Dispersion number studies in CMP of interlayer dielectric films

Determining the factors that cause flow nonidealities during chemical mechanical planarization (CMP) is critical for controlling and optimizing the process. This study explores aspects of the fluid dynamics of CMP on interlayer dielectric films. The residence time distribution of slurry under the wafer was experimentally determined and used to calculate the dispersion number of the fluid in the wafer-pad region based on a dispersion model for nonideal reactors. Furthermore, lubrication theory was employed to explain trends in flow behavior as operating conditions were varied. The results indicate that at low wafer pressure and high relative pad-wafer velocity, the slurry flow exhibits nearly ideal plug flow behavior. As pressure increases and velocity decreases, flow begins to deviate from ideal behavior and the slurry becomes increasingly more mixed beneath the wafer. These phenomena were found to be the result of variable slurry film thickness between the pad and the wafer, as measured by changes in the coefficient of friction between the pad and the wafer.