Study of the chemical stability of precursors used for ALD of lithium-containing films by structural and thermal analyses

Lithium-based batteries are well-known and robust energy storage solutions for portable devices. The widespread electrolyte material employed for microbatteries is a LiPON thin film. It can be deposited by atomic layer deposition (ALD) with organic precursors permanently heated into tanks. In this study, the impact of the ageing of lithium hexamethyldisilazide (LiHMDS) and diethylphosphoramidate (DEPA) precursors, i.e. the time elapsed in their tanks, on their chemical stability is reported. The results show that significant DEPA chemical degradation is detected after a fortnight, whereas LiHMDS remains quite stable up to forty days. Compared to the as-supplied DEPA, the aged one exhibits (i) a different thermal behavior as shown by TGA and DSC analyses, (ii) a structural change with the amine group degraded as shown by FTIR, (iii) polymerization as shown by 31 P NMR and (iv) a change in vapor composition. The ionic conductivity of the LiPON thin film is progressively and significantly affected by DEPA ageing.