2. Focus
• Materials since the midterm
• Be prepared to cover basic concepts from
material before the midterm
– Except for implantation (including diffusion),
not calculation intensive aspects from pre-
midterm material
– Crystal growth, wafering, etc
• You may bring two 8.5”x11” sheets of
paper with eqns, etc to the exam.
5. Mass Analyzer
- there may be many species in the beam generated. eg. BF3 B+, BF2
+,
BF+, F+ etc.
also may have been contamination ions O2
+, C+
- need to filter out unwanted species
- pass ions through a magnetic field.
- consider a singly charged (X+) particle with mass (m) and velocity (v)
- Only species with a certain mass (actually
mass / charge ratio) will possess the correct
curvature to travel through slit
2
1
2
2
2
1
2
1
q
mV
B
qB
mv
r
qV
mv
r
mv
ma
B
v
q
F
6. Amorphization
• A critical dose exists above which the
damage is so great that the material
becomes amorphous
• Good or bad?
species)
(implant
energy
ion
incident
(target)
energy
nt
displaceme
density
target
straggle
5
0
E
E
N
R
E
E
N
R
N
d
p
o
d
p
crit
7. Amorphization and annealing
• Amorphous Si
– “liquid” phase at low temperature
– Recrystallizes at 500-600 °C
– “liquid” nucleates at a/c interface and spreads up
toward surface
– Very high activation energies are achieved
• Some residual defect features remain
Wolf and Tauber
EOR damage
8. Dopant profile
- What happens to dopant distribution during anneal?
after implant:
2
2
1
exp
2
)
(
p
p
p
o
R
R
x
R
Q
x
C
2
2
2
2
2
2
2
exp
2
2
exp
2
2
)
(
Dt
R
R
x
R
R
Dt
R
R
x
R
R
DT
R
Q
x
C
p
p
p
p
p
p
p
p
o
p
after annealing:
?
9. Annealing and dopant diffusion
• Diffusion also may
occur during
annealing
• Predict dopant
profile as a function
of annealing
10. Mask Effectiveness
• Shaded region
– Fraction that penetrates through mask
Mask Semiconductor
d p
p
p
o
dx
R
R
x
R
Q
Q
2
2
1
exp
2
p
p
o
d
R
R
d
erfc
Q
Q
d
erfc
dx
x
2
2
1
2
exp 2
12. 12
The Deal-Grove model for oxidation
Cg Cs Co
Ci
F1
F2
F3
Fluxes:
F1 -- oxidant flux to the surface
F2 -- oxidant flux through the
oxide
F3 -- oxidant flux reacting with
the silicon substrate
Concentrations:
Cg -- gas-phase concentration
Cs -- oxygen conc. at surface
Co -- conc. inside oxide
surface
Ci -- conc. at Si/SiO2 interface
xox
15. 15
Examples
• Now, what about 60 min, 1000 °C dry oxidation of (001) Si
with a pre-existing oxide layer of 90 nm? How much thicker
will the oxide become?
90 nm + 60 nm = 150 nm ??? NO
Start with pre-existing oxide
t (from the eqn) is the time it
would have taken to grow the
pre-existing oxide. 2 hrs
according to the graph
Add one hour (from 2 to 3 hrs)
Total thickness is 126 nm
Added thickness is 36 nm
0.126 mm
16. Bandgap Engineering and Strain
• How much strain can be incorporated into
layer before defects form?
• Matthews-Blakeslee Critical Thickness
– Definitions of terms
1
ln
1
4
4
1
b
h
f
b
h c
c
18. N
c
k
h
k
h
R
g
s
g
s
g
CVD reaction kinetics
hg >> ks surface reaction limited
growth rate ks cg
hg << ks diffusion limited
growth rate hg cg
diffusion
controlled
surface reaction
controlled
1/T
growth
rate
R
• At “very high” temperatures,
thermodynamics plays a role
• At high temperatures, the growth rate is
usually diffusion controlled
• Mass transport in the gas phase is very
sensitive to pressure (as p, D so
R)
• Surface reaction rate limited growth is
very sensitive to temperature variations
thermodynamics
19. N
c
k
h
k
h
R g
s
g
s
g
• The steady-state flux can be converted to a growth rate:
number density of atoms
(Si: 51022 cm-3)
R [=] cm sec-1 (i.e. a growth velocity or growth rate); R
is obviously sensitive to hg, ks and cg
Kinetics of epitaxial growth
Temperature dependence of growth
rates of silicon from various
chlorosilanes
lower right: surface reaction limited
upper left: mass transfer limited
Growth rates:
Source determines
thermodynamic contribution
SiH4 vs. SiCl4
20. AlGaAs growth by the halide or hydride processes
• In general, one can determine the alloy
composition of a ternary alloy by knowing the
growth rate of each component:
This is an illustration of the effect:
Using the contrived graph… at 740 °C
AlAs growth rate is 40 µm/h
GaAs growth rate Is 32 µm/h
Composition is
AlAs mole fraction: 40/(40+32)
GaAs mole fraction: 32/(40+32)
Al0.56Ga0.44As
GaAs
AlAs
740 °C
21. MOCVD growth of GaAs
• The MOCVD growth of GaAs from TMG and AsH3
exhibits three distinct regimes:
• Low temperature growth rate
• limited by surface
decomposition of the
reactants
• Mid-range temperature regime
• limited by gas-phase
transport of reactants to
the surface
• High temperatures
• decrease in growth rate
due to desorption of the
reactants (low driving force)
and parasitic gas-phase
reactions
1
2
3
23. RHEED measurement of growth rates
The intensity of features in a RHEED pattern either
the specular spot or the diffracted streaks) vary
periodically with fractional coverage, thus allowing
the growth rate to be determined in situ
One monolayer
26. Positive photoresist
• Matrix:
– Novolac (similar to what holds together sheets of plywood)
• Class of phenol – formaldehyde resins
• PAC:
– (d)iazo(n)aphtho(q)uinone (DNQ)
• DNQ stabilizes (cross-links) Novolac except in the presence of
light
– Absorbs in vis-UV and breaks down resin
• Develop in a weak base
• Neutralize and wash away
• Small dimensions (no swelling)
– Common for fine features
27.
28. • Depth of focus: the distance along the optic axis the
wafer can be moved and still be kept in focus:
2
(NA)
Factors affecting focus and resolution
i
ii
With high NA – only focus on I or ii, not both
Planar surface better with high NA
29. Immersion Lithography
• Change ‘n’?
• Water at interface between lens and PR
• Also 2X improvement in depth of focus
• Keep bubbles, etc out of liquid
• Use 193 nm laser (ArF)
• http://www.icknowledge.com/misc_technology/Immersion%20Lithography.pdf
• “One issue that is likely to be significant for immersion lithography is temperature
control. Variations in temperature cause variations in n and therefore image
distortion. Maintaining temperature uniformity with a rapidly moving stage and a
pulsed laser passing through the fluid will likely be a significant challenge
min
61
.
0
sin
61
.
0 W
NA
n
31. GaAs examples
• Generally:
H
O
As
O
Ga
O
H
h
GaAs
O
H
O
As
H
AsO
H
AsO
OH
As
O
H
O
Ga
OH
Ga
OH
Ga
OH
Ga
As
Ga
h
GaAs
12
6
12
2
2
2
2
2
3
2
3
6
3
2
3
2
2
2
3
2
2
2
3
2
3
2
3
3
3
3
3
Dissolve oxides in
acid or base
solutions
34. CVD applications
• Insulating layers
– Planarization
– Encapsulate metal layers
• SiO2
– TEOS: tetraethylorthosilane
– Liquid at room temperature, non-toxic, high vapor
pressure
– 650 – 750 °C
– 400 °C
• 4% ozone added to oxygen source
• Same step coverage as higher T process w/o ozone
O
H
CO
SiO
O
O
H
C
Si 2
2
2
2
4
5
2 10
8
12
35. SiO2 by CVD
• Oxidation of silane
• 450 °C
• O2 (or N2O) absorbs strongly on Si
surface, reacts with silane
– Step coverage is not as favorable
2
2
2
2
4
2
2
2
4
2
2
2
2
N
H
SiO
O
N
SiH
H
SiO
O
SiH
36. Selective deposition of tungsten
• Either lower driving force reaction or
• Nucleation is dependent on surface
• Process is self limiting
– 10 nm film
– Continue growth with addition of H2
Si
SiO2
4
6 3
2
3
2 SiF
W
Si
WF
HF
W
H
WF 6
3 2
6
