Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2001-09-17
2002-06-11
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S31300R, C310S31300R, C310S364000
Reexamination Certificate
active
06404101
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave device such as a surface acoustic wave filter for use as a band filter in mobile communications equipment.
2. Description of the Related Art
Surface acoustic wave filters are more widely used as band filters in mobile communications equipment, since the filters can be reduced in size, in contrast to dielectric filters or other filters. The band filters for use in mobile communications equipment are required to have a low loss in the transmission bands. Accordingly, the surface acoustic wave filters have been variously designed and constructed to reduce the loss.
For example, a surface acoustic wave filter using one terminal-pair surface acoustic wave resonator shown in
FIG. 15A
has been proposed. Here, grating reflectors
202
and
203
each having a plurality of electrode fingers, are arranged on both of the sides in the surface acoustic wave propagation direction of an interdigital transducer
201
. The loss in the transmission band of the one terminal pair surface acoustic wave resonator is reduced by the grating reflectors
202
and
203
.
Moreover, a surface acoustic wave resonator having only one interdigital transducer
205
has been proposed as shown in FIG.
15
B. Here, the number of electrodes in the interdigital transducer
205
is large, for example, 200 electrodes. Thereby, surface acoustic wave energy can be trapped in the area where the interdigital transducer
205
is located without reflectors being provided. That is, a multi-pair type energy trapping surface acoustic wave resonator is formed.
Furthermore, a plurality of interdigital transducers
206
and
207
are arranged in the surface acoustic wave propagation direction in the resonator type surface acoustic wave filter shown in FIG.
16
C. Grating reflectors
208
and
209
are arranged on both sides in the surface acoustic wave propagation direction of the area where the interdigital transducers
206
and
207
are located, respectively.
Moreover, a surface acoustic wave filter having a ladder circuit configuration and a surface acoustic wave filter having a lattice circuit configuration, in each of which a combination of plural surface acoustic wave resonators is provided as described above and shown in FIGS.
15
A and
15
B, have been proposed.
As described above, the energy of an excited surface acoustic wave can be trapped by providing reflectors, or by increasing the number of the electrode finger pairs of an interdigital transducer. Thus, the Q value, which is a resonance characteristic, can be enhanced, and the loss can be reduced.
On the other hand, the electrode resistance of a surface acoustic wave device, the surface acoustic wave mode, the electrode capacity, and so forth are affected by the ratio of the width L
1
of each electrode finger
211
in an interdigital transducer shown in
FIG. 17
, based on the gap size L
2
between adjacent electrode fingers
211
in the surface acoustic wave propagation direction in the interdigital transducer, that is, the ratio of L
1
/(L
1
+L
2
) (hereinafter, referred to as duty, briefly), and moreover, the electrode film thickness h/&lgr; of the interdigital transducer (&lgr; is the wavelength of a surface acoustic wave, and h/&lgr; is a film thickness standardized by &lgr;. Thus, for design of the surface acoustic wave device, it is important to optimize these parameters.
The gap length L
2
represents the distance in the surface acoustic wave propagation direction of the gap.
As described above, conventionally, surface acoustic wave filters have been variously designed so as to enhance the filter characteristics. For example, Japanese Unexamined Patent Application Publication No. 7-28368 discloses a longitudinally coupled resonator type surface acoustic wave filter using a 36° Y-cut X-directional propagation LiTaO
3
piezoelectric substrate and moreover, utilizing coupling of modes in the horizontal direction relative to the surface acoustic wave propagation path. According to this publication, the ohmic resistance loss can be reduced, and the steepness of the filter characteristic can be increased by setting the electrode film thickness of the interdigital transducer to be in the range of 0.06 &lgr; to 0.10 &lgr;, and also, setting the duty of the interdigital transducer at about 0.6 or higher.
On the other hand, Japanese Unexamined Patent Application Publication No. 6-188673 discloses a ladder surface acoustic wave filter in which plural one terminal-pair surface acoustic wave resonators are formed on a 36° Y-cut X-directional propagation LiTaO
3
substrate.
FIG. 18
shows the ladder circuit. In
FIG. 18
, S
1
and S
2
represent series arm resonators, and P
1
to P
3
represent parallel arm resonators, respectively. In this conventional surface acoustic wave filter, the electrode film thickness h/&lgr; of the interdigital transducer is in the range of 0.4 &lgr; to 0.10 &lgr;, whereby an undesired spurious can be removed from the transmission band to improve the filter characteristic.
According to the above-described publications, the resistance loss can be reduced, and the spurious suppressing effect can be obtained by setting the film thickness of the interdigital transducer at 0.04 &lgr; or more and setting the duty at 0.5 or higher when the 36° Y-cut X-directional propagation LiTaO
3
is used.
Recently, mobile communication systems have been operated at higher frequencies, and the frequencies at which surface acoustic wave filters are operated in the systems become higher, that is, the frequencies are in the range of 800 MHz to 2.5 GHz. The acoustic velocities of surface acoustic waves are about several thousand meters per second. Thus, when a surface acoustic wave device is formed so as to operate at 800 MHz to 2.5 GHz, the wavelength of a surface acoustic wave is short, that is, about several &mgr;m. Accordingly, electrode patterns for defining the interdigital transducers and the reflectors must be very fine.
Therefore, the absolute value of the electrode film thickness become small, and the width of each electrode finger become small. As a result, the loss (ohmic loss), caused by the electrode resistance, cannot be made negligible.
Moreover, when the thickness of each electrode becomes small, the strength of the electrode is reduced. Accordingly, electrodes that are capable of being wire-bonded cannot be formed.
Thus, it has been attempted that the film thickness of portions of the electrodes, such as bus bar electrodes, turning-around electrodes, and wire bonding pads, excluding the electrode portions where a surface acoustic wave is excited in practice, is increased to reduce the ohmic loss as much as possible, whereby the strength required for wire-bonding is secured.
For example, Japanese Unexamined Patent Application Publication No. 62-47206 discloses a surface acoustic wave filter in which acoustic coupling of the component of a surface acoustic wave in the vertical direction to the surface acoustic wave propagation direction is caused. As described in this publication, in this surface acoustic wave filter, the thickness of each of the bas bar electrodes shared by the interdigital transducers adjacent to each other in the surface acoustic wave propagation direction is larger than that of each electrode finger of the interdigital transducers. Thus, the acoustic velocity can be controlled while the resistance is reduced. Therefore, a desirable filter characteristic can be obtained.
In the surface acoustic wave resonators shown in
FIGS. 15A and 15B
and in the resonator type surface acoustic wave filter shown in
FIG. 16
, the energy can be trapped by increasing the number of the electrode fingers of the reflectors, and increasing the number of electrode pairs of the interdigital transducer to reflect the surface acoustic wave substantially completely. However, the surface acoustic wave has not only an X-directional component but also a component in the vertical direction to the X-direction, that is, a Y-directional compon
Nagai Tatsurou
Takata Toshiaki
Taniguchi Norio
Yamato Shuji
Budd Mark O.
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
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