76
monosized, each asperity traps equal number of particles in CMP resulting in equal
removal rate by each asperity and each particle(Fig. 3-16b).
Wafer
Pad aspriilics
. .
\/
Pad asperities
(b)
Fig. 3-16. Schematic of pad, slurry and wafer in PERC I model. (a)Before contact. (b)
During CMP.
Whereas in the PERC II model (Fig. 3-17a), the spherical pad asperities are
assumed to have a height distribution as observed experimentally [116]. As the slurry is
assumed to be polydisperse, each asperity traps different number of particles of different
particle sizes in CMP according to pad asperity height and particle distribution (Fig. 3-
17b). Thus the PERC II model includes realistic contributions of pad and wafer
characteristics compared to the PERC I model. The model formulation is detailed in the
following section.
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