Thermal and electrical stability of TaN _x diffusion barriers for Cu metallization

Amorphous TaN _x thin films (14 and 62 nm) were deposited by reactive sputtering on Si substrates. Crystallization and the metallurgical failure mechanism for Si/TaN _x /Cu metallization stacks were investigated by resistivity measurements, X-ray diffraction analysis, detailed electron microscopy and elemental depth profiling on samples annealed in 5 %H₂/95 %N₂ gas for 30 min at various temperatures ranging from 300 to 900 °C. Amorphous TaN _x thin films crystallized at 600 °C to hexagonal Ta₂N by a polymorphous transformation. Depending on film thickness, polycrystalline Ta₂N diffusion barriers were effective up to 700–800 °C. Failure occurred by diffusion of Cu to the Si/TaN _x interface to form Cu₃Si particles followed by outdiffusion of Si and formation of Cu₃Si and TaSi₂ precipitates on the outer surface. The TaN _x barriers were integrated in metal–oxide–semiconductor devices (Cu/10 nm TaN _x /26 nm SiO₂/Si) to evaluate their electrical failure after bias-temperature-stress (BTS) testing using capacitance–voltage and current–voltage measurements. The shift in flat-band voltage and the leakage current were monitored before and after BTS. The electrical test results were compared with compositional and morphological information obtained from elemental depth profiling and electron microscopy. No evidence of Cu diffusion to SiO₂ was found for capacitors with large leakage currents.