Hi, I'm Jim McVittie with
the Stanford Nanofabrication
Facility here at
Stanford University.
This is Part Two of a four-part
lecture series on dry etching.
In this part, I'm going to
cover the basics of plasmas
and the types of
dry etching tools.
Previously, I talked about an
introduction to dry etching.
The following lectures will
cover dry etching mechanisms
and choosing dry etching
processes and tools.
In this presentation,
I'm going to cover
the basics of plasmas,
plasmas and sheath
regions, parallel
plate configurations,
inductive coupled plasmas--
or ICP configuration--
and finally, downstream
configuration.
Let's start with the
basics of plasmas.
Plasma is an ionized gas.
Plasma is commonly used
in fluorescent lighting.
Over here in this diagram,
I show basically a vacuum
chamber.
We have electrical power.
We're going to be limiting
our discussion to RF power,
since you can also
generate a plasma using DC,
but from a plasma
etching standpoint,
DC plasma causes damage
to wafers, device wafers.
So we're going to be
only talking about RF.
I show here gases coming in.
I show gases going out by
way of the vacuum pump.
I see light coming out.
And in the plasma,
I have neutrals.
There are going to be radicals.
In the initial
gases, I have ions.
And then I have electrons.
I already talked about radicals.
Plasmas are a steady
state a process
that results after gas
breakdown The electrons gain
energy from the applied field.
So I'm going to be
putting RF power that
can generate a field that is
going to heat up or give energy
to my electrons.
The higher energy
electrons, whose collisions
will have ionization and
excitation and fragmentation
of our source gas molecules.
As I said, plasma or
steady state process
results after breakdown.
In the plasma region,
we mainly see the glow.
We have a charge balance,
where the density of electrons
are going to be balanced
by or equal to the density
of positive ions.
And typically in the
processing plasmas,
we're talking about
partially ionized gases.
So there's less than 1%
of the molecules or atoms
in the chamber are ionized.
So it's mainly neutral species.
The glow is from the decay
of short-lifetime excited
species-- when I
say short lifetime,
I'm talking about nanoseconds.
So any place you see glow,
that's where electrons are hot.
They're basically breaking
down and fragmenting the gas.
And when it's
dark, that means we
don't have any hot electrons.
Typically, such a
plasma at 20 millitorr,
we're talking about neutrals
in the range of 10 to 15
per cubic centimeter, a
ratio of radicals to neutrals
from 50% down to 10%, and
ion to neutral ratios from 10
to the - 3 to 10 to the - 6.
So basically, there's mainly
neutrals in the chamber.
Now, I'm going to talk about
RF plasmas and sheath regions.
In this diagram here, I
show a parallel plate-type
configuration.
So basically, I'm showing I have
one of these electrodes that
can be grounded.
And the opposite electrode, I'm
going to be applying RF power.
And after the
breakdown of the gas,
at equilibrium what
I'm going to see
is that I'm going to have
a central glow region.
And I'm going to
have dark spaces
on both sides of the
plasma - between the plasma
and the electrodes
will basically
be plasma on all surfaces.
And the reason we develop
these dark spaces is
the electrons being very
small, travel much faster
than the ions.
And so their numbers are like
- electrons have velocities
of 100 times faster than ions.
So it means when we
have the glow region,
the first thing
that's going to happen
is the electrons are going
to start streaming out
of this region.
And they're going to charge
up all of the surfaces.
So I'm showing charge here
on the two electrodes.
As these surfaces
charge up, we're
going to develop an
electric field that
can be retarding for electrons.
So if the electric
field is this way,
electrons are going to want to
start turning around, and going
the opposite direction.
So I'm showing here
schematically I
have a high current of electrons
coming out of the plasma,
but most of those electrons
get returned to the plasma,
and only a small amount
make it to the surface.
But the amount that
makes it to the surface
is going to be balanced
by the same amount of ions
that make it to the surface.
So at the surface, the electron
current and ion current
will be equal.
And so we have the
electrons and ions
move from the plasma
towards all surfaces.
The electrons want to leave much
faster, because they're going,
let's say, 100 times faster.
And to maintain steady-state
balance in the plasma,
we form a boundary
layer that we're
going to call the "sheath"
or "depletion region."
And this makes the electron
and ion current, or loss rates,
from the plasma equal so we
can maintain our steady state.
Here again I'm showing
our diagram of the plasma
and the sheath region.
And so I talked
about the electrons,
and how the electric field
turns most of the electrons
back so we have this
balance at the electrodes.
But the ions, now,
are accelerated
by this electric field.
And so they accelerate
toward the surface.
And they gain both energy
and directionality.
So the ions come
in isotropically
into the boundary between
the plasma and the sheath.
And at the surface,
they're very directional.
In addition, they gain energy.
And this can be 100's of
eV coming from the ions
crossing the sheath.
So the ions gain,
the energy gain
depends on the voltage across
the sheath and the collisions.
So if I go to low
pressure, I'm going
to have a few collisions
or no collisions.
And the ions are going
to get the full energy
across the sheath.
And so if I integrate
the electric field here,
I have what's called the sheath
voltage across the sheath.
And that's basically the
voltage between the plasma
and the electrode.
And with no collisions, the ions
will basically gain the sheath
voltage in terms of energy.
So lower pressure,
less collisions,
more directionality,
more ion energy.
The sheath voltage depends
on the system configuration,
the applied power and pressure.
And I'm going to talk
a little bit more
of that on the next slide.
Or I'll talk about it actually
in a couple of slides.
So first, I want to give
you an overview of the four
configurations I'll be
going into more detail.
So first, I have what
we call the CCP-plasma.
And this means capacitive
coupled plasma.
And I'll talk about
the meaning of that
in the following slides.
And I have what we call
the plasma mode, where
you have equal
electrodes, RIE mode,
where you have one
electrode much smaller
than the other electrode.
And then I'll talk
about the ICP case,
inductive-coupled plasmas.
And finally, I'll talk
about downstream plasmas.
So let's start talking
about the CCP case.
So here you have the CCP, or
capacitive coupled plasmas.
And again, I'm showing
the parallel placed case,
where our plasma is sitting
between the electrodes,
with a sheath on both sides.
So the plasma is
driven by RF currents
going through the electrodes.
So we have the RF
supply here, there's
a RF current going
through the top electrode.
It goes across the sheath
region, through the plasma,
and then out the other side.
And this RF current
is what's going
to be keeping the plasma going.
So we never use DC supply for
etching because of the damage
issue.
RF - we normally
use 13.56 megahertz.
It's industrial plasma
frequency set up by the FCC,
although some of the time, there
are special cases where we use
lower frequency at the wafer.
We also use lower
frequency for the ICP,
but at the wafer,
typically 13 megahertz.
RF current flow determines
the sheath voltage.
Let's talk about
what this means.
So the electrons carry the RF
current in the plasma region.
So in the plasma I
have lots of electrons.
They're very mobile.
They carry the current.
But at the two sheath
regions / depletion regions,
there's not enough electrons
to carry the RF current.
And so how the RF current
gets across this region
is by what we call displacement
current or capacitive current.
And that way, our current
gets across the capacitor.
And there's where
we build up charge
on one side of a capacitor.
And on the two plates
of the capacitor--
the charge on one side induces
a charge on the other side.
And we have basically
an oscillating charge
across this capacitor.
So we take a look at
our sheath region.
We have an electrode
here, so it's
easy to have an oscillating
charge building up and going
away on the metal electrode.
But at the boundary layer,
I don't have any electrodes.
The way I get my
oscillating charge, dQ/dt,
is by the sheath oscillating.
I deplete into the plasma
and then I un-deplete.
And so the sheath will be
oscillating up and down.
And that's going to give
me a charge per unit time
that can allow the displacement
current, capacitive current,
to go across the sheath region.
Now, if we go in and
integrate that charge
that gives us a
voltage across it.
So what we're going
to get at here
is that the RF current, this
oscillating charge here,
is what gives us our
voltage across the sheath.
And basically, if you work
out the details of it,
you're going to get that
the sheath voltage goes
as the RF current density
divided by the electron
density.
So if I have a higher
RF current density,
I'm going to have
a higher voltage.
And this is where the
area comes into play.
Because now, if I change
the area of my electrode,
I can increase my
current density
and increase my voltage.
So let's talk about
it a little bit more.
So we're seeing that the
sheath voltage depends
on the RF current density.
Reducing the electrode area
increases the sheath voltage.
And therefore, I'm going to
be able to increase my ion
energy across the sheath.
There's going to be
two ways we do this.
So I forgot to mention--
this capacitive coupling
across the sheath,
this displacement
current or capacitive current,
is where we get the name.
Capacitive coupled plasma
refers to the capacitive current
across the sheath.
OK, let's talk
about plasma mode.
In the plasma mode, we've got
parallel plate electrodes,
but equal area.
So I have the two
electrodes with equal area.
And in this case, we get
relatively low voltage
across the sheath.
And so we get low ion energies.
The second case is the RIE mode.
This is known as
Reactive Ion Etching.
It's a name that goes back in
the early history of plasma
etching.
It ends up we actually don't
have very many reactive ions,
but the name has stuck.
And that basically
refers to the case
where we have one
small electrode,
and a large electrode.
And typically what we do is we
put the wafer on the smaller
electrode so that we can
then get this large voltage.
And with the
two-electrode region,
when we have a
large electrode, we
have a low RF current
density on the one side,
and a high RF current density
where the small electrode is.
So we get a large
voltage on one side.
And typically in this case,
we neglect the voltage here.
We say all the voltage
across the electrode
is across the sheath on the
small area electrode side.
And in this case, we get high
ion energies in the RIE mode.
And I want to have a definition
here of bias voltage.
And that's basically the voltage
between the two electrodes.
And so it's really the
difference between the two
sheath voltages.
For the RIE each case, the
one sheath voltage is small,
and so it dominates.
So for RIE, we normally say
the bias voltage is basically
approximately equal to the
sheath voltage, the larger
sheath voltage right
above the wafer.
OK, let's talk about
the limitations
of CCP configuration.
One, we only have
one power supply.
And what that means is
that one power supply
controls both the plasma
density and the ion energy.
So plasma density is -- either
we can look at the ion density
or the electron
density of the plasma.
And we can say the
plasma - so one, we use
- the RF in this one
power supply case
has to break down the plasma,
and that controls the ion flux
or the radical concentration.
So this has to do with how dense
my plasma is, so I get the ion
flux and radical generation.
But at the same time,
some of the power
is used up from the electrodes.
So we have a division of power.
Part of the power goes
into generating the plasma,
and part of the power goes
into the sheath acceleration,
basically giving
me my ion energy.
And so in the CCP
mode, we're dividing up
the power between
the power going
into the plasma and the power
going into the two sheaths,
or in the RIE mode,
it's the power
going into the sheath above
the wafer and power going
into the plasma.
So I have this division.
With the CCP mode, I
have one power supply.
And as I increase it, I increase
my energy across the sheath,
and I also increase
my plasma density.
And that ends up
being a limitation.
And this is why we've gone to
the ICP for our configuration
that I'm going to show next.
So here, Inductive Coupled
Plasma-- ICP-- in this case,
I have a vacuum chamber
where I have either a ceramic
or a glass wall.
And I have a coil
wrapped around that wall.
So I have RF going to a coil
on the outside of the plasma.
And what this does, it
induces a toroid of plasma.
So I have, let's
say, three to five
turns on the outside
of the plasma.
And I have a one-turn toroid
in the center of my plasma.
So basically, another name
for inductive coupled plasma
is what we call
transformer coupled.
But always think of it--
we're inducing a single toroid
of plasma in an ICP.
And I'll show you
how we use that.
We can get a uniform
plasma out of it,
and separately we can
control the ion energy.
So inductive plasma using
inductive or transformer
coupling with
minimum RF current.
So when we do this
configuration,
we don't have RF current
going across the sheath.
We don't have that depletion
region and all that.
And so we can put
power into the plasma
without developing a high what's
called "plasma potential."
The plasma can stay
at a low voltage.
We have low sheath
voltages because we're not
depending on RF current
going across a sheath.
So we use a multi-turn
coil on the outside,
and we get a single-turn
toroid of plasma in the inside.
And then the plasma is going
to diffuse out of this toroid.
And I'll show it
in the next slide.
It diffuses out.
And with the right geometry, we
can have a very uniform plasma
right at the wafer.
And so although we have a
toroid, non-uniform plasma
in the center, with the right
geometry and everything--
pressure and all that--
we can get a very
uniform plasma at the wafer.
But it's much lower.
The dense plasma is a toroid,
and it's a much lower density
at the wafer.
So here now I'm
going to show what's
called the ICP with the
HDP-- High Density Plasma.
And what I'm going to do is, we
have our ICP and we couple that
with what's called CCP action.
So we're going to add
RF bias to the wafer,
and more like a
CCP RIE mode, where
we can use the sheath at
the wafer to control the ion
energy, separate from the ICP
that's generating my power,
my plasma.
So I add a second RF power
supply used in a CCP mode
to control ion energy.
And this allows you to
separate the plasma density
into ion energy.
And this configuration is called
the high density plasma or HDP
configuration.
And so repeating
what I said before,
we have a dense toroid
of plasma in the center
that spreads out to the wafer.
There's a number of advantages
now of the ICP or HDP mode
compared to CCP mode, in
that we can operate, one,
at lower pressures.
And we get the separation
now of the plasma density
and the energy-- higher plasma
densities and larger range
of ion energy.
Now I want to talk
about, last part, is
downstream configuration.
In the downstream
configuration, we're
talking about having
a plasma chamber
separate from the wafer, where
we have no plasma interacting
with the wafer.
So we basically
have a plasma stop.
And this allows us to
get away from all the
- we get away from
plasma charging and ion
bombardment and all
that, so it's really
for reducing damage.
The wafer sits outside
the plasma chamber.
Active radicals
diffuse to the wafer.
So we can get the active
radicals, that have long life,
can go from the plasma
through the plasma stop
and down through the wafer.
And we set up the geometry so
everything is nice and uniform
at the wafer.
It eliminates damage.
So there's no ions or electron
bombardment, no charging,
no UV, but - and we get
no etch directionality.
It's going to be
totally isotropic as far
as the etching goes.
And the plasma generation
method, we can use CCP method.
We can do ICP.
We can do microwave.
There are lots of ways we
generate plasma in here.
And then once we
have the plasma,
we basically leak it
through to the wafer area.
And a common application is
for resist/polymer stripping
and for surface modification.
Sometimes plasma is useful
to affect the surface
chemistry of a wafer.
OK, summary-- plasmas are
a steady state balance
between generation and loss of
ions, electrons, and reactive
radicals.
We have plasma sheaths that
control ion bombardment or ion
energy and directionality.
The ion energy can be controlled
by the geometry of our system,
where we do RIE or plasma mode.
And we can control
it by wafer bias
in a HDP, ICP configuration.
And the configurations
include the CCP--
capacitive coupling cases, or
parallel plate configurations--
ICP, and downstream.
In the next video, I'm going
to talk about dry etching
mechanisms.
Thank you.
