Surface acoustic wave device

Details Surface acoustic wave device Surface acoustic wave device

2000-08-16

2002-08-06

Budd, Mark (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

Reexamination Certificate

active

06429570

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a surface acoustic wave device having an inter-digital electrode on a single crystal substrate, and, in particular, such a device enabling miniaturization, band-widening and excellent selectivity, namely, superior temperature characteristics.
DESCRIPTION OF THE PRIOR ART
In recent years, various kinds of mobile communication terminal, devices inclusive of cellular telephones, have come into widespead use. It is highly desirable to reduce this kind of terminal equipment in size and weight for enhanced portability.
In order to reduce the size and weight of terminal devices, their electronic parts must be substantially reduced in size and weight. For this reason, surface acoustic wave devices enabling size and weight reduction, namely, surface acoustic wave filters, are often used for high- and intermediate-frequency parts of terminal devices. Such devices are formed with an inter-digital electrode for exciting, receiving, reflecting and propagating surface acoustic waves on the surface of a piezoelectric substrate thereof.
Among characteristics important to a piezoelectric substrate used for surface acoustic wave devices are surface wave velocity (SAW velocity), temperature coefficient of center frequency in the case of filters or of resonant frequency in the case of resonators (the temperature coefficient of frequency: TCF), and electromechanical coupling factor (k
2
). The characteristics of typical piezoelectric substrates currently known for surface acoustic wave devices are set forth below in Table 1. For details regarding these characteristics, reference should be made to Yasutaka SHIMIZU, “Propagation characteristics of SAW materials and their current application”, the Transactions of The Institute of Electronics, Information and Communication Engineers A, Vol. J76-A, No.2, pp. 129-137 (1993). Hereinafter, the piezoelectric substrates for surface acoustic wave devices are referred to using the designations in Table 1.
TABLE 1
Propagation
SAW Velocity
TCF
Symbol
Composition
Cut Angle
Direction
(m/s)
k
2
(%)
(ppm/° C.)
128LN
LiNbO
3
128° -Rotated Y
X
3880~3920
5.6
−74
64LN
LiNbO
3
64° -Rotated Y
X
4330~4360
11
−79
LT112
LiTaO
3
X
112° -Rotated Y
3220~3260
0.72
−18
36LT
LiTaO
3
36° -Rotated Y
X
4100~4160
5.0
−45
ST Quartz
Quartz
ST
X
3130~3155
0.17
0
Crystal
Crystal
(first-order coeff.)
BGO
Bi
12
GeO
20
(100)
(011)
1681~1720
1.2
−122
As can be seen from Table 1, 64LN and 36LT have an SAW velocity of 4000 m/s or higher, and as described later, 64LN and 36LT are suitable to construct filters for high-frequency parts of mobile communication terminal devices.
Various systems are practically employed all over the world for mobile communications devices, typically cellular telephones, and are all used at frequencies of the order of 1 GHz. Therefore, filters used for high-frequency parts of terminal devices have a center frequency of approximately 1 GHz. A surface acoustic wave filter has a center frequency substantially proportional to the SAW velocity of the piezoelectric substrate used and almost inversely proportional to the width of electrode fingers formed on the substrate. To enable such filters to be operated at high frequencies, therefore, it is preferable to utilize substrates having high SAW velocities, for instance, 64LN and 36LT.
Also, a wide passband width of 20 MHz or more is required for filters used as high-frequency parts. To achieve such wide passband, however, it is essential for the piezoelectric substrate to have a large electromechanical coupling factor k
2
. For these reasons, much use is made of 64LN and 36LT.
On the other hand, mobile communication terminal devices use an intermediate frequency in the 70 to 300 MHz band. When a filter having a center frequency in this frequency band is constructed using a surface acoustic wave device, if the aforementioned 64LN or 36 LT is used as the piezoelectric substrate, the widths of the electrode fingers formed on the substrate have to be much larger than those of the aforementioned filter used as a high-frequency part.
More specifically, the following equation (1) roughly applies to the relationship among the width d of an electrode finger of a surface acoustic wave transducer that forms a surface acoustic wave filter, the center frequency f
0
of the surface acoustic wave filter, and the SAW velocity V of the piezoelectric substrate used.
f
0
=V/
(
4
d
)  (1)
If a surface acoustic wave filter having a center frequency of 1 GHz is constructed on the assumption that the SAW velocity is 4000 m/s, the width of the electrode finger thereof is calculated from the equation (1) to be
d=
4000(
m/s
)/(4×1000(MHz))=1 &mgr;m
On the other hand, when an intermediate-frequency filter having a center frequency of 100 MHz is constructed using this piezoelectric substrate having an SAW velocity of 4000 m/s, the width of the electrode finger required for this is given by
d=
4000(m/s)/(4×100(MHz))=10 &mgr;m
Thus, the required width of the electrode finger is ten times as large as that for the high-frequency part filter. A large width of the electrode finger means that the surface acoustic wave intermediate-frequency filter itself becomes large. Therefore, in order to make a surface acoustic wave intermediate-frequency filter small, it is necessary to use a piezoelectric substrate having a low SAW velocity V as can be appreciated from the equation (1).
BGO referred to in Table 1 is known as a piezoelectric substrate having a very low SAW velocity. However, since the temperature coefficient of frequency TCF of a BGO piezoelectric substrate is extremely large, the BGO piezoelectric substrate is not suitable for constructing an intermediate-frequency filter for extracting one channel signal alone. This is because a large TCF value means that the center frequency of the surface acoustic wave filter varies greatly with temperature. Thus, a large TCF is unsuitable for an intermediate-frequency filter because undesired signals may possibly be extracted from another channel adjacent to the desired channel.
As described above, one problem with the conventional surface acoustic wave device is that in the case where a piezoelectric substrate having a large electromechanical coupling factor such as 64LN and 36LT is used, it is possible to make the passband thereof wide but the device size becomes large since the substrate has a high SAW velocity. Another problem is that when the aforementioned BGO substrate having a low SAW velocity is used to achieve device size reduction, sufficient selectivity cannot be obtained because the absolute value of temperature coefficient of frequency TCF is too large. In either case, characteristics sufficient for any intermediate-frequency surface wave acoustic filter cannot be achieved.
On the other hand, ST quartz crystal referred to in Table 1 is known as a piezoelectric substrate having a relatively low SAW velocity. Although the effective SAW velocity of a piezoelectric substrate is influenced by the structure of the electrode finger formed on the substrate, it is known that the SAW velocity of ST quartz crystal is generally 3130 to 3155 m/s and since this value is approximately three-fourths the SAW velocity of 64LN or 36 LT, ST quartz crystal is suitable for miniaturization.
In view of these facts, most conventional intermediate-frequency surface acoustic wave devices for mobile communication terminal devices are constructed using an ST quartz crystal piezoelectric substrate.
However, as apparent from Table 1, the electromechanical coupling factor k
2
of ST quartz crystal is 0.17%, particularly small among piezoelectric substrates. Small k
2
means that only a filter having a narrow passband is achievable.
Nevertheless, main devices adopted so far for mobile communication, namely, cellular telephones, are analog systems with a very narrow channel width of, for instance, 12.5 kHz according to the Japanese NTT standard, 30 kHz according to the U.S. AMPS standard

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