A thin film transistor comprises a substrate; a gate electrode, a source electrode, a drain electrode and a semiconductor layer formed on the substrate; a gate insulating layer between the gate electrode and the semiconductor layer or between the gate electrode and the source and drain electrodes; an etching stop layer between the semiconductor layer and the source and drain electrodes having a source contact hole and a drain contact hole therein; and a source buffer layer between the source electrode and the semiconductor layer and a drain buffer layer between the drain electrode and the semiconductor layer. The source and drain electrodes are metal Cu electrodes, and the source and drain buffer layers are Cu alloy layer.