ULTRA THIN QUARTZ CRYSTAL FILTER ELEMENT OF MULTIPLE MODE

Abstract: An ultra thin quartz crystal filter element of multiple mode a quart crystal elementary plate of AT-cut constituted in a body by an ultra thin vibrating part and a thick annular part surrounding the vibrating part. Split electrodes are arranged on the vibrating part along the X-axis of the elementary plate, being close to each other to an extent that the electrodes are not short-circuited. Desired acoustical coupling can be obtained in spite of the closely arranged electrodes. Therefore, a filter element having a center frequency of tens to hundreds MHz and having a sufficient pass band can be obtained with good yield.

Description: The present invention relates to a multimode quartz crystal filter element using an ultrathin AT cut quartz crystal substrate which is capable of oscillation at high frequencies as high as tens to hundreds of megahertz through fundamental vibration. A quartz crystal plate to be used for a conventional ordinary AT cut quartz crystal resonator is flat, and hence its fundamental frequency obtainable for practical use is around 40 MHz at the highest from its manufacturing techniques and mechanical strength. On this account there has 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 an IC, besides the overtone oscillation circuit may sometimes be difficult to activate because a resonator to be included in the overtone oscillator has a large capacitance ratio and consequently a high impedance level. On the other hand, a surface acoustic wave (SAW) resonator, whose resonant frequency is determined by the pitch of electrode fingers of an interdigital transducer, has come to be able to resonate at a maximum of 1 GHz or so due to the progress in photolithography, but a piezoelectric substrate usable for the surface acoustic wave resonator is remarkably inferior to the AT cutquartz crystal in terms of temperature-frequency characteristic. To solve this problem. there has been proposed and is now under studyan ultrathin piezoelectric resonator which has a cavity provided, by etching or mechanical grinding, in one side or in both sides of a block of AT cut quartz crystal to form an ultrathin vibratory portion surrounded with a support frame so that the fundamental resonant frequency may extend from tens to hundreds of megahertz while retaining the mechanical strength of the vibratory portion. By fabricating a multimode filter element using such an ultrathin piezoelectric substrate as above, a filter of a center frequency in the range of tens to hundreds of megahertz could be easily obtained without utilizing the overtone techniques. Incidentally, as regards a conventional multimode piezoelectric filter element, it is said to be preferable that when the center frequency of the filter is 10 MHz, the thickness of an electrode to be evaporated onto the piezoelectric substrate be about 3000 Å so as to hold the degree of entrapping of the vibration energy at an appropriate value and an ohmic loss sufficiently low. On the other hand, in the cass where a filter element which has a center frequency of 100 MHz but is common to the above-mentioned filter element in the vibration energy entrapping characteristic and other basic characteristics is manufactured using an ultrathin AT cut quartz crystal plate, the electrode ought to be vapor-deposited to a thickness of 300 Å, i.e. 1/10 of the above-said value, in accordance with the theory of similarity, but it is evident that such an electrode thickness is so small that the ohmic loss will increase, making it impossible to obtain a sufficient attenuation of the filter. If the thickness of the electrode is set to around 1000 Å to keep the ohmic loss sufficiently low, the electrode is too thick as compared with the thickness of the AT cut quartz crystal substrate (about 17 µm), and consequently, the amount of vibration energy entrapping becomes excessive and satisfactory acoustic coupling between the devided electrodes cannot be obtained unless the inter-electrode gap is extremely reduced; accordingly, the passband width of the filter inevitably becomes very small. If the inter-electrode gap is reduced to such an extent as to provide desired acoustic coupling so as to avoid the above-noted problem, a highly accurate mask is needed for vapor deposition of the electrodes, besides the possibility of shorting betweeen the devided electrodes increases, introducing a difficulty in the formation of a filter of a passband wide enough for practical use. In addition, substantially no studies have been made so far as to what method should be used to obtain desired acoustic coupling between electrodes disposed closely to such an extent as not to cause shorting therebetween in the case of employing the resonator of the above-said type as a multimode filter, partly because the resonator has been confined to studies in laboratories and has not been mass-produced. The present invention is based on the fact that in the case of making a multimode filter element by forming split electrodes on an ultrathin AT cut quartz crystal plate as mentioned above, the passband of the filter element varies substantially with the direction of alignment of the electrodes, and it is an object of the invention to provide an ultrathin multimode quartz filter crystal element which is capable of obtaining desired acoustic coupling between split electrodes disposed closely to such an extent as not to cause shorting therebetween and hence well stands practical use. According to the present invention, an ultrathin multimode quartz crystal filter element which employs an AT cut quartz crystal plate including an ultrathin vibratory portion and a thick frame-like marginal portion supporting it and formed integrally therewith, is characterized in that split electrodes are disposed on said vibratory portion along the X-axis of said quartz crystal plate. In the accompanying drawings:- Figs. 1(a) and (b) are plan views illustrating different embodiments of an ultrathin multimode quartz crystal filter element according to the present invention; Figs. 2(a) and (b) are a diagram showing the relationship between the crystal axis of a quartz crystal plate and the direction of alignment of split electrodes and a graph showing experimental results about the degree of acoustic coupling obtained by changing the above-mentioned relationship; and, Figs. 3(a) and (b) are perspective and sectional views showing the construction of a conventional ultrathin piezoelectric multimode filter. Figs. 3(a) and (b) are a perspective view showing the construction of an ultrathin piezoelectric multimode filter studied heretofore and a sectional view showing the state in which the filter element is fixedly mounted in a case. The filter element has a cavity 2 formed by mechanical grinding or etching in one side of a block 1 of quartz crystal or similar piezoelectric material substantially centrally thereof so that the bottom of the cavity 2 forms a very thin vibratory portion 3 which is supported by a thick frame-like marginal portion 4 formed integrally therewith. A conductive film is evaporated onto the surface of the frame-like marginal portion 4, the inner wall surface of the cavity and the surface of the vibratory portion 3 to form an overall electrode 5 on the side of the piezoelectric block 1 where the caivity 2 is provided. Further, a pair of split electrodes 7a and 7b and electrode lead portions 8a and 8b extending therefrom to and along marginal edges of the block are provided on the flat surface 6 thereof opposite from the overall electrode 5. The conventional ultrathin multimode piezoelectric filter element, indicated generally by 9, has such a construction. It is advisable, in view of its structural feature, that the filter element of this type be fixedly mounted upside down--with the cavity 2 facing downward--in a so-called flat type case 10 which is a dish-like molding of an insulator, as shown in Fig. 3(b). Usually, in this instance one portion of the frame-like marginal portion 4 deposited with the overall electrode 5 and a conductive film 11 coated all over the bottom of the case 10 are bonded in only one edge of the marginal portion 4 and fixed to each other by a conductive adhesive 12 and the electrode lead portions 8a and 8b extending from the split electrodes 7a and 7b on the opposite side from the cavity 2 are each connected by a bonding wire 14 to a conductive film 13 coated on a stepped portion on the inner wall of the case 10, thereby reducing the restriction of the filter element 9 by the case 10 as much as possible and minimizing distortion which isapplied to the filter element due to the difference in the coefficient of thermal expansion between them. The conductive films 11 and 13 on the bottom of the case 10 and on the stepped portions on the inner wall of case 10 are electrically connected to external lead terminals 15 and 16 which hermetically pass through the case 10 and are exposed on the outside thereof. Such a piezoelectric device is completed by hermetically sealing the opening of the case 10 with a proper lid 17 (usually made of metal) after the resonator is fixedly mounted in the case 10. Also in the filter element of the type described above the degree of acoustic coupling between the split electrodes 7a and 7b appreciably varies according to the direction of their alignment with respect to the crystal axis of the quartz crystal plate used, as in conventional quartz crystal multimode filter elements. Figs. 2(a) and (b) shows the relationship between the direction of alignment of the split electrodes with respect to the x-axis of the AT cut quartz crystal plate and the degree of acoustic coupling between the electrodes, i.e. the bandwidth of the filter. As is evident from Figs. 2(a) and (b), the bandwidth of the quartz filter element varies about 1.26 times between the z-direction coupling by the alignment of the split electrodes 7a and 7b in the z-axis direction and the x-direction coupling by their alignment in the x-axis direction. Since the conventional multimode piezoelectric filter element usually employs a quartz crystal plate of a fundamental frequency of 30 MHz at the lowest, and of 50 MHz at the highest, however, there is no need of using the structure which provides the acoustic coupling in the x-direction. On the other hand, multimode quartz crystal filter elements shown in Figs. 1(a) and (b) each use a quartz crystal plate of about 100MHz fundamental vibration, and in this instance, the manufacture of such filter elements witha satisfactory bandwidth through utilization of the conventional z-direction coupling will encounter with difficulties in the formation of the split electrodes as referred to previously. In the ultrathin multimode quartz crystal filter element according to the present invention, the split electrodes are aligned in x-axis direction, that is, the x-direction coupling which essentially provides a wide bandwidth is utilized. The multimode quartz crystal filter element depicted in Fig. 1(a) has a construction in which the substrate is covered with a conductive film on the side where the cavity 2 is provided and the split electrodes 7a and 7b are aligned in the x-axis direction on the opposite side of the ultrathin vibratory portion 3. The circular multimode filter element shown in Fig. 1(b) has the split electrodes 7a and 7b aligned in the x-axis direction on the bottom of the cavity 2 and the overall electrode formed all over the substrate on the opposite side therefrom. Such constructions as mentioned above allow ease in mass production of multimode quartz crystal filter elements which have electrodes thick enough to secure conductivity and electrode spacing which does not introduce any particular difficulty in fabrication and which have very high center frequencies and hence possess sufficient bandwidth. Although the embodiments have been described only in connection with the case where a rectangular block of quartz crystal is used as the substrate and the split electrodes are provided on the vibratory portion on the side where the cavity is provided, the present invention need not be limited specifically thereto; for example, the quartz crystal block 1 may be disc-shaped as shown in Fig. 1(b) and the split electrodes 7a and 7b may also be vapor deposited on flat portion 6 of the block along the x-axis. With the present invention described above, it is possible to obtain desired acoustic coupling, even if the split electrodes are spaced wide apart to such an extent as not to cause shorting therebetween-this permits the fabrication of a filter element with a sufficiently large bandwidth. Hence, the present invention is remarkably effective in satisfying various characteristics of the multimode quartz crystal filter which employs a quartz crystal substrate of a fundamental frequency above 50 MHz and in improving the yield rate of product.

Claims: An ultrathin multimode quartz crystal filter element which employs an AT cut quartz crystal plate including an ultrathin vibratory portion and a thick frame-like marginal portion supporting it and formed integrally therewith, characterized in that split electrodes are disposed on said vibratory portion along the x-axis of said quartz crystal plate.

Inventors: ISHII OSAMU TOYO COMMUNICATION, KUROSAKI TAKEBUMI TOYO COMMUNI, MORITA TAKAO TOYO COMMUNICATIO, ISHII, OSAMU, TOYO COMMUNICATION EQUIPMENT CO., KUROSAKI, TAKEBUMI, TOYO COMMUNICATION EQUIPMENT, MORITA, TAKAO, TOYO COMMUNICATION EQUIPMENT CO.
Applicants: TOYO COMMUNICATION EQUIP, TOYO COMMUNICATION EQUIPMENT CO. LTD.
Application year: 1990
Main classification: H03H9
IPC codes: H03H9