1. Field of the Invention
The present invention relates to a surface acoustic wave device, and more particularly, it relates to a surface acoustic wave device which operates in a high frequency range of a GHz band.
2. Description of the Background Art
A surface acoustic wave device utilizing a surface acoustic wave which is propagated with energy concentrated on a surface of a solid body is applied to an intermediate frequency filter of a television receiver or the like due to its compactness and stability in performance.
Such a surface acoustic wave device is formed by a piezoelectric layer and interdigital electrodes. In general, an alternating electric field is applied to the piezoelectric layer through the interdigital electrodes to excite a surface acoustic wave.
The piezoelectric layer is prepared from a material such as a bulk single crystal of LiNbO.sub.3 or LiTaO.sub.3, or a ZnO thin film which is vapor-deposited on a substrate.
In general, an operation frequency f of such a surface acoustic wave device is determined as f=v/.lambda., where v represents the propagation velocity of the surface acoustic wave and .lambda. represents the wavelength of the surface acoustic wave device. In order to increase the operation frequency f, therefore, the propagation velocity v may be increased or the wavelength .lambda. may be reduced.
The value of the propagation velocity v depends on the materials for the piezoelectric layer and the substrate, as well as the mode of the surface acoustic wave. On the other hand, the wavelength .lambda. is determined by the pitch of the interdigital electrodes.
FIG. 1 is a plane view showing exemplary interdigital electrodes of the most standard structure. Referring to FIG. 1, a pair of interdigital electrodes, each having electrode tips of a width d which are integrally formed with each other at intervals of 3.times.d, are opposed to each other so that the electrode tips of the same polarities are alternately arranged. In the interdigital electrodes of such structure, the wavelength .lambda. is equal to 4.times.d.
FIG. 2 is a plane view showing another exemplary interdigital electrodes. In each of a pair of opposite interdigital electrodes shown in FIG. 2, pairs of electrode tips of a width d, which are spaced apart at an interval of d, are arranged at intervals of 5.times.d. In the interdigital electrodes of such structure, the wavelength .lambda. is equal to 8.times.d, and it is equal to 8.times.d/3 for the third resonance wave.
However, the value of the propagation velocity v is limited by the materials for the piezoelectric layer and the substrate, as hereinabove described. Further, the lower limit of the periodic size of the interdigital electrodes deciding the wavelength .lambda. is restricted by a limit in the technique of fine processing. Thus, the operation frequency of a currently available surface acoustic wave device is limited to 900 MHz.
On the other hand, a surface acoustic wave device which is applicable to a higher frequency range (GHz band) is required with increase in frequency in the field of communication such as satellite communication or mobile communication.
To this end, development is now being made on employment of a film of diamond which has the maximum sound velocity (transversal wave velocity: 13000 m/s; longitudinal wave velocity: 16000 m/s) among substances or diamond-like carbon, to be stacked on a piezoelectric material for forming a surface acoustic wave device.
In order to implement a surface acoustic wave device having a high operation frequency f, it is generally desirable to attain a large electromechanical coefficient K.sup.2 (index of conversion efficiency for converting electric energy to mechanical energy) in addition to the aforementioned high propagation velocity v. This electromechanical coefficient K.sup.2 must be greater than or equal to 0.1%, and is preferably greater than or equal to 0.5%, depending on the application.
When a thin piezoelectric film formed on a substrate is employed, the propagation velocity v and the electromechanical coefficient K.sup.2 remarkably depend not only on the physical properties of the materials for the piezoelectric film and the substrate, but on the thickness of the piezoelectric thin film.
When the substrate is in the form of a film, i.e., when the piezoelectric thin film is formed on a film-type substrate which is formed on a base material, the propagation velocity v and the electromechanical coefficient K.sup.2 also depend on the thickness of the film-type substrate.
In a surface acoustic wave device which is formed by a ZnO layer and a diamond or diamond-like carbon film, however, absolutely no recognition has been made as to the relation between the film thicknesses of the ZnO and diamond films and the propagation velocity and the electromechanical coefficient. Therefore, it has been impossible to properly design a high-efficiency surface acoustic wave device operating in a high frequency range.
As disclosed in Japanese Patent Laying-Open No. 3-198412 (1991), the inventors have implemented surface acoustic wave devices having high surface acoustic wave propagation velocities and large electromechanical coefficients by defining film thickness ranges of ZnO layers in relation to wavelengths while specifying modes of surface acoustic waves as to four types of devices shown in FIGS. 3 to 6.
When a surface acoustic wave device is formed by a piezoelectric thin film which is grown on a substrate having a higher sound velocity than the piezoelectric material, a plurality of surface acoustic waves having different propagation velocities v are generally excited in a zeroth order mode, a first order mode, a second order mode, . . . successively from the smaller velocity side.
The inventors have disclosed surface acoustic wave devices particularly utilizing zeroth, first and third order modes in detail in the aforementioned gazette. However, no sufficient disclosure has been made as to a surface acoustic wave device utilizing a second order mode.