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In this article, new results are presented about the structure of mixed-phase LPCVD silicon films from silane obtained by transmission electron microscopy (TEM) and Raman spectrometry. It is shown that (i) the crystalline fraction is strongly dependent not only on the operating conditions such as deposition temperature, pressure and gas flow rate, but also on the film thickness (which is equivalent to the deposition time) and the duration and temperature of any subsequent annealing. All these results prove that the microstructure of these mixed-phase films, like that of amorphous films, must be considered as a dynamic parameter, likely to evolve with time as soon as the layer is maintained at sufficiently high temperature. This new concept will have to be accurately considered by engineers in the future if they want to improve the control of LPCVD film microstructure. A new simple method is finally presented to measure the local crystalline fraction profile through the film thickness using Raman spectrometry: the so-obtained values have been validated by comparison with TEM micrographs. This new method will certainly facilitate the present tendency of increasing severity in terms of microstructural quality. |
