistance. These parameters were determined using the experimental frequency dependences of resonator impedance and the Butterworth-van Dyke model. The resonators under investigation had an operating frequency of 2.8…3.0 GHz; the frequency spread about 500 MHz. The quality factor of the resonators was 250…350 and the relative width of the resonant bandwidth of the resonators were equal to 0.2…0.4%.

Dynamic Magnonic Crystals for Measuring the Dispersion of Bulk Magnetostatic Spin Waves Caused by Magnetic Anisotropy in YIG Films

A dynamic magnonic crystal created by a surface acoustic wave that propagates in a GGG–YIG structure (gallium–gadolinium garnet–yttrium–iron garnet) is used to measure the dispersion of bulk magnetostatic waves due to the magnetic anisotropy in the YIG film. Such waves, which are called anisotropic-dipole magnetostatic waves (ADMSWs), can propagate together with surface spin waves in YIG films that are placed in a tangential magnetic field. It is shown experimentally that ADMSWs with a positive dispersion law occur in films that are grown by the liquid-phase epitaxy technique and not subjected to additional processing,. In the films that were subjected to additional processing the ADMSW dispersion law is negative when applying a piezoelectric zinc oxide film or bombarded with boron ions.

Development and characterization of confocal sputtered piezoelectric zinc oxide thin film

Thin film zinc oxide (ZnO) is often used as a piezoelectric material, but its material properties are dependent on the deposition method and parameters. Direct sputtering using reactive DC or RF sputtering are the most common deposition methods for thin film piezoelectric ZnO. This paper investigates the influence of varying sputtering parameters on piezoelectric and mechanical properties of ZnO deposited by confocal sputtering using pulsed DC on a silicon substrate. Parameters such as varying power, duty cycle and temperature were examined. Crystallinity of the films were investigated systematically by X-ray Diffraction. Mechanical properties such as hardness and Youngs Modulus of thin films were measured via nanoindentation. Piezoelectric coefficient d33 was also measured for the films. Results demonstrated that the ZnO thin film exhibited high c-axis (002) textured films with rocking curve full width half maximum values of 1.69°. It was observed that power, duty cycle and temperature significantly affected the crystallinity, mechanical properties and d33 values of the ZnO thin film. This paper demonstrates that pulsed DC confocal sputtering can be utilized to deposit piezoelectric ZnO films and demonstrates key sputtering parameters that influence the piezoelectric, mechanical and morphology properties of the film.

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.

Effect of the texture of a thin zinc oxide film on the volume acoustic emission

The influence of defects in piezoelectric zinc oxide film transducers on the lateral volume emission of hypersonic waves is considered for real and model piezoelectric activity distributions in the case when the wavelength is comparable with the material grain size and the emitter aperture is on the order of several wavelengths. For this purpose, the piezoelectric activity distribution in zinc oxide films on extended substrates is studied. Some possible mechanisms of the defect formation and the means of controlling these defects during the film synthesis in a gas-discharge plasma are analyzed. Directivity patterns of the piezoelectric transducers are calculated for real and model defect distributions.

Two-step deposition process of piezoelectric ZnO film and its application for film bulk acoustic resonators

In this article, a two-step deposition technique of piezoelectric zinc oxide (ZnO) film formation using radio-frequency (rf) sputtering and its successful applications for film bulk acoustic resonators (FBAR) are presented. Several critical sputtering process parameters such as deposition pressure, rf power, and O2 concentration were investigated to understand their impacts on the resulting crystal structures and surface morphologies of the ZnO films. The ZnO films formed by the two-step deposition have shown the growth characteristic of the strongly preferred orientation toward c axis. The FBAR 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 by simply controlling the resonance area of the resonator.

Stress control of piezoelectric ZnO films on silicon substrates

Piezoelectric zinc oxide (ZnO) films are widely used for the generation and detection of acoustic waves in non-piezoelectric substrates. The application of these films on silicon offers the possibility of realizing acoustic wave devices integrated with electronic circuits, e.g. sensors. It has been demonstrated that ZnO films can be combined with standard bipolar or CMOS IC processes. However, the stress in ZnO is still an important drawback and hinders the further development of silicon integrated acoustic wave devices, particularly if the ZnO layer is deposited on thin membranes, e.g. Lamb wave devices. In this paper we discuss aspects of stress in ZnO, methods to prevent or reduce the stress and we describe an IC-compatible method for the determination of the stress in ZnO films deposited on 100 mm diameter silicon wafers.

Silicon-based ultrasonic microsensors and micropumps

Abstract With the addition of a thin piezoelectric zinc oxide film and transducing electrodes, a thin membrane of low-stress silicon nitride formed in a silicon die can be made into a versatile ultrasonic sensor and actuator. We will describe the fabrication and application of this device, which typically operates in the low megahertz frequency range and can perform many functions: measuring the viscosity of microliter volumes of liquids; measuring the densities of gases and liquids; serving, with the addition of a thin polymer film, as a sensor for measuring the concentrations of organic gases; determining, when immersed in a liquid, the mass per unit membrane area of adsorbed protein molecules, cells and bacteria; pumping air at velocitities of centimerters per second and pumping water at hundreds of microns per second; and stirring microliter volumes of fluids.

Microfabrication of integrated scanning tunneling microscope

An integrated circuit-compatible process has been demonstrated for microfabricating arrays of scanning tunneling microscopes on silicon wafers. Each microfabricated scanning tunneling microscope (STM) incorporates a novel three-dimensional thin-film piezoelectric actuator which is capable of scanning the STM tip laterally over the sample surface and following surface topography. The actuator is in the form of a cantilever bimorph constructed from alternating layers of multiple metallic electrodes, dielectric films, and piezoelectric zinc oxide films. Each device is designed to scan a region 0.1×1 μm with low voltage drive, with a maximum vertical deflection of 15 μm, which simplifies sample approach. The dimensions of the device are 1000×200×8 μm. Operation of the device has been demonstrated by imaging graphite with atomic resolution. The piezoelectric actuator in this device is itself a significant contribution to the field of microactuators, with many non-STM applications.

Zinc oxide–silicon monolithic acoustic surface wave optical image scanner

An optical image is converted into an electrical signal by utilizing acoustic surface wave convolution in a monolithic structure composed of a piezoelectric zinc oxide film on a silicon substrate. Measurements are presented describing the variation of convolution efficiency with light intensity, input acoustic power, and convolver gate electrode dc bias. An initialization procedure is described that allows the device to operate either in an image memory mode or in a nonstorage mode which permits scanning of nonstationary images.