Wafer Level Modeling of Electrochemical-Mechanical Polishing (ECMP)
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Wafer Level Modeling of Electrochemical-Mechanical Polishing (ECMP) Daniel Truque, Xiaolin Xie, and Duane Boning Microsystems Technology Laboratories, Massachusetts Institute of Technology, Cambridge, MA, 02139 ABSTRACT In this work, we propose a wafer level dynamic ECMP model based on time-evolving current density distributions across the wafer. The wafer is discretized, and the potential and current density distributions are calculated based on the applied voltage zones and metal film thicknesses across the wafer. The copper removal rate is proportional to the current density, and thus the copper thickness (and conductance) can be calculated as a function of position on the wafer and polish time. Using a time-stepping simulation, the model is able to capture the wafer level non-uniformity and time-dependence of ECMP removal. The model is also able to capture the time-varying voltage zones used in ECMP, and can be used to find optimal voltage zone control schemes to achieve improved wafer-level uniformity. INTRODUCTION Over the past few years, electrochemical-mechanical polishing (ECMP) has become an accepted and robust process to remove bulk copper. As dimensions shrink and low-K dielectrics are introduced, polishing pressures must be decreased to prevent damage to increasingly fragile dielectrics, resulting in lower removal rates. ECMP is an alternative offering high removal rates for pressures under 1 psi, extending pad life and reducing consumables cost by up to 30%[1]. The electrolyte solution used instead of the slurry is less than half the price of the cost of slurry that would be needed[1]. It is believed to be a non-Prestonian process controlled by applied voltage, with an endpoint enabled by measuring the total charge delivered. Using multiple voltage zones enables control of removal rates throughout the wafer to account for the incoming copper thickness profile. At present, conventional CMP models[1] have been pressed into service to characterize this process, but the nature of ECMP is very different from conventional CMP. As the industry moves from hybrid ECMP approaches to full sequence ECMP tools, better models are needed at the wafer level, die level, and feature level. In this work, we propose a wafer level dynamic ECMP model based on time-evolving current density distributions across the wafer. The wafer is discretized, and the potential and current density distributions are calculated based on the applied voltage zones and metal film thicknesses across the wafer. These values are then used to calculate the removal rate based on copper dissolution. ECMP MODEL AND SIMULATION The model is a time-stepped finite element approach to calculate copper removal during ECMP. The model can be easily modified to fit different tool setups, but the present one was optimized for the Applied Materials Reflexion LK Ecmp tool. In this setup, the conducting polishing pad has three concentric voltage zones that affect the removal rate in different regions of the wafer. These zones can be tuned to ensure a
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