Embodiments of the present invention provide methods for forming fin structure with desired materials using a conversion process for three dimensional (3D) stacking of fin field effect transistor (FinFET) for semiconductor chips. In one embodiment, a method of forming a fin structure on a substrate includes performing an directional plasma process on a fin structure formed from a substrate comprising a first type of atoms, the directional plasma process dopes a second type of atoms on sidewalls of the fin structure, performing a surface modification process to form a surface modified layer on the sidewalls of the fin structure reacting with the first type of atoms, replacing the first type of the atoms with the second type of the atoms in the fin structure during the surface modification process, and forming the fin structure including the second type of the atoms on the substrate.