Dependence of photoresist surface modifications during plasma-based pattern transfer on choice of feedgas composition: Comparison of C 4 F 8 -and C F 4 -based dischargesPlasma-polymer interactions are important for the purpose of etching, deposition, and surface modification in a wide range of different fields. An Ar discharge from an inductively coupled plasma reactor was used to determine the factors in a simple plasma that control etch and surface roughness behavior for three styrene-based and three ester-based model polymers. The authors compared the etch behavior of polymers in Ar plasma discharges with low and high energy ions by changing the substrate bias, compared cooled and elevated substrate temperature conditions, and compared fully plasma-exposed conditions and vacuum ultraviolet ͑vuv͒-only conditions by employing a magnesium fluoride window to prevent ion bombardment in the vuv-only case. It was found that ions, vuv radiation, and temperature all had significant impact on the etch behavior of polymers. The dependence of polymer structure on etch and surface roughness was also compared. Polymers with styrene and ester side groups were compared and polymers with ␣-hydrogen and with ␣-methyl were compared. It was found that for styrene-based polymers, there was a large difference in material removal between ␣-hydrogen ͓poly͑4-methylstyrene͒͑P4MS͔͒ and ␣-methyl ͓poly͑␣-methylstyrene͒ ͑P␣MS͔͒ structures. This difference was highly temperature dependent, and the ceiling temperature of the polymers was found to be the most important property to consider. Below the ceiling temperature, the amount of material removed in P4MS and P␣MS was the same, but above it there was a dramatic material loss in P␣MS not seen in P4MS. For the ester-based polymers, it was established that oxygen depletion occurred before any other mechanism and the most important factor to consider was oxygen content in the polymer. By using in situ ellipsometry, it was also found that at temperatures below the ceiling temperature modification by vuv radiation of P␣MS created a slightly denser layer at the surface with higher index of refraction. This effect was not seen in P4MS. It was observed that in general, low energy ions contributed to material removal by physical sputtering at the polymer surface and the amount of material removal increased with oxygen content in the polymer. vuv radiation caused bulk depolymerization and oxygen depletion reactions that were highly polymer structure specific and temperature dependent. High energy ion bombardment was found to create an amorphous carbonlike damage layer with a thickness that was determined by the ion penetration depth. This damage layer could be characterized by ellipsometry. While for P4MS it was sufficient to model by ellipsometry the etch process using an ion-damaged layer on top of a bulk layer of unmodified polymer, the vuv effect needed to be added to the optical model in order to accurately characterize P␣MS. Finally, surface roughening of polymers only occurred under ion bomba...

Plasma based transfer of photoresist (PR) patterns into underlying films and substrates is basic to micro- and nanofabrication but can suffer from excessive surface and line edge roughness in the photoresist and resulting features. The authors have studied the interaction of a set of adamantyl methacrylate-based model polymers with fluorocarbon∕Ar discharges and energetic Ar+ ion beams. Through systematic variation of the polymer structure, the authors were able to clarify the contributions of several critical polymer components on the chemical and morphological modifications in the plasma environment. Etching rates and surface chemical and morphological changes for the model polymers and fully formulated 193 and 248nm photoresists were determined by ellipsometry, atomic force microscopy, time of flight static secondary ion mass spectrometry, and x-ray photoelectron spectroscopy. The polymer structure in the near surface region (∼10nm) of all materials is destroyed within the first seconds of exposure to a fluorocarbon∕Ar plasma. The plasma-induced changes include destruction of polymeric structure in the near surface region and oxygen and hydrogen loss along with fluorination. For the 193nm PR material, the initial densification of the near surface region was followed by the introduction of pronounced surface roughness. This change was not seen for 248nm PR processed under identical conditions. When comparing the responses of different polymer materials, the authors observed a strong dependence of plasma-induced surface chemical and morphological changes on polymer structure. In particular, the adamantane group of 193nm PR showed poor stability under plasma exposure. On the other hand, the plasma-induced changes for polymer resins with or without the low molecular weight chemicals required to make the photoresist system photoactive did not differ significantly. The behavior of the same materials during energetic argon ion beam bombardment was also investigated. No significant differences in etch yield and surface roughness evolution for the different materials were seen in that case.

Results are presented from molecular dynamics (MD) simulations of 100eV Ar+ bombardment of a model polystyrene (PS) surface. The simulations show that the system transitions from an initially high sputter yield (SY) for the virgin polymer to a drastically lower SY as steady state is approached. This is consistent with corresponding ion beam experiments. The MD indicates that this drop in SY is due to the formation of a heavily cross-linked, dehydrogenated damaged layer. The thickness and structure of this layer are also consistent with ellipsometry and x-ray photoelectron spectroscopy measurements of Ar plasma-exposed PS samples.

Molecular dynamics (MD) simulations have been carried out to examine the effects of Ar+, Ar+/H, and Ar+/F bombardment of a model polystyrene (PS) surface. For bombardment with 100 eV Ar+ only, the simulations show the formation of a heavily cross-linked dehydrogenated damaged layer in the near-surface region after some initial fluence, consistent with plasma and beam system experimental results. The 1–2 nm thick amorphous carbon-rich modified layer has a much lower sputter yield compared to that of the virgin PS, which has a H:C ratio of 1. Simultaneous bombardment of the damaged dehydrogenated PS layer with 300 K H or F radicals and 100 eV Ar+ can facilitate the removal of the layer as well as inhibit its initial formation. The development of the steady-state dehydrogenated layer under Ar+-only bombardment results from a competition between the breaking of carbon-hydrogen bonds (which leads to dehydrogenation and subsequent cross-linking) and the breaking of carbon-carbon bonds (which leads to sputtering of polymer fragments). For the conditions presented in this study, the loss of hydrogen eventually overtakes the removal of polymer fragments, resulting in the formation of the dehydrogenated cross-linked near-surface layer. The final properties of the dehydrogenated layer from the MD simulations are compared at steady state to ellipsometric data for plasma-exposed PS samples, and the initial and final sputter yields from MD are compared to experimental beam system data.

Photoresist modifications by plasma vacuum ultraviolet radiation: The role of polymer structure and plasma chemistry J. Vac. Sci. Technol. B 28, 993 (2010); 10.1116/1.3484249Real-time studies of surface roughness development and reticulation mechanism of advanced photoresist materials during plasma processing One recurring problem in nanoscale processing is roughening of photoresist ͑PR͒ materials during plasma etch. We studied the plasma etch behavior of 248 nm PR, 193 nm PR, and poly methyladamantyl methacrylate while changing the source power level ͑400-1200 W͒, adjusting the bias power to change the self-bias voltage V dc ͑−50 to − 150 V͒, and varying the pressure ͑10-80 mTorr͒ and the amount of fluorocarbon gas additive to the Ar discharge ͑0%-10% c-C 4 F 8 in Ar͒. The authors found that the PR removal is dominated by the ion energy and fluence. Surface fluorination enhanced the removal rates. Two linked mechanisms for the roughening behavior of the films during processing were identified. Changes of PR top surface roughening behavior in response to variations of bias power and pressure could be interpreted by a model of roughness formation which is dominated by a physical pattern transfer mechanism, i.e., roughness amplification through selective ion-induced transfer. When the plasma source power was varied, they observed that roughness formation was linked to the surface energy density deposited during processing. As the energy required to volatilize a volume element from the surface increased, the surface roughening rates grew proportionally. This conversion of excess energy into roughening was found to depend on the molecular structure of the polymer, with adamantyl polymers having a very high roughening constant. Additional effects on the etch behavior arise from fluorination of the samples, as quantified by x-ray photoelectron spectroscopy. High F 2s / F 1s intensity ratios, which indicate deeper fluorination, were measured for rough surface conditions. Smaller F 2s / F 1s ratios indicate near-surface fluorination and correspond to smoother top surfaces and feature sidewalls. Molecular compounds showed roughening behavior relative to the respective cross-linking behavior even when processed in pure Ar discharges, suggesting that the measured surface fluorination mirrors surface morphology. When plasma etching three-dimensional trenches and contact holes patterned in PR, the authors found that the sidewall roughness changed with process parameters in a fashion similar to that seen for blanket surface roughness introduction using the same etch conditions. A close correlation between the surface and sidewall roughness results was obtained, suggesting that their model of polymer surface roughening also applies to resist sidewall evolution during plasma etch.