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Digitally Controllable
RF MEMS Inductor
Atsushi Shirane, Yutaka Mizuochi, Shuhei Amakawa
Noboru Ishihara, Kazuya Masu
Solutions Research Laboratory, Tokyo Institute of Technology
AMC, Albany Oct. 7th, 2010
Outline
・Background and Purpose
・Conventional variable inductor
・Principle and design of proposed inductor
・Measurement results and discussion
・Conclusion
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Background
PA : Power Amplifier
LO : Local Oscillator
Frequency is decided by L and C
Requirement for
Wideband RF frontend
Variable L and C is
required
for wideband operation
Purpose
Variable inductor needs
Change real and imaginary
part independently
Ex.) Impedance matching circuit
antenna
7+j0 Ω→50+j0 Ω
freq
L
C
1GHz
2.6nH
7.9pF
6GHz
0.4nH
1.3pF
Both L and C
⇒variable
Purpose： Variable inductor for wideband RF frontend
Variable inductor requirements
・Wide inductance tunability for wideband operation
・High Q factor for lowering loss
・High self resonant frequency for higher available frequency
Conventional variable inductors
L varied by MOS switch
L varied by actuator
Tunability：187%
Qmax：3~5
Tunability：18%
Qmax:5~6
[1] P. Park ,et al.,2004
[2] V. M. Lubecke ,et al.,2001
※Tunability = （Lmax-Lmin）/Lmin
・Low resolution
・Low Q factor
・Continuous but narrow tunability
・Low Q factor
Integration of CMOS and MEMS
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・RF CMOS has realistic but CMOS is not suitable for RF inductor
・MEMS can provide thick metal, high substrate resistivity and
air suspended structure
・MEMS characteristic leads to higher Q and movable configuration
Integration of CMOS and MEMS realizes
higher performance RF frontend
Metal thickness
Substrate resistivity
Movable structure
CMOS process
CMOS
Thin
Low
MEMS
Thick
High
×
○
Principle of proposed variable inductor
Digitally controllable inductance
・Inductance is controlled by switching on or off
・Turning on a switch, total inductance decreases
・２N resolution can be obtained with N switches
・Wide tunability can be obtained with
small displacement because MEMS switches are used
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Design of the proposed inductor
・Two switches realize 2bit controlled inductance
・Solenoid inductor has 17-turn
・Solenoid inductor is suitable for linear inductance tuning
・Cut short 3-turn or 6turn practically by switching
Inductance tunability
8,11,14,17turn
17 turn solenoid inductor
sw2
6turn short-cut
sw1 3turn short-cut
Designed solenoid inductor
Thickness
MEMS process
■ Metal
：Au
■ Width
：20μm
■ Thickness
：15μm
■ Substrate
：1kΩ・cm
Width
8
Red：Metal1
Yellow：Via
Blue：Metal2
Thick metal and high
resistivity substrate
improve Q factor
Fabricated test inductors
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Without switch, we fabricated 4 kinds of
inductors that represent 4 status of switch
00:sw1 off, sw2 off
01:sw1 on, sw2 off
10:sw1 off, sw2 on
11:sw1 on, sw2 on
Measurement result
L VS status of switch
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・Inductance at 2GHz is varied from 1.7 to 2.2nH linearly
Measurement results L and Q vs frequency
・Q is higher than 10 from 1GHz to 6GHz
・Self resonant frequency is more than 10GHz
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Switch resistance influence
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0.2nH 0.5Ω 0.4nH 1.5Ω
If switch resistance is 1Ω, Q keeps higher than 10
Increasing number of switch, Q degradation isn’t serious
f = 5GHz
Status
Turn
L (nH)
Q
Measurement⇒
R (Ω)
Q’
Including RSW⇒
00
17
01
14
10
11
11
8
2.39
16.5
4.55
16.5
2.23
17.4
4.03
13.9
1.98
20.6
3.02
15.4
1.78
22.4
2.50
12.4
Conclusion
・Digitally controllable inductor was proposed
・L varied from1.7 to 2.2nH linearly with 2bit control
・High Q of more than 10 from 1GHz to 6GHz
・More than 10GHz self resonant frequency
・Switch resistance influence is investigated
Increasing the number of bit, wide tunability and
high resolution are expected
Acknowledgments
Thank you for your attention!!
This work was partially supported by STARC,
MIC.SCOPE, KAKENHI, NEDO, Special Coordination
Funds for Promoting Science and Technology, and
VDEC in collaboration with Agilent Technologies
Japan, Ltd., Cadence Design Systems, Inc., and
Mentor Graphics, Inc.
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