<s>
In	O
microfabrication	O
,	O
thermal	B-Algorithm
oxidation	I-Algorithm
is	O
a	O
way	O
to	O
produce	O
a	O
thin	O
layer	O
of	O
oxide	O
(	O
usually	O
silicon	O
dioxide	O
)	O
on	O
the	O
surface	O
of	O
a	O
wafer	B-Architecture
.	O
</s>
<s>
The	O
technique	O
forces	O
an	O
oxidizing	O
agent	O
to	O
diffuse	O
into	O
the	O
wafer	B-Architecture
at	O
high	O
temperature	O
and	O
react	O
with	O
it	O
.	O
</s>
<s>
The	O
rate	O
of	O
oxide	O
growth	O
is	O
often	O
predicted	O
by	O
the	O
Deal	B-Algorithm
–	I-Algorithm
Grove	I-Algorithm
model	I-Algorithm
.	O
</s>
<s>
Thermal	B-Algorithm
oxidation	I-Algorithm
may	O
be	O
applied	O
to	O
different	O
materials	O
,	O
but	O
most	O
commonly	O
involves	O
the	O
oxidation	O
of	O
silicon	O
substrates	B-Architecture
to	O
produce	O
silicon	O
dioxide	O
.	O
</s>
<s>
Thermal	B-Algorithm
oxidation	I-Algorithm
of	O
silicon	O
is	O
usually	O
performed	O
at	O
a	O
temperature	O
between	O
800	O
and	O
1200	O
°C	B-Operating_System
,	O
resulting	O
in	O
so	O
called	O
High	B-Algorithm
Temperature	I-Algorithm
Oxide	I-Algorithm
layer	O
(	O
HTO	O
)	O
.	O
</s>
<s>
Thermal	B-Algorithm
oxide	I-Algorithm
incorporates	O
silicon	O
consumed	O
from	O
the	O
substrate	B-Architecture
and	O
oxygen	O
supplied	O
from	O
the	O
ambient	O
.	O
</s>
<s>
Thus	O
,	O
it	O
grows	O
both	O
down	O
into	O
the	O
wafer	B-Architecture
and	O
up	O
out	O
of	O
it	O
.	O
</s>
<s>
According	O
to	O
the	O
commonly	O
used	O
Deal-Grove	B-Algorithm
model	I-Algorithm
,	O
the	O
time	O
τ	O
required	O
to	O
grow	O
an	O
oxide	O
of	O
thickness	O
Xo	O
,	O
at	O
a	O
constant	O
temperature	O
,	O
on	O
a	O
bare	O
silicon	O
surface	O
,	O
is	O
:	O
</s>
<s>
This	O
model	O
has	O
further	O
been	O
adapted	O
to	O
account	O
for	O
self-limiting	O
oxidation	O
processes	O
,	O
as	O
used	O
for	O
the	O
fabrication	O
and	O
morphological	O
design	O
of	O
Si	B-Algorithm
nanowires	I-Algorithm
and	O
other	O
nanostructures	O
.	O
</s>
<s>
If	O
a	O
wafer	B-Architecture
that	O
already	O
contains	O
oxide	O
is	O
placed	O
in	O
an	O
oxidizing	O
ambient	O
,	O
this	O
equation	O
must	O
be	O
modified	O
by	O
adding	O
a	O
corrective	O
term	O
τ	O
,	O
the	O
time	O
that	O
would	O
have	O
been	O
required	O
to	O
grow	O
the	O
pre-existing	O
oxide	O
under	O
current	O
conditions	O
.	O
</s>
<s>
Most	O
thermal	B-Algorithm
oxidation	I-Algorithm
is	O
performed	O
in	O
furnaces	O
,	O
at	O
temperatures	O
between	O
800	O
and	O
1200°C	O
.	O
</s>
<s>
A	O
single	O
furnace	O
accepts	O
many	O
wafers	B-Architecture
at	O
the	O
same	O
time	O
,	O
in	O
a	O
specially	O
designed	O
quartz	B-General_Concept
rack	O
(	O
called	O
a	O
"	O
boat	O
"	O
)	O
.	O
</s>
<s>
Historically	O
,	O
the	O
boat	O
entered	O
the	O
oxidation	O
chamber	O
from	O
the	O
side	O
(	O
this	O
design	O
is	O
called	O
"	O
horizontal	O
"	O
)	O
,	O
and	O
held	O
the	O
wafers	B-Architecture
vertically	O
,	O
beside	O
each	O
other	O
.	O
</s>
<s>
However	O
,	O
many	O
modern	O
designs	O
hold	O
the	O
wafers	B-Architecture
horizontally	O
,	O
above	O
and	O
below	O
each	O
other	O
,	O
and	O
load	O
them	O
into	O
the	O
oxidation	O
chamber	O
from	O
below	O
.	O
</s>
<s>
Unlike	O
horizontal	O
furnaces	O
,	O
in	O
which	O
falling	O
dust	O
can	O
contaminate	O
any	O
wafer	B-Architecture
,	O
vertical	O
furnaces	O
use	O
enclosed	O
cabinets	O
with	O
air	O
filtration	O
systems	O
to	O
prevent	O
dust	O
from	O
reaching	O
the	O
wafers	B-Architecture
.	O
</s>
<s>
Vertical	O
furnaces	O
also	O
eliminate	O
an	O
issue	O
that	O
plagued	O
horizontal	O
furnaces	O
:	O
non-uniformity	O
of	O
grown	O
oxide	O
across	O
the	O
wafer	B-Architecture
.	O
</s>
<s>
As	O
the	O
wafers	B-Architecture
lie	O
vertically	O
in	O
the	O
tube	O
the	O
convection	O
and	O
the	O
temperature	O
gradient	O
with	O
it	O
causes	O
the	O
top	O
of	O
the	O
wafer	B-Architecture
to	O
have	O
a	O
thicker	O
oxide	O
than	O
the	O
bottom	O
of	O
the	O
wafer	B-Architecture
.	O
</s>
<s>
Vertical	O
furnaces	O
solve	O
this	O
problem	O
by	O
having	O
wafer	B-Architecture
sitting	O
horizontally	O
,	O
and	O
then	O
having	O
the	O
gas	O
flow	O
in	O
the	O
furnace	O
flowing	O
from	O
top	O
to	O
bottom	O
,	O
significantly	O
damping	O
any	O
thermal	O
convections	O
.	O
</s>
<s>
Vertical	O
furnaces	O
also	O
allow	O
the	O
use	O
of	O
load	O
locks	O
to	O
purge	O
the	O
wafers	B-Architecture
with	O
nitrogen	O
before	O
oxidation	O
to	O
limit	O
the	O
growth	O
of	O
native	O
oxide	O
on	O
the	O
Si	O
surface	O
.	O
</s>
<s>
Mobile	O
metal	O
ions	O
can	O
degrade	O
performance	O
of	O
MOSFETs	B-Architecture
(	O
sodium	O
is	O
of	O
particular	O
concern	O
)	O
.	O
</s>
<s>
Thermal	B-Algorithm
oxidation	I-Algorithm
can	O
be	O
performed	O
on	O
selected	O
areas	O
of	O
a	O
wafer	B-Architecture
,	O
and	O
blocked	O
on	O
others	O
.	O
</s>
<s>
This	O
process	O
,	O
first	O
developed	O
at	O
Philips	O
,	O
is	O
commonly	O
referred	O
to	O
as	O
the	O
local	O
oxidation	O
of	O
silicon	O
(	O
LOCOS	B-Application
)	O
process	O
.	O
</s>
<s>
A	O
<100> wafer (see Miller indices )  oxidizes more slowly than a <111>	O
wafer	B-Architecture
,	O
but	O
produces	O
an	O
electrically	O
cleaner	O
oxide	O
interface	O
.	O
</s>
<s>
Thermal	B-Algorithm
oxidation	I-Algorithm
of	O
any	O
variety	O
produces	O
a	O
higher-quality	O
oxide	O
,	O
with	O
a	O
much	O
cleaner	O
interface	O
,	O
than	O
chemical	B-Algorithm
vapor	I-Algorithm
deposition	I-Algorithm
of	O
oxide	O
resulting	O
in	O
low	O
temperature	O
oxide	O
layer	O
(	O
reaction	O
of	O
TEOS	O
at	O
about	O
600°C	O
)	O
.	O
</s>
<s>
However	O
,	O
the	O
high	O
temperatures	O
required	O
to	O
produce	O
High	B-Algorithm
Temperature	I-Algorithm
Oxide	I-Algorithm
(	O
HTO	O
)	O
restrict	O
its	O
usability	O
.	O
</s>
<s>
For	O
instance	O
,	O
in	O
MOSFET	B-Architecture
processes	O
,	O
thermal	B-Algorithm
oxidation	I-Algorithm
is	O
never	O
performed	O
after	O
the	O
doping	O
for	O
the	O
source	O
and	O
drain	O
terminals	O
is	O
performed	O
,	O
because	O
it	O
would	O
disturb	O
the	O
placement	O
of	O
the	O
dopants	O
.	O
</s>
