An ex situ electrochemical impedance spectroscopy (EIS) method is used to evaluate the effectiveness of tetramethyl ammonium hydroxide (TMAH) in removing benzotriazole (BTA) layer from Cu surface after chemical mechanical polishing (CMP). Since the in situ EIS measurements could not be applied to characterize the cleaning process, ex situ EIS measurement is used in the present work. The impedance data are modeled by electrical equivalent circuit (EEC) analysis and the polarization resistance values are calculated. The BTA removal for various concentrations of TMAH is quantified from polarization resistance values. Scanning electron microscopy and X-ray photoelectron spectroscopy measurements were also conducted to complement the studies. The results show TMAH is effective in removing the BTA layer and that when part of the BTA is removed, the residual BTA appears as nodules. Some of the native oxides and hydroxides on the Cu surface are also removed by TMAH. When the slurry contains 0.1% (wt) BTA, a minimum concentration of 0.5% (wt) TMAH and a minimum cleaning time of 60 s are required for complete removal of BTA from the Cu surface. The suggested ex situ EIS technique is sensitive in evaluating removal efficiency of the organic contaminant from the metallic surface. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.014211jes] All rights reserved.