The authors investigated the contact resistance fluctuation caused by CoSix damage in plasma etching and ashing processes. They found that CoSix layers damaged by plasma process exposure are readily oxidized when exposed to air resulting in increased resistance. They also found that the contact resistance increases more when CH3F is used instead of CF4 during etching process. The lower the mass number of dominant ions becomes, the deeper the ions penetrate. Molecular dynamics simulation revealed that dissociated species from lighter ions penetrate deeper and that this stimulates deeper oxidation. They also found that contact resistance further increased by using postetch ashing plasma even in an H2/N2 ashing process in which O2 was not used. Here, too, the reason for this is that the ion penetration causes deep oxidation. They observed that the contact resistance has a linear relationship with the oxide concentration in CoSix. This leads to the conclusion that it is essential to precisely control the ion energy as well as to properly select the ion species in the plasma process in the fabrication of next-generation semiconductor devices.