Wave transmission lines and networks – Coupling networks – Electromechanical filter
Reexamination Certificate
2002-03-20
2004-04-13
Summons, Barbara (Department: 2817)
Wave transmission lines and networks
Coupling networks
Electromechanical filter
C333S150000, C333S133000, C331S1070DP, C331S155000, C310S31300R, C310S31300R
Reexamination Certificate
active
06720846
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a surface acoustic wave device having a piezoelectric thin film, a frequency filter, a oscillator, an electronic circuit and an electronic apparatus, and more specifically, to a surface acoustic wave device having a silicon substrate and a potassium niobate piezoelectric thin film, a frequency filter, a oscillator, an electronic circuit, and an electronic apparatus, which are employed in the telecommunications field.
2. Description of the Related Art
Surface acoustic wave devices employing a piezoelectric material with a high electromechanical coupling coefficient (denoted as “k
2
” hereinafter) have been desired in order to improve the performance of non-lead containing surface acoustic wave devices. Lithium niobate is known conventionally as a material with a high k
2
, demonstrating a k
2
of 5.5% using Rayleigh waves. However, it has been shown that k
2
can exceed 50% with potassium niobate (“KNbO
3
” hereinafter), as disclosed in
Electronics Letters
, Vol. 33, No. 3 (1997), p. 193, and much attention has been focused on this area in recent years. In addition, research has been conducted into surface acoustic wave devices employing a KNbO
3
thin film, such as disclosed in Japanese Unexamined Patent Application, First Publication No. 10-65488.
However, conventional surface acoustic wave devices have the following problems.
Namely, it is difficult to produce high quality, large KNbO
3
single crystals in a surface acoustic wave device that uses a KNbO
3
single crystal. As a result, this is not practical from the perspective of mass production. On the other hand, in a surface acoustic wave device employing a KNbO
3
thin film, acoustic velocity, k
2
and other such characteristics depend on the KNbO
3
crystal orientation. Thus, the orientation of the KNbO
3
thin film must be controlled. As disclosed in Japanese Unexamined Patent Application, First Publication No. 2000-278084, it is known that a KNbO
3
(010) epitaxial film can be obtained by using a (110) oriented substrate of strontium titanate (SrTiO
3
hereinafter). This indexing assumes that the b axis lattice constant is the largest. However, even if the orientation of the KNbO
3
thin film can be controlled using a SrTiO
3
substrate, it is difficult to form a SrTiO
3
substrate which is larger than two inches. Accordingly, this also is not suitable from the perspective of mass production. Furthermore, even if such a substrate could be produced hypothetically, it is not viewed to be practical in terms of cost.
A silicon (denoted as “Si” hereinafter) substrate would appear promising from the perspective of cost and capacity for mass production. Still, it is difficult to obtain a high-quality epitaxial thin film even when the KNbO
3
thin film is formed directly on top of the Si substrate, because of lattice mismatches and the like. As a result, a high k
2
cannot be obtained.
SUMMARY OF THE INVENTION
It is the objective of the present invention to resolve the above-described problems by providing a surface acoustic wave device having a high k
2
, which element is manufactured by employing a Si substrate that is advantageous in terms of cost and capacity for mass production, wherein a high quality KNbO
3
epitaxial thin film is formed onto the Si substrate.
The first aspect of the present invention is a surface acoustic wave device having a (110) silicon substrate, and a (010) potassium niobate piezoelectric thin film. This surface acoustic wave device has a first oxide thin film layer formed on top of the silicon substrate, a second oxide thin film layer formed on the first oxide thin film layer, a potassium niobate piezoelectric thin film formed on top of the second oxide thin film layer, and a protective thin film comprising an oxide or nitride formed onto the potassium niobate piezoelectric thin film.
As a result of the above design, the first oxide thin film layer and the second oxide thin film layer can be made to undergo epitaxial growth in sequence on top of the silicon substrate. A high quality KNbO
3
epitaxial thin film can then be formed on top of the aforementioned layers. Specifically, it becomes an easy matter to form a (010) KNbO
3
epitaxial thin film, making it possible to provide a surface acoustic wave device having a high k
2
which is advantageous with respect to cost and capacity for mass production.
The first oxide thin film layer is preferably formed from strontium oxide (denoted as “SrO” hereinafter) or magnesium oxide (denoted as “MgO” hereinafter). These first oxide thin films are capable of epitaxial growth on top of the (110) silicon substrate, ultimately enabling epitaxial growth of KNbO
3
. Specifically, a (010) oriented KNbO
3
is easily formed.
The second oxide thin film is preferably formed of SrTiO
3
.
SrTiO
3
is capable of epitaxial growth on the aforementioned first oxide thin film, and enables epitaxial growth of KNbO
3
on the SrTiO
3
film. Specifically, a (010) oriented KNbO
3
is easily formed.
The second aspect of the present invention is a surface acoustic wave device having a (100) silicon substrate and a (010) KNbO
3
piezoelectric thin film. This surface acoustic wave device has a first oxide thin film layer formed on top of the silicon substrate, a second oxide thin film layer formed on top of the first oxide thin film layer, a KNbO
3
piezoelectric thin film formed on top of the second oxide thin film layer, and a protective thin film comprising an oxide or nitride formed on top of the KNbO
3
piezoelectric thin film.
As a result of the above design, the first oxide thin film layer and the second oxide thin film layer can be made to undergo epitaxial growth in sequence on top of the silicon substrate, and a high quality KNbO
3
epitaxial thin film can then be formed on top of these layers. In particular, formation of a (010) KNbO
3
epitaxial thin film becomes an easy matter, making it possible to provide a surface acoustic wave device having a high k
2
which is advantageous with respect to cost and capacity for mass production.
The first oxide thin film layer is preferably formed from cerium oxide (denoted by “CeO
2
” hereinafter), zirconium oxide (denoted as “ZrO
2
” hereinafter), or yttrium-stabilized zirconia (denoted as “YSZ” hereinafter).
These first oxide thin films are capable of epitaxial growth on a (100) silicon substrate, so that epitaxial growth of KNbO
3
, and (010) KNbO
3
in particular, is made possible in the end.
The second oxide thin film layer is preferably formed from strontium titanate (denoted as “SrTiO
3
” hereinafter).
SrTiO
3
is capable of epitaxial growth on the first oxide, and enables epitaxial growth of KNbO
3
, and (010) KNbO
3
in particular, on the SrTiO
3
film.
The second aspect is preferred over the first aspect in that, during formation of the oxide layer, a relatively high-temperature, high-vacuum is not required, and because a (100) silicon substrate is more readily available and less expensive than a (110) silicon substrate.
In the present invention's surface acoustic wave device, an electrode is formed on top of the piezoelectric thin film or the protective thin film. However, when providing an electrode on top of the piezoelectric thin film, there is some concern that the piezoelectric thin film may be degraded by water or the like during the electrode forming process. Further, when forming the protective thin film on top of the piezoelectric thin film and the electrode, it is necessary to take out the electrode through the protective thin film. As a result, formation of the frequency filter becomes troublesome and costs rise. Accordingly, it is preferable that the electrode be formed on top of the protective thin film.
This is also true of the frequency filter and the oscillator that will be described below.
The third aspect of the present invention is a frequency filter characterized in the provision to any one of the above described surface acoustic wave devices of a first electrode, which is formed on top of the protective thin
Higuchi Takamitsu
Iwashita Setsuya
Miyazawa Hiromu
Harness & Dickey & Pierce P.L.C.
Seiko Epson Corporation
Summons Barbara
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