US 3858138 A
A flat inductor, one half of which is provided as a first set of parallel conductive strips printed on a substrate, each of which constitutes one half of each turn of a helix. A flat sleeve of insulating material is adherently disposed over the substrate strips leaving the ends of the strips exposed. The other half of the helix is provided as a second set of parallel conductive strips printed on the sleeve with the ends of the second set of strips contacting the exposed ends of the first set. A flat strip-like magnetic core member is slidably disposed within the sleeve.
Description (OCR text may contain errors)
United States Patent 1191 Gittleman et a1.
[ Dec. 31, 1974 [5 TUNEABLE THIN FILM INDUCTOR 3,344,237 9/1967 Gregg 336/200 x 3,413,716 12/1968 Schwertz et al. 336/200 X [751 Inventors: Isaac Trenton; 3,614,554 10/1971 Richardson 336/200 x Lawrence Matthew Zappulla, 3,638,156 1/1972 West 336/200 l-lazlet, both of NJ. 3,731,005 5/1973 Shearman 336/200 X 73 A RCA C t N Y k, N.Y. sslgnee orpom ew or Primary Examiner-Thomas J. Kozma Flledi y 1 1974 Attorney, Agent, or FirmG. H. Bruestle; W. S. Hill; 211 App]. No.2 468,932 Van Tmht Related U-S. Application Data  552252225 of 338l March 1973 A flat inductor, one half of which is provided as a first set of parallel conductive strips printed on a substrate, each of which constitutes one half of each turn of a 2 helix. A flat sleeve of insulating material is adherently I disposed Over the substrate Strips leaving the ends of  Fleld of Search 336/136, 200, 223 the strips exposed The other half of the helix is p vided as a second set of parallel conductive strips  References Cited printed on the sleeve with the ends of the second set UNITED STATES PATENTS of strips contacting the exposed ends of the first set. A 2,989,630 6/1961 Crooker 336/200 X flat stripdike magnetic core member is slidably dis- 3,000,079 9/ 1961 Howell et a1 336/200 X posed within the Sleeve 3,210,707 10/1965 Constantakes.... 336/200 3,305,814 2/ 1967 Moyer 336/200 5 Cl lms, 8 Drawing Flgures I2 6 8 40 4b 4c 4d 4e /--5 1 1 I l l A 3 r\-- h 1 3.
ll I I l I l8 7 101 101 10 101 #5 106 16 TUNEABLE THIN FILM INDUCTOR This is a continuation of application Ser. No. 338,1 l0, filed Mar. 5, 1973 now abandoned.
The invention herein described was made in the course of or under Contract 5882(25-313/71 )71R.
BACKGROUND OF THE INVENTION This invention is an inductor intended for use in miniaturized integrated circuits, especially circuits of the so-called hybrid type wherein conductors and some passive elements are printed or otherwise deposited on an insulating substrate. Active circuit elements and, usually, some passive elements, are separately fabricated, and attached to the substrate conductors by $01- dering. In this type of circuit, inductors have been a particular problem although several different types of inductors have been proposed. One type has been simply a flat spiral of conducting material. Fairly high inductances can be achieved with this type of inductor but at the expense of using up a relatively large area of substrate.
Another type of inductor proposed for miniature circuits is a flat helix with a flat bar of magnetic material for the core. This type provides higher inductances than the dielectric core type described above. An example of this type of inductor is described in US. Pat. No. 3,614,554 to Richardson et a]. It has been recog nized that it would be desirable to be able to tune this type of inductor but heretofore no method has been proposed for making a tuneable inductor of this type.
The present invention provides a film type inductor which is tuneable because it includes a moveable magnetic core.
THE DRAWING FIG. 1 is a plan view of an inductor of the present invention;
FIGS. 2-7 are similar views illustrating successive steps in making the device of FIG. 1; and FIG. 8 is a cross section view taken along the line 8-8 of FIG. 1.
DESCRIPTION OF PREFERRED EMBODIMENT As shown in FIG. 1, an embodiment of an inductor in accordance with the present invention may comprise an insulating substrate 2 on which is a first series of parallel metal film stripes 4a-4e. Covering all of these stripes except the ends thereof is a first film 6 of insulating material. On top of the first insulating film 6 is a second film of insulating material 8 which, at least along its central portion, is not joined to the first film 6. Together, these two films of material constitute a sleeve. There is a narrow space available between the two films 6 and 8 when the films are flexed. On top of the second film of insulating material 8 is a second se ries of parallel metal film stripes la-10e disposed at an angle to the stripes of the first series. Each of these stripes except 10a is joined to two of the lower series of stripes 4a-4d at the ends of the stripes. Stripes 4e and 10a are each joined to only one stripe in the opposite series because of their end positions in the series. The two series of metal stripes 4a4e and l0a-10e together constitute a flat helix. The stripes in each series do not need to be parallel to each other but the parallel arrangement is preferred.
Moveably disposed between the insulating films 6 and 8 is a core 11 composed of a strip of insulating material 12 having coatings of magnetic material 14 on both faces. The strip 12 may be moved to any desired position within the helix to vary the inductance of the device.
In order to connect the device in circuit, end metal stripe re is joined to a bonding pad 16 and end metal stripe 10a is joined to a bonding pad 18.
An example of a method of making an inductor of the present invention will now be given.
The substrate 2 may comprise a suitably sized ceramic or glass plate. In order to make the bottom series of metal stripes 4a-4e (FIG. 2), a thin film of gold may be deposited by evaporation or electroless deposition over the entire top surface of the substrate and the stripes may be delineated by conventional photoprocessing and etching techniques. Alternatively, a removable mask may be applied to the substrate and metal may be deposited only where the stripes are to be formed. Other suitable metals, such as copper, may also be used instead of gold. The thickness, pitch and width of the stripes are chosen to be consistent with the required number of turns of the completed coil and to maximize the coil Q.
Next (FIG. 3), the first film of insulating material 6 is desposited on the substrate 2 and over the stripes 4a-4e except over the ends thereof. The film may have a thickness of about 10 microns and may be composed of a synthetic resin such as polyimide. An example of a commercially available polyimide is Pyralin 5077" a product of E. I. Du Pont de Nemours. The film 6 can be deposited deposited by screen printing techniques or by using a stencil.
In order to fabricate the top insulating film 8 so that at least the central portion will not be adherent to the lower film 6, a mandrel 20 (FIG. 4), having dimensions such that it covers the central portion of film 6 over its entire length, is placed over the film 6. The mandrel may be made of a non-adherent material such as Teflon and may be 1 mil thick, for example, or it may be made of a metal such as copper or aluminum which can later be removed by etching.
The second film of insulating material 8 (FIG. 5), having the same dimensions as the first film 6, is then deposited over the mandrel 20 and over the edge portions of the film 6. The second film 8 adheres to the first film 6 along its edges but not, of course, where the film 6 is covered by the mandrel 20.
Next, the second series of metal film stripes 10a-10e (FIG. 6), is deposited over the second insulating film 8. These stripes are disposed at an angle such that their ends are joined to the ends of the first series of metal stripes 4a4e. That is, stripe 10a is joined to stripe 4a, stripe 10b is joined to stripes 4a and 4b, etc. This combination of metal stripes forms a flat helix.
At the completion of fabrication, the mandrel 20 is removed and a moveable core 11 is slipped between sired in the sputtered film, pressing the resultant mixture in the'shape of a disc target, and firing the disc at an elevated temperature, e.g., 1,100 C, for about an hour in a hydrogen atmosphere. For example, the mixture may consist of (in parts by weight) 1.0 part iron, 0.531 part nickel and 0.336 part silica. This mixture may be pressed at 12,000 psi into a disc 6 inches in diameter and about A inch thick.
A film sputtered from the above disc has the composition (Fe Ni (SiO in parts by volume. The permeability of the film is 150 and resistivity is 9 X 10 ohm-cm.
If a substrate is used which is heat-resistant, the film can be annealed to increase permeability.lf annealed at 250 C for 2 hours under vacuum and then cooled to room temperature, for example, the permeability increases to 270 and resistivity rises to 1.1 X 10 ohmif a magnetic film of variable composition is desired, a disc shaped target can be prepared by fabricating it of three different sectors, one sector containing iron, a second sector containing nickel and a third sector containing silica.
The magnetic film 14 can also be of constant composition but of variable thickness by exposing different portions of the substrate to the sputtering operation for different lengths of time.
The magnitude of the inductance for the present inductor configuration is given by the equation:
L 1.261O N (d, ad where L is in nanohenries, N is the number of turns, d, is the total insulation thickness of the films 6 and 8 and the Mylar strip 12, d is the thickness of the ferromagnetic coating of the core 11 and p. is the permeability. Thicknesses are in microns.
As an example, if N 20 turns, total insultion thickness of the films and core substrate is 40 microns, u 150 and the thickness of the magnetic coating on the core is 5 microns, a maximum inductance can be obtained as follows:
L= 1.26 X X 400 X (40 +150 X 5) The inductor is provided with terminals for connecting it in circuit by depositing on-the substrate 2 a metal pad 16 overlapping the free end of the stripe 4e and another metal pad 18 overlapping the free end of the stripe 10a. 8
1. A tuneable film type inductor comprising a first series of metal film stripes adhered to an insulating substrate, a first synthetic resinous film of insulating material covering said stripes except the ends thereof, a second synthetic resinous film of insulating material over said first film and having a central portion spaced therefrom, both of said films being joined along their edges such that both films together form a sleeve, a second series of metal film stripes on top of said second film of insulating material, each member of said second series of strips being connected to members of said first series of stripes to form a continuous flat helix, and a moveable member of magnetic material disposed between said insulating films, said moveable member comprising a thin strip of dielectric material with a coating of ferromagnetic material thereon.
2. An inductor according to claiml in which said ferromaganetic layer is a sputtered layer.
3. An inductor according to claim 1 in which said coating of ferromagnetic material is of variable composition.
4. An inductor according to claim 1 in which said coating of ferromagnetic material is of variable thickness.
5. An inductor according to claim 1 in which said coating of ferromagnetic material is on both sides of said strip.