| Recall that in order to achieve simultaneous
sputtering and deposition, it is necessary to apply RF bias power to the
wafer: 500-1000 W is typical for a 200 mm wafer. This power is mostly
coupled to the ions which bombard the wafer, and thus mostly dissipated at
the wafer surface. As we've seen in studying heat transfer in a showerhead
reactor, at the low pressures at which HDP reactors operate heat transfer
from the wafer will be almost solely through radiation. Even if the
chamber and chuck are cooled and absorptive, wafer temperatures exceeding
500 C are required to radiate this heat away. Such a temperature is too
high for many applications for HDP processes.
An obvious solution to this problem is to provide some gas (typically
helium due to its inertness and thermal conductivity) on the back of the
wafer to improve heat transfer to the chuck. A pressure of 5-10 Torr will
provide significantly improved heat transfer. However, this much pressure
also exerts a force much larger than the force of gravity on the wafer:
Some means of holding the wafer in place is necessary, lest it
"float" like an air hockey puck, while also filling the chamber
with the backside gas. Mechanical clamps have been widely used in etching,
and are fairly simple to implement. However, they are sources of
contamination and particles. In deposition reactions where the film will
be deposited at the contact between clamp and wafer, each time the clamp
is raised to release the wafer, the deposited film is shattered and
particles are spewed onto the wafer edge. Finally, since mechanical clamps
hold only the wafer edge, the wafers bow under the gas pressure, requiring
a specially machined chuck surface for uniform heat transfer. Most
commercial systems therefore employ an electrostatic chuck to clamp
the wafer in place.
[reference for heat transfer: "Low temperature
etch chuck: Modeling and experimental results of heat transfer and wafer
temperature" D. Wright, D. Hartman, U. Sridharan, M. Kent, T.
Jasinski and S. Kang J. Vac. Sci. Technol. A10 1065 (1992)]
Electrostatic Chucks
| The simplest electrostatic chuck consists of a
conducting electrode, an insulator, and the wafer (electrically
connected to the plasma) as a counter-electrode, forming a
capacitor: |
 |
| The energy stored in the capacitor depends on the
thickness of the insulator "d". |
 |
| The force exerted on the plates of the capacitor can
therefore be found by taking the derivative of the energy with
respect to the separation. Note that the result is quadratic in the
voltage and the thickness of the insulator. |
 |
| Let's look at some typical numbers, using aluminum
oxide as an example insulator. We see that typical voltages are
around 500 volts to achieve a force equal to that resulting from a
backside gas pressure of around 10 Torr (as calculated above). |
 |
Electrostatic Chuck Challenges
- Gas distribution: A flat chuck in
contact with a flat wafer held in place by applied electrostatic
forces forms an entirely too effective gas seal: the backside helium
won't spread uniformly under the wafer if dispensed only in one place.
It is necessary to carve channels in the chuck for the helium to
travel in. The insulator thus has to be thick enough to allow the
channels to be cut, but this means that the clamping force is reduced.
To make up for the reduced clamping force for an ideal insulator, an
electrically leaky insulator is employed: the surface of the insulator
becomes charged except very near the locations of actual contact with
the wafer, so that the "d" term in the force expression is
just the actual separation between the wafer and the chuck surface
(the "Johnson-Raybeck effect"). The leaky insulator is often
prepared by adding titanium to a ceramic, but the titanium would be
removed if the chuck surface were exposed to a fluorine plasma. The
chuck needs to be covered by a protective wafer or blank each time the
cleaning cycle is run: this adds administrative complexity and risk of
destroying a very expensive chuck by procedural error.
- Unclamping: Since the plasma is the
counter-electrode in a "unipolar" chuck, it is difficult to
remove the wafer if the plasma is turned off, either unintentionally
or because a problem is encountered elsewhere in the tool. Bipolar
chucks partially solve this problem by providing two electrodes with
voltage applied between them, but require more complex electrode and
insulator design. In either case, the use of the Johnson-Raybeck
effect means that residual charge remains on the insulator surface
even when all charges are removed from the electrodes, causing the
wafer to tend to stick in place. Some attention is therefore required
to unclamp the wafer: one approach is to apply one or more
reverse-polarity voltage steps (opposite of the clamping potential).
In automated handling systems, a failure is a significant annoyance:
lift pins pressing against a still-clamped wafer may cause it to
shatter, necessitating that the tool be taken off line, opened,
cleaned, and reconditioned before it can be returned to operation.
Note that the chuck design also has to support the uniform application
of RF bias values and provide cooling to remove the considerable resulting
heat. The surface must tolerate temperatures of several hundred degrees C,
and ideally provide access for temperature monitoring equipment. The
design of a reliable, inexpensive, serviceable chuck for HDP deposition is
challenging.
[Some electrostatic chuck references:
"Manufacturing issues of electrostatic chucks"
D. Wright, L. Chen, P. Federlin and K. Forbes, J. Vac. Sci. Technol. B13
1910 (1995)
Electrostatic Chuck Modeling Report SEMATECH 93061683A-ENG
R. Wright & B. Lane 6/24/93
"Electrostatic Clamping Applied to Semiconductor
Plasma Processing" I. Theoretical Modeling and II. Experimental
Results J.-F. Daviel , L. Peccoud and F. Mondon ; J. Electrochem. Soc. 140
3245 (1993)]
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