Laser Induced Damage Threshold Measurements at the Microscopic Level
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LASER INDUCED DAMAGE THRESHOLD MEASUREMENTS AT THE MICROSCOPIC LEVEL ALLEN D. ZWAN" AND DAVID R. MILLER "Hughes Missile Systems Company, San Diego, CA "'University of California at San Diego, Department of Applied Mechanics and Engineering Sciences, La Jolla, CA, 92093-0310 ABSTRACT We have (tudied the laser damage threshold to silver films (500A - 1000A) grown on single crystal silicon , in a newly developed laser damage UHV system at pressures of 10-1° torr. A 1.06Mm Nd:Glass laser is used to damage the mirror surfaces in 1-on-1 pulse studies. In-situ damage characterization includes Auger, reflectivity of the primary beam, diffuse scattering of a helium-neon laser, and mass spectrometry detection of desorbed surface species. External characterization includes optical microscopy and SEM. All in-situ damage probes are well correlated and baseline damage occurs at fluences near 3.4 MW/cm 2. Time-of-flight to the mass spectrometer shows ejected particles with energies in the 5 to 10 eV range indicating a plasma damage mechanism. Prior to typical thermally induced damage the external microscopy shows well defined precursor morphology changes which appear as feather-like microstructure at the submicron level. INTRODUCTION Successful development of damage resistant optic materials and coatings requires an understanding of the mechanism of laser induced breakdown. Pulsed laser induced damage of thin films is, in general, initiated by the absorption of laser radiation by imperfections and contamination of the thin film during growth and of the surface during cleaning, handling and polishing. We have developed a unique laser damage test facility that eliminates these cited 1' 2 initiators providing the capability for microscopic damage initiation studies. The novel feature of the system is that the thin films are grown in the same chamber in which the laser damage testing is performed. This insures that the thin film never leaves the ultra high vacuum (UHV) environment from growth through testing. The facility has recently become operational and our initial studies on silver mirrors on silicon substrates have resulted in reproducible data which shows featherlike microstructures as the precursor to the usual thermal macroscopic damage. EXPERIMENTAL FACILITY A facility that allows in-situ UHV coating application, controlled laser damage testing and detailed damage investigation has been designed, constructed and tested at the University of California at San Diego. A schematic of the system is shown in Figure 1. UHV Chamber The UHV chamber is a horizontal four inch diameter tubular design consisting of three axial stations surrounded by radial ports. Mat. Res. Soc. Symp. Proc. Vol. 285. 01993 Materials Research Society
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Multiple Sample Loadlock
Figure 1 Schematic of the Laser Damage Testing Facility
The first station provides the capability for film growth by various physical vapor deposition techniques. Current capabilities include water cooled crucibles for single layer coatings and a newly devel
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