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, Me3PO4) plasma, O-2 plasma, and tert-butylferrocene (TBF, Fe(C5H5)(C5H4C-(CH3)(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 degrees 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 FeP1.5O4.7 with 0.9% hydrogen and no detectable carbon residue. Trigonal FePO4 (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.