Effect of Tool Kinematics, Brush Pressure and Cleaning Fluid pH on Coefficient of Friction and Tribology of post-CMP PVA
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Effect of Tool Kinematics, Brush Pressure and Cleaning Fluid pH on Coefficient of Friction and Tribology of post-CMP PVA Brush Scrubbing Processes Ara Philipossian and Lateef Mustapha Department of Chemical and Environmental Engineering The University of Arizona, Tucson, AZ 85721 USA ABSTRACT Real-time coefficient of friction (COF) analysis is used to determine the extent of normal and shear forces during post-CMP PVA brush scrubbing and identify the tribology of the process. Fluid pH and applied pressure have pronounced effects on process tribology and the magnitude of COF. In cases where brush rotation is the only kinematic attribute of the system, low pH results in ‘mixed lubrication’ where increasing brush velocity causes a dramatic reduction in COF in accordance with classical tribological arguments. At lower pressures the tribological mechanism shifts to ‘hydrodynamic lubrication’ with significantly lower values of COF. Regardless of pressure, increasing the pH to 7 and 10.7 causes the tribology to remain in ‘hydrodynamic lubrication’ and results in even lower values of COF. Trends are explained qualitatively by considering the effect of pH on the solubility and gellation characteristics of silica in the silica-water system. Stribeck curves fail to shed light on the tribology of the system when complex ranges of tool kinematics (i.e. wafer rotation and brush oscillation superimposed on brush rotation) are employed. The results are counter-intuitive since they indicate nearly an order of magnitude increase in COF with increasing brush velocity. Nevertheless, results demonstrate the utility of having complex ranges of motion in PVA brush scrubbing such that very low or very high values of COF can be realized by simply increasing the rotational velocity of the brush while maintaining a constant wafer rotation and brush oscillation. INTRODUCTION Brush scrubbing for post-CMP cleaning applications involves direct contact between a soft PVA brush and the surface of the wafer. In these processes, particle removal is believed to occur via a series of steps as follows: • •
PVA brush is compressed when it contact a particle adsorbed on the surface of the wafer Pores and asperities present on the surface of the brush engulf the particle and cause the exposed surface of the particle to mechanically or chemically adsorb on the brush The torque created by the rotation of the brush dislodges the particle from the wafer The moving fluid present on the surface of the wafer, in conjunction with the fluid being pumped in and out of the pores of the brush (during compression and elastic recovery stages of the brush), carries the particle away from the wafer.
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One can therefore envision cleaning effectiveness to strongly depend on several parameters such as: • • • • • •
Mechanical properties of the brush Pressure applied by the brush on the wafer Physical and chemical properties of the fluid Flow rate of the fluid Overall kinematics of the tool Cleaning time
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