First Serial Rights, (c) 1999, Robert Widas -
April 21
Introduction
Types
Monopole
Bipolar
Tripole
Multi-polar
Materials
Anodized Aluminum
Aluminum Nitride
Polyimide
Ceramic
Flame spray
Johnsen-Rahbek
Issues
Clamping
Thermal Characteristics
RF Effects
Trouble-shooting
Goals
Introduction (c)
1999, Robert Widas - April 21 #38744
Electrostatic clamping is widely used on single substrate process equipment in
the semiconductor industry. In this article we will discuss the different
configurations, materials, and troubleshooting of Electrostatic chucks
"ESC's".
The evolution from mechanically clamped systems to ESC type systems was
predicated by device real estate, device yield and particle issues.
Monopole
An ESC is basically a capacitor, see Figure 1 for an example of a monopole ESC
and a capacitor, the two are similar. There are two plates with a dielectric
between them. The lower plate can be considered the metal mounting for the
dielectric. The dielectric is the insulating material between the two plates.
The upper plate in this configuration is the substrate. In Figure 1 the lower
plate is connected to a DC power supply. The ESC charges equal and opposite
signs across the dielectric to the substrate. The substrate will be securely
clamped to the dielectric when there is a return path for the ESC power supply.
The normal mode of operation for a monopole ESC is for the electrode to be
connected to the negative pole of the power supply. The substrate is connected
through the plasma to ground. This implies that the substrate is not securely
clamped prior to exposing the substrate to the plasma or other return path.
Bipolar
In the bipolar configuration the ESC can clamp the substrate irrespective of a
plasma. The return path is through the other pole of the ESC. Figure 2
illustrates a simple bipolar arrangement. This configuration uses the positive
and negative potential of the power supply to Electro-statically clamp the
substrate.
Tri-polar
A tri-polar ESC contains three poles. The inner two poles are similar to the
bipolar configuration, and are used to clamp the substrate. The outer pole can
be used as either a plasma shield or a substrate bias pickup point.
Multi-pole
Multi-pole ESC's use either AC or DC that is phased to each pole of the
electrode. The phasing of the voltage applied to the electrode permits
"rapid clamping and release". The supply and control circuitry
required for this type of ESC is more complicated than the monopole, bipolar or
Tripole configurations. The multi-pole configuration is used to minimize charge
build up on the substrate and also helps reduce de-clamping issues.
Anodized
The anodized aluminum ESC is used in plasma and deposition systems where the net
voltage delivered to the RF powered ESC is < 700 volts. The dielectric
breakdown of the anodized aluminum is dependent on the thickness of the
dielectric. A coating of 1 mil thickness experiences breakdown between 250 and
890 volts. This wide window is the result of the different process steps
performed during manufacture. All ESC process information by the manufacturers
is considered proprietary and will not be discussed in this article. A listing
of ESC suppliers is included at the end of this article. The dielectric constant
of anodized aluminum is 9-10.
Aluminum Nitride
Aluminum Nitride is used in high temperature process environments. It's thermal
conductivity is similar to Aluminum. The dielectric constant is approximately 10
which is slightly higher than the Anodized materials. The dielectric strength is
rated at 305 volts per mil. The Aluminum Nitride ESC is more expensive than the
other types.
Polyimide
Polyimide films have been used successfully for electrical isolation. This
permits these films to be used in ESC's. The material is available in various
thickness'. The Polyimide can be applied over a metal plate to form a capacitor
to the substrate. The Polyimide has a dielectric constant of 3.4 . The breakdown
voltage for the Polyimide coating is rated at 560 volts per mil.
Ceramic
Ceramic ESC's are thin plates of Alumina Oxide that has been fired and ground
flat. These plates are generally much thicker than their anodized counterparts.
Most ceramic ESC's are thicker than 4 mils. The problem with ceramic ESC's is
the large voltage necessary to clamp a substrate across the larger dielectric
thickness. This large voltage creates problems when it comes time to release the
substrate, and with perceived device damage. The ceramic ESC's transfer less
heat than their Anodized counterparts.
Flame spray
The flame-spray technique has gained renewed interest lately due to its ease of
application and its potential for customizing the performance of the ESC. The
flame-spray technique uses a "D" gun to ignite and sputter deposit
ceramic material on a surface. The material can be doped to control the
Johnsen-Rahbek effect. This type of dielectric manufacture is being tested in
semiconductor applications and results to date indicate it will successfully
meet device manufacturer requirements.
Johnsen-Rahbek
The ceramic is doped to control its resistivity to a specific value. The doping
causes the ceramic to exhibit the Johnsen-Rahbek effect. This effect permits the
ESC to be charged at a high voltage. This high voltage charging step drives the
charge towards one end of the dielectric. This reduces the effective distance
between the plates. The result is an increase in the capacitance due to a
decrease in the distance between the plates. The ESC can then be cycled to a
lower voltage for substrate processing. This gives increased clamping over a
non-doped device. The constraint on this type of clamping is that the dielectric
is severely polarized after the high voltage charging step. In order to release
a substrate reliably the ESC voltage potential needs to be reversed to
redistribute the charge throughout the dielectric.
Clamping
Substrate clamping is a function of the quality of the capacitor. The standard
formula for a capacitor is C=(A*K*Eo)/d where C = capacitance, A = area of the
surface, K = the dielectric constant of the material, Eo = permittivity of free
space, and d = the thickness of the dielectric. The capacitance can be raised by
increasing the area, or the dielectric constant. It can also be increased by
reducing d.
The force applied to the substrate is a function of F=(C*V2)/(2*d).
V = the voltage applied to the electrode. The force can be increased by
increasing the voltage or the capacitance. The voltage will have a much large
effect on the force due to the squared term.
The pressure the ESC should clamp to is a function of P=F/A. From this equation
it seems evident that whatever increases the force increases the substrate
release pressure.
The clamping of the ESC can be measured in a variety of ways. Our preferred way
of measuring the substrate release time is to introduce a controlled pressure
behind the substrate. The leak rate between the substrate and ESC is measured to
determine when the substrate has been released.
Thermal Characteristics
The main advantage an ESC has over mechanically clamped systems is its substrate
temperature control. The ESC is flat and the substrate conforms to the flat
surface finish much better than clamped systems with domed electrodes. The ESC
applies a uniform force against the substrate surface as opposed to maintaining
pressure around the outer edge of the substrate as in mechanically clamped
systems. Heat transfer is a combination of conduction, convection, and radiation
for the ESC. Testing has indicated that without a backside gas the heat transfer
is severely limited. This will result in uneven substrate temperature
distribution, device heating, and possible yield loss. The backside gas fills
the voids between the ESC and substrate. The heat transfer increases as the
pressure of the backside gas is increased.
RF Effects
In high frequency RF environments the substrate gains a net negative charge due
to the bias buildup on the electrode. This negative charge on the substrate
reduces the force "F" applied during clamping over any negatively
charged area. This reduction in charge must be compensated for in order to
maintain a consistent pressure of cooling gas on the backside of the substrate.
Different techniques are used to compensate for this substrate bias.
Older systems employed an offset voltage equal to the substrate voltage, or so
they believed. The limitation to a fixed voltage is that most production
machines run various recipes. The offset can only be adjusted optimally for one
recipe. All others will fluctuate around this voltage.
A newer concept called bias compensation varies the voltage on the negative
pole. This voltage corresponds to the negative pole ESC voltage plus whatever
bias voltage is built up on the substrate.
Goals for Next Generation Devices in Terms of Substrate Performance
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