<s>
In	O
photolithography	B-Algorithm
,	O
off-axis	B-Algorithm
illumination	I-Algorithm
is	O
an	O
optical	O
system	O
setup	O
in	O
which	O
the	O
incoming	O
light	O
strikes	O
the	O
photomask	B-Algorithm
at	O
an	O
oblique	O
angle	O
rather	O
than	O
perpendicularly	O
to	O
it	O
,	O
that	O
is	O
to	O
say	O
,	O
the	O
incident	O
light	O
is	O
not	O
parallel	O
to	O
the	O
axis	O
of	O
the	O
optical	O
system	O
.	O
</s>
<s>
The	O
advantages	O
of	O
off-axis	B-Algorithm
illumination	I-Algorithm
can	O
be	O
explained	O
in	O
the	O
context	O
where	O
the	O
pattern	O
on	O
the	O
photomask	B-Algorithm
is	O
a	O
diffraction	O
grating	O
with	O
a	O
small	O
pitch	O
.	O
</s>
<s>
The	O
result	O
is	O
that	O
no	O
pattern	O
is	O
created	O
on	O
the	O
wafer	O
,	O
since	O
the	O
zero-th	O
diffraction	O
order	O
only	O
contains	O
the	O
average	O
of	O
the	O
photomask	B-Algorithm
pattern	O
.	O
</s>
