Characterization of surface structure in sputtered Al films: Correlation to microstructure evolution

Abstract

Quantitative roughness and microstructural analysis of as-deposited Al films, 0.1–1.0 μm thick, were performed by atomic force microscopy (AFM), one-dimensional power spectral density analysis (1DPSD), transmission electron microscopy, and x-ray pole figure methods. The variation of grain size (d) with thickness (h) in the columnar grained film was d∝h0.9. The initial crystallographic texture was nearly random, with a strong Al (111) fiber texture evolving by ≈0.2 μm in deposited thickness. AFM imaging revealed a surface structure with hillocks, grains, and grain boundary grooves, and periodic within-grain ridges extending over entire grains. The root-mean-square surface height variation (RRMS) initially decreased during deposition but increased as RRMS∝h0.55 from 0.3 to 1.0 μm thickness. The 1DPSD analysis revealed three spatially resolved regimes of roughness evolution; a frequency independent regime at low frequency attributed to hillock growth, an intermediate frequency self-similar regime attributed to grains and grain boundary grooves, and a high frequency self-similar regime attributed to within-grain ridges. Two characteristic dimensions (CD) were defined at the inverse frequencies of transition between each 1DPSD roughness regime. CDI at high frequency was identified as the peak-to-peak ridge spacing which remained independent of film thickness. The ridge spacing is proposed to represent the upper limit of an effective surface diffusion length (λ0) due to the effects of surface diffusion and flux shadowing. The CDII at lower frequency was identified as the grain size which increased with thickness. The evolution and interactions of roughness and microstructural features are discussed in terms of surface diffusion, grain boundary motion, and flux shadowing mechanisms.