The interaction of fluorine atoms and ions with hydrogen-terminated Si(lll) thin film has been studied using periodic ab initio electronic structure calculations. Penetration of fluorine atoms into the Si surface is found to be quite easy, and an interstitial fluorine atom can be situated directly behind the Si-H bond while the original lattice structure, as well as the Si-H bond itself, remains intact. The fluorine atom in this position is negatively charged by accumulating electron charges from the nearest silicon atoms. The penetration reaction of the fluorine ions into the Si surface is also found to be easy.