Method of fabricating polysilicon thin film transistor

A method of forming a polycrystalline silicon thin film transistor, comprising

• forming a buffer layer over a substrate;
• forming an amorphous silicon layer over the buffer layer;
• applying a catalytic metal to a surface of the amorphous silicon layer;
• crystallizing the amorphous silicon layer having the catalytic metal thereon into a polycrystalline silicon layer;
• forming an island pattern on the polycrystalline silicon layer, thereby defining an active region underneath in the polycrystalline silicon layer;
• applying n-type ions to the polycrystalline silicon layer and then heat-treating the polycrystalline silicon layer to remove the catalytic metal from the active region underneath the island pattern;
• patterning the polycrystalline silicon layer using the island pattern as a mask to form an island-shaped active layer and to expose a surface of the adjacent buffer layer;
• removing the island pattern from the surface of the island-shaped active layer using an etchant;
• etching the exposed surface of the buffer layer to remove residual catalytic metal left on the buffer layer;
• sequentially forming a first insulation layer and a metal layer on the etched buffer layer to cover the island-shaped active layer;
• patterning the first insulation layer and the metal layer using a single mask to form a gate insulation layer on the first active layer of the island-shaped active layer and to form a gate electrode on the gate insulation layer thereby defining a first active area underneath the gate insulating layer and exposed second active areas in the island-shaped active layer;
• applying a dopant to the second active areas of the island-shaped active layer to form source and drain regions on respective sides of the first active area;
• forming a second insulation layer to cover the gate electrode and the source and drain regions;
• forming source and drain contact holes in the second insulation layer to expose portions of the source region and the drain region, respectively;
• and
• forming source and drain electrodes, the source electrode contacting the source region through the source contact hole, and the drain electrode contacting the drain region through the drain contact hole.