Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Patent
1998-06-17
2000-09-19
Ramirez, Nestor
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
310360, H01L 4104
Patent
active
061217131
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave device comprising an interdigital electrode on a single crystal substrate.
2. Discussion of the Background
In recent years, mobile communication terminal equipment inclusive of cellular telephones has been rapidly popularized. This terminal equipment is particularly desired to be reduced in size and weight for reason of portability. To achieve size and weight reductions for the terminal equipment, electronic parts used therewith, too, should be essentially reduced in size and weight. For this reason, surface acoustic wave devices favorable for size and weight reductions, i.e., surface acoustic wave filters are often used for high- and intermediate-frequency parts of the terminal equipment. A surface acoustic wave device has on the surface of a piezoelectric substrate an interdigital electrode for exciting, receiving, reflecting, and propagating surface acoustic waves.
Among characteristics important to a piezoelectric substrate used for surface acoustic wave devices, there are the surface wave velocity of surface acoustic waves (SAW velocity), the temperature coefficient of a center frequency in the case of filters and of a resonance frequency in the case of resonators (the temperature coefficient of frequency: TCF), and an electromechanical coupling factor (k.sup.2). Set out in Table 1 are the characteristics of various piezoelectric substrate known so far for surface acoustic wave devices. Hereinafter, these piezoelectric substrates will be referred to by the symbols used in Table 1. In this regard, it is to be noted that TCV (the temperature coefficient of SAW velocity) is a quantity representing the temperature dependence of the velocity of surface acoustic waves (the SAW velocity); that is, it has a value equivalent to that of the aforesaid TCF representing the temperature dependence of the center or resonance frequency. A large TCV value implies that the center frequency of a surface acoustic wave filter varies significantly with temperature.
TABLE 1 __________________________________________________________________________
Propagation
SAW Velocity
k.sup.2
TCV
Symbol
Composition
Cut Angle
Direction
(m/s)
(ppm/.degree. C.)
__________________________________________________________________________
128LN
LiNbO.sub.3
128.degree.-Rotation Y
X 3992 5.5
-74
64LN 64.degree.-Rotation Y
X -79.3
LT112
3288degree.-Rotation Y
0.64
-18
36LT 36.degree.-Rotation Y
X -45.7
ST Crystal
Quartz
0 (primary coef.)
BGO (100) GeO.sub.20
(011)
-1222
__________________________________________________________________________
As can be seen from Table 1, 64LN and 36LT have an SAW velocity of 4,000 m/s or higher, and so are suitable to construct filters for high-frequency parts of terminal equipment. Referring now to the reason for this, various systems are practically employed for mobile communications represented by cellular telephones all over the world, and are all used at frequencies of the order of 1 GHz. Accordingly, filters used for high-frequency parts of terminal equipment have a center frequency of approximately 1 GHz. Surface acoustic wave filters have a center frequency substantially proportional to the SAW velocities of piezoelectric substrates used but almost inversely proportional to the widths of electrode fingers formed on Ad substrates. To enable such filters to be operated at high frequencies, therefore, it is preferable to resort to substrates having high SAW velocities, for instance, 64LN, and 36LT. Also, wide passband widths of 20 MHz or more are required for filters used on high-frequency parts. To achieve such broad passbands, however, it is essentially required that piezoelectric substrates have a large electromechanical coupling factor k.sup.2. For these reasons, much use is made of 64LN, and 36LT.
On the other hand, a frequency band of 70 to 300 MHz is used as an intermediate frequency for mobile te
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Inoue Kenji
Sato Katsuo
Medley Peter
Ramirez Nestor
TDK Corporation
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