<s>
Thermocompression	B-Algorithm
bonding	I-Algorithm
describes	O
a	O
wafer	B-Algorithm
bonding	I-Algorithm
technique	O
and	O
is	O
also	O
referred	O
to	O
as	O
diffusion	O
bonding	O
,	O
pressure	O
joining	O
,	O
thermocompression	O
welding	O
or	O
solid-state	O
welding	O
.	O
</s>
<s>
The	O
most	O
established	O
materials	O
for	O
thermocompression	B-Algorithm
bonding	I-Algorithm
are	O
copper	O
(	O
Cu	O
)	O
,	O
gold	O
(	O
Au	O
)	O
and	O
aluminium	O
(	O
Al	O
)	O
because	O
of	O
their	O
high	O
diffusion	O
rates	O
.	O
</s>
<s>
It	O
also	O
immediately	O
forms	O
a	O
surface	O
oxide	O
which	O
can	O
,	O
however	O
,	O
be	O
removed	O
by	O
formic	O
acid	O
vapor	B-Application
cleaning	O
.	O
</s>
<s>
Oxide	O
removal	O
doubles	O
as	O
surface	B-Application
passivation	I-Application
.	O
</s>
<s>
The	O
diffusion	O
of	O
these	O
metals	O
requires	O
good	O
knowledge	O
of	O
the	O
CTE	O
differences	O
between	O
the	O
two	O
wafers	B-Architecture
to	O
prevent	O
resulting	O
stress	O
.	O
</s>
<s>
Therefore	O
,	O
the	O
temperature	O
of	O
both	O
heaters	O
needs	O
to	O
be	O
matched	O
and	O
center-to-edge	O
uniform	O
for	O
synchronized	O
wafer	B-Architecture
expansion	O
.	O
</s>
<s>
The	O
oxide	O
layer	O
removal	O
can	O
be	O
realized	O
by	O
various	O
oxide	O
etch	B-Algorithm
chemistry	I-Algorithm
methods	I-Algorithm
.	O
</s>
<s>
Dry	B-Algorithm
etching	I-Algorithm
processes	O
,	O
i.e.	O
</s>
<s>
formic	O
acid	O
vapor	B-Application
cleaning	O
,	O
are	O
preferred	O
based	O
on	O
the	O
minimization	O
of	O
the	O
immersion	O
in	O
fluids	O
and	O
the	O
resulting	O
etching	B-Algorithm
of	O
the	O
passivation	B-Application
or	O
the	O
adhesion	O
layer	O
.	O
</s>
<s>
Using	O
the	O
CMP	B-Algorithm
process	O
,	O
which	O
is	O
especially	O
for	O
Cu	O
and	O
Al	O
required	O
,	O
creates	O
a	O
planarized	O
surface	O
with	O
micro	O
roughness	O
around	O
several	O
nanometres	O
and	O
enables	O
the	O
achievement	O
of	O
void-free	O
diffusion	O
bonds	O
.	O
</s>
<s>
This	O
improvement	O
is	O
based	O
on	O
a	O
defined	O
height	O
Cu	O
,	O
Au	O
and	O
Sn	B-Protocol
.	O
</s>
<s>
The	O
metal	O
films	O
can	O
be	O
deposited	O
by	O
evaporation	B-Algorithm
,	O
sputtering	O
or	O
electroplating	O
.	O
</s>
<s>
Evaporation	B-Algorithm
and	O
sputtering	O
,	O
producing	O
high	O
quality	O
films	O
with	O
limited	O
impurities	O
,	O
are	O
slow	O
and	O
hence	O
used	O
for	O
micrometre	O
and	O
sub-micrometre	O
layer	O
thicknesses	O
.	O
</s>
<s>
The	O
gold	O
film	O
can	O
also	O
be	O
deposited	O
on	O
a	O
diffusion	B-Algorithm
barrier	I-Algorithm
film	O
,	O
i.e.	O
</s>
<s>
Further	O
,	O
the	O
applied	O
force	O
and	O
its	O
uniformity	O
is	O
important	O
and	O
depends	O
on	O
the	O
wafer	B-Architecture
diameter	O
and	O
the	O
metal	O
density	O
features	O
.	O
</s>
<s>
The	O
bonding	O
process	O
itself	O
takes	O
place	O
in	O
a	O
vacuum	B-General_Concept
or	O
forming	O
gas	O
environment	O
,	O
e.g.	O
</s>
<s>
The	O
pressure	O
atmosphere	O
supports	O
the	O
heat	O
conduction	O
and	O
prevents	O
thermal	O
gradients	O
vertically	O
across	O
the	O
wafer	B-Architecture
and	O
re-oxidation	O
.	O
</s>
<s>
Based	O
on	O
the	O
difficult	O
control	O
of	O
thermal	O
expansion	O
differences	O
between	O
the	O
two	O
wafers	B-Architecture
,	O
precision	O
alignment	O
and	O
high	O
quality	O
fixtures	O
are	O
used	O
.	O
</s>
<s>
The	O
bonding	O
settings	O
for	O
the	O
most	O
established	O
metals	O
are	O
following	O
(	O
for	O
200mm	O
wafers	B-Architecture
)	O
:	O
</s>
<s>
Thermocompression	B-Algorithm
bonding	I-Algorithm
is	O
well	O
established	O
in	O
the	O
CMOS	B-Device
industry	O
and	O
realizes	O
vertical	O
integrated	O
devices	O
and	O
production	O
of	O
wafer	B-Architecture
level	O
packages	O
with	O
smaller	O
form	O
factors	O
.	O
</s>
<s>
This	O
bonding	O
procedure	O
is	O
used	O
to	O
produce	O
pressure	O
sensors	O
,	O
accelerometers	O
,	O
gyroscopes	B-Application
and	O
RF	O
MEMS	O
.	O
</s>
