Thermo-electrical reflowing of pre-patterned thin films*

The electrohydrodynamic (EHD) instabilities are combined with thermocapillary (TC) instabilities to reflow the pre-patterned nanofilms aiming at creating high aspect ratio and well-ordered nanopillar arrays. Thin film (initial thickness of h₀) is sandwiched between two electrodes and the electric field and the thermal gradient is applied in the transverse direction. Using the long-wave approximation simplified the governing equations and boundary conditions leading to thin film equation that describes the dynamics and spatiotemporal evolution of the interface. Increasing the height of the protrusions, ξ, leads to a single columnar structure forming (with lower aspect ratio) whereas for the smaller values hierarchical patterns are formed. To have equally-sized pillars and to avoid a multi-scale pattern formation, using ξ/h₀≥2 is recommended. The effect of protrusions width is examined on the pattern fidelity and pillars’ aspect ratio.The optimum width ratio of w/Λ_LS =0.4 is found as for protrusions with smaller size leads to larger sized pillars with lower aspect ratios. Term Λ_LS is the characteristic wavelength for the growth of the instabilities found from linear stability analysis.