Abstract:
A method is provided for manufacturing a multilayer-type chip inductor having a small DC resistance without decreasing the inductance or the impedance. The method of manufacturing a multilayer-type chip inductor includes the steps of: preparing a ceramic green sheet; forming an electrode film on one surface of the green sheet; multilayering a plurality of the green sheets in such a way that the surfaces on which electrode films are formed face each other for pairs of the green sheets; contact-bonding the green sheets; and sintering the green sheets.

Description:
This application is a division of Ser. No. 09/028,748, filed Jan. 24, 1998, U.S. Pat. No. 6,223,422. 
     This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 9-39153 filed in Japan on Feb. 24, 1997, the entire content of which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of manufacturing a multilayer-type chip inductor, and in particular, a multilayer-type chip inductor having a small DC resistance. 
     2. Description of the Related Art 
     A method of reducing the DC resistance of a multilayer-type chip inductor involves increasing the cross-sectional area of an internal conductor. In order to increase the cross-sectional area of an internal conductor, the width and the thickness of the internal conductor may be increased. If the width of the internal conductor is increased, however, the inductance decreases. Increasing the cross-sectional area of the internal conductor also may cause various manufacturing problems. Therefore, it is difficult in practice to increase the cross-sectional area of the internal conductor. For this reason, a coil comprising parallel internal conductors has been conceived as a method for decreasing the DC resistance of an inductor. 
     First, a multilayer-type chip inductor of a first conventional example in which coils are connected in parallel will be described with reference to FIGS. 4 and 5. FIG. 5 is a cross-sectional view of the device shown in FIG.  4 . 
     With reference to both FIGS. 4 and 5, a multilayer-type chip inductor  1  is formed in such a way that green sheets  2   a  to  2   e  having electrode films  3   a  to  3   e  formed thereon, respectively, are multilayered in two upper and lower stages and sintered together. Further, external electrodes (not shown) are formed on both ends of this sintered body. 
     The first green sheets  2   a  to  2   e  are formed into sheets from an insulating ceramic slurry, such as ferrite or a dielectric. The electrode films  3   a  to  3   e , which become internal conductors, are formed on one surface of the sheets by printing or like technique. Furthermore, in the first green sheets  2   b  to  2   e , via holes  4   b  to  4   e  are provided at one end of each of the electrode films  3   b  to  3   e . The upper and lower stages of first green sheets  2   a  to  2   e  are multilayered in sequence, causing the electrode films  3   a  to  3   e  to conduct in order to form two inductors  5 . In parts of the electrode films  3   a  and  3   e , one end of each film is extended to the end of each of the green sheets  2   a  and  2   e  so that it connects to and provides conduction with the external electrode (not shown), forming extension electrodes  6   a  and  6   e , respectively. 
     The multilayer-type chip inductor  1  is obtained in the following way. As shown in FIG. 4, a predetermined number of dummy green sheets  2   f  on which no electrode film is formed are multilayered in sequence to form a bottom portion of the device. Next, the first green sheets  2   a  to  2   e  containing the electrode films  3   a  to  3   e  on their respective top faces are multilayered on top of the dummy green sheets  2   f . Further, in the same manner, another series of green sheets  2   a  to  2   e  are multilayered, and a predetermined number of dummy green sheets  2   f  are applied. Then, the body is contact-bonded and sintered. Then, external electrodes are formed at both ends (the right side and the left side in FIG. 4) of this sintered body. 
     Since the first green sheets  2   a  to  2   e  shown in FIG. 4 are formed with the electrode films  3   a  to  3   e  of a ¾ turn, respectively, two inductors  5  of 3.5 turns are formed inside the sintered body. 
     The external electrode on the right side is made to conduct with the extension electrodes  6   a  and  6   a  of the inductors  5  and  5 , and the external electrode on the left side is made to conduct with the extension electrodes  6   e  and  6   e  of the inductors  5  and  5 . Therefore, as shown in FIG. 5, the multilayer-type chip inductor  1  is such that the two upper and lower inductors  5  and  5  are connected in parallel. 
     Next, a multilayer-type chip inductor of a second conventional example comprising a coil of parallel internal conductors will be described with reference to FIGS. 6 and 7. Components in FIGS. 6 and 7 which are the same as those of the above-described first conventional example are given the same reference numerals and a detailed description thereof is omitted. 
     A multilayer-type chip inductor  11  is formed in such a way that first green sheets  2   a  to  2   e  have electrode films  3   a  to  3   e  formed thereon, respectively. First green sheets  12   a  to  12   e  are similar to the first green sheets  2   a  to  2   e . The green sheets  2   a  to  2   e  are alternately arranged (e.g., interleaved) in a multilayered fashion with the green sheets  12   a  to  12   e . These multiple layers are then sintered, and then external electrodes (not shown) are formed at both ends of this sintered body. 
     The first green sheets  12   a  to  12   e  are formed into sheets from an insulating ceramic slurry in the same manner as the first green sheets  2   a  to  2   e , and electrode films  13   a  to  13   e  are formed on one surface thereof. Further, in the first green sheets  12   b  to  12   e , via holes  14   b  to  14   e  are formed at the ends of the electrode films  13   b  to  13   e , respectively. In the first green sheets  12   a  to  12   d , via holes  17   a  to  17   d  are provided at the other ends of the electrode films  13   a  to  13   d , respectively. 
     The multilayer-type chip inductor  11  is obtained in the following way. As shown in FIG. 6, a predetermined number of dummy green sheets  2   f  are multilayered in sequence to from a bottom portion. Next, the first green sheets  2   a ,  12   a ,  2   b ,  12   b ,  2   c ,  12   c ,  2   d ,  12   d ,  2   e , and  12   e  are multilayered on top of the bottom portion, with each surface having an electrode formed on its top side. Further, a predetermined number of dummy green sheets  2   f  are applied on top of the body, and then the body is contact-bonded and sintered. Then, external electrodes are formed at both ends (the right side and the left side in FIG. 6) of this sintered body. 
     Therefore, in the multilayer-type chip inductor  11 , an inductor  15  of 3.5 turns which is made to branch into two lines via the respective via holes is formed within the multilayered body. The external electrode on the right side is made to conduct with the extension electrodes  6   a  and  16   a  of the inductor  15 , and the external electrode on the left side is made to conduct with extension electrodes  6   e  and  16   e  of the inductor  15 . 
     However, in the above-described conventional first and second examples, although the DC resistance of the inductor is reduced, the following problems are present. In the first conventional example, because the decrease in inductance is large, the number of windings of the coil must be increased to maintain the inductance at a desired value. In the second conventional example, although the decrease in inductance is small, the number of via holes corresponding to via holes  17   a  to  17   d  provided in the first green sheets  12   a  to  12   d  and the number of types of first green sheets increases, causing the manufacturing process to become more complex. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to solve at least the above-described problems. More specifically, an object of the present invention is to provide a method of manufacturing a multilayer-type chip inductor such that the DC resistance of the inductor is small without decreasing the inductance and the impedance. 
     To achieve the above-described object, according to the present invention, there is provided a method of manufacturing a multilayer-type chip inductor, comprising the steps of: preparing a ceramic green sheet; forming an electrode film on one surface of the green sheet; multilayering a plurality of the green sheets in such a way that the surfaces having electrode films formed thereon face each other for pairs of the green sheets; contact-bonding the green sheets; and sintering the green sheets. 
     More specifically, the present invention provides a method of manufacturing a multilayer-type chip inductor, comprising the steps of: preparing a ceramic green sheet; overlaying a first green sheet having a coil-like electrode film of less than one complete turn formed on one surface of the green sheet and a second green sheet having formed thereon a coil-like electrode film which is symmetrical with the first green sheet so that the electrode films face each other in order to be formed into a pair; multilayering a plurality of such pairs of the green sheets; and sintering the green sheets, wherein the respective coil-like electrode films are made to conduct by via holes provided at the ends of the coil-like electrode films, thereby forming an inductor. 
     Preferably, the ceramic is an insulating ceramic. 
     The method of manufacturing a multilayer-type chip inductor further includes a step of forming external electrodes which conduct to the ends of the inductor before or after being sintered. 
     As a result, as in the first and second conventional examples, it is possible to increase the cross-sectional area of the conductor and to reduce the DC resistance of the inductor. In the present invention, however, the design does not result in a decrease in the inductance and the impedance. Further, the manufacturing method of the present invention is less complex than the manufacturing method of the above-described second conventional example (e.g., because it reduces the number of via holes required). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and further objects, aspects and novel features of the invention will become more apparent from the following detailed description when read in connection with the accompanying drawings, in which: 
     FIG. 1 is a perspective view illustrating a method of manufacturing a multilayer-type chip inductor before multilayering according to an exemplary embodiment of the present invention; 
     FIG. 2 is a perspective view of the multilayer-type chip inductor shown in FIG. 1; 
     FIG. 3 is a longitudinal sectional view taken along the plane A of the multilayer-type chip inductor shown in FIG. 2; 
     FIG. 4 is a perspective view of a multilayer-type chip inductor of a first conventional example before multilayering; 
     FIG. 5 is a longitudinal sectional view corresponding to FIG. 3 of the multilayer-type chip inductor of the first conventional example; 
     FIG. 6 is a perspective view of a multilayer-type chip inductor of a second conventional example before multilayering; and 
     FIG. 7 is a longitudinal sectional view corresponding to FIG. 3 of the multilayer-type chip inductor of the second conventional example. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred embodiment of the present invention will be described below in detail with reference to FIGS. 1 to  3 . Components which are the same as those of the above-described conventional examples are given the same reference numerals. Detailed description of these common components is omitted. 
     A multilayer-type chip inductor  21  is formed in such a way that each one of first green sheets  2   a  to  2   e  has electrode films  3   a  to  3   e  formed respectively thereon, and each one of second green sheets  22   a  to  22   e  has electrode films  23   a  to  23   e  formed respectively thereon. These green sheets are alternately multilayered and sintered. External electrodes  28  and  29  are formed at both ends of this sintered body. 
     In the same manner as the first green sheets  2   a  to  2   e , the second green sheets  22   a  to  22   e  are formed into sheets from an insulating ceramic slurry, such as ferrite or a dielectric or other suitable material. These sheets have electrode films  23   a  to  23   e , which become internal conductors, respectively formed by printing or other suitable technique on one surface thereof. The respective electrode films  23   a  to  23   e  are formed symmetrically with respect to the electrode films  3   a  to  3   e  when they face the electrode films  3   a  to  3   e . Further, in the second green sheets  22   a  to  22   d , via holes  24   a  to  24   d  are formed at one end of each of the electrode films  23   a  to  23   e . In parts of the electrode films  23   a  and  23   e , one end of each film is extended to the ends of the green sheets  22   a  and  22   e  so as to conduct to the external electrodes, forming extension electrodes  26   a  and  26   e.    
     The multilayer-type chip inductor  21  is obtained in the following way. As shown in FIG. 1, a predetermined number of dummy green sheets  2   f , on the surface of which no electrode film is formed, are multilayered in sequence to form a bottom portion of the device. Next, the first green sheet  2   a , the second green sheet  22   a , the first green sheet  2   b , the second green sheet  22   b , . . . , the first green sheet  2   e , and the second green sheet  22   e  are overlaid and multilayered in such a way that each pair of electrode films (the electrode film  3   a  and the electrode film  23   a , . . . , the electrode film  3   e  and the electrode film  23   e ) face each other. Further, a predetermined number of dummy green sheets  2   f  are multilayered on top of the body, and then the body is contact-bonded and sintered. Then, external electrodes  28  and  29  are formed at both ends of this sintered body. Alternatively, the external electrodes can be applied before sintering. 
     Since the first green sheets  2   a  to  2   e  and the second green sheets  22   a  to  22   e  shown in FIG. 1 are formed with the electrode films  3   a  to  3   e  and the electrode films  23   a  to  23   e  of a ¾ turn, respectively, in the multilayer-type chip inductor  21 , an inductor  25  of 3.5 turns is formed inside the multilayered body. One external electrode  28  is made to conduct with the extension electrodes  6   a  and  6   a  of the inductor  25 , and the other external electrode  29  is made to conduct to the extension electrodes  6   e  and  6   e  of the inductor  25 . 
     Therefore, in the multilayer-type chip inductor  21 , as shown in FIG. 3, an inductor  25 , whose internal conductor is thicker than the thickness of the internal conductor of the multilayer-type chip inductors  1  and  11  shown in FIGS. 5 and 7, that is, having a larger cross-sectional area, is formed inside the multilayered body. 
     The multilayer-type chip inductor in accordance with the multilayer-type manufacturing method of the present invention is not limited to this embodiment, and various modifications are possible within the spirit and scope of the invention. For example, although an electrode film of a ¾ turn is shown, in addition to this, the electrode film may be of a ½ turn. Further, the shape of the electrode film is not limited to a coil shape, and may be a rectangular parallelepiped which connects the section between the two external electrodes by a straight line. 
     Further, the total number of windings of the inductor may be changed to any desired number of windings by increasing or decreasing the number of multilayers of the first and second green sheets. 
     As described above, in the method of manufacturing a multilayer-type chip inductor according to the present invention, since green sheets are multilayered so that surfaces on which electrode films are formed face each other in order to form an inductor, the thickness of the internal conductor is large, and the cross-sectional area increases accordingly. 
     Therefore, in the multilayer-type chip inductor according to the present invention, it is possible to reduce the DC resistance of the inductor without decreasing the inductance or the impedance. Further, the multilayer-type chip inductor according to the present invention becomes capable of withstanding a high-current load, and the allowable current value increases. 
     Many different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiment described in this specification. To the contrary, the present invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the invention as hereafter claimed. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications, equivalent structures and functions.