The characteristic behaviors of CH3Br are examined first for the dry etching of polysilicon in a Cl-2/CH3Br/O-2 plasma. CH3Br is revealed as one of the excellent additive gases to control anisotropy of the etching profile and to give no undercutting for various types of polysilicons. The fragments from CH3Br plasma would act as passivation precursors on the sidewalls in etch cavity. The decrease of etch selectivity due to the reaction of the C-containing species front CH3Br with the surface O atoms of SiO2 is overcome by the addition of O-2 into the plasma.