| AVS 54th International Symposium | |
| Surface Science | Thursday Sessions |
| Session SS-ThP |
| Session: | Surface Science Poster Session |
| Presenter: | S.R. John, Washington State University |
| Authors: | S.R. John, Washington State University J.A. Leraas, Washington State University S.C. Langford, Washington State University J.T. Dickinson, Washington State University |
| Correspondent: | Click to Email |
Polytetrafluoroethylene (PTFE) (C2F4)n is a model organic polymer with unique properties. In this work, we explore the mechanisms responsible for the superior etching others have demonstrated using 157-nm F2 excimer lasers. At fluences well below the threshold for plasma formation, the major neutral products are (CF2)x units. Thus decomposition is primarily from backbone scission. Mass selected time-of-flight signals for these neutral products show a fast component with energies on the order of 1.2 eV, as well as a slower, thermal component. We attribute the fast component to products formed from scission of C-C bonds due to electronic excitations along the surface. We attribute the slower component to products produced in the bulk, which then diffuse to the surface and desorb. The clean etching of PTFE at 157 nm is the result of dissociative electronic excitations. Nevertheless, analysis of the slow component indicates high surface temperatures. The F2 laser significantly heats the surface. Intense electron, positive and negative ion emissions are also observed. The high positive ion kinetic energies (3-10 eV) are consistent with an electrostatic emission mechanism. The negative ion signals are an order of magnitude weaker than the positive ions, but display similar kinetic energies (3-5 eV). We attribute negative ion formation to dissociative electron attachment of neutral monomers. This process requires high electron densities, which are observed in the cloud of positive ions. The presence of both positive and negative ions, in addition to reactive (CF2)x units, would promote the growth of high quality PTFE films by laser ablation deposition. We will briefly describe the ablation of a another fluorocarbon polymer, polyvenylidenefluoride (PVDF) (C2H2F2)n, at 157-nm. Initial exposure yields intense emissions of neutral HF due to side chain scission-in contrast to PTFE, where fragmentation is predominately due to backbone scission. The presence of hydrogen in the PVDF renders side chain bonds especially vulnerable. During prolonged irradiation, especially intense bursts of charged and neutral products accompany some laser pulses and not others. The bursts are presumably due to the accumulation of damaged material in the intervening laser pulses.
1This work was supported by the U.S. Department of Energy under Grant DE-FG02-04ER-15618.