Surface acoustic wave device

Details Surface acoustic wave device Surface acoustic wave device

2000-03-23

2001-10-30

Ramirez, Nestor (Department: 2839)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

C310S31300R

Reexamination Certificate

active

06310424

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to surface acoustic wave (SAW) devices.
2. Description of the Background Art
SAW devices include SAW oscillators, transversal SAW filters and oscillator type SAW filters, which are used for different purposes. In the following, a SAW filter will be described as one type of a SAW device.
FIG. 17
is a perspective view schematically showing a structure of a SAW filter. Referring to
FIG. 17
, SAW filter
15
basically includes a piezoelectric substrate
12
, and a four-terminal structure formed on a surface of piezoelectric substrate
12
and having two pairs of comb-shaped electrodes
13
respectively used for exciting and receiving surface waves. Such electrodes
13
are called interdigital electrodes, and this type of transducer is called an IDT (Interdigital Transducer).
Referring to
FIG. 18
, generally, when an impulse voltage is applied to comb-shaped electrodes
13
for excitation, strains of opposite phases are caused between adjacent electrodes
13
by a piezoelectric effect, thereby exciting a SAW. The SAW propagates on the surface of piezoelectric substrate
12
. The strains caused by the SAW produce electric charges at the surface of piezoelectric substrate
12
, which are, in turn, received as electric signals by comb-shaped electrodes
13
for reception.
Conventionally, the SAW device such as SAW filter
15
has a structure in which electrodes
13
in accordance with the purpose of the device are formed on the surface of piezoelectric substrate
12
, as shown in FIG.
19
. The characteristic of SAW device
15
depends largely on the characteristic of piezoelectric substrate
12
, which is also used according to the purpose of the device. Table 1 shows typical materials used for piezoelectric substrate
12
and characteristics of an SAW propagating on piezoelectric substrate
12
.
TABLE 1
Characteristics of Substrate for Typical SAW device
propagation
propa-
Eulerian angles
velocity
K
2
TCD
gation
substrate
&phgr;, &thgr;, &PSgr;
[m/s]
[%]
[ppm/° C.]
mode
quartz
 0°, 132.75°,
3159
0.12
0
Rayleigh
 0°
wave
 0°, 15°, 0°
3948
0.11
0
Leaky
wave
LiTaO
3
90°, 90°, 112°
3328
1.1
16.5
Rayleigh
wave
 0°, 126°, 0°
4211
4.7
45.1
Leaky
wave
LiNbO
3
 0°, 38°, 0°
4007
5.2
71.4
Rayleigh
wave
 0°, 154°, 0°
4731
10.9 
61.3
Leaky
wave
As shown in Table 1, a quartz substrate has a good temperature characteristic (value near zero), but the electromechanical coupling factor (K
2
) is disadvantageously small. For an LiNbO
3
(LN) substrate, although K
2
is sufficiently high, the temperature characteristic such as a temperature coefficient of a delay time (TCD), is disadvantageously high. Substrates using LiTaO
3
(LT) conventionally include an X-112° Y LT substrate (LT of 90°, 90°, 112°) in an Eulerian angles representation). The conventional LT substrate has a characteristic intermediate between those of a quartz substrate and the LN substrate.
Thus, the substrates have their own advantages and deficiencies, so that they are used according to the specific purposes of the device. Recently, with technological developments in the field of display units including televisions and telecommunication apparatuses including portable telephones, SAW devices used therefor are also required to have enhanced properties.
Now, Eulerian angles in the above Table 1 will be described with reference to FIG.
20
.
Referring to
FIG. 20
, the X axis is rotated by an angle &phgr; toward the Y axis about the Z axis, and the axis obtained is defined as the first axis.
Then, the Z axis is rotated counterclockwise by an angle &thgr; about the first axis, and the axis obtained is defined as the second axis. A material is cut in accordance with a surface orientation along a plane including the first axis and having the second axis as the normal, and used as a substrate. On the substrate which has been cut in accordance with the above mentioned surface orientation, the first axis is rotated counterclockwise by &psgr; about the second axis, and newly defined as the third axis. The third axis corresponds to a direction in which the SAW propagates. An axis orthogonal to the third axis on the plane is defined as the fourth axis. Thus, the Eulerian angles (&phgr;, &thgr;, &psgr;) are defined.
A center frequency f
0
of a SAW device is determined in accordance with the following equation:
f
0
=V/&lgr;
(V: propagation velocity of the SAW, &lgr;: electrode pitch of the IDT (FIG.
17
))
Thus, in producing devices having the same center frequency f
0
, if the LT substrate with relatively high propagation velocity V as compared with that of the quartz substrate or the like, the electrode pitch of the IDT &lgr; increases and the size of the SAW device is increased.
It is commonly believed that the higher K
2
a piezoelectric substrate has, the easier is the design of a device with a large bandwidth. However, the LN substrate with a high K
2
is accompanied by a high TCD, and is not suitable for a device which should have a good temperature characteristic.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a SAW device with a high K
2
and a good temperature characteristic and which can be reduced in size by lowering a propagation velocity.
After intense study to achieve the above mentioned object, the present inventors have found that a combination of a piezoelectric substrate, including LT, and a glass substrate makes it possible to reduce the size of a SAW device, and that high K
2
and a good temperature characteristic could be obtained if electrode pitch &lgr; and a thickness H of a piezoelectric substrate are set in a prescribed range.
Accordingly, the SAW device of the present invention is provided with a glass substrate, a piezoelectric substrate including LT formed on the glass substrate, and an electrode formed on the piezoelectric substrate. If an electrode pitch is &lgr;, a thickness of the piezoelectric substrate is H, and K equals to 2&pgr;/&lgr;, then a product of K and H (KH) is at least 0.5 and at most 1.5.
The sound velocity of the glass substrate is less than that of the piezoelectric substrate including LT. If the glass substrate and the piezoelectric substrate are combined, the piezoelectric substrate is affected by the glass substrate, whereby the sound velocity of the SAW decreases. For example, the thinner the piezoelectric substrate, the more significant is the effect of the glass substrate and the lower the sound velocity of the SAW. If the thickness of the piezoelectric substrate is gradually increased, the sound velocity of the SAW gradually converges to that of the piezoelectric substrate.
When the glass substrate and the piezoelectric substance are bonded, the sound velocity of the SAW can be lowered by adjusting the thicknesses of the substrates. Thus, the electrode pitch &lgr; of the IDT is reduced and a smaller SAW device is achieved.
An electric field distribution varies with thickness H of the piezoelectric substrate and the electrode pitch &lgr;. Here, the electric field distribution can be adjusted to concentrate in the piezoelectric substrate as the glass substrate and the piezoelectric substrate are combined and HK is set to at least 0.5 and at most 1.5. Thus, the SAW is efficiently excited, whereby K
2
is enhanced.
KH is set to at least 0.5 and at most 1.5 because if KH is below 0.5 or above 1.5, the sound velocity of the SAW increases, whereby it becomes difficult to reduce the size of the SAW device.
The signs of TCD and TCV (temperature coefficient of sound velocity (propagation velocity) of the glass substrate are opposite those of the piezoelectric substrate including LT. As to TCV, for example, the piezoelectric substrate including LT has a plus value, whereas the glass substrate has a minus value. Thus, if these substrates are bonded together, TCDs and TCVs thereof are mutually cancelled and the temperature characteristic is enhanced.
After intense study, the present inventors have found that setting E

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