A method for manufacturing a semiconductor device includes providing a substrate, forming an amorphous layer in the substrate, performing a first etching process on the substrate using the amorphous layer as an etch stop layer to form a plurality of first fins, performing a channel stop ion implantation process into the amorphous layer to form an impurity region, and performing an annealing process to activate implanted dopants in the impurity region, wherein the amorphous layer disappears during the annealing process. The method also includes performing a second etching process on a region of the substrate disposed between the first fins to form second fins from the first fins, and forming an isolation region between adjacent second fins by filling at least a portion of an air gap between the second fins with an insulating material. The method prevents dopants of the channel stop implant from diffusing into the channel.