Description

VIBRATOR AND METHOD OF MANUFACTURING THE SAME

Technical Field

[1] The present invention relates to a vibrator and a method of manufacturing the same, and more particularly, to a vibrator and a method of manufacturing the same in which piezoelectric elements are constructed with a multiple piezoelectric structure to provide good filter characteristics such as a wide bandwidth and a low insertion loss; and the piezoelectric elements with the multiple piezoelectric structure, vibrating space layers, and cover layers are unitedly stacked and sintered to provide good environment-proof characteristics such as moisture-proof characteristics. Background Art

[2] In general, PT-based or Pb(Zr ₅ Ti)O (PZT)-based ceramics are used as piezoelectric materials. Piezoelectric devices can be classified into two types. The first-type piezoelectric device generates a voltage when a mechanical force is applied thereto, while the second-type piezoelectric device generates a vibration when a signal is applied thereto. Piezoelectric materials are widely used in high- voltage generators, ultrasonic devices, audio devices, communication devices, and various sensors. In particular, a piezoelectric filter selects a predetermined frequency band using a vibration generated by the second-type piezoelectric material. Piezoelectric filters are used as intermediate frequency (IF) filters for mobile communication devices and electronic devices such as TVs, satellite settop boxes, audio devices, and portable phones.

[3] A conventional piezoelectric filter is constructed to include two piezoelectric elements, a vibrating space layer having an opening and mounted between the two piezoelectric elements, and two cover layers each having a vibrating groove formed therein. A divided electrode is formed on one side of the piezoelectric element, and a common electrode is formed on another side of the piezoelectric element. Also, a terminal electrode and a capacitor electrode are provided in a non-polarized portion in order to suppress unnecessary vibration.

[4] The conventional piezoelectric filter described above is manufactured by separately fabricating the piezoelectric elements, the vibrating space layer and cover layers, and then connecting them using an adhesive such as an epoxy.

[5] Such a conventional manufacturing method may have several drawbacks during practical fabrication processes. That is, entire fabrication processes are complicated, cutting and polishing processes for a piezoelectric sintered material are difficult and a handling damage is probable. Also, if substrates are not sealed with each other

completely, the piezoelectric filter may have poor environment-proof characteristics, e.g., poor moisture-proof characteristics, whereby long-term reliability may not be ensured. Also, the piezoelectric substrate can't be polished more thinly, uniformly and evenly by the conventional method while fabricating a filter having a relatively high center frequency, such as an IF filter for mobile communication devices, which has a frequency of 20 to 100 MHz that is more than twice as much as 10.7 MHz. Disclosure of Invention Technical Problem

[6] An aspect of the present invention provides a vibrator with a multiple piezoelectric structure and a method of manufacturing the same to ensure good filter characteristics such as a wide bandwidth and a low insertion loss.

[7] Another aspect of the present invention provides a vibrator and a method of manufacturing the same in which piezoelectric elements with the multiple piezoelectric structure, vibrating space layers, and cover layers are unitedly stacked and sintered to ensure good environment-proof characteristics such as moisture-proof characteristics.

[8] In accordance with the present invention, the piezoelectric element is constructed using a multiple piezoelectric structure including: a main piezoelectric substrate in which divided electrodes are formed on one surface and a common electrode is formed on another surface; and an auxiliary piezoelectric substrate in which a common electrode or divided electrodes are formed on only one surface or no electrode is formed. This provides good filter characteristics such as a wide bandwidth, a low insertion loss, and minimized unnecessary vibration.

[9] The multiple piezoelectric structure using the auxiliary piezoelectric substrate provides wide-bandwidth filter characteristics because a harmonic wave with a great difference between a resonant frequency and a semi-resonant frequency is generated strongly. The polarization efficiency at the divided electrodes and the common electrode of the main piezoelectric substrate can be increased by forming the common electrode on one surface of the auxiliary piezoelectric substrate, and as a result, the insertion loss can be reduced.

[10] A capacitor electrode connected to the common electrode of the main piezoelectric substrate or the auxiliary piezoelectric substrate may be removed. In this case, the unnecessary vibration due to the polarization at the capacitor electrode can be minimized. If the capacitor electrode connected to the common electrode of the main piezoelectric substrate is removed, the relay capacitance is provided by the capacitor electrode connected to the divided electrode of the main piezoelectric substrate and the capacitor electrode of the auxiliary piezoelectric substrate. The efficiency of the polarization between the electrodes is relatively reduced because an interval between the electrodes

is greater than an interval between the divided electrode and the common electrode of the main piezoelectric substrate by the thickness of the auxiliary piezoelectric substrate. Thus, unnecessary vibration can be suppressed by minimizing the polarization at an overlapping portion between the capacitor electrodes. Also, if the capacitor electrode of the auxiliary piezoelectric substrate is removed, the polarization efficiency at the capacitor electrode of the main piezoelectric substrate is relatively reduced and thus unnecessary vibration can be suppressed.

[11] In order to manufacture a vibrator in an integrated stacking method, a piezoelectric sheet with a finely-adjusted thickness is used to stack piezoelectric elements, vibrating space layers, and cover layers to have a desired thickness. Internal electrodes are formed in the piezoelectric element by using a thick film fabrication method such as screen printing or a thin film fabrication method such as sputtering, evaporation, and sol-gel coating. A vibrating groove with a desired size is formed in the vibrating space layer such that it corresponds to the position of the internal electrode formed in the piezoelectric element with precision. A guide line to be used for cutting after stacking is formed in the cover layer. The respective layers fabricated in such a way are stacked as a single body, cut and sintered to fabricate a chip-type sintered unit. Thereafter, external electrodes are formed for polarization, and thereby the vibrator is manufactured.

[12] The center frequency of the vibrator which is stacked as a single body according to the present invention can be finely controlled by forming the piezoelectric elements using the piezoelectric sheet with a finely adjusted thickness. Thus, the entire manufacturing process can be simplified. Also, because a thin piezoelectric substrate can be easily formed in the vibrator, the vibrator of the present invention may be suitable for a filter with a high center frequency of about 20 to 100 MHz. Further, since the respective layers are connected by sintering, there is no need for a separate connection process. Because the inside of the vibrator can be isolated from the external environments, environment-proof characteristics such as moisture-proof characteristics can be improved and thus the long-term reliability can be increased. Technical Solution

[13] In accordance with an embodiment of the present invention, a vibrator includes: a piezoelectric element that has a main piezoelectric substrate having first and second surfaces on which internal electrodes including divided electrodes and common electrodes are formed and an auxiliary piezoelectric substrate with piezoelectric characteristics that is connected to the main piezoelectric substrate.

[14] The main piezoelectric substrate may include: divided electrodes disposed in a predetermined region of a first surface of the main piezoelectric substrate such that they

are spaced apart from each other by a predetermined distance, and a common electrode disposed in a predetermined region of a second surface, corresponding to the divided electrodes.

[15] The main piezoelectric substrate may further include a capacitor electrode connected to the common electrode.

[16] The main piezoelectirc may further include a relay terminal electrode disposed in a region of the first surface corresponding to the capacitor electrode disposed on the second surface of the main piezoelectric substrate.

[17] The auxiliary piezoelectric substrate may have a first surface connected to the second surface of the main piezoelectric substrate, on which the common electrode is formed, and a second surface on which internal electrodes including a common electrode are formed.

[18] The auxiliary piezoelectric substrate may further include a capacitor electrode connected to the common electrode.

[19] The auxiliary piezoelectric substrate may have a first surface connected to the first surface of the main piezoelectric substrate and a second surface on which internal electrodes including a divided electrode is formed.

[20] The vibrator may include a plurality of auxiliary piezoelectric substrates.

[21] The vibrator may further include: a first/second vibrating space layer disposed over/ under the piezoelectric element, each of the first and second vibrating space layers having a vibrating groove; and a first/second cover layer disposed over/under the vibrating space layer.

[22] In accordance with another embodiment of the present invention, a vibrator includes: first and second piezoelectric elements each having a main piezoelectric substrate and an auxiliary piezoelectric substrate connected together, each of the main and auxiliary piezoelectric substrates having divided electrodes and a common electrode formed on a first surface and a second surface thereof; first, second and third vibrating space layers respectively disposed between, over and under the first and second piezoelectric elements, each of the first, second and third vibrating space layers having a vibrating groove; and first and second cover layers disposed over and under the second and third vibrating space layers, respectively.

[23] The auxiliary piezoelectric substrate and the main piezoelectric substrate of each of the first and second piezoelectric elements may be sequentially disposed from the center toward the surface of the vibrator. The auxiliary piezoelectric substrate may have electrodes including a common electrode formed only on one surface thereof.

[24] The main piezoelectric substrate and the auxiliary piezoelectric substrate of each of the first and second piezoelectric elements may be sequentially disposed from the center toward the surface of the vibrator. The auxiliary piezoelectric substrate may

have electrodes including a divided electrode formed only on one surface thereof.

[25] The vibrator in accordance with another embodiment of the present invention may inlcude a plurality of auxiliary piezoelectric substrates.

[26] The second auxiliary piezoelectric substrate, the first auxiliary piezoelectric substrate, and the main piezoelectric substrate of each of the first and second piezoelectric elements may be sequentially disposed from the center toward the surface of the vibrator. The second auxiliary piezoelectric substrate may have no electrode formed thereon, and the first auxiliary piezoelectric substrate may have electrodes including a common electrode formed only on one surface thereof.

[27] The main piezoelectric substrate may be one to two times thicker than the auxiliary piezoelectric substrate.

[28] In accordance with still another embodiment of the present invention, a method of manufacturing a vibrator includes: preparing a plurality of piezoelectric sheets; fabricating a main piezoelectric substrate and an auxiliary piezoelectric substrate, which have first and second surfaces on which internal electrodes are formed, vibrating space layers each having a vibrating groove, and cover layers using the piezoelectric sheets, and stacking the fabricated layers; cutting the stacked layers in a predetermined size and sintering the resulting structure; and forming external electrodes on a predetermined side region of the sintered stack structure.

Advantageous Effects

[29] According to an aspect of the present invention, a multiple piezoelectric filter is implemented using a main piezoelectric substrate in which divided electrodes are formed on one surface and a common electrode is formed on another surface, and an auxiliary piezoelectric substrate in which a common electrode or divided electrodes are formed on only one surface.

[30] A harmonic wave with a great difference between a resonant frequency and a semi- resonant frequency is generated due to the structural characteristics of the above multiple piezoelectric filter. The polarization efficiency can be increased by forming the common electrode on the auxiliary piezoelectric substrate. Also, the unnecessary vibration can be minimized by removing the capacitor electrode from the main piezoelectric substrate or the auxiliary piezoelectric substrate.

[31] By manufacturing the multiple piezoelectric substrates using the integrated stacking method, a surface-mounting type piezoelectric filter can be manufactured more easily and conveniently. The piezoelectric filter manufactured in such a way has a low insertion loss, a wide bandwidth, and a high long-term reliability. Brief Description of the Drawings

[32] FIGS. 1 and 2 illustrate a vibrator and a method of manufacturing the same in

accordance with an embodiment of the present invention.

[33] FIGS. 3 through 6 illustrate the structures of internal electrodes of a main piezoelectric substrate and an auxiliary piezoelectric substrate in a vibrator in accordance with an embodiment of the present invention.

[34] FIGS. 7 and 8 illustrate a vibrator and a method of manufacturing the same in accordance with another embodiment of the present invention.

[35] FIGS. 9 through 12 illustrate the structures of internal electrodes of a main piezoelectric substrate and an auxiliary piezoelectric substrate in the vibrator in accordance with another embodiment of the present invention.

[36] FIGS. 13 and 14 illustrate a vibrator and a method of manufacturing the same in accordance with still another embodiment of the present invention.

[37] FIGS. 15 through 20 illustrate the structures of internal electrodes of a main piezoelectric substrate and an auxiliary piezoelectric substrate in the vibrator in accordance with still another embodiment of the present invention.

[38] FIGS. 21 and 22 illustrate the structures of internal electrodes in accordance with a varied embodiment of the present invention.

[39] FIGS. 23 and 24 illustrate the structures of internal electrodes in accordance with another varied embodiment of the present invention. Best Mode for Carrying Out the Invention

[40] Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following description will be made exemplifying a piezoelectric filter including two piezoelectric substrates.

[41] FIGS. 1 and 2 illustrate the structure of a surface-mounting type piezoelectric filter with a double piezoelectric structure, as an example of a vibrator in accordance with the present invention, and a method of manufacturing the same.

[42] Referring to FIG. 1, a piezoelectric filter in accordance with an embodiment of the present invention includes first and second piezoelectric elements 100 and 200, first, second and third vibrating space layers 50, 60 and 70 for vibration, and first and second cover layers 80 and 90. The first piezoelectric element 100, the second vibrating space layer 60, and the first cover layer 80 are disposed over the first vibrating space layer 50 which is located at a central region of the piezoelectric filter, while the second piezoelectric element 200, the third vibrating space layer 70, and the second cover layer 90 are disposed under the first vibrating space layer 50. Hereinafter, for convenience of description, a side facing the surface of the filter will be referred to as a first main surface, while a side facing the center of the filter will be referred to as a second main surface.

[43] The first/second piezoelectric element 100/200 includes a main piezoelectric

substrate 10/30 and an auxiliary piezoelectric substrate 20/40, which are symmetrically disposed with respect to the first vibrating space layer 50. That is, the auxiliary piezoelectric substrate 20 and the main piezoelectric substrate 10 of the first piezoelectric element 100 are disposed over the first vibrating space layer 50, while the auxiliary piezoelectric substrate 40 and the main piezoelectric substrate 30 of the second piezoelectric element 200 are disposed under the first vibrating space layer 50.

[44] On the first main surface of the main piezoelectric substrate 10 of the first piezoelectric element 100, first and second divided electrodes 10a and 10b are formed to be spaced apart from each other by a predetermined distance, and an input terminal electrode 12a and a relay terminal electrode 12b are formed to be spaced apart from each other by a predetermined distance. The first/second divided electrode 10a/ 10b is connected to the input/relay terminal electrode 12a/ 12b through a thin lead electrode 1 la/1 Ib. Although they are not illustrated, a common electrode, a lead electrode, a plurality of ground terminal electrodes, and a capacitor electrode are formed on the second main surface of the main piezoelectric substrate 10. Although they are not illustrated, a common electrode, a lead electrode, a plurality of ground terminal electrodes, and a capacitor electrode are formed only on the second main surface of the auxiliary piezoelectric substrate 20.

[45] Likewise, first and second divided electrodes, an input terminal electrode, and a relay terminal electrode are disposed on the first main surface of the main piezoelectric substrate 30 of the second piezoelectric element 200, where the first/second divided electrode is connected to the input/relay terminal electrode through a lead electrode. Although they are not illustrated, a common electrode, a lead electrode, a plurality of ground terminal electrodes, and a capacitor electrode are formed on the second main surface of the main piezoelectric substrate 30. Also, a common electrode 40c, a lead electrode 41c, a plurality of ground terminal electrodes 42c, and a capacitor electrode 43c are formed only on the second main surface of the auxiliary piezoelectric substrate 40 of the second piezoelectric element 200.

[46] The above-described double piezoelectric structure including the main piezoelectric substrate and the auxiliary piezoelectric substrate generates a strong harmonic wave with a large difference between a resonant frequency and a semi-resonant frequency. Because an impedance is very low for the resonant frequency and very high for the semi-resonant frequency, the generated harmonic wave can provide a filter that has a wide bandwidth, a low insertion loss, and a good selectivity. Thicknesses of the main piezoelectric substrate 10/30 and the auxiliary piezoelectric substrate 20/40 may not be necessarily the same. However, when the main piezoelectric substrate 10/30 and the auxiliary piezoelectric substrate 20/40 have the same thickness, the generated harmonic wave is strongest. As the main piezoelectric substrate 10/30 becomes thicker

than the auxiliary piezoelectric substrate 20/40, a fundamental wave becomes strong while the harmonic wave becomes weak. If the main piezoelectric substrate 10/30 is thinner than the auxiliary piezoelectric substrate 20/40, diverse harmonic waves are generated, causing a difficulty in implementation. Therefore, it is preferable that the thickness of the main piezoelectric substrate 10/30 is similar to that of the auxiliary piezoelectric substrate 20/40.

[47] A vibrating groove 51/61/71 for vibration is formed at the center of the first/ second/third vibrating space layer 50/60/70. The vibrating grooves 51, 61 and 71 are formed corresponding to positions of internal electrodes.

[48] The first and second cover layers 80 and 90 are formed using a piezoelectric sheet or a non-piezoelectric ceramic sheet. An input terminal 91, an output terminal 92, a ground electrode 93, and a relay capacitor electrode 94 are formed on the second main surface of the second cover layer 90 in such a way that they extend to the side of the second cover layer 90.

[49] Hereinafter, a description will be given about a method of manufacturing the surface- mounting type piezoelectric filter with the double piezoelectric structure in accordance with the embodiment of the present invention shown in FIG. 1.

[50] First, a binder such as polyvinyl butyral (PVB) is mixed with a piezoelectric ceramic powder and alcohol to fabricate a piezoelectric ceramic slurry using a ball mill. Thereafter, bubbles are removed from the piezoelectric ceramic slurry by de-airing, the viscosity of the piezoelectric ceramic slurry is adjusted, and the piezoelectric ceramic slurry is shaped by a Dr. Blade method to thereby fabricate a plurality of piezoelectric sheets. Then, electrodes are formed on one or both sides of a selected piezoelectric sheet by sputtering, screen printing, evaporation, or sol-gel coating. Accordingly, a plurality of electrodes is formed on the first and second main surfaces of the main piezoelectric substrates 10 and 30 and the auxiliary piezoelectric substrates 20 and 40.

[51] The first, second and third vibrating space layers 50, 60 and 70 are fabricated by forming the vibrating grooves 51, 61 and 71 in the piezoelectric sheet, a non- piezoelectric ceramic sheet, or an insulating sheet using a punching machine. The vibrating grooves 51, 61 and 71 are formed in such a way that their sizes are larger than or equal to the sizes of the internal electrodes formed on the main piezoelectric substrates 10 and 30 and the auxiliary piezoelectric substrates 20 and 40. Although the vibrating grooves 51, 61 and 71 are illustrated as being square, the present invention is not limited to this. That is, the vibrating grooves 51, 61 and 71 may have any shape that is larger than or equal to the internal electrodes in size.

[52] The first and second cover layers 80 and 90 are fabricated using a piezoelectric sheet, a non-piezoelectric ceramic sheet, or an insulating sheet. The input terminal 91, the output terminal 92, the ground electrode 93, and the relay capacitor electrode 94 are

formed on the second main surface of the second cover layer 90 in such a way that they extend to the side of the second cover layer 90. Also, guide lines to be used for cutting after stacking are formed in the first and second cover layers 80 and 90.

[53] The first piezoelectric element 100, the second vibrating space layer 60, and the first cover layer 80 are sequentially stacked over the first vibrating space layer 50, while the second piezoelectric element 200, the third vibrating space layer 70, and the second cover layer 90 are sequentially stacked under the first vibrating space layer 50. In this way, a green bar is fabricated to have a plurality of unit chips. The green bar with the unit chips is pressed and cut in the size of a unit chip along the guide lines formed in the first and second cover layers 80 and 90. Thereafter, in order to remove organic materials, a de-binder process is performed at a predetermined temperature and a plurality of stacked layers is sintered simultaneously. As such, a piezoelectric unit is formed. Then, using metal such as Ag, Pd, and Pt, sputtering or screen printing is performed to form external electrodes 91a, 92a, 93a and 94a on the side of the piezoelectric unit, for polarization. In this way, a piezoelectric filter illustrated in FIG. 2 is manufactured.

[54] In the above method, the main piezoelectric substrates 10 and 30 and the auxiliary piezoelectric substrate 20 and 40 may be formed of a variety of piezoelectric single crystal materials or piezoelectric materials, as well as a piezoelectric ceramic sheet.

[55] FIGS. 3 through 6 illustrate the structures of the internal electrodes of the main piezoelectric substrate and the auxiliary piezoelectric substrate in the double piezoelectric structure of the surface-mounting type piezoelectric filter in accordance with the embodiment of the present invention shown in FIG. 1. FIG. 3 illustrates the structure of internal electrodes of the main piezoelectric substrate of the first piezoelectric element, and FIG. 4 illustrates the structure of internal electrodes of the auxiliary piezoelectric substrate of the first piezoelectric element. FIG. 5 illustrates the structure of internal electrodes of the main piezoelectric substrate of the second piezoelectric element, and FIG. 6 illustrates the structure of internal electrodes of the auxiliary piezoelectric substrate of the second piezoelectric element.

[56] Referring to FIG. 3, bar-type first and second divided electrodes 10a and 10b with a predetermined width are formed preferably on the center of the first main surface of the main piezoelectric substrate 10 of the first piezoelectric element 100 in such a way that they are spaced apart from each other by a predetermined distance, and a common electrode 10c is formed on the center of the second main surface of the main piezoelectric substrate 10 in such a way that it partially overlaps with the first and second divided electrodes 10a and 10b, thereby forming an energy trap type dual-mode filter. An input terminal electrode 12a is formed along portions of two sides of the first main surface of the main piezoelectric substrate 10, and the first divided electrode 10a

is connected through a thin lead electrode 1 Ia to the input terminal electrode 12a. A relay terminal electrode 12b with a predetermined width is formed on the opposite side with respect to the input terminal electrode 12a, and the second divided electrode 10b is connected through a thin lead electrode 1 Ib to the relay terminal electrode 12b, also serving as a capacitor electrode. For example, if the input terminal electrode 12a is formed along portions of first and second sides of the first main surface of the main piezoelectric substrate 10, the relay terminal electrode 12b is formed along the whole third side opposite to the first side. Because the relay terminal electrode 12b has the predetermined width, it is also formed along portions of the first and fourth sides. On the second main surface, the common electrode 10c is connected through a thin lead electrode 1 Ic to a capacitor electrode 13c that is disposed to face the relay terminal electrode 12b. The capacitor electrode 13c is formed along a portion of the third side of the second main surface. However, unlike the relay terminal electrode 12b, the capacitor electrode 13c is not formed along the first and fourth sides. Thus, a relay capacitor is formed between the relay terminal electrode 12b and the capacitor electrode 13c. A plurality of ground terminal electrodes 12c other than the capacitor electrode 13c are formed along portions of the remaining three sides of the second main surface along which the capacitor electrode 13c is not formed. Each ground terminal electrode 12c is connected through a thin lead electrode 1 Ic to the common electrode 10c.

[57] Referring to FIG. 4, a common electrode 20c, a plurality of lead electrodes 21c, a plurality of ground terminal electrodes 22c, and a capacitor electrode 23c are formed only on the second main surface of the auxiliary piezoelectric substrate 20 of the first piezoelectric element 100. The common electrode 20c is formed on the center of the second main surface. The capacitor electrode 23c is formed along one side of the second main surface, and the ground terminal electrodes 22c are formed along portions of the remaining three sides along which the capacitor electrode 23c is not formed. The ground terminal electrodes 22c and the capacitor electrode 23c are connected through the lead electrodes 21c to the common electrode 20c. The above structure of the auxiliary piezoelectric substrate 20 increases the polarization efficiency on the main piezoelectric substrate 10 and changes a vibration phase in order to generate a strong harmonic wave. Unlike the conventional art, the auxiliary piezoelectric substrate is inserted to provide filter characteristics by using a harmonic wave instead of a fundamental wave.

[58] Referring to FIG. 5, bar-type first and second divided electrodes 30a and 30b with a predetermined width are formed on the first main surface of the main piezoelectric substrate 30 of the second piezoelectric element 200 in such a way that they are spaced apart from each other by a predetermined distance, and a common electrode 30c is

formed on the second main surface of the main piezoelectric substrate 30 in such a way that it partially overlaps with the first and second divided electrodes 30a and 30b, thereby forming an energy trap type dual-mode filter. An input terminal electrode 32a is formed along portions of two sides of the first main surface of the main piezoelectric substrate 30, and the first divided electrode 30a is connected through a lead electrode 31a to the input terminal electrode 32a. A relay terminal electrode 32b with a predetermined width is formed along the opposite side with respect to the input terminal electrode 32a, and the second divided electrode 30b is connected through a lead electrode 31b to the relay terminal electrode 32b, also serving as a capacitor electrode. For example, if the input terminal electrode 32a is formed along portions of first and second sides of the first main surface of the main piezoelectric substrate 30, the relay terminal electrode 12b is formed along the whole third side opposite to the first side. Because the relay terminal electrode 32b has the predetermined width, it is also formed along portions of the first and fourth sides. On the second main surface, the common electrode 30c is connected through a lead electrode 31c to a capacitor electrode 33c facing the relay terminal electrode 32b. Unlike the relay terminal electrode 32b, the capacitor electrode 33c is formed along a portion of the third side of the second main surface but is not formed along the first and fourth sides. A relay capacitor is formed between the relay terminal electrode 32b and the capacitor electrode 33c. A plurality of ground terminal electrodes 32c are formed along portions of the remaining three sides of the second main surface along which the capacitor electrode 33c is not formed. Each ground terminal electrode 32c is connected through a lead electrode 31c to the common electrode 30c.

[59] Referring to FIG. 6, a common electrode 40c, a plurality of lead electrodes 41c, a plurality of ground terminal electrodes 42c, and a capacitor electrode 43c are formed only on the second main surface of the auxiliary piezoelectric substrate 40 of the second piezoelectric element 200. The common electrode 40c is formed on the center of the second main surface. The capacitor electrode 43c is formed along one side of the second main surface, and the ground terminal electrodes 42c are formed along portions of the remaining three sides along which the capacitor electrode 43c is not formed. The ground terminal electrodes 42c and the capacitor electrode 43c are connected through the lead electrodes 41c to the common electrode 40c.

[60] As described above, the common electrode 20c/40c is formed only on one surface of the auxiliary piezoelectric substrate 20/40. The forming of the electrodes on the auxiliary piezoelectric substrates 20 and 40 is advantageous to increase the polarization efficiency, and an increase in the area for increasing the polarization efficiency is advantageous to the device operation. Electrodes may not be formed on the auxiliary piezoelectric substrates 20 and 40. In this case, a generated harmonic wave has a

relatively high impedance for a resonant frequency and a relatively low impedance for a semi-resonant frequency, when compared to the case of the forming of the electrodes on the auxiliary piezoelectric substrates. Also, divided electrodes instead of the common electrodes may be formed on the auxiliary piezoelectric substrates 20 and 40 as described later.

[61] FIGS. 7 and 8 illustrate a surface-mounted piezoelectric filter with a double piezoelectric structure and a method of manufacturing the same, in accordance with another embodiment of the present invention. A detailed description of an overlap between the embodiment of FIGS. 1 and 2 and the embodiment of FIGS. 7 and 8 will be omitted for conciseness.

[62] In the embodiment of FIGS. 1 and 2, the common electrode is formed on the second main surface of the auxiliary piezoelectric substrate. However, in the embodiment of FIGS. 7 and 8, divdied electrodes are formed on the first main surface of the auxiliary piezoelectric substrate in order to increase the polarization efficiency. Because the divided electrodes are formed on the first main surface of the auxiliary piezoelectric substrate, it is preferable that the main piezoelectric substrate faces the inside of the filter and the auxiliary piezoelectric substrate faces the cover layer.

[63] Referring to FIG. 7, the piezoelectric filter in accordance with another embodiment of the present invention includes first and second piezoelectric elements 100 and 200, first, second and third vibrating space layers 50, 60 and 70, and first and second cover layers 80 and 90. The first piezoelectric element 100, the second vibrating space layer 60, and the first cover layer 80 are disposed over the first vibrating space layer 50, while the second piezoelectric element 200, the third vibrating space layer 70, and the second cover layer 90 are disposed under the first vibrating space layer 50.

[64] The first/second piezoelectric element 100/200 includes a main piezoelectric substrate 10/30 and an auxiliary piezoelectric substrate 20/40, which are symmetrically disposed with respect to the first vibrating space layer 50. That is, the main piezoelectric substrate 10 and the auxiliary piezoelectric substrate 20 of the first piezoelectric element 100 are disposed over the first vibrating space layer 50, while the main piezoelectric substrate 30 and the auxiliary piezoelectric substrate 40 of the second piezoelectric element 200 are disposed under the first vibrating space layer 50.

[65] First and second divided electrodes, lead electrodes, an input terminal electrode, and a relay terminal electrode are formed on the first main surface of each of the main piezoelectric substrates 10 and 30 of the first and second piezoelectric element 100 and 200, while a common electrode, a lead electrode, a plurality of ground terminal electrodes, and a capacitor electrode are formed on the second main surface. As illustrated in FIG. 7, a common electrode 30c, a plurality of lead electrodes 31c, a plurality of ground terminal electrodes 32c, and a capacitor electrode 33c are formed

on the second main surface of the main piezoelectric substrate 30 of the second piezoelectric element 200.

[66] First and second divided electrodes, lead electrodes, an input terminal electrode, and a relay terminal electrode are formed on the first main surface of each of the auxiliary piezoelectric substrates 20 and 40 of the first and second piezoelectric element 100 and 200. As illustrated in FIG. 7, first and second divided electrodes 25a and 25b, lead electrodes 26a and 26b, an input terminal electrode 27a, and a relay terminal electrode 27b are formed on the first main surface of the auxiliary piezoelectric substrate 20 of the first piezoelectric element 100.

[67] The shapes of the above internal electrodes are substantially the same as those described with reference to FIGS. 1 and 2, and thus their description will be omitted for conciseness. Also, the structures of the first, second and third vibrating space layers 50, 60 and 70 and the first and second cover layers 80 and 90 are substantially the same as those described with reference to FIGS. 1 and 2, and thus their description will be omitted for conciseness.

[68] Like the method of manufacturing the piezoelectric filter in accordance with the embodiment of FIGS. 1 and 2, a method of manufacturing the piezoelectric filter in accordance with the embodiment of FIGS. 7 and 8 sequentially stacks the first piezoelectric element 100, the second vibrating space layer 60, and the first cover layer 80 over the first vibrating space layer 50, sequentially stacks the second piezoelectric element 200, the third vibrating space layer 70, and the second cover layer 90 under the first vibrating space layer 50, presses/cuts/sinters the resulting structure, and forms external electrodes 91a, 92a, 93a and 94a for polarization, thereby manufacturing a piezoelectric filter as illustrated in FIG. 8.

[69] FIGS. 9 through 12 illustrate the structures of the internal electrodes of the main piezoelectric substrate and the auxiliary piezoelectric substrate in the double piezoelectric structure of the surface-mounted piezoelectric filter in accordance with another embodiment of the present invention shown in Fig. 7. FIG. 9 illustrates the structure of internal electrodes of the main piezoelectric substrate of the first piezoelectric element, and FIG. 10 illustrates the structure of internal electrodes of the auxiliary piezoelectric substrate of the first piezoelectric element. FIG. 11 illustrates the structure of internal electrodes of the main piezoelectric substrate of the second piezoelectric element, and FIG. 12 illustrates the structure of internal electrodes of the auxiliary piezoelectric substrate of the second piezoelectric element. A detailed description of an overlap between the embodiment of FIGS. 3 through 6 and the embodiment of FIGS. 9 through 12 will be omitted for conciseness.

[70] Referring to FIG. 9, first and second divided electrodes 10a and 10b, lead electrodes

11a and 1 Ib, an input terminal electrode 12a, and a relay terminal electrode 12b are

formed on the first main surface of the main piezoelectric substrate 10 of the first piezoelectric element 100, while a common electrode 10c, lead electrodes l ie, a plurality of ground terminal electrodes 12c, and a capacitor electrode 13c are formed on the second main surface of the main piezoelectric substrate 10.

[71] Referring to FIG. 10, first and second divided electrodes 25a and 25b are formed only on the first main surface of the auxiliary piezoelectric substrate 20 of the first piezoelectric element 100 in such a way that they are spaced apart from each other by a predetermined distance. An input terminal electrode 27a is formed along portions of two sides of the first main surface of the auxiliary piezoelectric substrate 20, and the first divided electrode 25a is connected through a lead electrode 26a to the input terminal electrode 27a. A relay terminal electrode 27b is formed on the opposite side with respect to the input terminal electrode 27 a, and the second divided electrode 25b is connected through a lead electrode 26b to the relay terminal electrode 27b.

[72] Referring to FIG. 11, first and second divided electrodes 30a and 30b, lead electrodes

31a and 31b, an input terminal electrode 32a, and a relay terminal electrode 32b are formed on the first main surface of the main piezoelectric substrate 30 of the second piezoelectric element 200, while a common electrode 30c, lead electrodes 31c, a plurality of ground terminal electrodes 32c, and a capacitor electrode 33c are formed on the second main surface of the main piezoelectric substrate 30.

[73] Referring to FIG. 12, first and second divided electrodes 45a and 45b are formed only on the first main surface of the auxiliary piezoelectric substrate 40 of the second piezoelectric element 200 in such a way that they are spaced apart from each other by a predetermined distance. An input terminal electrode 47a is formed along portions of two sides of the first main surface of the auxiliary piezoelectric substrate 40, and the first divided electrode 45a is connected through a lead electrode 46a to the input terminal electrode 47a. A relay terminal electrode 47b is formed on the opposite side with respect to the input terminal electrode 47 a, and the second divided electrode 45b is connected through a lead electrode 46b to the relay terminal electrode 47b.

[74] The impedance characteristics for resonant and semi-resonant frequencies are better when the divided electrodes are formed on the auxiliary piezoelectric substrates 20 and 40 as described above, than when the internal electrodes are not formed. That is, an impedance for the resonant frequency is relatively low and an impedance for the semi- resonant frequency is significantly high. The forming of the divided electrodes on the auxiliary piezoelectric substrates can increase the area for increasing the polarization efficiency of the piezoelectric substrates. However, when compared to the case of the forming of the common electrodes, an insertion loss slightly increases because the area for increasing the polarization efficiency is relatively small. Such a double piezoelectric structure is also applicable to a resonator as well as to a piezoelectric

filter.

[75] FIGS. 13 and 14 illustrate a surface-mounted piezoelectric filter with a multiple piezoelectric structure and a method of manufacturing the same, in accordance with still another embodiment of the present invention. In this embodiment, the filter includes a main piezoelectric substrate and two auxiliary piezoelectric substrates, wherein an internal electrode is formed only on one surface of one of the auxiliary piezoelectric substrates and an internal electrode is not formed on the other of the auxiliary piezoelectric substrates. A detailed description of an overlap between the foregoing embodiments and the embodiment of FIGS. 13 and 14 will be omitted for conciseness.

[76] Referring to FIG. 13, the piezoelectric filter in accordance with still another embodiment of the present invention includes first and second piezoelectric elements 100 and 200, first, second and third vibrating space layers 50, 60 and 70 for vibration, and first and second cover layers 80 and 90. The first piezoelectric element 100 includes a main piezoelectric substrate 10 and two auxiliary piezoelectric substrates 21 and 25. Likewise, the second piezoelectric element 200 includes a main piezoelectric substrate 30 and two auxiliary piezoelectric substrates 41 and 45. The first piezoelectric element 100, the second vibrating space layer 60, and the first cover layer 80 are disposed over the first vibrating space layer 50, while the second piezoelectric element 200, the third vibrating space layer 70, and the second cover layer 90 are disposed under the first vibrating space layer 50.

[77] The first/second piezoelectric element 100/200 includes the main piezoelectric substrate 10/30 and the auxiliary piezoelectric substrates 21 and 25/41 and 45, which are symmetrically disposed with respect to the first vibrating space layer 50. That is, the second auxiliary piezoelectric substrate 25, the first auxiliary piezoelectric substrate 21, and the main piezoelectric substrate 10 are disposed over the first vibrating space layer 50, while the second auxiliary piezoelectric substrate 45, the first auxiliary piezoelectric substrate 41, and the main piezoelectric substrate 30 are disposed under the first vibrating space layer 50.

[78] First and second divided electrodes, lead electrodes, an input terminal electrode, and a relay terminal electrode are formed on the first main surface of each of the main piezoelectric substrates 10 and 30 of the first and second piezoelectric element 100 and 200, while a common electrode, lead electrodes, a plurality of ground terminal electrodes, and a capacitor electrode are formed on the second main surface. Also, a common electrode, lead electrodes, a plurality of ground terminal electrodes, and a capacitor electrode are formed only on the second main surface of each of the first auxiliary piezoelectric substrates 21 and 41 of the first and second piezoelectric element 100 and 200. Meanwhile, internal electrodes are not formed on each of the

second auxiliary piezoelectric substrates 25 and 45 of the first and second piezoelectric element 100 and 200.

[79] Like the methods of manufacturing the piezoelectric filter in accordance with the foregoing embodiments, a method of manufacturing the piezoelectric filter in accordance with the embodiment of FIGS. 13 and 14 sequentially stacks the first piezoelectric element 100, the second vibrating space layer 60, and the first cover layer 80 over the first vibrating space layer 50, sequentially stacks the second piezoelectric element 200, the third vibrating space layer 70, and the second cover layer 90 under the first vibrating space layer 50, presses/cuts/sinters the resulting structure, and forms external electrodes 91a, 92a, 93a and 94a for polarization, thereby manufacturing a piezoelectric filter as illustrated in FIG. 14.

[80] FIGS. 15 through 20 illustrate the structures of the internal electrodes of the main piezoelectric substrate and the first and second auxiliary piezoelectric substrates in the multiple piezoelectric structure of the surface-mounted piezoelectric filter in accordance with still another embodiment of the present invention. FIG. 15 illustrates the structure of internal electrodes of the main piezoelectric substrate of the first piezoelectric element, FIG. 16 illustrates the structure of internal electrodes of the first auxiliary piezoelectric substrate of the first piezoelectric element, and FIG. 17 illustrates the structure of internal electrodes of the second auxiliary piezoelectric substrate of the first piezoelectric element. FIG. 18 illustrates the structure of internal electrodes of the main piezoelectric substrate of the second piezoelectric element, FIG. 19 illustrates the structure of internal electrodes of the first auxiliary piezoelectric substrate of the second piezoelectric element, and FIG. 20 illustrates the structure of internal electrodes of the second auxiliary piezoelectric substrate of the second piezoelectric element. A detailed description of an overlap between the foregoing embodiments and the embodiment of FIGS. 15 through 20 will be omitted for conciseness.

[81] Referring to FIG. 15, first and second divided electrodes 10a and 10b, lead electrodes

11a and 1 Ib, an input terminal electrode 12a, and a relay terminal electrode 12b are formed on the first main surface of the main piezoelectric substrate 10 of the first piezoelectric element 100, while a common electrode 10c, lead electrodes l ie, a plurality of ground terminal electrodes 12c, and a capacitor electrode 13c are formed on the second main surface of the main piezoelectric substrate 10.

[82] Referring to FIG. 16, a common electrode 20c, a plurality of lead electrodes 21c, a plurality of ground terminal electrodes 22c, and a capacitor electrode 23c are formed only on the second main surface of the first auxiliary piezoelectric substrate 21 of the first piezoelectric element 100. The common electrode 20c is formed on the center of the second main surface. The capacitor electrode 23c is formed along one side of the

second main surface, and the ground terminal electrodes 22c are formed along portions of the reaming three sides where the capacitor electrode 23c is not formed. The ground terminal electrodes 22c and the capacitor electrode 23c are connected through the lead electrodes 21c to the common electrode 20c.

[83] Referring to FIG. 17, internal electrodes are not formed on the second auxiliary piezoelectric substrate 25 of the first piezoelectric element 100.

[84] Referring to FIG. 18, first and second divided electrodes 30a and 30b, lead electrodes

31a and 31b, an input terminal electrode 32a, and a relay terminal electrode 32b are formed on the first main surface of the main piezoelectric substrate 30 of the second piezoelectric element 200, while a common electrode 30c, lead electrodes 31c, a plurality of ground terminal electrodes 32c, and a capacitor electrode 33c are formed on the second main surface of the main piezoelectric substrate 30.

[85] Referring to FIG. 19, a common electrode 40c, a plurality of lead electrodes 41c, a plurality of ground terminal electrodes 42c, and a capacitor electrode 43c are formed only on the second main surface of the second auxiliary piezoelectric substrate 41 of the second piezoelectric element 200. The common electrode 40c is formed on the center of the second main surface. The capacitor electrode 43c is formed along one side of the second main surface, and the ground terminal electrodes 42c are formed along the reaming three sides where the capacitor electrode 43c is not formed. The ground terminal electrodes 42c and the capacitor electrode 43c are connected through the lead electrodes 41c to the common electrode 40c.

[86] Referring to FIG. 20, internal electrodes are not formed on the second auxiliary piezoelectric substrate 45 of the second piezoelectric element 200.

[87] In the still another embodiment of the present invention, the auxiliary piezoelectric substrate is divided into the second auxiliary piezoelectric substrate and the first auxiliary piezoelectric substrate contacting the main piezoelectric substrate. The common electrode is formed on the second main surface of the first auxiliary piezoelectric substrate, while the internal electrodes are not formed on the second auxiliary piezoelectric substrate, thereby forming a triple structure that is an application example of a dual piezoelectric structure and can maximize the polarization efficiency of the common electrode on the main piezoelectric substrate. That is, the polarization efficiency increases with a decrease in an interval between the electrode of the main piezoelectric substrate and the electrode of the auxiliary piezoelectric substrate, and thus the impedance for a resonant frequency decreases. In this way, the multiple piezoelectric structure may be a triple or more piezoelectric structure for maximization of the polarization efficiency.

[88] Also, in the case of a resonator, the polarization efficiency can be increased by inserting auxiliary piezoelectric substrates on the top and bottom surfaces of a main

piezoelectric substrate. In this case, if the two auxiliary piezoelectric substrates have the same thickness, a harmonic wave is generated at a relatively high frequency. On the other hand, a fundamental wave is generated at a low frequency in inverse proportion to the total thickness of the piezoelectric substrates, regardless of the thickness ratio. In such a triple piezoelectric substrate as well as the double piezoelectric substructure, with an increase in the thickness of the main piezoelectric substrate, a fundamental wave increases in strength and a harmonic wave decreases in strength. Thus, in order to secure the filter performance by the harmonic wave, it is preferable that the thickness of the main piezoelectric substrate is similar to the thickness of the first and second auxiliary piezoelectric substrates.

[89] The piezoelectric filter manufactured by the integrated stack method of the present invention can minimize a unnecessary vibration by suppressing the polarization at the relay capacitor, as illustrated in FIGS. 7 and 8.

[90] FIGS. 21 and 22 illustrate the structures of internal electrodes of a piezoelectric filter in accordance with a modified embodiment of the present invention. In this embodiment, the capacitor electrodes are removed from the second main surfaces of the main piezoelectric substrates in the first and second piezoelectric elements. In this case, it is possible to suppress the polarization at the relay capacitor electrode formed on the first main surface of the main piezoelectric substrate.

[91] Referring to FIG. 21, first and second divided electrodes 10a and 10b, lead electrodes

11a and 1 Ib, an input terminal electrode 12a, and a relay terminal electrode 12b are formed on the first main surface of the main piezoelectric substrate. A common electrode 10c is formed on the center of the second main surface of the main piezoelectric substrate. Two round terminal electrodes 12c are formed along portions of two sides of the second main surface that face the sides of the first main surface along which the input terminal electrode 12a and the relay terminal electrode 12b are not formed. The common electrode 10c and the two ground terminal electrodes 12c are connected by lead electrodes l ie.

[92] Referring to FIG. 22, a common electrode 20c, a plurality of lead electrodes 21c, a plurality of ground terminal electrodes 22c, and a capacitor electrode 23c are formed only on the second main surface of the auxiliary piezoelectric substrate. The common electrode 20c is formed on the center of the second main surface. The capacitor electrode 23c is formed along one side of the second main surface, and the ground terminal electrodes 22c are formed along portions of the reaming three sides where the capacitor electrode 23c is not formed. The ground terminal electrodes 22c and the capacitor electrode 23c are connected through the lead electrodes 21c to the common electrode 20c.

[93] FIGS. 23 and 24 illustrate the structures of internal electrodes of a piezoelectric filter

in accordance with another modified embodiment of the present invention. In this embodiment, the capacitor electrodes are removed from the second main surfaces of the auxiliary piezoelectric substrates in the first and second piezoelectric elements. In this case, the polarization efficiency at the relay capacitor electrode disposed on the first main surface of the main piezoelectric substrate is not increased.

[94] Referring to FIG. 23, first and second divided electrodes 10a and 10b, lead electrodes

11a and 1 Ib, an input terminal electrode 12a, and a relay terminal electrode 12b are formed on the first main surface of the main piezoelectric substrate, while a common electrode 10c, lead electrodes l ie, a plurality of ground terminal electrodes 12c, and a capacitor electrode 13c are formed on the second main surface.

[95] Referring to FIG. 24, a common electrode 10c is disposed on the center of the second main surface of the auxiliary piezoelectric substrate, and ground terminal electrodes 12c are formed along two sides of the second main surface that correspond to two sides of the first main surface of the main piezoelectric substrate along which the input terminal electrode 12a and the relay terminal electrode 12b are not formed. The common electrode 10c and the two ground terminal electrodes 12c are connected by lead electrodes l ie.

[96] As described above, if the capacitor electrode is removed, the polarization efficiency increases at the divided electrode and the common electrode but does not increase at the relay capacitor electrode, so that the unnecessary vibration can be suppressed.

[97] The construction of the main piezoelectric substrate and the auxiliary piezoelectric substrate of the first and second piezoelectric elements is not limited to the above embodiments, but can be implemented by the combination of the above embodiments. For example, a piezoelectric element may include a main piezoelectric substrate and an auxiliary piezoelectric substrate on which internal electrodes are not formed. Also, auxiliary piezoelectric substrates may be formed on the top and bottom surfaces of a main piezoelectric substrate. Also, the frequency characteristics can be adjusted by adjusting the thicknesses of the components of the piezoelectric filter, particularly the main piezoelectric substrate and the auxiliary piezoelectric substrate.