Example Copper-Titanium Interaction

Copper and titanium polish rates were first measured independently on freshly dressed pads. Different new pads were used for copper and titanium polishing to prevent unwanted contamina- 

test wafers were fabricated with copper deposited on titanium. The copper was subsequently etched from half of the wafer, leaving titanium exposed on the etched half. The wafers were polished for 30 seconds to determine if the copper polished from one half of the wafer interacted with the titanium on the other half. Copper and titanium polish rates, measured on the same wafer, were found to be 520 nm/min and 252 nm/min, respectively, giving a selectivity of 2.1. The reduction in selectivity suggests that copper polished from the wafer interacted with titanium to increase the polish rate of the titanium. In addition, the titanium appears to have caused a decrease in the polish rate of copper. 

Recently Evans et al have reported an extreme pattern dependence in the copper polish rate over a tantalum overlayer. Features with large perimeter to area ratios were completely removed while features with lower ratios remained. This behavior was not observed when TiN was used as the underlayer. The behavior, which is attributed to electrochemically induced interaction caused by the difference in the electrochemical potentials of tantalum and copper, is similar to the case of interaction seen between titanium and copper. 

Galvanic Interaction Between Titanium Metal and Ions 

The discussion in Section 4.2.1 suggested that titanium dissolution may be driven by the reduction of H, the reduction of Oj, or the reduction of oxidizing agents. When titanium is polished in the presence of copper ions, copper ion reduction leads to the oxidation of the titanium. A galvanic couple is formed between the Cu/Cu and the Ti/Ti systems such that the copper ions are reduced and plated onto the titanium surface. At the same time, titanium is dissolved into solution. Thus, copper replaces the titanium on the surface. The reactions governing such a process are  

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ELECTROCHEMICAL PHENOMENA 4.6.5 Example Copper-Titanium Interaction... 

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