Formation of cationic silicon clusters in a remote silane plasma and their contribution to hydrogenated amorphous silicon film growth

The formation of cationic silicon clusters ${\mathrm{Si}}_n{\mathrm{H}}_m^{+}$ by means of ion–molecule reactions in a remote ${\mathrm{Ar–H}}_{\mathrm{2}}{\mathrm{–SiH}}_{\mathrm{4}}$ plasma is studied by a combination of ion mass spectrometry and Langmuir probe measurements. The plasma, used for high growth rate deposition of hydrogenated amorphous silicon (a-Si:H), is based on ${\mathrm{SiH}}_{\mathrm{4}}$ dissociation in a downstream region by a thermal plasma source created ${\mathrm{Ar–H}}_{\mathrm{2}}$ plasma. The electron temperature, ion fluence, and most abundant ion emanating from this plasma source are studied as a function of ${\mathrm{H}}_{\mathrm{2}}$ admixture in the source. The electron temperature obtained is in the range of 0.1–0.3 eV and is too low for electron induced ionization. The formation of silicon containing ions is therefore determined by charge transfer reactions between ions emanating from the plasma source and ${\mathrm{SiH}}_{\mathrm{4}}.$ While the ion fluence from the source decreases by about a factor of 40 when a considerable flow of ${\mathrm{H}}_{\mathrm{2}}$ is admixed in the source, the flux of cationic silicon clusters towards the substrate depends only slightly on this ${\mathrm{H}}_{\mathrm{2}}$ flow. This implies a strong dissociative recombination of silicon containing ions with electrons in the downstream region for low ${\mathrm{H}}_{\mathrm{2}}$ flows and it causes the distribution of the cationic silicon clusters with respect to the silicon atoms present in the clusters to be rather independent of ${\mathrm{H}}_{\mathrm{2}}$ admixture. The average cluster size increases, however, strongly with the ${\mathrm{SiH}}_{\mathrm{4}}$ flow for constant plasma source properties. Moreover, it leads to a decrease of the ion beam radius and due to this, to an increase of the ion flux towards the substrate, which is positioned in the center of the beam. Assuming unity sticking probability the contribution of the cationic clusters to the total growth flux of the material is about 6% for the condition in which solar grade a-Si:H is deposited. Although the energy flux towards the film by ion bombardment is limited due to the low electron temperature, the clusters have a very compact structure and very low hydrogen content and can consequently have a considerable impact on film quality. The latter is discussed as well as possible implications for other (remote) ${\mathrm{SiH}}_{\mathrm{4}}$ plasmas.