This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in JOURNAL OF APPLIED PHYSICS. 99(8):08P508 (2006) and may be found at https://doi.org/10.1063/1.2172580 .ArticleJOURNAL OF APPLIED PHYSICS. 99(8):08P508 (2006)journal articl

Akie,  M

Miura,  Y

Miyake,  Y

Mizoguchi,  Y

Munakata,  M

Nakayama,  H

Nakazawa,  M

Sato,  T

Uehara,  Y

Yagi,  M

Yamamoto,  Y

Yamasawa,  K

Journal of Applied Physics

English

Shinshu University Institutional Repository

Copyright (2006) American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.  The following article appeared in J. Appl. Phys. 99, 08P508 (2006) and may be found at http://dx.doi.org/10.1063/1.2172580.Suppression of Insertion Loss by Slit-Patterning of a Magnetic Film in a CoFeB/Polyimide Hybrid Thin-Film Coplanar-Line for a RF Im-pedance Matching Device  Hidetoshi Nakayama Faculty of Engineering, Shinshu University, Nagano 380-8553, Japan Nagano National College of Technology, Nagano 381-8550, Japan  Tomohiro Yamamoto, Yuko Mizoguchi, Masahiro Nakazawa, Toshiro Sato, Kiyohito Yamasawa, and Yoshimasa Miura Faculty of Engineering, Shinshu University, Nagano 380-8553, Japan  Yuko Miyake, Masanori Akie, and Yuji Uehara Fujitsu Limited, Nagano 381-8501, Japan  Makoto Munakata and Masaaki Yagi Energy Electronics Laboratory, Sojo University, Kumamoto 860-0082, Japan  (Presented on ; published online )  A hybrid thin-film coplanar-line with a slit-patterned CoFeB magnetic film for a RF impedance matching was fabricated, and an effect of the loss reduction by introducing a slit-patterned magnetic film was investigated. The fabricated hybrid thin-film coplanar-line device consisted of the top and bottom 5 µm thick copper conductors, an inner (0.5 µm thick polyimide)/(0.15 µm thick CoFeB)/(0.5 µm thick polyimide) trilayer. A signal line width was 6 µm. Total width of the slit-patterned magnetic film was about 200 µm. The devices with some kinds of the width of the magnetic film stripe were fabricated. As a result, insertion loss per unit line length decreased greatly above 1.5 GHz with de-creasing width of the magnetic film. The loss suppression was due to an effect of reducing the in-plane eddy current of the magnetic film. On the other hand, the propagation signal-wavelength of the device increased with decreasing width of the magnetic film, which was due to a decrease of distributed in-ductance. When applying the device to a quarter-wavelength impedance matching device, the slit-patterned magnetic film was effective for decreasing the insertion loss at high frequencies beyond 1.8 GHz.   I. INTRODUCTION Recently, wireless communication equipments such as cellu-lar phones and wireless-LAN have increased rapidly. For example, RF front-end circuit in the cellular phones consists of various devices such as power amplifier, LNA, LC impedance matching circuit, filter, duplexer, antenna, and so on. Although the low temperature co-fired ceramics (LTCC) chip LC devices consisting of the plural passives are currently used for the RF impedance matching circuit, it is difficult to integrate it into RF IC. On the other hand, the monolithic microwave integrated circuit (MMIC) including thin film passives such as integrated spiral air core in-ductors and capacitors has been developed.1 However, it is not so easy to reduce the chip size of MMIC, because of a difficulty of the miniaturization for the spiral inductor. Yamaguchi et al.2 de-veloped an RF integrated magnetic thin film inductor and demon-strated that the inductor can be miniaturized by introducing a magnetic thin film to the air core spiral. As well known, a quarter-wavelength transmission-line func-tions as an impedance conversion transformer. The authors re-ported a (CoFeB magnetic film3)/(polyimide dielectric film) hy-brid transmission-line with a large effect of the wavelength short-ening.4,5 If the device has the acceptable size and insertion loss for practical use, the hybrid transmission-line would be used for an integrated RF impedance matching device. In a previous paper,5 insertion loss of the fabricated hybrid thin-film coplanar-line with a CoFeB magnetic film for a RF im-pedance matching device was about 0.8 dB, which was larger than the target specification of below 0.5 dB at frequency of 1.8 GHz range used in the GSM cellular phone. One cause of the large loss was owing to an in-plane eddy current due to the perpendicular magnetic flux component to the magnetic film plane. On the other hand, it has been reported that a slit-patterned magnetic film is effective to the loss suppression of the high frequency band except ferromagnetic resonance (FRM) frequency.6The purpose of this study is to develop the impedance match-ing device for GHz frequency band using hybrid thin-film trans-mission-line. In this study, to reduce an in-plane eddy current loss, an effect of insertion loss decrease by using a slit-patterned mag-netic film for a hybrid thin-film coplanar-line for a RF impedance matching device was investigated.  II. FABRICATION OF THE HYBRID THIN-FILM COPLA-NAR-LINE The magnetic/dielectric hybrid thin film transmission-line op-erates as a distributed constant circuit with a large effect of wave-length shortening, because of an enhancement of inductance due to the magnetic film. When a quarter wavelength λ/4 is equal to a line length l at a specific frequency fλ/4, the device functions as an impedance conversion transformer, that is an impedance matching device. When a signal source impedance is Zs and a load imped-ance is ZL, the impedance matching can be done by inserting the quarter wavelength device between Zs and ZL, where the quarter wavelength device should have a characteristic impedance Zc, and s follows. a )1(sLc ZZZ =   For example, in the power amplifier circuit of the cellular phones, Zs of the amplifier is a few ohms, and ZL of the antenna is from fifty to a few hundred ohms. Therefore, the characteristic imped-ance Zc of the device should have a few ten ohms. Fig. 1 shows the structure of the fabricated hybrid thin film coplanar-line device. The fabricated device consisted of the top and bottom 5 µm thick copper conductor layers, and an inner (0.5 µm thick polyimide)/(0.15 µm thick CoFeB)/(0.5 µm thick poly-imide) trilayer. The device was 1mm in width and 5 mm in length. The signal line and ground plane width were 6 µm and 397 µm, respectively. The Total width of a continuous film or slit-patterned magnetic film was about 200 µm. As shown in Fig. 2, the slit-patterned magnetic film consisting of some magnetic film bars (stripe width wm) and spaces (10 µm wide). The longitu-dinal direction of the magnetic film bar was same as the signal line direction. A plain CoFeB amorphous magnetic film had a FMR fre-quency of about 4 GHz and a static relative permeability of 180. A    FIG. 1  Structure of the fabricated hybrid thin-film coplanar-line.   FIG. 2  Structure of the magnetic film. (a) Continuous magnetic film and (b) slit-patterned magnetic film. polyimide dielectric film had a relative permittivity of 3.5. The specifications of the fabricated devices such as each film thickness and dimensions are shown in Fig. 1. The hybrid thin-film coplanar transmission-lines were fabricated through the following methods: (1) the top and bottom copper conductor films were deposited by electroplating, (2) the polyimide dielectric film was deposited by spin- coating and baking, (3) the CoFeB mag-netic film was deposited by RF magnetron sputtering with rotating a cylindrical electrode3 and was patterned by lift-off process. The devices with magnetic film stripe width wm of 10, 20, 40, and 200 µm were fabricated. Fig. 3 shows the photograph of the fabricated device with 10 µm wide magnetic film stripes.  III. EXPERIMENTAL RESULTS AND DISCUSSION The transmission characteristics of the fabricated devices were evaluated by using a network analyzer (HP 8720D) and GSG RF probe. The propagation signal wavelength λ, characteristic im-pedance Zc and insertion loss L were estimated by measured S parameters.5The fabricated devices with each magnetic film structure had the characteristic impedance of 30 to 40 Ω. Fig. 4 shows the frequency dependences of insertion loss per unit line length Lmm of the fabricated devices. In this figure, the insertion loss Lmm was almost constant 0.1dB/mm below 1.2 GHz in any case. It was considered that the constant loss below 1.2 GHz was due to a dc resistance of a signal line. The loss Lmm of the device with a continuous magnetic film increased rapidly be-yond 1.5 GHz. The increase of loss beyond 1.5 GHz was sup-pressed by slit-patterning of the magnetic film. Although not shown in detail here, it was found that the magnetic flux perpen-dicular to the magnetic film plane gives rise to the in-plane eddy current,5 hence it was considered that the rapid increase of loss of above 1.5 GHz is mainly due to the in-plane eddy current of the magnetic film, and the in-plane eddy current was suppressed by introducing the slit-patterned structure for the magnetic film. On the other hand, the peak of the loss per unit line length of each fabricated device occurred at around 6 GHz. The peak fre-quency related to FMR was shifted to high frequency side, which was due to the increase of shape anisotropy field with decreasing width of a magnetic film bar in Ref. 6. However, the peak fre-    FIG. 3  Photograph of the fabricated hybrid thin-film coplanar-line with 10 µm wide magnetic film stripes.   quency of the fabricated device was hardly changed with magnetic film stripe width in Fig. 4. It might be a cause that the effect of a shape anisotropy magnetic field is relatively small, because of CoFeB film with large intrinsic anisotropy magnetic field of about 10 kA/m. Fig. 5 shows the frequency dependences of the quarter wave-length λ/4 devices. The propagation signal-wavelength of the fabricated devices was almost in inverse proportion to the fre-quency. Signal-wavelength of the device increased with decreas-ing magnetic film stripe width of the slit-patterned magnetic film. It was considered that the increase of wavelength was influenced by increasing shape anisotropy magnetic field and decreasing magnetic film volume. Fig. 6 shows the frequency dependences of insertion loss of the four kinds of the quarter-wavelength devices with a continu-ous film, wm=40, 20 and 10 µm. In the frequency range that loss per unit line length was almost constant in Fig. 4, the insertion loss Lλ/4 decreased linearly with increasing frequency, which was due to a decrease of signal-wavelength. The insertion loss Lλ/4 exhibited a minimum value at around 1.2 to 2.5 GHz. When de-creasing magnetic film stripe width in the slit-patterned magnetic film, the frequency for minimum insertion loss became higher, which was due to the suppression of the in-plane eddy current loss of magnetic film. In this case, the slit-patterned magnetic film was effective for decreasing the insertion loss at high frequencies be-yond 1.8 GHz. FIG. 4  The frequency dependences of loss per unit line length Lmm of the fabricated devices. The labels show total width and stripe width of mag-netic film.    IV. CONCLUSIONS The CoFeB/polyimide hybrid thin-film coplanar transmis-sion-line for impedance matching was fabricated and investigated. (1) Loss per unit line length of the fabricated device decreased greatly above 1.5 GHz with decreasing the magnetic film stripe width of the patterned magnetic film, which was due to an effect of reducing the in-plane eddy current of the magnetic film. (2) The propagation signal-wavelength of the device increased with decreasing the magnetic film stripe width, which was due to an increase of a shape magnetic anisotropy and decrease of magnetic film volume. (3) When applying the device to a quar-ter-wavelength impedance matching, the slit-patterned magnetic film was effective for decreasing the insertion loss at high fre-quencies beyond 1.8 GHz. FIG. 5  The frequency dependence of quarter wavelength λ/4 of the fab-ricated device. The labels are similar to Fig. 4.    1 M. Aikawa, T. Ohira, T. Tokumitsu, T. Hirota, M. Muraguchi: Mono-lithic Microwave Integrated Circuit, Corona Publishing Co., Edited by IEICE of Japan, 224 (1997). 2 M. Yamaguchi, K.-I. Arai: J. Magn. Soc. Jpn. 25, 59 (2001). 3 M. Munakata, M. Namikawa, M. Motoyama, M. Yagi, Y. Shimada, M. Yamaguchi, and K. Arai: Trans. Magn. Soc. Jpn. 2, 388 (2002). 4 H. Nakayama, T. Yamamoto, T. Sato, K. Yamasawa, Y. Miura, M. Munakata, and M. Yagi: J. Magn. Soc. Jpn. 28, 157 (2004). 5 H. Nakayama, T. Yamamoto, Y. Mizoguchi, T. Sato, K. Yamasawa, Y. Miura, Y. Miyake, M. Akie, Y. Uehara, T. Endo, M. Munakata, and M. Yagi, J. Magn. Soc. Jpn., 29, 667 (2005). 6 S. Ikeda, K. H. Kim, and M. Yamaguchi, J. Appl. Phys., 97, 10F912 (2005). FIG. 6  The frequency dependence of insertion loss per a quarter wave-length Lλ/4 of the fabricated device. The labels are similar to Fig. 4.  

Suppression of insertion loss by slit patterning of a magnetic film in a CoFeB/polyimide hybrid thin-film coplanar line for a rf impedance matching device

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Suppression of insertion loss by slit patterning of a magnetic film in a CoFeB/polyimide hybrid thin-film coplanar line for a rf impedance matching device

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