CP-MAS NMR and in-situ FTIR study of an AlN surface reactivity

The hydrolytic stability of aluminum nitride (AlN) is an important issue from the standpoint of technology development of AlN as an electronic packaging material. Although numerous studies have been made of the hydrolysis of AlN and characterization of the reaction products, an understanding of the interaction between surface sites and molecular species is still lacking. A combination of Magic-Angle-Spinning Nuclear Magnetic Resonance (MAS NMR) and in-situ Fourier Transform Infrared (FTIR) spectroscopies was used to study the surface reactivity of AlN. The specimens used were specially fabricated self-supported AlN films of high-surface-area (approximately 25 m²/g), made by nitridation of sol-gel derived pseudoboehmite (AlO(OH)). The in-situ FTIR spectra suggest the formation of a surface layer on the AlN films upon storage. MAS NMR was used to further characterize the surface layer. Using the chemical shifts obtained by ¹H-²⁷Al Cross Polarization (CP) MAS NMR experiments, we were able to identify the surface cation sites. The assignment of these sites to Al³⁺ cations in either tetrahedral coordination with nitrogen or octahedral coordination with oxygen was used to monitor the evolution of the surface upon exposure to the atmosphere. The results indicate that high-surface-area AlN rapidly acquires a surface layer (probably AlO(OH)/Al(OH)₃) upon exposure to the atmosphere.