Electrochemical behavior of cobalt in post-via etch cleaning solutions

The integration of CoWP and CoWB self-aligned barriers (SAB) for 32 nm technology nodes allows improving copper interconnections reliability [1, 3]. However the introduction of such materials in copper interconnection levels drives new challenges for plasma dry etch and wet clean processes. Indeed, during the post-via-etch cleaning step, cobalt and copper can be altered by corrosion. Moreover, a galvanic coupling between cobalt, the major component of SAB, and copper can thermodynamically occur. In this way, the cleaning solution acts as ionic medium providing a contact between the two metals. Thus, both metals polarize to a mixed potential comprised between the individual open circuit potentials (OCP) of cobalt and copper. As a result, the less noble metal can suffer from accelerated corrosion, and the more noble metal corrodes with slower rate. According to thermodynamic aspects, cobalt in contact with copper is the less noble metal. Consequently, Co is susceptible to undergo galvanic corrosion which may enhance the dissolution of the SAB.