| For thin polycrystalline films, especially those with columnar structure, once the diameter of grains on the film surface is larger than the film thickness, the total area of above (free surface) and under basal planes (film-substrate interface) will be larger than that of side plane. And if the film precipitates on the substrate with thermal expansion coefficient different from that of the film, great thermal stress will be introduced during annealing. Therefore besides the minimization of boundary energy in traditional bulk material, the anisotropy of surface energy, interface energy and strain energy can drive the grain growth in thin films. The magnitude of these three kinds of energy, however, depends strongly on the orientation of grains, namely, they are anisotropic. It is noticeable that their respective dependence relationship is different markedly, that is to say, the orientation of grains with lowest surface energy needn't correspond to the lowest interface energy and strain energy. For the minimization of energy, the grains with lowest total energy involving surface energy, strain energy and interface energy will be preferable to grow during annealing so that the films will show corresponding texture ultimately.In this paper we have calculated the surface energy of various crystalline planes as film surface for ten face-centered cubic (FCC) metals, ten body-centered cubic (BCC) metals and three diamond-structure (Diamond) crystals with Modified Embedded Atom Method (MEAM). According to the theoretical analysis, the driving forces for abnormal grain growth from anisotropy of surface energy and consequently texture evolution of films have been discussed. At last, the driving effect in thin Cu and Ag films is proved experimentally, which is in good agreement with theoretical analysis. The principal theoretical analysis and experimental results can be concluded as:(1) Using compiled programs for surface geometrical factors in "C", the related parameters of 38 FCC, 24 BCC, and 24 Diamond-structure crystalline planes as surfaces have been determined including geometrical factors , the number of neighbors to an atom in any planes Zd, and the area occupied by one atomin surface A(x). The results can also be applied to the theoretical investigation of other surface properties (such as electron states, segregation, oxidation, catalysis, adsorption and reconstructure in surface).(2) With Modified Embedded Atom Method (MEAM) we have calculated the surface energy of various crystalline planes as surfaces for ten FCC metals Cu, Ag, Au, Ni, Pd, Pt, Al, Pb, Rh, Ir and ten BCC metals Li, Na, K, V, Nb, Ta, Cr, Mo, W, Fe, as well as three Diamond-structure crystals C, Si, Ge. The results suggest that the surface energy of closely-packed planes (such as (111) plane of FCC and Diamond-structure crystals, (110) plane of BCC crystals) is the lowest. Especially for FCC metals, the surface energy of any other planes (hkl) as surfaces increases with the including angle between them and (111) plane, according to which, the relative value of surface energy of any (hkl) planes can be estimated. From the calculated results, we can predict, if surface energy is taken into account only, (111) of FCC and Diamond-structure, as well as (110) of BCC are the preferable orientations respectively. During annealing the anisotropy of surface energy can provide additional driving force for abnormal grain growth in thin polycrystalline films, consequently corresponding textures will emerge or be strengthened.(3) By X-ray diffraction (XRD) and transmission electron microscopy (TEM) abnormal grain growth and texture evolution in Ag and Cu films are studied, the comparative investigation between deposited films and free-standing films as well as attaching films after annealing shows that deposited Cu (Ag) films without annealing are polycrystalline and of no preferable orientations, and the size of grains is less than 100 nm. After annealed at 300℃ for 3 hours, (100) and (HO)-oriented abnormal grains about 1.4 and 2.0//W i...
|
PDF Full Text Request