Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2001-09-27
2003-07-15
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06593679
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave element, an electronic circuit, a frequency filter, a frequency oscillator and an electronic apparatus, and more specifically, to a surface acoustic wave element which uses a piezoelectric thin film on a sapphire single crystal substrate, and an electronic circuit, a frequency filter and a frequency oscillator provided with said surface acoustic wave element, and an electronic apparatus equipped therewith.
2. Description of Related Art
Accompanying the remarkable progress of the telecommunications field focusing primarily on cellular telephones and other mobile communications, the demand for surface acoustic wave elements is increasing rapidly. In addition, accompanying the development of an information society in recent years, the demand for not only mobile communication devices, but also for tuners capable of receiving broadcast satellite (BS) and commercial satellite (CS) broadcasts is also growing, and the proliferation of computer networks has accelerated the pace of HUB and other electrical equipment.
During the development of surface acoustic wave elements, it is important to make improvements in the three areas of compact size, high efficiency and high frequency, and in order to accomplish this, it is necessary to realize a larger electromechanical coupling coefficient (to be abbreviated as k
2
), a central frequency temperature coefficient that is closer to zero (to be abbreviated as TCF) and a larger surface acoustic wave propagation velocity (to be abbreviated as V
s
).
In the past, a structure in which an inter-digital transducer (to be abbreviated as IDT) was formed on a piezoelectric single crystal was primarily used as a surface acoustic wave element. Typical examples of piezoelectric single crystals include quartz crystal, lithium niobate (to be abbreviated as LiNbO
3
) and lithium tantalate (to be abbreviated as LiTaO
3
). For example, in the case of RF filters that require a broader band and reduced loss of the pass band, LiNbO
3
having a large k
2
is used. On the other hand, in the case of IF filters requiring stable temperature characteristics even in a narrow band, quartz crystal having a small TCF is used. Moreover, LiTaO
3
, in which k
2
and TCF are between those of LiNbO
3
and quartz crystal, is used in intermediate applications.
However, since elements that satisfy all requirements of a high k
2
, zero TCF and high V
s
cannot be realized with piezoelectric single crystal alone, a surface acoustic wave element has been proposed that uses a laminated structure including a substrate and piezoelectric thin film. This element combines a substrate and thin film material, and allows the obtaining of characteristics superior to single crystal by controlling the orientation direction of the thin film and film thickness. For example, by forming a piezoelectric thin film having a large k
2
on a substrate having a TCF near zero, it is possible to realize a surface acoustic wave element for which the overall characteristics include a high k
2
and zero TCF.
Here, in the case of considering the use of a surface acoustic wave element as an oscillator in particular, although specific examples of the oscillators include high order increase types, phase synchronization types, and direct oscillation types, the direct oscillation types are desirable in order to reduce the size of the element and improve jitter. Therefore, it is important to increase the oscillation frequency. In the case of increasing the oscillation frequency, an element structure that realizes a higher frequency by laminating a piezoelectric layer with a hard layer having a high V
s
is promising even in consideration of limits on the design rule of the electrode pitch of the IDT. In Japanese Unexamined Patent Application, First Publication No. Hei 6-164294, a diamond film is formed as a hard layer on an Si substrate, onto which a ZnO piezoelectric layer and SiO
2
protective layer are additionally formed, by which an extremely important structure satisfying all of the requirements of high k
2
, zero TCF and high V
s
can be obtained.
However, the surface acoustic wave elements having the hard layer and the piezoelectric layer of the prior art had the following problems.
In the case of using the diamond thin film hard layer described in Japanese Unexamined Patent Application, First Publication No. Hei 6-164294, although high V
s
can be obtained, surface flatness cannot be obtained. Under the present circumstances, in addition to the polishing process being difficult and requiring considerable time due to the hardness of the diamond thin film, since the diamond film is polycrystalline, surface flatness is considerably inferior to that of ordinary piezoelectric single crystal substrates. Thus, the crystallinity of the piezoelectric thin film deposited on the diamond thin film also decreases, thereby preventing the obtaining of an epitaxial film. The characteristics of the piezoelectric thin film as surface acoustic wave elements are greatly dependent on its crystallinity, and in the case of using the polycrystalline diamond thin film hard layer, problems occur including a decrease in k
2
in particular.
Thus, at the time of practical application, it is important to attempt to increase V
s
by a single crystal hard layer other than diamond. Sapphire is one example of a hard layer and a substrate material other than diamond which is a single crystal, is easily obtainable, and has a large V
s
. Although the V
s
of sapphire is inferior to that of diamond, which has a V
s
of about 10000 m/s, its V
s
is still large, and its single crystal substrate is inexpensive and is used universally. Since various types of piezoelectric thin films also demonstrate epitaxial growth on such substrates, it is an extremely important material. Here, since the propagation velocity of a surface acoustic wave (V
s
) is Vs ∝ (E/&rgr;)
½
when Young's modulus is represented with E and density is represented with &rgr;, if it were possible to increase the Young's modulus and decrease the density of sapphire through the use of additives, it would be possible to increase its propagation velocity to a level comparable to that of diamond.
SUMMARY OF THE INVENTION
The present invention is accomplished in view of the above circumstances, and aims at providing a surface acoustic wave element which uses a sapphire hard layer having a large Vs and which is able to accommodate higher frequencies of the band used, and providing an electronic circuit, a frequency filter, and a frequency oscillator, each of which is provided with said surface acoustic wave element, and an electronic apparatus equipped therewith.
In order to solve the above problems, the present invention provides a surface acoustic wave element including a sapphire single crystal substrate, a hard layer formed on said sapphire single crystal substrate and having a composition containing (Al
1-x
M1
x
)
2
O
3
(0≦x≦0.5)) in which at least one element M1 (M1=B, Ga, In, Ti, V, Cr, Mn, Fe, Co) is added to sapphire, and a piezoelectric layer formed on said hard layer.
According to the above composition, Vs of the surface acoustic wave is increased by either increasing Young's modulus or decreasing density of the hard layer, thereby enabling the surface acoustic wave element to be used in high frequency regions.
In addition, the above hard layer can have a corundum crystal structure.
According to this composition, as a result of conforming the lattice to a single crystal substrate in which the hard layer has the same crystal structure, the crystallinity of the thin film is improved and k
2
is improved, thereby making it possible to conserve the electrical power of the surface acoustic wave element.
In addition, the above piezoelectric layer can have a composition containing (Al
1-x
B
x
)N (0≦x≦0.5) in which boron nitride is added to aluminum nitride, and also can have a wurtzite crystal structure.
According to this compos
Higuchi Takamitsu
Iwashita Setsuya
Miyazawa Hiromu
Dougherty Thomas M.
Oliff & Berridg,e PLC
Seiko Epson Corporation
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