Surface Engineering of Metals - Principles, Equipment and Technologies Part 19 pdf

The mechanism of formation of crystallization nuclei in ion plating is, of course, different from that in classical vapour deposition in vacuum or in an atmosphere of non-ionized gas. High energy particles, i.e. ions and at- oms, neutralized in the discharge reaction, are implanted shallow in the substrate. Particles of lower energy, i.e., neutral atoms which did not re- ceive energy directly from the electric field during any phase of its move- ment in the direction of the substrate, are subject to the same mechanisms of nucleation as in vapour deposition. However, in the case of ion plating, the stream of vapours moving through the plasma zone receives energy through collisions with plasma particles. As a result, the mean energy of deposited particles rises with a rise in the number of collisions, i.e., with a rise in the distance between particle source and substrate, and this enhances adhe- sion of the layer to the substrate. Due to fairly uniform ion bombardment of the entire substrate, as well as uniform distribution of surface defects gener- ated by this bombardment, the distribution of crystallization nuclei is also more uniform, with smaller nuclei (10 -2 µm ) occurring more densely. A columnar structure is formed on these nuclei and a continuous coating is obtained already at thicknesses as small as 1.3·10 -2 µm, i.e., smaller that in the case of vapour deposition [13, 16-18, 21]. Great variety of modifications of PVD techniques and a lack of uni- form terminology are responsible for the fact that these techniques are named differently by different authors. Later in this chapter, the most important PVD techniques will be dis- cussed, using original terminology. 6.2.3 Discussion of more important PVD techniques 6.2.3.1 Techniques utilizing simultaneous evaporation of substrate from entire liquid surface This group includes those techniques which utilize the vapours of the deposited material (substrate), heated in the evaporator until it melts. Evaporation occurs from the entire liquid surface. The means of heating does not change the principle of the method itself, although sometimes has an effect on the design of the equipment and its service parameters. Most often, electron beam heating is used, less often resistance heating (due to low effectiveness of vapors and difficulty in application to materi- als with a high melting point) and sometimes induction heating [1]. Diagrams of the more important techniques are shown in Fig. 6.8. Activated Reactive Evaporation - ARE. This classical technique, de- scribed by R.F. Bunshah [17, 18], utilizes the electron beam and was first applied in 1963 by D.M. Mattox [16] to evaporate material (Fig. 6.8a). The surface of the molten metal serves two purposes: as a source of vapours and an emitter of electrons. Metal vapours levitating above the molten metal surface are ionized by low energy electrons, emitted also by that surface which serves as a thermal cathode. Into the thus formed plasma a © 1999 by CRC Press LLC different voltage [18-20]. These techniques allow an increase in effectiveness of ionization by over 50%. This technique is offered in equipment manufac- tured by Balzers and by the Institute of Electron Technology from Wroc≈aw Technical University in Poland [21]. Thermo-ionic Arc Evaporation - TAE. This technique was developed in 1977 by E. Moll, working with the Balzers company, and used to depositing TiN coatings [29]. A pot with the metal, constituting the anode, is heated by an electron beam. The electrons are emitted from a thermal cathode. Voltage between the two electrodes is approximately 50 V, while the beam current is approximately 100 A. Ions emitted by the anode are trapped in a magnetic trap, formed by solenoids wound around a vacuum chamber, and are depos- ited on the surface of the load (Fig. 6.8d) [22]. Hot Hollow Cathode Discharge - HCD. In 1968, J.R. Morley proposed the utilization of magnetic deflection of a beam of electrons emitted by a resistance heated hollow cathode in the presence of a neutral gas (e.g., argon) introduced into its interior and the melting, with their aid, of a metal anode, in a water-cooled pot (40 V, 400 A). The evaporated metal is partially ionized during collisions with beam electrons, and reacting with a reactive gas, supplied through additional heads, forms a chemical com- pound, deposited on the negatively biased (approximately 100 V) load [17, 23-25]. This technique features a high degree of plasma ionization (10-50%). This technique was used on a large scale in equipment mar- keted by the Japanese company Ulvac, while in Europe, it was utilized in “Tina” equipment by the once East German manufacturer - VEB Hochvakuum Dresden (Fig. 6.8e). Ionized Cluster Beam Deposition - ICB or ICBD. This technique was developed in 1972 by I. Yamada and T. Takagi from Kyoto University [26, 27] from which come several designs of equipment. It involves melting (by induc- tion or resistance) of a metal inside a pot, adiabatic decompression of the evaporated metal during its flow through a head to a high vacuum zone (133.3·10 -6 Pa), resulting in the partial formation of a beam of atom clus- ters, i.e., conglomerations of 500 to 2000 intercombined atoms. After leaving the pot, these clusters are partially ionized in the ionizer by a lateral electron flux. Usually, up to 40% of the clusters are ionized. Positively charged clus- ters are then accelerated by voltage of approximately 10 kV to a supersonic velocity and directed toward the load. The load is bombarded with clusters that are ionized (with energy of several eV per atom) and non-ionized (ap- proximately 0.1 eV per atom), as well as single atoms and ions. The current density at the load surface is a value resultant from the geometry of the source and electrical parameters and varies from fractions to tens of µA per cm 2 . Usually, reactive gas is introduced into the chamber and then the work- ing pressure in the chamber is higher by 1 to 2 orders of magnitude than in deposition without the gas and a chemical compound is formed at the load surface (Fig. 6.8f). At the moment of striking the load surface the cluster is broken and liberated atoms gain, among others, a transverse component of momentum, conducive to a rise in the density of packing of the coating mate- © 1999 by CRC Press LLC rial. The basic advantage of this technique is the high rate of deposition, ranging from fraction of to several nm per s, which can be attributed to the ratio of cluster mass to the charge, which is greater by several hundred to several thousand times with respect to corresponding values for ions of the given element [28]. 6.2.3.2 Techniques utilizing local evaporation In this group of techniques, the vapour source as a whole has temperature which is too low for thermal evaporation. Evaporation takes place locally, from small zones (usually changing their position on the surface of the source) of several µm to several mm 2 area, and temperature of several thou- sand degrees, evolved as the result of a strong-current electric arc, pulse discharge or subjection to the action of a laser beam. Arc Evaporation or Cathode Spot Arc Evaporation - AE. This tech- nique was developed in the early 1970’s at the Physico-Technical Institute in Kharkov and by way of license and sub-license purchase has been broadly propagated by US companies like Multi-Arc and Vec-Tec System, as well as Plasma und Vakuum Technik from Germany [29-37]. Depending on the size and designation of the equipment, the vapour deposition contains from 1 to 12 sources with cathodes made from the evaporated material. At the surface of the cathode a high current, low pres- sure arc discharge is generated. The current intensity is 35 to 100 A and current density 10 6 to 10 8 A/cm 2 , and the power is usually several kilo- watts. The discharge takes place between the thick, water cooled target and the ring anode which is also water cooled. The main discharge is initiated by an auxiliary anode. The discharge has no fixed spatial character and is localized within the zone of so-called cathode spots which, due to sublima- tion, constitute a source of highly ionized material vapours. The degree of ionization of the plasma flux is 30 to 100% and depends on the type of evaporated material. The direction, size and rate of displacement (reaching 100 m/s) of cathode spots of diameter reaching 100 µm are all controlled with the help of electrostatic screens or electromagnetic systems (Fig. 6.9). The occurrence of multiple ions, their high kinetic energy Fig. 6.9 Schematic diagrams showing techniques of electric arc evaporation with lo- calization of electron spot: a) electrostatically; b) electromagnetically; c) electromag- netically with movable magnetic system; d) electrostatic- electromagnetically. © 1999 by CRC Press LLC (10 to 100 eV), the possibility of ionic cleaning of the substrate and of mak- ing cathodes of different materials in one equipment, combined with the possibility of evaporation in a mixture of reactive gases, all render AE the most often utilized technique [36-38]. One disadvantage of this method is the presence in metallic plasma of drops of evaporated material and their participation in the formation of the coating which is limited by appropri- ate cathode design, controlled by the movement of cathode spots and plasma filtration [39]. Pulsed Plasma Method - PPM. This technique was developed by the M. and A. Soko≈owski husband and wife team at the Institute for Material Engineering of Warsaw University of Technology in the 1970s [40]. It con- sists of evaporation from the solid phase of an electrode, made from coating material and placed centrally in a plasma generator. Evaporation is accom- plished as the result of a strong current (100 kA) pulse discharge of a series of condensers of 1 to 10 kV voltage [41, 42]. At the moment of discharge, a current layer is formed which is displaced in the direction of the outlet from the plasma generator, driven by a magnetomotive force, collects the gas ahead of it (may be reactive) and causes ablation of consecutive ring-shaped fragments of the central electrode. By controlling the shape of the current layer it is possible to influence evaporation of the central electrode and the transportation of consecutive packages of plasma (and its decomposition) in the direction of the load. The time of crystallization from ionized por- tions of metallic vapours (plasma packages) and time of heating of the substrate by plasma at a temperature of approximately 2000 K does not exceed 100 µs when the rate of substrate temperature rise is approximately 10 7 K/s and cooling rate approximately 10 5 K/s, and the interval between two successive pulses is approximately 5 s. These phenomena may be con- trolled with the aid of its own (Fig. 6.10a, b) [43, 44] or external magnetic field (Fig. 6.10c) [45, 46]. Own magnetic field may be made dependent on the application of ferromagnetic materials for external electrodes of the pulse generator (surrounding the central electrode) which causes lamination the plasma flux in the generator zone [47]. In industrial units, more than one plasma generator may be utilized. Such equipment is especially well suited for coating of big loads in the form of tooling. Fig. 6.10 Schematic diagrams showing techniques of pulsed-plasma evaporation: a) with non-standardized own magnetic field; b) with standardized own magnetic field; c) with magnetic field situated externally relative to the generator; d) with external magnetic field and additional power supply to generator by direct current. © 1999 by CRC Press LLC Laser Beam Evaporation - LBE. This technique was developed in the early 1980’s and involves evaporation of material by a pulsed laser beam, focused on the surface of the material. Similarly, material may also be evaporated by a pulsed electron beam. The vapours of the material are ionized in the zone of the laser spot and the generated ions are extracted in the direction of the negatively biased substrate. This technique has not yet widely reached the phase of industrial application. It gives the possibility of obtaining submi- cron coatings of practically any chosen composition: ceramic oxide materi- als, metals, biomaterials, diamond-like carbon, semiconductor superlattices. From 1988 laser beam evaporation techniques are named: Pulsed Laser Depo- sition - PLD techniques [48-50]. 6.2.3.3 Techniques utilizing direct sputtering In these techniques the material constituting the chemical substrate for the coating, in this case called the target, is sputtered by ions of gas, generated in the zone between the plasma and the load. The sputtered atoms pass through the plasma zone where they are ionized and, possibly reacting with ions and atoms of the reactive gas, are deposited in the form of a chemical compound on the load (Figs. 6.11 and 6.12) [1]. Fig. 6.11 Schematic diagrams showing selected techniques of direct sputtering: a) diode; b) triode; c) in hollow cathode; d) cyclotron; e) ion; f) magnetron. Diode Sputtering - DS. This technique takes its roots from the works by W.R. Groove on glow discharge, almost 150 years ago [32, 51]. Diode sputtering, also commonly known as cathode sputtering, occurs as the result of sputtering of the negative electrode (cathode - target) by positive ions of gas, due to the application of high voltage between the electrodes, separated by gas at 1 to 10 Pa pressure (Fig. 6.11a). The load always forms the positive electrode. We distinguish direct current sputtering and alter- © 1999 by CRC Press LLC nating current, radio frequency diode sputtering (RFDS) [52, 53]. Pres- ently, diode sputtering is carried out as a reactive process. Triode Sputtering - TS. This technique consists of the introduction into the system of a third, auxiliary electrode, usually in the form of a thermocathode. This is aimed at forming inside the working chamber of two-zones: ion generation (situated near the cathode) and cathode sput- tering. From the first zone ions are extracted in the direction of the cath- ode to the second zone, in order to sputter the material of the cathode. Atoms (and possibly ions) of the sputtered material are ionized while pass- ing through the plasma zone and as the result of a chemical reaction with the reactive gas, are deposited on the load (Fig. 6.11b). Triode sputtering may be generated by direct or alternating current [54, 55]. Hollow Cathode Sputtering - HCS. In this technique, the cathode target takes the form of a big, cylindrical cavity, which also constitutes a big part of the working chamber of the unit (Fig. 6.11c). Similarly to the F.M. Penning cylindrical cathode, this design forces electron oscillations in the working volume of the cathode, thus allowing the obtaining of a higher degree of plasma ionization. The majority of units is supplied by high frequency alter- nating current and only some units (those employed in initial ion cleaning) are supplied by direct current [56]. Electron Cyclotron Resonance Sputtering - ECRS. A fundamental characteristic of this technique is gradual acceleration of ionizing elec- trons in a portion of the chamber, with the aid of an alternating electric field of constant frequency, in a magnetic field of constant intensity. This takes place until cyclotron resonance is reached where the frequency of the variable component of electron velocity is equal to the frequency of excitation (Fig. 6.11d). Such conditions are assured by the appropriate selection of the value of induction of the magnetic field and of the fre- quency of the high frequency generator. In such conditions, a change in parameters (power, pressure) allows control of the degree of ionization of the gas. This technique is one of the newest and is successfully applied in deposition of diamond coatings [57, 58]. Ion Sputtering - IS, Ion Beam Sputter Deposition or simply Sputter Deposition. The classical form of this technique consists of depositing a coating on the load by sputtering the material of the target by an ion beam generated by an ion source of any design and the reaction of sput- tered atoms with ions from the beam and by ionized atoms (Fig. 6.11e). Modifications of the technique consist of additional introduction of reac- tive gas, the application of two sources of ions (sputtering of the target and ionizing sputtered atoms). The second ion beam may possibly react chemically with the sputtered material [28]. The ion beam may be em- ployed to sputter any material with precise control of composition of the deposited coating [59-61]. Magnetron Sputtering - MS. The beginnings of this technique date back to 1936 when Penning, in an effort to increase plasma concentration of glow discharge, proposed the application of a transverse magnetic field © 1999 by CRC Press LLC are known designs with radiant heating of the load prior to deposition in order to improve the connection between coating and substrate. The shape and size of deposition zones, as well as spatial shaping of plasma, de- pend on the power supplied to the magnetrons, the intensity of the mag- netic field and on gas pressure. The technique of magnetron sputtering is one of the most broadly used techniques (over 20% of all applications) - besides arc sputtering (approximately 25% of all applications) - mainly on account of the high rate of target sputtering (1 to 2 orders of magnitude higher than in cathode sputtering) and the reduced range of operating pressures [1, 67, 72-77]. 6.2.3.4 Techniques utilizing deposition from ion beams In this group of techniques, the deposited material, constituting the sub- strate of the coating, is initially evaporated or sputtered in any preferred way and next ionized, usually outside of the deposition zone. Ions of the material are formed into a flux of low energy, lower than that required for implantation. In the vicinity of the surface or on the load surface, chemical reactions take place between ions and atoms of the reactive gas supplied to the chamber and ions of the beam material, as a result of which, a coating crystallizes on the surface of the load (Fig. 6.13). The potential of the load is negative [1]. Fig. 6.13 Schematic diagrams showing techniques of deposition from ion beams: a) simple deposition; b) self-mixing; c) ion mixing with sputtering of target with nega- tive potential and of substrate by ion beam; d) ion mixing with sputtering of target by ion beam; e) ion mixing with cathode sputtering; f) ion mixing with thermal (laser) evaporation. Ion Beam Deposition - IBD. This technique consists of direct aiming of the low energy ion beam at the load being coated and of depositing the coating in this way on its surface (Fig. 6.13a). It is characterized by simple © 1999 by CRC Press LLC control of the deposition process, as well as possibility of control of structure and chemical composition [28, 78-83]. Ion Mixing - IM. This method differs from the implantation technique of ion mixing (see Section 4.5) by lower energy of the ion beam. An inter- esting version of this technique constitutes simultaneous sputtering of the substrate (load surface) and deposition of the coating. As the result of so- called self-mixing, an intermediate coating, strongly adhering to the sub- strate, is formed at the surface of the load (Fig. 6.13d) [84]. In the majority of techniques, a low energy ion beam and a flux of evaporated or sput- tered material (Fig. 6.13 c,d,e) or evaporated material (Fig. 6.13f) react chemically with each other or with the substrate material and crystallize on its surface [85]. 6.3 Equipment for coating deposition by PVD techniques All equipment used for coating deposition by PVD techniques, which could be termed vapour depositors (evaporative - resistance, electron, laser, arc or pulsed plasma, or sputtering - diode, triode, cathode, ion, magnetron and cyclotron) regardless of the technique employed, comprise the fol- lowing basic functional elements of design: – vacuum chamber, of rectangular or cylindrical shape or a combina- tion of both, usually made of stainless steel and serving to place deposi- tion heads together with their auxiliary components, as well as elements used for fixturing and displacement of the load relative to the heads. Often, the internal surface of the vacuum chamber is covered with re- movable (after several work cycles) aluminum foil which protects the chamber walls from coating deposition. It is not possible to deposit par- ticles exclusively on the load surface; to a lesser or greater extent they cover all the internal elements of the chamber. Some units have several vacuum chambers; – deposition heads (correspondingly: evaporative or sputtering) for formation and direction, with the utilization of electric and magnetic fields, of ions or atoms into the ionization and crystallization zones. The latter is situated near or at the load surface; – systems for formation and sustaining of vacuum, comprising oil and diffusion vacuum pumps. Usually, these systems are equipped with vacuum valves and instruments to measure vacuum (vacuum gauges); – systems for supply of reactive gases (cylinders, valves, pressure and flow gages); – electrical and possibly magnetic systems supplying the heads and aux- iliary electrodes and polarizing the electrodes and load; – auxiliary components, e.g., for preheating of the load or for water cool- ing of the radiator elements; – systems for fixturing and displacement (sliding, rotation) of the load, comprising one or many elements, relative to the deposition heads. From © 1999 by CRC Press LLC the design point of view, these systems feature a varied degree of complica- tion, dependent on the type of technique employed and on the size (mass may range from several grams to several hundred kilograms) and on the number of pieces in the load (from one to several hundred, e.g., 600 twist drills of 3 to 8 mm diameter). Such systems range from the simplest sliding or rotational stages to complicated planetary systems, equipped with indi- vidual, strip or jaw-type fixturing grips. Their job is always to effect such spatial positioning of the load relative to the head or heads, that regardless of the direction of particles deposited on the load, maximum uniformity of coverage is ensured; – control systems, usually computerized, for controlling the process of coating deposition. Besides the computer, they comprise the optical load ob- servation system, systems for measurement of parameters, of plasma, degree of ionization, of the coating process. Usually the vacuum chamber, together with its equipment, constitutes a separate design sub-assembly. Supply and control systems constitute sepa- rate sub-assemblies (power supply cabinet, control console). Often, vapor depositors, together with systems for load cleaning, constitute complete pro- duction lines. Fig. 6.14 Schematic diagrams showing designs of vapour depositors for some PVD techniques: a) Activated Reactive Evaporation (ARE); b) Reactive Ion Plating (RIP); c) Reactive Arc Ion Plating (RAIP); d) Simple Sputtering; 1 - coated object; 2 - coating metal; 3 - electron gun; 4 - glowing cathode; 5 - sparking electrode. (From Michalski, A. [6]. With permission.) © 1999 by CRC Press LLC a) b) Fig. 6.15 Schematic diagrams showing designs of depositors for most frequently used PVD techniques: a) Bias Activated Reactive Evaporation (BARE); b) Hollow Cathode Discharge (HCD); c) Arc Evaporation (AE); d) Magnetron Sputtering (MS). © 1999 by CRC Press LLC [...]... [min] 150 140 120 70 140 - 0.1 0.2 0.102 0.072 source power: 6 kW 100 50 120 90 - 0. 5-0 .66 0.2 5-0 .3 0.4 0.2 5-0 .3 - 70 0-8 00 600 - - - - 4 0-6 0 - 10 - load mass: 600 kg load mass: 600 kg - - Deposition pressure [Pa] Load bias (polarization of substrate or target) [V] Substrate preheating Working chamber volume [m3] or linear dimensions [m] Energy consumption per cycle [kVAh] Equipment cost in [mln £ £]... Univeristy of Technology, Warsaw, Poland Bulat NNW6. 6-1 2 Technique of generating vapours RIP - Reaktywna ImpulsowoPlazmowa, or PPMPulsed Plasma Method Number of sources (deposition heads) high temperature sublimation by pulsed arc discharges 1 1 1 3 1 3 2. 6-1 0 -4 0. 4-0 .8 1 0-5 1 0-3 5-5 0 5-5 0 400 1000 10 0-5 00 n/a n/a Bombardment by Ar ions Bombardment by Ti ions Bombardment by Ti ions n/a n/a - 35 0-5 00 400 -. .. yes yes Typical layer thickness [ m] - 2-4 3-3 .5 5-4 0 m/h 5 5 Process duration [min] - 140 2 0-3 0 - 80 200 Working chamber volume [m 3] or linear dimensions [m] 0.9 dia x 0.9 0.14 0.3 0.4 0.6 dia 0.6 0.04 0.35 35 kVA (chamber) + 27 kVA (cabinet) 50 30 kW 4 50 0.25 0.1 5-0 .17 - 0.25 - 1000 - - - 40 - load mass: 300 kg Deposition pressure [Pa] Load bias (polarization of substrate or target) [V] Substrate... Mechanism of tribological wear - weak adhesion of coating to material of tribological mating pair adhesive - appropriate hardness abrasive middle layer - high hardness - high mechanical and fatigue strength surface fatigue internal layer - appropriate hardness - character of chemical bonds akin to bonds in substrate material - good adhesion to substrate abrasive - good adhesion to coating - good mechanical... deposited on the substrate surface and - unfortunately - on elements of the vacuum chamber of the depositor This occurs as the result of attraction to the surface by the action of dipole moments of surface atoms of the substrate, as well as other electrical forces (e.g., caused by negative polarization of the substrate) Due to surface diffusion, atoms (ions) migrate across the surface When they encounter... 2- and 3-dimensional nuclei (clusters of several or more atoms) which grow, expand across the surface and create the coating The flux density of the atoms (ions) reaching under the surface may be high or low When a high density flux of atoms (ions) reaches the cold substrate, many nuclei are formed on the substrate surface These nuclei form cen- © 199 9 by CRC Press LLC ters of crystallization, expand,... the type of chemical bonds and morphology [8 9-9 3] Strong structural defects are probably the cause of the majority of excellent properties of hard coating materials [89] The compounds forming them are non-stoichiometric, their chemical composition has a broad range of variation and the concentration of defects reaches 50% [92] Double carbides and nitrides of transient metals, in the majority of cases,... ratio range of 0.2 . kW 100 50 120 90 Equipment cost in [mln £ ]- 0. 5-0 .66 0.2 5-0 .3 0.4 0.2 5-0 .3 Maximum load twist drills 6 mm dia - 70 0-8 00 600 - - milling cutters 100 100 mm 4 0-6 0 - 10 Comments - load mass: 600. discharges Number of sources (deposition heads) 1 11313 Deposition pressure [Pa] 2. 6-1 0 -4 0. 4-0 .8 10 -5 10 -3 5-5 0 5-5 0 Load bias (polarization of substrate or target) [V] 400 1000 10 0-5 00 n/a n/a Substrate. (cabinet) 50 30 kW 4 50 Equipment cost [mln £]0.25 0.1 5-0 .17 - 0.25 0.10 Maximum load twist drills 6 mm dia - 1000 - - - - milling cutters 100 100 mm -4 0 225 Comments: - load mass: 300 kg 2 chambers operating alternately