Atomic-layer-deposited alumina (ALD Al2O3) can be utilized for passivation, structural, and functional purposes in electronics. In all cases, the deposited film is usually expected to maintain chemical stability over the lifetime of the device or during processing. However, as-deposited ALD Al2O3 is typically amorphous with poor resistance to chemical attack by aggressive solutions employed in electronics manufacturing. Therefore, such films may not be suitable for further processing as solvent treatments could weaken the protective barrier properties of the film or dissolved material could contaminate the solvent baths, which can cause cross-contamination of a production line used to manufacture different products. On the contrary, heat-treated, crystalline ALD Al2O3 has shown resistance to deterioration in solutions, such as standard clean (SC) 1 and 2. In this study, ALD Al2O3 was deposited from four different precursor combinations and subsequently annealed either at 600, 800, or 1000 °C for 1 h. Crystalline Al2O3 was achieved after the 800 and 1000 °C heat treatments. The crystalline films showed apparent stability in SC-1 and HF solutions. However, ellipsometry and electron microscopy showed that a prolonged exposure (60 min) to SC-1 and HF had induced a decrease in the refractive index and nanocracks in the films annealed at 800 °C. The degradation mechanism of the unstable crystalline film and the microstructure of the film, fully stable in SC-1 and with minor reaction with HF, were studied with transmission electron microscopy. Although both crystallized films had the same alumina transition phase, the film annealed at 800 °C in N2, with a less developed microstructure such as embedded amorphous regions and an uneven interfacial reaction layer, deteriorates at the amorphous regions and at the substrate-film interface. On the contrary, the stable film annealed at 1000 °C in N2 had considerably less embedded amorphous regions and a uniform Al-O-Si interfacial layer.