A method for manufacturing a semiconductor device includes forming a fin structure extending in a first direction on a substrate, forming a sacrificial gate pattern extending in a second direction to intersect the fin structure, forming a gate spacer layer covering the fin structure and the sacrificial gate pattern, providing a first ion beam having a first incident angle range and a second ion beam having a second incident angle range to the substrate, patterning the gate spacer layer using the first ion beam and the second ion beam to form gate spacers on sidewalls of the sacrificial gate pattern, forming source/drain regions at both sides of the sacrificial gate patterns, and replacing the sacrificial gate pattern with a gate electrode.