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 ͑PR͒ ashing processes are attractive because of the ease of process integration with plasma etching processes. The authors have examined the performance of carbon dioxide ͑CO 2 ͒ as a source gas for in situ PR ashing processes compatible with ultralow k ͑ULK͒ materials and compared it with the results obtained using O 2 . 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 ϳ−400 V. To increase line-of-sight etching of PR by inert ion bombardment, Ar/ CO 2 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-SIMS͒. 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 ͑AE͒ 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 CO 2 was about three times greater than AE with O 2 for the same process conditions. When a 75% Ar/ CO 2 gas mixture was used and a Ϫ100 V substrate bias was applied during PR ashing, a PR ashing rate of 200 nm/min could be achieved for a 10 mTorr Ar/ CO 2 plasma. For this process, the measured AE was 230, more than 10ϫ greater than AE achieved with O 2 discharges using the same conditions. The authors found that ULK damage showed a direct dependence on the atomic oxygen densities of both CO 2 and O 2 discharges which was characterized by optical emission of discharges. The question whether in-diffusion of carbon species from CO 2 discharges into ULK material was significant was also examined. For this the authors substituted 13 CO 2 for 12 CO 2 and performed ToF-SIMS analysis of the exposed ULK films. No significant amount of 13 C from 13 CO 2 plasmas was detected either on the surface or in the bulk of the 13 CO 2 plasma-exposed ULK.

Modification of Ultra Low-k Dielectric Films by O2and CO2Plasmas

“…6 Ming-Shu Kuo et al and Hualiang Shi extensively studied the surface modification of ultralow-k materials by CO 2 plasma and the results are published in their PhD dissertations 7,8 and in several papers. [9][10][11][12][13] They reported that i) CO 2 and O 2 plasma damage to ultralow-k films are comparable ii) there is lower atomic oxygen density in CO 2 discharge than O 2 discharge because of higher activation energy (11.5 eV) required to liberate atomic oxygen from CO 2 than from O 2 molecules (6 eV) -this is why there is supposedly a reduction in damage by CO 2 discharge with respect to O 2 for same operating conditions iii) the ashing rate of CO 2 increases with the addition of Ar at higher pressure (about 100 mTorr) than at lower pressure hence the addition of Ar is beneficial and dependent on pressure. Adding Ar brings about the dilution of the concentration of O 2 atoms that are liberated from CO 2 hence a lower damage to low-k.…”

Process Technology for Copper Interconnects

“…Acceptable resist etch rates (100 nm/min) can be achieved by using a high temperature (260°C) downstream H 2 plasma. The CO-or CO 2 -based resist strip approaches are generally run in RIE tools [55,65,66]. The downstream H 2 plasma only reacts with H in the low-k film, in a replacement reaction, without altering the stoichiometry of the film [64].…”

“…It has been reported that residues are left after a downstream H 2 plasma strip, which must be removed with a wet clean [63]. Argon is often added to the strip chemistry and a bias is applied to the wafer to enable a high removal rate of resist [66]. Hence, there is no change in the film thickness or dielectric constant.…”

“…Hence, there is no change in the film thickness or dielectric constant. The low damage associated with CO 2 -based resist strips is at least partly due to the lower amount of atomic oxygen present in the plasma compared to O 2 resist strips [66]. Argon is often added to the strip chemistry and a bias is applied to the wafer to enable a high removal rate of resist [66].…”

Large-radius neutral beam enhanced chemical vapor deposition process for non-porous ultra-low-k SiOCH

“…Major problems are delamination caused by chemical mechanical planarization and drastic change in the dielectric due to pore-induced damage occurring in the wet cleaning and ashing processes. Since porous SiOCH occupies a lot of space in a film, it is fragile and readily absorbs any solutions or gases [1][2][3][4][5]. Moreover, Cu atoms easily diffuse into the SiOCH through pores.…”