In situ measurement of pressure and friction during CMP of contoured wafers

In situ fluid film pressure and interfacial friction measurements during chemical mechanical planarization (CMP) are reported over a range of applied loads (27.6-41.4 kPA or 4-6 psi) and relative pad/wafer velocities (0.35-0.58 m/s). The slurry film pressure beneath contoured test wafers was measured using a novel experimental setup that enables dynamic data collection. The friction data have a repeatability of similar to 10%. The uncertainty of the pressure measurements and the computed down forces were +/- 2.1 kPas (+/- 0.3 psi) and 20%, respectively. The data indicate that wafer shape, specifically global curvature, is a significant factor in determining the lubrication regime during CMP. Full hydrodynamic lubrication, in which the slurry fluid film supports the entire applied load, was not realized for either concave (center high) or convex (center low) wafers. The data for concave wafers show that -6% to 37% of the applied load is supported by the slurry film, where the negative sign indicates suction conditions that were obtained at the lowest applied load condition. CMP of convex wafers is found to operate closer to full hydrodynamic lubrication, with the fluid layer supporting 36% to 64% of the applied downforce. In all cases, the measured friction coefficient decreased as the support of the fluid layer increased (higher positive pressures). CMP of concave wafers is more sensitive to changes in applied downforce, while the convex wafer type was most affected by changes in the wafer/pad rotation speed, which in turn determines effective slurry film velocity beneath the wafer. Overall, the CMP conditions seen in these scaled experiments operate primarily in the partial lubrication regime shifting closer to hydrodynamic lubrication for convex wafers at the high load, high speed conditions. (c) 2005 The Electrochemical Society. All rights reserved.