<s>
Electron-beam	B-Architecture
lithography	I-Architecture
(	O
often	O
abbreviated	O
as	O
e-beam	B-Architecture
lithography	I-Architecture
,	O
EBL	O
)	O
is	O
the	O
practice	O
of	O
scanning	O
a	O
focused	O
beam	O
of	O
electrons	O
to	O
draw	O
custom	O
shapes	O
on	O
a	O
surface	O
covered	O
with	O
an	O
electron-sensitive	O
film	O
called	O
a	O
resist	O
(	O
exposing	O
)	O
.	O
</s>
<s>
The	O
purpose	O
,	O
as	O
with	O
photolithography	B-Algorithm
,	O
is	O
to	O
create	O
very	O
small	O
structures	O
in	O
the	O
resist	O
that	O
can	O
subsequently	O
be	O
transferred	O
to	O
the	O
substrate	O
material	O
,	O
often	O
by	O
etching	O
.	O
</s>
<s>
The	O
primary	O
advantage	O
of	O
electron-beam	B-Architecture
lithography	I-Architecture
is	O
that	O
it	O
can	O
draw	O
custom	O
patterns	O
(	O
direct-write	O
)	O
with	O
sub-10	O
nm	O
resolution	O
.	O
</s>
<s>
This	O
form	O
of	O
maskless	B-Algorithm
lithography	I-Algorithm
has	O
high	O
resolution	O
but	O
low	O
throughput	O
,	O
limiting	O
its	O
usage	O
to	O
photomask	B-Algorithm
fabrication	B-Architecture
,	O
low-volume	O
production	O
of	O
semiconductor	O
devices	O
,	O
and	O
research	O
and	O
development	O
.	O
</s>
<s>
Electron-beam	B-Architecture
lithography	I-Architecture
systems	O
used	O
in	O
commercial	O
applications	O
are	O
dedicated	O
e-beam	O
writing	O
systems	O
that	O
are	O
very	O
expensive	O
(	O
>US	O
$1M	O
)	O
.	O
</s>
<s>
For	O
research	O
applications	O
,	O
it	O
is	O
very	O
common	O
to	O
convert	O
an	O
electron	O
microscope	O
into	O
an	O
electron	B-Architecture
beam	I-Architecture
lithography	I-Architecture
system	O
using	O
relatively	O
low	O
cost	O
accessories	O
(	O
<	O
US$100K	O
)	O
.	O
</s>
<s>
Electron-beam	B-Architecture
lithography	I-Architecture
systems	O
can	O
be	O
classified	O
according	O
to	O
both	O
beam	O
shape	O
and	O
beam	O
deflection	O
strategy	O
.	O
</s>
<s>
This	O
is	O
a	O
factor	O
of	O
about	O
10	O
million	O
times	O
slower	O
than	O
current	O
optical	B-Algorithm
lithography	I-Algorithm
tools	O
.	O
</s>
<s>
It	O
is	O
clear	O
that	O
throughput	O
is	O
a	O
serious	O
limitation	O
for	O
electron	B-Architecture
beam	I-Architecture
lithography	I-Architecture
,	O
especially	O
when	O
writing	O
dense	O
patterns	O
over	O
a	O
large	O
area	O
.	O
</s>
<s>
E-beam	B-Architecture
lithography	I-Architecture
is	O
not	O
suitable	O
for	O
high-volume	O
manufacturing	O
because	O
of	O
its	O
limited	O
throughput	O
.	O
</s>
<s>
The	O
smaller	O
field	O
of	O
electron	O
beam	O
writing	O
makes	O
for	O
very	O
slow	O
pattern	O
generation	O
compared	O
with	O
photolithography	B-Algorithm
(	O
the	O
current	O
standard	O
)	O
because	O
more	O
exposure	O
fields	O
must	O
be	O
scanned	O
to	O
form	O
the	O
final	O
pattern	O
area	O
( ≤mm2	O
for	O
electron	O
beam	O
vs.	O
≥40mm2	O
for	O
an	O
optical	B-Algorithm
mask	I-Algorithm
projection	O
scanner	O
)	O
.	O
</s>
<s>
The	O
electron	O
beam	O
field	O
is	O
small	O
enough	O
that	O
a	O
rastering	O
or	O
serpentine	O
stage	O
motion	O
is	O
needed	O
to	O
pattern	O
a	O
26mm	O
X	O
33mm	O
area	O
for	O
example	O
,	O
whereas	O
in	O
a	O
photolithography	B-Algorithm
scanner	O
only	O
a	O
one-dimensional	O
motion	O
of	O
a	O
26mm	O
X	O
2mm	O
slit	O
field	O
would	O
be	O
required	O
.	O
</s>
<s>
Currently	O
an	O
optical	O
maskless	B-Algorithm
lithography	I-Algorithm
tool	O
is	O
much	O
faster	O
than	O
an	O
electron	O
beam	O
tool	O
used	O
at	O
the	O
same	O
resolution	O
for	O
photomask	B-Algorithm
patterning	O
.	O
</s>
<s>
Shot	O
noise	O
is	O
a	O
significant	O
consideration	O
even	O
for	O
mask	O
fabrication	B-Architecture
.	O
</s>
<s>
Despite	O
the	O
high	O
resolution	O
of	O
electron-beam	B-Architecture
lithography	I-Architecture
,	O
the	O
generation	O
of	O
defects	O
during	O
electron-beam	B-Architecture
lithography	I-Architecture
is	O
often	O
not	O
considered	O
by	O
users	O
.	O
</s>
<s>
Physical	O
defects	O
are	O
more	O
varied	O
,	O
and	O
can	O
include	O
sample	O
charging	O
(	O
either	O
negative	O
or	O
positive	O
)	O
,	O
backscattering	B-Algorithm
calculation	O
errors	O
,	O
dose	O
errors	O
,	O
fogging	O
(	O
long-range	O
reflection	O
of	O
backscattered	B-Algorithm
electrons	I-Algorithm
)	O
,	O
outgassing	O
,	O
contamination	O
,	O
beam	O
drift	O
and	O
particles	O
.	O
</s>
<s>
Since	O
the	O
write	O
time	O
for	O
electron	B-Architecture
beam	I-Architecture
lithography	I-Architecture
can	O
easily	O
exceed	O
a	O
day	O
,	O
"	O
randomly	O
occurring	O
"	O
defects	O
are	O
more	O
likely	O
to	O
occur	O
.	O
</s>
<s>
Photomask	B-Algorithm
defects	O
largely	O
originate	O
during	O
the	O
electron	B-Architecture
beam	I-Architecture
lithography	I-Architecture
used	O
for	O
pattern	O
definition	O
.	O
</s>
<s>
This	O
point	O
was	O
driven	O
home	O
by	O
a	O
2007	O
demonstration	O
of	O
double	O
patterning	O
using	O
electron	B-Architecture
beam	I-Architecture
lithography	I-Architecture
in	O
the	O
fabrication	B-Architecture
of	O
15nm	O
half-pitch	O
zone	O
plates	O
.	O
</s>
<s>
This	O
is	O
due	O
to	O
the	O
fact	O
that	O
below	O
the	O
ionization	O
potential	O
the	O
only	O
energy	O
loss	O
mechanism	O
is	O
mainly	O
through	O
phonons	B-Language
and	O
polarons	O
.	O
</s>
<s>
The	O
travel	O
distance	O
of	O
secondary	O
electrons	O
is	O
not	O
a	O
fundamentally	O
derived	O
physical	O
value	O
,	O
but	O
a	O
statistical	O
parameter	O
often	O
determined	O
from	O
many	O
experiments	O
or	O
Monte	B-Algorithm
Carlo	I-Algorithm
simulations	I-Algorithm
down	O
to	O
<	O
1eV	O
.	O
</s>
<s>
A	O
study	O
by	O
the	O
College	O
of	O
Nanoscale	O
Science	O
and	O
Engineering	O
(	O
CNSE	O
)	O
presented	O
at	O
the	O
2013	O
EUVL	B-Algorithm
Workshop	O
indicated	O
that	O
,	O
as	O
a	O
measure	O
of	O
electron	O
blur	O
,	O
50-100	O
eV	O
electrons	O
easily	O
penetrated	O
beyond	O
10nm	O
of	O
resist	O
thickness	O
in	O
PMMA	O
or	O
a	O
commercial	O
resist	O
.	O
</s>
<s>
More	O
recent	O
studies	O
have	O
indicated	O
that	O
20	O
nm	O
resist	O
thickness	O
could	O
be	O
penetrated	O
by	O
low	O
energy	O
electrons	O
(	O
of	O
sufficient	O
dose	O
)	O
and	O
sub-20	O
nm	O
half-pitch	O
electron-beam	B-Architecture
lithography	I-Architecture
already	O
required	O
double	O
patterning	O
.	O
</s>
<s>
These	O
electrons	O
are	O
called	O
backscattered	B-Algorithm
electrons	I-Algorithm
and	O
have	O
the	O
same	O
effect	O
as	O
long-range	O
flare	O
in	O
optical	O
projection	O
systems	O
.	O
</s>
<s>
A	O
large	O
enough	O
dose	O
of	O
backscattered	B-Algorithm
electrons	I-Algorithm
can	O
lead	O
to	O
complete	O
exposure	O
of	O
resist	O
over	O
an	O
area	O
much	O
larger	O
than	O
defined	O
by	O
the	O
beam	O
spot	O
.	O
</s>
<s>
The	O
smallest	O
features	O
produced	O
by	O
electron-beam	B-Architecture
lithography	I-Architecture
have	O
generally	O
been	O
isolated	O
features	O
,	O
as	O
nested	O
features	O
exacerbate	O
the	O
proximity	B-Algorithm
effect	I-Algorithm
,	O
whereby	O
electrons	O
from	O
exposure	O
of	O
an	O
adjacent	O
region	O
spill	O
over	O
into	O
the	O
exposure	O
of	O
the	O
currently	O
written	O
feature	O
,	O
effectively	O
enlarging	O
its	O
image	O
,	O
and	O
reducing	O
its	O
contrast	O
,	O
i.e.	O
,	O
difference	O
between	O
maximum	O
and	O
minimum	O
intensity	O
.	O
</s>
<s>
The	O
proximity	B-Algorithm
effect	I-Algorithm
is	O
also	O
manifest	O
by	O
secondary	O
electrons	O
leaving	O
the	O
top	O
surface	O
of	O
the	O
resist	O
and	O
then	O
returning	O
some	O
tens	O
of	O
nanometers	O
distance	O
away	O
.	O
</s>
<s>
Proximity	B-Algorithm
effects	I-Algorithm
(	O
due	O
to	O
electron	O
scattering	O
)	O
can	O
be	O
addressed	O
by	O
solving	O
the	O
inverse	O
problem	O
and	O
calculating	O
the	O
exposure	O
function	O
E(x,y )	O
that	O
leads	O
to	O
a	O
dose	O
distribution	O
as	O
close	O
as	O
possible	O
to	O
the	O
desired	O
dose	O
D(x,y )	O
when	O
convolved	B-Language
by	O
the	O
scattering	O
distribution	O
point	O
spread	O
function	O
PSF(x,y )	O
.	O
</s>
<s>
However	O
,	O
it	O
must	O
be	O
remembered	O
that	O
an	O
error	O
in	O
the	O
applied	O
dose	O
(	O
e.g.	O
,	O
from	O
shot	O
noise	O
)	O
would	O
cause	O
the	O
proximity	B-Algorithm
effect	I-Algorithm
correction	O
to	O
fail	O
.	O
</s>
<s>
However	O
,	O
for	O
a	O
quartz	O
substrate	O
such	O
as	O
a	O
photomask	B-Algorithm
,	O
the	O
embedded	O
electrons	O
will	O
take	O
a	O
much	O
longer	O
time	O
to	O
move	O
to	O
ground	O
.	O
</s>
<s>
The	O
range	O
of	O
low-energy	O
secondary	O
electrons	O
(	O
the	O
largest	O
component	O
of	O
the	O
free	O
electron	O
population	O
in	O
the	O
resist-substrate	O
system	O
)	O
which	O
can	O
contribute	O
to	O
charging	O
is	O
not	O
a	O
fixed	O
number	O
but	O
can	O
vary	O
from	O
0	O
to	O
as	O
high	O
as	O
50nm	O
(	O
see	O
section	O
New	O
frontiers	O
and	O
extreme	B-Algorithm
ultraviolet	I-Algorithm
lithography	I-Algorithm
)	O
.	O
</s>
<s>
Due	O
to	O
the	O
scission	O
efficiency	O
generally	O
being	O
an	O
order	O
of	O
magnitude	O
higher	O
than	O
the	O
crosslinking	O
efficiency	O
,	O
most	O
polymers	O
used	O
for	O
positive-tone	O
electron-beam	B-Architecture
lithography	I-Architecture
will	O
crosslink	O
(	O
and	O
therefore	O
become	O
negative	O
tone	O
)	O
at	O
doses	O
an	O
order	O
of	O
magnitude	O
than	O
doses	O
used	O
for	O
positive	O
tone	O
exposure	O
.	O
</s>
<s>
Another	O
alternative	O
in	O
electron-beam	B-Architecture
lithography	I-Architecture
is	O
to	O
use	O
extremely	O
high	O
electron	O
energies	O
(	O
at	O
least	O
100keV	O
)	O
to	O
essentially	O
"	O
drill	O
"	O
or	O
sputter	O
the	O
material	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
it	O
is	O
a	O
slow	O
process	O
,	O
requiring	O
much	O
longer	O
exposure	O
times	O
than	O
conventional	O
electron	B-Architecture
beam	I-Architecture
lithography	I-Architecture
.	O
</s>
<s>
Interference	B-Algorithm
lithography	I-Algorithm
using	O
electron	O
beams	O
is	O
another	O
possible	O
path	O
for	O
patterning	O
arrays	O
with	O
nanometer-scale	O
periods	O
.	O
</s>
<s>
Despite	O
the	O
various	O
intricacies	O
and	O
subtleties	O
of	O
electron	B-Architecture
beam	I-Architecture
lithography	I-Architecture
at	O
different	O
energies	O
,	O
it	O
remains	O
the	O
most	O
practical	O
way	O
to	O
concentrate	O
the	O
most	O
energy	O
into	O
the	O
smallest	O
area	O
.	O
</s>
