<s>
Photolithography	B-Algorithm
is	O
a	O
process	O
in	O
removing	O
select	O
portions	O
of	O
thin	O
films	O
used	O
in	O
microfabrication	O
.	O
</s>
<s>
Microfabrication	O
is	O
the	O
production	O
of	O
parts	O
on	O
the	O
micro	O
-	O
and	O
nano	O
-	O
scale	O
,	O
typically	O
on	O
the	O
surface	O
of	O
silicon	B-Architecture
wafers	I-Architecture
,	O
for	O
the	O
production	O
of	O
integrated	O
circuits	O
,	O
microelectromechanical	B-Architecture
systems	I-Architecture
(	O
MEMS	B-Architecture
)	O
,	O
solar	O
cells	O
,	O
and	O
other	O
devices	O
.	O
</s>
<s>
Photolithography	B-Algorithm
makes	O
this	O
process	O
possible	O
through	O
the	O
combined	O
use	O
of	O
hexamethyldisilazane	O
(	O
HMDS	O
)	O
,	O
photoresist	O
(	O
positive	O
or	O
negative	O
)	O
,	O
spin	O
coating	O
,	O
photomask	B-Algorithm
,	O
an	O
exposure	O
system	O
and	O
other	O
various	O
chemicals	O
.	O
</s>
<s>
By	O
carefully	O
manipulating	O
these	O
factors	O
it	O
is	O
possible	O
to	O
create	O
nearly	O
any	O
geometry	O
microstructure	O
on	O
the	O
surface	O
of	O
a	O
silicon	B-Architecture
wafer	I-Architecture
.	O
</s>
<s>
The	O
chemical	O
interaction	O
between	O
all	O
the	O
different	O
components	O
and	O
the	O
surface	O
of	O
the	O
silicon	B-Architecture
wafer	I-Architecture
makes	O
photolithography	B-Algorithm
an	O
interesting	O
chemistry	O
problem	O
.	O
</s>
<s>
Current	O
engineering	O
has	O
been	O
able	O
to	O
create	O
features	O
on	O
the	O
surface	O
of	O
silicon	B-Architecture
wafers	I-Architecture
between	O
1	O
and	O
100μm	O
.	O
</s>
<s>
Silicon	B-Architecture
wafers	I-Architecture
are	O
cut	O
from	O
a	O
solid	O
ingot	O
of	O
nearly-pure	O
(	O
99.9999999	O
%	O
)	O
silicon	O
.	O
</s>
<s>
Using	O
miller	O
indexes	O
to	O
denote	O
the	O
different	O
plane	O
orientations	O
,	O
the	O
(	O
1	O
,	O
0	O
,	O
0	O
)	O
and	O
the	O
(	O
1	O
,	O
1	O
,	O
1	O
)	O
faces	O
are	O
typically	O
used	O
in	O
silicon	B-Architecture
wafers	I-Architecture
(	O
see	O
image	O
)	O
.	O
</s>
<s>
The	O
silicon	O
ingot	O
is	O
oriented	O
and	O
cut	O
along	O
one	O
these	O
planes	O
to	O
expose	O
that	O
surface	O
for	O
processing	O
through	O
photolithography	B-Algorithm
.	O
</s>
<s>
The	O
reason	O
to	O
use	O
either	O
of	O
these	O
planar	O
faces	O
depends	O
on	O
the	O
application	O
for	O
which	O
the	O
silicon	B-Architecture
wafer	I-Architecture
will	O
be	O
used	O
,	O
or	O
how	O
it	O
will	O
be	O
processed	O
.	O
</s>
<s>
In	O
photolithography	B-Algorithm
,	O
photoresist	O
compounds	O
are	O
used	O
create	O
a	O
mask	O
on	O
the	O
surface	O
of	O
a	O
silicon	B-Architecture
wafer	I-Architecture
.	O
</s>
<s>
The	O
mask	O
allows	O
for	O
precise	O
control	O
over	O
the	O
doping	O
and	O
etching	O
processes	O
used	O
to	O
form	O
devices	O
on	O
silicon	B-Architecture
wafers	I-Architecture
.	O
</s>
<s>
The	O
compound	O
is	O
applied	O
to	O
the	O
silicon	B-Architecture
wafer	I-Architecture
in	O
liquid	O
form	O
and	O
polymerization	O
is	O
controlled	O
through	O
exposure	O
to	O
light	O
.	O
</s>
<s>
In	O
order	O
to	O
reliably	O
create	O
small	O
features	O
,	O
the	O
surface	O
of	O
the	O
silicon	B-Architecture
wafer	I-Architecture
must	O
be	O
made	O
hydrophobic	O
to	O
promote	O
photoresist	O
adhesion	O
.	O
</s>
<s>
The	O
surface	O
energy	O
characteristics	O
of	O
the	O
silicon	B-Architecture
wafer	I-Architecture
can	O
be	O
measured	O
by	O
placing	O
a	O
drop	O
of	O
deionized	O
water	O
or	O
ethylene	O
glycol	O
and	O
measuring	O
the	O
contact	O
angle	O
of	O
the	O
droplet	O
.	O
</s>
<s>
Positive	O
resists	O
are	O
better	O
for	O
producing	O
small	O
feature	O
size	O
,	O
but	O
does	O
not	O
adhere	O
to	O
silicon	B-Architecture
wafers	I-Architecture
as	O
well	O
as	O
negative	O
resist	O
.	O
</s>
<s>
A	O
common	O
method	O
to	O
increase	O
adhesion	O
of	O
photoresist	O
on	O
the	O
silicon	B-Architecture
wafer	I-Architecture
surface	O
is	O
to	O
treat	O
the	O
wafer	B-Architecture
with	O
Hexamethyldisilazane	O
(	O
HMDS	O
)	O
.	O
</s>
<s>
A	O
new	O
silicon	B-Architecture
wafer	I-Architecture
has	O
a	O
polar	O
surface	O
and	O
has	O
some	O
adsorbed	O
water	O
on	O
the	O
surface	O
.	O
</s>
<s>
The	O
wafer	B-Architecture
can	O
undergo	O
a	O
dehydration	O
bake	O
to	O
remove	O
adsorbed	O
water	O
,	O
and	O
followed	O
by	O
the	O
HMDS	O
treatment	O
also	O
known	O
as	O
the	O
priming	O
stage	O
.	O
</s>
<s>
HMDS	O
can	O
be	O
dispensed	O
in	O
liquid	O
form	O
onto	O
the	O
wafer	B-Architecture
using	O
a	O
syringe	O
while	O
the	O
wafer	B-Architecture
is	O
attached	O
to	O
a	O
vacuum	O
chuck	O
in	O
a	O
spin	O
coater	O
.	O
</s>
<s>
The	O
HMDS	O
promotes	O
good	O
photoresist-to-wafer	O
adhesion	O
because	O
it	O
ensures	O
the	O
wafer	B-Architecture
surface	O
is	O
hydrophobic	O
.	O
</s>
<s>
The	O
more	O
viscous	O
the	O
solvent	O
the	O
thicker	O
the	O
film	O
will	O
be	O
and	O
the	O
faster	O
the	O
wafer	B-Architecture
is	O
spun	O
the	O
thinner	O
the	O
film	O
will	O
be	O
.	O
</s>
<s>
A	O
common	O
issue	O
in	O
spin	O
coating	O
is	O
a	O
"	O
beading	O
"	O
up	O
of	O
solvent	O
at	O
the	O
edge	O
of	O
the	O
silicon	B-Architecture
wafer	I-Architecture
.	O
</s>
<s>
A	O
process	O
known	O
as	O
backside	O
washing	O
is	O
most	O
commonly	O
used	O
to	O
spin	O
this	O
bead	O
off	O
of	O
the	O
wafer	B-Architecture
.	O
</s>
<s>
Currently	O
engineers	O
and	O
scientist	O
are	O
working	O
on	O
figuring	O
out	O
a	O
better	O
way	O
to	O
apply	O
photoresist	O
to	O
the	O
substrate	B-Architecture
of	O
a	O
silicon	B-Architecture
wafer	I-Architecture
.	O
</s>
<s>
Spin	O
coating	O
can	O
result	O
in	O
issues	O
with	O
wafer	B-Architecture
topography	O
such	O
as	O
non-round	O
substrates	B-Architecture
,	O
oversized	O
substrates	B-Architecture
,	O
fragile	O
substrates	B-Architecture
and	O
material	O
consumption	O
.	O
</s>
<s>
By	O
spraying	O
the	O
photoresist	O
onto	O
the	O
surface	O
of	O
the	O
wafer	B-Architecture
as	O
opposed	O
to	O
spin	O
coating	O
the	O
photoresist	O
,	O
a	O
lot	O
of	O
photoresist	O
is	O
saved	O
,	O
and	O
smaller	O
and	O
more	O
precise	O
parts	O
can	O
be	O
made	O
.	O
</s>
