This study investigates the fluorocarbon based plasma etching (FBPE) of ultra low dielectric constant (ULK) carbon-doped oxide (CDO) films, which have a k value of 2.4. The influence of the presence of a thin fluorocarbon layer on the carbon concentration in the underlying CDO film during etching was investigated. Different plasma treatment conditions were explored including different RF powers and a range Of C4F8/Ar/O-2 gas ratios. X-ray photoelectron spectroscopy (XPS) was used to analyse the chemical composition of the post-etched low-k CDO films, while spectroscopic ellipsometry (SE) was used to determine the overall film thickness. Plasma conditions which resulted in the absence of a fluorocarbon layer showed a 50% reduction in the carbon concentration in the upper part of the low-k layer. However, the depletion of carbon from the CDO was minimized when plasma conditions resulted in the presence of a fluorocarbon layer on the CDO surface during etching of the low-k. The etch rate of the low-k layer was found to depend on the thickness of the fluorocarbon film which could be controlled by varying the C4F8:O-2 gas ratio. (c) 2006 Elsevier B.V. All rights reserved.