Monitoring techniques have been developed for direct/indirect determination of metal etching bath components and for managing their replenishment. The disclosed methods have been successfully employed to make TiW etching a robust process that provides minimized and controlled undercutting of ball limited metallurgy and mechanical reliable C4s. A metal etching solution is monitored and replenished by measuring the sulfate concentration of a hydrogen peroxide, soluble salt, and soluble EDTA salt etchant. Turbidimetric titration conditions are used to measure and compare opaqueness of liquids by viewing light through them and determining how much light is cut off. Additional sulfate is added to maintain the sulfate concentration. Water and/or fresh etchant is added to compensate for evaporation or drag.
E. Israel Cooper, Dr. Madhav Datta, Jr, T. Edward Din, Kanarsky, T. Safron, Pike, M. Barry, and Shenoy, R. Vaman, “Methods for monitoring components in the TiW etching bath used in the fabrication of C4s”, U.S. Patent US 09/138,4422001.