Investigation of the silicon-plasma silicon nitride interface with in situ transient photoconductivity measurements

During the deposition of silicon nitride films from a silane/ammonia plasma at 350 degrees C on crystalline silicon there is a drastic change of the electronic properties of the SiNx/Si interface. In order to achieve an on-line quality control of the deposited films the change of the photoconductivity during the deposition is studied with time resolved microwave conductivity measurements. It is shown that at the start of the deposition then is a strong decrease of the lifetime of the measured transient signal due to plasma induced damage at the silicon surface. With continuous deposition an increase of the lifetime to a much higher value is observed due to passivation of the interface. The interface recombination velocity can be reduced to less than 100 cm/s which is more than two orders of magnitude smaller than before the deposition. Ex situ measurements on films of different thickness show analogous results: For thin (<20 nm) silicon nitride films, the film thickness has a strong impact on the surface recombination velocity and the refractive index. Furthermore Rutherford backscattering measurements show a drastic change of the film stoichiometry within this range of thickness.