In polycrystalline silicon (polysilicon) etching for gate electrodes in high-performance complementary metal-oxide-semiconductor large scale integration fabrication, extremely high selectivity of polysilicon etching to the gate oxide is needed because the thickness of the gate oxide reduces. It is also important to overcome the well-known shape distortion problem, called "notching" or "local side etching," where the gate side-wall is locally side-etched at the interface between the polysilicon and the gate oxide. We examine the origin of notching and the selectivity to the gate oxide in chlorine-and oxygen-based reactive ion etching of the phosphorous-doped polysilicon gate with an SiO2 hard mask. It is found that the etching product returning to the surface plays an important role in both side-wall and gate-oxide protection, which determines the shape of the electrode and the selectivity. Based on our findings, we developed a novel step-etching method that provides high-performance gate electrode etching that is infinitely selective without notching. This method uses the addition of a SiCl4 gas, which is equivalent to the etching product, as well as oxygen during overetching when the generation of etching product is reduced.