A number of different high-entropy sublattice nitrides have been investigated in the past as diffusion barriers between Cu and Si. These investigations were performed by depositing several nanometer thin barriers on single crystalline Si substrates, followed by a thick Cu layer on top, and subsequent vacuum annealing. In this work we report on the barrier performance of a nitride, and also three oxynitrides of the system Al-Cr-Nb-Ta-Ti by depositing a reversed stacking sequence. 10 nm of (Al,Cr,Nb,Ta,Ti)-O-N (between 0.5 and 63.7 at% O, obtained by Elastic Recoil Detection Analysis) were deposited on polished polycrystalline Cu substrates by magnetron sputtering at room temperature with -100 V bias, followed by deposition of 200 nm Si. The samples were then vacuum annealed at 600, 700, 800 and 900 °C for 30 min. All four investigated coatings perform similar. While Secondary Ion Mass Spectrometry depth profiling in high-current-bunched (HCBU) mode with a high mass resolution (of >12.000 amu, lateral resolution ±1 µm) shows breakthrough of Si even at 600 °C. 3D constructed images with Burst Alignment (BA, lateral resolution of ±2 nm) depth profiles reveal that this failure is a highly localized phenomenon. The failure is likely related to recrystallization effects at the Cu grain boundaries, leading to punctuation of the diffusion barrier, as the diffusing Si stays confined in columnar regions within the Cu. 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 essentially stays intact up to 800 °C and fails completely at 900 °C.