The removal of an oxygen-containing plasma-modified layer on top of porous low dielectric constant (low-k) materials by treatment with aqueous HF has been investigated. Our study demonstrates that estimation of the thickness of a damaged layer of porous low-k film requires careful consideration of the HF concentration and immersion time. High HF concentrations result in an enhanced etch rate that is difficult to control for the film thicknesses of interest here. Ellipsometry measurements before and after immersion in HF provide a good means of internal comparison, for instance between different materials after a plasma process step or between different processes for the same material. For a given low-k material, we report the appropriate HF concentration for estimation of the thickness of the damaged layer induced by the plasma treatment. Using ellipsometric porosimetry, it was found that after a 15 s plasma treatment, the thickness of the sealing layer is estimated to be between 2 and 4 nm, which is much less than the thickness of the modified layer induced by the plasma of about 12-14 nm. (c) 2005 The Electrochemical Society. All rights reserved.