Patent ID: 7626125

Claim:
A thin-film transistor comprising: an island-shaped silicon layer which is provided on an insulating substrate; a source region and a drain region which are provided with an interval on the silicon layer on the insulating substrate; a gate insulator layer which is provided over the silicon layer between the source region and the drain region; a gate electrode which is provided on the gate insulator layer; and a source electrode and a drain electrode which are provided on the source region and the drain region, respectively, wherein the gate electrode comprises: a first copper diffusion-preventing layer formed by an electroless metal plating method on the gate insulator layer; a copper seed layer in which an undesired portion is removed on the first copper diffusion-preventing layer; a copper layer formed on the copper seed layer of which the undesired portion is removed, the copper layer being formed by the electroless metal plating method and a film thickness of the copper layer being greater than that of the copper seed layer; and a second copper diffusion-preventing layer surrounding an exposed surface including side upper surfaces of a multilayered structure having the copper seed layer and the copper layer, the second copper diffusion-preventing layer being formed by the electroless metal plating method, and wherein the copper seed layer and the copper layer are surrounded by the first copper diffusion-preventing layer and the second copper diffusion-preventing layer, and have a forward tapered cross section.