Pads and pressure

Experimental [66] and theoretical [47] studies of the actual contact area Ac show that during CMP it is only a few tenths of a percent of the total wafer area Aw, and increase with applied pressure P. Stiff pads have a smaller contact area than compliant ones at any given pressure. This has been modeled [47], for pads with Young s modulus E, as Equation 5.1 

Equation 5.1 has some important implications. One is that the effective pressure of the pad pushing the abrasive into the wafer depends on Young s modulus of the pad but is independent of the applied polishing pressure. This can be shown as follows The force applied by the pad is E = EAw = Eeff Reusing Equation 5.1 to eliminate Ac leads to the following expression for the average effective pressure  

FIGURE 5.13 Fraction of pad surface in contact with the wafer as a function of pressure for two different pads. Data points are from Yu et al. [47]. Lines are fit to Eq. 5.1 with a = 0.00925. 

Qualitatively, since the contact area increases linearly with applied pressure, the effective pressure is constant for a given pad. Soft, compliant pads have a larger contact area and lower effective pressure whereas hard, stiff pads have a smaller contact area and higher effective pressure. Thus soft pads push abrasive particles against the wafer over a larger area but with less force than hard pads do. 

Since the abrasive-wafer surface contact area increases with the force applied, harder pads will remove more material per abrasive than soft pads, but soft pads—with larger contact areas—can remove more material than hard pads at low pressures. 

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