A low-k dielectric film is deposited on the wafer. A metal layer is then deposited over the low-k dielectric film. A resist pattern is formed over the metal layer. The resist pattern is then transferred to the underlying metal layer to form a metal pattern. The resist pattern is stripped off. A through hole is plasma etched into the low-k dielectric film by using the metal pattern as a hard mask. The plasma etching causes residues to deposit within the through hole. A wet treatment is then performed to soften the residues. A plasma dry treatment is carried out to crack the residues.