Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
2000-11-06
2002-08-27
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S321000
Reexamination Certificate
active
06441539
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to piezoelectric resonators, and more specifically, the present invention relates to piezoelectric resonators, filters and electronic apparatuses utilizing the elastic vibration of piezoelectric layers.
2. Description of the Related Art
Resonant frequencies of piezoelectric thin film resonators that are adapted to utilize the thickness mode vibration of piezoelectric substrates, have an inverse proportional relationship relative to the thickness of the piezoelectric substrates, in very high frequencies. Therefore, piezoelectric substrates included in such piezoelectric resonators must be extremely thin. However, there is a limit to how think the piezoelectric substrates can be. In most piezoelectric resonators of this type, there is a limit of several 100 MHz of high frequencies in practice due to restrictions in the mechanical strength, treatment steps, manufacturing processes and other factors relating to the piezoelectric substrates.
To solve these problems, piezoelectric thin film resonators have been proposed as filters and resonators, etc. The piezoelectric thin film resonator
1
shown in
FIG. 1
, is manufactured by partially etching a Si substrate
2
using a fine processing method to form a thin film support portion
3
, having a thickness of several &mgr;m or less, on a portion of the Si substrate
2
, and by providing a ZnO piezoelectric thin film
4
, having a pair of excitation electrodes
5
a
and
5
b
on both sides, on the support portion
3
.
The aforementioned piezoelectric thin film resonator
1
has a possibility to extend its high frequency characteristics to as high as several 100 MHz to several 1000 MHz, because the thin film support portion
3
can be made thin, using the fine processing technique, and the piezoelectric thin film
4
can also be made thin by sputtering.
However, a temperature coefficient of resonant frequency (TCF) of ZnO is about −70 ppm/° C., and a temperature coefficient of resonant frequency of Si is about −30 ppm/° C. Both ZnO and Si have negative temperature coefficients of resonant frequency, and, therefore, a combination of the piezoelectric thin film
4
, made of ZnO, and the thin film support portion
3
, made of Si, has the disadvantage that temperature characteristics of resonant frequency in the dominant mode become inferior.
In a piezoelectric thin film resonator
6
, shown in
FIG. 2
, an SiO
2
thin film
7
is formed on the surface of an Si substrate
2
by thermal oxidation. A thin film support portion
3
is formed from the SiO
2
thin film
7
by partially etching the Si substrate
2
, and a ZnO piezoelectric thin film
4
, having a pair of excitation electrodes
5
a
and
5
b
on both sides, is formed on the support portion
3
.
A temperature coefficient of resonant frequency of ZnO is about −70 ppm/° C., and a temperature coefficient of resonant frequency of SiO
2
is about +100 ppm/° C. ZnO and SiO
2
have temperature coefficients of resonant frequency having opposite signs from each other. By adjusting a ratio of a film thickness of the piezoelectric thin film
4
, made of ZnO, to a film thickness of the thin film support portion
3
, made of SiO
2
, at a ratio of about 2:1, it is possible to make the temperature coefficient of resonant frequency, in the dominant mode, small, and to make the temperature characteristics of resonant frequency stable. This is described in Japanese Unexamined Patent Application Publication No. 58-121817.
FIG. 3
is a sectional view illustrating a piezoelectric thin film resonator
9
having another structure. This is the piezoelectric thin film resonator
9
, having a floating construction or air bridge construction, manufactured by forming a thin film support portion
12
, made of SiO
2
, on a Si substrate
10
via an air gap
11
, and providing a ZnO piezoelectric thin film
13
, having excitation electrodes
14
a
and
14
b
on both sides, on the support portion
12
that is arranged to be free from the Si substrate
10
.
In the piezoelectric thin film resonator
9
, similarly to the piezoelectric thin film resonator
6
shown in
FIG. 2
, by adjusting a ratio of a film thickness of the ZnO piezoelectric thin film to a film thickness of the SiO
2
thin film support portion
12
at a proper value, it is possible to make the temperature coefficient of resonant frequency small and to make the temperature characteristics of resonant frequency stable.
In the aforementioned second piezoelectric thin film resonator
6
, by a combination of the ZnO piezoelectric thin film
4
and the SiO
2
thin film support portion
3
, temperature coefficients of resonant frequency can offset each other. In the aforementioned third piezoelectric thin film resonator
9
, by a combination of the ZnO piezoelectric thin film
13
and the SiO
2
thin film support portion
12
, temperature coefficients of resonant frequency can offset each other.
However, ZnO is a piezoelectric, whereas SiO
2
is not a piezoelectric. Therefore, in these piezoelectric thin film resonators, resonant responses have been very small and resonant characteristics have been inferior.
SUMMARY OF THE INVENTION
In order to overcome and solve the above-described problems, preferred embodiments of the present invention provide piezoelectric resonators having a very stable temperature coefficient of resonant frequency, a very large resonant response, and excellent resonant characteristics.
According to one preferred embodiment of the present invention, a piezoelectric resonator includes a laminated member, at least one pair of electrodes and a substrate. The laminated member includes a piezoelectric laminate body, the piezoelectric laminate body including at least one first piezoelectric layer which has a positive temperature coefficient of a resonant frequency and at least one second piezoelectric layer which has a negative temperature coefficient of a resonant frequency. The pair of electrodes interpose at least one of the first and second piezoelectric layers. The substrate supports the laminated member and holds a support portion of the laminated member such that the support portion vibrates in response to application of a voltage across the pair of electrodes.
According to the structure, by properly adjusting the thickness of each piezoelectric layer, the temperature coefficient of resonant frequency of the entire laminate member becomes nearly zero. Furthermore, because all layers except for electrodes are made of piezoelectric materials, the resonant response of the piezoelectric resonator is excellent, and the resonant characteristics are also excellent. Therefore, piezoelectric resonators, having very stable temperature characteristics, very large resonant responses, and excellent resonant characteristics, are provided.
The laminated structure may also include an insulating layer located between the substrate and the piezoelectric laminate.
According to such a structure, the insulator layer is disposed on the substrate, and because, generally, insulator layers are difficult to be etched with etching liquids used for substrates and layers intended to be etched, the processing in the manufacturing procedures is much easier.
Furthermore, because the insulating layer, and two or more kinds of piezoelectric layers are laminated on the substrate, material parameters of the vibration portion become three or more, and it becomes possible to easily and accurately adjust electromechanical coefficients and piezoelectric characteristics.
Therefore, it is possible to stabilize temperature coefficients of resonant frequency, to greatly increase resonant responses, to achieve excellent resonant characteristics, to greatly simplify the etching process for floating the insulation layer above the substrate, and to greatly increase the design flexibility for other characteristics.
The pair of electrodes may interpose the at least one first piezoelectric layer and the at least one second piezoelectric layer.
According to this unique structure,
Inoue Kazuhiro
Kitamura Hidekazu
Takeuchi Masaki
Addison Karen
Keating & Bennett LLP
Murata Manufacturing Co. Ltd.
Ramirez Nestor
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