Study of ion and vacuum ultraviolet-induced effects on styrene- and ester-based polymers exposed to argon plasma

Abstract

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...

Relationship between nanoscale roughness and ion-damaged layer in argon plasma exposed polystyrene films

“…[4][5][6][11][12][13][14][15][16] We have previously shown that under energetic Ar + ion bombardment during plasma etching, a dense, amorphous carbonlike modified layer is formed at the surface of a wide range of polymers ͓polystyrene ͑PS͒, poly͑␣-methylstyrene͒, poly͑4-methylstyrene͒, PMMA, poly͑hydroxyadamantyl acrylate͒, and poly͑hydroxyadaman-tyl methacrylate͔͒ with a thickness of a few nanometers. 17 This modified layer forms within the first few seconds of plasma exposure ͑corresponding to an ion fluence ϳ4 ϫ 10 16 cm −2 ͒, concurrent with a period of rapid surface roughening. 17 The bilayer structure formed by ion bombardment in the polymer films is reminiscent of similar bilayer structures composed of a compressed, stiff, thin film constrained to a much softer underlayer, such as SiO 2 ͑Refs.…”

Effect of ion irradiation on the thermal stability of thin polymer films

“…The substantial loss of side-chain methoxycarbonyl groups at high fluences transform PMMA on a disordered polyethylene-like polymer, for which the probability of cross-linking is higher than scission [17,18]. The creation of a dense cross-linked network on the surface of polymers that undergo preferentially chain scission has also been observed under plasma bombardment at high fluences [20].…”

Optical Observation of Deep Bulk Damage in Amorphous Perfluorocarbon Films Produced by UV Photons Emitted from Low-Pressure Argon Plasma

“…The fluorescence spectrum of the sample exposed directly to the plasma showed a single broad peak ranging from approximately 500 to 700 nm, while no clear fluorescence peaks were detected from the sample prepared with a synthetic quartz window. Although emission spectrum of low-pressure argon plasma contains many peaks that are dominated by atomic Ar lines in the blue light region (due to 4s-5p transitions) and in the red/near-infrared spectral region (due to 4s-4p transitions) [33,34], few emission lines are emitted in UV and VUV region; resonance lines at 104.8 nm (ArI, 1s 0 -1s 2 ) and 106.7 nm (ArI, 1s 0 -1s 4 ) [10,11,14,16,[34][35][36]. Photon energies of these VUV emission lines are high enough (11.8 and 11.6 eV) to cause breakage of C-C, C-O and C-F bonds in the perfluorocarbon polymer, whose bond energies are 3.60, 3.64 and 4.57 eV, respectively [37].…”

“…Compared with other energetic particles in plasma such as ions and electrons, UV photons have a much larger depth of penetration into transparent materials and can induce defects in the bulk region several hundred nanometers below the surface [18][19][20]. In past studies, various analytical methods have been employed for the detection and characterization of plasma-induced damage, including X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and ellipsometry [4,[9][10][11][12][13][14]19,20]. Indeed, these approaches are very useful for studying near-surface damage produced by ion bombardment, but not for detecting very slight damage generated deep in the bulk.…”

“…As with electrical charge-up and ion bombardment, photons in the UV and especially in the vacuum ultraviolet (VUV) regions emitted from plasma also induce damage on device materials [9][10][11][12][13][14][15][16][17]. Although the number of reports on UV-induced damage in LSI fabrication is much smaller than those on damage induced by electrical charge-up and ion bombardment, the recent increase in the use of organic materials in electronic devices, e.g., for low-k dielectric barrier and flexible semiconductors, has highlighted the ever-growing importance of the study on UV-induced damage in plasma processing.…”