Dishing and Erosion

Erosion is another artifact of CMP.  The physical nature of the flow dynamics, coupled with the mechanical aspects of the system cause turbulence.  Turbulence varies across the CMP disk, proportional to the relative speed of rotation.  This is countered by having the wafer also move, relative to the CMP polishing disk.  However, it cannot be completed nullified.  Causes of dishing are the variation in hardness of materials.  Oxide is harder (less malleable) than copper or aluminum, therefore, when CMP breaks through the overlying oxide to expose the metal interconnect, the interconnect can be dished.

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