Slurry Mean Residence Time Analysis and Pad-Wafer Contact Characterization in Chemical Mechanical Planarization

Abstract

This dissertation presents a series of studies related to the slurry mean residence time analysis and the pad-wafer contact characterization in Chemical Mechanical Planarization (CMP). The purpose of these studies is to further understand the fundamentals of CMP and to explore solutions to some of CMP's challenges. Mean residence time (MRT) is a widely used term that is mostly seen in classical chemical engineering reactor analysis. In a CMP process, the wafer-pad interface can be treated as a closed system reactor, and classical reactor theory can be applied to the slurry flow through the region. Slurry MRT represents the average time it takes for fresh incoming slurry to replace the existing slurry in the region bound between the pad and the wafer. Understanding the parameters that have an impact on MRT, and therefore removal rate, is critical to maintain tight specifications in the CMP process. In this dissertation, we proposed a novel slurry injection system (SIS) which efficiently introduced fresh slurry into the pad-wafer interface to reduce MRT. Results indicated that SIS exhibited lower slurry MRT and dispersion numbers but higher removal rates than the standard pad center slurry application by blocking the spent slurry and residual rinse water from re-entering the pad-wafer interface during polishing. Another study in this dissertation dealt with the effect of pad groove width on slurry MRT in the pad-wafer interface as well as slurry utilization efficiency (η). Three concentrically grooved pads with different groove widths were tested at different polishing pressures to experimentally determine the corresponding MRT using the residence time distribution (RTD) technique. Results showed that MRT and η increased significantly when the groove width increased from 300 to 600μm. On the other hand, when the groove width increased further to 900μm, MRT continued to increase while n remained constant. Results also indicated that MRT was reduced at a higher polishing pressure while η did not change significantly with pressure for all three pads. In the last study of this dissertation, the effect of pad surface micro-texture on removal rate during tungsten CMP was investigated. Two different conditioner discs ("Disc A" and "Disc B") were employed to generate different pad surface micro-textures during polishing. Results showed that "Disc B" generated consistently lower removal rates and coefficients of friction than "Disc A". To fundamentally elucidate the cause(s) of such differences, pad surface contact area and topography were analyzed using laser confocal microscopy. The comparison of the pad surface micro-texture analysis on pad surfaces conditioned by both discs indicated that "Disc A" generated a surface having a smaller abruptness (λ) and more solid contact area which resulted in a higher removal rate. In contrast, "Disc B" generated many large near-contact areas as a result of fractured and collapsed pore walls.