In this work we probe the structural properties of amorphous hydrogenated carbon thin films prepared by plasma-enhanced chemical vapor deposition in a low pressure inductively coupled plasma using X-ray reflectometry in order to study the effect of varying the ion energy on the density of these films. The ion energy is varied by varying the RF power used to bias the substrate. It is shown that a very low ion energy is already sufficient to obtain a dense diamond-like carbon (DLC) film, in contrast with other deposition techniques where much higher ion energies are required to obtain a dense DLC film. The results of this study are corroborated by Raman spectroscopy and ellipsometry measurements. The X-ray reflectometry data analysis is detailed in order to highlight some methodological problems encountered during the data fitting which could lead to an incorrect interpretation of the measured curves.