Highly selective etching of polycrystalline silicon on silicon dioxide at low wafer temperature, employing magnetron plasma

A mechanism for highly selective etching of phosphorus-doped polycrystalline silicon (n+ poly-Si) on SiO2 by employing a Cl2 magnetron plasma reactor at low wafer temperatures was investigated. Only the SiO2 etch rate drops rapidly in the magnetron plasma at low wafer temperatures below 0 °C. X-ray photoelectron spectroscopy analysis revealed that only the SiO2 surface 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 SiO2 than on Si because the SiO2 bond is also ionic, and thus attracts a dipole molecule by Coulomb force. Thus, only the SiO2 surface was protected by a thin film from chlorine ion bombardment at a certain temperature range in the magnetron plasma. This protection film suppressed SiO2 etching, and a high selectivity of n+ poly-Si/SiO2 has been achieved.