Raman microprobe studies of laser induced damage in dielectric films
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M. Bowden, N. M. Dixon, and D. J. Gardiner Newcastle upon Tyne Polytechnic, Ellison Place, Newcastle upon Tyne, NEI8ST, England (Received 27 April 1990; accepted 28 September 1990) Raman microprobe studies of pulsed laser damaged TiO2 films deposited using three different methods are reported. Phase transformation and redeposition of coating materials were observed in selected regions of amorphous films deposited by ion beam sputtering and electron beam evaporation. Preferential removal of a specific phase or transformation to a second phase were observed in reactively sputtered films. Some damage sites exhibited regions of stress heterogeneity which can be explained in terms of the return electron stream model of plasma/target interaction and rapid quenching. I. INTRODUCTION
Dielectric thin films are used in the production of optical filters, mirrors, and anti-reflection coatings. When irradiated with high power lasers, these coatings may incur irreversible damage characterized by alteration of microstructure, material ablation, and perturbation of inherent film stress, all of which can be determined from measured Raman spectra.1"4 Careful examination of damage sites under a microscope provided useful data from which mechanism(s) responsible for laser damage in each particular coating have been proposed.5 More quantitative information concerning the changes in a coating associated with the onset of laser damage may be obtained from Raman spectra.6"8 Through the use of an optical microprobe, Raman spectroscopy has been used in this study to characterize phase distributions and localized residual stress levels within laser damage sites and in the surrounding area. Measured Raman data provide the basis for a fundamental understanding of the damage morphologies which have been observed for many years. Two stable phases of TiO2, anatase and rutile, exhibit distinct Raman spectra and allow unambiguous characterization of the phase composition and homogeneity which can be related to the thermal history of the film.9 Raman microprobe studies are well suited for investigation of laser damage, since the spatial resolution offered by this technique, defined by the effective spot size of the focused laser beam at the sample, can probe both morphology, phase composition, and perturbations to localized stress within the film. Thus, oba)
Current Address, Rocketdyne Division, 6633 Canoga Ave., MS FA16, Canoga Park, California 91303. b) Author to whom correspondence should be addressed. ''Operated for the United States Department of Energy by Battelle Memorial Institute under contract DE-AC06-76RLO 1830. 126 http://journals.cambridge.org
J. Mater. Res., Vol. 6, No. 1, Jan 1991 Downloaded: 14 Mar 2015
served morphologies can be related directly to phase composition. Residual stress in materials is manifested by frequency shifts of allowed phonon modes as a result of changes in lattice dimensions and force constants. Correlation of the frequency shifts for a given phase with an applied stress may be derived from diamond anvil ce
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