Hertzian Indentation vs. Fluid-Based Wear

Whether CMP occurs as Hertzian indentation or fluid-based wear is not clear and has been the subject of some debate. The difference between the two wear modes is in the slurry fluid layer between the pad and wafer. As discussed in Section 4.2, if the fluid layer is not continuous, then pad-wafer contact occurs. Note, however, that the pad does not contact the wafer surface directly, but rather the pad presses abrasive particles against the surface. In such instances, the pad will drag the abrasives across the surface, resulting in Hertzian indentation. 

If the fluid layer is continuous, then the pad does not contact the wafer surface, and Hertzian indentation will not occur. Instead, the collisions between abrasive particles and the pad accelerate the abrasive particles. The particles then impinge on the wafer surface, resulting in fluid-based wear. The velocity and angle of approach of the abrasive particles will determine the kinetic 

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