Papers

ICMCTF2008 Session G3: Atmospheric Plasma, Hollow Cathode, and Hybrid Plasma Processing

Tuesday, April 29, 2008 8:00 AM in Room Sunset

Tuesday Morning

Start	Invited?	Item
8:00 AM		G3-1 Hollow Cathode Arc Activated Reactive High Rate Deposition - State of the Art and New Developments H. Morgner (Fraunhofer-Institut Elektronenstrahl- und Plasmatechnik, Germany); Chr. Metzner (Fraunhofer Institut Elektronenstrahl- und Plasmatechnik, Germany) Hollow cathode (HC) arc discharge plasma source is well known for high density plasma and high electron temperature since long time. Feature of HC plasma is high portion of directed electrons called as Low Voltage Electron Beam (LVEB). Typically, application of HC plasma source in vacuum coating technology has utilized both, LVEB electrons for heat generation in crucible and plasma for vapor activation. Experiments to use the HC arc discharge plasma exclusively for vapor activation in high rate deposition process has been performed in a roll coater machine in the early nineties. The activation of very dense vapor by HC arc discharge has enabled the condensation of dense layers on plastic film in a reactive process. Relatively high self bias potential in HC plasma allows deposition of non conductive layers and coating on insulating substrates. Different evaporator configurations with magnetic plasma guidance have been investigated for coating on plastic web and metal strip or sheets. Al₂O₃ and SiO_x layers could be deposited on plastic substrates with dense amorphous, glassy structure. SiO_x layers with nano crystalline Si on metal substrates exhibit high hardness up to 15 GPa. Deposition rates range between 30 - 600 nm/s. HC sources are arranged side by side close to the substrate in these configurations. The disturbance of vapor density distribution by localized plasma sources results in layer thickness uniformity deviations of about 10%. Latest HC plasma source development has increased the efficiency enormously. A cathode - ring anode configuration with strong axial magnetic field of a solenoid coil is operated with low working gas flow. It generates LVEB electrons with essential higher energy. This results in two effects. Higher ionization cross section leads to more dense plasma and the maximum of plasma density has found in 0.5 - 1 m distance from the plasma source. With this plasma source the door is open for high rate deposition of oxide layers with optical quality.
8:40 AM		G3-3 Metallic and Reactive Sputter Deposition Using a Ti- Hollow Cathode Discharge V.H. Baggio-Scheid (Genneral-Command of Aerospace Technology, Brazil) In this work deposition processes of metallic titanium and TiN coatings using a Ti-hollow cathode discharge were investigated. A cylindrical cathode with an internal diameter of 17 mm and a length of 70 mm was used in the experiments. Typical discharge parameters during depositions were pressures between 50 and 200 Pa and current densities up to 10 mA cm^-2. The discharge was characterized by means of emission spectroscopy and a thermal probe. The total energy influx from the plasma towards the probe surface was measured in the metallic and reactive sputtering mode. Energy fluxes higher than 0.1 J cm^-2 s^-1 were obtained. The coatings were deposited onto cylindrical high speed steel substrates inserted inside the cathode at different discharge parameters. The properties of the films were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and by microhardness and wear testing. Influences of the process parameters on the coating properties are analyzed.
9:00 AM		G3-4 Optimal Dimensions of the Atmospheric Hollow Cathodes L. Bárdos, D. Söderström, H. Baránková (Uppsala University, Sweden) The operation principle of hollow cathodes is based on electron exchange between opposite space-charge sheaths at the opposite walls in the cathode, well known as the hollow cathode effect. To generate such a regime it is necessary to create suitable geometry inside the cathode, where the widths of opposite sheaths don’t coalesce and enable oscillations of electrons between sheaths with consequent ionizations leading to high plasma densities characteristic for hollow cathodes. However, a decreasing sheath thickness with an increasing gas pressure requires smaller distances between opposite walls for the ignition and maintaining the hollow cathode discharge. Using a special hollow electrode construction with an adjustable distance between the opposite walls the starting and operation conditions for the hollow cathode plasma at atmospheric pressure were studied experimentally. Both, the atomic gases and molecular nitrogen gas were used and compared in the radio frequency and pulsed DC powered hollow cathodes to find an optimal distance between cathode walls.
9:20 AM		G3-5 Chemical and Morphological Modification of Polymers Under a Helium-Oxygen Dielectric Barrier Discharge D.D. Pappas, A.A. Bujanda, J.A. Orlicki, R.E. Jensen (United States Army Research Laboratory) In this work, we present results from the surface modification of various polymer films due to the exposure to a helium-oxygen dielectric barrier discharge (DBD), operating under atmospheric pressure. The polymer films studied were: ultra high molecular weight polyethylene, low density polyethylene, polyamide, polytetrafluoroethylene and polyimide. Contact angle, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) data are presented and the dependence of the surface chemical and topological changes on the discharge parameters such as treatment time, interelectrode gap and gas composition is investigated. Experimental results reveal improved hydrophilicity of the plasma exposed polymers and an increase of their surface energy. This can be attributed to the presence of oxygen containing groups, grafted on the surface during plasma treatment, as confirmed by XPS analysis. SEM data show the appearance of micro depressions, the size of which depends on the chemical structure and the treatment time, suggesting that mild etching occurs in a controlled fashion. The above observations suggest that the plasma treatment leads to an improvement of the surface properties such as wettability and adhesion and can be therefore used in a variety of applications. Most importantly, this uniform modification occurs within a few seconds of exposure, time comparable to continuous on-line industrial processing.
9:40 AM		G3-6 Deposition of Transparent Conductive Indium and Tin Oxides by Atmospheric Pressure Plasma R.A. Sailer, A. Wagner, N. Klaverkamp, C. Schmit, D.L. Schulz (North Dakota State University) Transparent conductive oxides (TCO) have found broad application over the past few decades in photovoltaics, flat panel displays, and other electronic applications. Traditionally, TCO deposition has been performed at moderate temperature using vacuum-based growth systems which is not readily applicable to deposition on inexpensive polymer substrates (e.g., Mylar) nor roll-to-roll manufacturing. The goal of this project has been to demonstrate a system capable of depositing durable conductive transparent oxides at relatively low temperature and atmospheric pressure. Sn(II) and In(III) beta-diketonate complexes were employed as the solid precursor sources with He carrier gas, O₂ reactant gas and growth temperatures from ambient to 300°C. Thickness measurements by ellipsometry and x-ray reflectivity give film thicknesses of 30-70 nm over an area of 30 cm² after a 20 minute growth cycle. The as-deposited films exhibit light transmittance in excess of 90% over the visible spectrum while maintaining resistivities on the order of 10^-2 ohm-cm . Improved electrical properties (i.e., ρ ~ 10^-3 ohm-cm) were observed after thermal treatment (T ~300°C) in a controlled gas ambient tube furnace. Additional commercial and novel precursors and doping methods are also under investigation and will be reported in our presentation.
10:00 AM		G3-7 Effects of the DC Bias in a Molten Silicon Bath on its Purification by a RF Thermal Plasma. M. Benmansour, S. Rousseau (Université Pierre et Marie Curie, France); D. Morvan (Laboratoire de Génie des Procédés Plasmas et Traitements de Surfaces, France); J. Amouroux (Université Pierre et Marie Curie, France) A RF thermal Plasma process was developed for the purification of metallurgical grade silicon. In this process, the elimination of impurities is due to chemical reactions at the interface between the plasma and the molten silicon. These reactions lead to the formation of high volatile molecules which are eliminated by evaporation phenomena. In this work, we combined a DC bias of the liquid silicon to the chemical effect of the plasma treatment as a new parameter. The aim is to increase the impurities extraction kinetics by inducing electrochemical reactions at the plasma - liquid silicon interface. The silicon sample, deposited on a graphite substrate, is firstly molten by the plasma source, and secondly polarized by the dedicated circuit, whose voltage ranges from -10V to 120 V. This system consisted of a DC supply connected to the graphite electrode via tungsten wire and a RF filter. The effect of the bias voltage on the impurities extraction was followed using two kinds of diagnostic techniques: On-line by Optical Emission Spectroscopy, and ex-situ by LIBS (Laser Induced Breakdown Spectroscopy) and ICP. On line measurement showed that positive bias conditions enhance metallic impurities (Ca, Fe, and Al) evaporation. These observations were confirmed by ex-situ analyses (LIBS and ICP), which show an increase by a factor 10 in the refining effectiveness when a strong positive bias (110V) is applied. On the contrary, a negative bias (-10V) has a limiting effect on the evaporation of metallic species. Nevertheless, a negative bias enhances the hydrogenation of the silicon when a small volume of hydrogen is mixed with argon plasma flows. All these results can be due to a modification of the transport properties of charged particles in the boundary layer and / or to a change of the electrochemical reaction rates at the interface plasma / liquid silicon.
10:20 AM		G3-8 Controlling Deposiion Rates in an Atmospheric Pressure Plasma System J.D. Albaugh, L. O'Neill, C. O'Sullivan (Dow Corning) Activating, coating and curing processes are accomplished in-situ on rigid and flexible substrates using an atmospheric pressure plasma system. Liquid based precursors are injected into an ambient plasma jet whereby machine parameters allow for film thickness control. In addition, recent experiments show that this technology produces coatings durable enough for textile applications, and can also be used with sensitive electronic devices with no detrimental effects. This paper summarizes the effect of varying process parameters on the deposition rate of a polymeric coating using a plasma jet system. A series of designed experiments were undertaken to monitor the deposition rate and coating chemistry of both fluorocarbon (heptadecafluorodecyl acrylate) and siloxane (Z-TOMCATS) films. Using a combination of ellipsometry, FT-IR, XPS and surface energy measurements, it is clearly shown that highly functional coatings can be deposited using this system. It was found that the plasma power, precursor flow rate and gap height between the substrate and the plasma outlet are significant factors which affect the deposition rate and the chemistry of the coatings.
10:40 AM		G3-9 Synthesis of Amorphous Carbon Film on Polymer Substrates Under Atmospheric Pressure H. Kodama, K. Takano, S. Suemitsu (Kanagawa Academy of Science and Technology, Japan); A. Hotta, T. Suzuki (Keio University, Japan) Amorphous carbon film (a-C:H), well known as diamond-like carbon (DLC), is expected to use in many fields because of its unique properties. It is now practically used for polyethylene terephthalate (PET) bottles for high gas barrier film. DLC is generally synthesized under low pressure but conventional vacuum technique costs much for cheap products such as plastic containers. Therefore, we started to study cost-effective atmospheric pressure plasma CVD technique. In our previous study, we successfully synthesized high gas barrier a-C:H films on PET substrate which can be applied for food and beverage container. For further application, it is necessary to synthesize films not only PET but also other plastic films such as polyethylene or polypropylene. In this study, we synthesize a-C:H films on conventional polymer substrates and evaluated the gas barrier properties. Furthermore, we analyzed structure of the films and compared with general DLC synthesized under low pressure.
11:00 AM		G3-10 Deposition of Functional Coatings Using an Atmospheric Pressure Dielectric Barrier Discharge A.J. Beck (University of Sheffield, United Kingdom); R.D. Short (University of South Australia); A. Matthews (University of Sheffield, United Kingdom) A parallel plate dielectric barrier glow discharge operating at around atmospheric pressure has been used to deposit functional coatings onto steel substrates. The plasma apparatus has a mixing chamber where the volatile organic precursor is introduced along with helium. The helium is required to sustain a diffuse and even plasma at atmospheric pressure. The laboratory scale apparatus can coat thin (< 3 mm) sheets of the substrate material. Precursors such as acrylic acid and octamethylcyclotetrasiloxane (OMCTS) have been studied. X-ray photoelectron spectroscopy (XPS) has been used to characterise the surface chemistry of the coatings. Sessile drop contact angles with water have been measured to ascertain how wettable the surfaces are. XPS has shown that using suitable plasma conditions, the chemical composition of the precursor can be retained in the coating. This shows that under these conditions the monomer is only slightly fragmented during the plasma process. For example, coatings prepared using acrylic acid can be tailored to retain much of the carboxylic acid. These functional groups are useful for promoting adhesion or for additional chemical modification of the surface. In addition, the surfaces with high concentrations of carboxylic acid groups have low (<20 degrees) contact angles with water so are highly wettable. Coatings can be prepared that have similar elemental concentrations (C, Si and O) to the OMCTS. The plasma conditions can be varied to produce both hydrophilic and hydrophobic coatings.
11:20 AM		G3-11 Atmospheric Pressure Plasma CVD of Silicon Nitride Films for Solar Cells C.C. Amato-Wierda, S. Hu (University of New Hampshire); P.S. Raghavan, C. Chartier, Y.P. Wan, C Khattak (GT Solar, Inc. Merrimack, NH) This paper will present our efforts to design and develop an atmospheric pressure plasma chemical vapor deposition technique for producing hydrogenated silicon nitride thin films for solar cell applications. The silicon nitride serves as an anti-reflection and passivation layer, and is currently deposited by various low pressure plasma techniques which are batch-type, require cumbersome vacuum equipment, and limit throughput. A simpler, continuous, fast throughput, non-vacuum deposition process is critical for the solar cell industry to meet upcoming high growth and low cost goals. A large reactor that can handle 4 inch square, multi-crystalline silicon solar cell wafers has been designed and fabricated. Atmospheric pressure plasma is achieved using a dielectric barrier discharge. Preliminary tests have been performed using this reactor, and the system demonstrates a stable and uniform plasma with different gas mixtures, including argon, nitrogen, ammonia and hydrogen and silane. Silicon nitride films are being deposited under various reaction parameters, including frequency, relative flow rates of silane and ammonia, power, and gap distance between electrodes. The silicon nitride thin films properties will be described based x-ray photoelectron spectroscopy, scanning electron microscopy, and infrared spectroscopy.
11:40 AM		G3-12 Atmospheric-Pressure Plasma Pretreatment for Direct Bonding of Silicon and Glass Wafers at Low Temperatures M. Eichler (Fraunhofer Institute for Surface Engineering and Thin Films IST, Germany); B. Michel (Technical University of Braunschweig, Germany); M. Thomas (Fraunhofer Institute for Surface Engineering and Thin Films IST, Germany); M. Gabriel (Suss MicroTec, Germany); C.-P. Klages (Fraunhofer Institute for Surface Engineering and Thin Films IST, Germany) Wafer-level packaging with silicon and glass wafers is state of the art. Especially direct bonding is appropriate, since no intermediate layer is needed. In order to achieve high bond strength, a void free interface, and high yield, the required annealing temperatures are higher than 900 °C. For temperature sensitive materials and bonds between materials with different expansion coefficient, there is a strong demand for low-temperature bonding processes. This paper presents atmospheric-pressure plasma pretreatment processes, which allow the reduction of annealing temperatures down to 200 °C. Measured bond strengths after pretretament and low temperature annealing are compareable to conventionally bonded wafer pairs annealed at 1000 °C. Furthermore bond defects at the interface can be avoided. The suitability for industrial production was shown by SUSS MicoTec with the successful integration of this new process into a commercial bond cluster. Main advantage of the activation at atmospheric-pressure compared to low-pressure is, that there is no vacuum equipment necessary and treatment takes only a few seconds. Therefore the new process is more cost efficient. Because of the short free path length, plasmas at 1 bar offer the unique opportunity to realize patterned surface modifications in a very easy way by localizing the discharge to selected substrate regions. Plasma Printing and Local Plasma Treatment processes based on this principle are presently under development at Fraunhofer IST. These new methods can be used to either improve bond strength in bond regions or prevent sticking between touching surfaces like membranes, valves and inert masses.