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Emerging Nanotechnology Devices. Outline. Introduction Nano Scale MOSFET Carbon Nanotube FETs Solid State Quantum Devices Molecular Electronics Challenges and the Current State of the Art Conclusion. Introduction. Feature size nearing the physical limits - PowerPoint PPT Presentation
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Emerging Nanotechnology Devices
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Outline
Introduction
Nano Scale MOSFET
Carbon Nanotube FETs
Solid State Quantum Devices
Molecular Electronics
Challenges and the Current State of the Art
Conclusion
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Introduction
Feature size nearing the physical limits
Fabrication process approaching limits
Power consumption – a concern
Quantum effects need to be accounted for
Solution? Nanotechnology
We present an overview of new devices and outline some open problems.
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What is Nanotechnology?Switching devices of nanometer (below 100nm, typically 10nm) dimensions define nanotechnology.
DNA strands as Bits
Molecular orientations as Bits
CNFETsSETs
Self assembled CNT using DNA
Quantum Dots
CNT arrays
DNA self assembly
Logic
(Our Focus)
Memory
Fabrication
RTDMolecular Nano CMOS
Molecules in Solution
Emerging Nanotechnology Drivers
Emerging Nanotechnology Solutions
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Computing Devices
CMOS Devices
Solid State Devices Molecular Devices
Nano CMOS
Quantum Dot
RTD
Quantum Devices
CNFET SET
Electro-
mechanicalPhotoactiveQuantum
Electro-
chemical
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Nano-Scale MOSFET
Metal Oxide Semiconductor Field Effect TransistorThree terminal deviceSource, gate and drainVg controls the conduction from source to drainHalf thickness of the gate is called “Feature size λ”Current feature sizes in production – 90nm (Intel Pentium 5)Demonstrated feature sizes up to 20nm (IBM).
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Challenges
DifficultiesHigh electric fieldsPower supply vs. threshold voltageHeat dissipationInterconnect delaysVanishing bulk propertiesShrinkage of gate oxide layerToo many problems to continue miniaturization as physical limits approachProposed solutions are short term
Open ProblemsImprove lithographic precision (eBeam)Explore new materials (GaAs, SiGe, etc.)As a long term goal explore new devices
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Outline
Introduction
Nano scale MOSFET
Carbon Nanotube FETs
Solid State Quantum Devices
Molecular Electronics
Challenges and current state of the art
Conclusions
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Carbon Nanotubes
Carbon nanotubes are long meshed wires of carbonLongest tubes up to 1mm long and few nanometers thick made by IBM.
Property Carbon Nanotubes Comparatively
Size 0.6-1.8 nm in diameter Si wires at least 50nm thick
Strength 45 Billion Pascals Steel alloys have 2 Billion P.
Resilience Bent and straightened without damage Metals fracture when bent and restraightened
Conductivity Estimated at 109 A/cm2 Cu wires burn at 106 A/cm2
Cost $2500/gram by BuckyUSA in Houston Gold is $15/gram
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Electrical Properties of CNT
Carbon nanotubes can be metallic or semiconductor depending on their chirality.Chiral Vector C is defined as the vector from one open end of the tube to the other after it is rolled.If (n-m) is divisible by 3, the tube is metallicIf (n-m) is not divisible by 3, the tube is semiconducting.
C = n a1 + m a2
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Carbon Nanotube FET
CNT can be used as the conducting channel of a MOSFET.These new devices are very similar to the CMOS FETs.All CNFETs are pFETs by nature.nFETs can be made through
Annealing Doping
Very low current and power consumptionAlthough tubes are 3nm thick CNFETs are still the size of the contacts, about 20nm.
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CNT Fabrication
Controlling the conductivity of the tubes (Constructive Destruction)
All tubes laid on the contactMetallic tubes are destroyed
Controlling diameter of the tubeStart with MWNTs.Destroy the outer layers one by one to reduce diameter.
Placing exactly at the required location. Yet to be demonstrated convincingly to exploit complete advantage using Lithography.Using DNA for self assembly
Demonstrated by Techion-Israel very recently (Nov’2003).
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Summary and ChallengesCNTs are flexible tubes that can be made conducting or semiconducting.Nano-scale, strong and flexible.Challenges:
Multilevel interconnects not availableChip density still limited to the density of contacts.Tube density not entirely exploitedFabrication is still a stochastic processAlternatives to gold contacts need to be found.
Open Problems and Initiatives:Fabrication using DNA for self assembly (Technion-Israel; Science, Nov 2003)Memory array of nanotubes using junctions as bit storages (Lieber at Harvard)Using nanotube arrays to make computing elements (DeHon at Caltech)Fabricate FPGAs using CNFETs and STM (Avouris at IBM)
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Outline
Introduction
Nano scale MOSFET
Carbon Nanotube FETs
Solid State Quantum Devices
Molecular Electronics
Challenges and current state of the art
Conclusions
177 Jan 2004 17th Int'l Conference on VLSI Design
Solid State Quantum Devices
Quantum effects used to build devices.Electrons confined on an island
Island can be created by using different band-gap devices in successionIsland has certain allowed energy levelsIf allowed energy levels are filled then the device is in conduction
Types of devicesResonant Tunneling Diode (RTD)Single Electron Transistor (SET)Quantum Dot (QD)
Blocking conduction due to unavailable energy levels is called coulomb blockade
En
erg
y
Occupied Energy Levels
Occupied Energy Levels
Allowed Energy Levels
Source Island Drain
Bar
rier
Distance
Bar
rier
187 Jan 2004 17th Int'l Conference on VLSI Design
Principle of Conduction
Conduction can occur byIncreasing source to drain voltage
Applying Gate Bias
Allowed Energy Levels
Source Island Drain
En
erg
y
Occupied Conduction
Band
Allowed Energy Levels
Source Island Drain
En
erg
y
Occupied Conduction
BandGate bias
Occupied Conduction
Band
Conduction Conduction
197 Jan 2004 17th Int'l Conference on VLSI Design
Single Electron Transistors (SET)
Conductance changes in spurts as energy levels are discreteTo go from conducting to non-conducting stage, it requires voltage sufficient for one electron to cross
This is achieved by applying gate bias enough for just one electron charge -- hence the name SETBias required for conduction is coulomb gap voltage
Same device can act as pFET or nFET based on the barrier strengthApplications:
Extra sensitive charge metersCMOS style conducting devices
Drain
Source
GateCg
Island
207 Jan 2004 17th Int'l Conference on VLSI Design
Quantum Dots and Arrays
3-dimensional island tunneling barrierState determined by presence of electron and not by conduction.Quantum cell array (QCA) is a lattice of these cells with 2 electrons confined.Occupied electrons are furthest from each other due to repulsive forces.
Courtesy: vortex.tn.tudelft.nl/ grkouwen/kouwen.html
Inter-dot Barriers
Outer Barriers
Dot occupied by Electron
Dot unoccupied
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Quantum Cellular Automata
2 states – “1” and “0”.Electrostatic interaction of nearby cells makes the bits flip.Input to the cell is by manipulating the Inter-dot barriers.Logic gates can be constructed.
“1” “0”
1 1
QCA Wire
1 0
QCA Inverter
Stable
Unstable
227 Jan 2004 17th Int'l Conference on VLSI Design
Summary and ChallengesSummary
Electrons confined on an island.Allowed energy levels are discrete and allow the device to fluctuate between conducting and non-conducting states.SET – 2 dimensional device with gate bias control.QD – device with electron presence as state.QCA – Arrays of QDs used for computing.
ChallengesBackground charge may offset states (noise sensitivity)Sensitivity of tunneling current to barrier width (lithographic accuracy)Sensitivity to barrier widthsCryogenic operation
Open ProblemsLithographic methods with guaranteed accuracySelf assembly of systemsBackground charge eliminationSynthesis and verification techniques neededTesting of these devices as stuck-at models may be inadequate.
237 Jan 2004 17th Int'l Conference on VLSI Design
Outline
Introduction
Nano scale MOSFET
Carbon Nanotube FETs
Solid State Quantum Devices
Molecular Electronics
Challenges and current state of the art
Conclusions
247 Jan 2004 17th Int'l Conference on VLSI Design
Molecular Electronics
IncentivesMolecules are nano-scaleSelf assembly is achievableVery low-power operationHighly uniform devices
Quantum Effect DevicesBuilding quantum wells using molecules
Electromechanical DevicesUsing mechanical switching of atoms or molecules
Electrochemical DevicesChemical interactions to change shape or orientation
Photoactive DevicesLight frequency changes shape and orientation.
257 Jan 2004 17th Int'l Conference on VLSI Design
Molecular Electronics
Mechanical synthesisMolecules aligned using a scanning tunneling microscope (STM)Fabrication done molecule by molecule using STM
Chemical synthesisMolecules aligned in place by chemical interactionsSelf assemblyParallel fabrication
Benzene ring
Acetylene linkageThiol
267 Jan 2004 17th Int'l Conference on VLSI Design
An Atomic Relay
277 Jan 2004 17th Int'l Conference on VLSI Design
Summary and ChallengesSummary
Parallel self assemblyVery regular structuresMany alternatives proposed but inherent problemsVery low energy operation
ChallengesSignal restoration and gainFinding non-interacting chemicalsChemical reactions stochastic with by-productsSlow operating speeds
Open ProblemsSelf assembling of devicesIncreased speed of operationGuaranteed switching of molecules (HP- UCLA devices)Simulation models and CAD
287 Jan 2004 17th Int'l Conference on VLSI Design
Conclusion
CMOS technology is approaching saturation – problems in the nanometer range
Several new possibilities emerging Carbon nanotubes (CNT) Single-electron transistor (SET) and
quantum dots (QD) Molecular computing devices