Role of the Coreactant\non the Dual-Source Behavior\nof Lithium Hexamethyldisilazide for ALD Li-Containing Films

Atomic layer deposition (ALD) of Li-containing thin films\nis deemed\nas highly relevant for the development of next-generation Li-ion batteries.\nLithium hexamethyldisilazide (LiHMDS), as Li-containing precursor,\nis preferred over the widely used lithium <i>tert</i>-butoxide\nbecause of its lower melting point of 70 °C. However, the presence\nof silyl groups in the LiHMDS chemical structure can result in the\nundesired incorporation of Si in the film. Therefore, understanding\nthe reaction mechanism of LiHMDS is required to control its dual-source\nbehavior and grow Si-free Li-containing thin films. For this purpose\nLiHMDS was combined with O<sub>2</sub> plasma or water as coreactant. <i>In situ</i> spectroscopic ellipsometry (SE) and X-ray photoelectron\nspectroscopy (XPS) revealed that using O<sub>2</sub> plasma as coreactant\nresults in linear growth and Si-containing films, whereas using H<sub>2</sub>O as coreactant leads to fast, nonsurface-reaction-limited\ngrowth and Si-free films. To shed light on the role of the coreactant\non the reaction mechanism of LiHMDS, <i>in situ</i> studies\nby time-resolved quadrupole mass spectrometry (QMS) were performed\non the O<sub>2</sub> plasma and H<sub>2</sub>O-based ALD processes.\nMeasurements taken during full ALD cycles and half-cycles were carefully\ncompared to identify which half-cycle surface reaction products lead\nto silicon incorporation in the film. The QMS results of the LiHMDS\n+ H<sub>2</sub>O process showed that LiHMDS both chemisorbs and physisorbs.\nFurthermore, it is concluded that Si incorporation occurs during the\nO<sub>2</sub> plasma step, when the physisorbed ligands are combusted\nand Si-containing products are redeposited. This work also demonstrates\nthat the incorporated Si can be abstracted from the film by means\nof a H<sub>2</sub> plasma step following the O<sub>2</sub> plasma\nstep. These insights on the role of the coreactant in the synthesis\nof Li-containing films contribute to the development of LiHMDS-based\nALD processes for Li-ion battery applications.