This study investigates the fluorocarbon-based plasma etching for ultra-low-k carbon doped oxide (CDO) dielectric layers which have a k value of 2.4; the CDO film is made up of SiOCH. The thickness of fluorocarbon layers deposited on the surface of the underlying CDO was controlled by independently varying the flow of C4F8 and O-2 feed gases into the chamber. X-ray photoelectron spectroscopy and spectroscopic ellipsometry have been used to analyse the post-etched low-k CDO films. These studies have identified the conditions that lead to the formation of the thinnest fluorocarbon layers compatible with efficient etching, but that do not result in carbon depletion from the CDO layer. The results indicate that once the plasma gas mixture results in the presence of an extremely thin fluorocarbon layer, the carbon depletion is significantly suppressed. The thickness of the fluorocarbon film can be controlled by the C4F8/O-2 gas ratio and the presence of this film has a large impact on the etch rate.