LOW-TEMPERATURE DIFFUSION OF ALKALI-EARTH CATIONS IN THIN, VITREOUS SIO2-FILMS

Ionic diffusion of two mobile alkali earth impurities, calcium and mag nesium, has been observed in thin vitreous silicon dioxide (silica) fi lms at temperatures as low as 80 degrees C. A metal oxide semiconducto r MOS capacitor has been utilized as the test structure for this devic e investigation. Controlled amounts of each impurity were introduced o nto thermally oxidized surfaces of p-type silicon wafers prior to devi ce contact metallization through immersion in strongly basic solutions , then the impurities were driven into the silica films during final m etal sintering. Initially, transient ion diffusion currents have been measured during elevated temperature device stressing under both unbia sed (shorted) and biased stress conditions; then the currents were int egrated to determine the time dependence of mobile charge transferred from the gate interface to the substrate interface. Capacitor C-V flat band voltage shifts have also been examined to verify the amount of mo bile charge transferred through the silicon dioxide films under biased as well as unbiased stress conditions. Negative flatband voltage shif ts have been observed under unbiased (shorted) stress conditions, indi cating the calcium and magnesium were present in the silica films as m obile cations. These observations were subsequently supported by secon dary ion mass spectroscopy impurity concentration profiles within the silica films. Finally, impurity diffusion activation energies have bee n determined for both ions from time dependent charge flux curves betw een 80 and 180 degrees C. Both activation energies were observed to ex hibit strong dependencies upon applied electric field intensity during device stressing. These results are in agreement with an existing mob ile ion transport model that includes both an emission-limited (interf ace boundary layer) activation energy term as well as a drift-limited (bulk trapping) term.