STRUCTURE OF ELECTRODE AND LEAD THEREOF OF ULTRA THIN PLATE PIEZOELECTRIC RESONATOR

Abstract: A piezoelectric resonator in which an ultra thin vibration part and a thick ring-like surrounding part supporting the periphery of the vibration part are formed in a body. An electrode and an electrode lead pattern extending from the electrode are formed on one side of the vibration part. Another electrode facing the former is formed on the other side of the vibration part except for the region facing the lead pattern. Thus, a capacitance is formed from the mutually facing electrodes and the lead patterns, preventing the increase of the parallel capacitance of the resonator. The reduction of ohmic loss and the suppression of spurious response are realized by making the electrode lead long and narrow, etc. Defect of conduction between the conductive film of the bottom of a package and the electrode of a recessed part is prevented by making a through hole through the ring-like surrounding part in the direction of its thickness and by connecting electrically the electrode lead part extending on one surface of the ring-like surrounding part with the terminal pad provided on the surface opposed thereto via the through hole.

Description: The present invention relates to electrodes and their lead structure of an ultrathin piezoelectric resonator whose fundamental frequency is as high as tens to hundreds of megahertz. In recent years there has been a strong and growing demand for high frequency operation and high frequency stability of various pieces of electronic and communication equipment. An ordinary AT cut quartz crystal resonator, which has heretofore been used widely as a piezoelectric device (such as a resonator or filter), has a very excellent temperature-frequency characteristic; however, since its resonance frequency is in inverse proportion to the thickness of AT cut quartz crystal plate ,the fundamental frequency of this kind of resonator with a mechanical strength sufficient for practical use is around 40 MHz at the highest. There has also been widely employed what is called overtone oscillation means which extracts a higher order harmonic mode vibration of an AT cut quartz crystal resonator to obtain a frequency which is an odd multiple of the fundamental resonance frequency,but its oscillation circuit calls for an LC tuning circuit including a coil, and hence is not suitable for fabrication as a semiconductor IC, besides the overtone oscillation circuit may sometimes be difficult to activate because such a resonator has a large capacitance ratio and consequently a high impedance level. On the other hand, a surface acoustic wave resonator,whose oscillation frequency is determined by the pitch of electrode fingers of an interdigital transducer, has come to be able to output a maximum of 1 GHz or so due to the progress in photolithography, the resonator of this kind possesses a problem as a piezoelectric substrate usable therefor is remarkably inferior to the AT cut quartz crystal in terms of temperature-frequency characteristic. To solve the above-mentioned problems, study has been given such a piezoelectric resonator as shown in Figs. 1(a) and 1(b). The piezoelectric resonator has a cavity 2 provided by machining or etching in one side of a block of AT cut quartz crystal 1 centrally thereof to form a vibratory portion 3, which has a thickness of about 17 µm, if a fundamental resonance frequency of, say, 100 MHz is desired to obtain. On the side of the block where the cavity 2 is provided,the ultrathin vibratory portion 3 is edged with and mechanically supported by a thick frame-like marginal portion (or rib) 4 formed integrally therewith. By depositing a conductive film 5 all over the piezoelectric substrate on the side where the cavity 2 is provided and by depositing a partial electrode 6 and an electrode lead 7 extending therefrom, through use of, for example, vacuum evaporation on the flat surface of the vibratory portion 3 opposite from the cavity 2, it is possible to obtain a resonator of a very high resonance frequency substantially equal to the fundamantal resonant frequency of the ultrathin vibratory portion 3 or a filter element of such a high center frequency. As can be imagined from its structure, the above-said piezoelectric resonator is suitable for housing in such a flat package as depicted in Fig. 2. It is customary and technically advantageous to house the piezoelectric resonator upside down--with the cavity 2 of the quartz crystal block 1 facing downward--in the bottom of ceramic or similar dish-like case 8 which is concave centrally thereof and to mechanically bond and electrically connect the overall electrode 5 and a conductive film 10 exposed on the bottom of the concavity of the case 8, by a conductive adhesive 9 laid down in line on the top of one side of the frame-like marginal portion 4. Incidentally, the conductive film 10 on the bottom of the concavity of the case 8 is connected to a terminal 11 formed at one corner of the case 8 on the underside thereof via a connecting conductor hermetically passing through the case 8. A pad 7a at one end of the electrode lead 7 extending from the partial electrode 6 of the resonator has a minimum area necessary for connection, by a bonding wire 12, to a pad 14 formed on a stepped portion 13 on the inside of the case 8. The pad 14 is also connected to a terminal 15 provided at another corner of the case 8 on the underside thereof via a connecting conductor which hermetically passes through the case 8. The above-described ultrathin resonator studied so for has the overall electrode 5 on the side of the piezoelectric block (i.e.the piezoelectric plate) I where the cavity 2 is provided, and hence is easy to manufacture, but the overall electrode 5 and the electrode lead portion 7 extending from the partial electrode 6 are deposited on both sides of the ultrathin vibratory portion 3 and they constitute a capacitor of a large capacity, which causes an increase in a parallel capacitance C o in an equivalent circuit of the piezoelectric resonator shown in Fig. 3. The increase in the parallel capacitance C o means an increase in the capacitance ratio γ (= C o /C 1 ) of the piezoelectric resonator; consequently, in the case of applying the piezoelectric resonator to a voltage controlled quartz crystal oscillator (VCXO), the variable width of its oscillation frequency is limited, and in the case of using it as a band-pass filter, its passband is limited, as is well-known in the art. It is also self-evident that the reduction of the width of the electrode lead portion 7 with a view to avoiding the increase in the capacitance ratio γ will cause an increase in an ohmic loss, directly leading to a reduction in the Q of the resonator or an increase in the insertion loss of the filter. The present invention is intended to solve the above-mentioned problems of the ultrathin piezoelectric resonator which have been left unsolved so far, and it is an object of the invention to provide an electrode structure of an ultrathin piezoelectric resonator which is free from various drawbacks attributable to an increase in an equivalent parallel capacitance of the resonator which results from the formation of a capacitor between the overall electrode and the electrode lead portion deposited on both sides of the ultrathin vibratory portion forming the bottom of the cavity made in one side of a block of a piezoelectric material such as quartz crystal. If the width (the sectional area) of the electrode lead portion 7 extending from the partial electrode 6 deposited opposite the overall electrode 5 on the bottom of the cavity of the piezoelectric plate 1 is selected large enough to make the ohmic loss(i.e. the electrode resistance) negligible, then a resonance is caused by the electrode lead portion 7 serving as a dummy electrode in the vicinity of the point of resonance by the afore mentioned true electrode and the resonance becomes a high-level spurious . It is therefore an object of the present invention to provide an electrode lead structure of such an ultrathin piezoelectric resonator which is free from the generation of such a spurious resonance in the case where the width of the electrode lead portion extending from the partial electrode deposited opposite the overall electrode is wide enough to reduce the ohmic loss. JP-A-60 210018 discloses an ultrathin piezoelectric resonator wherein an overall electrode is provided over the entire area of one surface of a piezoelectric plate including an ultrathin vibratory portion and a surrounding thick frame-like marginal portion formed integrally therewith for supporting the vibratory portion; and a partial electrode and an electrode lead extending therefrom to one marginal edge of the piezoelectric plate are provided on the other surface of the piezoelectric plate and, according to the present invention, such a resonator is characterised in that the electrode lead comprises two or more narrow lead patterns. In the accompanying drawings: Figs. 1(a) and (b) are a perspective view illustrating the construction of a conventional ultrathin piezoelectric resonator and a sectional view taken on the line X-X in Fig. 1(a). Fig. 2 is a sectional view showing its packaging method; Fig. 3 is an equivalent circuit of a resonator; and Fig. 4 is a plan view showing an ultrathin piezoelectric resonator having an electrode lead of about the same width as that of the partial electrode. Fig. 5 is a plan view illustrating the basic structure of an embodiment of the present invention; Fig. 6 is a plan view showing the application of the embodiment to such an ultrathin block of the piezoelectric material; Fig. 7 is a plan view illustrating an embodiment of an ultrathin multimode filter element in accordance with the invention; and, Figs. 8(a) and 8(b) are plan and sectional views of a resonator of a type to which the invention is applicable. The scope of the invention is defined by the appended claims. To facilitate a better understanding of the present invention, a brief description will be given first of the reason for which the ultrathin piezoelectric resonator, on which the present invention is based, employs the electrode structure which has the overall electrode deposited on the piezoelectric substrate on the side where the cavity is provided and the partial electrode on the opposite side therefrom. Firstly, it is very difficult, from the viewpoint of vacuum vapor deposition techniques, to form the partial electrode on the vibratory portion in the cavity of the piezoelectric substrate and the narrow electrode lead which extends from the partial electrode climbing over the inner wall of the cavity and a stepped portion contiguous thereto, although the formation of the partial electrode and its lead portion can be carried out by a troublesome method which involves tilting the piezoelectric substrate during the vapor deposition, and in this instance it is not sure to secure the conduction of the electrode lead portion. As a solution to this problem, the overall electrode is formed on the side where the cavity is provided. Secondly, the resonator of this type is intended primarily for microminiaturization and it is desirable that the piezoelectric substrate has a size of 3 mm by 3mm or less, for example. Accordingly, the present invention utilizes a method by which a number of chips are formed on one piezoelectric wafer by batch processing and the wafer is cut into individual chips. In this case, since the above-said electrode structure is employed , the overall electrode can be formed simply by evaporating a conductive film all over one side of the wafer and no photomask or mask for photolithography is needed-this improves the efficiency of production, i.e. the yield rate of product, and hence affords reduction of manufacturing costs. It must be noted that, for the reasons given above, the ultrathin piezoelectric resonator studied by the inventors so far has the overall electrode deposited on the substrate on the side where the cavity is formed. With the above-mentioned electrode structure, however, the vibratory portion is so thin that a large capacitance is formed between the overall electrode and the lead portion extending from the partial electrode deposited on opposite sides of the vibratory portion, and as a result, the capacitance ratio of the resonator becomes large, introducing various disadvantages. If the area of the lead portion 7 of the partial electrode 6 opposite the overall electrode 5 is made large to prevent an increase in the ohmic loss in the lead portion 6, the width of the electrode lead portion 7 on the ultrathin vibratory portion 3 becomes about the same as the width of the partial electrode 6 as shown in Fig. 4, because the partial electrode 6 itself is very small. As a result, the overall electrode 5 and the electrode lead portion 7b deposited on both sides of vibratory portion 3 act as dummy electrodes and a spurious resonance of large level is caused by the electrode lead portion 7b in the vicinity of the point of resonance by the true electrode 6 as referred to previously. To solve this problem, the ultrathin piezoelectric resonator according to the present invention uses such an electrode structure as described below. Fig. 5 is a plan view showing the basic structure of the present invention, wherein that portion of the electrode lead 7 extending from the partial electrode 6 deposited opposite the overall electrode 5 on the piezoelectric block 1 which lies on the ultrathin vibratory portion 3 is formed by a plurality of narrow electrode lead patterns 7c, 7c, ... in such a manner that the sum of their sectional areas may be sufficiently large enough to prevent an increase in the ohmic loss. With such an electrode structure, a spurious resonance between the overall electrode 5 and each of the divided electrode lead patterns 7c, 7c, ... develops at a considerable distance from the point of resonance by the true electrode and its level is also low; hence, various characteristics of the resonator do not suffer any particular deterioration. In this embodiment the three electrode lead patterns 7c are extended from the partial electrode 6 to the pad 7a on the marginal portion of the piezoelectric block 1 but this is intended as being merely illustrative and the number of electrode lead patterns 7c may be two or more. Moreover, the electrode lead patterns need not always be parallel to each other. The area of the pad 7a can be set arbitrarily, because it lies on the thick frame-like marginal portion 4 and has nothing to do with the resonance of the vibratory portion 3 (even if it resonates, its resonance frequency is very lower than that of the resonance of the vibratory portion). According to the present invention, it is thus possible essentially to preclude the possibility of spurious resonance or cause it to develop at a considerable distance from the resonance point of the resonator, simply by slightly modifying the shape of an evaporation mask or photomask for use in the formation of the partial electrode 6. Accordingly, the electrode structure of this embodiment is markedly effective in maintaining the spurious resonance characteristic of the easy-to-manufacture ultrathin piezoelectric resonator with the overall electrode deposited over one side of the piezoelectric block 1 without increasing its equivalent resistance. Fig. 6 shows an application of the electrode structure of Fig. 5 to the ultrathin piezoelectric block 1-including such unetched portions 20 and 22 as depicted in Figs. 8(a) and (b). As shown in Figs. 8 (a) and (b), the narrow electrode lead portion 7, which extends from the partial electrode 6 deposited on the side opposite from the overall electrode 5 formed on the ultrathin piezoelectric block 1 on the side where the cavity 2 is provided, is shortened to a position corresponding to a boundary portion 21 between an inclined unetched portion of the inner wall of the cavity 2 and the vibratory portion 3 and a wide conductor pad 7a is provided between the boundary portion 21 and the marginal edge of the piezoelectric block 1. The reason for which this embodiment adopts such an electrode structure is as follows. In the formation of the cavity 2 in the AT cut quartz crystal block 1 by etching, unetched portions 20 and 22 are formed on the inner wall of the cavity 2 at a substantially constant angle of inclination in +x and -x directions, respectively, as shown in Fig.8 (a) . Consequently, the inner wall of the cavity 2 formed by etching does not form a vertical wall from the frame-like marginal portion 4 to the vibratory portion 3, but instead the unetched inclined portion of widths ℓ 1 and ℓ 2 are formed between the vibratory portion 3 and the frame-like marginal portion 4. These unetched portions meet the vibratory portion 3 at boundaries 21 and 23. Incidentally, the slope of the unetched portion 20 in the +x direction is gentler than the slope of the unetched portion 22 in the -x direction, that is, ℓ 1 > ℓ 2 . By depositing the wide conductor pad 7a on the same side as the partial electrode 6 over the area between the unetched inclined portion of the width ℓ 1 and the marginal edge of the frame-like marginal portion 4, the electrode lead pattern 7 extending between the conductor pad 7a and the partial electrode 6 is narrow but is shortened by ℓ 1 . This affords further reduction of the ohmic loss of the electrode lead portion 7 and permits holding low the capacitance between the electrode lead portion 7 and the overall electrode 5. Thus, as shown in Fig. 6, the pad 7a lying on the frame-like marginal portion 4 is extended to the portion corresponding to the unetched portion 20 and the electrode lead portion 7 on the vibratory portion 3 is formed by a plurality of narrow electrode lead patterns 7b. In this instance, the area of the electrode lead pattern 7b is shortend--this affords further reduction of the level of the spurious resonance by the electrode lead portion 7b and, at the same time, decreases the ohmic loss of the electrode lead portion and the equivalent parallel capacitance of the resonator. The ultrathin piezoelectric resonator is suitable for housing in, for example, a ceramic-sintered dish-like package such as shown in Fig. 2. That is, it is effective in simplifying the manufacturing process of the device to house the piezoelectric block 1 with the cavity 2 facing down toward the bottom of the package 8 and electrically connect and mechanically , fix the aluminum electrode film 5 on the frame-like marginal portion 4 of the block 1 and the aluminum film 10 on the bottom of the package 8 through use of the conductive adhesive 9. Fig. 7 shows an example of application of the embodiment of the present invention to a dual-mode filter. It is self evident that, also in the multimode filter element, if the ohmic loss of the electrode lead extending from each split electrode 19,19 is large, the insertion loss of the filter increases accordingly, and if the electrode lead is made wide, a spurious resonance is caused by the electrode lead in the vicinity of the passband of the filter, posing a problem that a desired attenuation cannot be obtained. In view of this, those portions of the electrode leads 17,17 extending from the split electrodes 19, 19 which are formed on the vibratory portion 3 are each formed by a plurality of narrow electrode lead patterns 17c as shown in Fig. 7. With such an electrode structure, it is possible to reduce the spurious resonance which occurs in the vicinity of the passband of the filter having a center frequency of tens to hundreds of megahertz. Incidentally, the area indicated by 16 in Fig. 7 is a blank area where the conductive film forming the overall electrode 5 is selectively removed from the area corresponding to the lead pattern 17c to form the blank area 16, thereby decreasing the parallel capacitance C o .

Claims: An ultrathin piezoelectric resonator wherein an overall electrode (5) is provided over the entire area of one surface of a piezoelectric plate (1) including an ultrathin vibratory portion (3) and a surrounding thick frame-like marginal portion (4) formed integrally therewith for supporting the vibratory portion; and a partial electrode (6) and an electrode lead (7) extending therefrom to one marginal edge of the piezoelectric plate are provided on the other surface of the piezoelectric plate; characterised in that the electrode lead (7) comprises two or more narrow lead patterns (7b, 7c, 17c). A resonator according to claim 1, wherein a wide conductive pad (7a) is provided on the other surface of the piezoelectric plate (1) in an area from one marginal edge (4) thereof to the boundary (21) between an inner side wall (20) of a cavity defined by the frame-like marginal portion (4) and the vibratory portion; and the conductive pad (7a) and the partial electrode (6) are coupled together by the narrow electrode lead patterns. A resonator according to claim 2, wherein the inner side wall (20) is gradually inclined to the vibratory portion.

Inventors: ISHII OSAMU, KUROSAKI TAKEHUMI, MORITA TAKAO, ISHII, OSAMU, KUROSAKI, TAKEHUMI, MORITA, TAKAO, ISHII, OSAMU, TOYO COMMUNICATION EQUIP. CO., LTD., KUROSAKI, TAKEHUMI, TOYO COMM. EQUIPMENT CO., LTD., MORITA, TAKAO, TOYO COMMUNICATION EQUIP. CO. LTD.L
Applicants: TOYO COMMUNICATION EQUIP, TOYO COMMUNICATION EQUIPMENT CO. LTD.
Application year: 1991
Main classification: H03H9
IPC codes: H03H9