High-frequency Surface Acoustic Wave Filters Research Articles

Deposition of high-quality zinc oxide thin films on diamond substrates for high-frequency surface acoustic wave filter applications

Surface acoustic wave (SAW) device based on diamond is attracting much interest because diamond has the highest elastic constant of all substrates and the SAW velocity is more than 10,000 m/s. Although diamond is not piezoelectric, its high acoustic propagation makes it a desirable substrate for SAW device when coupled with piezoelectric thin films such as zinc oxide (ZnO) thin film. It is well known that, for these devices, performance is critically dependent on the quality of ZnO films. In this paper, highly-oriented, dense, and fine-grain polycrystalline ZnO thin films with excellent surface flatness and high resistivity were produced on diamond substrate by reactive magnetron sputtering. The properties of ZnO films were characterized by X-ray diffraction, high-energy electron diffraction, scanning electron microscopy, and atomic force microscopy. The influences of deposition condition are discussed. Interdigital-transducer/ZnO/diamond layered structure was fabricated, which are expected to be capable of wide bandwidth application in SAW devices at a high frequency of about 2.5 GHz.

Simulation of characteristics of a SiO/sub 2//c-axis-oriented LiNbO/sub 3//diamond surface acoustic wave

High-frequency surface acoustic wave (SAW) devices based on diamond that have been realized to date utilize c-axis-oriented ZnO as the piezoelectric thin film. This material, with SiO2 overlay, shows excellent characteristics of a high phase velocity of over 10,000 m/s and a zero temperature coefficient, and it has been successfully applied to high-frequency SAW filters and resonators. To expand on materials used on diamond, the theoretical calculation has been carried out for LiNbO3/diamond, and a high electromechanical coupling coefficient up to 9.0% is expected. In this work, the characteristics of SiO2/LiNbO3/diamond were studied by computer simulation, emphasizing a zero temperature coefficient with a high coupling coefficient. Calculations are carried out for the phase velocity, the electromechanical coupling coefficient, and the temperature coefficient of the Rayleigh wave and its higher mode Sezawa wave. As a result, SiO2/IDT/LiNbO3/diamond is found to offer a zero temperature coefficient with a very high coupling coefficient up to 10.1% in conjunction with a high phase velocity of 12,100 m/s.

MPACVD diamond films for surface acoustic wave filters

Diamond, due to its high surface acoustic wave (SAW) velocity, offers much attraction for realisation of high frequency SAW filters. To launch elastic waves in solid, the interdigital transducers (IDTs) and a thin layer of suitable piezoelectric material such as ZnO are deposited on diamond film. The most significant requirements of diamond films for SAW filters concern a high resistivity, a high purity, a surface roughness of approximately 10 nm and a large thickness to obtain good characteristics of frequency response (insertion loss, rejection,…). In this work, diamond films deposited on silicon by microwave plasma assisted chemical vapour deposition (MPACVD) in CH 4H 2 gas mixture, are characterised as a function of time deposition and N 2 adjunction in order to determine their capacity to be used as substrates for SAW layered structure filters. The purity, morphology and roughness of the films are characterised by Raman spectroscopy, scanning electron microscopy (SEM), and profilometry. The electrical properties are determined with the four points probe. It is first shown that the films resistance and roughness increase with thickness when the diamond purity, in regard to Raman spectroscopy, remains constant. Secondly, we point out that nitrogen addition in CH 4H 2 gas mixture improves the resistance and the thickness of diamond films while the roughness drastically decreases. These results have a crucial importance for the realisation of good functioning SAW filters.

Software for a Multilayered Surface Acoustic Wave Device Based on the Finite Element Method and Coupling-of-Modes Theory

A software package that provides an interactive and graphical environment for the design and modeling of multilayered surface acoustic wave (SAW) devices is described. The software is based on the finite element method (FEM) and the coupling-of-modes theory (COM). FEM is used to calculate the dispersion diagram of an infinite periodic array and the capacitance of one pair of interdigital transducers (IDTs). COM parameters are then determined using these calculated data. As a result of COM calculations, a 6-port circuit representation is obtained for basic device elements such as IDTs, grating or IDT reflectors and delay lines. The frequency response of SAW devices composed of these elements can be systematically evaluated by cascading the 6-port circuits. As an example, a high frequency SAW filter with a ZnO/diamond/Si structure is designed.

Epitaxial growth of LiNbO3 thin films by excimer laser ablation method and their surface acoustic wave properties

We have formed epitaxial LiNbO3 thin films of high quality on sapphire substrates (001) using an excimer-laser ablation technique and evaluated the surface acoustic wave (SAW) properties of these films. It is demonstrated that the LiNbO3 thin films on the sapphire are the candidates for a high frequency SAW filter because of their high velocities of propagation (5300–6600 m/s) and good temperature coefficients (−34 to −80 ppm/ °C).

Epitaxial growth of LiNbO3 thin films by excimer laser ablation method and their surface acoustic wave properties

ABSTRACTWe have formed epitaxial LiNbO3 thin films on sapphire substrates (001),(110) and (012) using an excmer-laser ablation technique. We have fabricated surface acoustic wave (SAW) filters on the films and evaluated SAW properties. It is demonstrated that the LINbO3 thin films on the sapphire substrates are candidates for a high frequency SAW filter because of their high velocities of propagation (5300–5600m/s).

Rare-earth substituted piezoelectric PbTiO3 ceramics for acoustic wave applications

Electromechanical and surface acoustic wave (SAW) properties of PbTiO3, ceramics modified by partial substitution of rare-earth for the Pb, specifically (Pb, Ln) (Ti, Mn)O3 (Ln=La, Ce, Pr, Nd, Sm, Eu, or Gd) ceramics, are extensively investigated. The SAW delay time temperature coefficient is found to be minimum when Ln=Nd. A zero temperature coefficient can be achieved in the (Pb, Nd) (Ti, In, Mn)O3 system with appropriate compositions. High frequency SAW filters are fabricated using these ceramics. It is also found that (Pb, Sm) (Ti, Mn)O3 ceramics have exceptionally large electromechanical anisotropy (ratio of electromechanical coupling factor for thickness vibration to that for planar vibration is above 15). A 7.5MHz linear array ultrasonic probe employing these ceramics is developed for medical use.