Expanding thermal plasma for low-k dielectrics : engineering the film chemistry by means of specific dissociation paths in the plasma

As the need for low-k dielectrics in the ULSI technology becomes urgent, the research focuses on the deposition of novel materials with appropriate electrical properties and on the challenges concerning their integration with subsequent processing steps. Here, we address the remote expanding thermal plasma as a novel technique for the deposition of low-k carbon-doped SiO2 films from Ar/ hexamethyldisiloxane/ oxygen mixtures. We have obtained films with k values in the range 2.8–3.4 and with still fairly good mechanical properties (hardness 1 GPa, Young's modulus 10 GPa). This outcome is surprising because literature, in general, reports on low-k films deposited from precursors with 2 Si–O and 1 Si–C bonds per Si atom (e.g., diethoxymethylsilane), in order to reach a compromise between dielectric and mechanical film properties. Our approach, on the contrary, utilizes a precursor characterized by a relatively high Si–CH3: Si–O bond ratio. The plasma chemistry investigation by means of Mass Spectrometry has highlighted that the HMDSO molecule preferably dissociates at the Si–O bond. The loss of Si(CH3)3 radicals, therefore, is expected to account for the major removal of CH3 radicals from the deposition precursors, leading to the deposition of low-carbon content low-k dielectrics.