Disclosed herein is a planar magnetic element comprising a substrate, a first magnetic layer arranged over the substrate, a first insulation layer arranged over the first magnetic layer, a planer coil formed of a conductor, having a plurality of turns, arranged over the first insulation layer and having a gap aspect ratio of at least 1, the gap aspect ratio being the ratio of the thickness of the conductor to the gap between any adjacent two of the turns, a second insulation layer arranged over the planar coil, and a second magnetic layer arranged over the second insulation layer. When used as an inductor, the planar magnetic element has a great quality coefficient Q. When used as a transformer, it has a large gain and a high voltage ratio. Since the element is small and thin, it is suitable for use in an integrated circuit, and can greatly contribute to miniaturization of electronic devices.
These planar elements are disclosed in K. Yamasawa et al, High-Frequency of a Planar-Type Microtransformer and Its Application to Multilayered Switching Regulators, IEEE Trans. Mag., Vol. 26, No. 3, May 1990, pp. 1204-1209. As is described in this thesis, the planar elements have a large power loss. Similar planar magnetic elements are disclosed also in U.S. Pat. No. 4,803,609.
It has been proposed that the thin-film process, is employed in order to miniaturize these planar magnetic elements.
Planar inductors of the structure specified above need to have a sufficient quality coefficient Q in the frequency band for which they are used. Planar transformers of the structure described above must have a predetermined gain G which is greater than 1 for raising the input voltage or less than 1 for lowering the input voltage, and must also minimize voltage fluctuation.
The value Q of a planar inductor is:
Q=.omega.L/R
where R is the resistance of the coil, and L is the inductance of the inductor.
The voltage gain G of a planar transformer without load is:
G=k(L.sub.2 /L.sub.1).sup.1/2 {Q/(1+Q.sup.2).sup.1/2 }
where k is the coupling factor between the primary and secondary windings, L.sub.1 and L.sub.2 are the inductances of the primary and secondary windings, respectively, the quality coefficient Q is .omega. L.sub.1 /R.sub.1, and R.sub.1 is the resistance of the primary-winding coil. The gain G is virtually proportional to Q when Q<<1, and has a constant value k (L.sub.2 /L.sub.1).sup.1/2 when Q>>1.
To increase the quality coefficient Q of the inductor, and to increase the gain G of the transformer thereby to limit the voltage fluctuation, it is necessary to reduce the resistance of, and increase the inductance of, the coil, as much as possible. In the conventional planar magnetic elements made by means of the thin-film process, however, the coil conductors, which need to be formed in a plane, cannot have a large cross-sectional area. Therefore, these elements cannot help but have a very high resistance and an extremely small inductance. Consequently, the conventional planar inductor has an insufficient quality coefficient Q, and the conventional planar transformer has an insufficient gain G and a great voltage fluctuation. These drawbacks of the conventional planar magnetic elements have been a bar to the practical use of these elements.
Of planar coils which can be used in planar inductors, spiral coils are the most preferable due to their great inductance and their great quality coefficient Q. In fact, planar inductors, each having a spiral planar coil, have have been manufactured, one of which is schematically illustrated in FIG. 1. As FIG. 1 shows, the planar inductor comprises a spiral planar coil shaped like a square plate, two polyimide films sandwiching the coil, and two Co-base amorphous alloy ribbons sandwiching the coil and the polyimide films and prepared by cutting a Co-based amorphous alloy foil made by rapidly quenching cooling the melted alloy. This planar inductor is incorporated in an output choke coil for use in a 5 V-2 W DC-DC converter of step-down chopper-type, as is disclosed in N. Sahashi et al, Amorphous Planar Inductor for Small Power Supplies, the National Convention Record, the Institute of Electrical Engineers of Japan 1989, S. 18-5-3. As is evident from the graph of FIG. 2A, two currents flow through this choke coil. The first current is a DC current which corresponds to the load current. The second current is an AC current which has been generated by the operation of a semiconductor switch. As the DC current increases, the operating point of the soft magnetic core, shifts into the saturation region of the B-H curve. As a result, the magnetic permeability of the magnetic alloy lowers, whereby the inductance abruptly decreases as is illustrated in FIG. 2B. As is evident from FIG. 3, the AC current becomes too large at the time the inductance sharply decreases. This excessive AC current is a stress to the semiconductor switch, and may break down the switch in some cases.
It is desired that the choke coil have its electric characteristics, such as inductance, unchanged even if a superimposed DC current flows through it. FIG. 4 is a graph representing the typical superimposed DC current characteristic of the choke coil, which is the relationship between the inductance of an inductor and a superimposed DC current flowing through the inductor.
In the case of a planar inductor, the conductor coil is very close to the soft magnetic cores and, hence, generates an intense magnetic field even if the current flowing through it is rather small. Thus, the soft magnetic cores are likely to undergo magnetic saturation. It will be explained how such magnetic saturation occurs in, for example, a planar inductor which comprises an Al--Cu alloy spiral planar coil, two insulation layers sandwiching the coil, and two magnetic layers clamping the coil and the insulation layers together.
The planar coil of this planar inductor is made of an conductor having a width of 50 .mu.m and a thickness of 10 .mu.m. The coil has 20 turns, and the gap between any two adjacent turns is 10 .mu.m. Each insulation layer has a thickness of 1 .mu.m, and either magnetic layer has a thickness of 5 .mu.m. The planar coil has a saturated magnetic flux density B.sub.S of 15 kG and a magnetic permeability .mu..sub.s of 5000.
Assuming that the Al--Cu alloy conductor has a permissible current density of 5.times.10.sup.8 A/m.sup.2, the permissible current Imax is 250 mA. The present inventors tested the planar inductor in order to determine the relationship between the current flowing through the coil and the intensity of the magnetic field generated in the surface of either magnetic layer from the current. The results of the test revealed that both magnetic layers were magnetically saturated when a current of 48 mA or more flowed through the Al-Cu alloy coil. It follows that, if this planar inductor is used as a choke coil, the maximum DC superimposed current is limited to 48 mA. This value is no more than about one fifth of the permissible coil current Imax. Inevitably, the magnetic layers will be readily saturated magnetically.
The limited DC superimposed current is a drawback which is serious, not only in the planar inductor used as a choke coil, but also in a planar transformer. In a planar transformer incorporated in, for example, a DC-DC converter of forward type or fly-back type, a pulse voltage of one polarity is applied to the primary coil. The magnetic layers are thereby saturated magnetically, abruptly decreasing the inductance of the transformer.
Hence, attempts have been made to provide a planar inductor and a planar transformer, which are designed such that the influence of the saturation of the magnetic layers is reduced, thereby to increase the maximum DC superimposed current of the device comprising the planar or transformer and to make an effective use of the magnetic anisotropy of the magnetic layers.
Planar coils can be classified into various types such as zig-zag type, spiral type, zig-zag/spiral type, and so on, in accordance with their patterns. Of these types, the spiral type can be provided with the greatest inductance. Hence, a spiral planar coil can be smaller than any other type having the same inductance. To form the terminals of a spiral planar coil, however, it is necessary to connect two spiral coils positioned in different planes by means of a through-hole conductor, or to use conductors for leading the terminals outwards. Hence, the process of manufacturing a spiral planar coil is more complex than those of manufacturing the other types of planar coils.
For electronic circuit designers it is desirable that planar magnetic elements to be incorporated in an electronic circuit have so-called "trimming function" so that their characteristics may be adjusted to values suitable for the electronic circuit. A magnetic element having a trimming function has indeed been developed, which has a screw and in which, as the screw is rotated, its position with respect of the core of the coil, thereby to vary the inductance of the magnetic element continuously. However, most conventional planar magnetic elements have no trimming function, for the following reason.
As is known in the art, the characteristics of planar magnetic elements greatly depend on their structural parameters and the characteristics of the planar coils and magnetic layers. These factors determining the characteristics of the magnetic elements depend on the steps of manufacturing the elements. Since these steps can hardly be performed under the same conditions, the resultant elements differ very much in their characteristics. Naturally it is desired that the elements be provided with trimming function. However, they cannot have trimming function because of their specific structural restriction.
Transformer with large output power is disclosed in A.F. Goldberg et al., Issues Related to 1-10-MHz Transformer Design, IEEE Trans. Power Electronics, Vol. 4, No. 1, January 1989, pp. 113-123.
As has been pointed out, planar magnetic elements small enough to be integrated with other circuit elements have not been produced, making it practically impossible to manufacture sufficiently small integrated LC-circuit sections, a typical example of which is a power-supply section.
Since the Multilayered planar inductors essentially have an open magnetic circuit, it is difficult to achieve the following two requirements:
(1) They have no leakage fluxes, and only slightly influence the other components of the IC in which they are in corporated.
(2) They have a large inductance.
Therefore, the multilayered planar inductors cannot serve to provide sufficiently small integrated LC-circuit sections, such as a power-supply section.
Hence, there is still great demand for planar magnetic elements for use in a circuit section, which only slightly influence the other components of the circuit, influence other components. Further, the conventional planar magnetic elements can hardly have trimming function, due to the structural restriction imposed on them.
SUMMARY OF THE INVENTION
It is a first object of the present invention to provide a planar magnetic element which is small enough to be integrated with electric elements of other types;
It is a second object of the invention to provide a planar magnetic element which has a sufficiently great inductance;
It is a third object of this invention to provide a planar magnetic element which has but only a few leakage fluxes
It is a fourth object of the invention to provide a planar magnetic element which excels in high-frequency characteristic and superimposed DC current characteristic;
It is a fifth object of the present invention to provide a planar magnetic element which has large current capacity and, hence, great inductance;
It is a sixth object of the invention to provide a planar magnetic element wherein it is easy to lead terminals outwards;
It is a seventh object of this invention to provide a planar magnetic element which has a trimming function, so that its electric characteristics can be adjusted externally.
The invention will accomplish the above objects by the following six aspects of the invention. According to the invention, the elements of different aspects, each having better characteristics than the conventional ones, can be used in any possible combination, thereby to provide new types of planar elements which have still better characteristics and which have better operability.
According to a first aspect of this invention, there is provided a planar magnetic element which comprises: a spiral planar coil having a gap aspect ratio (i.e., the ratio of the width of the conductor to the gap among the conductors) of at least 1; insulation members laminated with the spiral planar coil; and magnetic members laminated with the insulation members. The coil of this planar magnetic element has a relatively low resistance. Therefore, it will have a large quality coefficient Q when used as an inductor, and will have a great gain when used as a transformer. In other words, the element has a sufficient operability.
According to a second aspect of the present invention, there is provided a planar magnetic element which comprises a planar coil formed of a conductor which has a conductor aspect ratio (i.e., the ratio of the width of the conductor to the thickness thereof) of at least 1. In this regard, it should be noted that when this element is used as an inductor, its ability is determined by its permissible current and inductance. The permissible current is, in turn, determined by the cross-sectional area of the conductor. Hence, the permissible current can be increased by making the conductor broader. If the conductor is made broader, however, it will inevitably occupy a greater area in a plane, which runs counter to the demand for miniaturization of the planar magnetic element. On the other hand, the inductance of the planar magnetic element can indeed be increased by bending the conductor more times, thus forming a coil having more turns. The more turns, the larger the area the coil occupies. This also runs counter to the demand for miniaturization. The planar magnetic element according to the invention can have a sufficiently large permissible current since the conductor has an aspect ratio of at least 1.
According to a third aspect of the invention, there is provided a multilayered planar inductor comprising a spiral planar coil and magnetic members sandwiching the planar coil. The magnetic members have a width w greater than the width a.sub.0 of the spiral planar coil by a value more than 2.alpha.. It should be noted that the value .alpha. is [.mu..sub.s g t/2].sup.1/2 where .mu..sub.s is the relative permeability of the magnetic members, t is the thickness of the magnetic members, and g is the distance between the magnetic members. Since w>a.sub.0 +2.alpha., this planar inductor has a great inductance. When w=a.sub.0 +2.alpha., for example, the inductance is at least 1.8 times greater than in the case where w=a.sub.0. The planar inductor not only has a great inductance, but also has small leakage flux. In view of this, this planar inductor is suitable for use in an integrated circuit, and serves to make electronic devices thinner.
According to a fourth aspect of the present invention, there is provided a planar magnetic element comprising a planar coil and magnetic layers sandwiching the coil. The magnetic layers are magnetically anisotropic in a single axis which extends at right angles to the direction of the magnetic field generated by the coil. Owning to the uniaxial magnetic anisotropy of the magnetic layers, the planar magnetic element excels in superimposed DC current characteristic and high-frequency characteristic. It is suitable for use in high-frequency circuits such as DC-DC converters. In addition, it can be made small and integrated with electric elements of other types, thereby to form an integrated circuit.
According to a fifth aspect of this invention, there is provided a planar magnetic element comprising a planar coil and magnetic layers sandwiching the coil. The planar coil consists of a plurality of one-turn planar coils located in the same plane, having different sizes, and each having an outer terminal. This planar magnetic element can be electrically connected to an external circuit with ease, and can be trimmed by an external means to have its electric characteristics adjusted. Hence, this is a very useful magnetic element, finding use in step-up chopper-type DC-DC converters, resonant DC-DC converters, and very thin RF circuits for use in pagers.
According to a sixth aspect of the present invention, there is provided a planar magnetic element comprising a conductive layer and a magnetic layer. The magnetic layer surrounds the conductive layer, thus forming a closed magnetic circuit. The current flowing in the conductor layer magnetizes the magnetic layer in the direction of the closed magnetic circuit. This planar magnetic element has small leakage flux and a great current capacity. It can, therefore, serve to render electronic devices thinner when incorporated into these devices.
The planar magnetic elements of the invention, described above, can not only be small but also have improved characteristics generally required of magnetic elements such as inductors.
The planar inductors and transformers according to the invention, which comprise planar micro-coils, are small and can be formed on a semiconductor substrate. Therefore, they can be integrated with active elements (e.g., transistors) and passive elements (e.g., resistors and capacitors), thereby constituting a one-chip semiconductor device. In other words, they help to provide small-sized electronic devices containing inductors and transformers. In addition, the planar inductors and transformers of the invention can be fabricated by means of the existing micro-technique commonly applied to the manufacture of semiconductor devices.
As can be understood from the above, the present invention serve to provide small and thin LC-circuit sections for use in various electronic devices, and ultimately contributes to the miniaturization of the electronic devices.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an exploded view of a conventional planar inductor comprising amorphous magnetic ribbons and square spiral planar coil;
FIGS. 2A and 2B illustrate the waveforms of the currents flowing through the output choke coils of conventional DC-DC converters;
FIG. 3 is a graph representing the B-H curve of the soft magnetic core shown in FIG. 1;
FIG. 4 is a graph showing the superimposed DC current characteristic of the planar inductor shown in FIG. 1;
FIGS. 5 to 11 are diagrams and graphs showing and explaining the first aspect of the invention;
FIG. 5 is an exploded view illustrating a planar inductor according to the first aspect of the present invention;
FIG. 6 is a sectional view schematically showing the planer inductor shown in FIG. 5;
FIG. 7 is a plan view showing a planar transformer according to the first aspect of the invention;
FIG. 8 is a sectional view schematically showing the planar transformer shown in FIG. 7;
FIG. 9 is a graph representing the relationship between the gap aspect ratio of the inductor of FIG. 5 to the coil resistance thereof, and also to the inductance thereof;
FIG. 10 is a graph showing the relationship between the gap aspect ratio of the inductor of FIG. 5 to the L/R value thereof;
FIG. 11 is a graph explaining the relationship between the gap aspect ratio of the transformer of FIG. 7 to the gain thereof;
FIGS. 12A to 22 are diagrams and graphs showing and explaining the second aspect of the invention;
FIG. 12A is an exploded view showing a magnetic element according to both the first aspect and the second aspect of the invention, having not only a high conductor aspect ratio but also a high gap aspect ratio;
FIG. 12B is a sectional view, taken along line 12B--12B in FIG. 12A;
FIG. 13A to 13D, and FIG. 14 are diagrams explaining how cavities are formed among the turns of the coil conductor incorporated in the magnetic element shown in FIGS. 12A and 12B;
FIG. 15 is a perspective view illustrating a planar capacitor according to the second aspect of the invention, which comprises capacitor with parallel electrodes;
FIG. 16 is a graph representing the k-dependency of the value C/Co of the planar capacitor illustrated in FIG. 15;
FIG. 17 is a sectional view showing a magnetic element according to the second aspect of the present invention, which comprises a single planar coil;
FIG. 18 is a sectional view showing a magnetic element according to the second aspect of the invention, which comprises a plurality of planar coils laminated together;
FIGS. 19A and 19B are plan views showing two modifications of the planar coil used in the magnetic elements shown in FIGS. 17 and 18;
FIG. 20 is a sectional view illustrating a magnetic element according to the second aspect of the invention, which comprises a planer coil, a substrate, and a bonding layer interposed between the coil and the substrate;
FIG. 21 is a sectional view showing a microtransformer according to the second aspect of the present invention;
FIG. 22 is a diagram illustrating two types of planar coils according to the second aspect of the present invention;
FIGS. 23 to 28 are diagrams and graphs showing and explaining the third aspect of the invention;
FIGS. 23 and 24 are exploded views showing two types of inductors according to the third aspect of the invention;
FIGS. 25A to 25C are sectional views of the inductor shown in FIG. 23, explaining how magnetic fluxes leak from the inductor;
FIG. 26 is a diagram explaining the distribution of magnetic field at the ends of the planer spiral coil incorporated in the inductor shown in FIG. 23;
FIG. 27 is a graph representing the relationship between the width w of the magnetic members used in the inductor of FIG. 23 and the leakage of magnetic fluxes;
FIG. 28 graph showing the relationship between the width w of the magnetic members used in the inductor of FIG. 23 and the inductance of the inductor;
FIGS. 29 to 48 are diagrams and graphs showing and explaining the fourth aspect of the invention;
FIG. 29 is an exploded view showing a first planar inductor exhibiting a uniaxial magnetic anisotropy, according to the fourth aspect of the invention;
FIG. 30 is a diagram explaining the relationship between the direction of the magnetic field generated by the coil used in the inductor (FIG. 29) and the easy axis of the magnetization of the the magnetic cores;
FIG. 31 is a graph showing a curve of magnetization in the axis of easy magnetization of the inductor (FIG. 29) and a curve of magnetization in the hard axis of magnetization of the magnetic cores;
FIG. 32A is a diagram showing the distribution of the magnetic fluxes in those regions of the magnetic members used in the inductor (FIG. 29), where the magnetic field extends parallel to the axis of easy magnetization;
FIG. 32B is a diagram showing the distribution of the magnetic fluxes in those regions of the magnetic members used in the inductor (FIG. 29), where the magnetic field extends at right angles to the axis of easy magnetization;
FIG. 33 is an exploded view illustrating a second planar inductor according to the fourth aspect of the present invention;
FIG. 34 is a graph representing the superimposed DC current characteristic of the planar inductor illustrated in FIG. 33;
FIG. 35 is an exploded view showing a modification of the planar inductor illustrated in FIG. 33;
FIG. 36 is an exploded view illustrating a third planar inductor according to the fourth aspect of the invention;
FIG. 37 is a graph representing the superimposed DC current characteristic of the planar inductor shown in FIG. 36;
FIG. 38 is an exploded view showing a fourth planar inductor according to the fourth aspect of the present invention;
FIG. 39 is a perspective view showing the surface structure of either magnetic layer incorporated in the inductor shown FIG. 38;
FIG. 40 is a graph representing the relationship between the parameters of the surface structure of either magnetic layer of the inductor (FIG. 38) and the second term of the formula defining Uk;
FIG. 41 is a graph representing the superimposed DC current characteristic of the planar inductor shown in FIG. 38;
FIG. 42A is a graph showing a curve of magnetization in the easy axis of magnetization of the inductor (FIG. 38) and a curve of magnetization in the hard axis of magnetization of the magnetic material;
FIG. 42B is a graph illustrating the permeability-frequency relationship in the axis of easy magnetization, and also the permeability-frequency relationship in the hard axis of magnetization
FIGS. 43A and 43B are a plan view and a sectional view, respectively, illustrating a fifth planar inductor according to the fourth aspect of the invention;
FIG. 44 is a plan view showing a modification of the planar inductor illustrated in FIGS. 34A and 43B;
FIG. 45 is a plan view illustrating a sixth planar inductor according to the fourth aspect of the present invention;
FIGS. 46A and 46B are a plan view and a sectional view, respectively, showing another type of a planar inductor according to the fourth aspect of the present invention;
FIGS. 47A and 47B are a plan view and a sectional view, respectively, illustrating a seventh planar inductor according to the fourth aspect of the present invention;
FIGS. 48A and 48B are a plan view and a sectional view, respectively, showing an eighth planar inductor according to the fourth aspect of the invention;
FIGS. 49 to 61 are diagrams and graphs showing and explaining the fifth aspect of the invention;
FIG. 49 is a plan view showing a first magnetic element according to the fifth aspect of the invention;
FIG. 50 is a plan view illustrating a second magnetic element according to the fifth aspect of the present invention;
FIG. 51 is a plan view showing a third magnetic element according to the fifth aspect of the invention, which is a modification of the element shown in FIG. 49 by connecting outer terminals in a specific manner;
FIG. 52 is a plan view showing a third magnetic element according to the fifth aspect of the invention, which is a modification of the element shown in FIG. 49 by connecting outer terminals in another manner;
FIG. 53 is a plan view showing a third magnetic element according to the fifth aspect of the invention, which is a modification of the element shown in FIG. 49 by connecting outer terminals in still another manner;
FIG. 54 is a diagram representing the relationship between the inductance of the magnetic element shown in FIG. 49 and the manner of connecting the outer terminals;
FIG. 55 is a plan view showing a planar transformer made by connecting the outer terminals of the magnetic element of FIG. 49 in a specific manner;
FIG. 56 is a plan view illustrating a planar transformer made by connecting the outer terminals of the magnetic element of FIG. 49 in another way;
FIG. 57 is a plan view showing another planar transformer made by connecting the outer terminals of the element of FIG. 49 in still another manner;
FIG. 58 is a graph representing the relationship between the voltage and current ratios of the magnetic element shown in FIG. 49, on the one hand, and the manner of connecting the outer terminals, on the other;
FIG. 59 is a sectional view showing a device comprising a semiconductor substrate, an active element formed on the substrate, and a magnetic element according to the fifth aspect of the invention, formed on the semiconductor substrate;
FIG. 60 is a sectional view showing another device comprising a semiconductor substrate, an active element formed in the substrate, and magnetic elements according to the fifth aspect of the invention, located above the active element;
FIG. 61 is a sectional view illustrating a device comprising a semiconductor substrate, magnetic elements according to the fifth aspect of the invention, formed on the substrate, and a magnetic element located above the magnetic elements;
FIGS. 62A to 64 are diagrams and graphs showing and explaining the sixth aspect of the invention;
FIG. 62A is a sectional view showing a one-turn coil according to the sixth aspect of the invention;
FIG. 62B is a partly sectional, perspective view showing the one-turn coil of FIG. 62A;
FIG. 63A is a sectional view illustrating one-turn coils of the type shown in FIG. 62A which are connected in series, forming a coil unit;
FIG. 63B is a sectional view showing a magnetic element according to the sixth aspect of the invention, which comprises a combination of two coil units of the type shown in FIG. 63A;
FIG. 64 is a sectional view illustrating a magnetic element according to the sixth aspect of the invention, which comprises a one-turn coil of the type shown in FIG. 62A, magnetic layers, and insulation layers;
FIG. 65 is a diagram explaining the criterion of selecting a material for magnetic layers, and representing the relationship between the number of turns of a spiral planar coil, on the one hand, and the maximum coil current Imax and the intensity (H) of the magnetic field generated by supplying the current Imax to the spiral planar coil, on the other hand;
FIGS. 66 to 72 are diagrams illustrating various devices incorporating the magnetic elements of the invention;
FIG. 66 is a diagram schematically showing a pager comprising a magnetic element according to the present invention;
FIG. 67 is a plan view showing a 20-pin IC chip of single in-line package (SIP) type, comprising magnetic elements according to the invention;
FIG. 68 is a perspective view of a 40-pin IC chip of dual in-line package type (DIP);
FIG. 69 is a circuit diagram showing a DC-DC converter of step-up chopper type;
FIG. 70 is a circuit diagram illustrating a DC-DC converter of step-down chopper type;
FIG. 71 is a diagram showing an RF circuit for used in an very small portable telephone;
FIG. 72 is a circuit diagram showing a resonant DC-DC converter; and
FIG. 73 is a section of a planar coil for one embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Various aspects of the present invention will now be described in detail. Although these aspects will be explained, one by one, they can be combined, thereby to provide a variety of magnetic elements which fall within the scope of the invention. Since the materials of the magnetic elements are substantially common to the aspects of the invention, they will be described at the very end of this description.
The first aspect of the invention will be described, with reference to FIGS. 5 to 11.
FIG. 5 is an exploded view showing a planar inductor according to the first aspect of the invention. As is shown in the FIG. 5, the planar inductor comprises a semiconductor substrate 10, three insulating layers 20A, 20B and 20C, two magnetic layers 30A and 30B, a spiral planar coil 40, and a protection layer 50. The insulation layer 20A is formed on the substrate 10. The magnetic layer 30A is formed on the layer 20A. The insulation layer 20B is formed on the magnetic layer 30A. The coil 40 is mounted on the layer 20B. The insulation layer 20C covers the coil 40. The magnetic layer 30B is formed on the layer 20C. The protection layer 50 is formed on the magnetic layer 30B. FIG. 6 is a sectional view, taken along line 6--6 in FIG. 5, illustrating a portion of the planar inductor. In FIG. 6, the components identical to those shown in FIG. 5 are designated by the same numerals.
FIG. 7 is an exploded view showing a planar transformer according to the first aspect of the invention. This transformer is characterized in that the primary and secondary coils have the same number of turns. As is illustrated in FIG. 7, the transformer comprises a semiconductor substrate 10, four insulation layers 20A to 20D, two magnetic layers 30A and 30B, two spiral planar coils 40A and 40B, and a protection layer 50. The layers 20A, 30A, and 20B are formed, one upon another, on the substrate 10. The primary coil 40A is mounted on the insulation layer 20B. The insulation layer 20C is laid upon the primary coil 40A. The secondary coil 40B is mounted on the insulation layer 20C. The insulation layer 20D is laid on the secondary coil 40B. The magnetic layer 30B is formed on the layer 20D. The protection layer 50 is formed on the magnetic layer 30B. FIG. 8 is a sectional view, taken along line 8--8 in FIG. 7, illustrating a portion of the planar transformer. In FIG. 8, the components identical to those shown in FIG. 7 are denoted by the same numerals.
In both the planar inductor of FIGS. 5 and 6 and the planar transformer of FIGS. 7 and 8, the substrate 10 is made of silicon. The silicon substrate 10 can be replaced by a glass substrate. When a glass substrate is used in place the silicon substrate 10, the insulation layer 20A, which is beneath the magnetic layer 30A, can be dispensed with.
The spiral planar coil 40 used in the inductor of FIG. 5 and the spiral planar coils 40A and 40B used in the transformer of FIG. 7 have a gap aspect ratio h/b of at least 1, where h is the thickness of the coil conductor and b is the gap between any adjacent two turns. Two alternative methods can be employed to form a spiral planar coil having this high gap aspect ratio h/b. The first method is to perform deep etching on a conductor layer, thus forming a spiral slit in the plate, and then fill the spiral slit with insulative material. The second method is to layer dry etching on an insulative layer, thus forming a spiral slit in the layer, and then fill this slit with conductive material.
There are two variations of the first method. In the first variation, the spiral slit is filled up with the insulative material. In the second variation, the slit is partly filled, such that a cavity is formed in the resultant coil conductor. The first variation falls within the first aspect of the invention, whereas the second variation falls within the second aspect of the present invention.
More specifically, according to the first aspect of the invention, the spiral planar coil is formed in the following way. First, a conductor layer is formed on an insulation layer. Then a mask layer is formed on the conductor layer. The mask layer is processed, thereby forming a spiral slit in the mask layer using this mask layer, high-directivity dry etching, such as ion beam etching, ECR plasma etching, reactive ion etching, is performed on the conductor layer, thus forming a spiral slit in the conductor layer and, simultaneously, a coil conductor having a gap aspect ratio h/b of 1 or more. It is required that the etching speed of the mask layer be much different from that of the conductor layer, so that vertical anisotropic etching may be accomplished.
To form an insulation layer on the coil conductor having a high gap aspect ratio h/b, it is desirable that the gap between the turns with insulative material having small dielectric coefficient and that the mass of the insulative material be processed to have a flat top surface. When the insulative material is an inorganic one, such as SiO.sub.2 or Si.sub.3 N.sub.4, CVD method or sputtering (e.g., reactive sputtering or bias sputtering) is employed to form the insulation layer. When the insulative material is an organic one, it is preferably polyimide (including a photosensitive one). Instead, resist can be utilized. The insulative material, either organic or inorganic, is mixed with a solvent, thus forming a solution. The solution is spin-coated on a substrate. The resultant coating is cured by an appropriate method, whereby an insulation layer is formed. The insulation layer, thus formed in the gap between the turns of the coil conductor, is subjected to etch-back process and is caused to have a flat top surface.
The second method of forming a spiral planar coil, which falls within the second aspect of the invention will be described. In this method, an insulation layer is first formed. A patterned resist is formed on the insulation layer. Using the resist as a mask, selective dry etching is performed on the insulation layer, thus forming a spiral slit in the insulation layer. Then, a conductor layer is formed on the patterned resist and in the spiral slit, by means of sputtering, CVD method, vacuum vapor-deposition, or the like. Next, the resist is removed from the insulation layer and the conductor layer by means of a lift-off method. Simultaneously, those portions of the conductor layer, which are on the resist, are also removed. As a result, a spiral planar coil is formed.
Whether the first method or the second method should be used to form the spiral planar coil depends upon the pattern of the planar coil.
The advantages of the magnetic elements according to the first aspect of the invention will be explained.
FIG. 9 represents the relationship between the gap aspect ratio of the planar inductor of FIG. 5 to the coil resistance thereof, and also to the inductance thereof. The parameter of the inductance L is .mu..sub.s t, where .mu..sub.s is the relative permeability of the magnetic layers 30A and 30B, and t is the thickness thereof. In this instance, .mu..sub.s t=5000 .mu.m or 1000 .mu.m. As is evident from FIG. 9, the inductance L of the planar inductor is almost constant, not depending on the gap aspect ratio h/b. The resistance of the spiral planar coil 40 is inversely proportional to the gap aspect ratio h/b, and remains virtually constant if the gap aspect ratio h/b exceeds 5.
FIG. 10 shows the relationship between the gap aspect ratio of the inductor of FIG. 5 to the L/R value thereof. L/R is a physical quantity proportional to the quality coefficient Q of the inductor, which is given as: Q=2.pi. f L/R where f is frequency (Hz). In FIG. 10, the relationship is shown for two parameters, i.e., relative permeabilities .mu..sub.s of 10.sup.4 and 10.sup.3 of either magnetic layer. As is evident from FIG. 10, L/R increases with the gap aspect ratio h/b, but not over 5 even if the ratio h/b further increases.
The inventors hereof made planar inductors of the type shown in FIG. 5, which had different gap aspect ratios of 0.3, 0.5, 1.0, 2.0, and 5.0. Some of these inductors had a parameter .mu..sub.s t of 5000 .mu.m, and the rest of them had a parameter .mu..sub.s t of 1000 .mu.m, where s is the the relative permeability of either magnetic layer, and t is the thickness thereof. The inventors tested these planar inductors to see how their quality coefficients Q depended on their gap aspect ratios. The results of the test were as is shown in the following table:
______________________________________
Q (f = 5 MHz)
.mu..sub.s (.mu.m)
Ratio h/b 5 .times. 10.sup.3
1 .times. 10.sup.3
______________________________________
0.3 5.5 1.4
0.5 13.5 3.3
1.0 19.8 4.9
2.0 22.9 5.7
5.0 25.0 6.3
______________________________________
As can be understood from the table, the coefficient Q of the planar inductor having a gap aspect ratio of 1 is about 3.5 times greater than that of the inductor having a gap aspect ratio of 0.3, and about 1.5 times greater than that of the inductor having a gap aspect ratio of 0.5. Obviously, any planar inductor of the type shown in FIG. 5 can have a sufficiently great quality coefficient Q if its gap aspect ratio is 1 or more.
FIG. 11 explains the relationship between the gap aspect ratio of the planar transformer of FIG. 7 to the gain thereof. As this figure reveals, the transformer can have a sufficient large coefficient Q and, hence, a sufficiently great gain, if its gap aspect ratio is 1 or more.
One of the determinants of the ability of a magnetic element is the material of the element. Hence, material used is important for forming the magnetic element. This point will be described at the end of the present description.
Various planar magnetic elements according to the second aspect of the invention, which are characterized by their specific conductor aspect ratio h/d (h is the height of the coil conductor, and d is the width thereof), will now be described with reference to FIG. 12A through FIG. 22.
FIG. 12A is an exploded view showing a planar magnetic element. FIG. 12B is a sectional view, taken along line 12B--12B in FIG. 12B. The planar magnetic element has not only a high conductor aspect ratio but also a high gap aspect ratio. In view of this, it falls within both the first aspect and the second aspect of the present invention.
As is shown in FIGS. 12A and 12B, the planar magnetic element comprises a substrate 10 and a spiral planar coil 40 directly mounted on the substrate 10. The coil conductor 42 (FIG. 12B) can be formed by the known process commonly employed in forming the wiring of semiconductor devices. The smaller the gap between the turns of the coil conductor 24, the smaller the planar magnetic element. However, the smaller the gap, the more difficult for the element to have a sufficiently high conductor aspect ratio. Hence, it is required that a gap be first set at the value most suitable for the use of the element, and then the conductor aspect ratio h/d be then determined. According to the second aspect of the invention, the conductor aspect ratio h/d is at least 1. In other words, the coil conductor 42 has a height equal to or greater than the width d. In order to miniaturize the planar magnetic element, it is of course desirable that the gap aspect ratio h/b be as large as possible. In practice, however, it would be most recommendable that both the width d of the conductor 42 and the gap b between the turns thereof be both about 10 .mu.m ore less.
In order to produce a coil conductor having a high aspect ratio h/d, it is necessary to etch a narrow spiral portion of a thick conductive layer. Hence, preferred as such a conductive layer is a crystal film having a plane of easy etching which is parallel to the layer itself. Needless to say, a single crystal film is the most preferable.
Despite its structure, the planar magnetic element shown in FIGS. 12A and 12B may have an insufficient inductance if it is made small. Nonetheless, its reactance .omega.L (.omega. is drive angular frequency) can be increased by driving the element at high switching frequency. Recently, magnetic elements are driven at higher and higher switching frequencies. The reactance of the planar magnetic element shown in FIGS. 12A and 12B, if insufficient due to the miniaturization of the element, does not suffer from any drawbacks. The inductance can perform its function in a high-frequency region (e.g., several MHz) even if its inductance is as low as nH.
When the turns of a coil conductor having high aspect ratio h/d are close to one another, the inter-turn capacitance is large, due to the narrow gap between any two adjacent turns and the large opposing faces thereof. Because of this great inter-turn capacitance, the planar magnetic element can be incorporated in an LC circuit. In most cases, however, the use of the element decreases the LC resonant frequency (generally known as "cutoff frequency"), and the element can no longer work as an inductor. It is therefore necessary to decrease the inter-turn capacitance to a minimum. This capacitance can be reduced by forming an insulation layer (e.g., a SiO.sub.2 layer) which has a cavity extending between the turns of the coil conductor and which decreases the inter-turn dielectric coefficient. The cavity may be vacuum or filled with the material gas used for forming the insulation layer. In either case, the inter-turn dielectric coefficient is far smaller than in the case where the gap between the turns is filled with the insulative material.
To form an insulation layer having such a cavity, it suffices to employ the CVD method used in manufacturing semiconductor devices. The gap between the turns of the coil conductor is not completely filled with the insulative material (e.g., Si0.sub.2) as in manufacturing semiconductor devices. Rather, an insulation layer grows thicker, first on the top surface of the coil conductor and then on the sides of the upper portion of each turn. The layer on the sides of each turn is made to grow thicker until it closes up the opening of the gap between the turns. To grow the insulation layer in this specific way, it suffices to set the gas-feeding speed at an appropriate value.
More specifically, as is illustrated in FIG. 13A, the material gas 82 is applied onto the coil conductor 42 formed on the substrate 10. It is difficult for the gas 82 to flow to the bottom of the gap between the coil turns. Hence, an insulation layer 80 grows fast on the top of each turn 42, and grows less on the sides of the upper portion of thereof, as is illustrated in FIGS. 13B. The layer 80 fast grows thicker on the top of each turn 42 and slowly grows on the sides of the upper portion thereof. As is shown in FIG. 13C, the layer 80 contacts the layer formed on the next turn. The layer 80 keeps on growing thicker, closing up the openings among the turns 42. As a result, as is shown in FIG. 13D, a cavity 70 is formed which extends between the turns of the coil conductor 42.
An insulation layer having a cavity can be formed by means of sputtering, as is illustrated in FIG. 14. More specifically, particles of insulative material are applied slantwise to a coil conductor 42, at an angle .theta. to the top surface of the conductor 42. The insulation layer formed by the sputtering is less smooth than the insulation layer formed by the CVD method. In view of this, the sputtering method is not desirable.
The reduction of the inter-turn capacitance, which has resulted from the cavity 70 extending between the turns of the coil conductor 42, will be explained, with reference to FIG. 15 illustrating a planar capacitor according to the second aspect of the invention, which comprises two parallel capacitor units.
The upper unit comprises an insulation member 20 and an electrode 60B formed on the upper surface of the member 20. The lower unit comprises an insulation member 20 and an electrode 60B formed on the lower surface of the member 20. The capacitor units have the same size of r(m).times.t(m). The insulation members 20 have a dielectric coefficient .epsilon.. They are spaced apart by distance s. Were the gap s.sub.0 between the electrodes 60A and 60B filled with the same insulative material as the members 20, this capacitor should have capacitance C.sub.0 given as:
C.sub.0 =.epsilon..sub.0 .epsilon.t/s.sub.0
where .epsilon..sub.0 is vacuum dielectric coefficient.
The ratio of the capacitor C of this capacitor to the capacitance C.sub.0 is given as follows:
C/C.sub.0 =1/[k(.epsilon.-1)+1]
where k is s/s.sub.0, i.e., the ratio of the volume of a cavity to the space s.sub.0).
FIG. 16 represents how the ratio C/C.sub.0 depends on the ratio K when the insulating members 20 are made of SiO.sub.2 whose specific dielectric coefficient is about 4. Assuming k is 1/3 or less, the capacitance C will be about 1/2 C.sub.0 or less. No matter whether the gap 70 between the insulation members 20 is filled with gas or maintained virtually vacuum, this gap will be desirable about 1 or more of the gap s.sub.0.
The planar coil 40 (FIG. 12A) is incorporated in a planar inductor. This coil 40 has but an insufficient inductance. Hence, it is desirable that a magnetic layer be arranged as close as possible to the planar coil 40 so that the magnetic layer may serve as magnetic core. In order to reduce leakage flux to a minimum, the coil 40 should better be interposed between two magnetic layers, as is shown in FIG. 17.
As is shown in FIG. 17, this planar inductor comprises an insulative substrate 10 made of, for example, silicon, a magnetic layer 30A formed on the substrate 10, an insulation layer 20A formed on the magnetic layer 30A, a planar coil 40 mounted on the insulation layer 20A, an insulation layer 20B covering the top of the coil 40, and a magnetic layer 30B. The magnetic layers 30A and 30B function as magnetic shields as well, reducing leakage flux to almost nil. Since virtually no magnetic fluxes leak from the planar inductor, other electronic elements can be arranged very close to the planar inductor. The planar inductor of the type shown in FIG. 17 therefore contributes to the miniaturization of electronic devices.
For some specific use, the planar inductor shown in FIG. 17 can be modified by removing one or both of the magnetic layers 20A and 20B which serve as cores.
FIG. 18 shows a modification of the planar inductor illustrated in FIG. 17. This inductor is characterized in two respects. First, the coil 40 consists of three units 42 placed one upon another. Second, two additional insulation layers 20C are used, each interposed between the adjacent two coil units 42. Obviously, the planar coil 40 has more turns than the coil 40 used incorporated in the planar inductor of FIG. 17. Hence, the inductor of FIG. 18 can have a higher inductance than the planar inductor shown in FIG. 17.
Planer coils of various shapes can be incorporated into the planar magnetic elements according to the present invention. One of them is the spiral planar coil illustrated in FIG. 19A. Another of them is the meandering planar coil shown in FIG. 19B. The spiral coil is more recommendable for use in planar magnetic elements which need to have high inductance.
Generally, coil conductors 42 for use in planar magnetic elements have a height far greater than the conductors used in semiconductor devices. Thus, some measures must be taken to secure a coil conductor 42 firmly to a substrate. A bonding layer can be used to secure the conductor 42 to the substrate, as is shown in FIG. 20. As is shown in FIG. 20, a bonding layer 25, such as a Cr layer, of the same pattern as a oil conductor 42 is formed on a substrate 10, and the conductor 42 is formed on the bonding layer 25. This method can be applied also to the planar elements according to the first, third, fourth and fifth aspects of the invention.
Needless to say, the coil conductor 42 must be designed in accordance with the use of the planar magnetic element in which it is to be incorporated. Hence, the turn pitch, the aspect ratio h/d, and other features of the conductor 42 must be determined in accordance with the purpose for which the planar magnetic element will be used. To help reduce the size of the element, it is required that the gap b between any adjacent two turns be less than the width d of the conductor 42. There is no particular limitation to the gap b, but a gap b of 10 .mu.m or less is recommendable, for the elements according to not only the second aspect but also other aspects of the present invention.
The description of the second aspect of this invention has been limited to planar inductors each having one planar coil. Nevertheless, the second aspect of the invention is not limited to planar inductors having one coil only. Microtransformers, each having two planar coils, also fall within the second aspect of the present invention.
Such a microtransformer is illustrated in FIG. 21. This microtransformer comprises a substrate 10, three insulation layers 20A, 20B and 20C, two magnetic layers 30A and 30B, and two planar coils 40A and 40B. The substrate 10 is made of silicon or the like. The magnetic layer 30A is formed on the substrate 10, and the insulation layer 20A is formed on the layer 30A, The planar coil 40A, which function as primary coil, is mounted on the layer 20A. The insulation layer 20B covers the coil 40A. The planar coil 40B, which functions as secondary coil, is mounted on the insulation layer 20B. The insulation layer 20C covers the coil 40B. The magnetic layer 30B is formed on the insulation layer 20C. The magnetic layers 30A and 30B sandwich the unit comprising of the primary and secondary coils.
The primary coil 40A and the secondary coil 40B can be located in the same plane, as is illustrated in FIG. 22A. The secondary coil 40B extends between the turns of the primary coil 40B. Alternatively, the secondary coil 40B can be placed in the area surrounded by the primary coil 40A, as is illustrated in FIG. 22A.
The third aspect of the present invention will now be described, with reference to FIGS. 23 to 28.
FIG. 23 iS an exploded view showing a planar inductor according to the third aspect. As is shown in FIG. 23, this inductor comprises two insulation layers 20A and 20B, two magnetic layers 30A and 30B, and a spiral planar coil 40. The coil 40 is sandwiched between the insulation layers 20A and 20B. The unit consisting of the layers 20A and 20B and the coil 40 is sandwiched between the magnetic layers 30A and 30B. The spiral planar coil 40 is square, each side having a length a.sub.0. The magnetic layers 30A and 30B are also square, each side having a length w. They have the same thickness t. They are spaced apart from each other by a distance g.
FIG. 24 is also an exploded view illustrating another type of a planar inductor according to the third aspect of the invention. This planar inductor comprises three insulation layers 20A, 20B and 20C, two magnetic layers 30A and 30B, two spiral planar coils 40A and 40B, and a through-hole conductor 42. The insulation layer 20C is interposed between the coils 40A and 40B. The unit consisting of the layer 20C and the coils 40A and 40B is sandwiched between the insulation layers 20A and 20B. The unit consisting of the layers 20A, 20B and 20C and the coils 40A and 40B is sandwiched between the magnetic layers 30A and 30B. The through-hole conductor 42 extends through the insulation layer 20C and electrically connects the spiral planar coils 40A and 40B. The spiral planar coils 40A and 40B are square, each side having a length a.sub.0. The magnetic layers 30A and 30B are also square, each side having a length w, and have the same thickness t. The layers 30A and 30B are spaced apart from each other by a distance g.
Both planar inductors shown in FIGS. 23 and 24, respectively, can be advantageous in the following two respects when appropriate values are selected for a.sub.0, w, t, and g:
(1) They have an effective magnetic shield, and the leakage flux is therefore very small.
(2) They have a sufficiently high inductance.
Either planar inductor according to the third aspect can be formed on a glass substrate, by means of thin-film process described above. Alternatively, it can be formed on any other insulative substrate (e.g., a substrate made of a high-molecular material such as polyimide).
The magnetic fluxes generated by the spiral planar coil or coils must be prevented from leaking from the planar inductors shown in FIGS. 23 and 24. Otherwise, the leakage fluxes of either inductor adversely influence the other electronic components arranged very close to the inductor and formed on the same chip, thus forming a hybrid integrated circuit. According to the third aspect of the invention, the ratio between the width w of either magnetic layer and the width a.sub.0 of the square planar coil or coils should is set at an optimum value so that the magnetic fluxes generated by the coil or coils are prevented from leaking.
FIGS. 25A to 25C are sectional views of three planar inductors of the type shown in FIG. 23 which have different values w for the magnetic layers, and explain how magnetic fluxes 100 leak from these planar inductors. In the inductor shown in FIG. 25A, the width w of either magnetic layer is substantially equal to the width a.sub.0 of the spiral coil 40. In the inductor shown in FIG. 25B, the width w is slightly greater than the width a.sub.0 of the coil 40. In the inductor of FIG. 25C, the width w is much greater than the width a.sub.0 of the spiral coil 40. As is evident from FIGS. 25A, 25B, and 25C, the broader either magnetic layer, the less the leakage fluxes.
FIG. 26 is a diagram explaining the distribution of magnetic fluxes at the edges of the spiral planar coil 40 used in the inductor shown in FIG. 23. As can be understood from FIG. 26, the magnetic field is about 0.37 time less at a point at distance a from any edge of the coil 40, than at the edge of the coil 40. The distance .alpha. is: .alpha.=[.mu..sub.s g t/2].sup.1/2, where .mu..sub.s is the relative permeability of the magnetic layers 30, t is the thickness of thereof, and g is the distance therebetween. Thus, in the planar inductor shown in FIG. 23, the width w of either magnetic layer is 2.alpha. or more, thereby reducing the leakage fluxes drastically. The coil conductor 42 forming the coil 40 has a width d of 70 .mu.m and a inter-turn gap b of 10 .mu.m, the distance g between the magnetic layers is 5 .mu.m, and the coil current is 0.1 A.
FIG. 27 represents the relationship between the width w of the magnetic members used in the inductor of FIG. 23 and the leakage of magnetic fluxes from the edge of either magnetic layer. As is evident from FIG. 27, the greater the width w, the less the flux leakage. It is desirable that the width w be a.sub.0 +10.alpha. or more. When the width w is a.sub.0 +10.alpha., almost no magnetic fluxes leak from the planar inductor.
It is demanded that the planar inductor have as high an inductance as possible. The planar inductor according to the third aspect of the invention can have a high inductance only if the magnetic layers have a width w which is greater than the width a.sub.0 of the spiral planar coil by 2.alpha. or more. FIG. 28 represents the relationship between the width w and the inductance of the inductor shown in FIG. 23. As can be understood from FIG. 28, the inductance increases 1.8 times or more if the width w is increased from a.sub.0 to a.sub.0 +2.alpha. or more.
Planar magnetic elements according to the fourth aspect of the invention will now be described, with reference to FIGS. 29 to 48. Although the elements which will be described are planar inductors only, the planar magnetic elements according to the fourth aspect include planar transformers, too. Any planar transformer that belongs to the fourth aspect is essentially identical in structure to the planar inductor, except that the primary planar coil and the secondary planar coil are arranged one above the other.
FIG. 29 is an exploded view showing a first planar inductor according to the fourth aspect of the invention. As is shown in FIG. 29, this inductor comprises two magnetic layers 30, two insulation layers 20, and a spiral planar coil 40. The coil 40 is sandwiched between the insulation layers 20. The unit formed of the layers 20 and the coil 40 is sandwiched between the magnetic layers 30. The magnetic layers 30 exhibit a uniaxial magnetic anisotropy. They have an axis of easy magnetization, which is indicated by an arrow.
When a current flows through the spiral planar coil 40, the coil 40 generates a magnetic field. This magnetic field which extends through either magnetic layer 30 in four directions indicated by arrows in FIG. 30. In the regions A shown in FIG. 30, the magnetic field extends in lines parallel to the axis of easy magnetization of the magnetic layer 30. In the regions B, the magnetic field extends in lines which intersect the axis of easy magnetization, or which are parallel to the hard axis of magnetization of the magnetic layer.
FIG. 31 shows a B-H curve of magnetization in the axis of easy magnetization of either magnetic layer 30 incorporated in the inductor shown in FIG. 29, and also a B-H curve of magnetization in the hard axis of magnetization of the magnetic layer. As can be seen from FIG. 31, the magnetic layer exhibits a very high permeability in the axis of easy magnetization, and hence can easily be saturated in the axis of easy magnetization and can hardly be saturated in the hard axis of magnetization. It follows that the regions A (FIG. 30) can easily be saturated magnetically, whereas the regions B (FIG. 30) can hardly be saturated magnetically. When the magnetic field generated by the coil 40 is intense, the regions A of either magnetic layer 30 are saturated, and some magnetic fluxes leak from the layer 30, as is illustrated in FIG. 32A. The remaining magnetic fluxes extend through the regions B (FIG. 30), as is shown in FIG. 32B. Obviously, the inductance of this planar inductor depends on the density of magnetic fluxes which extend along the hard axis of magnetization of either magnetic layer 30.
To solve the problem of saturation of the magnetic layers, the planar inductors according to the fourth aspect of the invention have one of the following three structures:
First Structure
Two groups of magnetic layers are located below and above a spiral planar coil, respectively. The magnetic layers of either group are arranged, one above another, such that their axes of easy magnetization intersect.
Second Structure
Two square magnetic layers are located below and above a spiral planar coil, respectively. Each of the magnetic layers consists of four triangular pieces, each having an axis of easy magnetization which extends parallel to the base.
Third Structure
Two magnetic layers are located below and above a spiral planar coil, respectively. Either magnetic layer has a spiral groove which extends, exactly along the spiral conductor of the coil.
FIG. 33 is an exploded view illustrating a planar inductor having the first structure defined above. As is evident from FIG. 33, this inductor comprises two laminates and a spiral planar coil 40 sandwiched between the laminates. The laminates are identical in structure.
Each of the laminates comprises two insulation layers 20A and 20B and two magnetic layers 30A and 30B. The insulation layer 20A is mounted on the coil 40, the magnetic layer 30A is mounted on the layer 20A, the insulation layer 20B is formed on the magnetic layer 30A, and the magnetic layer 30B is formed on the insulation layer 20B. The magnetic layers 30A and 30B are arranged such that their axes (arrows) of easy magnetization intersect at right angles.
In either laminate, those regions of the magnetic layer 30A located close to the coil 40, which corresponds to the region A shown in FIG. 30, are easily saturated magnetically, and some magnetic fluxes leak from these saturated regions. These leakage fluxes extend through those regions of the magnetic layer 30B, which correspond to the regions B shown in FIG. 30. As a result, the magnetic fluxes extend along the hard axis of magnetization in both magnetic layers 30A and 30B, and magnetic saturation can hardly take place in either magnetic layer.
FIG. 34 represents the superimposed DC current characteristic of the planar inductor shown in FIG. 33. More precisely, the solid-line curve shows the superimposed DC current characteristic of the inductor, whereas the broken-line curve indicates the superimposed DC current characteristic of the planar inductor shown in FIG. 29. As is evident from FIG. 34, the inductance of the inductor shown in FIG. 34, which has two sets of magnetic layers, is twice has high as that of the inductor shown in FIG. 29 which has only one set of magnetic layers. In addition, as FIG. 34 clearly shows, the DC current, at which the inductance of the inductor shown in FIG. 33 starts decreasing, is greater than the DC current at which the inductance of the inductor shown in FIG. 29 begins to decrease.
FIG. 35 is an exploded view showing an modification of the inductor shown in FIG. 33. This planar inductor is different from the inductor of FIG. 33, in that either laminate comprises four magnetic layers 30A, 30B, 30C and 30D. The four magnetic layers of either laminate are arranged such that the axes of easy magnetization of any adjacent two intersect at right angles.
It will be explained briefly how the planar inductors shown in FIGS. 33 and 35 are manufactured. First, soft magnetic layers made of amorphous alloy, crystalline alloy, or oxide and having a thickness of 3 .mu.m or more are prepared. Then, these magnetic layers are processed, imparting a uniaxial magnetic anisotropy to them. The magnetic layers are orientated, such that the axes of easy magnetization of any adjacent two intersect with each other at right angles. Insulation layers are interposed among the magnetic layers thus orientated. A planar coil is interposed between the two innermost insulation layers. Finally, the coil, the magnetic layers, and the insulation layers, all located one upon another, are compressed together.
The magnetic layers can be formed by means of thin-film process such as vapor deposition or sputtering. When they are made by the thin-film process, they come to have uniaxial magnetic anisotropy while they are being formed in an electrostatic field or while they are undergoing heat treatment in a magnetic field. The less magnetostriction, the better. Nonetheless, a magnetic layer, if made of material having a relatively large magnetostriction, can have a uniaxial magnetic anisotropy by virtue of the inverse magnetostriction effect, only if the stress distribution of the layer is controlled appropriately.
FIG. 36 is an exploded view illustrating a planar inductor having the second structure defined above. As is evident from FIG. 36, this inductor comprises two insulation layers 20, two square magnetic layers 30, a spiral planar coil 40. The coil 40 is sandwiched between the insulation layers 20. The unit formed of the layers 20 and the coil 40 is sandwiched between the magnetic layers 30. Either magnetic layer 30 consists of four triangular pieces, each having an axis of easy magnetization which extends parallel to the base. The axis of easy magnetization of the each triangular piece intersects at right angles with the magnetic fluxes generated by the coil 40. Therefore, the magnetic layers 30 have no regions which are readily saturated magnetically.
FIG. 37 represents the superimposed DC current characteristic of the inductor shown in FIG. 36. More precisely, the solid-line curve shows the superimposed DC current characteristic of the inductor, whereas the broken-line curve indicates the superimposed DC current characteristic of the planar inductor shown in FIG. 29. As is evident from FIG. 34, the inductance of the inductor of FIG. 29 is very high in the small-current region, but abruptly decreases with the superimposed DC current, and remains almost constant thereafter until the superimposed DC current increase to a specific value. By contrast, the inductance of the inductor shown in FIG. 36, wherein the magnetic layers have no regions that can readily be saturated, is about two times higher than that of the inductor shown in FIG. 29, and remains almost constant, irrespective of the superimposed DC current, until the superimposed DC current increases to a specific value.
It will be explained how the planar inductor shown in FIGS. 36 is manufactured. First, soft magnetic layers made of amorphous alloy, crystalline alloy, or oxide and having a thickness of 3 .mu.m or more are prepared. These layers are cut into triangular pieces, each having a base longer than the width of the spiral planar coil 40. The triangular pieces are heat-treated in a magnetic field which extends parallel to the bases of the triangular pieces. As a result, each piece will have an axis of easy magnetization which extends parallel to its base. Four of these triangular pieces, now exhibiting uniaxial magnetic anisotropy, are arranged and connected together, such that their axes of easy magnetization extend parallel to the spiral conductor of the planar coil 40.
Alternatively, the magnetic layers 30 can be formed by means of thin-film process such as vapor deposition or sputtering. When they are formed by the thin-film process, triangular masks are utilized for forming triangular pieces. More specifically, two triangular resist masks are formed on two triangular region B of a square substrate. Then a magnetic layer having a predetermined thickness is formed on the substrate and the resist masks, while a magnetic field extending parallel to the bases of the regions A is being applied. Next, the resist masks are removed from the substrate, and the magnetic layers on these masks are simultaneously lifted off. As a result, two triangular ma
