Investigation of the Mechanical Strength of Nanocrystalline Au-si Thin Films

Abstract

A study on the effects of deposition rate and Si content on the mechanical and electrical properties of Au-Si films has been performed. Au-Si films were synthesized by electron beam evaporation on (100) Si substrates at the Center for Integrated Nanotechnologies at Los Alamos National Laboratory. The Au and Si were simultaneously codeposited to create films with a nominal thickness of 1 m, with deposition rates from 10 /s to 60 /s and Si content up to 21 at. %. The films deposited at 10 /s were then investigated by transmission electron microscopy (TEM), selected area diffraction (SAD), and particle-induced X-ray emission (PIXE). All films were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), nanoindentation, and four point resistivity probe. Nanotwinned Au grains were observed in the TEM images of the films deposited at 10 /s. In addition, Si nanoparticles were observed at the grain boundaries in the films containing 6 and 21 at. % Si deposited at 10 /s. Diffraction peaks associated with crystalline Si were not observed by SAD or XRD, which suggests the Si particles are amorphous. The reduced elastic modulus was found to decrease with increasing Si content for films deposited at 10 /s and 40 /s. This is consistent with the shift observed by in the diffraction intensities of the (111) and (200) Au diffraction peaks. Pure Au films were observed to have a strong (111) preferred orientation, however increasing Si content corresponded to an increase in the intensity of the (200) Au diffraction peak relative to the (111) Au diffraction peak. The hardness of the films was observed to increase with increasing Si content. Increases in measured hardness were shown to be described by a combination of the Hall-Petch relationship and Orowan strengthening. Grain size estimates from XRD results as well as TEM images were used to predict the strength of the films using the Hall-Petch relation. Electrical resistivity was observed to increase linearly with Si content. Nanotwinned Au grains were observed in the TEM images of the films deposited at 10 /s. In addition, Si nanoparticles were observed at the grain boundaries in the films containing 6 and 21 at. % Si deposited at 10 /s. Diffraction peaks associated with crystalline Si were not observed by SAD or XRD, which suggests the Si particles are amorphous. The reduced elastic modulus was found to decrease with increasing Si content for films deposited at 10 /s and 40 /s. This is consistent with the shift observed by in the diffraction intensities of the (111) and (200) Au diffraction peaks. Pure Au films were observed to have a strong (111) preferred orientation, however increasing Si content corresponded to an increase in the intensity of the (200) Au diffraction peak relative to the (111) Au diffraction peak. The hardness of the films was observed to increase with increasing Si content. Increases in measured hardness were shown to be described by a combination of the Hall-Petch relationship and Orowan strengthening. Grain size estimates from XRD results as well as TEM images were used to predict the strength of the films using the Hall-Petch relation. Electrical resistivity was observed to increase linearly with Si content.