Papers

ICMCTF1998 Session B4: Emerging Technologies & Critical Issues in Vapor Deposition

Thursday, April 30, 1998 8:30 AM in Room Golden West

Thursday Morning

Start	Invited?	Item
8:30 AM		B4-1 High-rate Reactive Sputtering Comes of Age. R.P. Howson (Loughborough University, U.K.) Recent advances in the techniques of the reactive PVD creation of compound thin films have utilised plasmas to provide activation of the process and of the structure. This work has led to processes being universally available for the preparation of many materials with outstanding properties at economical rates for commercial applications. It is perhaps not too widely appreciated how these new techniques have led to processes which are straightforward and simple to apply in large scale systems. The outstanding advantages of a vacuum process lie in the ability to use ions, which can be provided with much more energy than can be done by simply raising the temperature. Ions can be given energy easily by the application of an electric field, providing that the pressure of the residual atmosphere is sufficiently low to allow a mean-free-path of sufficient length so that high energy is not dissipated in gas-phase collisions. This energy can be directed to the surface of a growing thin film to give actuation of adhesion, reaction, cleaning and structural effects. Ion assisted processes then are naturally used with physical vapour phase (PVD) techniques. In PVD techniques material is transported to a growing film surface, through the vacuum, after being vaporised by a process such as evaporation from a heated source or by being ejected from a cold target through a collision with a high energy inert gas ion, the process of sputtering. The appearance of low pressure sputtering processes has demonstrated the advantages obtained with a higher energy depositing species, the films are generally more adhesive with a more dense structure. Placing the insulating or isolated substrate in a dense dc plasma results in the surface acquiring the self-bias, that appears on the surface due to the need for electron and ion currents to equalize. This causes the ions to be accelerated through voltages of up to 100 volts into the growing film surface. This plasma has been produced in several ways: it can be created by passing a large current of electrons through a low-pressure inert gas the current required is in the region of 200 A, at about 80 V, which is close to the optimum required for ionization of the Argon gas that is used. The pressure required is below that which would cause significant collisions of evaporating material with the constituents of the residual atmosphere and is most conveniently used with in conjunction with low-voltage, electron-beam guns in evaporation systems. In arc evaporation the high arc current passes through the plasma of the evaporating material creating ions of that rather than any residual atmospere. Re-directing the plasma created in a planar magnetron onto the substrate, by unbalancing the magnetic field which confines it, has proved another effective way of directing a plasma and providing ion-bombardment of the growing film. This plasma has proved to be easily manipulated by small magnetic fields because at the pressures that are used the electron mean-free-path is sufficiently long for electrons to follow field lines without scattering and the ions in the plasma follow these by electro-static attraction. This technique has been used to direct the ionized product of arc evaporation, so as to avoid large particulate contamination, where it is called a filtered arc. A version of an unbalanced post magnetron, with a hot lanthanum hexaboride cathode, has recently combined the techniques to provide a plasma in an electron-beam evaporation system. Such deposition processes can incorporate reactions. Ions in the plasma provide energy which activates reactions as well as growth mechanisms. These reactions occur on surfaces where energy and momentum conservation is provided. Commonly the admission of the reactive gas in a reactive sputtering process is done by control of the flow, with a simple mechanical valve or an electronic feedback device, which can indicate the magnitude of that flow. In high rate reactive sputtering with a planar magnetron consumption of the reactive gas by the process is greater than that being pumped by the system and conditions are often created where the process is unstable, switching between "metallic" and "oxide" sputtering modes without allowing access to the intermediate point, which is the one where the two constituents, the metal from the sputtering process and the gas from the residual atmosphere are supplied in the proportions which are generally required. The sputtering of an oxide covered target is generally not desirable because the sputtering rate of the oxide is much lower than that of the metal and the appearance of an insulating surface on the cathode causes arcs if dc is used and rf power is required. It has been found that fast feedback can be provided by using an observation of the spectral line emission of the gas or the sputtering metal (PEM) to control the admission of the reactive gas or in some cases from the voltage seen on the target when sputtering at constant power. The object of the process is to create films of closely controlled stoichiometry and structure. All these techniques have proved difficult when used for the preparation of insulating materials because the compound is formed in regions of the metal cathode during the balanced process and arcs happen as charge builds up on that surface. Compensation of the static charge that causes these arcs to appear can be achieved through the use of rf power, but this has difficulties which are even more limiting. It has recently been realised that these compound films are very thin, which means that they have a high electrical-capacitance, and much lower frequencies, than those used with thick high-resistivity targets can be used to neutralise the charge build-up. Medium frequencies [20-100 kHz] are used to eliminate this effect. They have the advantage of being as simple to handle as dc, and do not require the load matching and tuning that has to be used with the much higher frequencies of the radio band. A further operation advantage has emerged when the power is fed to two magnetrons, acting as cathode and anode alternately, the operating anode required by a single magnetron can no longer be covered with an insulating film, and long term operation can be maintained without problems. It has further been observed that the plasma produced in such a system is denser than that obtained with dc. An alternative to the simultaneous supply of reactants to the growing film surface is to supply the materials in sequence, with the layers that are formed being of monolayer dimensions to allow easy access of the materials to each other. Such a process has proved to be very effective in certain circumstances. The use of pulsed or medium-frequency ac power to prevent arcing allows systems to operate in a poisoned mode when driven by simple power supplies, making available simple techniques of successive deposition and oxidisation of metals using the same magnetron. We have used such a process which we have called successive pulsed plasma anodisation, anodisation, because the process is driven by the potential appearing on the insulating surface when the substrate is immersed in a plasma directed at it by the unbalanced magnetron source unit. I believe that the existance of these techniques of partial-pressure control, plasma creation and direction, and non-arcing simple power supplies will create a new era in the preparation of thin films for large area, low-cost applications.
9:10 AM		B4-3 Deposition and Optimization of Amorphous Silicon Carbide Thin Film Characteristics for Tribological Applications J.J. Nainaparampil, J.S. Zabinski (Air Force Research Laboratory) Amorphous silicon carbide is a material that is studied as a protective coating and as an electronic material. The significance of this material is that its electrical and mechanical properties can be controlled by varying the carbon and silicon ratio as well as the concentration of doping elements like hydrogen and nitrogen. Typically, substrates are to be heated to very high temperatures so that crystalline SiC is formed. In this work, silicon carbide is formed by simultaneous sputtering of silicon and laser ablation of graphite onto substrates at room temperature. By keeping the sputtering power and the reactive gas pressure at a predetermined optimum value and increasing the laser pulse frequency, the carbon content in the film is varied to select the tribological characteristics. The advantage of this method lies in the individual selection of species accelerated to optimum energy which permits the formation of selected phases in the film. Films are formed on M50 steel substrates at biasing varied from -100 to -300 Volts. The bias voltage causes variations in the deposition rates of carbon and silicon. The hardness of these films lies between 15 and 25 GPa with critical load varying up to 30 N from scratch measurements. Chemical bonding measured to be SiC like is correlated to microstructure and mechanical properties. Special emphasis is given to film stoichiometry control and Si/C composition ratio to achieve optimum tribological properties.
9:30 AM		B4-4 Engineered Thin Film Microstructures using Glancing Angle Deposition (GLAD) J.C. Sit, K. Robbie, M.J. Brett (University of Alberta, Canada) A new vapour deposition process called Glancing Angle Deposition (GLAD)¹ has been developed which combines high incidence angle deposition with computer controlled substrate motion to control film microstructure evolution on the 10 nm size scale. The GLAD technique has been implemented with both evaporation and, recently for the first time, sputtering techniques to produce films of a large selection of materials including ZrO₂, Ti, Cr, MgF₂, and Si. Coordination of deposition and substrate motion enables fabrication of many different microstructures including helical structures (i.e. chiral films or beds of "microsprings") and zig-zag or chevron structures. These novel shapes may be grown coincident with film porosities tunable from 10 to 94%. As an example of one such structure, ZrO₂ films have been formed of closely packed parallel helices, with each helix comprised of 4 turns of 200 nm pitch for a total film thickness of 800 nm. Structured GLAD thin films have applications as thermal barrier coatings, porous radiative films, and optical coatings². The GLAD process will be presented, demonstrating the versatility of this technique, including new results with sputtered GLAD and progress in applications of the coatings. ¹K. Robbie, M. J. Brett, Journal of Vacuum Science and Technology A 15(3), 1460-1465 (1997). ²K. Robbie, M. J. Brett, A. Lakhtakia, Nature 384, 616 (1996).
9:50 AM		B4-5 Deposition of Boron Carbide Thin Film by Supersonic Plasma Jet CVD with Secondary Discharge O. Postel, J.V.R. Heberlein (University of minnesota) Boron carbide is of a significant interest for wear resistant coatings where high material strength is required. Nowadays this material is being deposited with expensive techniques only and at very low growth rate, hence reducing the scope of its industrial applications. We have developed a technique to deposit boron carbide films at high growth rates and high hardness. A dc arcjet thruster developed by NASA is used to generate at low pressure -1 to 100 Torr- an argon/hydrogen supersonic plasma jet of moderately high temperature. The reactive chemicals -boron trichloride and methane- are injected into the plasma jet and are fully dissociated and transported in few tenths of microseconds onto a silicon substrate, the temperature of which is accurately controlled. The chemically frozen flow conditions and the strongly compressed boundary layer in front of the substrate are believed to be ideal for thermal plasma chemical vapor deposition. The substrate temperature is varied from 450 to 700 degrees Celsius, the growth rate reaching a maximum in the range 550 to 650 degrees Celsius. A positive bias can be applied to the substrate, drawing an electron current of up to 3 Amperes between the grounded torch and the substrate holder. The resulting secondary discharge allowed a further increase in deposition rate, as well as a reduction in substrate temperature. The resulting films have been characterized with respect to their uniformity, their growth rate, their crystallinity and their stoichiometry. Amorphous films have been deposited at growth rates of up to 80 microns per hour, with a hardness of 28 GPa. Our results indicate that the combination of mass transport with a supersonic jet and continued dissociation using a secondary discharge represent a new process for deposition of hard boron carbide films at high rates which is valuable to industrial production.
10:10 AM		B4-6 Magnetically Enhanced Ionized PVD using ICP. J.H. Joo (Kunsan National University, Korea) High density plasmas, like ICP, ECR, can be used as an assistant plasma source to conventional planar magnetron sputtering machine to deposit various metals and compounds at lower substrate temperature. In this study, Al, Cu and Ag were deposited on glasses using internal ICP assisted magnetron sputtering for use as electrode materials of flat panel displays. For larger and uniform plasma source generation, two types of external magnetic fields were tried and various effects on plasma uniformity and preferred orientation of deposited films as well as fundamental plasma diagnostics will be addressed. And this plasma source was used to make mass modulated ion beam by modified quadrupole mass filter system. Technical aspects of construction and some fundamental results of ion beam scattering experiment will be also addressed.
10:30 AM		B4-7 Anode Mass Loss During Pulsed Air Arc Deposition R.L. Boxman, N. Parkansky, I. Beilis, S. Goldsmith, Y. Rosenberg (Tel Aviv University, Israel) A new criterion, labled, K for the direction of the mass transfer between electrodes and coating formation in Pulsed Air Arc Deposition (PAAD) is proposed. K is the ratio of the cathode critical heat flux (q_c) to the anode critical heat flux (q_a) needed to reach the melting or boiling temperatures of the electrodes. A model is proposed in which the heat flux from the discharge is sufficient to heat up both electrodes to a critical temperature defined by either the melting or boiling points of the electrode. The model considers different but comparable anode and cathode heat fluxes under PAAD conditions. When K = q_c/q_a>1 the coating is formed on the cathode. If K<1 the coating is formed on the anode. With K*1 it is possible to obtain coatings on both electrodes. The criterion K depends only on the thermo-physical material constants of the electrode materials. Mass transfer rates were measured under the following PAAD conditions: pulse duration - 150 micros, repetition rate - 100 Hz , and current amplitude - 75-500 A. Calculated K values were used to explain observed mass transfer between electrodes during PAAD deposition of Al, Ag, Zn, and WC on steel cathodes.
10:50 AM		B4-8 Deposition of Hard Crystalline Al₂ O₃ Coatings by Bipolar Pulsed D.C. PACVD Ch. Taeschner (Institute for Solid State and Materials Research Dresden, Germany); B. Ljungberg, V. Alfredsson (AB Sandvik Coromant, Sweden); I. Endler, A. Leonhardt (Institute for Solid State and Materials Research Dresden, Germany) A bipolar pulsed D.C. PACVD technique was used to deposit crystalline Al₂O₃ coatings onto WC/Co cemented carbide cutting inserts or steel substrates using a AlCl₃, H₂, Ar, O₂ gas mixture. Different deposition conditions were tested especially substrate temperatures between 500 oC and 700 oC and gas mixture ratios of O/AlCl₃ in the range from 3 to 10. At T_S = 600 and 650 oC crystalline coatings of the g-Al₂O₃ phase was obtained with a best crystallinity for the O/Al ratio = 4 and 6. At a substrate temperature of 700 oC the a-Al₂O₃ phase or a mixture of g-Al₂O₃ and a-Al₂O₃ phases were formed. The hardness HV[0.02] of the coatings was found to be in the range 20 - 22 GPa. All deposited coatings appeared transparent with smooth surfaces. The wear properties of the different PACVD-coatings were evaluated and compared with a thermal CVD k-Al₂O₃ coating in a machine cutting test in a ball bearing steel. From the machine test it could be concluded that the PACVD-Al₂O₃ coatings in this particular ball bearing steel showed wear properties very similar to the thermal CVD k-Al₂O₃ coatings.
11:10 AM		B4-9 Titanium Nitrade Hard Coatings Deposited on Steel by Pulsed Plasma A. Devia, P. Arango, E. Restrepo, M. Arroyave (Universidad Nacional de Colombia Sede Manizales, Colombia); H. Bruzzone (Universidad de Buenos Aires-CONICET, Argentina); J.L. Heiras (Universidad Nacional Autònoma de Mèxico) We have deposited hard coatings of TiN on 304 Stainless Steel samples by the technique of arc pulsed plasma discharge (300 A peak current, 35 ms half width time duration). Distance between electrodes have been changed from 3 mm up to 10 mm and the working (Nitrogen) pressure ranged between .5 mbar to 5 mbar. Good quality coatings as thick as one micron have been deposited in four discharges. This is a very rapid way to obtain hard coatings since each discharge take only a few tens of ms and the time between discharges, necessary to charge up the bank of capacitors, takes about 5 min. and could be further reduced The structure and composition of the coatings have been analyzed by Electron Microscopies (TEM, SEM and Auger) and X-ray Diffraction. Studies of hardness and resistance to corrosion have also been performed. Both the deposition technique and the characteristics of the coatings will be discussed and compared with the results obtained in coatings prepared by other techniques.
11:30 AM		B4-10 The Simultaneous Influence of Process Parameters on the Properties of the Arc-Ion-Plated ZrN Coatings B.F. Chen (Materials Research Laboratories and National Tsin Hua University, ROC); W.C. Lih (Materials Research Laboratories, ROC); J. Hwang (National Tsin Hua University, ROC); C. Law (Pratt & Whitney Aircraft Group) An extensive investigation on the simultaneous influence of four process parameters on the structures and properties of arc-ion-plated ZrN coatings was performed. A series of experiment that incorporated four process parameters, i.e. nitrogen partial pressure, substrate bias voltage, substrate initial temperature and undercoating time, coupled with three levels of variation on these parameters was carried out in accordance with the array of the orthogonal Table L₉. The structures of the coatings were characterized by OM, SEM and XRD techniques, and the mechanical properties including adhesion strength, microhardness, rolling contact fatigue life and wear were subsequently evaluated. The data of microhardness and deposition rate were analyzed according to the Taguchi’s method for analysis of means (ANOM). The outcome of the analysis reveals a set of optimum process parameters of nitrogen partial pressure = 3.0 p Pa, bias voltage = 150 V, initial substrate temp = 100 deg. C and undercoating time = 10 min that is supposed to produce a coating with maximum hardness along with heighest deposition rate. The analysis also reveals that the deposition rate was nearly proportional to the partial pressure of nitrogen, f rom 0.5 ~ 3.0 Pa. Furthermore, the results of other tests performed are summarized as follows. 1) The adhesion strengths (Sabastian pull test) of all the nine sets of specimen tested surpass approximately 10,260 psi (70.743 MPa), 2) The results of the XRD performed for all the 9 sets of specimen show that all the specimens contained Zr and ZrO₂ as minor phases, in addition to the major phase, ZrN. 3) The rolling contact fatigue properties derived from specimens pertaining to two representative deposition conditions are found to have no improvement over that of the uncoated substrate material (M50 high speed steel)
11:50 AM		B4-11 Pulsed High Current Electron/Ion/Cluster Source for Deposition of Films and Coatings with High Adhesion S.A. Korenev (New Jersey Institute of Technology); M.V. Altaisky (Joint Institute for Nucelar Research, Russia); A.J. Perry (A.I.M.S. Marketing); A. Kalmykov (Joint Institute for Nuclear Research, Russia) In previous work we presented an intense electron-ion source based on explosive emission for surface modification and coating deposition ¹ and demonstrated its application to depositing different coatings and the effectiveness of post-treatment in the same unit ¹ ². A compact source of this type has now been built at NJIT and is now operational with the additional capability of producing clusters in the ion beam mode. The main operating parameters are: accelerating voltage 100-300kV; electron beam current 500-2000A; ion beam current for many materials 10-150A; pulse length 300-500nsec; repetition rate 1-50Hz. In the present work, three aspects are presented: the fractal nature of the surfaces after intense electron or ion bombardment, the production of clusters in comparison with other methods using an additional ionizer, and, the production of well adhering thick coatings on substrates with different coefficients of linear thermal expansion using beam mixing of the coating and substrate. ¹ S.A. Korenev and A.J. Perry, Vacuum, 47 (1996) 1089. ² S.A. Korenev, B.F. Coll and A.J. Perry, Surf. Coat. Technol., 86-87 (1996) 292.