A Megahertz Switching DC/DC Converter Using FeBN ... - IEEE Xplore

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Ki Hyeon Kim, Jongryoul Kim, Hee Jun Kim, Suk Hee Han, and Hi Jung Kim, Member, IEEE. Abstract—A dual spiral sandwiched thin film inductor with a di-.
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 38, NO. 5, SEPTEMBER 2002

A Megahertz Switching DC/DC Converter Using FeBN Thin Film Inductor Ki Hyeon Kim, Jongryoul Kim, Hee Jun Kim, Suk Hee Han, and Hi Jung Kim, Member, IEEE

Abstract—A dual spiral sandwiched thin film inductor with a dimension of 5 mm 5 mm was fabricated by a LIGA-like micromachined process. FeBN magnetic films were used as a core material. The inductance and quality factor value of the inductor were measured approximately 1 H and 4 up to 5 MHz, respectively. Using the FeBN film inductor, a hybrid dc/dc converter (15 mm 12 mm 1.5 mm) was designed and fabricated. The circuit topology of the converter was a zero voltage switching clamp voltage (ZVS-CV) buck converter. An input of 3.6 V was bucked to 2.7 V at a switching frequency of 1.8 MHz. The maximum power was 1.5 W. The measured efficiency of the buck converter reached a maximum value of 80% and kept stable up to 300 mA of load currents at 1.8 MHz. Index Terms—DC/DC converter, magnetic core, permeability, thin film inductor.

I. INTRODUCTION

M

AGNETIC devices have been confronted with a strong demand for further miniaturization and high-frequency operation. To meet the demand, it is generally accepted that thin film magnetic devices should be substituted for conventional bulk devices. For example, planar inductive devices showed excellent high-frequency characteristics compared with their conventional counterparts [1], [2]. Thus, the application of planar inductive devices to power electronics, such as switching converters and inverters, may promise to enhance the miniaturization and efficiency of electronic products. However, the characteristics of inductor and converter in the power electronics are strongly dependent on the magnetic core materials. Therefore, the development of high inductive magnetic films with high-frequency characteristics and fabrication of excellent magnetic devices are strongly required [2]. In this paper, a hybrid dc/dc converter was fabricated to examine the applicability of a thin film inductor to power electronics. The inductor was a dual spiral sandwiched thin film inductor that was fabricated by a LIGA-like micromachined process. FeBN was used as a core material of the inductor and the deposition parameters of FeBN films were varied to improve the high-frequency characteristics. Using this thin film inductor, Manuscript received February 19, 2002; revised April 28. 2002. This work was supported by the Postdoctoral Fellowship Program of Korea Science & Engineering Foundation (KOSEF). K. H. Kim is with the Research Institute of Electrical Communication, Tohoku University, 980-8577 Sendai, Japan (e-mail: [email protected]). J. Kim and H. J. Kim are with the Department of Metallurgy and Materials Engineering and the School of Electrical and Computer Science, Hanyang University, 425-791 Ansan, Korea (e-mail: [email protected]; [email protected]). S. H. Han and H. J. Kim are with the Nano Device Research Center and the Division of Materials Science, KIST, 130-650 Seoul, Korea (e-mail: [email protected]; [email protected]). Digital Object Identifier 10.1109/TMAG.2002.802401.

Fig. 1. Schematic diagram of a dual spiral sandwich type magnetic thin film inductor.

a zero-voltage switching clamp voltage (ZVS-CV) buck converter was fabricated. The performance of the ZVS-CV buck converter was evaluated in a frequency range of 1 to 10 MHz. II. EXPERIMENT The structure of a rectangular dual spiral inductor fabricated in this experiment is schematically illustrated in Fig. 1. The m wafer, grown inductor was fabricated on a Si–SiO by the thermal and plasma oxidation method and composed of magnetic layer (FeBN)/insulation layer (polyimide)/spiral coil (Cu)/insulation layer (polyimide)/magnetic layer (FeBN), as shown in Fig. 1. The dimensions of the planar inductor were 5 mm 5 mm with about 26- m thickness excluding Si substrate. As-deposited FeBN films by N reactive radio-frequency (RF) magnetron sputtering were used as lower and upper magnetic layers with a thickness of 3 m. During the deposition of the magnetic films, a magnetic field using magnetic bars was applied to control the anisotropy direction of the films. As a result, the hard magnetization axis of the magnetic layers was aligned normal to the direction of coil conductor current, as shown in Fig. 1. In order to deposit Cu coil, Cr(200 ) was deposited as a seed layer by RF magnetron sputtering and then 15- m-thick photoresist was applied for coil patterns. After masking of the patterns, Cu coils were selectively electroplated to a thickness of 10 m. The electrolyte for Cu electroplating was composed of CuSO , H SO and DI water. After the photoresist was removed, the seed layers were eliminated by wet etching. The polyimide as a insulating layer was spin-coated between the magnetic layer and the Cu coil. The polyimide upon the electrode part was removed by reactive ion etching

0018-9464/02$17.00 © 2002 IEEE

KIM et al.: DC/DC CONVERTER USING FeBN THIN FILM INDUCTOR

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Fig. 5. Circuit diagram of the ZVS-CV buck converter. Fig. 2. Saturation magnetization and coercivity of FeBN films with the increment of N partial pressure.

(a)

(b)

Fig. 3. Frequency dependency of the effective permeability and loss of FeBN films with the increment of N partial pressure.

Fig. 6. Top view and cross-sectional view of a dc/dc converter with a chip inductor (a) and a thin film inductor (b).

(RIE). The saturation magnetization (4 ) and effective ) of magnetic films were measured by a pearmeability ( vibrating sample magnetometer (VSM) and HP 4396 network analyzer, respectively. The values of inductance were measured using HP 4192A impedance analyzer from 1 to 10 MHz. III. RESULTS AND DISCUSSION

Fig. 4. Frequency dependency of the inductance and quality factor for a FeBN thin film inductor.

As shown in Fig. 2, the range of 4 and coercivity ) of Fe–B–N films is 16.7–20.8 kG, 0.2–30 Oe ( with the variation of N partial pressure (PN ), respectively. The coercivity shows a minimum critical point at a PN of 3.5%. Fig. 3 shows that the frequency dependence of permeabilities are maintained between 100 and film 500 MHz. Especially, a Fe B N PN is 19.5 kG, magnetic anisotropy field shows that 4

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IEEE TRANSACTIONS ON MAGNETICS, VOL. 38, NO. 5, SEPTEMBER 2002

Fig. 8. Efficiency of ZVS-CV with the increment of load current. (a)

(b) Fig. 7. (a) Waveform of inductor current I (upper), V (b) waveform of subswitch, I (upper), V (lower).

Oe,

(lower) and

(at 100 MHz). In addition, a PN film exhibits excellent high-freFe B N with high quency characteristics up to 500 MHz kG and Oe. The electrical resistivity film shows twice ( 100 cm) of the Fe B N film. These in comparison with that of the Fe B N high resistivity and magnetic anisotropy are contributed to improve the high-frequency characteristics [3]. In addition, ferromagnetic resonance (FMR), which means loss due to spin precession in high-frequency region, should be desirable to enhance high-frequency characteristics. The FMR frequency [4], [5] is expressed as where is the gyromagnetic constant (if value of spectrofactor is 2, MHz/Oe) and scopic splitting is the magnetic in-plane anisotropy field. The calcuand lated FMR frequency of Fe B N PN PN films are about 0.7 and Fe B N 1.3 GHz, respectively. As aforementioned, the as-deposited nanocrystalline FeBN films showed the excellent soft magnetic properties and highfrequency characteristics in comparison with FeXN(X Hf, Al, PN film Ti, etc.) [6]. However, the Fe B N was used as a core material of an inductor to ensure the perfor-

mance. The desired inductance and quality factor was obtained 1 H, a using this film. As shown in Fig. 4, inductance of quality factor of 5, and coil resistance of 1 are achieved m, at 8 MHz using a thin film inductor with coil width m, thickness m, coil turns . gap A hybrid ZVS-CV buck converter was designed which could recompense the efficiency decline by ZVS. The circuit topology of the converter was a ZVS-CV buck converter (Fig. 5). The circuitry enclosed by dashed lines is the film inductor. Fig. 6 shows a photograph of the dc/dc converter using chip inductor (a) and thin film inductor (b). A thin film inductor was connected to circuit board by the wire bonding method. An input of 3.6 V was bucked to 2.7 V in the 1.8 MHz switching frequency (Fig. 7). The maximum power was 1.5 W. Measured efficiency of ZVS-CV buck converter reaches a maximum value of about 80% (Fig. 8), which is stable up to a load current of 300 mA at 1.8 MHz. IV. CONCLUSION Fabricated inductors employing FeBN films showed excellent properties, such as an inductance of 1 H and a quality factor of 5 (at 8 MHz). These inductors were used to fabricate a hybrid ZVS-CV buck converter. The converter showed about 80% of the conversion efficiency at 1.2 MHz, which could withstand load current up to 300 mA. REFERENCES [1] T. Sato, H. Tomita, A. Sawabe, T. Inoue, T. Mizoguchi, and M. Sahashi, “A magnetic thin film inductor and its application to a MHz switching dc–dc converter,” IEEE Trans. Magn., vol. 30, pp. 217–223, 1994. [2] K. H. Kim, D. W. Yoo, J. H. Jeong, J. Kim, S. H. Han, and H. J. Kim, “Dual spiral sandwiched magnetic thin film inductor using Fe–Hf–N soft magnetic films as a magnetic core,” J. Magn. Magn. Mater., vol. 239, no. 1–3, pp. 579–581, 2002. [3] K. H. Kim, J. H. Jeong, J. Kim, S. H. Han, and H. J. Kim, “High moment and high frequency permeability Fe–B–N nanocrystalline soft magnetic films,” J. Magn. Magn. Mater., vol. 239, no. 1–3, pp. 487–489, 2002. [4] E. van de Riet and F. Roozeboom, “Ferromagnetic resonance and eddy currents in high-permeable films,” J. Appl. Phys., vol. 81, p. 350, 1997. [5] B. Lax and K. J. Button, Microwave Ferrites and Ferrimagnetics. New York: McGraw-Hill, 1962, p. 159. [6] K. H. Kim, H. W. Choi, J. Kim, S. R. Kim, K. Y. Kim, S. H. Han, and H. J. Kim, “Magnetic properties and reliabilities of FeXn(X Ti, Al, Hf, CoHf, CrHf) nanocrystalline thin film head materials,” IEEE Trans. Magn., vol. 36, pp. 2656–2659, 2000.

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