NOVELTY - Fabricating copper-indium-gallium-diselenide (CIGS) compound thin film (320) involves forming adhesive layer (302) over substrate (300); forming metal electrode layer (304) over adhesive layer; forming precursor stacked layer over metal electrode layer, where precursor stacked layer comprises copper-gallium (CuGa) alloy layers and copper-indium (CuIn) alloy layer sandwiched between CuGa alloy layers; performing annealing process where precursor stacked layer is converted into copper-indium-gallium (CuInGa) alloy layer; and performing selenization process.    USE - For fabricating a copper-indium-gallium-diselenide (CIGS) compound thin film (claimed) used in semiconductors or solar cell.    ADVANTAGE - The precursor stacked layer for the fabricating CIGS compound layer is formed with a surface roughness not greater than 200 Ra. Therefore, a surface roughness of the formed CIGS compound layer is improved and cell efficiency and photovoltaic conversion efficiency of a thin film solar cell using the CIGS compound layer is improved.    DETAILED DESCRIPTION - Fabricating a copper-indium-gallium-diselenide (CIGS) compound thin film (320) involves: providing a substrate (300); forming an adhesive layer (302) over the substrate; forming a metal electrode layer (304) over the adhesive layer; forming a precursor stacked layer over the metal electrode layer, where the precursor stacked layer comprises several copper-gallium (CuGa) alloy layers and at least one copper-indium (CuIn) alloy layer sandwiched between the CuGa alloy layers; performing an annealing process, converting the precursor stacked layer into a copper-indium-gallium (CuInGa) alloy layer; and performing a selenization process, converting the CuInGa alloy layer into a copper-indium-gallium-diselenide (CuInGaSe) compound thin film.    DESCRIPTION OF DRAWING(S) - The figure shows cross sectional view of fabricated copper-indium-gallium-diselenide (CIGS) thin film.    Substrate (300)    Adhesive layer (302)    Metal electrode layer (304)    Copper-indium-gallium-diselenide compound thin film (320)