Structure of mixed-phase LPCVD silicon films as a function of operating conditions

In this article, new results are presented about the structure of mixed-pha se LPCVD silicon films from silane obtained by transmission electron micros copy (TEM) and Raman spectrometry. It is shown that (i) the crystalline fra ction is strongly dependent not only on the ope:rating conditions such as d eposition temperature, pressure and gas flow rate, but also on the film thi ckness (which is equivalent to the deposition time) and the duration and te mperature of any subsequent annealing. All these results prove that the mic rostructure 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 w ant to improve the control of LPCVD film microstructure. A new simple metho d is finally presented to measure the local crystalline fraction profile th rough the film thickness using Raman spectrometry; the so-obtained values h ave been validated by comparison with TEM micrographs. This new method will certainly facilitate the present tendency of increasing severity in terms of microstructural quality.