In-Situ Chronoamperometry for CMP Slurry Investigations
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In-Situ Chronoamperometry for CMP Slurry Investigations Jian Zhang, Steven Grumbine, Phillip W. Carter, and Thomas Werts Cabot Microelectronics 870 N. Commons Dr., Aurora, IL 60504 USA ABSTRACT A new application of the electrochemical method, in situ chronoamperometry, has been developed and used as a tool for CMP slurry characterization. The measured current response, i, resulting from a small applied potential is fit to an exponential equation, i = io + A1exp(-t/tD). The fitting parameters serve as a measure of surface reactivity (io), surface film robustness (A1), and the kinetics of film formation (tD). A description of the method and correlations to polishing performance are discussed in this paper. INTRODUCTION Chemical Mechanical Planarization (CMP) is one of the key processes in manufacturing semiconductor devices and the CMP slurry plays an important role in achieving the desired polishing performance1. With the advancement of semiconductor structures, such as copper interconnect, the performance requirements for a copper CMP slurry have become more challenging. Mechanistic understanding of metal polishing by CMP slurries is critical for meeting future device planarization requirements.2 Electrochemistry polarization has been widely applied to gain an understanding of electrochemical dissolution during metal CMP processes.3, 4, 5 Polarization studies and Tafel plot analyses are useful for studying the mechanisms of CMP involving interfacial charge transfer reactions. Such studies are typically limited by (1) precision errors in quantification of Tafel slopes, and (2) a lack of information about the kinetics of film formation on the metal surface. Chronoamperometry is an electrochemical technique in which the current transient is measured after a potential pulse and is a quantitative indicator for interfacial electrochemical reaction processes.6 This technique “probes” the electron transfer processes at the metal-liquid interface and therefore can provide valuable information about the film properties of that interface. Previous chronoamperometry work in CMP focused on solution and interfacial characterization where the working electrode metal surfaces were not under dynamic polishing conditions. The previous work characterized the electrochemical properties of the slurry-metal interface using nonreactive metals such as platinum7 or reactive metals such as copper, tantalum, and tungsten.8 One study has examined different copper crystal planes under dynamic polishing conditions by chronoamperometry.9 In this work, a special electrochemical cell has been designed to study metal surfaces under dynamic polishing conditions in order to correlate the surface reactions and kinetics obtained from chronoamperometry to CMP performance. EXPERIMENTAL As shown in Figure 1, electrochemical polarization and chronoamperometry were done using a PAR EG&G 273A potentiastat. A three electrode electrochemical cell was introduced during the experiments consisting of the working electrode (e.g. copper, tantalum etc), count
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