Chlorine etching of heavily doped single-crystalline Si(100) using electron cyclotron resonance plasma has been investigated focusing on surface adsorption and reaction and impurity segregation. The etch rate of undoped and P-doped Si is nearly equal to the square of the concentration of Cl atoms adsorbed on the etched surface. On the other hand, in the case of B-doped Si, B segregation up to the monatomic layer concentration on the etched surface is observed, and the etching is suppressed by the segregated B atoms. Thus, the difference in the etching characteristics of doped Si and undoped Si is caused by the concentration difference of the adsorbed Cl atom and the impurity segregation on the etched surface.