Microphysical investigation of low temperature annealed LPCVD polysilicon thin films

In this paper we present the morphological and structural properties of the low pressure chemically vapor deposited (LPCVD) silicon films after annealing at 600°C for 26 hours in nitrogen, in comparison with the properties of as-deposited layers. For the films found in the polycrystalline state after deposition (i.e. those prepared at temperatures below and above 550°C), XRD spectra have indicated a supplementary (111) diffraction peak after annealing. In addition, from XRD data, we obtained a dependence of grain size of annealed Si layers on initial CVD conditions. The spectroellipsometry (SE) and Ultra-Violet (UV) spectroscopy have indicated an important increase of the crystalline/amorphous silicon ratio for all annealed films. The signature of initial CVD conditions in optical properties investigated by SE and UV was found only for as-deposited Si films. From AFM measurements we have found that the surface roughness has increased by annealing, with a higher value for the annealed films, which were in the poly-crystalline state, immediately after deposition. In connection to our earlier results we bring here further support (by SE and UV data) for the crystalline state detected in as-deposited LPCVD films prepared at temperatures as low as 500°C. The value of the amorphous fraction present in the as-deposited film can explain the structural changes of the LPCVD Si films after annealing, as a function of initial CVD conditions.