Transient Electrochemical Measurements During Copper Chemical Mechanical Polishing
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Transient Electrochemical Measurements During Copper Chemical Mechanical Polishing Seung-Mahn Lee, Wonseop Choi, Valentin Craciun, and Rajiv K. Singh Department of Materials Science and Engineering and Particle Engineering Research Center University of Florida, Gainesville, FL 32611, U.S.A. ABSTRACT Chronoamperometry was used to investigate the reaction/passivation kinetics and thickness of the chemically modified surface layer on the copper during chemical mechanical polishing (CMP). The result showed that the reaction/passivation kinetics and the thickness of the chemically modified surface layer are strongly dependent on the chemistry of CMP slurry in the chemical aspect of CMP and play critical keys in the selection of the chemistry and its concentration. BTA and H2O2 enhanced the passivation kinetics, resulting in thinner layer on the copper surface. In addition, the reaction kinetics increased as pH decreased.
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INTRODUCTION Chemical mechanical polishing (CMP) has been recognized as the only technology to eliminate topographic variation and achieve wafer level global planarization for ULSI circuits. However, despite its extensive utilization, the process controls for CMP remain at an empirical stage because the fundamental mechanisms underlying the polishing process have yet to be fully understood. Since Preston presented the first empirical mechanical model, researchers have devoted their efforts to modify the standard Preston equation [1-3]. To delineate the CMP removal mechanism and to further develop more quantitative and predictive models, it is necessary to understand the mechanical and chemical interactions encountered during CMP. The mechanical interactions have been investigated extensively [4-6]. However, the chemical interaction has not been deliberated due to its complexity. The investigation of the chemical interaction between the substrate (e.g., copper) and chemicals included in the slurry has been carried out in two aspects: the formation of the chemically modified surface layer on metal typically by x-ray photoelectron spectroscopy (XPS) [7] and the passivation study by potentiodynamic electrochemical measurements [8]. However, these measurements have a limitation of thickness resolution and time-resolved information. The removal mechanism of metal during CMP involves the formation of a passivation layer on the metal surface due to the oxidizing nature of the slurry, and its mechanical abrasion from the abrasives included in the slurry. The polishing pad subsequently removes the abraded material. Simultaneously, the pristine metal, immediately exposed to the slurry, is repassivated. Figure 1 shows the schematic of the CMP removal mechanism. Obviously, as indicated by this mechanism, the passivation of metal in the slurry plays a very important role in CMP performance, either by forming native metal oxides on the surface using oxidizers, or by forming chemically adsorbed layers (e.g., Cu-BTA layer) using organic corrosion inhibitors. As a proof of the formation of the passivatio
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