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UNIT IV LECTURE 3 1 LECTURE 3 Sol – Gel method, Chemical Vapour Deposition, Physical Vapour Deposition

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Page 1: UNIT IV LECTURE 31 LECTURE 3 Sol – Gel method, Chemical Vapour Deposition, Physical Vapour Deposition

UNIT IV LECTURE 3 1

LECTURE 3 Sol – Gel method, Chemical Vapour Deposition, Physical Vapour Deposition

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Material Processing by Sol-Gel MethodIntroduction

The sol-gel process is very long known since the late 1800s. The versatility of the technique has been rediscovered in the early 1970s when glasses where produced without high temperature melting processes. This made possible the organic modification of silicon compounds (ORMOSIL), which cannot withstand high temperatures. Sol-gel is a chemical solution process used to make ceramic and glass materials in the form of thin films, fibers , or powders . A sol is a colloidal (the dispersed phase is so small that gravitational forces do not exist; only Van der Waals forces and surface charges are present) or molecular suspension of solid particles of ions in a solvent. A gel is a semi-rigid mass that forms when the solvent from the sol begins to evaporate and the particles or ions left behind begin to join together in a continuous network

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Typical precursors are metal alkoxides and metal chlorides, which undergo hydrolysis (Hydrolysis is a chemical reaction or process in which a chemical compound is broken down by reaction with water) and polycondensation reactions.(A chemical reaction in which two or more molecules combine upon the separation of water or some other simple substance) to form a colloid, a system composed of solid particles (size ranging from 1 nm to 1 μm) dispersed in a solvent. The sol evolves then towards the formation of an inorganic network containing a liquid phase (gel). Formation of a metal oxide involves connecting the metal centers with oxo (M-O-M) or hydroxo (M-OH-M) bridges, therefore generating metal-oxo or metal-hydroxo polymers in solution. The drying process serves to remove the liquid phase from the gel thus forming a porous material, then a thermal treatment (firing) may be performed in order to favor further polycondensation and enhance mechanical properties.

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The precursor sol can be either deposited on a substrate to form a film (e.g. by dip-coating or spin-coating), cast into a suitable container with the desired shape (e.g. to obtain a monolithic ceramics, glasses, fibers, membranes, aerogels), or used to synthesize powders (e.g. microspheres, nanospheres).

In essence, the sol-gel process usually consists of 4 steps:(1) The desired colloidal particles once dispersed in a liquid to form a sol.(2) The deposition of sol solution produces the coatings on the substrates by spraying, dipping or spinning.(3) The particles in sol are polymerized through the removal of the stabilizing components and produce a gel in a state of a continuous network.(4) The final heat treatments pyrolyze the remaining organic or inorganic components and form an amorphous or crystalline coating.

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The sol-gel approach is interesting in that it is a cheap and low-temperature technique that allows for the fine control on the product’s chemical composition, as even small quantities of dopants, such as organic dyes and rare earth metals, can be introduced in the sol and end up in the final product finely dispersed. An overview of the sol-gel process is presented in a simple graphic work below.

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Sol-Gel process overview

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Advantages of Sol-Gel Technique:Can produce thin bond-coating to provide excellent adhesion between the metallic substrate and the top coat.Can produce thick coating to provide corrosion protection performance.Can easily shape materials into complex geometries in a gel state.Can produce high purity products because the organo-metallic precursor of the desired ceramic oxides can be mixed, dissolved in a specified solvent and hydrolyzed into a sol, and subsequently a gel, the composition can be highly controllable.Can have low temperature sintering capability, usually 200-600°C.Can provide a simple, economic and effective method to produce high quality coatings.

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ApplicationsIt can be used in ceramics manufacturing processes, as an investment casting material, or as a means of producing very thin films of metal oxides for various purposes. Sol-gel derived materials have diverse applications in optics, electronics, energy, space, (bio)sensors, medicine (e.g. controlled drug release) and separation (e.g. chromatography) technology. One of the more important applications of sol-gel processing is to carry out zeolite synthesis. Other elements (metals, metal oxides) can be easily incorporated into the final product and the silicalite sol formed by this method is very stable.Other products fabricated with this process include various ceramic membranes for microfiltration, ultrafiltration, nanofiltration, pervaporation and reverse osmosis.

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Chemical Vapour DepositionIntroduction

Chemical vapour deposition or CVD is a generic name for a group of processes that involve depositing a solid material from a gaseous phase.Microfabrication processes widely use CVD to deposit materials in various forms, including: monocrystalline, polycrystalline, amorphous, and epitaxial. These materials include: silicon, carbon fiber, carbon nanofibers, filaments, carbon nanotubes, SiO2, silicon-germanium, tungsten, silicon carbide, silicon nitride, silicon oxynitride and titanium nitride. CVD process is also used to produce synthetic diamonds.

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Working Concept

• Chemical vapor deposition (CVD) results from the chemical reaction of gaseous precursor(s) at a heated substrate to yield a fully dense deposit.

• Thermodynamics and kinetics drive both precursor generation and decomposition.

• Control of thermodynamics and kinetics through temperature, pressure, and concentrations yields the desired deposit.

• A simplified concept diagram is shown as Fig • Metal deposition metal halide (g)  →  metal(s) + byproduct (g)• Ceramic deposition metal halide (g) + oxygen/carbon/nitrogen/boron source (g) →

ceramic(s) + byproduct (g) g- gas; s-solid

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CVD REACTION

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A basic CVD process consists of the following steps:  a predefined mix of reactant gases and diluent inert gases are introduced at a specified flow rate into the reaction chamber;    the gas species move to the substrate;  the reactants get adsorbed on the surface of the substrate; the reactants undergo chemical reactions with the substrate to form the film; andthe gaseous by-products of the reactions are desorbed and evacuated from the reaction chamber.  

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During the process of chemical vapor deposition, the reactant gases not only react with the substrate material at the wafer surface (or very close to it), but also in gas phase in the reactor's atmosphere.  Reactions that take place at the substrate surface are known as heterogeneous reactions, and are selectively occurring on the heated surface of the wafer where they create good-quality films.  Reactions that take place in the gas phase are known as homogeneous reactions.  Homogeneous reactions form gas phase aggregates of the depositing material, which adhere to the surface poorly and at the same time form low-density films with lots of defects.  In short, heterogeneous reactions are much more desirable than homogeneous reactions during chemical vapor deposition.

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A typical CVD system consists of the following parts: 

 sources of and feed lines for gases;

mass flow controllers for metering the gases into the system;

a reaction chamber or reactor;

a system for heating up the wafer on which the film is to be deposited; and

temperature sensors.

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Types of chemical vapor deposition

A number of forms of CVD are in wide use. These processes differ in the means by which chemical reactions are initiated (e.g., activation process) and process conditions. For instance, a reactor is said to be 'hot-wall' if it uses a heating system that heats up not only the wafer, but the walls of the reactor itself, an example of which is radiant heating from resistance-heated coils.  'Cold-wall' reactors use heating systems that minimize the heating up of the reactor walls while the wafer is being heated up, an example of which is heating via IR lamps inside the reactor.  In hot-wall reactors, films are deposited on the walls in much the same way as they are deposited on wafers.so this type of reactor requires frequent wall cleaning.

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Another way of classifying CVD reactors is by basing it on the range of their operating pressure.  Atmospheric pressure CVD (APCVD) reactors operate at atmospheric pressure, and are therefore the simplest in design.  Low-pressure CVD (LPCVD) reactors operate at medium vacuum (30-250 Pa) and higher temperature than APCVD reactors.  Plasma Enhanced CVD (PECVD) reactors also operate under low pressure, but do not depend completely on thermal energy to accelerate the reaction processes. They also transfer energy to the reactant gases by using an RF-induced glow discharge.

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The glow discharge used by a PECVD reactor is created by applying an RF field to a low-pressure gas, creating free electrons within the discharge region.  The electrons are sufficiently energized by the electric field that gas-phase dissociation and ionization of the reactant gases occur when the free electrons collide with them. Energetic species are then adsorbed on the film surface, where they are subjected to ion and electron bombardment, rearrangements, reactions with other species, new bond formation, and film formation and growth.  Table compares the characteristics of typical APCVD, LPCVD, and PECVD reactors.

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CVD Process Advantages Disadvantages Applications

APCVDSimple, Fast Deposition,Low Temperature

Poor Step Coverage,Contamination

Low-temperature Oxides

LPCVD

Excellent Purity,Excellent Uniformity,Good Step Coverage,Large Wafer Capacity

High Temperature,Slow Deposition

High-temperature Oxides, Silicon Nitride, Poly-Si, W, WSi2

PECVDLow Temperature,Good Step Coverage

Chemical and Particle Contamination

Low-temperature Insulators over Metals, Nitride Passivation

APCVD, LPCVD, and PECVD Comparison

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CVD Coatings containing

On to various substrates Properties

Chromium Solid solution alloys(i) with Iron, Nickel and Cobalt(ii) on Iron as carbides and nitrides

(i) Corrosion / oxidation resistance(ii) Wear / corrosion resistance

Aluminium As Aluminides with Iron, Cobalt and Nickel High temperature oxidation resistance

Boron As Borides with Iron, Cobalt and Nickel Wear / erosion resistance

Silicon As Silicides with Iron, Tungsten and Molybdenum High temperature oxidation resistance

Titanium As Carbides, nitrides and carbonitride on ferrous and non-ferrous alloys

Wear resistance

Manganese Solid solution alloys on carbon steels Wear resistance

The range of CVD coatings are diverse and consequently this generates a wide range of properties as indicated in the following table

CVD Coatings and their properties.

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Advantages of CVD• Can be used for a wide range of metals and ceramics• Can be used for coatings or freestanding structures• Fabricates net or near-net complex shapes• Is self-cleaning—extremely high purity deposits (>99.995% purity)• Conforms homogeneously to contours of substrate surface• Has near-theoretical as-deposited density• Has controllable thickness and morphology• Forms alloys• Infiltrates fiber preforms and foam structures• Coats internal passages with high length-to-diameter ratios• Can simultaneously coat multiple components• Coats powders

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Applications• CVD processes are used on a surprisingly wide range of industrial

components, from aircraft and land gas turbine blades, timing chain pins for the automotive industry, radiant grills for gas cookers and items of chemical plant, to resist various attacks by carbon, oxygen and sulphur.

• Some important applications are listed below.• Surface modification to prevent or promote adhesion • Photoresist adhesion for semiconductor wafers Silane/substrate

adhesion for microarrays (DNA, gene, protein, antibody, tissue) • MEMS coating to reduce stiction • BioMEMS and biosensor coating to reduce "drift" in device

performance• Promote biocompatibility between natural and synthetic materials

Copper capping Anti-corrosive coating

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Physical Vapour Deposition(PVD)Introduction1.Physical vapour deposition (PVD) is fundamentally a vaporisation coating technique, involving transfer of material on an atomic level. It is an alternative process to electroplating2.The process is similar to chemical vapour deposition (CVD) except that the raw materials/precursors, i.e. the material that is going to be deposited starts out in solid form, whereas in CVD, the precursors are introduced to the reaction chamber in the gaseous state.

Working ConceptPVD processes are carried out under vacuum conditions. The process involved four steps:1.Evaporation2.Transportation3.Reaction4.Deposition

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EvaporationDuring this stage, a target, consisting of the material to be deposited is bombarded by a high energy source such as a beam of electrons or ions. This dislodges atoms from the surface of the target, ‘vaporising’ them.

TransportThis process simply consists of the movement of ‘vaporised’ atoms from the target to the substrate to be coated and will generally be a straight line affair.

ReactionIn some cases coatings will consist of metal oxides, nitrides, carbides and other such materials. In these cases, the target will consist of the metal. The atoms of metal will then react with the appropriate gas during the transport stage. For the above examples, the reactive gases may be oxygen, nitrogen and methane.In instances where the coating consists of the target material alone, this step would not be part of the process.

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DepositionThis is the process of coating build up on the substrate surface.Depending on the actual process, some reactions between target materials and the reactive gases may also take place at the substrate surface simultaneously with the deposition process.Fig. shows a schematic diagram of the principles behind one common PVD method. The component that is to be coated is placed in a vacuum chamber. The coating material is evaporated by intense heat from, for example, a tungsten filament. An alternative method is to evaporate the coating material by a complex ion bombardment technique. The coating is then formed by atoms of the coating material being deposited onto the surface of the component being treated.

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The vacuum evaporation PVD process

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Variants of PVD include, in order of increasing novelty: Evaporative Deposition: In which the material to be deposited is heated to a high vapor pressure by electrically resistive heating in "high" vacuum.Electron Beam Physical Vapor Deposition: In which the material to be deposited is heated to a high vapor pressure by electron bombardment in "high" vacuum.Sputter Deposition: In which a glow plasma discharge (usually localized around the "target" by a magnet) bombards the material sputtering some away as a vapor.Cathodic Arc Deposition: In which a high power arc directed at the target material blasts away some into a vapor.Pulsed Laser Deposition: In which a high power laser ablates material from the target into a vapor.

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Method Merits Demerits

E-Beam Evaporation

1. high temp materials2. good for liftoff3. highest purity

1.  some CMOS processes sensitive to radiation

2. alloys difficult3. poor step coverage

Filament Evaporation

1. simple to implement2. good for liftoff

1. limited source material (no high temp)2. alloys difficult3. poor step coverage

Sputter Deposition

1. better step coverage2. alloys3. high temp materials4. less radiation damage

1. possible grainy films2. porous films3. plasma damage/contamination

Summary of Merits and Demerits of evaporation methods

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Importance of PVD Coatings• PVD coatings are deposited for numerous reasons. Some

of the main ones are:• Improved hardness and wear resistance• Reduced friction• Improved oxidation resistance• The use of such coatings is aimed at improving efficiency

through improved performance and longer component life. • They may also allow coated components to operate in

environments that the uncoated component would not otherwise have been able to perform.

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Advantages• Materials can be deposited with improved properties compared to the

substrate material• Almost any type of inorganic material can be used as well as some kinds of

organic materials• The process is more environmentally friendly than processes such as

electroplatingDisadvantages• It is a line of sight technique meaning that it is extremely difficult to coat

undercuts and similar surface features• High capital cost• Some processes operate at high vacuums and temperatures requiring

skilled operators• Processes requiring large amounts of heat require appropriate cooling

systems• The rate of coating deposition is usually quite slow

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Applications PVD coatings are generally used to improve hardness, wear resistance and oxidation resistance. Thus, such coatings use in a wide range of applications such as:AerospaceAutomotiveSurgical/MedicalDies and moulds for all manner of material processingCutting toolsFire arms 3030