Microfabrication of toroidal inductors integrated with nanolaminated ferromagnetic metallic cores

Transcription

1 IOP PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING J. Micromech. Microeng. 23 (2013) (9pp) doi: / /23/11/ Microfbriction of toroidl inductors integrted with nnolminted ferromgnetic metllic cores Jooncheol Kim, Jung-Kwun Kim, Minsoo Kim, Florin Herrult nd Mrk G Allen School of Electricl nd Computer Engineering, Georgi Institute of Technology, Atlnt, GA, USA E-mil: mllen@gtech.edu Received 2 April 2013, in finl form 10 June 2013 Published 25 October 2013 Online t stcks.iop.org/jmm/23/ Abstrct We report microfbricted toroidl inductors with nnolminted ferromgnetic metllic cores for chip-scle, high-power switching converters. The fbriction process of the toroidl inductor is bsed on individul mnufcturing of prtil windings (i.e. bottom nd verticl conductors) nd nnolminted mgnetic core, nd integrting them by mens of drop-in pproch. The nnolminted ferromgnetic metllic cores presented in this pper consist of mny multilyers of electrodeposited CoNiFe films, ech lyer with sub-micron thickness, with totl core thickness exceeding tens of microns. The beneficil mgnetic properties (i.e. high sturtion flux density nd low coercivity) of CoNiFe lloys re well suited for chip-scle inductors s they chieve both lrge energy storge cpcity s well s minimized volumetric core losses t high operting frequencies due to their nnolminted structure. A drop-in integrtion pproch, introduced to combine the microfbricted toroidl inductor windings with the mgnetic cores, llows ese of integrtion. An dvntge of this hybrid pproch over monolithic fbriction in this ppliction is the potentil use of wide vriety of core mterils, both microfbricted nd bulk-fbricted, nd which my or my not ultimtely be CMOS-comptible. Exploiting this drop-in pproch, 30-turn- nd 50-turn-toroidl inductors integrted with nnolminted CoNiFe cores, hving 10 mm outer dimeter nd 1 mm thickness, hve been successfully developed. Both types of inductors exhibit inductnces higher thn 1 µh tfrequenciesuptotensofmhz,showingtentimestheinductnceofnir core device with the sme nominl geometry. The pek qulity fctor of the 30-turn-toroidl inductor reches 18 t 1 MHz. (Some figures my pper in colour only in the online journl) 1. Introduction Compct, multi-functionl personl electronic devices (e.g., smrt phones nd tblet PCs), which often employ multiple internl voltge levels nd require the rechrging of internl btteries, should employ high-power density switching converters to mintin their compctness. However, the minituriztion of dc/dc converters is often mde more chllenging by their need for pssive components, especilly inductors, which cn consume lrge physicl volumes [1, 2]. Consequently, significnt efforts hve been mde to develop chip-scle inductors using dvnced microfbriction technology, exploiting the trends of incresing switching frequencies s well s incorportion of inductor mgnetic mterils to reduce physicl size [3 5]. Although conventionl ferrites hve been mostly used in these pplictions, soft ferromgnetic lloys (e.g., NiFe, NiFeMo nd CoNiFe) re gining incresed ttention due to their superior mgnetic properties (i.e. higher sturtion flux density nd lower coercivity) s well s their potentil for CMOS-comptible integrtion. However, s switching frequencies increse, such ferromgnetic lloys cn possess significnt eddy-current losses t high-frequency opertion, limiting the use of the mgnetic cores to smll overll thicknesses [6, 7]; this is /13/ $ IOP Publishing Ltd Printed in the UK & the USA

2 in contrst to non-electriclly conducting ferrites. Therefore, most microfbricted inductors integrted with mgnetic cores re either plnr spirl windings [8, 9] or solenoid windings [10, 11] withlimitedtotlmgneticcorethicknesses. Such smll thicknesses re typiclly insufficient to meet the power hndling needs of portble electronics, e.g., in the W rnge. To relize microfbricted inductors tht cn hndle such high power, it is required to develop (1) mgnetic mterils with sufficiently lrge volume nd minimized losses, (2) microfbricted windings providing lrge mgnetic flux pth where the lrge volume core could be plced nd (3) fbriction technique to integrte the lrge volume core into the windings. We recently developed highly lminted ferromgnetic metllic cores vi utomted sequentil electrodeposition, demonstrting sufficient mgnetic volumes with significnt reduction in eddy-current losses up to 10 MHz opertion frequency [12]. In ddition, 650 µm tll ir core toroidl inductors hve been fbricted bsed on metl-encpsulted polymer vis, yielding lrge cross-sectionl re, thereby lrge mgnetic flux pth [13]. In this pper, we provide fbriction technique to incorporte these types of lrge volume mgnetic cores into toroidl windings by mens of core drop-in pproch, where pre-fbricted cores re incorported s n intermedite step in the fbriction of toroidl windings. This fbriction technology llows not only the use of the thick mgnetic core for high-power hndling, but it lso llevites process comptibility issues in the monolithic fbriction of toroidl inductors with highly lminted mgnetic cores (e.g., defect-free insultion of copper coils during the lminted core fbriction nd multiple processing of thick photoresist lyers potentilly cusing stress issues) [14]. Although the fbriction technology is illustrted by mens of the incorportion of lminted CoNiFe cores in the microfbricted windings, it should be noted tht the technique cn be generlized to other core mterils, including conventionl ferrites, if desired. 2. Fbriction The fbriction process for the integrted toroidl inductor with drop-in nnolminted core cn be divided into three min steps s illustrted in figure 1. First,thennolminted ferromgnetic metllic cores nd prtilly formed windings (i.e. bottom nd verticl windings) re individully prepred (figure 1). Second, the cores re integrted with the prtilly formed windings by mens of drop-in pproch (figure 1). In order to prevent electricl shorting between the metllic cores nd the bottom windings, 100 µm thickinsulting spcer is plced within the windings prior to core insertion. Third, top windings re fbricted to complete the toroidl inductor (figure 1(c)). The top windings cn be built either on scrificil insulting lyer, which will be removed fter top conductor fbriction (temporry core embedding pproch), or on non-scrificil insulting lyer to reinforce the mechnicl stbility of the toroidl windings nd core (permnent core embedding pproch). Tble 1. Electrodeposition conditions for nnolminted CoNiFe core. Component Quntity CoSO 4 7H 2 O 0.08(mol L 1 ) NiSO 4 6H 2 O 0.2 (mol L 1 ) FeSO 4 7H 2 O 0.03 (mol L 1 ) NH 4 Cl 0.3 (mol L 1 ) Boric cid 0.4 (mol L 1 ) Sodium scchrin 0.02 (mol L 1 ) Sodium luryl sulfte (mol L 1 ) Anode Nickel sheet ph 2.8 Current density 20 (ma cm 2 ) 2.1. Nnolminted metllic core fbriction Nnolminted ferromgnetic metllic cores re btchfbricted bsed on utomted sequentil electrodeposition of lternting ferromgnetic mteril nd scrificil copper lyers on sputtered titnium/copper seed lyer [15]. In this sequentil electrodeposition system, robotic rm, which holds substrte (cthode), moves lterntely between CoNiFe bth nd commercil copper bth (Grobet, Clen Erth Cu-mirror solution) with two steps of DI wter rinsing in between these bths. The electroplted CoNiFe typiclly exhibits high sturtion flux density (>1.8 T) nd low coercivity (<2Oe),ndseverlelectrodepositionpproches hve been estblished [16, 17]. The electrodeposition conditions for this work re detiled in tble 1. At current density of 20 ma cm 2 for both the CoNiFe nd the commercil copper bths, deposition rtes re 0.25 µm min 1 nd 0.5 µm min 1, respectively. The thickness of ech electrodeposited lyer is precisely controlled by monitoring the electrodeposition time. Furthermore, the thickness of the mgnetic lyer is designed to be smller thn the skin depth (δ), expressed s δ = 1 π f µσ, (1) where f is the operting frequency (Hz), µ is the mgnetic mteril permebility (µ r µ o ) (H m 1 ), nd σ is the conductivity of the mgnetic mteril (S m 1 ). Considering typicl properties of n electrodeposited CoNiFe film (reltive permebility of pproximtely 1000 nd conductivity of pproximtely S cm 1 )[16, 17], CoNiFe/copper multilyer core is fbricted with single lmintion thickness less thn 500 nm (i.e. below the skin depth of the mteril, 1 µm t 30 MHz), while the totl mgnetic lyer thickness (i.e. sum of the individul mgnetic lyer thicknesses) exceeds 30 µm. With the electrodeposition conditions mentioned bove, it tkes pproximtely 4.5 h to chieve the totl mgnetic lyer thickness. After sequentil electrodeposition, the cores re seprted from the substrte by removing the titnium seed lyer in 49% hydrofluoric cid solution. Then, the freed multilyer cores re ffixed using cynocrylte dhesive onto 150 µm thick polyester insultor film tht hs been ptterned in the sme shpe s the toroidl core by use of lser micromchining s illustrted in figure 2. The insultor film cts s 2

3 (c) Figure 1. Conceptul integrtion pproch for microfbricted toroidl inductor with drop-in core. physicl spcer between the nnolminted metllic core nd the bottom conductors. Once the on-insultor CoNiFe/copper multilyer cores re prepred, it is criticl to selectively remove the copper lyers without dmging the mgnetic CoNiFe lyers. A sturted solution of copper sulfte in mmonium hydroxide (NH 4 OH + CuSO 4 )isutilizedscopperetchnt since it provides excellent selectivity to nickel-bsed soft mgnetic lloys (e.g., CoNiFe nd NiFe) [15, 18]. The scrificil copper mteril is prtilly removed in the copper etchnt, leving lterl trenches between the CoNiFe lyers. SU-8 insulting polymer is then pplied through the support holes s shown in figure 2, underfilling spce formerly occupied by the prtilly etched copper. After the SU-8 curing, the remining scrificil copper lyers re entirely removed to form ir-insulted CoNiFe lmintions in which ech lmintion is supported by SU-8 insulting polymer s depicted in figure 2(c). The detiled fbriction process to crete the ir-insulted lminted cores is described in [12]. The nnolminted CoNiFe cores re further pckged in polydimethylsiloxne (PDMS) to improve mechnicl reinforcement nd prevent stiction between lmintions s shown in figure 2(d). In order to ensure the infiltrtion of PDMS into the nnolminted structure, the core is immersed in uncrosslinked PDMS nd vcuum is pplied for pproximtely 10 min. The infiltrted PDMS is fully crosslinked in 48 h t room temperture. As the PDMS is firmly confined between the CoNiFe lyers due to the vcuum process, excess PDMS round the core is esily stripped mnully, providing PDMSlminted CoNiFe multilyer core. Figure 3 shows vrious imges of nnolminted CoNiFe toroidl cores. Figure 3 shows opticl imges of nnolminted toroidl cores nd polyester insultor films. The cores hve n outer dimeter of 10 mm nd n inner dimeter of 6 mm. Figure 3 showsninclinedtop-view scnning electron microscope (SEM) imge. Note tht support holes re utilized to fcilitte prtil removl of the scrificil copper lyers s well s to pply the SU-8 supports. Figure 3(c) shows cross-sectionl view of the nnolminted core, comprising 70 CoNiFe lyers, ech with thickness of 500 nm. (The scrificil copper lyers re prtilly etched to enhnce their visibility.) A mgnified view of the 500 nm CoNiFe lmintions is shown in figure 3(d), demonstrting tht ech CoNiFe lyer hs uniform thickness nd exhibits no interlyer stiction. (c) Figure 2. Fbriction sequence for PDMS-encpsulted CoNiFe core. Electrodeposited CoNiFe/copper multilyer cores on n insultor film, multilyercorefterprtilremovlofcopper, (c)multilyercoresupportedbysu-8fterentireremovlof copper, (d) multilyer core encpsulted with PDMS Winding fbriction nd core drop-in Atypiclmicrofbrictedtoroidlinductorwindingconsists of bottom, verticl nd top conductors. For the proposed dropin pproch, the verticl conductors re bsed on copper metlliztion of high-spect-rtio SU-8 pillrs, followed by lithogrphic ptterning [13]. Figure 4showsfbricted50- turn bottom nd verticl conductors on the glss substrte. For the bottom windings, pproximtely 250 µm wide, 30 µm thick conductors re positioned with 100 µm interconductor lterl spcing. For the verticl windings, 1 mm tll SU-8 pillrs with 100 µmdimeterrelithogrphicllyformednd coted with 30 µmthickelectrodepositedcopperlyer.these high spect rtio verticl windings enble plcement of lrge volume mgnetic cores. Once the prtilly fbricted windings (i.e. bottom nd verticl windings) re prepred, the PDMSinfiltrted CoNiFe multilyer cores re mnully pick-ndplced nd ffixed s shown in figure 4. No observble winding dmge ws cused by this drop-in core pproch. In figure 4, the thickness of the PDMS-infiltrted CoNiFe lmintions including the insultor film is pproximtely 300 µm, which is lower thn the height of verticl windings. (d ) 3

4 (c) (d) Figure 3. Imges of nnolminted CoNiFe cores. Toroidl cores nd insultor films with outer dimeter of 10 mm, SEM imge of inclined top view, (c) SEM cross-sectionl imge of 70 lyers of 500 nm thick CoNiFe, (d)mgnifiedviewof(c). Figure 4. Btch fbricted 50-turn prtil windings on glss substrte. Mgnetic cores integrted with the prtil windings Core integrtion Prior to top winding fbriction, it is criticl to form n insulting lyer on top of the nnolminted core where the top windings will be deposited nd ptterned. The insulting lyer should possess plnr surfce for top conductor deposition s well s sufficient thickness to void prsitic cpcitnce between the metllic core nd top windings. Two methods, temporrycoreembeddingpprochndpermnentcore embedding pproch, re explored s shown in figure 5.Also, both pproches re summrized in tble 2. The temporry core embedding pproch utilizes non-photoptternble EPON SU-8 epoxy pellets (Miller- Stephenson, Inc.), which will be removed in cetone fter top winding fbriction. In this pproch, epoxy pellets in sufficient quntity to cover the nnolminted core re distributed on the prtilly fbricted inductors (i.e. bottom nd verticl windings, nd mgnetic cores) nd melted t 130 Conhotpltesshowninfigure5. In this step, it is criticl to obtin n pproprite insulting lyer thickness ( µm from the bottom winding)so s not to cover the top of the verticl windings since the SU-8 epoxy pellet is not photoptternble. However, due to the lck of solvent nd crosslinker in the SU-8 epoxy pellet, long softbking times re not required even for thick films, resulting in reduced process time. After melting, the SU-8 epoxy is brought to room temperture for pproximtely 1 h until it is re-solidified. Figure 6shows the inductor fter the epoxy scrificil lyer step hs been completed. Note tht the top of the verticl windings is exposed, while the lminted metllic core is embedded within the SU-8 scrificil epoxy. In contrst, the permnent core embedding pproch utilizes photo-definble SU (MicroChem). After csting SU-8 on the smple by weight s shown in figure 6, 4

5 Figure 5. Core integrtion pproches fter core drop into the prtilly fbricted windings. (e)temporrycoreembedding pproch, (e) permnent core embedding pproch. Figure 6. Opticl imges of fbricted dditionl lyers with EPON SU-8 epoxy using temporry core embedding pproch, nd photoptternblesu-82025usingpermnentcoreembeddingpproch. 5

6 Tble 2. Comprison of temporry core embedding nd permnent core embedding pproches. Temporry core embedding pproch Permnent core embedding pproch Insulting mteril EPON SU-8 epoxy pellets SU (Miller-Stephenson, Inc.) (MicroChem) Process sequence Melted t 130 CSolidifiedt23 Cfor1h Softbket95 Cfor12h Expose t 30 mw cm 2 for 30 min Postbke t 95 Cfor30min Develop for 25 min in Thinner P Top conductor seed lyer deposition E-bem evportion Sputtering or E-bem evportion Advntges Fst process time Mechnicl reinforcement Disdvntges Non-photoptternble More complex process steps it is plnrized nd softbked t 95 Cfor12honhotplte. Note tht 1 g of cst SU-8 corresponds to 1 mm thickness on 6.45 cm 2 glss substrte fter the softbking. Since the softbked SU-8 covers the verticl windings, it is ptterned to expose the verticl windings to connect with top windings. After 4 h cooling, the SU-8 is exposed with UV light intensity of 30 mw cm 1 for 30 min s shown in figure 5, followed by post-bke t 95 Cfor30min.Thesmpleis then developed in PGMEA (propylene glycol methyl ether cette)-bsed Thinner P (MicroChem) for 25 min leving the verticl windings exposed s shown in figure 5(c).Figure6 shows tht ptterned SU-8 insulting lyers encpsulte the lower nd verticl windings s well s the core, nd tht the tops of the verticl conductors protrude. Since the SU-8 is fully crosslinked, it potentilly provides higher mechnicl robustness for the inductor Top winding fbriction For the two pproches, top windings re fbricted in similr process. First, 300 nm thick copper seed lyer is deposited by e-bem evportion. Then, 20 µmthickphotoresistlyer is spry-coted on the seed lyer nd photolithogrphiclly ptterned to serve s mold for electrodeposition of top copper conductors s shown in figures 5(c) nd(d).after 30µm thickcopperelectrodeposition,thephotoresistnd the copper seed lyer re removed to complete the toroidl inductor fbriction s shown in figures 5(d) nd(e).for the temporry core embedding pproch, the embedding SU-8 epoxy cn be removed in cetone s shown in figure 5(e). 3. Fbricted inductors nd chrcteriztion 3.1. Fbricted toroidl inductors integrted with nnolminted cores Opticl imges of integrted toroidl inductors with nnolminted CoNiFe cores re shown in figure 7.Figure 7 shows 1 mm tll, 50-turn-toroidl inductor fbricted by mens of the temporry core embedding pproch nd dense winding msk. The scrificil SU-8 epoxy hs been removed upon completion of the top winding fbriction. The lminted mgnetic core inside the toroid is comprised of 100 lyers of CoNiFe with individul lmintion thickness of 300 nm. Figure 7 shows 1 mm tll, 30-turn-toroidl inductor fbricted using the permnent core embedding pproch nd Tble 3. Prmeters of integrted inductor with nnolminted cores. 50-turn inductor 30-turn inductor A ir (m 2 ) A core (m 2 ) r i (m) r o (m) (m) µ e N sprse winding msk. The crosslinked SU-8 underlying the top windings enhnces the mechnicl robustness of the inductor Chrcteriztion of toroidl inductors integrted with nnolminted cores The inductnce, resistnce nd qulity fctor of the inductors fbricted using two different pproches were chrcterized s function of frequency t typicl core mgnetic flux densities between 2 nd 10 mt using n impednce nlyzer (HP 4194 A). Figure 8 shows the mesured result from 50 turn microfbricted inductor integrted with nnolminted CoNiFe core using the temporry core embedding pproch, nd n ir core inductor with the sme nominl geometry. The nnolminted core consists of 100 lyers of 300 nm thick CoNiFe lmintions with 300 nm tll interlmintion gps. Mesured inductnces of these two inductors re shown in figure 8. The microfbricted ir core inductor exhibits constnt inductnce of pproximtely 210 nh (inductnce density of µhmm 2 )upto 30 MHz. To vlidte this mesured inductnce, the theoreticl inductnce of the 50 turn toroidl inductor hs been estimted using the theoreticl expression [19] L ir = µ 0 A ir N 2 π(r i + r o ), (2) where µ 0 is the permebility of vcuum, 4π 10 7 Hm 1, A ir is the cross-sectionl re of the mgnetic flux pth (m 2 ), N is the number of windings, nd r i nd r o re the inner nd outer rdii of the toroid (m), respectively. With the prmeters presented in tble 3, the theoreticl ir core inductnce is estimted s 280 nh. The clculted nd mesured inductnces gree within 25% up to 30 MHz. The lower mesured inductnce from the microfbricted inductor could be due to 6

7 Figure 7. Microfbricted toroidl inductors integrted with nnolminted CoNiFe cores by mens of temporry core embedding pproch nd permnent core embedding pproch. Note tht in the encpsulting SU-8 possesses rim tht extends to the periphery of the picture, resulting in dditionl mechnicl stbility. lekge flux between the windings nd fbriction tolernces between the microfbricted inductor nd the originl design. The mesured inductnce of the integrted inductor with lminted CoNiFe core exceeds 1.6 µh(inductnce density of µhmm 2 )upto30mhz,showingnpproximtely ten times inductnce increse from the ir core inductor. The mesured inductnce is lso nlyzed by compring with theoreticl prediction. To estimte the frequency-dependent inductnce of the integrted inductor with nnolminted CoNiFe core, the effective permebility of the CoNiFe core is first extrcted using theoreticl expression of low-frequency inductnce of integrted inductor with mgnetic core, where eddy-current loss is negligible: L core, dc = µ 0 µ e A core N 2, (3) π(r i + r o ) where µ e is the effective permebility of the mgnetic core nd A core is the cross-sectionl re of the mgnetic core (m 2 ). Considering tht the totl mgnetic thickness of the nnolminted core (30 µm) occupies pproximtely 3% of the totl inductor thickness (1 mm), the effective permebility of the core is estimted s 250, which is in the rnge of typicl soft mgnetic mteril permebility ( ) [4]. Then, the frequency-dependent inductnce (L core )cnbeclculted bsed on one-dimensionl nlysis of the electromgnetic diffusion in lminted core with c sinusoidl excittion [6, 7]ndisexpresseds ( ) ( sinh ( ) ( + sin ) ) L core = L core, dc cosh ( ) ( + cos ), (4) where is the thickness of single lmintion lyer (m) nd is the skin depth (m) of the mgnetic mteril t the opertion frequency s expressed in eqution (1). Finlly, the overll inductnce (L totl )isexpressedbyddinginductncesfromir nd mgnetic core: ( L totl = L ir + L core = µ 0 N 2 + A core µ e ( π(r i + r o ) ) ( sinh ( cosh ( A ir ) ( + sin ) )) ) ( + cos ). (5) From equtions (1) nd(5), the single lmintion thickness (300 nm) of the core is well below the skin depth t Figure 8. Chrcteriztion of 50-turn-integrted inductors with nnolminted mgnetic cores s well s n ir core inductor by mens of temporry core embedding pproch. Mesured inductnce, theoreticl inductnce, (c) mesured resistnce, (d)mesuredqulityfctor. 30 MHz ( 1 µm); thereby, the theoreticl totl inductnce of the integrted inductor with the nnolminted CoNiFe (c) 7

8 core exhibits the constnt inductnce of 2.1 µh up to 30 MHz. However, use of bulk (non-lminted) core with the sme mgnetic volume (30 µm) would cuse significnt inductnce decrese fter 300 khz due to the eddy currents flowing in the mgnetic core. Compred with the theoreticl inductnce, the mesured inductnce shows slight inductnce decrese s frequency increses. Since the theoreticl eqution ssumes perfect insultion of identicl CoNiFe lmintions, the decresing inductnce of fbricted inductor is possibly due to (1) lmintion thickness nd mteril composition uniformity, (2) potentilly collpsed CoNiFe lyers cusing electricl short, (3) prsitic cpcitnce between lmintion lyers, s well s between the dense 50-turn windings nd the core. It is lso shown tht the mesured inductnce tends to increse fter 20 MHz, implying self-resonnce of the integrted inductor with nnolminted CoNiFe core. In figure 8, the resistnce of both inductors t low opertion frequency is ner 1, ndincresesthigherfrequencywith growing winding nd core losses. The different resistnces t low frequency, where the losses from the mgnetic core should be negligible, re minly due to the different winding losses from the microfbriction winding tolernces. The incresing resistnce of the integrted inductor with nnolminted CoNiFe core t higher frequency is ttributed to frequencydependent mgnetic core losses including hysteresis losses, eddy-current losses nd nomlous losses [7]. However, the qulity fctors of the integrted inductor with nnolminted core is pproximtely 12 t 6 MHz nd higher thn tht of the ir core inductor up to 15 MHz s shown in figure 8(c), indicting n effective energy storge/trnsfer cpcity by utilizing nnolminted mgnetic cores. Figure 9 shows the mesurement result from 30-turn microfbricted inductor integrted with nnolminted CoNiFe core consisting of 70 lyers of 500 nm thick CoNiFe lmintions (nd 500 nm tll interlmintion gp) using the permnent core embedding pproch, s well s n ir core inductor with the sme nominl geometry. Mesured inductnces of these two types of inductors re shown in figure 9. The microfbricted ir core inductor exhibits constntinductnceofpproximtely96nh(inductnce density of µh mm 2 ) up to 30 MHz. The theoreticl inductnce of the 30-turntoroidl inductor using eqution (2)isestimteds104nH,demonstrtingresonble greement with mesured inductnce up to 30 MHz. The 30-turn inductor integrted with lminted CoNiFe core exhibits constnt inductnce of 1.15 µh (inductncedensity of µh mm 2 ) up to 30 MHz, showing n pproximtely 12 times inductnce increse over the ir core inductor. From eqution (3), the effective permebility of the nnolminted CoNiFe core is estimted s 330. The mesured inductnce is lso compred with theoreticl prediction using equtions (1) nd(5). Since the single lmintion thickness (500 nm) of the core is still below the skin depth t 30 MHz ( 1 µm), the theoreticl totl inductnce of the integrted inductor with nnolminted CoNiFe core predicts constnt inductnce of 1.15 µhupto30mhz.thegoodgreement of mesured inductnce with theoreticl prediction indictes improved insultion from thicker interlmintion gp s well Figure 9. Chrcteriztion of 30-turn-integrted inductors with nnolminted mgnetic cores, s well s n ir core by mens of permnent core embedding pproch. Inductnce, resistnce, (c)qulityfctor. s uniform lmintion thickness, resulting in suppressed eddycurrent flow in the nnolminted core. It is lso expected tht the sprsely distributed 30-turn-windings compred to the densely distributed 50-turn-windings minimize the cpcitive effect between the core nd the windings. In figure 9, the resistnces of both inductors t 500 khz re less thn 1 due to the fewer number of turns compred with figure 8, nd increses s function of opertion frequency due to winding nd core losses. The similr resistnce of both inductors in the low-frequency region indictes the chievement of the sme winding thickness nd height nd improved mechnicl integrity between verticl nd top windings. In figure 9(c),the qulity fctor of the integrted inductor with nnolminted CoNiFe core is pproching 20 t 5 MHz, nd is greter thn tht of the ir core inductor up to 15 MHz. Compred to the pek qulity fctor frequency ( 6 MHz)from the 100 lyers (c) 8

9 of 300 nm thick CoNiFe lmintions shown in figure 8(c), the pek qulity fctor ppers t lower frequency ( 4.5 MHz) due to the thick lmintion (500 nm) indicting tht the opertionl frequency cn be djusted by controlling single lmintion thickness. Also, the higher qulity fctor implies tht optimiztion of winding geometry (e.g., winging width, thickness nd height) s well s core design (e.g., lmintion thickness, gp between the lmintions) cn improve the qulity fctors of the integrted inductors with nnolminted mgnetic core. 4. Conclusion The design, fbriction nd chrcteriztion of integrted toroidl inductors with nnolminted ferromgnetic metllic cores by mens of drop-in pproch re presented. Nnolminted CoNiFe, potentilly usble t higher opertion fluxes thn conventionl ferrites, is utilized s demonstrtion core mteril. Two different pproches to core integrtion, the temporry core embedding pproch nd the permnent core embedding pproch, re provided nd compred. Thirty-turn nd 50-turn-toroidl inductors integrted with the nnolminted CoNiFe cores re fbricted nd tested. The resulting toroidl inductors exhibit n pproximtely 10 inductnce increse from nominlly identicl ir core inductors t opertion frequencies up to 30 MHz. The inductor pek qulity fctor pproches 20 t low MHz frequency. Since the CoNiFe nnolminted mteril hs high sturtion flux density compred to ferrites nd low-eddycurrent losses due to lmintion, microfbricted inductors bsed on these mterils hve the potentil to enble ultrcompct dc/dc power conversion opertion t high power nd high frequency. Although the integrtion pproch is illustrted using nnolminted lloys, it is lso potentilly pplicble to commercil ferrites nd silicon steels with suitble geometries. Acknowledgments This work ws supported by the Advnced Reserch Projects Agency-Energy under the Agile Delivery of Electricl Power Technology progrm (Awrd DE-AR ). Microfbriction ws crried out in prt in Georgi Tech s Institute for Electronics nd Nnotechnology. The uthors would like to thnk Richrd Shfer for his help on the electroplting robot opertion. References [1] Lee C F nd Li Q 2013 High-frequency integrted point-of-lod converters: overview IEEE Trns. Power Electron [2] Li Q, Lim M, Sun J, Bll A, Ying Y, Lee C F nd Ngo K D T 2010 Technology rod mp for high frequency integrted dc dc converter APEC: Applied Power Electronics Conf. nd Exposition pp [3] Sullivn C R, Hrburg D V, Qui J, Levey C G nd Yo D 2013 Integrting mgnetics for on-chip power: perspective IEEE Trns. Power Electron [4] Mthun C O, Wng N, Kulkrni S nd Roy S 2012 Review of integrted mgnetics for power supply on chip (PwrSoC) IEEE Trns. Power Electron [5] Flynn D, Toon A, Allen L, Dhriwl R nd Desmulliez M P Y 2007 Chrcteriztion of core mterils for microscle mgnetic components operting in the meghertz frequency rnge IEEE Trns. Mgn [6] Lmmerner J nd Stfl M 1966 Eddy Current (London: Iliffe Books) [7] Bertotti G 1998 Hysteresis in Mgnetism (London: Acdemic) [8] Grdner D S, Schrom G, Pillet F, Jmieson B, Krnik T nd Borkr S 2009 Review of on-chip inductor structures with mgnetic films IEEE Trns. Mgn [9] Wng N, O Donnell T, Roy S, McClosky P nd O Mthun C 2007 Micro-inductors integrted no silicon for power supply on chip J. Mgn. Mgn. Mter. 316 e233 7 [10] Frommberger M, Schmutz C, Tewes M, McCord J, Hrtung W, Losehnd R nd Qundt E 2005 Integrtion of crossed nisotropy mgnetic core into toroidl thin-film inductors IEEE Trns. Microw. Theory [11] Lee D W, Hwng K-P nd Wng S X 2008 Design nd fbriction of integrted solenoid inductors with mgnetic cores ECTC: Electronic Components nd Technology Conf. pp [12] Kim J, Kim M, Glle W P, Herrult F, Shfer R, Prk J Y nd Allen M G 2013 Nnolminted permlloy core for high-flux, high-frequency ultrcompct power conversion IEEE Trns. Power Electron [13] Kim J, Herrult F, Yu X, Kim M, Shfer R H nd Allen M G 2013 Microfbriction of ir core power inductors with metl-encpsulted polymer vis J. Micromech. Microeng [14] Kim M, Herrult F, Kim J nd Allen M G 2013 Monolithiclly-fbricted lminted inductors with electrodeposited silver windings MEMS 2013: 26th Int. Conf. on Micro Electro Mechnicl Systems (Tipei, Tiwn) pp [15] Herrult F, Glle W P, Shfer R H nd Allen M G 2011 Electroplting-bsed pproches for volumetric nnomnufcturing Proc. Tech. Dig. Technol. Future Micro-Nno Mnuf. pp 71 4 [16] Osk T, Tki M, Hyshi K, Ohshi K, Sito K nd Ymd K 1998 A soft mgnetic CoNiFe film with high sturtion mgnetic flux density nd low coercivity Nture [17] Sverdlov Y, Rosenberg Y, Rozenberg Y I, Zmood R, Erlich R, Ntn S nd Shchm-D Y 2004 The electrodeposition of coblt nickel iron spect rtio thick film structures for mgnetic MEMS pplictions Microelectron. Eng [18] Kim M, Kim J, Herrult F, Shfer R H nd Allen M G 2013 A MEMS lmintion technology bsed on sequentil multilyer electrodeposition J. Micromech. Microeng. t press [19] Kiser C J 1996 The Inductor Hndbook (Olthe: CJ Publishing) 9