Structural study of nanocrystalline nickel thin films

Nanocrystalline nickel (nc-Ni) is a very interesting material due to its enhanced mechanical and corrosion properties. However, being metastable, it transforms into large-grain nickel upon heating. Here we present the results of a GISAXS study of the structure of nc-Ni thin films deposited onto substrates at elevated temperatures. Nickel thin films were deposited by dc magnetron sputtering onto fused silica substrates. The base pressure in the process chamber was 10-6 – 10-5 Pa, and the working gas was argon at 1.3 Pa. The deposition rate was about 7-8 nm/min, and the film thickness was about 400 nm. In order to examine the effects upon the film structure, the substrate temperature was varied in the room temperature (RT) - 700  C range. The film structure has been investigated using Grazing incidence small angle X-ray scattering (GISAXS) and the XRD method. 2D GISAXS patterns were recorded with a two-dimensional CCD-detector, placed perpendicular to the incident beam. The scattering intensities along a line parallel to the specular plane, (and distanced enough to minimize the surface roughness contribution) are shown in Fig. 1 for different substrate temperatures. We have found that the nc-Ni films deposited onto substrates held at temperatures up to 80º C contain inhomogenities up to 8 nm in size. Above 80º C their size is gradually reduced to about 6 nm. Only at 700º C there is a strong SAXS signal from about 2nm large inhomogenities. These inhomogenities are ascribed to intergranular matter/ grain boundaries, since the grain size (as determined from the XRD patterns) of the examined sample was found to increases from 12 nm in samples deposited at room temperature to about 90 nm in samples deposited at 150 C, or higher temperature.The change of ratio between the two phases as observed by GISAXS and XRD, respectively, corresponds well to the variation of the electric resistance of the nc-Ni samples deposited at different temperatures.