Effects of pad grooves on chemical mechanical planarization

Chemical mechanical planarization (CMP) has played an enabling role in producing near-perfect planarity of interconnection and metal layers in ultralarge scale integrated devices. For stable and high performance of CMP, it is important to ensure uniform slurry flow at the pad-wafer interface, hence necessitating the use of grooved pads that help discharge debris and prevent subsequent particle loading effects. Here, using two-dimensional lubrication theory and contact mechanics models, we examine the effects of pad groove designs (viz. their width, depth, and spacing) on slurry flow in CMP. It is found that the presence of pad grooves generally increases the slurry flow rate (which clearly facilitates debris discharge) and the magnitude of the subambient fluid pressure (i.e., suction) on the pad-wafer interface. The increased suction implies higher contact stress on the pad-wafer interface, and hence the local material removal rate is expected to increase as well. However, our numerical results suggest that, as a grooved pad has less contact area for effective interaction with the wafer, the overall material removal rate is expected to increase as well. However, our numerical results suggest that, as a grooved pad has less contact area for effective interaction with the wafer, the overall material removal rate is decreased by the presence of pad grooves. There is therefore a trade-off between slurry flow rate enhancement and material removal rate reduction in pad groove design.