ROLE OF IONS IN PECVD OF AMORPHOUS-SILICON

Recently, it has become clear that the interpretation of the role of i ons in the r.f. plasma-deposition process of amorphous silicon has to be revised. Instead of relating deposition rates and other material pr operties solely to the type and amount of radicals in the plasma, the influence of ions on the growing layer has to be considered. This is c onfirmed by results obtained for deposition at very high frequency, up to 100 MHz: the deposition rate is increased, whereas the radical con centration hardly changes, as measured by mass spectrometry. At the sa me time, the energy of the ions impinging on the growing surface is de creased while the ion flux is increased. This implies that the balance between the competing processes at the surface strongly shifts toward s deposition. The influence of ions also appears to be very important in so-called ''chemical annealin'' experiments. They reveal that on ex posure to a pure hydrogen plasma, the amorphous material can be crysta llized, a process which will also occur during continuous deposition w ith a silane-hydrogen plasma.