The influences of Ge and C impurities on the reactions between atomic hydrogen and Si(001) surfaces have been analyzed using the quantum chemical calculations based on the cluster model. In the case of the Ge impurity, Ge-Si back-bonds were shown to be easily broken by attacks of atomic hydrogen: the activation energies were estimated to be 0.2-0.4eV. On the other hand, in the case of the C impurity. it as shown that the C Si dimer bond and C-Si back-bonds were hardly broken by attacks of atomic hydrogen, because the activation energies for these reactions were estimated to be similar to1.5 eV. Therefore, it as considered that C impurities disturb the etching, whereas Ge impurities do not. It us also assumed that SiC is hardly etched by atomic hydrogen at room temperature. (C) 2002 Elsevier Science B.V. All rights reserved.