A method of passivating silicon-oxide based low-k materials using a supercritical carbon dioxide passivating solution comprising a silylating agent is disclosed. The silylating agent is preferably an organosilicon compound comprising organo-groups with five carbon atoms such as hexamethyldisilazane (HMDS), chlorotrimethylsilane (TMCS), trichloromethylsilane (TCMS) and combinations thereof. In accordance with further embodiments of the invention, a post ash substrate comprising a dielectric material is simultaneously cleaned and passivated using a supercritical carbon dioxide cleaning solution.