Nanomagnetic Structures for Inductive Coupling and Shielding in Wireless Charging Applications

Dibyajat Mishra, Srikrishna Sitaraman, Saumya Gandhi, Sun Teng, P.M.Raj, Himani Sharma and Rao Tummala

T. N. Arunagiri*, Z. Dordi* and R. Mullapudi* 3D Systems Packaging Research Center

Georgia Institute of Technology Atlanta, GA USA

*Tango Systems San Jose, CA USA

Email: dmishra3@gatech.edu

Abstract

This paper presents materials modeling, design, processing, integration and characterization of a new class of nanomagnetic structures for coupling and shielding in wireless charging and power conversion applications. Wireless power transfer applications such as wireless charging, operating at 6.78 MHz, require high-performance magnetic materials for enhancing the coupling between transceiver and receiver coils as well as for suppressing electromagnetic interference (EMI) shielding. This research describes two novel magnetic structures for coupling inductors and ultra-thin EMI shields. A novel vertically aligned magnetic composite structure was demonstrated for the coupling inductor. This structure is shown to result in permeabilities of above 500 and loss tangent of 0.01, which enhances the coupling inductance by 3-5x at 6.78 MHz, and also enhances the power-transfer efficiency by 2x. The second part of this paper presents the modeling, design and fabrication of nanomagnetic structures for ultra-thin EMI shields in wireless power transfer applications. The ultra-thin EMI shields for wireless power transfer described in this research can achieve greater than 20dB attenuation at 6.78 MHz even for 3-5µm shield thickness. Introduction

The rapid evolution of smartphones and wearable electronic systems is driving new advances in high-efficiency power storage, conversion and transfer. Today’s wireless power transfer applications operate at frequencies of 5-10 MHz and require high-performance magnetic materials for enabling critical functions such as inductive coupling and EMI shielding of the stray RF power from rest of the system.

Inductive coupling is the near field wireless transmission of electrical energy between two magnetically coupled coils that can resonate at the same frequency. Inductive coupling is widely used in wireless power systems. In this approach a transmitter coil in one device transmits electric power across a short distance to a receiver coil in other device. The inductive coupling between the coils can be enhanced by using high permeability magnetic materials as shown in Equation 1.

∆V=  -­‐µμA  ∂H/∂t (1)

where V is the induced voltage, H is the magnetic field, A is the area of the coil and µ is the permeability of the magnetic material. The magnetic materials used should also have high

Q-factor and low losses in the operating frequency range so as to enable effective coupling.

Ferrites are currently being explored as magnetic materials in the inductive coupling layers for wireless charging applications. They combine extremely high resistivity with reasonably good magnetic properties. They have significantly lower eddy current losses compared to metal cores, resulting in higher Q-factors at moderate frequencies of 100kHZ-1 MHz. Because of their low saturation flux density (Ms), they have fundamental limitations in achieving high permeability and low loss beyond 6 MHz[1-3]

Metal-based magnetic materials have been traditionally considered unsuitable for microwave applications due to their frequency instabilities from eddy current losses. However, recent advances in high-frequency magnetic materials have established the suitability of metal nanocomposites for microwave applications by demonstrating high permeability with frequency stability and suppression of losses. Nanoscale composite thinfilms have been successfully demonstrated by co-sputtering techniques. These nanocomposites are typically composed of (<10 nm) magnetic nanoparticles surrounded by an amorphous insulating matrix. The Fe and Co-based nanocomposite thinfilms can have high permeabilities (>100) with essentially flat frequency response up to 1 GHz due to exchange coupling [4, 5], significantly better than those of conventional ferrite and metal powder cores. These advanced films formed from sputtering processes, however, cannot be made into the required film thickness of 100µm for inductive links economically. There is a critical need, therefore, for novel magnetic materials that address the limitations of today’s ferrites and metal nanocomposite approaches.

Electromagnetic shielding is another major requirement for wireless power transfer applications. The transmitter and receiver modules house several active and passive components in close proximity to the magnetic coils. It is essential, therefore, to isolate these components and interconnections from the strong fields of the transmitter. At an operating frequency of 6 MHz, since the wavelength is 49 meters, these components fall under the near field (distance < λ/2π) in both transmitter and receiver modules. Traditional shielding approaches use thick metal cans or metal over-molds to address the EMI issues, adding constraints to system miniaturization. This paper investigates a novel concept based on nanomagnetic stacked or sandwiched films to create thinner and more effective EMI isolation structures. Such stacks could also be extended to provide isolation in both

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vertical and horizontal directions, enabling unique opportunities for 3D integrated modules in wireless charging systems.

The focus of this paper is to model, design, fabricate and demonstrate two novel nanomagnetic structures for inductive coupling and EMI shielding in wireless charging applications. The first part of the paper describes vertically aligned multilayered composite structures for inductive coupling. The second part of the paper demonstrates novel nanomagnetic structures to form ultra-thin EMI shields.

Multi-layered Composite Structure for Wireless Power Transfer

The composite structure consists of multiple layers of ferromagnetic films stacked alternately, separated by ultra-thin insulating polymer dielectric layers. The ferromagnetic layers are made of high-permeability, high-Ms and low-coercivity magnetic materials. The structure is vertically aligned between the transmitter and receiver coils. Fig. 1 shows the schematic concept for such a vertically aligned composite structure. The vertical metal layers give high permeability in the Z direction while thickness of each magnetic layer is maintained such that it is thinner than the skin depth at the desired frequency regime. This reduces the eddy current losses and ensures frequency stability of permeability. Different magnetic materials are chosen for this study and effects of their resistivity and thickness on the frequency of operation are analyzed via modeling approaches. The polymer-dielectric layer acts as both an insulating layer as well as an adhesive to stack the magnetic layers. Thus, the composite structure addresses the fundamental challenge of eddy current losses at high frequencies so as to achieve high µ with low losses in adequate thickness for the inductive coupling layer.

Fig. 1. Schematic of coupling structure.

Modeling and Design

The composite approach is modeled for high permeability, and low loss at the desired frequency of operation. The first step in material modeling is to compute the skin depth of the ferromagnetic layer in the composite structure in the frequency range of interest. This decides the thickness of the magnetic layer in the multilayered structure for a stable permeability at high frequencies. Three magnetic materials, NiFe, NiFeMo and CoZrO are chosen as candidate materials for the ferromagnetic layer, because of their high µ, high Ms and low coercivity. Table 1 shows the computed skin depth for each of these magnetic materials at 6.78 MHz.

Table 1. Skin depth analysis of magnetic materials

Material Permeability

Resistivity (micro-ohm-

cms) Skin depth (microns)

NiFe 1000 20 2.25 NiFeMo 800 59 4.83 CoZrO 200 200 27.57

Based on the above analysis, the thickness of the

ferromagnetic layer was selected to be µm. The effective DC permeability µeff can be easily calculated using Wiener’s law as (Equation 2)

µeff=q(µi-1) +1 (2)

where q is the volume fraction of the magnetic material and µI is the intrinsic permeability of the ferromagnetic layer. To ensure a high volume fraction of magnetic layers for high Ms, the thickness of the ultra-thin polymer layers was chosen to be 1.5 µm, based on Wiener’s law.

The frequency-dependent permeability of the composite structure is estimated through the extraction of reflection coefficients for a shorted strip transmission line using full-wave electromagnetic (EM) simulations. The expression for the reflection coefficient of a transmission line section is given by the basic Equation 3[6].

R=Roe-2γl (3)

where γ is the propagation constant, l is the length of a section and RO is the reflection coefficient of the strip line at the termination. The propagation constant, valid for a TEM mode, is defined by the general solution of the Maxwell’s equations [7].   According to a quasi-transverse electromagnetic wave approximation, the propagation constant of an inhomogeneous medium (air + material) can be modeled by an effective homogenous expression in Equation 4.

γ = iω√ (εeff µeff) /c0 (4)

where εeff is the effective permittivity and c0 is the velocity of light in vacuum. Thus, it can be seen that that the effective permeability of a material directly influences the reflection coefficient of a shorted strip transmission line.

The EM simulation of the shorted strip-line using SONNET outputs the scattering parameter S11 for different frequencies. The parameter S11 represents the reflection coefficient R of the strip line. Using analytical methods described in [8], the frequency-dependent effective permeability of the composite structure can then be extracted from the measured S11 plot by correlating the model to hardware measurements. The simulated multi-layered composite structure consists of three layers of 2µm thick NiFeMo film separated by 1.5µm thick polymer-dielectric layers. Fig. 2 shows the plot of effective permeability of the composite structure vs. frequency, extracted using EM

Copper&Spiral&

Coupling&Structure&

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simulations. Thus, it can be seen from material modeling that the proposed design of the composite structure has stable high permeability till 10 MHz.

Fig. 2. Plot of effective permeability vs. frequency for

composite structure via EM simulation. Fabrication of Composite Structure

Cold-rolled magnetic foils of ~2µm thickness are laminated together using ultra-thin polymer adhesives to form the multilayered composite structure. Cold-rolled magnetic foils of molypermalloy (NiFeMo) that are 2µm thick are commercially obtained for this purpose.

To form multi-layered structures, Co-PET polymer dry films of 1.5µm thickness were commercially obtained from SKC Corp. The PET film is then individually laminated on the magnetic foils. The PET laminated foils are then stacked together in a hot press and high-temperature bonding is performed at 250°C for 60 minutes to ensure wrinkle-free lamination. The foil-laminate is then diced and reassembled to form vertically aligned composite structure. Fig. 3 shows the image of a free-standing three-layered foil laminate. Fig. 4 shows the cross section of a multi-layered composite formed by foil-lamination.

   Fig. 3. Free-standing foil

laminate. Fig. 4. Cross-section of

multi-layered composite.

VSM measurements are performed to obtain B-H hysteresis loop used to characterize DC magnetic properties.Fig. 5 shows the B-H loop of a four-layered foil laminate. It can be seen that the multi-layered composite structure formed by foil lamination has high Ms, high µ and low coercivity (~4.4 Oe).

The frequency-dependent effective permeability of the composite structure is estimated from the measured S11 parameters for a shorted-strip transmission line using a network analyzer. The extraction of permeability from S11 parameters has been described earlier. The permeability measurement was made in three steps, the first one with an empty strip line, and the second one with the strip line loaded with kapton tape as substrate only. The third step was made with the strip line loaded with the multilayered composite film laminated on the substrate kapton tape. This procedure eliminates frequency dependent errors associated with conducting losses and dielectric losses as well as strip line fabrication and connection mismatches. The effect of permittivity of substrate and sample on reflection losses is also taken into account.

Fig. 6 shows the plot of effective permeability vs frequency of a four-layered composite structure. It can be seen that the composite structure has stable high permeability (>500) till 10 MHz. This enhances the coupling inductance over ferrite sheets by a factor of 3-5x as they have low permeabilities of 100-200.

Fig. 6. Plot of effective permeability vs. frequency for

composite structure via measurement.

Nanomagnetic EMI Shielding Structures Reflection loss and absorption loss are two key mechanisms through which electromagnetic shielding is effected [9]. The space around an electromagnetic radiation source at distances up to a sixth of the wavelength (λ/2π) is

Frequency(Hz)

Perm

eabi

lity

Perm

eabi

lity

Frequency(Hz)

Fig.5. B-H loop of composite structure

B (T

)

Field (Oe) !

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termed as the near-field region. In the near-field region, the electric and magnetic fields are decoupled, and the magnetic waves have lower wave impedance. Since reflection loss is dependent on the wave impedance, the shield effectiveness due to reflection loss is lower for magnetic waves. Further, absorption loss of a shield is lower as frequency decreases, and increases with increasing thickness of the shield. It is therefore important to utilize electromagnetic shield materials and material stacks such that the required shield effectiveness can be achieved at the least thickness. To this end, nanomagnetics provide the ability to design and create EMI isolation films to effectively replace the metallic cans or spray-coated paints employed today.

Here, two nano-magnetic materials, Cobalt Zirconium (CoZr) and Nickel Iron (NiFe) are considered for EMI shield design. The properties [5, 10] of these systems are summarized in Table 2.

Table 2. Nanomagnetic systems for EMI shielding. Material   Perm.   Resistivity [µ−ohm-­‐cm]  CoZr   200   100  NiFe   1000   20  

The shielding effectiveness of these materials was

compared based on analytical simulations [9, 11]. It was assumed that the distance between source and shield of 10mm average separation distance between transmitter and receiver coils in loosely-coupled wireless power transfer systems. The simulation is performed from 1-10MHz since the ISM band at which such systems operate is 6.78 MHz. Since this analysis is focussed on comparing the shield effectivenesses of different material combinations, the effect of shield geometry and apertures on the shield effectiveness are ignored.

The comparison was performed for different single and

multi-layer stack scenarios. For the nanomagnetic materials systems, the shielding effectiveness of single-layer and multi-layer (10 layers) stack are studied. For the multi-layer stacks a 20nm-thick layer of insulator (alumina) is used to isolate two successive conductive layers. The insulator layer helps increase the reflection loss since the wave impedance of insulators is high compared to that of metals.

Fig.7 shows the comparison of shield effectiveness between

single layer of NiFe and CoZr. It shows that for a single layer of thickness 1microns, nickel iron has more shield effectiveness arising from its higher perbeability and lower resistivity.

Fig. 7. Comparison of shield effectiveness of single layer NiFe and Co-Zr.

Extending this to multiple layers of NiFe, a case where 5

layers of 1micron-thick NiFe layers were stacked, separated by insulator of thickness 20nm. It was observed that the absorption loss and the reflection loss were five times as much as the single layer case. However, due to the phenomenon of multiple-reflections (indicated by ‘B’), in shields thinner than one skin depth [9], the shield effectiveness was lowered. This multi-layer scenario was compared with a 5-micron-thick single layer. The comparison is shown in Figure 8. It was observed that using 5 layers of 1-micron thickness each resulted in higher shield effectivenss due to higher reflection loss, despite the higher B..

Fig. 8. Shield effectiveness: single-layer and multi-layer NiFe.

Thus it can be observed that for the same thickness, a multi-layer stack can be designed to offer better EMI shielding performance than a single thick layer. Such a multi-layer strcuture consisting of alternating thin layers of NiFe and insulator (alumina) was fabricated through sequential sputtering and the SEM image of the same is shown in Fig. 9.

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Fig. 9. NiFe /alumina multilayered stack fabricated by sequential sputtering

Summary

Modeling, design, fabrication and characterization of novel nanomagnetic structures were studied to demonstrate inductive links and EMI shields in wireless power transfer applications. The vertically aligned multilayered composite structures have stable high permeabilities (>500) and low losses upto 10 MHz and they enhance coupling inductance by 3-5X over prior art. Multi-layered stack comprising alternating magnetic-insulating thin-films was shown to have excellent shielding effectiveness compared to monolithic metal shields. Such ultra-thin EMI shields using the multilayered nanomagnetic structure were also demonstrated.

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