Pads, Dishing, and Erosion

When dishing occurs in a patterned wafer, the ILD beside the dished area polishes faster than the ILD in isolated field regions. The ILD height difference between patterned and field areas, shown in Fig. 5.20, is called erosion. Experimentally, erosion depends on the pattern density. 

FIGURE 5.20 Surface of a wafer (a) before CMP, (b) at the ideal endpoint, and (c) during overpolishing. 

Slurry selectivity a is commonly defined as the ratio of the polishing rate of blanket wafers at a given polishing pressure and speed, a = Large cr 

Pad expansion into a dished region of height h leads to a pressure difference AP between the top and the bottom. This pressure difference is shown in Equation 5.7 as the sum of two terms. 

The first term is for the conventional Hooke s law—Young s modulus expansion/compression of an elastic pad material with Young s modulus E, 

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