1. Section 4: Thermal Oxidation
Jaeger Chapter 3
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2. Properties of SiO2 SiO2 <Si> Thermal SiO2 is amorphous.
Weight Density = 2.20 gm/cm3
Molecular Density = 2.3E22 molecules/cm3
SiO2
Crystalline SiO2 [Quartz] = 2.65 gm/cm3
<Si>
(1) Excellent Electrical Insulator
Resistivity > 1E20 ohm-cm Energy Gap ~ 9 eV
(2) High Breakdown Electric Field
> 10MV/cm
(3) Stable and Reproducible Si/SiO2 Interface
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3. Properties of SiO2 (cont’d)
(4) Conformal oxide growth on exposed Si surface
(5) SiO2 is a good diffusion mask for common dopants
e.g. B, P, As, Sb.
*exceptions are Ga
(a p-type dopant) and some
metals, e.g. Cu, Au
SiO2 Si
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4. Properties of SiO2 (cont’d)
(6) Very good etching selectivity between Si and SiO2.
SiO2
HF dip Si Si
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5. Thermal Oxidation of Silicon
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6. Thermal Oxidation Equipment
Horizontal Furnace
Vertical Furnace
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7. Kinetics of SiO2 growth Oxidant Flow (O2 or H2O) Gas Flow
Stagnant Layer
Gas Diffusion
Solid-state
Diffusion
SiO2
SiO2
Formation
Si-Substrate
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8. Silicon consumption during oxidation
SiO2 Si
2.17 mm
1mm
1mm Si oxidized
2.17 mm SiO2
molecular density of SiO2
atomic density of Si
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9. The Deal-Grove Model of Oxidation
stagnant
layer CG Cs
SiO2 Si
Note
Cs  Co Co Ci
X0x F1 F2 F3
gas
transport
flux
diffusion
flux
through SiO2
reaction
flux
at interface
F: oxygen flux – the number of oxygen molecules that crosses a plane of a certain area in a certain time
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10. The Deal-Grove Model of Oxidation (cont’d)
Mass transfer coefficient [cm/sec].
“Fick’s Law of Solid-state Diffusion”
Diffusivity [cm2/sec]
Oxidation reaction rate constant
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11. Diffusivity: the diffusion coefficient
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12. The Deal-Grove Model of Oxidation (cont’d)
CS and Co are related by Henry’s Law
CG is a controlled process variable (proportional to the input oxidant gas pressure)
Only Co and Ci are the 2 unknown variables
which can be solved from the steady-state condition:
F1 = F2 =F3 ( 2 equations)
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13. The Deal-Grove Model of Oxidation (cont’d)
Henry’s Law
Henry’s
constant
partial pressure of oxidant
at surface [in gaseous form].
from ideal gas law PV= NkT 
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14. The Deal-Grove Model of Oxidation (cont’d)
Define
Similarly, we can set up equations for F2 and F3
Using the steady-state condition:
We therefore can solve for Co and Ci 1 2
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15. The Deal-Grove Model of Oxidation (cont’d)
We have:
At equilibrium: F1=F2=F3
Solving, we get:
Where h=hg/HkT
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16. The Deal-Grove Model of Oxidation (cont’d)
We can convert flux into growth thickness from:
Oxidant molecules/unit volume required
to form a unit volume of SiO2.
SiO2 Si F
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17. The Deal-Grove Model of Oxidation (cont’d)
Initial Condition: At t = 0 , Xox = Xi
xox
SiO2
SiO2 Si Si
Solution
Note: hg >>ks for typical
oxidation condition
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18. Note : “dry” and “wet” oxidation have different N1 factors
Dry / Wet Oxidation
Note : “dry” and “wet” oxidation have different N1 factors
for O2 as oxidant
for H2O as oxidant
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19. Summary: Deal-Grove Model
Xox t
Oxide Growth Rate slows
down with increase of oxide thickness
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20. Solution: Oxide Thickness Regimes
(Case 1) Large t [ large Xox ]
(Case 2) Small t [ Small Xox ]
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21. Thermal Oxidation on <100> Silicon
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22. Thermal Oxidation on <111> Silicon
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23. Thermal Oxidation Example
A <100> silicon wafer has a 2000-Å oxide on its surface
(a) How long did it take to grow this oxide at 1100o C in dry oxygen?
(b)The wafer is put back in the furnace in wet oxygen at 1000o C. How long will it take to grow an additional 3000 Å of oxide?
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24. Thermal Oxidation Example Graphical Solution
(a) According to Fig. 3.6, it would take 2.8 hr to grow 0.2 mm oxide in dry oxygen at 1100o C.
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25. Thermal Oxidation Example Graphical Solution
(b) The total oxide thickness at the end of the oxidation would be 0.5 mm which would require 1.5 hr to grow if there was no oxide on the surface to begin with. However, the wafer “thinks” it has already been in the furnace 0.4 hr. Thus the additional time needed to grow the 0.3 mm oxide is = 1.1 hr.
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26. Thermal Oxidation Example Mathematical Solution
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27. Thermal Oxidation Example Mathematical Solution
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28. Effect of Xi on Wafer Topography1 2 3
SiO2
SiO2 Xi Si
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29. Effect of Xi on Wafer Topography1 2 3
SiO2
SiO2 Xi
more oxide grown
more Si consumed Si 1 3 2
less oxide grown
less Si consumed
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30. Factors Influencing Thermal Oxidation
Temperature
Ambient Type (Dry O2, Steam, HCl)
Ambient Pressure
Substrate Crystallographic Orientation
Substrate Doping
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31. High Doping Concentration Effect
Coefficients for dry oxidation at 900oC
as function of surface Phosphorus concentration
Dry oxidation, 900oC
SiO2 n+ n+ n n
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32. Transmission Electron Micrograph of Si/SiO2 Interface
Amorphous
SiO2
Crystalline Si
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33. Thermal Oxide Charges
potassium
sodium
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34. Oxide Quality Improvement
To minimize Interface Charges Qf and Qit
Use inert gas ambient (Ar or N2) when cooling down at end of oxidation step
A final annealing step at oC is performed with 10%H2+90%N2 ambient (“forming gas”) after the IC metallization step.
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35. Oxidation with Chlorine-containing Gas
Introduction of halogen species during oxidation
e.g. add ~1- 5% HCl or TCE (trichloroethylene) to O2
reduction in metallic contamination
improved SiO2/Si interface properties
SiO2
Na+ K+ Si
Cl2
Na+ or K+ in SiO2 are mobile!
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36. Effect of HCl on Oxidation Rate
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37. Local Oxidation of Si [LOCOS]
~100 A SiO2 (thermal) - pad oxide
to release mechanical stress between nitride and Si.
Oxidation
Nitride Etch
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38. Local Oxidation of Silicon (LOCOS)
Standard process suffers for significant bird’s beak
Fully recessed process attempts to minimize bird’s beak
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39. Dopant Redistribution during Thermal Oxidation
conc. Si
e. g. B, P, As, Sb.
SiO2 Si C2 CB C1
CB (uniform) x
Segregation Coefficient
equilibrium dopant conc. in Si
equilibrium dopant conc. in SiO2
Fixed ratio
(can be>1 or <1)
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40. Four Cases of Interest (A) m < 1 and dopant diffuses slowly in SiO2CB
e. g. B (m = 0.3) D C1
flux loss through SiO2 surface not considered here.
B will be depleted near
Si interface.
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41. Four Cases of Interest (B) m > 1, slow diffusion in SiO2.
e.g. P, As, Sb CB C2
dopant piling up near Si interface
for P, As & Sb
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42. Four Cases of Interest (C) m < 1, fast diffusion in SiO2 e. g. B,
oxidize with
presence of H2
SiO2 Si C2 CB C1
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43. Four Cases of Interest (D) m > 1, fast diffusion in SiO2 SiO2 Si CB
e. g. Ga (m=20) C1 C2
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44. Polycrystalline Si Oxidation
poly-Si
SiO2
grain boundaries (have lots of defects).
fast
slower
SiO2 a
roughness
with Xox b
Overall growth rate
is higher than
single-crystal Si
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45. 2-Dimensional oxidation effects
(100)
(110)
Thinner oxide
Thicker
oxide
(100)
(100)
(110) Si
cylinder
Top
view
Mechanical stress created by SiO2 volume expansion also affects oxide growth rate
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