Effects of retaining ring on contact stress uniformity in chemical mechanical planarization

Chemical mechanical planarization (CMP) plays a critical role in achieving near-perfect planarity of interconnection and metal layers in ultralarge-scale integrated (ULSI) semiconductor devices. In practice, during the CMP process there is a retaining ring that keeps the wafer being planarized in place. To understand how the retaining ring's existence affects the uniformity of the contact stress-and hence the uniformity of the material removal rate-on the wafer surface, here we use two-dimensional fluid film lubrication and contact mechanics models to calculate the contact stress and fluid (i.e., slurry) pressure distributions on the wafer-pad interface. In particular, the effects of the retaining ring width and its back pressure on the contact stress uniformity are examined. Our numerical results indicate that the presence of a retaining ring generally decreases the exceedingly high contact stress (due to stress concentration) near the wafer edge, and hence improves the contact stress uniformity. Meanwhile, it also increases the slurry flow resistance and therefore reduces the slurry flowrate. The reduction in slurry flowrate, however, is relatively insignificant. Through systematicparameter studies, we deduce certain guidelines for choosing the optimal retaining ring width and back pressure that minimize the contact stress non-uniformity on the wafer surface.