# Patent

## Title
Small size and fully integrated power converter with magnetics on chip

## Abstract

An integrated circuit has a semiconductor die provided in a first IC layer and an inductor fabricated on a second IC layer. The inductor may have a winding and a magnetic core, which are oriented to conduct magnetic flux in a direction parallel to a surface of a semiconductor die. The semiconductor die may have active circuit components fabricated in a first layer of the die, provided under the inductor layer. The integrated circuit may include a flux conductor provided on a side of the die opposite the first layer. PCB connections to active elements on the semiconductor die may progress through the inductor layer as necessary.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. patent application Ser. No. 14/011,182, filed on Aug. 27, 2013, which is a continuation of U.S. patent application Ser. No. 13/273,726, filed Oct. 14, 2011, now U.S. Pat. No. 8,558,344, which claims priority to U.S. Provisional Patent Application No. 61/531,459, filed Sep. 6, 2011, each of which are hereby incorporated by reference in their entireties.


BACKGROUND
The subject matter of this application is directed to magnetic circuits implemented on an integrated circuit for providing functionality derived from magnetic circuits, e.g., applications for resistor-inductor-capacitor (commonly, “RLC”) circuits.
Traditional switched power converters require discrete inductors that are large and expensive. For many portable applications such as handsets, size and cost are critical. For a single battery system, there are usually many voltage domains so that power is optimized for each voltage domain. Such systems require efficient power conversion from the voltage of the source battery to the other voltage domains to optimize power consumption and thus extend battery life. Linear regulators can be used without the need of inductors, but they are very inefficient, especially for large step down ratios. It is desired to have switched converters for step down and step up conversions for efficient power transfer, however, discrete inductors required for the switching power converters are large and heavy, not desirable for portable applications. Also, for portable applications, it is desired that the converters have good load transient response and thus fast switching frequency. Discrete inductors are disadvantageous for such applications because they become lossy at high frequencies. It is desired to have inductors that are small, light weight and have good high frequency efficiency.
Air core inductors have limitations due, in part, to high resistance and low inductance. For example, power may be radiated back to the power plant or ground plane which may affect the electromagnetic interference (EMI). Designers must concentrate a great deal of effort to using high frequency signals and switching to mitigate the effects of EMI. EMI is proportional to frequency. Printed circuit board (PCB) designers must be concerned with EMI effects due to high currents that are generated. Radiated power is also a problem as it may interfere with other circuits that are not connected to the PCB.
In addition, when manufactured within an integrated circuit die (“IC”), air core inductors are not efficient with small inductance and high resistance, which causes users to limit power available due to thermal limits for packaging. On chip power dissipation limits the power that may be provided to an on-chip inductor. These effects can limit the applications for which air core IC inductors can be used.
The addition of magnetic cores to inductors increases winding inductance and power conversion efficiency resulting in lower inductor peak current, reduced power consumption and also reduced interference to other components. It can lead to use of lower switching frequencies among driving signals. Further, magnetic flux is more constrained by a magnetic core which limits EMI corruption to circuit components that would be co-located with the magnetic core inductor. Increased inductance per unit area also leads to high energy density and device miniaturization.
Magnetic core-based inductors have been used on integrated circuit dies with only limited success. Usually planar spiral coils are used with the addition of a single magnetic layer above or below them. The inductance enhancement from such implementation over the air core spirals is very limited, at most 100%. To achieve the inductance needed, it occupies a large die area. Its size mismatch with power switching circuits makes the integration not economically viable. Magnetic core-based inductors tend to occupy large areas when laid out on integrated circuit die, which interferes with design attempts to make smaller chips. Such layout issues become exacerbated when designers attempt to find configurations that allow such integrated circuits to be mounted on larger components, for example, a printed circuit board (PCB). No known inductor configuration adequately meets these design needs.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of an integrated circuit according to an embodiment of the present invention.
FIG. 2 illustrates an exemplary inductor layer of an integrated circuit according to an embodiment of the present invention.
FIG. 3 is a diagram of an exemplary circuit that may find application with an integrated circuit according to an embodiment of the present invention.
FIG. 4 is a sectional view of an integrated circuit according to an embodiment of the present invention.
FIGS. 5-9B illustrate configurations of inductors and cores according to various embodiments of the present invention.
FIG. 10 is a sectional view of an integrated circuit according to another embodiment of the present invention.



DETAILED DESCRIPTION
Embodiments of the present invention may provide for an integrated circuit having a semiconductor die provided in a first IC layer and an inductor fabricated on a second IC layer. The inductor may have a winding and a magnetic core, which are oriented to conduct magnetic flux in a direction parallel to a surface of a semiconductor die. The semiconductor die may have active circuit components fabricated in a first layer of the die, provided under the inductor layer. The integrated circuit may include a flux conductor provided on a side of the die opposite the first layer. The integrated circuit also may include a plurality of chip connectors, such as solder balls, to mount the integrated circuit to a larger structure, for example a PCB, in a chip scale package (CSP) configuration to minimize PCB area needed for the devices, thus make them more compact and light weight. PCB connections to active elements on the semiconductor die may progress through the inductor layer as necessary.
The magnetic core may be a solid bar with the winding provided around it. Alternatively, the magnetic core may be formed from a plurality of magnetic bars separated by dielectric spacers with the winding provided around the collection of bars. In a further embodiment, the core may be provided as a pair of cores with the winding provided around the first core in a first sub-winding then extending to and winding around the second core.
A single bar core has the most area efficiency as a pair of cores on the same surface will occupy larger area, but there is concern on EMI due to leakage flux with single bar core. A magnetic layer on the opposite of the die surface where the inductors are fabricated can help to close the flux loop without the need for extra surface area. This added magnetic layer needs not to be patterned so it can simply be a ferrite loaded epoxy layer or other films with magnetic permeability larger than one deposited or coated.
In another embodiment, an integrated circuit may have a semiconductor die provided in a first IC layer and an inductor fabricated on a second IC layer. The inductor may have a winding and a magnetic core, which are oriented to conduct magnetic flux in a direction parallel to a surface of a semiconductor die. The semiconductor die may have active circuit components fabricated in a layer of the die which provided on an opposite side from a side on which the inductor layer is mounted. The integrated circuit may include a plurality of chip connectors, such as solder balls, to mount the integrated circuit to a larger structure, for example a PCB, in a chip scale package (CSP) configuration to minimize PCB area needed for the devices, thus make them more compact and light weight.
FIG. 1 illustrates an integrated circuit 100 according to an embodiment of the present invention. The integrated circuit 100 may include a semiconductor die 110 having an inductor layer 112 formed adjacent to a face of the semiconductor die 110 on which active devices are fabricated (shown as layer 114). The integrated circuit 100 may be mounted on a PCB 120 via an interconnect such as a plurality of solder balls 130.1-130.n (referred generally as interconnect 130). The interconnect 130 may be covered by a protective encapsulant 140 that can enhance thermal conduction between the die 110 and PCB 120. The integrated circuit 100 may include a flux conductor 150 provided on a second surface of the die 110 away from the active layer 114. The flux conductor 150 may be provided as a film of magnetic material sputtered onto the second surface of the die 110.
Inductor(s) of the inductor layer 112 and component(s) of the active device layer 114 each will be configured for specific applications of the integrated circuit. The semiconductor die 110 may have dimensions sized to accommodate the dimensions of the inductor layer 112 and active device layer 114. Interconnect structures 130.1-130.n may be provided within a spatial area defined by a perimeter of the semiconductor die 110. Thus, the interconnect 130 need not expand the “footprint” of the die 110 when the die 110 is mounted on the PCB 120.
FIG. 1 is a schematic view of the components of the integrated circuit 100. Relationships between the die 110, interconnect 130 and the various layers 112, 114 are not shown to scale.
FIG. 2 illustrates a view of an exemplary inductor layer 112, viewed from a PCB layer. The inductor layer 112 may include a pair of inductors 150, 160 each having a winding 152, 162 provided in a spiral around a respective magnetic core 155, 165. Each winding is shown having a solder ball 130.1, 130.3 providing a first electrical connections for the inductor winding 152, 162 and a through silicon via (TSV) representing an electrical connection between the respective winding 152, 162 and a component of the active layer.
The inductor layer 112 is illustrated as including traces 172-178 connected to other solder balls 130.2 and 130.4-130.n for connection to circuit components of the active layer (not shown). These traces may penetrate through the inductor layer 112 without electrically engaging with either the winding 150 or the magnetic core 152. The traces may carry supply voltages (e.g., VDD or ground) or information signals for components of the active layer.
The example of FIG. 2 illustrates a pair of inductors in the inductor layer 112. Of course, the inductor layer may have fewer inductors (one) or more inductors as design needs require.
The example of FIG. 2 illustrates the inductor has having a linear magnetic core 152, 162. This is but one available configuration. Other configurations are provided in the ensuing discussion, including multi-segment cores, toroidal cores and cores with voids.
Moreover, the example of FIG. 2 illustrates solder ball connections 130.1, 130.3 to provide external contacts to the inductors—to connect the inductor to the PCB. Again, this is but an example. Depending on circuit requirements, inductors may be connected solely to components of the active layer instead of to the PCB. Alternatively, one inductor may be connected solely to the active layer and another inductor may be connected solely to the PCB as design needs dictate.
FIG. 3 illustrates an exemplary circuit 300 suitable for application with the inductors of the present invention. The circuit as illustrated is a buck converter but, of course, different circuit systems may find application with the present invention. The circuit of FIG. 3 includes switching transistors Mp and MN, capacitors COUT and Cc, an inductor L, resistors RC, RL, R1 and R2, various amplifiers, a comparator CMP and a controller CTRL. When the circuit 300 is manufactured as an integrated circuit according to embodiments of the present invention, the inductor L may be fabricated in an inductor layer 112 (FIG. 1) and the remaining components may be fabricated in an active layer 114 (also FIG. 1).
FIG. 4 illustrates a sectional view of the integrated circuit of FIGS. 1 and 2. FIG. 4 is a schematic illustration only; components are not drawn to scale. As illustrated, the integrated circuit 400 may include a semiconductor die 410 having a layer for active elements 414 and a dielectric layer 416 separating an inductor layer 420 from the die 410.
The inductor layer 420 may include an inductor winding 430 having a plurality of spiral turns, each turn formed by a pair of traces 432, 434 formed on two parallel sub-layers of the inductor layer 420, and a pair of winding posts 436, 438 extending upwardly through the sub-layers to connect to the traces 432, 434. When the inductor layer 420 is manufactured in an integrated circuit, the winding 430 may include several sets of posts 436, 438 and traces 432, 434 arranged in a multi-turn spiral as shown in FIG. 2. The inductor may include a magnetic core 440 provided in a center area formed by the inductor spiral. Dielectric material 442 may be provided within the inductor layer 420 to electrically isolate all turns of the winding 430 from the magnetic core 440.
The inductor may be encased in various layers of dielectric insulating material 452-458 to prevent electrical engagement between the inductor winding 430 and any other circuit component.
Since FIG. 2 illustrates a pair of inductors, FIG. 4 illustrates structures 482-488 of a second winding 480 and a second magnetic core 490. FIG. 4 also illustrates connection between a winding 480 and an external interconnect formed by a solder ball 460 and an under bump metallization layer 462. Again, such interconnect structures are appropriate only if design requirements call for electrical connection between the winding 480 and a component external to the integrated circuit such as on a PCB.
FIG. 4 further illustrates a connection 470 formed between a winding 430 and the active elements layer 414 formed by a post 472, a metal trace 474 and a via 476. The post 472 may be connected to a trace 434 of the winding 430 and may extend through dielectric layer 452 toward the die 410. The trace 474 may provide an electrical path between the inductor winding 430 and a via 476. The via 476 may extend through the dielectric layer 416 to an appropriate position of the active element layer 414. Thus, the connection 470 may provide electrical connection between the inductor and a component in the active element layer 414.
FIG. 4 illustrates a layer of a flux conductor 495 provided on a second surface of the die 410 away from the active elements 414. The flux conductor 495 may be fabricated from the same material as used for the magnetic core(s) 440, 490.
The orientation of the magnetic core 440 and winding 430 allows the inductors to be manufactured according to conventional integrated circuit manufacturing techniques. Using semiconductor masks and photolithography, the windings 430, 480 dielectrics 442 and 454-458 and magnetic cores 440, 490 may be built up in multiple layers of material depositions. In one example, winding traces 434, 484 that form a rear surface of the windings 430, 480 may be built up in a first stage of manufacture on top of dielectrics 452 that isolate the inductors from the die 410 and from any die-oriented interconnect traces 474-474. Thereafter, a dielectric layer 454 may be applied to fill in interstitial regions between the traces 434, 484 and also to cover them. In another stage, materials representing the magnetic cores 440, 490 may be laid upon the first dielectric layer 452. Additionally, materials representing the winding posts 436, 438, 486, 488 may be built up from appropriate connection points of the rear surface traces 434, 484 to build lateral sides of the respective windings 430, 480. An additional layer 456 of dielectric material may be applied to encase the magnetic cores 440, 490 and winding posts 436, 438, 486, 488 in the dielectric. Further metallic material may be deposited on the dielectric-covered front side of the magnetic cores 440, 490 to build up front traces 432, 484 to complete the windings 430, 480. Thereafter a final layer 458 of dielectric may be deposited on the windings 430, 480 with accommodation made for any interconnect structures 460, 462 that are needed.
In an embodiment, the dielectric materials may be high dielectric breakdown materials such as polyimide, silicon dioxide, silicon nitride and the like. The magnetic core layers 440, 490 and flux conductor layer 495 can be made of materials of high permeability such as CoTaZr (cobalt tantalum zirconium) NiFe (nickel ferrite) and FeCo (ferrite cobalt)-based alloys. The windings and metal interconnect structures may be formed of an appropriate conductive metal such as gold or copper.
FIG. 5 illustrates a configuration of an on-chip inductor with a magnetic core according to another embodiment of the present invention. In this configuration, a core 510 is provided in a segmented, linear configuration. The magnetic core 510 may be formed as a plurality of core bars 510.1-510.n have having a elongated direction parallel to the direction of magnetic flux that will be induced by currents flowing through the winding 520. Voids 515 may occur between the magnetic bars 510.1-510.n and may be filled by a dielectric. The voids may be as narrow as practical (say, 1-10 micrometers) to minimize the reduction of the total core cross-sectional area and yet isolate each of the bars from its neighbor. The voids 515 may alter the shape anisotropy of the magnetic core 510 and provide enhanced permeability. The voids 515 also may limit the generation and transmission of eddy currents in the magnetic core 510 due to magnetic flux.
The winding 520 may spiral around the entire core structure, including all bars 510.1-510.n and voids. It may have input and output terminals 522, 524 to connect the winding to other circuit structures.
The inductor may be mounted within a semiconductor substrate such that conductivity of magnetic flux carried by the core extends in a direction parallel to a surface of the substrate.
FIG. 6 illustrates another configuration of an on-chip inductor according to an embodiment of the present invention. In this embodiment, the inductor 600 includes a multi-element core 610 and a winding 620. The core 610 is provided as two linear segments 610A and 610B, each made of a magnetic material.
The winding 620 may extend between input and output terminals 622, 624. The winding 620 may spiral around the first core element 610A, then extend to and spiral around the second core element 610B. The orientation of the winding 620 may be reversed between the first and second core elements 610A, 610B to reduce flux leakage from the inductor 600. In this manner, a driving current may induce flux in the two core elements having opposite direction from each other. This configuration helps provide a flux return path, and reduce flux leakage into surrounding components and EMI radiation. The inductor 620 may be mounted within a semiconductor substrate such that conductivity of magnetic flux carried by the core extends in a direction parallel to a surface of the substrate.
During manufacture, the hard axis of the magnetic core material may be controlled to align to the direction of magnetic flux that will be generated by the inductor during operation. Aligning the hard axis with the direction of flux is expected to reduce switching losses that may occur during operation of the inductor.
The embodiment of FIG. 6 may be used in IC applications that do not employ flux conductors on rear surfaces of a semiconductor die.
FIG. 7 illustrates an on-chip inductor 700 with a magnetic core according to another embodiment of the present invention. The inductor 700 may include a magnetic core 710 and a winding 720. The winding 720 may be provided as a pair of sub-windings 720A, 720B extending between input and output terminals 722, 724. The magnetic core 710 may formed as a closed loop extending between centers of each of the sub-windings 720A, 720B. Magnetic flux may travel circularly through the ring-shaped core. During manufacture, the anisotropic direction may be controlled such that the easy axis is along the Y direction and hard axis is along the X direction. Flux generated by the windings may travel easily with the core along the hard axis (X direction).
The flux traveling along the easy axis (Y-direction) can be lossy. Thus, flux may tend to escape through the top of the core instead of following the shape of the magnetic core 710 along the Y axis. In this sense, the embodiment may exhibit more losses than the embodiment of FIG. 6.
The inductor may be mounted within a semiconductor substrate such that conductivity of magnetic flux carried by the core extends in a direction parallel to a surface of the substrate.
In a first embodiment, the magnetic core 710 may be a solid magnetic core. In another embodiment, the magnetic core may have voids 716 provided in a volume formed by the sub-windings 720A, 720B. The voids 716 may be filled with insulating material or a dielectric material that may change anisotropy and enhance magnetic permeability.
The embodiment of FIG. 7 may be used in IC applications that do not employ flux conductors on rear surfaces of a semiconductor die.
FIG. 8 illustrates another configuration of an inductor 800 according to an embodiment of the present invention. This embodiment is similar to the FIG. 7 embodiment but the shape of the core is provided as a generally hexagonal shape rather than a rectangular shape as illustrated in FIG. 7. The configuration of the core may be modified to include any number of closed-loop shapes including circular configurations, octagonal configurations and the like. The embodiment of FIG. 8 may be used in IC applications that do not employ flux conductors on rear surfaces of a semiconductor die.
FIGS. 9A and 9B illustrate another configuration of an inductor 900 according to an embodiment of the present invention. The inductor 900 may include a magnetic core 910 and a winding 920. In this embodiment, the core may be provided as multi-element core in which the winding 920 spirals around a center core element 910B, and edge core elements 910A, 910C may be provided external to the winding. Each of the core elements 910A, 910B, 910C may be provided as solid core elements (not shown) or may be provided with voids 912. The inductor may be mounted within a semiconductor substrate such that conductivity of magnetic flux carried by the core extends in a direction parallel to a surface of the substrate.
During operation, flux may be developed in the center core element 910B in response to a driving current. The edge core elements 910A, 910C may develop a return path for the flux, which helps reduce flux leakage outside the core.
The embodiment of FIGS. 9A and 9B may be used in IC applications that do not employ flux conductors on rear surfaces of a semiconductor die.
FIG. 10 illustrates layout of an integrated circuit with an integrated inductor layer according to another embodiment of the present invention. In the FIG. 10 embodiment, active elements are provided on a face of a semiconductor die facing away from the inductor layer. FIG. 10 is a schematic illustration only; components are not drawn to scale.
As illustrated, the integrated circuit 1000 may include a semiconductor die 1010 having a layer for active elements 1012 on a first face of the die 1010 and a dielectric layer 1014 provided on a second face of the die 1010. The inductor assembly 1020 may be provided on the second face of the die 1010 and may include inductor winding(s) 1030, 1080 having a plurality of spiral turns. The integrated circuit may include a variety of through silicon vias (TSVs) to facilitate electrical connection between inductor(s) and components of the active element layer 1012 as circuit needs require.
With the inductor assembly 1020, windings 1030 may include a plurality of turns formed by traces 1032, 1034 formed on two parallel layers of the inductor assembly 1020 and winding posts 1036, 1038 extending upwardly between the layers to connect to the traces 1032, 1034. Inductors further may include a magnetic core 1040 provided in a center of the windings and a dielectric material 1042 electrically isolating all turns of the winding 1030 from the magnetic core 1040. FIG. 10 also illustrates structures 1082-1088 of a second winding 1080 and a second magnetic core 1090.
The inductor assembly 1020 may be encased in various layers of dielectric insulating material 1052-1058 to prevent electrical engagement between the inductor winding 1030 and any other circuit component.
FIG. 10 also illustrates connection between a winding 1080 and an external interconnect formed by a solder ball 1060 and an under bump metallization layer 1062. Again, such interconnect structures are appropriate only if design requirements call for electrical connection between the inductor 1020 and a component external to the integrated circuit such as on a PCB.
FIG. 10 further illustrates a connection 1070 formed between a winding 1030 and the active elements layer 1012 formed by a post 1072 extending through dielectric layer 1052 toward the die 1010 and a metal trace 1074 providing routing between the inductor winding 1030 to a through silicon via (TSV) 1076. The TSV 1076 may extend through the semiconductor die 1010 to an appropriate position of the active element layer 1012. A metal may occupy the TSV 1070 to provide electrical connectivity between the winding 1030 and a component of the active elements layer 1012.
The orientation of the magnetic core 1040 and winding 1030 allows the inductors to be manufactured according to conventional integrated circuit manufacturing techniques. Using semiconductor masks and photolithography, the windings 1030, 1080 dielectrics 1042 and 1054-1058 and magnetic cores 1040, 1090 may be built up in multiple layers of material depositions. In one example, winding traces 1034, 1084 that form a rear surface of the windings 1030, 1080 may be built up in a first stage of manufacture on top of dielectrics 1052 that isolate the inductors from the die 1010 and from any die-oriented interconnect traces 1074-1074. Application of a dielectric layer 1054 may occur in a subsequent manufacturing stage to fill in interstitial regions between the traces 1034, 1084 and also to cover them. In another stage, materials representing the magnetic cores 1040, 1090 may be laid upon the first dielectric layer 1052. Additionally, materials representing the winding posts 1036, 1038, 1086, 1088 may be built up from appropriate connection points of the rear surface traces 1034, 1084 to build lateral sides of the respective windings 1030, 1080. An additional layer 1056 of dielectric material may be applied to encase the magnetic cores 1040, 1090 and winding posts 1036, 1038, 1086, 1088. Further, metallic material may be deposited on the dielectric-covered front side of the magnetic cores 1040, 1090 to build up front traces 1032, 1084 to complete the windings 1030, 1080. Thereafter a final layer 1058 of dielectric may be deposited on the windings 1030, 1080 with accommodation made for any interconnect structures 1050, 1052 that are needed.
In an embodiment, the dielectric materials may be high dielectric breakdown materials such as polyimide, silicon dioxide, silicon nitride and the like. The magnetic core layers 1040, 1090 and flux conductor layer 1095 can be made of materials of high permeability such as CoTaZr (cobalt tantalum zirconium) NiFe (nickel ferrite) and FeCo (ferrite cobalt)-based alloys. Finally, the windings and metal interconnect structures may be formed of an appropriate conductive metal such as gold or copper.
Several embodiments of the invention are specifically illustrated and/or described herein. However, it will be appreciated that modifications and variations of the invention are covered by the above teachings and within the purview of the appended claims without departing from the spirit and intended scope of the invention.

## Claims

1. A microfabricated inductor with a magnetic core for an integrated circuit, comprising:
a semiconductor die;
a multi-element magnetic core formed on a surface of the semiconductor die and comprising at least three magnetic core bars, there being a plurality of voids disposed between adjacent magnetic core bars of the at least three magnetic core bars; and
a conductive winding coiled around the multi-element magnetic core such that the at least three magnetic core bars are all within a single loop of the conductive winding.

2. The microfabricated inductor of claim 1, wherein the at least three magnetic core bars are parallel to each other.

3. The microfabricated inductor of claim 1, wherein at least one void of the plurality of voids is filled with a dielectric material.

4. The microfabricated inductor of claim 1, wherein the surface of the semiconductor die is a first surface, and the microfabricated inductor further comprises a flux conductor formed on a second surface of the semiconductor die, wherein the flux conductor is magnetically coupled to the multi-element magnetic core.

5. The microfabricated inductor of claim 1, wherein the multi-element magnetic core further comprises a plurality of magnetic core portions connecting the at least three magnetic core bars to each other.

6. The microfabricated inductor of claim 1, wherein a first magnetic core bar of the at least three magnetic core bars comprises a single solid layer of magnetic material.

7. The microfabricated inductor of claim 1, wherein the multi-element magnetic core comprises a magnetic material having a hard axis parallel with a direction of a magnetic flux produced in the at least three magnetic core bars.

8. The microfabricated inductor of claim 1, further comprising active circuit components formed on the semiconductor die and coupled to the multi-element magnetic core.

9. The microfabricated inductor of claim 8, wherein the active circuit components include at least one transistor.

10. A microfabricated converter circuit having an inductor with a magnetic core, comprising:
a semiconductor die in which at least one component of the converter circuit is formed;
a multi-element magnetic core, having first and second magnetic core elements, formed on a surface of the semiconductor die and comprising a magnetic material having a hard axis; and
a conductive winding coiled around the first and second magnetic core elements, coupled to the at least one component of the converter circuit, and configured to produce a magnetic flux in the first magnetic core element parallel to the hard axis.

11. The microfabricated converter circuit of claim 10, wherein at least one of the first or second magnetic core elements is formed from a single solid layer of the magnetic material.

12. The microfabricated converter circuit of claim 10, wherein the multi-element magnetic core further comprises one or more magnetic core portions connecting the first magnetic core element with the second magnetic core element, the one or more magnetic core portions extending along a second direction different from the first direction and parallel to an easy axis of the magnetic material.

13. A microfabricated inductor with a magnetic core for an integrated circuit, comprising:
a semiconductor die;
a multi-element magnetic core formed on a surface of the semiconductor die and comprising at least three magnetic core bars and a plurality of voids disposed between adjacent magnetic core bars of the at least three magnetic core bars; and
a conductive winding coiled around the multi-element magnetic core such that the at least three magnetic core bars are all within a single loop of the conductive winding;
wherein the conductive winding comprises a first plurality of conductive traces adjacent a first side of the multi-element magnetic core, a second plurality of conductive traces adjacent a second side, opposite the first side, of the multi-element magnetic core, and a plurality of conductive posts connecting the first and second pluralities of conductive traces.

14. The microfabricated inductor of claim 13, wherein at least one magnetic core bar of the at least three magnetic core bars comprises a single solid layer of magnetic material.

15. The microfabricated inductor of claim 13, wherein the plurality of conductive posts are elongated along a direction perpendicular to the surface of the semiconductor die on which the multi-element magnetic core is formed.

16. The microfabricated inductor of claim 13, wherein at least one void of the plurality of voids is filled with a dielectric material.

17. A microfabricated converter circuit having an inductor with a magnetic core, comprising:
a semiconductor die in which at least one component of the converter circuit is formed; and
means for conducting a continuous current at least partially between first and second separate magnetic core elements and a surface of the semiconductor die, the first and second magnetic core elements occupying different areas of the semiconductor die;
wherein the first magnetic core element and the second magnetic core element include a magnetic material having a hard axis parallel with directions of portions of first and second magnetic fluxes produced in the first and second magnetic core elements.

18. The microfabricated converter circuit of claim 17, wherein the first and second magnetic core elements occupy parallel first and second areas of the semiconductor die.

19. The microfabricated converter circuit of claim 17, wherein the means for conducting produces the first and second magnetic fluxes that at least partially cancel each other outside of the first and second magnetic core elements.

20. The microfabricated converter circuit of claim 17, wherein at least one of the first or second magnetic core elements includes a plurality of solid layers of the magnetic material separated by a plurality of dielectric spacers.

21. A microfabricated inductor with a magnetic core for an integrated circuit, comprising:
a semiconductor die;
a multi-element magnetic core formed on a surface of the semiconductor die and comprising at least three magnetic core bars and a plurality of voids disposed between adjacent magnetic core bars of the at least three magnetic core bars, wherein at least one void of the plurality of voids is configured to separate a first magnetic core bar of the at least three magnetic core bars from a second magnetic core bar of the at least three magnetic core bars by a distance that is between 1 μm and 10 μm; and
a conductive winding coiled around the magnetic core such that the at least three magnetic core bars are all within a single loop of the conductive winding.

22. The microfabricated inductor of claim 21, wherein at least one void of the plurality of voids is filled with a dielectric material.

23. A method of operating a converter circuit having an inductor with a magnetic core, comprising:
generating an input signal using a microfabricated converter component disposed in a semiconductor die; and
converting the input signal to an output signal on the semiconductor die at least in part by conducting a current in an inductor formed on a surface of the semiconductor die, the inductor including a continuous conductive winding formed at least partially between first and second separate magnetic core elements and the surface of the semiconductor die, the first and second magnetic core elements occupying different areas of the semiconductor die;
wherein the first magnetic core element and the second magnetic core element include a magnetic material having a hard axis parallel with directions of portions of first and second magnetic fluxes produced in the first and second magnetic core elements.

24. The method of claim 23, wherein the first and second magnetic core elements occupy parallel first and second areas of the semiconductor die.

25. The method of claim 23, wherein conducting the current comprises conducting current through a portion of the winding about the first magnetic core element to produce the first magnetic flux and conducting current through a portion of the winding about the second magnetic core element to produce the second magnetic flux, and wherein the first magnetic flux at least partially cancels the second magnetic flux outside of the first and second magnetic core elements.

26. The method of claim 23, wherein at least one of the first or second magnetic core elements includes a plurality of solid layers of magnetic material separated by a plurality of dielectric spacers.