Thin film deposition
For more details, refer to book on reserve: Fundamentals of Microfabrication, M. J.
Madou; CRC Press, New York: 2002.
Chemical vapor deposition (CVD): reactant gas + carrier gas react on a hot surface to
deposit a solid film
Materials deposited with CVD tech:
 Metals: Al, Cu, W
 Insulators (dielectrics): oxides and nitrides
 Semiconductors: a-Si. poly-Si, InP, GaAs
 Silicides: TiSi2, WSi2
homogeneous reaction
 reaction occurs in gas phase
 poor adhesion
 low density
 high-defect density
heterogeneous reaction
 occurs on or close to substrate surface
 preferred over homogeneous phase reactions
Mass transport limited: Reaction cannot proceed any faster than rate at which reactant
gases are supplied to substrate by mass transfer
Reaction rate limited: Reaction rate depends on temperature (thermally driven),
independent of mass flow rate
Growth rate dependence on temperature:
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Description of reaction limited regime: Arrhenius relationship:
r  Ao exp(  Ea / RT )
r: reaction rate
T: temperature
R: gas constant
Ao: constant
Ea: Activation energy
Energy is needed to drive a reaction. Sources of energy:
 thermal – most common
 photons
 electrons
Plasma source
 ions
Plasma basics
Plasma (with regard to fabrication techs): area of high energy electric and magnetic fields
that rapidly dissociate a feed gas to produce radicals, neutrals, electrons, ions, and
photons.
Generating a plasma (glow discharge: weakly ionized)
 Apply a high voltage potential between two electrodes (dc-discharge)
 Apply a radio-frequency (RF) voltage between two electrodes. Free electrons
collide with gas molecules – easier to sustain than dc-discharge
Plasma density: number of charged species/unit volume
RF plasma density: ~109 /cm3
Inductively coupled plasma (ICP) systems: uses ICP power source for creating highdensity (~1011/cm3), low-pressure and low-energy plasma
Plasma-enhanced chemical vapor deposition (PECVD) – high density plasma provides
additional source of energy allowing for deposition at relatively lower temp. (~300 °C)
 Provides good step coverage
 High deposition rates (~100 nm/min)
 Non-stoichiometric films can be made (SiNx)
VINSE PECVD
 Silicon Lab: SiO2, SiNx, SiOxNy, a-Si
 Carbon Lab: diamond, carbon nanotubes
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Process parameters and film properties

Reaction pressure
o ↓P, ↑ion bombardment: ↑film density, ↑film quality; ↓compressive stress,
↓ deposition rate
o ↑P: ↑gas phase reaction and defect density

RF frequency – use to control film stress and density; typically no control

RF power
o ↑Power: ↑deposition rate and ↑ion bombardment, thus film density

Temperature – influences deposition rate and structure
o Reaction rate limited regime: ↑T: ↑deposition rate
o ↑T: crystalline structure
o ↓T: amorphous structure

Flow rate
o increase flow to operate in reaction rate limited regime
o change film stoichiometry by changing flow ratio of reactants
Physical Vapor Deposition (PVD) – Deposition technique in which some form of energy
is used to transfer material from target to the substrate, where it condenses.
Types of PVD
 Evaporation
o Thermal evaporation
o Electron beam evaporation
 Sputtering
 Pulsed laser deposition
Evaporation
 Some source of energy is used to vaporize material to be deposited
 Deposition rate depends of vapor pressure of material
 Reasonable deposition rate obtained for temperature at which vapor pressure is at
least 10 mTorr
 Refractory metals (W, Ta, Mo, Ti) need to be heated in excess of 3000 °C
 Deposition is line-of-sight (good for use with masks)
Thermal evaporation
 Heated filament used to boil off material
 Depositing alloys is difficult
 Poor adhesion
 Poor step coverage
 Not possible for refractory metals (limited choice of materials)
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Electron beam evaporation
 High intensity electron beam focused on target material causes evaporation
 Deposition rates (10’s nm/min)
 Wider choice of materials
 Higher purity films
 Can cause x-ray and/or ion damage to substrate
VINSE E-beam evaporator: Al, Au, Ag, Cr, Co, Mo, and others….
Lower deposition rate increases uniformity, but increases risk of contamination
Pulsed laser deposition
 Like e-beam evaporation, but laser is used instead for removing target material
 Wide choice of target materials
 High purity
 Slow dep. rates
Sputter deposition
 Plasma creates ions that are accelerated toward target. Momentum transfer from
ions to target causes target material to be ejected toward surface (sputtering),
where it condenses
 High purity films over large area are possible
 Just about any material can be sputtered – including compounds, but used mainly
for metal deposition
 Changing target material is difficult
 Better step coverage than evaporated films, but not always as smooth
 Deposition rate: 10’s nm/min
VINSE sputter tool: Al, Mo, Hf, and V
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