Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices
Reexamination Certificate
1999-12-27
2002-08-20
Budd, Mark O. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Piezoelectric elements and devices
C310S334000
Reexamination Certificate
active
06437484
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a piezoelectric resonator equipped with a resonator element which comprises a laminate of a plurality of piezoelectric ceramic layers and a plurality of electrode layers which are alternately laminated one upon the other.
2. Description of the Prior Art
The trend toward higher frequencies of electric signals used in radio communications and electric circuits, has urged the development of filters adapted to electric signals of high frequencies.
In the radio communication, for example, microwaves of around 2 GHz are becoming a main stream and, besides, there is a move toward establishing the standards for obliging the use of high frequencies of not lower than several gigahertz. It has therefore been demanded to provide a filter which is cheap in cost and has high performance to meet high frequencies.
An SAW filter is drawing attention in recent years using an elastic surface wave resonator (SAW resonator) which transmits acoustic waves along the surfaces of a solid. This filter utilizes the resonance of an elastic surface wave and a high-frequency electric field applied across the comb-shaped electrodes formed on the surface of a solid, and features a high selectivity of frequencies, and is widely used as an excellent band-pass filter.
In recent years, there has been proposed a resonator utilizing a thickness longitudinal (extensional) oscillation mode of a thin film that exhibits piezoelectric property. In this resonator, the piezoelectric thin film produces thickness longitudinal oscillation which produces resonance in the direction of thickness of the thin film. This resonator therefore is called bulk elastic wave resonator (BAW resonator).
Referring, for example, to
FIG. 18
, U.S. Pat. No. 4,320,365 discloses a BAW resonator comprising a substrate
61
, a support film
63
formed on the surface of the substrate
61
, a buffer layer
65
formed on the support film
63
, a first electrode
66
formed on the buffer layer
65
, a piezoelectric thin film
67
formed on the first electrode
66
, and a pair of second electrodes
68
formed on the piezoelectric thin film
67
. In this BAW resonator, an oscillation body (resonator element) is formed by the buffer layer
65
, first electrode
66
, piezoelectric thin film
67
and second electrode
68
. The support film
63
is formed on the upper surface of the substrate
61
so as to cover an oscillation space A formed in the substrate
61
, and a portion contacting to the oscillation space A in the support film
63
oscillates due to oscillation of the oscillation body (resonator element).
There has further been proposed an acoustic impedance converter as shown in FIG.
19
. This acoustic impedance converter includes a support film
72
formed on the surface of a substrate
71
and, further, includes, formed on the support film
72
, an oscillation body (resonator element) comprising a first electrode
74
, a piezoelectric thin film
73
formed on the first electrode
74
, and a second electrode
75
formed on the piezoelectric thin film
73
. The support film
72
is a multi-layer film in which thin layers a and b of two kinds of materials having dissimilar acoustic impedances are laminated in many number one upon the other. The thin layers a and b each have a thickness of one-fourth the wavelength of a standing wave produced by the oscillation (resonator element). In such an acoustic impedance converter, ultrasonic waves are effectively reflected by the layers in the support film toward the oscillation body (resonator element), to suppress the leakage of energy into the substrate
71
and to acoustically insulate the oscillation body (resonator element) from the substrate
71
(W. E. Newell, “Face-Mounted Piezoelectric Resonators”, Proceeding of the IEEE, pp. 575-581, June, 1965 and U.S. Pat. No. 5,373,268).
The resonance frequency of the BAW resonator varies in inverse proportion to the thickness of the film. By using a thin film as a piezoelectric member, therefore, it is allowed to form a resonator of the GHz band. Besides, since the thin film can be directly formed on a semiconductor substrate such as of Si, GaAs or the like, the BAW resonator is drawing attention as an element that can be integrated.
In either the SAW resonator or the BAW resonator, the resonance frequency varies in reverse proportion to the gap between the electrodes. Therefore, a high frequency is obtained by decreasing the gap between the electrodes. In the SAW resonator, however, resonance of the first degree takes place when the gap between the comb-shaped electrodes is one-fourth the wavelength and, hence, the gap between the electrodes inversely varies into four times of the resonance frequency. In a frequency region in excess of 1 GHz, therefore, the gap between the comb-shaped electrodes becomes of the order of submicrons, and it becomes very difficult to form the electrodes (for example, when the comb-type electrodes are to be formed on the board of an LiTaO
3
single crystal, the gap between the comb-shaped electrodes becomes about 0.5 microns to obtain a piezoelectric resonance of 2 GHz). Besides, since the gap between the electrodes is of the order of submicrons, lack of resistance against the high-frequency electric power becomes a serious problem in the SAW resonator. In the high-power applications such as the transmission filters, the high-frequency SAW filter of a 2 GHz band has not yet been realized due to the lack of breakdown voltage since the gap between the electrodes is as very small as in the order of submicrons.
In the BAW resonator, on the other hand, resonance of the first degree takes place when the gap between the electrodes is one-half the wavelength, and the gap between the electrodes varies in inverse proportion to the two folds of the resonance frequency. Therefore, when the BAW resonator and the SAW resonator using the piezoelectric member exhibiting the same elastic property and having the same gap between the electrodes, are compared with each other, the BAW resonator makes it possible to obtain a resonance frequency two times as high as that of the SAW resonator. Further, the BAW resonator makes it possible to obtain the same resonance frequency with the gap between the electrodes which is twice as large as that of the SAW resonator and, hence, exhibits excellent resistance against the electric power compared with the SAW resonator.
At present, however, the frequency of the electric signals is sharply increasing, and it has been desired to obtain frequencies higher than high frequencies accomplished by the existing BAW resonators. It has further been desired to provide a filter that exhibits excellent resistance against the electric power in high-frequency regions.
High frequencies are obtained by two methods of either decreasing the thickness of the film or utilizing the resonance of a high degree. When it is attempted to increase the frequency by decreasing the thickness of the film, the gap between the electrodes becomes of the order of submicrons at frequencies of the level of several gigahertz even when the BAW resonator is used as described above, leaving a problem of precision in controlling the film thickness and resistance against the electric power. When the resonance of a high degree is utilized, frequencies which are two folds or three folds as high as the fundamental wave can be utilized while maintaining a given thickness of the film, enabling the resonance of a high frequency to be utilized maintaining a large film thickness. It is therefore possible to provide a resonator which can be used at frequencies of two folds, three folds or four folds as high as that of the conventional resonator that uses the primary standing waves (fundamental waves). In the resonance of a high degree, however, oscillation attenuates with the degree of resonance, resulting in a great reduction in the electro-mechanical coupling coefficient Kt that determines the bandwidth of the filter. Though the frequency could be increased, therefore, a wi
Kamigaki Kousei
Nishimura Michiaki
Tsuyoshi Hirotaka
Yoshimura Ken-ichi
Budd Mark O.
Hogan & Hartson L.L.P.
Kyocera Corporation
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