Transcript
EE243 Sp2011 Lec 5
Polycrystalline Si Oxidation
poly-Si
grain boundaries (have lots of defects).
SiO2
roughness
with Xox
fast slower
a
b
Overall growth rate
is higher than
single-crystal Si
SiO
2
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Solution: (i) Grow sacrificial gate oxide to consume the nitride spots,
(ii) Strip sacrificial oxide ,
(iii) Regrow good gate oxide
KOOI Effect
“White
Ribbon”
Si3N4 + 6 H20 3SiO2 + 4 NH3
3 Si + 4 NH3 Si3N4 + 6 H2
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Bird’s Beak and Kooi Effect
Kooi (White Ribbon)
effect that requires the
gate oxide to be grown
twice. (N related; etch
hot H3PO4)
LOCOS Bird’s Beak
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Sealed - Interface Local Oxidation (SILO)
Key Ideas: Use mask-stack engineering to increase the stiffness of the oxidiation mask.
More resistance to bending during oxide expansion will give less lateral oxide growth.
Nitride, oxynitride, poly can all be used as the stiffeningg layer. It is important that the
stiffing layer be keep thin so as not to generate too much stress during oxidation.
Oxide Encroachment 0.2m/side
Can be fully recessed (etching Si
substrate ) or semi-recessed.
Mask Stack Engineering
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(1) no pad oxide
Si3N4
Si crystal defects
Silicon Nitride as oxidation mask
Si3N4
Si
Si3N4
Si
SiO2oxidation
Nitride can be
oxidized at high temp(2) nitride too thin
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SideWAll Masked Isolation (SWAMI)
nitride1
nitride2Pad oxide
Si(100)
(1)
(2)
(3)
(4)
•Can grow thick oxide with little lateral encroachment
•High thermal budget still has dopant diffusion problem
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Solutions:
Use oxidation temp > 960°C to relieve stress by viscous flow of SiO2. High pressure
oxidation also helps.
Use thinner nitride and rounded etch corners (stress reduction)
Use groove sidewall angle < 75 °. This enhances the oxide viscous flow.
Defects with recessed LOCOS
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Oxidation of Metal Silicides
• SiO2 formed on top of silicide in most cases.
Exceptions : MOx forms on top for HfSi2 and TiSi2 if oxidized
below 900oC
•Silicide layer is conserved : Si supplied by poly-Si or silicide
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Oxynitridation by Nitrous oxide N2O
Gas Phase reactions
Interface reactions
Si +O2 SiO2 reaction constant k1
Si +NO SiOxNy reaction constant k2
Film reaction
SiOxNy +NO SiO2 +N2 reaction constant k3
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Growth rate slows
down attributed
to the
incorporation of
nitrogen
at silicon/silicon
oxynitride
interface.
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Nitrogen accumulation at
the Si–SiO2 interface
improves:
•Radiation hardness
•Barrier properties to
impurity penetration,
•Stress stability
•Smaller charge trapping
•Large charge to
breakdown. (QBD)
50% N20
40% N20
Singhvi and Takoudis
JAP, Vol82, No.1, 442 (1997)
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Oxidation Affects DiffusionOxidation Enhanced
Diffusion (OED):
Interstitials created by
excess Silicon from
oxidation diffuse rapidly
and act as additional
stepping stones for the
dopant atoms.
Oxidation Reduced
Diffusion (ORD): The
dopant uses vacancies
and the population of
intersitials reduces
the poplulation of
vacancies.EE243 Sp2011 Lec 5
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Thermal Oxidation creates volume change
(For 1-D growth, 1 m SiO consumes 0.46 m Si).
There will be mechanical stress generation and oxide flow.
1-D Deal-Grove Model: parameters ( D and ks); process variables (
pressure, oxidant, temperature, time, dopant conc, orientation).
2-D Deal-Grove model : has to include curvature and mechanical stress
effects
Thin oxide and oxynitride growth: need extended models
Interface Charges can be minimized by H annealing
Issues with nitride as oxidation mask
Other Topics: Oxidation enhanced diffusion, dopant redistribution, oxidation
induced stacking fault will be discussed in Diffusion Module
Summary of key concepts in thermal oxidation
top related