Process interactions between fluorocarbon (FC) plasma ultralow k (ULK) dielectrics etching followed by carbon dioxide (CO(2)) in situ photoresist (PR) ashing on ULK damage have been studied in a dual frequency, capacitively coupled plasma reactor. Introduction of ULK trench/via sidewall damage was simulated employing blanket ULK films by exposing them in a non-line-of-sight fashion in a small gap structure to the plasma environment. ULK damage was quantified using the dilute hydrofluoric acid (0.5%/15 s) selective etching method. CO(2) in situ ashing processes showed a chamber memory effect due to prior FC plasma etching, significantly increasing damage of pristine ULK films. For ULK plasma etching/PR ashing process sequences, ULK material surfaces were modified by FC plasma etching prior to the CO(2) plasma exposure. X-ray photoelectron spectroscopy studies showed that the modifications consisted primarily of 1-2 nm FC coverage of the ULK. This FC deposit remained on the ULK surfaces during in situ CO(2) processing and provided protection of the underlying ULK material. PR-patterned ULK structures were also processed employing the same processing conditions. The results obtained from the characterization of the resulting trench structures support the findings obtained with blanket films. CO(2) in situ PR ashing processes performed at low pressure (10 mTorr) and enhanced by rf biasing provided a good combination of high PR stripping rate and low ULK damage introduction. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3482353]