The uncontrolled development of nanoscale roughness during plasma exposure of polymer surfaces is a major issue in the field of semiconductor processing. In this paper, we investigated the question of a possible relationship between the formation of nanoscale roughening and the simultaneous introduction of a nanometer-thick, densified surface layer that is formed on polymers due to plasma damage. Polystyrene films were exposed to an Ar discharge in an inductively coupled plasma reactor with controllable substrate bias and the properties of the modified surface layer were changed by varying the maximum Ar + ion energy. The modified layer thickness, chemical, and mechanical properties were obtained using real-time in situ ellipsometry, x-ray photoelectron spectroscopy, and modeled using molecular dynamics simulation. The surface roughness after plasma exposure was measured using atomic force microscopy, yielding the equilibrium dominant wavelength and amplitude A of surface roughness. The comparison of measured surface roughness wavelength and amplitude data with values of and A predicted from elastic buckling theory utilizing the measured properties of the densified surface layer showed excellent agreement both above and below the glass transition temperature of polystyrene. This agreement strongly supports a buckling mechanism of surface roughness formation.
The roles of ultraviolet/vacuum ultraviolet (UV/VUV) photons, Ar+ ion bombardment and heating in the roughening of 193nm photoresist have been investigated. Atomic force microscopy measurements show minimal surface roughness after UV/VUV-only or ion-only exposures at any temperature. Simultaneous UV/VUV, ion bombardment, and heating to surface temperatures of 60–100°C result in increased surface roughness, and is comparable to argon plasma-exposed samples. Ion bombardment creates a modified near-surface layer while UV/VUV radiation results in loss of carbon-oxygen bonds up to a depth of ∼100nm. Enhanced roughness is only observed in the presence of all three effects.
We have identified a synergistic roughening mechanism of 193 nm photoresist, where simultaneous ion bombardment, vacuum ultraviolet (VUV) radiation, and moderate substrate heating in a well‐characterized beam system results in a similar level of surface roughness observed during conditions typical of plasma etching. VUV radiation (147 nm) results in bulk modification of the photoresist polymer, witnessed by the loss of carbon–oxygen bonds through transmission FTIR. Ion bombardment (150 eV) results in the formation of a densified surface layer on the order of a few nanometers in depth. We have shown that elevated levels of roughness are observed only during simultaneous exposure and that sequential exposure is not sufficient to produce surface roughness. In addition, through the use of transmission FTIR we have shown that an etching synergy does not exist and that etch rates are nearly independent of temperature. We propose that the observed roughness could be due to the drastically different mechanical properties of the ion‐modified near‐surface region and VUV‐modified bulk photoresist, where the difference is exaggerated at elevated temperatures. A more complete understanding of plasma‐induced surface roughness will require further study, resulting in the improvement of existing pattern transfer technologies and possibly novel new technologies as well.
The mechanism of modified layer formation for 193nm photoresist (PR) during a short time (up to ∼10s) fluorocarbon plasma exposure was investigated. We employed a shutter approach to achieve rapidly steady-state plasma condition when processing PR surfaces. The time evolution of the optical constants and the thickness of the modified layer on the PR surface were obtained using two layer optical modeling of ellipsometric data for the processed PR material. This enabled us to determine the time-resolved etching rate of the PR and the kinetics of modified layer formation. The change in the surface chemical composition of the PR materials was determined by x-ray photoelectron spectroscopy (XPS). A graphitic layer with a higher refractive index as compared to the bulk PR material was formed on the PR surface within a few (∼3s) seconds of plasma exposure. The XPS data revealed that before a fluorinated surface developed, cleavage at ester groups of the side chain in the polymer and dangling bond formation took place, leading to cross-linking. To investigate the influence of the oxygen content of the polymer on surface roughness formation, we compared the surface evolution of oxygen-rich 193 and 248nm PRs, which have a smaller oxygen content. Remarkable differences in the etching behavior during the initial plasma interaction period were observed for the two materials. Whereas for 193nm PR, etching was observed immediately and the material exhibited higher surface roughness, for the 248nm PR material fluorocarbon film deposition took place initially. Once a fluorinated surface had developed, steady-state etching took place, but the 248nm PR exhibited lower surface roughness than the 193nm material. XPS measurements showed that when comparing the fluorine content of the surface layer to the oxygen content, the fluorine content was relatively more important for the 248nm PR than for the 193nm PR. For the latter, oxygen in the side groups of the bulk PR enhanced the PR etching rate initially and led to a rapid surface roughness formation. This coincides with the development of a fluorinated surface.
While vacuum ultraviolet (VUV) photon irradiation has been shown to significantly contribute to material modifications of polymers during plasma exposures, the impact of radiation-induced material alterations on roughness development during plasma processing has remained unclear. The authors have studied the interaction of the radiation of Ar and C4F8/Ar plasma discharges with 193 and 248 nm advanced photoresists (PRs). Optical filters were used to vary the radiation exposure wavelength range in the ultraviolet (UV) and VUV emission spectra. This enables clarification of the respective roles of plasma photon radiation wavelength and PR polymer structure on the chemical and structural changes produced in the materials. Chemical changes in polymer composition at the film surface and in the material bulk were determined by vacuum transfer x-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy. Morphological changes, film thickness reduction, and changes in surface and pattern morphology were characterized by ellipsometry, scanning electron microscopy, and atomic force microscopy. The exposure of methacrylate based 193 nm PR to photon radiation in the UV/VUV spectral range (λ=112–143 nm) leads to detachment and removal of oxygen containing polymer pendant groups to a depth of about 200 nm. This causes changes in the polymer structure by chain scission, significant film thickness reduction, and reduced pattern critical dimensions and line edge roughness. Chain-scission reactions and residual detached polymer pendant groups are expected to effectively soften layers of 193 nm PR. In contrast to 193 nm PR, styrene based 248 nm PR was found to be significantly more stable under plasma-produced irradiation due to the low oxygen content, low ester linkage concentration, and absence of lactone. Small thickness reduction, reduced oxygen loss, and cross-linking were observed in the surface region of 248 nm PR. Radiation-induced material modifications of both PR materials decreased with increasing photon wavelength in Ar discharges. Increasing modification of 193 nm PR was observed for increasing photon flux at higher wavelengths (λ=143–300 nm) by the emission characteristic of fluorocarbon containing plasmas. In C4F8/Ar plasma, the authors observed strongly increased loss of oxygen at the film surface and in the COC and CO lactone bonds in the material bulk along with film thickness reduction compared to pure Ar discharges. These modifications are directly relevant to plasma processes used for pattern transfer, which often contain fluorocarbon species.
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.