Surface acoustic wave device

Details Surface acoustic wave device Surface acoustic wave device

2000-03-23

2001-10-30

Dougherty, Thomas M. (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

C310S31300R

Reexamination Certificate

active

06310423

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 according to different purposes. In the following, an SAW filter will be described as one type of SAW devices
FIG. 11
is a perspective view schematically showing a structure of an SAW filter. Referring to
FIG. 11
, 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. 12
, 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 an SAW. The SAW propagates on the surface of piezoelectric substrate
12
. The strains caused by the SAW produces 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.
13
. 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
propa-
gation
Eulerian angles
velocity
K
2
TCD
propagation
substrate
&phgr;, &thgr;, &psgr;
[m/s]
[%]
[ppm/° C.]
mode
quartz
0°, 132.75°, 0°
3159
0.12
0
Rayleigh 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 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 delay time (TCD) is disadvantageously high.
Thus, the substrates have their own advantages and deficiencies, so that they are used according to the specific purposes of the device. Recently, with a technological development 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.
14
.
Referring to
FIG. 14
, 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 an SAW device is determined in accordance with the following equation.
f
0
=V/&lgr;
(V: propagation velocity of SAW, &lgr;: electrode pitch of IDT (FIG.
11
)) Thus, in producing devices having the same center frequency f
0
, if the IL substrate with relatively high propagation velocity V as compared with that of the quartz substrate or the like is used, electrode pitch &lgr; of IDT increases and the size of SAW device is increased.
It is commonly believed that with the higher K
2
a piezoelectric substrate has, the easier the design of a device with a large bandwidth is. However, the LN substrate with high K
2
is accompanied by 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 an SAW device which can be reduced in size by lowering a propagation velocity and which is provided with high K
2
and a good temperature characteristic.
After intensively studying to achieve the above mentioned object, the present inventors have found that a combination of a piezoelectric substrate including LN and a glass substrate makes it possible to reduce the size of an 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 LN 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 1.0.
The sound velocity of the glass substrate is lower than that of the piezoelectric substrate including LN. 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 is, the more significantly the glass substrate affects and the lower the sound velocity of the SAW is. If the thickness of the piezoelectric substrate is gradually increased, the sound velocity of the SAW gradually converges at that of the piezoelectric substrate.
When the glass substrate and the piezoelectric substrate are bonded, the sound velocity of the SAW can be lowered by adjusting the thicknesses of the substrates. Thus, electrode pitch &lgr; of the IDT is reduced and a smaller SAW device is achieved.
An electric field distribution varies with thickness H of piezoelectric substrate and 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 KH is set to at least 0.5 and at most 1.0. Thus, the SAW is efficiently excited, whereby K
2
is enhanced.
KH is set to at least 0.5 and at most 1.0 because if KH is below 0.5 or above 1.0, 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 to those of the piezoelectric substrate including LN. As to TCV, for example, the piezoelectric substrate including LN 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 intensively studying, the present inventors have found that setting Eulerian angles in a prescribed range could produce high K
2
and good temperature characteristic.
Accordingly, in the above described SAW device, LN is preferably (30°, 80° to 100°, 130° to 145°) in an Eulerian angles representation.
As a result, a substrate of the SAW device is obtained w

Affiliated with

Also associated with

Rating

Surface acoustic wave device does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Surface acoustic wave device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Surface acoustic wave device will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2591268