AVS 50th International Symposium
    Surface Science Monday Sessions
       Session SS1-MoA

Paper SS1-MoA2
Defect-driven Photodesorption at UV and VUV Excimer Laser Wavelengths

Monday, November 3, 2003, 2:20 pm, Room 326

Session: Stimulated Processes at Surfaces
Presenter: L. Cramer, Washington State University
Authors: L. Cramer, Washington State University
S.C. Langford, Washington State University
J.T. Dickinson, Washington State University
Correspondent: Click to Email
The optical properties of single crystal calcium fluoride make it useful lens material for the ultraviolet to vacuum-ultraviolet region, including possible use at 157 nm (7.8 eV) for high resolution microlithography. Of concern is possible degradation of these properties with prolonged exposure to laser light. Both surface and bulk modification of the material are of interest. Our interest is to obtain fundamental understanding of laser induced emissions (e.g., ions, electrons, and neutrals) from insulators with wide bandgaps (CaF2 has a band gap of ~11.2 eV) and radiation induced surface/bulk modifications We report and compare the measured rates of laser desorption from cleaved surfaces of CaF2 of the observable ionic and neutral species at three excimer laser wavelengths: 157, 193, and 248 nm with pulse lengths ~20-30 ns. Mass selected time-of-flight techniques allows both species and kinetic energies to be determined. At laser fluences well below threshold for plume formation, only positive ions are observed. Ca+ is seen at all of the wavelengths; CaF+ is seen only at 157 nm. The Ca+ intensity vs. fluence shows essentially no dependence on wavelength. Ion energies show slight wavelength dependencies but all emissions are consistent with a multiple-photon electrostatic repulsion model. Neutral emission is dominated by Ca0 and the neutral energy distributions correspond to thermal distributions at relatively high temperatures (> 2500 K). Finally, we report on the role of simultaneous keV electron irradiation co-focused on the laser spot. We observe substantial increases in the laser desorbed products which we contribute to the resulting halide vacancy formation (F-centers).