The electrochemical behavior of chemically vapor deposited tungsten films in solutions of interest to tungsten chemical mechanical polishing has been investigated using dc potentiodynamic polarization, linear polarization, and Tafel methods. It was found that in the absence of an oxidizer, the tungsten surface was passivated most effectively at acidic pH values. At pH 2 or 4, a W02/W03 duplex oxide layer of less than 50 A thickness was detected over the tmgsten layer by x-ray photoelectron spectroscopy. The oxide layer formed at pH 2 was much thicker, and had better passivity compared to the oxide formed at pH 4. Addition of H202 at pH 2 or 4 resulted in a dramatic increase in tungsten dissolution. * Electrochemical Society Student Member. * * Electrochemical Society Active Member. tential values. In his investigation of the anodic oxidation

A polishing tool and a potentiostat were used to simultaneously polish and measure the direct current (dc) open-circuit potential and anodic polarization behavior of chemical vapor deposited tungsten films in the presence of various oxidants. Of the different oxidants tested at pH 1.5 or pH 4.4, (NH,),MoO,, 2 4 formed the most protective passive layer on tungsten. Even in the presence of the most aggressive oxidant, Fe(NO 3 ) 3 , the dissolution rates of chemical vapor deposited tungsten were approximately 3 nm/min during abrasion, which is a very small fraction of typical removal rates reported for chemical mechanical polishing of tungsten. This indicates that electrochemical oxidation followed by abrasive removal of the oxidation product and dissolution may not be the primary mechanism for tungsten removal. Atomic force microscopy scans of polished tungsten films indicate that corrosion assisted fracture may be an important removal mechanism for tungsten during chemical mechanical polishing.

An electrochemical direct current polarization method was used to investigate characteristics of copper deposition onto silicon from dilute and buffered hydrofluoric acid solutions. The corrosion current density and corrosion potential of silicon were not very sensitive to the Cu2 concentration, up to 1000 parts per billion, in buffered hydrofluoric acid. However, the extent of copper deposition, as measured by total reflection X-ray fluorescence, increased as the Cu' concentration in solution increased. In dilute hydrofluoric acid, Cu2 addition had a significant and systematic effect on the corrosion potential and corrosion current density of silicon. However, in both types of solution, the cathodic current calculated from the measured copper deposition was found to be only a small fraction of the corrosion current (less than 1%). This indicates that the primary cathodic reaction is not copper ion reduction but hydrogen ion reduction. Illumination affected the electrochemical behavior of both p-and n-type silicon in Cu' spiked dilute hydrofluoric acid, but only that of p-type silicon in buffered hydrofluoric acid.