Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1998-12-28
2001-09-18
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06291923
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave device having a quartz substrate.
2. Description of the Related Art
There has been a great demand for filters which have a middle frequency bandwidth for use in mobile communication equipment and other consumer electronic devices. In general, a bandwidth of a surface acoustic wave device is proportional to a square of an electromechanical coefficient K (K
2
) of the device.
A surface acoustic wave device having a quartz substrate is desirable because a temperature coefficient of group delay time (TCD) thereof is nearly zero. However, quartz substrates essentially have a low K
2
value, and a band width of a surface acoustic wave device including a quartz substrate is therefore narrow. More specifically, a component including a plurality of conventional surface acoustic wave devices each having an interdigital transducer (IDT) and disposed on a conventional quartz substrate has a K
2
value of about 0.14%.
Substrates made of materials such as LiTaO
3
have large values of K
2
, but the bandwidth of a surface acoustic wave device having such a substrate is too wide to satisfy the above-described demand, since the K
2
value is too large. In addition, the substrates made of LiTaO
3
and the like have a high TCD value, which causes the characteristics to change greatly depending on temperature.
To solve these problems, a surface acoustic wave device having an enhanced K
2
value has been proposed. This surface acoustic wave device is produced by forming a piezoelectric thin film on a quartz substrate and forming an interdigital transducer on the piezoelectric thin film.
This conventional surface acoustic wave device
110
will be now described with reference to
FIGS. 10 and 11
.
FIG. 10
is a schematic plan view of the conventional surface acoustic wave device
110
.
FIG. 11
is a cross-section taken along line W—W of FIG.
10
.
As shown in
FIGS. 10 and 11
, the conventional surface acoustic wave device
110
is constructed by forming a piezoelectric thin film
112
such as a ZnO film and the like on a quartz substrate
111
. On the piezoelectric thin film
112
, an interdigital transducer
113
having a pair of comb-shaped electrodes which are interdigitated with each other is formed of Al. A pair of reflectors
114
are provided on opposite sides of the interdigital transducer
113
. The reflectors
114
are provided to reflect surface acoustic waves propagated thereto and to confine the energy of the surface acoustic waves. Thus, the surface acoustic wave device
110
functions as a resonator. One or a plurality of the surface acoustic devices are used to form a band-pass filter. The K
2
value of the surface acoustic wave device
110
can be enhanced by use of the quartz substrate
111
having a cut angle and a propagation direction selected to achieve a negative TCD value. By using the piezoelectric thin film
112
which has a positive TCD value, the TCD values of the quartz substrate
111
and the piezoelectric thin film
112
can be canceled. Accordingly, the TCD value of the surface acoustic wave device
110
can be adjusted to be close to zero.
The K
2
value of the surface acoustic wave device
110
can be enhanced by providing a short circuiting electrode made of Al or the like, between the quartz substrate
111
and the piezoelectric thin film
112
.
Thus, the conventional surface acoustic wave device
110
has a desired K
2
value of about 1% in addition to a TCD value of about zero, thereby realizing a surface acoustic wave device having a middle bandwidth.
Although the conventional surface acoustic wave device
110
has successfully realized a middle bandwidth as explained above, there is another major problem associated with the conventional surface acoustic wave device
110
. That is, the conventional surface acoustic wave device
110
has a small impedance ratio Za/Zr. This small impedance ratio presents a problem in that, when the surface acoustic wave device
110
is used as a resonator, the surface acoustic wave device
110
cannot oscillate easily. Moreover, when a filter is constructed using the surface acoustic wave device
110
, the filter experiences the disadvantages of a large insertion loss and gradual frequency characteristics at the band ends.
SUMMARY OF THE INVENTION
To overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device having a quartz substrate and a piezoelectric film provided thereon and a large impedance ratio Za/Zr, thereby achieving excellent oscillation characteristics, a small insertion loss and abrupt frequency characteristics at the band ends.
In one preferred embodiment of the present invention, a surface acoustic wave device includes a quartz substrate, a reflector disposed on the quartz substrate, a piezoelectric thin film disposed on an area of the quartz substrate which excludes at least a part of the quartz substrate on which the reflector is disposed and an interdigital transducer disposed in contact with the piezoelectric thin film.
The interdigital transducer may be disposed on the piezoelectric thin film. Alternatively, the interdigital transducer may be disposed between the piezoelectric thin film and the quartz substrate.
The surface acoustic wave device may further include a short-circuiting electrode disposed on the side of the piezoelectric thin film opposite to the side thereof on which the interdigital transducer is disposed.
The quartz substrate preferably has a cut angle and a propagation direction selected so as to achieve a negative temperature characteristic of group delay time. Further, the piezoelectric thin film is preferably made of a material selected from the group consisting of ZnO, AlN, Ta
2
O
5
, and CdS.
According to the surface acoustic wave device of preferred embodiments of the present invention, the reflectors are preferably disposed directly on the quartz substrate. Thus, it is possible to eliminate the reduction of the reflection coefficient and reduce the propagation loss at the reflector, thereby increasing the impedance ratio Za/Zr of the surface acoustic wave device. As a result, the surface acoustic wave device achieves excellent oscillation characteristics, and a filter constructed to include the surface acoustic wave device
10
exhibits a small insertion loss and abrupt frequency characteristics at the band ends.
For the purpose of illustrating the invention, there are 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: 3828283 (1974-08-01), Daniel
patent: 3952268 (1976-04-01), Schulz et al.
patent: 4223286 (1980-09-01), Ono et al.
patent: 4409570 (1983-10-01), Tanski
patent: 4501987 (1985-02-01), Mitsuyu et al.
patent: 5895996 (1999-04-01), Takagi et al.
patent: 0 602 666 (1994-06-01), None
patent: 0 616 426 (1994-09-01), None
patent: 0 762 640 A1 (1997-03-01), None
Budd Mark O.
Keating & Bennett LLP
Murata Manufacturing Co. Ltd
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