A quaternary phase diagram for Ti–W–N–Al has been calculated from existing thermodynamic data, and is used to predict the Al/TiW-nitride reaction. The predicted reaction products-TiAl3, WAl5, WAl12, and AlN—were observed by XRD and TEM in annealed Al/TiW(-nitride) thin films. The sheet resistance of Al/TiW films increased by an order of magnitude at 550°C, whereas the increase in the case of the Al/TiW-nitride films was not even two-fold. The formation of an interfacial AlN layer was observed in the Al/TiW-nitride metallization. This AlN layer limits the interaction between Al and TiW-nitride, thus providing good thermal stability.