Chemical Vapor Transformation of Lithium Carbonate during Plasma-Assisted ALD Growth of Lithium Phosphate

The research on interface engineering in Li-ion batteries by atomic layer deposition (ALD) is presently shifting from binary metal oxides toward Li-containing oxides, phosphates, and fluorides. In parallel, significant attention is devoted to the development of ALD processes based on volatile Li precursors, for future industrial process upscaling. In this work, we employ the novel Li precursor Lider, characterized by a vapor pressure of 1 Torr at 129 °C and Si-free ligands, for the ALD synthesis of lithium phosphate (LiPO). Combining Lider with an O2 plasma (O2*) or O3 as a coreactant results in the growth of Li2CO3 films with growth per cycle values of 0.25 and 0.29 Å, respectively, with differences in film crystallinity and mass density. Then, LiPO is synthesized by combining the O2*-based Lider process, leading to Li2CO3, with trimethyl phosphate (TMPO) in the supercycle approach, leading to a growth per supercycle of 0.6 Å and excellent uniformity over an 8 in. wafer. Remarkably, XPS shows that the Li2.7PO3.7 film does not contain carbonate impurities. The adoption of quadrupole mass spectrometry (QMS) reveals that a chemical vapor transformation reaction occurs between TMPO and the Li2CO3 surface: similarly to trimethyl aluminum (TMA), TMPO is shown to abstract surface carbonate species as CO2. This work demonstrates that this chemical vapor transformation mechanism, so far only investigated for surface cleaning of battery electrodes, can also be utilized in ALD supercycles of Li compounds to obtain films without carbonate impurities.