Examining Leucine As an Environmentally Friendly Corrosion Inhibitor for Cu and Co Chemical-Mechanical Planarization (CMP)
Authors: Gichovi Juster , Tran Hoang Thi Thuy , Seo JIhoon , Podlaha Elizabeth J 
Published: September 2023

Metal chemical-mechanical planarization (CMP) is a key step in semiconductor processing of microchips and devices. The slurries used in CMP contain abrasives, oxidizers, chelating agents, and corrosion inhibitors. Despite the desired removal of metal, corrosion inhibitors are used to protect the underlying metal features from aggressive corrosion, and to this end, azoles, such as benzotriazole (BTA), are the most commonly used corrosion inhibitor in Cu CMP,1-3 and Co CMP4-5 slurries, although it was recently found that the surface interaction between BTA and Co is weaker than BTA and Cu.6 When Co is used as a barrier layer with Cu interconnects the addition of benzotriazole (BTA) has been shown to eliminate galvanic corrosion between Cu and Co.1,7 Spent slurries, however, pose an environmental hazard as the azole corrosion inhibitors do not readily degrade in conventional wastewater treatment processes.8 Thus, amino acids, such as leucine, is examined as a possible BTA substitute, being likely to readily biodegrade. Leucine is a known corrosion inhibitor of steel, nickel, nickel-copper, copper and cobalt in acid,9 and is examined here under typical CMP alkaline conditions.
Linear sweep voltammetry with ohmic corrosion was used to assess the corrosion current density, icorr, and the corrosion potential, Ecorr, of Cu and Co, as well as the galvanic corrosion between Co and Cu, using a rotating disk electrode. Varying amounts of L-leucine (5- 40 mM) in a pH 8 and 10 KOH electrolyte containing H2O2 as the oxidant, with and without arginine as the complexant, was used as a representative model polishing slurry without abrasive particles. Leucine was an effective inhibitor of Cu at pH 8. At a pH of 10 the icorr of Cu was higher, compared to the pH 8 electrolyte, indicating the importance of the protonation of the amine group of leucine on corrosion inhibition, where the pH < pKa. Leucine was less beneficial for the Co corrosion inhibition. When arginine was present there was a larger difference in the corrosion potential between Co and Cu, making cobalt prone to galvanic corrosion, compared to a case when arginine was not present. Eliminating arginine, in an excess of leucine, favorably inhibited copper and reduced the galvanic corrosion of cobalt, but enhanced the corrosion of Co alone. The adsorption energy was determined for Cu by varying the amount of leucine in the electrolyte and found to be predominately physisorbed compared to chemisorbed.
Acknowledgement: This work was supported by the Semiconductor Research Corporation grant (Task 3100.001) under the Global Research Collaboration program.

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