The Role of Pad Topography in Chemical-Mechanical Polishing

In this paper, the role of pad topography on material removal rate (MRR) in chemical-mechanical polishing (CMP) is investigated. First, based on the mechanics of pad/particle and particle/wafer sliding contacts at an asperity of the polishing pad a new MRR model is developed. The model is then extended to multi-asperity contacts, taking into account the statistics of the asperity heights. The single-asperity model reveals that the removal rate at relatively low pressure strongly depends on the pressure and the area at a sliding asperity contact. However, removal rate per asperity becomes independent of the contact pressure once it exceeds a critical value, which is determined by the asperity hardness and the particle concentration. Material removal by multi-asperity sliding contacts increases due to the increase in real contact area, provided a large number of asperity contacts at pressures greater than the critical. The plasticity index is identified as a key parameter that determines the contact area ratio and proportion of asperities in contact at pressures greater than the critical, thus the overall MRR. The model suggests that MRR in CMP can be greatly increased by controlling the surface topography of the pads. Results of polishing experiments on Cu thin films validate the model.