Abstract: A process for formation of a thin film transistor liquid crystal display is disclosed, in which an etch-back type 3-mask process or an etch stopper type 4-mask process is applied, so that the semiconductor layer of the thin film transistor can be isolated from the data line. Consequently, the optical leakage current which aggravates the performance of the transistor is inhibited. Further, the data line is composed of a material which has a low chemical reactivity with ITO, so that a corrosion due to a chemical reaction between the data line and ITO can be eliminated.