Tribological Experiments Applied to Tungsten Chemical Mechanical Polishing
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ABSTRACT Tungsten CMP involves a synergistic interaction of electrochemical and tribological (wear) phenomena. So far, numerous studies have been conducted using static electrochemical measurements as well as some polishing experiments. In this study, we present some results obtained from carrying out potentiodynamic measurements and tribological experiments in a reciprocating sphere-on-plate tribometer, which allowed a precise control of mechanical and electrochemical conditions. In addition, anodic current-time transient measurements were also used to characterize the kinetics of tungsten passivation reaction. These results indicate that the presence of an passive film is essential for wear of tungsten to take place. INTRODUCTION Chemical Mechanical Polishing (CMP) still remains a poorly understood process, although it has been integrated quite successfully into the industry. CMP of metals like tungsten involves strong interactions between electrochemical phenomena and mechanical abrasion. The most widely accepted model for tungsten CMP was developed by Kaufman et al. [1], which postulates that material removal takes place by the formation of a passive oxide film, which is then abraded by the particles in the polishing slurry. Several electrochemical studies have also
been performed on the tungsten system [2],[3], along with measurements carried out during CMP [4], [5]. Many of these studies report contradictory results, with Kneer et al. [4] claiming that removal during CMP is the result of corrosion assisted fracture, and Stein et al. [5] concluded that a passive film on the tungsten surface is not required for CMP. These different
conclusions have left a lot of unresolved questions about the nature of the removal process. To shed some additional light on the tribochemical phenomena occurring during tungsten CMP, we conducted electrochemical measurements and wear experiments using a sphere-on-plate tribometer system.
EXPERIMENT The sliding wear experiments were conducted by rubbing an alumina sphere (6 mm diameter) against a tungsten disk (1.3 cm diameter and 0.25 cm thickness). The experiments were carried out at the EPFL (Ecole Polytechnique Fed~rale de Lausanne or Swiss Federal Institute of Technology) in the laboratory LMCH (Laboratoire de M6tallurgie Chimique). The samples were cut from a tungsten rod of 1.3cm diameter and 99.95% purity procured from Alfa Aesar®. The disks were embedded in a plastic resin (Technovit') with an electrical contact allowing the connection. Then the disks were mirror polished using 1 pm diamond paste. 97 Mat. Res. Soc. Symp. Proc. Vol. 566 ©2000 Materials Research Society
Potentiodynamic polarization curves were measured to characterize the electrochemical behavior of tungsten in 0.5 M H 2SO 4 . A tungsten stationary disk electrode of 1.27 cm diameter (1.266 cm 2 surface area) was used in the setup described below (Fig. 1). The reference electrode was Ag/AgCI and the scan rate was 5 mV/s. In addition, anodic current-time transients were recorded to measure passive film for
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