Molecular dynamics simulations of near-surface modification of polystyrene: Bombardment with Ar+ and Ar+/radical chemistries

Abstract

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.

Surface Roughening of Polystyrene and Poly(methyl methacrylate) in Ar/O2 Plasma Etching

“…While the dependence of surface roughness on ion energy is opposite for PS and PMMA etched in Ar/O 2 , the surface roughness features are similar in size and form [12] suggesting that the same roughening mechanism is involved. The role of cross-linking is further explored through the use of alternate etch gas mixtures that are expected to suppress cross-linking [19]. Surface roughness of Ar/H 2 and Ar/F 2 etched PS and PMMA samples with four substrate bias voltages: −30 V to −150 V (ion energy: 45 eV to 165 eV) are shown in Figure 7.…”

“…In the case of PS, surface cross-linking is prevalent, while in PMMA, chain scissioning and depolymerization dominate [13,14,16]. While chemical structures such as the aromatic ring in PS and the methyl ester group in PMMA are commonly believed to play important roles in etching behaviors such as etch rate, cross-linking and chain scission, most studies reporting a correlation between chemical structures and etch behaviors of polymers are based on indirect observations, including change in molecular weight [16], etch rate [13,19], interface adhesion [14,16] or diffusion coefficient [20]. Details of the mechanisms involved have yet to be elucidated.…”

“…In addition, while O 2 plasmas are found to enhance cross-linking during PS etching through formation of C-O-C bonds, H and F atoms were shown to suppress cross-linking during plasma etching [19]. H and F atoms form a single bond with C atoms, terminating dangling bonds and preventing cross-linking between C atoms in neighboring chains.…”

“…Another set of experiments with Ar/H 2 and Ar/F 2 etching of PS and PMMA are designed to suppress the effects of cross-linking through the termination of dangling bonds by F or H atoms [19]. Post-etch roughness of PS samples with different MWs are also compared to investigate the contribution of intrinsic aggregation in surface roughening.…”

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

“…poly͑methyl methacrylate͒ ͑PMMA͒, 8 193 and 248 nm photoresists 5,6,9,10 ͔ and has been described as a thin, highly crosslinked and graphitized layer. [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.…”