Study of fluorine stability in fluoro-silicate glass and effects on dielectric properties

Abstract

The stability of PECVD fluorosilicate glass (FSG) layers deposited with a TEOS/O 2/C 2F 6 chemistry was investigated. The FSG water absorption was found to increase with the fluorine concentration. At a fluorine content above 4 at.%, the fluorine seems to react with the water present in the film and then outgasses. In this case the film properties are altered: the stress increases drastically (compressive to tensile) and the dielectric constant rises. But stable layers, with low absorptivity of water and no fluorine outgassing can be obtained at 3.5 at.% fluorine. These stable layers enable a dielectric constant of 3.6 to be reached, no corrosion is detected on metal lines (Ti/TiN/AlCu/TiN) capped with 1 μm of such FSG, after a moisture absorption test (168 H, 85°C, 85%HR) and annealing at 450°C. Good adhesion of the metal lines directly deposited on the FSG, using a TiN barrier, was observed. Furthermore, the gap filling capability of these FSG films is significantly improved compared with standard PECVD TEOS oxide films.