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CHEMICAL MECHANICAL POLISHING

CHEMICAL MECHANICAL POLISHING 17.5

17.2.3 Shallow Trench Isolation
Lastly, Ibarra and 3M have developed fixed abrasive polishing pads. These ceria-based pads generate
abrasives as they are worn during CMP. The process of generating abrasives as a pad wears is called
friability. Process lubrication is provided by basic KOH solutions, while the cerium oxide particles
abrade higher spots within the wafer die. Ceria slurries are also non-prestonian in behavior (pressure),
offering high removal rates for rough wafer surfaces and almost none for a smooth film or die. Friable-
fixed abrasive pads and self-planarizing ceria-based polishing abrasives work very well for STI appli-
cations, since erosion through the thin nitride layers applied directly to the silicon wafer surface will
result in contamination of the substrate (typical within a die range of less than 100 Å). When the feature
surface becomes smooth, these systems stop polishing quickly, almost like magic. Polishing friction
increases dramatically with some ceria-based systems as the wafer surface planarizes.

17.3 REVIEW OF CMP PROCESS CONTROL

CMP processes are conceptually simple, but can be rather difficult to control. The most common
process issues faced in the wafer fab are within wafer polishing uniformity degradation, feature pla-
narity changes, and bulk material removal rate drift. With today’s more sophisticated equipment,
process control has become a bit easier due to the use of closed-loop systems. In addition, over the
past few years the quality of CMP process consumables has improved dramatically. Still to this day,
slight changes in CMP polishing pads and/or slurries will have a dramatic impact on CMP process
stability. The key to CMP process control is to make sure that each wafer processed is subject to the
exact same conditions, both chemically and mechanically. This is the challenge of CMP in a pro-
duction environment.

17.3.1 Removal Rate Control
Basic process tuning for CMP involves adjusting the table or platen speed, carrier angular velocity,
and applied polishing pressure (downforce). It is typical for the carrier rotation rate to be similar to
the platen rotation rate (in the same direction) for most rotational CMP processes. When the carrier
and platen angular velocities are the same, the entire wafer surface will be subjected to globally uni-
form linear velocity. By increasing the carrier angular velocity relative to the platen speed, the wafer
will polish the edge fast, since linear velocities at the wafer edge will be relatively higher than the
center. Conversely, as one reduces the carrier angular velocity the wafer will polish the center fast. To
increase blanket film removal rates, one just has to increase the polishing pressure and/or platen angular
velocities (remember to keep the angular velocity of the carrier about the same as the platen for the
most uniform polish). This relationship holds well for lower linear velocities and prestonian systems.
At very high linear velocities or platen speeds, the wafer can hydroplane when the polishing down-
force is inadequate to overcome hydrodynamic pressures. During hydroplaning the wafer will polish
with a poor polishing removal rate and uniformity control. If the polishing downforce is increased too
much, the platen drive system may not be able to overcome the polishing friction forces required to
rotate the platen. In addition, within the die, the planarity will be negatively impacted as one increases
the polishing downforce. This is due to more die level pad/film interaction between features, reduc-
ing geometric selectivity and therefore planarization efficiency. Typical platen speeds for 24 in rota-
tional polishing tools are 25 to 90 rpm. Polishing pressures for oxide CMP range from 3 to 8 psi.
WCMP and Cu CMP processes use similar polishing pressures and speeds (tool dependent).

17.3.2 Within Wafer Uniformity Control
Process tuning can also be influenced by the construction of the wafer carrier. Older CMP systems
use back referencing carriers. These carriers comprise a steel or ceramic plate with an applied carrier

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