Dependence of Crystal Orientation on Composition in Silver-Silicon Composite Thin Films

1. 21% Si 31% Si 37% Si 47% Si 61% Si 76% Si Resistivity rises exponentially from 46% to 64% Si 35% Si 48% Si 56% Si 66% Si 30% Si Transmission electron micrographs of silver-silicon films Pole figure plots showing loss of consistent orientation Substrate Holder Silicon Substrate Si Ag Deposition Sources Orientation of sputter targets and substrate Dependence of Crystal Orientation on Composition in Silver-Silicon Composite Thin Films Don Carlson, Dana Filoti, Amanda Brown, James M.E. Harper Resistivity A plot of film resistivity versus composition shows that between 46% and 64% silicon there is an exponential rise in resistivity. This indicates tunneling conduction with the silver no longer interconnected at these compositions. This exponential increase in resistivity is evidence of phase segregation and shows that the silver is forming independent islands in the insulating silicon. Electron Microscopy Transmission electron microscopy (TEM) offers a direct look at this behavior. The TEM images below show films with increasing silicon from left to right. The silver (dark spots) loses its interconnectedness and forms small independent regions, showing a change to more distinct silver segregation. This is consistent with the exponential rise in resistivity from 46% to 64%. The TEM images also show that with increasing silicon, the silver regions become smaller. INTRODUCTION RESULTS Metal-insulator thin films have been studied for making self-patterning nano-templates and for controlling attachment strength on template surfaces. These films can form a phase segregated structure where the metal separates into independent, non-adjoining regions within the insulator. X-ray diffraction pole figure analysis of these composite films has suggested that the texture quality of the film microstructure varies with changes in the ratio of insulator to metal. This study examines silver-silicon thin films to verify phase segregation and to track the changes in microstructural silver crystal orientation over a range of silicon/silver compositional ratios. EXPERIMENTAL METHODS A dual source magnetron sputtering system was used to deposit silver and silicon onto a stationary substrate. Variations in deposition rate due to distance from the sources created a compositional gradient across the substrate. XRD/Pole Figure Analysis XRD measurements of films with compositions ranging from 20-80% silicon confirmed the presence of crystalline silver with 111 planes producing the most intense diffraction peak. Pole figure analysis of the 111 diffraction peak shows that for films mostly composed of silver (< 35% Si) the 111 planes are consistently oriented parallel to the substrate surface (intense peak at the center of the polar pole figure plot). However, for increasingly higher percentages of silicon, the orientation becomes more random. This is likely due to smaller islands of phase segregated silver buried in the amorphous silicon. Without access to the substrate surface, they have no preferred orientation. This loss of consistent texture between 37% and 47% is supported by the resistivity measurements and TEM images covering the same compositional region. The completed films were analyzed using three methods: • The resistivity of the composite film was measured at 2 mm intervals along the substrate using a 4-point probe, and film composition was calculated from the thicknesses of separate silver and silicon calibration films. An exponential rise in resistivity versus composition indicates tunneling conduction. • Electron Microscopy provided a picture of how the silver was distributed through the silicon insulator. • X-ray diffraction (XRD)/pole figure analysis was used to study the crystal structure of the composite films. XRD identifies any crystalline substance in the film while pole figure analysis of a diffraction peak is used to measure the orientation of the associated crystal planes with respect to the substrate. Sputter deposited poly-crystalline silver films typically form with their 111 planes parallel to the substrate. This consistent orientation shows up as an intense peak on a pole figure plot. If the silver crystallites have a random orientation, then the pole figure plot would be fairly homogenous with no strong peaks. CONCLUSION Measurement of resistivity and electron microscopy images are both used to confirm phase segregation and the formation of independent metal islands in metal-insulator composite thin films. Measurements of polycrystalline orientation using pole figure analysis has shown that it can identify the same behavior. In the silver-silicon films studied, resistivity measurement showed independent silver islands forming at 46% silicon and TEM images showed increasingly distinct silver segregation for compositions above 35% Si. Pole figure analysis agreed with these results by suggesting island formation between 37% and 47% Si. Use of resistivity to measure phase segregation only works with metal-insulator films. Pole figure analysis could potentially verify phase segregation and island formation in metal-metal composite films. Since each crystalline material would have a unique diffraction signature, pole figure analysis could be used to measure the polycrystalline orientation of each component individually. This is an avenue for further study on the use of pole figure analysis for composite film characterization.