U.S. patent application number 09/953720 was filed with the patent office on 2002-05-16 for surface acoustic wave device.
This patent application is currently assigned to Murata Manufacturing Co., Ltd.. Invention is credited to Nagai, Tatsurou, Takata, Toshiaki, Taniguchi, Norio, Yamato, Shuji.
Application Number | 20020057036 09/953720 |
Document ID | / |
Family ID | 18773860 |
Filed Date | 2002-05-16 |
United States Patent
Application |
20020057036 |
Kind Code |
A1 |
Taniguchi, Norio ; et
al. |
May 16, 2002 |
SURFACE ACOUSTIC WAVE DEVICE
Abstract
A surface acoustic wave device is arranged such that surface
acoustic wave energy is trapped substantially perpendicularly to
the surface acoustic wave propagation direction. At least one
interdigital transducer having a plurality of electrode fingers and
first and second bus bar electrodes is located on a piezoelectric
substrate on which a surface acoustic wave is excited, and has an
anisotropy index .gamma. of less than about -1 in the propagation
direction. The electrode fingers each have a film thickness of not
less than about 0.04.lambda. in which .lambda. is the wavelength of
the surface acoustic wave. At least a portion of the first and
second bus bar electrodes have a thickness that is larger than that
of each electrode finger.
Inventors: |
Taniguchi, Norio;
(Shiga-ken, JP) ; Takata, Toshiaki; (Kanazawa-shi,
JP) ; Nagai, Tatsurou; (Kanazawa-shi, JP) ;
Yamato, Shuji; (Ishikawa-ken, JP) |
Correspondence
Address: |
Keating & Bennett LLP
10400 Eaton Place, Suite 312
Fairfax
VA
22030
US
|
Assignee: |
Murata Manufacturing Co.,
Ltd.
Nagaokakyo-shi
JP
|
Family ID: |
18773860 |
Appl. No.: |
09/953720 |
Filed: |
September 17, 2001 |
Current U.S.
Class: |
310/313B ;
310/313R |
Current CPC
Class: |
H03H 9/6483 20130101;
H03H 9/14538 20130101; H03H 9/0222 20130101; H03H 9/6436 20130101;
H03H 9/25 20130101 |
Class at
Publication: |
310/313.00B ;
310/313.00R |
International
Class: |
H01L 041/04 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 25, 2000 |
JP |
2000-290661 |
Claims
What is claimed is:
1. A surface acoustic wave device comprising: a piezoelectric
substrate at which a surface acoustic wave having an anisotropy
index .gamma. in the propagation direction of less than about -1 is
excited; and at least one interdigital transducer disposed on the
piezoelectric substrate and including first and second bus bar
electrodes and a plurality of electrode fingers, the plurality of
electrode fingers being connected to the first and second bus bar
electrodes so as to define a pair of comb-shaped electrodes that
are interdigitated with each other, the plurality of electrode
fingers containing Al as a major component; wherein each of the
electrode fingers has a film thickness of not less than about
0.04.lambda., where .lambda. is the wavelength of the surface
acoustic wave, and at least a portion of the first and second bus
bar electrodes has a thickness larger than that of each of the
electrode fingers so that the energy of the surface acoustic wave
is trapped substantially perpendicularly to the propagation
direction of the surface acoustic wave.
2. A surface acoustic wave device according to claim 1, wherein at
least a portion of the first and second bus bar electrodes have a
multi-layer structure in which a plurality of electrode films are
laminated to each other, whereby at least a portion of the first
and second bus bar electrodes have a thickness larger than that of
each of the electrode fingers.
3. A surface acoustic wave device according to claim 2, wherein the
electrode film defining the lowest layer in each of the bus bar
electrodes having a multi-layer structure is arranged so as to be
connected to the electrode fingers, and the electrode films
defining the second and succeeding layers are made of a metal that
is different from that used to form the electrode film defining the
lowest layer.
4. A surface acoustic wave device according to claim 3, wherein, in
each of the bus bar electrodes having a multi-layer structure, at
least one layer of the electrode films defining the second and
succeeding layers is made of a metal having a relatively high
density compared to the electrode film defining the lowest
layer.
5. A surface acoustic wave device according to claim 3, wherein, in
each of the bus bar electrodes having a multi-layer structure, at
least one layer of the electrode films defining the second and the
succeeding layers has a lower resistivity and a larger thickness
compared to the electrode film defining the lowest layer.
6. A surface acoustic wave device according to claim 3, wherein, in
each of the bus bar electrodes having a multi-layer structure, an
insulating film is disposed between electrode films constituting
the multi-layer structure so as to secure electrical connection
between the upper and lower electrode films.
7. A surface acoustic wave device according to claim 3, wherein, in
each of the bus bar electrodes having a multi-layer structure, a
distance g of the boundary between the bus bar electrode and the
electrode fingers in the electrode film of the lowest layer to the
edge on the electrode finger side of the electrode film made of Al
defining the second layer, and the film thickness M of the
electrode film defining the second layer are in the range defined
by the formula of M.gtoreq.0.159 g-0.094, in which values of g and
M are integral multiples of the wavelength .lambda. of the surface
acoustic wave.
8. A surface acoustic wave device according to claim 3, wherein, in
each of the bus bar electrodes having a multi-layer structure, the
electrode film thickness Ma of the second layer is in the range
defined by the formula of Ma.times.(d0/da).gtoreq.0.159 g- 0.094,
in which g is the distance of the boundary between the bus bar
electrode and the electrode fingers in the electrode film of the
lowest layer to the edge on the electrode finger side of the
electrode film made of Al defining the second layer the values of g
and M are expressed by integral multiples of the wavelength
.lambda. of the surface acoustic wave, respectively, the second
layer is made of metal excluding Al, da is the density of the metal
of the second layer, and d0 is the density of Al.
9. A surface acoustic wave device according to claim 1, wherein the
piezoelectric substrate at which a surface acoustic wave is a
LiTaO.sub.3 substrate at which a pseudo surface acoustic wave is
excited.
10. A surface acoustic wave device comprising: a piezoelectric
substrate at which a surface acoustic wave having an anisotropy
index .gamma. in the propagation direction of less than about -1 is
excited; and at least one interdigital transducer disposed on the
piezoelectric substrate and including first and second bus bar
electrodes and a plurality of electrode fingers, the plurality of
electrode fingers being connected to the first and second bus bar
electrodes so as to define a pair of comb-shaped electrodes that
are interdigitated with each other, the plurality of electrode
fingers containing Al as a major component; wherein an electrode
finger width L1 of the interdigital transducer and a gap length L2
between adjacent electrode fingers in the surface acoustic wave
propagation direction satisfy the formula of L1/(L1+L2).gtoreq.0.5,
and at least a portion of the first and second bus bar electrodes
has a thickness larger than that of each of the electrode fingers
so that the energy of the surface acoustic wave is trapped
substantially perpendicularly to the propagation direction of the
surface acoustic wave.
11. A surface acoustic wave device according to claim 10, wherein
at least a portion of the first and second bus bar electrodes have
a multi-layer structure in which a plurality of electrode films are
laminated to each other, whereby at least a portion of the first
and second bus bar electrodes have a thickness larger than that of
each of the electrode fingers.
12. A surface acoustic wave device according to claim 11, wherein
the electrode film defining the lowest layer in each of the bus bar
electrodes having a multi-layer structure is arranged so as to be
connected to the electrode fingers, and the electrode films
defining the second and succeeding layers are made of a metal that
is different from that used to form the electrode film defining the
lowest layer.
13. A surface acoustic wave device according to claim 12, wherein,
in each of the bus bar electrodes having a multi-layer structure,
at least one layer of the electrode films defining the second and
succeeding layers is made of a metal having a relatively high
density compared to the electrode film defining the lowest
layer.
14. A surface acoustic wave device according to claim 12, wherein,
in each of the bus bar electrodes having a multi-layer structure,
at least one layer of the electrode films defining the second and
the succeeding layers has a lower resistivity and a larger
thickness compared to the electrode film defining the lowest
layer.
15. A surface acoustic wave device according to claim 12, wherein,
in each of the bus bar electrodes having a multi-layer structure,
an insulating film is disposed between electrode films constituting
the multi-layer structure so as to secure electrical connection
between the upper and lower electrode films.
16. A surface acoustic wave device according to claim 12, wherein,
in each of the bus bar electrodes having a multi-layer structure, a
distance g of the boundary between the bus bar electrode and the
electrode fingers in the electrode film of the lowest layer to the
edge on the electrode finger side of the electrode film made of Al
defining the second layer, and the film thickness M of the
electrode film defining the second layer are in the range defined
by the formula of M.gtoreq.0.159 g-0.094, in which values of g and
M are integral multiples of the wavelength .lambda. of the surface
acoustic wave.
17. A surface acoustic wave device according to claim 12, wherein,
in each of the bus bar electrodes having a multi-layer structure,
the electrode film thickness Ma of the second layer is in the range
defined by the formula of Ma.times.(d0/da).gtoreq.0.159 g-0.094, in
which g is the distance of the boundary between the bus bar
electrode and the electrode fingers in the electrode film of the
lowest layer to the edge on the electrode finger side of the
electrode film made of Al defining the second layer the values of g
and M are expressed by integral multiples of the wavelength
.lambda. of the surface acoustic wave, respectively, the second
layer is made of metal excluding Al, da is the density of the metal
of the second layer, and d0 is the density of Al.
18. A surface acoustic wave device according to claim 10, wherein
the piezoelectric substrate at which a surface acoustic wave is a
LiTaO.sub.3 substrate at which a pseudo surface acoustic wave is
excited.
19. A surface acoustic wave device comprising: a piezoelectric
substrate at which a surface acoustic wave having an anisotropy
index .gamma. in the propagation direction of less than about -1 is
excited; and at least one interdigital transducer disposed on the
piezoelectric substrate and including first and second bus bar
electrodes and a plurality of electrode fingers, the plurality of
electrode fingers being connected to the first and second bus bar
electrodes so as to define a pair of comb-shaped electrodes that
are interdigitated with each other, the plurality of electrode
fingers containing Al as a major component; wherein a film
thickness h1 of each of the electrode fingers, a electrode finger
width L1, a gap length L2 between adjacent electrode fingers in the
surface acoustic wave direction, a wavelength .lambda. of the
surface acoustic wave satisfy one of the following formulae (1) to
(6);L1(L1+L2).gtoreq.0.55 and h/.lambda..gtoreq.0.100
(1)L1(L1+L2).gtoreq.0.60 and h/.lambda..gtoreq.0.090
(2)L1(L1+L2).gtoreq.0.65 and h/.lambda..gtoreq.0.080
(3)L1(L1+L2).gtoreq.0.70 and h/.lambda..gtoreq.0.070
(4)L1(L1+L2).gtoreq.0.75 and h/.lambda..gtoreq.0.065
(5)L1(L1+L2).gtoreq.0.80 and h/.lambda..gtoreq.0.055 (6),andat
least a portion of the first and second bus bar electrodes having a
thickness larger than that of each of the electrode fingers so that
the energy of the surface acoustic wave is trapped substantially
perpendicularly to the propagation direction of the surface
acoustic wave.
20. A surface acoustic wave device according to claim 19, wherein
at least a portion of the first and second bus bar electrodes have
a multi-layer structure in which a plurality of electrode films are
laminated to each other, whereby at least a portion of the first
and second bus bar electrodes have a thickness larger than that of
each of the electrode fingers.
21. A surface acoustic wave device according to claim 20, wherein
the electrode film defining the lowest layer in each of the bus bar
electrodes having a multi-layer structure is arranged so as to be
connected to the electrode fingers, and the electrode films
defining the second and succeeding layers are made of a metal that
is different from that used to form the electrode film defining the
lowest layer.
22. A surface acoustic wave device according to claim 21, wherein,
in each of the bus bar electrodes having a multi-layer structure,
at least one layer of the electrode films defining the second and
succeeding layers is made of a metal having a relatively high
density compared to the electrode film defining the lowest
layer.
23. A surface acoustic wave device according to claim 21, wherein,
in each of the bus bar electrodes having a multi-layer structure,
at least one layer of the electrode films defining the second and
the succeeding layers has a lower resistivity and a larger
thickness compared to the electrode film defining the lowest
layer.
24. A surface acoustic wave device according to claim 21, wherein,
in each of the bus bar electrodes having a multi-layer structure,
an insulating film is disposed between electrode films constituting
the multi-layer structure so as to secure electrical connection
between the upper and lower electrode films.
25. A surface acoustic wave device according to claim 21, wherein,
in each of the bus bar electrodes having a multi-layer structure, a
distance g of the boundary between the bus bar electrode and the
electrode fingers in the electrode film of the lowest layer to the
edge on the electrode finger side of the electrode film made of Al
defining the second layer, and the film thickness M of the
electrode film defining the second layer are in the range defined
by the formula of M.gtoreq.0.159 g-0.094, in which values of g and
M are integral multiples of the wavelength .lambda. of the surface
acoustic wave.
26. A surface acoustic wave device according to claim 21, wherein,
in each of the bus bar electrodes having a multi-layer structure,
the electrode film thickness Ma of the second layer is in the range
defined by the formula of Ma.times.(d0/da).gtoreq.0.159 g-0.094, in
which g is the distance of the boundary between the bus bar
electrode and the electrode fingers in the electrode film of the
lowest layer to the edge on the electrode finger side of the
electrode film made of Al defining the second layer the values of g
and M are expressed by integral multiples of the wavelength
.lambda. of the surface acoustic wave, respectively, the second
layer is made of metal excluding Al, da is the density of the metal
of the second layer, and d0 is the density of Al.
27. A surface acoustic wave device according to claim 19, wherein
the piezoelectric substrate at which a surface acoustic wave is a
LiTaO.sub.3 substrate at which a pseudo surface acoustic wave is
excited.
28. A surface acoustic wave device comprising: a piezoelectric
substrate at which a surface acoustic wave is excited, having an
anisotropy index .gamma. in the propagation direction of less than
about -1 ; and at least one interdigital transducer disposed on the
piezoelectric substrate, having a plurality of electrode fingers
each containing Al as a major component and first and second bus
bar electrodes, in which the energy of the surface acoustic wave is
trapped substantially perpendicularly to the propagation direction
of the surface acoustic wave; the film thickness of each of the
electrode fingers in the interdigital transducer being not less
than about 0.04.lambda., in which .lambda. is the wavelength of the
surface acoustic wave; an insulating film being formed on each of
the bus bar electrodes so that the thickness of the bus bar
electrodes is larger than that of each electrode fingers.
29. A surface acoustic wave device according to claim 28, further
comprising an insulating film disposed on the electrode fingers,
said insulating film having a thickness larger than that of the
insulating film disposed on each of the electrode fingers.
30. A surface acoustic wave device according to claim 28, wherein
the piezoelectric substrate at which a surface acoustic wave is a
LiTaO.sub.3 substrate at which a pseudo surface acoustic wave is
excited.
31. A surface acoustic wave device comprising: a piezoelectric
substrate at which a surface acoustic wave is excited, having an
anisotropy index .gamma. in the propagation direction of less than
about -1; and at least one interdigital transducer disposed on the
piezoelectric substrate, having a plurality of electrode fingers
each containing Al as a major component and first and second bus
bar electrodes, in which the energy of the surface acoustic wave is
trapped substantially perpendicularly to the propagation direction
of the surface acoustic wave; the electrode finger width L1 and the
gap length L2 between adjacent electrode fingers in the surface
acoustic wave propagation direction satisfying the formula of
L1/(L1+L2).gtoreq.0.5; an insulating film being disposed on each of
the bus bar electrodes so that the thickness of the bus bar
electrodes is larger than that of each of the electrode
fingers.
32. A surface acoustic wave device according to claim 31, further
comprising an insulating film disposed on the electrode fingers,
said insulating film having a thickness larger than that of the
insulating film disposed on each of the electrode fingers.
33. A surface acoustic wave device according to claim 31, wherein
the piezoelectric substrate at which a surface acoustic wave is a
LiTaO.sub.3 substrate at which a pseudo surface acoustic wave is
excited.
34. A surface acoustic wave device comprising: a piezoelectric
substrate at which a surface acoustic wave is excited, having an
anisotropy index .gamma. in the propagation direction of less than
about -1 ; and at least one interdigital transducer disposed on the
piezoelectric substrate, having a plurality of electrode fingers
each containing Al as a major component and first and second bus
bar electrodes, in which the energy of the surface acoustic wave is
trapped substantially perpendicularly to the propagation direction
of the surface acoustic wave; the film thickness h1 of each of the
electrode fingers, the electrode finger width L1, the gap length L2
between adjacent electrode fingers in the surface acoustic wave
direction, the wavelength .gamma. of the surface acoustic wave
satisfying one of the following formulae (1) to
(6);L1(L1+L2).gtoreq.0.55 and h/.lambda..gtoreq.0.100
(1)L1(L1+L2).gtoreq.0.60 and h/.lambda..gtoreq.0.090
(2)L1(L1+L2).gtoreq.0.65 and h/.lambda..gtoreq.0.080
(3)L1(L1+L2).gtoreq.0.70 and h/.lambda..gtoreq.0.070
(4)L1(L1+L2).gtoreq.0.75 and h/.lambda..gtoreq.0.065
(5)L1(L1+L2).gtoreq.0.80 and h/.lambda..gtoreq.0.055 (6)and further
includes an insulating film disposed on the bus bar electrodes.
35. A surface acoustic wave device according to claim 34, further
comprising an insulating film disposed on the electrode fingers,
said insulating film having a thickness larger than that of the
insulating film disposed on each of the electrode fingers.
36. A surface acoustic wave device according to claim 34, wherein
the piezoelectric substrate at which a surface acoustic wave is a
LiTaO.sub.3 substrate at which a pseudo surface acoustic wave is
excited.
37. An antenna sharing device includes at least one of the surface
acoustic wave device according to claim 1.
38. A communications equipment apparatus including the antenna
sharing device of claim 16.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] 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.
[0003] 2. Description of the Related Art
[0004] 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.
[0005] 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.
[0006] Moreover, a surface acoustic wave resonator having only one
interdigital transducer 205 has been proposed as shown in FIG. 15B.
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.
[0007] 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.
16C. 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.
[0008] 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 FIG. 15A and 15B, have been proposed.
[0009] 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.
[0010] 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 L1 of
each electrode finger 211 in an interdigital transducer shown in
FIG. 17, based on the gap size L2 between adjacent electrode
fingers 211 in the surface acoustic wave propagation direction in
the interdigital transducer, that is, the ratio of L1/(L1+L2)
(hereinafter, referred to as duty, briefly), and moreover, the
electrode film thickness h/.lambda. of the interdigital transducer
(.lambda. is the wavelength of a surface acoustic wave, and
h/.lambda. is a film thickness standardized by .lambda.. Thus, for
design of the surface acoustic wave device, it is important to
optimize these parameters.
[0011] The gap length L2 represents the distance in the surface
acoustic wave propagation direction of the gap.
[0012] 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.degree. Y-cut X-directional propagation LiTaO.sub.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.lambda. to 0.10.lambda., and also, setting the duty of the
interdigital transducer at about 0.6 or higher.
[0013] 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.degree. Y-cut X-directional
propagation LiTaO.sub.3 substrate. FIG. 18 shows the ladder
circuit. In FIG. 18, S1 and S2 represent series arm resonators, and
P1 to P3 represent parallel arm resonators, respectively. In this
conventional surface acoustic wave filter, the electrode film
thickness h/.lambda. of the interdigital transducer is in the range
of 0.4.lambda. to 0.10.lambda., whereby an undesired spurious can
be removed from the transmission band to improve the filter
characteristic.
[0014] 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.lambda. or more and setting the duty at 0.5 or
higher when the 36.degree. Y-cut X-directional propagation
LiTaO.sub.3 is used.
[0015] 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 .mu.m. Accordingly, electrode patterns for defining the
interdigital transducers and the reflectors must be very fine.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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 component in the vertical
direction to the main plane of the piezoelectric substrate. Thus,
the surface acoustic wave propagates while the Y-directional
component extends in a beam shape. For this reason, it is necessary
to sufficiently trap the energy of the surface acoustic wave in the
Y-axial direction. Unless the energy is not sufficiently trapped,
the diffraction loss will increase, so that the Q value is
deteriorated.
[0021] As described in the Journal of the Acoustical Society of
Japan, 3-1-1, 77-78 (1979/6), the anisotropy index .gamma. is less
than -1 on a 36.degree. Y-cut propagation LiTaO.sub.3. The
anisotropy index .gamma. is a constant in the following formula by
which the acoustic velocity (.theta.), obtained when the
propagation direction is deviated by an angle .theta. from the
X-axis, is expressed. In the formula, V0 is the acoustic velocity
when .theta. is 0.degree..
V(.theta.)=V0.times.(1+.gamma./2.times..theta..sup.2)
[0022] In the case in which the anisotropy index .gamma. is less
than -1, the energy is trapped when the velocity in the wave guide
is lower than that outside the wave guide. That is, Vs/Vm>1 is
the condition required for energy trapping, in which Vs is the
velocity of a surface acoustic wave in the area where the electrode
fingers are provided, and Vm is the velocity of the surface
acoustic wave propagating on each bus bar electrode.
[0023] On the other hand, it has been found that the ratio Vs/Vm,
that is, the ratio of Vs representing the velocity of a surface
acoustic wave propagating on the area where the electrode fingers
are meshed with each other, to Vm representing the velocity of the
surface acoustic wave propagating on each bus bar electrode is
significantly varied depending on the duty and the electrode film
thickness, when the film thickness of the electrode fingers and
that of the bus bar electrode are equal to each other.
[0024] In particular, when the electrode film thickness is small,
and the duty is low, the ratio Vs/Vm>1 is satisfied. When the
electrode film thickness and also the duty are increased, the ratio
Vs/Vm is decreased. The ratio Vs/Vm reaches Vs=Vm on a certain
condition. When the duty or the electrode film thickness is further
increased, the ratio Vs/Vm becomes less than 1. That is,
substantially no energy can be trapped in the Y-axial
direction.
[0025] FIG. 19 shows a relationship between the electrode film
thickness h/.lambda. and the ratio Vs/Vm, obtained when the
interdigital transducer made of Al is formed on a 36.degree. Y-cut
X-directional propagation LiTaO.sub.3 substrate, and the duty is
0.5. As seen in FIG. 19, the ratio Vs/Vm has a maximum value when
the electrode film thickness h/.lambda. is in the range of 3% to
4%, namely, in the range of 0.03 to 0.04. When the electrode film
thickness h/.lambda. become larger, the ratio Vs/Vm is decreased,
changing along the parabolic curve. Especially, it is observed that
the ratio Vs/Vm is rapidly decreased when the electrode film
thickness h/.lambda. exceeds 0.06.lambda..
[0026] If the length in the Y-axial direction of each bus bar
electrode is infinite, the energy can be trapped, provided that the
ratio Vs/Vm is less than 1. In the case in which the length in the
Y-axial direction of each of the bus bars is definite, the energy
trapping effect will be reduced, if the ratio Vs/Vm is not
sufficiently large. Thus, the loss in the filter characteristic is
increased.
[0027] FIG. 20 shows a relationship between the duty and the ratio
Vs/Vm, obtained when the interdigital transducer is made of Al, and
the electrode film thickness is constant, that is, 0.06.lambda. on
a 36.degree. Y-cut X-directional propagation LiTaO.sub.3
substrate.
[0028] As seen in FIG. 20, when the duty is low, the ratio Vs/Vm is
large. As the duty is increased, the ratio Vs/Vm is reduced.
Especially, when the duty exceeds 0.8, the ratio Vs/Vm becomes less
than 1. Thus, the energy trapping condition is not satisfied.
[0029] Furthermore, the following Table 1 shows change of the ratio
Vs/Vm, obtained when the duty and the electrode film thickness are
varied.
1 TABLE 1 duty 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 electrode
film thickness (h/.lambda.) % 1 1.0111 1.0098 1.0084 1.0071 1.0058
1.0047 1.0035 1.0026 1.0016 1.5 1.0114 1.0100 1.0086 1.0073 1.0060
1.0048 1.0036 1.0026 1.0016 2 1.0116 1.0102 1.0089 1.0075 1.0062
1.0050 1.0037 1.0027 1.0017 2.5 1.0118 1.0103 1.0089 1.0076 1.0062
1.0050 1.0037 1.0027 1.0016 3 1.0119 1.0105 1.0090 1.0076 1.0062
1.0050 1.0037 1.0027 1.0016 3.5 1.0120 1.0105 1.0090 1.0075 1.0061
1.0048 1.0036 1.0025 1.0014 4 1.0121 1.0105 1.0090 1.0075 1.0060
1.0047 1.0034 1.0023 1.0012 4.5 1.0120 1.0104 1.0088 1.0073 1.0058
1.0044 1.0031 1.0020 1.0008 5 1.0119 1.0102 1.0086 1.0070 1.0055
1.0042 1.0028 1.0016 1.0004 5.5 1.0118 1.0110 1.0083 1.0067 1.0051
1.0037 1.0023 1.0011 0.9999 6 1.0116 1.0098 1.0079 1.0063 1.0046
1.0032 1.0018 1.0006 0.9994 6.5 1.0112 1.0092 1.0073 1.0056 1.0039
1.0024 1.0009 0.9997 0.9986 7 1.0107 1.0087 1.0067 1.0050 1.0032
1.0016 0.9999 0.9988 0.9977 7.5 1.0103 1.0082 1.0061 1.0042 1.0023
1.0007 0.9992 0.9980 0.9968 8 1.0099 1.0077 1.0054 1.0034 1.0014
0.9999 0.9984 0.9972 0.9960 8.5 1.0091 1.0070 1.0049 1.0025 1.0001
0.9985 0.9969 0.9956 0.9944 9 1.0083 1.0063 1.0043 1.0015 0.9987
0.9971 0.9954 0.9941 0.9928 9.5 1.0075 1.0056 1.0037 1.0005 0.9972
0.9955 0.9939 0.9925 0.9912 10 1.0066 1.0049 1.0031 0.9994 0.9957
0.9940 0.9923 0.9909 0.9896
[0030] As seen in TABLE 1, as the film thickness and the duty are
increased, the ratio Vs/Vm is decreased. Especially, the ratio
Vs/Vm is less than 1 when the relationships satisfying the
following formulae (1) to (6) are obtained, that is, in the
conditions where the values listed in the columns on the right side
from the thick lines in TABLE 1 can be obtained. Thus, the wave
mode in the Y-axial direction cannot be satisfied
substantially.
L1(L1+L2).gtoreq.0.55 and h/.lambda..gtoreq.0.100 (1)
L1(L1+L2).gtoreq.0.60 and h/.lambda..gtoreq.0.090 (2)
L1(L1+L2).gtoreq.0.65 and h/.lambda..gtoreq.0.080 (3)
L1(L1+L2).gtoreq.0.70 and h/.lambda..gtoreq.0.070 (4)
L1(L1+L2).gtoreq.0.75 and h/.lambda..gtoreq.0.065 (5)
L1(L1+L2).gtoreq.0.80 and h/.lambda..gtoreq.0.055 (6)
[0031] 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 electrode resistance loss can be reduced, and
an undesirable spurious can be eliminated by increasing the
electrode film thickness and also the duty. This was estimated to
be preferable.
[0032] Referring to the energy trapping effect in the Y-axial
direction of the surface acoustic wave, the trapping effect becomes
maximum at an electrode film thickness of 0.04.lambda., and is
reduced when the electrode film thickness becomes 0.04.lambda. or
more.
[0033] Moreover, similar phenomena are observed when the duty is
increased. The energy trapping effect is reduced at a duty of 0.5
or higher.
[0034] Especially, the energy trapping condition cannot be
satisfied in the range where the relationship between the electrode
film thickness and the duty fulfills a certain condition. Thus, the
loss in the filter characteristic is increased.
[0035] Accordingly, it is preferable that the electrode film
thickness is up to 0.04.lambda., and the duty is up to 0.5 to
obtain the greatest energy trapping effect.
[0036] However, when the electrode film thickness is small, and the
duty is 0.5 or less, the filter characteristic is deteriorated for
a reason other than the above-described one, as seen in the
above-described Japanese Unexamined Patent Application Publication
No. 7-283682 and the Japanese Unexamined Patent Application
Publication No. 6-188673.
[0037] In other words, the optimum electrode structure of a surface
acoustic wave filter for obtaining the preferred filter
characteristic thereof and the optimum electrode structure from the
standpoint of the above-described energy trapping effect in the
Y-axial direction are different from each other. Both of the
electrode structures have a trade-off relationship.
[0038] Moreover, Japanese Unexamined Patent Application Publication
No. 62-47206 describes that the acoustic coupling degree between
the interdigital transducers can be enhanced, and the bandwidth can
be increased by increasing the thickness of each bus bar electrode
shared by interdigital transducers adjacent to each other in the
surface acoustic wave propagation direction to be larger than that
of each electrode finger, until the acoustic velocity Vs of the
surface acoustic wave propagating on the electrode fingers is equal
to the acoustic velocity Vb of the surface acoustic wave
propagating on each bus bar electrode.
[0039] The above-described phenomena are caused in the
configuration of the surface acoustic wave filter in which the
interdigital transducers are acoustically coupled to each other
perpendicularly to the surface acoustic wave propagation direction.
When Vs is equal to Vb, the above-described energy trapping effect
in the Y-axial direction is reduced to the contrary.
SUMMARY OF THE INVENTION
[0040] In order to overcome the problems described above, preferred
embodiments of the present invention provide a surface acoustic
wave device which can efficiently trap the energy of an excited
surface acoustic wave, and moreover, can reduce the loss and
improve the filter characteristic.
[0041] According to a first preferred embodiment of the present
invention, a surface acoustic wave device includes a piezoelectric
substrate at which a surface acoustic wave is excited, having an
anisotropy index .gamma. in the propagation direction of less than
about -1, and at least one interdigital transducer disposed on the
piezoelectric substrate, having a plurality of electrode fingers
each containing Al as a major component and first and second bus
bar electrodes, in which the energy of the surface acoustic wave is
trapped substantially perpendicularly to the propagation direction
of the surface acoustic wave, the electrode fingers each having a
film thickness of not less than about 0.04.lambda. in which
.lambda. is the wavelength of the surface acoustic wave, at least a
portion of the first and second bus bar electrodes having a
thickness larger than that of each electrode finger.
[0042] According to a second preferred embodiment of the present
invention, a surface acoustic wave device includes a piezoelectric
substrate at which a surface acoustic wave is excited, having an
anisotropy index .gamma. in the propagation direction of less than
about -1, and at least one interdigital transducer disposed on the
piezoelectric substrate, having a plurality of electrode fingers
each containing Al as a major component and first and second bus
bar electrodes, in which the energy of the surface acoustic wave is
trapped substantially perpendicularly to the propagation direction
of the surface acoustic wave, the electrode finger width L1 of the
interdigital transducer and the gap length L2 between adjacent
electrode fingers in the surface acoustic wave propagation
direction satisfying the formula of L1/(L1+L2).gtoreq.0.5, at least
a portion of the first and second bus bar electrodes having a
thickness larger than that of each electrode finger.
[0043] According to a third preferred embodiment of the present
invention, a surface acoustic wave device includes a piezoelectric
substrate at which a surface acoustic wave is excited, having an
anisotropy index .gamma. in the propagation direction of less than
about -1, and at least one interdigital transducer disposed on the
piezoelectric substrate, having a plurality of electrode fingers
each containing Al as a major component and first and second bus
bar electrodes, in which the energy of the surface acoustic wave is
trapped substantially perpendicularly to the propagation direction
of the surface acoustic wave, the film thickness h1 of each
electrode finger, the electrode finger width L1, the gap length L2
between adjacent electrode fingers in the surface acoustic wave
direction, the wavelength .lambda. of the surface acoustic wave
satisfying one of the following formulae (1) to (6);
L1(L1+L2).gtoreq.0.55 and h/.lambda..gtoreq.0.100 (1)
L1(L1+L2).gtoreq.0.60 and h/.lambda..gtoreq.0.090 (2)
L1(L1+L2).gtoreq.0.65 and h/.lambda..gtoreq.0.080 (3)
L1(L1+L2).gtoreq.0.70 and h/.lambda..gtoreq.0.070 (4)
L1(L1+L2).gtoreq.0.75 and h/.lambda..gtoreq.0.065 (5)
L1(L1+L2).gtoreq.0.80 and h/.lambda..gtoreq.0.055 (6)
[0044] at least a portion of the first and second bus bar
electrodes having a thickness larger than that of each electrode
finger.
[0045] Preferably, at least a portion of the first and second bus
bar electrodes have a multi-layer structure in which a plurality of
electrode films are laminated to each other, whereby at least a
portion of the first and second bus bar electrodes have a thickness
larger than that of each electrode finger.
[0046] Also, preferably, in each bus bar electrode having a
multi-layer structure, the electrode film defining the lowest layer
is arranged so as to be connected to the electrode fingers,
respectively, and the electrode films defining the second and the
succeeding layers are made of a metal different from that used to
form the electrode film defining the lowest layer.
[0047] Also, preferably, in each bus bar electrode having a
multi-layer structure, at least one layer of the electrode films
defining the second and the proceeding layers is made of a metal
having a relatively high density compared to the electrode film
defining the lowest layer.
[0048] Also, preferably, in each bus bar electrode having a
multi-layer structure, at least one layer of the electrode films
defining the second and the proceeding layers has a lower
resistivity and a larger thickness compared to the electrode film
defining the lowest layer.
[0049] Preferably, in each bus bar electrode having a multi-layer
structure, an insulating film is disposed between electrode films
constituting the multi-layer structure so as to secure electrical
connection between the upper and lower electrode films.
[0050] Moreover, preferably, in each bus bar electrode having a
multi-layer structure, the distance g of the boundary between the
bus bar electrode and the electrode fingers in the electrode film
of the lowest layer to the edge on the electrode finger side of the
electrode film made of Al defining the second layer, and the film
thickness M of the electrode film defining the second layer are in
the range determining by the formula of M.gtoreq.0.159 g-0.094, in
which values of g and M are integral multiples of the wavelength
.lambda. of the surface acoustic wave.
[0051] Also, preferably, in each bus bar electrode having a
multi-layer structure, the film-thickness Ma of the second layer is
in the range defined by the formula Ma.times.(d0/da).gtoreq.0.159
g-0.094, in which g is the distance from the boundary between the
bus bar electrode and the electrode fingers to the edge on the
electrode finger side of the electrode film defining the second
layer, Ma is the electrode film thickness of the second layer, the
values of g and M are expressed by integral multiples of the
wavelength .lambda. of the surface acoustic wave, respectively, the
second layer is made of metal excluding Al, da is the density of
the meal of the second layer, and d is the density of Al.
[0052] According to a fourth preferred embodiment of the present
invention, a surface acoustic wave device includes a piezoelectric
substrate at which a surface acoustic wave is excited, having an
anisotropy index .gamma. in the propagation direction of less than
about -1, and at least one interdigital transducer disposed on the
piezoelectric substrate, having a plurality of electrode fingers
each containing Al as a major component and first and second bus
bar electrodes, in which the energy of the surface acoustic wave is
trapped substantially perpendicularly to the propagation direction
of the surface acoustic wave, the film thickness of the electrode
fingers in the interdigital transducer being not less than about
0.04.lambda., in which .lambda. is the wavelength of the surface
acoustic wave, an insulating film being disposed on each bus bar
electrode so that the thickness of the bus bar electrode is larger
than that of each electrode finger.
[0053] According to a fifth preferred embodiment of the present
invention, a surface acoustic wave device includes a piezoelectric
substrate at which a surface acoustic wave is excited, having an
anisotropy index .gamma. in the propagation direction of less than
about -1, and at least one interdigital transducer disposed on the
piezoelectric substrate, having a plurality of electrode fingers
each containing Al as a major component and first and second bus
bar electrodes, in which the energy of the surface acoustic wave is
trapped substantially perpendicularly to the propagation direction
of the surface acoustic wave, the electrode finger width L1 and the
gap length L2 between adjacent electrode fingers in the surface
acoustic wave propagation direction satisfying the formula of
L1/(L1+L2) .gtoreq.0.5, an insulating film being disposed on each
bus bar electrode so that the thickness of the bus bar electrode is
larger than that of each electrode finger.
[0054] According to a sixth preferred embodiment of the present
invention, a surface acoustic wave device including a piezoelectric
substrate at which a surface acoustic wave is excited, having an
anisotropy index .gamma. in the propagation direction of less than
about -1, and at least one interdigital transducer disposed on the
piezoelectric substrate, having a plurality of electrode fingers
each containing Al as a major component and first and second bus
bar electrodes, in which the energy of the surface acoustic wave is
trapped substantially perpendicularly to the propagation direction
of the surface acoustic wave, the film thickness h1 of each
electrode finger, the electrode finger width L1, the gap length L2
between adjacent electrode fingers in the surface acoustic wave
direction, the wavelength .lambda. of the surface acoustic wave
satisfying one of the above-described formulae (1) to (6), and
further including an insulating film disposed on the bus bar
electrode.
[0055] Preferably, the surface acoustic wave device according to
various preferred embodiments of the present invention further
includes an insulating film disposed on the electrode fingers,
whereby the thickness of each bus bar electrode portion including
the insulating film is larger than the electrode finger portion
including the insulating film.
[0056] Also, preferably, the piezoelectric substrate at which a
surface acoustic wave can be excited, having an anisotropy index
.gamma. in the propagation direction of less than about -1 is
preferably a LiTaO.sub.3 substrate at which a pseudo surface
acoustic wave can be excited, for example, a 36.degree. Y-cut
X-directional propagation LiTaO.sub.3 substrate.
[0057] According to another preferred embodiment of the present
invention, an antenna sharing device includes at least one of the
surface acoustic wave devices according to the above-described
preferred embodiments of the present invention.
[0058] In addition, according to yet another preferred embodiment
of the present invention, a communications equipment apparatus
includes at least one antenna sharing device according to the
preferred embodiment described in the preceding paragraph.
[0059] Other features, elements, characteristics and advantages of
the present invention will become more apparent from the following
detailed description of preferred embodiments of the present
invention with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0060] FIG. 1 is a schematic plan view showing the electrode
configuration of a one terminal-pair surface acoustic wave
resonator according to a first preferred embodiment of the present
invention;
[0061] FIG. 2 is a graph showing a relationship between the film
thickness of each bus bar electrode and the acoustic velocity Vm of
a surface acoustic wave propagating on the bus bar portion;
[0062] FIG. 3 is a schematic plan view of a surface acoustic wave
device according to the first preferred embodiment of the present
invention;
[0063] FIG. 4 is a graph showing the filter characteristics of the
surface acoustic wave device of the first embodiment and an
comparative example thereof;
[0064] FIG. 5A is a graph showing a relationship between a gap
length g and the bandwidth at M=840 nm;
[0065] FIG. 5B is a graph showing a relationship between the gap
length g and the bandwidth at M=840 nm;
[0066] FIG. 6 is a graph showing a relationship between the gap
length g and the bandwidth at M=280 nm;
[0067] FIG. 7 is a graph showing a relationship between the gap
length g and the film thickness M, obtained when energy trapping is
effective;
[0068] FIG. 8 is a plan view showing the surface acoustic wave
resonator according to a second preferred embodiment of the present
invention;
[0069] FIG. 9 is a graph showing the filter characteristics of the
surface acoustic wave device of the second preferred embodiment and
a comparative example thereof;
[0070] FIG. 10 is a schematic plan view showing the surface
acoustic wave device according to a third preferred embodiment of
the present invention;
[0071] FIG. 11 is a cross sectional view showing the bus bar
electrode of the surface acoustic wave device of the third
preferred embodiment and an insulating film disposed on the bus bar
electrode;
[0072] FIG. 12 is a graph showing the filter characteristics of the
surface acoustic wave deices of the third preferred embodiment and
a comparative example thereof;
[0073] FIG. 13 is a circuit diagram showing a sharing device
configured including the surface acoustic wave device of various
preferred embodiments of the present invention;
[0074] FIG. 14 is a schematic block diagram of a communication
system including a sharing device according to various preferred
embodiments of the present invention;
[0075] FIG. 15A is a schematic plan view showing a conventional one
terminal-pair surface acoustic wave resonator;
[0076] FIG. 15B is a schematic plan view showing another
conventional one terminal-pair surface acoustic wave resonator;
[0077] FIG. 16 is a plan view of a conventional resonator type
surface acoustic wave filter;
[0078] FIG. 17 is a cross sectional view of a main portion of the
conventional surface acoustic wave device;
[0079] FIG. 18 is a ladder type circuit diagram;
[0080] FIG. 19 is a graph showing a relationship between the film
thickness of each electrode and Vs/Vm of the conventional surface
acoustic wave device; and
[0081] FIG. 20 is a graph showing a relationship between the duty
of each electrode and Vs/Vm of the conventional surface acoustic
wave device.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0082] The present invention will be more apparent from the
following description of preferred embodiments of the surface
acoustic wave device of the present invention made with reference
to the drawings.
[0083] FIG. 3 is a schematic plan view of a surface acoustic wave
device according to a first preferred embodiment of the present
invention. A surface acoustic wave device 1 preferably includes a
substantially rectangular piezoelectric substrate 2. The
piezoelectric substrate 2 is preferably defined by a 36.degree.
Y-cut X-directional propagation LiTaO.sub.3 substrate.
[0084] A plurality of one terminal-pair surface acoustic wave
resonators are arranged on the piezoelectric substrate 2 so as to
define a ladder circuit configuration. That is, series arm
resonators 3 and 4, parallel arm resonators 5 to 7, electrode pads
8 to 12, and a wiring electrode connecting them are formed by
photolithography and an etching process.
[0085] In the surface acoustic wave device 1 of this preferred
embodiment, the electrode pads 8 and 9 are used as input and output
terminals. The series combination of the series arm resonators 3
and 4 is connected to the series arm between the input and output
terminals. The electrode pads 10 to 12 are connected to the ground,
respectively. The parallel arm resonators 5 to 7 are connected
between the series arm and the ground, respectively, to define a
ladder filter.
[0086] Each of the series arm resonators 3 and 4 and the parallel
arm resonators 5 to 7 is a one terminal-pair surface acoustic wave
resonator, which includes an interdigital transducer arranged in
the approximate center in the surface acoustic wave propagation
direction, and grating reflectors arranged on both of the sides in
the surface acoustic wave propagation direction of the interdigital
transducer.
[0087] In this preferred embodiment, the electrode finger meshing
width of each of the interdigital transducers of the series arm
resonators 3 and 4 is preferably about 50 .mu.m, the number of the
electrode pairs is 100, and the number of the electrode fingers of
each reflector is 100. The electrode finger pitch in each of the
series arm resonators 3 and 4 is preferably about 2.31 .mu.m.
Accordingly, the wavelength of a surface acoustic wave is
preferably about 4.63 .mu.m. The series arm resonators 3 and 4
preferably have the same configuration as described above.
[0088] The parallel arm resonators 5 to 7 preferably have
substantially the same configuration. In particular, the electrode
finger meshing width of each interdigital transducer is preferably
about 55 .mu.m. The number of the electrode finger pairs is 85, and
the number of the electrode fingers of each reflector is 100. The
electrode finger pitch is about 2.41 .mu.m. That is, the wavelength
of the surface acoustic wave is preferably about 2.81 .mu.m.
[0089] In the parallel arm resonator 6, the electrode finger
meshing width of the interdigital transducer is preferably about
110 .mu.m. The number of the electrode finger pairs is 85, and the
number of the electrode fingers of each reflector is 100. The
electrode finger pitch is preferably about 2.15 .mu.m (the
wavelength of the surface acoustic wave is preferably about 4.30
.mu.m).
[0090] It should be pointed out that in FIG. 3, the resonators 3 to
7 are schematically shown, and the number of the electrode fingers
and the electrode finger meshing width ratios are different from
those of the resonators used in practice, respectively.
[0091] The series arm resonators 3 and 4, the parallel arm
resonators 5 to 7, the electrode pads 8 to 12, and the wiring
electrode connecting them are preferably made of Al. The film
thickness of each of these electrodes excluding the bus bar
electrodes described below is preferably about 420 nm. The
wavelength of a surface acoustic wave on the series arm resonators
3 and 4 is preferably about 4.63 .mu.m. Thus, the ratio (h/.lambda.
(%)) of the film thickness of each of the electrode fingers of the
interdigital transducers of the series arm resonators 3 and 4 to
the wavelength is about 9.1%.
[0092] Thus, the wavelength of a surface acoustic wave on the
parallel arm resonators 5 and 7 is preferably about 4.81 .mu.m.
Thus, the ratio (h/.lambda. (%)) of the film thickness of each of
the electrode fingers of the interdigital transducers of the
parallel arm resonators 5 and 7 to the wavelength is preferably
about 8.7%.
[0093] Moreover, the duties of the electrode fingers of the
interdigital transducers in the series arm resonators 3 and 4 and
the parallel arm resonators 5 to 7 are about 0.5, respectively.
[0094] Concerning the bus bar electrodes of the interdigital
electrodes in the series arm resonators 3 and 4 and the parallel
arm resonators 5 to 7, an electrode film having a thickness of
about 840 nm defining the second layer is laminated to each bus bar
electrode film made of Al having a thickness of about 420 nm, not
shown in FIG. 3. This will be described with reference to FIG.
1.
[0095] FIG. 1 is a plan view showing the electrode configuration of
one of the one terminal-pair surface acoustic wave resonators used
as the series arm resonators 3 and 4 and the parallel arm
resonators 5 to 7. An interdigital transducer 14 is arranged in the
center of the one terminal-pair surface acoustic wave resonator 13,
and grating reflectors 15 and 16 are arranged on both of the sides
in the surface acoustic wave propagation direction of the
interdigital transducer 14, respectively. The interdigital
transducer 14 contains a plurality of electrode fingers 14a and
14b. A plurality of the electrode fingers 14a and a plurality of
the electrode fingers 14b are arranged so as to be interdigitated
with each other. The one-side ends of the plurality of the
electrode fingers 14a are connected to the bus bar electrode 14c.
The plurality of the electrode fingers 14b are electrically
connected to the bus bar electrode 14d which is arranged on the
opposite side to the bus bar electrode 14c, whereby a pair of
comb-shaped electrodes interdigitated with each other are
provided.
[0096] The reflectors 15 and 16 are arranged so that the both ends
of a plurality of electrode fingers 15a and also, the both ends of
a plurality of electrode fingers 16a are short-circuited to each
other, respectively.
[0097] In this preferred embodiment, in the one terminal-pair
surface acoustic wave resonators 13 constituting the series arm
resonators 3 and 4 and the parallel arm resonators 5 to 7,
respectively, electrode films 17 and 18 defining second layers,
shown by hatching, are laminated to at least a portion of the bus
bar electrodes 14c and 14d, respectively. That is, the bus bar
electrodes 14c and 14d each have a multi-layer structure. The
second electrode films 17 and 18 are made of Al. Each thickness is
preferably about 840 nm. Accordingly, the film-thickness based on
the wavelength of the surface acoustic wave is about 17%.
[0098] Moreover, the electrode films 17 and 18 defining the second
layers are arranged on the bus bar electrodes 14c and 14d so as to
be positioned outside the boundaries between the bus bar electrodes
14c and 14d and the electrode fingers 14a and 14b connected to the
bus bar electrodes 14c and 14d, substantially perpendicularly to
the surface acoustic wave propagation direction, respectively. In
other words, the edges B of the electrode films 17 and 18 defining
the second layers, which are on the sides of the electrode fingers
14a and 14b, are positioned outside the boundaries A between the
bus bar electrodes 14c and 14d and the electrode fingers 14a and
14b substantially perpendicularly to the surface acoustic wave
propagation direction, respectively. The distances g between the
boundaries A and the edges B are preferably set approximately at 4
.mu.m, that is, at about 0.8.lambda. to about 0.9.lambda.,
respectively.
[0099] The series arm resonators 3 and 4 and the parallel arm
resonators 5 to 7 each includes the one terminal-pair surface
acoustic wave resonator 13 shown in FIG. 1 in the surface acoustic
wave device 1 of this preferred embodiment.
[0100] The surface acoustic wave device 1 of this preferred
embodiment can be operated as a ladder filter by using the
electrode pad 8 as the input terminal and the electrode pad 9 as
the output terminal, and connecting the electrode pads 10 to 12 to
the ground. The ratio of the film thickness of each of the
electrode fingers of the series arm resonators 3 and 4 and the
parallel arm resonators 5 and 7 to the wavelength is preferably
about 9.1% and 8.7%, respectively. The duty of each interdigital
transducer is preferably about 0.5.
[0101] Thus, if the film-thickness of each of the electrode fingers
14a and 14b of the interdigital transducer 14 is substantially
equal to the film thickness of each of the bus bar electrodes 14c
and 14d, the energy trapping effect in the Y-axial direction will
be reduced.
[0102] However, in this preferred embodiment, the electrode films
17 and 18 defining the second layers are laminated to the bus bar
electrodes 14c and 14d, respectively. Accordingly, the acoustic
velocity of a surface acoustic wave propagating on the bus bar
electrodes 14c and 14d become lower by about 140 m/second.
[0103] As a result, the ratio Vs/vm, that is, the ratio of the
acoustic velocity Vs of the surface acoustic wave propagating on
the electrode finger meshing area to the propagation velocity Vm of
the surface acoustic wave propagating on the bus bar electrodes, is
increased. Thus, the energy trapping effect in the Y-axial
direction is increased.
[0104] FIG. 2 is a graph showing changes in the acoustic velocity
of a surface acoustic wave propagating on the bus bar electrodes
14c and 14d, obtained when the thickness of the whole of each of
the bus bar electrodes 14c and 14d is varied without the electrode
films 17 and 18 defining the second layers being formed. The
interdigital transducer and the reflectors are formed similarly to
the above series arm resonator 3 except that the thickness of each
bus bar electrode is varied.
[0105] As seen in FIG. 2, when the thickness of each bus bar
electrode is increased by about 0.01.lambda., in which .lambda. is
the wavelength of a surface acoustic wave, the acoustic velocity Vm
of the surface acoustic wave becomes lower by about 8.4
m/second.
[0106] In this preferred embodiment, the electrode films 17 and 18
defining the second layers are laminated to the bus bar electrodes
14c and 14d, so that the acoustic velocity Vm of a surface acoustic
wave propagating on the bus bar electrodes 14c and 14d becomes low.
Thereby, the ratio Vs/Vm is set to be less than about 1.
Accordingly, the energy trapping in the Y-axial direction can be
effectively carried out, and the loss in the filter characteristic
can be reduced, as seen in the results of FIG. 2.
[0107] In FIG. 4, the solid lines show the filter characteristic of
the surface acoustic wave device 1 of this preferred embodiment.
Moreover, in FIG. 4, the broken lines show the filter
characteristic of a comparative example of the surface acoustic
wave device configured similarly to that of this preferred
embodiment except that the electrode films defining the second
layers are not provided. The filter characteristic curves of the
insertion losses magnified on the scale shown on the right side of
the ordinate are depicted in the lower portion of FIG. 4.
[0108] In the surface acoustic wave device of this preferred
embodiment, the filter characteristic in the transmission band is
considerably improved, although the minimum of the insertion loss
is not changed substantially, as seen in FIG. 4. Probably, this is
because the multi-layer structures of the bus bar electrodes 14c
and 14d reduce the electrode resistance and moreover, significantly
improve the surface acoustic wave energy trapping effect.
[0109] In this embodiment, the 36.degree. Y-cut X-directional
propagation LiTaO.sub.3 substrate is used as the piezoelectric
substrate. However, a Y-cut X-directional propagation LiTaO.sub.3
substrate having another cut angle of about 33.degree. to about
46.degree., for example, may be used. In this case, similar
advantages can be obtained. Furthermore, other appropriate
piezoelectric single crystal substrates may be used.
[0110] In this preferred embodiment, the electrode films 17 and 18
defining the second layers are also preferably made of Al. The
electrode films defining the second layers may be made of a metal
material different than that used to form the first layers.
Moreover, as the electrode material, not only Al but also Al
containing alloys may be preferably used.
[0111] Moreover, both of the electrode films defining the first
layers and the electrode films defining the second layers may be
made of metal other than Al and an Al containing alloy. Moreover,
each electrode film itself defining the first layer may be a
multi-layer film including a plurality of metal films laminated
together.
[0112] In this preferred embodiment, each of the distances g from
the boundaries A between the bus bar electrodes 14c and 14d and the
electrode fingers to the edges B of the electrode films 17 and 18
defining the second layers on the sides of the electrode fingers
14a and 14b is preferably about 4 .mu.m, that is, about 0.8.lambda.
to about 0.9.lambda.. Thus, sufficient attention should be paid to
the gap length g to obtain a satisfactory energy trapping
effect.
[0113] When the gap length g of each of the series arm resonators
and the parallel arm resonators in the surface acoustic wave device
1 of this preferred embodiment is varied, the filter characteristic
is changed. The inventor of this patent application has discovered
that the energy trapping effect is changed when the gap length g
and the film thickness M of each of the electrode films 17 and 18
defining the second layers are varied. Basically, the trapping
effect can be obtained when each electrode film thickness of the
reflectors is larger than that of each bus bar. However, if the gap
length g is excessively larger, sufficient trapping effects can not
be obtained in some cases.
[0114] Thus, the bandwidth of the surface acoustic wave device was
investigated by changing the gap length g and the film thickness M
of each of the electrode films 17 and 18 defining the second
layers. FIGS. 5A and 5B, FIG. 6, and the following Table 2 show the
results.
[0115] FIG. 5A shows the results obtained when the film thickness
of each of the electrode films 17 and 18 defining the second layers
is about 840 nm (0.188.lambda.). FIG. 5B shows the results obtained
when the film thickness is about 560 nm (0.126.lambda.). FIG. 6
shows the results obtained when the film thickness is about 280 nm
(0.063.lambda.). As seen in FIGS. 5A, 5B, and 6, the bandwidth
tends to be reduced when the gap length g is increased. This
tendency becomes remarkable when the film thickness M is small.
[0116] For comparison, the bandwidths of the electrodes each having
no two-layer structure are shown by the broken lines in FIGS. 5A,
5B, and 6. As seen in FIGS. 5A, 5B, and 6, the bandwidths are
reduced to the same level as those of the electrodes each having no
two layer structure in the vicinity of the gap length g of about 8
.mu.m at a film thickness M of about 840 nm, in the vicinity of the
gap length g of about 6 .mu.m at a film thickness M of about 560
nm, and in the vicinity of the gap length g of about 4.5 .mu.m at a
film thickness M of about 280 nm, respectively.
[0117] These results are shown by the graph of FIG. 7 obtained by
the first-order approximation. The obtained approximation equation
is M.gtoreq.0.159 g-0.094.
[0118] The values of M and g are expressed by integral multiples of
.lambda..
[0119] Thus, a desirable energy trapping effect can be rendered to
the surface acoustic wave device by forming the device so as to
satisfy the formula of M.gtoreq.0.159 g-0.094. Thus, the bandwidth
of the device can be increased.
[0120] In the case in which the metal films defining the second
layers are made of metal excluding Al, the surface acoustic wave
device is constructed so as to satisfy the formula of
Ma.times.(d0/da).gtoreq.0.159 g-0.084, in which Ma is the metal
film thickness of each of the second layers, and do is the density
of Al.
[0121] FIG. 8 is a schematic plan view showing a one terminal-pair
surface acoustic wave resonator for use in a second preferred
embodiment of the present invention.
[0122] In the second preferred embodiment, a one terminal-pair
surface acoustic wave resonator 21 shown in FIG. 8 is preferably
used. The surface acoustic wave device 21 of the second preferred
embodiment is configured substantially similarly to that of the
first preferred embodiment, except that the one terminal-pair
surface acoustic wave resonator 21 is used as each of the series
arm resonators 3 and the parallel arm resonators 5 to 7.
Accordingly, the surface acoustic wave device of the second
preferred embodiment is a ladder type filter containing the two
series arm resonators and the three parallel arm resonators.
[0123] The one terminal-pair surface acoustic wave resonator 21
contains three interdigital transducers 23 to 25 arranged in the
surface acoustic wave propagation direction on a piezoelectric
substrate 22. In this preferred embodiment, the piezoelectric
substrate 22 is also formed of a 36.degree. Y-cut X-directional
propagation LiTaO.sub.3 substrate.
[0124] Grating reflectors 26 and 27 are arranged on both of the
sides in the surface acoustic wave propagation direction of the
area where the interdigital transducers 23 to 25 are provided.
[0125] The electrode finger meshing width in the interdigital
transducers 23 to 25 is preferably about 122 .mu.m. The number of
the electrode finger pairs of the interdigital transducer 24
arranged in the approximate center of the interdigital transducers
23 to 25 is preferably 18. The number of the electrode finger pairs
of each of the interdigital transducers 23 and 25 arranged on both
of the sides is preferably 11. The number of the electrode finger
pairs of each of the reflectors 26 and 27 is preferably 120. The
pitch between the electrode fingers in the interdigital transducers
23 to 25 is about 2.1 .mu.m. The wavelength of a surface acoustic
wave is about 4.2 .mu.m.
[0126] The interdigital transducers 23 to 25 and the reflectors 26
and 27 are made of Al. The film thickness of each of the
interdigital transducers 23 to 25 which are the electrode films
formed underneath the electrode films defining the second layers
described later is about 320 nm. That is, the film thickness of
each electrode finger is about 7.4% of the wavelength of the
surface acoustic wave. The duty of each of the interdigital
transducers 23 to 25 is about 0.72.
[0127] The interdigital transducers 23 to 25 include a plurality of
electrode fingers 23a, 23b, 24a, 24b, 25a, and 25b, and first and
second bus bar electrodes 23c, 23d, 24c, 24d, 25c, and 25d,
respectively. Also, in this preferred embodiment, the electrode
films 17 and 18 defining the second layers are laminated to the bus
bar electrodes 23c, 23d, 24c, 25c, and 25d. The areas where the
electrode films 17 and 18 defining the second layers are laminated
are hatched for illustration.
[0128] The electrode films 17 and 18 defining the second layers are
made of Al similarly to those of the first preferred embodiment,
and the film thickness thereof is preferably about 840 nm.
[0129] Each of the gap lengths g from the boundaries between the
bus bar electrodes and the electrode fingers to the edges on the
electrode finger sides of the electrode films 17 and 18 defining
the second layers is about 2 .mu.m, that is, about 0.5.lambda..
[0130] In FIG. 9, the solid lines represent the filter
characteristic of the surface acoustic wave device formed according
to this preferred embodiment. For comparison, the broken lines
represent the filter characteristic of the surface acoustic wave
device configured similarly to the surface acoustic wave device of
this preferred embodiment except that no second electrode films are
laminated on the bus bar portions. The characteristics shown in the
lower part of FIG. 9 are the loss insertions magnified on the scale
shown on the right side of the ordinate.
[0131] As seen in FIG. 9, the filter characteristic in the
bandwidth is significantly improved, although the minimum insertion
loss is not changed, by laminating the electrode films defining the
second layers according to this preferred embodiment, compared to
the surface acoustic wave device having no electrode films defining
the second layers.
[0132] That is, the film thickness of each electrode finger of the
interdigital transducers 23 to 25 is about 7.4% of the wavelength
of the surface acoustic wave, and the duty of each of the
interdigital transducers 23 to 25 is about 0.72. As described
above, the energy of the surface acoustic wave in the Y-axial
direction can not be trapped by the electrode films defining the
first layers only. However, such a filter characteristic as
represented by the broken lines in FIG. 9 can be obtained even when
the electrode films defining the first layers only are provided,
since the electrode finger meshing width of each interdigital
transducer is large, that is, about 30.lambda..
[0133] However, the laminated electrode films 17 and 18 defining
the second layers cause the filter characteristic to increase
considerably, as described above.
[0134] In particular, low-loss filter characteristics can be
obtained, provided that at least a part of the bus bar electrodes
have a thickness larger than that of each electrode finger, namely,
Vs/Vm>1 is satisfied, even if the conditions under which the
wave mode is substantially present in the Y axial-direction are not
satisfied, that is, the film thickness h1 of each electrode finger,
the electrode finger width L1, and the length L2 of the gap between
adjacent electrode fingers in the surface acoustic wave propagation
direction satisfy one of the formulae (1) to (6).
[0135] FIG. 10 is a schematic plan view of a surface acoustic wave
device according to a third preferred embodiment of the present
invention. A surface acoustic wave device 31 of the third preferred
embodiment is configured in the same manner as that of the first
preferred embodiment. Thus, similar parts are designated by the
same reference numerals. The repeated description is omitted by
invoking the relevant explanation of the first preferred
embodiment.
[0136] The present preferred embodiment is different from the first
preferred embodiment in that after the electrode arrangement shown
in FIG. 10 is formed, an SiO.sub.2 film (not shown) is formed so as
to have a thickness of approximately 500 nm by sputtering on the
overall upper surface of the piezoelectric substrate 2. Thereafter,
a resist is applied thereon excluding the series arm resonators 3
and 4, the parallel arm resonators 5 to 7, and the electrode pads 8
to 12. In this state, the SiO.sub.2 film on the electrode fingers
and the electrode pads is removed by etching. Thus, the reliability
of the electrical connection between bonding wires and the
electrode pads 8 to 12 can be secured since the SiO.sub.2 film on
the electrode pads 8 to 12 is removed therefrom.
[0137] The surface acoustic wave propagation velocity Vs in the
area where the electrode fingers are meshed with each other becomes
higher than the acoustic velocity of the surface acoustic wave
propagating on the bas bar electrodes having the SiO.sub.2 film
laminated thereto, since the SiO.sub.2 film on the electrode
fingers are removed. In other words, the ratio Vs/Vm becomes larger
than 1.
[0138] In particular, in this preferred embodiment, the SiO.sub.2
film 33 defining an insulating film is laminated to the overall
surfaces of the bus bar electrodes 32, as shown in the cross
sectional view of FIG. 11. Thus, the bus bar electrodes each have a
multi-layer structure. According to the present invention, an
insulating film excluding a metal film may be laminated when at
least a part of the bus bar electrodes are thicker compared to the
electrode fingers. In this case, the same advantages as those of
the first preferred embodiment can be also obtained, since the
acoustic velocity Vm of the surface acoustic wave propagating on
the bus bar electrodes becomes low.
[0139] In this preferred embodiment, the SiO.sub.2 film 33
preferably defines the insulating film. The thickness of the
insulating film is about 500 nm, which is about 11% of the
wavelength of the surface acoustic wave. The density of the
SiO.sub.2 film is about 2.21 g/cm.sup.3, and is slightly smaller
compared to the density of about 2.69 g/cm.sup.3, of the Al film
constituting the electrodes, since the SiO.sub.2 film is formed by
sputtering. On the other hand, the gap length g is small, namely,
about 0.1.lambda.. Accordingly, the energy trapping effect for the
surface acoustic wave in the Y-axial direction is as much as that
in the first preferred embodiment.
[0140] In FIG. 12, the solid lines represent the filter
characteristic of the surface acoustic wave device of the third
preferred embodiment, which is configured as described above. The
broken lines represent the filter characteristic of a surface
acoustic wave device for comparison configured in the same manner
as that of the third preferred embodiment except that no SiO.sub.2
film is formed. The filter characteristics in the lower part of
FIG. 12 are the insertion losses magnified on the scale shown on
the right side of the ordinate.
[0141] As seen in FIG. 12, the energy trapping effect is also
enhanced, due to the formation of the SiO.sub.2 film, in the
surface acoustic wave device of the third preferred embodiment.
Thus, desirable filter characteristics can be obtained.
[0142] In the third preferred embodiment, the insulating film on
the electrode fingers is removed, so that the SiO.sub.2 film is
formed as the insulating film on the bus bar electrodes only. The
insulating film may be formed on the electrode fingers, in which
the thickness of the insulating film on the electrode fingers is
smaller than that on the bus bar electrodes. In this case, the
acoustic velocity Vm of the surface acoustic wave propagating on
the bus bar electrodes can be controlled to be smaller than the
propagation velocity Vs of the surface acoustic wave propagating on
the electrode fingers by adjusting the difference in thickness
between the insulating films. Thus, the filter characteristic can
be improved similarly to that of the third preferred
embodiment.
[0143] Moreover, a film made of an appropriate insulating material
other than the SiO.sub.2 film can be used. For film-formation,
vapor deposition methods, CVD methods, and so forth can be
used.
[0144] Furthermore, similarly to the third preferred embodiment,
the insulating film may be formed on the bus bar electrodes which
is formed on the piezoelectric substrate having the electrode
arrangement formed thereon in the same manner as the second
preferred embodiment except that the electrode films 17 and 18
defining the second layers are not formed. In this case, the
velocity Vm can be controlled to be low similarly to the third
preferred embodiment, so that the surface acoustic wave energy can
be trapped.
[0145] The surface acoustic wave devices each having a ladder
circuit configuration are described in the first to third preferred
embodiments. According to various preferred embodiments of the
present invention, the energy trapping effect for the surface
acoustic wave in the Y-axial direction in a one terminal-pair
surface acoustic wave resonator is greatly improved, and thereby,
the filter characteristic of a filter, which is formed by using the
one terminal-pair surface acoustic wave resonator, can be greatly
improved, and so forth. Thus, the present invention may be applied
not only to surface acoustic wave filters each having a ladder
circuit configuration but also various types of surface acoustic
wave filters and surface acoustic wave resonators.
[0146] Next, an example of an antenna sharing device including the
surface acoustic wave filter according to various preferred
embodiments of the present invention will be described with
reference to FIG. 13.
[0147] FIG. 13 is a circuit diagram illustrating an antenna sharing
device of the present preferred embodiment. An antenna sharing
device 70 of this embodiment includes one pair of ladder filters 61
each of which is similar to the ladder surface acoustic wave filter
shown in FIG. 3 except that the number of stages is different from
that of the filter shown in FIG. 3. In particular, the input
terminals 62 of the respective ladder filters 61 are connected to
each other to define a first port 71. On the other hand, the output
terminals 63 of the respective ladder filters 61 are used as they
are to form the second and third ports of the antenna sharing
device of this preferred embodiment.
[0148] The antenna sharing device can be constructed to include one
pair of the ladder filters 61 as described above.
[0149] Moreover, communications equipment can be formed by using
the antenna sharing device described above. FIG. 14 shows an
example of such communications equipment.
[0150] A communications equipment apparatus 81 of this preferred
embodiment preferably includes the antenna sharing device 70 and
transmission-reception circuits 82 and 83. The first port 71 of the
antenna sharing device 70 is connected to an antenna 84. The output
terminals 63 and 63 constituting the second and third ports are
connected to the transmission-reception circuits 82 and 83,
respectively.
[0151] In the antenna sharing device 70, the one pair of the ladder
filters 61 are configured so as to have different transmission
bands, and thereby, the antenna 84 can be used as transmission and
reception antennas.
[0152] In the surface acoustic wave according to a preferred
embodiment of the present invention, the film thickness of each
electrode finger of the interdigital electrode is not less than
about 0.04.lambda.. Even if the device is in the condition that the
energy trapping effect tends to be reduced, the energy trapping
effect can be greatly improved, since at least a portion of the
first and second bus bar electrodes having a thickness larger than
that of each electrode finger, so that the acoustic velocity Vm of
a surface acoustic wave propagating on the bus bar electrodes
becomes low compared to the sound velocity Vs of the surface
acoustic wave propagating on the electrode fingers. Thus, the
surface acoustic wave device, if it is used in a surface acoustic
wave filter having a ladder circuit configuration, provides a low
loss filter characteristic.
[0153] In the surface acoustic wave according to another preferred
embodiment of the present invention, the ratio of L1/(L1+L2) is
satisfied, that is, the duty is not less than about 0.5. Even if
the device is in the condition that the energy trapping effect
tends to be reduced, the energy trapping effect is greatly
improved, since at least a portion of the first and second bus bar
electrodes having a thickness larger than that of each electrode
finger, so that the acoustic velocity Vm of a surface acoustic wave
propagating on each bus bar electrode becomes low compared to the
sound velocity Vs of the surface acoustic wave propagating on the
electrode fingers. Thus, the surface acoustic wave device, if it is
used in a surface acoustic wave filter having a ladder circuit
configuration, has a low loss filter characteristic.
[0154] According to another preferred embodiment of the present
invention, one of the above-described formulae (1) to (6) is
satisfied, and substantially, the conditions under which no wave
mode in the Y-axial direction is present are not satisfied
substantially. However, in this case, the acoustic velocity Vm of a
surface acoustic wave propagating on each bus bar electrode becomes
low compared to the sound velocity Vs of the surface acoustic wave
propagating on the electrode fingers, since at least a portion of
the first and second bus bar electrodes having a thickness larger
than that of the electrode fingers. Accordingly, the Vs/Vm>1 is
satisfied, so that the energy trapping in the Y-axial direction can
be performed. Thus, if the device is used in a filter, a low loss
filter characteristic can be obtained.
[0155] According to preferred embodiments of the present invention,
at least a portion of the bus bar electrode has a thickness larger
than that of each electrode finger. Various methods can be used to
increase the film thickness of each bus bar electrode. This can be
realized by forming at least a portion of the bus bar electrodes so
as to have a multi-layer structure including a plurality of films
laminated together.
[0156] In the case in which at least a portion of each bus bar
electrode has a multi-layer structure, the multi-layer structure
may be formed by laminating at least one electrode film on the
electrode film, or the multi-layer structure may be formed by
laminating an insulating film onto the electrode film. In the case
in which the plurality of electrode films are laminated to obtain
the multi-layer structure, the electrode film defining the lowest
layer is formed so as to be connected to the electrode fingers. In
the case in which the second layer and the proceeding layers are
made of a metal different from that of the electrode film defining
the lowest layer, the electrode film defining the lowest layer can
be formed by the same process for forming the electrode fingers.
Moreover, since the second layer and the proceeding layers are made
of the metal different from that of the electrode film defining the
lowest layer, the type of the metal can be selected so that a high
energy trapping effect can be obtained.
[0157] In the case in which at least one layer of the second layer
and the proceeding layers is made of a metal having a relatively
high density, compared to the electrode film defining the lowest
layer, a large mass-addition effect can be obtained, and thereby,
the acoustic velocity of the surface acoustic wave propagating on
the bus bar electrodes can be more reduced. For example, when the
first layer is made of Al or an Al containing alloy, a large energy
trapping effect for the surface acoustic wave can be obtained by
forming at least one layer of the second and the proceeding layers
by use of a metal having a relatively high density, such as Au, Ag,
W, Ti, Ni, or other suitable material.
[0158] Moreover, in the case in which at least one layer of the
electrode films defining the second and the proceeding layers has a
lower resistivity and a larger thickness compared to the electrode
film defining the lowest layer, the acoustic velocity of a surface
acoustic wave propagating on the bus bar electrode can be
effectively controlled to be low. Thus, the surface acoustic wave
energy trapping effect can be greatly improved. For example, the
energy trapping effect can be improved by forming at least one
layer of the second layer and the proceeding layers by use of Au,
Ag, Cu, or other suitable material, as described above.
[0159] In each bus bar electrode having a multi-layer structure, an
insulating film may be formed between electrode films constituting
the multi-layer structure so as to secure electrical connection
between the upper and lower electrode films. In this case, the
acoustic velocity of the surface acoustic wave propagating on the
bus bar electrode can be controlled to be low, due to the mass
addition effect of the insulation film. Thus, a desirable energy
trapping effect can be obtained.
[0160] In the surface acoustic wave device according to various
preferred embodiments of the present invention, the surface
acoustic wave energy trapping effect can be greatly improved, and
the bandwidth can be increased when M.gtoreq.0.159 g-0.094 or
Ma.times.(d0/da).gtoreq.0.159 g-0.094 is satisfied.
[0161] In the surface acoustic wave device according to another
preferred embodiment of the present invention, the film thickness
of the electrode fingers in the interdigital transducer is
preferably not less than about 0.04.lambda.. Under this condition,
the surface acoustic wave energy trapping in the Y-axial direction
can not satisfactorily be performed. However, since the insulating
film is formed on the bus bar electrode, the propagation velocity
Vm of the surface acoustic wave propagating on the bus bar
electrode becomes low, due to the mass addition effect of the
insulating film. Thus, a sufficient surface acoustic wave energy
trapping effect can be obtained. Thus, the surface acoustic wave
device, when it is used for a surface acoustic wave filter, can
provide a low loss filter characteristic.
[0162] Similarly, in the surface acoustic wave device according to
another preferred embodiment of the present invention, the duty is
not less than about 0.5. Since the insulating film is formed on the
bus bar electrode, the acoustic velocity of the surface acoustic
wave propagating on each bus bar electrode can be controlled to be
low, even if the surface acoustic wave device is under the
condition that the surface acoustic wave energy trapping effect in
the Y-axial direction can not sufficiently be performed. Thus,
Vs/Vm>1 is satisfied. Accordingly, the surface acoustic wave
energy tapping in the Y-axial direction can be performed similarly
to the surface acoustic wave device according to the fourth
preferred embodiment of the present invention. When the surface
acoustic wave device is used to define a filter, for example, a low
loss filter characteristic can be obtained.
[0163] In the surface acoustic wave device according to another
preferred embodiment of the present invention, one of the
above-described formulae (1) to (6) is satisfied. The condition
under which a wave mode in the Y-axial direction is present is not
substantially satisfied. Also, in this case, an insulating film is
formed on the bus bar electrode according to various preferred
embodiments of the present invention. Thus, the acoustic velocity
of the surface acoustic wave propagating on the bus bar electrode
is reduced. Thus, Vs/Vm>1 is satisfied. Accordingly, the surface
acoustic wave energy trapping in the Y-axial direction can be
performed similarly to the surface acoustic wave device according
to fourth or fifth preferred embodiments of the present invention.
When the surface acoustic wave device is used to define a filter,
for example, a low loss filter characteristic can be obtained.
[0164] In the surface acoustic wave device according to various
preferred embodiments of the present invention, insulating films
are disposed on the electrode fingers and the bus bar electrodes.
In the case in which the thickness of the insulating film provided
on each bus bar electrode is larger than that of the insulating
film formed on each bus bar electrode, the acoustic velocity Vm of
the surface acoustic wave propagating on the bus bar electrode
becomes low. Thus, the ratio Vs/Vm>1 is satisfied. The surface
acoustic wave in the Y-axial direction can be effectively trapped.
When the surface acoustic wave device is used to define a filter,
for example, a low loss filter characteristic can be obtained.
[0165] In the antenna sharing device including one of the surface
acoustic wave devices according to various preferred embodiments of
the present invention, the loss in the antenna sharing device is
minimized, since the surface acoustic wave loss is low.
[0166] Moreover, in the communication equipment including the
antenna sharing device of preferred embodiments of the present
invention, the overall loss in the communication equipment is
minimized, since the equipment includes the antenna sharing device
having a low loss as described above.
* * * * *