In Situ Electrochemical Investigation of Tungsten Electrochemical Behavior during Chemical Mechanical Polishing

The electrochemical behavior of tungsten during chemical mechanical polishing (CMP) was observed in order to investigate a proposed blanket passivation and abrasion mechanism for tungsten removal. The experiments were performed in a cell that allowed electrochemical measurements to be made during polish. Polish rates were determined from the same samples used in the cell. Alumina‐based polish slurries containing potassium iodate, ferric nitrate, or ammonium persulfate were used. DC polarization experiments show no evidence of passive film formation on the tungsten during polish. Tungsten oxidation rates measured during polish account for removal rates that are 1 to 2 orders of magnitude below the measured polish rate. Values of the charge‐transfer resistance (measured by ac impedance spectroscopy) during polish are 1 to 2 orders of magnitude higher than expected from the polish rate, thus corroborating the dc‐based data. Polish rates under potentiostatic conditions were also measured. The current required to maintain the metal anodic of the open‐circuit potential is well below the current expected from measured polish rates, assuming complete oxidation of the tungsten. The polish rate during cathodic potentiostatic conditions (−0.5 V with regard to the open‐circuit potential) was similar to the polish rate at open circuit. We conclude that the formation of a blanket passive layer does not significantly contribute to tungsten removal during CMP.