Conformality of atomic/molecular layer deposited lithium terephthalate thin films

The atomic/molecular layer deposition (ALD/MLD) technique is believed to provide unparallelled benefits for the fabrication of thin-film components for flexible 3D lithium-ion microbatteries, for which the nanoscale conformality of the active battery material layers is a critical requirement. For such an ALD/MLD-fabricated microbattery assembly, lithium-terephthalate (Li-TPA) is a prominent anode material candidate. Here we demonstrate the role of the lithium precursor in controlling the conformality of Li-TPA films. We first develop two new ALD/MLD processes for Li-TPA by pairing the Li precursors, lithium tert-butoxide (LiO^tBu) and lithium bis(trimethylsilyl)amide (Li-HMDS) with terephthalic acid (TPA); in previous studies, Li-TPA thin films have been grown from lithium 2,2,6,6-tetramethyl-3,5-heptanedionate (Li-THD). Then, using state-of-the-art lateral high-aspect-ratio (LHAR) test structures, we evaluate the three ALD/MLD processes: LiO^tBu + TPA, Li-HMDS + TPA and Li-THD + TPA, for both the essentially constant film-growth range (conformality) and the ultimate extent of the film growth (penetration depth) along the LHAR cavities; these parameters are determined using a novel imaging ellipsometry approach in addition to scanning electron microscopy characterization. The results highlight the importance of the size and nature of the lithium precursor. Notably, among the three Li precursors, Li-HMDS yielded Li-TPA films of superior conformality and penetration depth characteristics. Through a detailed computational investigation of the precursors, including conformer screening, DFT optimizations and accurate DLPNO-CCSD(T) energy calculations, we correlate the experimentally observed trends with the most likely oligomeric forms of the precursors at reactor conditions.