Amorphous carbon was evaluated as a diffusion barrier between Cu and Si. Samples were annealed at 500 ºC for 1 hour in vacuum. Barrier properties were characterized for metallurgical failure with RBS and XTEM, and electrical properties were characterized with pn junction diodes and MOS capacitors. RBS and X-TEM showed that a barrier 13.5 nm thick prevented Cu and Si diffusion during the anneal. The diodes remained rectifying after annealing, although the series resistance went up. C-V measurements of the capacitors did not show the hysteresis that is typical of mobile charge in the oxide. The barrier breakdown mechanism is diffusion through the amorphous structure.