Instability in molybdenum/aluminum taper etching for large, high-resolution, thin-film transistor (TFT) liquid-crystal display (LCD) gate line was analyzed for stable TFT LCD manufacturing. The gate line edge shape is influenced by the passivity period that decreases the overetching period that is necessary for taper creation. Passivity is mainly created soon after molybdenum just-etching time when the aluminum/molybdenum area ratio is huge and the electric current concentrate to the molybdenum edge. Bubbles on the substrate play an important role, to decrease the cathode/anode area ratio to decrease the electric current in molybdenum to suppress passivity formation. Wire breakdown measurement is effective to maintain substrate-wide taper gentle shape profile by sampling inspection.