  In this work, we studied amorphous carbon ($a$-C) thin films deposited using
direct current (dc) and high power impulse magnetron sputtering (HiPIMS)
techniques. The microstructure and electronic properties reveal subtle
differences in $a$-C thin films deposited by two techniques. While, films
deposited with dcMS have a smooth texture typically found in $a$-C thin films,
those deposited with HiPIMS consist of dense hillocks surrounded by a porous
microstructure. The density of $a$-C thin films is a decisive parameter to
judge their quality. Often, x-ray reflectivity (XRR) has been used to measure
the density of carbon thin films. From the present work, we find that
determination of density of carbon thin films, specially those with a thickness
of few tens of nm, may not be accurate with XRR due to a poor scattering
contrast between the film and substrate. By utilizing neutron reflectivity (NR)
in the time of flight mode, a technique not commonly used for carbon thin
films, we could accurately measure differences in the densities of $a$-C thin
films deposited using dcMS and HiPIMS.
