Plasma-based ashing of photoresist masks after pattern transfer is a common processing step in the fabrication of integrated circuits. In this work we investigated damage mechanisms of nanoporous ultra low k (ULK) materials with different overall porosities due to the ashing process. Oxygen-, nitrogen- and hydrogen-based photoresiststripping using direct and remote plasma processes were examined. Ellipsometry, x-ray photoelectron spectroscopy, secondary ion mass spectroscopy, and transmission electron microscopy were utilized to study the damage layer thickness, physical (pore morphology), and chemical modifications of the nanoporous silica thin films after exposure to the O2-, N2- or H2-based ashing processes. As a result of the plasma exposure, carbon groups in nanoporous silica can be removed from the ULK layers which is also accompanied by material densification. We find severe ashing damage of ULK materials after O2-based ashing using both direct and remote discharges. N2 and H2 discharges also damage ultralow k materials for direct plasma ashing processes which are accompanied by low energy ion bombardment of the substrates. The introduction rate and degree of the ULK materials modifications correlates with the overall porosity. We show that the pore interconnectivity is one of the key parameters that determine ashing damage. ULK damage is greatly reduced for remote N2 or H2 discharges, but the resist removal rates are impractically low if the substrate is at room temperature. We show that both acceptable photoresist stripping rates and ULK damage levels can be achieved for remote H2 plasma ashing processes if the substrate temperature is 250°C and higher.
Surface roughness development of photoresist (PR) films during low pressure plasma etching has been studied using real-time laser light scattering from photoresist materials along with ellipsometric and atomic force microscopy (AFM) characterization. We show that evolution of the intensity of light scattered from a film surface can be used to study the development of surface roughness for a wide range of roughness starting from subnanometer to few hundred nanometers. Laser light scattering in combination with ellipsometry and AFM is also used to study the reticulation mechanism of 193 and 248 nm PRs during argon plasma processing. We employ a three-layer model (modified layer, rough layer, and bulk film) of the modified PR surface (193 and 248 nm PRs) to simulate and understand the behavior of ellipsometric Ψ-Δ trajectories. Bruggeman’s effective medium approximation is employed to study the roughness that develops on the surface after reticulation. When the glass transition temperature of the organic materials is reached during Ar plasma processing, the PR films reticulate and roughness develops rapidly. Roughness development is more pronounced for 248 nm PR than for 193 nm PR. Simulation of Ψ-Δ shows that the growth of roughness is accompanied by strong expansion in the materials, which is stronger for 248 nm PR than 193 nm PR. The leading factors responsible for reticulation are found to be compressive stress that develops in the modified surface layer as it is created along with strong molecular chain motion and expansion of the material when the temperature is increased past the glass transition temperature. Reticulation leads to a significantly different surface morphology for 248 nm PR as compared to 193 nm PR and can be related to differences in molecular structure and composition leading to different responses when a modified surface layer is formed by ion bombardment accompanying plasma etching.
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.
Articles you may be interested inMechanistic study of ultralow k -compatible carbon dioxide in situ photoresist ashing processes. I. Process performance and influence on ULK material modification J. Vac. Sci. Technol. B 28, 952 (2010); 10.1116/1.3482343 Numerical simulation of dual frequency etching reactors: Influence of the external process parameters on the plasma characteristics J. Appl. Phys. 98, 023308 (2005); 10.1063/1.1989439 Study of C 4 F 8 / CO and C 4 F 8 / Ar/CO plasmas for highly selective etching of organosilicate glass over Si 3 N 4 and SiCThe authors evaluated photoresist ͑PR͒ stripping processes that are compatible with ultralow dielectric constant ͑ULK͒ materials using H 2 -based remote plasmas generated in an inductively coupled plasma reactor. The materials used were 193 nm PR and nanoporous SiCOH-based ULK ͑JSR LKD 5109͒. PR ashing rates and ULK damage ͑carbon depletion͒ were measured for H 2 , H 2 / N 2 , and H 2 / Ar discharges as a function of substrate temperature over the range of 200-275°C. They employed ellipsometry, x-ray photoelectron spectroscopy ͑XPS͒, optical emission spectroscopy, and time-of-flight secondary ion mass spectroscopy ͑ToF-SIMS͒ for analysis. For their H 2 remote plasmas and a substrate temperature in the range of 200-275°C, the PR ashing rate varied from 270 to 880 nm/min, whereas 3-5 nm of ULK damage was measured for 20 s remote plasma exposure. As a useful process metric, they defined ashing efficiency as the thickness of PR removed over the thickness of ULK simultaneously damaged. PR stripping processes can be optimized to an ashing efficiency of ϳ60 for substrate temperatures above 250°C, if pure H 2 discharges are employed. The addition of N 2 or Ar to H 2 did not improve the ashing rate and, especially for N 2 , such additions dramatically increased ULK damage. This resulted in reduced ashing efficiency for these cases. To clarify the impact of etching/ashing process interactions on ULK modification, they exposed blanket ULK film to C 4 F 8 / Ar-based etching plasmas employing a dual frequency ͑40.68/4 MHz͒ capacitively coupled plasma ͑CCP͒ reactor. Plasma exposures of the ULK were performed utilizing a silicon roof, which shielded the ULK film located underneath from direct ion bombardment. Since the aspect ratio of the small gap structure was selected to be equal to that of an actual trench structure, trench sidewall-like surface modifications induced by etching processes along with their impact on ashing damage that were introduced during a subsequent PR stripping process can be simulated and studied on blanket films with appropriate size. XPS revealed fluorocarbon ͑FC͒ deposition together with ϳ3 nm of ULK damage on the ULK film surface after the FC plasma etching process. Most of the deposited FC material was removed during a subsequent H 2 -based remote plasma treatment at 275°C. The influence of surface modifications introduced by the prior C 4 F 8 / Ar-based etching exposure on hydrogen permeation of the ULK material during a subsequent H 2 remote pla...
The authors describe the temporal evolution of the surface and near-surface regions of a porous SiCOH ultralow k (ULK) dielectric during exposure under sidewall-like exposure conditions to various plasma processing environments. The authors studied the exposure of the ULK material to Ar plasma, C4F8/Ar-based etching plasma, and O2 or CO2 ashing plasmas, as well as various sequences of these processes. Real-time monitoring of the ULK surfaces during plasma processing was performed by in situ ellipsometry employing a novel gap structure. Additionally, changes in ULK surface properties were characterized by x-ray photoelectron spectroscopy and selective dilute hydrofluoric acid wet etching in combination with ex situ ellipsometry measurements. Pristine ULK material exposed to O2 plasma without ion bombardment shows the formation of a near-surface porous layer. For exposure of the ULK to CO2 plasma operated at comparable plasma operation conditions, the modification depth for a given exposure time is reduced relative to O2, but otherwise an identical ellipsometric trajectory is followed. This is indicative of a similar ULK damage mechanism for the two discharges, although at different rates. Energetic (∼400 eV) ion bombardment on the surface of ULK with line-of-sight Ar plasma exposure introduced a ∼12 nm thick SiO2-like densified layer on the ULK surface meanwhile sputtering off the ULK material. The sidewall-like modifications of ULK due to metastable Ar, if present, were too subtle to be measurable in this article. For ULK exposed under sidewall-like geometry to C4F8/Ar-based etching plasma, fluorocarbon quickly permeated into the subsurface region and showed saturation at a mixed layer thickness of about 14 nm. For additional exposure to O2 or CO2 discharges, a strong decrease of the CO2 plasma induced ULK surface modifications with increasing fluorocarbon (FC) film thickness was found, indicative of surface protection by FC surface deposition along with pore-sealing by the FC material. Attempts to increase the protective nature of the FC film by additional plasma processing, e.g., by exposure to Ar or He plasma after FC plasma etching, did not reduce CO2 plasma induced ULK surface modifications further.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.