CH5716

Processing of Materials

Ceramic Processing

Lecture MC3 – Thin Film Processing

Thin Films vs Thick Films

Defined not just by thickness but also processing method

Thin films often use vacuum based techniques

•Chemical Vapour Deposition (CVD)

•Physical Vapour Deposition (PVD)

Often form final ceramic in-situ with little or no further thermal processing required

Thick films generally based around wet processes

Slurry, paste suspension,

Further thermal processes, drying & sintering required to form final film

Thin films generally sub-micron

Thick films 5-100’s μm

Length Scales in Ceramic Film Processing

10’s nm 100’s nm micron 10’s micron 100’s micron mm

Bulk

processing

Tape Casting

Screen

Printing

Colloidal

Processing

PVD Techniques

Thick Film Processing

CVD Techniques

Human hair

50-75µm

Thin Film Processing

Thin Film Overview

•Thin Films generally ≤ 500nm

•Although not just defined by thickness also deposition method

•Generally produced from vapour phase

•Form final ceramic film in-situ with no further firing required •In contrast thick films based on slurry or solution deposition

•Which require heat treatment to form ceramic (firing or sintering)

•Vapour phase may be

•Reactive precursors (Chemical Vapour Deposition)

•Excitation of a solid source (target) (Physical Vapour Deposition)

•Thin films can be crystallographically orientated to the substrate •Epitaxial growth

•CVD generally used for thicker films

•PVD favoured for thinner films •Often linked to growth rates

•CVD – 1-2µm/min

•PVD – 0.01-0.03µm/min

•Although not always the case

Requirements for Thin Films Four main film variables to control

Crystal Structure •Crystalline, epitactic, polycrystalline, amorphous

•Exploitation or control of specific property (electronic, optical, magnetic)

•These are often anisotropic in nature

•Hence control of orientation or morphology of crystal structure important

Stoichiometry •Some properties may exist only over narrow, well defined compositional range

•May be looking to introduce specific defect or deficiency

•Eg YBa2Cu3O6+x superconductor when x~1Tc~90K when x=0.3 Tc~30K

Phase •Closely linked to stoichiometry

•Specific phases may be required for desired functionality

•Eg ferroelectric form of BaTiO3 is tetragonal

•Small changes in stoichiometry may lead to phase instability

Surface Morphology/Topography •Important when multilayer devices where further layers will be depositied

•Generally smooth films are desired

Which type of deposition depends on material and functionality needs

Some specialist requirements (eg epitaxy) may not be attainable by any

other means

CVD General principles •CVD involves one of 2 main processes

•Chemically reacting a volatile compound of a material to be deposited with another gas to form

a non volatile solid

•Pyrolysis (decomposition) of a compound at high temperature to produce a solid

•Either way solid must be deposited onto a substrate •Reaction must take place on or close to substrate surface

•Otherwise particles form resulting in low density

•Thermodynamics must be favourable for reaction •eg TiCl4(g) + CH4 (g) → TiC(s) + 4HCl(g)

•At 1200K ΔGr= -11kJ, whereas at 298K ΔGr= 109kJ

•Kinetics also important •If reaction proceeds too slowly it may not be commercially feasible

•Again strong temperature dependence

•Reaction temperatures must be sufficient to permit reaction •If epitaxial growth required temperature must also permit diffusion to allow for lattice alignment

•Reactant species must be in a readily volatile form

Important example is electronic grade silicon (EGS)

Formed from decomposition of siloxane (SiH4)

Decomposition takes place onto heated single crystal silicon rods

Deposit forms columner featherlike crystals

Basis for production of high grade silicon wafers for semiconductor industry

Some examples of Ceramic Films Produced by CVD

C.B. Carter. M.G. Norton, Ceramic Materials Science and Engineering Springer Science 2007

Typical Reactor Arrangements Various different reactor designs

Substrates generally of graphite slabs- susceptors for R-F induction heating

Reactant gases sometimes introduced with inert carriers eg argon

Pancake reactor

Barrel reactor

Horizontal reactor

Low pressure CVD reactor

C.B. Carter. M.G. Norton, Ceramic Materials Science and Engineering Springer Science 2007

Variations on A Theme Several different variations of CVD exist- Below are a brief selection

Atmospheric Pressure CVD (APCVD) Basically the general form of CVD

Partial pressures of reactant and carrier gases at atmospheric pressure

Reaction chamber still needs evacuation before process starts

Low Pressure CVD (LPCVD) Pressures in the range 0.5-1 Torr (7x10-4-10-3 bar)

Substrates generally vertically mounted

Plasma Enhanced CVD (PECVD) Plasma used create ions or free radicals that recombine to form the film

Allows lower substrate temperature due to the addition of energy from the plasma

Important application are SiN films for passivation/encapsulation of semiconductor devices

Often in these cases substrates may not exceed 300°C

Metal Organic CVD (MOCVD) Precursors based on metal organic compounds

Mainly a semiconductor fabrication technique

Common example is AlN from trimethyl aluminium ((CH3)3Al and ammonia

Laser Enhanced CVD (LECVD) Laser used to enhance reactions substrate surface

Very localised deposition can be used

Laser can “write” onto the surface of the substrate

CVD In Solid Oxide Fuel Cell Fabrication • Large scale tubular system

•Tubes 1-2m in length

•Developed by Siemens Westinghouse Corporation

• Pressurised systems up to 220kW demonstrated

• Tube durability demonstrated repeatedly over

10’sK hours

• Electrolyte layers deposed by electrochemical vapour

deposition • Forms a dense layer without the need for a sinter step • Two stage process

– Pore closure – Film growth

• Growth rates typically between 0.5-2micron.min-1 • Requires high temperatures 1200-1350°C and low

pressures 0.4-2 torr • HCl by product • Large capital and running costs • Unable to work these out of the system

Other Thin film Methods have also been applied to SOFC – both CVD and PVD gererally for

electrolyte or thin barrier layers yet to see a full cost effective route to manufacture proven

CVD in Optical Fibres •Successful application of CVD techniques

•Provides a unique solution not easily attained by any other method

•Can provide a higher refractive index core to the fibre

•Greatly improves performance of the fibre

•Process starts with pure SiO2 tube

•Rotated in special lathe

•High RI material applied as coating to the

inside of tube

•Reactants and O2 introduced inside tube

•Oxy-hydrogen flame on outside of tube

promotes reactant to oxidise

SiCl4 +O2 → SiO2 +2Cl2

GeCl4 +O2 → GeO2 +2Cl2

•Oxides coat inside of tube

•Tube is then collapsed and drawn to a fibre

•Several km fibre can be drawn from m’s of

original tube

CVD Safety & Environmental •A significant issue with CVD processes

•Many precursors, toxic, flammable, corrosive, pyrophoric or combinations

of multiples of these

•Many of the products share these issues and are difficult to handle and

dispose of

•This does add cost and complexity to the processing

Summary of Safety issues with some CVD precursors & products

C.B. Carter. M.G. Norton, Ceramic Materials Science and Engineering Springer Science 2007

PVD General Principles Many PVD techniques based around evaporation

Here a source is heated to provide a flux of atoms or molecules

Low pressures required for adequate vapour pressures

In case of many metals molten source also required

Metals are straight forward as they tend to evaporate in atoms or clusters

However many compounds will disassociate on evaporation eg ZrO2(s) →ZrO(g) and½O2(g)

Affects final film stoichiometry

Needs oxygen rich environment ( sometimes referred to as Reactive Evaporation RE)

Other examples are SiO2, GeO2, TiO2, SnO2

Some ceramics sublime maintaining stoichiometry between solid and vapour

phase eg AlN(s)→AlN(g)

Other examples are B2O3, GeO, SnO

Some ceramics have very high sublimation temperature and require special

heating

One method is via a focussed electron beam (known as e-beam evaporation)

Sputtering Here atoms are dislodged from a solid target by impact from energetic gaseous ions

Argon often used as high mass creates high momentum and so more particles ejected from target

Energy depends on magnitude of applied electrical field

Wide range of metals and ceramics can be sputtered Commonly used in SEM preparation

Does not involve melting so refractory materials can be deposited

Relatively slow process

Stoichiometric control difficult in multi target systems due to differential sputtering

rates

Occasionally growing film can be damaged by species in the plasma

Sputtering Variations

Reactive Sputtering

Use of metal target in reactive gas atmosphere

eg Al target with either N2 or NH3 to form AlN

RF Sputtering

Use of an alternating RF signal between

electrodes (5-30MHz typical)

Avoids charge build up during deposition

Sputtering Targets & Applications

C.B. Carter. M.G. Norton, Ceramic Materials Science and Engineering Springer Science 2007

Molecular Beam Epitaxy

Evaporation technique Mainly used for compound semiconductors

Has also been used for high temperature

superconductors

Requires ultra high vacuum Little interaction between atoms until substrate

surface

Very high purity films can be attained

Difficult to control oxygen stoichiometry in

complex oxides

Good for layered structures Allows deposition of monolayers in a precise order

Single crystal Gallium Arsenide is a good example

Very expensive capital costs (>1M$) Requires high added value

Very low deposition rates ≤ 1µm per hour

typical

Typical applications are semiconducting

lasers, LEDs

Pulsed Laser Deposition

Laser used to ablate material from target Produces plasma with high kinetic energy

This allow epitactic growth at low substrate temperatures

Complex film stoichiometries can be obtained

Relatively high deposition rates >0.5µm/min

Relatively inexpensive

Large area deposition problematic – lack of

uniformity Line of sight technique

Difficult to coat many substrates at once

Larger particles can be ejected – leading to flaws

in final film

Substrates These form an important part of the system and need to be considered along side

the film in several aspects

Chemical compatibility No deleterious reactions with film materials

At higher deposition temps ( around1000°C in some cases) this can be a challenge

Coefficient of Thermal Expansion (CTE) Close matching of CTE advantageous

Large mismatch can cause delamination or cracking especially in brittle ceramics

CTE film < CTE substrate preferred as this puts film in compression

Better stress/strain mode for ceramics

Thermal Stability Avoid phase changes on heating and cooling

Surface Quality Any surface defects (scratched etc) can lead to nucleation of grain growth

Important when crystal morphology important

Even emerging defects in single crystals can nucleate crystal orientation

Known as geoepitaxy

Cleanliness Not just dust but also surface reaction species

eg Mg(OH)2 on surface of MgO

Many labs have carefully controlled cleaning and preparation procedures

For epitaxial growth

Crystal Structure and Lattice Mismatch Ensure high coincidence of lattice sites

Minimal dislocations in deposited film

Summary Thin Film methods are important scientific and commercial process techniques

Often requires complex high cost equipment

Semiconductor industry is build on fabrication of multiple, layered structures

Miniaturisation has driven these finer & finer

Often can only be achieved via thin film techniques

Specific technical need and high added value

Results in commercial viability

Often two routes to commercial viability

high volumes to compensate for initial investments

Low volume, niche application at high cost

Careful selection of process route required to ensure commercial viability