Study on low temperature facetting growth of polycrystalline silicon thin films by ECR downstream plasma CVD with different hydrogen dilution

Polycrystalline silicon films with grain size of 1 mu m have been successfu lly deposited on glass substrates using electron cyclotron resonance chemic al vapor deposition (ECR-CVD) with hydrogen dilution method at 250 degrees C and without any thermal annealing. The deposited poly-Si films exhibited severe 'hill and valley' surface roughness and facets structures. The X-ray diffraction spectra showed that the dominant crystal textures are [220] an d [111] orientations. The leaf-like two-fold symmetrical. grain shape and t he corresponding crystallography diffraction pattern indicated the orientat ion of largest grain is [110]. The dark field TEM image also showed the ups ide octahedral facets shape. Considering the effect of orientation on depos ition rate and symmetry, the possible facets orientation should be [311]. M oreover, the grain sizes of poly-Si thin films deposited on bare Si wafers and on oxidized Si dr glass substrates were almost the same. In situ optica l emission spectroscopy (OES) and mass spectrometry were applied to study t his peculiar growth processes. Emission lines from the Balmer series of ato mic hydrogen (H-alpha: 656.3 nm, H-beta: 486.4 nm), SiH (412.8 nm), Si (251 .6 nm, 288.2 nm), and Ar (750.4 nm) were monitored. The correlation of gas phase species with the structure properties of poly-Si thin films indicated that the SiH3 radicals are the dominant precursors at high hydrogen diluti on ratio and SiH2 radicals dominate the lower hydrogen dilution growth proc esses. Based on the facts that SiH2 precursors can be inserted into any sil icon-hydrogen bonds, whereas SiH3 can react only with sites associated with dangling bonds. Therefore, the surface mobility or the effective diffusion length of SiH2 precursors will he shorter and the anisotropy of surface st icking coefficient will not be so serious. The grain shape will be more sym metrical and grain orientation will be dependent on the atomic arrangement of the substrates. Due to the three-dimensional upward growth characteristi cs, the most likely orientation will be [111] for poly-Si thin film growth on amorphous substrates. The growth surface of thin films deposited with hi gh hydrogen dilution is covered by high-density atomic hydrogen and the dan gling bonds are terminated with the atomic hydrogen. Thus, the SiH3 precurs ors will search the sites without hydrogen termination. Their sticking coef ficient will be very small and the effective diffusion length will be long. Moreover, due to the geometrical characteristics of diamond structure, the [110] surface (low index surface with least dangling bond density) will be the fastest growth direction. The size of grains is enhanced by surface di ffusion of SiH3 and exhibits facetting growth and textural phenomena. (C) 1 999 Elsevier Science B.V. All rights reserved.