Atomic bonds of carbon thin films prepared by negative-ion beam deposition were investigated with respect to the dependence on incident ion energy. Mass-separated carbon negative ions of C-and C-2(-) were deposited on Si(100) and Si(111) at a low ion energy of 50-100 eV with a current density of about 20 mu A/cm(2) under 1 x 10(-6) Pa for 5 h after removing surface oxide layer on the substrate by heating up to 900 degrees C for 15 min. Deposited films were found from their broad Raman band in 1400-1600 cm(-1) to be diamond-like carbon films. From decomposed XPS C-1 spectra into three bonds of C-C (sp(3) bonds), C=C (sp(2)) and C=O, it was found that the fraction of sp(3) bonds increased with an increase in ion energy, and that it decreased gradually with the further increase in ion energy for both ion species. It is considered that the kinetic energy of ions contributes to form the metastable bond like sp3 bond but that the excessive kinetic energy results damage in the films. As a result, the sp(3) bond fraction of more than 80 % was obtained at ion energy of 75 eV per a carbon atom for both C- and C-2(-) depositions.