In-situ AFM study of pitting corrosion of Cu thin films

We applied an in situ electrochemical atomic force microscope (AFM) to investigate the pitting corrosion and the effect of benzotriazole (BTA) on corrosion inhibition of Cu films in 0.01 M NaHCO3. The Cu films are 800 Angstrom thick deposited on polished Si(100) wafers. Pit growth was observed in a range from hundreds of nanometers to millimeters using the combination of atomic force microscopy (AFM) and optical microscopy. The use of flat thin Cu films on inert substrates simplifies the three-dimensional problem to two dimensions allowing the observation of the detailed structure at initial stages. Our study has found that pits grow by two mechanisms depending upon the applied potential. Stepping the potential from the open-circuit potential to the region between the repassivation potential (E-r) and the pitting potential (E-p) generates pits with irregular perimeters. These pits range from hundreds of nanometers to a few microns in diameter. If the potential is stepped above E-p, large pits greater than 10 mu m in diameter were observed with smoother perimeters. The pitting rate at this potential is probably quite high so that corrosion products accumulate at pit edges to form ridges observed by AFM. On the other hand, if the potential is linearly ramped at 10 mV s(-1) to values well above E-p and then ramped back to the open-circuit potential at the same rate, large pits with diameters over 10 mu m were formed. The perimeters of these pits are smooth and circular, indicating a mass-transport controlled growth mechanism. The edges of these pits are clean and flat, suggesting that materials quickly dissolve into the solution and the corrosion process is in equilibrium with the local electrolyte concentration within the pits. The addition of 0.2 mM BTA in 0.01 M NaHCO3 solution results in the formation of a BTA film on the Cu surface, which effectively protects Cu from corrosion. Fitting potential of Cu surfaces covered with BTA films is dramatically increased by more than 700 mV, so that, practically, pitting corrosion is not a concern. (C) 1999 Elsevier Science B.V. All rights reserved.