Amorphous carbon (a-C:H) is widely used as a hardmask in semiconductor device manufacturing to enable patterning of sub-65 nm features. Key criteria for a good hardmask include the ability to transfer patterns into underlayers with tightest dimension control. In order to achieve this goal, the hardmask film must possess superior etch selectivity during reactive ion etching of the underlayers. In this article, the authors show that etch selectivity of a-C:H hardmask (to underlayers) is directly related to its film density. Fourier transform infrared and composition data are analyzed to determine the aggregate bond energy per unit volume of hardmask. It is shown that for a-C:H films, this energy increases with density as a result of the bonding configurations, thereby explaining the correlation between etch selectivity and density.