<s>
In	O
integrated	B-Architecture
circuit	I-Architecture
manufacturing	I-Architecture
,	O
photolithography	B-Algorithm
or	O
optical	B-Algorithm
lithography	I-Algorithm
is	O
a	O
general	O
term	O
used	O
for	O
techniques	O
that	O
use	O
light	O
to	O
produce	O
minutely	O
patterned	O
thin	O
films	O
of	O
suitable	O
materials	O
over	O
a	O
substrate	B-Architecture
,	O
such	O
as	O
a	O
silicon	B-Architecture
wafer	I-Architecture
,	O
to	O
protect	O
selected	O
areas	O
of	O
it	O
during	O
subsequent	O
etching	B-Algorithm
,	O
deposition	B-Algorithm
,	O
or	O
implantation	O
operations	O
.	O
</s>
<s>
Typically	O
,	O
ultraviolet	B-Application
light	O
is	O
used	O
to	O
transfer	O
a	O
geometric	O
design	O
from	O
an	O
optical	B-Algorithm
mask	I-Algorithm
to	O
a	O
light-sensitive	O
chemical	O
(	O
photoresist	O
)	O
coated	O
on	O
the	O
substrate	B-Architecture
.	O
</s>
<s>
Photolithography	B-Algorithm
processes	O
can	O
be	O
classified	O
according	O
to	O
the	O
type	O
of	O
light	O
used	O
,	O
such	O
as	O
ultraviolet	B-Application
,	O
deep	O
ultraviolet	B-Application
,	O
extreme	B-Algorithm
ultraviolet	I-Algorithm
,	O
or	O
X-ray	B-Algorithm
.	O
</s>
<s>
Photolithography	B-Algorithm
is	O
a	O
subclass	O
of	O
microlithography	O
,	O
the	O
general	O
term	O
for	O
processes	O
that	O
generate	O
patterned	O
thin	O
films	O
.	O
</s>
<s>
Other	O
technologies	O
in	O
this	O
broader	O
class	O
include	O
the	O
use	O
of	O
steerable	O
electron	O
beams	O
,	O
or	O
more	O
rarely	O
,	O
nanoimprinting	B-Algorithm
,	O
interference	B-Algorithm
,	O
magnetic	B-Algorithm
fields	I-Algorithm
,	O
or	O
scanning	O
probes	O
.	O
</s>
<s>
Photolithography	B-Algorithm
shares	O
some	O
fundamental	O
principles	O
with	O
photography	O
in	O
that	O
the	O
pattern	O
in	O
the	O
photoresist	O
is	O
created	O
by	O
exposing	O
it	O
to	O
light	O
—	O
either	O
directly	O
by	O
projection	B-Algorithm
through	O
a	O
lens	O
,	O
or	O
by	O
illuminating	O
a	O
mask	O
placed	O
directly	O
over	O
the	O
substrate	B-Architecture
,	O
as	O
in	O
contact	O
printing	O
.	O
</s>
<s>
The	O
name	O
originated	O
from	O
a	O
loose	O
analogy	O
with	O
the	O
traditional	O
photographic	O
method	O
of	O
producing	O
plates	O
for	O
lithographic	O
printing	O
on	O
paper	O
;	O
however	O
,	O
subsequent	O
stages	O
in	O
the	O
process	O
have	O
more	O
in	O
common	O
with	O
etching	B-Algorithm
than	O
with	O
traditional	O
lithography	O
.	O
</s>
<s>
Photolithography	B-Algorithm
is	O
the	O
most	O
common	O
method	O
for	O
semiconductor	B-Architecture
fabrication	I-Architecture
of	O
integrated	O
circuits	O
(	O
"	O
ICs	O
"	O
or	O
"	O
chips	O
"	O
)	O
,	O
such	O
as	O
solid-state	B-Device
memories	I-Device
and	O
microprocessors	B-Architecture
.	O
</s>
<s>
It	O
provides	O
precise	O
control	O
of	O
the	O
shape	O
and	O
size	O
of	O
the	O
objects	O
it	O
creates	O
and	O
can	O
create	O
patterns	O
over	O
an	O
entire	O
wafer	B-Architecture
in	O
a	O
single	O
step	O
,	O
quickly	O
and	O
with	O
relatively	O
low	O
cost	O
.	O
</s>
<s>
In	O
complex	O
integrated	O
circuits	O
,	O
a	O
wafer	B-Architecture
may	O
go	O
through	O
the	O
photolithographic	B-Algorithm
cycle	O
as	O
many	O
as	O
50	O
times	O
.	O
</s>
<s>
It	O
is	O
also	O
an	O
important	O
technique	O
for	O
the	O
fabrication	B-Architecture
of	O
microscopic	O
structures	O
in	O
general	O
,	O
such	O
as	O
microelectromechanical	B-Architecture
systems	I-Architecture
.	O
</s>
<s>
However	O
,	O
photolithography	B-Algorithm
cannot	O
be	O
used	O
to	O
produce	O
masks	O
on	O
surfaces	O
that	O
are	O
not	O
perfectly	O
flat	O
;	O
and	O
,	O
like	O
all	O
chip	O
manufacturing	O
processes	O
,	O
it	O
requires	O
extremely	O
clean	O
operating	O
conditions	O
.	O
</s>
<s>
As	O
suggested	O
by	O
the	O
name	O
compounded	O
from	O
them	O
,	O
photolithography	B-Algorithm
is	O
a	O
printing	O
method	O
(	O
originally	O
based	O
on	O
the	O
use	O
of	O
limestone	O
printing	O
plates	O
)	O
in	O
which	O
light	O
plays	O
an	O
essential	O
role	O
.	O
</s>
<s>
In	O
1958	O
,	O
during	O
the	O
IRE	O
Professional	O
Group	O
on	O
Electron	O
Devices	O
(	O
PGED	O
)	O
conference	O
in	O
Washington	O
,	O
D.C.	O
,	O
they	O
presented	O
the	O
first	O
paper	O
to	O
describe	O
the	O
fabrication	B-Architecture
of	O
transistors	O
using	O
photographic	O
techniques	O
and	O
adopted	O
the	O
term	O
"	O
photolithography	B-Algorithm
"	O
to	O
describe	O
the	O
process	O
,	O
marking	O
the	O
first	O
published	O
use	O
of	O
the	O
term	O
to	O
describe	O
semiconductor	O
device	O
patterning	O
.	O
</s>
<s>
Despite	O
the	O
fact	O
that	O
photolithography	B-Algorithm
of	O
electronic	O
components	O
concerns	O
etching	B-Algorithm
metal	O
duplicates	O
,	O
rather	O
than	O
etching	B-Algorithm
stone	O
to	O
produce	O
a	O
"	O
master	O
"	O
as	O
in	O
conventional	O
lithographic	O
printing	O
,	O
Lathrop	O
and	O
Nall	O
chose	O
the	O
term	O
"	O
photolithography	B-Algorithm
"	O
over	O
"	O
photoetching	O
"	O
because	O
the	O
former	O
sounded	O
"	O
high	O
tech.	O
"	O
</s>
<s>
A	O
year	O
after	O
the	O
conference	O
,	O
Lathrop	O
and	O
Nall	O
's	O
patent	O
on	O
photolithography	B-Algorithm
was	O
formally	O
approved	O
on	O
June	O
9	O
,	O
1959	O
.	O
</s>
<s>
Photolithography	B-Algorithm
would	O
later	O
contribute	O
to	O
the	O
development	O
of	O
the	O
first	O
semiconductor	O
ICs	O
as	O
well	O
as	O
the	O
first	O
microchips	O
.	O
</s>
<s>
A	O
single	O
iteration	O
of	O
photolithography	B-Algorithm
combines	O
several	O
steps	O
in	O
sequence	O
.	O
</s>
<s>
Modern	O
cleanrooms	O
use	O
automated	O
,	O
robotic	O
wafer	B-Architecture
track	O
systems	O
to	O
coordinate	O
the	O
process	O
.	O
</s>
<s>
The	O
photolithography	B-Algorithm
process	O
is	O
carried	O
out	O
by	O
the	O
wafer	B-Architecture
track	O
and	O
stepper/scanner	O
,	O
and	O
the	O
wafer	B-Architecture
track	O
system	O
and	O
the	O
stepper/scanner	O
are	O
installed	O
side	O
by	O
side	O
.	O
</s>
<s>
Wafer	B-Architecture
track	O
systems	O
have	O
been	O
replaced	O
by	O
wafer	B-Architecture
coater/developer	O
systems	O
,	O
which	O
perform	O
the	O
same	O
functions	O
.	O
</s>
<s>
If	O
organic	O
or	O
inorganic	O
contaminations	O
are	O
present	O
on	O
the	O
wafer	B-Architecture
surface	O
,	O
they	O
are	O
usually	O
removed	O
by	O
wet	O
chemical	O
treatment	O
,	O
e.g.	O
</s>
<s>
the	O
RCA	B-Algorithm
clean	I-Algorithm
procedure	O
based	O
on	O
solutions	O
containing	O
hydrogen	O
peroxide	O
.	O
</s>
<s>
The	O
wafer	B-Architecture
is	O
initially	O
heated	O
to	O
a	O
temperature	O
sufficient	O
to	O
drive	O
off	O
any	O
moisture	O
that	O
may	O
be	O
present	O
on	O
the	O
wafer	B-Architecture
surface	O
;	O
150°C	O
for	O
ten	O
minutes	O
is	O
sufficient	O
.	O
</s>
<s>
Wafers	B-Architecture
that	O
have	O
been	O
in	O
storage	O
must	O
be	O
chemically	O
cleaned	O
to	O
remove	O
contamination	O
.	O
</s>
<s>
A	O
liquid	O
or	O
gaseous	O
"	O
adhesion	O
promoter	O
"	O
,	O
such	O
as	O
Bis(trimethylsilyl )	O
amine	O
(	O
"	O
hexamethyldisilazane	O
"	O
,	O
HMDS	O
)	O
,	O
is	O
applied	O
to	O
promote	O
adhesion	O
of	O
the	O
photoresist	O
to	O
the	O
wafer	B-Architecture
.	O
</s>
<s>
The	O
surface	O
layer	O
of	O
silicon	O
dioxide	O
on	O
the	O
wafer	B-Architecture
reacts	O
with	O
HMDS	O
to	O
form	O
tri-methylated	O
silicon-dioxide	O
,	O
a	O
highly	O
water	O
repellent	O
layer	O
not	O
unlike	O
the	O
layer	O
of	O
wax	O
on	O
a	O
car	O
's	O
paint	O
.	O
</s>
<s>
This	O
water	O
repellent	O
layer	O
prevents	O
the	O
aqueous	O
developer	O
from	O
penetrating	O
between	O
the	O
photoresist	O
layer	O
and	O
the	O
wafer	B-Architecture
's	O
surface	O
,	O
thus	O
preventing	O
so-called	O
lifting	O
of	O
small	O
photoresist	O
structures	O
in	O
the	O
(	O
developing	O
)	O
pattern	O
.	O
</s>
<s>
The	O
wafer	B-Architecture
is	O
covered	O
with	O
photoresist	O
by	O
spin	O
coating	O
.	O
</s>
<s>
Thus	O
,	O
the	O
top	O
layer	O
of	O
resist	O
is	O
quickly	O
ejected	O
from	O
the	O
wafer	B-Architecture
's	O
edge	O
while	O
the	O
bottom	O
layer	O
still	O
creeps	O
slowly	O
radially	O
along	O
the	O
wafer	B-Architecture
.	O
</s>
<s>
The	O
photoresist-coated	O
wafer	B-Architecture
is	O
then	O
prebaked	O
to	O
drive	O
off	O
excess	O
photoresist	O
solvent	O
,	O
typically	O
at	O
90	O
to	O
100°C	O
for	O
30	O
to	O
60	O
seconds	O
on	O
a	O
hotplate	O
.	O
</s>
<s>
A	O
post-exposure	O
bake	O
(	O
PEB	O
)	O
is	O
performed	O
before	O
developing	O
,	O
typically	O
to	O
help	O
reduce	O
standing	O
wave	O
phenomena	O
caused	O
by	O
the	O
destructive	O
and	O
constructive	O
interference	B-Algorithm
patterns	O
of	O
the	O
incident	O
light	O
.	O
</s>
<s>
In	O
deep	B-Algorithm
ultraviolet	I-Algorithm
lithography	I-Algorithm
,	O
chemically	O
amplified	O
resist	O
(	O
CAR	O
)	O
chemistry	O
is	O
used	O
.	O
</s>
<s>
However	O
,	O
sodium	O
is	O
considered	O
an	O
extremely	O
undesirable	O
contaminant	O
in	O
MOSFET	B-Architecture
fabrication	B-Architecture
because	O
it	O
degrades	O
the	O
insulating	O
properties	O
of	O
gate	O
oxides	O
(	O
specifically	O
,	O
sodium	O
ions	O
can	O
migrate	O
in	O
and	O
out	O
of	O
the	O
gate	O
,	O
changing	O
the	O
threshold	O
voltage	O
of	O
the	O
transistor	O
and	O
making	O
it	O
harder	O
or	O
easier	O
to	O
turn	O
the	O
transistor	O
on	O
over	O
time	O
)	O
.	O
</s>
<s>
The	O
resulting	O
wafer	B-Architecture
is	O
then	O
"	O
hard-baked	O
"	O
if	O
a	O
non-chemically	O
amplified	O
resist	O
was	O
used	O
,	O
typically	O
at	O
120	O
to	O
180°C	O
for	O
20	O
to	O
30	O
minutes	O
.	O
</s>
<s>
The	O
hard	O
bake	O
solidifies	O
the	O
remaining	O
photoresist	O
,	O
to	O
make	O
a	O
more	O
durable	O
protecting	O
layer	O
in	O
future	O
ion	O
implantation	O
,	O
wet	O
chemical	O
etching	B-Algorithm
,	O
or	O
plasma	B-Algorithm
etching	I-Algorithm
.	O
</s>
<s>
From	O
preparation	O
until	O
this	O
step	O
,	O
the	O
photolithography	B-Algorithm
procedure	O
has	O
been	O
carried	O
out	O
by	O
two	O
machines	O
:	O
the	O
photolithography	B-Algorithm
stepper	O
or	O
scanner	O
,	O
and	O
the	O
coater/developer	O
.	O
</s>
<s>
In	O
etching	B-Algorithm
,	O
a	O
liquid	O
(	O
"	O
wet	O
"	O
)	O
or	O
plasma	O
(	O
"	O
dry	O
"	O
)	O
chemical	O
agent	O
removes	O
the	O
uppermost	O
layer	O
of	O
the	O
substrate	B-Architecture
in	O
the	O
areas	O
that	O
are	O
not	O
protected	O
by	O
photoresist	O
.	O
</s>
<s>
In	O
semiconductor	B-Architecture
fabrication	I-Architecture
,	O
dry	O
etching	B-Algorithm
techniques	O
are	O
generally	O
used	O
,	O
as	O
they	O
can	O
be	O
made	O
anisotropic	O
,	O
in	O
order	O
to	O
avoid	O
significant	O
undercutting	O
of	O
the	O
photoresist	O
pattern	O
.	O
</s>
<s>
Wet	O
etch	O
processes	O
are	O
generally	O
isotropic	O
in	O
nature	O
,	O
which	O
is	O
often	O
indispensable	O
for	O
microelectromechanical	B-Architecture
systems	I-Architecture
,	O
where	O
suspended	O
structures	O
must	O
be	O
"	O
released	O
"	O
from	O
the	O
underlying	O
layer	O
.	O
</s>
<s>
The	O
development	O
of	O
low-defectivity	O
anisotropic	O
dry-etch	O
process	O
has	O
enabled	O
the	O
ever-smaller	O
features	O
defined	O
photolithographically	O
in	O
the	O
resist	O
to	O
be	O
transferred	O
to	O
the	O
substrate	B-Architecture
material	O
.	O
</s>
<s>
After	O
a	O
photoresist	O
is	O
no	O
longer	O
needed	O
,	O
it	O
must	O
be	O
removed	O
from	O
the	O
substrate	B-Architecture
.	O
</s>
<s>
This	O
usually	O
requires	O
a	O
liquid	O
"	O
resist	O
stripper	O
"	O
,	O
which	O
chemically	O
alters	O
the	O
resist	O
so	O
that	O
it	O
no	O
longer	O
adheres	O
to	O
the	O
substrate	B-Architecture
.	O
</s>
<s>
This	O
process	O
is	O
called	O
ashing	O
and	O
resembles	O
dry	O
etching	B-Algorithm
.	O
</s>
<s>
Exposure	O
systems	O
typically	O
produce	O
an	O
image	O
on	O
the	O
wafer	B-Architecture
using	O
a	O
photomask	B-Algorithm
.	O
</s>
<s>
The	O
photomask	B-Algorithm
blocks	O
light	O
in	O
some	O
areas	O
and	O
lets	O
it	O
pass	O
in	O
others	O
.	O
</s>
<s>
(	O
Maskless	B-Algorithm
lithography	I-Algorithm
projects	O
a	O
precise	O
beam	O
directly	O
onto	O
the	O
wafer	B-Architecture
without	O
using	O
a	O
mask	O
,	O
but	O
it	O
is	O
not	O
widely	O
used	O
in	O
commercial	O
processes	O
.	O
)	O
</s>
<s>
Exposure	O
systems	O
may	O
be	O
classified	O
by	O
the	O
optics	O
that	O
transfer	O
the	O
image	O
from	O
the	O
mask	O
to	O
the	O
wafer	B-Architecture
.	O
</s>
<s>
Photolithography	B-Algorithm
produces	O
better	O
thin	O
film	O
transistor	O
structures	O
than	O
printed	O
electronics	O
,	O
due	O
to	O
smoother	O
printed	O
layers	O
,	O
less	O
wavy	O
patterns	O
,	O
and	O
more	O
accurate	O
drain-source	O
electrode	O
registration	O
.	O
</s>
<s>
A	O
contact	O
printer	O
,	O
the	O
simplest	O
exposure	O
system	O
,	O
puts	O
a	O
photomask	B-Algorithm
in	O
direct	O
contact	O
with	O
the	O
wafer	B-Architecture
and	O
exposes	O
it	O
to	O
a	O
uniform	O
light	O
.	O
</s>
<s>
A	O
proximity	O
printer	O
puts	O
a	O
small	O
gap	O
between	O
the	O
photomask	B-Algorithm
and	O
wafer	B-Architecture
.	O
</s>
<s>
In	O
both	O
cases	O
,	O
the	O
mask	O
covers	O
the	O
entire	O
wafer	B-Architecture
,	O
and	O
simultaneously	O
patterns	O
every	O
die	O
.	O
</s>
<s>
Contact	O
printing	O
is	O
liable	O
to	O
damage	O
both	O
the	O
mask	O
and	O
the	O
wafer	B-Architecture
,	O
and	O
this	O
was	O
the	O
primary	O
reason	O
it	O
was	O
abandoned	O
for	O
high	O
volume	O
production	O
.	O
</s>
<s>
Both	O
contact	O
and	O
proximity	O
lithography	O
require	O
the	O
light	O
intensity	O
to	O
be	O
uniform	O
across	O
an	O
entire	O
wafer	B-Architecture
,	O
and	O
the	O
mask	O
to	O
align	O
precisely	O
to	O
features	O
already	O
on	O
the	O
wafer	B-Architecture
.	O
</s>
<s>
As	O
modern	O
processes	O
use	O
increasingly	O
large	O
wafers	B-Architecture
,	O
these	O
conditions	O
become	O
increasingly	O
difficult	O
.	O
</s>
<s>
Hence	O
,	O
except	O
for	O
projection	B-Algorithm
lithography	O
(	O
see	O
below	O
)	O
,	O
contact	O
printing	O
offers	O
the	O
best	O
resolution	O
,	O
because	O
its	O
gap	O
distance	O
is	O
approximately	O
zero	O
(	O
neglecting	O
the	O
thickness	O
of	O
the	O
photoresist	O
itself	O
)	O
.	O
</s>
<s>
In	O
addition	O
,	O
nanoimprint	B-Algorithm
lithography	I-Algorithm
may	O
revive	O
interest	O
in	O
this	O
familiar	O
technique	O
,	O
especially	O
since	O
the	O
cost	O
of	O
ownership	O
is	O
expected	O
to	O
be	O
low	O
;	O
however	O
,	O
the	O
shortcomings	O
of	O
contact	O
printing	O
discussed	O
above	O
remain	O
as	O
challenges	O
.	O
</s>
<s>
Very-large-scale	O
integration	O
(	O
VLSI	O
)	O
lithography	O
uses	O
projection	B-Algorithm
systems	O
.	O
</s>
<s>
Unlike	O
contact	O
or	O
proximity	O
masks	O
,	O
which	O
cover	O
an	O
entire	O
wafer	B-Architecture
,	O
projection	B-Algorithm
masks	O
(	O
known	O
as	O
"	O
reticles	O
"	O
)	O
show	O
only	O
one	O
die	O
or	O
an	O
array	O
of	O
dies	O
(	O
known	O
as	O
a	O
"	O
field	O
"	O
)	O
.	O
</s>
<s>
Projection	B-Algorithm
exposure	O
systems	O
(	O
steppers	O
or	O
scanners	O
)	O
project	O
the	O
mask	O
onto	O
the	O
wafer	B-Architecture
many	O
times	O
to	O
create	O
the	O
complete	O
pattern	O
.	O
</s>
<s>
The	O
difference	O
between	O
steppers	O
and	O
scanners	O
is	O
that	O
,	O
during	O
exposure	O
,	O
a	O
scanner	O
moves	O
the	O
photomask	B-Algorithm
and	O
the	O
wafer	B-Architecture
simultaneously	O
,	O
while	O
a	O
stepper	O
only	O
moves	O
the	O
wafer	B-Architecture
.	O
</s>
<s>
Mask	O
aligners	O
preceded	O
steppers	O
and	O
do	O
not	O
move	O
the	O
photomask	B-Algorithm
nor	O
the	O
wafer	B-Architecture
during	O
exposure	O
.	O
</s>
<s>
Immersion	B-Algorithm
lithography	I-Algorithm
scanners	O
use	O
a	O
layer	O
of	O
Ultrapure	O
water	O
between	O
the	O
lens	O
and	O
the	O
wafer	B-Architecture
to	O
increase	O
resolution	O
.	O
</s>
<s>
An	O
alternative	O
to	O
photolithography	B-Algorithm
is	O
nanoimprint	B-Algorithm
lithography	I-Algorithm
.	O
</s>
<s>
This	O
data	O
file	O
is	O
converted	O
to	O
a	O
series	O
of	O
polygons	O
and	O
written	O
onto	O
a	O
square	O
of	O
fused	O
quartz	O
substrate	B-Architecture
covered	O
with	O
a	O
layer	O
of	O
chromium	B-Language
using	O
a	O
photolithographic	B-Algorithm
process	O
.	O
</s>
<s>
A	O
laser	O
beam	O
(	O
laser	O
writer	O
)	O
or	O
a	O
beam	O
of	O
electrons	O
(	O
e-beam	O
writer	O
)	O
is	O
used	O
to	O
expose	O
the	O
pattern	O
defined	O
by	O
the	O
data	O
file	O
and	O
travels	O
over	O
the	O
surface	O
of	O
the	O
substrate	B-Architecture
in	O
either	O
a	O
vector	O
or	O
raster	O
scan	O
manner	O
.	O
</s>
<s>
The	O
ability	O
to	O
project	O
a	O
clear	O
image	O
of	O
a	O
small	O
feature	O
onto	O
the	O
wafer	B-Architecture
is	O
limited	O
by	O
the	O
wavelength	O
of	O
the	O
light	O
that	O
is	O
used	O
,	O
and	O
the	O
ability	O
of	O
the	O
reduction	O
lens	O
system	O
to	O
capture	O
enough	O
diffraction	O
orders	O
from	O
the	O
illuminated	O
mask	O
.	O
</s>
<s>
The	O
minimum	O
feature	O
size	O
that	O
a	O
projection	B-Algorithm
system	O
can	O
print	O
is	O
given	O
approximately	O
by	O
:	O
</s>
<s>
The	O
minimum	O
feature	O
size	O
can	O
be	O
reduced	O
by	O
decreasing	O
this	O
coefficient	O
through	O
computational	B-Algorithm
lithography	I-Algorithm
.	O
</s>
<s>
The	O
depth	O
of	O
focus	O
restricts	O
the	O
thickness	O
of	O
the	O
photoresist	O
and	O
the	O
depth	O
of	O
the	O
topography	O
on	O
the	O
wafer	B-Architecture
.	O
</s>
<s>
Chemical	B-Algorithm
mechanical	I-Algorithm
polishing	I-Algorithm
is	O
often	O
used	O
to	O
flatten	O
topography	O
before	O
high-resolution	O
lithographic	O
steps	O
.	O
</s>
<s>
The	O
image	O
of	O
two	O
points	O
separated	O
by	O
less	O
than	O
1.22	O
wavelength/NA	O
will	O
not	O
maintain	O
that	O
separation	O
but	O
will	O
be	O
larger	O
due	O
to	O
the	O
interference	B-Algorithm
between	O
the	O
Airy	O
discs	O
of	O
the	O
two	O
points	O
.	O
</s>
<s>
As	O
light	O
consists	O
of	O
photons	B-Application
,	O
at	O
low	O
doses	O
the	O
image	O
quality	O
ultimately	O
depends	O
on	O
the	O
photon	B-Application
number	O
.	O
</s>
<s>
This	O
affects	O
the	O
use	O
of	O
extreme	B-Algorithm
ultraviolet	I-Algorithm
lithography	I-Algorithm
or	O
EUVL	B-Algorithm
,	O
which	O
is	O
limited	O
to	O
the	O
use	O
of	O
low	O
doses	O
on	O
the	O
order	O
of	O
20	O
photons/nm2	O
.	O
</s>
<s>
This	O
is	O
due	O
to	O
fewer	O
photons	B-Application
for	O
the	O
same	O
energy	O
dose	O
for	O
a	O
shorter	O
wavelength	O
(	O
higher	O
energy	O
per	O
photon	B-Application
)	O
.	O
</s>
<s>
With	O
fewer	O
photons	B-Application
making	O
up	O
the	O
image	O
,	O
there	O
is	O
noise	O
in	O
the	O
edge	O
placement	O
.	O
</s>
<s>
Historically	O
,	O
photolithography	B-Algorithm
has	O
used	O
ultraviolet	B-Application
light	O
from	O
gas-discharge	O
lamps	O
using	O
mercury	O
,	O
sometimes	O
in	O
combination	O
with	O
noble	O
gases	O
such	O
as	O
xenon	B-Protocol
.	O
</s>
<s>
These	O
lamps	O
produce	O
light	O
across	O
a	O
broad	O
spectrum	O
with	O
several	O
strong	O
peaks	O
in	O
the	O
ultraviolet	B-Application
range	O
.	O
</s>
<s>
The	O
commonly	O
used	O
deep	O
ultraviolet	B-Application
excimer	O
lasers	O
in	O
lithography	O
systems	O
are	O
the	O
krypton	O
fluoride	O
(	O
KrF	O
)	O
laser	O
at	O
248nm	O
wavelength	O
and	O
the	O
argon	O
fluoride	O
laser	O
(	O
ArF	O
)	O
at	O
193nm	O
wavelength	O
.	O
</s>
<s>
Furthermore	O
,	O
insulating	O
materials	O
such	O
as	O
silicon	O
dioxide	O
,	O
when	O
exposed	O
to	O
photons	B-Application
with	O
energy	O
greater	O
than	O
the	O
band	O
gap	O
,	O
release	O
free	O
electrons	O
and	O
holes	O
which	O
subsequently	O
cause	O
adverse	O
charging	O
.	O
</s>
<s>
Optical	B-Algorithm
lithography	I-Algorithm
has	O
been	O
extended	O
to	O
feature	O
sizes	O
below	O
50nm	O
using	O
the	O
193nm	O
ArF	O
excimer	O
laser	O
and	O
liquid	O
immersion	O
techniques	O
.	O
</s>
<s>
Also	O
termed	O
immersion	B-Algorithm
lithography	I-Algorithm
,	O
this	O
enables	O
the	O
use	O
of	O
optics	O
with	O
numerical	O
apertures	O
exceeding	O
1.0	O
.	O
</s>
<s>
The	O
liquid	O
used	O
is	O
typically	O
ultra-pure	O
,	O
deionised	O
water	O
,	O
which	O
provides	O
for	O
a	O
refractive	O
index	O
above	O
that	O
of	O
the	O
usual	O
air	O
gap	O
between	O
the	O
lens	O
and	O
the	O
wafer	B-Architecture
surface	O
.	O
</s>
<s>
These	O
were	O
once	O
targeted	O
to	O
succeed	O
193nm	O
lithography	O
at	O
the	O
65nm	O
feature	O
size	O
node	O
but	O
have	O
now	O
all	O
but	O
been	O
eliminated	O
by	O
the	O
introduction	O
of	O
immersion	B-Algorithm
lithography	I-Algorithm
.	O
</s>
<s>
High-index	O
immersion	B-Algorithm
lithography	I-Algorithm
is	O
the	O
newest	O
extension	O
of	O
193nm	O
lithography	O
to	O
be	O
considered	O
.	O
</s>
<s>
By	O
setting	O
up	O
multiple	O
lasers	O
and	O
mirrors	O
,	O
the	O
amount	O
of	O
energy	O
loss	O
is	O
minimized	O
,	O
also	O
since	O
the	O
lenses	O
are	O
coated	O
with	O
antireflective	O
material	O
,	O
the	O
light	O
intensity	O
remains	O
relatively	O
the	O
same	O
from	O
when	O
it	O
left	O
the	O
laser	O
to	O
when	O
it	O
hits	O
the	O
wafer	B-Architecture
.	O
</s>
<s>
Lasers	O
have	O
been	O
used	O
to	O
indirectly	O
generate	O
non-coherent	O
extreme	O
UV	O
(	O
EUV	O
)	O
light	O
at	O
13.5nm	O
for	O
extreme	B-Algorithm
ultraviolet	I-Algorithm
lithography	I-Algorithm
.	O
</s>
<s>
The	O
EUV	O
light	O
is	O
not	O
emitted	O
by	O
the	O
laser	O
,	O
but	O
rather	O
by	O
a	O
tin	O
or	O
xenon	B-Protocol
plasma	O
which	O
is	O
excited	O
by	O
an	O
excimer	O
or	O
laser	O
.	O
</s>
<s>
Fabrication	B-Architecture
of	O
feature	O
sizes	O
of	O
10nm	O
has	O
been	O
demonstrated	O
in	O
production	O
environments	O
,	O
but	O
not	O
yet	O
at	O
rates	O
needed	O
for	O
commercialization	O
.	O
</s>
<s>
However	O
vacuum	O
systems	O
and	O
a	O
number	O
of	O
novel	O
technologies	O
(	O
including	O
much	O
higher	O
EUV	O
energies	O
than	O
are	O
now	O
produced	O
)	O
are	O
needed	O
to	O
work	O
with	O
UV	O
at	O
the	O
edge	O
of	O
the	O
X-ray	B-Algorithm
spectrum	O
(	O
which	O
begins	O
at	O
10nm	O
)	O
.	O
</s>
<s>
As	O
of	O
2020	O
,	O
EUV	O
is	O
in	O
mass	O
production	O
use	O
by	O
leading	O
edge	O
foundries	B-Algorithm
such	O
as	O
TSMC	O
and	O
Samsung	O
.	O
</s>
<s>
Theoretically	O
,	O
an	O
alternative	O
light	O
source	O
for	O
photolithography	B-Algorithm
,	O
especially	O
if	O
and	O
when	O
wavelengths	O
continue	O
to	O
decrease	O
to	O
extreme	O
UV	O
or	O
X-ray	B-Algorithm
,	O
is	O
the	O
free-electron	O
laser	O
(	O
or	O
one	O
might	O
say	O
xaser	O
for	O
an	O
X-ray	B-Algorithm
device	O
)	O
.	O
</s>
<s>
Photolithography	B-Algorithm
has	O
been	O
defeating	O
predictions	O
of	O
its	O
demise	O
for	O
many	O
years	O
.	O
</s>
<s>
New	O
techniques	O
such	O
as	O
immersion	B-Algorithm
lithography	I-Algorithm
,	O
dual-tone	O
resist	O
and	O
multiple	B-Algorithm
patterning	I-Algorithm
continue	O
to	O
improve	O
the	O
resolution	O
of	O
193nm	O
lithography	O
.	O
</s>
<s>
Meanwhile	O
,	O
current	O
research	O
is	O
exploring	O
alternatives	O
to	O
conventional	O
UV	O
,	O
such	O
as	O
electron	B-Architecture
beam	I-Architecture
lithography	I-Architecture
,	O
X-ray	B-Algorithm
lithography	I-Algorithm
,	O
extreme	B-Algorithm
ultraviolet	I-Algorithm
lithography	I-Algorithm
and	O
ion	B-Algorithm
projection	I-Algorithm
lithography	I-Algorithm
.	O
</s>
<s>
Extreme	B-Algorithm
ultraviolet	I-Algorithm
lithography	I-Algorithm
is	O
in	O
mass	O
production	O
use	O
as	O
of	O
2020	O
by	O
Samsung	O
.	O
</s>
<s>
In	O
2001	O
NIST	O
publication	O
has	O
reported	O
that	O
photolithography	B-Algorithm
process	O
constituted	O
about	O
35%	O
of	O
total	O
cost	O
of	O
a	O
wafer	B-Architecture
processing	O
costs	O
.	O
</s>
<s>
In	O
2021	O
,	O
the	O
photolithography	B-Algorithm
industry	O
was	O
valued	O
over	O
8	O
bn	O
USD	O
.	O
</s>
