Analysis of (Al,Cr,Nb,Ta,Ti)-nitride and -oxynitride diffusion barriers in Cu-Si interconnects by 3D-Secondary Ion Mass Spectrometry

We report on the barrier performance of a nitride, and three oxynitrides of the system Al-Cr-Nb-Ta-Ti between Cu and Si. Different high-entropy sublattice nitrides have been tested before as diffusion barriers in this system, by depositing thin barriers on single crystalline Si substrates, followed by a thick Cu layer on top, and subsequent vacuum annealing. We investigated a reversed stacking sequence, by sputtering 15-30 nm of (Al,Cr,Nb,Ta,Ti)-O-N (between 0.5 and 63.7 at.% O) on polished polycrystalline Cu substrates, followed by 200 nm of Si. The samples were then vacuum annealed at 600, 700, 800 and 900 degrees C for 30 min. All four investigated coatings perform similar. Secondary Ion Mass Spectrometry depth profiling in high-current-bunched mode (lateral res-olution +/- 1 mu m) shows breakthrough of Si even at 600 degrees C. But 3D constructed images with Burst Alignment mode (lateral resolution of +/- 2 nm) reveal that this failure is a highly localized phenomenon, likely related to coarsening effects at the Cu grain boundaries, leading to punctuation of the diffusion barrier. Aside from this penetration, the majority of the area of each barrier coating retains its function. This in-depth analysis shows that the barrier function of the nitride and oxynitride coatings mostly stays intact up to 800 degrees C and fails completely at 900 degrees C.