We have studied the etching of low dielectric constant polymers in halogen- and sulfur-free chemistries under various plasma operating conditions. The polymer graphitization phenomenon, which corresponds to the transformation of the aromatic hydrocarbon network into an amorphous carbon backbone, has been fully investigated under several plasma conditions by in situ mass spectrometry and quasi-in situ x-ray photoelectron spectroscopy (XPS). Profile control in high aspect ratio contact holes is obtained thanks to the formation of a passivation layer on the polymer sidewalls preventing the spontaneous chemical attacks by the oxygen reactive species of the plasma. XPS studies show that the passivation layer only forms under conditions where the plasma induces a polymer graphitization. Strong correlations are observed between plasma conditions leading to the polymer graphitization, the presence of heavy carbon byproducts detected in the plasma gas phase, and the passivation layer formation.