Thin Film Coils And Method And Apparatus For Making The Same

Abstract

Thin film inductance coils, separately or in transformer combinations, are made by depositing on a substrate from vapor sources directed through selected apertured masks, superimposed layers of alternate films of electrically conductive metal and electrical insulating material in successive segments of each of a plurality of coil turns.

Description

BACKGROUND OF THE INVENTION

This invention relates to inductance coils and more particularly to thin film coils and to a method and apparatus for making them.

In the present state of micro circuit technology, interconnections between chips are made by welding, soldering, or mechanical fastening of metal wires or ribbons. It is generally recognized that such forms of connections represent significant sources of circuit failures. The present invention contemplates minimizing such problems and providing other advantages by the use of thin film inductance coils. For example, such coils afford the introduction of inductor components into micro-circuitry. Still further, by appropriate combination of coils the provision of strong and dependable interconnections, transformers and switches are made available.

Thin film coils have been made experimentally heretofore by directing vapors of metal and insulating material through separate apertures onto a rotating substrate to form a continuous coil of a desired number of turns. This procedure imposes certain significant structural limitations on the coils produced thereby. For example, only one coil may be produced at one time. Moreover, the coils must of necessity be circular in shape and therefore the inside area defined by the coil constitutes a significant waste of valuable substrate surface. Still further, the procedure limits the deposition of insular material to but a single film, with consequent high incidence of coil failure due to electrical shorting between turns. In addition the procedure does not accommodate the making of multiple coils and coupling cores and therefore cannot be employed for the fabrication of transformers.

SUMMARY OF THE INVENTION

In its basic concept the present invention provides for the production of coils in which each of a plurality of turns of superimposed layers of conductor metal and insulating material is formed in successive interconnected segments.

It is by virtue of the foregoing basic concept that the principle objective of this invention is achieved; namely, to overcome the above enumerated disadvantages associated with micro-circuit technology and with prior thin film coils.

Another important object of this invention is to provide a method and apparatus by which coils may be formed with a plurality of concentric coils of any desired shape, size and number of turns.

Still another important object of this invention is to provide a method and apparatus by which coils may be provided with multiple layers of insulating material between conductor metal turns to insure proper insulation.

A further object of this invention is to provide a method and apparatus by which multiple coils may be formed simultaneously on a single substrate to form transformers of various types and configurations.

A still further important object of this invention is to provide a simplified method and apparatus by which to produce coils and transformers of high quality at minimum cost.

The foregoing and other objects and advantages of this invention will appear from the foregoing detailed description taken in connection with the accompanying drawings of preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary, foreshortened vertical elevation, partly in section, showing in some what schematic form apparatus embodying the features of this invention.

FIG. 2 is a sectional view taken along the lines 2--2 in FIG. 1.

FIGS. 3-7 are plan views of various apertured masks for use in fabricating an inductance coil.

FIGS. 8-14 are plan views illustrating the sequential steps in fabricating an inductance coil by use of the masks illustrated in FIGS. 3-7.

FIG. 15 is a plan view showing a plurality of concentric coils mounted on a single substrate.

FIG. 16 is a vertical section illustrating a transformer constructed from two inductance coils of FIG. 14.

FIGS. 17-23 are plan views of various apertured masks for use in fabricating a multiple secondary transformer.

FIGS. 24-31 are plan views illustrating the sequential steps in fabricating a multiple secondary transformer by use of the masks illustrated in FIGS. 17-23.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 and 2 of the drawings, the apparatus includes a housing adapted to be evacuated. In the embodiment illustrated this housing comprises a base 10 and a bell jar 12, preferably transparent, supported removably thereon. A seal 14 is interposed between the base and bell jar to provide a vacuum tight seal, as will be understood. A conduit 16 extends through the base for connection to a vacuum pump (not shown) for evacuating the housing.

A supporting framework, including sub-base 18 and spaced vertical posts 20, is mounted on the base within the housing. The upper inturned ends of the posts mount a substrate heater unit 22 and underlying substrate support 24 for a substrate 26. The substrate is secured removably to the support by spaced clamps 28, although other conventional means such as set screws, spring clips, or other suitable form of retainer may be employed.

The heater unit is provided for heating the substrate support and hence the substrate mounted thereon. In the embodiment illustrated the heater unit includes an electric heater 30 connected to a suitable source of electric potential by means of electrical conductors 32 which extend through suitably sealed openings in the base 10.

Means is provided in the housing for supplying vapors of electrically conductive metal and electrical insulation material selectively for deposition on a substrate. In the embodiment illustrated such means comprise a pair of spaced crucibles 34 and 36 supported above the subbase. Each crucible is provided with means for heating to vaporization the metal or insulating material contained therein. Any of a variety of heating means may be employed, such as an electron beam, a laser beam, an electric arc, or an induction heater. Sputtering also may be employed as the technique for vaporizing the materials. The source illustrated merely for this explanation is an electric heater element 38 contained within each crucible and connected through a pair of supporting bus bars 40 to a suitable source of electric potential, by electrical conductors which extend through suitably sealed openings in the base.

Electrically conductive metal to be deposited on a substrate may be selected from a wide variety of metals such as gold, nickel, aluminum, chromium, silver, tantalum and others. The form of the metal selected is dependent in part upon the type of heating employed to vaporize it. In the illustrated embodiment the heater element is a wire of tungsten or other suitable metal heated by high current, and the vaporizable metal is provided in a form capable of being wrapped around the wire in crucible 34.

The electrical insulating material to be vaporized and deposited on the substrate may be aluminum oxide, silicon dioxide, tantalum pentoxide and various other oxides well known in the art for their dielectric properties. In the embodiment illustrated the material is placed in the crucible 36 for heating by the resistance wire heater.

The substrate 26 preferably is made of the same dielectric material as the electrical insulating material in order to minimize differences in thermal expansion. However, it will be recognized that the substrate may be made of other suitable materials.

It will be understood that vapors of the materials in the crucibles progress through the evacuated space of the housing along optical paths between the crucibles and substrate. Accordingly, means is provided for intercepting said optical paths selectively to prevent the passage of one or both of the vapors from the associated crucibles to the substrate. In the embodiment illustrated such means is provided by a shield plate 42 which is supported in horizontal position above the crucibles by means of an operating rod 44. This rod extends downward through a suitably sealed opening in the base. The lower end of the rod projecting downward from the base provides a handle by which the rod may be rotated on its vertical axis. By this means the shield plate may be swung on said vertical axis to intercept or expose the optical path between the substrate and each of the crucibles.

In accordance with the present invention an inductance coil is fabricated by depositing on a substrate, in superimposed layers, alternate films of electrically conductive metal and electrical insulating material in successive segments of each of a plurality of coil turns. Accordingly, means is provided for directing the vaporized materials onto the substrate in predetermined patterns to produce said successive segments. In the embodiment illustrated this means is provided by a plurality of apertured masks which are described more fully hereinafter. Each mask is mounted, preferably removably, on a mask holder for movement between a retracted position away from the substrate and an operative position interposed between the substrate and the crucibles.

In the embodiment illustrated, mask holders are disposed about the substrate at 90.degree. intervals. In order to accommodate up to eight different masks, the mask holders are arranged in cooperating pairs, as best illustrated in FIG. 1. Thus, both mask holders 46 and 48 of a pair are mounted for pivotal movement on a common pivot shaft 50 supported by bearing tabs 52 extending laterally from the substrate support 24. A projecting arm 54 on each holder is connected pivotally through a link 56 to the upper end of an operating rod 58. The rod extends downward slidably through a guide opening in a lateral bracket 60 on a vertical post 20, thence through an aligned guide opening in the sub-base 18, and thence through a suitably sealed opening in the base 10. The lower end of the rod provides a handle by which it may be moved vertically to effect movement of the associated mask holder between the aforementioned retracted and operative positions of the supported mask.

The arrangement of cooperating pairs of mask holders allows the mask carried by the inner holder 46 to be moved between retracted and operative positions independently of the mask carried by the associated outer holder 48. Movement of the mask carried by the outer holder to its operative position results in simultaneous movement of the associated inner mask to operative position interposed between the outer mask and the substrate 26. The cooperation between the inner and outer masks of an associated pair of holders is explained more fully hereinafter.

It will be understood that if the number of masks required for the fabrication of a particular type of coil or transformer are fewer than eight, each unused mask holder has a sufficiently large opening 62 so as not to interfere with the proper function of the mask carried by the associated holder. Alternatively, the number of mask holders may be reduced correspondingly, as will be understood.

The method of this invention and the operation of the apparatus described hereinbefore as best explained by the following descriptions of the fabrication of inductance coils and transformers.

Considering first the fabrication of a single inductance coil, reference is made to FIGS. 3-14.

The substrate 26 first is provided with a pair of spaced electrically conductive terminals 64 and 66 (FIG. 8). These may be formed by vapor deposition, by printed circuit technique, or by any other suitable means. One of the terminals, for example terminal 64, extends farther inward from the margin of the substrate than the other terminal 66, as illustrated.

The substrate is mounted in the substrate support 24, the bell jar 12 is mounted in place on the base 10 and the housing then is evacuated to at least 1 .times. 10.sup.-.sup.4 torr. The substrate then is heated by heater 30 to its appropriate deposition temperature.

The first deposition on the substrate is a film of electrically conductive metal to form the first segment of the first turn of the coil, Accordingly, the mask 68 illustrated in FIG. 3 is moved to operative position adjacent the substrate. With the shield 42 positioned across both crucibles, the heater 38 for the crucible 34 containing the vaporizable metal is adjusted to the desired conductor metal deposition rate, the metal in the crucible is outgassed and the shield then is moved out of the optical path between the crucible 34 and the substrate 26. The metal vapors pass through the opening 70 in the mask 68 and are deposited on the substrate to form the segment 72 illustrated in FIG. 9. It is to be noted that this segment is in electrical contact with the longer terminal 64.

When the desired thickness of the conductor segment is attained, for example about 1000 Angstroms, the shield 42 is moved back to cover both crucibles, the mask 68 is retracted and the mask 74 illustrated in FIG. 4 is moved to operative position. The insulating material in the crucible 36 is outgassed, the heater 38 associated therewith is adjusted to proper operating temperature to provide the desired deposition rate, and the shield then moved out of the optical path between the crucible 36 and the substrate 26. The vapors of insulation material pass through the aperture 76 in the mask 74 and deposit on the substrate to form the insulation segment 78 illustrated in FIG. 10. It is to be noted that this segment of insulating material covers all but one end portion 72' of the underlying conductor segment 72.

Although the film of insulating material may be provided as a single layer of predetermined thickness, the preferred procedure is as follows: The deposition is stopped when about one half of the total desired film thickness is deposited, and the pressure of the vacuum system is increased to about 100 milli-torr by such means as a variable leak valve. Gasses such as argon and nitrogen may be used to backfill the vacuum chamber. The leak valve then is turned off and the pressure is allowed to return to its original operating level, after which the second half of the insulator film is deposited.

After the insulator film has been deposited, for example to a total thickness of about 1000 Angstroms, the shield 42 is returned over the crucible 36 and the mask 74 is retracted. The pair of masks 80 and 82 illustrated in FIGS. 5 and 6 then are moved to operative position, with the mask 80 interposed between the mask 82 and the substrate 26. This is achieved by mounting masks 80 and 82 on holders 46 and 48, respectively, of a pair. The shield now is moved to expose the conductive metal in the crucible 34, whereupon metal vapors are caused to pass first through the aperture 84 in mask 82 and then through the aperture 86 in mask 80. The vapors deposit on the substrate to form the second segment 88 of the first turn of the inductance coil, as illustrated in FIG. 11. It is to be noted that one end of this second segment overlaps the exposed end portion 72' of the first segment, and therefore is in electrical contact therewith, and that the opposite end of the second segment overlaps a portion of the underlying first segment 78 of insulating material. It is to be noted further that the use of the mask 82 illustrated in FIG. 6 in combination with the mask 80 illustrated in FIG. 5 has caused the lower portion 86' of the aperture 86 of the mask 80 to be covered. Thus, conductive metal vapors have not been allowed to deposit on the substrate in contact with the shorter terminal 66.

In the embodiment illustrated, each coil turn is made of two segments. It will be apparent that each turn may be made of more than two segments, by provision of appropriate numbers of masks provided with appropriately shaped apertures.

When the desired thickness of conductor metal has been deposited to complete the second segment 88 of the first turn, the shield 42 is returned to cover the crucible 34, the masks 80 and 82 are retracted and the mask 90 illustrated in FIG. 7 is moved to operative position. The shield then is moved to expose the crucible 36, thereby allowing the passage of insulation material vapors through the aperture 92 in mask 90, for depositing the second segment 94 of insulating material, as illustrated in FIG. 12. This film of insulating material overlaps the first segment 78 of insulating material and all but one end portion 88' of the second segment 88 of conductive metal, as illustrated.

The foregoing deposition steps are repeated sequentially until all but the last of the desired number of coil turns and insulator films have been deposited in superimposed layers. When the first segment of the last turn of conductive metal and insulating material have been deposited, in the manner illustrated in FIG. 10, the second segment 98 of the last turn of conductive metal is deposited by using only the mask 80 illustrated in FIG. 5, i.e. without combination with the mask 82 illustrated in FIG. 6. The deposition of the conductive metal segment 98 resulting therefrom is illustrated in FIG. 13. It can be seen that this last segment of conductive metal includes the extension 98' which makes electrical contact with the shorter terminal 66. Thereafter, the final insulating segments are deposited by sequential use of the masks 74 and 90 illustrated in FIGS. 4 and 7 to complete the coil, as shown in FIG. 14.

It should be noted here that the foregoing method and apparatus are capable of producing inductance coils of any desired shape, size and number of turns. Although the coil configuration illustrated in FIGS. 3-14 is rectangular, it may be circular, triangular, square, or any other shape desired. The size of the coil may be less than 1/8 inch in its longest dimension, while a few thousandths of an inch total thickness accommodates several thousand coil turns.

Another advantage attending the case of apertured masks for the deposition of coil segments, resides in the ability to provide masks for producing simultaneously a plurality of concentric coils on a single substrate. Such a multiple coil arrangement is illustrated in FIG. 15, wherein there is shown three concentric coils supported on a single substrate.

Although the coils are illustrated as being interconnected to form a single coil of the sum of all three coil turns, it will be understood that the coils may be separated, each with its own pair of terminals, to provide a plurality of separate coils on a single substrate. These separate coils may be produced simultaneously by masks provided with multiple apertures.

Coils produced in the manner described hereinbefore may be utilized as inductor components in micro-circuitry. On the other hand, two or more such coils may be combined to provide a transformer. One such form of transformer is illustrated in FIG. 16. Therein is shown two such coils 100 and 102 arranged in spaced apart relation with their respective substrates 104 and 106 secured to a support 108 of dielectric material. Underlying this support is ferro-magnetic plate 110. Aligned openings are provided through the substrates, support and plate in registry with the central openings defined by the coils. A bar 112 of ferro magnetic material supports a pair of spaced legs 114 of similar material which extend removably through the aligned openings and make physical contact with the underlying plate 110. The plate, bar and legs thus form a ferro-magnetic core which couples the coils together. One coil is usable as a primary winding of a transformer and the other as a secondary. Since the assembly of bar 112 and legs 114 is removable, the transformer may serve the function of a switch. It will be apparent to those skilled in the art that if the plate 110, bar 112 and legs 114 are formed as a fixed, continuous ferro-magnetic loop coupling the coils 100 and 102 together, the resulting transformer may serve as an electrical interconnection between the substrate 104 and 106.

FIGS. 17-31 illustrate the masks and method steps by which to fabricate a transformer having a permanent core which couples to a primary winding a step-down secondary winding and another secondary winding having the same number of turns as the primary. In fabricating this transformer, the substrate 120 first is provided (FIG. 24) with a pair of electrically conductive terminals 122 and 124 for the primary winding, a pair of terminals 126 and 128 for the step-down secondary winding and a pair of terminals 130 and 132 for the equal turns secondary winding. The substrate also is provided with magnetic core segments 134 and 136 for coupling the primary winding to the secondary windings. The intermediate portion of each of these core segments is overlaid with a film 138 of electrical insulating material such that the opposite end portions of the core segment are exposed. The substrate 120 then is mounted in the substrate support.

The first deposition on the substrate is provided by moving the pair of masks 140 and 142 illustrated in FIGS. 17 and 18 into operative position across the substrate, with mask 142 interposed between mask 140 and the substrate. It is to be noted that the aperture 144 of mask 142 does not interfere with, or modify, the apertures 146, 148 and 150 of mask 140. However, it is interposed between mask 142 and the substrate because it is used by itself in a later deposition step. Vapors of electrically conductive metal then are directed through the apertures of mask 140, in the manner previously described. This results in the deposition of the first electrically conductive metal segments 152, 154 and 156 of the first turns of all three windings, as illustrated in broken lines in FIG. 25. The masks 140 and 142 then are retracted.

It is to be noted that these first segments of electrically conductive metal are deposited in electrical contact with one of the associated pair of terminals, for example terminals 122, 126 and 130, respectively, whereby to provide electrical connection at one end of each coil. It is to be noted further, by comparison of FIGS. 24 and 25, that these first segments are spaced from the exposed ends of the core segments 134 and 136 by depositing a portion of the segments on the intermediate layer 138 of insulating material and the remainder on the substrate 120.

The second deposition is provided by moving the mask 158 illustrated in FIG. 20 to operative position and directing vapors of insulating material through its apertures 160, 162 and 164 onto the substrate. This results in the deposition of the first segments 166, 168 and 170 of insulating material over the respective first segments 152, 154 and 156 of conductor metal, as illustrated in FIG. 25. It is to be noted that each of these insulator segments covers all but one end portion of the associated underlying segment of conductor metal. The mask 158 then is retracted.

The third deposition is provided by moving the masks 172 and 174 illustrated in FIGS. 21 and 22 into operative position, with mask 174 interposed between mask 172 and the substrate 120. The aperture 176 of mask 172 serves to expose all of the apertures 178, 180 and 182 of mask 174 except the projecting portions 178', 180' and 182' thereof. Vapors of conductive metal then are directed through the mask apertures onto the substrate to provide the deposition of the second segments 184, 186 and 188 of the first turns of conductive metal for the coils, as illustrated in FIG. 26. It is to be noted, by comparison of FIGS. 25 and 26, that one end of each of these second segments overlaps the exposed end portion of the associated first segment of conductive metal and that the opposite end portion of the second segment overlaps the corresponding end portion of the first segment of insulating material.

The fourth deposition is provided by moving the mask 190 illustrated in FIG. 23 into operative position and directing vapors of insulating material through its apertures 192, 194 and 196 onto the substrate. This provides the second segments 198, 200 and 202 of insulating material for the first turn of each coil, as illustrated in FIG. 26. These insulating segments cover all but one end portion of the underlying second segments of conductor metal, as illustrated. Thus, the first complete turn of each coil is completed.

The foregoing four depositions are repeated sequentially until there has been deposited on the substrate the desired number of coil turns for the step-down secondary winding 204 (FIG. 29).

The fifth deposition is provided by moving the masks 140 and 142 illustrated in FIGS. 17 and 18 into operative position and directing therethrough vapors of electrically conductive metal. This provides the deposition of the first segments 206 and 208 (FIG. 27) of conductive metal for the next subsequent turn of the primary winding 210 and equal turns secondary winding 212, respectively (FIG. 29). It also provides a deposition of a corresponding segment 214 of conductive metal overlaying the step-down secondary winding 204, but this does not constitute an additional winding, as will become apparent.

The sixth deposition is provided by moving the masks 216 and 158 illustrated in FIGS. 19 and 20 into operative position with the mask 158 interposed between mask 216 and the substrate. Vapors of insulating material then are directed through the masks to provide the deposition of segments 218 and 220 illustrated in FIG. 27. It is to be noted that these insulating film segments cover only the respective conductive metal segments 206 and 208 associated with the primary winding and equal turns secondary winding. This is evident from FIGS. 19 and 20 wherein the apertures 222 and 224 are shown to register only with apertures 160 and 164 of mask 158 and no aperture is provided in mask 216 for registry with aperture 162 of mask 158. Accordingly, mask 216 has prevented deposition of insulating material over the segment 214 of conductive metal deposited over the step-down secondary winding.

The seventh deposition is provided by moving the masks 172 and 174 illustrated in FIGS. 21 and 22 into operative position and directing vapors of conductive metal therethrough. The depositions thus provided are the same as those illustrated in FIG. 26.

The eighth deposition is provided by moving the mask 190 illustrated in FIG. 23 into operative position and directing vapors of insulating material therethrough. These depositions of insulating films also are the same as those illustrated in FIG. 26.

The foregoing fifth, sixth, seventh and eighth depositions are repeated sequentially until there has been deposited all but the last one of the desired number of turns for the primary winding 210 and the equal turns secondary winding 212. The first segment of conductive metal and insulating material for the last turn then are provided by repeating the fifth and sixth depositions.

The ninth deposition is provided by moving the mask 174 illustrated in FIG. 22 into operative position while retaining the associated mask 172 illustrated in FIG. 21 in retracted position. Vapors of conductive metal then are directed through the mask apertures to provide deposition of the conductive metal segments 226, 228 and 230 illustrated in FIG. 28. It is to be noted that each of these segments includes an extension which makes electrical contact with the associated second terminal 124, 128 and 132, respectively, of each pair. Each pair of terminals thus provides electrical connection to the opposite ends of the associated coil.

The tenth and eleventh depositions are provided by moving the masks 158 and 190 illustrated in FIGS. 20 and 23 sequentially into operative position and directing vapors of insulating material therethrough. These depositions provide an insulating film 232 covering the completed windings, as illustrated in FIG. 29.

The twelfth deposition is provided by moving the mask 172 illustrated in FIG. 21 into operative position while retaining the mask 174 illustrated in FIG. 22 in retracted position. Vapors of magnetic metal then are directed through the mask to provide the deposition 234 illustrated in FIG. 30. This deposition completes the transformer, as shown in FIG. 31. It is to be noted that this deposition constitutes a second segment of the transformer core and it makes electrical contact with the exposed opposite ends of the underlying first segments 134 and 136 of the core to provide continuous loops of metal which couple together the primary winding 210 and each of the secondary windings 204 and 212.

It will be apparent from the foregoing that various other transformer configurations may be fabricated, as desired. For example, additional secondary windings may be included by appropriate provision at different masks. Step-up secondary coils may be provided by providing masks to shield the other windings from additional depositions. By omitting the core segments and providing removable cores, in the manner illustrated in FIG. 16, such multiple secondary transformers also may be employed as switches.

The foregoing and many other variations may be made within a wide range of coil shapes and sizes, as will be understood. As an illustration, the double secondary transformer illustrated in FIG. 31 may be fabricated on a substrate that is one-half inch square. Smaller or larger substrates may be employed, as will be apparent.

It will be further apparent to those skilled in the art that various other changes may be made in the size, shape, number and arrangement of parts of the components of the coils, transformers and apparatus, as well as the number, type and arrangement of method steps described hereinbefore, without departing from the spirit of this invention.

Claims

Having now described my invention and the manner in which it may be used, I claim:

1. The method of making an inductance coil, comprising depositing on a planar surface of a substrate in superimposed layers alternate films of electrically conductive metal and electrical insulating material, to form a plurality of superimposed coil turns of said metal separated by interposed films of said insulating material, each metal coil turn being formed by depositing successive segments of said metal, with the leading end portion of each segment being overlapped by the trailing end portion of the next succeeding segment and the leading end portion of the last segment of each turn overlapping the insulating material covering the trailing end portion of the first segment of the underlying coil turn.

2. The method of claim 1 wherein the plurality of coils are supported on the substrate in spaced, concentric relation.

3. The method of claim 1 wherein the films of metal and insulating material are deposited from vapors thereof.

4. The method of claim 1 including

a. providing the substrate with a pair of spaced terminals,

b. depositing the first coil segment of metal in electrical contact with one of said terminals, and

c. depositing the last coil segment of metal in electrical contact with the other of said terminals.

5. The method of claim 1 including

a. depositing on a substrate a film of electrically conductive metal in the form of a segment of a turn of a coil,

b. depositing over all but one end portion of said segment of metal a film of electrical insulating material;

c. depositing on the substrate a film of electrically conductive metal in the form of a second segment of said turn of a coil, with one end of the second segment of metal in contact with the exposed end of the first segment of metal and the opposite end of the second segment of metal overlapping the film of insulating material covering the first segment of metal,

d. depositing over all but one end portion of the second segment of metal a film of electrical insulating material, and

e. repeating said deposition steps sequentially to form a predetermined number of coil turns.

6. The method of claim 1 for making a transformer having a primary coil and at least one secondary coil, the method including

a. the initial step of depositing on the substrate for each secondary coil a film of magnetic metal in the form of a segment of a core for coupling the primary coil and each secondary coil,

b. depositing over an intermediate portion of said metal core segment a film of electrical insulating material,

c. depositing said successive segments of coil turns for the primary coil and each secondary coil about and spaced from the exposed end portions of the associated metal core segment, and

d. after forming the primary and secondary coils, depositing over the last film of insulating material a film of magnetic metal in the form of a second segment of said core in electrical contact with the first segment of the core.

7. The method of claim 6 wherein the films of metal and insulating material are deposited from vapors thereof.