Growth of Ag/Ni Thin Films and Multilayers

Abstract: Fundamental studies concerning the relation between growth conditions, microstructure and internal stresses of polycrystalline Ag/Ni thin films and multilayers, as well as the relation between growth conditions and the structure of highly oriented Ni films, were performed. All films were depositied using ultra high vacuum based de magnetron sputtering. The polycrystalline Ag/Ni thin films and multilayers were deposited onto oxidized Si(001) substrates held at ambient temperature. The Ni films were depositedonto MgO(111) and MgO(001) substrates kept at temperatures between 20 °C and 700°C. The structural characterization were performed mostly using atomic force microscopy (AFM), different x-ray diffraction (XRD) techniques, transmission electron microscopy (TEM) and also time-resolved in-situ relection high energy electron diffraction (RHEED). The stress evolution were studied using an in-situ wafer curvature technique.For Ag/Ni multilayers, it is shown that energetic particle bombardment, i.e. sputtering using low pressures results in a reduction of the surface roughness, compared to films grown at higher pressures. It is also shown that the definition of the layer interfaces and the crystallinity of the films are improved when the pressure is lowered and when using Ar instead of Kr. Short post-deposition annealing at 280 °C resulted in improved layer definitions as well as improved intralayer order whereas prolonged annealing or annealing at 390 °C resulted in agglomeration of Ag precipitates and loss of the compositional modulation. It is also shown that, the stresses in the Ni layers are controlled mostly by interface effects and large thermal effects due to un-intentional heating during the deposition, and thus they are always tensile after the deposition. In order to obtain compressive stresses in the Ni layers at 1 mTorr, the layer thickness must be larger than ∼30 nm. At 8 mTorr the stresses will always be tensile. The results also give some evidence for intermixing of Ag into the Ni layers. For Ag, the resulting stresses after deposition are controlled mostly by the large thermal stresses present. The temperature rise is estimated to 15-20 °C at 1 mTorr and 5-10 °C at 8 mTorr, for both Ag and Ni.For Ni films on MgO, the results show that it is possible to grow smooth, high quality single domain Ni films, with the same orientation as the underlying substrate, on both MgO(111) and MgO(001). The substrate temperatures for the single domain formation was 300 °C and 100 °C, for the two orientations respectively. For Ni onMgO(111), pole-figure analysis shows that the films are built up of two domains for temperatures ≤200°C and at 600°C, while at 300°C and 400°C only one domain is formed. The in-situ RHEED results, show that whether a single- or a two domain [111] oriented film forms is decided already during the deposition of the first two or three monolayers of Ni, and that the Ni nucleate as strained islands with the same stacking sequence as the underlying substrate during the initial growth. Ni on MgO(001) shows a more complex evolution of the texture with temperature. At ambient temperature the texture is dominated by <022> oriented grains that co-exists with <14̅1> and also some traces of <002> texture. At temperatures between 100 °C and 200 °C the layers are very smooth with a single domain <001> texture. All higher temperatures produces a <751̅ > texture that is four-fold degenerated and twinned with a very faceted surface. The increase in deposition temperature allows the initially nucleated island to rearrange to the <002> texture. At even higher temperatures the increased mobility of the Ni atoms allows for further rearrangement to a <751̅ > texture that better accomodates both the strong Ni-Ni bonds and places each interfacial Ni atom on the preferred position on top of an oxygen atom, with a minimal mismatch between the lattices.