Chemical Vapor Deposition
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Introduction to Chemical Vapor Deposition
A) Chemical Vapor Deposition
CVD Types
CVD Uses
CVD Process
General CVD Reactor Concept
General CVD Process Advantages
General CVD Process Applications
B) Dealing with Engineering Science of CVD Reactions
Transport Processes
Laminar Flow Boundary Layer Concept
Other Susceptor to Flow Axis Options
Thermodynamics
Reaction Kinetics
C) Operational Overview
Polycrystaline Silicon
Silicon Dioxide
Nitride Films
Chemical Vapor Deposition
Current Options
LPCVD
APCVD
Atmospheric Pressure CVD
PECVD
Plasma Enhanced CVD
Low Pressure CVD
Chemical Vapor Deposition
CVD Applications
Customized Surfaces
Epitaxial Layers
Insulator
Silicon dioxide
CVD
Barriers
Silicon Nitride
Conductors
Polycrystalline
Silicon
Chemical Vapor Deposition
CVD Process
Surface Reaction
Film
Arrival Flow
Rate
Growth Rate
Input Flow Rate
Surface Reaction Rate
r
g
r g = Growth Rate of Film
Substrate
CVD Reactor Concept
Reaction Chamber
Susceptor
Controlled Thermal Environment
Film Surface
Controlled Pressure Environment
Hydrogen Carrier Gas
With additional film significant containing gas components
General CVD Process Advantages
Excellent Step Coverage
Large Throughput (100 A/min film growth)
Low Temperature Processing (450 to 1000 C)
Applicable to any Vaporization Source Technology
(Laser CVD for direct Writing)
General CVD Process Applications
Epitaxial Films
Enhance performance of Discreet and Integrated Bipolar Devices
Allow Fabrication of RAM’s and CMOS in Bulk Substrate
Dielectrics
Insulation between Conducting Layers
Diffusion and Ion Implant Masks
Capping Dopant Films
Extracting Impurities
Passivation to Protect Structures from
Impurities
Moisture
Scratches
Polysilicon Conductors
Gate Electrodes
Conductors for Multilevel Metalizations
Contacts for Shallow Junction Devices
B) Dealing with Engineering Science of CVD Reactions
Transport Processes
Thermodynamics
Reaction Kinetics
Transport Processes
Turbulent Flow
No, to Many Particles.
Molecular Flow
No, to Low a Throughput
Laminar Flow ( Only One Left, Make Do)
Set Conditions For Laminar Flow ( Low Reynolds Number Value)
Tube Diameter
Gas Density
Gas Viscosity
R#= D V( D µ )
Reynolds Number
Linear Velocity
Laminar Flow Conditions
Diameter and velocity in tens of cm and cm/s will give
Reynolds numbers in laminar flow regime
Reagent Partial Pressure
R
5
Growth
1.67
= 1.76 x 10 ( $
D /R) (1/ T ) (
T/ y ) (Z) P)
Boundary Layer
Thickness
$' " % 0.33
Reagent’s Gas Phase Coefficient
of Thermal Diffusion
Input Reactant Gas Flow
X
X
X
2
3
X
4
Boundary layer develops along susceptor flow axis
X
1
X
2
X
X
3
4
Under developed
flow pattern at this
position along
susceptor
Distance Above Susceptor
Susceptor
1
Velocity Gradient Profiles at Discrete Points along Flow Axis
Graphic Exaggerated for Visual Effect
X
1
X
2
X
X
3
4
Under developed
flow pattern at this
position along
susceptor
Trends in Gradients
Velocity Values
Increase Along Susceptor
Increase Above Susceptor
Temperature Values
Increase Along Susceptor
Decrease Above Susceptor
Reactant Concentration Value
Decrease Along Susceptor
Increase Above Susceptor
Velocity Gradient Profiles at Discrete Points along Flow Axis
Other Susceptor to Flow Axis Options
Design Factors Include Flow Direction and Wafer Angle
A) Input gas flow
B) Input gas flow
C) Input gas flow
D) Input gas flow
E) Input gas flow
Thermodynamics
CVD Phase Diagram
Give range of input conditions for CVD that could produce specific
condensed phases.
.
Presented as Function of Temperature or Pressure vs Mole Fraction
1200
1000
0.01 Atm
o
1.0 Atm
C
o
C
TiB 2 Phase
o
C
TiB2 & B Phase
1400
Boron codeposit only in High
Boron Mole Fractions in input
stream
0.6
Boron codeposition favored at
higher pressures.
H/HCl = 0.95
Reactant Gas Mole Fraction
B/(Ti + B)
Use Graphic for Educational Value Only
K.E. Spear
7 th Conference on CVD 1979
Electrochemical Society Vol 79
Boron-Carbon CVD Phase Diagrams
10
BCl 3 /CH
-1
4
=4
B4 C + B
10
-2
B
10
1600 0 C
B4 C + C
1.0 Atm
-3
B4C
10
-4
Vapor
10
-4
Carbon
10
-3
10
-2
10
-1
Partial Pressure for Methane
10
-0
Bernard Ducarroir
J. Electrochem. Soc. 123 ,136, 1976
Use Graphic for Educational Value Only
Vanadium-Silicon-Hydrogen-Chloride CVD Phase Diagrams
1200 oC
1100 oC
1000 oC
V5Si3
900 oC
VCl2 + V5Si3
VCl2
0.6
Input Reactant Gas Mole Fraction
H/HCl = 0.95
P = 0.25 atm
Si /(Si + V)
Use Graphic for Educational Value Only
K.E. Spear
7th Conference on CVD 1979
Electrochemical Society Vol 79
Vanadium-Silicon-Hydrogen-Chloride CVD Phase Diagram
Compos ition ratios for input gases of VCl 4 /SiC l4 /H2 are not equilibrium values
Trans port Processes vs Thermo dynamics
Task:
Make a V5 Si3 film.
Procedure:
From C VD Phase D iagram for a 900 o C deposition, an input gas mole
fraction of 0.20 can be used.
Problem:
As V5 Si3 forms on surface, actual reagent gas Si mole fraction consumed
at surface is higher (0.375) than the input reactant gas ratio supplied
(0.20). Thus Si at surface is depleted, more Vanadium is ava ilable at the
surface and actual equilibrium shifts to production of V3 Si.
Procedure:
Hold temperature constant but shift the input gas mole fraction to 0.5.
Problem:
As V5 Si3 forms on surface, actual reagent vanadium gas mole fraction
consumed (0.625) is higher than the input gas mole fraction for vanadium.
Thus Vanadium at surface is depleted, more Silicon is available at the
surface and actual equilibrium shifts to production of VSi 2 .
Reaction Kinetics
Titanium Diboron Deposition Arrhenius Plot
Reaction Temperatures (2000 K to 1000 K)
P = 0.263 Atm.
Input flow Rate = 462 cc /min
10.0
1.0
B/(B + Ti) = 0.66
Cl/(Cl + H) = 0.33
5.0
Besmann ,J. Electrochem. Soc.
124 , 790 (1979)
Use Graphic for Educational Value Only
6.0
7.0
-1
1/T (x 10 / K)
8.0
9.0
Input Gases
TiCl 4
BCl3
H2
Arrhenius Rate Profiles
10.0
(f)
(a)
1.0
1/T
Higher Surface Reaction Rates
Use Graphic for Educational Value Only
Lower Surface Temperatures
Arrhenius Isotherms
Surface Reaction Limiting Growth Rate
(f)
10.0
1.0
(a)
Partial Pressure Reactant Gas
Use Graphic for Educational Value Only
Operational Line for Deposition at Higher Pressure
Desired Growth
Rate
Best Fit Model Behavior based
On 5 Calibration Runs
rg2
rg1
Current
Growth Rate
1/ T 2
New Operating
Temperature
ln (r
g2
1/T
/r
g1
1/ T1
Current
Operating Temperature
&T / T T )
) ' (q +
/ k ) (T
act
2
1
2 1
C) Operational Overviews
Polycrystalline Silicon (Polysilicon)
H
Si
APCVD
o
575 to 650 C
Si
High Exposure Limit
Pyrophoric
Toxic ( 1 Atm but 90% N 2 )
H H H
LPCVD
o
575 to 650 C
25 PA to 130 PA
100% Silane
Si
25 PA to 130 PA
20% to 30% Silane
Si
Considerations
Temperature
At high temperatures get gas phase reactions that produce rough, loosely
adhering deposits and poor uniformity.
At low temperatures deposition rates are to slow for industrial situations.
Zone heating rear of furnace up to 15 oC hotter. (Better film uniformity)
Pressure (LPCVD)
Four popular ways to alter pressure.
Change gas flow rate but keep pumping speed constant.
Change pumping speed with constant flow rate
Change reacting gas or carrier gas with other held constant
Change both gases but keep there ratio constant.
Silicon dioxide
H
Films Contain Hydrogen as
Si
(Oxidation)
400 - 450 C
SiO2
H
H
O2
H
Silanol (SiOH)
Hydride (SiH)
Or Water
Amorphous Structure of SiO4 Tetrahedra
Low Temperature
Loose adhering deposits on side walls of reactor. ( Particles that can
contaminate the film.
At high silane pressures allows for gas phase reactions. ( Promotes
particle contamination and hazy films)
Fair step coverage
Low film density ( 2. 0 g/cm 3 )
Deposition rate complex function of Oxygen concentration
Easy chemical reaction. ( Low activation energy, 0.4 ev (10 kcal/mole) )
Film depends on gas phase transport of material to surface
Low temperature allows production of films that will serve as
insulation between aluminum levels in device.
Medium Temperature
Silane
Tetraethoxysilane
OCH 2CH3
H
Si
H
H
H
H
H
H
650 to 750 C
NO
SiO 2
C
C
Si
O
H
(LPCVD)
O
C
O
TEOS
H
H
C
H
H
CH2CH3
650 to 750 C
100 to 1000 std. cc / min
30 PA to 250 PA
SiO 2
High Temperature
Dichlorosilane
Cl
Si
Cl
H
H
Nitrous Oxide
850 to 900 C
(N 2O )
LPCVD
SiO 2
Nonlinear pressure dependence that is function of wafer position.
Small amounts of Chlorine in films that tends to cause cracking in a poly layer)
Reagent depletion problems
Phosphorus doping is difficult. ( The phosphorus oxides are volatile at high
deposition temperatures.)
Excellent Uniformity
Cl
Cl
Cl
H
H
Si
Cl
Cl
H
N
H
H Si
Cl H
Si
H
N
H
H
H
H
H
Precursor
H
N
Si
Cl
Cl
Cl
H
Cl
Cl
First Monolayer
of Silicon Nitride
Pad Silicon Dioxide
Except for epi and parallel plate processes both sides of wafer are coated.
Equipment
Furnace with or without vacuum capability
Plasma Chamber
CVD is Crucial to Fabrication of IC's, Especially MOSFETS
(The Bottom Line)