Method of manufacturing a High-frequency inductor

Abstract: A high-freguency inductor comprising an insulating substrate; and a conductive land formed on the insulating substrate, the conductive land comprising at least two thin films formed of different compositions of metals, each of the metals is to be etched by such an etching agent that substantially never removes the other metals, the thin films being accumulated vertically.

Description: BACKGROUND OF THE INVENTION (1) Field of the Invention This invention relates to a method of manufacturing a high-frequency inductor comprising a conductive land formed on an insulating substrate, especially one realizing a high value of the Q factor. (2) Description of the Prior Art Fig. 1 shows a conventional high-frequency inductor used for a microwave circuit or the like; and Fig. 2 is a cross sectional view thereof. A high-frequency inductor 20 comprises an insulating substrate 21, a spiral conductive land 22 formed on a surface of the substrate 21, and terminal electrodes 23a and 23b along opposed side ends of the substrate 21. An outer end 22a of the spiral conductive land 22 is connected with the terminal electrode 23a, and an inner end 22b is connected with the terminal electrode 23b. The conductive land 22 and the terminal electrodes 23a and 23b are covered with an insulating film 24, which has a connecting electrode 25 formed thereon. An end of the connecting electrode 25 is connected to the inner end 22b through a through-hole electrode 26, and the other end of the connecting electrode 25 is connected to the terminal electrode 23b, whereby the inner end 22b and the terminal electrode 23b are connected to each other. The inductor 20 having the above construction is manufactured in the following way. The insulating substrate 21 is coated with a metal film by sputtering, evaporation or the like. Then, the metal film is photo-etched, whereby the spiral conductive land 22 and the terminal electrodes 23a and 23b are formed. The terminal electrode 23a and the outer end 22a are connected to each other. The insulating substrate 21 having the spiral conductive land 22 and the terminal electrodes 23a and 23b is coated with polyimide to form the insulating film 24. A portion of the film 24 corresponding to the inner end 22b is hollowed to make a through hole; and portions of the film 24 corresponding to the terminal electrodes 23a and 23b are removed. The insulating film 24 is coated with a metal film, which is then photo-etched to form the connecting electrode 25 and the through-hole electrode 26. The above conventional inductor has a problem of having a low value of the Q factor due to a large resistance value. This occurs because the conductive land and the terminal electrodes are manufactured by the thin film technologies such as sputtering and evaporation. This problem can be solved by increasing the thickness of the conductive land and the terminal electrodes. However, a thicker metal film takes longer to etch. IF etching lasts long, the metal film is etched not only vertically but horizontally. As a result, even a portion 30 (Fig. 3) which should be left to be the conductive land or the terminal electrode is etched despite that the portion 30 is covered with a mask 28. Consequently, the value of the Q factor is decreased, or the formation of the spiral conductive land cannot be realized. In Patent Abstracts of Japan, volume 10, No 132 (P-456) (2189), there is an abstract of JP-A-60 254 403 (Fujitsu K.K.) which discloses forming a thin-film spiral on a substrate by (a) forming a base layer on a substrate, the materials of the base layer and the substrate not being mentioned in the abstract, (b) forming a mask on the base layer, (c) copper plating on to the base layer through apertures in the mask, (d) removing the mask and the exposed parts of the base layer, leaving a thin-film copper spiral on the remaining parts of the base layer, (e) applying insulating material to the assembly to fill the gaps between the turns of the spiral, (f) repeating steps (b) and (c), the plated-on material presumably again being copper, although this is not explicitly stated in the abstract. It is not clear whether in step (d) the base layer is etched by a fluid which does not etch copper or by a fluid which does etch copper, this etching being slight and tolerable. In Patent Abstracts of Japan, volume 12, No 385 (E-668), there is an abstract of JP-A-63 129 604 (Fujitsu Ltd) which discloses forming a spiral on a ceramic substrate by (a) forming a Cu layer on one face of the substrate and a Cr layer on the opposite face, (b) coating the Cr layer with Au, (c) using a mask and etching fluid to etch the Au so that a spiral-form part of the Au remains and between the turns thereof the Cr is exposed and is not then etched away. In Patent Abstracts of Japan, volume 12, No 265 (E-637) (3112) there is an abstract of JP-A-63 050 004 (Mitsubishi) which discloses a spiral conductor on a substrate, but not how it is produced. To connect the inner end of the spiral to an external circuit, there is a connecting conductor which lies in a groove in the substrate and passes beneath the turns of the spiral, being separated from them by a layer of insulation which fills the groove and is flush with the surface of the substrate. In Patent Abstracts of Japan, volume 12, No 234 (E-629) (3081) there is an abstract of JP-A-63 028 008 (Matsushita) which again discloses a spiral conductor on a substrate, but not how it is produced. To connect the inner end of the spiral to an external circuit there is a connecting conductor which appears from the drawings to pass under the turns of the spiral. In DE-A-3524832, there is disclosed a process for selective etching of metals in which different layers of metals are etched by means of different etching agents. In particular, an etching agent is selected which is able to etch one metal present but is unable to etch another metal present. OBJECT OF THE INVENTION A primary object of the invention is to produce a method of manufacturing a high-frequency inductor having a large thickness with no possibility of under-etching (such as is described above with reference to Figure 3) and with the inductor having a high Q factor. SUMMARY OF THE INVENTION Both the prior art method described above with reference to Figures 1 to 3 and the method according to the present invention can be described as a method of manufacturing a high-frequency inductor in which a first metallic film is deposited on a substrate and is etched to form a land comprising a spiral portion, a first terminal connected to the outer end of the spiral portion and a second terminal, an insulating film is formed over the spiral portion and a second metallic film is deposited on the insulating film and is etched to form a connecting conductor connecting the inner end of the spiral to the second terminal. The method according to the invention is characterised in that in a first step a first layer of a first metal is deposited on the substrate and over that is deposited at least a second layer of a second metal, in a second step the second layer is etched by a first etching agent which is substantially unable to etch the first layer and then the first layer is etched by a second etching agent which is substantially unable to etch the second layer, with the result that a first composite body is formed comprising a spiral portion, a first terminal connected by a portion to the outer end of the spiral portion and a second terminal and with the further result that the substrate is exposed other than at the composite body, in a third step an insulating film is formed on a portion of the first composite body extending between the inner end of the spiral portion and the second terminal, in a fourth step a first layer of a first metal is deposited on the assembly of layers resulting from the first to third steps and over that is deposited at least a second layer of a second metal, in a fifth step parts of the metal layers of the fourth step are removed, leaving a second composite body on the first composite body and registering with the latter except where the insulating film is disposed and also leaving a connecting conductor on the insulating film connecting the inner end of the spiral portion to the second terminal. This method is clearly not disclosed in the prior art discussed and it would not be obvious to any person skilled in the art who reads any or all of that prior art. The metals of the first and second layers may advantageously be selected from Ti, Ag, Cu, Al, Ni, Cr and Pd. Preferably, in the first step or in each of the first and fourth steps the metal layers are of Ti, Ti-Ag and Ag, in which case, preferably Ti is etched by an aqueous solution of hydrofluoric nitric acid and Ag is etched by an aqueous solution of ferric nitrate. The metals of the first and second layers, and the intermediate layer or layers may, for example, be applied by evaporation, sputtering or ion-plating. BRIEF DESCRIPTION OF FIGURES 4 TO 7 OF THE DRAWINGS Fig.4 is a plan view a high-frequency inductor made by a method according to this invention; Fig. 5 is a cross sectional view of the inductor taken along line V-V; Figs. 6a to 6j show first, second and third steps in manufacturing the high frequency inductor of Figs. 4 and 5; and Figs. 7a to 7d show fourth and fifth steps in manufacturing the high-frequency inductor of Figs. 4 and 5. DESCRIPTION OF A PREFERRED EMBODIMENT As shown in Fig. 4, a high-frequency inductor 1 comprises an insulating substrate 2 and a conductive land 3 formed on the substrate 2. The conductive land 3 comprises terminal electrodes 4a and 4b formed along opposed side ends of the substrate 2 and also a spiral portion 5 at the center of the substrate 2. The outer end of the spiral portion 5 is connected by a conductor 5a with the terminal electrode 4a. Between the inner end 5b of the spiral portion 5 and the terminal electrode 4b, the conductive land 3 is covered with an insulating film 9 formed of polyimide or polyamide. Formed on the insulating film 9 is a connecting electrode 6; the ends thereof are connected with the inner end 5b and with the terminal electrode 4b, respectively. As shown in Fig. 5, the conductive land 3 comprises a first layer 7 formed on the substrate 2 and a second layer 8 formed on the first layer 7. Although not shown in Fig. 5, the first and second layers 7 and 8 each comprise three superposed metal thin films, the three films being formed of Ti (nearest the substrate 2), Ag (furthest from the substrate) and Ti-Ag (between the Ti and the Ag). The first and the second layers 7 and 8 are each produced by forming the three metal thin films by the thin film technology such as evaporation, sputtering and ion-plating and then by etching with an etching agent which removes only Ag followed by an etching agent which removes only Ti. The high-frequency inductor 1 is manufactured as shown in Figs. 6 and 7. Step 1 The insulating substrate 2 (Fig. 6a) formed of glass, crystallised glass, alumina or the like, is subjected to glass abrasion, and a polyimide film (not shown) having a thickness of approx. 5µm is formed on an upper surface of the substrate 2, whereby smoothing the upper surface. Then, a Ti film 10a having a thickness of approx. 200Å is formed on an upper surface of the substrate 2 by sputtering (Fig. 6b). A Ti-Ag film 10b having a thickness of approx. 1,200Å is formed on an upper surface of the Ti film 10a by sputtering Ti and Ag from two different sources simultaneously (Fig. 6c). An Ag film 10c having a thickness of approx. 3µm is formed on an upper surface of the Ti-Ag film 10b by sputtering (Fig. 6d). In this way, a three-layer conductive film 10 is completed. The Ti film 10a contributes to the better adherence between the conductive film 10 and the substrate 2. <Step 2> An upper surface of the conductive film 10 is masked in accordance with the shape of the conductive land 3. Then, the conductive film 10 is etched by an etching agent that removes only Ag (for example, an aqueous solution of ferric nitrate), whereby non-masked portions of the films 10c and 10b are deprived of Ag. As a result, the film 10c is left in the shape of the conductive land 3 (7c of Fig. 6e). The conductive film 10 is further etched by an etching agent that removes only Ti (for example, an aqueous solution of hydrofluoric nitric acid), whereby the non-masked portions of the films 10b and 10a are deprived of Ti. As a result, the films 10b and 10a are left in the shape of the conductive land 3 (7b and 7a of Fig. 6e) and the surface of the substrate 2 between turns of the spiral is exposed. Then, the conductive film 10 is unmasked to complete the first composite body or layer 7 (Figs. 6f and 6g). <Step 3> A surface of the first layer 7 is coated with a photoconductive polyimide resin to form an insulating film 9a, and the film 9a is dried (Fig. 6h). The insulating film 9a is exposed only between the inner end 5b and the terminal electrode 4b and the whole area of the insulating film 9a is developed, whereby the insulating film 9a is removed except for the exposed portion to form the insulating film 9 (Figs. 6i and 6j). The insulating film 9 covers the corresponding portion of the conductive land 3. <Step 4> A Ti film 11a having a thickness of approx. 200Å is formed on the first layer 7 and the insulating film 9 by sputtering (Fig. 7a). A Ti-Ag film 11b having a thickness of approx. 1,200Å is formed on the film 11a in the same way as in Step 1, and an Ag film 11c having a thickness of approx. 3µm is formed on the film 11b by sputtering. In this way, a three-layer conductive film 11 is completed (Fig. 7b). The conductive film 11 is masked in a portion thereof covering the layer 7 and a portion corresponding to the insulating film 9, with the shape of the connecting electrode 6. <Step 5> The conductive film 11 is etched by an etching agent that removes only Ag, whereby non-masked portions of the films 11c and 11b are deprived of Ag. The film 11c is left in the shape of the first layer 7 and the connecting electrode 6 (8c of Fig. 7c). Since the Ag film 7c of the first layer 7 is covered with the Ti film 11a, the film 7c is not etched. The conductive film 11 is further etched by an etching agent that removes only Ti, whereby the non-masked portions of the films 11b and 11a are deprived of Ti. The films 11b and 11a are also left in the shape of the first layer 7 and the connecting electrode 6 (8b and 8a of Fig. 7d). Since the Ti film 10a of the first layer 7 is covered with the Ag film 10c, the film 10a is not etched. The conductive film 11 is unmasked to complete the second composite body or layer 8 and the connecting electrode 6 (Fig. 7d). This completes the conductive land 3 and also the high-frequency inductor 1. The two-layer construction of the high-frequency inductor 1 increases the total thickness of the inductor, which greatly contributes to the improvement of the Q factor. The inductor 1 attained the Q factor of as high as 30 at 400MHz and L=18nH and further attained Q=32 at L=69nH. In terms of the resonance frequency, the inductor 1 realized 3.2GHz when L=18nH and 2.0GHz when L=68nH. According to the above manufacturing method, the Ti film 11a, the Ti-Ag film 11b and the Ag film 11c are formed in this order and then the assembly of the three films is etched first by an etching agent that removes only Ag and then by another etching agent that removes only Ti. The Ag film 10c of the first layer 7, which is covered with the Ti film 11a, is never etched by the etching agent that removes Ag. Also, the Ti film 10a, which is covered with the Ag film 10c, is never etched by the etching agent that removes Ti. Therefore, the conductive land 3 can be obtained in a desirable size and a desirable shape without fail. For comparison, a Ti film having a thickness of 200A was formed on the substrate 2 and an Ag film having a thickness of 6µm (twice as thick as the Ag film of the embodiment of this invention) was formed on the above Ti film. When the assembly of the Ti film and the Ag film was masked and etched in the same way as in the embodiment, some masked portions were etched. Although the conductive land 3 of the above embodiment comprises two layers 7 and 8, the land 3 can be formed of three or more layers by repeating the above steps. Although the first and the second layers 7 and 8 each comprise three films of Ti, Ti-Ag and Ag in the above embodiment, the Ti-Ag film can be eliminated. Other metals such as Cu, Al, Ni, Cr and Pd can be used instead of the above three if each metal is etched by an etching agent which removes only that metal. This invention can also apply to an inductor comprising a plurality of conductive land layers and a plurality of insulating layers superposed alternately.

Claims: A method of manufacturing a high-frequency inductor in which a first metallic film (10a) is deposited on a substrate (2) and is etched to form a land comprising a spiral portion (5), a first terminal (4a) connected to the outer end of the spiral portion and a second terminal (4b), an insulating film (9) is formed over the spiral portion and a second metallic film (8) is deposited on the insulating film and is etched to form a connecting conductor (6) connecting the inner end of the spiral to the second terminal (4b) wherein in a first step a first layer (10a) of a first metal is deposited on the substrate (2) and over that is deposited at least a second layer (10c) of a second metal, in a second step the second layer (10c) is etched by a first etching agent which is substantially unable to etch the first layer (10a) and then the first layer (10a) is etched by a second etching agent which is substantially unable to etch the second layer (10c), with the result that a first composite body (7) is formed comprising a spiral portion (5), a first terminal (4a) connected by a portion (5a) to the outer end of the spiral portion (5) and a second terminal (4b) and with the further result that the substrate (2) is exposed other than at the composite body (7), in a third step an insulating film (9) is formed on a portion of the first composite body (7) extending between the inner end of the spiral portion (5) and the second terminal (4b), in a fourth step a first layer (lla) of a first metal is deposited on the assembly of layers resulting from the first to third steps and over that is deposited at least a second layer (llc) of a second metal, in a fifth step parts of the metal layers (11a and 11c) of the fourth step are removed, leaving a second composite body (8) on the first composite body (7) and registering with the latter except where the insulating film (9) is disposed and also leaving a connecting conductor (6) on the insulating film (9) connecting the inner end (5b) of the spiral portion (5) to the second terminal (4b). A method according to claim 1, characterised in that the metals of the first and second layers (10a, 10c, 11a, 11c) are selected from Ti, Ag, Cu, Al, Ni, Cr and Pd. A method according to any preceding claim, characterised in that in the first step or in each of the first and fourth steps an intermediate layer (10b or 11b) of Ti-Ag is deposited after the first layer (10a or 11a) of Ti and before the second layer (10c or 11c) of Ag. A method according to claim 3, characterised in that Ti is etched by an aqueous solution of hydrofluoric nitric acid and Ag is etched by an aqueous solution of ferric nitrate.

Inventors: KANOH OSAMU, SASAKI YUTAKA, SENDA ATSUO, KANOH, OSAMU, SASAKI, YUTAKA, SENDA, ATSUO
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1990
Main classification: H01F17
IPC codes: H01F41, H01F17