Analysis of the properties of radio frequency ion plated Cu films based on the calculation of the ion energy available per film atom

Abstract

During plasma assisted film deposition (PAD), the ion energy available per film atom plays an important role in film growth and morphology development. Consequently, it influences the film properties such as stress, adhesion, and dc resistivity. Calculation of the ion energy available per film atom is a little difficult in PAD because of a lack of data on the flux and energy distribution of ions (IED) incident on the film during deposition. In this article, the nature of the IED, in a rf ion plating system used for Cu film deposition, is determined with the help of a theoretical model available in literature and the plasma parameters obtained from Langmuir probe data. Then the ion energy available per Cu film atom (ECu) is calculated to be in the range of 0.5–5 eV/atom. This is further used to explain the variation of film morphology and dc resistivity with the ion plating process variables, viz., pressure, Cu evaporation rate and substrate bias. This range of ECu is insufficient to influence the film thickness significantly via resputtering. The film thickness is influenced to a much greater degree by the evaporation rate due to collisional scattering of the film vapor in the relatively high pressure used during ion plating. The film morphology is shown to improve with increase in the ECu but shows the presence of micro cracks at pressure higher than 2 mTorr, probably due to increased neutral gas adsorption. The data on ECu and morphology is then used to show that the dc resistivity of the films is influenced by grain boundary scattering and partial oxidation during deposition.