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

The formation of cationic silicon clusters SinHm+ by means of ion-molecule reactions in a remote Ar-H-2-SiH4 plasma is studied by a combination of ion mass spectrometry and Langmuir probe measurements. The plasma, used for hi gh growth rate deposition of hydrogenated amorphous silicon (a-Si:H), is ba sed on SiH4 dissociation in a downstream region by a thermal plasma source created Ar-H-2 plasma. The electron temperature, ion fluence, and most abun dant ion emanating from this plasma source are studied as a function of H-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 formatio n of silicon containing ions is therefore determined by charge transfer rea ctions between ions emanating from the plasma source and SiH4. While the io n fluence from the source decreases by about a factor of 40 when a consider able flow of H-2 is admixed in the source, the flux of cationic silicon clu sters towards the substrate depends only slightly on this H-2 flow. This im plies a strong dissociative recombination of silicon containing ions with e lectrons in the downstream region for low H-2 flows and it causes the distr ibution of the cationic silicon clusters with respect to the silicon atoms present in the clusters to be rather independent of H-2 admixture. The aver age cluster size increases, however, strongly with the SiH4 flow for consta nt plasma source properties. Moreover, it leads to a decrease of the ion be am radius and due to this, to an increase of the ion flux towards the subst rate, which is positioned in the center of the beam. Assuming unity stickin g 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 bombard ment is limited due to the low electron temperature, the clusters have a ve ry compact structure and very low hydrogen content and can consequently hav e a considerable impact on film quality. The latter is discussed as well as possible implications for other (remote) SiH4 plasmas. (C) 1999 American I nstitute of Physics. [S0021-8979(99)00319-9].