Atomic Layer Deposition of Ruthenium Dioxide Based on Redox Reactions between Alcohols and Ruthenium Tetroxide

Atomic layer deposition (ALD) of ruthenium dioxide (RuO2) thin films using metalorganic precursors and O2 can be challenging because the O2 dose needs to be precisely tuned and significant nucleation delays are often observed. Here, we present a low-temperature ALD process for RuO2 combining the inorganic precursor ruthenium tetroxide (RuO4) with alcohols. The process exhibits immediate linear growth at 1 Å/cycle when methanol is used as a reactant at deposition temperatures in the range of 60–120 °C. When other alcohols are used, the growth per cycle increases with an increasing number of carbon atoms in the alcohol chain. Based on X-ray photoelectron spectroscopy (XPS) and conventional X-ray diffraction, the deposited material is thought to be amorphous RuO2. Interestingly, pair distribution function (PDF) analysis shows that a structural order exists up to 2–3 nm. Modeling of the PDF suggests the presence of Ru nanocrystallites within a predominantly amorphous RuO2 matrix. Thermal annealing to 420 °C in an inert atmosphere crystallizes the films into rutile RuO2. The films are conductive, as is evident from a resistivity value of 230 μΩ·cm for a 20 nm film grown with methanol, and the resistivity decreased to 120 μΩ·cm after crystallization. Finally, based on in situ mass spectrometry, in situ infrared spectroscopy, and in vacuo XPS studies, an ALD reaction mechanism is proposed, involving partial reduction of the RuO2 surface by the alcohol followed by reoxidation of the surface by RuO4 and concomitant deposition of RuO2.