МММ / 04_lecture5
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Oxidation of Si
Why spend a whole lecture on oxidation of Si?
Ge has high μe, μh , Ge stable… |
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… but no oxide |
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GaAs has high μe and direct band… |
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… no oxide |
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Why SiO2? |
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SiO |
is stable down to 10-9 Torr , T > 900°C |
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2 |
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SiO2 can be etched with HF which leaves Si unaffected |
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SiO2 |
is a diffusion barrier for B, P, As |
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SiO |
is good insulator, ρ > 1016 Ωcm, E |
g |
= 8 eV! |
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2 |
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O2 |
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SiO2 |
has high dielectric breakdown field, 500 V/μm |
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SiO2 dtOxide
SiO2 growth on Si fi clean Si / SiO2 interface
Si
because DSi through SiO2 << DOxy through SiO2
Sept. 19, 2003 |
3.155J/6.152J |
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O2 |
So SiO2 growth occurs at inside surface |
SiO2 dtOxide |
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Si + O2 SiO2 |
Si |
or |
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Si + 2H2O = SiO2 + 2H2
(faster growth, more porous, lower quality)
2
Sept. 19, 2003 |
3.155J/6.152J |
O2
SiO2 dtOxide
Si
Extra free volume
in dangling bonds of
amorphous SiO2 =>
Implications different for field vs. patterned oxide.
Sept. 19, 2003 |
3.155J/6.152J |
3 |
Cleaning station for removing organic contaminants and native oxide (by HF-dip) from Si wafers.
Oxidation furnaces for controlled growth of oxide layer on Si:
1050 C and steam for field oxide.
Sept. 19, 2003 |
3.155J/6.152J |
4 |
Probably safe to say that
entire course of semiconductor industry would be different without SiO2.
Device fabrication, especially MOS,
more difficult.
Depositing SiO2 or Al2O3 is not clean.
Sept. 19, 2003 |
3.155J/6.152J |
5 |
It’s no accident that the world leader in Si chip technology, Intel, has been led by the flamboyant Hungarian, Andy Grove.
As a young researcher at Fairchild Semiconductor, he wrote the book on SiO2 growth: the Deal-Grove model.
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3.155J/6.152J |
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Deal-Grove model of silicon oxidation
SiO2 growth occurs
at Si / SiO2 interface
because DO2 (SiO 2) >> DSi (SiO 2)
Growth Process limited by
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O2 |
1. P(O2) = Pg Cg |
Concentration |
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Cg |
2.Transport O2 to SiO2 surface across dead layer J1
3.Adhesion of Cs(O2) at SiO2 surface C0
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Diffusion O2 through SiO2 |
J2 |
5. |
Chemical reaction rate |
J3 |
O2
SiO2
Si
dead |
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layer |
SiO |
Si |
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J1 |
Cs |
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Co |
J2 |
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Ci |
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J3 |
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x |
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3.155J/6.152J |
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Deal-Grove model of silicon oxidation |
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Oxide growth rate |
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J1 |
= J2 |
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= J3 |
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Ideal gas law: P V = NkT |
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O2 |
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dead |
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layer |
SiO |
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Si |
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g |
Concentration |
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Cg |
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P |
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N |
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Cs |
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= Cg |
= |
g |
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J1 |
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V |
kT |
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Co |
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J2 |
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C = HP |
= Hk |
TC |
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Ci |
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0 |
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B |
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(C - C ) |
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(Cgg− Css ) |
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J1 > D |
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Henry’s law |
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3 |
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tdead layer |
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C − C |
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Turbulence => |
J2 = DO2 (SiO |
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= k |
C |
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2 ) Cs0 - C0 i |
i |
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xox |
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i |
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rate constant |
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J1 = hg(Cg |
- Cs) |
Diffusion (D cm2/s) |
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ki |
(cm/s) |
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Equate ideal gas + J1 |
Equate J2 + Henry to |
J3 |
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to J2 + Henry |
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fi Ci = fn (Pg ,hg,H,DO2 , xoxide,ki) |
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Sept. 19, 2003 |
3.155J/6.152J |
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Deal-Grove model of silicon oxidation
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J1 = J2 = J3 |
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O2 |
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dead |
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layer |
SiO |
Si |
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Concentration |
C |
g |
2 |
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= fn (Pg ,hg,H,DO2 , xoxide |
,ki) |
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fi Ci |
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J1 |
Cs |
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Co |
J2 |
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Ci |
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Ci |
= |
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HPg / ki |
1 |
hg |
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J3 |
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+ |
oxO |
+ |
h = |
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h |
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ki |
HkT |
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D 2 |
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mass |
Diffusion |
Reaction |
(ki = ks in text) |
transport |
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J3 |
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J1 |
J2 |
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Slowest process controls concentration of oxygen at interface…
Sept. 19, 2003 |
3.155J/6.152J |
9 |
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Ci |
= |
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HPg /ki |
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1 |
+ |
xox |
+ |
1 |
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h |
DO2 |
ki |
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Limits: |
Growth |
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mass |
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diffusion |
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limited by: transport |
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Diffusion limited: |
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DO2/x < |
k , h |
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ox |
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g |
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O2
Ci = HPg D ki xox
h = |
hg |
very large |
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HkT |
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reaction
Reaction-rate limited:
ki < hg, DO2/xox
Ci = HPg
Sept. 19, 2003 |
Slower process controls concentration of oxygen at interface, |
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which in turn controls growth rate… |
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