WO2010001336A1

WO2010001336A1 - Inductors and methods of manufacture thereof

Info

Publication number: WO2010001336A1
Application number: PCT/IB2009/052832
Authority: WO
Inventors: Johannes Wilhelmus Weekamp; Hendrik J. Bergveld; Eric Cornelis Egbertus Van Grunsven; Yann Pierre Roger Lamy; Franciscus Adrianus Cornelis Maria Schoofs
Original assignee: Nxp B.V.
Priority date: 2008-07-01
Filing date: 2009-06-30
Publication date: 2010-01-07

Abstract

An inductor core (6) is described along with methods for its manufacture. The core comprises a body (20) formed of electrically insulating magnetic material,and a stack (22) of layers over the body, formed alternately of electrically conductive magnetic material (24) and insulating material(26), wherein the magnetic saturation level of the magnetic material of the body is lower than that of the magnetic layer material.Thus, two magnetic paths are provided in parallel, one through the stack of layers and one through the body material. This results in a higher inductance at low currents and maintenance of a lower inductance at higher currents. The core (6) is particularly suitable for use in inductors integrated inside a device package for example in an integrated DC/DC converter.

Description

DESCRIPTION

INDUCTORS AND METHODS OF MANUFACTURE THEREOF

The present invention relates to inductors and methods for manufacturing inductors. More particularly, it relates to inductor configurations suitable for integration inside a package together with active electronics.

With the increasing sophistication of mobile devices, the number of conversions of one DC voltage to another required in a device is increasing. Switched-mode DC/DC converters use either capacitors or inductors to temporarily store energy that is transferred from the device battery to the load. Such converters are particularly preferred for larger voltage differences, as their efficiency is then superior to that of linear regulators.

Inductive converters, using an inductor for temporal energy storage, have an advantage over capacitive converters, using capacitors for temporal energy storage, in terms of the number of passive components needed to ensure proper control of the output voltage at high efficiency over load and line variations. With only one inductor and one output buffer capacitor, the output voltage can be controlled over a range of input voltages and output loads by simply changing the duty cycle at which the power switches are addressed. For this reason, inductive converters are used in a wide range of applications, including many battery-powered devices. As the number of voltage conversions needed in mobile devices increases, the volume occupied by known implementations increases dramatically. Therefore, there is a need for small form-factor inductive DC/DC converters, where the inductor is implemented inside the package of the converter. As energy storage takes up space, the amount of energy that can be stored in a package-integrated inductor per switching cycle will be lower. In order to transfer the same amount of power as a DC/DC converter with external passive components, the switching frequency of the integrated inductive DC/DC converter must be increased. Within the small volume available, it is also desirable to achieve the largest inductance possible with the lowest series resistance.

Most existing package-integrated inductors use core material to increase the inductance and to guide the flux lines to improve electromagnetic interference (EMI) behavior. Several magnetic materials have been used to form the core, each one having advantages and disadvantages. A general characteristic of magnetic materials used for power management is that they are soft-magnetic. This implies a small area of the BH loop and low values for the coercivity and retentivity.

When selecting the magnetic material to build an integrated inductor, the following material characteristics are preferred:

• A high B_sat value. The higher the B_sat value, the higher the winding current at which the core material saturates. When core saturation occurs, the inductance of the inductor will drop quickly to that of an equivalent air coil, leading to additional losses, so the saturation current should be higher than the maximum coil current occurring in the DC/DC converter.

• A high resistivity p. The AC currents through the inductor winding will lead to eddy currents in other conducting materials near the winding. When the resistivity of the core material is high, eddy currents will be low and therefore losses will remain low. This is desirable to obtain maximum efficiency for the integrated DC/DC converter.

• A high μ'.f bandwidth. As the inductance of an integrated inductor will always be relatively small, the inductor will be used at higher switching frequencies than for external counterparts. Depending on the material used, the initial permeability will roll off at elevated frequencies. The higher the μ'.f bandwidth is, the higher the frequency at which the material sustains its permeability and therefore the higher the switching frequency at which the inductor can be used. Therefore, an inductor formed with a material with a higher μ'.f bandwidth will enable a higher output power at the same volume than a material with a lower μ'.f bandwidth.

• A high f.B_max product. The inductor current in a DC/DC converter comprises a DC current and a current ripple. This means that during a switching cycle, the x-axis of the BH loop shows a certain excursion, which translates into a magnetisation excursion on the vertical axis. The area of the so-called minor loop that is cycled once per switching cycle represents an amount of energy lost which translates into power loss when multiplied with the switching frequency. Bmax is the peak magnetisation excursion. Data graphs of core material for power management applications list the f. B_max values for a certain dissipated power in the core per unit volume. This dissipated power translates into a certain core material temperature based on its thermal resistivity. Since for integrated power management, the inductors will be relatively small, the current ripple will be large, which implies a large B_max. At the same time, to transfer certain power with the DC/DC converter, the switching frequency will be high, therefore, a high f.B_max product is desirable for a given power loss.

Some of the characteristics mentioned above of applied soft-magnetic materials are related as follows: B_8at ^« μ'.f

This implies that a material with a high B_sat will have a high μ'.f bandwidth. The magnetisation B_sat cannot exceed the maximum value found in nature, 2.4T. Moreover, high B_sat tend to have a low resistivity. Most of the Fe- or Co- materials have a B_sat above 2.0T, but their resistivities do not exceed a few μ Ω.cm. Furthermore, they do not exhibit soft-magnetic properties in their bulk form which makes them unsuitable for high-frequency applications.

Soft-magnetic oxide materials have been considered as ferhtes for example, NiZn ferrite, as they offer high resistivity and good soft-magnetic properties. The drawbacks are a relatively low B_sat, low μ'.f bandwidth, and a low integration capability in devices. "High-efficiency DC/DC converter chip size module with integrated soft ferrite" by Z. Hayashi et al, IEEE Trans. Magn., Vol. 39, No. 5, September 2003, pp 3068-3072 describes a solenoid inductor based on using 525 μm thick NiZn ferrite core material.

The present invention provides an inductor core comprising a body formed of electrically insulating magnetic material; and a stack of layers over the body, comprising alternate layers of electrically conductive magnetic material and insulating material, wherein the magnetic saturation level of the magnetic material of the body is lower than that of the magnetic layer material. The inductor core is formed of one or more magnetic thin films laminated with an insulator on top of a solid magnetic body. By placing the two magnetic paths (one through the laminated layer(s) and one through the solid body material) in parallel, the resulting inductance of an inductor including this core will be high for low currents through its winding. When the current increases, the inductance will drop to a value associated with the laminated material, rather than to an inductance associated with an air coil with the same winding, leading to additional losses as will be the case with only a solid core formed from a single material present. In this way the saturation behaviour is enhanced and the inductance does not drop as fast as it would otherwise do. These characteristics are particularly beneficial for the design of integrated DC/DC converters.

This inductor core configuration is suitable for use in package-integrated inductors, and is relatively simple and cost-effective to manufacture.

The core may be fabricated independently ready for incorporation in a package-integrated micro-inductor that can be combined with an active die inside a power-supply system-in-package product.

The effective permeability of the core may be adjusted by modifying the materials, number of layers and/or dimensions which allows the inductance of the resulting inductor to be determined without changing the overall design of its winding (such as the number of turns included).

The stack of layers may be provided over one side of the body. Alternatively, the stack may extend around the circumference of the body. This may be achieved for example by using barrel sputtering to deposit each layer in turn.

The core may have a substantially square cross-section for example. In a preferred embodiment, opposite sides of the core taper towards each other when viewed in transverse cross-section with respect to the planes of the layers. More particularly, the cross-section may resemble a trapezium. Where bond wires are used to form the winding, this configuration allows the wires to be placed closer to the core material owing to the larger internal angles at the upper corners of the core. This serves to reduce stray flux which would otherwise be present around the turns, and so improve EMI behaviour.

In some embodiments, the insulating material layers of the stack may be formed of magnetic material, such as magnetic oxide. This enhances the magnetic properties of the core in comparison to non-magnetic insulation layers and thereby more effectively makes use of the volume occupied by the core.

Preferably, the magnetic saturation level of the body material may be around 0.5T or less. The body may be formed of a ferrite material for example, such as NiZn-ferrite.

By using a magnetic material such as ferrite for the body of the core, the body not only acts as a carrier for the laminated thin films, but also enhances the magnetic properties of the inductor core. Compared to using glass or silicon as a carrier, the inductance is significantly enhanced at low currents for a given core volume. At low current, the magnetic carrier will not saturate, and will offer a significant increase in flux area, increasing the inductance. At higher currents, the body will start to saturate and the inductor behaviour will tend towards the behaviour of an inductor using glass, silicon or any other nonmagnetic carrier material. Therefore, using magnetic material as the carrier enhances the inductance at low currents by offering a parallel flux path.

In preferred embodiments, a magnetic body material is selected which has a high resistivity, of the order of 10³, or more preferably 10⁵ Ω.m or more. AC currents through the winding of an inductor including the core will lead to eddy currents in conducting material of the core. When the resistivity of the core body material is high, eddy currents will be low and therefore the associated losses will remain low, thereby improving the efficiency of devices incorporating the inductor. Ferrite material for example may be used as the body material to enhance the inductance at low currents without a penalty in terms of high losses at high frequencies as it may provide a high resistivity.

The magnetic saturation level of the material used to form the magnetic layers of the stack is higher than that of the material of the body, and preferably around 1T or more. Its resistivity may be lower than that for the body, for example around 100-200 μΩ.cm. They may be formed of permalloy thin film, such as polycrystalline NiFe permalloy, or amorphous thin films such as CoNbZr.

The present invention further provides a method of manufacturing an inductor core as described herein, comprising the steps of:

(a) providing a substrate formed of electrically conductive magnetic material;

(b) depositing a stack of layers over the substrate, comprising alternate layers of electrically conductive magnetic material and insulating material; and

(c) patterning the stack of layers and substrate to define an inductor core of a desired shape. In a preferred embodiment, step (c) includes the steps of:

(c)(i) providing a layer of masking material over the stack of layers; (c)(ii) patterning the layer of masking material to form a mask; and (c)(iii) patterning the stack of layers and substrate using the mask to define an inductor core of a desired shape. The stack of layers may be pattered in steps (c)(iii) by a physical erosion process such as sandblasting. Such processes will tend to form tapered sides on the inductor core which may be advantageous as discussed above.

In some cases the mask may be retained on top of the core to protect the underlying structure. Also, if a non-conducting masking material is used, this prevents a short circuit between the windings of the inductor and the top layer of the stack of layers. An embodiment of the invention will now be described by way of example and with reference to the accompanying schematic drawings, wherein:

Figure 1 is a perspective view of a toroidal package-integrated micro- inductor;

Figures 2 to 4 are side cross-sectional views at successive stages in the manufacture of an inductor including an inductor core embodying the invention; and

Figure 5 is a schematic graph of inductance against current for an inductor embodying the invention in comparison with other configurations.

It should be noted that the figures are diagrammatic and not drawn to scale. Relative dimensions and proportions of parts of these figures have been shown exaggerated or reduced in size, for the sake of clarity and convenience in the drawings. The same reference signs are generally used to refer to corresponding or similar features in modified and different embodiments.

An example of an inductor configuration which may include a core embodying the present invention is depicted in Figure 1. The inductor 2 is a toroidal micro-inductor suitable for integration inside a package alongside active electronics, such as the power stage, driver and controller forming an integrated

DC/DC converter. The winding 4 around the core 6 consists of copper tracks 8 beneath the core and bond wires 10 running from one track to the next over the top of the core 6.

Fabrication of an inductor core in accordance with an embodiment of the invention will now be described with reference to Figures 2 to 4.

Initially, a ferrite carrier layer or plate 20 is provided. A stack 22 of layers is then provided on the plate 20. For example, laminated thin films may be placed on top of the plate 20, alternating electrically conductive magnetic material layers 24 and insulating material layers 26. Alternatively, the laminated films may be provided by means of a deposition process to grow successive layers on top of the ferrite carrier 20. For example, physical vapour deposition using two targets (such as CoNbZr and SiO₂) may be employed. Next a layer 28 of masking material is provided on top of the stack.

Preferably, an insulating material is used and masked material remaining after patterning is included in the finished device to prevent a short circuit between the inductor windings and the uppermost thin-film layer of the core.

The layer of masking material is then patterned to define a mask 30, by means of photolithography. The mask is employed to pattern the stack 22 of layers and the substrate layer 20 to define the inductor core. This may be achieved using sandblasting for example, such that the core cross-section is substantially trapezium-shaped, having tapered sides 23,25.

As shown in Figure 4, the inductor core so formed may then be bonded over copper tracks 8 on a substrate (not shown) with bond wires 10 provided over the core to define an inductor winding.

By way of illustration, the total height of an inductor core embodying the invention may be around 300 microns and its width may be of a similar order.

Each thin film layer 24,26 may be a few microns thick. The number of layers in the stack 22 can be chosen to suit particular requirements, with more layers increasing the flux area of the thin film stack.

In another fabrication method embodying the invention, the shape of the core body 20 may be defined prior to addition of the thin-film stack. Again, the stack may be formed using a deposition process. In this embodiment, it may be advantageous to use barrel sputtering such that the layers extend around the body on all sides. This would increase the cross-section of the core and therefore its magnetic properties, providing a higher inductance and saturation current.

The permeances (inverse of the reluctances) of the several magnetic paths in parallel add together. The effective permeability of multiple magnetic materials can be written as: ^y core V""¹

L^ ^■

where in, and t, are the permeability and thickness of each material, respectively. The effective permeability of a dual inductor core embodying the invention, having a ferrite carrier and a thin-film of CoNbZr on top of it is therefore:

μ =i^ ferrite + μ_Co. NbZr ' CoNbZr

V^ core

* ferrite ~*~ * CoNbZr

The ferrite carrier should preferably have a high enough permeability not to lose the benefit of using a high-B_sat thin film. Ideally, V-_ferrue - t_ferrue ~ V-c_oMz_r - tc_oMz_r - The effective permeability and thus the initial inductance can be set by choosing an adequate ratio between the thicknesses of the magnetic material layers, tf_emte and t_CθNbzr, which gives greater design freedom compared to a single material core.

Figure 5 is a schematic graph to illustrate the saturation behaviour of an inductor including a core embodying the present invention (plot 40) in comparison to a laminated inductor core using glass as a carrier (plot 42) or a core formed of ferrite material only (plot 44).

When a non-magnetic material such as glass or silicon is used as a carrier in a laminated core, the inductance at low currents remains limited. This is because the available flux area is low as the magnetic material present is in the form of thin films. Using magnetic materials as ferrite instead as the carrier, the inductance at low currents increases significantly. In comparison to a core made of ferrite material only, the saturation behaviour is extended to high currents as the magnetic material thin-film layers have a higher saturation level than the body material. Thus, at higher currents, a low magnetic-reluctance path is provided, leading to a higher inductance, compared to the reluctance of the flux through a saturated ferrite core behaving as air. The "step-wise saturation" performance indicated in Figure 5 is desirable in several applications which require a high inductance at lower winding currents and a smaller inductance at higher currents. For example, for a DC/DC converter working in continuous conduction modes (CCM) a higher inductance at lower DC current decreases the ripple current. This extends the current range at which the converter can still be operated in CCM. Also, ohmic losses are reduced due to the lower ripple current and the inductor current will no longer become negative, which leads to higher efficiency.

For a self oscillating power supply (SOPS), working on the boundary of CCM and discontinuous conduction mode (DCM), the switching frequency goes up for a lower DC output current. If the inductance of the used inductor goes up for a lower DC current, this upper frequency remains limited. On the other hand, a lower inductance at high output current increases the switching frequency, preventing the switching frequency from going too low. In other words, using a non-linear step-wise saturation inductor is useful here to reduce the switching frequency range of the SOPS, which is advantageous in terms of EMI regulations.

Furthermore for applications in mains-harmonic reduction circuits, using non-linear inductors has several advantages. In most existing applications, step-wise saturation is achieved by using an air gap with variable length, which complicates the construction of the inductor.

From reading the present disclosure, other variations and modifications will be apparent to persons skilled in the art. Such variations and modifications may involve equivalent and other features which are already known in the art, and which may be used instead of or in addition to features already described herein.

Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention.

Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination.

The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.

Claims

1. An inductor core (6) comprising: a body (20) formed of electrically insulating magnetic material; and a stack (22) of layers over the body, comprising alternate layers of electrically conductive magnetic material (24) and insulating material (26), wherein the magnetic saturation level of the magnetic material of the body is lower than that of the magnetic layer material.

2. A core of claim 1 wherein the stack (22) of layers extends around the body (20).

3. A core of claim 1 or claim 2 having tapered sides (23,25) in transverse cross-section.

4. A core of any preceding claim wherein the insulating material layers (26) are magnetic.

5. A core of any preceding claim wherein the magnetic saturation level of the body material (20) is around 0.5T or less.

6. A core of any preceding claim wherein the body (20) is formed of a ferrite material.

7. A core of any preceding claim wherein the resistivity of the body material (20) is around 10⁵ Ω.m or more.

8. A core of any preceding claim wherein the magnetic saturation level of the magnetic layer material (24) is around 1T or more.

9. An integrated circuit including an inductor having a core (6) of any preceding claim.

10. A method of manufacturing a core (6) of any preceding claim, comprising the steps of:

(a) providing a substrate (20) formed of electrically insulating magnetic material;

(b) depositing a stack (22) of layers over the substrate, comprising alternate layers of electrically conductive magnetic material (24) and insulating material (26); and

(c) patterning the stack of layers and substrate to define an inductor core (6) of a desired shape.

11. A method of claim 10 wherein step (c) includes the steps of: (c)(i) providing a layer of masking material (28) over the stack of layers;

(c)(ii) patterning the layer of masking material to form a mask (30); and (c)(iii) patterning the stack of layers and substrate using the mask to define an inductor core (6) of a desired shape.