Thin Film Deposition
Thin Film Deposition
Thin Film Deposition
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Four growth modes
• Layer by layer growth (Frank ‐ van der Merwe): film atoms more strongly
bound to substrate than to each other and/or fast diffusion
• Island growth (Volmer ‐ Weber): film atoms more strongly bound to each
other than to substrate and/or slow diffusion.
• Mixed growth (Stranski ‐ Krastanov): initially layer by layer then forms three
dimensional islands.
(wetting properties)
• Noble metals don’t bond (“wet”) to Si/SiO2 substrate, so tend to have island growth.
• Ag always form island (not continuous film); Au is better than Ag.
• Adhesion layer Ti or Cr can reduce island formation, but for Ag, surface is still very rough.
• Here, higher adhesion is because Ti or Cr bond chemically to O in SiO2. 9
Effect of substrate temperature on the lateral grain size
100 Å thick Au films deposited at 100,
100oC
200, and 300℃ by vacuum evaporation
200oC
CVD steps:
• Introduce reactive gases to the chamber.
• Activate gases (decomposition) by heat or plasma.
• Gas absorption by substrate surface .
• Reaction take place on substrate surface, film firmed.
• Transport of volatile byproducts away form substrate.
• Exhaust waste.
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Chemical vapor deposition (CVD) systems
Figure 9-4
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CVD advantages and disadvantages
(as compared to physical vapor deposition)
Advantages:
• High growth rates possible, good reproducibility.
• Can deposit materials which are hard to evaporate.
• Can grow epitaxial films. In this case also termed as “vapor phase epitaxy (VPE)”. For
instance, MOCVD (metal-organic CVD) is also called OMVPE (organo-metallic VPE).
• Generally better film quality, more conformal step coverage (see image below).
Disadvantages:
• High process temperatures.
• Complex processes, toxic and corrosive gasses.
• Film may not be pure (hydrogen incorporation…).
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Types of CVD reactions
• Thermal decomposition
AB(g) ---> A(s) + B(g)
Si deposition from Silane at 650oC: SiH4(g) → Si(s) + 2H2(g)
Ni(CO)4(g) → Ni(s) + 4CO(g) (180oC)
• Reduction (using H2)
AX(g) + H2(g) → A(s) + HX(g)
W deposition at 300oC: WF6(g) + 3H2(g) → W(s) + 6HF(g)
SiCl4(g) + 2H2(g) → Si(s) + 4HCl (1200oC)
• Oxidation (using O2)
AX(g) + O2(g) → AO(s) + [O]X(g)
SiO2 deposition from silane and oxygen at 450oC (lower temp than thermal
oxidation): SiH4(g) + O2(g) ---> SiO2(s) + 2H2(g)
2AlCl3(g) + 3H2(g) + 3CO2(g) → Al2O3 + 3CO + 6HCl (1000oC)
(O is more electronegative than Cl)
• Compound formation (using NH3 or H2O)
AX(g) +NH3(g) → AN(s) + HX(g) or AX(g) + H2O(g ) → AO(s) + HX(g)
Deposit wear resistant film (BN) at 1100oC: BF3(g) + NH3(g) → BN(s) + 3HF(g)
(CH3)3Ga(g) + AsH3(g) → GaAs(s) + 3CH4 (650 – 750oC)
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Chemical reactions for silicon epitaxial growth
SiH 4 Si + 2 H 2
SiCl4 + H 2 SiHCl3 + HCl
SiHCl3 + H 2 SiH 2Cl2 + HCl
T(K)
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Thermal (not plasma-enhanced) CVD films
(Al2O3)
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CVD sources and substrates
• Types of sources
o Gasses (easiest)
o Volatile liquids
o Sublimable solids
o Combination
• Source materials should be
o Stable at room temperature
o Sufficiently volatile
o High enough partial pressure to get good growth rates
o Reaction temperature < melting point of substrate
o Produce desired element on substrate with easily removable
by-products
o Low toxicity
• Substrates
o Need to consider adsorption and surface reactions
o For example, WF6 deposits on Si but not on SiO2
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Types of CVD
APCVD (Atmospheric Pressure CVD), mass transport limited growth rate, leading to non-
uniform film thickness.
LPCVD (Low Pressure CVD)
• Low deposition rate limited by surface reaction, so uniform film thickness (many
wafers stacked vertically facing each other; in APCVD, wafers have to be laid
horizontally side by side.
• Gas pressures around 1-1000mTorr (lower P => higher diffusivity of gas to substrate)
• Better film uniformity & step coverage and fewer defects
• Process temperature 500°C
PECVD (Plasma Enhanced CVD)
• Plasma helps to break up gas molecules: high reactivity, able to process at lower
temperature and lower pressure (good for electronics on plastics).
• Pressure higher than in sputter deposition: more collision in gas phase, less ion
bombardment on substrate
• Can run in RF plasma mode: avoid charge buildup for insulators
• Film quality is poorer than LPCVD.
• Process temperature around 100 - 400°C.
MOCVD (Metal-organic CVD, also called OMVPE - organo metallic VPE), epitaxial growth
for many optoelectronic devices with III-V compounds for solar cells, lasers, LEDs,
photo-cathodes and quantum wells. 20
Types of CVD
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Chapter 9 Thin film deposition
Gas stream
1 7
2 6
Reaction rate may be limited by:
3 4 5 • Gas transport to/from surface.
Figure 9-5 Wafer
• Surface chemical reaction rate that
Susceptor
depends strongly on temperature.
F2 = flux
of reactant consumed by the surface reaction (steps 3-
5) = surface reaction flux,
Figure 9-6 F2 F2 = k S C S (2)
• ks increases with temperature.
(Arrhenius with EA depending on the particular
reaction, e.g. 1.6 eV for single crystal silicon
deposition).
• hG ≈ constant
(diffusion through boundary layer is insensitive
to temperature)
Higher T. Lower T. 25
CVD film growth rate
Actually hG is not constant (depends on T)
Figure 9-8
Deposition rate vs. gas glow rate
Figure 9-8 Growth or deposition rate for
silicon by APCVD. The partial pressure of the
reactant gas is 0.8Torr (1atm=760Torr!!).
H2 is used as the carrier or diluent gas for
the solid curves.
For SiH4, using N2 carrier gas increases the
growth rate, because the carrier gas H2 is a
reaction product of SiH4 decomposition,
thus slowing down the reaction. 26
Chemical Vapor Deposition (CVD) growth rate
E
• kS limited deposition is VERY temperature sensitive. k s = k 0 exp − a
kT
• hG limited deposition is VERY geometry (boundary layer) sensitive.
• Si epitaxial deposition is often done at high T to get high quality single crystal
growth. It is then hG controlled, and horizontal reactor configuration is needed for
uniform film thickness across the wafer.
• When a high film quality is less critical (e.g. SiO2 for inter-connect dielectric),
deposition is done in reaction rate controlled regime (lower temperature). Then
one can greatly increase the throughput by stacking wafers vertically (for
research, usually 25 wafers per run; 100-200 for industry).
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Other factors affecting growth rate: thickness
of boundary layer and source gas depletion
• Doping is usually done for epitaxial (thus single crystal) film during film growth.
• Dopant will be grown directly onto crystalline site (no need of dopant activation).
• Doping is realized by adding gas containing the dopant. Such as PH3, B2H6, AsH3 (all
gas phase at room temperature); or PCl3, BCl3, AsCl3 (all liquid at RT).
• They will go through: dissociation, lattice site incorporation, and burying of dopants
by other atoms in the film.
• The dopant concentration C: (P is partial pressure of he dopant species, and v growth
rate)
C Pi for low growth rates
P
C i for high growth rates
v
• However, there is also unintentional doping process:
o Out-diffusion of dopant from heavily doped substrate into the epi-layer.
o Auto-doping – dopant from substrate diffuses into gas stream first, then back
into epi-layer.
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Auto-doping and out-diffusion in CVD film growth
Figure 9-11 Auto-doping processes in an epitaxial reactor. Illustrated are evaporation from 1)
the wafer front side; 2) the wafer backside or edges; 3) other wafers; and 4) the susceptor.
Out-diffusion:
Auto-doping:
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Chapter 9 Thin film deposition
Hot-wall
Possible disadvantages:
• For too low temperature, deposition rates may be too low, film quality decreases.
• Shadowing (less gas-phase collisions) due to directional diffusion to the surface, so
deterioration of the step coverage and filling.
• Use RF-induced plasma to transfer energy into the reactant gases, forming radicals that is
very reactive. (RF: radio-frequency, typically 13.56MHz for PECVD)
• Low temperature process (<300oC), as thermal energy is less critical when RF energy exists.
• Used for depositing film on metals (Al…) and other materials that cannot sustain high
temperatures. (APCVD/LPCVD at such low temperatures gives increased porosity and poor
step coverage)
• Surface reaction limited deposition, thus substrate temperature control is important to
ensure uniformity.
• At low T, surface diffusion is slow, so one must supply kinetic energy for surface diffusion –
plasma (ion bombardment) provides that energy and enhances step coverage.
• Disadvantages: plasma damage, not pure film (often lots of H incorporated into film). 37
PECVD process parameter
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High Density Plasma (HDP) CVD
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Miscellaneous: selective deposition and laser CVD
Selective deposition:
• Especially important in microelectronics, surface
Laser CVD
patterning and 3D-growth. (energy provided by laser)
• Reaction rate of precursor is limited on a non-growth
surface. E.g. deposition of Cu from (hfac)Cu(PMe3)
occur on Cu, Pt… but not on SiO2.
• Growth surface acts as co-reactant, and is selectively
consumed. E.g. Si reacts with WF6 or MoF6, while
reaction at SiO2 or Si3N4 is slower.
• A chemical reaction of a gaseous co-reactant occur on
the growth surface. E.g. H2 dissociation on a metal
surface, but not on SiO2 or metal oxide surfaces.
Tungsten spring
grown by laser CVD.
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CVD reactor types: quick summary
According to the LPCVD slides, APCVD growth rate should be lower, which is not true. Because:
(?? I think)
• In APCVD reactive gas partial pressure could be set much higher than that in LPCVD.
• Its pressure could be much lower (by 10) than 1atm and is still called APCVD.
• Gas transport actually increases with T as T3/2 (APCVD is usually done at higher T than LPCVD).
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• When putting wafer side-by-side facing the gas, more exposed to gas, thus faster transport.