Transcript Emerging Nanotechnology Devices

A Tutorial on
Emerging Nanotechnology Devices
Tezaswi Raja, Rutgers University
Vishwani D. Agrawal, Auburn University
Michael Bushnell, Rutgers University
7 Jan 2004
17th Int'l Conference on VLSI Design
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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.
Emerging Nanotechnology
Solutions
Quantum
Dots
Molecular
SETs
CNFETs
RTD
Nano
CMOS
CNT arrays
Molecular orientations
as Bits
DNA self
assembly
Molecules in
Solution
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Emerging Nanotechnology
Drivers
DNA strands
as Bits
Self assembled
CNT using DNA
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Logic
(Our Focus)
Memory
Fabrication
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Computing Devices
Solid State Devices
CMOS Devices
Nano
CMOS
CNFET
Quantum
Molecular Devices
Quantum Devices
Quantum
Dot
Electro-
RTD
Photoactive
mechanical
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SET
Electrochemical
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Photo Courtesy: Fujitsu Labs
Nano-Scale MOSFET
Metal Oxide Semiconductor Field Effect Transistor
Three terminal device
Source, gate and drain
Vg controls the conduction from source to drain
Half 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
Difficulties
High electric fields
Power supply vs. threshold voltage
Heat dissipation
Interconnect delays
Vanishing bulk properties
Shrinkage of gate oxide layer
Too many problems to continue miniaturization as physical
limits approach
Proposed solutions are short term
Open Problems
Improve 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 carbon
Longest 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.
C = n a1 + m a2
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 metallic
If (n-m) is not divisible by 3, the tube is
semiconducting.
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Courtesy: IBM
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 consumption
Although tubes are 3nm thick CNFETs are still the size of the
contacts, about 20nm.
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Courtesy: IBM
Courtesy: IBM
CNT Fabrication
Controlling the conductivity of the tubes (Constructive
Destruction)
All tubes laid on the contact
Metallic tubes are destroyed
Controlling diameter of the tube
Start 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 Challenges
CNTs are flexible tubes that can be made conducting
or semiconducting.
Nano-scale, strong and flexible.
Challenges:
Multilevel interconnects not available
Chip density still limited to the density of contacts.
Tube density not entirely exploited
Fabrication is still a stochastic process
Alternatives 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
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Allowed
Energy
Levels
Barrier
Occupied
Energy
Levels
Barrier
Energy
Solid State Quantum Devices
Occupied
Energy
Levels
Distance
Source
Island
Drain
Quantum effects used to build devices.
Electrons confined on an island
Island can be created by using different band-gap devices in succession
Island has certain allowed energy levels
If allowed energy levels are filled then the device is in conduction
Types of devices
Resonant Tunneling Diode (RTD)
Single Electron Transistor (SET)
Quantum Dot (QD)
Blocking conduction due to unavailable energy levels is called
coulomb blockade
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Conduction
Occupied
Conduction
Band
Allowed
Energy
Levels
Occupied
Conduction
Band
Energy
Energy
Principle of Conduction
Conduction
Occupied
Conduction
Band
Allowed
Energy
Levels
Gate bias
Source
Island
Drain
Source
Island
Drain
Conduction can occur by
Increasing source to drain voltage
Applying Gate Bias
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Single Electron Transistors (SET)
Source
Gate
Cg
Island
Drain
Conductance changes in spurts as energy levels are discrete
To 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 SET
Bias required for conduction is coulomb gap voltage
Same device can act as pFET or nFET based on the barrier strength
Applications:
Extra sensitive charge meters
CMOS style conducting devices
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Quantum Dots and Arrays
Dot occupied
by Electron
Dot unoccupied
Inter-dot
Barriers
Outer
Barriers
Courtesy: vortex.tn.tudelft.nl/ grkouwen/kouwen.html
3-dimensional island tunneling barrier
State 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.
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Quantum Cellular Automata
1
“1”
1
QCA Wire
“0”
Stable
1
0
Unstable
QCA Inverter
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.
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Summary and Challenges
Summary
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.
Challenges
Background charge may offset states (noise sensitivity)
Sensitivity of tunneling current to barrier width (lithographic
accuracy)
Sensitivity to barrier widths
Cryogenic operation
Open Problems
Lithographic methods with guaranteed accuracy
Self assembly of systems
Background charge elimination
Synthesis and verification techniques needed
Testing of these devices as stuck-at models may be inadequate.
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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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Molecular Electronics
Incentives
Molecules are nano-scale
Self assembly is achievable
Very low-power operation
Highly uniform devices
Quantum Effect Devices
Building quantum wells using molecules
Electromechanical Devices
Using mechanical switching of atoms or molecules
Electrochemical Devices
Chemical interactions to change shape or orientation
Photoactive Devices
Light frequency changes shape and orientation.
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Molecular Electronics
Thiol
Acetylene linkage
Benzene ring
Mechanical synthesis
Molecules aligned using a scanning tunneling
microscope (STM)
Fabrication done molecule by molecule using STM
Chemical synthesis
Molecules aligned in place by chemical interactions
Self assembly
Parallel fabrication
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An Atomic Relay
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Summary and Challenges
Summary
Parallel self assembly
Very regular structures
Many alternatives proposed but inherent problems
Very low energy operation
Challenges
Signal restoration and gain
Finding non-interacting chemicals
Chemical reactions stochastic with by-products
Slow operating speeds
Open Problems
Self assembling of devices
Increased speed of operation
Guaranteed switching of molecules (HP- UCLA devices)
Simulation models and CAD
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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
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