Surface acoustic wave device

Details Surface acoustic wave device Surface acoustic wave device

2002-10-31

2004-04-06

Dougherty, Thomas M. (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

Reexamination Certificate

active

06717327

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave device using a quartz substrate, and more particularly relates to a greatly improved a surface acoustic wave device using a surface acoustic wave substrate that is formed by laminating a piezoelectric thin film on a quartz substrate.
2. Description of the Related Art
In the past, surface acoustic wave devices have been widely used, for example, for bandpass filters of mobile communication devices. A surface acoustic wave (hereafter “SAW”) device has a structure in which at least one interdigital transducer (hereafter “IDT”) composed of at least one pair of comb electrodes is formed so as to contact the piezoelectric body.
Furthermore, various types of SAW devices using a piezoelectric thin film have also been proposed in recent years. Specifically, SAW devices using a surface acoustic wave substrate composed of a piezoelectric thin film formed on an elastic substrate such as a glass substrate and a piezoelectric substrate have been proposed.
The four types of structures shown in FIGS.
22
(
a
),
22
(
b
) and
23
(
a
), and
23
(
b
) are known as structures using a surface acoustic wave substrate formed by laminating the above-mentioned piezoelectric thin film and elastic substrate. Specifically, in the SAW device
101
shown in FIG.
22
(
a
), a piezoelectric thin film
103
is formed on an elastic substrate
102
, and IDTs
104
are formed on the piezoelectric thin film
103
, while in the SAW device
105
shown in FIG.
22
(
b
), the IDTs
104
are formed on the lower surface of the piezoelectric thin film
103
, i.e., in the interface between the elastic substrate
102
and the piezoelectric thin film
103
.
Furthermore, in the SAW device
106
shown in FIG.
23
(
a
), a short-circuiting electrode
107
is formed on the elastic substrate
102
, and the piezoelectric thin film
103
is laminated on top of this short-circuiting electrode
107
. The IDTs
104
are formed on the piezoelectric thin film
103
. In other words, the structure of the SAW device
106
corresponds to the structure of the SAW device
101
shown in FIG.
22
(
a
) with the short-circuiting electrode
107
inserted in the interface between the elastic substrate
102
and the piezoelectric thin film
103
.
In the SAW device
108
shown in FIG.
23
(
b
), the short-circuiting electrode
107
is formed on the piezoelectric thin film
103
. Furthermore, the IDTs
104
are formed in the interface between the elastic substrate
102
and the piezoelectric thin film
103
. Therefore, the structure of the SAW device
108
corresponds to the structure of the SAW device
105
shown in FIG.
22
(
b
) with the short-circuiting electrode
107
formed on the upper surface of the piezoelectric thin film
103
.
FIG. 24
shows the electromechanical coupling coefficients of the above-mentioned SAW devices
101
,
105
,
106
, and
108
in a case where the structures of these devices are only differentiated by the formation position of the IDTs
104
and the presence or absence of the short-circuiting electrode
107
, and other structures are kept the same, with a ZnO thin film used as the piezoelectric thin film, and a glass substrate used as the elastic substrate.
FIG. 24
illustrates changes in electromechanical coupling coefficients with respect to the normalized thickness H/&lgr; of the ZnO thin film in the above-mentioned four types of SAW devices. In the present specification, H indicates the thickness of the piezoelectric thin film, and &lgr; indicates the wavelength of the surface acoustic wave to be excited (units are the same in both cases).
Furthermore, the solid line A indicates the results for the SAW device
101
, the broken line B indicates the results for the SAW device
105
, the one-dot chain line C indicates the results for the SAW device
106
, and the two-dot chain line D indicates the results for the SAW device
108
.
As is clearly seen from
FIG. 24
, larger electromechanical coupling coefficients can be obtained with the SAW devices
105
and
108
than with the SAW devices
101
and
106
by selecting H/&lgr;.
Accordingly, it has conventionally been thought that larger electromechanical coupling coefficients can be obtained when the IDTs
104
are formed in the interface between the glass substrate
102
and the ZnO thin film
103
in a structure in which the ZnO thin film
103
is formed on the glass substrate
102
. Furthermore, the waves indicated as Sezawa waves in
FIG. 24
are a higher-order mode of surface acoustic waves of the Rayleigh type.
In addition, various characteristics of the surface acoustic wave in the case of using a surface acoustic wave substrate in which a ZnO thin film is formed on a quartz substrate are described by the present inventors in IEEE ULTRASONICS SYMPOSIUM (1997), pp. 261-266 and in the research data from the 59th Acoustic Wave Device Technology No. 150 Committee Meeting (1998) of Japan Society for the Promotion of Science, pp. 23-28 (hereinafter referred to as “Reference
1
”). These characteristics are described with reference to FIGS.
25
(
a
),
25
(
b
), and
26
. In this prior art, it is theoretically and experimentally confirmed that a surface acoustic wave substrate with the temperature coefficient of frequency (TCF) of zero can be obtained by forming a ZnO thin film that has a negative value of the TCF on a quartz substrate with a cut angle and propagation direction which are such that the TCF has a positive value.
Furthermore, the theory in this Reference
1
is based on IEEE Trans. Sonics & Ultrasonic. Vol. SU-15, No. 4 (1968), page 209.
FIG.
25
(
a
) shows the ZnO film thickness dependence of the TCF of the SAW device shown in FIG.
22
(
a
) using the quartz substrate described in Reference
1
mentioned above, which is made of a 29°45′ rotated Y-cut 35° X propagating plate, and which has the Euler angles of (0°, 119°45′, 35°). FIG.
25
(
b
) shows the ZnO film thickness dependence of the TCF of the SAW device shown in FIG.
22
(
a
) using the quartz substrate described in Reference
1
mentioned above, which is made of a 42°45′ rotated Y-cut 35° X propagating plate, and which has the Euler angles of (0°, 132°45′, 35°). Furthermore,
FIG. 26
shows the electromechanical coupling coefficients of the Rayleigh waves and the Sezawa waves constituting the spurious waves of the SAW devices that use a ZnO thin film as the piezoelectric thin film and a quartz substrate as the elastic substrate. The solid lines A through C in
FIG. 26
indicate the electromechanical coupling coefficients of the Rayleigh waves in the SAW device structures shown in FIGS.
22
(
a
),
22
(
b
), and
23
(
a
), respectively, while the broken lines A″, C″, D″ indicate the changes in the electromechanical coupling coefficients of the Sezawa waves constituting the spurious waves in the SAW devices having the structures shown in FIGS.
22
(
a
),
23
(
a
), and
23
(
b
), respectively.
It is seen from FIGS.
25
(
a
) and
25
(
b
) that the TCF becomes zero by selecting the normalized thickness of the ZnO film in the SAW device of FIG.
22
(
a
).
Table 1 below shows the comparison between the SAW device of FIG.
22
(
a
) (Al/ZnO/quartz laminated structure), which is described in the above-mentioned prior art, and a conventionally known SAW device having a favorable TCF.
TABLE 1
Frequency-
Acoustic
Temperature
Euler Angles
Velocity
K
2
Deviation at −20
Substrate
of Substrate
(m/s)
(%)
to 80° C. (ppm/° C.)
Al/ST-X
(0°,
3158
0.14
0.9
quartz
132° 45′, 0°)
Al/
(0°, 143°,
2756
0.42
1.63
La
3
Ga
5
SiO
14
24°)
Al/Li
2
B
4
O
7
(110°, 90°,
3480
1
6.8
90° )
Al/ZnO/
(0°,
2900
1.0-
1.1
quartz
119°45′,
1.1
35°)
(0°,
132°45′,
35°)
It is seen from FIG.
26
and Table 1 that with the SAW device of FIG.
22
(
a
), approximately 1% of the electromechanical coupling coefficient K
2
is obtained, which is larger than in the case of the ST-X quartz substrate or La
3
Ga
5
SiO
14
substrate, and the acoustic velocity is lower by about 2

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