PIEZO-ELECTRIC RESONATOR FOR GENERATING OVERTONES

Abstract: To utilize characteristics of a mode for confining vibra­ tion energy of symmetrical or asymmetrical mode of higher orders generated when a piezo-electric responator is ex­ cited, portions (3, 3′) having a cut-off frequency f₂ (f₁ < f₂) are provided near the energy confining portion (2, 2′, 20, 20′, 30, 30′) having a cut-off frequency f₁, symmetrical or asym­ metrical mode vibration of higher orders is excited intense­ ly, vibration energy of overtones of orders higher than the n-th overtone of said mode of vibration is almost confined in the vicinity of said energy confining portion (2, 2′, 20, 20′, 30, 30′), and the size of said energy confining portion (elec­ trode), plate back amount (f₂ - f₁)/f₂ and the thickness of the substrate are so selected that the vibration energy of over­ tone vibrations of orders smaller than the (n - 2)-th overtone including a fundamental wave vibration is diffused over the whole surfaces of the substrate (1).

Description: This invention relates to a piezoelectric resonator, and more particularly a piezoelectric resonator for overtone oscillation capable of oscillating at a desired overtone frequency without requiring a special oscillating circuit. In recent years, demand for higher frequency operation and miniaturization has become further increased in electric communication apparatus and various electronic apparatus. To satisfy such demand, in addition to the utilization of overtone vibrations of conventional piezoelectric resonator such as a quartz resonator, a surface acoustic wave (SAW) resonator is now being used in many fields of application. The piezoelectric oscillator is designed such that a desired output is extracted through an LC resonance circuit that resonates to a desired overtone frequency or that a LC resonance circuit is inserted into a portion of an oscillation circuit to make the negative resistance of the oscillation circuit sufficiently large at a desired overtone frequency region. In any case, however, it is necessary to use a coil which is extremely inconvenient for the oscillation circuit to be incorporated into an integrated circuit. On the other hand, as is well known in the art, the oscillation frequency of a SAW resonator is principally determined by the material constituting a piezoelectric substrate, and the pitch of interdigital transducer (IDT) electrodes formed on the surface of the piezoelectric substrate so that not only the resonator itself but also above described circuit problems can be obviated. However, this type of resonator is defective in that its frequency-temperature characteristic is much inferior than that of an AT cut crystal resonator. Persons skilled in manufacturing crystal resonators have noticed a phenomenon that a crystal resonator after oscillates in 3rd or 5th order overtones when the ratio between the length and the width of a crystal substrate, and the ratio of the side length with respect to the thickness of the substrate fall within a specified range as a result of miniaturization of the crystal substrate required for a small resonator. To suppress such phenomenon has been subject to be studied. US-A-4 114 062 discloses technology for utilizing the phenomenon so as to produce crystal resonators for overtone oscillation suitable for use with a periodic oscillation circuits, which suppress the fundamental and the third harmonic by means of the structure shown on Fig. 3a and 3c, on which the energy entrapping portion, the energy propagating portion and the energy absorbing portion of Claim 1 can be defined. The known resonator is unable to serve as a practical use because it has several problems as follows: Considering a disc shaped substrate which is most typical shape for a resonator substrate, the diameter of the substrate is extremely small, merely 20-30 times of the substrate thickness, and the diameter of the electrodes is about 50-85% of that of the substrate. Since the distortion of the support around the periphery of the substrate affects the oscillating portion, there arise problems that the frequency-temperature characteristic becomes unstable, resonant frequency varies before and after the thermb-cycle, and the resonant frequency greatly changes with its aging. To solve these problems, US-A-4 188 557 proposes machining a piezoelectric substrate used in a small piezoelectric resonator for overtone oscillation into a convex shape. However, merely entrapping overtone vibration energy of higher order than a desired order within excitation electrodes while forming a crystal substrate of a specific dimensional ratio, or merely providing a dot or line shaped dumping member at the periphery of the resonator substrate is not sufficient at all for vibration energy whose overtone vibration order is lower than the desired order to be consumed. Accordingly, a resonator of the above-mentioned type has a defect that it cannot provide a stable oscillation in overtone vibration of a desired order, and therefore no further study has been made on this type of resonators. Further, it has been known a resonator which generates high-frequency vibration by utilizing the fundamental mode of a thin film. For example, VHF/UHF COMPOSITE RESONATOR ON A SILICON SUBSTRATE by Kitayama et al., Japanese Journal of Applied Physics, Vol. 22 (1983) Supplement 22-3 pp. 139-141 describes a typical example of the composite resonator utilizing the fundamental mode of a piezoelectric thin film. However, every conventional composite resonators utilize the fundamental mode of a piezoelectric thin film and do not intend to generate the overtone thereof. It is the object of the present invention to provide a piezoelectric resonator for an overtone oscillation utilizing a vibration of a higher order symmetrical or asymmetrical mode, which enables oscillation of a desired overtone frequency with substantially the same form as the conventional piezoelectric resonator so that it is not necessary to substantially change the manufacturing steps and to add an LC resonance circuit to the oscillation circuit. This object is solved, according to the invention, with the features of claim 1, whereby the substrate and a vibration medium block are bonded together, which contributes to overcome the problems of the overtone resonator described in US-A-4 114 062 without substantially changing the manufacturing steps used in traditional resonator manufacture for producing the substrate. More particularly, for the purpose of utilizing the characteristic in a vibration energy entrapping mode of a higher order symmetric or asymmetric mode vibration which is generated at the time of exciting a piezoelectric resonator, according to the overtone oscillation resonator embodying the invention, a portion having a cutoff frequency f 2 is formed around an energy entrapping portion having a cutoff frequency f 1 (f 1 < f 2 ) on a piezoelectric substrate so as to strongly excite a higher order symmetric or asymmetric mode vibration, thereby substantially entrapping the vibration energy of an overtone vibration of the order higher than the n-th overtone among the vibrations of the higher order symmetric or asymmetric mode vibration near the energy entrapping portion, and the size of the energy entrapping portion (electrode), an amount of plate back (f 2 - f 1 )/f 2 and the substrate thickness are selected such that the vibration energy of an overtone vibration of less than n-2 order including the fundamental frequency is caused to be dispersed over the entire surface of the substrate. Further, a vibration energy absorbing portion whose cutoff frequency is lower than f 2 is applied to a suitable portion near the outer periphery of the piezoelectric substrate so as to efficiently dissipate of the vibration energy of an overtone energy of an order lower than (n-2)th overtone vibration including the fundamental wave vibration spreading over the entire surface of the substrate. A resistor having a suitable resistance value may be connected across electrodes attached to both sides of the piezoelectric substrate near the-outer periphery thereof so as to facilitate dissipation of the vibration energy spread over the entire surface of the substrate. The overtone oscillation resonator according to this invention can be also utilized as a resonator for a GHz band by utilizing a composite material. More particularly, the overtone oscillation resonator of this invention is constructed such that a recess or opening is formed at a proper portion of a block made of silicon, quartz, glass, metal, etc., the recess or opening is covered by a thin layer having a desired thickness and made of the same material as the block, and a piezoelectric portion made of ZnO, etc. and provided with appropriate electrodes is applied onto the thin layer. The electrode portion (actually, contact portion to the piezoelectric portion) is used as the vibration energy entrapping portion having a cutoff frequency f 1 , the interface between the periphery of the electrode portion and the recess or opening of the block is utilized as a vibration energy propagating portion having a cutoff frequency f 2 , and the block is utilized as a vibration energy absorbing portion having a cutoff frequency f 3 . The relation among the cutoff frequencies of the various portions are made to be f 3 < f 1 < f 2 , and various parameters such as electrode size, amount of plate back etc. are suitably selected in accordance with the percentage of entrapping of the vibration energy in the vibration mode being utilized. Furthermore, the invention provides an overtone oscillation resonator not depending upon the characteristic of the oscillation circuit. To provide such resonator, in the resonator of this invention, the cutoff frequency of the absorbing energy absorption portion near the periphery of the piezoelectric substrate is made to be smaller, preferably much smaller than that of the vibration energy entrapping portion attached with exciting electrodes. Furthermore, the invention provides an overtone oscillation resonator capable of compensating for the degradation of the resonator characteristic caused by the shift of the position of an additional vapor deposition necessary for a fine adjustment of the oscillation frequency, greatly decreasing the influence upon the resonator characteristics caused by the difference in the thickness of the piezoelectric substrate, and greatly increasing the upper limit of the resonance frequency. For this reason, in the resonator of this invention, the electrode area on the rear surface is made to be larger than the area corresponding to the electrode on the front surface. In the accompanying drawings: Fig.1a is a diagrammatic sectional view of a piezoelectric substrate useful to explain the energy entrapping theory of Schockley Fig.1b shows vibration spectra in the symmetric and asymmetric modes; Fig. 2a is a model of a substrate for proving the fact that the vibration spectra hold even on a substrate of a limited area; Fig.2b are graphs showing the result of theoretical analysis; Fig.3a is a sectional view showing the basic construction of a resonator shown for comparison; Fig.3b is a graph showing a method of setting parameters; Fig.3c is a graph showing vibration energy distributions; Fig.4a and 4b are sectional views and graphs showing the result of experiment for determining the size of a gap at the vibration energy propagating portion; Fig.5 is a graph for explaining the relationship among the impedance and the negative resistance of the oscillation circuit at respective overtone frequencies of the resonator; Fig.6a is plan view of a further resonator; Fig.6b is a side view of the resonator shown in Fig.6a; Figs. 6c - 6e are plan views showing resonators having different plan configurations; Figs. 7a through 7f are sectional views showing different sectional configurations of resonators Fig.8a is a sectional view showing the basic construction of another overtone oscillation resonator; Fig.8b shows a vibration energy distribution in various vibration modes generated by the resonator shown in Fig.8a; Fig.8c is a graph for explaining the basic method of selecting various parameters of the resonator; Fig.9 is a graph for explaining another method of setting parameters necessary for constituting an overtone oscillation resonator Fig.10a shows a charge distribution generated by a resonator utilizing an asymmetric mode oscillation of the lowest order; Fig.10b shows a series connection of the electrodes; Fig.10c shows a parallel connection of the electrodes; Fig.11a and 11b shows examples of the series connection and parallel connection of the electrodes of a resonator utilizing a S 1 mode oscillation; Fig.12a and 12b are perspective views showing series connection and a parallel connection of the electrode divided into two sections; Fig.12c and 12d are perspective views showing a series connection and a parallel connection of the electrode divided into three sections; Fig.13a through fig.13c are sectional views showing different constructions of the vibration energy absorbing portion; Fig.14a and 14b are plan views showing the constructions of the resonators utilized for the confirmation experiments in which Fig.14a shows a resonator not provided with a vibration energy absorbing portion, and Fig.14b shows a resonator provided with a vibration energy absorbing portion; Figs.15a and 15b are graphs showing the results of experiments showing the characteristics of the resonators respectively shown in Fig.14a and Fig.14b; Fig.16a through 16d are sectional views showing different constructions of resonators Fig.17 is a sectional view showing the basic construction of the other overtone oscillation piezoelectric resonator Fig.18 is a plan view showing the basic principle of a resonator in which there are a plurality of overtone vibrations, the vibration energies thereof are to be absorbed; Fig.19a is a plan view showing another resonator ; Fig.19b is a sectional view taken along a line A-A in Fig.19a; Fig.20 is a plan view showing still another resonator ; Fig.21a is a sectional view showing an overtone oscillation piezoelectric resonator according to this invention; Fig.21b is a plan view of the embodiment shown in Fig.21a; Fig.22 and 23 are sectional views showing further embodiments of this invention; Fig.24 is a graph showing the relationship between the resonator impedance and the characteristic of the oscillation circuit of overtone frequencies of respective orders of resonators; Fig.25 is a plan view of a resonator utilized for the experiment made for investigating the film thickness ratio of the vibration energy absorbing portion; Fig.26 is a graph showing the result of experiment for investigating a CI value regarding an overtone oscillation of an order at which oscillation is desired for a film thickness ratio of the vibration energy absorbing portion and the CI value of the overtone vibration of a lower order to be suppressed; Fig.27a through 27c are sectional views showing different constructions of the vibration energy absorbing portion; Figs.28a and 28b show different electrode constructions; Fig.29a and 29b are sectional views for explaining the fact that the characteristic does not vary even when the frequency is adjusted where the size of the piezoelectric substrate of the resonator varies; Fig.30a and 30b are front and rear views of an overtone oscillation piezoelectric resonator ; Fig.31a and 31b are plan views showing different electrode constructions utilized in the resonator shown in Figs.30a and 30b; and Fig.32a through 32c are sectional views respectively showing a two port resonator or a dual mode filter, an overtone oscillator utilizing an asymmetric (a 0 ) mode oscillation at the lowest order, and an overtone oscillator utilizing a symmetric (S 1 ) mode oscillation of the highest order. In the following, the theory of the resonators shown in the drawings will be described. Prior to the description of the preferred embodiments, the theory of piezoelectric resonators will firstly be discussed to gain better understanding of the invention. The facts on which the resonators described herein are based are as follows. (1) The theory of entrapping energy by Schockley and other scientists is a pure elastic analysis in which among various waves, a SH wave is analyzed. As the vibration medium, metal, guartz lithium tantalate, lithium neobate, or a dense material such as piezoelectric ceramic is employed. Even when the vibration energy propagation direction and the order of the overtone are different for all vibration modes including a thickness slip, a thickness twist, and thickness longitudinal, etc., it has been known that this theory is applicable for the qualitative performance. (2) According to the energy entrapping theory of Schockley, where a portion having.a cutoff frequency f 2 presents around a portion having a cutoff frequency f 1 as shown in Fig.la, the vibration energy will be entrapped in a portion (a portion of f 1 ) having a low cutoff frequency and the extent of entrapping will be shown by Fig. 1b. In Fig.1b, the ordinate shows normalized resonance frequency expressed by an equation ψ = (f - f 1 )/(f 2 - f 1 ), and wherein ψ = 1 means f 1 = f 2 which is not influenced by a portion having a cutoff frequency f 1 as shown in Fig.1a. In other words, this means absence of the portion, in which case the oscillation energy spreads over the entire surface of the substrate and no entrapping occurs. On the other hand, a case wherein ψ = 0 means contrary to the above description that all vibration energy is entrapped in a portion having a cutoff frequency f 1 . A parameter na√Δ/H along the abscissa designates an entrapping coefficient which is determined by the order of the overtone n, the size a and thickness H of a portion having a cutoff frequency f 1 and the amount of frequency decrease Δ. Considering Fig.1b, it can be noted that where the entrapping coefficient na√A/H is less than a given value, no entrapping mode presents in the vibration of the symmetric modes (S 1 , S 2 --- ) of higher orders and the vibrations of the asymmetric modes (a 1 , a 2 --- ). This characteristic does not present in the symmetric mode (S 0 ) of the lowest order. (3) In the entrapping coefficient described above, where a, √Δ and H are suitable fixed values, the degree of energy entrapping becomes large as the overtone order number n increases. However, as the entrapping coefficient becomes large, the degree of energy entrapping reaches a large value not to be substantially influenced by the order of the overtone. (4) Where the order n of the overtone and the piezoelectric substrate frequency (that is by H) are given, the parameter that varies the entrapping coefficient corresponds to the size of a portion (generally, electrode contacts) having a cutoff frequency f 1 the amount of frequency decrease Δ being caused by an addition of a certain mass. However, where such materials having high bonding properties as quartz is used as the oscillation medium, the effect of frequency decrease caused by the piezoelectric nature of the material becomes remarkable so that where it is impossible to sufficiently vary Δ, variation of the size a of a portion having the cutoff frequency f 1 is effective. (5) Regarding the vibration whose energy is entrapped:as the order of overtone increases, the equivalent resistance becomes large. As can be clearly noted from Fig. 1b, the vibration energy is not perfectly entrapped in a portion having a small cutoff frequency but more or less leaks to the peripheral portions having high cutoff frequencies, but if a portion which absorbs the leakage energy and converts the same into heat presents extremely close to a portion having a small cutoff frequency, the equivalent resistance for the vibration becomes large. In this regard, in a conventional quartz oscillator, sufficiently thick electrodes are attached to the center of the quartz substrate so as to increase the difference in the cutoff frequency so as to entrap in the electrode portion the vibration energy of the overtones of all orders. Accordingly, the vibration at the fundamental frequency at which the CI value becomes minimum is the easiest to occur. As above described, if one wants to cause the crystal resonator to vibrate in an overtone mode, some modification is necessary for the oscillation circuit. (6) The characteristic of a higher order symmetric or asymmetric mode shown in Fig.lb is the result of theoretical analysis regarding an infinite area piezoelectric substrate. However, when this characteristic can be preserved substantially, even in a substrate of a limited area, since the vibration energy spreads over the entire surface of the substrate, the energy will leak from a holding portion for the substrate edges or dissipate in the holding portion. Accordingly, the equivalent resistance for the vibration will become extremely high. Based on the facts described above, let us consider that the characteristic of the vibration of the higher order symmetric or asymmetric mode on the infinite are a substrate described in items (2) and (3) can be retained in what degree on the limited area. Fig.2a is a diagrammatic sectional view showing an energy entrapping type resonator having an infinite length in the x direction, a limited width 2b in the z direction and electrodes applied to the lateral center of the substrate and having a width of 2a. Among the wave motions, it is necessary to consider only a SH wave having a displacement parallel to the x axis, and the displacement U can be expressed by an equation U = H·cos (nπy/H).exp (j ωt) according to Onoue and Jumonji's paper entitled "Analysis of an energy entrapping type piezoelectric resonator" (1965, September, Journal of the Institute of Electronics and Communication Engineers of Japan, Vol. 48, No. 9) where U = f(z) which can be rewritten as follows by using k and k' defined such that the propagation constant B will always be a real number at respective regions (I) (II) and (III) of the substrate shown in Fig.2a. U I = B ch sh k'z - C sh ch k'z (-b ≦ z ≦ -a) U II = A cos sin kz (-a ≦ z ≦ a) U III = B ch sh k'z + C sh ch k'z (a ≦ z ≦ b) where A, B and C are constants. Let us assume that f 1 represents a natural resonance frequency of a thickness slip oscillation where it is assumed that the electrode portion (II) has an infinite width, and that f 2 represents the natural resonance frequency when it is assumed that portions (I) and (III) not provided with electrodes have infinite widthes, the propagation constants k and k' of respective regions II, and I and III are expressed as follows k = nπ f/f 1 - 1 /H k'= nπ 1- f/f 2 /H The upper portion of the double notation of equations (2) (3) and (4) gives symmetric vibration in which a displacement U becomes an even function with respect to the original, whereas the lower portion gives an asymmetric oscillation that becomes an odd function. In equations (1) through (6) described above, the following equation of frequency can be derived out from the boundary condition that the displacement and stress are continuous when z = ± a, and that the end surface is free when z = ± b. tan h k' (b - a) = k/k' tan -cos ka Let us determine an equation of approximation of equation (7) when f 1 is slightly lower than f 2 . Let us denote the observable frequency, that is the resonance frequency by f where the substrate thickness is varied in region (II) shown in Fig.2a or where electrodes are attached. Then two normalizing frequencies δ and Δ that satisfy the following equation are introduced. f = ( 1 + δ ) f 1 f 1 = (1 + Δ) f 2 From equations (5) and (8), we obtain (kH) 2 = (nπ) 2 ·2δ From equations (6) and (9), we obtain (k'H) 2 = (nπ) 2 ·2( Δ - δ ) From equations (10) and (11), we obtain k'H/kH = Δ - δ /δ By denoting δ/Δ as ψ, then k'/k = 1 - Ψ /Ψ Further, from equations (10) and (13), we obtain ka = nπ a 2δ /H = n a 2ΨΔ /H Further, from equations (11) and (13), we obtain k'a = nπ a 2Δ 1 - Ψ /H Substitution of equations (14), (15) and (16) into equation (7) gives an equation tan h nπ a 2Δ 1 - Ψ x (b/a - 1)/H = 1 - Ψ /Ψ tan -cot nπ a 2ΨΔ /H This equation shows the relationship between the normalized resonance frequency Ψ utilizing b/a as a parameter and the resonator dimension na √Δ/H, and corresponds to a general equation resulting in a vibration energy entrapping state on a limited area substrate. The frequency spectra of the higher order symmetric and asymmetric mode vibrations where b/a = 4 are calculated by using equation (17) and the calculated spectra are compared with those of a indefinite area substrate and the result of comparison is shown in Fig. 2b. As can be clearly noted from Fig.2b, the degree of energy entrapping of the lowest order symmetric mode vibration varies greatly depending upon the dimension of the substrate, whereas the degree of entrapping higher order symmetric and asymmetric mode vibrations can be deemed substantially equal for the infinite area substrate and the limited area substrate. More particularly, the above-mentioned preconditions (2) and (3) can be applied to the limited area substrate as they are. When compared with the characteristic of the lowest order symmetric (S 0 ) mode, the inclinations of the other modes are steep and this tendency becomes remarkable as the mode becomes higher order. This characteristic too is retained in the limited area substrate. The above-mentioned facts (1) through (5) constitute one of the premises of the invention which is defined in the appended Claim 1. Fig.3a through Fig.3c are sectional views showing the fundamental construction of resonators and graphs for explaining the principles. As shown in Fig.3a, electrodes 2 and 2' each having a diameter 2a are attached to both surfaces of the central portion of a piezoelectric substrate 1 having a thickness H for decreasing the cutoff frequency of the central portion to f 1 thereby providing a difference (f 2 - f 1 ) in the cutoff frequencies between the central portion and the surrounding portion having a cutoff frequency f 2 . The portions at which the electrodes 2 and 2' are attached are used as the vibration energy entrapping portions and the portions 3 and 3' not attached with electrodes are used as the vibration energy propagating portion of unwanted vibration. On the periphery of the portions 3 and 3' are formed vibration energy absorbing electrodes 4 and 4' having a cutoff frequency f 3 (where f 3 < f 2 ). When this resonator is used as an overtone vibration resonator of the fifth order, for example, the foregoing principle teaches that the percentage of the vibration energy entrapping the overtone vibration of the fundamental wave (first order) and of the third order should be small and that the entrapping percentage of the overtone vibration of the fifth and higher order should be large. For this reason, as shown in Fig.3b, the percentage T 5 of entrapping the vibration energy of the fifth order overtone vibration is set to about 80%, for example. As can be noted from Fig.3b, the entrapping coefficient na√Δ/H at T 5 = 80% is about 0.53. In the equation na√Δ/H = na f 2 - f 1 /f 2 /H, as n, H and f 2 are given conditions, it is easy to calculate what value f 1 should be selected to. When f 1 is determined, since the difference in the cutoff frequencies (f 2 - f 1 ) is a quantity directly related to so-called plate back, and since the amount of electrode attached satisfying this quantity is already known, the resonator as shown in Fig.3a can readily be manufactured. When such material of high bonding property as lithium niobate, lithium tantalate or a piezoelectric ceramic is used as the piezoelectric substrate, as an extremely small amount of the electrode attached results in an extremely large frequency decrease, it is advantageous to vary the electrode size on the assumption that the amount of frequency decrease has already given. The cutoff frequency f 3 of the vibration energy absorbing portions 4 and 4' may be the same as f 1 , but if desired the frequency f 3 is increased or decreased thereby increasing, as far as possible, the consumption of the vibration energy in the portions 4 and 4'. In the resonator manufactured as above described, the vibration energies of the fifth, seventh ... overtone oscillations are entrapped in the electrode portions as shown in Fig. 3c, the vibration energies of the overtone vibrations of the fundamental wave (first order) and the third order will not be entrapped in any appreciable amount, and these energies are leaked to the outer periphery of the piezoelectric substrate thereby causing the piezoelectric substrate 1 to vibrate in the vibration energy absorbing portions 4 and 4'. Thus the energy is converted into heat in these portions and dissipated, the impedance of the lower order overtone vibrations including the fundamental vibration as viewed from the side of the oscillation circuit becomes higher than that of the higher order overtone vibration in which the energy is entrapped. This phenomenon can readily be understood if one imagines the energy distribution of the free electrode mounting portions of the so-called multimode piezoelectric monolithic filter in which a plurality of closely adjacent free electrodes are disposed between electrodes. On the other hand, the overtone vibrations of the fifth order, seventh order ... overtone vibrations whose vibration energies are entrapped in the electrode portions as above described, since the vibration impedance of the fifth order is the minimum, this resonator is one in which the fifth order overtone vibration is the most easy to excite. When the lengths of the vibration energy propagating portions 3 and 3' between the vibration energy entrapping portions 2 and 2' and the vibration energy absorbing portions 4 and 4' are extremely small, a portion of the portions 4 and 4' are extremely small, a portion of the entrapped vibration energy is consumed by the vibration energy absorbing portions. Conversely when the lengths are too large, the leakage energy propagate without any loss, so that theoretically, it can be deemed that the oscillation energy absorbing elements do not present as has been described above. Accordingly, there must be optimum values for the lengths. However, as it is difficult to qualitatively determine the loss, let us determine the same by experiments. Fig. 4b shows the result of experiments when the vibration energy propagating gap is varied at the time of manufacturing a quartz resonator having the third order overtone vibration (50 MHz). When the gap (D-a) shown in Fig. 4a is too small, the impedance for the vibration of the fundamental wave (first order) and that for the third order overtone increases proportionally. Conversely, when the gap is too large, there is no difference among the impedances for the first and third overtone vibrations. Consequently, it was found that it is advantageous to select a value of about 1.5 for the ratio D/a so as to give a condition under which the third order overtone vibration is most easy to occur. More particularly, as the frequency of an oscillation circuit increases, the gain of the amplifier decreases, the negative resistance of the circuit varies in reverse proportion to the square of the frequency. Accordingly, it is not always true that the oscillation at an overtone frequency at which the resonator impedance as seen from the side of the oscillation circuit becomes a minimum, is easiest. For example, in Fig.3b, when the percentage T of entrapping the vibration energy of the fifth order overtone vibration at a desired vibration frequency is set to a value, for example, about 60%, which is considerably lower than 80%. Qualitatively, the impedance for the seventh order overtone vibration may be somewhat smaller than the impedance for the fifth order overtone vibration. In such a case too, due to the characteristic of the oscillation circuit, the oscillating circuit does not oscillate at the seventh order overtone frequency at which the resonator impedance is the minimum, but at the fifth order overtone frequency the oscillation circuit may happen to oscillate. Accordingly, the term "desired overtone vibration" does not always mean an overtone vibration at which the resonator impedance as seen from the side of an oscillating circuit becomes minimum. Thus it is sufficient that the impedance at the "desired overtone vibration" is greatly smaller than that for the lower order overtone vibration including the fundamental wave vibration. The resonator that satisfies the conditions described above may have a plane configuration as shown in Figs. 6a - 6c. More particularly, as shown in Figs. 6a and 6b, main vibration energy entrapping electrodes 2 and 2' are disposed at the central portion of a piezoelectric substrate 1, nonelectrode portions 3 and 3' (vibration energy propagating portions) are disposed about the electrodes, vibration energy absorbing electrodes 4 and 4' of a suitable configuration are disposed about the non-electrode portions, both electrodes 2, 4 and 2',4' are interconnected by lead wires 5 and 5' disposed bout the periphery of the substrate 1, and the vibration absorbing electrodes 4 and 4' are coupled together to have the same potential. Thus, by connecting the vibrators and the circuit at the vibration energy absorbing electrode contacts, the electrodes can be utilized as tub electrodes so that the mechanical strength of the lead connections can be ensured. As shown in Fig.6c the lead pattern extending from the vibration energy entrapping portions may be directly connected to the vibration energy absorbing electrodes 4 and 4' through the vibration energy propagating portions 3 - and 3'. With this construction, however, it was experimentally confirmed that the desired overtone vibration energy too tends to slightly leak through the lead patterns 5 and 5'. However, this construction is advantageous in that since the lead wires are short, not only the ohmic loss can be reduced, but also the position alignment between the piezoelectric substrate and a vapor deposited pattern is not required to be accurate. Furthermore, it is not always necessary to integrate the lead wires 5 and 5' from the vibration energy entrapping electrodes 2 and 2' with the vibration energy absorbing electrodes 4 and 4', but instead they may be separated as shown in Fig.6d, or as shown in Fig. 6e, the vibration energy absorbing electrodes 4 and 4' may be formed to completely enclose the vibration energy entrapping portions 2 and 2'. This modified construction increases the consumption of the leakage energy thereby increasing the equivalent resistance for the vibration. More particularly, in the constructions shown in Figs. 7a and 7b, the vibration energy propagating portion of the piezoelectric substrate 1 are removed by etching, for example, on both or single surface, for increasing the cutoff frequency of these portions. This construction reduces the thickness of the attached electrodes which not only reduces the electrode vapor deposition time but also prevents peeling off of thick electrodes. As the frequency is increased, the thickness of the main vibration entrapping electrodes 2 and 2' becomes extremely thin whereby the electroconductivity is reduced and the performance of the electrode is lost. To obviate this problem, the portions of the piezoelectric substrate corresponding to the electrodes are slightly recessed as shown in Fig.7c for ensuring necessary electrode thickness. It is not always necessary to attach the electrodes to the vibration energy absorbing portions on the outermost periphery of the piezoelectric substrate, but as shown in Fig.7d, the thickness of the substrate at that portion is increased for consuming unwanted vibration energy by thick end portions 4". To attain the above described object, it is not always necessary to increase the thickness of the piezoelectric substrate 1 at the outermost periphery, but a material having a large acoustic loss, for example, lead, tin, gold, etc. may be attached, or organic materials 6,6' and 7,7' such as epoxy resin may be attached with thick film printing technique or the like. As shown in Fig.7f, a dot shaped material 8,8' and 9,9' of a large acoustic loss such as an electroconductive binder may be applied onto the electrodes 4,4' for consuming unwanted vibration energy. The resonator can be supported at said dot shaped material. The result of experiment made on the characteristics of the resonator of this embodiment will be described in brief. In this experiment, a resonator having a construction as shown in Fig.4 and the following specification was used. Quarts (AT cut) substrate dia. 7 mm Vibration frequency (overtone order) 50 MHz (third order) Vibration entrapping portion gold electrode, film thickness 350 Å electrode size 2mm x 2mm Gap between vibration entrapping electrode and absorbing electrode 0.5 mm Direction of arrangement of vibration energy absorbing electrode Z' direction In this resonator, the CI values for the fundamental wave vibration and the third order overtone vibration are respectively 150Ω and 520Ω, while the CI value after bonding the lead wire to the resonator using an electroconductive bonding agent are respectively 280Ω and 53Ω which proves the correctness of the theory. Fig. 8a through 8c are sectional views showing another fundamental construction of a resonator utilizing the asymmetric mode and a graph for explaining the theory thereof. As shown in Fig.8a, split electrodes 20,20' and 30,30' are applied to both surfaces of the central portion of the piezoelectric substrate 1. When the resonator is excited, an asymmetric (a 0 ) vibration of the lowest order will be strongly excited. Denoting the order of the overtone at which the resonator is desired to vibrate by n, where the parameters a and Δ are selected such that the position of the entrapping coefficient (n-2) a √Δ/H of the (n-2)th order shown on the abscissa of Fig.2b will be positioned at a position at which the percentage of the energy entrapping of the a 0 mode curve is zero that is a position at which the entrapped mode does not present, and that the position of na √Δ/H will be the position at which the percentage of entrapping the a 0 mode curve will be a maximum, the vibration energy of the overtone vibration higher than the n-th order will be entrapped near the peripheries of the split electrodes 20,20'; and 30,30' as shown in Fig.8b, while the vibration energy of the overtone vibration of lower than (n-2)th order will spread over the entire surface of the piezoelectric substrate 1 and a substantial amount of the energy will leak and consumed through the substrate supporting portion supporting the periphery of the substrate. The vibration energy of the overtone vibration higher than the n-th order and entrapped near the peripheries of the split electrodes 20,20' and 30,30' has a higher equivalent resistance as the order of the overtone increases as above described, so that the resonator can vibrate at the n-th overtone frequency without modifying the oscillation circuit. Fig.8c shows a basic method of selecting parameters where it is desired to obtain a third order overtone vibration resonator by utilizing an excitation of a 0 mode. The percentage T of entrapping the a 0 mode vibration energy becomes zero when the entrapping coefficient na √Δ/H is less than 04 so that, for the fundamental wave (n=1) vibration and the third order (n-3) overtone vibration, it is sufficient to determine the parameters a and Δ such that a √Δ/H ≒ 0.4, and 3a√Δ/H ≒ 1.2 The spacing between the split electrodes may be selected to a suitable value so long as short circuiting does not occur between electrodes. Where the interelectrode spacing is extremely small, as is well known in the art, as a result of an acoustic coupling of the asymmetric mode vibration with a symmetric mode vibration which are simultaneously excited, two types of different vibration frequencies are generated. However, in the resonator of this embodiment, it is sufficient to derive out one of the frequencies, that is in this example only the resonance frequency of the asymmetric mode vibration, so that there is no problem. Where it is desired to utilize the vibration of the higher mode, for example, S 1 mode or a 1 mode, each electrode is divided into three or four sections. In this case, as shown in Fig.2b, the gradient of each mode frequency spectrum becomes steeper as the order becomes higher so that this is advantageous because it is possible to set large the difference in the percentages of entrapping the n-th order overtone vibration energy and that of the order lower than (n-2)th order. More particularly, as the frequency of an oscillating circuit increases, the gain of its amplifier generally decreases so that the negative resistance of the circuit varies in reverse proportion to the square of the frequency. Accordingly, it is not always true that the vibration at an overtone frequency at which the resonator impedance as seen from the side of the oscillation circuit becomes a minimum is the most easy. For example, in Fig.8c for the purpose of manufacturing the fifth order overtone vibration resonator by utilizing the vibration of the lowest order asymmetric (a 0 ) mode vibration, when the parameters are selected such that the percentages of entrapping the vibration energies of the first order (fundamental wave vibration) and the third order overtone vibration are both zero, as shown in Fig.9, the percentage of entrapping the vibration energy of the fifth order overtone vibration will not become sufficiently large with the result that a case may occur in which the equivalent resistance for the fifth order overtone vibration is higher than that for the seventh order overtone vibration. In such a case, where the difference between the equivalent resistances for the fifth and seventh order overtone vibrations is small, it can be considered that the vibration of the fifth order overtone frequency is possible in view of the relation to the above described characteristic of the oscillation circuit. However, when the difference between the equivalent resistances for the fifth and seventh order overtone vibrations is large, the oscillating circuit may oscillate at the frequency of the seventh order overtone. In order to avoid this unstability, the percentage of entrapping the third order overtone vibration energy is made to be slightly larger than zero, and the percentage of entrapping the fifth order overtone vibration energy is selected such that the equivalent resistance for a desired fifth order overtone vibration becomes extremely smaller than that for the third order overtone vibration and that the equivalent resistance is slightly higher than that for the seventh order overtone vibration. It should be noted that, in this invention, it is not always necessary to make the vibration energy of an overtone vibration (including the fundamental wave vibration) of the order lower than the order of the overtone desired to vibration to be a perfectly not entrapped mode. In the foregoing, resonators were described by taking a case utilizing the lowest order asymmetric (a 0 ) mode vibration as an example, parameters a and Δ can be selected in the same manner as a case wherein a higher order asymmetric mode (a 1 , a 2 ...) vibration or a higher order symmetric mode (S 1 , S 2 ...) vibration. Having completed the description of the basic concept of resonators, the concrete electrode connection and wirings, necessary for manufacturing a resonator will be described in detail as follows. An electrode of a conventional resonator is required to be arranged for efficiently deriving out the charge created by the displacement of the piezoelectric substrate. As a consequence, in the resonator of this invention too, it is necessary to coincide the number and positions of the hills or valleys of the charge generated by the vibration mode utilized with those of the split electrode, and to interconnect such that the charges generated at respective electrodes will not cancel with each other. Where the vibration of the lowest order asymmetric (a 0 ) mode is to be utilized, as shown in Fig.10a, split electrodes 20,20' and 30,30' are attached to the front and rear surfaces of the piezoelectric substrate 1 for generating electric charge having a level as shown in Fig.10b and the electrodes are connected in series as shown in Fig.10b or in parallel as shown in Fig.10c. Since the impedance of the parallel connection is 1/4 of that of the series connection, where it is desired to decrease the parallel resistance, parallel connection is preferred. The spurious of a resonator utilizing a split electrode will now be considered. Where the utilized vibration mode is a higher order symmetric (S 1 , S 2 ...) mode, an electrically asymmetric (a 0 , a 1 , a 2 ...) mode will not be excited, while where the vibration of a higher order asymmetric mode is to be utilized, the symmetric mode will not be excited. However, due to manufacturing errors of the resonator, some other modes are actually excited. Among the vibration modes utilized, a mode of different order, for example, a 2 mode with respect to a mode will be sufficiently excited. In the former case, however, the response is inherently very small and in the latter case the response is also small because the number of hills or valleys of the generated charge pattern does not coincide with the electrode division pattern so that the response is also small. In any case, the spurious caused by these facts does not reach a level causing a trouble. For the sake of reference, methods of connecting an electrode split into three sections and utilizing the S 1 mode are shown in Figs.11a and 11b. Examples of the wiring pattern to be formed on the piezoelectric substrate of a resonator utilizing an electrode split into two or more sections are shown in Figs. 12a through 12d. Figs. 12a and 12b show examples of series and parallel connections of the electrode split into two sections, while Figs. 12c and 12d show examples of series and parallel connections of the electrode split into 3 sections. Where the wiring patterns 50 and 50' --- are made around the edge of the piezoelectric substrate, a conductor pattern is vapor deposited on edge too or an electroconductive point or bonding agent is coated for interconnecting the patterns on the front and rear surfaces. Since the resonator of this embodiment is constructed to cause the vibration energy of an overtone vibration of the order lower than that of the desired overtone containing the fundamental wave vibration to leak and spread over the entire surface of the piezoelectric substrate, the leakage energy propagates without loss through portions of a large cutoff frequency thereby leaking out through a resonator support provided for the end of the piezoelectric substrate as described before. However, it is very important to consume the leakage energy effectively in order to increase the equivalent resistance of the vibration. For this purpose, a plurality of vibration energy absorbing portions 60 having an extremely small cutoff frequency f 3 are provided on a part or the whole of the edge portion of the piezoelectric substrate 1 as shown in Fig. 13a. With this construction, the amplitude of the leaking vibration energy is increased at this portion where the vibration energy is converted into thermal energy which is consumed thereat. The energy absorbing portions 60 may be provided either by thickening the piezoelectric substrate 1 or by attaching a conduction film or a resin film of an appropriate mass. Alternatively, as shown in Fig. 13b, a mass such as adhesive agent 70 may be attached on the outer surface. Although the cutoff frequency f 3 of the energy absorbing portions 60 may be equal to or less than f 3 , it is preferable to increase or decrease it so as to maximize the consumption of undesired vibration energy at this portion. Further, in order to positively consume the leakage energy, conductor films 80 may be attached at appropriate positions on the front and back surfaces of the edge portion of the piezoelectric substrate 1 with resistors 9 connecting therebetween as shown in Fig. 13c. The resistance value r of the resistors can be basically determined as follows: r = (2πf n C o ) -1 where f n : frequency of vibration to be converted into heat C o : capacitance between the conductor films provided on front and back surfaces. When there are a plurality of vibration frequencies whose energy is to be absorbed, a plurality of different resistors of (2πf a C o ) -1 , (2πf b C o ) -1 --- can be applied to the periphery of the resonator. Alternatively, a resistors of (2πf m C o ) -1 may be applied where f m represents the mean value of f a , f b --- . The resistor is not necessarily to be limited to that utilized to construct an electronic circuit. For example, the resistor may take the form of a coating of an electroconductive bonding agent for fixing the support of a conventional quartz vibrator. This measure is advantageous in that it can be used without affecting the productivity of the resonator. Two different constructions of the overtone vibration resonator utilizing a lower order asymmetric (a 0 ) mode vibration and manufactured based on the principle described above and their characteristics will be described hereunder. Fig.14a shows a plan view of a basic third order overtone vibration resonator not provided with a vibration energy absorbing portion according to this invention. As shown, lead wires 50 led out from the main electrode 20 and 20' split into two section are provided at the center of a circular quarts substrate for connecting in parallel, the front and rear electrodes having the same construction. In Fig.14b, vibration energy absorbing portions 60 are provided for the outer periphery of a quartz substrate. The results of measurement of the characteristics of these two types of resonators show that, in the former, the CI values of the third order and the first order (fundamental wave) overtone frequencies are 70Ω and 175Ω respectively. When a small quantity of an electroconductive bonding agent as applied these values become 72Ω and 380Ω respectively which proves the correctness of the theory. The CI values of the third order and the first order (fundamental wave) overtone frequencies are 65Ω and 250Ω respectively. Where an electroconductive bonding agent is' applied to the holder of the quartz substrate, these values become 65Ω and 1330Ω respectively, thus confirming the effectiveness of the vibration energy absorbing portion. In the resonator shown in Fig.14a, when the quartz substrate holder is fixed by the electroconductive bonding agent and then the electrode film thickness is varied. The difference between the CI values at the third order and the first order overtone frequencies decrease as shown in Fig.15a. This means that in the construction shown in Fig.12a, when Δ is caused to vary depending upon the electrode film thickness and both of the first order'and the third order overtone vibrations are entrapped, the difference in the CI values decreases. Consequently, it will be clear that various parameters including the electrode film thickness should be set to maintain sufficiently large the difference between their CI values. Fig.15b shows the variation in the CI values at the first order and third order overtone frequencies where the gap between the exciting electrode and the energy absorbing portion of a resonator provided with the vibration energy absorbing portions shown in Fig.14b is varied. As can be noted, as the gap increases, the leaked vibration energy of the first order (fundamental wave) frequency is not sufficiently consumed by the energy absorbing portions thus decreasing the effect thereof. Accordingly, it is essential to set the gap between the energy absorbing portion and the exciting electrode to an optimum value. In the construction shown in Fig.16a, resin films 70' are applied to the vibration energy absorbing portions 60 on the periphery of the piezoelectric substrate for decreasing the cutoff frequency of the portions 60. Fig.16b shows a construction in which, for the purpose of obtaining a sufficiently large cutoff frequency difference, the thicknesses of the substrate between the main electrodes 20 and 30 at the center of the substrate and the vibration energy absorbing portions 60 are reduced by etching for increasing the cutoff frequency thereof. Fig.16c shows a construction in which the thickness of the substrate is reduced between the main electrodes 20 and 30 and electrodes 80 are attached to the vibration energy absorbing portions 60. Thereby enabling electro-thermic conversion of the vibration energy. In Fig.16d, for the purpose of preventing increase in the electric resistance due to the decrease in the film thickness of main electrodes 20,20' and 30,30', only the electrode contacts to the substrates are etched to reduce their thicknesses. Although in the foregoing, a resonator utilizing the lowest order asymmetric (a 0 ) mode vibration and having simplest construction was described, it should be understood that the invention is not limited to this construction but can use higher order symmetric (S 1 , S 2 ---) mode vibrations as well as higher order asymmetric mode vibrations. The vibration mode of an illustrated resonator is the lowest order asymmetric vibration mode of a quartz substrate excited in Z direction, but the quartz substrate can also be excited in X directions. Furthermore, it is possible to divide the electrode in Z and X directions for exciting vibrations in both directions. Although in Fig.14b, the vibration energy absorbing portions were provided in the Z direction, these portions can be provided in the X direction, or around substantially the entire periphery of the substrate or the portions can be divided into a plurality of sections. The vibration energy absorbing portions becomes more efficient as the area thereof covering the periphery of the substrate increases. The invention is not limited to a thickness twist vibration as shown in Figs. 14a and 14b but is also applicable to such vibration mode as thickness longitudinal and thickness slip vibrations. In addition, various types of the piezoelectric material can also be used. Although not shown in the drawing, the resonator may be a two port resonator having a three terminal electrode construction. This construction makes easy vibration in high frequency bands. When the invention is applied to a monolithic crystal filter, a filter characteristic having a small spurious over a wide frequency range can be expected. resonator Fig.17 shows still another resonator in which resistors R having suitable values are connected across electrodes 4 and 4' on both surfaces of the energy absorbing portions near the outer periphery of the piezoelectric substrate. With this modification, the vibration energy of an order lower than the desired overtone order propagates to the vibration energy absorbing portions excites the piezoelectric substrate 1 at the vibration absorbing portions, and the electric charge generated in the electrodes 4 and 4' is converted into heat energy by resistors R and then consumed, whereby the impedance for the lower order overtone vibration containing the fundamental wave vibration increases sufficiently. The value of resistor R is set to be substantially equal to (2π f n C o ) -1 where C o represents the capacitance between electrodes 4 and 4' and fn the frequency of the overtone vibration, the energy thereof is to be converted into heat and consumed. This condition corresponds to the impedance matching of an alternating current circuit so that it is a most efficient measure for absorbing unwanted vibration energy. Where it is necessary to consume a plurality of vibration energies of the fundamental (first order) and the third order overtones such as the fifth order overtone vibration resonator, a frequency f m intermediate of the fundamental wave frequency f n1 and the third order overtone frequency f n3 is used for setting the value of resistor to R = (2π f m C o ) -1 . Alternatively, as shown in Fig.18, the vibration energy absorbing electrodes 4 and 4' are divided and resistors having values of R 1 = (2πf n1 C o ) -1 and R 2 = (2πf n3 C o ) -1 respectively are connected across respective divided electrodes. Fig.19a is a plan view showing a modification based on this principle and Fig.19b is a sectional view of this modification taken along a line A-A. In this modification, vibration entrapping electrodes 2 and 2' are provided for both surfaces at the center of a quartz substrate 1, and the electrodes 2 and 2' are connected to vibration energy absorbing electrodes 4 and 4' at suitable portions on the outer periphery of the substrate 1 via patterns 5 and 5'. The electrodes 4 and 4' are secured to resonator holding portions 6 upwardly extending from a base, not shown, for supporting the resonator by using an electroconductive bonding agent 7 for making electrical connection and mechanical fixing. In this case, care should be taken not to short circuit the front and rear electrodes 4 and 4' by the electroconductive bonding agent 7. By applying a bonding agent 8 having a desired resistance value to suitable portions of the electrodes 4 and 4', absorption of the unwanted vibration energy can be increased. As the bonding agent 8 can be used an epoxy resin containing a controlled quantity of dispersed carbon particles. Where there are a plurality of wave motions whose vibration energies are to be absorbed, the vibration energy absorbing electrodes 4,and the peripheral surface of the substrate 1 are split as shown in Fig.20, and the split electrodes 9 and 10 are bonded to the front and rear surfaces with the resistive bonding agent 8 described above. Of course, the resistance value of the bonding agent should correspond to the frequency of the wave motions whose vibration energies are to be absorbed. Figs. 21a and 21b show another resonator utilizing a composite material for use in a GHz band. As shown, boron (B) is doped into one surface of a silicon block 500, and a recess 600 of a desired diameter is formed by etching the opposite surface. Then a flat thin layer 700 having a thickness equal to that of the layer doped with boron and cannot be etched will remain. Then a lower electrode 800 and its lead electrode are formed by vapor deposition. After that, ZnO 2 is sputtered to a desired thickness, and a upper electrode 100 and its lead electrode are vapor deposited on the film of ZnO 2 . The portions covered by the electrodes 800 and 100 are utilized as the vibration energy entrapping portions having a cutoff frequency f 1 . The thin layer 700 extending about the outer peripheries of the electrodes is used as the vibration energy transmitting portion having a cutoff frequency f 2 , and the not etched portion 500 of the silicon block is used as a vibration energy absorbing portion having a cutoff frequency f 3 . Then a relation f 3 > f 1 > f 2 holds automatically so that by suitably controlling the ratio between f 1 and f 2 , the absolute value of f 2 and the diameters of the electrodes 800 and 100, an overtone vibration piezoelectric vibrator capable of vibrating at a desired order of overtone vibration can be obtained since these values correspond respectively to the plate-back amount Δ, substrate thickness H and the electrode size a which are the parameters of the entrapping coefficient. As is well known in the art as it is possible to manufacture extremely thin layer 700 and sputtered Z n O, and hence having a high fundamental wave vibration frequency, it will be clear that it is possible to provide a high frequency oscillation circuit having 3, 5 or 7 times frequency and not requiring any special circuit, for example, a LC tuning circuit. The resonator is not limited to a silicon block. According to a preferred embodiment of the invention, as shown in Fig. 22, an opening having a desired diameter is formed at the center of an artificial quartz or glass block 110 by mechanical machining or etching, and a quarts or glass plate 130 is strongly bonded to one surface of the block by a three layer alloy bonding agent 120 consisting of Aℓ-Ag-Jn. After lapping the quartz or glass plate to a desired thickness, upper and lower electrodes 800 and 100 and a piezoelectric portion 900 made of Zn O and sandwiched between the upper and lower electrodes are formed. Alternatively, according to a further preferred embodiment of the invention, an opening is formed through the central portion of a metal block 140 as shown in Fig.23, and a metal plate 150 of the same material is seam welded to one surface of the metal block 140. Then the metal plate 150 is lapped to a desired thickness thereby forming a vibration portion. When such vibration portion is used as the vibration portion, it can be used as the lower electrode, the piezoelectric substrate Can be directly sputtered onto the thin metal plate 150 and an insulating film 160 is applied onto only the lead pattern wiring of the upper electrode attached to the piezoelectric portion 900. Thereafter, the lead pattern is formed. In addition to Z n O, Aℓ N and C d S can also be used the piezoelectric material, and other method than sputtering can be used as the method of attaching the piezoelectric material. However, as the sputtering deposition method is most suitable for preparing extremely thin piezoelectric layer, this method can satisfy the object of this invention for preparing a resonator of an extremely high frequency. In this case, usually since the crystal grows in the axial direction, the vibration mode is limited to the thickness longitudinal vibration. Also, in this embodiment, a thickness slip vibration mode and a thickness twist vibration mode can be used. In the vibration energy absorbing portion 4 shown in Fig.3a, the consumption of an overtone vibration on energy of an order lower than that of the desired overtone vibration after converting it into heat, is not sufficient as that the impedance at a corresponding frequency does not rise sufficiently. Thus a desired overtone vibration may become impossible in view of the negative resistance characteristic of the oscillation circuit as shown in Fig.24. Accordingly, an experiment was carried out by using an overtone vibration resonator shown in Fig. 25 in which the amount of attaching the vibration energy absorbing electrode 4 was varied for determining the difference between the CI value of the fundamental wave vibration and the CI value of the desired third order overtone vibration. Fig.26 is a graph showing the result of experiment in which after fixing the size of the exciting electrode 2 made of Ag, the film thickness is varied for varying the entrapping coefficient na√Δ/H. For eight entrapping coefficients, the amount of Ag of the vibration energy absorbing electrodes 4 was varied to 1, 2, 4 and 8 times of that of the exciting electrode, and the relation between the CI value of the fundamental wave vibration to be suppressed and the CI value of the desired third order overtone vibration was plotted. As can be noted from Fig.26, the CI value of the fundamental wave vibration to be suppressed has a peak value except a case wherein the ratio (film thickness ratio) between the thickness of the energy absorbing electrode mode of Ag and that of the excitation electrode is unify, the peak value increasing with the increase in the film thickness ratio. The same tendency was observed in the fifth order and the seventh order overtone vibration resonators although figures were omitted for the sake of simplicity. Above described result of experiment shows that it is necessary to make lower the cutoff frequency of the vibration energy absorbing portion than that of the vibration energy absorbing portion attached with an excitation electrode. Otherwise, it is impossible to have a sufficient difference between the CI value of the overtone vibration of an order at which the vibration is desired and that of the lower order overtone vibration containing the fundamental wave vibration to be suppressed, thus disenabling stable vibration at a desired frequency. When designing an overtone vibration piezoelectric resonator as shown in Fig.3a, it is essential to select the relation between a film thickness ratio at which the CI value for an overtone vibration of an order at which the vibration is desired to be extremely low while the CI value of the overtone vibration of a lower order including the fundamental wave vibration is extremely high, and the value of the entrapping coefficient by taking into consideration the result of experiment described above. The result of experiment shown in Fig.26 shows that optimum parameters for the third order overtone vibration resonator can be obtained when the film thickness ratio of the energy absorbing portion to the excitation electrode is selected to be 4 and the entrapping coefficient na√Δ/H is selected to be about 4.5. For decreasing the cutoff frequency of the vibration energy absorbing portion, it is not only possible to elongate only the vapor deposition time of a conductor to the portion, and to form a recess at the central portion of the piezoelectric substrate 1 as shown in Figs. 27a through 27c for creating a difference between cutoff frequencies. Alternatively, as shown in Fig.27a, the same material a" as the electrode can further be vapor deposited onto the vibration energy absorbing portion 400 thus formed. The cutoff frequency can be made more lower by using a vapor deposited material having a higher specific gravity than that of the electrodes 2 and 2' (Fig. 27b). Further, as shown in Fig.27c, for the purpose of obtaining a higher overtone vibration frequency, where the portions of the piezoelectric substrate on which the electrodes are to be attached are made particularly thin, it is sufficient to increase the thickness of the portions of the substrate which constitute the vibration energy absorbing portions. The cutoff frequency f 2 of the piezoelectric substrate about the electrodes 2 and 2' is not always necessary to be perfectly the same on both surfaces and a slight difference is permissible. Even when the cutoff frequency f 3 of the vibration absorbing portions 400 at the periphery of the piezoelectric substrate varies substantially so long as the condition f 3 < f 1 is satisfied, there is no problem. An electrode construction of the piezoelectric resonator will now be described that enables to compensate for the deterioration of the resonator characteristic caused by the deviation of the position of additional vapor deposition effected for the purpose of fine adjustment of the vibration frequency, and to greatly decrease the influence upon the resonator characteristic caused by the difference in the thickness of the piezoelectric substrate thereby increasing the upper limit of the resonance frequency. Fig.28a is a sectional view showing the fundamental construction of the electrode of such piezoelectric resonator. More particularly, a surface electrode 2 is attached near the central portion of the surface of a piezoelectric substrate 1 and a rear electrode 2' having the same or wider area is attached to the rear surface, and the frequency is adjusted by applying an additional vapor deposition to the electrode 2'. With this measure, even when the position of the additional vapor deposition for the fine adjustment of the frequency displaces a little, since the area of the additional vapor deposition is substantially wider than the area of the surface electrode, for example by about 1.5 times, there is no possibility of resulting in a new spurious. Even when the spurious is generated it is possible to limit it to an extremely low level, thereby causing no problem. Fig.28b shows a construction wherein the rear electrode 2' is extended to cover the entire rear surface of the piezoelectric substrate. This construction has the same advantage as that of the embodiment shown in Fig.28a. However, since the position of the vapor deposition mask utilized at the time of manufacturing the resonator may be rough, the construction shown in Fig.28b is advantageous from the point of produce ability. The piezoelectric vibrator having the electrode construction just described is advantageous for the following application. It is inevitable that the thickness of a piezoelectric substrate varies more or less during the manufacturing steps. In such case, in the prior art vibrator, the resonance frequency is matched by applying an additional vapor deposited film onto the surface electrode. This measure can eliminate variation in the resonance frequency, but since the amounts of plate back are different, the amount of entrapping the vibration energy is also different. As a consequence, the state of occurrence of the spurious caused thereby is different for respective resonators. When this electrode construction is applied to such case, there arises an effect as shown in Figs. 29a and 29b. More particularly, when an additional vapor deposition film 300 is added to the entire surface of the rear electrode 2' covering the entire rear surface as shown in Fig.29b for matching the resonance frequency to that of Fig.29a, the overall thickness of the piezoelectric substrate 1 increases so that an effect equivalent to that obtained when the natural frequency of the substrate itself is adjusted. Consequently, not only the adjustment of resonance frequency of the resonator is possible but also vibrators of less variation in the characteristic can be mass produced. This construction is advantageous for obtaining a resonator required to have a high resonance frequency. More particularly, when preparing a resonator described above, the electrode is desired to have a film thickness sufficient to provide a desired percentage of vibration energy entrapping. The parameter that governs the percentage of vibration energy entrapping is called an entrapping coefficient which is given by na √Δ/H, where n designates an order number of the overtone, a the electrode size, Δ the amount of plate feed back and H the substrate thickness. Where a resonator having a high resonance frequency is desired, the designer is requested to select parameters of the entrapping coefficient for obtaining desired energy entrapping percentage on a mode curve irrespective of what mode of vibration is used. There is an optimum value of the entrapping coefficient that minimizes accompanying auxiliary resonances. When investigating respective parameters of the entrapping coefficient, the substrate thickness H is substantially determined by the desired resonance frequency and the electrode size a is obliged to be determined to a predetermined value due to an equivalent inductance etc., so that only the remaining parameter, that is the amount of plate feed back, that is the electrode film thickness (sum of the surface and rear film thicknesses) presents a problem. As can be noted from the definition of the entrapping coefficient, the electrode film thickness becomes thin as the resonance frequency increases. Consequently, in the prior art vibrator, the thin electrode film is divided into equal sections on the front and rear surfaces, the respective sections can not act as the electrodes from the standpoint of electroconductivity. As is well known in the art, the thickness of the electrode film should be larger than 350 Å from the standpoint of electroconductivity irrespective whether what type of metal is used. Assuming that the film thickness of the surface electrode that determines the resonator characteristic, for example, the frequency of generating the auxiliary resonances is set near the limit of the electrode thickness, for example about 400 Å, this thickness determines the upper limit of the resonance frequency which is possible to design. Since the film thickness of the large are a electrode covering the entire rear surface has no influence upon such characteristic as the auxiliary resonance, the film thickness can be exclusively used to determine the frequency. For this reason, the thickness of the front and rear electrodes may be 400 Å and more. This measure enables to realize a piezoelectric resonator having a high resonance frequency which has been impossible in the past. A trially manufactured AT cut quartz vibrator had a substrate diameter of 5 mm, substrate thickness of 19 µm, a diameter of the surface and rear electrodes of 3 mm, a film thickness of 700 Å, a diameter of the surface electrode of 1.4 mm, a film thickness of 500 Å and a resonance frequency of 88 MHz. It was found that there was no problem of the auxiliary resonance. Assuming a diameter of 5 mm, a quartz substrate having a thickness of about 20 µm can be manufactured according to the present technique of polishing quartz. However, a resonator having diameter of 5 mm with electrodes of a thickness of about 500 Å close to the limit that satisfies the electrical characteristic attached to both surfaces of the substrate will have a substrate thickness of about 24 microns and its resonance frequency is limited to about 68 MHz. Thus, by using the electrode construction of this invention, the upper limit of the resonance frequency that has been considered possible can be increased by 30%. Furthermore this electrode construction is advantageous for the electrode of an overtone vibration piezoelectric vibrator requiring a delicate adjustment. Fig.30a is a plan view showing an overtone resonator which is the most fundamental resonator utilizing the lowest order symmetric (S 0 ) mode. As shown, a vibration energy entrapping excitation electrode 2 is provided at the center of the piezoelectric substrate 1 and leaked vibration energy absorbing electrodes 4 and 4' are attached to suitable positions on the outer periphery of the piezoelectric substrate, the electrodes 2, 4 and 4' being formed as conductor vapor deposited patterns. In the prior art, the construction of the rear electrode is the same as that shown in Fig.30a. In an overtone resonator having above described electrode construction, unless the gap between the exciting electrode 2 and the vibration energy absorbing portion 4 is maintained at a suitable value, that is when the gap is too large, the leaked vibration energy will not be consumed sufficiently. Consequently, when the gap is too small the vibration energy of a desired overtone vibration too is absorbed and consumed, thus disenabling vibration at the desired frequency. Consequently, where the position of the additional vapor deposition to the excitation electrode 2 for the purpose of frequency adjustment becomes different, a serious problem occurs. The problem described can be eliminated and the adjusting steps can be simplified by providing a rear electrode 6 on the rear surface of the substrate, which covers both of the excitation electrode 2 and the vibration energy absorbing portions 4, as shown in Fig.30b. Since the overtone resonator is constructed to generate an overtone vibration (a high frequency vibration) by utilizing the inherent characteristic of the resonator without utilizing any special means, for example a LC tuning circuit, the fact that the fundamental frequency of the vibrator is high is a desirable characteristic. Accordingly, when the electrode construction just described is applied to this overtone vibrating vibrator not only the problem occurring at the time of adjusting the vibrator characteristic can be solved but also the resonance frequency of the resonator can be increased, which are extremely advantageous. In a resonator having a single exciting electrode 2, whether the rear electrode 2' has a small area (Fig.31a) or a large area covering the entire rear surface (Fig.31b) it is necessary to lead out a lead pattern 5. However, the relative size of the rear electrode 2' and the lead pattern 5 causes an unbalance of the mechanical vibration of the piezoelectric substrate. To alleviate such unbalance, a balancing pattern 5a acting as a counter balance weight is added to a suitable portion of the exciting electrode. This construction can also be applied to a two port resonator wherein the exciting electrode is divided into a plurality of sections or to a dual mode filter having a similar electrode construction. Fig.32a shows the electrode construction when the present invention is applied to a two port vibrator or a dual mode filter. Fig.32b shows the electrode construction when the present invention is applied to an overtone vibration vibrator utilizing the lowest order asymmetrical (a 0 ) mode vibration, and Fig.32c shows the electrode construction when the present invention is applied to an overtone vibration vibrator utilizing a higher order symmetric (S 1 ) mode vibration With these electrode constructions, the problem of spurious caused by the displacement of the vapor deposition portion can be eliminated with simplified adjustment, etc. by merely varying the area or thickness of the electrode on the front or rear side of the vibrator. In addition, variation in the resonator characteristic caused by the variation in the percentage of vibration energy entrapping caused by the variation in the thickness of the piezoelectric substrate, when the vibration frequency thereof is adjusted. Moreover, as it is possible to increase the upper limit of the frequency of the resonator it has an excellent temperature characteristic and capable acting as a frequency source for various electronic apparatus and devices, the operating frequency increasing in recent years. Since the resonator of this invention is constructed as described in claim 1 the resonator can readily generate an overtone frequency without adding to the oscillation circuit any specific circuit, for example, a LC tuning circuit. Accordingly, the resonator of this invention can be advantageously used as a frequency source of various electronic apparatus and devices which are desired to operate at higher frequencies and to be constructed as integrated circuits of higher densities. Further in a conventional piezoelectric resonator, especially in such thickness vibration type resonator as an AT cut quartz resonator having an excellent temperature characteristic, when it is desired to obtain a high frequency at the fundamental wave vibration, the thickness of the piezoelectrics substrate becomes extremely small, thus making it difficult to manufacture. However, according to the invention as defined in claim 1, the resonator can be readily excited at an overtone frequency so that the thickness of the piezoelectric substrate can be selected in a range of ready manufacture thereby decreasing the manufacturing cost.

Claims: An overtone oscillation piezoelectric resonator in which: a vibration medium is provided, a piezoelectric material (900) including a pair of excitation electrodes (100,800) is attached to said vibration medium for providing a vibration energy entrapping portion having a cutoff frequency f 1 , said vibration medium includes a thin-layer portion and a vibration medium block (110;140) surrounding said thin-layer portion, said excitation electrodes (100,800) are attached substantially to the center of said thin-layer portion, a vibration energy propagating portion extends about the outer periphery of the excitation electrodes (100,800) having a cutoff frequency f 2 , and a vibration energy absorbing portion having a cutoff frequency f 3 is adapted to absorb the vibration energy propagated through said vibration energy propagating portion, the configuration of the excitation electrode attached to the piezoelectric material (900) is selected to excite a symmetric or an asymmetric mode vibration, and an entrapping coefficient na√Δ/H in said selected mode is selected to cause the oscillation of the desired overtone to occur more easily than the oscillation caused by the overtone vibration of an order lower than said desired overtone vibration, characterized in that a substrate (130;150) providing said thin-layer portion and said vibration medium block (110;140) are bonded to each other, and the relation among the cutoff frequencies (f 1 , f 2 , f 3 ) of said vibration energy entrapping portion, vibration energy propagating portion and vibration energy absorbing portion is established as f 3 <f 1 <f 2 . The resonator according to claim 1 wherein said vibration medium block is made of silicon. The resonator as set forth in claim 1 wherein said vibration medium block (110) is made of quartz crystal or glass, and a quartz crystal or glass substrate (130) is bonded to an opening formed at a predetermined portion of said vibration medium block (110) with a bonding agent, and said substrate is ground to have a predetermined thickness. The resonator as set forth in claim 1 wherein said vibration medium block (140) is made of metal, and a substrate (150) made of the same metal is welded to an opening formed at a predetermined portion of said vibration medium block (140), and said substrate (150) is ground to have a predetermined thickness. The resonator according to either one of claims 1 through 4 wherein said piezoelectric material (900) comprises ZnO, AlN and CdS. The resonator according to either one of claims 1 through 5 wherein said thin-layer portion is of the same material as a vibration medium block (110;140).

Inventors: HIRAMA KOUICHI, SHOUZI TOMOKAZU, TANAKA YOSHIAKI, HIRAMA, KOUICHI, SHOUZI, TOMOKAZU, TANAKA, YOSHIAKI, HIRAMA, KOUICHI TOYO COMMUNICATION EQUIPMENT, SHOUZI, TOMOKAZU TOYO COMMUNICATION EQUIPMENT, TANAKA, YOSHIAKI TOYO COMMUNICATION EQUIPMENT
Applicants: TOYO COMMUNICATION EQUIP, TOYO COMMUNICATION EQUIPMENT CO.,LTD.
Application year: 1986
Main classification: H03H9
IPC codes: H03B5, H03H9