The absolute concentrations of SiCl_X (X = 0–2), SiF_X (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: SiCl_2–4 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 SiOCl_X layers on the chamber walls) or be etched back into the gas phase by Cl atoms. Therefore, the deposition rate of SiOCl_X 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 SiCl_2–4 radicals when the O₂ flow is low. When CF₄ is added to the plasma, the concentration of SiCl_X etch products decreases in favour of SiF_X etch products. In addition, the presence of F atoms and CF_X radicals and fluorine-based ions prevents the deposition of a SiOCl_X layer on the reactor walls.