Abstract
The wide applications of ultrathin group IV metal oxide films (TiO2, ZrO2 and HfO2) probably expose materials to potentially reactive etchants and solvents, appealing for extraordinary chemical stability and corrosion resistance property. In this paper, TiO2 ultrathin films were deposited on Si at 200 °C while ZrO2 and HfO2 were grown at 250 °C to fit their growth temperature window, by thermal atomic layer deposition (TALD) and plasma-enhanced ALD (PEALD). A variety of chemical liquid media including 1 mol/L H2SO4, 1 mol/L HCl, 1 mol/L KOH, 1 mol/L KCl, and 18 MΩ deionized water were used to test and compare chemical stability of all these as-deposited group IV metal oxides thin films, as well as post-annealed samples at various temperatures. Among these metal oxides, TALD/PEALD HfO2 ultrathin films exhibit the best chemical stability and anti-corrosion property without any change in thickness after long time immersion into acidic, alkaline and neutral solutions. As-deposited TALD ZrO2 ultrathin films have slow etch rate of 1.06 nm/day in 1 mol/L HCl, however other PEALD ZrO2 ultrathin films and annealed TALD ones show better anti-acid stability, indicating the role of introduction of plasma O2 in PEALD and post-thermal treatment. As-deposited TiO2 ultrathin films by TALD and PEALD are found to be etched slowly in acidic solutions, but the PEALD can decrease the etching rate of TiO2 by ~41%. After post-annealing, TiO2 ultrathin films have satisfactory corrosion resistance, which is ascribed to the crystallization transition from amorphous to anatase phase and the formation of 5% Si-doped TiO2 ultrathin layers on sample surfaces, i.e. Ti-silicate. ZrO2, and TiO2 ultrathin films show excellent corrosion endurance property in basic and neutral solutions. Simultaneously, 304 stainless steel coated with PEALD-HfO2 is found to have a lower corrosion rate than that with TALD-HfO2 by means of electrochemical measurement. The pre-treatment of plasma H2 to 304 stainless steel can effectively reduce interfacial impurities and porosity of overlayers with significantly enhanced corrosion endurance. Above all, the chemical stability and anti-corrosion properties of IV group metal oxide coatings can be improved by using PEALD technique, post-annealing process and plasma H2 pre-treatment to substrates.
Highlights
Group IV metal oxide films deposited via atomic layer deposition (ALD) including titania (TiO2), zirconia (ZrO2) and hafnia (HfO2) have been widely investigated due to their excellent properties in electrical, optical, photocatalytical, biological and mechanical fields[1,2,3,4,5,6]
TiO2, ZrO2 and HfO2 thin films were deposited on p-type Si (100) with nature oxide layer by the technique of thermal ALD (TALD) and plasma-enhanced ALD (PEALD)
As-deposited TiO2 films are relatively unstable in acidic solutions of H2SO4 and HCl, where the etch rate is 1.39 nm/day and 0.82 nm/day for TALD ones, and 0.81 nm/day and 0.48 nm/day for PEALD ones, respectively
Summary
Group IV metal oxide films deposited via atomic layer deposition (ALD) including titania (TiO2), zirconia (ZrO2) and hafnia (HfO2) have been widely investigated due to their excellent properties in electrical, optical, photocatalytical, biological and mechanical fields[1,2,3,4,5,6]. Compared with the conventional thermal ALD (TALD), plasma-enhanced ALD (PEALD) is an energy-assisted method for fabrication of thin films, where plasma species are utilized as reactive gas during one step of the cyclic deposition process[9]. The chemical stability of TALD Al2O3 and TiO2 films in different acidic, basic and neutral media, and the influence of post-deposition thermal treatment on different samples have been discussed[20] These valuable results are helpful to expansive applications of organic electronic devices[21], stabilization of semiconductor photoanodes for water oxidation[22] and many other fields. The corrosion endurance of HfO2 coating for 304 stainless steel (SS) in 1 mol/L KCl solution has been studied by in-situ electrochemical characterization
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