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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