Spintronics Integrating magnetic materials with semiconductors

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Radio-Frequency MEMS (RF-MEMS)
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Technology Trends One example
Picture courtesy M.C. Wu
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Potential Applications of RF MEMS
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MEMS enabled wireless transceiver

• MEMS for on-chip capacitors (C) and inductors (L)
• Need high Q-(quality) factors

http//www.eecs.umich.edu/ctnguyen/mtt99.pdf
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Q-factors for a R-L-C circuit
Modeling a micro- electro-mechanical resonator
Bandwidth
http//www.eecs.umich.edu/ctnguyen/mtt99.pdf
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Voltage tunable high-Q capacitor inductor
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Traditional SAW devices vs. MEMS
SAW Surface Acoustic Wave
gt SAW devices, for generating frequencies are
off-chip gt MEMS offers the same high-frequency
selectivity at a much smaller size
Picture Courtesy C. Nguyen
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Wrist Communicator
Slide courtesy Al Pisano, DARPA
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Case study MEMS in Biochemistry Medicine
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Electro-kinetic effects
- Electrophoresis and electroosmosis - Used in
bio-separation technologies
Electrophoresis Migration of ions in a
separation medium under the influence of
an electric field (e.g. for DNA sequencing)
f friction co-efficient uE electrophoretic
speed of ion h viscosity of medium r radius
of particle
q electric charge E electric field
Accelerating force Frictional force
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Electro-Osmotic Flow
Debye length
Charge density

• Electroosmosis
• Motion of electrolytic solutions
• under the influence of an
• electric field
• Used in micro-pumping
• (EOF Electro-Osmotic Flow)

Cathode -
V
www.capitalanalytical.co.uk/
Anode
Rectangular
Flow profiles
dielectric constant
zeta potential
Circular
Flow velocity
- Better for analysis, as there is less
band-broadening
viscosity
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AC electro-kinetic effects
Particles having dielectric properties experience
different forces
Dielectrophoresis
Dielectric constant of particle medium
ar is proportional to
Source M. Madou, Principles of Microfabrication
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Dielectrophoretic separation

• Separation of bio-molecules, cells by the
  application of electric fields

E 0
E gt 0
M. Madou and M. Heller
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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
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DNA Amplification by PCR
PCR is used in molecular biology, genome
sequencing, evolutionary studies
Exponential increase in DNA, 1 million after 20
cycles, 1 billion after 30 cycles
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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)

Schabmueller, J. Micromech. Microeng., 11, 329
(2001)
Gene-Amp PCR system 9700
Applied Biosystems
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DNA Analysis
D. Devoe, Univ. of Maryland
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DNA Analysis, Point of Care (POC)
Analyte Specific Reagents Cardiovascular
DiseaseHypertensionDrug Metabolism/CancerCan
cerDeafness
www.nanogen.com
The NanoChip from Nanogen Inc.
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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

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Principles of Electro-wetting

• The solid-liquid interfacial tension (gsl) can be
  controlled by electric
• potential across the interface

Young equation
Lippmann equation
glv
gsv
gsl
Applying V ? Reducing gsl ?reducing q ? more
wetting and vice versa
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A digital micro-fluidic circuit
S.K.Cho et al, Journal of MEMS, vol. 12, no. 1,
page 72, 2003
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LabCD A Bio-analytic m-TAS
Radial distance from center
Angular velocity of CD platform
Pumping force
Density of liquid
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BIOSENSORS Cantilever based sensing
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Bio-molecule sensing
Detection of biomolecules by simple mechanical
transduction - cantilever surface is
covered by receptor layer (functionalization) -
biomolecular interaction between receptor
and target molecules (molecular
recognition) - interaction between adsorbed
molecules induces surface stress change ?
bending of cantilever
target molecule
receptor molecule
gold
SiNx cantilever
target binding
deflection ?d
B. Kim et al, Institut für Angewandte Physik -
Universität Tübingen
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A cantilever as a mass-sensitive detector
f1
f1
?m
f2
f2
A mass sensitive resonator transforms an
additional mass loading into a resonance
frequency shift ? mass sensor
B. Kim et al, Institut für Angewandte Physik -
Universität Tübingen
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Surface Stress induced bending
? ? surface stress change t thickness of the
beam L length of the beam E Youngs modulus
of the material ? Poisson ratio of the
material ? d deflection of the end of the beam
Stoney formula
Cantilever bending can potentially detect single
molecules, however they are noise limited
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Optical detection of analyte binding
Detection scheme
B. Kim et al, Institut für Angewandte Physik -
Universität Tübingen
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NANO-ELECTRO MECHANICAL SYSTEMS
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Everything is going to get smaller
AVIONICS ROADMAP
Thinking spacecraft Smart dust
(Toomarian, NASA Jet Propulsion Laboratory, 2000)
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What does the future hold in micro-systems?
Are NEMS the next wave of technology?

• Nano-Electro-Mechanical Systems
• - Carbon Nanotubes as Mechanical elements
• 2. Electro-mechanical cantilevers
• 3, Is there a molecular future?

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Nano-Electro-Mechanical Systems

• Differs from classical mechanical systems
• A new mode of thinking and operation

NEMS, S. E. Lyshevski, 2001
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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.
N. Taniguchi, "On the Basic Concept of
'Nano-Technology'," Proc. Intl. Conf. Prod. Eng.
Tokyo, Part II, Japan Society of Precision
Engineering, 1974
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NEMS (Nano-Electro-Mechanica
l Systems)

• wo Vibrational frequency of
  system
• keff effective force constant a l
• meff effective mass a l3
• wo increases as l (linear dimension) decreases
• Faster device operation
• Si cantilever MEMS (100 X 3 X 0.1 mm) 19
  KHz
• NEMS (0.1 X 0.01 X 0.01 mm) 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)
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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) Transdu
ction mechanisms Electrostatic, mainly Indirect
means, e.g. piezo-electric, thermal Mechanics of
Materials Continuum mechanics sufficient Atomisti
c mechanics necessary?
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NEMS (Nano-Electro-Mechanica
l Systems)
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)
Carbon nanotube as a electromechanical resonator
f 0.97 MHz, m 226 fg, E 92 GPa
(Poncharal et al, Science, 283, 1513, 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)

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(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 (1mm
possible, currently 1 mm)
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Carbon Nanotubes
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
(Martel, 1998 Smalley, 2002)
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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.)
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Single-Wall Carbon Nanotube Properties
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Carbon Nanotubes A few applications
Frictionless bearings
Drag-free flow through the tubes
(Tuzun et al, Nanotechnology, 1996)
(Han, NASA Ames, 2003 Fennimore, Nature, 424,
408, 2003 )
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Carbon nanotubes for Space Elevators
100 times stronger than steel
www.space.com and Los Alamos National Lab.
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A nano-cantilever
Displacement sensitivity 0.2 Å (0.1 atomic
diameter) - can be used for single molecule
sensing (NEMS)
Mechanical displacement using an electrical
voltage
Spring
Carbon nanotube
Voltage source

Q
V
- - - -
-Q
Applied voltage (Electrostatics) causes a
Mechanical force which moves the cantilever
Fmech k Dx Felectrostatic Q2
2eA
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Electrostatic deflection of a CNT based cantilever
Restoring force (Fmec) - k x
Displacement proportional to V2
P. Poncharal et al., Science, vol. 283, pp.
1513-1516, 1999.
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Mechanical resonances of the CNT cantilever
Resonance frequencies (For a cylindered
cantilever beam)
Elastic modulus
L. Meirovich, Elements of Vibration Analysis
density
Length Outer dia. Inner dia.
For the jth harmonic (b1 1.875, b2 4.694)
Other bending modes activated at decreased
bending radius
P. Poncharal et al., Science, vol. 283, pp.
1513-1516, 1999.
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A nano-balance at the femto-gram level
1 femto gram 106 Oxygen molecules
Displacement proportional to V2
This method can be used to detect single viruses
and bacteria
P. Poncharal et al., Science, vol. 283, pp.
1513-1516, 1999.
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Looking at the atomic structure of surfaces
Atomic Force Microscopy (AFM)
Surface Profiling
Principle of operation
Biological imaging of Immunoglobin G
Schmitt group, Denmark
(C.M. Lieber, 2001)
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Scanning Tunneling Microscope (STM)

• For probing and positioning individual atoms

Silicon surface
Catalytic Converter surface
Rh bright atoms Pt dark atoms
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STM can be used to position atoms
Quantum Corral
Atom in Kanji
Carbon-Monoxide man
Or look at atom movements
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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?

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The mechanical detection of charge History
The Resonant Gate Transistor (RGT) -Nathanson,
1965

• The first application of micro-mechanics as a
  Silicon based technology
• For a high-Q electromechanical filter (1-132
  kHz, Q 500)

• Not widely accepted, as
• Reproducibility and predictability of resonance
  frequencies
• Potential lifetime limitations, due to fatigue
  and creep
• Was the concept too far ahead of its time?

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Towards Nano-science and Technology
Reduced dimensionality Quantum effects in
semiconductor and magnetic materials
i-GaAs
i-Al0.3Ga0.7As
n-Al0.3Ga0.7As
i-Al0.3Ga0.7As
Thin film epitaxy
GaAs
S/L buffer
EF
Two-dimensional electron gas ( 1000 electrons /
mm2)
Al0.3Ga0.7As / GaAs
Quantum WIRES (10-30 electrons)
200 nm
40 nm
Quantum BOXES / DOTS ( 1 electron !)
Single electron box
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Semiconductor heterostructures- the
two-dimensional electron gas
Extremely high electron mobilities (107 cm2/V-s)
can be achieved in a 2DEG configuration (c.f.
bulk GaAs 8000 cm2/V-s)
1.42 eV
High mobility 2-dimensional electron gas
(2DEG)
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The mechanical detection of a single charge
- Detection through a 2-dimensional electron gas
Cleland and Roukes, Nature, 392, 160, 1998.
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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)

Roukes, NEMS, Proc. of SSAC, Hilton Head, 2000
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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 NEMS
• The 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

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Electron-beam lithography
Electrons have wavelengths of lt 0.005 nm ( 40
kV), c.f. UV photons 200 nm ? finer features
possible
An e-beam lithography system
The worlds smallest guitar
(Cornell University)
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It is very difficult to make predictions,
especially about the future
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Single Molecule, Single Atom and Single Electron
Transistors
Single Co atom
Gold electrodes
(P. McEuen, 2003)
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Will bottom-up fabrication replace top-down
technology?
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

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References

• Nanosystems Molecular machinery, Manufacturing
  and Computation
• - K.E. Drexler
• 2. Journals
• 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

62

• Review Topics for the Mid-Term Exam (May 8,
  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
• 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)?
• gt The issue of stiction and how to avoid it (the
  principle of critical point drying).
• gt Dry etching processes DRIE
• (What is unique to the Bosch process over
  conventional dry etching techniques?)

63

• (c) Materials in MEMS
• gt Advantages of using poly-Si. Stress in Poly-Si.
  
• gt Growth by Chemical Vapor Deposition (CVD).
• gt What are the mechanisms of grain growth in
  poly-Si?
• gt Use of Silicon oxide (Wet and dry oxidation
  The Deal-Grove model)
• gt Use of Silicon nitride
• gt Common methods of depositing materials
• (e.g., Physical vapor deposition vs. Chemical
  Vapor Deposition vs. Electroplating)
• gt What is a MEMS foundry? What is a MUMPS
  process?
• gt What are the common wafer bonding methods?
  Which sensors use wafer bonding?

64

• (3) Case studies in MEMS
• gt Why is electrostatics more common in MEMS than
  magnetostatics?
• gt What is Paschens law and where is it
  applicable?
• 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).
• gt How can you measure capacitance (electrostatics
  formulae)?
• The principle of differential capacitive
  sensing.
• gt Why is a Comb-Drive-Actuator better than a
  parallel plate capacitor element?
• (Force-displacement and operating characteristics
  for all the above actuators)

65

• Optical MEMS The operating principles of a
  Digital Micro-Mirror Device (DMD)
• and a Grating Light Valve (GLV), the relative
  advantages and disadvantages.
• gt Force-displacement and operating
  characteristics for the torsion mirror in the
  DMD.
• gt 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,
• gt Microfluidics (Laminar vs. Turbulent flow),
• gt Principles of Electrophoresis and
  Electro-osmosis.
• gt 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?
• gt The sensitivity of cantilevers as bio-sensors.