Abstract: An etchant and a method for fabricating a substrate for an electronic device using the etchant where the etchant contains a predetermined additive to control the etch rate of a Cu deposition layer (containing Cu, Cu/Ti, or Cu/Ta) over passage of time. Some examples of the additive may include a chelate having the —COOH group, a chemical compound containing a Cu ion, and a deoxidizer containing sulfur (S). The method includes forming a metal thin film containing copper (Cu) on a substrate, selectively exposing the metal thin film, and etching at least one of the exposed and the unexposed portions on the metal thin film with the additive-containing etchant to control the Cu etch rate over time against the number of sheets of processed substrates. The use of the additive-containing etchant results in improved yield and reduction in production costs because of less frequent etchant replacements.