The ionic diffusion of the 2 alkali earth impurities was monitored in thin vitreous silica films, at temperatures as low as 80C, by using a metal oxide semiconductor capacitor as the test structure. Controlled amounts of each impurity were deposited onto thermally oxidized surfaces of p-type Si wafers before contact metallization. The impurities were then driven into the silica films during the final metal sintering. Transient ion diffusion currents were measured during high-temperature device stressing under unbiased and biased stress conditions. The currents were then integrated in order to determine the time dependence of mobile charge transfer from the gate interface to the substrate interface. Capacitor voltage shifts were also used to determine the amount of mobile charge which was transferred through the oxide films under biased and unbiased stress conditions. Negative flat-band voltage shifts were detected under unbiased stress conditions; thus indicating that the Ca and Mg were present as mobile cations in the silica films. These deductions were confirmed by secondary ion mass spectroscopic measurements of the impurity profiles in the silica films. Impurity diffusion activation energies were deduced from time-dependent charge flux curves between 80 and 180C. It was found that both activation energies exhibited marked dependences upon the applied electric field intensity during stressing. The results were in agreement with a mobile-ion transport model that included an emission-limited (interface boundary layer) activation energy term and a drift-limited (bulk trapping) term.

C.K.Williams, R.W.Hamaker, S.G.Ganesan, R.T.Kuehn, K.R.Swartzel, J.O’Sullivan: Journal of the Electrochemical Society, 1995, 142[1], 303-11