Nanofabrication: Self Assembly

ME584 Introduction to NanotechnologyNick Bublavi, Kevin Voss, Greg Behrmann

April 29, 2005

“The spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates”

Outline

• Introduction

•Important Properties at Small Scales

• Some Self Assembly Mechanisms

•Macro Scale Self Assembly Demonstrations

•Technical Challenges and Future Work

•Some Companies Involved in Self Assembly Based Nanotechnology

Introduction

Self Assembly is analogous to the assembly of a puzzle where the characteristics of each piece and the surrounding environment allow each piece to only fit in its proper location.

see Soft Machines: Nanotechnology and Life, Richard A. L. Jones, Oxford Press, 2004

Introduction

Why is Self Assembly an important fabrication method for nanotechnology?

Semiconductor Fabrication Methods are “Top-Down” and require a largenumber of steps, an expensive infrastructure, and feature sizes are limitedby diffraction limits of exposure wavelength.

see www.mcgill.ca / chemistry / undergrad / courses / 534.html

Introduction

Why is Self Assembly an important fabrication method for nanotechnology?

While offering high resolution and nanometer scale feature sizes, Direct Write Methods such as e-beam lithography, focused ion beam, and atom force manipulation operate over small areas in a serial manner making them slow and economically undesirable for production in large volumes.

E-beam Write Areas are~0.5 µm to 2 mm squarewith stitching errors betweenwrite fields ~20-40 nm

see www.raith.com Nanolithography products

Introduction

Why is Self Assembly an important fabrication method for nanotechnology?

As opposed to the “Top Down” methods of the semiconductor industry, Self assembly exploits the naturally existing effects of Brownian Motion, Intermolecular Forces, and the 2nd Law of Thermodynamics to producestructures in a “Bottom Up” fashion.

Puzzle Analogy One Pot Cooking

Properties at Small Scales

The random movement of microscopic particles by thermal energy and collisions with molecules of the surrounding medium.

Brownian Motion

See http://www.phy.ntnu.edu.tw/java/gas2D/gas2D.html

Gas Molecules in a Container

Properties at Small ScalesBrownian Motion

• Observed in 1827 by Robert Brown in observation of pollen molecules.

• Mathematically described by Albert Einstein in 1905.

•A single water molecule at room temperature has an average velocity of 600 miles per hour.

•A 100 nm size particle such as a bacterium has an average velocity of 120 miles per hour.

•A 1 micron size particle has an average velocity of 4 miles per hour.

•Random Walk/Diffusion- implies that the motion is undirected and does not cover large distances. An oxygen molecule in water will move 10 nm in a few milliseconds, 1 minute to move a micron, and 100 years to move a centimeter.

•Brownian Motion is also used in financial modeling.

see Soft Machines: Nanotechnology and Life, Richard A. L. Jones, Oxford Press, 2004

Properties at Small ScalesIntermolecular Forces

See http://itl.chem.ufl.edu/2045/lectures/lec_g.html

All matter is held together by forces. The force between atoms within a molecule is a chemical or intramolecular force. The force between molecules is a physical or intermolecular force.

Properties at Small ScalesIntermolecular Forces

Intermolecular forces can be considered electromagnetic in nature and weaker than the forces within molecules. The physical properties of melting point, boiling point, viscosity, surface tension, and solubility are related to the strength of attractive forces between molecules. At the nanoscale, intermolecular forces cause particles to want to “stick” to each other.

See http://www.science.uwaterloo.ca/~cchieh/cact/c123/intermol.html

Essence of Self Assembly• Successful self assembly engineering requires the balance of Brownian Motion and Intermolecular Forces.

• Brownian Motion contributes by moving components into correct location and orientation. Intermolecular Forces are utilized to hold components in position.

•Process is massively parallel and “self-correcting”. If a component is assembled incorrectly, it is likely that Brownian Motion will shake it loose until it assumes the proper configuration.

A Reversibly Switching Surface

Self Assembly MethodsHydrophilic/Hydrophobic Molecules

Molecules that are uncharged and contain no polar groups (non-polar) and are therefore unable to form hydrogen bonds to water are hydrophobic. e.g. engine oil, olive oil.

Molecules containing polar charges are able to form hydrogen bonds to water. These molecules are hydrophilic. e.g. sugar, acetic acid, aspirin

Lipids are molecules that possess a hydrocarbon tail that is hydrophobic, or insoluble in water and a polar head group that is hydrophilic, or water soluble. The total length of the lipid molecules is 2-4 nm. As building blocks, supramolecular structures can be built that are hundreds of nanometres to hundreds of microns in size.These structures are known as amphiphiles.

See http://distance.stcc.edu/AandP/AP/AP1pages/Units1to4/epitissmol/lipids.htmAnd http://www.chems.msu.edu/classes/s03/891/003/

Self Assembly MethodsHydrophilic/Hydrophobic Molecules

At MIT, researchers have developed self assembly techniques that producenanotubes and spheres from peptides by exploiting hydrophobic/hydrophilic properties with ionic bonding.

See Zhang, S., Materials Today, May 2003.

Self Assembly MethodsSAMS: Self Assembled Monolayers

Self assembly techniques have been developed that use Soft Lithography as mechanism for depositing self-assembling layers on the nanoscale.

Kumar, et al “Patterning Self Assembled Monolayers: Applications in Material Science,” Langmuir, Vol. 10, No. 5, 1994

Self Assembly MethodsSelf Assembly of Magnetic Particles

Techniques have been developed that use iron nano particles that self assemble into 3D arrays .

See http://www.nanonet.go.jp/english/mailmag/2004/030b.html

Self Assembly MethodsSelf Assembly by Centrifugation and Spin-annealing

Spin coating has been used to produce three dimensional close packed lattices used as photonic crystals.

See http://www.ece.udel.edu/~dprather/research/pbg/fabrication/3dpbgfab_selfassemble.pdf

Macro Scale Self Assembly DemosDISCLAIMER: The following demonstrations are Macro-Scale and their purposeis only to illustrate the how self assembly mechanisms work. At the nanoscale,due to size, the physics can be quite different.

Polarity causes magnets toassemble in alternating colors

Two atomic crystal configurations gives NiTi shape memory

Hydrophobic grape skin floats.Hydrophilic peeled grape sinks in soda .

Future Work

• Control of in-plane dimensions for single layers less than 20 nm.

• Development of the theory of designable self assembling machine structures.

• Develop computational modeling of the self assembly process.

• Develop and incorporate self assembly capability at the interface of different nanotechnology building blocks.

• Refine self assembly processes into “Directed Techniques” that can provide nanostructures with short to long range order.

The Self Assembly Business

• IBM researchers have coaxed two different polymers – stringy moleculesto assemble themselves into a honeycombed template of 20-nanometer holes. The honeycomb pattern, known as a diblock copolymer, was applied as a layer in the conventional process used to make transistors in a flash memory device, the thin cards used in digital cameras and handheld computers.

• NanoSonic, Inc., is refining a molecular self-assembly process, called electrostatic self-assembly (ESA), that produces thin films with material properties that can be precisely controlled.

Nestle Foods and Proctor and Gamble is studying the application of self-assemblyprinciples in food.

NanoOpto Corporation is making optical components radically smaller, less expensive, denser, more integrated, and better performing by nano imprint pattern transfer.