A reliable gap-filling process is performed in the manufacturing of a semiconductor device. An apparatus for performing the gap-filling process includes a chamber in which a wafer chuck is disposed, a plasma generator for generating plasma used to etch the wafer, an end-point detection unit for detecting the point at which the etching of the wafer is to be terminated, and a controller connected to the end-point detection unit. The end-point detection unit monitors the structure being etched at a region outside the opening that is to be filled, and generates in real time data representative of the layer that is being etched. As soon as an underlying layer is exposed and begins to be etched, an end-point detection signal is generated and the etching process is terminated. In the case in which the layer being etched is an oxide layer, a uniform etching is achieved despite any irregularity that exists in the thickness to which the oxide layer is formed.