Mechanically robust silica-like coatings deposited by microwave plasmas for barrier applications
Tension and compression strain applied on the coating during bending tests.
(Color online) (a) Nanoindentation of 1 μm-thick SiOx coating deposited on silicon wafers at plasma duty cycle = 100%, 75%, and 50%. The coatings deposited by pulsed plasmas are softer than the ones deposited by continuous discharge. (b) Creep evolution as a function of time. During the creep time, the load applied on the material is constant; therefore, the variation in depth is due to the viscoelasticity of the coating.
Progressive loading scratch (0–10 mN) onto SiOx deposited at 50% DC (a) and 100% DC (b). SiOx coating deposited on silicon wafer with a plasma duty cycle of 50% exhibits a delamination point ∼ 7 mN while the SiOx deposited continuously has a delamination point ∼ 3 mN. Delaminations on the scratch profile can be related to the adhesion strength of the SiOx/Si interface. (c) and (d) are SEM images of the delamination points for the coating deposited at 50% of DC (c) and 100% DC (d).
Fragmentation test of thin SiOx layer. Scanning electrons micrographs of 100 nm SiOx fragments evolution shown at different tensile strains.
(Color online) Fragmentation test of thin SiOx layer. (a) The crack density data plotted as a function of the applied tensile strain. SiOx delamination point on PET is found to occur for a tensile strain in the range of 20–25%. (b) The evolution of the critical tensile strain as a function of SiOx thickness. The data have been fitted with a (1/h)1 / 2 dependence and the agreement between the model and the data shows the reliability of the critical strain measurements.
(Color online) Comparison of the critical tensile strain variation as a function of the coating thickness obtained for the MW-plasma coatings deposited for this study, the SiOx coatings deposited by RF plasmas (Ref. 24) and the Al2O3 (Ref. 22) coatings deposited by ALD.
(Color online) Crack depth profile detected with profilometer for the samples deposited at DC = 100 and 50%. The profiles of (a) are taken after bending the sample to 2 mm as radius of curvature, while in (b) the radius of curvature used was 3 mm. Pulsed layers (50% DC) exhibit better crack recovery according to the depth and width of the residual crack.
(Color online) RMS roughness of the SiOx coating deposited at different DC, as a function of the coating thickness. The surface roughness increases with the coating thickness, while for the coating deposited at 50% DC it remains essentially constant.
(Color online) WVTR (measured at 25 °C and 85%RH) of SiOx coating deposited at different DC, as a function of the coating thickness. WVTR rapidly decreases in the range of 0–100 nm. At very high thicknesses, it slightly increases for the coating deposited at DC = 100% and slightly decreases for the case DC = 50%.
Mechanical properties (elastic modulus, E; hardness, H) measured by nanoindentation as a function of the plasma duty cycle (% DC). Values in this range are in agreement with the mechanical properties of thermal deposited silicon dioxide layers, reported in the last row.
Critical tensile strain, critical bending radius, and saturation crack density calculated from the fragmentation test for the SiOx coating deposited at DC = 100% and 50% as a function of the thickness. High critical tensile strain, high saturation crack density, and low critical bending radius demonstrate the high flexibility of the coatings and good adhesion and cohesion to the substrate.
SiOx bending failure points as a function of the thickness after bending in compression and in tension. The radius indicated in the table is the bending radius at which cracks appear on the surface. The strain corresponding to each bending radius has been calculated.
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