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Plasma lab Oxford Instruments Plasma Technology System Manual 3.2.10.4 Typical OES endpoint wavelengths Material etched Gas species Wavelength Rise/fall at endpoint detected nm Si F 704 Rise Si SiF 440, 777 Fall Si SiCI …
System
Manual
Oxford
Instruments Plasma Technology
Plasma
lab
3.2.10.2
Laser
interferometry
PC
Signal
Process
Chamber
Process
chamber
Time
• In-situ etch
rate
monitoring
•
Endpoint
does
not
require
etch stop layer
•
Endpoint
can be chosen
anywhere
within
the
layer once etch
rate
has been established.
3.2.10.3
Comparison
of
OES
and
laser
endpoint
techniques
A laser
interferometer
(L1)
endpoint
system has
the
benefits
that
it
gives very precise measurement
of
etch
depth
in
the
etched layer
or
layers and can be used on very small pieces
of
wafer.
It
works
best
with
a
flat
transparent
layer (or stack
of
layers) on a reflective substrate.
It
can be used
to
determine
when
the
etch reaches an interface
between
differing
materials (by
detecting
a change
in
slope
of
the
laser reflectance signal
with
time),
or
can measure
the
etch
depth
when
partially
etching
through
a layer (by
counting
interference
ripples).
It
can also
often
be used
to
identify
multiple
interfaces
when
etching
through
different
layers in a
multi-
layer stack
of
materials
(through
the
changes in reflectance
of
the
materials in question).
The disadvantage
is
that
the
laser
spot
needs
to
be aligned every
time
to
a suitable measurement
point
on
the
wafer
(i.e. an etched area,
not
a masked area). Also,
it
only
measures a single
point.
so
any process
non-uniformity
will
result
in a range
of
etch depths across-wafer
or
across batch.
The optical emission spectroscopy
(OES)
system has
the
benefit
that
it
does
not
require
alignment
for
every run,
it
simply looks
at
optical emission
from
the
whole
plasma. This however, means
that
it
needs
larger
wafers
or
a larger etched area (>2cm
2
)
to
effectively
determine
endpoint.
The size
of
the
etched
area needed
for
good
OE
endpoint
is
also
dependent
on
the
materials being etched since
the
emission
lines
for
certain materials can be very
faint.
Also,
if
the
etch rate
of
the
material
is
low
then
the
concentration
of
its etch species
will
be
low.
OE
can
only
detect
a change in
the
strength
of
a
particular
emission line (or
group
of
emission lines),
so
can
only
detect
when
the
etch
passes
through
an interface
between
differing
materials.
OE
can give a
qualitative
idea
of
uniformity,
since
the
length
of
the
transition
of
the
signal
from
before
endpoint
to
after
endpoint
will
indicate
the
quality
of
the
etch
uniformity.
Also
the
endpoint
is
more
accurate
for
the
whole
(average)
of
the
wafer
rather
than
a single
point
on
the
wafer.
Process
Information
(Information
contained in this
document
is
confidential)
Printed: 08
January
2006 09:37 Page 15
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Issue
1:
December 03
Plasma
lab
Oxford
Instruments Plasma Technology
System Manual
3.2.10.4
Typical
OES
endpoint
wavelengths
Material
etched Gas species
Wavelength
Rise/fall
at
endpoint
detected
nm
Si
F 704
Rise
Si
SiF
440, 777 Fall
Si
SiCI
287
Fall
Si02 F 704
Rise
Si02
CO
483
Fall
Resist,
polyimide
0
843
Rise
Resist,
polyimide
CO
483 Fall
Resist,
polyimide
OH
309
Fall
Resist,
polyimide
H 656
Fall
Si3N4
N2
337
Fall
Si3N4
CN
387
Fall
Si3N4 N 674 Fall
W F 704
Rise
AI
AI
391,394,396
Fall
3.2.10.5
Endpoint
algorithm
examples
Endpoint
trace
- (signal,
falling)
%
level
END
of
process
Normalisation
level
Threshold
level
Signal
crosses
its
Signal
endpoint
threshold
level.
Endpoint
capture
timer
l
~-
--I
.......
--------------------------------------------------------~---J-------------t----
I
',-
~
I
_______ , ,
__
" :
_·_-::··_.tc,
=.,_~~_-.::_~~.j
_
Normalisation
time
)
Endpoint
closed
time
(no
false
endpoint
can
be
found)
:~(
):Time
:
Endpoint
Overetch
I capture
time
I
time
Process
Information
(Information
contained
in this
document
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confidential)
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1:
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System
Manual
Oxford
Instruments
Plasma Technology
Plasma
lab
Endpoint
trace
-
(derivative,
less
than)
Time
)
process
ENOof
Overetch
time
:~(
,
:
Endpoint
I
capture
time
Derivative
crosses
its
endpoint
threshold
level.
Endpoint
captLR'e
timer
started.
Sigmd
d
Endpoint
closed
time
(nofals8
endpoint
can
be
found)
Derivative
\
\
:
\ i
~
i
/~'"
i
:0:~~~::::/~:::::::~---~";:'ir~z-"
\~
:
,
,
,
,
,
,
,
,
,
Normalisation
time
%
level
+/-
derivative
3.2.11
Gas
calibration
factors
See
the
MKS OEM manual
for
details
of
gas correction factors. However,
it
is
worth
pointing
out
that
for
certain gases (e.g.
H2
or
He)
it
is
recommended
that
the
MFC
is
calibrated
for
that
particular
gas, since
they
have very
different
gas properties compared
to
other
gases, and hence
the
errors on calibrations
factors
is
large.
3.2.12
Exhaust
emissions
The gas
emitted
by a plasma etch process
will
be mostly made
up
of
the
input
gases.
However,
there
will
be a small
but
significant
component
of
etch
or
plasma by-products
(say
up
to
-10%
in an
RIE
tool,
possibly
more
for
ICP).
The exact
amounts
will
depend
on process
type
and conditions. These can be any
combination
of
etch gas
material
and etched material.
For example:
Si
+
CF
4
=
SiFx,
CFx,
Fetc
Si0
2
+
CHF
3
=
SiFx,
COx,
CFx,
F,
HF,
CHx,
SiOFx
etc
Resist
+ O
2
=
COx,
0 etc
As
many
of
these by-products are toxic,
it
is
a
minimum
requirement
that
these gases are exhausted in an
enclosed
extraction
system
to
the
roof
of
the
building
-
following
health
and safety regulations. In
addition
to
this,
depending
on
local regulations,
it
may be necessary
to
have some
form
of
gas scrubbing
before
releasing these materials
to
the
atmosphere.
Even
if
we
were
not
running
gases
through
the
system,
we
would
recommend
that
the
system exhaust
is
extracted correctly, since
the
pump
exhaust
will
contain small
droplets
of
pump
oil
which
are in
themselves
harmful
to
lung
function.
Another
important
consideration
is
the
gas absorbed in
the
pump
oil. Since
the
exhaust gases contain
HF
there
will
be a
build
up
of
HF
in
the
pump
oil. Therefore,
it
is
important
to
use
the
correct
protective
equipment
when
servicing
the
pump
or
changing
pump
oil, i.e. suitable gloves, protective
clothing,
filtered
facemask
or
breathing
apparatus.
It
is
also
worth
remembering
that
when
using O
2
processes
the
pump
oil should be Fomblin
oil
and NOT
mineral
oil
to
avoid risk
of
fire
or
explosive reaction
between
O
2
and mineral oil.
Process
Information
(Information
contained
in
this
document
is
confidential)
Printed: 08 January 2006 09:37 Page
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1:
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