Mechanistic study of ultralow k-compatible carbon dioxide in situ photoresist ashing processes. I. Process performance and influence on ULK material modification

Abstract

In situ photoresist (PR) ashing processes are attractive because of the ease of process integration with plasma etching processes. The authors have examined the performance of carbon dioxide (CO2) as a source gas for in situ PR ashing processes compatible with ultralow k (ULK) materials and compared it with the results obtained using O2. They performed measurements of 193 nm PR ashing rates in a dual frequency capacitively coupled plasma reactor. The damage to porous ULK feature sidewalls was simulated by exposing blanket ULK films in a non-line-of-sight fashion in a small gap structure to the plasma-generated reactants. The pressure for the in situ ashing processes was varied from 10 to 100 mTorr, and the self-bias voltages ranged from floating potential to ∼−400 V. To increase line-of-sight etching of PR by inert ion bombardment, Ar/CO2 mixtures with up to 75% Ar were investigated. The ULK material modifications were analyzed by x-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). Plasma-damage of the ULK material primarily is detected as the removal of carbon from the SiCOH ULK films. To compare the performance of different ashing processes for PR stripping from ULK material, the authors introduced an ashing efficiency (AE) parameter which is defined as the thickness of PR removed over the thickness of ULK simultaneously damaged, and can be considered a process figure of merit. AE with CO2 was about three times greater than AE with O2 for the same process conditions. When a 75% Ar/CO2 gas mixture was used and a −100 V substrate bias was applied during PR ashing, a PR ashing rate of 200 nm/min could be achieved for a 10 mTorr Ar/CO2 plasma. For this process, the measured AE was 230, more than 10× greater than AE achieved with O2 discharges using the same conditions. The authors found that ULK damage showed a direct dependence on the atomic oxygen densities of both CO2 and O2 discharges which was characterized by optical emission of discharges. The question whether in-diffusion of carbon species from CO2 discharges into ULK material was significant was also examined. For this the authors substituted C13O2 for C12O2 and performed ToF-SIMS analysis of the exposed ULK films. No significant amount of C13 from C13O2 plasmas was detected either on the surface or in the bulk of the C13O2 plasma-exposed ULK.