Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-03-20
2003-03-25
Feild, Lynn D. (Department: 2839)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
Reexamination Certificate
active
06538359
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to surface acoustic wave devices such as filters and convolvers using special effects of surface acoustic waves, and particularly to surface acoustic wave devices that use KNbO
3
single crystals having superior electromechanical coupling factors as piezoelectric layers.
This application is based on Patent Application No. Hei 11-80553 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
In general, surface acoustic wave devices convert electric signals to surface acoustic waves propagating on surfaces of elastic substances, so that signals of specific frequencies are being extracted. Since scientists and engineers have discovered that surface acoustic waves can be subjected to excitation and signal reception efficiently on a piezoelectric substrate, they have studied and developed a variety of applications for signal function components such as filters and convolvers using the superior properties of surface acoustic waves, which are not provided by electromagnetic waves. So, the surface acoustic wave devices are used in a wide variety of fields in practice. Conventionally, the surface acoustic wave devices are manufactured by forming interdigital transducers, which function by transduction between electric signals and surface acoustic waves, on piezoelectric single crystals made of LiNbO
3
, LiTaO
3
, etc.
The surface acoustic wave device is designed such that an operational frequency f is determined based on an acoustic velocity v of surface acoustic waves propagating on a surface of an elastic substance and an electrode width w of an interdigital transducer in accordance with an equation (1), as follows:
f
=
v
_
=
y
4
w
(
1
)
where is a wavelength of a surface acoustic wave.
That is, as the electrode width w becomes small while the velocity v becomes large, the surface acoustic wave device can be used in higher frequencies. In order to obtain higher frequencies of a gigahertz order which is needed in future communications fields, however, it is necessary to select specific materials for elastic substances which allow propagation (or transmission) of surface acoustic waves at higher velocities because the present manufacturing techniques have limits in further narrowing electrode widths. An example of the material allowing propagation of acoustic waves at a high velocity is diamond. Japanese Unexamined Patent Publication No. Sho 64-62911 discloses an example of a surface acoustic wave device having a laminated structure in which a piezoelectric layer and an electrode layer are sequentially formed on a diamond layer.
Another factor which is required for selection of the material used for the surface acoustic wave device is an electromechanical coupling factor K
2
representative of a capability of transduction between electric signals and surface acoustic waves. That is, it is necessary that the selected material has a large electromechanical coupling factor K
2
. As K
2
becomes large, it is possible to obtain a surface acoustic wave device having a higher efficiency in transduction. From this point of view, the scientists and engineers have discovered that KNbO
3
, which is conventionally known as a ceramic material having piezoelectric properties, has an extremely large electromechanical coupling factor K
2
. Such a fact is confirmed through experiments. That is, results of the experiments show that a KNbO
3
single crystal has a larger value of K
2
than LiNbo
3
, which is conventionally believed to have a large electromechanical coupling factor K
2
. In particular, the KNbO
3
single crystal at a specific crystal plane (e.g., plane (001)) in a specific direction (e.g., [100]) has an electromechanical coupling factor K
2
=0.053, which is approximately ten times greater than K
2
=0.055 in LiNbO
3
. This is disclosed in a monograph entitled “Surface Acoustic Wave Substrate with Super High Electro-Mechanical Couplings Using KNbO
3
Single Crystal” written in a report of research no. 50 on pp. 27-31 (published on Nov. 27, 1996), which is provided for the No. 150 Symposium for SAW techniques at the Japan Academy Promotion Society. Reference is made to Japanese Unexamined Patent Publication No. Hei 10-65488, which discloses a surface acoustic wave substrate using potassium niobate (KNbO
3
) to obtain K
2
=0.5.
In addition, some documents disclose techniques for formation of KNbO
3
thin films which are not always related to techniques of surface acoustic wave devices, as follows:
(1) Document 1: a monograph of PIONEER R&D Vol. 7 No. 1, entitled “Growth of Nonlinear Optical Crystal Films for SHG Devices by Vapor Phase Deposition”, which discloses a method for forming KNbO
3
thin films, used as waveguides of SHG (Second Harmonic Generation) light emission elements, on SrTiO
3
substrates by using MOCVD (an abbreviation for “Metal Organic Chemical Vapor Deposition”).
(2) Document 2: a monograph of Mat. Res. Soc. Symp. Proc. Vol. 271 for 1992 Materials Research Society, entitled “THE GROWTH OF SINGLE CRYSTAL-LIKE AND POLYCRYSTAL KNbO
3
FILMS VIA SOL-GEL PROCESS”, which discloses a method for forming KNbO
3
thin films on SrTiO
3
substrates by sol-gel processing.
As described above, if the KNbO
3
single crystals are used as piezoelectric materials, it is possible to actualize surface acoustic wave devices which have large electromechanical coupling factors K
2
and high efficiencies in propagation of acoustic waves. In addition, it has been proven that values of K
2
are changed in various manners in response to the propagation directions of surface acoustic waves in crystal structures of the KNbO
3
single crystals. So, there is a strong demand to obtain a desired value of K
2
or control K
2
in the manufacture of surface acoustic wave devices. To cope with such a demand, it is necessary to match a propagational plane of the surface acoustic waves with a specific crystal orientation plane of KNbO
3
. That is, an interdigital transducer is formed to suit the specific crystal orientation plane, so that the propagational plane of the surface acoustic waves are matched with the specific crystal plane.
Suppose that a KNbO
3
single crystal having a perovskite crystal structure shown in
FIG. 8
is used as bulk material. To obtain a specific crystal orientation, it is necessary to perform very troublesome operations for cutting out a specific crystal plane from a KNbO
3
single crystal whose crystal orientation is known. In addition, the KNbO
3
single crystal is difficult to grow, and therefore is very expensive as an industrial material. For the reasons described above, it is very hard to use the aforementioned bulk material made of the KNbO
3
single crystal as the material for the surface acoustic wave device.
The aforementioned difficulties challenge engineers to develop a concept in which the KNbO
3
single crystal is not used as a bulk material but is used as a thin film being formed on some substrate in a laminated manner. However, even if a KNbO
3
single crystal thin film is directly formed on general-purpose substrate materials such as MgO, Pt, Al
2
O
3
, GaAs and Si, it is impossible to obtain sufficient lattice matching between crystals, so KNbO
3
is hardly subjected to epitaxial growth. In addition, the formed KNbO
3
thin film itself is relatively low in crystallinity. As a result, it is impossible to obtain a good property (i.e., large value of K
2
). Both of the aforementioned documents 1, 2 are related to a method for forming a thin KNbO
3
film directly on a SrTiO
3
substrate. Even in such a method, it is impossible to obtain sufficient lattice matching between crystals of KNbO
3
(lattice constants: a=5.70, b=5.72, c=3,97) and SrTiO
3
(lattice constants: a=b=c=3.91).
SUMMARY OF THE INVENTION
It is an object of the invention to provide a surface acoustic wave device, which can be manufactured without troublesome operations for cutting out a specific crys
Hiraku Tatsuya
Imanishi Masao
Kaneko Akira
Okada Masuhiro
Suzuki Yukitoshi
Feild Lynn D.
Yamaha Corporation
Zarroli Michael C.
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