A barrier layer forming method includes providing a porous dielectric layer over a substrate, the dielectric layer having a surface with exposed pores, and treating the dielectric layer with a plasma formed from a methane-containing gas. The treating seals the exposed pores. The method includes depositing a barrier layer over the surface, the barrier layer being continuous over the sealed pores. The porous dielectric may be low K. The plasma may be formed at a bias of at least about 100 volts.