Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2002-12-27
2004-11-23
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S364000, C310S31300R
Reexamination Certificate
active
06822371
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to surface acoustic wave devices, such as surface acoustic wave resonators or surface acoustic wave filters, and manufacturing methods therefor, and more particularly, relates to an electrode structure of a surface acoustic wave device and a forming method therefor.
2. Description of the Related Art
As has been well known, surface acoustic wave devices are electronic elements including a surface acoustic wave in which mechanical vibration energy is concentrated only in the vicinity of surfaces of a solid material and is then propagated. In addition, the surface acoustic wave devices are each generally composed of a piezoelectric substrate having piezoelectric properties and electrodes, such as interdigital electrodes and/or grating electrodes, disposed on the piezoelectric substrate for processing electrical signals and surface acoustic waves.
In the surface acoustic wave devices described above, as an electrode material, aluminum (Al) having a low electrical resistivity and a low specific gravity or an alloy thereof has been used.
However, since Al has poor stress migration resistance, when a large electrical power is applied thereto, hillocks and/or voids are formed in the electrodes, and short-circuiting or disconnection of the electrodes may occur in some cases, resulting in breakage of the surface acoustic wave device.
In order to solve the problems described above, a method for improving electrical power resistance has been disclosed in Japanese Unexamined Patent Application Publication No. 7-162255 (patent publication 1) in which the crystal orientation is improved by an ion beam sputtering method used as a method for forming electrodes.
In addition, another method for improving electrical power resistance has been proposed in Japanese Unexamined Patent Application Publication No. 3-48511 (patent publication 2) in which an Al crystal is oriented in a predetermined direction by an epitaxial growth method.
Japanese Unexamined Patent Application Publication No. 6-6173 (patent publication 3) has disclosed that electrical power resistance of electrodes can be improved as crystal grain size is decreased.
Furthermore, in “Technical Handbook of Surface Acoustic Wave Device” edited by the 150th Committee on Technology of Surface Acoustic Wave Device of Japan Society for the Promotion of Science, published by Ohmsha, Ltd., p. 267 (non-patent publication 1), a phenomenon has been disclosed in which the electrical power resistance is improved when copper (Cu) is added to Al.
Patent publication 1: Japanese Unexamined Patent Application Publication No. 7-162255
Patent publication 2: Japanese Unexamined Patent Application Publication No. 3-48511
Patent publication 3: Japanese Unexamined Patent Application Publication No. 6-6173
Non-patent publication 1: “Technical Handbook of Surface Acoustic Wave Device” edited by the 150th Committee on Technology of Surface Acoustic Wave Device of Japan Society for the Promotion of Science, published by Ohmsha, Ltd., p. 267.
However, by the traditional techniques disclosed in patent publications 1 and 3, recent higher frequency and larger electrical power requirements cannot satisfactorily be fulfilled, and hence, when the techniques described above are used in high-frequency or large electrical power applications, insufficient electrical power resistance becomes a serious problem.
In addition, according to the traditional technique disclosed in patent publication 2, an epitaxial film having superior crystallinity can be actually grown only on a quartz substrate. However, on a substrate composed of a piezoelectric crystal, such as LiTaO
3
or LiNbO
3
, used for filters which have superior piezoelectric properties and are advantageously used in a broad band, it has been difficult to grow an epitaxial film having superior crystallinity by the technique disclosed in patent publication 2, and as a result, the traditional technique described above cannot practically be applied to a surface acoustic wave device including a LiTaO
3
or LiNbO
3
substrate.
According to the traditional technique disclosed in non-patent publication 1, by adding Cu to Al, the electrical power resistance can actually be improved. However, a level of this improvement has not been satisfactory in practice.
SUMMARY OF THE INVENTION
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave device and a manufacturing method therefore, in which the novel surface acoustic wave device achieves superior electrical power resistance by using an epitaxial Al film having a twin structure for an Al electrode layer which is primarily composed of Al and which constitutes an electrode provided on a piezoelectric substrate formed, for example, of a 64° Y-X cut LiNbO
3
. In this case, it was understood that the epitaxial Al film grows in a particular manner, that is, the (
111
) plane thereof is oriented with respect to a Z axis of the piezoelectric substrate and has a twin structure which is grown in the (
111
) plane.
Compared to a single crystal, mechanical strength of an epitaxial film having a twin structure is very high, and as a result, the plastic deformation is unlikely to occur. Accordingly, a significant advantage can be obtained in that electrode breakage of surface acoustic wave devices, which is frequently caused by stress migration, is prevented from occurring.
Through intensive research by the inventors of the present invention regarding the epitaxial Al film having the twin structure described above, it was understood that, in some cases, crystal growth may occur according to a mechanism which is totally different from that in which the epitaxial film grows while the (
111
) plane of the Al film is oriented with respect to the Z axis as described above. In this case, the Al(
111
) plane is not oriented along the Z axis of the piezoelectric substrate, and very particular crystal growth occurs in which the Al(
111
) are oriented in a plurality of directions. The crystal growth described above is observed in particular when a Y-cut piezoelectric single crystal is used as a piezoelectric substrate, and in more particular, when 36° to 42° Y-cut LiTaO
3
substrate is used. Other suitable substrates may also be used.
According to the information thus obtained, a preferred embodiment of the present invention provides a surface acoustic wave device including a piezoelectric substrate made of a piezoelectric single crystal and at least one electrode provided on the piezoelectric substrate, and the at least one electrode has an electrode layer which is an oriented electrode layer formed by epitaxial growth, and the electrode layer is a polycrystalline thin film having a twin structure in which a diffraction pattern observed in an X-ray diffraction pole figure has a plurality of symmetry centers.
The electrode layer described above preferably includes Al as a primary component.
The electrode described above may further include an underlying electrode layer provided between the electrode layer and the piezoelectric substrate for improving the crystallinity of the electrode layer. This underlying electrode layer may include at least one of titanium (Ti) and chromium (Cr) as a primary component.
In addition, the electrode may further include an intermediate electrode layer provided between the Al electrode layer and the underlying electrode layer so as to cause a crystal face present the surface of the underlying layer to be in a cleaner state.
The piezoelectric substrate preferably includes a LiNbO
3
or a LiTaO
3
single crystal and, more preferably, is a &thgr; rotation Y-cut (&thgr; is between 36° and 42°) LiTaO
3
substrate.
Concerning the crystal orientation of the electrode layer provided for the surface acoustic wave device of preferred embodiments of the present invention, in X-ray diffraction in which X-rays are incident on the (
200
) plane of the crystal constituting the electrode layer, the [
111
] direction of the cr
Inoue Kazuhiro
Nakagawara Osamu
Saeki Masahiko
Dougherty Thomas M.
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
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