Thickness Dependence of Electrical and Structural Properties of Nb Thin Films

Nb thin films 10 to 1000 nm thick, deposited by rf sputtering on Coming glass substrates, are widely characterized both from the electrical and structural points of view by measuring resistivity, grain size, lattice parameter, and surface roughness. The strong dependence of these properties, especially resistivity, on film thickness is evidenced, and the effect of sputter etching of the substrate before deposition is also evaluated. Size effects alone are unable to account for the observed thickness dependence. In this paper, it is shown that electron scattering at the grain boundaries, analyzed through the Reiss conduction model, can be considered to be the main reason of the nonbulk-like behaviour of these films. A comparison between theoretical and experimental data suggests, however, that the transmission coefficient at the grain boundaries is not constant at all thicknesses. Impurities diffusion and stress at the grain boundaries modifying intragrain potential may be the reasons of this inhomogeneity.