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

Abstract

Polycrystalline silicon films with grain size of 1 μm have been successfully deposited on glass substrates using electron cyclotron resonance chemical vapor deposition (ECR-CVD) with hydrogen dilution method at 250°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〉 and 〈111〉 orientations. The leaf-like two-fold symmetrical grain shape and the corresponding crystallography diffraction pattern indicated the orientation of largest grain is 〈110〉. The dark field TEM image also showed the upside octahedral facets shape. Considering the effect of orientation on deposition rate and symmetry, the possible facets orientation should be 〈311〉. Moreover, the grain sizes of poly-Si thin films deposited on bare Si wafers and on oxidized Si or glass substrates were almost the same. In situ optical emission spectroscopy (OES) and mass spectrometry were applied to study this peculiar growth processes. Emission lines from the Balmer series of atomic hydrogen (H α: 656.3 nm, H β: 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 SiH 3 radicals are the dominant precursors at high hydrogen dilution ratio and SiH 2 radicals dominate the lower hydrogen dilution growth processes. Based on the facts that SiH 2 precursors can be inserted into any silicon–hydrogen bonds, whereas SiH 3 can react only with sites associated with dangling bonds. Therefore, the surface mobility or the effective diffusion length of SiH 2 precursors will be shorter and the anisotropy of surface sticking coefficient will not be so serious. The grain shape will be more symmetrical and grain orientation will be dependent on the atomic arrangement of the substrates. Due to the three-dimensional upward growth characteristics, the most likely orientation will be 〈111〉 for poly-Si thin film growth on amorphous substrates. The growth surface of thin films deposited with high hydrogen dilution is covered by high-density atomic hydrogen and the dangling bonds are terminated with the atomic hydrogen. Thus, the SiH 3 precursors will search the sites without hydrogen termination. Their sticking coefficient 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 diffusion of SiH 3 and exhibits facetting growth and textural phenomena.