Oxford-100-Manual.pdf - 第154页
Plasma lab Oxford Instruments Plasma Technology System Manual 3.2.8 Use of helium backing for effective process temperature control 3.2.8.1 Scope For all systems with wafer clamping and helium backing for wafer temperatu…
System
Manual
Oxford
Instruments
Plasma Technology
Plasma
lab
(c)
For processes
which
deposit
a
combination
of
etched material and mask layer, e.g. GaAs and
sputtered
photoresist
during
GaAs 'via
hole
etching'
it
is
common
to
use a mixed
Chlorine/fluorine
chemistry:
RIE
chamber:
SF
6
85
sccm
CI
2
50
sccm
Pressure 45mT
Power
150VV
Temperature
20 C
Quartz
carrier
plate
ICP
chamber:
Step1:
40sccm
C1
2
,
20sccm
SF
6
,
50mT,
500VV
ICP,
200VV
RF,
22C,
OTorr He, 20mins
to
remove GaAs
and
PR
residues (may need
to
be
longer
after
lots
of
'via
hole
etching').
Step2:
50sccm
°
2
,
20mT,
2000VV
ICP,
200VV
RF,
22C,
OTorr He, 30mins
Step3:
50sccm
°
2
,
60mT,
2000VV
ICP,
200VV
RF,
22C,
OTorr He, 30mins
3.2.7
Sample
cooling
I
gluing
It
is
quite
a
common
requirement
to
process small samples
or
pieces
of
wafer.
If
the
process requires
cooling
to
improve
the
etch
profile
or
to
allow
use
of
resist mask
at
high
power
levels,
then
the
small
pieces
of
wafer
must be
glued/fixed
to
a carrier
wafer
which
is
clamped and
helium
cooled. There are
several ways
of
attaching
the
small pieces
of
wafer
to
the
carrier:
(a)
Vacuum grease
(after
etching
has been
completed
the
vacuum grease can be removed
from
back
of
wafer
using IPA
or
acetone).
(b) Thermal
compound.
(c)
Photoresist (i.e. spin a
few
microns
of
resist
onto
a carrier
wafer,
place
the
sample on
top
while
the
resist
is
still
wet,
push sample
down
well
into
resist, and
then
bake resist).
(d)
Use
a
thermally
conductive elastometer pad
(see
EMI Shielding and Thermal
Manaqement
Solutions).
VVith
methods
(a), (b) and (d)
it
is
important
that
the
sample
completely
covers
the
bonding
material,
so
that
no
bonding
material
is
exposed
to
the
plasma and
therefore
cannot
be re-deposited on
the
wafer.
VVith
all these
methods
it
is
necessary
to
also clamp
the
carrier
wafer
and apply
helium
pressure
to
the
back
of
the
carrier
wafer
to
provide
cooling
to
the
sample
(there
is
no
cooling
effect
simply
from
gluing
the
sample
to
the
carrier
if
there
is
no
cooling
of
the
carrier).
If
the
process does
not
need
cooling
(as
with
most
low
power
RIE-only processes)
then
it
is
not
necessary
to
bond
the
sample
to
carrier.
If
the
sample
is
liable
to
slide
off
the
carrier
during
transfer,
it
is
often
better
to
glue
pieces
of
Si
to
the
carrier
wafer
to
act
as
locating pieces
to
hold
the
sample in place. This
avoids
the
need
to
glue
the
sample and
therefore
keeps
the
sample cleaner.
Process
Information
(Information
contained
in
this
document
is
confidential)
Printed: 08 January 2006 09:37 Page
11
of
30
Issue
1: December 03
Plasma
lab
Oxford Instruments Plasma Technology
System Manual
3.2.8
Use
of
helium
backing
for
effective
process
temperature
control
3.2.8.1 Scope
For all systems
with
wafer
clamping and
helium
backing
for
wafer
temperature
control.
i.e. Plasmalab
System 100
with
ICP
65,
180 and 380 sources. Also, occasionally
RIE
133 systems and
RIE
80
Plus.
3.2.8.2
Purpose
It
is
important
to
ensure
that
the
helium
is
sealed adequately
behind
the
wafer.
If
the
helium
is
leaking
out
past
the
wafer
with
a
poor
seal against
the
table,
the
thermal
contact
to
the
temperature-controlled
table
will
be degraded. The
wafer
will
then
heat
up
more
than
expected and
the
process results may
suffer. For example, in
SiOz
etching
the
profile
may become
partially
isotropic and/or any photoresist
masking used may
burn
too
easily.
3.2.8.3
Simple
Method
to
check
Helium
backing
(a)
If
the
wafer
is
sealing
the
helium
effectively,
the
measured He
flow
will
be
less
than
that
when
no
wafer
is
present.
(b) Set a range
of
He pressures and
note
the
measured
helium
flows
with
no
wafer
loaded. (Set all
process
gases,
RF
and pressure
to
zero and
work
in
'manual'
mode.)
(c)
Load a
blank
Si
wafer
of
the
correct size
(if
the
system
is
a standard single
wafer
type) and
note
the
He
flows
for
the
same range
of
set He pressures.
(d) Load a typical customer
wafer
(e.g.
with
a
thick
SiOz
layer) and
note
the
He
flows
for
the
same
range
of
set He pressures.
If
a carrier
is
appropriate
for
the
system, use
that.
(e)
Fill results
in
the
following
table.
(If
you
do
not
have
the
capability
to
measure Helium
flow
then
measure CM
gauge
chamber pressure
with
APC
fully
open,
no
other
gases
flowing).
Set
Herrorr He flow/seem He flow/seem He flow/seem
No
wafer
Si
wafer
Customer
wafer
7
10
15
20
(f) The
larger
the
difference
between
'No
wafer'
and
'With
wafer'
flows,
the
better
the
seal.
'With
wafer'
values should be
less.
Pass
criteria are still being evaluated
but
a recent example
with
acceptable results
is
as
follows.
(g) Recent acceptable example:
Set
He
pressure
7Torr
10Torr
No
wafer
He
flow
4.2sccm
7.2sccm
With
wafer
He
flow*
<3.9sccm
<6.5sccm
*These
were
the
maximum
values observed (usually occurring
for
wafers
with
thick
SiOz
layers)
and
cooling
was
thought
to
be adequate because profiles
were
acceptable.
If
there
is
little
or
no
difference
between
the
'No
wafer'
and
'with
wafer'
flows,
then
the
seal
is
ineffective.
Process
Information
(Information
contained in this
document
is
confidential)
Issue
1:
December 03 Page 12
of
30 Printed: 08 January 2006 09:37
System
Manual
(h)
Troubleshooting:
Oxford
Instruments
Plasma Technology
Plasma
lab
Check
the
backs
of
wafers
for
excessive
contamination,
scratching
or
curvature/bowing.
Vent
chamber and check electrode
for
particles, scratches,
or
erosion. Check
wafer
clamp
integrity
and
wafer
clamping
force
i.e. can
you
move
the
wafer
by
finger
pressure
when
clamped?
Compare results
with
blank
Si
if
possible.
If
blank
Si
is
OK,
there
is
a
problem
with
the
customer
wafers, i.e.
they
are
warped
or
too
flexible
or
too
thin
(thin
wafers may
require
reduced He
pressure
to
avoid
flexing
of
wafers),
or
the
clamp
ring
does
not
have
sufficient
clamping points
to
maintain
wafer
flatness.
Also check
that
the
measured He pressure
is
correct -
if
the
Helium pressure
gauge
is
faulty,
the
actual pressure could be
far
too
high. Typical CM
gauge
pressure
when
wafer
is
clamped and
helium
pressure
applied
(APC
fully
open and
no
other
gases
flowing)
is
in
the
range 0.3-2mT
for
the
range
of
Helium
pressures given above.
For
the
range
of
helium
pressures given previously: Typical CM
gauge
pressure
when
the
wafer
is
clamped and
the
helium
pressure
applied
(with
the
APC
fully
open and
no
other
gases
flowing)
is
in
the
range
0.3mTorr
to
2mTorr.
Checks
with
the
system
vented:
(1)
Ensure
that
the
electrode
is
very
flat
and clean (no bumps
or
grooves
eroded
into
it) and
that
the
back
of
the
wafer
is
clean and
smooth
(no resist
or
glue
or
anything
else
adhering
to
the
back),
and
is
mechanically
strong
so
that
it
does
not
buckle
or
bow.
(2)
Check
that
the
wafer
lift
star (or pins) retracts
fully
below
the
surface
of
the
electrode. This can be
checked
with
a
flat
edge placed
on
top
of
the
star -
if
it
wobbles
then
the
star
is
too
high.
If
the
star sticks
up
above
the
electrode,
the
helium
will
escape and
the
cooling efficiency
will
be
severely reduced (also, because
of
the
increased
gap
between
wafer
and electrode surface,
which
needs
to
be a
few
tens
of
microns
for
best cooling).
(3)
Check
that
the
clamping
ring
is
actually clamping
the
wafer
to
give
maximum
clamp force. Often,
there
can be a discrepancy
between
the
wafer
clamp
recess
height
and
the
wafer
thickness,
meaning
that
the
wafer
is
not
clamped and 'rattles'
about
inside
the
clamp ring. The clamping
force should
therefore
be adjusted
as
described in Section
6.
of
the
system manual.
You should
try
to
move
the
wafer
with
your
finger,
if
you
can
move
it
then
it
is
not
clamped
properly
and
you
may need
to
temporarily
modify
the
ring
by
adding
strips
of
PTFE
or
aluminium
foil
to
make
it
press
down
on
the
wafer.
(4)
The
wafer
should be placed centrally
in
the
clamp ring.
(5)
Press
down
on
the
wafer
in various places and
see
if
it
moves. This
will
indicate
that
the
wafer
is
not
sitting
down
flat
on
the
electrode. Try polishing
away
any bumps.
Checks
with
the
system
under
vacuum:
(1)
A
good
test
of
whether
the
wafer
is
being
clamped
properly
is
to
measure
the
helium
pressure in
the
chamber (measured on CM gauge)
both
with
and
without
a
wafer
in place
for
a variety
of
helium
pressure setpoints.
There should
be
a clear
difference
between
helium
pressure
with
and
without
wafer.
If
there
is
no
difference
then
it
indicates
that
there
is
a
helium
leak caused by incorrect clamping.
Process
Information
(Information
contained
in
this
document
is
confidential)
Printed: 08 January 2006 09:37 Page
13
of
30
Issue
1:
December
03