1. Chemical Vapor Deposition
Section II
Chemical Vapor Deposition
© 1997, Angus Rockett

2. Basic Ideas
Chemical vapor deposition processes form a solid phase from a vapor containing constituents of the solid. The reactants may be gases or high vapor pressure liquids or solids.
Temperature controls reaction thermodynamics and kinetics
Temperature Control
Reactor Wall
Reactant Source
Heat
Reactants and products are transported in the vapor phase
Product Exhaust
Gas Phase Diffusion
Adsorption
Reaction
of Reactant(s)
Volatile Product
A boundary layer separates these species from the reaction surface
Desorption
Surface
Diffusion
Growth
© 1997, Angus Rockett

3. Typical Apparatus Flow Controllers & Vents Computer Control
Source Gases
Buffer
Volume
Load
Lock
Valve
Reactor
Pumps
Scrubber
Safety Enclosure
Toxic Gas
Alarm System
© 1997, Angus Rockett

4. Typical Apparatus Important components of a CVD reactor include:
Mass flow controllers:
These control the flow of gas through the system.
Gas diversion valves:
Rather than shutting off gas that is not being sent to the reactor, better stability is achieved by simply diverting a fixed flow to the scrubber.
Vent
Vent
Vent
A mixing volume:
Better uniformity can result by mixing the reactants before they enter the reactor.
To the reactor
© 1997, Angus Rockett

5. Typical Apparatus Atmospheric pressure reactors Low pressure reactors
Advantages: Disadvantages:
Simple Poor step coverage
High deposition rate Reaction in the gas phase
Low temperature possible.
Low pressure reactors
Advantages: Disadvantages:
Excellent uniformity Lower deposition rate
High purity films More complex & expensive
Good step coverage Higher deposition temperature
© 1997, Angus Rockett

6. Reactor Configurations
Substrates
Reactant
feed
Pumps
Hot Wall Reactor
Uniform
and
temperature
scrubbers
Furnace for temperature control
Gas flow perpendicular to the wafer surface
Substrates
Cold Wall Reactor
Reactant
Pumps
Feed
rf Cavity
Susceptor
and
Scrubbers
Gas flow parallel to the wafer surface
© 1997, Angus Rockett

7. Gas composition approximately constant
Reactor Configurations
Reactors with closely spaced wafers require the following to achieve uniform deposition on the wafers:
Surface reaction rate limited growth
Low deposition rates
Low pressure
Hot wall
Heated Wafers
Gas composition approximately constant
Reaction limited
Substrates
Reactant
Thin boundary
layers
feed
Pumps
Uniform
and
temperature
scrubbers
Furnace for temperature control
© 1997, Angus Rockett

8. Reactor Configurations
Barrel-type Reactor
Vertical Reactor
Reactant supply
Reactant supply
rf source
rf source
or quartz
or quartz
lamps
lamps
Pumps &
scrubbers
Pumps &
scrubbers
Gas flow parallel to the wafer surface
Gas flow perpendicular to the wafer surface
© 1997, Angus Rockett

9. Sources of Reactants Solid Sources
Transport/Deposition driven
by a temperature gradient
Closed-space vapor transport is the simplest form of CVD in which a solid is evaporated from one region and precipitated in another region. Transport is driven by the gradient in the temperature which determines the reactant vapor pressure at source and substrate.
Without a carrier gas
CdTe
Diffusion
Sealed
Substrate
Source
of CdTe
Volume
1100 °C
950 °C
With a carrier gas
Diffusion
Si+2I SiI2
SiI Si+2I
of SiI2
Iodine Vapor
1100 °C
950 °C
© 1997, Angus Rockett

10. Sources of Reactants Gas Sources
Gases are the most flexible source materials. The reaction thermodynamics and kinetics can be engineered by design of the precursor. Partial pressures can be varied over wide ranges with accurate control.
Pressure regulator
Shut off valve
Double-walled tubing
Main valve
To mixing volume or reactor
Flow-restricting orifice
Gas storage tank
Toxic Gas
Safety cabinet
Alarm System
© 1997, Angus Rockett

11. Sources of Reactants
Some source gases require purification before being useful.
gas in
Heated Ti
Sponge
gas out
Inert gases such as Ar can be cleaned by passing them through a hot Ti sponge.
Passing some reactive gases such as triethyl Al across graphite plates stimulates reactions with residual gases such as oxygen before reaching the substrate.
© 1997, Angus Rockett

12. Source Gases Toxicity & Hazard
Hydrides are often toxic, flammable or both.
Storage is an issue. Some source gases can be synthesized as needed from less toxic precursors.
Special safety enclosures and leak detectors are required.
Purity
Typically better for hydrides than for metal organics.
Must be carefully considered when choosing gases.
Thermodynamics and kinetics
Deposition reaction must occur in preference to reverse etching reaction at convenient temperatures.
Parasitic undesirable reactions must be suppressed.
Reaction on the substrate not in the gas or reactor walls.
© 1997, Angus Rockett

13. Source Gases Organometallic CVD
Organometallic source materials are metal atoms bonded to organic ligands. Examples:
Trivalent Metals
Divalent Metals
Trimethyl Ga (TMG)
Hydrogen
Carbon
Gallium
Triethyl Ga (TEG)
Dimethyl Zn (DMZ)
Zinc
One carbon ligand
Two carbon ligand
Analogous sources include longer carbon chains and other metals
© 1997, Angus Rockett

14. Source Gases Thermodynamics -- bond enthalpies in metalorganic gases.70
Bond enthalpy influences the reaction pathway.
For example, trimethyl Al (TMAl) tends to decompose into carbides while triethyl Al decomposes to Al ethylene and hydrogen. 60 50 40
Bond enthalpy (kcal/mole) 30 20 10
methyl
ethyl Zn Cd Hg Al Ga In
Divalent metals
Trivalent metals
© 1997, Angus Rockett

15. Source Gases
Thermodynamics -- bond enthalpies in metalloid source gases.
100 80 60
Bond enthalpy (kcal/mole) 40
Hydrides 20
Methyls N P As Sb Bi
Group V Element
© 1997, Angus Rockett

16. Sources of Reactants Liquid Sources
Liquid source materials are typically vaporized by bubbling a carrier gas such as hydrogen through the source.
Standard flow controller
Carrier gas inlet (typically H2)
Temperature probe
To reactor
Thermal mass
Heater
Temperature (°C)
100 60 40 20 80 1 10
100
1000
Vapor Pressure (Torr)
DMTe
TMAs
Liquid source materials should have vapor pressures in a convenient range at temperatures below ~150°C.
TMGa
DEZn
TEAl
TEIn
TEGa
2.4
2.6
2.8
3.0
3.2
3.4
3.6
3.8
1000/T (K -1 )
© 1997, Angus Rockett

17. Standard gas flow controls
Mixed solid & gas source processes
Example process: GaAs formation from Ga and AsH3 (arsine)
Ga volatilization reaction: 2 Ga + 6 HCl GaCl H2
800°C
Reaction zone: 4 AsH As4 + 6 H2
Deposition reaction: 4 GaCl3 + As4 + 6 H GaAs + 12 HCl
750°C
Reaction zone
Hydride source gases
Ga volatilization
Deposition
AsH3 + H2
HCl
Substrate
HCl + H2
GaCl3
Standard gas flow controls
Liquid Ga
800 °C
850 °C
750 °C
Scrubber & pumps
Reaction is catalyzed by the substrate surface.
© 1997, Angus Rockett

18. Mixed liquid & gas source processes
Example reaction process: GaAs by the halide process
Upstream of Ga:
4 AsCl3 + 6 H As HCl
425°C
As4 dissolves in Ga to form GaAs at the source
H2 Carrier
Gas
Liquid
Substrate Ga
825 °C
750 °C
Liquid
Scrubber & pumps
AsCl3
0-20 °C
HCl etches the GaAs at the Ga source at high temperature and deposits GaAs by reverse reaction at the substrate at lower temperature
© 1997, Angus Rockett

19. Equilibrium & Rate for a Reaction
The net rate for the reaction depends upon how far from equilibrium it is.
Most reactions are far from equilibrium and the rate is determined by the concentration of reactants (or products) only.
For example, the reaction rate R in:
4 GaCl3 + As4 + 6 H GaAs + 12 HCl
forward
reverse
is:
Where:
k0 is the equilibium constant of this reaction
[C] is the concentration of species C R = k [ G a C l 3 ] 4 A s H 2 6
© 1997, Angus Rockett

20. Gas Phase Transport of Reactants
Turbulent Flow
Laminar Flow
Lines indicate gas flow.
Reynolds Number:
Turbulent for Re > 2000
where:
 is the gas density
L is the distance along the reactor
u is the gas velocity
 is the gas viscosity
Gas out
Gas in L
© 1997, Angus Rockett

21. Height above the susceptor, y
Gas Phase Transport of Reactants
Boundary layer formation:
Heated Susceptor (Reaction Surface)
Gas Velocity, u
Height above the susceptor, y
Distance, x
Constant velocity
Variable velocity
Boundary layer (x) y u F f d x =  y u m a
Ff: frictional force a: gas acceleration
: viscosity m: gas mass
: gas density
substitute
for m and a
from which
(x) is found
© 1997, Angus Rockett

22. Gas Phase Transport of Reactants
The result of solving for d(x):  ( x ) =  u 
As velocity increases or x decreases the boundary layer thickness decreases.
Tilting the substrate with respect to the gas flux compresses the gas along the substrate which increases the velocity. This compensates the increase in x and keeps the boundary layer constant along the substrate.
© 1997, Angus Rockett

23. Reactant Concentration, C
Gas Phase Transport of Reactants
The reaction rate R depends on the concentration in the gas phase Cg, the boundary layer thickness (x), and the reaction rate k0.
When k0 << hg the reaction is surface reaction rate limited.
When k0 >> hg the reaction is diffusion transport limited.
Reactant Concentration, C k h Cg 1 g R = C N k + h g s g Cs
Ns: atom concentration in the solid
k0: reaction rate constant
hg=Dg/(x) Dg: gas phase diffusivity
(x)
© 1997, Angus Rockett

24. Gas Phase Transport of Reactants- ∆ G / k T
∆G: free energy of reaction
kB: Boltzmann constant
T: reaction temperature (in K)
ks: constant
K ~ 1
: viscosity : density B k = k e
Reaction rate: s 
Gas diffusivity: D = K  g
Log (Reaction Rate)
Inverse Temperature
Diffusion limited
Reaction limited
(power law)
(exponential)
High T
Low T D g T
proportional to x
0.5<x<1.75
When the flow is parallel to the wafer surface, best uniformity results are for diffusion limited growth with a tilted susceptor. This allows control of boundary layer thickness.
© 1997, Angus Rockett

25. Gas Phase Transport of Reactants
The reduced concentration of reactants as the reaction occurs along the substrate can be compensated by tilting the susceptor. This decreases the thickness of the boundary layer and places the susceptor in a more reactant rich gas flow region.
Optimized boundary layer
shape
© 1997, Angus Rockett

26. Stimulated CVD
Stimulated CVD reactions supply energy to the reactants at or above the surface to accelerate the reaction.
Purpose: To change the reaction kinetics or thermodynamics
Reduction in growth temperature
for example, CVD of SiO2 at 250°C by plasma-enhanced CVD rather than 700°C or higher for conventional CVD.
Allowing reactions which are not thermodynamically possible
for example precracking reactants such as N2.
Using ion bombardment during deposition
to modify film morphology
form metastable phases.
© 1997, Angus Rockett

27. Stimulated CVD
Reaction kinetics and thermodynamics can be modified by various methods.
Photon-stimulation
Plasma-stimulation
Lamp or
laser
Local plasma
normal
incidence
parallel
incidence
Remote plasma
Options:
Resonant reaction
Monochromatic light
Sample orientation
© 1997, Angus Rockett

28. Stimulated CVD: Enhancement Mechanism
Plasma excitation accelerates the reaction kinetics by providing energy sufficient to make the reaction spontaneous.
System Energy
Reaction
Coordinate
Endothermic
Activation
Energy
Exothermic
Stimulated reaction
Normal reaction
+ voltage
Plasma
Electron collisions with reactant molecules create reactive species
electrons
Cathode
- voltage
The excited species generated in the plasma have a higher starting energy than thermal molecules. Thus the reaction may become spontaneous.
© 1997, Angus Rockett

29. Stimulated CVD: Reaction Rate
In a stimulated reaction:
The reactant supply is diffusion limited,
The optimal temperature is generally determined by surface diffusion necessary for good film morphology.
PECVD
Diffusion
limited
Log (Reaction Rate)
Reaction limited
Conventional CVD
Inverse Temperature
© 1997, Angus Rockett

30. Selective CVD Low reactant supersaturation on the “mask”
Deposition only on selected parts of the substrate is desirable because it eliminates a patterning step and is self aligned.
Requirements and methods for obtaining selectivity:
Low reactant supersaturation on the “mask”
Adsorbing flux =  ka
 : arriving gas flux
ka : adsorption rate
Desorbing flux =  kd
 : surface coverage
kd : desorption rate
1 >> (substrate) >> (mask) ~ 0
thus kd (mask) >> kd (substrate)
© 1997, Angus Rockett

31. Selective CVD Requirements and methods for obtaining selectivity:
Reaction with the substrate required
for example, formation of a silicide by deposition of a metal by CVD at elevated temperatures at which a reaction with the exposed substrate occurs.
Species “A”
Product: AB
Mask
Species “B”
Pretreatment of the substrate surface to accelerate nucleation.
Reactant
Nucleation promoter
Mask
Species “B”
© 1997, Angus Rockett

32. Selective CVD Requirements and methods for obtaining selectivity:
Etching of deposits increase effective critical nucleus size
All nuclei are etched to some extent. This favors nucleation where more nuclei form more often.
Etching corrects for accidental nucleation on the mask.
Small nuclei are etched completely
Nuclei remain only on the substrate
Selectivity is improved
© 1997, Angus Rockett

33. Selective CVD Problems:
For moderate adsorbed reactant mobility, reactant from the mask accelerates growth near the mask edge.
If adsorbed reactant mobility is very high, local growth rate can depend on local mask area.
Regions near large mask areas grow faster than regions near smaller mask areas.
© 1997, Angus Rockett

34. Selective CVD Deposition of W on Si and not on SiO2:
Step 1: Selective nucleation W
2 WF6 + 3 Si W + 3 SiF4
≤ 300 °C
SiO2 Si
Self-limiting reaction stops at ~20 nm thick W film. Highly selective due to a nucleation rate difference.
Step 2: Selective growth on previously deposited W:
WF6 + 3 H W + 6 HF
SiO2 W
This reaction runs much faster where W nuclei/thin films already exist and is not self-limited in thickness. Si
See E. Broadbent, J. Vac. Sci. Technol. B5, 1661 (1987)
© 1997, Angus Rockett