A method for fabricating a semiconductor device includes: forming a first multi-layer stack including liner layers and a source sacrificial layer over a lower structure; forming a second multi-layer stack including dielectric layers and sacrificial layers over the first multi-layer stack; forming a vertical contact recess extending through the second multi-layer stack and the source sacrificial layer; replacing the source sacrificial layer with a source contact layer; forming a carbon-containing spacer on sidewall of the vertical contact recess; replacing the sacrificial layers with conductive layers; and forming a source contact plug in the vertical contact recess.