The first step in copper CMP is to remove the bulk of the copper, typically stopping on the underlying Ta/TaN diffusion barrier (Fig. 1). At this point, since tantalum has quite different polishing properties than copper, it's often desirable to switch to a different polishing slurry and perhaps a different polishing pad in an effort to obtain high selectivity between the barrier and copper. This is particularly important in cases where the copper thickness is not uniform across the wafer, leading to areas where the barrier is exposed for long periods of time while the thicker areas of copper continue to be polished. This time — from when the diffusion barrier is first exposed to when the last remaining copper is cleared — is called the overpolish time.
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