NanoComputers

May 10, 2001 Michael Pan MeiYa Li Rebecca Stadler

Introduction

Moore’s Law Lower limit for Transistor Size

1 micrometer, or 50 nanometers Loss of functionality

New technological field required Nanotechnology

Nanotechnology

Manipulation of atoms 1 at a time Decrease in the size of transistors

Increase in density Concurrent advances in biology and

chemistry 1990’s

Ability to position single atoms

Types of Nanocomputers

Mechanical Nanocomputers Chemical Nanocomputers Quantum Nanocomputers Electrical Nanocomputers

Does not require a fundamental change in the operating principles of the transistor

Potential Technologies for use in a Nanocomputer

Devices are based on the principles of quantum mechanics

Include: Resonant Tunneling Transistor Single Electron Transistor Quantum Dot

Resonant Tunneling Device

RTD is comprised of 2 insulating barriers in a semiconductor heterostructure Creates an island Island is about 10 nms wide

Potential Well Finite, integral number of “quantized” energy levels Electrons are able to pass through the device by

tunneling through 2 barriers Depends on the energy of the incoming electrons as

compared to the device’s internal energy level Needs to be in resonance for current to flow

Resonant Tunneling Transistor

Incorporate an RTD into emitter of a Bipolar Junction Transistor Three terminal device, similar to MOSFET Current controlled, rather than voltage controlled

RTT as a 2 state device RTD serves as a filter, allowing current to flow to BJT at

certain base-emitter voltages only Transistor “on” or “off”

Problem: Nanometer device integrated with a microelectronic device Size limitations

Single Electron Transistor

SET operates by moving single electrons Consists of a source, island, and drain

Electrons can enter the island one at a time Tunnel onto the island from source, exit via drain

Control the number of electrons entering and exiting the drain Electron flow continues, causing a current flow

through the island

SET

Use a metal gate electrode near the island Increase gate voltage, an additional electron

can tunnel on and off island, creating measurable current

Step-wise function Limitations

Low Temperatures to avoid thermal energy

Quantum Dot Cells

Small potential well or box Electrostatic field to determine the number

of electrons in the quantum dot Holds 0 – 100’s of electrons

Rely on specific quantum effects Cannot store and retrieve information

Exact number of electrons is not known, due to low resistivity of the device

QDs

Quantum dots can effect one another Even if they are not wired together Due to long-range electrostatic interactions

One dot’s electric field can change the number of electrons in another dot Quantum dots can be lined up to cause the movement of

electrons Two state device corresponding to occupancy of the dot

by 0 or 1 electron Wireless because of communication through electric field

Limitations of fabrication and low temperature

Architectures for Molecular Electronic Computer Logic

Background

Diode AND OR XOR Half-adder Molecular-scale electronic devices

Diode

Two-terminal switch On: closed, forward

bias Off: open, reverse bias

AND Gate

Output = 1: All inputs are 1

Output = 0

OR Gate

Output = 0: All inputs are 0

Output = 1: Any input is 1 All inputs are 1

XOR Gate

Output = 0: All inputs are 0; All inputs are 1

Output = 1: Only one input is 1

Half-Adder Gate

Combinational circuit Two inputs and two

binary outputs The output variables

produce: Sum, S: least

significant bit Carry, C: output =1 ->

both inputs are 1

Molecular-Scale Electronic Device

Polyphenylene-based molecular backbone chains

Carbon nanotubes

Polyphenylene-based molecular-scale electronic devices

chains of organic aromatic benzene rings

Aromatic Organic Molecules Serves as conductor Benzene ring:

Chemical formula: C6H6

Phenyl group: C6H5

Phenylene group: C6H4

Polyphenylene: Binding phenylenes to each

other, terminatin the result chain-like structures with phenyl groups

Different types of molecular groups: aliphatic, ethenyl, ethynyl.

Aliphatic Organic Molecules

Serves as insulators Forms a barrier: middle of conductive

polyphenylene chain

Polyphenylene-based molecular rectifying diodes switch

Two intramolecular dopant group: X, Electron donating Y, Electron

withdrawing Separate by R:

aliphatic groups

Molecular Electronic AND Gate

Using Diode-Diode Logic Dimension: 3x4nm2

Molecular Electronic OR Gate

Using Diode-Diode Loigc Dimension: 3x4nm2

Molecular Electronic XOR Gate

Using Diode-Base Logic

N or Z: represnts an RTD

Dimension: 5x5nm2

RTD

Resonant Tunneling Diodes

Molecular Electronic Half Adder

Combinational logic: Several molecular logic

gate to bond together Molecular XOR and AND

gate

Conclusion

The area of the molecular electronic logic structures is one million times smaller than analogous logic structures.

Both molecular AND and OR gates are using Diode-Diode logic structure.

Different between the molecular AND and OR gate is the orientation of the molecular diodes is reversed.

Conclusion (cont.)

XOR uses Diode-Based logic structure. Molecular XOR gate is very similar to

molecular OR gate, except for the addition of the molecular RTD.

The most well-known combinational circuit for a binary half adder design is implemented with an XOR and an AND gate.

Fabrication and Future Studies

Abstract Introduction Fabrication techniques Future challenges Conclusion

Abstract

the laws of quantum mechanics and the limitations of fabrication techniques may soon prevent further reduction in the size of today’s conventional field-effect transistors

the devices will become more difficult and costly to fabricate

Introduction

Improved fabrication technologies are the key to progress in nanotechnology and nanoelectronics.

No matter how small a proposed electronic device can or should be built in theory, the limitations in fabrication processes determine how small the device can be built in practice.

Present Fabrication Techniques

Lithography Molecular Beam Epitaxy (MBE) Mechanosynthesis Chemosynthesis

Lithography

Lithography uses a beam of light or matter to make a pattern on a surface. There are several lithography techniques that are currently being used in the industry; including UV lithography, X-ray lithography, atom lithography and Electron-beam lithography.

UV Lithography

Most modern integrated circuits are produces by photolithography. Photolithography is a process that beams visible or ultraviolet light through a reusable mask and onto a thin coating of photoresistive material covering a silicon wafer.

X-ray Lithography

X-ray lithography is a further refinement of lithographic techniques using ultraviolet light. This refinement provides a more precise tool with which to carve out a pattern on a substrate. The smaller wavelengths of X-rays allow feature sizes from 500 to 30 nm.

Electron-beam lithography

Electron-beam lithography replaces the light beam and masks used in photolithography with a direct beam of electrons. It works well with for high resolution features because electrons have much shorter wavelengths than light and can be focused very precisely using electric field.

Atom Lithography

Atom lithograph actually writes the atom directly onto the substrate. It uses the standing wave of light as mask to guide a beam of atoms to desired resting places on the surface of a wafer.

MBE

MBE is an advanced fabrication technique for creating layered surfaces. Molecular beam epitaxy uses a beam of molecules under low pressure that collides with a heated single-crystal surface to create epitaxial layers of molecules.

Mechanosynthesis

Nanoelectronic devices maybe one day be assembled by the mechanical positioning of atoms or molecular building blocks one atom or molecule at a time, a process known as mechanosynthesis

Chemosynthesis

Chemosynthesis is also an emerging fabrication of the components for nao-scale electronics.

Chemical self assembly is the spontaneous orientation of a number of molecules. It usually occurs in non-covalent bonding among molecules. One advantage of this method is the error correction process. It corrects the wrong type of molecules, and wrong positioned molecules in the assembly process. Another type of chemosynthesis is Hybrid Chemosynthesis, it combines the use of atom beams with some techniques of self-assembly.

Future Challenges

I. Demonstration of a molecular electronic rectifier or transistor

We need to increase the density and raise the temperature in which nanoelectronic devices can operate above the cryogenic range, it is very important to fabricate nanoelectronic devices on the same scale as a single molecule. One proposed method is to design and synthesis of single molecule.

Future Challenges(Cont.)

II Fabricate working electronic device from molecular transistors

Even if we know how to make molecular transistors, the assembly of these components into a working logic structure still presents a problem.

One possible method to the assemble such a device is to use a scanning-tunneling electron microscope to arrange the molecular components on a surface

Future Challenges(Cont.)

III Demonstration of a nanoscale Silicon quantum heterojunction

For us to reduce the size of modern electronic devices down to the nanometer scale, it is apparent that we need to construct quantum wells of that dimension. Knowing that, we must build very tiny layers of solid structures, where each layers are made of different semiconductors with different energies. These layered structures as we know are semiconductor heterojunctions. We need to make them reliably on the nanometer scale, and make them on the nanometer scale out of silicon compounds.

Future Challenges(Cont.)

IV Demonstration of nanometer-scale quantum dot cells and wireless logic.0

The design for constructing wireless quantum dot computer logic is a very promising idea for implementing nanoelectronic computers. In order to make nanometer-scale devices of this type, we need to come up with a method to fabricate and test this device.

Future Challenges(Cont.)

V Demonstration of Terabit quantum-effect electronic memory chip.

If we were to build nanoelectronic logic devices, it is very possible to assemble from them is terabit (10^12 bit) memory array. With terabit memory array, we would have a much larger storage. Also, we will have a much faster access and no moving mechanical parts. Storage of a movie on a such chip is on example.

Future Challenges(Cont.)

VI. Nanofabrication with a micro-STM or micro-AFM

It is very difficult to mechanically assemble nanoscopic structures and devices with macroscopic probes. Using microelectromechanical systems (MEMS) devices will permit more efficient mechanical manipulation of nanometer-scale structures. We will need to apply micro-STMs and micro-AFMs to practical nanofabrication.

Future Challenges(Cont.)

VII. Parallel nanofabrication with a micro-STM or micro -AFM arrays

For one thing, if nanoelectronics is to become practical and reliable, we must fabricate nanometer-scale structures by the billions and with high effieniency. Now, we fabricate nanostructures one at a time with a micromechanical STM or AFM is simply not enough.

Future Challenges(Cont.)

IIIV. Responsive virtual environment for realistic, stimulated nanomanipulation.

We need to be able to simulate nanometer-scale experiment in real time on a digital computer, then use that computer simulation to generate a virtual environment.

The quantum simulations required for this type of simulated virtual environments are well beyond our current quantum simulation technology. We need to work and address this problem.

Future Challenges(Cont.)

IX. The Interconnect Problem Even all the other challenges to fabricate nanometer-

scale electronic devices are overcome. We still need to find a way to get information in and out of a dense computational structure with trillions of electrical elements. Nanocomputers will store a tremendous amount of information in a very tiny and limited space, and the computer will generate information extremely fast. We will need to control and coordinate the elements of the computer.

Conclusion

It is evident that the conventional semiconductor technology and photographic etching techniques will reach its theoretical limits.

It is necessary to come up with new approaches to build the computers of next generation.

Whether or not nanocomputers can be built will depend upon several factors; including device speed, power dissipation, reliability, and methods of fabrication.

Applying the methods of quantum dots, single electron transistor, and resonant tunneling devices, and the method of fabrication techniques, we should be able to achieve the high expectation for the next generation nanocomputers.

Reference M. Belohradsky, C. P. Collier, J. R. Heath,

P. J. Kuekes, F. M. Raymo, J. F. Stoddart, R. S. Williams, E. W. Wong, “Electronically Configurable Molecular-Based Logic Gates”, Science magazine, Vol. 285, July 1999.

James C. Ellenbogen, J. Christopher Love, David Goldhaber-Gordon, Michael S. Montemerlo, and Gregory J. Opiteck, “Technologies and Designs for Electronic Nanocomputers”, MITRE Technical Report No. 96w0000044, The MITRE Corporation, McLean, VA, July 1996.

Reference (cont.) James C. Ellenbogen, J. Christopher Love,

David Goldhaber-Gordon, Michael S. Montemerlo, and Gregory J. Opiteck, “Overview of Nanoelectronic Devices”, MITRE Technical Report No. 96w0000136, The MITRE Corporation, McLean, VA, April 1997.

James c. Ellenbogen, J. Christopher Love, “Architectures for Molecular Electronic Computers: 1. Logic Structures and an Adder Built from Molecular Electronic Diodes”, MITRE Technical Report No. 98W0000183, The MITRE Corporation, McLean, VA, July 1999.

Reference (cont.)

Mark N. Horenstein, Microelectronic Circuits and Devices, Prentice Hall, Inc., New Jersey, 1996.

M. Morris Mano, Digital Design, Prentice Hall, Inc., New Jersey, 1991.

Adel Sedra and Smith, Microelectronic Circuits. Oxford Press. New York, 1998.