Drift of copper (Cu) ions into methyl-doped silicon oxide film, a promising low-dielectric-constant (low-kappa) material for inter-layer dielectrics applications in ultralarge scale integrated circuit (ULSI) interconnects, was investigated using bias temperature stressing (BTS)/capacitance-voltage (C-V), current-voltage (C-V) at elevated temperatures and time-dependent dielectrics breakdown (TDDB) tests. Standard and control metal-insulator-semiconductor capacitor test samples with "sandwich" dielectric layer structures were used in the tests. BTS in nitrogen (N-2) ambient at an electric field magnitude up to 1.5 MV/cm and with temperatures between 175 and 275 degreesC was used to accelerate the Cu ion drift. By studying flat band voltage shift in C-V tests, leakage current magnitude in I-V tests and time to fail in TDDB tests, it is demonstrated that Cu ions readily drift into methyl-doped silicon oxide under electric fields at elevated temperatures. Results obtained using different techniques correlate well to each other. The activation energy of Cu drift is determined to be 0.76 eV in N-2 ambient. The film is found to have poorer resistance to Cu drift than silicon oxide (SiO2) deposited using plasma-enhanced chemical vapor deposition (PECVD). Therefore, good Cu drift barrier layers are required for reliable ULSI interconnects using this low-kappa material. A thin layer of PECVD SiC is proven to be a good Cu drift barrier layer.