Amro M. Elshurafa
May 2013
RF MEMS Capacitors and Variable
Capacitors – The Future of Wireless
Communication
Before we begin…
You are free to use this presentation as you see fit. Any publications, in the form of slides,
conference papers, journal articles, technical reports, or otherwise, in which these slides
will be used (in their original or modified format) should cite one or more of the following
references:
[+] Amro M. Elshurafa et al., "Low voltage puzzle-like fractal MEMS variable capacitor suppressing pullin," IET/IEEE Micro & Nano Letters, Vol. 7, No. 9, pp. 965-969, 2012.
[+] Amro M. Elshurafa et al., "Differential RF MEMS Interwoven Capacitor Immune to Residual Stress
Warping," IET/IEEE Micro & Nano Letters, Vol. 7, No. 7, pp. 658-661, 2012.
[+] Amro M. Elshurafa et al., "Two-Layer RF MEMS Fractal Capacitors in PolyMUMPS for S-Band
Applications," IET/IEEE Micro & Nano Letters, Vol. 7, No. 5, pp. 419-421, 2012.
[+] Amro M. Elshurafa et al., "A Low Voltage RF MEMS Variable Capacitor with a Linear C-V
Response," IET/IEEE Electronics Letters, Vol. 48, No. 7, pp. 392-393, 2012.
[+] Amro M. Elshurafa et al., "RF MEMS Fractal Capacitors with High Self Resonant Frequencies," IEEE
JMEMS, Vol. 21, No. 1, pp. 10-12, 2012.
[+] Amro M. Elshurafa et al., "MEMS Variable Capacitance Devices Utilizing the Substrate: I. Novel Devices
with Customizable Tuning Range," Journal of Micromechanics and Microengineering, Vol. 20, No. 4, 045027
(8pp), 2010.
[+] Amro M. Elshurafa et al., "Effects of Non-uniform Nanoscale Deflections on Capacitance in RF MEMS
Parallel Plate Variable Capacitors," Journal of Micromechanics and Microengineering, Vol. 18, No. 4, 040512
(11pp), 2008.
[+] Amro M. Elshurafa et al., "Finite Element Modeling of Low Stress Suspension Structures and
Applications in RF MEMS Parallel Plate Variable Capacitors,“ IEEE Transactions of Microwave Theory and
2
Techniques, Vol. 54, No. 5, pp. 2211-2219, 2006.
Before we begin…
This presentation was prepared in May 2013. Publications, research, and results reported
afterwards will not be reflected herein.
About the author:
Amro Elshurafa obtained his PhD in 2008 in electrical engineering with a focus on RF
MEMS variable capacitors. Amro is a registered professional engineer (PEng) and is a
senior member of the IEEE. He can be contacted via email at elshurafa@ieee.org.
3
Agenda
What is MEMS and RF MEMS
RF MEMS Capacitors
RF MEMS Variable Capacitors
Simulation
Measurements
State of the Art
4
What is MEMS?
• MEMS abbreviates Micro Electro Mechanical Systems
• Integrating sensors and actuators possessing
dimensions ranging from 1mm to 1μm and relying on
electrical, mechanical, optical, chemical, etc, phenomena.
• You can hear NEMS, (similar to RF, it is no longer radio
frequencies; microelectronics also is nano now!).
• Hair diameter thickness is ~100μm.
• Strong emphasis on fabrication.
• If coupled with IC, the sky is the limit.
5
Most Famous Applications
• Printer Ink Jet Nozzles
• Inertial sensors
– Accelerometers (1D acceleration meter)
– Gyroscope (rotation rate meter)
– Applications in: air bags, Wii, game arcades, iPhones, Samsung
phones, satellites, missiles, etc.
• Biomedical medication dispensers (microfluidics).
• RF switch array in mobile phones (2012).
6
Most Famous Companies
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
AD
HP
TI
Siemens
Bosch
Xerox
GE
STMicroelectronics
Qualcomm
Cavendish
WiSpry
Omron
Raytheon
Intel (classified)
This list is different than pure MEMS design, fab, and fabless companies.
7
Fabrication – Briefly
Deposit and pattern
sacrificial layer
Deposit and pattern
structural layer
Some Substrate
1
2
Etch away sacrificial layer to
get free standing structures
5
3
Repeat steps 2 and 3 again
4
8
Foundry –
Standard vs. Non-standard
• The PolyMUMPS process
MEMSCAP, NC, USA.
from
• Has been in business since 1992.
• Very reliable and robust.
• Used by hundreds of groups
throughout the world in countless
applications.
• What about CMOS? Can we have
that?
9
Foundry –
Standard vs. Non-standard
10
Why RF MEMS?
Inductors
Capacitors
Resistors
IC’s
It is the high-Q
passive
components that
are hindering
miniaturization!
Slide by
Dr. Clark Nguyen at
University of California
at Berkeley.
11
Why RF MEMS?
• Ceramic filters:
• Made of piezoelectric ceramics
• Frequency is adjusted by thickness and size of the
ceramic element
• Typical dimensions are: ~20mm × ~10mm × ~5mm
• Extremely dimensions sensitive: a ±0.1mm
dimension tolerance yields a frequency accuracy of
±220MHz → Expensive
• No further opportunities for further
miniaturization
12
MEMS Benefit: General
Less power consumption
Discrete
electronics
Better performance
ICs
High volume fabrication
Less power consumption
Off-chip
passives
and filters
Better performance
MEMS
High volume fabrication
13
What Can MEMS Offer?
High Q filters:
fo = 8.5MHz
Qvac = 8,000
Qair ~ 50
Lr = 40μm
F. Bannon, J. Clark, and C. Nguyen, “High Frequency Microelectromechanical IF Filters," IEEE
International Electron Device Meetings, pp. 773-776, 1996.
14
What Can MEMS Offer?
High Q resonators:
-84
Transmission (dB)
20μm
Polysilicon
Electrode
CVD Diamond
mMechanical Disk
Resonator
Ground
Plane
-86
-88
Q = 10,100 (air)
-90
-92
-94
-96
-98
-100
1507.4 1507.6 1507.8
1508
1508.2
Frequency (MHz)
J. Wang, J. Butler, T. Feygelson, and C. Nguyen, “1.51GHz nanocrystaline Diamond Micromechanical Disk Resonator with Material
Mismatched Isolating Support,” IEEE Conference on Microelectromechanical Systems, pp. 641-644, 2004.
15
What Can MEMS Offer?
High Q inductors:
J. Zou, J. Nickel, D. Trainor, C. Liu, and J. Schutte-Aine,
“Development of Vertical Planar Coil Inductors Using
Plastic
Deformation
Magnetic
Assembly,”
IEEE
International Microwave Symposium, pp. 193-196, 2001.
Jun-Bo Yoon, Byeong-I1 Kim, Yun-Seok Choi, and Euisik Yoon, “3-D
Lithography and Metal Surface Micromachinig for RF and Microwave
MEMS,” IEEE Microelectromechanical Systems, pp. 673-676, 2002.
16
Varactors: CMOS vs. MEMS
CMOS Varactors
(reverse biased diodes)
MEMS Varactors
Leakage currents exists
No leakage current
Typical Q is 30-40, but can reach to
50-60
Q can reach up to 200 – 300
Due to continuous downscaling, the
tuning range (Cmax/Cmin) is
decreasing – Maximum ratio is 3 at
millimeter-wave range.
Tuning ranges are high (~5 for
varactors and ~50 for switches and
even higher)
Good at low frequency (inductor
loss dominates for LC tank), but
lossy at millimeter-wave range
No real concern
K. Kwok and J. Long, “A 23-to-29 GHz Transconductor-Tuned VCO MMIC in 0.13um CMOS,” IEEE Journal of Solid State Circuits,
Vol. 42, No. 12, pp. 2878-2886, 2007.
17
Why MEMS Varactors?
• Internal antenna, form factor, touch screens, etc, pose real
challenges (remember the death grip in iPhone4).
• Technologies changing rapidly, 2.5G, 3G, 4G: many carriers and
bands. Hence, antennas, filters, power amplifiers need to tune to
these bands.
• Dennis Yost, CEO of Cavendish, states: Theoretical 4G limit is
80Mbps, though testing shows ~8Mbps at best.
• Gabriel Rebeiz: a tunable front end is the holy grail of advanced
multi-mode multi-frequency mobile devices.
• Paratek shipped a tunable device to Samsung (thin-film based
varactor). Interestingly, RIM bought Paratek in 2012!
18
Why MEMS Varactors
• The RF filters are mostly ceramic or SAW filters, and are very
bulky and expensive (off-chip).
• Typical dimensions are 2cm x 1cm x 0.5 cm!
19
Why MEMS Varactors
• Add MEMS filters and receive whatever you want (no size
limitation). You can add many filters or a tunable filter.
• Quality factors > 15,000 at 1.4GHz  BW = 100kHz; better than
the current BW of 35MHz found in today’s phones.
20
The Ultimate Goal is:
A complete MEMS-based transceiver:
21
However…
• Fabrication and integration challenges: CMOS and
MEMS.
• Actuation voltages for MEMS varactors: 10V ~ 40V.
• Lifetime and reliability despite tests have been performed
for billions of cycles in lab conditions.
• Temperature stability and drift.
• Modeling vs. trial and error fabrication.
C. T. C. Nguyen, “Mechanical Radio,” IEEE Spectrum, December 2009.
22
Publications: Numbers
1000
Number of Publications
900
800
RF MEMS Publications
884
882 862
814 826
Variable Capacitor Publications
782
750
737
700
600
537
500
400
313
358
300
200
100
253
113
59
157
316 298 305
349
135 147
82
324
368
342
378
197
116
0
1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012
Year
Source: Engineering Village
23
Publications – Where?
3000
Number of Publications
2750
2627
2500
2250
2000
1750
1500
1250
1000
808
750
621
500
462
450
400
299
272
240
189
Canada
Italy
India
Belguim
250
0
United
States
China
France
Japan
Korea
Germany
Country
Source: Engineering Village.
24
Agenda
What is MEMS and RF MEMS
RF MEMS Capacitors
RF MEMS Variable Capacitors
Simulation
Measurements
State of the Art
25
A MEMS Perspective
• Typical capacitors in MEMS are of the parallel-plate type:
Criterion
CMOS
MEMS
Etching Holes
No Concern
Concern
Residual-Stress
Warping
No Concern
Concern
Availability of Metal
Layers
No Concern
Concern
Parasitics/balanced
capability
Concern
Concern
26
Etching Holes
When a sacrificial layer is present
between two structural layers, it has
to be removed. One way is to do wetetching: submerge wafer in an
etcher.
How much time will the etching take
for the example here if we assume
that the etching rate is 10μm/min?
27
Etching Holes
• Now, by adding etching holes
throughout the large structure,
the etchant will have more
opportunities to penetrate through
the structure. Hence, reducing the
etching
time
required
significantly.
• Also called release
access holes.
holes and
• Concerns:
affect
capacitance,
mechanical
performance,
and
optical performance.
• What about CMOS?
28
Metal Layer Scarcity
• Self explanatory!
• Most MEMS processes possess
several polysilicon layers but a
single metal layer  obtaining
high Q is difficult.
• What about CMOS?
• In
CMOS processes, there are ~9
metal layers  both capacitor
terminals can be metal and hence
not affect Q.
Faraday Technology Corporation
www.design-reuse.com
29
Substrate Parasitics:
Balanced/Differential Capability
• In a typical parallel-plate capacitor, the bottom-plate/substrate
parasitic is larger than the top-plate/substrate parasitic.
• Connecting transistors to capacitors is very strict (TSMC) –
many prohibited configurations.
• The output of the circuit is NOT the same if the terminals of the
capacitor are swapped (similar to a polarized capacitor).
•Exists also in discrete capacitors.
• For both MEMS and CMOS. So, what to do?
30
Residual Stress – Examples
http://mems.ece.dal.ca/research.php
R. Al-Dahleh and R. Mansour, “High Capacitance Ratio Warped-Beam Capacitive
MEMS Switch Designs,” IEEE Journal of Microeletromechanical Systems, Vol. 19,
No. 3, pp. 538-547, 2010.
What about CMOS?
31
Residual Stress – 1
•What is it?
•First kind is: bending, or warping,
taking place when two materials
with different thermal expansion
coefficients, are on top of each other.
•During fabrication, heating then
cooling!
MATERIAL 1
α1 > α2
MATERIAL 2
32
Residual Stress – 2
•Second kind is: bending, or warping,
taking place in a single layer while
cooling.
•During cooling, the molecules
reorient themselves in such a way
that the Young’s Modulus is not the
same
everywhere
within
the
material (moving parts).
•Different for fixed-free, fixed-fixed,
etc beam/plate orientations.
•Severe in larger plates and longer
beams.
33
Calculate Capacitance with Warping
A. M. Elshurafa and E. I. El-Masry, “Effects of Nonuniform Nanoscale Deflections on Capacitance in RF MEMS Parallel
Plate Variable Capacitors,” Journal of Micromechanics and Microengineering, Vol. 18, No. 4, 2008
34
Calculate Capacitance with Warping
Elliptic paraboloid
Hyperbolic paraboloid
A. M. Elshurafa and E. I. El-Masry, “Effects of Nonuniform Nanoscale Deflections on Capacitance in RF MEMS Parallel
Plate Variable Capacitors,” Journal of Micromechanics and Microengineering, Vol. 18, No. 4, 2008
35
Calculate Capacitance with Warping
Capacitance evaluation in 2D
Capacitance evaluation in 3D
A. M. Elshurafa and E. I. El-Masry, “Effects of Nonuniform Nanoscale Deflections on Capacitance in RF MEMS Parallel
Plate Variable Capacitors,” Journal of Micromechanics and Microengineering, Vol. 18, No. 4, 2008
36
Closed Form Expressions
A. M. Elshurafa and E. I. El-Masry, “Effects of Nonuniform Nanoscale Deflections on Capacitance in RF MEMS Parallel
Plate Variable Capacitors,” Journal of Micromechanics and Microengineering, Vol. 18, No. 4, 2008
37
What are Fractals?
http://www.evl.uic.edu/aej/488/lecture13.html
http://www.bathsheba.com/gallery/assorted/
http://www.evl.uic.edu/aej/488/lecture13.html
Fractal Capacitors
• Initially introduced in 1998 in ISSCC by
Samavati et al. while working with Tom Lee
at Stanford.
• Main Concern in capacitance density.
• Benefit from the lateral downscaling.
• Obtain lateral and vertical capacitances
(if two layers are used).
• Increase fringing.
H. Samavati, A. Hajimiri, A. Shahani, G. Nasserbakht, and T. Lee,
“Fractal Capacitors,” IEEE International Solid State Circuits
Conference, 256-257, 1998.
39
One Solution: Fractals
Moore’s Fractal
A. M. Elshurafa, A. Radwan, A. Emira, K. N. Salama, “RF MEMS Fractal Capacitors with High Self Resonant Frequencies,” IEEE
Journal of Microelectromechanical Systems, Vol. 21, No. 1, pp. 10-12, 2012.
40
4th and 5th Iteration
These capacitors are single-layer capacitors
A. M. Elshurafa, A. Radwan, A. Emira, K. N. Salama, “RF MEMS Fractal Capacitors with High Self Resonant Frequencies,” IEEE
Journal of Microelectromechanical Systems, Vol. 21, No. 1, pp. 10-12, 2012.
41
Addressing Etching Holes
A separation exists already throughout the structure!
42
Addressing Residual Stress
The segments are small, and when
long they can be easily anchored.
43
Residual Stress Warping
Parallel Plate
5th order fractal
A. M. Elshurafa, A. Radwan, A. Emira, K. N. Salama, “RF MEMS Fractal Capacitors with High Self Resonant Frequencies,” IEEE
Journal of Microelectromechanical Systems, Vol. 21, No. 1, pp. 10-12, 2012.
44
Addressing Metal Scarcity
Use the only available metal layer.
A. M. Elshurafa, A. Radwan, A. Emira, K. N. Salama, “RF MEMS Fractal Capacitors with High Self Resonant Frequencies,” IEEE
Journal of Microelectromechanical Systems, Vol. 21, No. 1, pp. 10-12, 2012.
45
Addressing ‘Balance-ness’
• Black: Signal Terminal
• Gray: Ground Terminal
Almost same area for both
terminals, hence it is balanced.
A. M. Elshurafa, A. Radwan, A. Emira, K. N. Salama, “RF MEMS Fractal Capacitors with High Self Resonant Frequencies,” IEEE
Journal of Microelectromechanical Systems, Vol. 21, No. 1, pp. 10-12, 2012.
46
Measurements
A. M. Elshurafa, A. Radwan, A. Emira, K. N. Salama, “RF MEMS Fractal Capacitors with High Self Resonant Frequencies,” IEEE
Journal of Microelectromechanical Systems, Vol. 21, No. 1, pp. 10-12, 2012.
47
SRF Measurements
Self resonant frequency:
the frequency at which the impedance
of the capacitor becomes purely real
(resistive).
C Band: 4 – 8 GHz.
X Band: 8 – 12 GHz.
Ku Band: 12 – 18 GHz.
48
Parallel Plate vs. Fractal
49
More Designs – in 2 Layers
Woven Design
Interleaved Design
A. M. Elshurafa and K. N. Salama, "Two-Layer RF MEMS Fractal Capacitors in PolyMUMPS for S-Band Applications," IET
Micro & Nano Letters, Vol. 7, No. 5, pp. 419-421, May 2012
50
More Designs – in 2 Layers
Woven Design
Interleaved Design
A. M. Elshurafa and K. N. Salama, "Two-Layer RF MEMS Fractal Capacitors in PolyMUMPS for S-Band Applications," IET
Micro & Nano Letters, Vol. 7, No. 5, pp. 419-421, May 2012
51
Measurements
Woven Design
Interleaved Design
A. M. Elshurafa and K. N. Salama, "Two-Layer RF MEMS Fractal Capacitors in PolyMUMPS for S-Band Applications," IET
Micro & Nano Letters, Vol. 7, No. 5, pp. 419-421, May 2012
52
Interwoven Design
Amro M. Elshurafa and K. N. Salama, "Differential RF MEMS Interwoven Capacitor Immune to Residual Stress Warping," IET Micro &
Nano Letters, Vol. 7, No. 7, pp. 658-661, July 2012.
53
Interwoven Design
J. de Jong and S. Baler, “Integrated Capacitor with Alternating Layered Segments,” US Patent 7,944,732, 2011.
54
Interwoven Design
A. M. Elshurafa and K. N. Salama, "Differential RF
MEMS Interwoven Capacitor Immune to Residual
Stress Warping," IET Micro & Nano Letters, Vol. 7,
No. 7, pp. 658-661, July 2012.
Comparison*
Criterion
PP Interleaved Woven Interwoven
Moore’s
C, pF
4.7
1.2
1.1
0.7
0.58
Q
7
3.5
6
9
10
SRF
(GHz)
5.5
10
10
>20
>20
* Measurement results at 2GHz
A. M. Elshurafa and K. N. Salama, "Differential RF MEMS Interwoven Capacitor Immune to Residual Stress Warping," IET Micro & Nano Letters,
Vol. 7, No. 7, pp. 658-661, July 2012.
A. M. Elshurafa and K. N. Salama, "Two-Layer RF MEMS Fractal Capacitors in PolyMUMPS for S-Band Applications," IET Micro & Nano Letters,
Vol. 7, No. 5, pp. 419-421, May 2012.
A. M. Elshurafa, A. Radwan, A. Emira, K. N. Salama, “RF MEMS Fractal Capacitors with High Self Resonant Frequencies,” IEEE Journal of
Microelectromechanical Systems, Vol. 21, No. 1, pp. 10-12, 2012.
56
Fractal Cookbook
Criterion
PP
Could be
created in a
Single Layer?
✗
✔
✗
✗
✔
Balanced
✗
✔
✗
✔
✔
Capacitance
Value
✔✔
✔
✔
✗
✗
High SRF
✗
✔
✔
✔✔
✔✔
✗
✔
Could it be
used in a
varactor?
✔
Interleaved Woven Interwoven
✔
✔
not
readily*
Moore’s
in two layers*
Capacity Limits
Cmax = Cmax,x + Cmax,y + Cmax,z
R. Aparicio and A. Hajimiri, “Capacity Limits and Matching Properties of Lateral Flux Integrated Capacitors,” IEEE Custom
Integrated Circuits Conference, pp. 365-368, 2001.
58
Agenda
What is MEMS and RF MEMS
RF MEMS Capacitors
RF MEMS Variable Capacitors
Simulation
Measurements
State of the Art
59
Variable Capacitors
• What are they? Simply capacitors with a varying capacitance!
•They could be called variable capacitor, varactor, or varicap.
• For a capacitor, figures of merit (FOM): C, Q, SRF.
• For a varactor actuated electrostatically, we add:
• actuation voltage (VDC)
• linearity (R2)
• tuning range (TR)
60
Variable Capacitor vs. Switch!
Variable Capacitor
Switch or Switched Capacitor
Analog tunability (Linearity)
Two distinct capacitances only
(digital: High and Low)
Low Tuning Range (~5:1)
High Tuning Range (>~50:1 for
switches and ~15:1 for switched
capacitors)
Usually air separates both terminals
only
Mostly an insulator layer in
addition to air separates both
terminals
Dielectric charging is not a concern
because no physical contact occurs
usually
Dielectric charging is a concern
because physical contact occurs
usually
Pull-in limits performance
Pull-in is not a real limitation
Actuation Voltage
• A DC voltage is used to achieve movement, i.e. actuation.
• The change in the separating distance governs the change in
capacitance.
• Ideally, want VDC to be
as low as possible.
Typically however, it is at
least 10V, but can reach
50V and more, and very
few designs use low
voltage (~4V).
62
Linearity: C-V
• The capacitance-voltage relation in a parallel plate capacitor
when d is varying is inherently nonlinear; C = εA/d.
• One of the solutions is to use a comb-drive capacitor.
• The catch: requires  area to obtain usable C and  V for
actuation.
Rockwell Labs Varactor
63
Tuning Range (TR)
• Definition: the ratio of the maximum capacitance to the
minimum capacitance; i.e. TR = Cmax/Cmin
• Ideally, want TR to be as high as possible.
• Usually, minimum capacitance takes place when V = 0.
• Usually, maximum capacitance takes place when V = VDC:pull-in
64
TR – Pull In
• Parallel plate variable capacitors suffer from the pull-in
phenomenon, or snap-in phenomenon.
65
Deriving Pull–in: 1
A
Initial Capacitance
C
Electrostatic Force
1 C 2
1 AV 2 DC
Fe 
V DC 
2 x
2 ( xo  x ) 2
Electrostatic
Spring Constant
Mechanical Force
What if
x = (1/3)xo?
xo  x
Fe
CV 2 DC
ke 

x ( xo  x ) 2
CV 2 DC
1
km x 
 ke ( xo  x )
2( xo  x ) 2
2km x
ke 
( xo  x )
66
Deriving Pull–in: 2
Electrostatic and
Mechanical Forces
1 AV 2 DC
FE 
 k M x  FM
2
2 ( xo  x)
Solve for the Voltage
2k M x( xo  x) 2
V
A
1
2
1

 2k   2
V     x ( xo  x)
 A  

1. Take the derivative
2. Equate to 0
3. Solve for x
1
2
  2k 
V  1
  x ( xo  x)  x     0
x  2
  A 
1

2
1
2
1
1  12
 x ( xo  x)  x 2
2
x
x o
3
67
Maximum TR
eps.A
d
eps.A
Cmax 
2
d
3
Cmax 3
TR 
  1.5
Cmin 2
Cmin 
Ideally:
Practically however: TR1

C max  C f
C min  C f

C max  0.4C max
 1.3125
C min  0.4C max
68
How to increase TR – 1
TR = 2.83
A. Dec and K. Suyama, “Micromachined Varactor with Wide Tuning Range,” Electronics Letters, Vol. 33, No. 11, pp. 922-944, 1997.
69
How to increase TR – 1
TR = 2.83
1. Dec and K. Suyama, “Micromachined Varactor with Wide Tuning Range,” Electronics Letters, Vol. 33, No. 11, pp. 922-944, 1997.
2. A. Dec and K. Suyama “Micromachined Electro-mechanically Tunable Capacitors and their Applications to RF IC’s” IEEE
Transactions on Microwave Theory and Techniques, Vol. 46, No. 12, 1998.
70
How to increase TR – 2
TR = 4.1
Maher Bakri-Kassem and R. R. Mansour, “High Tuning Range Parallel Plate MEMS Variable Capacitors with Arrays of Supporting
Beams,” IEEE Conference on Microelectromechanical Systems, pp. 666-669, 2006.
71
How to increase TR – 2
TR = 4.1
Maher Bakri-Kassem and R. R. Mansour, “High Tuning Range Parallel Plate MEMS Variable Capacitors with Arrays of Supporting
Beams,” IEEE Conference on Microelectromechanical Systems, pp. 666-669, 2006.
72
How to increase TR – 3
Theoretical TR = ∞
J. Zou, J. Aine, J. Chen, and S. Kang, “Development of a Wide Tuning Range MEMS Tunable Capacitor for Wireless Communication,”
IEEE International Electron Device Meeting, pp. 403-406, 2000.
73
How to increase TR – 3
Theoretical TR = ∞
T. Tsang and M. El-Gamal, “Very Wide Tuning Range Microelectromechanical Capacitors in the MUMPS Process for RF
Applications,” IEEE VLSI Symposium, pp. 33-36, 2003.
74
How to increase TR – 4
TR = 1.5
G. Ionis, A. Dec, and K. Suyama, “A Zipper Action Differential Micromechanical Tunable Capacitor,” IEEE Conference on
Microelectromechanical Systems, pp. 29-32, 2001.
75
How to increase TR – 5
A. M. Elshurafa and E. I. El-Masry, "MEMS Variable Capacitance Devices
Utilizing the Substrate: I. Novel Devices with Customizable Tuning
Range," Journal of Micromechanics and Microengineering – JMM, Vol. 20,
No. 4, 045027 (8pp), April 2010.
76
How to increase TR – 5
A. M. Elshurafa and E. I. El-Masry, "MEMS Variable Capacitance Devices
Utilizing the Substrate: I. Novel Devices with Customizable Tuning
Range," Journal of Micromechanics and Microengineering – JMM, Vol. 20,
No. 4, 045027 (8pp), April 2010.
77
How to increase TR – 6
TR = 2.2
C. Han, D. Choi, and J. Yoon, “Parallel Plate MEMS Variable Capacitor with Superior Linearity and Large Tuning Ratio using a Levering
Structure,”ّّ IEEE J. Microelectromechanical Systems, Vol. 20, No. 6, pp. 1345-1354, 2011.
78
How to increase TR – 6
C. Han, D. Choi, and J. Yoon, “Parallel Plate MEMS Variable Capacitor with Superior Linearity and Large Tuning Ratio using a Levering
Structure,”ّّ IEEE J. Microelectromechanical Systems, Vol. 20, No. 6, pp. 1345-1354, 2011.
79
How to increase TR – 7
• Make use of residual stress
•After fabricating the varactor, use ALD
• Tuning range: 5:1
• Q= 29 at 1GHz
M. Bakri-Kassem and R. R. Mansour, “Linear Bilayer ALD Coated
MEMS Varactor with High Tuning Capacitance Ratio,” IEEE J. of
Miroelectromechanical Sytems, Vol. 18, No. 1, pp. 147-153, 2009.
80
How to increase TR – 7
•A comment on the relatively high quality factor: the substrate was
etched under the bottom plate
M. Bakri-Kassem and R. R. Mansour, “Linear Bilayer ALD Coated MEMS Varactor with High Tuning Capacitance
Ratio,” IEEE J. of Miroelectromechanical Sytems, Vol. 18, No. 1, pp. 147-153, 2009.
81
How to Increase TR – 8
A. M. Elshurafa, P. H. Ho, and K. N. Salama, "Modeling and Fabrication of an RF MEMS
Variable Capacitor with a Fractal Geometry," IEEE International Symposium on Circuits
and Systems – ISCAS, 2013.
82
How to increase TR – 8
F = Force
F subscript = Fringing
V Subscript = Vertical
H Subscript = Horizontal
S Subscript = Substrate
A. M. Elshurafa, A. G. Radwan. P. H. Ho, M. H. Ouda, K. N. Salama, "Low voltage puzzle-like fractal MEMS variable capacitor
suppressing pull-in," IET Micro & Nano Letters, Vol. 7, No. 9, pp. 965-969, September 2012.
83
Actuation
Before Actuation
After Actuation
A. M. Elshurafa, P. H. Ho, and K. N. Salama, "Modeling and Fabrication of an RF MEMS Variable Capacitor with a
Fractal Geometry," IEEE International Symposium on Circuits and Systems – ISCAS, 2013.
84
Optical Profiler and CV Curve
85
Comparison
86
Other Requirements: Linearity
A. M. Elshurafa, P. H. Ho, and K. N. Salama, "A Low Voltage RF MEMS Variable Capacitor with a Linear C-V Response," IET
Electronics Letters, Vol. 48, No. 7, pp. 392-393, March 2012.
87
Other Requirements: Linearity
A. M. Elshurafa, P. H. Ho, and K. N. Salama, "A Low Voltage RF MEMS Variable Capacitor with a Linear C-V Response," IET
Electronics Letters, Vol. 48, No. 7, pp. 392-393, March 2012.
88
Close-up
A. M. Elshurafa, P. H. Ho, and K. N. Salama, "A Low Voltage RF MEMS Variable Capacitor with a Linear C-V Response,"
IET/IEEE Electronics Letters, Vol. 48, No. 7, pp. 392-393, March 2012.
89
Performance
A. M. Elshurafa, P. H. Ho, and K. N. Salama, "A Low Voltage RF MEMS Variable Capacitor with a Linear C-V Response,"
IET/IEEE Electronics Letters, Vol. 48, No. 7, pp. 392-393, March 2012.
90
Agenda
What is MEMS and RF MEMS
RF MEMS Capacitors
RF MEMS Variable Capacitors
Simulation
Measurements
State of the Art
91
Modeling of MEMS
Before that, how is CMOS modeling performed:
Cadence: Virtuoso, Hspice, Spectre, Encounter: One stop shop!
• DC/AC/Transient analysis
•Steady State
• Periodic Steady State
• Digital Flow
• Layout
• LVS/DRC
• System Level Simulation
• Temperature Analysis
• Inductor design
• Capacitors
• Frequency Response
• Filter Design
• Noise Analysis
• Leakage
• Monte Carlo
Generally,
the
results
acquired from Cadence are
reasonably accurate, and
simulations do predict the
behavior of the fabricated
chip very well.
92
Modeling of MEMS
Let’s look at a typical MEMS problem – a thermal actuator:
Pad – Anchored
Thin Arm – Suspended
5V
Current
Current
GND
Pad – Anchored
Thick Arm – Suspended
Movement Direction
93
Modeling of MEMS
• In thermal actuators: three physics are
involved:
a. Electric currents
b. Thermal losses
c. Structural interaction
COMSOL ©
• Electrostatic problems:
a. Electrostatic force
b. Mechanical deflection
• Microfluidic problems
• Magnetic actuation
• Gyroscopes/accelerometers
• RF performance!
94
Modeling of MEMS
LEdit
Layout
Cadence
Clewin
ANSYS
COMSOL
Multiphysics
MEMS
Modeling
Coventorware
Sugar
Intellisuite
HFSS
MatLab/Simulink
Equations
Maple
Mathematica
3D Drawing
SolidWorks
AutoCAD
95
Typical Flow
96
Finite Element Modeling
• Given the interdisciplinary nature of MEMS, the FEM
method seems to be the most suitable way of solving
problems.
• Divide the structure to elements (i.e. mesh the structure).
• Specify boundary conditions for each physics.
• Know the solution for one element, then add all
independent solutions for a global solution.
97
Real Example
Heat Flux
Physics: Thermo
DC Voltage
Physics: Electrical
Fixed Boundaries
Physics: structural
Ground
98
Physics: Electrical
Tips and Tricks
• Start with a coarse mesh first, then
refine. More elements,
better
accuracy,
memory.
but
more
time
and
• Start with a single physics first,
then add.
• Verify
with a known simple problem to verify your model, then
do yours.
• Make use of Symmetry.
•Do you always get a more accurate result with more elements?
99
Agenda
What is MEMS and RF MEMS
RF MEMS Capacitors
RF MEMS Variable Capacitors
Simulation
Measurements
State of the Art
100
Measurements
• In order to characterize RF MEMS varactors and/or
switches, we can do a 1-port or a 2-port measurement,
and for that we need:
a.
b.
c.
d.
e.
f.
g.
h.
Vector network analyzer (VNA)
DC Voltage source
Appropriate Probe (1 or 2)
Calibration Impedance Substrate
Contact Substrate
Bias-T network
Correct Layout/Pads
Cables and adapters (VERY IMPORTANT).
• Can I use an LCR meter?
101
Microwave Probes
•The most popular probe is a ground-signal-ground probe, or a
GSG probe.
G
Source: Gavin Fisher; Cascade Microtech
S
G
102
Preparing Probes - Planarity
Not planarized
Use contact
substrate to
perform this
task.
Planarized
Source: Gavin Fisher; Cascade Microtech
103
Probe Preparation - Alignment
Recommended over-travel (skidding or skating)
Pitch (150μm usually)
In addition to planarization, we need to perform alignment:
• Arrive at the reference (not landed yet).
• Land (vertical)
• Skid/skate (horizontal)
• Repeat to adjust
104
Source: Gavin Fisher; Cascade Microtech
Probes are ready – Calibrate!
•We can now calibrate. Let’s start with a 1-port calibration,
also could be named SOL calibration. We will:
Define a short
Define an Open
Define a load (50Ω)
Land on load
Land on a line that
shorts all tips
Stay in air!
This is done using a calibration substrate.
105
For a 2-port Calibration:
•The only difference is that you need a Thru and you do
calibration for both probes (hence SOLT).
• For SOL, you will obtain information regarding S11 only
(i.e. reflection), while for the SOLT you obtain information
for S11 and S21 (i.e. reflection and transmission).
•You need calibration only for
RF frequencies.
106
Setup
VNA
VDC
+
_
GND
No DC
RF
No RF
DC
RF+DC
107
Setup
VNA
VDC
+
_
GND
DC
To Check:
1. Connectors
(male or female,
size, etc)
2. Cables
3. Frequency
range
4. Max Power
5. Max voltage
RF
RF+DC
108
A Few Tips on Pads
• Too much current, and you blow your probes away!
• Your boss will not be happy.
109
A Few Tips on Pads
• Ensure you design pitch is the
same as the probes you have!
• Small pads: less parasitics.
• Big pads mean: easier
landing.
• 70um is reasonable or use an
insulator substrate.
Pitch
110
Smith Chart Considerations
• Top half of the Smith
Chart is Inductive, and
bottom half is capacitive.
• Resonance is the middle
horizontal line.
L
• Be on the outer sides for
high quality factor Q =
imaginary/real.
• Reasonable method up to
Qs of 100 or maybe 200.
C
111
A Real Measurement
112
Agenda
What is MEMS and RF MEMS
RF MEMS Capacitors
RF MEMS Variable Capacitors
Simulation
Measurements
State of the Art
113
WiSpry – Tuner Board
•Published in January
2013 in IEEE TMTT by
Gu and Morris.
•Built
using
MEMS
tunable capacitors
•Operates from 300MHz
to 500MHz.
Q. Gu and A. Morris, “A New Method for Matching Network Adaptive Control,” IEEE Transactions on Microwave Theory
and Techniques, Vol. 61, No. 1, pp. 587-595, 2013.
114
WiSpry – Antenna Tuner
•World’s smallest antenna
tuning developers’ kit for
smart phones and tablets.
•Designed for
optimization during the
development stages.
•Fits inside a true formfactor smartphone or
tablet products.
115
Omron and Radant Switches
•Omron’s switch. Professor Rebeiz
describes it as ‘amazing’. ‘The
best RF MEMS switch in the
world’.
•Radant Switch.
•Difference DC requirement between handheld devices and base
stations
116
Classic CMOS MEMS-VCOs (0.5um)
A. Dec and K. Suyama “Micromachined Electro-mechanically Tunable
Capacitors and their Applications to RF IC’s” IEEE Transactions on
Microwave Theory and Techniques, Vol. 46, No. 12, 1998.
A. Dec and K. Suyama, “A 1.9GHz CMOS VCO with Micromachined
Electromechanically Tunable Capacitors,” IEEE Journal of Solid
State Circuits, Vol. 35, No. 8, pp. 1231-1237, 2000.
117
Classic CMOS-MEMS Oscillator
•Developed at UC Berkeley.
•Wang developed the resonator first circa 1989.
•Nguyen integrated both in circa 1994 (PhD
dissertation) but published later in 1999.
C. Nguyen and R. Howe, “An
Integrated CMOS
Micromechanical Resonator HighQ Oscillator,” IEEE Journal of
Solid State Circuits, Vol. 34, No.
4, pp. 440-455, 1999.
CMOS-MEMS Variable Capacitor
•Benefit from Residual Stress.
• Thermal actuation first, then electrostatic.
J. Reinke, G. Fedder, T. Mukherjee, “CMOS MEMS 3-bit Digital Capacitors with Tuning Ratios Greater Than 60:1,” IEEE
Transactions on Microwave Theory and Techniques, Vol. 59, No. 5, pp. 1238-1248, 2011.
119
CMOS-MEMS Variable Capacitor
• Maskless, post-CMOS etching.
• TR = 63:1!
• Q = 160 at 1 GHz.
J. Reinke, G. Fedder, T. Mukherjee, “CMOS MEMS 3-bit Digital Capacitors with Tuning Ratios Greater Than 60:1,” IEEE
Transactions on Microwave Theory and Techniques, Vol. 59, No. 5, pp. 1238-1248, 2011.
120
CMOS-MEMS Variable Capacitor
M. Bakri-Kassem, S. Fouladi, and R. Mansour, “Novel High Q MEMS Curled-Plate Variable Capacitors Fabricated in 0.35um
CMOS Technology,” IEEE Transactions on Microwave Theory and Techniques, Vol. 56, No. 2, pp. 530-541, 2008.
121
CMOS-MEMS Variable Capacitor
• Maskless, post-CMOS etching.
• TR = 6:1!
• Q = ~300 at 1.5 GHz.
M. Bakri-Kassem, S. Fouladi, and R. Mansour, “Novel High Q MEMS Curled-Plate Variable Capacitors Fabricated in 0.35um
CMOS Technology,” IEEE Transactions on Microwave Theory and Techniques, Vol. 56, No. 2, pp. 530-541, 2008.
122
Temperature Insensitivity
• Tuning Range: 3
• Q is very high > 100!
• Stable up to +125 C and processed in a university.
R. Mahameed and G. Rebeiz, “Electrostatic RF MEMS Tunable Capacitors with Analog Tunability and Low Temperature
Sensitivity,” IEEE International Microwave Symposium, pp. 1254-1257.
123
UCSD: 10W Switch!
• Tuning Range: 7
• Q is very high: >>200!
• Could handle 10W of power.
H. Zareie and G. M. Rebeiz, “High Power (>10W) RF
MEMS Switched Capacitors,” IEEE International
Microwave Symposium, 2012.
124
Tunable Filter
•Bandwidth from 1.5GHz to 2.5GHz.
•Q is around 100 using variable devices.
M. El-Tanani and G. Rebeiz “High Performance 1.5-2.5GHz RF MEMS Tunable Filters for Wireless Applications,” IEEE
Transactions on Microwave Theory and Techniques, Vol. 58, No. 6, pp. 1629-1637, 2010.
125
Tunable Filters - Michigan
A. Abbaspour-Tamijani, L. Dussopt, G. M. Rebeiz, “Miniature and Tunable Filters Using MEMS Capacitors,”
IEEE Transactions on Microwave Theory and Techniques, Novl. 51, No. 7, pp. 1878-1885, 2003.
126
To Integrate or Not to Integrate -1
• IMEC:
(Interuniversity
Microlectronics
Center) provide
the SiGe-MEMS
process.
www.imec.be
• Same wafer
processing
• Why SiGe?
127
To Integrate or Not to Integrate - 2
• Invensense Nasiri Fabrication Platform: www.invensense.com
• Different wafers – superb bonding (their competitive edge).
128
To Integrate or Not to Integrate - 3
• DALSA MEMS process with CMOS 0.8: www.dalsa.com
• Same wafer, achieved through wafer packaging, vias, bonding.
• Structural layer is metal.
129
Future Outlook
• Challenges that need to be overcome are:
– Packaging: SiP, SoC, SoP, Bonding, Seamless,
Hermetic.
– Temperature Drift: Material selection and
optimization.
– Mechanical
Reliability:
Material
selection,
actuation techniques, and fatigue.
– Voltage Actuation Requirement: intelligent MEMS
designs or high performance charge pumps*.
– CMOS Opportunities: MEMS interface circuits.
* A. Emira, M. AbdelGhany, M. Elsayed, A. M. Elshurafa, S. Sedky, and K. N. Salama, "50V All-PMOS Charge Pumps Using Low-Voltage
Capacitors," IEEE Transactions on Industrial Electronics, 2013, (10.1109/TIE.2012.2213674).
130
Conclusion
• Variable/Tunable devices are needed for
next
generation
wireless
communication, and RF MEMS variable
capacitance devices can satisfy the
requirements.
• To integrate seamlessly or not to
integrate: two views and I like to stand
in the middle.
131
Further Reading
1. RF MEMS: Theory, Design, and Technology (Textbook).
2. Tuning in to RF MEMS, IEEE Microwave Magazine, 2009.
3. Handling RF Power: The Latest advances in RF MEMS Tunable Filters,
IEEE Microwave Magazine, 2013.
4. RF MEMS-CMOS Device integration: An Overview of the Potential for
RF Researchers, IEEE Microwave Magazine, 2013.
5. The Search for a Reliable MEMS Switch???: Metal-Contact Switches,
IEEE Microwave Magazine, 2013.
6. Mechanical Radio, IEEE Spectrum, 2009.
7. MEMS Technology for Timing and Frequency Control, IEEE
Transactions on Ultrasonics, Ferroelectrics, & Frequency Control, 2007.
8. RF MEMS for Ubiquitous Wireless Connectivity: Parts I and II, IEEE
Microwave Magazine, 2004.
132
Thank You
References: Variable Capacitors
1. A. M. Elshurafa, P. H. Ho, and K. N. Salama, "A Low Voltage RF MEMS Variable Capacitor with a Linear CV Response," IET/IEEE Electronics
Letters, Vol. 48, No. 7, pp. 392-393, March 2012.
2. A. M. Elshurafa and E. I. El-Masry, "MEMS Variable Capacitance Devices Utilizing the Substrate: II. Zipping Varactors," Journal of
Micromechanics and Microengineering, Vol. 20, No. 4, 045028 (7pp), April 2010.
3. A. M. Elshurafa and E. I. El-Masry, "MEMS Variable Capacitance Devices Utilizing the Substrate: I. Novel Devices with Customizable Tuning
Range," Journal of Micromechanics and Microengineering, Vol. 20, No. 4, 045027 (8pp), April 2010.
4. A. M. Elshurafa and E. I. El-Masry, "Effects of Non-uniform Nanoscale Deflections on Capacitance in RF MEMS Parallel Plate Variable
Capacitors," Journal of Micromechanics and Microengineering, Vol. 18, No. 4, 040512 (11pp), April 2008.
5. A. M. Elshurafa and E. I. El-Masry, "Finite Element Modeling of Low Stress Suspension Structures and Applications in RF MEMS Parallel
Plate Variable Capacitors," IEEE Transactions of Microwave Theory and Techniques, Vol. 54, No. 5, pp. 2211-2219, May 2006.
6. A. M. Elshurafa and E. I. El-Masry, "A Novel 3-in-1 MEMS Variable Capacitance Device with Customizable Tuning Ranges," IEEE
International Device and Test Workshop, November 2009.
7. A. M. Elshurafa and E. I. El-Masry, "Design Considerations in MEMS Parallel Plate Variable Capacitors," IEEE Midwest Symposium on
Circuits and Systems (INVITED), pp. 1173-1176, August 2007.
8. A. M. Elshurafa and E. I. El-Masry, "Effects of Etching Holes on Capacitance and Tuning Range in MEMS Parallel Plate Variable
Capacitors," IEEE International Workshop on System on Chip, pp. 221-224. December 2006.
9. A. M. Elshurafa and E. I. El-Masry, "Quality Factor Estimation of Fabricated MEMS Parallel-Plate Variable Capacitors in MUMPS," IEEE
Proceedings of the 2nd Northeast Workshop on Circuits and Systems, pp. 109-112, June 2004.
10. Y. Shim, Z. Wu, and M. Rais-Zadeh, "A Multimetal Surface Micromaching Process for Tunable RF MEMS Passives," IEEE Journal of
Microelectromechanical Systems, pp. 867-874, 2012.
11. J. Gauvin, F. Barriere et al., "Design, Fabrication, and Measurements of Reliable Low Voltage RF MEMS Switched Varactors," IEEE Microwave
Integrated Circuits Conference, pp. 434-437, 2011.
12. R. Stefanini, B. Chen, A. Yu, and J. Shi, "Miniature RF MEMS Metal Contact Switches for DC – 20GHz Applications," IEEE Solid State Sensors,
Actuators, and Microsystems Conference, pp. 406-409, 2011.
13. U. Shah, M. Sterner, and J Oberhammer, "Basic Concepts of Moving Side Tuneable Capacitors for RF MEMS Reconfigurable Filters," IEEE
European Microwave Conference, pp. 1087-1090, 2011.
14. Maher Bakri-Kassem and R. R. Mansour, “An Improved Design for Parallel Plate MEMS Variable Capacitors,” IEEE International Microwave
Symposium, pp. 865-868, 2004.
15. Maher Bakri-Kassem and R. R. Mansour, “High Tuning Range Parallel Plate MEMS Variable Capacitors with Arrays of Supporting Beams,”
IEEE Conference on Microelectromechanical Systems, pp. 666-669, 2006.
16. H. Hsu and D. Peroulis "A CAD Model for Creep Behavior of RF-MEMS Varactors and Circuits," IEEE Transactions on Microwave Theory and
Techniques, Vol. 59, No. 7, pp. 1761-1768, 2011.
134
References: Variable Capacitors
17. U. Shah, M. Sterner, and J Oberhammer, "Basic Concepts of Moving Side Tuneable Capacitors for RF MEMS Reconfigurable Filters," IEEE
European Microwave Conference, pp. 1087-1090, 2011.
18. J. Reinke, G. Fedder, T. Mukherjee, “CMOS MEMS 3-bit Digital Capacitors with Tuning Ratios Greater Than 60:1,” IEEE Transactions on
Microwave Theory and Techniques, Vol. 59, No. 5, pp. 1238-1248, 2011.
19. Maher Bakri-Kassem and R. R. Mansour, “An Improved Design for Parallel Plate MEMS Variable Capacitors,” IEEE International
Microwave Symposium, pp. 865-868, 2004.
20. A. Abbaspour-Tamijani, L. Dussopt, and G. Rebeiz, “Miniature and Tunable Filters using MEMS Capacitors,” IEEE Transactions on
Microwave Theory and Techniques, Vol. 51, No. 7, pp. 1878-1885.
21. I. Reines and G. Rebeiz, “A Robut Power Handling (>10W) RF MEMS Switched Capacitor,” IEEE Microelectromechanical Systems, pp. 764767, 2011.
22. M. Bakri-Kassem, S. Fouladi, and R. Mansour, “Novel High Q MEMS Curled-Plate Variable Capacitors Fabricated in 0.35um CMOS
Technology,” IEEE Transactions on Microwave Theory and Techniques, Vol. 56, No. 2, pp. 530-541, 2008.
23. A. Dec and K. Suyama, “A 1.9GHz CMOS VCO with Micromachined Electromechanically Tunable Capacitors,” IEEE Journal of Solid State
Circuits, Vol. 35, No. 8, pp. 1231-1237, 2000.
24. H. Zareie and G. M. Rebeiz, “High Power (>10W) RF MEMS Switched Capacitors,” IEEE International Microwave Symposium, pp. 1-3, 2012.
25. Z. Yao et al, “Micromachined Low-loss Microwave Switches,” Journal of Microelectromechanical Systems, Vol. 8, No. 2, pp. 129-134, June
1999.
26. A. Dec and K. Suyama, “Micromachined Electro-mechanically Tunable Capacitors and Their Applications to RF IC’s,” IEEE Transactions on
Microwave Theory and Techniques, Vol. 46, No. 12, pp. 2587-2596.
27. Q. Gu and A. Morris, “A New Method for Matching Network Adaptive Control,” IEEE Transactions on Microwave Theory and Techniques,
Vol. 61, No. 1, pp. 587-595, 2013.
28. M. Bakri-Kassem and R. R. Mansour, “Linear Bilayer ALD Coated MEMS Varactor with High Tuning Capacitance Ratio,” IEEE J. of
Miroelectromechanical Sytems, Vol. 18, No. 1, pp. 147-153, 2009.
29. C. Han, D. Choi, and J. Yoon, “Parallel Plate MEMS Variable Capacitor with Superior Linearity and Large Tuning Ratio using a Levering
Structure,”ّّ IEEE J. Microelectromechanical Systems, Vol. 20, No. 6, pp. 1345-1354, 2011.
30. G. Ionis, A. Dec, and K. Suyama, “A Zipper Action Differential Micromechanical Tunable Capacitor,” IEEE Conference on
Microelectromechanical Systems, pp. 29-32, 2001.
31. T. Tsang and M. El-Gamal, “Micromechanical Variable Capacitors for RF Applications,” IEEE Midwest Symposium on Circuits and Systems,
pp. 25-28, 2002.
32. J. Zou, J. Aine, J. Chen, and S. Kang, “Development of a Wide Tuning Range MEMS Tunable Capacitor for Wireless Communication,” IEEE
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135
33. A. Dec and K. Suyama, “Micromachined Varactor with Wide Tuning Range,” Electronics Letters, Vol. 33, No. 11, pp. 922-944, 1997.
References: Fixed Capacitors
1.
2.
3.
4.
5.
6.
A. M. Elshurafa and K. N. Salama, "Two-Layer RF MEMS Fractal Capacitors in PolyMUMPS for S-Band Applications," IET Micro & Nano
Letters, Vol. 7, No. 5, pp. 419-421, May 2012.
A. M. Elshurafa, A. G. Radwan, A. Emira, and K. N. Salama, "RF MEMS Fractal Capacitors with High Self Resonant Frequencies," IEEE J.
Microelectromechanical Systems - JMEMS, Vol. 21, No. 1, pp. 10-12, February 2012.
H. Samavati, A. Hajimiri, A. Shahani, G. Nasserbakht, and T. Lee, “Fractal Capacitors,” IEEE International Solid State Circuits Conference , 256257, 1998.
J. de Jong and S. Baler, “Integrated Capacitor with Alternating Layered Segments,” US Patent 7,944,732, 2011.
R. Aparicio and A. Hajimiri, “Capacity Limits and Matching Properties of Lateral Flux Integrated Capacitors,” Custom Integrated Circuits
Conference, pp. 365-368, 2001.
A. M. Elshurafa and K. N. Salama, “Differential RF MEMS Interwoven Capacitor Immune to Residual Stress Warping,” IET Micro and Nano
Letters, Vol. 7, No. 7, pp. 658-661, 2012.
136
References: Resonators and Filters
1.
W. Chen, W. Fang, and S. Li, "High Q Integrated CMOS MEMS Resonators With Submicrometer Gaps and Quasi-Linear Frequency
Tuning," IEEE Journal of Microelectromechanical Systems, Vol. 21, No. 3, pp. 688-701, 2012.
2. J. Wang, J. Butler, T. Feygelson, and C. Nguyen, “1.51GHz nanocrystaline Diamond Micromechanical Disk Resonator with Material
Mismatched Isolating Support,” IEEE Conference on Microelectromechanical Systems, pp. 641-644, 2004.
3. F. Bannon, J. Clark, and C. Nguyen, “High Frequency Microelectromechanical IF Filters,” IEEE International Electron Device
Meeting, pp. 773-776, 1996.
4. V. Sekar, M. Armendariz, and K. Entesari, "A 1.2 – 1.6 GHz Substrate integrated Waveguide RF MEMS Tunable Filter," IEEE
Transactions on Microwave Theory and Techniques, Vol. 59, No. 4, pp. 866-876, 2011.
5. K. Chan, S. Fouladi, R. Ramer, and R. Mansour, "RF MEMS Switchable Interdigital Bandpass Filter," IEEE Microwave and
Wireless Components Letters, Vol. 22, No. 1, pp. 44-46, 2012.
6. K. Chan, S. Fouladi, R. Ramer, and R. Mansour, "RF MEMS Switchable Interdigital Bandpass Filter," IEEE Microwave and
Wireless Components Letters, Vol. 22, No. 1, pp. 44-46, 2012.
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