Development of two-level porosity during glancing angle deposition

Abstract

Porous Ta and Al layers that exhibit 280-nm-wide micropores, which are interconnected by 5–50-nm-wide nanopores, were grown by glancing angle deposition (GLAD). The micropore nucleation is facilitated by patterning Si(001) substrates with inverted-pyramidal pit arrays using anisotropic etching through lithographic masks obtained by colloidal self-assembly. The microstructure consists of vertical nanorods with an average width w that increases with layer thickness t, following a power law w∝tβ. This indicates a self-similar growth mode which is controlled by purely geometric long-range atomic shadowing interactions. However, statistical analyses show a larger growth exponent βAl=0.58±0.07 for Al than for Ta with βTa=0.46±0.08, suggesting a secondary effect where the surface curvature of the high adatom mobility Al rods is lowered by diffusion-mediated lateral growth which exacerbates inter-rod competition and, in turn, leads to an enhanced rod broadening. The broadening in Al causes a close-up of the microscopic pores, a decrease in the porosity to 54% at t=750nm, and a reduction in the rod number density n, where n∝tγ and the extinction exponent γAl=−1.02±0.01. In contrast, the Ta porosity remains constant at 70%, the pore width is independent of t, and the extinction rate decreases from γ=−2.5 to −0.5. This is attributed to a transition from two- to one-dimensional shadowing, associated with the microscopic pores that initially enhance but later suppress growth competition in comparison to conventional GLAD on flat substrates where γ is expected to be ∼−1. These results provide insight into columnar competition under anisotropic shadowing conditions and also demonstrate a path to create layers with a controlled bimodal pore structure.