TRAPPED-ENERGY MODE RESONATOR AND METHOD OF MANUFACTURING THE SAME

Abstract: The application discloses a trapped-energy mode re­ sonator comprising a plate (20) of piezoelectric material having a Poisson's ratio less than one third, a pair of central electrodes (21,21′) embedded within said piezoelectric plate so as to oppose each other; and short-circuited peripheral electrodes (22, 22′) opposedly arranged within or on the surface of said plate (20) and formed so as to substantially not overlap said central electrodes in the thickness direction of said plate; the distance between said central electrodes being shorter than the distance between said short-circuited peripheral electrodes. The resonator can be manufactured by sintering together a stack of green sheets which have the central electrodes and opposed electrodes printed as con­ ductive layers on their respective surfaces.

Description: BACKGROUND OF THE INVENTION This invention relates to a piezoelectric resonator of a thickness-extensional vibration mode according to the preamble of claim 1. Thickness-extensional mode piezoelectric resonators are composed of a piezoelectric plate which is polarized in the thickness direction and sandwiched between electrodes. In such resonators, however, a number of spurious vibrations are excited near the main resonance as a consequence of mode-coupling with high-order radial vibrations. These spurious responses resulting from contour vibrations can be excluded through so called forward-wave energy-trapping technique using a partially-electroded piezoelectric plate. In this case, thickness-extensional vibration is confined to an electroded region and does not couple with radial vibrations. To this end, the cutoff frequency is lowered relatively to the surrounding region by electrode mass-loading and a piezoelectric effect. This forward-wave energy-trapping technique can be applied to only those piezoelectric material having a Poisson's ratio of one third or more. U.S. Pat. No. 3,891,872 discloses a new type of thickness-extensional trapped-energy mode piezoelectric resonator using a piezoelectric plate having a Poisson's ratio of less than one third, in which the so called backward-wave energy-trapping technique is used to realize the energy-trapping by slightly reducing the plate thickness locally in the electroded region so as to make a cutoff frequency in the electroded region higher than that in the surrounding region. However, this technique requires extremely precise control to realize an appropriate thickness ratio between the electroded portion and surrounding region. Another method of realizing the backward-wave energy-trapping is described in "Energy-Trapping for Backward-Wave Mode Thickness-Vibrations by Controlling Piezoelectric Reaction" in the transactions of the Institute of Electronics and Communication Engineers of Japan, Vol. J. 62 - A, No. 1, p.p. 8 - 15 (January, 1979) in which a piezoelectric plate of uniform thickness is used and the surrounding region as well as the central trapping region of the plate is electroded, the electrodes in the surrounding region being short-circuited. The inner and outer electrodes are insulated by an annular unelectroded gap of small width. In addition, a capacitor element is connected in series with the central electrodes. The cutoff frequency at the electroded regions are lowered by the effect of piezoelectric reaction. However, since the series capacitor reduces the magnitude of the reduction of the cutoff frequency at the central electroded region, the backward-wave energy-trapping can be realized without resorting to the thickness difference of the piezoelectric plate. Although this method facilitates the realization of the backward-wave mode energy-trapping, the usage of an additional capacitor element deteriorates the stability of resonance frequency due to the difficulty of matching the temperature coefficient of the capacitor element and the piezoelectric plate. A trapped-energy mode resonator according to the preamble of claim 1 is disclosed in "Patents Abstracts of Japan", Vol. 7, No. 182 (E-192) (1327), August 11, 1983". As in this resonator only central electrodes are provided undesired vibrations will occur. Furthermore, a backward-wave energy-trapping is not achieved. A conventional resonator of forward-wave engergy-trapping is described in DE-B-1 081 065. In this resonator additional electrodes are provided to dampen undesired vibrations. It is an object of the invention to provide a trapped-energy mode resonator which facilitates the backward-wave mode energy-trapping condition and which is operable with a stable resonant frequency. This obejct is achieved by a trapped-energy mode resonator as defined in claim 1; the dependent claims are related to further developments of the invention. In the resonator according to the invention the resonant frequency at energy-trapped electrodes is made higher than at peripheral regions by using a pair of short-circuited peripheral electrodes. The backward-wave energy-trapping is achieved by selecting the distance between the energy-trapping electrodes smaller than that of the short-circuited peripheral electrodes. By this mean an energy-trapped resonator is achieved which has a stable resonant frequency. Furthermore, the backward-wave mode energy-trapping condition is facilitated without necessity of an additional capacitor element. In addition, in the resonator according to the invention the resonant frequency can be easily determined. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1A and 1B are sectional view and a plan view respectively of an example of the conventional trapped-energy mode resonator; Fig. 2A and 2B are a sectional view and a plan view respectively of an embodiment of a trapped-energy mode resonator according to the invention; Fig. 3 is an exploded perspective view of the laminated structure of the embodiment of a resonator according to the invention; Fig. 4 is a sectional view of another embodiment of a resonator showing the state in which the short-circuited electrodes are exposed to the surfaces; and Fig. 5 is a characteristic curve of the frequency response of a resonator according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Figs. 1A and 1B, a prior art trapped-energy mode resonator is composed of a polarized lead titanate piezoelectric ceramic plate 10, and energy-trapping electrodes 11 provided in recesses 12 formed on the top and bottom surfaces of the ceramic plate 10. In Fig. 1 the symbol t shows the plate thickness of the portion of an energy-trapping electrode, t' the plate thickness of the ceramic plate, and the arrow the direction of polarization. The principle of energy-trapping shown in Fig. 1 is that backward-wave mode energy-trapping is achieved by making the plate thickness t of the electrode portions thinner than the plate thickness t' of the peripheral portions by utilizing the fact that the plate thickness is inversely proportional to cutoff frequency, and thereby heightning the cutoff frequency at the electroded portions in comparison with that at the peripheral portions. However, the relationship of t'> t is not sufficient for good energy-trapping and it is not achieved until t and t' have a predetermined ratio. In case of lead titanate piezoelectric ceramic, for example, it is desired to chose t/t' to be in the range from 0.859 to 0.908 as mentioned in aforementioned U.S. Patent. Accordingly it is necessary to extremely precisely control the plate thickness of the recesses 12 provided on the piezoelectric ceramic plate 10, and the recesses 12 must have parallel surfaces with respect to each other. Such precise processing is difficult from the view point of industry. Since the electrodes 11 are provided in the recesses 12 in the resonator shown in Fig. 1, frequency control appears to be easily carried out by control the thickness of the ceramic plate 10 by means of parallel plane lapping operation, but it is in fact difficult, because the lapping operation makes the ratio of thickness t/t' larger than the optimal ratio for good energy-trapping. Referring to Figs. 2A and 2B, the basic structure of a trapped energy mode resonator according to the invention is shown. Within a piezoelectric ceramic plate 20 which is polarized uniformly in the thickness direction, energy-trapping central electrodes 21, 21' are embedded at predetermined intervals and at opposing positions, and lead electrodes extend from these electrodes 21, 21' to the side surfaces of the ceramic plate. Opposing short-circuited peripheral electrodes 22, 22' are provided so as not to overlap the energy-trapping electrodes in the thickness direction. Short-circuiting of the peripheral electrodes is easily executed by providing end surface electrodes 24, 24' on the side surfaces of the resonator by a method such as deposition, plating or baking. Electric input/output terminals may be drawn from the lead electrodes 23, 23' provided on the side surfaces. The material of the piezoelectric plate can be any mono-crystalline piezoelectric material or piezoelectric ceramic material having a Poisson's ratio less than one third. For example, lead titanate piezoelectric ceramics are suitable for the piezoelectric plate of the resonator of the invention. The reasons for a resonator according to the invention being able to achieve good energy-trapping are as follows. When there are no opposing electrodes on the piezoelectric ceramic plate, the cutoff frequency of energy-trapping is approxinately equal to the mechanical resonance frequency f a (which electrically corresponds to anti-resonant frequency) of the piezoelectric ceramic plate. When there are short-circuited electrodes on the top and bottom surfaces of the piezoelectric ceramic plate, the cutoff frequency is lowered to a far lower value than f a chiefly by piezoelectric reaction, namely coupling coefficient K t , and agrees with electric resonance frequency f r . When electrodes are embedded in the piezoelectric ceramic plate as in this invention, the cutoff frequency takes the intermediate value between f a and f r , and the shorter the interval between the opposing electrodes in the piezoelectric ceramic plate in comparison with the plate thickness of the ceramic plate, the smaller becomes the influence of the piezoelectric reaction on the lowering of cutoff frequency. In a resonator according to the invention, the cutoff frequency at the portions of energy-trapping electrodes 21, 21' is made higher than that at the peripheral portions by controlling the lowering of cutoff frequency caused by piezoelectric reaction by making the distance between the energy-trapping electrodes 21, 21' shorter than the distance between the short-circuited electrodes 22, 22'. As a result, good energy-trapping is constantly achieved in the resonator according to the invention. Accordingly, since the resonator itself according to the invention can constantly achieve good energy-trapping without using any additional capacitance element, the problem of the remarkable degradation of stability of resonance frequency which would be produced when the additional capacitance element is used can be avoided. According to experimental results using a lead titanate as a piezoelectric ceramic material, it is desirable that the ratio of the distance d between the energy-trapping electrodes and the distance d' between the short-circuited electrodes is ranged from 0.2 to 0.92. If the ratio exceeds 0.92, additional series capacitor is required to achieve good energy-trapping. When the ratio is lower than 0.2, on the other hands, it becomes difficult to isolate the pair of opposed energy-trapping electrodes. From a theoretical point of view, the above lower limit of the distance ratio can be selected smaller than 0.2 but larger than zero. According to the present invention, therefore energy-trapping condition is not critical compaired with the resonator of Fig. 1. In addition, since the surface of the energy-trapping electrode is covered with a ceramic layer in a resonator according to the invention, frequency control is possible by lapping the ceramic plate so as to control the thickness thereof. For the above-described reasons, a resonator according to the invention involves no factor leading to obstruction of energy-trapping when frequency is controlled by lapping the ceramic plate. There is no critical relation for the diameter of the energy-trapping electrodes, as there is for the thickness relation. A method of manufacturing a resonator according to the invention will be explained in the following. This resonator can be manufactured by utilizing a well known manufacturing technique used for multilayer laminated ceramic capacitors, Fig. 3 is an exploded perspective view of the laminated structure of the resonator. The energy-trapping central electrodes 21, 21' and the short circuited peripheral electrodes 22, 22' are formed on each of the green sheets 30 which contain piezoelectric ceramic powder and organic binder by screen printing process or the like using conductive paste. The green sheet having the electrodes printed thereon and the green sheet without electrodes shown in Fig. 3 are laminated in the thickness direction by contact bonding, and are sintered in an electric furnace at a temperature of about 1200 o C. This is polarized thereafter uniformly in the thickness direction, whereby a resonator according to the invention as shown in Fig. 2 is produced. In this manufacturing process, accurate positioning of the upper and lower electrodes 21, 21', 22 and 22' is achieved by using a known technique for a ceramic multi-layer substrate. This method according to the invention enables the manufacture of a multiplicity of resonator tips as shown in Fig. 2 in one wafer at high accuracy, and is thus efficient in terms of mass productivity. In the above-described embodiment of trapped-energy mode resonator, the short-circuited electrodes are within the piezoelectric plate, but the efficiency of the resonator is not lessened at all when the short,circuited electrodes are exposed to the surface at the stage of parallel plane lapping process, as is shown in Fig. 4. Incidentally in the case of that stric frequency control is not always necessary during use, it is also possible to manufacture a resonator by lapping the surface of a ceramic plate having built-in energy-trapping electrodes and thereafter forming short-circuited electrodes on the surface of the ceramic plate by a method such as deposition or plating. (Example) A typical example of a fundamental thickness-extensional mode backward-wave type trapped-energy mode resonator has the structure shown in Fig. 2 and uses Pb 0.85 {Ti 0.941 (Mn 1/3 Sb 2/3 ) 0.05 Ga 0.012 } O₃ as a piezoelectric ceramic material. The density of which is 7.38 x 10³ kg/m³ the thickness extensional coupling coefficient k t is 0.47, and the relative dielectric constant ε₃₃T/ε₀ is 196. The distance between the energy-trapping electrodes 21, 21' is 25 µm, the distance between the short-circuited peripheral electrodes 95 µm, and the plate thickness of the ceramic plate 120 µm. The diameter of each energy-trapping electrode is 5 mm. When a resonance frequency of the resonator is controlled by parallel plate polishing of the polarized ceramic plate 20, a resonance frequency of just 21.4 MHz is obtained when the plate thickness is reduced into 103 µm. Fig. 5 shows the resonance anti-resonance frequency response at this time. It is clear from Fig. 5 that good energy-trapping is being achieved. As is described in detail, a trapped-energy mode resonator according to the invention is very advantageous in that it enables good energy-trapping in the fundamental thickness-extensional vibration mode, and facilitates frequency control, thereby allowing efficient mass productivity.

Claims: A trapped-energy mode resonator comprising a plate (20) of piezoelectric material, a pair of energy-trapping central electrodes (21, 21') embedded within said piezoelectric plate (20) so as to oppose each other, characterized by a pair of short-circuited peripheral electrodes (22, 22') being opposedly arranged within or on the surface of said plate (20) and formed to substantially not overlap said central electrodes (21, 21') in the thickness direction of said plate (20); wherein the piezoelectric material has a Poisson's ratio less than one third and the distance between said central electrodes (21, 21') is shorter than the distance between said short-circuited electrodes (22, 22') such that the resonant frequency of the fundamental thickness-extention vibration of said plate (20) at said central electrodes (21, 21') region is higher than the resonant frequency of said plate (20) at said short-circuited electrodes (22, 22') region. A trapped-energy mode resonator as claimed in claim 1, further comprising a pair of lead electrodes (23, 23') on two side surfaces of said plate (20), respectively, said lead electrodes being connected to said energy-trapping electrodes (21, 21'), respectively, and being arranged so as do not overlap each other in the thickness direction of said plate (20), and said short-circuited electrodes (22, 22') having slits at the region opposing said lead electrodes (23, 23') such that said lead electrodes substantially not overlap said short-circuited electrodes (22, 22') in the thickness direction of said plate (20). A trapped-energy mode resonator as claimed in claim 1, wherein said piezoelectric plate (20) is of a lead titanate piezoelectric ceramic and the ratio of the distance of said energy-trapping electrodes and the distance of said short-circuited electrodes is from 0,2 to 0,92.

Inventors: INOUE, TAKESHI C/O NEC CORPORATION, SUZUKI, MASANORI C/O NEC CORPORATION
Applicants: NEC CORPORATION
Application year: 1985
Main classification: H03H9
IPC codes: H03H3, H03H9