<s>
Etching	B-Algorithm
is	O
used	O
in	O
microfabrication	O
to	O
chemically	O
remove	O
layers	O
from	O
the	O
surface	O
of	O
a	O
wafer	B-Architecture
during	O
manufacturing	O
.	O
</s>
<s>
Etching	B-Algorithm
is	O
a	O
critically	O
important	O
process	O
module	O
,	O
and	O
every	O
wafer	B-Architecture
undergoes	O
many	O
etching	B-Algorithm
steps	O
before	O
it	O
is	O
complete	O
.	O
</s>
<s>
For	O
many	O
etch	O
steps	O
,	O
part	O
of	O
the	O
wafer	B-Architecture
is	O
protected	O
from	O
the	O
etchant	O
by	O
a	O
"	O
masking	O
"	O
material	O
which	O
resists	O
etching	B-Algorithm
.	O
</s>
<s>
In	O
some	O
cases	O
,	O
the	O
masking	O
material	O
is	O
a	O
photoresist	O
which	O
has	O
been	O
patterned	O
using	O
photolithography	B-Algorithm
.	O
</s>
<s>
If	O
the	O
etch	O
is	O
intended	O
to	O
make	O
a	O
cavity	O
in	O
a	O
material	O
,	O
the	O
depth	O
of	O
the	O
cavity	O
may	O
be	O
controlled	O
approximately	O
using	O
the	O
etching	B-Algorithm
time	O
and	O
the	O
known	O
etch	O
rate	O
.	O
</s>
<s>
More	O
often	O
,	O
though	O
,	O
etching	B-Algorithm
must	O
entirely	O
remove	O
the	O
top	O
layer	O
of	O
a	O
multilayer	O
structure	O
,	O
without	O
damaging	O
the	O
underlying	O
or	O
masking	O
layers	O
.	O
</s>
<s>
The	O
etching	B-Algorithm
system	O
's	O
ability	O
to	O
do	O
this	O
depends	O
on	O
the	O
ratio	O
of	O
etch	O
rates	O
in	O
the	O
two	O
materials	O
(	O
selectivity	O
)	O
.	O
</s>
<s>
Some	O
etches	O
undercut	B-Algorithm
the	O
masking	O
layer	O
and	O
form	O
cavities	O
with	O
sloping	O
sidewalls	O
.	O
</s>
<s>
Etchants	O
with	O
large	O
bias	O
are	O
called	O
isotropic	O
,	O
because	O
they	O
erode	O
the	O
substrate	B-Architecture
equally	O
in	O
all	O
directions	O
.	O
</s>
<s>
The	O
first	O
etching	B-Algorithm
processes	O
used	O
liquid-phase	O
(	O
"	O
wet	O
"	O
)	O
etchants	O
.	O
</s>
<s>
This	O
process	O
is	O
now	O
largely	O
outdated	O
,	O
but	O
was	O
used	O
up	O
until	O
the	O
late	O
1980s	O
when	O
it	O
was	O
superseded	O
by	O
dry	O
plasma	B-Algorithm
etching	I-Algorithm
.	O
</s>
<s>
The	O
wafer	B-Architecture
can	O
be	O
immersed	O
in	O
a	O
bath	O
of	O
etchant	O
,	O
which	O
must	O
be	O
agitated	O
to	O
achieve	O
good	O
process	O
control	O
.	O
</s>
<s>
For	O
instance	O
,	O
buffered	B-Algorithm
hydrofluoric	I-Algorithm
acid	I-Algorithm
(	O
BHF	O
)	O
is	O
used	O
commonly	O
to	O
etch	O
silicon	O
dioxide	O
over	O
a	O
silicon	O
substrate	B-Architecture
.	O
</s>
<s>
Wet	O
etchants	O
are	O
usually	O
isotropic	O
,	O
which	O
leads	O
to	O
large	O
bias	O
when	O
etching	B-Algorithm
thick	O
films	O
.	O
</s>
<s>
However	O
,	O
the	O
photographic	O
developer	O
used	O
for	O
photoresist	O
resembles	O
wet	O
etching	B-Algorithm
.	O
</s>
<s>
As	O
an	O
alternative	O
to	O
immersion	O
,	O
single	O
wafer	B-Architecture
machines	O
use	O
the	O
Bernoulli	O
principle	O
to	O
employ	O
a	O
gas	O
(	O
usually	O
,	O
pure	O
nitrogen	O
)	O
to	O
cushion	O
and	O
protect	O
one	O
side	O
of	O
the	O
wafer	B-Architecture
while	O
etchant	O
is	O
applied	O
to	O
the	O
other	O
side	O
.	O
</s>
<s>
This	O
etch	O
method	O
is	O
particularly	O
effective	O
just	O
before	O
"	O
backend	O
"	O
processing	O
(	O
BEOL	B-Algorithm
)	O
,	O
where	O
wafers	B-Architecture
are	O
normally	O
very	O
much	O
thinner	O
after	O
wafer	B-Algorithm
backgrinding	I-Algorithm
,	O
and	O
very	O
sensitive	O
to	O
thermal	O
or	O
mechanical	O
stress	O
.	O
</s>
<s>
Etching	B-Algorithm
a	O
thin	O
layer	O
of	O
even	O
a	O
few	O
micrometres	O
will	O
remove	O
microcracks	O
produced	O
during	O
backgrinding	O
resulting	O
in	O
the	O
wafer	B-Architecture
having	O
dramatically	O
increased	O
strength	O
and	O
flexibility	O
without	O
breaking	O
.	O
</s>
<s>
silicon	B-Architecture
wafers	I-Architecture
)	O
,	O
this	O
effect	O
can	O
allow	O
very	O
high	O
anisotropy	O
,	O
as	O
shown	O
in	O
the	O
figure	O
.	O
</s>
<s>
The	O
term	O
"	O
crystallographic	O
etching	B-Algorithm
"	O
is	O
synonymous	O
with	O
"	O
anisotropic	O
etching	B-Algorithm
along	O
crystal	O
planes	O
"	O
.	O
</s>
<s>
The	O
authors	O
employs	O
multistream	O
laminar	O
flow	O
that	O
contains	O
etching	B-Algorithm
non-etching	O
solutions	O
to	O
fabricate	O
a	O
glass	O
groove	O
.	O
</s>
<s>
The	O
etching	B-Algorithm
solution	O
at	O
the	O
center	O
is	O
flanked	O
by	O
non-etching	O
solutions	O
and	O
the	O
area	O
contacting	O
etching	B-Algorithm
solutions	O
is	O
limited	O
by	O
the	O
surrounding	O
non-etching	O
solutions	O
.	O
</s>
<s>
Thereby	O
,	O
the	O
direction	O
of	O
etching	B-Algorithm
is	O
mainly	O
vertical	O
to	O
the	O
surface	O
of	O
glass	O
.	O
</s>
<s>
EDP	O
(	O
an	O
aqueous	O
solution	O
of	O
ethylene	O
diamine	O
and	O
pyrocatechol	O
)	O
,	O
displays	O
a	O
<100>/<111>	O
selectivity	O
of	O
17X	O
,	O
does	O
not	O
etch	O
silicon	O
dioxide	O
as	O
KOH	O
does	O
,	O
and	O
also	O
displays	O
high	O
selectivity	O
between	O
lightly	O
doped	B-Algorithm
and	O
heavily	O
boron-doped	O
(	O
p-type	O
)	O
silicon	O
.	O
</s>
<s>
Use	O
of	O
these	O
etchants	O
on	O
wafers	B-Architecture
that	O
already	O
contain	O
CMOS	B-Device
integrated	O
circuits	O
requires	O
protecting	O
the	O
circuitry	O
.	O
</s>
<s>
Etching	B-Algorithm
a	O
(	O
100	O
)	O
silicon	O
surface	O
through	O
a	O
rectangular	O
hole	O
in	O
a	O
masking	O
material	O
,	O
for	O
example	O
a	O
hole	O
in	O
a	O
layer	O
of	O
silicon	O
nitride	O
,	O
creates	O
a	O
pit	O
with	O
flat	O
sloping	O
{111}-oriented	O
sidewalls	O
and	O
a	O
flat	O
(	O
100	O
)	O
-oriented	O
bottom	O
.	O
</s>
<s>
The	O
{111}-oriented	O
sidewalls	O
have	O
an	O
angle	O
to	O
the	O
surface	O
of	O
the	O
wafer	B-Architecture
of	O
:	O
</s>
<s>
If	O
the	O
etching	B-Algorithm
is	O
continued	O
"	O
to	O
completion	O
"	O
,	O
i.e.	O
</s>
<s>
The	O
undercut	B-Algorithm
,	O
δ	O
,	O
under	O
an	O
edge	O
of	O
the	O
masking	O
material	O
is	O
given	O
by	O
:	O
</s>
<s>
Modern	O
very	O
large	O
scale	O
integration	O
(	O
VLSI	O
)	O
processes	O
avoid	O
wet	O
etching	B-Algorithm
,	O
and	O
use	O
plasma	B-Algorithm
etching	I-Algorithm
instead	O
.	O
</s>
<s>
Plasma	B-Algorithm
etchers	I-Algorithm
can	O
operate	O
in	O
several	O
modes	O
by	O
adjusting	O
the	O
parameters	O
of	O
the	O
plasma	O
.	O
</s>
<s>
Ordinary	O
plasma	B-Algorithm
etching	I-Algorithm
operates	O
between	O
0.1	O
and	O
5	O
Torr	O
.	O
</s>
<s>
The	O
plasma	O
produces	O
energetic	O
free	O
radicals	O
,	O
neutrally	O
charged	O
,	O
that	O
react	O
at	O
the	O
surface	O
of	O
the	O
wafer	B-Architecture
.	O
</s>
<s>
Since	O
neutral	O
particles	O
attack	O
the	O
wafer	B-Architecture
from	O
all	O
angles	O
,	O
this	O
process	O
is	O
isotropic	O
.	O
</s>
<s>
Plasma	B-Algorithm
etching	I-Algorithm
can	O
be	O
isotropic	O
,	O
i.e.	O
,	O
exhibiting	O
a	O
lateral	O
undercut	B-Algorithm
rate	O
on	O
a	O
patterned	O
surface	O
approximately	O
the	O
same	O
as	O
its	O
downward	O
etch	O
rate	O
,	O
or	O
can	O
be	O
anisotropic	O
,	O
i.e.	O
,	O
exhibiting	O
a	O
smaller	O
lateral	O
undercut	B-Algorithm
rate	O
than	O
its	O
downward	O
etch	O
rate	O
.	O
</s>
<s>
Such	O
anisotropy	O
is	O
maximized	O
in	O
deep	B-Algorithm
reactive	I-Algorithm
ion	I-Algorithm
etching	I-Algorithm
(	O
DRIE	B-Algorithm
)	O
.	O
</s>
<s>
The	O
use	O
of	O
the	O
term	O
anisotropy	O
for	O
plasma	B-Algorithm
etching	I-Algorithm
should	O
not	O
be	O
conflated	O
with	O
the	O
use	O
of	O
the	O
same	O
term	O
when	O
referring	O
to	O
orientation-dependent	O
etching	B-Algorithm
.	O
</s>
<s>
Ion	O
milling	O
,	O
or	O
sputter	O
etching	B-Algorithm
,	O
uses	O
lower	O
pressures	O
,	O
often	O
as	O
low	O
as	O
10−4	O
Torr	O
(	O
10	O
mPa	O
)	O
.	O
</s>
<s>
It	O
bombards	O
the	O
wafer	B-Architecture
with	O
energetic	O
ions	O
of	O
noble	O
gases	O
,	O
often	O
Ar+	O
,	O
which	O
knock	O
atoms	O
from	O
the	O
substrate	B-Architecture
by	O
transferring	O
momentum	B-Algorithm
.	O
</s>
<s>
Because	O
the	O
etching	B-Algorithm
is	O
performed	O
by	O
ions	O
,	O
which	O
approach	O
the	O
wafer	B-Architecture
approximately	O
from	O
one	O
direction	O
,	O
this	O
process	O
is	O
highly	O
anisotropic	O
.	O
</s>
<s>
Reactive-ion	B-Algorithm
etching	I-Algorithm
(	O
RIE	O
)	O
operates	O
under	O
conditions	O
intermediate	O
between	O
sputter	O
and	O
plasma	B-Algorithm
etching	I-Algorithm
(	O
between	O
10−3	O
and	O
10−1	O
Torr	O
)	O
.	O
</s>
<s>
Deep	B-Algorithm
reactive-ion	I-Algorithm
etching	I-Algorithm
(	O
DRIE	B-Algorithm
)	O
modifies	O
the	O
RIE	O
technique	O
to	O
produce	O
deep	O
,	O
narrow	O
features	O
.	O
</s>
