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CHEMICAL MECHANICAL POLISHING

17.2 WAFER PROCESSING

Slurry supply
Carrier pad

Chuck Slurry Polishing pad
Wafer

Carrier Polishing table

FIGURE 17.1 The basic CMP process.

17.1.3 Preston’s Law
†
For the CMP process, this relationship is characterized by Preston’s equation that linearly relates
film removal rates to polishing velocity and down force.
R = K*PV
where R = material removal rate
K = Preston’s coefficient
P = applied pressure
V = linear velocity
For a true prestonian system, the exponents M and N are equal to one. Preston’s equation is a
simple tool for characterizing the wafer/slurry/polishing pad interaction and is a fairly good
approximation of relatively mechanical processes such as shallow trench isolation (STI) and
interlevel dielectric (ILD) CMP. Some CMP processes like Cu and STI do not necessarily follow
Preston’s law. In general, it is safe to estimate material removal rates by Preston’s law most of
the time. However, if the abrasive chemistry or abraded materials change during the process, the
system will not follow Preston’s law.
17.1.4 Other CMP Terminology

Along with material removal, the objective of CMP is to flatten the wafer surface both globally
(within the wafer) and locally (within a die). Planarization efficiency is defined by the amount of
material removed in comparison to the step height reduction of a nonplanar feature on a wafer. In
general, larger features require more material removal to planarize. Smaller features can actually
planarize too quickly, resulting in feature erosion. In metal CMP processes such as tungsten and
copper, overpolishing can also manifest itself in feature dishing. Dishing occurs when a large metal-
lic feature is over polished in the center, resulting in a dish-shaped profile when viewed from the
film cross section. Lastly, control in CMP requires stopping the polishing process at the right time.
Much like RIE systems, CMP processes require good selectivity and process endpoint control. The
best CMP processes produce a planar surface with minimal material removal. This is the most dif-
ficult part of CMP process optimization. In general, processes that generate globally uniform pol-
ishing uniformity yield poor local planarization. This is one of the true engineering tradeoffs of the
CMP process.

† Preston F., Journal of the Society of Glass Technology 11, no. 214 (1927).

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