The barrier effect for oxygen diffusion is studied in TaSiN layers. The TaSiN is deposited by reactive sputtering employing Ta and Si targets. The composition of the layer ranges from Ta0.16Si0.27N0.57 to Ta0.55Si0.07N0.38 by varying the Ta target power from 100 to 400 watts (W). A resistivity of 210 mu Omega cm is obtained for the Ta0.55Si0.07N0.38 layer. The surface oxidation and in-diffusion of oxygen to a depth of 15 nm into the Ta0.30Si0.17N0.53 layer are observed by annealing in O-2 at 650 degrees C. However, the oxygen diffusion is suppressed in the Ta0.55Si0.07N0.38 layer. No out-diffusion of oxygen occurs from the Ta2O5 dielectric layer to the amorphous barrier layer. This result shows that a low Si concentration layer for instance, Ta0.55Si0.07N0.38 is a promising barrier layer for oxygen diffusion and is useful for charge storage capacitors for MOS memory devices.