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Plasmalab Oxford Instruments Plasma Technology System Manual 3.2.3 Low-pressure strike facility "Low pressure strike" is a necessary software facility, since for low pressure RIE or ICP processes, the concentra…

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System
Manual
Oxford
Instruments
Plasma Technology
Plasma
lab
3.2.2
ICP
operating
parameter
ranges
For an
ICP
180
or
ICP
380
the
typical process
operating
ranges are:
Total
gas
flows
==
10
to
200sccm. The
maximum
flow
depends
on
type
of
turbo
pump, i.e. its maximum
flow
capacity, and
the
required
operating
pressure.
If
you
need
to
use a
low
pressure, you may have
to
limit
the
flow
rate
to
achieve this.
Pressure
==
1
to
60mTorr.
Below
5mTorr and above 20mTorr
the
plasma may
not
strike easily (or
with
sufficient
stability)
for
certain gases and
power
levels,
so
you
need
to
check this and adjust
the
process
accordingly, since
operating
the
system
without
a plasma
either
on
the
substrate electrode
or
in
the
ICP
tube
could cause damage. This
is
because
it
is
likely
to
cause a
high
reflected power,
or
dumping
of
power
into
matching
unit.
It
is
always essential
to
check
for
a plasma in
both
regions. You can use
the
'low
pressure strike'
feature
in
the
software
to
allow
easier
striking
for
low
pressure processes. For certain
flow/pressure combinations,
the
pressure
controller
may have
difficulty
in
maintaining
a constant
pressure,
therefore
this
may
also be a
determining
factor
in
the
flow/pressure used.
ICP
power
==
approximately
200W
to
2500W (or 4000W
for
ICP
380). The
minimum
power
level
will
be
dependent
on
how
easily
the
plasma strikes
for
certain gases. You
will
need
to
check this and adjust
process accordingly, since
operating
system
without
a plasma
either
on
the
substrate electrode
or
in
the
ICP
tube
could cause damage.
The
maximum
ICP
power
limit
is
set
by
the
power
rating
of
the
RF
generator. However, most processes
perform
well
with
only
moderate
ICP
power
levels. This also helps
to
avoid excessive substrate heating.
Substrate
electrode
RF
power
==
typically
5W
to
400W. A plasma
may
not
strike easily
for
low
power
levels
for
certain
gases.
You
will
need
to
check this and adjust
the
process accordingly, since
operating
the
system
without
a plasma
either
on
the
substrate electrode
or
in
the
ICP
tube
could cause damage.
Helium
pressure
==
0
to
30Torr. Depends
on
the
cooling
efficiency
required
(some processes
benefit
from
no
cooling) and
the
maximum
tolerable
helium
leakage.
Temperature
is
limited
by
the
operating
range
of
the
electrode
or
its heaterlchiller,
depending
on
type
of
electrode
or
heater/chiller used.
NOTES:
(A) The system base pressure
will
be
of
approaching
10-
6
Torr
or
better
when
measured using
the
Penning gauge. However,
the
time
taken
to
reach this pressure
will
depend
on
whether
the
chamber has recently been
vented
to
atmosphere and
the
cleanliness
of
the
chamber walls.
If
the
process chamber / electrodes are anodised,
the
time
will
increase
as
the
anodised surfaces
will
take
longer
to
outgas compared
with
bare metal surfaces.
(B)
Operating
with
chlorine
based processes can cause damage
to
the
electrode unless
it
is
protected
with
a
dummy
wafer.
(C)
Operating
with
a
high-reflected
power
(>5%
of
forward
power)
is
not
advised,
as
this
will
cause
damage
to
the
matching
unit
or
RF
generator.
To reduce
the
high-reflected
power,
adjust
the
process parameters
or
re-tune
the
matching
unit.
Process
Information
(Information
contained
in
this
document
is
confidential)
Printed: 08 January 2006 09:37 Page 7
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Issue
1:
December 03
Plasmalab
Oxford
Instruments Plasma Technology
System
Manual
3.2.3
Low-pressure
strike
facility
"Low
pressure
strike"
is
a necessary
software
facility, since
for
low
pressure
RIE
or
ICP
processes,
the
concentration
of
free electrons being produced simply isn't
high
enough
to
start
and
maintain
the
ionisation 'chain reaction'
or
avalanche
which
is
required
to
initiate
the
plasma.
So
it
is
necessary
to
use
higher
pressures
during
the
first
few
seconds
of
the
process step. The
software
therefore
allows
the
operator
to
raise
the
pressure
briefly
at
the
start
of
the
plasma process
to
enable
the
plasma
to
strike.
This does
not
cause any problems
for
the
processes involved, because
the
time
taken
to
strike
the
plasma
at
the
higher
pressure
is
very short, and
will
be a very small percentage
of
the
total
process time.
3.2.4
DC
bias
With
any plasma etch system
it
is
important
to
remember
that
DC
bias readings can be affected by surface
coatings
on
the
lower
electrode. These can include:
(a)
Quartz
cover
plate
(or any
other
insulating cover
plate
material),
(b) Electrode surface anodisation,
(c)
Polymer coating
on
electrode surface
generated
by process,
(d)
Any
other
insulating coating generated by plasma.
In all
of
these
cases,
the
DC
bias reading
will
be inaccurate
due
to
the
lack
of
DC
contact
to
the
plasma.
Quite
often,
the
measured
DC
bias
will
be close
to
zero
if
there
is
a
complete
insulation
of
the
electrode
or
if
the
process conditions are such
that
there
is
minimal
contact
between
the
exposed areas
of
the
electrode and
the
plasma.
This
is
not
to
say
that
the
sheath
potential
(or
ion
energy) has actually reduced
to
zero; in
fact
it
has
not
changed
at
all
from
the
non-insulated
case.
It
is
simply
that
the
measurement
of
this value via
DC
bias
is
no
longer
possible.
It
is
also
quite
common in these
cases
for
the
DC
bias reading
to
vary sharply
throughout
the
run.
It
is
therefore
recommended
that
the
DC
bias
for
a
particular
process
condition
be measured
with
a bare
electrode (e.g.
prior
to
loading
the
quartz
cover plate). In load locked single
wafer
systems,
it
is
necessary
to
measure
the
DC
bias
without
a
wafer
loaded,
as
this
will
expose
the
central
wafer
lift
pin
to
provide
accurate
DC
bias measurement.
This
is
the
only
way
of
obtaining
a reliable/stable
DC
bias measurement.
If
the
electrode
is
anodised,
it
may also
be
necessary
to
ensure
that
the
wafer
lift
pin
is
exposed and clean
as
this
will
be
the
only
conductive
path
for
the
DC
bias measurement.
If
there
is
no
wafer
lift
pin (e.g.
RIE
80 Plus),
it
may be necessary
to
use
the
central locating pin
for
the
DC
bias measurement
or
even
to
scratch away a small area
of
anodisation.
If
it
is
suspected
that
there
has been
polymer
deposition
on
the
electrode,
it
may be necessary
to
clean
off
the
polymer
(with
an O
2
plasma
or
by
mechanical cleaning)
to
allow
an accurate measurement
of
the
DC
bias.
3.2.4.1
Electronegative
gas
mixtures
Strongly electronegative mixtures, such
as
SF
6
gas above 10
Pa
(70 mTorr), may give close-to-zero
DC
bias.
This
is
not
a
fault,
but
is
due
to
the
formation
of
negative ions in
the
plasma. A
DC
bias exists
only
because
of
the
difference
in
mobility
between
the
negative and positive charges in a
normal
plasma.
When
both
charge carriers are heavy ions,
the
plasma does
not
rectify
and
the
dc bias collapses.
Process
Information
(Information
contained
in
this
document
is
confidential)
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System
Manual
Oxford Instruments Plasma Technology
Plasma
lab
3.2.4.2
ICP sources
The
DC
bias on
the
lower
electrode
can be a strong
function
of
the
power
in
any auxiliary plasma source,
for
a
fixed
lower
electrode
RF
power.
At
low
ICP
power,
there
can be a rise in
DC
bias reading, because
of
the
increase
in
the
effective
area
of
the
grounded
electrode.
As
the
ICP
power
rises,
the
DC
bias
is
reduced,
as
ions
from
the
source begin
to
dominate
the
ion
flux
at
the
electrode. This
reduction
in
DC
bias
is
a
good
sign
of
plasma
from
the
source reaching
the
lower
electrode.
3.2.4.3
DC
bias
polarity
The surface
of
the
RF
driven electrode takes a
negative
bias
with
respect
to
the
plasma. However,
the
literature
and
the
industry
tend
to
refer
to
DC
bias
as
a positive
quantity,
and
we
follow
this
convention
in
our
equipment.
3.2.4.4
DC
bias
control
The
DC
bias value
will
depend
on
all
the
process parameters and several aspects
of
the
machine condition.
When
the
DC
bias value
is
important
to
the
correct
operation
of
the
process,
it
is
often
possible
to
use
DC
bias
as
the
recipe parameter, and have
the
RF
bias
power
as
a free parameter. The
control
mode
(power
or
bias)
is
selectable
on
the
PC
screen
where
this
feature
has been provided. However,
if
you
are
planning
to
use
DC
bias
control
mode,
it
is
worth
noting
the
points raised in
the
preceding sub-sections
about
potential
causes
of
inaccuracies I
variability
in
DC
bias readings.
3.2.4.5
DC
bias
reproducibility
DC
bias
is
a very sensitive
indicator
of
the
state
of
the
plasma
tool.
While
this makes
it
a useful
parameter
to
measure and record,
it
also makes
it
difficult
to
ensure
that
the
value
is
consistent
from
day-to-day on
the
same machine, and
between
nominally
identical systems.
It
is
occasionally requested
that
the
DC
bias reading
is
adjusted
to
make
the
reading
the
same across
different
tools,
but
we
have
taken
the
view
that
it
is
better
to
know
the
actual value.
The main
hardware
causes
for
DC
bias changes are:
(a) Electrical
conductivity
of
the
cooling
medium
for
the
electrode
or
automatch. Check this
by
running
briefly
with
the
cooling
fluid
removed completely. Shifts in
DC
bias
between
dry
and cooled states
of
up
to
5%
are common. A
shift
of
more
than
10%
indicates
the
fluid
is
too
conducting.
(b)
Inadequate
cooling
of
a
fluid-cooled
automatch.
This
is
sometimes
linked
with
fluid
conductivity,
with
an electrochemical reaction
depositing
material in
the
stainless steel
bulkhead
fitting
of
the
automatch.
(c)
Changes in
dark
space shield distance.
If
the
table
is
not
seated correctly,
the
shield
gap
changes and
the
DC
bias value
is
altered
(d)
Oxidation
of
components
in
the
RF
delivery path. The connection
between
the
automatch
and
the
electrode
carries several amperes
of
RF
current. The connection must be sound,
or
it
can become heated,
with
a progressive
addition
to
the
losses
(e)
Loss
of
the
ground
path.
RF
current
driving
the
plasma
flows
in
a closed
loop
circuit. High
resistance
or
breaks
in
the
ground
return
path
will
alter
the
DC
bias -
but
usually manifest
themselves
first
as
RF
interference
problems.
Pay
particular
attention
to
any straps
securing
the
dark
space screen,
the
RF
shielding
under
the
lower
electrode, and
the
mounting
of
the
automatch.
Process
Information
(Information
contained in this
document
is
confidential)
Printed: 08 January 2006 09:37 Page 9
of
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Issue
1:
December
03