Deposition of Diamond-Like Carbon (DLC) With Picosecond Laser Pulses
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ABSTRACT We have deposited unhydrogenated diamond-like carbon (DLC) thin films by laser ablation of graphite, using a high power Ti: Sapphire solid state laser system. DLC films were deposited onto single crystal silicon substrates at room temperature with picosecond laser pulses, at peak power densities in the 5x1 011 12 - 8x10 W/cm 2 range. A variety of techniques, including scanning and transmission electron microscopy (SEM and TEM), Raman spectroscopy, spectroscopic ellipsometry (SE), and electron energy loss spectroscopy (EELS) have been used to analyze the film quality. Smooth, partially transparent films were produced, distinct from the graphite target. Sp 3 volume fractions were found to be in the 50 - 60% range, with optical bandgaps ranging from 0.6 tol.2 eV, depending on the laser power density. INTRODUCTION Diamond-like carbon (DLC) can generally be defined as hard, amorphous carbon with different degrees of sp 2 , and sp 3 hybridization. DLC films possess properties close or similar to that of diamond, and a wide variety of applications have been found for these films, primarily in the microelectronics, optics and tribology industries. There are two basic types of DLC films, one is the hydrogenated DLC (C:H) prepared by chemical vapor depositions (CVD), in which various kinds of hydrocarbon agents, such as methane gas1 and ethylene 2 are used. The other is the hydrogen free DLC, fabricated mainly through physical vapor depositions (PVD), such as ion beam sputtering, 3 cathodic arc deposition, 4 and pulsed laser depositions (PLD). 5 In these PVD techniques, carbon species arrive at substrates with kinetic energies significantly higher than that presented by the substrate temperatures (leV=l .1x10 4 K), resulting in a highly condensed bonding structure with high volume fraction of sp 3 bonded carbon atoms. An inherent advantage common to these techniques is that much lower substrate temperatures are allowed in these deposition processes, making it possible to deposit high quality DLC films on heat sensitive materials, such as optical components and polymers. Recently, pulsed laser deposition (PLD) has shown tremendous promise in making high quality DLC films with sp 3 volume fractions higher than 70%,6 and 'amorphic diamond' films almost as hard as natural diamond, 7 having been reported. Common features in PLD techniques include laser generated microexplosion effects and the formation of high temperature, high pressure
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plasmas. The microexplosion effects lead to the ejection of target particulates onto the surface of deposited films, while the laser induced plasma gives rise to highly activated and ionized target species, which move toward the substrate in a strongly forward-directed manor, carrying high kinetic energies with them. It has been demonstrated that, as a kinetic condensation process, pulsed laser deposited DLC film quality is closely related to its deposition parameters. Among these the kinetic energies of the carbo
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