Influence of reactor walls on plasma chemistry and on silicon etch product densities during silicon etching processes in halogen-based plasmas
The absolute concentrations of SiClX (X = 0-2), SiFX (X = 1-2), SiClF, SiBr and SiO radicals were measured in an industrial silicon gate etching reactor with various halogen gas mixtures (HBr, Cl2, O2, CF4 mixtures). The influence of O2 gas flow in the HBr/Cl2 mixture, of the HB/Cl2 ratio, and of CF4 addition in HBr/Cl2 have been analysed systematically. These experimental results are the first quantitative information on the densities of silicon etch products in a high density Cl2-based plasma, and are useful to validate and improve numerical models. In addition, these results are correlated with the 200 mm diameter silicon wafer etch rate, and with the deposition rate of the layers deposited on the plasma chamber walls and with their chemical compositions. This allows us to discuss the production and loss mechanisms of silicon etching by-products during silicon etching processes. In Cl2-based plasmas, we show that the chamber walls are efficient for recycling silicon: SiCl2-4 radicals, initially produced from the etching of the wafer are ionized and dissociated by the plasma into reactive products Si, SiCl, Si+, SiCl+. These species can then undergo chemisorption on the reactor walls, where they can be either oxidized by O atoms from the plasma (leading to the formation and growth of SiOClX layers on the chamber walls) or be etched back into the gas phase by Cl atoms. Therefore, the deposition rate of SiOClX layers on the chamber wall results from a competition between these two reactions and is then limited by the amount of O atoms available. The other consequence of this competition is that reactor walls produce large amounts of SiCl2-4 radicals when the O2 flow is low. When CF4 is added to the plasma, the concentration of SiClX etch products decreases in favour of SiFX etch products. In addition, the presence of F atoms and CFX radicals and fluorine-based ions prevents the deposition of a SiOClX layer on the reactor walls.