Plasma-Enhanced Atomic Layer Deposition of Iron Phosphate as a Positive Electrode for 3D Lithium-Ion Microbatteries

A new plasma-enhanced atomic layer deposition process was developed to deposit iron phosphate by using a sequence of trimethyl phosphate (TMP, Me 3 PO 4 ) plasma, O 2 plasma, and tert -butylferrocene (TBF, Fe(C 5 H 5 )(C 5 H 4 C(CH 3 ) 3 )) exposures. Using in situ spectroscopic ellipsometry and ex situ X-ray reflectometry, the growth linearity, growth per cycle (GPC), and density of the resulting thin films was investigated as a function of the pulse times and the substrate temperature. At a substrate temperature of 300 °C and using saturated pulse times, an exceptionally high GPC of 1.1 nm/cycle without nucleation delay was achieved, resulting in amorphous films with an empirical stoichiometry of FeP 1.5 O 4.7 with 0.9% hydrogen and no detectable carbon residue. Trigonal FePO 4 (Berlinite) was formed upon annealing in air. Remarkably, annealing in helium resulted in the formation of elemental phosphorus. The as-deposited, amorphous material became active as a Li-ion cathode after an initial irreversible electrochemical lithiation, showing insertion and extraction of Li + around a potential of 3.1 V vs Li/Li + . By conformally depositing the same material on a 3D-microstructured substrate consisting of Pt-coated Si micropillars, the capacity could be drastically increased without sacrificing rate performance.