Micro-scale scratching by soft pad asperities in chemical-mechanical polishing

Abstract

In the manufacture of integrated circuits (IC) and micro-electromechanical systems (MEMS), chemical-mechanical polishing (CMP) is widely used for providing local and global planarization. In the CMP process, polishing pads, typically made of polyurethanes, play a key role. Due to the random, rough surface of the pad, only the tall asperities contact the wafer and transmit the necessary down force and motion to the abrasive particles for material removal. As the applied pressure is concentrated under few asperities, however, the asperities themselves, even though softer, may generate unintended micro-scratches on relatively hard surfaces under certain conditions. This thesis investigates the effects of topographical, mechanical, and tribological properties of the pad and of the wafer surfaces on pad scratching in CMP. The generation and probability of scratching by soft pad asperities on hard monolithic layers are modeled. At single-asperity sliding contact, the asperity contact pressure along with the interfacial friction that can induce surface layer yielding are first derived, for different asperity deformation modes: elastic, elastic but at the onset of yielding, elastic-plastic, and fully-plastic. Under multi-asperity sliding contact, the probability of scratching asperities is determined taking into account the asperity height variation of the rough pad surface. The models are further advanced for scratching of patterned Cu/dielectric layers. As a result, the conditions for and probability of scratching are presented in terms of the asperity-to-layer hardness ratio, friction coefficient, asperity modulus-hardness ratio and ratio of asperity radius to standard deviation of asperity heights. The scratching models are validated by performing sliding experiments using solid polymer pins and CMP pads. For scratch mitigation, especially, a novel, cost-effective asperity-flattening method is introduced to control the pad topography, i.e., to increase the ratio of asperity radius to standard deviation of asperity heights. Finally, the role of asperities in material removal is studied based on contact mechanics and abrasive wear models. A new material removal rate model is developed in terms of pad surface properties, and polishing experiments are conducted on Cu to validate the theoretical prediction that the asperity-flattened pads not only reduce the pad scratching but also improve the material removal rate.