Chemical-Mechanical Polishing of a Soft Pad Asperity with Protective Hard Thin Film

Abstract

CMP is an indispensable process in semiconductor manufacturing, which utilizes polyurethane pads as a mean to deliver the necessary forces to the individual abrasive nanoparticles for surface planarization. Despite the importance of the pad surface properties, the process relies on irregular pad surfaces having large geometrical and mechanical variation. Such randomness hinders our understanding toward process physics and therefore the CMP process control yet remains highly empirical. In this work, we study the CMP process at a single contact point of a polyurethane asperity with well-defined geometrical and mechanical properties. We first fabricate ~50 μm sized single asperities in a hemispherical shape using thermal reflow technique followed by microreplication molding. Then, we used a customized tribometer, to perform the CMP process using a single micro-asperity. Three types of hemispherical single asperities are prepared and tested: hard, soft, and the multi-layered polyurethane asperity. The multi-layered asperity is the soft hemispherical asperity coated with the hard and thin polyurethane film. We characterize and compare the polishing traces using atomic force microscope. While the soft asperity results in a large contact width of the polishing trace, the material removal rate is low due to the small contact pressure. On the other hand, the hard asperity leads to small contact area, but the contact pressure is greater and therefore may increase the material removal rate. However, the deep but shallow polishing behavior of the hard asperities have the risk to develop as a larger-scale scratching defect. To compromise these two, we designed a novel multi-layered asperity which not only compliantly deforms to enlarge the contact width but also delivers high and uniform pressure to enable sufficient material removal. We also provide theoretical models based on contact mechanics and elucidate the role of asperity size and hardness on the polishing results. Finally we discuss how our novel multi-layered design can be further developed and implemented to the typical CMP pads.