Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
1999-08-12
2002-03-12
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S195000, C333S133000, C310S31300R, C310S360000
Reexamination Certificate
active
06356167
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave resonator, a surface acoustic wave filter, a duplexer, and a communications apparatus including the same. More particularly, the present invention relates to a surface acoustic wave resonator having a piezoelectric substrate made of a single crystal material including langasite (La
3
Ga
5
SiO
14
).
2. Description of the Related Art
Conventionally, surface acoustic wave resonators have been used widely in band pass filters or other electronic components used in mobile communications equipment. As an example of such a surface acoustic wave resonator, a surface acoustic wave resonator or a surface acoustic wave filter having a configuration wherein an interdigital transducer (IDT) including a comb-shaped electrode is provided on a piezoelectric substrate is well known.
A piezoelectric single crystal, such as lithium niobate (LiNbO
3
), lithium tantalate(LiTaO
3
), quartz, lithium tetraborate (Li
2
B
4
O
7
) or the like, is used as the material for the piezoelectric substrate of such a surface acoustic wave resonator or surface acoustic wave filter.
The surface acoustic wave resonator and the surface acoustic wave filter are required to have a maximum electromechanical coupling coefficient (K
2
), which represents the conversion efficiency of the electricity and the mechanical energy, and a minimum temperature coefficient of group delay time (TCD), which represents the fluctuation ratio of the frequency by the temperature.
Since surface acoustic wave filters using the above-mentioned LiNbO
3
or LiTaO
3
material have a large K
2
, such surface acoustic wave filters have a large difference between the resonance frequency and the anti-resonance frequency, thereby achieving a wide band width. However, since such filters have a TCD which is larger than that of quartz, there is a problem that the operation frequency fluctuates greatly due to the temperature change.
Moreover, surface acoustic wave filters using a quartz substrate are advantageous in that they have a small frequency shift in temperature because of an extremely small TCD. However, since the quartz substrate surface acoustic wave filters have a small K
2
, they are disadvantageous in that they have a small difference between the resonance frequency and the anti-resonance frequency and a narrow band width.
For providing a material having a TCD smaller than that of LiNbO
3
or LiTaO
3
, and a K
2
larger than that of quartz, Li
2
B
4
O
7
has been used in surface acoustic wave filters and surface acoustic wave resonators. However, since Li
2
B
4
O
7
creates problems with handling and processing because of its deliquescence property and because the growth rate of the single crystal is low, use of Li
2
B
4
O
7
causes very poor productivity. Moreover, although Li
2
B
4
O
7
has a good temperature characteristic with respect to the frequency, it has a poor temperature characteristic with respect to K
2
so that the band undesirably changes according to the temperature. Thus, there are many problems with using Li
2
B
4
O
7
as a substrate material for a surface acoustic wave filter.
Recently, as a material for solving the above-mentioned problems, La
3
Ga
5
SiO
14
has been proposed. La
3
Ga
5
SiO
14
does not have the deliquescence property like Li
2
B
4
O
7
, but has a high growth rate of a single crystal compared with that of Li
2
B
4
O
7
. Moreover, La
3
Ga
5
SiO
14
has characteristics including a TCD which is smaller than that of LiNbO
3
or LiTaO
3
, and a K
2
which is larger than that of quartz. A large number of reports have been made concerning the theoretical analyses or the experimental results relating to the prospective Euler angle or propagation direction of such an La
3
Ga
5
SiO
14
single crystal substrate.
However, the analyses and measured results described in such reports relate only to alone such that the TCD is optimum. The reports and analyses are not related to and do not describe devices in which an La
3
Ga
5
SiO
14
single crystal substrate is provided with other elements. Thus, the TCD is not optimum when other elements are added thereto.
SUMMARY OF THE INVENTION
To overcome the problems described above, preferred embodiments of the present invention provides a surface acoustic wave resonator which includes a langasite piezoelectric and has a greatly improved TCD.
According to one preferred embodiment of the present invention, a surface acoustic wave resonator includes a piezoelectric substrate made of a langasite single crystal, and an interdigital transducer including Al and located on the surface of the piezoelectric substrate. The Euler angle (&phgr;, &thgr;, &phgr;) of the piezoelectric substrate is preferably about (0°, 140° to 150°, 24°+1°), and the film thickness H of the interdigital transducer is preferably within the range of about 0.005 to about 0.15 with respect to the wavelength &lgr; of a surface acoustic wave to be excited on the piezoelectric substrate.
According to another preferred embodiment of the present invention, a surface acoustic wave device includes a piezoelectric substrate made of a langasite single crystal, and an interdigital transducer including Al and located on the surface of the piezoelectric substrate. The Euler angle (&phgr;, &thgr;, &phgr;) of the piezoelectric substrate is preferably about (0°, &thgr;, 24°±1°), and a cut angle &thgr; of the Euler angle and a normalized film thickness H/&lgr; of the interdigital transducer preferably fall within an area surrounded by the straight lines linking the points represented by:
A (&thgr;=140°, H/&lgr;=0.005)
B (&thgr;=143°, H/&lgr;=0.005)
C (&thgr;=147°, H/&lgr;=0.15)
D (&thgr;=150°, H/&lgr;=0.15)
on a coordinate with respect to the cut angle &thgr; and the normalized thickness H/&lgr;, where H represents a film thickness of the interdigital transducer and &lgr; represents a wavelength to be excited on the piezoelectric.
According to still another preferred embodiment of the present invention, a method of manufacturing a surface acoustic wave device including a piezoelectric substrate made of a langasite single crystal and an interdigital transducer including Al and located on the surface of the piezoelectric substrate is provided. The method preferably includes the step of selecting a film thickness of the interdigital transducer in accordance with the Euler angle (&phgr;, &thgr;, &phgr;) of the piezoelectric substrate such that fluctuation of an operation frequency of the surface acoustic wave resonator is substantially zero between about 20° C. to about 30° C.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.
REFERENCES:
patent: 5917265 (1999-06-01), Naumenko et al.
patent: 6005325 (1999-12-01), Inoue et al.
patent: 6137207 (2000-10-01), Inoue et al.
patent: 0 098 154 (1984-01-01), None
patent: 0 874 455 (1998-10-01), None
patent: 2328 815 (1999-03-01), None
patent: 10-190407 (1998-07-01), None
patent: 11-27089 (1999-01-01), None
patent: 11-136083 (1999-05-01), None
patent: WO 97/25776 (1997-07-01), None
K. Inoue et al., “Propagation Characteristics of Surface Acoustic Waves on Langasite,” Feb. 4, 1998; Japanese Journal of Applied Physics; vol. 37, No 5B. pp 2909-2913.
H. Satoh et al., “Surface Acoustic Wave Propagation Characteristics on a langasite Crystal Plate,” Mar. 14, 1997; Japanese Journal of Applied Physics; vol. 36, No 5B, pp 3071-3073.
Kadota Michio
Kumatoriya Makoto
Nakanishi Jun
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Pascal Robert
Summons Barbara
LandOfFree
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