Akita Research Institute of Advanced Technology, Akita 010-1623, Japan. Abstract â Closed magnetic circuit type ferromagnetic RF integrated inductors have ...
Ferromagnetic RF Integrated Inductor with Closed Magnetic Circuit Structure Masahiro Yamaguchi1, Seok Bae1, Ki Hyeon Kim1, Kenji Tan2, Takayuki Kusumi2, Kiyoshi Yamakawa2 1
Department of Electrical and Communication Engineering, Tohoku University, Sendai 980-8579, Japan 2 Akita Research Institute of Advanced Technology, Akita 010-1623, Japan
Abstract — Closed magnetic circuit type ferromagnetic RF integrated inductors have been fabricated based on MEMS-like micro fabrication techniques. The taper etching process greatly helped to endure sufficient magnetic flux flow at the edge of the top and bottom magnetic layers. Air cores and three different sandwich type ferromagnetic inductos are also microfabricated. Measured results exhibited the quality factor Q=12, being highest among the published data of ferromagnetic RF integrated inductor at 1 GHz. Index Terms — integrated inductor, magnetic film, electromagnetic field simulation, quality factor.
Fig. 1 Closed magnetic circuit type RF integrated inductor.
I. INTRODUCTION Miniaturized and high-Q on-chip spiral inductors are required for high performance RF analogue devices and circuits as integrated passives. While most efforts are done to reduce ohmic/eddy-current losses and stray capacitance, this paper tries to enhance the inductance by means of introducing permeable materials to the spiral structure. This work is on the series of our study to use ferromagnetic thin film to spiral inductor as the on-top type [1] and the sandwich type [2] inductors. This paper demonstrates the first experimental results on the closed magnetic circuit type GHzdrive integrated spiral inductors. The closed magnetic circuit contributes to endure sufficient magnetic flux flow at the edge of the top and bottom magnetic layers.
(a) Outlook of the inductor with ground guard. (Type A) (Type Al)
(Type S)
(Type C)
II. 2. STRUCTURE AND DESIGN Fig. 1 shows the structure of the closed magnetic circuit type inductor discussed in this work. The inductor consists of a stack of the 0.2-µm-thick RF-sputter deposited CoNbZr film, the RF-sputter deposited SiO2, the 3-µm-thick electrodeposited Cu coil, the SiO2 with surface planarization, and the 0.2-µm-thick RF-sputter deposited CoNbZr. Lead line was also processed to complete the four turn and 380x380 µm2 two-port type inductors with ground guards. Fig. 2 and Table I show the list of the fabricated inductors. Closed magnetic circuit type inductor is denoted as “C Type.” Other designs were also studied to investigate the influence of ferromagnetic resonance (FMR) frequency of magnetic film, eddy current losses and stray capacitance. The name of the air-core inductors begin from A, while the ferromagnetic inductors have names with either of P, Al, S or C depending
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(b) Cross sectional view of the inductors. Fig. 2 Outlook and cross sectional view of (a) Schematic description for inductor structure, (b) Cross sectional view of right part of inductor coil with magnetic aligned, shifted and closed designs
on the structure of magnetic films. The P type is without slits on the magnetic film, and the edges of the top and the bottom magnetic films are not terminated. The Al and S types are similar to the P except that these two types have slits on the magnetic film [3]. Each magnetic film specimen of the Al type faces a leg of spiral coil exactly while the magnetic film specimen of the S type faces the gap of each leg of the spiral.
Table I List of the fabricated inductors ~GO༁P
nGO༁P
hT
wG
h
z
j
XW
Z
hTX
wTX
hTX
zTX
jTX
XW
\
hTY
wTY
hTY
zTY
jTY
YW
Z
hTZ
wTZ
hTZ
zTZ
jTZ
YW
\
hT[
wT[
hT[
zT[
jT[
Fig. 3 shows the simulated inductance and Q-factor of the closed type inductor as a function of real part permeability. Imaginary part permeability was set to zero in this simulation. The inductance becomes higher with the permeability increasing while the Q-factor degrades in the permeability>100 range. This is because of the amount of leakage flux associated with the structure and dimension. Permeabity of 100 would be the best design to have the highest inductance with keeping the Q-factor at the highest
The C type is based on the Al type with the top and the bottom magnetic films are terminated to each other at the edge of themselves. Design parameters are as follows. Coil: Spiral 4 turns made of Cu, 20 µm wide with 3µm wide spacing, 2 µm thick and 20µm wide. Magnetic film: 0.2µm thick Co85Nb12Zr3 (at.%.) Insulator: 1µm thick SiO2. The in-plane distance between ground guard and outer most leg of the coil was fixed as 75µm. Spiral area was 300 µm by 300 µm.
6.0 8.5
5.8
Inductance (nH)
The HFSS (v. 8.5 by Ansoft. Co) simulation program have been used to study the application of patterned magnetic films in RF integrated inductors. Eddy currents were took into account for all objectives and mesh planes divided the magnetic films and coil lines into 0.2µm and 0.5µm thinner than the skin depth at 2 GHz, that was aimed for mobile handset applications. As well, frequency property of magnetic materials was used that calculated values. The L, R, Q values were extracted from S parameters of simulation results using well known electric circuit theory [4]. Table II shows the simulation results. The inductance can be more than doubled by the no-slit magnetic film while the Q-factor degrades down to less than one-half by the no-slit magnetic film. The quality factor can be improved by using patterned magnetic film of any design and the closed design (C type) could produce similar Q-factor as the air core of 10 with the advantage of inductance by 30% higher than the air core’s.
Air-core
Enhance of L (%) 0
R (ȍ) 5.46
No slit
10.01
129.6
27.6
Aligned
5.15
18.1
9.40
6.8
Shifted
4.59
5.3
10.28
5.7
Closed
5.70
30.7
7.23
9.9
5.4
7.5
5.2
7.0
5.0
6.5
4.8
6.0
4.6 4.4
air-core's R
5.5
air-core's L
5.0
4.2 4.0
4.5 10
100
1000
10000
Permeability (a) Inductance 12.5 12.0
Q factor
11.5
Table II. L, R, Q at 2 GHz of air-core, aligned, shifted and closed magnetic core inductor L (nH) 4.36
8.0
5.6
Resistance (¥Ø)
III. ELECTROMAGNETIC FIELD SIMULATION
11.0 10.5 air-core's Q
10.0 9.5
Q 9.0
10.3
8.5
4.6
10
100
1000
10000
Permeability (b) Q-factor Fig. 3 Calculated L and Q value at 2 GHz as a function of real part of relative permeability
f=2GHz
level.
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IV. MICRO-FABRICATION PROCESS Closed magnetic circuit type ferromagnetic RF integrated inductors have been fabricated based on MEMS-like microfabrication techniques. Air cores and ferromagnetic inductors with three different geometrical design of patterned magnetic films are also microfabricated. In Table I, The top-side SiO2 layer was applied special taper etching process to make sure the closed magnetic circuit structure, Fig. 4 shows the test pattern of the taper etching process, showing the cross section of .the tapered lines with different trench widths. The SiO2 thickness was set to either of 1 Pm or 4 Pm. The line/space [Pm] of the coil was either of 10/3, 10/5, 20/3, or 20/5. The CoNbZr film was applied slit to shift the ferromagnetic resonance (FMR) frequency. Its line/space was identical to those of the Cu. Fig. 5 shows the patterned bottom magnetic layers, and Fig. 6 shows the completed inductors. The pad line was drawn from the center of the spiral to the left.
XUYG༁
\WÝXWG༁
XYU^G༁ \WÝYWG༁
YYUXG༁
\WÝZWG༁
IV. MEASUREMENTS
Fig. 4 Tapered SiO2 layers with different window width.
Two GSG type RF wafer probes (Cascade Microtech, Inc., ACP40-A-GSG) were used to measure scattering parameters of S11, S21, S21 and S22, respectively, by using a network analyzer (HP 8720D). A equivalent circuit analysis we on-top type ferromagnetic RF integrated spiral inductor [3] was applied to extract the lumped element constants. The definition of the quality factor, Q, of the inductor in this work is the ratio of the impedance of the series inductance to the
series resistance (Q= ZL/R.)
Table III Microfabrication process flow Outline
Details
Cleaning
Sample dicing, cleaning
Bottom CoNbZr layer
Ti/CoNbZr/Ti sputtering Photo lithography, ion milling, P.R. removing
insulator
SiO2 sputtering
Fig. 5 Patterned bottom magnetic layers integrated
Ti/Cu seed sputtering, align mark opening Coil
Photo lithography, Cu electroplating, Ti P.R. removing, Ion milling
insulator
SiO2 sputtering Photo lithography, Taper etching, P.R.
Top CoNbZr layer
Ti/CoNbZr/Ti sputtering, align mark opening
Insulator
SiO2 sputtering
Pad open
Photo lithography, RIE, P.R. removing
Pad line connection
Photo lithography, Ti/Cu sputtering
Annealing
Magnetic field annealing
Photo lithography, ion milling, P.R. removing
Lift off
Fig. 6 Tapered SiO2 layers with different window
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Fig. 8 shows the inductance (L) and quality factor (Q) at 1 GHz with the different type of ferromagnetic inductor. The Q factor is linearly decreased in turn from 14.7 of air core, 11.8 of S type , 11.1 of A type, 8 of C type to 3.05 of plain film (no slit) inductor at 1 GHz as stacking of magnetic film. Q=14.7 of air core is highest among the published data at 1 GHz [4]. This was achieved through: (a) The narrower Cu coil design and the thinner SiO2 design yielded higher magnetic field at the magnetic film area. (b)The magnetic slit design of 10/3 well shifted the FMR frequency beyond 1 GHz. (c) Stray capacitance between the coils legs, and between the coil and magnetic film were negligible at 1GHz. VII. CONCLUSION Fig. 7 Inductance of each type inductor with the increase of frequency
RF electromagnetic field simulation of four types of ferromagnetic RF integrated inductors have been performed, followed by actual microfabrication and measurements. The closed magnetic circuit type ferromagnetic RF integrated inductor exhibited the quality factor Q=12, being highest among the published data at 1 GHz.
ACKNOWLEDGEMENT The authors thanks Prof. K.I. Arai, Prof. Y. Nakamura, Laboratory for Nanoelectronics and Spintronics in RIEC, and Venture Business Laboratory, Tohoku University, and also by ITIM for supported equipments. REFERENCES [1] M. Yamaguchi, K. Suezawa, K. I. Arai, Y. Takahashi, S. Kikuchi, Y. Shimada, W. D. Li, S. Tanabe, and K. Ito, “Microfabrication and characteristics of magnetic thin-film inductors in the ultrahigh frequency region,” Journal of Applied Physics, vol.85,pp. 7919-7922,1999 [2] Masahiro Yamaguchi, Makoto Baba and Ken-Ichi Arai, “Sandwich Type Ferromagnetic RF Integrated Inductor,” IEEE Trans. on Microwave Theory and Techniques, vol.49,pp.23312335,2001 [3] Masahiro Yamaguchi, Yoshisato Yokoyama1, Shinji Ikeda, Takashi Kuribara, Kazuya Masu1 and Ken-Ichi Arai, “Equivalent Circuit Analysis of RF-Integrated Inductors with/without Ferromagnetic Material,” Japanese Journal of Applied Physics, vol.42,pp.2210-2213,2003 [4] K. Ikeda, K. Kobayashi, K. Ohta, R. Kondo, T. Suzuki, M. Fujimoto, “Thin-film inductor for gigahertz band with CoFeSiO-SiO/sub 2/ multilayer granular films and its application for power amplifier module,” IEEE Transactions on Magneitcs, vol.39,pp.3057-3061,2003
Fig. 8 Inductance and Q values (at 1 GHz) with the various type of the inductor.
V. EXPERIMENTAL RESULTS Fig. 7 shows the frequency dependence of inductance (L) from 100 MHz to 10 GHz including air core type. The P type inductor shows inductance of 13.7 nH, that is enhanced 71 % higher than 7.97 nH of the air core inductor. It implies that the higher inductance in gigahertz range can be obtained by using the magnetic film. Among the patterned magnetic film inductors, the inductance is increased gradually from 7 % of S type (8.5 nH), 13 % of A type (9.0 nH) and 23 % of C type (9.8 nH) than that of air core nductor.
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