Effect of polyatomic ion structure on thin-film growth: Experiments and molecular dynamics simulations

Abstract

The experiments described here examine 25-100 eV CF 3 ϩ and C 3 F 5 ϩ ion modification of a polystyrene ͑PS͒ surface, as analyzed by x-ray photoelectron spectroscopy. The molecular dynamics computer simulations probe the structurally and chemically similar reactions of 20-100 eV CH 3 ϩ and C 3 H 5 ϩ with PS. CF 3 ϩ and C 3 F 5 ϩ each form a distribution of different fluorocarbon ͑FC͒ functional groups on PS in amounts dependent upon the incident ion energy, structure, and fluence. Both ions deposit mostly intact upon the surface at 25 eV, although they also undergo some crosslinking upon deposition. Fragmentation of the two ions increases as the ion energies are increased to 50 eV. Both ions show increases in total fluorine and fluorinated carbon content when changing the ion energy from 25 to 50 eV. The simulations predict that CH 3 ϩ and C 3 H 5 ϩ behave in a similar fashion to their FC analogs, remaining mostly intact and either embedding or scattering from the surface without reacting at 20 eV. At 50 and 100 eV, the simulations predict fragmentation most or all of the time. The simulations also show that the chemical products of the collisions depend significantly on the structure of the incident isomer. The simulations further illustrate how the maximum penetration depth of ion fragments depends on ionic structure, incident energy, and the identity of the penetrating fragment. These ion-surface results are discussed in terms of their possible role in plasmas.

On the importance of ions and ion-molecule reactions to plasma-surface interface reactions

“…Moreover, ion beam experiments for CF x ϩ ions (x ϭ 1-3) have shown that both etching and deposition are sensitive to the F/C ratio as well as the incident ion energy [8,9,14]. Molecular dynamics simulations by Graves and coworkers have also focused on CF x ϩ (x ϭ 1-3) ion is also formed on the Si surface during the simulations, and net deposition results in the case of CF ϩ bombardment [16].…”

“…Most recently, we have focused on C 3 F 8 and C 4 F 8 plasmas as these systems are finding increased use in the microelectronics industry because of their lower global warming potentials [48]. Many of the theoretical [15,16 ] and experimental [8,9, 49 ] studies of FCP's have shown that plasma ions have considerable influence over resulting film chemistries. Recently, we have shown that ions can also significantly influence molecule-surface interactions in these systems [39].…”

“…Although early workers in the field of plasma polymerization identified ions as important film precursors [6], radical species are most often cited as polymerization building blocks. Recently, Hanley and coworker have shown that ion beams of hyperthermal ions can produce thin films and can be used to modify material interfaces [7,8,9]. The size and structure of the ions used (i.e., CF 3 ϩ vs. C 3 F 5 ϩ ) can strongly affect film composition, structure, and surface reactivity of the ion.…”

Molecular Dynamics Simulations of the Chemical Modification of Polystyrene through CxFy+Beam Deposition

“…It is therefore somewhat surprising that there is much that is not known about the chemistry by which treated surfaces are modified, largely because of the difficulties in monitoring the process experimentally. [1][2][3][4][5] Computational methods such as molecular dynamics (MD) simulations are therefore an important approach to the enhancement of our understanding of the chemical processes involved in plasma surface treatments, especially to determine the effect of major components of the complex plasma environment. 4,[6][7][8][9][10][11][12][13] Fluorocarbons are one of the most commonly used materials in plasma treatments of surfaces.…”

“…[1][2][3][4][5] Computational methods such as molecular dynamics (MD) simulations are therefore an important approach to the enhancement of our understanding of the chemical processes involved in plasma surface treatments, especially to determine the effect of major components of the complex plasma environment. 4,[6][7][8][9][10][11][12][13] Fluorocarbons are one of the most commonly used materials in plasma treatments of surfaces. For instance, fluorocarbon plasma treatment has been used for reactive ion etching of silicon dioxide during the fabrication of semiconductor devices and for fluorination of polymer surfaces to produce films with high thermal and chemical resistance, high dielectric constants, and low friction coefficients.…”