Thermodynamic calculations of the equilibrium compositions of condensed- and gaseous-reaction products from the reaction between Cu(s) and Cl-2(g) has been performed to understand the fundamental reactions during the Cl-based dry etching of Cu thin film. Total pressure was fixed at 5 mTorr, which is typical for high-density plasma etching. The final phase equilibrium state of Cu(s)-Cl-2(g) system was determined by using the minimization of the total free energy of the system as a function of temperature and initial Cl-2(g)/Cu(s) molar ratio, and the calculated phase equilibrium results were discussed in relation to the Cl-based Cu dry etch reaction. (C) 2002 Elsevier Science B.V. All rights reserved.