Transcript September 2001

Micromachining for Integrated Electronics
Chang Liu
Micro Actuators, Sensors, Systems Group
University of Illinois at Urbana-Champaign
Power and Energy Systems Seminar, ECE 490 I
9/24/2001
Chang Liu
MASS
UIUC
Outline
•
•
•
•
Chang Liu
Overview - MEMS for circuit applications
Micromachined tunable capacitors
Micromachined high-Q inductors
Conclusions
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MEMS Applications
Chang Liu
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UIUC
A Future Wireless World …
Small, Low Power and Low Cost
Wireless LAN
Chang Liu
Collision avoidance radar
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Ultimate Miniaturization
GPS
Cellular Phone
30 GB memory
True color display
DC-200KHZ microphone
Pressure sensor
Pulse sensors
Heart monitor
Personal digital assistance
Digital camera and movie
Large screen projection display
Chang Liu
MASS
UIUC
MEMS for Integrated Electronics
• RF Circuits
– high performance, integrated components including
•
•
•
•
•
capacitors,
inductors,
resonators,
filters,
switches.
– High performance integrated probes for circuit characterization
• ability to interrogate sub-micrometer structures
• Power electronics and energy systems
–
–
–
–
Chang Liu
Capacitors
Inductors
Relays and switches
Transformers
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Chang Liu
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Integrated Power Generation and Conversion
Portable generation
and conversion
for
High power density
High voltage
applications
Chang Liu
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UIUC
BioMicrofluidics Applications for Integrated Lab-ona-Chip
s
Chang Liu
MASS
UIUC
Outline
•
•
•
•
Overview - MEMS for circuit applications
Micromachined tunable capacitors
Micromachined high-Q inductors
Conclusions
Moore’s law for integrated circuits
Chang Liu
MASS
UIUC
•
Background & Motivation
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
•
Design, Fabrication and Testing
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

•
Chang Liu
Current tunable capacitors
Performance limiting “pull-in” effect in micromachined
parallel-plate tunable capacitors
Design How to overcome the “pull-in” effect and
achieve a wide tuning range?
Fabrication How to make it ?
Testing and measurement
Conclusions
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Existing Tunable Capacitor Overview
•
Solid-state Varactors


•
Diode
High substrate resistive loss
Limited tuning range (<10%)
MOS
Capacitor
MEMS tunable capacitors



Lower loss
Wider tuning range
Typical example – electrostatically actuated
parallel-plate tunable capacitor
Suspended plate
VDC
Chang Liu
C
Fixed plate
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Pull-in Effect in Electrostatically Actuated Parallel-Plate Tunable
Capacitors
Pull-in effect is an intrinsic phenomenon to
electrostatically actuated devices, which greatly
limits the tuning range of the tunable capacitor.
Chang Liu
MASS
UIUC
Pull-in Effect in Electrostatically Actuated Parallel-Plate Tunable
Capacitors
Suspended plate
x0
C
Spacing
Fixed plate
Capacitance
x0
A
x0
0
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VDC
0
VDC
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Pull-in Effect in Electrostatically Actuated Parallel-Plate Tunable
Capacitors
Suspended plate
VDC
C
Spacing
Fixed plate
Capacitance
x0
A
x0
0
Chang Liu
VDC
0
VDC
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Pull-in Effect in Electrostatically Actuated Parallel-Plate Tunable
Capacitors
Suspended plate
VPI
Spacing
Fixed plate
Controllable
displacement
Capacitance
Unstable
Snap-in
x0
2 x0
3
3A
2 x0
A
x0
0
VPI
VDC
0
VPI
VDC
Max. controllable tuning range = 50%
Chang Liu
MASS
UIUC
To Extend Tuning Range …
• Require full-gap positioning
Capacitance
Spacing
3A
2 x0
x0
2 x0
3
A
x0
0
Chang Liu
VPI
VDC
0
VPI
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Design of the Novel Tunable Capacitor
Conventional design
New design
Suspended plate
Suspended plate
x
VDC
C
x0/3
VPI
Fixed plate
d2
VDC=0V
C
d1
Suspended plate
Suspended plate
C
Fixed plates
VPI
2d2/3
Fixed plate
Fixed plates
d1 -d2/3
C0 
A
x0
Cmax 
3A
 1.5C0
2 x0
Max. C/C0 = 50%
Chang Liu
C0 
A
d1
d1  2m
Cmax 
A
(d1  d 2 / 3)
d2  3m
Max. C/C0 = 100%
US Patent Pending
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Fabrication Process
Cu
Gold (0.5m)
2m
E3
E2
E3
3m
(d)
(a)
Ni-Fe (2m)
Cu (1m)
(e)
(b)
E1
d1(2m)
E3
(c)
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E2
d2(3m)
E3
(f)
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SEM Micrograph of the Tunable Capacitor
E1
Contact Pads
3m
2m
d1-x
Cantilever beam
Suspension
E3
E2
E3
To E3
Top Plate
E1
To E2
E1
Etch hole
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Simulation/Optimization
•
Electromechanically coupled simulation (MEMCAD*)
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Base capacitance (C0)
Pull-in voltage (VPI)
Maximum Tuning range (C/C0)
Dynamic characteristic of the movable plate suspension
High frequency performance (Sonnet em Suite*)


Loss
Capacitive behavior
* MEMCAD - Microcosm Inc, MA
* Sonnet em Suit - Sonnet Software, Inc, NY
Chang Liu
MASS
UIUC
MEMCAD Model for C-V Simulation
Suspended plate
x
Fixed plates
E1
E2
E3
MEMCAD model showing 3 plates of
the wide tuning range tunable
capacitor (See from the bottom)
Chang Liu
Travel distance (x) of the top plate
(E1) when a DC driving voltage (VDC)
is applied between E1 and E3
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-0.6
S11 Magnitude in dB
Model used in SonnetSimulation
E1
E2
Simulated
-0.5
Measured
-0.4
-0.3
-0.2
-0.1 0
1
2
3
4
5
6
7
8
9
10
0
Frequency (GHz)
-50
45MHz
Simulated
-40
Phase (degree)
Measured
Simulated
Measured
-30
-20
-10
0
1
2
3
4
5
6
7
8
9
10
0
Frequency (GHz)
10GHz
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
Return loss < 0.6dB@10GHz

Linear phase-frequency relationship
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Experiment
Pull-in
VDC=0V

d1 decreases continuously from 2 m to
1.2 m. Then d1 decreases abruptly from
1.2 m to 0.6m at VDC=17.2V.

The top plate travels 0.8 m (>d1/ 3)
before the Pull-in occurs.
VDC=16V
Chang Liu
MASS
UIUC
Results achieved
Chang Liu

A new design concept for parallel-plate tunable
capacitor to achieve arbitrary tuning range

A maximum tuning range of 70% achieved
experimentally

Low loss (<0.6dB@10GHz) and excellent capacitive
behavior at high frequencies
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UIUC
Outline
•
•
•
•
Chang Liu
Overview - MEMS for circuit applications
Micromachined tunable capacitors
Micromachined high-Q inductors
Conclusions
MASS
UIUC
Conventional Spiral Inductors
2nd Metal layer
1st Metal layer
SiO2
Si
Chang Liu

Occupy relatively large substrate space (~100100m2)

Suffer loss and parasitics from lossy substrate
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Limited quality factor (Q)
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Limited self-resonant frequency
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Limitations of Current Micromachined Planar Coil Inductors
Si
Si
Completely removing substrate
material underneath (Ozgur)
Polyimide
Si
Applying a thick polyimide layer
underneath (Kim)
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
Chang Liu
Si
Partially removing substrate material
underneath (Yeh)
Air Gap
Glass
Levitating the inductor structure
above the substrate (Park, Yoon)
Still requiring relatively large substrate real estate
(~100100m2)
Involving complex microfabrication steps, possibly
incompatible with IC fabrication foundry
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Fabrication Process of Vertical Spiral Inductors
Chang Liu
•
Fabrication
begun with a
IC chip
•
Deposition of
sacrificial layer
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Fabrication of
spiral inductor
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Deposition of
Permalloy
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•
Sacrificial
layer etching
Inductor
assembly using
PDMA
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Assembly Using Micro Hinged Structures
Main flap
Secondary flap
Cantilever beam spring
loading mechanism

Require additional substrate estate for
supporting structures or actuators
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Difficult to create electrical path between the
3D structures and the substrate

Electrical connection to
substrate
Major application: Optical MEMS
**-Yong Yi and C. Liu, IEEE J. Microelectromechanic. Syst. vol. 8, no. 1, pp .10-17, 1999.
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MASS
UIUC
Assembly Using Phase Changing Materials
Phase changing material
Micro flap
Heat

Phase changing material: solder or photo resist

Requires heating to melt bulky phase changing material

Requires delicate control to attain uniform assembly
**- R. R. A. Syms, IEEE J. Microelectromechanic. Syst. vol. 4, no. 4, pp. 177-184, 1995.
**- K. F. harsh, V. M. Bright, and Y. C. Lee, Sensors & Actuators A., vol. 77, pp. 237-244, 1999.
Chang Liu
MASS
UIUC
Motivation of This Work
To develop an alternative generalpurpose 3D assembly process
Chang Liu

High density, uniform and efficient assembly

Solid electrical path between the 3D structure
and the substrate

Room temperature assembly

Compatible with IC Foundry
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Plastic Magnetic Deformation Assembly
Magnetic
material

Flexible
region
Micro flap
Substrate
(a)
(b)
Hext
(a1) Surface-micromachining of the structure to be assembled
(a2) Deposition of the magnetic material piece
(b)
Application of an external magnetic field (strength & direction)
to create required plastic deformation in the flexible region
Magnetic material can be removed afterwards if necessary.
Chang Liu
MASS
UIUC
Plastic Magnetic Deformation Assembly
Before PDMA
Chang Liu
After PDMA

Supporting structure not required  High density

Magnetic actuation  Room temperature process
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3D structures aligned to the external magnetic filed  Uniformity

Metal used as bending material  Solid electrical path

IC compatible
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Plastic Magnetic Deformation Assembly


Flexible
region
(c)
(b)
Hext

Chang Liu
Structures may fall back
to a certain angle () after
Hext is removed due to the
elastic energy stored in
microstructure during the
bending.
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Theoretical Analysis
For a specific PDMA implementation, we
need to know the relationship between
Hext and the bending angle ()
beforehand, so that the targeted final
rest angle () can be achieved.
Chang Liu
MASS
UIUC
Theoretical Analysis
tp
Tm
tg
Permalloy
lp
Gold
lg
lg


tg
Hext
Magnetic Force: Tm = MwptplpHextcos=MVpHextcos
The magnetic force tries to align the cantilever beam to the magnetic field
M - Magnetization of Permalloy
wp, tp and lp – width, thickness and length of Permalloy
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UIUC
Theoretical Analysis
When Hext is increased, the bending experiences two phases.
Phase 1: Elastic bending (at small s)
Tm  Mw p t p l p H ext cos  
H ext 
Eg I g

l g Mw p l p t p cos 

Eg I g
lg
lg

Eg I g
H ext
Chang Liu


l g MV p cos 
Phase 2: Plastic bending (at large s)
 y wg t g2
Tm  Mw p t p l p H ext cos 
4
 y wg t g2
Tm
tg
 y wg t g2 1
1


4Mw p l p t p cos
4MV p cos
Eg
M
y
Ig
lg
wg
tg
lp
wp
tp
Vp
– Young’s Modulus of gold
– Magnetization of the Permalloy piece
– Yield stress of gold
– Moment of inertia of the gold beam
– Length of the gold beam
– Width of the gold beam
– Thickness of the gold beam
– Length of the Permalloy
– Width of the Permalloy
– Thickness of the Permalloy
– volume of the Permalloy piece
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Theoretical Analysis
When Hext is increased, the bending experiences two phases.
Phase 1: Elastic bending
Tm  Mw p t p l p H ext cos  
H ext 
Eg I g

l g Mw p l p t p cos 

Eg I g
lg

Eg I g
Plastic

l g MV p cos 
Phase 2: Plastic bending
 y wg t g2
Tm  Mw p t p l p H ext cos 
4
H ext
Chang Liu
Elastic
 y wg t g2
 y wg t g2 1
1


4Mw p l p t p cos
4MV p cos
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Theoretical Analysis
Plastic
Yielding
Elastic
Chang Liu

The bending is first elastic and then plastic.

The bending angle () saturates when Hext increases since
the magnetic force tries to align the cantilever beam to the
magnetic field.

The final rest angle () is determined by the bending angle
occurring in the plastic regime.
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Measured Displacement Vs. Hext
80
60
40
M easured
M odel
20
0
0
Chang Liu
10000
20000
30000
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Experimental Results
Tm
tg
lg

80
Measured
60
Model
40
20
0
10
Chang Liu
30
50
70
90
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Other Issues
Creating vertical structure
> 90
o
 90
o
Flexible
region
(a)
(b)
Hext
Post-assembly Strengthening
3 m Parylene coating
Chang Liu
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Scanning Electron Micrographs of Fabricated Prototype Devices
Test pads
Gold Bottom
conductor
CYTOP
Dielectric Bridge
Copper Top
conductor
Before PDMA
Chang Liu
After PDMA
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Experimental Results
Inductor on Silicon
5
Vertical inductor
14
Q Factor
Q Factor
4
3
2
Model
1
Measurement
0
Measurement
8
6
4
2
0
0
0.2
0.4
0.6
Frequency (GHz)
0.8
1
0
Inductor on Glass
14
12

Model
1
2
3
Frequency (GHz)
4
Inductance = 4.5nH
Measurement
10
8
Q Factor
Model
12
10

Max Q inductor on Silicon = 3.5
6
4

Max Q vertical inductor = 12
2
0

Max Q inductor on glass = 12
0
Chang Liu
1
2
3
Frequency (GHz)
4
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3D Solenoid
Chang Liu
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3D Solenoid
Chang Liu
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UIUC
Outline
•
•
•
•
Chang Liu
Overview - MEMS for circuit applications
Micromachined tunable capacitors
Micromachined high-Q inductors
Conclusions
MASS
UIUC
Conclusions
• Potentials for applying micromachining technology to circuit
and power electronics applications
• Developed variable gap tunable capacitor architecture and
demonstrated large tuning range;
• Developed three-dimensional assembly technique and realized
high qualify factor inductor elements;
• Current and future work:
– Develop integrated power convertors for on-chip power regulation.
– Develop tunable, lockable and resettable micro capacitors.
– Develop high-Q inductors and demonstrate integration onto IC
chips.
Chang Liu
MASS
UIUC
Acknowledgements
• This research is supported by the Grainger Center and DARPA.
• Thanks for discussions from Professor Krein and Professor
Chapman.
Chang Liu
MASS
UIUC