ICMCTF1998 Session B6: Laser-Assisted Deposition and Surface Treatment

Tuesday, April 28, 1998 1:30 PM in Room Golden West

Tuesday Afternoon

Start	Invited?	Item
1:30 PM	Invited	B6-1 Review of Ion-Assisted Pulsed Laser Deposition of Cubic-BN and Other Hard Materials T.A. Friedmann (Sandia National Laboratories) My presentation will focus on a review of ion-assisted pulsed-laser deposition (IAPLD) of cubic-BN (cBN) thin films. Work will be presented both from our laboratory and the literature to highlight developments in the field over the past 5-7 years. IAPLD uses concurrent bombardment of the growth substrate by fluxes from both a PLD ablated BN target and a broad beam ion gun. The ion gun is typically run with a combination of nitrogen and a noble gas (Ar, Kr, or Xe). For the case of a BN target with no ion-assist, there have been no reproducible reports of films with a significant cubic fraction. Clearly, the additional ion bombardment is necessary to nucleate and grow cBN. The main advantage of IAPLD for cBN film growth is the independent control over ion and ablation fluxes. This allows the ion-to-atom arrival ratio to be independently varied as well as the incoming ion energy and current density. The laser energy density on the target can be adjusted to vary the energy of the ablated species. In addition, noble gases can be added to the ion gun in order to vary the momentum/atom, another important film growth parameter. Finally, the substrate temperature also plays a significant role in film nucleation and growth. All of these parameters must be controlled in order to grow films with high cBN fractions. Another practical advantage of IAPLD is rapid film growth. This is important since the phase space of possible film growth conditions is rather large. Recent x-ray reflectivity results from our laboratory show that it is possible to produce fully dense films with columnar grain structures and high cBN fractions.
2:10 PM		B6-3 PLD Growth of Boron Nitride Using an Inductively Coupled RF Nitrogen Plasma Source G.E. Triplett, Jr., S.M. Durbin (Florida State University) The III-V compound boron nitride is very similar in many respects to carbon. In particular, the cubic phase is strikingly like diamond in terms of hardness, thermal conductivity and large optical bandgap energy. For these reasons, a number of groups in recent years have been attempting thin film growth of the cubic phase using a variety of physical and chemical vapor techniques. Unfortunately, achieving single-phase films remains the primary obstacle, although many groups have reported the existence of cubic or cubic-like sp³ bonded boron nitride within their films. In the majority of the reproducible reports, the cubic phase was achieved only through ion-based techniques, and there is some controversy over the exact growth mechanisms involved. There is considerable evidence that suggests that type conversion plays some role in achieving the cubic phase, and that it results from compressive strain induced by ion bombardment. This paper describes the modification of an inductively coupled RF nitrogen plasma source originally designed for use in molecular beam epitaxy systems to fit a pulsed laser deposition chamber. In contrast to the many types of nitrogen ion sources available, this particular type of plasma source produces a bright orange glow suggestive of efficient production of atomic nitrogen, as opposed to violet-colored plasmas which are indicative of ionized molecular nitrogen. Using the 266 nm line of a frequency-quadrupled Nd:YAG laser, films have been grown using either hexagonal boron nitride or elemental boron targets. Films of up to 1000 Å thickness have been deposited on a variety of materials, including one-inch square polycrystalline diamond substrates. Results of SEM, ESCA, Raman and FTIR spectroscopy will be presented.
2:30 PM		B6-4 Enhancement of the Properties of Pulsed Laser Deposited Carbon Nitride by the Synchronization of Laser and N₂ Gas Jet Pulses. I. Alexandrou (University of Liverpool, United Kingdom); I. Zergioti (Institute of Electronic Structure and Laser, Greece); G.A.J. Amaratunga, C.J. Kiely (University of Liverpool, United Kingdom); C. Fotakis (Institute of Electronic Structure and Laser, Greece) The improvement of properties in carbon nitride films deposited by the pulsed laser ablation method is reported. During laser ablation the carbon species emitted from the target are very energetic, a factor anticipated to help in a high percentage of nitrogen bonded strongly to carbon. However in reactive ablation the carbon species are slowed down because of the gas, setting therefore an upper limit to the working pressure and in most cases to the reactive gas percentage in the film. Additionally for metastable phase (e.g β-C₃N₄) formation in the films energetic deposition species are needed, which is difficult to achieve in conventional reactive pulsed laser deposition. In this paper we report a method for obtaining a high nitrogen content in the films while keeping the working pressure low enough for the depositing species to have relatively high kinetic energies. The method is based on the synchronized pulsing of a reactive gas jet with the laser pulse. Microintendation measurements show that the film hardness increases while the nitrogen concentration monitored by composition analysis remains high. The dependence of film microstructure and the sp³ to sp² ratio on the deposition conditions is studied by HREM and PEELS respectively.
2:50 PM		B6-5 Pulsed Laser Deposition Plume Species Effects Due to Double Excimer Laser Pulses S.J.P. Laube, A.A. Voevodin (Air Force Research Laboratory) Pulsed laser deposition of carbon has been performed with multiple excimer laser pulses. The effects of double pulses on laser induced florescence optical emission spectroscopy was observed. The effects due to varying the time interval between pulses shows an increase in ion density similar to that observed by electrode discharge. The increase in carbon ion density species velocity has been linked to increases in film hardness. This increase leads to diamondlike carbon films that exhibit higher hardness and more consistent structures.
3:10 PM		B6-6 Fluorine-doping of ZnO Films in Reactive Dual-laser Ablation S. Witanachchi, A.M Miyawa, Y. Ying, J. Cuff, P. Mukherjee (University of South Florida) Electrical and optical properties of ZnO films deposited by laser ablation depend on the ambient oxygen pressure. In vacuum and pressures below about 20 mTorr of oxygen the deposited films contain carrier densities of 3-5x10¹⁹ cm ^-3 and show high free carrier absorption at long wavelengths. Deposition of ZnO films by dual-laser ablation in an SF₆/O₂ ambient facilitated fluorine-doping of the films. The high density of hot electrons produced in the dual-laser process aids the dissociative electron attachment reaction of SF₆ leading to the generation of fluorine free radicals. The fluorine-doped films grown on glass substrates by this method are highly c-axis oriented and show a considerable improvement in the electron mobility and optical absorption properties. As a result of the enhanced radial expansion of the plume, the areas of the dual-laser deposited films are about a factor of four larger than the single laser ablated films. Optical emission spectroscopy of the laser-generated plume has been used to study the dynamics of the fluorine radicals in the gas phase. A correlation between the emission results and the electrical properties of the deposited films will be discussed.
3:30 PM	Invited	B6-7 High-rate Laser Deposition of Ceramic Coatings B. Schultrich, T. Wittke (Fraunhofer Institute - IWS, Germany) Starting with the current state of pulse laser deposition of thin films, the pulse laser deposition (PLD) is compared with conventional PVD processes from the technological point of view. The technological potential of PLD and its present restrictions (especially concerning the deposition rate) are discussed. Various attempts to overcome these difficulties by special laser sources or by combining the laser with other techniques are presented. A very promising way towards high rate laser deposition was opened by the development of the pulse laser cluster deposition (PLCD). By adapting technological muIti-kW-CO2 pulse laser for thin film deposition, mean deposition rates of 100 nm/s and more have been realized. The process is based on atomizing shallow laser induced melt pools by the recoil action of the vapor pulse and laser heating of the emitted microdroplets. Due to the high droplet temperature of some thousand K in combination with the accompanying vapor completely dense ceramic films are deposited. The influence of deposition parameters on film structure and surface morphology is discussed.
4:10 PM		B6-9 Investigation Into 3-Dimensional Laser Processing Of Tribological Coatings A.A. Voevodin (Air Force Research Laboratory); J. Bultman (University of Dayton Research Institute); J.S. Zabinski (Air Force Research Laboratory) Tribological coating design has evolved from single layer/single phase to multilayer and composite architectures. The early single-layer coatings were not efficient for arresting cracks, distributing loads, relaxing stress, and preventing adhesive failures. Second generation coatings with multilayer, gradient, and composite architectures added another dimension to the coating design and permitted much better accommodation of stresses and crack braking. Precise laser beam processing provides further evolution of coating designs to three dimensions (3-D), where lateral property variation is added to cross-thickness property variation. The 3-D laser processing can considerable improve tribological characteristics of the coatings, by creating solid lubricant replenishment inside contact zones on hard coatings. A functionally gradient Ti-TiC-TiC/diamond-like carbon coating with an upper layer of a tough nanocrystalline/amorphous composite was used for load supporting, crack preventing, and stress equalizing. This coating was produced by magnetron-assisted pulsed UV laser ablation and was then processed with IR laser irradiation to form grooved tracks along wear paths, which were filled with MoS₂. This provided a solid lubricant reservoir in the lateral dimension of the coating. The coating was tested in long duration (up to 1 million cycles) sliding tests at fixed and cycled humidity. The 3-D coating exhibited environmental adaptation with friction coefficients of 0.15 in humid air and 0.02 in dry nitrogen. The coating wear life was at least one order of magnitude longer in comparison to that for a hard gradient coating with a top layer of MoS₂ without 3-D laser processing. The impact of 3-D laser processing on the coating tribological properties is discussed.
4:30 PM		B6-10 High Wear Resistant Surface Coatings on Ti-Alloys Prepared by Combination of Laser-assisted Gas-alloying and Arc Deposition H.-J. Scheibe, B. Brenner, S. Bonss, H. Ziegele (Fraunhofer Institute - IWS, Germany); R. Franke (IMA-Dresden, Germany) The Ti6Al4V-alloy has a very high specific strength, good biocompatibility, high corrosion resistance and is the most frequently used Ti-alloy in aeronautics, medicine and chemical industry. A serious problem of applying is their low resistance against sliding and abrasive wear. To overcome this problem a new technology is presented. On a laser gas alloyed surface a superhard amorphous carbon film is deposited by the Laser-Arc. A high power laser is applied for the gas-alloying process to create a TiN containing surface layer with a thickness of about 1 mm and a hardness of about 700 HV0.1. This new technology guarantees total exclution of oxygen and a constant partial pressure of Nitrogen using a mixture of Nitrogen and Argon in a new developed inert gas bell instead of a vacuum chamber. After mechanical finishing of the functional surface a superhard carbon coating is deposited by the Laser-Arc. The thickness of the film is about 400 nm and its hardness is in the order of 50 GPa. Results of different tribological tests demonstrate the excellent friction behavior and wear resistance of this coating combination even under unlubricated test conditions and high contact loads.
4:50 PM		B6-11 Low Temperature Pulsed Laser Deposition of Titanium Carbide Thin Films on Bearing Steels G. Radhakrishnan, P.M. Adams (The Aerospace Corporation) Thin films of titanium carbide have been successfully deposited on steel substrates at low temperatures using the technique of pulsed laser deposition. The low temperature process has been aimed at avoiding any thermal damage to heat sensitive bearing steels used in space applications. The morphology, composition, and crystallinity of these films have been examined. Hardness measurements have been made using a nano-indenter, and the performance of these films in a vacuum wear-tester have been evaluated. The deposition process has been optimized with particular focus on improving the critical tribological properties of these films. A significant improvement in film properties has been achieved. The specifics of the deposition of high-quality TiC films and their performance will be discussed.
5:10 PM		B6-12 The Effects of Substrate Properties on Metal Deposition from Liquid Media by Laser Chemical Vapor Deposition (LCVD) B.S. Park, A. Muyshondt, A.P. Malshe, W.D. Brown (University of Arkansas) Laser chemical vapor deposition (LCVD) of metallic films from liquid media is known to be a simple and cost effective micro-patterning technique. LCVD technology is potentially attractive for electronic packaging applications, multi-chip modular interconnection and repair. Chen et. al.¹, have deposited aluminum lines on silicon substrates and Gupta et. al.², have demonstrated copper lines on silicon and quartz substrate by LCVD. Thus, it seems that there is no limitation on materials processed using LCVD. However, Mini et. al.³, have reported that, in the case of LCVD, increasing the substrate temperature and having an electrically conducting surface are the essential conditions for metal deposition. They deposited conducting layers of copper and chromium on the substrate using the electron beam prior to the LCVD of copper. The effects of thermal and electrical conductivity of the substrate on LCVD in liquid media, however, has not been studied systematically. In this paper, using the LCVD technique, it has been demonstrated that gold deposition is possible using an Ar+ laser at 700 mW in a gold solution. The results were obtained on different substrates such as aluminum, silicon, and diamond. It was observed that the higher the thermal conductivity of the substrate, the higher is the laser power density required to initiate metal deposition. Thus, for silicon and diamond substrates having thermal conductivities of 1.49 and 12 W/cm-K, the required power densities for successful gold deposition are 180 and 500 W/mm², respectively. However, it is also shown that for a laser power density of less than 500 W/mm², gold deposition is impossible even if an electrically conductive layer exists on the diamond substrate. ¹ Q. Chen, S. D. Allen, Mat. Res. Symp. Proc. Vol 397 (1996) 636 ² A. Gupta, R. Jagannathan, Appl. Phys. Lett. 51(26) (1987) 2254 ³ L. Mini, C. Giaconia, C. Arnone, Appl. Phys. Lett. 64(25) (1994) 3404