Low Temperature Diffusion of Alkali Earth Cations in Thin, Vitreous SiO2 Films

Abstract

Ionic diffusion of two mobile alkali earth impurities, calcium and magnesium, has been observed in thin vitreous silicon dioxide (silica) films at temperatures as low as 80°C. A metal oxide semiconductor MOS capacitor has been utilized as the test structure for this device investigation. Controlled amounts of each impurity were introduced onto thermally oxidized surfaces of p‐type silicon wafers prior to device contact metallization through immersion in strongly basic solutions, then the impurities were driven into the silica films during final metal sintering. Initially, transient ion diffusion currents have been measured during elevated temperature device stressing under both unbiased (shorted) and biased stress conditions; then the currents were integrated to determine the time dependence of mobile charge transferred from the gate interface to the substrate interface. Capacitor C‐V flatband voltage shifts have also been examined to verify the amount of mobile charge transferred through the silicon dioxide films under biased as well as unbiased stress conditions. Negative flatband voltage shifts have been observed under unbiased (shorted) stress conditions, indicating the calcium and magnesium were present in the silica films as mobile cations. These observations were subsequently supported by secondary ion mass spectroscopy impurity concentration profiles within the silica films. Finally, impurity diffusion activation energies have been determined for both ions from time dependent charge flux curves between 80 and 180°C. Both activation energies were observed to exhibit strong dependencies upon applied electric field intensity during device stressing. These results are in agreement with an existing mobile ion transport model that includes both an emission‐limited (interface boundary layer) activation energy term as well as a drift‐limited (bulk trapping) term.