NANOFABRICATION TECHNOLOGIES Nanotechnology Products Introduction to Nanoscience Nanofabrication Processes ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Nanotechnology Fabrication and application of entities whose feature sizes are in the range from about 1 nm to 100 nm 1 nm = 10-3 m = 10-6 mm = 10-9 m Entities include structures, films, coatings, dots, lines, tubes, and systems Nanoscience – the field of scientific study that is concerned with objects in the 1 to 100 nm range Nanoscale – refers to dimensions within this range and slightly below ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Some Examples of Nanoscale Entities Helium atom – diameter about 0.1 nm Uranium atom – diameter about 0.22 nm Molecules tend to be larger because they consist of multiple atoms Molecules made up of about 30 atoms are roughly 1 nm in size Depending on the atoms involved ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Nanotechnology Products Cosmetics and sun lotions Car polishes and waxes Coatings for eyeglass lenses Scratch-resistant paints Carbon nanostructures Buckyballs Carbon nanotubes ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Buckyballs (Fullerines) Carbon molecules containing exactly 60 atoms (C60) and shaped like a soccer ball Originally named buckministerfullerene, after R. Buckminister Fuller, designer of the geodesic dome (shortened to fullerene) Can be bonded together to form crystals whose lattice structure is face-centered cubic ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Buckyballs Structure of C60 molecule 12 pentagonal faces 20 hexagonal faces ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Properties and Applications of Fullerines Electrical properties of insulator, but can be doped (e.g., K3C60) to become conductor Exhibits properties of a superconductor and very low temperatures (around 18K) Possible medical uses because they possess many possible attachment points for drugs Other medical applications include antioxidants, burn creams, and diagnostic imaging ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Carbon Nanotubes Another nanostructure of interest, consisting of carbon atoms bonded together in the shape of a long tube Depending on structure and diameter, can have conducting or semiconducting properties Conductivity superior to copper due to fewer defects that increase electrical resistance Thus, high currents do not increase temperature as in metals Elastic modulus and tensile strength of carbon nanotubes much greater than steel ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Carbon Nanotubes Carbon nanotube structures: (a) armchair and (b) zigzag ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Introduction to Nanoscience Nanoscience and nanotechnology are interdisciplinary fields Chemistry, physics, various engineering disciplines, computer science, biology, and medical science Biology operates in the nanoscale range Proteins – large molecules ranging in size between about 4 nm and 50 nm Chlorophyll in plants – about 1 nm Hemoglobin in blood – about 7 nm ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Size Matters As the size of an object becomes smaller and smaller, approaching nanoscale, the surface molecules become increasingly important relative to internal molecules Because of the increasing proportion of surface molecules relative to internal molecules Thus, the surface properties of materials of nanoscale objects become more influential in determining the behavior of the objects And the influence of bulk properties is reduced ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Some Size Effects: Atomic Bonding Two types of atomic bonds: Primary bonds –combining atoms into molecules Secondary bonds – attraction between molecules to form bulk materials Secondary bonds become more important for nanoscale objects because their shapes and properties depend on these secondary bonding forces Thus, material properties and behavior of nanoscale objects are different from those of much larger objects ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Size Effects: Quantum Mechanics Branch of physics concerned with the notion that all forms of energy occur in discrete units when observed on a small enough scale Example: electricity is conducted in units of electrons Quantum mechanics are significant for nanoscale entities One implication: As microelectronic devices reach nanoscale, we approach the limits of technological feasibility of current fabrication processes for integrated circuits ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Scanning Probe Microscopes Conventional optical microscopes Magnification ~ 1000 times Resolutions ~ 200 nm Electron microscopes Magnification ~ 1,000,000 times Resolutions ~ 1 nm Scanning probe microscopes Magnifications ~ 10 times greater than electron microscopes ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Scanning Probe Microscopes Utilize a very sharp needle probe to scan a surface from a distance about 1 nm above it Types of scanning probe microscopes: Scanning tunneling microscope (STM) Atomic force microscope (AFM) ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Scanning Tunneling Microscope A voltage is applied to the probe, causing electrons on the surface to tunnel up to the probe tip Tunneling electrons can be measured as current As probe is moved across surface, current is higher at locations immediately above surface atoms and lower in between surface atoms These variations in current can be used to create topographical maps of surface on an atomic scale ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Atomic Force Microscope Limitation of scanning tunneling microscope - it can only be used on surfaces of conducting materials Atomic force microscope can be used on any material In the AFM, the probe is attached to a delicate cantilever that deflects due to forces exerted by surface atoms as the probe traverses the specimen surface Forces include van der Waals, capillary, magnetic Vertical deflection can be measured to construct topographical image of surface ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Atomic Force Microscope Image AFM image of silicon dioxide letters on a silicon substrate; the oxide letters are about 20 nm wide (photo courtesy of IBM Corp.) ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Nanofabrication Processes Top-down approaches Adaptation of microfabrication techniques to make nanoscale objects Bottom-up approaches Atoms and molecules are manipulated and combined to form larger nanoscale structures ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Top-Down Processing Approaches Mostly based on lithographic techniques Extreme UV – UV wavelengths down to 13 nm Electron-beam lithography – resolutions ~ 10 nm X-ray lithography - resolutions ~ 20 nm Micro-imprint lithography – uses flat mold with desired pattern that physically deforms resist surface to create regions that will be etched Nano-imprint lithography – same as micro-imprint but adapted to nanoscale ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Micro-Imprint Lithography (1) Flat mold positioned above resist, (2) mold is pressed into resist surface, (3) mold is lifted, (4) remaining resist removed by etching to expose substrate surface ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Bottom-Up Processing Approaches Production of carbon nanotubes Nanofabrication by scanning probe techniques Self-assembly ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Production of Carbon Nanotubes Laser evaporation method Carbon arc techniques Chemical vapor deposition ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Laser Evaporation Method Starting material is graphite with traces of Co and Ni that act as nucleation sites in formation of nanotubes Graphite workpiece is placed in quartz tube filled with argon and heated to 1200°C A pulsed laser beam is focused on surface, causing carbon atoms to evaporate from the bulk graphite Argon moves carbon atoms to cool copper surface, where they condense, forming nanotubes with diameters 10 to 20 nm and lengths ~ 100 m ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Carbon Arc Technique If no catalyst – multi-walled nanotubes form Uses two carbon electrodes that are separated by 1 mm and located in a partial vacuum 25 V is applied across the electrodes, causing carbon atoms to be ejected from positive electrode and carried to negative electrode where they form nanotubes If no catalyst – multi-walled nanotubes form If cobalt used as catalyst, single-walled nanotubes with diameters 1 to 5 nm and lengths ~ 1 m ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Chemical Vapor Deposition Starting material is hydrocarbon gas such as methane (CH4) Gas is heated to 1100°C, causing it to decompose and release carbon atoms Atoms condense on cool substrate to form nanotubes Substrate surface may contain metallic traces that act as nucleation sites for nanotubes CVD process can be operated continuously, making it attractive for mass production ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Scanning Probe Techniques The scanning tunneling microscope (STM) can be used to manipulate individual atoms or molecules that adhere to a substrate surface by forces of adsorption (weak chemical bonds) If the probe tip is moved close enough to the adsorbed atom so that its force of attraction is greater than the adsorption force, the atom will be dragged along the surface In this way, individual atoms or molecules can be manipulated to create nanoscale structures ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Manipulation of Individual Atoms Using STM Techniques (a) Probe tip is maintained a distance from the surface that is sufficient to avoid disturbing the adsorbed atom (b) Probe tip is moved closer to the surface so that the adsorbed atom is attracted to the tip ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Manipulation of Individual Atoms Can be classified as: Lateral manipulation – atoms or molecules are transferred along the substrate surface by the attractive (or repulsive) forces of the probe tip Vertical manipulation – atoms or molecules are lifted from the substrate surface and deposited at a different location to form a structure ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Limitations of STM Manipulation Must be carried out in a very high vacuum to prevent stray atoms or molecules from interfering with the process Surface of substrate must be cooled to temperatures near absolute zero in order to reduce thermal diffusion that would gradually distort the atomic structure being formed These limitations make STM manipulation of individual atoms and molecules very slow and expensive ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Dip-Pen Lithography (DPN) An alternative scanning probe technique that does show promise for practical applications In DPN, the tip of an atomic force microscope is used to transfer molecules to a substrate surface by means of a solvent meniscus Process is analogous to using an old-fashioned quill pen to transfer ink to a paper surface via capillary forces ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Dip-Pen Lithography Tip of an atomic force microscope is used to deposit molecules through the water meniscus that forms naturally between the tip and the substrate ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Self-Assembly A fundamental process in nature Natural formation of a crystalline structure during slow cooling of molten minerals is an example of nonliving self-assembly Growth of living organisms is an example of biological self-assembly In both instances, entities at the atomic and molecular level combine on their own into larger entities, proceeding in a constructive manner toward the creation of some deliberate thing ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Self-Assembly in Nanotechnology Attempts to emulate nature’s self-assembly process to produce materials and systems that have nanoscale features or building blocks But the final product may be larger than nanoscale May be micro- or macro-scale, at least in some of its dimensions ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Self-Assembly in Nanotechnology Desirable features of atomic or molecular self-assembly processes in nanotechnology: They can be carried out rapidly They occur automatically and do not require central control They exhibit massive replication They can be performed under mild environmental conditions (at or near atmospheric pressure and room temperature) ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Principle of Minimum Energy Physical entities such as atoms and molecules seek out a state such that the total energy of the system of which they are components is minimized ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Implications for Self-Assembly There must a mechanism for the movement of the entities, thus causing them to come into close proximity Diffusion, convection, electrical fields There must be some form of molecular recognition among the entities Tendency for one atom or molecule to be attracted to and bind with another The molecular recognition among the entities causes them to join in such a way that the resulting physical arrangement achieves a state of minimum energy ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Current Processes That Use Self-Assembly Czochralski process - growing single crystal silicon boule from molten pool of silicon Lattice structure of the crystal is of nanometer size Polymerization - joining of individual monomers (e.g., ethylene, C2H4) to form very large molecules (macromolecules such as polyethylene) in the form of long chains, each with thousands of repeating units Repeating units are of nanometer size ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Self-Assembled Monolayers (SAMs) Two dimensional array (surface film) that is one molecule thick Molecules are organized in some orderly fashion Multi-layered structures also possible that are two or more molecules thick ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Self-Assembled Monolayers Substrate materials include gold, silver, copper, silicon, silicon dioxide Substrate material must not form an oxide surface film that would interfere with the layering process Layering materials include thiols, sulfides, and disulfides The layering material must be capable of being adsorbed (adhering in the form of a very thin film) onto the substrate surface ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Sequence in Formation of SAMs Layering molecules move freely above substrate surface and are adsorbed onto the surface Contact occurs between adsorbed molecules on surface, forming islands The islands grow and gradually join together through the addition of more molecules on the surface, until the substrate is completely covered In some cases, SAMs can be formed into desired patterns on the substrate, using techniques such as nano-contact printing and dip-pen lithography ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e

Sequence in Formation of SAMs Formation of thiol monolayer on gold substrate: (1) some molecules are attracted to surface, (2) they are adsorbed on surface, (3) they form islands, and (4) islands grow until surface is completely covered ©2010 John Wiley & Sons, Inc. M P Groover, Fundamentals of Modern Manufacturing 4/e