Radio-Frequency MEMS
(RF-MEMS)
Technology Trends: One example
Picture courtesy: M.C. Wu
Potential Applications of RF MEMS
MEMS enabled wireless transceiver
http://www.eecs.umich.edu/~ctnguyen/mtt99.pdf
-MEMS for on-chip capacitors (C) and inductors (L)
- Need high Q-(quality) factors
Q-factors for a R-L-C circuit
LC2π
1f
Δf
f
R
f2πQ
o
oo Bandwidth
Modeling a micro-
electro-mechanical resonator
http://www.eecs.umich.edu/~ctnguyen/mtt99.pdf
Voltage tunable high-Q capacitor &
inductor
Traditional SAW devices vs. MEMSSAW: Surface Acoustic Wave
Picture Courtesy: C. Nguyen
> SAW devices, for generating frequencies are off-chip
> MEMS offers the same high-frequency selectivity at a much
smaller size
Wrist Communicator
Slide courtesy: Al Pisano, DARPA
Case study
MEMS in Biochemistry & Medicine
Electro-kinetic effects
Electrophoresis: Migration of ions in a separation medium
under the influence of an electric field
(e.g. for DNA sequencing)
- Electrophoresis and electroosmosis
- Used in bio-separation technologies
EfuqE
Accelerating force = Frictional force
q: electric charge
E: electric field
f: friction co-efficient
uE: electrophoretic speed of ion =
viscosity of medium
r: radius of particler
qE
6
- +---
-
-
Electro-Osmotic Flow
Electroosmosis:Motion of electrolytic solutions
under the influence of an
electric field
-Used in micro-pumping
(EOF: Electro-Osmotic Flow)
Flow profiles
- Better for analysis, as there is less band-broadening
Anode: +
Cathode: - ww
w.cap
italanaly
tical.co.u
k/
EuEOF4
dielectric constant zeta potential
viscosity
Flow velocity
EL
V Dw
V
Charge densityDebye length
3
12
wh
LQP
4
128
d
LQP
Rectangular
Circular
Dielectrophoresis
Source: M. Madou, Principles of Microfabrication
AC electro-kinetic effectsParticles having dielectric properties experience different forces
232 ErF rmDEP
)2(
)(
mp
mpr is proportional to
Dielectric constant of
particle medium
Dielectrophoretic separation
- Separation of bio-molecules, cells by the application of electric fields
E = 0 E > 0
M. Madou and M. Heller
DNA (De-oxy Ribonucleic Acid)- The “molecule of life”
DNA RNA Proteins
Each cell contains 1.5 GB of information, through DNA
Source: M. Madou, Principles of Microfabrication
PCR is used in molecular biology, genome sequencing, evolutionary studies …
DNA Amplification by PCR
Exponential increase in DNA, 1 million after 20 cycles, 1 billion after 30 cycles
Sch
abm
uel
ler,
J. M
icro
mec
h. &
Mic
roen
g., 1
1, 3
29
(2
00
1)
Applied Biosystems
Gene-Amp PCR system 9700
Microsystems based PCR
- Faster heating (~ 35 oC/sec) and heat removal more rapid assays
(Time reduced from 6 hours to a few minutes)
- Smaller samples needed
Advantages
Issues - The Si surface is incompatible with Taq enzyme
- Mixing is an issue, (low dimensions laminar flow)
DNA Analysis
D. Devoe, Univ. of Maryland
The NanoChip® from Nanogen Inc.
DNA Analysis, Point of Care (POC)
Analyte Specific Reagents
Cardiovascular Disease
Hypertension
Drug Metabolism/Cancer
Cancer
Deafness
ww
w.n
an
ogen
.com
Electro-wetting for liquid transport
- Instead of pumping, electric fields may be used to move fluids
- Tailoring hydro-phobicity/-philicity of a surface
- Surface tension scales as “l”, while mass scales as “l3”
Principles of Electro-wetting
2)0()(
2CVVV slsl
Young equation:
lv
svsl
Lippmann equation
The solid-liquid interfacial tension sl can be controlled by electric
potential across the interface
lv
slsv
γ
γγcos
cosθγγγ lvslsv
Applying V Reducing slreducing more wetting and vice versa
A digital micro-fluidic circuit
S.K
.Ch
o e
t al,
Jou
rnal
of
ME
MS
, vol.
12, n
o. 1, p
age
72, 2003
LabCD: A Bio-analytic -TAS
rdt
dPc 2
rdt
dPc 2
Pumping forceDensity of liquid
Angular velocity of CD platform
Radial distance from center
BIOSENSORS
Cantilever based sensing
Detection of biomolecules bysimple mechanical transduction:
- cantilever surface is coveredby receptor layer(functionalization)
- biomolecular interactionbetween receptor andtarget molecules(molecular recognition)
- interaction between adsorbed molecules induces surface stress change
bending of cantilever
target molecule
receptor molecule
gold
SiNx cantilever
deflection d
target binding
Bio-molecule sensing
B. Kim et al, Institut für Angewandte Physik - Universität Tübingen
f
A
A
f
eff
1m
k
2
1f
mm
k
2
1f
eff
2
m
f2
A mass sensitive resonator transforms an additional mass loading into a resonance frequency shift mass sensor
f1
f1
f2
B. Kim et al, Institut für Angewandte Physik - Universität Tübingen
A cantilever as a mass-sensitive detector
surface layersubstrate
Stoney formula = surface stress change
t = thickness of the beam
L = length of the beam
E = Young’s modulus of the material
= Poisson ratio of the material
d = deflection of the end of the beam
dL
tE2
2
)1(3
Surface Stress induced bending
Cantilever bending can potentially detect single molecules, however they are
noise limited
B. Kim et al, Institut für Angewandte Physik - Universität Tübingen
Detection scheme
Optical detection of analyte binding
NANO-ELECTRO MECHANICAL SYSTEMS
Everything is going to get smaller
AVIONICS ROADMAP
(Toomarian, NASA Jet Propulsion Laboratory, 2000)
“Thinking spacecraft”
“Smart dust”
What does the future hold in micro-systems?
1. Nano-Electro-Mechanical Systems
- Carbon Nanotubes as Mechanical elements
2. Electro-mechanical cantilevers
3, Is there a molecular future?
Are NEMS the next wave of technology?
Nano-Electro-Mechanical Systems
NE
MS
, S
. E
. Lysh
evski, 2
00
1
• Differs from “classical” mechanical systems
• A new mode of thinking and operation
N. Taniguchi, "On the Basic Concept of 'Nano-Technology',"
Proc. Intl. Conf. Prod. Eng. Tokyo, Part II, Japan Society of Precision Engineering, 1974
Nano-technology
“Nano-technology' mainly consists of the processing of
separation, consolidation, and deformation of materials by
one atom or one molecule.” - N. Taniguchi, 1974.
NEMS
(Nano-Electro-Mechanical Systems)
Vibrational frequency of system
keff: effective force constant l
meff: effective mass l3
increases as l (linear dimension) decreases
Faster device operation
Si cantilever MEMS (100 X 3 X 0.1 m): 19 KHz
NEMS (0.1 X 0.01 X 0.01 m): 1.9 GHz
Promise true Nano-technology !
better force sensitivities (10-18 N)
larger mechanical factors (10-15 g)
higher mass sensitivity (molecular level)
heat capacities, below a yoctocalorie
than MEMS
(Roukes, NEMS, Hilton Head 2000)
eff
eff
om
k
MEMS vs. NEMS
Fabrication
Essentially micro-electronics (CMOS) Involves molecular scale
Based, photo-lithography manipulation, electron-lithography
Materials
Silicon based SiC, GaAs (III-V semiconductors)
Transduction mechanisms
Electrostatic, mainly Indirect means, e.g. piezo-electric, thermal
Mechanics of Materials
Continuum mechanics sufficient Atomistic mechanics necessary?
NEMS
(Nano-Electro-Mechanical Systems)
f: 0.97 MHz, m: 22 6 fg, E: 92 GPa
(Poncharal et al, Science, 283, 1513, 1999)
Carbon nanotube as a electromechanical resonator
SiC/Si wires as electro-mechanical resonators
f: 380 MHz, 90 nm wires(Yang et al, J.Vac. Sci. and Tech B, 19, 551 2001)
(Carr et al, APL, 75, 920, 1999)
Nanometer scale mechanical electrometer
f: 2.61 MHz, Q: 6500(Cleland et al, Nature, 392, 160, 1998)
Bio-motors
F1-ATPase generates ~ 100pN
(Montemagno et al, Science, 290, 1555, 2000)
(Roukes, NEMS, Hilton Head 2000)
Why are higher frequencies important?
- higher Quality (Q)-factors (NEMS have higher Q compared to electrical circuits)
One Application: Greater resolution MRI (1 m possible, currently ~1 mm)
)(22l
tEo
ltwm
l
wEtk
m
k
eff
eff
eff
eff
o
735.0
323
3
Carbon Nanotubes
(Martel, 1998; Smalley, 2002)
Are they the new wonder materials of the 21st century?
* The strongest fiber that will ever be made.
* Electrical Conductivity of Copper or Silicon.
* Thermal Conductivity of Diamond.
* The size and perfection of DNA.
Rolled up sheets of graphite, properties
comparable to pyrolytic graphite
Graphite
Types of Carbon Nanotubes
• Single-walled (SWNT) & multi-walled (MWNT)
• SWNT: seamless cylinder, wall thickness = 1 atom, circumference = tens of atoms, typical diameter = 1.4nm
• MWNT: concentric cylinders
Carbon Nanotubes: Synthesis, Structure, Properties and Application
Mildred S. Dresselhaus, Gene Dresselhaus, Phaedon Avouris (Eds.)
Single-Wall Carbon Nanotube Properties
Property Carbon Nanotube Comparison
Size 0.6 – 1.8 nm Diameter E-Beam lithography: 50nm
wide, few nm thick
Density 1.33 – 1.4 g/cm3 Aluminum – 2.7 g/cm3
Elastic modulus ~ 1 TPa High-strength steel alloys: 2
GPa
Current Carrying
Capacity
1 billion amperes per cm^2 Copper burns @ 1 million
A/cm^2
Temperature Stability 2800 oC in vacuum,
750 oC in air
Metal wire in microchips
melt @ 600 – 1000 C
Cost $100/gram Gold - $10/g
Carbon Nanotubes: A few applications
Frictionless bearings
(Han, NASA Ames, 2003
Fennimore, Nature, 424, 408, 2003 )
Drag-free flow through the tubes
(Tuzun et al, Nanotechnology, 1996)
Carbon nanotubes for “Space Elevators”100 times stronger than steel
www.space.com and Los Alamos National Lab.
A nano-cantilever
Mechanical displacement using an electrical voltage
Voltage
source
Applied voltage (Electrostatics) causes a Mechanical force which moves the cantilever
V
Spring
+ + + +
- - - -
Fmech = k x; Felectrostatic = Q2
+Q
-Q
2 A
Displacement sensitivity: 0.2 Å (0.1 atomic diameter)
- can be used for single molecule sensing (NEMS)
Carbon nanotube
P. Poncharal et al., Science, vol. 283, pp. 1513-1516, 1999.
Electrostatic deflection of a CNT based cantilever
Electrostatic force (Fele) = - V2
2
1 ∂C
∂xRestoring force (Fmec)= - k x
Displacement proportional to V2
P. Poncharal et al., Science, vol. 283, pp. 1513-1516, 1999.
Mechanical resonances of the CNT cantilever
ρ
E)D(D
L
1
8π
βν b2
i
2
2
j
j
Resonance frequencies (For a cylindered cantilever beam)
L. M
eiro
vic
h, E
lem
en
ts o
f Vib
ratio
n A
na
lysis
For the jth harmonic = 4.694)
Elastic modulus
density
Length Outer dia. Inner dia.
Other bending modes
activated at decreased
bending radius
P. Poncharal et al., Science, vol. 283, pp. 1513-1516, 1999.
A nano-balance at the femto-gram level
Displacement proportional to V2
1 femto gram ~ 106 Oxygen molecules
This method can be used to detect single viruses and bacteria
Atomic Force Microscopy (AFM)
Principle of operation
Schmitt group, Denmark
Biological imaging of Immunoglobin G
(C.M. Lieber, 2001)
Surface Profiling
Looking at the atomic structure of surfaces
Scanning Tunneling Microscope (STM)-For probing and positioning individual atoms
Catalytic Converter surface
Rh
: “b
righ
t a
tom
s”
Pt:
“da
rk”
ato
ms
Silicon surface
“Atom” in Kanji Carbon-Monoxide manQuantum Corral
STM can be used to position atoms
Or look at atom movements
The mechanical detection of charge
For detecting a few electronic charges, we currently have
Single Electron Transistors (SETs)
Limited bandwidth, low temperature (mK) operation
Can we use mechanical elements instead?
The mechanical detection of charge: History
•The first application of micro-mechanics as a Silicon based technology
• For a high-Q electromechanical filter (1-132 kHz, Q ~ 500)
The Resonant Gate Transistor (RGT)
-Nathanson, 1965
Not widely accepted, as:
(1)Reproducibility and predictability of resonance frequencies
(2) Potential lifetime limitations, due to fatigue and creep
(3) Was the concept too far ahead of its time?
Towards Nano-science and TechnologyReduced dimensionality & Quantum effects in semiconductor and
magnetic materials
Thin film epitaxy
Two-dimensional electron gas
( ~1000 electrons / m2)
Quantum WIRES
(10-30 electrons)
Quantum BOXES / DOTS
(~ 1 electron !)
+++
2-DEGAl0.3Ga0.7As / GaAs
200 nm
Single
electron box
EF
40 nm
GaAs substrateS/L buffer
i-Al0.3Ga0.7AsGaAs
n-Al0.3Ga0.7Asi-Al0.3Ga0.7As
i-GaAs
Semiconductor heterostructures- the two-dimensional electron gas
1.4
2 eV
High mobility
2-dimensional electron gas (2DEG)
Al0.3 Ga0.7 As
InP
GaAs
In0.53 Ga0.47 As
Fermi
energyEF
CB
VB
CB
VB
e- flow
h+ flow
Extremely high electron mobilities (107 cm2/V-s) can be achieved in a 2DEG configuration
(c.f. bulk GaAs: 8000 cm2/V-s)
EF
Conduction BandConduction Band
Valence Band
Valence Band
1.8
0 eV
+
++
Cleland and Roukes, Nature, 392, 160, 1998.
The mechanical detection of a single charge
- Detection through a 2-dimensional electron gas
Principal Engineering Challenges in NEMS
• The pursuit of a Ultra-high Q:
Dissipation (~ 1/Q) limits force sensitivity and broadens linewidth,
and determines power levels
Extrinsic losses: Air damping, clamping and coupling losses
Intrinsic losses: Materials related (bulk defects, interfaces, adsorbates)
and anelastic losses.
• Surfaces: play a central role in NEMS
(Upto 10% of atoms can be on the surface)
Rou
ke
s, N
EM
S, P
roc. o
f SS
AC
, Hilto
n H
ea
d, 2
00
0
Principal Engineering Challenges in NEMS
• The problem of suitable transducers:
Electrostatic transduction does not scale well into NEMS
Parasitic capacitances dominate the overall capacitance
Optical Transduction: e.g. Fiber-optic schemes do not scale into NEMSThe optical beam size (633 nm for He-Ne) is larger than the device!
• Reproducible nanofabrication is not trivial:
New fabrication techniques required, e.g. electron-beam lithography
The world’s smallest guitar
(Cornell University)
Electron-beam lithography
* Electrons have wavelengths of < 0.005 nm (~ 40 kV), c.f. UV photons ~ 200 nm
finer features possible
An e-beam lithography system
“It is very difficult to make predictions,
especially about the future”
Single Molecule, Single Atom and Single Electron Transistors
(P. McEuen, 2003)
Single Co atom
Gold electrodes
“Our machines will come to resemble biological systems in their complexity,
adaptability, and agility...it is instructive to cast these directed replicating machines in the
light of a new form of intelligent life" (Bishop).
Shape Shifters or Matter Compilers
“Nanotechnology”, E. Drexler “Diamond Age”, Neal Stephenson
- Construct products atom by atom (100% recycling)
- Use the Sun as an energy source
Will bottom-up fabrication replace top-down technology?
References1. Nanosystems: Molecular machinery, Manufacturing and Computation
- K.E. Drexler
2. Journals:
(1) Advanced Materials,
(2) Asia Pacific Nanotechnology Forum,
(3) Chemical Abstracts on CD-ROM,
(4) Colloids And Surfaces A
(5) Forbes-Wolfe Nanotech Report,
(6) IEEE Proceedings – Nanobiotechnology,
(7) IEEE Transactions On Nanobioscience
(8) IEEE Transactions On Nanotechnology
(9) International Journal Of Nanoscience,
(10) Journal Of Metastable And Nanocrystalline Materials
(11) Journal Of Nanoscience And Nanotechnology,
(12) Lab on a Chip: miniaturization for chemistry and biology
(13) Langmuir, Macromolecules
(14) Micro Nano, Microengineering And Nanotechnology News
(15) Nano Et Micro Technologie,
(16) Nano Letters
(17) Nano-bio-info Technology Convergence News,
(18) Nanomagazine
(19) Nanoparticle Technology News,
(20) Nanostructured Materials
(21) Nanotechnology,
(22) Nanoweek:
(23) Physica - Section E : Low-Dimensional Systems And Nanostructures
(24) Small Times, Surface Science
(25) Technology Review: Nanotechnology and Materials
(26) Virtual Journal of Nanoscale Science & Technology
Review Topics for the Mid-Term Exam (May 5, Tuesday)
(1) Scaling laws
Scaling of mass, acceleration, force, power, voltage, electric and
magnetic fields, heat flow in micro-systems, as a function of length.
Broadly, when is miniaturization useful and when is it not?
(2) Principles of Micro-fabrication
(a) Bulk-micromachining: Isotropic vs. Anisotropic etching,
which chemicals are used for each? What is the mechanism involved?
What kinds of etching profiles are created? Name devices which are based on
anisotropic etching, e.g. (100) etching for a pressure sensor
(b) Surface micromachining: When is this used?
Why is it better/worse than bulk micromachining?
What are the steps involved here (e.g. for making a cantilever)?
> The issue of stiction and how to avoid it (the principle of critical point drying).
> Dry etching processes: DRIE
(What is unique to the Bosch process over conventional dry etching techniques?)
> Gas phase Si (XeF2) etching.
(c) Materials in MEMS:
> Advantages of using poly-Si. Stress in Poly-Si.
> Growth by Chemical Vapor Deposition (CVD).
> What are the mechanisms of grain growth in poly-Si?
> Use of Silicon oxide (Wet and dry oxidation: The Deal-Grove model)
> Use of Silicon nitride
> Common methods of depositing materials
(e.g., Physical vapor deposition vs. Chemical Vapor Deposition vs. Electroplating)
> What is a MEMS foundry? What is a MUMPS process?
> What are the common wafer bonding methods? Which sensors use wafer bonding?
(3) Case studies in MEMS:
> Why is electrostatics more common in MEMS than magnetostatics?
> What is Paschen’s law and where is it applicable?
(a) Piezoresistance: Tensor nature and its relation to both stress and current.
How does it vary in Si (as a function of doping and temperature?
What is the principle of using a piezoresistive sensor? Where is it used?
(b) Electrostatics: What is an Accelerometer?
How does it work? Three applications of accelerometers.
A capacitor as a mechanical and an electrical element
(simple modeling as in Homework 2).
> How can you measure capacitance (electrostatics formulae)?
The principle of differential capacitive sensing.
> Why is a Comb-Drive-Actuator better than a parallel plate capacitor element?
(Force-displacement and operating characteristics for all the above actuators)
Optical MEMS: The operating principles of a Digital Micro-Mirror Device (DMD)
and a Grating Light Valve (GLV), the relative advantages and disadvantages.
> Force-displacement and operating characteristics for the torsion mirror in the DMD.
> Single side vs. push-pull drives
(c) Radio-Frequency MEMS: Applications. Frequency dependent elements: R, L, C.
The concept of the Q-factor. Why is a high Q-factor necessary?
(d) Bio-MEMS & Micro-fluidics: Areas of application of Bio-MEMS,
> Microfluidics (Laminar vs. Turbulent flow),
> Principles of Electrophoresis and Electro-osmosis.
> Surface tension plays a big role in MEMS as it scales “only” linearly with length.
How is it exploited in electrowetting to design microfluidic circuits?
> The sensitivity of cantilevers as bio-sensors.