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.