Comparison of chemical stability and corrosion resistance of group IV metal oxide films formed by thermal and plasma-enhanced atomic layer deposition

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.

Figure 1

Narrow-scan Ti 2p, Zr 3d and Hf 4f XPS spectra of as-deposited TALD and PEALD (a) TiO₂, (b) ZrO₂ and (c) HfO₂ films on Si. (d) Typical O 1s XPS spectra of as-deposited TALD and PEALD HfO₂ films on Si.

Figure 2

(a) Si 2p narrow-scan XPS spectra of as-deposited at 200 °C, 450 °C and 900 °C annealed TALD and PEALD TiO₂ films surface on Si. XPS depth profiles for (b) as-deposited at 200 °C, (c) 450 °C and (d) 900 °C annealed TALD-TiO₂ films on Si using Ar⁺ ions etching.

Figure 3

FTIR spectra of as-deposited at 200°, 450 °C and 900 °C annealed TALD TiO₂ films.

Figure 4

Cross-section FESEM images of TALD TiO₂ films before chemical test. (a) as-deposited at 200 °C. (b) 450 °C anneal. (c) 900 °C anneal.

Figure 5

AFM images of as-deposited and annealed TALD and PEALD TiO₂ films. TALD: (a) as-deposited at 200 °C, (b) 450 °C anneal and (c) 900 °C anneal; PEALD: (d) as-deposited at 200 °C, (e) 450 °C anneal and (f) 900 °C anneal.

Figure 6

AFM images of as-deposited and annealed TALD ZrO₂ and HfO₂ films. (a,c) As-deposited at 250 °C; (b,d) 600 °C anneal.

Figure 7

GIXRD patterns of as-deposited and annealed TALD TiO₂, ZrO₂ and HfO₂ ultrathin films.

Figure 8

The thickness variation vs. immersion time for all as-deposited and post-annealed TALD/PEALD TiO₂ films in various chemical liquid media: (a) 1 mol/L H₂SO₄, (b) 1 mol/L HCl, (c) 1 mol/L KOH, (d) 1 mol/L KCl and (e) 18 MΩ water.

Figure 9

The thickness variation vs. time for all 250 °C as-deposited, 600 °C-annealed TALD/PEALD-ZrO₂ films in various chemical liquid media: (a) 1 mol/L H₂SO₄, (b) 1 mol/L HCl, (c) 1 mol/L KOH and (d) 18 MΩ water.

Figure 10

The thickness variation vs. time for all 250 °C as-deposited, 600 °C-annealed TALD/PEALD-HfO₂ films in various chemical liquid media: (a) 1 mol/L H₂SO₄, (b) 1 mol/L HCl, (c) 1 mol/L KOH and (d) 18 MΩ water.

Figure 11

(a) Polarization curves and (b) Bode plots of untreated bare 304 SS and TALD- and PEALD-HfO₂ coated 304 SS with and without H₂ plasma pretreatments.

Figure 12

Experimental setup for electrochemical measurements including (a) electrochemical workstation, (b) three-electrode test and (c) sample package picture.