Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1999-11-03
2001-03-27
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06208063
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave device, and more specifically, to a surface acoustic wave device which generates BGS waves therein for use in high-frequency resonators and high-frequency filters.
2. Description of the Related Art
A surface acoustic wave device is generally made of a substrate having piezoelectricity (“a piezoelectric substrate”) and a pair of comb-shaped electrodes having a plurality of electrode fingers arranged on the substrate so that the electrode fingers are interdigitated with one another. Such a surface acoustic wave device generates a surface acoustic wave when an electric signal is applied to the interdigital transducer.
As a surface acoustic wave, Rayleigh waves are well known, but BGS waves (Bleustein-Gulyaev-Shimizu waves or piezoelectric surface shear waves) and Love waves have also been used.
In these surface acoustic waves, a resonant frequency and electrical and mechanical characteristics thereof, such as an electrical or mechanical quality factor “Q” and an electromechanical coupling factor “k,” are roughly determined by the material quality of the piezoelectric substrate and the configuration of the interdigital transducer. Uses of such surface acoustic waves for high-frequency resonators and high-frequency filters are possible.
A surface acoustic wave literally is an elastic surface acoustic wave that propagates concentrated energy in the vicinity of the surface of a substrate. There are only two components of a displacement of the surface acoustic wave propagating along a surface on an isotropic substrate; one component in the wave progress direction and one component in the direction of the substrate thickness. There is no surface acoustic wave (an SH wave or a horizontally-polarized shear wave), corresponding to a transverse wave, having only one component in a direction that is perpendicular to the wave propagation direction and parallel to the substrate surface in the isotropic substrate.
However, since the piezoelectric substrate is anisotropic, it can propagate the SH wave (including a pseudo SH wave) with concentrated energy in the vicinity of the surface thereof. This surface acoustic wave is called a BGS wave. Since the BGS wave is completely reflected by an edge of the piezoelectric substrate in the wave propagation direction, it is not required that a reflecting device (a reflector) be disposed on the substrate as in a surface acoustic wave device propagating Rayleigh waves. Therefore, the BGS wave has an advantage over the Rayleigh wave with respect to miniaturizing a surface acoustic wave device.
However, the BGS wave using a substrate having a small electromechanical coupling factor “k” cannot achieve sufficient filter characteristics when a narrow band filter is formed because an impedance ratio Za/Zr, which is a ratio between an impedance Za at an anti-resonant frequency and an impedance Zr at a resonant frequency, is comparatively small. On the other hand, the BGS wave using a substrate having a large electromechanical coupling factor “k” has very poor resistance to external conditions, although the impedance ratio Za/Zr is large so that a wide band filter can be provided.
In addition, it is desirable to be able to select various piezoelectric materials used to form the piezoelectric substrate so as to increase design choices associated with forming a surface acoustic wave device.
SUMMARY OF THE INVENTION
To overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device which achieves a large impedance ratio, excellent resistance to external conditions, and increased freedom of choice of materials to be used to form the piezoelectric substrate incorporated therein.
According to preferred embodiments of the present invention, in a surface acoustic wave device generating BGS waves, a piezoelectric substrate includes a piezoelectric polycrystalline body and an interdigital transducer disposed on a surface of the piezoelectric substrate, wherein the surface acoustic wave device includes at least two layers disposed on the piezoelectric substrate, the at least two layers having different electrical characteristics and mechanical characteristics in a direction that is substantially perpendicular to the surface of the piezoelectric substrate on which the interdigital transducer is disposed, so that the degree of surface acoustic wave energy concentration in the surface is controlled.
As a result of this novel arrangement and construction, problems caused by the degree of wave concentration in the surface of the substrate are solved.
For example, one problem that is solved by preferred embodiments of the present invention occurs when a piezoelectric substrate having a small electromechanical coupling factor “k” is used. In such a case, the degree of surface acoustic wave energy concentration in the surface is reduced to decrease an impedance ratio. Another problem that is solved occurs when a piezoelectric substratewith a large electromechanical coupling factor “k” is used, the moisture resistance ability is deteriorated by excess surface acoustic wave energy concentration in the substrate surface.
Furthermore, in accordance with preferred embodiments of the present invention, a desired surface acoustic wave device can be comparatively easily formed at low cost by forming the above-described multi-layer structure or gradient structure using the piezoelectric substrate, which is preferably formed of ceramics.
For the purpose of illustrating the present invention, there is shown in the drawings several forms and embodiments which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
REFERENCES:
patent: 3943389 (1976-03-01), Hickernell et al.
patent: 4564782 (1986-01-01), Ogawa
patent: 4568848 (1986-02-01), Ogawa
patent: 4952832 (1990-08-01), Imai et al.
patent: 4967113 (1990-10-01), Mitsutsuka
patent: 5446330 (1995-08-01), Eda et al.
patent: 5920143 (1999-07-01), Tarui et al.
patent: 5977686 (1999-11-01), Kadota et al.
patent: 5998907 (1999-12-01), Taguchi et al.
patent: 0 833 446 A2 (1998-04-01), None
patent: 56031213 (1981-03-01), None
patent: 59218028 (1984-12-01), None
Michio Kadota et al., Japanese Journal of Applied Physics, Proc. 12th Symp. on Ultrasonic Electronics 31 Sullp. 31-1, Yokohama, Japan, “Ceramic Resonators Using BGS Waves,” Dec. 1991.
Hideaki Adachi et al., Japanese Journal of Applied Physics, 24 suppl. No. 24, Part 1, Tokyo, Japan, “SAW Properties of PLZT Epitaxial Thin Films,” Dec. 1984.
Ando Akira
Horikawa Katsuhiro
Dougherty Thomas M.
Keating R. Bennett, LLP
Murata Manufacturing Co. Ltd.
LandOfFree
Surface acoustic wave device having polycrystalline... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Surface acoustic wave device having polycrystalline..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Surface acoustic wave device having polycrystalline... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2541708