A method of fabricating an organic light emitting diode (OLED) display device, the method comprising: forming a substrate having a thin film transistor region (a) and a capacitor region (b); forming a buffer layer on the substrate; forming protrusions in the buffer layer; forming a capacitor (145,150,163) on the buffer layer in the capacitor region (b), wherein the capacitor (145,150,163) follows the shape of the protrusions of the buffer layer; and characterised in that the step of forming protrusions in the buffer layer comprises: forming an amorphous silicon layer on the buffer layer; forming a metal catalyst layer on the amorphous silicon layer; annealing the substrate to crystallize the amorphous silicon layer into a polysilicon layer; removing the metal catalyst layer; and patterning the polysilicon layer to form a semiconductor layer disposed on the buffer layer in the thin film transistor region (a); and forming the protrusions in the buffer layer except for the portion of the buffer layer on which the semiconductor layer is formed, wherein when the polysilicon layer is patterned to form the semiconductor layer, grain boundaries on which metal silicides of the polysilicon layer crystallized by the metal catalyst gather and a seed region are not completely removed from the surface of the etched buffer layer and remain as said protrusions. forming a substrate having a thin film transistor region (a) and a capacitor region (b); forming a buffer layer on the substrate; forming protrusions in the buffer layer; forming a capacitor (145,150,163) on the buffer layer in the capacitor region (b), wherein the capacitor (145,150,163) follows the shape of the protrusions of the buffer layer; and characterised in that the step of forming protrusions in the buffer layer comprises: forming an amorphous silicon layer on the buffer layer; forming a metal catalyst layer on the amorphous silicon layer; annealing the substrate to crystallize the amorphous silicon layer into a polysilicon layer; removing the metal catalyst layer; and patterning the polysilicon layer to form a semiconductor layer disposed on the buffer layer in the thin film transistor region (a); and forming the protrusions in the buffer layer except for the portion of the buffer layer on which the semiconductor layer is formed, wherein when the polysilicon layer is patterned to form the semiconductor layer, grain boundaries on which metal silicides of the polysilicon layer crystallized by the metal catalyst gather and a seed region are not completely removed from the surface of the etched buffer layer and remain as said protrusions. forming an amorphous silicon layer on the buffer layer; forming a metal catalyst layer on the amorphous silicon layer; annealing the substrate to crystallize the amorphous silicon layer into a polysilicon layer; removing the metal catalyst layer; and patterning the polysilicon layer to form a semiconductor layer disposed on the buffer layer in the thin film transistor region (a); and forming the protrusions in the buffer layer except for the portion of the buffer layer on which the semiconductor layer is formed, wherein when the polysilicon layer is patterned to form the semiconductor layer, grain boundaries on which metal silicides of the polysilicon layer crystallized by the metal catalyst gather and a seed region are not completely removed from the surface of the etched buffer layer and remain as said protrusions.