<s>
Evaporation	B-Algorithm
is	O
a	O
common	O
method	O
of	O
thin-film	O
deposition	O
.	O
</s>
<s>
The	O
source	O
material	O
is	O
evaporated	B-Algorithm
in	O
a	O
vacuum	B-General_Concept
.	O
</s>
<s>
The	O
vacuum	B-General_Concept
allows	O
vapor	O
particles	O
to	O
travel	O
directly	O
to	O
the	O
target	O
object	O
(	O
substrate	O
)	O
,	O
where	O
they	O
condense	O
back	O
to	O
a	O
solid	O
state	O
.	O
</s>
<s>
Evaporation	B-Algorithm
is	O
used	O
in	O
microfabrication	O
,	O
and	O
to	O
make	O
macro-scale	O
products	O
such	O
as	O
metallized	O
plastic	O
film	O
.	O
</s>
<s>
Evaporation	B-Algorithm
involves	O
two	O
basic	O
processes	O
:	O
a	O
hot	O
source	O
material	O
evaporates	O
and	O
condenses	O
on	O
the	O
substrate	O
.	O
</s>
<s>
Evaporation	B-Algorithm
takes	O
place	O
in	O
a	O
vacuum	B-General_Concept
,	O
i.e.	O
</s>
<s>
In	O
high	O
vacuum	B-General_Concept
(	O
with	O
a	O
long	O
mean	B-Algorithm
free	I-Algorithm
path	I-Algorithm
)	O
,	O
evaporated	B-Algorithm
particles	O
can	O
travel	O
directly	O
to	O
the	O
deposition	O
target	O
without	O
colliding	O
with	O
the	O
background	O
gas	O
.	O
</s>
<s>
At	O
a	O
typical	O
pressure	O
of	O
10−4	O
Pa	O
,	O
a	O
0.4-nm	O
particle	O
has	O
a	O
mean	B-Algorithm
free	I-Algorithm
path	I-Algorithm
of	O
60	O
m	O
.	O
Hot	O
objects	O
in	O
the	O
evaporation	B-Algorithm
chamber	O
,	O
such	O
as	O
heating	O
filaments	O
,	O
produce	O
unwanted	O
vapors	O
that	O
limit	O
the	O
quality	O
of	O
the	O
vacuum	B-General_Concept
.	O
</s>
<s>
Evaporated	B-Algorithm
atoms	O
that	O
collide	O
with	O
foreign	O
particles	O
may	O
react	O
with	O
them	O
;	O
for	O
instance	O
,	O
if	O
aluminium	O
is	O
deposited	O
in	O
the	O
presence	O
of	O
oxygen	O
,	O
it	O
will	O
form	O
aluminium	O
oxide	O
.	O
</s>
<s>
Evaporated	B-Algorithm
materials	O
deposit	O
nonuniformly	O
if	O
the	O
substrate	O
has	O
a	O
rough	O
surface	O
(	O
as	O
integrated	O
circuits	O
often	O
do	O
)	O
.	O
</s>
<s>
Because	O
the	O
evaporated	B-Algorithm
material	O
attacks	O
the	O
substrate	O
mostly	O
from	O
a	O
single	O
direction	O
,	O
protruding	O
features	O
block	O
the	O
evaporated	B-Algorithm
material	O
from	O
some	O
areas	O
.	O
</s>
<s>
When	O
evaporation	B-Algorithm
is	O
performed	O
in	O
poor	O
vacuum	B-General_Concept
or	O
close	O
to	O
atmospheric	O
pressure	O
,	O
the	O
resulting	O
deposition	O
is	O
generally	O
non-uniform	O
and	O
tends	O
not	O
to	O
be	O
a	O
continuous	O
or	O
smooth	O
film	O
.	O
</s>
<s>
Any	O
evaporation	B-Algorithm
system	O
includes	O
a	O
vacuum	B-General_Concept
pump	O
.	O
</s>
<s>
Alternatively	O
the	O
source	O
material	O
is	O
placed	O
in	O
a	O
crucible	B-General_Concept
,	O
which	O
is	O
radiatively	O
heated	O
by	O
an	O
electric	O
filament	O
,	O
or	O
the	O
source	O
material	O
may	O
be	O
hung	O
from	O
the	O
filament	O
itself	O
(	O
filament	O
evaporation	B-Algorithm
)	O
.	O
</s>
<s>
Molecular	B-Algorithm
beam	I-Algorithm
epitaxy	I-Algorithm
is	O
an	O
advanced	O
form	O
of	O
thermal	O
evaporation	B-Algorithm
.	O
</s>
<s>
In	O
flash	O
evaporation	B-Algorithm
,	O
a	O
fine	O
wire	O
or	O
powder	O
of	O
source	O
material	O
is	O
fed	O
continuously	O
onto	O
a	O
hot	O
ceramic	O
or	O
metallic	O
bar	O
,	O
and	O
evaporates	O
on	O
contact	O
.	O
</s>
<s>
Resistive	O
evaporation	B-Algorithm
is	O
accomplished	O
by	O
passing	O
a	O
large	O
current	O
through	O
a	O
resistive	O
wire	O
or	O
foil	O
containing	O
the	O
material	O
to	O
be	O
deposited	O
.	O
</s>
<s>
The	O
heating	O
element	O
is	O
often	O
referred	O
to	O
as	O
an	O
"	O
evaporation	B-Algorithm
source	O
"	O
.	O
</s>
<s>
Wire	O
type	O
evaporation	B-Algorithm
sources	O
are	O
made	O
from	O
tungsten	B-Application
wire	O
and	O
can	O
be	O
formed	O
into	O
filaments	O
,	O
baskets	O
,	O
heaters	O
or	O
looped	O
shaped	O
point	O
sources	O
.	O
</s>
<s>
Boat	O
type	O
evaporation	B-Algorithm
sources	O
are	O
made	O
from	O
tungsten	B-Application
,	O
tantalum	O
,	O
molybdenum	O
or	O
ceramic	O
type	O
materials	O
capable	O
of	O
withstanding	O
high	O
temperatures	O
.	O
</s>
<s>
Purity	O
of	O
the	O
deposited	O
film	O
depends	O
on	O
the	O
quality	O
of	O
the	O
vacuum	B-General_Concept
,	O
and	O
on	O
the	O
purity	O
of	O
the	O
source	O
material	O
.	O
</s>
<s>
At	O
a	O
given	O
vacuum	B-General_Concept
pressure	O
the	O
film	O
purity	O
will	O
be	O
higher	O
at	O
higher	O
deposition	O
rates	O
as	O
this	O
minimises	O
the	O
relative	O
rate	O
of	O
gaseous	O
impurity	O
inclusion	O
.	O
</s>
<s>
The	O
thickness	O
of	O
the	O
film	O
will	O
vary	O
due	O
to	O
the	O
geometry	O
of	O
the	O
evaporation	B-Algorithm
chamber	O
.	O
</s>
<s>
Wire	O
filaments	O
for	O
evaporation	B-Algorithm
cannot	O
deposit	O
thick	O
films	O
,	O
because	O
the	O
size	O
of	O
the	O
filament	O
limits	O
the	O
amount	O
of	O
material	O
that	O
can	O
be	O
deposited	O
.	O
</s>
<s>
Evaporation	B-Algorithm
boats	O
and	O
crucibles	B-General_Concept
offer	O
higher	O
volumes	O
for	O
thicker	O
coatings	O
.	O
</s>
<s>
Thermal	O
evaporation	B-Algorithm
offers	O
faster	O
evaporation	B-Algorithm
rates	O
than	O
sputtering	O
.	O
</s>
<s>
Flash	O
evaporation	B-Algorithm
and	O
other	O
methods	O
that	O
use	O
crucibles	B-General_Concept
can	O
deposit	O
thick	O
films	O
.	O
</s>
<s>
In	O
order	O
to	O
deposit	O
a	O
material	O
,	O
the	O
evaporation	B-Algorithm
system	O
must	O
be	O
able	O
to	O
vaporize	O
it	O
.	O
</s>
<s>
This	O
makes	O
refractory	O
materials	O
such	O
as	O
tungsten	B-Application
hard	O
to	O
deposit	O
by	O
methods	O
that	O
do	O
not	O
use	O
electron-beam	O
heating	O
.	O
</s>
<s>
Electron-beam	O
evaporation	B-Algorithm
allows	O
tight	O
control	O
of	O
the	O
evaporation	B-Algorithm
rate	O
.	O
</s>
<s>
Evaporation	B-Algorithm
is	O
commonly	O
used	O
in	O
microfabrication	O
to	O
deposit	O
metal	O
films	O
.	O
</s>
<s>
Alternatives	O
to	O
evaporation	B-Algorithm
,	O
such	O
as	O
sputtering	O
and	O
chemical	B-Algorithm
vapor	I-Algorithm
deposition	I-Algorithm
,	O
have	O
better	O
step	O
coverage	O
.	O
</s>
<s>
Sputtering	O
tends	O
to	O
deposit	O
material	O
more	O
slowly	O
than	O
evaporation	B-Algorithm
.	O
</s>
<s>
Evaporated	B-Algorithm
atoms	O
have	O
a	O
Maxwellian	O
energy	O
distribution	O
,	O
determined	O
by	O
the	O
temperature	O
of	O
the	O
source	O
,	O
which	O
reduces	O
the	O
number	O
of	O
high-speed	O
atoms	O
.	O
</s>
