A method includes depositing a plurality of first semiconductor layers and a plurality of second semiconductor layers over a substrate, wherein the first semiconductor layers and the second semiconductor layers are stacked alternately; patterning the first and second semiconductor layers to form a fin structure; supplying a first bias to the substrate after patterning the first and second semiconductor layers; and etching the second semiconductor layers when the semiconductor substrate is supplied with the first bias, wherein etching the second semiconductor layers is performed such that the first semiconductor layers are suspended above the substrate.