Perpendicular etching profiles of n+-Si(100) are obtained with a supersonic Cl2 beam at substrate temperature of 900°C. Although small undercuts are observed just below the SiO2 mask, the side wall etching caused by the background Cl2 is almost negligible. An aspect ratio of greater than 6 and selectivity of greater than 8000 are obtained with 0.5 μm line & space mask pattern. From Arrhenius plots of etch rates, an effective activation energy of the nozzle beam etching is determined to 0.53 eV. Assuming that the reaction product is SiCl2, the reaction probability is estimated to be 19% at 900°C. Highly anisotropic etching of the Si(100) obtained here is due to the large reaction probability.