A semiconductor structure includes a plurality of first semiconductor layers interleaved with a plurality of second semiconductor layers. The first and second semiconductor layers have different material compositions. A dummy gate stack is formed over an uppermost first semiconductor layer. A first etching process is performed to remove portions of the second semiconductor layer that are not disposed below the dummy gate stack, thereby forming a plurality of voids. The first etching process has an etching selectivity between the first semiconductor layer and the second semiconductor layer. Thereafter, a second etching process is performed to enlarge the voids.