Papers

ICMCTF2000 Session B6: Laser Assisted Deposition

Friday, April 14, 2000 8:30 AM in Room Golden West

Friday Morning

Start	Invited?	Item
8:30 AM		B6-1 Laser-CVD - Current Status and Potential for Industrial Applications V. Hopfe (Fraunhofer Institute for Material and Beam Technology Dresden, Germany) Laser activated CVD (L-CVD) can be applied in different ways: (i) wide area coating having the potential of continuously forming homogeneous layers on temperature sensitive substrates at atmospheric pressure (e.g. optical coatings on glass/ plastics, web-coating), (ii) coating on shaped parts, e.g. fibers, (iii) micro structuring/ patterning, e.g. for rapid prototyping or repair of wafers, (iv) build-up of 3-dimensional microstructures on substrates under CAD control for designing/ making microsystems, (v) forming nano-powders with narrow size distribution, e.g. for advanced ceramics. Extremely high deposition rates up to several tens of microns/s have been demonstrated and a wide variety of materials can be deposited. With L-CVD the deposition region is restricted to the volume of the impinging laser beam forming a "wall-less" reactor. In contrast to conventional CVD, additional process variables in L-CVD comprise the wavelength and power density of the laser, the absorptivity of the precursor gases and/or the substrates at laser wavelength, and the geometrical configuration of the laser beam with respect to the substrate (parallel or perpendicular to surface). Playing around the different types of laser sources (IR lasers, excimer laser) the activation of the CVD process can be tailored from a photolytical mode to a thermal mode including different hybride stages. Results will be presented in more detail on high speed coating of fibers which are used for making damage-tolerant fiber reinforced ceramic composites. Based on a industrial 6 kW cw-CO2 laser, an atmospheric pressure L-CVD process has been established which performs a continuous coating of fiber bundles. The laser based method is characterized by several significant advantages, among them deposition rates of typically >1 µm/s, small volume cold-wall reactors, and short residence time of the fibers in the deposition chamber, which avoids fiber degradation. As another feature favoring industrial application, the process can be performed at atmospheric pressure in an open reactor with a continuous air-to-air coiling of the fiber bundles.
9:10 AM		B6-3 Ion-Beam Assisted Pulsed Laser Deposition of Al-O-N Films A.A. Voevodin, J.G. Jones, J.S. Zabinski (Air Force Research Laboratory) Ion-Beam Assisted Pulsed Laser Deposition (IPLD) was known to be highly effective in a low temperature deposition of films with desirable microstructure and chemistry, such as Zr₂O/Y₂O₃ or C₃N₄. In this study, we found that interaction effects between laser ablated plasma plumes and ion beam source can be critical for the film property formation. In particular, interactions between plasmas produced by a nitrogen ion-beam source and pulsed laser ablation of Al₂O₃ were studied. Plasma emission imaging and spectroscopy analyses using an ICCD camera and a spectrometer were used to investigate plasma development and chemistry in real time using the initial laser pulse for synchronization. In the study, the N₂ background pressure was varied in the range from 0.08 to 4 Pa and film elemental compositions were correlated with plasma chemistry. Two significant plasma interaction effects were discovered. One resulted in a considerable activation of N and O and formation of NO molecules in a near substrate region, which then reacted with Al to from Al-O-N. A maximum plasma excitation was observed at reduced 0.1-0.2 Pa N₂ pressures and provided the highest amount of N in the films. Above 1 Pa of N₂, the Al-O-N films had lower nitrogen content, even though more nitrogen was available for the deposition. Another interaction effect was observed in the 2-4 Pa pressure region, when formation of short lived plasma channels connecting ion-beam and laser ablated plasmas was detected. These channels resulted in plasma bending and shifting from the substrate surface, affecting film composition and influencing ion beam current extracted from an ion beam-source. The study suggested that the interaction of ion-beam and laser ablation plumes in IPLD may considerably affect plasma chemistry, excitation stages, and spatial distribution, providing new opportunities for the control of resulting film properties. Al-O-N Film structural and mechanical properties will be presented.
9:30 AM		B6-4 CW CO₂-Laser Plasmatron: Main Features and CVD Diamond Applications A.P. Bolshakov (General Physics Institute, Russia); F. Dausinger (IFSW, Germany); V.I. Konov, S.A. Uglov (General Physics Institute, Russia) The new type of plasma CVD reactor has been proposed and experimentally realized for diamond film deposition. It is based on maintenance by continuous wave CO₂-laser radiation of stationary plasma in the super-atmospheric pressure gas mixture stream, exhausting over the substrate into the air. Such a non-vacuum (not demanding vacuum chambers) technique, called laser plasmatron, provides the highest temperature and power density among existing plasma devices. Besides, it is clean because of lack of electrodes and is a strong source of ultraviolet radiation. The results of the experiments performed with 2,5 and 8 kW CO₂-laser and Ar(Xe):CH₄:H₂ gas mixtures will be presented. The dependence of plasma maintenance threshold beam intensity on gas stream composition, stream velocity and beam orientation (vertical or horizontal) was investigated and optimal diamond growth conditions were found. The possibility to increase active gas concentration by application of the supplementary dc arc discharge coupled to the laser produced plasma is studied. Special attention is paid to the problem of laser beam effective absorption in the CW optical discharge region.
10:30 AM		B6-7 Phase Transformations Induced by Pulsing Multiple Laser Fluxes A. Badzian, R. Roy, W. Drawl, E. Breval, T. Badzian (The Pennsylvania State University); P. Mistry, M. Turchan (QQC, Inc.) The idea that a laser pulse can cause reconstructive quenchable phase transitions was given an enormous boost by the discovery by Fedoseev and Derjaguin. That graphite could be converted to diamond in the open air by dropping a fine powder through a CO₂ laser beam. The phase transformations demonstrated with CO₂ lasers are not limited to the graphite-diamond transition. Even more amazing was the fact that quartz was converted to stishovite by this simple process. Confirmation and extension of the Fedoseev and Derjaguin experiments was carried out at the Materials Research Laboratory of the Pennsylvania State Univesity. Roy and coworkers using a continuous wave CO₂ laser reproduced the graphite to diamond and quartz-stishovite transformations. In 1995 Mistry and co-workers of QQC company announced formation of diamond on WC/Co shielded by CO₂/N₂ gas mixture in the air lab ambient. The radical difference in this approach and that of all previous laser-material interaction work was the use of three pulsed lasers of different wavelength simultaneously impacting the work place. Since the initial announcement the Materials Research Laboratory has studied the results of the “QQC process.” Diamonds, metals, ceramics and plastics has been exposed to the laser fluxes. No melting appears at these processes. Polycrystalline diamond coating has been amorphasized under laser beam as is shown by disappearance of diamond signature in the Raman spectrum taken on the cross-section of the coating. Fuel nozzle injectors which are made of ferrosilicon (3% Si) were subject of cladding and transformation treatment. Below the Ti(NC) clad layer the 10-15 microns layer transformed to a martensite-like phase. Inside the solid body, but below that layer, there is a 30-50 microns layer of metal transformed completely to a non-crystalline state as was shown by x-ray diffraction. A ceramic product zirconia ball made of tetragonal phase under laser fluxes transformed at the surface to the cubic phase to a depth of 20 microns.
10:50 AM		B6-8 Investigation of Thermochemical Instability in Laser Ablation J.S. Kapat, Z. Hao (University of Central Florida) Pulsed Laser Deposition (PLD) is a simple yet versatile method for thin film deposition. However, this method is yet to have a wide-spread acceptance by the industry, and a cost-effective tool for process optimization will be important in making this method more economical and viable for industrial application. Numerical simulation can provide a cost-effective alternative to experimentation for possible process optimization. This paper presents partial results from a research effort on developing a numerical model to predict all aspects of laser solid interaction. Laser-induced heating can cause chemical reactions. The rates of these reactions increase with increase in temperature. If the products of the reactions have larger absorption coefficient than the original material, then there is a positive feedback, leading to an instability in the process. For example, in laser induced oxidation of metals, where the original metal is highly reflective (i.e. with poor absorptivity) but the metal oxide is highly absorptive of the laser radiation, this type of instability can occur. Also, in laser heating of polymers, as long molecular chains break down under laser irradiation, the products are typically more absorptive than the original material. This again causes instability leading to the formation of soot. In both cases, ways to control the instability will be of fundamental as well as practical importance. The details of these instabilities are not predicted accurately by conventional computer simulations because of the large phase or dispersion error in a typical finite-volume or h-type finite element formulation. For these reasons, a spectral method-based computation has been performed. The computational results are discussed in the context of the experimental findings of the other researchers. The important non-dimensional parameters that govern this instability are identified.