Comparison of the cohesive and delamination fatigue properties of atomic-layer-deposited alumina and titania ultrathin protective coatings deposited at 200 °C

Abstract

The fatigue properties of ultrathin protective coatings on silicon thin films were investigated. The cohesive and delamination fatigue properties of 22 nm-thick atomic-layered-deposited (ALD) titania were characterized and compared to that of 25 nm-thick alumina. Both coatings were deposited at 200 °C. The fatigue rates are comparable at 30 °C, 50% relative humidity (RH) while they are one order of magnitude larger for alumina compared to titania at 80 °C, 90% RH. The improved fatigue performance is believed to be related to the improved stability of the ALD titania coating with water compared to ALD alumina, which may in part be related to the fact that ALD titania is crystalline, while ALD alumina is amorphous. Static fatigue crack nucleation and propagation was not observed. The underlying fatigue mechanism is different from previously documented mechanisms, such as stress corrosion cracking, and appears to result from the presence of compressive stresses and a rough coating-substrate interface.

Keywords: ALD; Alumina; Fatigue; Harsh environment; Titania; Ultrathin coatings.

Figure 1

Inclined SEM images of (a) Si microresonator coated with 22 nm of ALD TiO₂ and (b) notch area before test. The scalloping results from the deep reactive ion etching process. The defects observed in (b) slightly away from the notch root at the top of the film's thickness (see white arrow) are related to the Si device fabrication process [22]. No fatigue damage was observed in that particular area, where stresses are much lower than at notch root [13]. (c) High magnification inclined SEM image of the sidewall coated with ALD TiO₂, showing that the TiO₂ coatings are likely to be crystalline.

Figure 2

(a) Representative f₀ evolution plots for four fatigue tests on ALD titania-coated Si micro-resonators (initial f₀ ∼ 40 kHz). (b) Total decrease in f₀, Δf_0,max, as a function of ε_a for ALD titania and alumina coatings.

Figure 3

Predicted (lines) and measured (circles) number of cracks in ALD titania coatings, as a function of ε_a, at (a) 30 °C, 50% RH and (b) 80 °C, 90% RH.

Figure 4

Inclined SEM images of notch area after fatigue test at 30 °C, 50% RH (ALD titania): (a), (b) ε_a = 1.68%, (c), (d) ε_a = 1.84%, and (e), (f) ε_a = 2.07%. Inclined SEM images of notch area after fatigue test at 80 °C, 90% RH (ALD titania): (g), (h) ε_a = 1.05% and (i), (j) ε_a = 1.47%. Inclined SEM images of notch area after fatigue test at 80 °C, 90% RH (ALD alumina): (k), (l) ε_a = 2.19%.

Figure 5

Calculated fatigue crack channeling growth rates for ALD titania (h_TiO2 = 22 nm) and ALD alumina (h_Al2O3 = 25 nm) as a function of (a) ε_a and (b) G_a,ch.

Figure 6

Calculated fatigue delamination rates for ALD titania (h_TiO2 = 22 nm) and ALD alumina (h_Al2O3 = 25 nm) as a function of (a) ε_a and (b) G_a,del.