The degradation of porous methyl silsesquioxane (MSQ) films (k = 2.2) by plasma irradiation has been investigated using several kinds of gas chemistries, which are used for photoresist strip. The porous MSQ is easily degraded by O-2 plasma ash process and the dielectric constant increases largely, even by using the condition that minimizes the damage to nonporous SiOCH films. As the results of several kinds of plasma irradiation, it is clarified that N-2 plasma minimizes the degradation of porous MSQ. O-2 and H-2 plasmas extract methyl (CH3) groups from the porous MSQ films and degrade even the deep part from the film surface. On the other hand, N-2 plasma changes the quality of only the film surface and suppresses the degradation of the inside of the film. Furthermore, the small amount of H-2 addition to N-2 plasma is effective in the increase of the ash rate and the removal of the deposition on the sidewall without the remarkable increase of the dielectric constant. Finally, the optimized N-2/H-2 ash process minimizes the damaged layer thickness of the sidewall with single damascene structures. (C) 2004 American Vacuum Society.