A study on both etching characteristics and mechanism of $Cl_2/O_2$ gas were investigated. We obtained that the change of $O_2/(Cl_2+O_2)$ mixing ratio from 0 to 0.2 leads to the increase of electron average energy, electron drift rate and Electron Energy Distribution Function deformation due to a "transparency" effect and rate coefficients of electron impact processes also depend on a mixture content. The comparison of rate coefficients and rates of elementary processes have shown that the main mechanisms of Cl and O atoms formation are the direct electron impact dissociation of corresponding molecules while the contribution of all possible "secondary" processes is not significant in the case of a relatively low $O_2$ addition. The main mechanisms of Cl and O atom formation are the direct electron impact dissociation of corresponding molecules while the contribution of all possible "secondary" processes is not significant in the case of a relatively low $O_2$ addition.