HIGH FREQUENCY COIL AND METHOD OF MANUFACTURING THE SAME

Abstract: A high frequency coil having a structure in which strip-like coil conductors are formed on the surfaces of an insulating substrate, and a method of manufacturing the coil. An object of the invention is to increase the Q value of the coil without increasing the film thickness and wire width of the coil conductors. A pair of coil conductors (3, 4) are so formed that the coil conductors sandwich an insulating substrate (1). The paired coil conductors (3, 4) are so connected in parallel to each other that currents flow through them in the same direction. The ends of the paired coil conductors (3, 4) are connected electrically with an input and an output electrode (5, 6) formed on the insulating substrate (1).

Description: The present invention relates generally to a high frequency coil having a structure in which strip-shaped coil conductors are formed on the surface of an insulating substrate and a method of fabricating the same, and more particularly, to a high frequency coil whose Q can be increased without increasing the thickness and the line width of coil conductors and a method of fabricating the same. Conventionally, a high frequency coil used in a microwave circuit or the like has had the following structure. More specifically, the high frequency coil has a structure in which a coil conductor, for example, of a spiral type, is formed on the surface of an insulating substrate, an input electrode and an output electrode are respectively formed in side edge portions opposed to each other of the insulating substrate, and the input electrode and the output electrode are electrically connected to an outer end and an inner end of the above coil conductor, respectively. The above described high frequency coil is disclosed in, for example, British Patent Publication GB2,223,624A. In such a high frequency coil, a coil conductor is formed by a thin film forming technique such as sputtering or vacuum evaporation. Therefore, the high frequency coil has the disadvantage in that the electrical resistance of the coil conductor is increased because the thickness of the coil conductor is not made too large, resulting in decreased Q of the coil. Therefore, attempts have been conventionally made to lower the electrical resistance of the coil conductor by increasing the thickness and the line width of the coil conductor so as to increase Q of the coil. However, the conventional high frequency coil has the disadvantage in that if the line width of the coil conductor is increased, the substrate is increased in size by the amount of the increase, to make it impossible to meet the recent requirement of miniaturization of electronic components. Furthermore, if the thickness of the coil conductor is increased, additional time is required to make etching by the amount of the increase. As a result, the high frequency coil has the disadvantage in that there occurs a undesirable phenomenon referred to as under etching. Consequently, there is a limitation on the decrease in thickness of the coil conductor, so that there is actually a limitation on the improvement in Q of the coil. In addition, it is considered that the thickness of a coil conductor is increased by superimposing on the upper surface of one coil conductor another coil conductor. However, the line width of the coil conductor and the spacing between portions where the coil conductors are wound are very small, i.e., several tens of micrometers, therby to make it very difficult in the fabrication to further superimpose on one coil conductor another coil conductor having the same size with high precision. DE-U-88 01 879 discloses an inductor comprising an electrical insulating support member, two spiral-type conductors being mounted on the two opposing surfaces of the insulating support member, and two central contact elements as well as two external contact elements. By means of a through hole the two centre contact elements of the two spiral-type conductors are connected to each other, and the external contact members represent the external terminals for this inductor. GB-A-2 083 952 discloses a microcoil assembly comprising a spiral-patterned flat conductor 1 which is formed on both surfaces of an insulating substrate. The conductors on both surfaces of the substrate are arranged so that electric current can flow through the two spiral-patterned conductors in the same directions. The two conductors comprise inner ends which are connected to each other by a through hole, and outer ends which form the external lead terminals of the microcoil assembly. In the above two documents, the two conductors are serially connected, thus increasing the resistance R of this assembly, wherein as a result the Q is decreased. JP-A-2-123 706 discloses a high frequency coil comprising a substrate, one spiral-patterned conductor being positioned on one of the surfaces of said substrate, and two terminal electrodes. This document does not deal with the reduction of the resistance of an inductor comprising two coil conductors being positioned on opposite surfaces of a insulating substrate. OBJECT OF THE INVENTION Accordingly, an object of the present invention is to eliminate the above described disadvantages of the conventional high frequency coil and to provide a high frequency coil having a structure in which Q can be increased without increasing the thickness and the line width of coil conductors and a method of fabricating the same. DISCLOSURE OF THE INVENTION The inventors of the present application have found that if a pluarlity of high frequency coils are electrically connected in parallel so as to decrease the conductor resistance of coil conductors, the conductor resistance can be decreasd but the inductance is similarly decreased so that Q of the coil cannot be improved. As the result of further examining a structure in which a plurality of coil conductors are connected in parallel, the inventors have found that the amount of the decrease in conductor resistance is larger than the amount of the decrease in inductance if a pair of coil conductors are formed such that the directions of currents flowing through the coil conductors are the same and so as to be opposed to each other with the insulating layer being interposed therebetween and have found that Q of the coil can be increased because the decrease in inductance can be restrained, thereby to make the present invention. More specifically, a high frequency coil according to the present invention has a structure comprising at least one insulating layer, a pair of coil conductors formed on both major surfaces of the insulating layer so as to be opposed to each other with the above insulating layer being interposed therebetween, and an input electrode and an output electrode formed on the above insulating layer, in which one end of each of the above pair of coil conductors is electrically connected to the input electrode and the other end thereof is electrically connected to the output electrode such that the directions of currents flowing through the above coil conductors are the same. Furthermore, the high frequency coil according to the present invention includes one having the following multilayer structure in addition to the above described structure in which a pair of coil conductors is formed on both major surfaces of one insulating layer. More specifically, the present invention also includes a structure comprising a plurality of insulating layers, in which the insulating layers and pairs of coil conductors are alternately laminated such that the pair of coil conductors is arranged on both major surfaces of each of the insulating layers laminated. Meanwhile, an insulating layer made of an insulating material having sufficient mechanical strength to support coil conductors, for example, glass or ceramics is generally used as the above described insulating layer. When a plurality of insulating layers are laminated, however, the insulating layers may be made of a flexible insulating material provided that the lowermost insulating layer is made of such a rigid insulating material. Furthermore, in forming coil conductors according to the present invention, the known thin film forming technique such as a sputtering process, an evaporation process, an ion plating process or a screen printing process can be utilized. That is, a method of forming the coil conductors is not particularly limited. Additionally, a method of fabricating a high frequency coil according to the present invention comprises the steps of preparing an insulating layer, forming a conductor on the entire surface of the above insulating layer, etching the above conductor to form first and second coil conductors formed so as to be opposed to each other while being separated by the insulating layer and input and output electrodes on both major surfaces of the above insulating layer, coating both the entire major surfaces of the above insulating layer with photosensitive synthetic resin to form photosensitive synthetic resin films, disposing masks on portions, which face the above input and output electrodes and one end of each of the coil conductors, of the above photosensitive synthetic resin films to expose and develop the photosensitive synthetic resin films, removing portions other than the portions of the photosensitive synthetic resin films hardened by the exposure after the development to expose the input and output electrodes and to form a through hole in the portion facing the above one end of each of the coil conductors, and forming a conductor film on the above insulating layer to electrically connect the output electrode to the one end of each of the coil conductors exposed to an inner surface of the above through hole. In the high frequency coil according to the present invention, a pair of coil conductors is formed so as to be opposed to each other while being separated by an insulating layer, and the directions of currents flowing through the coil conductors are the same. Accordingly, the decrease in conductor resistance is made larger than the decrease in inductance. Consequently, inductance L is relatively increased, as compared with that of the conventional high frequency coil having decreased conductor resistance. In the high frequency coil according to the present invention, therefore, Q of the coil can be increased by the amount of the relative increase in the inductance L. Accordingly, in the high frequency coil according to the present invention, Q of the coil can be increased without increasing the thickness and the line width of the coil conductors. Consequently, the increase in size of components of the high frequency coil can be avoided, and no undesired phenomena such as under etching occur at the time of the fabrication of the high frequency coil. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic perspective view for explaining a structure of a high-frequency coil according to an embodiment of the present invention; Figs. 2 (a) to (c) are diagrams for explaining the high frequency coil according to the embodiment of the present invention, where Figs. 2 (a) and (b) are respectively a plan view and a bottom view showing the high frequency coil, and Fig. 2 (c) is a cross sectional view; Figs. 3 (a) to (e) are cross sectional views for explaining a method of fabricating the high frequency coil according to the embodiment shown in Fig. 1, where Fig. 3 (a) is a cross sectional view showing an insulating substrate used, 3 (b) is a cross sectional view showing a state where a conductive film having a multilayer structure is formed on the entire surface of the insulating substrate, Fig. 3 (c) is a cross sectional view showing a state where a pair of coil conductors and the like are formed by etching, Fig. 3 (d) is a cross sectional view showing a state where insulating films are formed by coating, and Fig. 3 (e) is a cross sectional view showing a state where the insulating films are exposed and developed to form insulating layers and through holes; and Fig. 4 is a schematic cross sectional view for explaining another embodiment of the present invention. BEST MODE FOR CARRYING OUT OF THE INVENTION Embodiments of the present invention will be described with reference to the accompanying drawings. Figs. 1 to 3 are diagrams for explaining a high frequency coil according to one embodiment of the present invention. Referring to Fig. 1, a chip-type high frequency coil 1 according to the present embodiment is constructed by pattern formation of spiral-shaped first and second coil conductors 3 and 4 on both major surfaces 2a and 2b of an insulating substrate 2 made of glass or ceramics such as alumina. In addition, an input electrode 5 and an output electrode 6 are formed on both side surfaces of the above insulating substrate 2 in the longitudinal direction. Outer ends 3a and 4a of the above first and second coil conductors 3 and 4 are connected to the input electrode 5. Furthermore, the surfaces of the first and second coil conductors 3 and 4 excluding regions on the input and output electrodes 5 and 6 on both major surfaces of the above insulating substrate 2 are coated with insulating layers 7 made of polyimide or polyamide resin. Through holes 8 are formed in portions, which face inner ends 3b and 4b of the above first and second coil conductors 3 and 4, of the insulating layers 7. In addition, lead electrodes 9 are respectively formed on the upper surfaces of the above insulating layers 7, and one ends of the lead electrodes 9 are respectively connected to the inner ends 3b and 4b of the first and second coil conductors 3 and 4 through the above through holes 8 and the other ends thereof are respectively connected to the output electrode 6. The above described first and second coil conductors 3 and 4 are so constructed that the directions of currents flowing through the first and second coil conductors 3 and 4 are the same. A method of fabricating a chip-type high frequency coil 1 according to the present embodiment will be described with reference to Figs. 2 and 3. (1) A Ti film 10a for improving adhesion to an insulating substrate 2 subjected to mirror polishing is first formed on the entire outer surface of the insulating substrate 2 by a sputtering process. Subsequently, a Ti-Ag film 10b is formed on the surface of the Ti film 10a by a dual sputtering process (simultaneous sputtering process), and an Ag film 10c is further formed on the surface of the Ti-Ag film 10b similarly by the sputtering process, to form a conductor 10 having a three-layer structure (see Figs. 3 (a) and (b)). (2) Both major surfaces 2a and 2b of the above insulating substrate 2 are coated with masks, which are not shown, designed in shapes corresponding to the shapes of first and second coil conductors and input and output electrodes and then, are subjected to etching processing. Consequently, portions which are not coated with the masks are removed, to form first and second coil conductors 3 and 4 and input and output electrodes 5 and 6. In Fig. 3 (b), each of the coil conductors 3 and 4 and the input and output electrodes 5 and 6 is illustrated for easy understanding such that it is formed of a single layer. The above first and second coil conductors 3 and 4 have such a structure that they are opposed to each other with the substrate 2 being interposed therebetween (see Figs. 2 (a) and (b) and Fig. 3 (c)). (3) Subsequently, both the entire major surfaces 2a and 2b of the above insulating substrate 2 are coated with photosensitive polyimide resin to form insulating films 7a and dry them (see Fig. 3 (d)). Then, masks are disposed on portions, which face the above input and output electrodes 5 and 6 and inner ends 3b and 4b of the coil conductors 3 and 4, of the insulating films 7a to expose and develop portions other than the portions of the insulating films 7a. Consequently, the exposed portions of the insulating films 7a remain, while the other portions of the insulating films 7a are removed, to form insulating layers 7. Accordingly, the input and output electrodes 5 and 6 are exposed, and through holes 8 are formed in the portions facing the inner ends 3b and 4b of the coil conductors 3 and 4 in the insulating layers 7 (see Fig. 3 (e)). (4) Finally, conductive films are formed on the upper surfaces of both the above insulating layers 7 by the sputtering process, and lead electrodes 9 are formed in the same manner as the above described step (2) to connect the inner ends 3b and 4b to the output electrode 6. Accordingly, a high frequency coil 1 according to the present embodiment is formed (see Fig. 2 (c)). Meanwhile, in the above described step (4), when the inner ends 3b and 4b of the coil conductors 3 and 4 are connected to the output electrode 6, a method of connecting the inner ends 3b and 4b and the output electrode 6 by wire bonding using an Au line and fastening them with nylon or adhesives of an epoxy resin system may be adopted. Description is now made of the function and the effect of the present embodiment. In the chip-type high frequency coil 1 according to the present embodiment, the first and second coil conductors 3 and 4 are formed on both the major surfaces 2a and 2b of the insulating substrate 2 so as to be opposed to each other with the substrate 2 being interposed therebetween, and the outer ends 3a and 4a of the coil conductors 3 and 4 are connected to the input electrode 5 and the inner ends 3b and 4b thereof are connected to the output electrode 6, thereby to make the directions of currents flowing through the coil conductors 3 and 4 the same. Accordingly, the conductor resistance can be reduced to approximately half, and Q of the coil can be improved because the decrease in inductance can be restrained. When only one coil conductor is formed on one major surface of an insulating substrate having a thickness of 0.64 mm, the value of Q is 30 (at 400 MHz) if the value of L is 18 nH. On the other hand, in the structure according to the present embodiment, the value of Q is 35 (at 400 MHz) if the value of L is 10.5 nH, thereby to make it possible to improve Q by approximately 15 %. Furthermore, in the present embodiment, the first and second coil conductors 3 and 4 may be merely formed opposed to each other on both the major surfaces of the insulating substrate 2 by the thin film forming technique. Accordingly, the thickness and the line width of the coil conductors need not be increased unlike the conventional example. Consequently, the increase in size of components can be avoided, and there arise no problems such as under etching. Accordingly, if the high frequency coil 1 according to the present embodiment is employed when the thickness and the line width of the coil conductors are made as large as possible and the conductor resistance is made smaller to improve Q, a larger effect is obtained. Although in the above described embodiment, description was made of a case where the first and second coil conductors 3 and 4 are formed with the insulating substrate 2 being interposed therebetween, the present invention may be applied to a multilayer coil constructed by forming one high frequency coil and then, forming an insulating layer in a portion excluding input and output electrodes and forming coil conductors on the upper surface of the insulating layer, or further repeatedly forming insulating layers and coil conductors. More specifically, as shown in Fig. 4, a multilayered high frequency coil may be constructed by further forming an insulating layer 11 and a third coil conductor 12 on a coil conductor 3 formed on the upper surface of one insulating substrate 2. Alternatively, a multilayered high frequency coil having more layers than those in the example as shown in Fig. 4 may be constructed by further laminating insulating layers and coil conductors. Additionally, although in the above described embodiment, description was made of spiral-type coil conductors by way of example, it goes without saying that the present invention is not limited to the same. For example, the present invention is also applicable to a high frequency coil having Mianda-type coil conductors.

Claims: A high frequency coil comprising: at least one insulating layer (2); at least two coil conductors (3,4), being arranged on two opposite surfaces of said insulating layer (2) opposed to each other; and an input electrode (5) and an output electrode (6); characterized in that said coil conductors (3, 4) are arranged such that currents flowing through said two coil conductors (3,4) have the same direction; and said coil conductors (3,4) are connected to said input electrode (5) and said output electrode (6), such that said coil conductors (3,4) are being electrically connected in parallel. The high frequency coil according to claim 1, characterized by a plurality of insulating layers (2,11) laminated, said insulating layers (2,11) and pairs of coil conductors (3,4,12) being alternately laminated such that the pair of coil conductors (3,4,3,12) is arranged on both major surfaces of each of the insulating layers (2,11) laminated. The high frequency coil according to claim 1 or 2, characterized in that said input electrode (5) and said output electrode (6) are formed so as to cover a pair of side surfaces, which are opposed to each other, of said insulating layer (2). The high frequency coil according to one of claims 1 to 3, characterized in that said insulating layer (2,11) is a rectangular insulating substrate made of an insulating material. The high frequency coil according to one of claims 1 to 4, characterized by insulating resin layers (7) formed so as to respectively cover said pair of coil conductors (3,4). The high frequency coil according to one of claims 1 to 5, characterized by a plane shape of the coil conductor (3,4,12) is spiral. A method of fabricating a high frequency coil according to one of the claims 1 to 6, characterized by the steps of: preparing an insulating layer (2); forming a conductor on the entire outer surface of said insulating layer (2); etching said conductor to form first and second coil conductors (3,4) formed so as to be opposed to each other while being separated by the insulating layer (2) and input and output electrodes (5,6) on both major surfaces of said insulating layer (2); coating of the entire major surfaces of said insulating layer (2) with photosensitive synthetic resin to form photosensitive synthetic resin films; disposing masks on portions, which face said input and output electrodes (5,6) and one end of each of the coil conductors (3,4), of said photosensitive synthetic resin films to expose and develop the photosensitive synthetic resin films (7); removing portions other than the portions of the photosensitv resin films hardened by the exposure after the development, thereby to expose the input and output electrodes (5,6) and to form a through hole (8) in the portion facing said one end of each of the coil conductors (3,4); and forming a conductor film (9) on said resin film (7), thereby to electrically connect the output electrode (6) to the one end of each of the coil conductors (3b,4b) exposed to an inner surface of said through hole (8).

Inventors: KANOU OSAMU, MISAKI KATSUHIRO, SENDA ATSUO, KANOU, OSAMU, MISAKI, KATSUHIRO, SENDA, ATSUO
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1991
Main classification: H01F17
IPC codes: H01F41, H01F17