A device comprises a first dielectric layer, a first conductor, a carbon-containing etch stop layer, a second dielectric layer, and a second conductor. The first conductor has a lower portion in the first dielectric layer. The carbon-containing etch stop layer wraps an upper portion of the first conductor. The second dielectric layer is over the carbon-containing etch stop layer. An interface formed by the second dielectric layer and the carbon-containing etch stop layer is higher over the first conductor than over the first dielectric layer. The second conductor is in the second dielectric layer.