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.
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...
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.
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.
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