<s>
X-ray	B-Algorithm
lithography	I-Algorithm
is	O
a	O
process	O
used	O
in	O
semiconductor	B-Architecture
device	I-Architecture
fabrication	I-Architecture
industry	O
to	O
selectively	O
remove	O
parts	O
of	O
a	O
thin	O
film	O
of	O
photoresist	O
.	O
</s>
<s>
It	O
uses	O
X-rays	B-Library
to	O
transfer	O
a	O
geometric	O
pattern	O
from	O
a	O
mask	O
to	O
a	O
light-sensitive	O
chemical	O
photoresist	O
,	O
or	O
simply	O
"	O
resist	O
,	O
"	O
on	O
the	O
substrate	O
to	O
reach	O
extremely	O
small	B-Architecture
topological	I-Architecture
size	I-Architecture
of	O
a	O
feature	O
.	O
</s>
<s>
It	O
's	O
less	O
commonly	O
used	O
in	O
commercial	O
production	O
due	O
to	O
prohibitively	O
high	O
costs	O
of	O
materials	O
(	O
such	O
as	O
gold	O
used	O
for	O
X-rays	B-Library
blocking	O
)	O
etc	O
.	O
</s>
<s>
X-ray	B-Algorithm
lithography	I-Algorithm
originated	O
as	O
a	O
candidate	O
for	O
next-generation	B-Algorithm
lithography	I-Algorithm
for	O
the	O
semiconductor	O
industry	O
,	O
with	O
batches	O
of	O
microprocessors	B-Architecture
successfully	O
produced	O
.	O
</s>
<s>
Having	O
short	O
wavelengths	O
(	O
below	O
1nm	O
)	O
,	O
X-rays	B-Library
overcome	O
the	O
diffraction	O
limits	O
of	O
optical	B-Algorithm
lithography	I-Algorithm
,	O
allowing	O
smaller	O
feature	O
sizes	O
.	O
</s>
<s>
If	O
the	O
X-ray	B-Library
source	O
is	O
n't	O
collimated	O
,	O
as	O
with	O
a	O
synchrotron	O
radiation	O
,	O
elementary	O
collimating	O
mirrors	O
or	O
diffractive	O
lenses	O
are	O
used	O
in	O
the	O
place	O
of	O
the	O
refractive	O
lenses	O
used	O
in	O
optics	O
.	O
</s>
<s>
The	O
X-rays	B-Library
illuminate	O
a	O
mask	O
placed	O
in	O
proximity	O
of	O
a	O
resist-coated	O
wafer	O
.	O
</s>
<s>
The	O
X-rays	B-Library
are	O
broadband	O
,	O
typically	O
from	O
a	O
compact	O
synchrotron	O
radiation	O
source	O
,	O
allowing	O
rapid	O
exposure	O
.	O
</s>
<s>
Deep	O
X-ray	B-Algorithm
lithography	I-Algorithm
(	O
DXRL	O
)	O
uses	O
yet	O
shorter	O
wavelengths	O
on	O
the	O
order	O
of	O
and	O
modified	O
procedures	O
such	O
as	O
the	O
LIGA	O
process	O
,	O
to	O
fabricate	O
deep	O
and	O
even	O
three-dimensional	O
structures	O
.	O
</s>
<s>
The	O
mask	O
consists	O
of	O
an	O
X-ray	B-Library
absorber	O
,	O
typically	O
of	O
gold	O
or	O
compounds	O
of	O
tantalum	O
or	O
tungsten	B-Application
,	O
on	O
a	O
membrane	O
that	O
is	O
transparent	O
to	O
X-rays	B-Library
,	O
typically	O
of	O
silicon	O
carbide	O
or	O
diamond	O
.	O
</s>
<s>
The	O
pattern	O
on	O
the	O
mask	O
is	O
written	O
by	O
direct-write	O
electron	B-Architecture
beam	I-Architecture
lithography	I-Architecture
onto	O
a	O
resist	O
that	O
is	O
developed	O
by	O
conventional	O
semiconductor	O
processes	O
.	O
</s>
<s>
Most	O
X-ray	B-Algorithm
lithography	I-Algorithm
demonstrations	O
have	O
been	O
performed	O
by	O
copying	O
with	O
image	O
fidelity	O
(	O
without	O
magnification	O
)	O
on	O
the	O
line	O
of	O
fuzzy	O
contrast	O
as	O
illustrated	O
in	O
the	O
figure	O
.	O
</s>
<s>
However	O
,	O
with	O
the	O
increasing	O
need	O
for	O
high	O
resolution	O
,	O
X-ray	B-Algorithm
lithography	I-Algorithm
is	O
now	O
performed	O
on	O
what	O
is	O
called	O
the	O
"	O
sweet	O
spot	O
"	O
,	O
using	O
local	O
"	O
demagnification	O
by	O
bias	O
"	O
.	O
</s>
<s>
X-rays	B-Library
generate	O
secondary	O
electrons	O
as	O
in	O
the	O
cases	O
of	O
extreme	B-Algorithm
ultraviolet	I-Algorithm
lithography	I-Algorithm
and	O
electron	B-Architecture
beam	I-Architecture
lithography	I-Architecture
.	O
</s>
<s>
While	O
the	O
fine	O
pattern	O
definition	O
is	O
due	O
principally	O
to	O
secondaries	O
from	O
Auger	O
electrons	O
with	O
a	O
short	O
path	O
length	O
,	O
the	O
primary	O
electrons	O
will	O
sensitize	O
the	O
resist	O
over	O
a	O
larger	O
region	O
than	O
the	O
X-ray	B-Library
exposure	O
.	O
</s>
<s>
The	O
sidewall	O
roughness	O
and	O
slopes	O
are	O
influenced	O
by	O
these	O
secondary	O
electrons	O
as	O
they	O
can	O
travel	O
few	O
micrometers	O
in	O
the	O
area	O
under	O
the	O
absorber	O
,	O
depending	O
on	O
exposure	O
X-ray	B-Library
energy	O
.	O
</s>
<s>
Another	O
manifestation	O
of	O
the	O
photoelectron	O
effect	O
is	O
exposure	O
to	O
X-ray	B-Library
generated	O
electrons	O
from	O
thick	O
gold	O
films	O
used	O
for	O
making	O
daughter	O
masks	O
.	O
</s>
<s>
Secondary	O
electrons	O
have	O
energies	O
of	O
25	O
eV	O
or	O
less	O
,	O
and	O
can	O
be	O
generated	O
by	O
any	O
ionizing	O
radiation	O
(	O
VUV	O
,	O
EUV	O
,	O
X-rays	B-Library
,	O
ions	O
and	O
other	O
electrons	O
)	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
the	O
secondaries	O
follow	O
a	O
different	O
trend	O
below	O
≈30	O
eV	O
:	O
the	O
lower	O
the	O
energy	O
,	O
the	O
longer	O
the	O
mean	B-Algorithm
free	I-Algorithm
path	I-Algorithm
though	O
they	O
are	O
not	O
then	O
able	O
to	O
affect	O
resist	O
development	O
.	O
</s>
<s>
What	O
matters	O
for	O
X-ray	B-Algorithm
lithography	I-Algorithm
is	O
the	O
effective	O
range	O
of	O
electrons	O
that	O
have	O
sufficient	O
energy	O
to	O
make	O
or	O
break	O
chemical	O
bonds	O
in	O
negative	O
or	O
positive	O
resists	O
.	O
</s>
<s>
X-rays	B-Library
do	O
not	O
charge	O
.	O
</s>
<s>
The	O
relatively	O
large	O
mean	B-Algorithm
free	I-Algorithm
path	I-Algorithm
(	O
~	O
20nm	O
)	O
of	O
secondary	O
electrons	O
hinders	O
resolution	O
control	O
at	O
nanometer	O
scale	O
.	O
</s>
<s>
In	O
particular	O
,	O
electron	B-Architecture
beam	I-Architecture
lithography	I-Architecture
suffers	O
negative	O
charging	O
by	O
incident	O
electrons	O
and	O
consequent	O
beam	O
spread	O
which	O
limits	O
resolution	O
.	O
</s>
<s>
The	O
combined	O
electron	O
mean	B-Algorithm
free	I-Algorithm
path	I-Algorithm
results	O
in	O
an	O
image	O
blur	O
,	O
which	O
is	O
usually	O
modeled	O
as	O
a	O
Gaussian	O
function	O
(	O
where	O
σ	O
=	O
blur	O
)	O
that	O
is	O
convolved	O
with	O
the	O
expected	O
image	O
.	O
</s>
<s>
As	O
the	O
desired	O
resolution	O
approaches	O
the	O
blur	O
,	O
the	O
dose	O
image	O
becomes	O
broader	O
than	O
the	O
aerial	O
image	O
of	O
the	O
incident	O
X-rays	B-Library
.	O
</s>
<s>
X-rays	B-Library
carry	O
no	O
charge	O
,	O
but	O
at	O
the	O
energies	O
involved	O
,	O
Auger	O
decay	O
of	O
ionized	O
species	O
in	O
a	O
specimen	O
is	O
more	O
probable	O
than	O
radiative	O
decay	O
.	O
</s>
<s>
Usually	O
,	O
excepting	O
X-ray	B-Library
steppers	O
,	O
charging	O
can	O
be	O
further	O
controlled	O
by	O
flood	O
gun	O
or	O
resist	O
thickness	O
or	O
charge	O
dissipation	O
layer	O
.	O
</s>
