EP1030320A1

EP1030320A1 - Split geometry inductor

Info

Publication number: EP1030320A1
Application number: EP00301135A
Authority: EP
Inventors: Robert C. Potter
Original assignee: Knowles Electronics LLC
Current assignee: Knowles Electronics LLC
Priority date: 1999-02-16
Filing date: 2000-02-15
Publication date: 2000-08-23

Abstract

A miniaturized inductor of the thin-film type utilized in miniaturized electrical circuits is disclosed. The inductor includes two thin-film spiral inductive portions with a substrate positioned therebetween. The first inductive portion is wound in a clockwise direction and has a distal end. Conversely, the second inductive portion is wound in a counterclockwise direction and has a distal end connected to the distal end of the first inductive portion.

Description

Technical Field

The present invention relates to electronic components, and in particular to an inductor having two oppositely wound thin-film spirals that are physically, and magnetically, coupled to each other.

Background of the Invention

Hybrid technology provides the means for assembling most major categories of electronic components in relatively small enclosures to satisfy system size requirements that cannot be met by more conventional packaging techniques. Analog and digital circuits as well as microwave modules are typically made in hybrid form. Such circuits usually consist of an insulating substrate with deposited networks, generally conductors and resistors, to which semiconductor devices, integrated circuits, and passive elements are attached in chip form.
Several steps are implemented in the design and fabrication of circuits for fitting within a small enclosure. First, the design is analyzed and thick- and/or thin-film materials are chosen, along with the discrete components, which include uncased active and passive chips. The geometry that defines the dominate parameter of each film-type circuit and the position of each component are determined next. After establishing the deposition sequence for the individual layers of film materials, master patterns for each layer are prepared. From them, photoreduced transparencies are made for photolithographic fabrication of the masks used to deposit or define the film on the substrate, after which the chip parts are attached and interconnected to the film circuit. Maximum circuit yield with related cost effectiveness necessitates an optimum number of add-on components per substrate, a limitation on the range of resistance and capacitance values and tolerances, and, in the past, a minimum use of inductors and transformers because of their relatively low inductance and extensive size.
As indicated above, and known by those skilled in the art, thick- and thin-film networks involve the deposition of passive circuit elements in a predetermined geometric pattern on the surface of the insulating substrate. Deposited thin-films can be made with precision and stability for the diverse requirements of linear circuits and can be fabricated with finer lines than thick films.
In the past, as shown in FIGURE 1, an inductor 1 was typically fabricated by printing or defining conductor patterns of an appropriate spiral design 2 directly onto the top of a substrate 3. The distal end 4 of the spiral design 2 was connected, via a lead 5, to another conductive path 6 on the top of the substrate 3.
Accordingly, such an inductor requires extensive substrate area to achieve high inductance values since its inductance is directly proportional to the overall size of the spiral design and the number of turns. Accordingly, the use of these inductors were avoided.
However, in many circuit designs it is advantageous to include one or more inductors. For example, reducing EMI (ElectroMagnetic Interference) in transducers supports the fabrication of monolithic inductors. When an inductor is used with a high-quality capacitor, a low pass filter can be fabricated which works up to high frequencies (∼6 Ghz or greater).
Besides the prior art inductor of FIGURE 1 being relatively large, the presence of a ground plane opposite the inductor (i.e., on the opposite side of the substrate) can have a detrimental effect on the characteristics of the inductor. In fact, any structure that has a large extended metallization such as the bottom plate of a capacitor would have a similar detrimental effect.
These effects are a result of induced eddy currents forming in the back metallization. In addition, the shunt capacitance that exists between the inductor and the back metallization also degrades inductor performance. These degrading effects become more important as the substrate thickness is reduced.
Hence, prior to the present invention, a need existed for a relatively small thin-film inductor having a relatively high inductance without occupying an inordinate amount of board area. The present invention satisfies these needs.

Summary of the Invention

According to the present invention, a compact inductor of the thin-film type for utilization in miniaturized electrical circuits has been developed. The inductor provides a relatively high inductance value while utilizing little substrate area.
Generally, the inductor of the present invention includes first and second thin-film spiral inductive portions with a planar substrate positioned therebetween. The first spiral inductive portion is wound in a clockwise direction, for example, and has a distal end. Conversely, the second spiral inductive portion is wound in a counterclockwise direction (opposite that of the first) and has a distal end connected to the distal end of the first inductive portion. Both spiral inductive portions have magnetic lines of force that are linked together to provide a significant mutual inductance.
Other advantages and aspects of the present invention will become apparent upon reading the following description of the drawings and detailed description of the invention.

Brief Description of the Drawings

FIGURE 1 is a greatly enlarged perspective view of a prior art inductor attached to a substrate;
FIGURE 2 is a greatly enlarged top view of a square spiral portion of an inductor in accordance with the present invention attached to the top of a substrate;
FIGURE 3 is a greatly enlarged top view (through the substrate) of the bottom of the substrate of FIGURE 2 with another square spiral portion of the inductor attached thereon;
FIGURE 4 is a cross sectional view of FIGURES 2 and 3 along plane 6-6;
FIGURE 5 is a cross sectional view of FIGURES 2 and 3 along plane 5-5;
FIGURE 6 is a greatly enlarged plan view of another embodiment of an inductor in accordance with the present invention wherein the substrate is removed and the thickness of one spiral inductive portion is reduced for illustrative purposes;
FIGURE 7 is a greatly enlarged plan view of yet another embodiment of an inductor in accordance with the present invention depicting one of two continuously arcuate spiral inductive portions mounted on a substrate; and
FIGURE 8 is a greatly enlarged plan view of the bottom of the substrate of FIGURE 7 and depicting the other spiral inductive portion of the same inductor.

Detailed Description

While this invention is susceptible of embodiments in many different forms, there is shown in the drawings and will herein be described detail preferred embodiments of the invention. The embodiments are to be considered as an exemplification of the principles of the invention and are not intended to limit the broad aspect of the invention to the embodiments illustrated.
Referring now to the drawings, and particularly to FIGURES 2-5, a split geometry inductor 10 is disclosed having square spiral inductive portions 12 and 14 deposited on opposite sides 16 and 18, respectively, of a substrate 20. Preferably, the inductive portions 12 and 14 are defined by placing conductor patterns 22 and 24, respectively, on the substrate 20 by using thin-film technology. As such, conductive material is deposited in a vacuum by electro-beam evaporation or, alternatively, sputtering. The film can consist of, for example, gold, nickel-chromium, or aluminum.
The inductive spiral portions 12,14 on each side of the substrate 20 can have many shapes besides the square spirals depicted in FIGURES 2 and 3. For example, as shown in FIGURES 7 and 8, the inductive spirals 112 and 114, respectively, can have a continuously arcuate shape that recedes from a center point similar to a clock spring.
In each of the square spiral inductive portion 12 and 14 depicted in FIGURES 2 and 3, a continuous trace 26 is organized into a plurality of integrally connected linear segments 28 that extend perpendicularly from at least one other linear segment. Accordingly, a plurality of the linear segments 28 that form each square spiral inductive portion 12,14 are in spaced parallel relationship to each other.
Each inductive spiral portion 12 and 14 has a proximal end 30 and 32, respectively, with the proximal ends connected to other circuitry (not shown), or ground, via conductive leads 34,36 mounted on side 16 of the substrate 20.
Preferably, lead 34 is integrally connected to the proximal end 30 of inductive spiral portion 12. Moreover, as shown in FIGURE 4, the proximal end 32 of inductive spiral 14 is conductively coupled to lead 36 via an integrally connected lead 38 passing through an aperture 40 in the substrate 20. The material forming lead 38 consists of the same material forming the inductive portions 12,14 on each side of the substrate 20 or, alternatively, another type of conductive material.
As shown in FIGURES 2, 3 and 5, each inductive spiral portion 12 and 14 also includes a distal end 42 and 44, respectively. The distal ends 42,44 are integrally connected together via a center aperture 46 in the substrate 20. The aperture 46 is filled with conductive material that is integrally attached to the distal ends 42,44 of the inductive portions 12,14. Preferably, the material filling aperture 46 is the same as the material forming inductive portions 12,14.
As indicated above, the surface areas on the substrate 20 supporting the spiral inductive portions 12,14 are preferably coplanar with a dielectric constant of between about 9.0 and 9.5. The substrate can consist of, for example, alumina, quarts, or sapphire.
For each of the square spiral inductive portions 12,14, inductance increases roughly with the number of turns (N) as: L ∝ N^5/3. Accordingly, because the two conductive portions 12, 14 are connected together in series, the inductor 10 has a series inductance (L_S)of ∝ 2 N^5/3 or 2L. For the circular spiral inductor of FIGURES 7 and 8, the series inductance L_S is 2N² since inductive portions 112 and 114 are connected together at ends 142 and 144, respectively.
Referring back to FIGURES 2-5, in addition to the series inductance L_S, there is also a significant mutual inductance (M) between the two inductive portions 12, 14 caused by their magnetic lines of force being linked together when carrying current. This results in the inductor 10 having a coupled inductance (L_T) of 2L + M. To maximize the mutual inductance, it is preferred that the spiral inductive portions 12, 14 are symmetrical and in alignment with respect to each other about a plane defined by the substrate 20. However, as shown in FIGURE 6, there can be differences between the spiral designs, but these differences will reduce the mutual inductance between the inductive portions 212, 214.
The mutual inductance M is also increased as the thickness (e) of the substrate 20 is reduced. In the embodiment having square spiral inductive potions 12 and 14, the coupled inductance (i.e., 2L+M) approaches 3.2L, which is 1.6 times that of just two isolated inductors. In the other embodiment having circular spiral inductive portions 112 and 114, the coupled inductance approaches 4L, which is 2 times that of just two isolated inductors.
As indicated previously, in the presence of a back metallization, the effective inductance (L_EFF ) of each inductive portion is reduced by about 35%. Accordingly, because the two inductive portions 12, 14 are connected together in series, the inductor 10 has an effective series inductance (L_S-eff) of about 2·(1-.35)L = 1.3L depending on the distance (d) between adjacent linear segments 28 and the thickness e of the substrate 20. By including the effects of the mutual inductance between the inductive portions 12, 14, the inductor 10 has a coupled inductance of

where β represent the effect of the back metallization and ranges from 2 - 3, depending on the substrate thickness e and the linear spatial extent d of the linear segments. Accordingly, the net increase of the inductance of inductor 10 over that of a conventional design is about 1.5 to 2.3 times greater.
While the coupling between the inductive portions greatly increases the effective inductance of inductor 10, this effect also provides for reducing the substrate area required to realize a given inductance. For inductor 10, the substrate area occupied is about 35 percent of the area required for placing two inductors, similar to inductive portion 12, on the top surface 16 of substrate 20. This is a significant savings in board area. Only the top surface 16 is counted in the above comparison for the inductor 10 since the bottom surface 18 is considered "free", since most of the other components arc fabricated on the top side of the substrate 20.
While the specific embodiments have been illustrated and described, numerous modifications come to mind without significantly departing from the spirit of the invention and the scope of protection is only limited by the scope of the accompanying Claims.

Claims

An inductor attached to a substrate comprising:

a first thin-film spiral portion wound in a first direction and having a distal end;

a second thin-film spiral portion wound in a second direction opposite the first direction and having a distal end connected to the distal end of the first spiral portion; and
wherein the substrate is positioned between the spiral portions.
The inductor of claim 1, wherein the first spiral portion is a square spiral inductor having a plurality of linear segments that are in parallel spaced relationship to each other.
The inductor of claim 2, wherein the first spiral inductor and the second spiral portion are symmetrical with respect to each other about a plane defined by the substrate.
The inductor of claim 2, wherein the first spiral portion is a circular spiral inductor having an elongated arcuate portion.
The inductor of claim 1, wherein the first spiral portion and the second spiral portion are symmetrical with respect to each other about a plane defined by the substrate.
The inductor of claim 1, wherein the first spiral portion has an isolated inductance of L and the first spiral portion and the second spiral portion have an inductance of between about 3.2L and 4L.
The inductor of claim 1, wherein portions of the first spiral portion and the second spiral portion are in alignment with each other.
An inductor attached to a substrate comprising:

a first thin-film square spiral inductor wound in a clockwise direction and having a plurality of linear segments that are in parallel spaced relationship to each other and a distal end;

a second thin-film square spiral inductor wound in a counterclockwise direction and having a distal end connected to the distal end of the first spiral inductor; and
wherein the substrate is positioned between the spiral inductors.
The inductor of claim 8, wherein the first inductor and the second inductor are symmetrical with respect to each other about a plane defined by the substrate.
The inductor of claim 8, wherein the first spiral inductor has an isolated inductance of L and the first spiral portion and the second spiral inductor have a coupled inductance of about 3.2L.
The inductor of claim 8, wherein linear segments of the first spiral inductor and the second spiral inductor are in parallel alignment with each other.
An inductor attached to a substrate comprising:

a first thin-film circular inductor wound in a clockwise direction and having a distal end;

a second thin-film inductor wound in a counterclockwise direction and having a distal end connected to the distal end of the first inductor; and
wherein the substrate is positioned between the spiral inductors.
The inductor of claim 12, wherein the first inductor and the second inductor are symmetrical with respect to each other about a plane defined by the substrate.
The inductor of claim 12, wherein the first inductor has an isolated inductance of L and the first inductor and the second inductor have a coupled inductance of about 4L.