Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2002-02-28
2004-09-14
Budd, Mark (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S31300R
Reexamination Certificate
active
06791237
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave substrate having a LiNbO
3
substrate propagating piezoelectric leaky surface wave along an X-axis direction on a cut plane of a rotated Y plate, and a thin film of SiO
2
or the like formed on the cut plane. More particularly, the invention relates to a surface acoustic wave substrate and a surface acoustic wave functional element having superior temperature characteristics.
2. Description of the Related Art
A surface acoustic wave functional element provided with an interdigital electrode on a surface of a piezoelectric substrate, has been widely used for a filter of an intermediate frequency band of a television set, a filter of a mobile communication equipment and so forth. The surface acoustic wave functional element has an electrode for exciting a surface acoustic wave and an electrode for receiving the surface acoustic wave on a surface of a substrate having piezoelectric function.
As a piezoelectric substrate to be used for the surface acoustic wave functional element in the prior art, a material having large electromechanical coupling coefficient k
2
has been used. However, the surface acoustic wave functional element using the substrate made of material having large electromechanical coupling coefficient k
2
generally has bad temperature characteristics and thus encounters a problem of lacking of temperature stability.
On the other hand, the surface acoustic wave functional element employing a monocrystalline piezoelectric substrate, such as ST-cut quartz, LST-cut quartz and so forth, demonstrates superior temperature stability, but has small electromechanical coupling coefficient k
2
. Therefore, when it is used as the filter, insertion loss becomes large. Also, such surface acoustic wave functional element cannot be used for a filter having wide band width.
Therefore, there have been devised a SiO
2
/LiNbO
3
substrate and a SiO
2
/LiTaO
3
substrate, which are fabricated by using a LiNbO
3
substrate and a LiTaO
3
substrate as the substrates having superior temperature stability and large electromechanical coupling coefficient k
2
, and depositing thereon SiO
2
films having small linear expansion coefficient and opposite temperature characteristics, respectively. Effectiveness of such SiO
2
/LiNbO
3
substrate and SiO
2
/LiTaO
3
substrate has been discussed in Yamanouchi, Iwahashi and Shibayama, “Temperature Dependence of Rayleigh Waves and Piezoelectric Leaky Surface Waves in Rotated Y-Cut LiTaO
3
and SiO
2
/LiTaO
3
Structures” Wave Electronics, Vol. 3, December, 1979, or Yamanouchi and Hayama, “SAW Properties of SiO
2
/128° Y-X LiNbO
3
Structure Fabricated by Magnetron Sputtering Technique”, IEEE Transactions on Sonics and Ultrasonics, Vol. SU-31, No. 1, January, 1984. These substrates are recommended to apply as high stability oscillator and filters using normally bi-directional interdigital electrode.
However, substrate having further greater electromechanical coupling coefficient k
2
than the conventional substrate and having superior temperature stability has been required.
SUMMARY OF THE INVENTION
The present invention has been worked out in view of the shortcomings in the prior art. Therefore, it is an object of the present invention to provide a surface acoustic wave substrate and a surf ace acoustic wave functional element having an electromechanical coupling coefficient k
2
greater than the prior art and good temperature characteristics.
According to a first aspect of the present invention, there is provided a surf ace acoustic wave substrate comprising:
a piezoelectric or electrostrictive substrate having large electromechanical coupling coefficient; and
a thin film formed on the substrate and having variation characteristics of frequency of a surface acoustic wave relative temperature variation opposite to that of the substrate,
wherein the substrate is a LiNbO
3
substrate having a cut angle of rotated Y plate within a range greater than or equal to −10° and smaller than or equal to +30° and propagating a piezoelectric leaky surface wave having a propagation velocity higher than that of a Rayleigh type surface acoustic wave along X-axis direction or within a range of ±5° with respect to X-axis direction, and
a value of H/&lgr; falls within a range from 0.05 to 0.35, where H is the film thickness of the thin film, and &lgr; is the wavelength of operating center frequency of the piezoelectric leaky surface wave.
It is preferred that the cut angle of rotated Y plate of the substrate is in a range greater than or equal to 0° and smaller than or equal to +20°, and the value of H/&lgr; falls within a range from 0.1 to 0.35. In the alternative, it is preferred that the cut angle of rotated Y plate of the substrate is in a range greater than or equal to +20° and smaller than or equal to +30°, and the value of H/&lgr; falls within a range from 0.15 to 0.35. By selecting the cut angle of rotated Y plate and H/&lgr; within the foregoing ranges, the temperature coefficient of frequency (TCF) as measured at 25° C. can be zero or quite small.
In the surface acoustic wave substrate of the invention, preferably, the temperature coefficient of frequency (TCF) as measured at 25° C. is in a range from −30 ppm/° C. to +30 ppm/° C. In addition, it is preferred that the electromechanical coupling coefficient k
2
of the piezoelectric leaky surface wave is greater than or equal to 0.155 and the electromechanical coupling coefficient k
R
2
of a Rayleigh wave component is smaller than or equal to 0.01.
The ranges of the temperature coefficient of frequency (TCF) and the electromechanical coupling coefficient k
2
may be achieved by selecting the cut angle of rotated Y plate and H/&lgr; within a range set forth in any of the following (1) to (5):
(1) the cut angle of rotated Y plate of the substrate is in a range greater than or equal to −10° and smaller than or equal to −5°, and the value of H/&lgr; falls within a range from 0.07 to 0.31;
(2) the cut angle of rotated Y plate of the substrate is in a range greater than or equal to −5° and smaller than or equal to +10°, and the value of H/&lgr; falls within a range from 0.115 to 0.31;
(3) the cut angle of rotated Y plate of the substrate is in a range greater than or equal to +10° and smaller than or equal to +15°, and the value of H/&lgr; falls within a range from 0.16 to 0.31;
(4) the cut angle of rotated Y plate of the substrate is in a range greater than or equal to +15° and smaller than or equal to +20°, and the value of H/&lgr; falls within a range from 0.2 to 0.31; and
(5) the cut angle of rotated Y plate of the substrate is in a range greater than or equal to +20° and smaller than or equal to +30°, and the value of H/&lgr; falls within a range from 0.25 to 0.31.
According to a second aspect of the present invention, there is provided a surface acoustic wave functional element comprising a surface acoustic wave substrate as set forth above, the element including:
an exciting or receiving region having an interdigital electrode for exciting or receiving the piezoelectric leaky surface wave formed at an interface between the surface of the substrate and the thin film; and
a propagating region having a structure for electrically shorting between the substrate and the thin film or a shorting type grating electrode structure formed at an interface between the surface of the substrate and the thin film.
According to a third aspect of the present invention, there is provided a surface acoustic wave functional element comprising a substrate including: a piezoelectric or electrostrictive substrate having large electromechanical coupling coefficient; and a thin film formed on the substrate and having variation characteristics of frequency of a surface acoustic wave relative temperature variation opposite to that of the substrate,
wherein the substrate is a LiNbO
3
substrate having a cut angle of rotated Y plate
Budd Mark
Osha & May L.L.P.
LandOfFree
Surface acoustic wave substrate and surface acoustic wave... 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 substrate and surface acoustic wave..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Surface acoustic wave substrate and surface acoustic wave... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3202621