Alloys of copper with small amounts of aluminum have recently been investigated as the interconnect metal directly on SiO2 or as the low resistivity diffusion barrier/adhesion promoter between copper and SiO2. In the present work the electrical stability of the metal-oxide-semiconductor (MOS) capacitors of such alloys on thermal oxide on p-silicon was investigated. MOS samples preannealed at 250 degreesC and subjected to bias temperature stressing (BTS) at 200 and 250 degreesC under an electrical held of 1.5 or 2 MV/cm showed stable C-V behavior with no observed shifts along the voltage axis. However, an observed decrease in the C-ox by about 4-5% was associated with the higher BTS temperature or the field. Such changes were not observed if the preanneal was performed at 300 degreesC instead of 250 degreesC. Our X-ray photoelectron spectroscopy (XPS) analyses of the metal/SiO2 interface suggest that the observed instabilities in samples preannealed at 250 degreesC (not observed in those preannealed at 300 degreesC) are related to the interface interactions between Al (in the alloy) and SiO2. The higher temperature preanneal or higher BTS conditions led to stabilization of this interface leading to stable MOS behavior as is also supported by the results of the I-V measurements which is also presented and discussed in this work. (C) 2001 The Electrochemical Society. All rights reserved.