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Oxidation -
National Chiao-Tung UniversityDepartment of Electronics Engineering &
Institute of electronics
Thermal Oxidation
Oxidation method wet oxidation
dry oxidation
rapid thermal oxidation
high pressure oxidation plasma enhanced oxidation
Oxide hardness technology post oxidation anneal
nitrogen passivation
N2O or NO oxidation, N2O or NO anneal, nitrogen implantation fluorine passivation
HF dip, VHF clean, NF3 additive, fluorine implantation
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National Chiao-Tung UniversityDepartment of Electronics Engineering &
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Atomic Structures
SiO2 Density=2.26(2.4) g/cm3
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National Chiao-Tung UniversityDepartment of Electronics Engineering &
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Thermal Oxidation
Dry Oxidation Si (s) + O2 (g) SiO2 (s)
Wet Oxidation 2H2 (g) + O2 (g) H2O (g)
Si (s) + H2O (g) SiO2 (s) +2H2 (g)
0.44d Si to 1d SiO2
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National Chiao-Tung UniversityDepartment of Electronics Engineering &
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Deal-Grove Model-1
Oxidants must be transported from the bulk of the gas tothe oxide surface.
Cg : oxidant concentration in bulk of gas
Cs
: oxidant concentration right next to the oxide surface
hg : gas phase mass-transfer coefficient
Henrys law In equilibrium, the concentration of a species within a solid is proportional
to the partial pressure of that species in the surrounding gas.
C=Hp, where H is the Henrys law constant and p is the gas pressure
C* = H pg(equilibrium concentration in bulk SiO2)
Co= H ps(equilibrium concentration at bulk gas/SiO2interface)
)(1 sgg CChF
HkThhCChF
p/kTC
go /where,)(
gasidealFor
*
1
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National Chiao-Tung UniversityDepartment of Electronics Engineering &
Institute of electronics
Deal-Grove Model-2
Oxidants must diffuse across the oxide layer already
present.
D is the diffusitivity of oxidant in bulk oxide
Ci is the oxidant concentration in bulk oxide
at the oxide/silicon interface
xo is the thickness of oxide layer already present
Oxidants must react at the
oxide/silicon interface
ks is the chemical surface-reaction rate constant
o
io
x
CCDF
2
isCkF 3
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National Chiao-Tung UniversityDepartment of Electronics Engineering &
Institute of electronics
Deal-Grove Model-3
Steady state : F=F1=F2=F3
h
k
CCCk
CCCD
D
xk
h
k
CD
xk
C
D
xk
h
k
CC
soi
oi
oss
os
ooss
i
1
0As
and00As
1
1
and
1
*
s
*
*
*
Diffusion control
Reaction control
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National Chiao-Tung UniversityDepartment of Electronics Engineering &
Institute of electronics
Deal-Grove Model-4
N : The number of oxidant molecules incorporated into a
unit volume of oxide. O2 oxidation : N=2.2x10
22 molecules/cm3
H2O oxidation : N=4.4x1022 molecules/cm3
B
Axx
N
DCB
hkDA
tBAxx
xx
D
xk
h
kCkF
dtdxN
ii
s
oo
iooss
so
2
*
2
*
2
112
0conditioninitial,
1
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National Chiao-Tung UniversityDepartment of Electronics Engineering &
Institute of electronics
Deal-Grove Model-5
constantratelineartheis
iprelationshlinearcalledist
4tAs
constantrateparabolictheis
iprelationshparaboliccalledis
4As
1
4
1
2
From
*
2
2
2
2
2
N
C
hk
hk
A
BA
Bx
BA
B
Btx
BAt
BA
t
A
x
tBAxx
s
s
o
o
o
oo
Reaction control Diffusion control
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National Chiao-Tung UniversityDepartment of Electronics Engineering &
Institute of electronics
Initial Oxidation
Massouds experimental model:
Apply to either (111) or (100) oriented Si.
The first term is the Deal-Grove Model.
The second term represents an additional oxidation mechanism.
The actual mechanism is still not clear.
nmLeVEhrmC
kT
E
CC
L
xC
Ax
B
dt
dx
Ao
A
o
o
o
o
7and,35.2,/106.3
exp
exp2
8
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National Chiao-Tung UniversityDepartment of Electronics Engineering &
Institute of electronics
1
Oxidation Equipments
Furnace oxidation
(Hot wall)
Rapid thermal oxidation
(Cold wall)
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1
Oxidation Furnaces
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National Chiao-Tung UniversityDepartment of Electronics Engineering &
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1
Oxidation Recipe
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National Chiao-Tung UniversityDepartment of Electronics Engineering &
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1
Measurement of Oxide
Color chart
Physical methods Scanning Electron Microscope (SEM), Transmission Electron Microscope
(TEM), Surface Profiler, Atomic Force Microscope (AFM)
Detect : thickness, interface structure, surface roughness
Optical methods Ellipsometer
Detect : thickness (T), refraction index (n), absorption index (k)
Electrical methods Capacitance-Voltage curve, electrical strength, interface state density, etc.
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National Chiao-Tung UniversityDepartment of Electronics Engineering &
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1
Color Chart
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National Chiao-Tung UniversityDepartment of Electronics Engineering &
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1
Scanning Electron Microscopy
Principle : image contrast obtained by electron from different material or different
topography
Signal to Form Images : Secondary Electron (SE)
Ee
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Oxidation -
National Chiao-Tung UniversityDepartment of Electronics Engineering &
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1
Transmission Electron Microscopy
Principle : image contrast obtained by electron transmitted through thin materials
Signal to Form Images : bright field image
direct transmitted beam to provide micro-structure and morphology
dark field image diffracted beam image to obtain crystallographic information
lattice image (high resolution TEM, HREM) a combination of diffracted and direct beams to yield images with atomic resolution
diffraction diffraction yields crystallographic and orientation information
x-ray spectroscopy (EDX/WDX)
electron energy loss spectroscopy (EELS)
energy analysis of transmitted electron to analyze low-Z element
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Oxidation -
National Chiao-Tung UniversityDepartment of Electronics Engineering &
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1
TEM Sample Preparation
Plane-View Sample circular sample cut to diameter 3mm
chemical or mechanical thinning to 100 um
chemical or ion-milling to form ultra-thin area
Cross-Sectional Sample rectangular sample cut to 2mm x 3 mm
stack samples
mechanical polish to 20 um
fixed by Cu-ring
ion-milling to form ultra thin area
Precision Sample FIB cut
3 mm
ultra thin area
3 mm
2 mm
E-beam
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Oxidation -
National Chiao-Tung UniversityDepartment of Electronics Engineering &
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1
SiO2/Si Interface
Si Lattice
Image
SiO2
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Oxidation -
National Chiao-Tung UniversityDepartment of Electronics Engineering &
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1
Atomic Force Microscopy
Principle : a sharp tip is vibrated perpendicular to the sample surface at the resonant
frequency.
Van der Waals force between probe tip and sample surface modify the
resonant frequency.
The change of resonance provides a corresponding displacement.
Application : surface morphology measurement
sensitivity ~ atomic level
three dimensional topography measurement accuracy +-1 nm, repeatability +-10 nm
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Oxidation -
National Chiao-Tung UniversityDepartment of Electronics Engineering &
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2
Ellipsometer
layeriththeofthicknessthet
layeriththeofindexabsorptionthek
layeriththeofindexrefractionthen
airofindexrefractionthen
1allwith),,,,,,,
1allwith),,,,,,,
.measurableare
)3600(and)900(anglesricellipsometThe
)tan(ratioreflectioncomplexThe
)(
)(;
(
)(
i
i
i
o
ikntknnf(
ikntknnf(
eR
R
IncidentE
relectedER
incidentE
reflectedER
ssiiio
ssiiio
j
s
p
s
ss
p
p
p
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Oxidation -
National Chiao-Tung UniversityDepartment of Electronics Engineering &
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2
Spectrum Ellipsometer
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Oxidation -
National Chiao-Tung UniversityDepartment of Electronics Engineering &
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2
Effect of Oxidation Ambient
Wet oxidation rate is much higher than dry oxidation rate
because the oxidant solubility in SiO2 (C*) is much higher
for H2O than for O2.
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Oxidation -
National Chiao-Tung UniversityDepartment of Electronics Engineering &
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2
Effect of Crystal OrientationDry O2/1000C/1hr
Effect of crystal orientation is explained by the differences
in the surface density of silicon atoms on the various
crystal faces.
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Oxidation -
National Chiao-Tung UniversityDepartment of Electronics Engineering &
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2
Effect of Pressure
The concentration of oxidant just inside the oxide at thegas/SiO2 interface C
* is proportional to pg, then both B and
B/A are proportional to pg.
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National Chiao-Tung UniversityDepartment of Electronics Engineering &
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2
Boron-doped
Effect of Impurity Concentration
The mechanism associates with the faster oxidation at theSi/SiO2 interface due to higher vacancy concentration, i.e.
B/A is affected. The differences of oxidation time is more pronounced for n+ Si than for p+
Si and is more pronounced for low temperature oxidation compared to
high-temperature oxidations.
Phosphorus-doped
N i l Chi T U i i
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2
Chlorine Incorporation
O2 + HCl or TCA or DCE Effective for dry oxidation only
Lower interface state density
Lower defect density
Gettering of metal ions in oxides
Enhanced oxidation rate due to
Rough interface at high Cl concentration (TCA>8%)
OHClOHCl 222 24
N ti l Chi T U i it
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2
Fluorine Incorporation
Methods HF surface immersion
F2 surface treatment
Oxidation with NF3 additive
F+ implantation into Si or oxide
Fluorine effects Enhanced oxidation rate
Lower dielectric constant
Better oxide quality
Enhanced boron penetration Poorer dielectric reliability with excess fluorine
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National Chiao-Tung UniversityDepartment of Electronics Engineering &
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Nitrogen Incorporation
Higher low field mobility Lower high field mobility
Lower defect density
Higher reliability Stronger radiation immunity
Slower boron penetration
National Chiao-Tung University
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Oxidation -
National Chiao-Tung UniversityDepartment of Electronics Engineering &
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2
Nitrogen Incorporation
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Oxidation -
National Chiao Tung UniversityDepartment of Electronics Engineering &
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3
Oxide Charges
Flat-band voltage (Vfb) and threshold voltage (Vth)
F
ox
Sifbth
T
m
oxoxox
f
ox
it
ox
otmsfb
C
QVV
dxxT
x
CC
Q
C
Q
C
QV
ox
2
1
0
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Oxidation -
g yDepartment of Electronics Engineering &
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3
Oxide Charges
Interface charge (Qit) Magnitude depends on Si orientation : (111)>(110)>(100)
Magnitude depends on surface impurity and interface bonding
Magnitude can be reduced by annealing at 400-500oC in H2 ambient
Fixed charge (Qf)
Immobile and independent of surface potential Depends on Si orientation and oxidation condition
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3
Oxide Charges
Mobile ion (Qm) Comes from metal contamination
Drift rate is thermally activated and is field dependent
Trapped charge (Qot)
Maybe positive or negative Maybe caused by ionization radiation or carrier injection
Trap site comes from defect in oxide layer
Low temperature annealing can not remove trap site but cause
neutralization or compensation of trapped charge.
Polarization charges (Qp) Comes from electric field induced dipole
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3
Effect of Mobile Ions
SiO2
HfO2
EOT 1
Q(x)
EOT
EOT 1
EOT 2
0
X
HfO2
SiO2
P-type
v
EOT
2
2
2 2
,
2
,
1[ 1* *
1 11 * ( 1 )* ]
2 2
it S iOit High
Hig
FB ms
ox
S iOh
V EOT EOT
EOT EOT EOT
Q
Q
Q
Q
0
1 1( )dFB
o
V xQ x dxC d
- 12 -
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3
Application of Thermal Oxides
National Chiao-Tung UniversityD t t f El t i E i i &
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3
ITRS Roadmap (2003)
National Chiao-Tung UniversityDepartment of Electronics Engineering &
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3
Carrier Injection
Tunneling Mechanism Schottky Emission
Frenkel-Pool Emission
Fowler-Nordheim Tunnelingor Field Emission
Ionic Conduction
Space Charge Limited
Ohmic
Direct Tunneling
kT
q
T
VaT
kT
qEqTAJ BIB
exp~
4/exp
22*
kT
q
T
VaV
kT
qEqEJ BIB
2exp~
/exp
V
b
VEq
qm
EJB
exp~3
24
exp2
2/3*2
T
c
T
V
kT
E
T
EJ a exp~exp
T
cV
kT
EEJ a exp~exp
23
23
23
22
1
2
exp
1 s
oxss
oxsoxs
ox qV
E
C
qV
BEJ
2
3
2
~4
VL
VJ
S.E.
F-P
F-N
D.T.
Ionic
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3
Issues of Thin Oxide
Direct tunneling current
Maximum tolerable Igate
2
321
2
3
23
23
23
2
21
2
2
3
4
16
exp
1
sox
s
s
oxss
ox
s
oxs
oxDT
q
mC
qB
qV
E
C
qV
BEJ
2
gate
/1
m0.1//1
m0.1LAs
/1
cmA
mnAI
mnAII
gate
offgate
1um
0.1 um
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3
Issues of Thin Oxide
Poly-Si depletion
Influence Constant field & constant Npoly
=> constant Vpoly
An 0.3-0.5nm thick EOT loss is
inevitable.
polySi
gox
accoxinvox
poly
Sipoly
polyoxSifbg
Nq
V
TT
N
QV
VVVVV
22
,
2
,
2
2
meas
oox
C
AEOT
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p g g
Institute of electronics
3
Issues of Thin Oxide
Quantum effect Because of the conduction band triangular like well at the surface,electron states are a series of discrete levels above the edge of the
conduction band.
The surface potential is larger than classical predicted.
Charge is located 0.5-1.0 nm away from surface than classical predicted.
Consequences A 0.3 nm increase of effective oxide thickness
An increased depletion layer charge density
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Issues of Thin Oxide
Boron penetration Boron may diffuse through gate oxide
from p+ poly-Si gate of PMOSFET. Positive flat-band voltage shift
Degradation of oxide wearout properties
Thinner oxides result in severer boronpenetration effect.
Fluorine and hydrogen enhance boron
diffusion.
Crystallized a-Si retards boron diffusion.
Nitrided oxides reduce boron diffusitivity.
Trade-off between boron
penetration and poly depletion
Tox=3.3nm
IEDM, 1995, p.85
VLSI-TSA, 1997, p.167
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Advantages of High K Dielectric
Thicker thickness
Lower EOT
Lower tunneling current
ox
ox
HKHK TT
National Chiao-Tung UniversityDepartment of Electronics Engineering &
I tit t f l t i
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Requirement of High K Dielectric
Criteria Requirement
EOT scalability < 1 nm Dielectric constant > 15
Negligible FIBL effect Dielectric constant < 60
Leakage current < 1A/cm2 Band gap > 5eV
Barrier height > 1eV
Thermal stability No silicidation and reduction
Hysteresis < 20mV
Dispersion 10 years
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Band Alignment Consideration
Energy gap of dielectric depends on dielectric constant.
Oxide Dielectric
Constant
SiO2 3.9
Si3N
4 7-9
HfO2 ~30
ZrO2 ~25
Al2O
3 9-13
Y2O
3 11-17Ta
2O
5 25-45
La2O
3 21
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