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Tunable on-chip inductors up to 5 GHz using patterned permalloy laminations
Conference Paper in Electron Devices Meeting, 1988. IEDM '88. Technical Digest., International · January 2006
DOI: 10.1109/IEDM.2005.1609516 · Source: IEEE Xplore
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Tunable On-Chip Inductors Up to 5 GHz Using Patterned Permalloy Laminations
James Salvia, James A. Bain, and C. Patrick Yue
Department of Electrical & Computer Engineering
Carnegie Mellon University, Pittsburgh, PA 15213, USA
Abstract
This paper presents the design criteria, processing techniques,
and measurement results for an on-chip inductor that uses a
permalloy (Ni80Fe20) film to achieve a tunable reactance.
Eddy currents and ferromagnetic resonance are suppressed in
the thin permalloy film using patterning and laminations to
enable operation in the RF range. Measurements results show
a 40% increase in inductance, a 15% tuning range, and a Q
between 5 and 11 up to 5 GHz.
Introduction
High-quality variable reactance is an essential component in
tunable RF filters and oscillators. In existing tunable RF ICs,
varactors are used exclusively. Tunable on-chip inductors
have been demonstrated using active circuits [1] and MEMS
technologies [2-3]. However, excess noise and processing
complexity has prevented the wide adoption of these
techniques. Using magnetic materials for boosting on-chip
inductor performance has been reported. However, previous
attempts have been limited to fixed inductances operating
below 1 GHz [4] or with low quality factors [5]. In this work,
tunable on-chip inductors utilizing high-µ permalloy
(Ni80Fe20) thin films are reported for the first time. The
permeability of permalloy is adjusted through the application
of external DC magnetic fields, thereby making the
inductance tunable.
Tunable Inductor Design
A. Magnetic Circuit Model
The tuning range and improvement in inductance for a
conductor is proportional to the portion of its magnetic flux
path that is filled with magnetic material. To help visualize
this concept, consider the magnetic circuit shown in Fig. 1(a),
which models the magnetic flux generated by a single
conductor that is surrounded by four different materials
(Fig. 1(b)). In this circuit, the magnetomotive force (MMF)
produced by current flowing in the conductor (Io) generates
magnetic flux (Φ). This flux flows through four reluctances
(R ) which are inversely proportional to the corresponding
relative permeabilities (µr’s) of the materials surrounding the
conductor. The inductance is related to the µr’s as follow:
L=
∝
Φ 1
MMF
=
I o I o ℜ top + ℜ right + ℜ bottom + ℜ left
1
1
µ top
+
1
µ right
+
1
µ bottom
+
1
µ left
µtop
1
R top ~ µ
top
R left
MMF
Φ
R bottom
R right
µleft
Io
µright
Φ
µbottom
(a)
(b)
Figure 1. Equivalent magnetic circuit model of a conductor surrounded by
four materials with different µr.
In the case that only the space on top of the conductor is
occupied by a high permeability material (µtop >> 1), R top is
essentially shorted. As a result, the flux generated by the
conductor would increase by 33.3%, producing in the same
enhancement in inductance. However, other factors such as
the finite permeability and finite thickness of the magnetic
material significantly reduce the actual improvements that
can be realized in practice. An 11% improvement in
inductance is demonstrated in [6] with the addition of a single
layer of magnetic material to one side of a conductor.
Surrounding the conductor on all four sides with high-µ
material would yield the largest improvement in inductance
by shorting all four reluctances. However, to facilitate postprocessing on standard ICs, in our design, the permalloy film
is deposited on the surface of the inductor traces. As a result,
each conductor is wrapped around on three sides by the
permalloy, thus shorting R top, R left, and R right.
B. Processing Steps
As shown schematically in Fig. 2, a conformal NiFe
deposition followed by a photoresist lift-off results in a
permalloy film that wraps around three sides of the
conductors. This design simplifies the fabrication in
comparison to using the high-µ material as the inductor core
[4] or as an underlying shield [5]. Figure 3 shows the loop
inductor test structure with Signal-Ground-Signal RF probe
pads. SEM photos of the top and cross-sectional view of the
inductor trace covered with permalloy are shown in Fig. 4
and Fig. 5, respectively.
C. Permalloy with Tunable Permeability
.
(1)
r
Permalloy has a preferred magnetic orientation ( M ) which
lies in the plane of the film along its easy axis (Fig. 6). High
r
permeability, which stems from the rotation of M , is attained
only when the permalloy film is excited by the inductor’s
0-7803-9269-8/05/$20.00 (c) 2005 IEEE
r
magnetic field that is perpendicular to M and in the plane of
the film. Based on this insight, the loop inductor design with
two long sides was chosen to allow the majority of the
magnetic material to operate with a single preferred
magnetization direction that is parallel to the long dimension
of the inductor. During the sputter deposition of the
permalloy film, the NiFe crystal structure is oriented using a
r
in-situ DC magnetic field such that the easy axis ( M ) of the
r
film is perpendicular to the RF magnetic field ( H RF )
r
associated with the RF currents ( I RF ) in the long sides of the
inductor (Fig. 6). This arrangement in turn enables high
frequency domain rotation. As a result, by applying an
r
r
external DC magnetic bias field ( H Bias ) parallel to M , the
permeability of the permalloy can be varied, thereby tuning
the inductance, as described by the model in [7]. The tunable
permeability as a function of the bias field magnitude (HBias )
is given by
Ms
,
(2)
µr = 1+
H k + H Bias
where Ms is the saturation magnetization and Hk is the
internal shape-induced anisotropy field of the permalloy film.
From measured results, Ms and Hk are extracted to be
800 kA/m and 28 kA/m, respectively.
D. Eddy Currents
Because of the high-µ permalloy’s high electrical
conductivity, eddy currents tend to circulate in a permalloy
film when it is driven in the RF range. The eddy currents
generate resistive losses in the film and significantly lower
the permalloy’s effective µr. Two techniques are employed to
suppress eddy currents and thus extend the inductor usable
frequency range. First, instead of a single layer of permalloy,
ten thin laminations (120 nm) separated by nine laminations
of silicon dioxide (10 nm) were used. Second, to prevent
displacement currents from flowing between individual
laminations through capacitive coupling (C in Fig. 6), the
r
laminations are patterned in the direction perpendicular to M
(along d in Fig. 6) to cut off the eddy current path. The width
of the patches is designed to be 29 µm (Fig. 4). This
dimension was chosen using the eddy current model of
patterned laminated films described in [7] and verified using
the Ansoft HFSS field solver.
E. Ferromagnetic Resonance
Permalloy films undergo ferromagnetic resonance (FMR) at
high frequencies [8]. Beyond the frequency of FMR (fFMR),
the permeability of the film drops below zero. Patterning the
r
permalloy stripes in the direction parallel to M (along w in
Fig. 6) is used to create a shape-induced anisotropy field
r
r
( H k ). The presence of H k increases fFMR at the expenses of
establish DC magnetic bias fields for tuning the inductance.
In practical IC implementation, the DC current flowing
through the inductor itself or another closely coupled inductor
could be used as the source of magnetic bias field. The
frequency response of the inductance and resistance for
inductors with and without permalloy under the influence of a
magnetic bias field are plotted in Fig. 8 and Fig. 9,
respectively. A 40% improvement in inductance with the
addition of the permalloy is observed up to 5 GHz without
any bias field. A tuning range of 15% is achieved by varying
the magnetic field strength from 0 to 80 kA/m. The measured
inductance tuning range agrees very well with EM simulation
as shown in Fig. 10. The roll-off in inductance (Fig. 8) and
peaking in resistance (Fig. 9) with increasing frequency are
due to FMR in the permalloy. At fFMR, a peak in the
imaginary part of the complex permeability, which
corresponds to the real part of the impedance of an inductor,
creates a peak in the series resistance as observed in Fig. 9.
Figure 11 shows that the applied magnetic field pushes the
fFMR to a higher value and thus improves the inductor Q by
negating the increase in resistance in the frequency range of
interest. The linear dependence of fFMR2 on the bias field
strength is shown in Fig. 12. This is used to extract the
internal magnetic field, Hk, and thus the permeability of the
NiFe film that is deposited on the samples. The permeability
of the patterned film differs from that of a uniform film
(measured with a permeameter) because of shape-induced
r
demagnetizing fields ( H k ). As seen in Fig. 13, these fields
reduce the dependence of the films permeability on bias field
strength.
Conclusions
This work has identified the key physical phenomena that
govern the behaviors of tunable on-chip inductors employing
high-µ permalloy. The effects of eddy currents and FMR
have been analyzed in detail and confirmed with measured
results. By proper patterning and laminations of the
permalloy film, the usable frequency range of inductors with
high-µ material is extended into the 5-GHz range for the first
time while achieving tunable inductance with reasonable Q’s.
Acknowledgement
The authors would like to thank ITRI (Taiwan) for funding
support, Professor Tamal Mukherjee for advice on processing
issues, and Cascade Microtech for donating an impedance
standard substrate.
References
[1]
[2]
lower µr and smaller tuning range under bias fields.
Results and Discussion
On-wafer RF measurements were performed using the setup
shown in Fig. 7. Two rows of NdFeB magnets are used to
[3]
D.R. Pehlke, A. Burstein and M.F. Chang, “Extremely high-Q tunable
inductor for Si-based RF integrated circuit applications,” IEDM
Technical Digest, pp. 63–66, Dec. 1997.
H. Sugawara, H. Ito, K. Okada, K. Itoi, M. Sato, H. Abe, T. Ito and K.
Masu, “High-Q Variable Inductor Using Redistributed Layers for Si RF
Circuits,” Topical Meeting on Silicon Monolithic Integrated Circuits in
RF Systems, pp. 187-190, Sept. 2004.
I. Zine-El-Abidine, M. Okoniewski and J.G. McRory, “RF MEMS
Tunable Inductor,” 15th International Conference on Microwaves,
Radar and Wireless Communications, vol. 3, pp. 817-819, May 2004.
[4]
[5]
[6]
[7]
[8]
Y. Zhuang, M. Vroubel, B. Rejaei, and J.N. Burghartz, “Ferromagnetic
RF Inductors and Transformers for Standard CMOS/BiCMOS,” IEDM
Tech. Dig., pp. 475-478, Dec. 2002.
A.M. Crawford and S.X. Wang, “Effect of Patterned Magnetic Shields
on High-Frequency Integrated Inductors,” IEEE Transactions on
Magnetics, vol. 40, no. 4, pp. 2017-2019, July 2004.
M. Yamaguchi, K. Suezawa, Y. Takahashi, K. I. Arai, S. Kikuchi, Y.
Shimada, S. Tanabe, and K. Ito, “Magnetic thin-film inductors for RF
integrated circuits,” J. Magn. Magn. Mater., vol. 215, pp. 807, 2000.
W.P. Jayasekara, J.A. Bain, and M.H. Kryder, “High frequency initial
permeability of NiFe and FeAlN,” IEEE Transactions on Magnetics,
vol. 34, no. 4, pp. 1438, July 1998.
J.S.Y. Feng and D.A. Thompson, “Permeability of narrow permalloy
stripes,” IEEE Transactions on Magnetics, vol. 13, no. 5, pp. 1521,
Sept. 1977.
SiO2
Si
a)
Photoresist
NiFe
Al
Al
SiO2
Si
b)
SiO2
Si
c)
Photoresist
Al
Al
SiO2
Si
d)
SiO2
Si
e)
Cross section (Fig. 5)
Figure 4. SEM image of an inductor trace with patterned permalloy
laminations.
Photoresist
Al
29 µm
NiFe
Al
Laminated NiFe
SiO2
Si
f)
1.3 µm
~4 µm
Al
.4 µm
SiO2
Figure 2. Post-processing steps: a) original BiCMOS chip cross section, b)
after anisotropic oxide etch, c) after photoresist coating, d) after photoresist
patterning, e) after sputtering ten 120-nm layers of NiFe separated by 10-nm
layers of SiO2, f) after liftoff.
13 µm
Figure 5. SEM cross section of an inductor trace with patterned permalloy
laminations.
d
SEM image (Fig. 4)
w
HBias
M
C
HRF
IRF
Inductor Trace
NiFe SiO2
Figure 6. 3D view of an inductor trace with permalloy laminations.
Figure 3. Image of an inductor with (top) and without (bottom) patterned
permalloy laminations.
Inductance (nH) at 1 GHz
Microscope
1
0.8
0.6
0.4
Simulated
0.2
Measured
0
40
60
Bias field (kA/m)
Figure 10. Dependence of inductance on bias field strength.
Chip
Bias Field Direction
Probe
0
20
80
20
Without NiFe
Figure 7. On-wafer RF measurement setup with magnets to provide external
magnetic bias field.
1.2
Quality Factor
Magnets
With NiFe, no bias field
15
With NiFe, 80 kA/m (1000 Oe) bias field
10
5
Without NiFe
0
With Nife, no bias field
0
10
Inductance (nH)
With NiFe, 80 kA/m (1000 Oe) bias field
1
10
Frequency (GHz)
Figure 11. Frequency response of inductor Q with and without permalloy
under bias fields.
1
0.8
19
15
x 10
0.4
0
10
1
10
Frequency (GHz)
Figure 8. Frequency response of inductance with and without permalloy
under bias fields.
35
5
Measured
Trendline
40
60
80
Bias field (kA/m)
Figure 12. The dependence on bias field strength of the resonance peaks in
the resistance (from Fig. 9).
With NiFe, no bias field
With NiFe, 80 kA/m
(1000 Oe) bias field
0
20
20
Modeled, uniform film
1000
15
10
5
0
0
10
1
Measured, uniform film
Modeled, patterned film
300
Measured, patterned film
100
30
10
Frequency (GHz)
Figure 9. Frequency response of resistance with and without permalloy under
bias fields.
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Relative permeability
Resistance (Ohms)
25
10
0
Without NiFe
30
2
f FMR
(Hz2 )
0.6
10
0
20
40
60
80
Bias field (kA/m)
Figure 13. The effect of bias field on the permeability of NiFe laminations
versus the prediction by the model in [7].