A method of fabricating a semiconductor device is provided. A dielectric layer is formed on a barrier layer. A first opening is formed in the dielectric layer and exposes a portion of the barrier layer. A protection layer is formed on the barrier layer at the bottom of the first opening. The protection layer is thicker at the central portion while thinner at the edge portion thereof. A portion of the exposed barrier layer is removed by using the protection layer as a mask to form a second opening. The second opening has at least one sub-opening disposed in the barrier layer adjacent to the sidewall of the second opening. A semiconductor device formed with the method is also provided.