Femtosecond Laser Deposition of Diamond-Like Carbon Films
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FEMTOSECOND LASER DEPOSITION OF DIAMOND-LIKE CARBON FILMS F. QIAN*, R. K. SINGH*, S. DUTTA**, P.P. PRONKO** and W.H. WEBER*** *Department of Materials Science & Engineering, the University of Florida, Gainesville, FL 32611. "**Centerfor Ultrafast Optical Science, the University of Michigan, Ann Arbor, MI 48109. * * *Ford Research Laboratory, Dearborn, MII 48121. ABSTRACT We have deposited unhydrogenated diamond-like carbon (DLC) films with 100 femtosecond laser pulses, at intensities in the 3x 1014 - 6.5x 1015 W/cm 2 range. Film surface topography, optical property, and bonding structure were examined, respectively, with atomic force microscopy (AFM), spectroscopic ellipsometry (SE) and Raman spectrometry. The femtosecond pulse generated plasma was studied through time-of-flight (TOF) experiment. The most probable kinetic energy of carbon ions was estimated to be in the 300 - 2000 eV range, increasing with laser intensity. In addition, a unique 'suprathermal' component with kinetic energy ranging from 4 to 40 keV was observed in the TOF spectrum. This high energy peak is believed to originate from fast ions in a solid density plasma created during the absorption of each femtosecond laser pulse. INTRODUCTION Pulsed laser deposition (PLD) has, over the past decade, established itself as one of the most versatile techniques in depositing thin film materials. Largely due to the non-thermal equilibrium nature of the laser induced plasma, this technique has found particular success in the deposition of hydroden-free diamond-like carbon (DLC) thin films. Formations of DLC with high sp 3 content and films with microhardness approaching that of natural diamond have been reported. 1,2 It was found that, in order to obtain the best possible DLC film properties, certain deposition criteria have to be met. In earlier days, it was the Nagel criteria,3 i.e., a critical laser intensity of 5x10l10 W/cm 2 is required to form hard, diamond-like films, as opposed to soft, graphitic ones obtained at lower intensities. In recent years, extensive studies on this subject have yielded a more universally accepted conclusion that, as a kinetic condensation process, DLC film quality is a function of the kinetic energies carried by carbon particles in the laser induced plasma. Higher laser intensity produces a plasma containing carbon ions with higher kinetic energies, giving rise to better film quality. This observation is supported by the model proposed by Stevefelt and Collins.4 Successful DLC depositions have mostly been carried out by excimer (XeCl, ArF, KrF, etc.) and various frequency modified Nd:YAG lasers. Depending on laser type, power densities in the 101 - 1011 W/cm2 range are often used in making films. Most probable ion kinetic energies ranging from several eV up to a couple of hundred eV are detected in these plasmas, as a function of laser intensity and wavelength. One common feature among these techniques is that they all use laser pulses in the nanosecond range. The most significant aspect of nanosecond laser pulse interaction
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