A mechanism for highly selective etching of phosphorus‐doped polycrystalline silicon (n+poly‐Si) on SiO2by employing a Cl2magnetron plasma reactor at low wafer temperatures was investigated. Only the SiO2etch rate drops rapidly in the magnetron plasma at low wafer temperatures below 0 °C. X‐ray photoelectron spectroscopy analysis revealed that only the SiO2surface etched at lower temperatures was covered with silicon chloride or/and oxychloride compounds. The highly dense magnetron plasma decomposes the etched products into unsaturated molecules such as SiCl, SiCl2, and their oxides. These species have a large dipole moment and a higher sticking probability on SiO2than on Si because the SiO2bond is also ionic, and thus attracts a dipole molecule by Coulomb force. Thus, only the SiO2surface was protected by a thin film from chlorine ion bombardment at a certain temperature range in the magnetron plasma. This protection film suppressed SiO2etching, and a high selectivity ofn+poly‐Si/SiO2has been achieved.