Pad surface management as a strategy to reduce the cost of ownership for CMP

Abstract

It is well recognized that one of the most expensive unit processes in the fabrication of semiconductors is chemical mechanical planarization (CMP), mainly due to the inefficiency of typical rotary tools. With each new logic technology node, new structures, materials, or processes have been introduced which required planarization, resulting in the number of CMP steps increasing exponentially. Dependence on CMP is also increasing for memory with the introduction of copper metallization. While memory has used CMP for tungsten plugs for many generations, the introduction of Cu poses a stiff challenge to control fabrication costs to allow memory manufacturers to remain cost competitive. Cu CMP is significantly more expensive than tungsten or oxide CMP, potentially exceeding $15 per wafer pass. Cost of consumables (CoC) dominates the cost of ownership (CoO) with pad and slurry costs typically accounting for more than 60%. Consequently, a 2 or 3X increase in pad life and a reduction in slurry consumption of 30-70% can yield a significant reduction in the CoO for CMP processes. A large portion (>90%) of the slurry delivered to the pad during a typical polishing process on a rotary tool never touches the wafer and is sent to waste. Pads are over dressed since diamond conditioning disks are used to mix fresh slurry with water, spent slurry, and process debris on the pad. This mode of operation can best be described as a “dilution” mode since fresh slurry delivered to the pad is diluted by the increasing concentration of process waste on the pad. To reach a “quasi-stable” operating point, one must flood the pad with fresh slurry to achieve pseudo-equilibrium for removal rates. Confluense has developed a pad surface manager (PSM) that removes process debris from the pad, in situ. PSM has been shown to significantly improve slurry utilization, by decreasing slurry residence time, and greatly extend pad life. Equivalent or higher material removal rates (MMRs) were achieved with significantly lower slurry flow rates when using PSM with perforated or k-grooved pads. The noticeably smaller decrease in removal rate with a decrease in slurry flow rate, when using PSM, was attributed to removal of process debris and spent slurry from the pad pores; thereby minimizing dilution effects and allowing fresh slurry to fill the vacated pores, providing fresh, chemically active slurry to contact the wafer with nearly every pad revolution. This can be described as a “replenishment” mode of operation since one is replacing the working fluid on the surface of the pad. The amount of slurry required to replenish surface pores is notably less than that required to fill pad macrostructures, which are reservoirs for non-working slurry and process debris. Consequently, lower slurry flow rates were needed with PSM to achieve acceptable removal rates. Oxide slurry consumption was reduced by at least 50%, independent of pad type. Since PSM is removing process waste in situ, there are reduced requirements for inter-wafer pad rinsing. Consequently, water consumption is also reduced and a small improvement in throughput is achievable. Pad staining due to build up of Cu byproducts decreases pad life and results in an unstable Cu CMP process. PSM has been shown to capture a large fraction (>90%) of the Cu by-products removed during CMP, significantly reducing or eliminating pad staining. Lower wafer defectivity has also been reported when using PSM and since waste is removed from the pad in situ, PSM allows controlling the residence time of wafer defect-inducing debris. Inserting experimental results for reduced slurry and water consumption, extended pad life, and a 15% throughput improvement into an acknowledged CoO model from Wright, Williams, and Kelley indicated that a reduction of at least 45% in CoO was achievable with PSM compared to that for a standard process of record (POR) without PSM.