We present first-principles calculations for the initial stages of etching of Si(100) by H atoms. We find that adsorbed SiH3 species [(SiH3(a)] on the H-saturated Si(100)1×1 surface have a negative formation energy. Our results also indicate that the Si-Si backbond breaking which gives rise to the formation of these SiH3(a) defects is possibly promoted by a bonding state between hydrogen and silicon atoms in the second layer.