We demonstrated that the quality of siloxane spin-on glass (SOG) films, widely used as interlevel planarization dielectrics, is improved significantly by curing in argon plasma. The wet etch rate of SOG film decreases with increasing plasma treatment temperature or treatment time, and is much lower than that cured in a furnace. Long-time plasma treatment reduces the density of silanols (Si-OH) and methyl (−CH3) group, which act as adsorption sites of water. The results were compared with those obtained from the N2O (or H2 ) plasma treated SOG films. The modification of the SOG film by Ar plasma is related to the radiation damage and the reconstruction of the atomic structure during the plasma exposure. The role of metastable Ar (Ar*) appears to be very important to improve the SOG film; SOG film is more relaxed by the energy released from the conversion of Ar* to Ar.