Hard coatings by plasma-assisted PVD technologies: Industrial practice

Surface and Coatings Technology, 37(1989) 483

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HARD COATINGS BY PLASMA-ASSISTED PVD TECHNOLOGIES: INDUSTRIAL PRACTICE EBERHARD MOLL and ERICH BERGMANN Balzers AG, FL -9496 Balzers, (Liechtenstein) (Received July 3, 1988; in revised form October 25, 1988)

Summary The historical background to the main plasma-assisted PVD processes is presented, emphasizing the methods in current commercial use for the production of hard coatings. Industrial practices with regard to cleaning and processing receive special attention.

1. Introduction 1.1. General criteria How is it that thin hard films deposited onto much less hard substrates can withstand local deformations without breaking like thin ice layers on snow? In the mid-seventies the following three criteria were tentatively proposed by one of the authors (E. M.). (i) The hard film should be under extremely high internal compressive stress at room temperature. When an external pressure is applied to a small area of the surface of such a film, the internal compressive stress and thus also the danger of cracking is actually decreased instead of increased, as might be expected. Here, of course, we assume that the force of this pressure is not high enough to change the internal compressive stress to tensile stress. Thinking along the same lines we assumed that the film should be designed with a lower thermal expansion coefficient than the substrate and should be deposited at a high temperature in order to achieve a reduction of the stress instead of an increase when the tool heats up during use. (ii) The adhesion to the substrate should be high enough to compensate for the forces resulting from the compressive stress in the film. For a given stress these forces are proportional to the thickness of the film. Beyond a certain maximum film thickness, the film flakes away from the substrate in the form of small dust particles. This process begins at the sharp edges. This flaking off can begin either during deposition or during the cool—

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down phase after deposition. The higher the adhesive strength, the thicker it is possible to make the hard film, But, in spite of any increase in the adhesive strength, there is always a maximum achievable thickness defined by the mechanical strength of the substrate material itself. (iii) The film should be chemically inert in order to avoid any coldwelding tendency even under high local loads and high temperatures. The primary task of the film is to avoid local or asperity welding. Low friction can be regarded as a consequence of the low fretting tendency but is conversely the reason for the film withstanding wear and continuing to avoid fretting. A technology for the production of films that could fulfill these three criteria was not known at the time, but a promising possibility seemed to be a combination of two originally independent technologies: ion plating (IP) and activated reactive evaporation (ARE).

1.2. Ion plating The term ion plating, first coined by Mattox [1, 2], is applied to deposition processes in which the substrate surface is subjected to a flow of ions before and during film formation [3]. Ion plating in its purest sense concerns only the substrate and can be combined with any vapor source. Applied to a sputtering source it is known as bias sputtering. In plasma-enhanced CVD, even gaseous sources are used for ion plating. Historically, Berghaus [4] was the first to propose the exposure of a negatively biased substrate to plasma in order to obtain a perfectly structured film with high adhesive strength. The amelioration of the adhesive strength can be attributed in part to an interfacial pseudodiffusion zone [3, 5, 6]. The fact that film deposition onto cold substrates using energetic atoms or ions can yield high compressive stresses in the film, in addition to those stresses which result from the difference in thermal dilatation between the substrate and the film, has been shown for ion plating as well as for sputtering [6, 7]. The first industrial evaporation systems incorporating ion plating were those with diode discharges developed by Battelle [8] and Airco Temescal [9 11]. Such systems needed a relatively high argon pressure of about 10 mbar and therefore, when used with electron beam evaporators, a pressurereducing stage at the electron beam source. The high gas pressure in the vicinity of the substrates caused scattering of the vapor and argon ions, resulting in relatively low particle energies in comparison with the bias voltage [12] yet increased throwing power. A potential problem of the d.c. diode is the formation of hollow cathodes in three-dimensional substrate arrangements [13]. An important improvement in this type of process was the step to the triode configuration with a heated filament cathode and a thermionicemission-supported glow discharge in the vicinity of the substrates [14 19]. The negatively biased substrate exhibited the typical current—voltage characteristic of a Langmuir probe. The increased ionization efficiency of -

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the triode has been emphasized in the recent review papers of Teer [20] and Matthews [21]. Another family of ion plating processes [22] uses r.f. biased substrate holders, r.f. coil electrodes near the vapor source [23, 24], an inductionheated source [25, 26], or recently even microwave devices [27]. These r.f. systems also increase the ionization efficiency in the argon gas atmosphere and, with the possible exception of the simple r.f. bias, contribute to more effective ionization in the dense vapor region above the source. For dielectric films, the use of negative self bias in an r.f. plasma is widespread [281. Less well known seems to be the fact that dielectric films can also be ion plated using a d.c. bias applied to a grid in front of the substrate surface [29, 30]. 1.3. Activated reactive evaporation This term was introduced by Bunshah for evaporation processes utilizing an electron beam evaporator and a positive electrode close to this vapor source [31 -34]. In this arrangement the vapor is directly ionized by the electrons emitted from the source material acting as the cathode. In the following, we use the word “activation” as a synonym for ionization plus dissociation plus excitation of the vapor (not argon), and we use the word “activated” for plasma processes with cathodic or anodic vapor sources. Reactive plasma-assisted processes have an additional gas inlet, such as for oxygen if oxide films are to be produced. An activated and at the same time reactive deposition method was described as early as 1932 by Berghaus [35]. Reactive evaporation of optical coatings in oxygen became a standard process after Auwärter’s invention [36]. Most hard wear-resistant coatings used today are carbides, borides, nitrides or oxides. Only in a few cases can these be deposited economically by non-reactive methods. Examples of nonreactive processes are the early work on TiN sputtering, the use of sputtered TiB2 coatings and work on evaporated oxides. Today, the vast majority of companies using sputtering to produce hard wear-resistant coatings use reactive methods. 1.4. Activated reactive ion plating The combination of ion plating (IP) and activated reactive evaporation (ARE) results in a technology which could be called activated reactive ion plating (ARIP). ARIP includes not only the biased activated reactive evaporation (BARE) process [37], which was used as early as 1975 by Kobayashi and Doi at Sumitomo Electric [38 40], but primarily those processes utilizing anodic and cathodic arc sources especially developed for the combination of high activation of the vapor and high vapor ion bombardment of substrates. In fact, the critical parameter for reactivity is not the density of argon ions, but the density of metal ions; in other words, the proportion of the ionized vapor hitting the substrates. It can be shown that the plasma density in the vessel and close to the substrate is far less -

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important than the electron density over the molten metal pool. In our opinion, this explains why the following three systems with very different plasma configurations give similar results: thermionic arc

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2. Industrial practice It should be emphasized that in industrial practice [41] the process for depositing hard coatings is only one step in a sequence of operations: mechanical preparation of surfaces, wet cleaning, heating, etching, coating, cooling, conditioning. The hard coating can only be as good as the weakest link in this chain. The very first step the substrate production already determines the maximum quality achievable. Only a few products are coated industrially by processes in which one or more of the above-mentioned steps are omitted. The methods and parameters for these steps vary enormously for different components, depending on the coating material and process used as well as on the final performance specifications. —.

2.1. Mechanical substrate preparation Mechanical preparation of the substrate may involve, for example, deburring of the cutting edges of cutting tools or cleaning of the bracelet attachment holes in a watch case made of brushed-over brass. Various sand and/or grit blasting techniques as well as reaming are employed. Only clean substrates can be coated effectively by PVD technology. In this context one must distinguish between the cleaning capabilities of the coater and the conditions that must be met by the manufacturer of a part to be coated. Figure 1 gives a tool overview prepared several years ago by the WZL group in Aachen. This can be applied to other parts when other surface finishing steps are used. Many tool manufacturers still have a tendency to discuss these problems as an unnecessary fuss, but one must be aware of the following facts. (i) The interface of a hard coating has to transmit the compressive stress of the hard film to the substrate material. Therefore, the interface must resist high shear and compressive (becoming tensile) stresses. The best measure for the strength of the interface is the strength of the substrate material itself. High quality coatings do not flake off if they are bent or stretched, i.e. they do not break at the interface.

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(ii) The interface certainly cannot transmit the compressive stress of the hard film to the substrate material if burrs are present on functional edges or if there is ploughed-away material on the borders of the microcuts produced in grinding operations, i.e. when grinding was essentially microforming rather than microcutting. Figure 2 shows a burr on the cutting edge of a twist drill. Figure 3 compares a grinding finish suitable for coating with a grinding finish unsuitable for coating. Up to now most investigations have been carried out on ground high-speed steel surfaces and polished ballbearing races. But similar problems exist in other areas. (iii) Thermochemical surface treatments lead to a top white layer of iron nitrides or carbides under most common process conditions. These

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hard white layers are an advantage in many applications where they provide a certain amount of abrasive wear protection. But for PVD one would prefer to have a pure diffusion layer, because this brittle white layer fractures under high compressive stress. Small losses due to spalling are not important for uncoated parts, but they are detrimental to a thin PVD top coating. (iv) Current methods of electric discharge machining (EDM) lead to white layers which are of the order of twice the maximum surface roughness. These white layers can be kept very thin for the highly finished operations like AGIE-brill® available today. However, the hard and brittle nature of these white layers does lead to problems under high load. These problems are increased further by the fact that the white layers produced by EDM are under tension, which gives a poor match with the PVD coating under compression. (v) Beilby layers, produced in the grinding and polishing operations, can be very fine (see Fig. 4) but they can cause detachment of coatings from tools. These Beilby layers were recognized to be the main factor determining coating life under dynamic load conditions.

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(vi) A further important feature is the effect such surface roughness changes have on tribological factors such as boundary film lubrication, build-up edge formation, etc. So far, we have only treated that aspect of the surface finish problem that shows up on use. It could be called the functional integrity of the final composite structure. The other aspect is the contamination of the PVD process resulting from outgassing of white layers, grinding features, Beilby layers etc. Most of the cavities and cracks are finer than the wavelength of the ultrasonic generators in U.S. cleaning systems and hence they are not cleaned at all. Moreover, they often act as capillaries for the drying fluid. This is probably the only important quality problem in modern hardcoating plants. Many different techniques are employed for the mechanical cleaning of surfaces. But since they are specific to the shape, function and condition of the part supplied, their discussion is beyond the scope of this paper. 2.2. Wet cleaning Wet cleaning is designed to remove all soluble contaminants. It can be extended to include some etching or electropolishing, but any material removal must be checked carefully for its compatibility with the requirements of end use. Figure 5 is an example of a cleaning system for use in job coating centers. Its most extensive version would incorporate the following steps: (material removal and rinsing) acid cleaning, ultrasonically assisted

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rinsing (multistage) alkaline cleaning, ultrasonically assisted rinsing drying For heavily contaminated parts, the cleaning system must be equipped with more than one stage. Choice of cleaning solutions and drying fluids is determined by the substrate material and local environmental protection laws. Wet cleaning processes must be compatible with the substrate materials, their mechanical properties, and the shape of the parts to be coated. They must be adapted to the preceding and subsequent treatment steps. In some coating processes the requirements for initial/final substrate temperature stem to a certain extent from the residual contamination that can not be removed by wet agents. However, a good wet cleaning system can reduce the problems of the plasma etching process to a manageable scale. 2.3. Heating Heating is the first operation carried out inside the high-vacuum coating system. In principle one can choose radiation heating, resistance heating, electron-beam heating or ion-bombardment heating. Radiation heating is used in most load lock systems, but is also used in some batch coaters. Resistance heating is a possibility for long broach-type tools. Almost all industrial tool coaters use either electron-beam or ionbombardment heating. The economic advantages of using the coating sources for heating are obvious and are a major reason for the wide use of these sources for high temperature (e.g. 550 °C)coating processes. In thermionic arc evaporation, the low-energy electron beam that will be used later on for etching and evaporation, also serves as the heater. Problems of uniformity have been solved by magnetic dispersion [421. Control in mixed loads is easy, but requires well-thought-out loading. In cathode spot arc evaporation, a bombardment of titanium ions is the method most frequently used to heat the substrates. For some substrates such as heavy parts or decorative components, glow discharge and/or radiation heating is added to titanium ion heating. These additional heating methods can boost or replace the ion heating. In some processes where the ion bombardment onto the cathode of a glow discharge is used as the etching process, the glow discharge is manipulated so that it provides additional heating. This procedure is not suitable for mixed loads. 2.4. Etching Instead of sputter cleaning [5, 43] we prefer the term sputter etching. This process removes material, but it does not create a clean substrate surface, because the coating material already deposited on the substrate holders is sputtered off at the same time and is redeposited on the new batch

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of substrates. A successful etching process leads to a very thin pseudodiffusion layer and thus to a suitable interface for the hard film. Glow discharges, thermionic arc triodes and cathode spot arcs are used industrially to sputter-etch the substrates prior to coating. While it is standard practice h-i the semiconductor industry, as far as we know little work has been done to explore the potential of reactive etching for steel substrates. Mattox and his collaborators [44] found that the use of carbon compounds in d.c. glow discharge etching has negative effects on adhesion. The idea of adding hydrogen to the discharge did not fulfil the expectations. 2.4.1. Glow discharge d.c. diode glow discharge is used in many sputtering systems. The substrates form the cathode of an abnormal glow discharge with the vacuum chamber wall acting as the anode. While this method can give excellent results, it is not suitable for mixed loads as it is not possible to achieve a uniform discharge. An additional disadvantage is that the process parameters must be adapted to the load which often requires expensive pretests. A further problem is the frequent occurrence of hollow cathode glow discharges that concentrate the current and destroy the substrates involved. Avoiding this problem requires very careful design of the substrate holders and operators skilled in plasma physics. The use of radio frequency facilitates the removal of oxide layers. Glow discharge etching has two further problems. (i) Poor throwing power. The etch does not penetrate into the flutes of a twist drill. High pressure is no remedy because it aggravates the redeposition problem of etched-off material. (ii) Economy. Glow discharge etching is expensive as it requires highvoltage and high-power power supplies on big machines. r.f. power supplies cost roughly fifteen times more than d.c. power supplies. Poor throwing power and high voltages can be avoided if magnetic fields are applied to enhance the glow discharge in the vicinity of the substrates. Magnetron-enhanced sputter etching is used in some industrial sputter systems. 2.4.2. Thermionic arc The thermionic arc can be used in a triode arrangement in which argon ions are drawn from the arc discharge to the substrates. The great advantage of this method is that the substrates can be “bathed” in a very dense plasma, and the etching voltage can be adjusted independently to provide gentle or more intensive etching. At the same time, the low noble gas pressure ensures that the mean-free-path lengths are long in comparison with the detail of the substrates, thus providing excellent penetration. Control is easy and there are fewer problems with mixed loads. No high-voltage power supply is necessary. 2.4.3. Cathode spot arc Argon, cathode material and nitrogen ions are used to etch the substrates. Again, penetration and suitability for mixed loads should not be a

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problem, but since one must bias the substrates at a sufficiently high level to avoid cathode material deposition, the method may turn out to be less simple than the preceding technique, but it has demonstrated its viability in everyday practice. 2.5. Coating by activated reactive ion plating ARIP processes can be classified according to the kind of discharge they use and which part of the discharge is used for the atomization and activation of the coating material (Fig. 6). Figure 7 compares the configuration of the various arc sources studied so far. 2.5.1. Thermionic arc evaporation This process was invented at Balzers in 1977 [451and, right from the beginning, was mainly applied for the production of titanium nitride films [45 471. All important details are shown in Fig. 8: the evaporation metal is the anode of a non-self-sustaining arc discharge. Its cathode is a resistanceheated filament situated in a separate chamber under a relatively high noble gas pressure. The arc plasma is confined by a small aperture mounted between the cathode chamber and the evaporation chamber, and by a magnetic field produced by two large coils. To prevent the electrons from losing their energy through collisions with gas atoms on the way to the anodic crucible, a lower pressure, e.g. 2 X i0~ mbar, is maintained in the -

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evaporation chamber. The direction of the magnetic field is chosen to guide the electrons in the confined plasma beam in a straight line to the anodic crucible. When a sufficiently high power density is achieved for example, using an aperture with a diameter less than 10 mm and an arc current of more than 100 A at an arc voltage of about 50 V an evaporation material such as titanium will melt and evaporate. Due to the anodic source, a highly ionized, local vapor cloud is formed which, together with the noble gas in the cathode chamber, becomes the medium for the arc discharge. This is evidenced in the decrease of the arc voltage when evaporation begins, which results in an additional concentration of the beam by anodic spot formation, an effect which can also be achieved and used for activation without magnetic confinement (see below). The arc voltage is determined essentially by the geometric conditions and by the ionization potential of the noble gas which is fed into the cathode chamber. Normally a negative d.c. bias is applied to the substrates. The substrate current attains its saturation value of about 20 A (presuming an arc current of about 100 A) at bias voltages as low as 50 V. At higher voltages it only increases slowly. This is the favorable characteristic of a Langmuir probe which allows independent adjustment of the substrate voltage. The deposition rate is approximately proportional to the substrate current because of the high proportion of vapor ions. For reactive processes, a reactive gas such as nitrogen is fed into the evaporation chamber. The nitrogen pressure there is maintained at a constant level, e.g. iO~mbar. Thus when the evaporation rate is changed by changing the arc current, the nitrogen flow is automatically adjusted according to (i) the amount of nitrogen “pumped off” by the film formation, and (ii) the nitrogen not included in the film which is pumped off by the high-vacuum pump at a constant pumping speed. —

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Thermionic arcs can also be applied in less symmetric configurations (compared with that illustrated in Fig. 8) such as those using horizontal thermionic arcs with or without magnetic guidance [48 50]. Such arrangements fit well into cubic coating systems with a front door. One can get an even more flexthle system by combining an electron beam source with a thermionic arc source. In this case the crucible of the electron beam gun is insulated and used as the anode of the arc discharge [51]. Such systems are used in cubic coaters for the production of TiN coatings on parts with delicate relief structures. When two electron-beam evaporators are mounted, this thermionic arc system can produce, at low substrate temperatures, extremely dense oxide coatings with alternating refractive indices for optical applications [52 54]. -

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2.5.2. Hot hollow cathode discharge (HCD) evaporation Unlike the cold hollow cathode with its high-voltage low-current discharge, the hot hollow cathode is well suited for producing high vapor activation [55]. In this process, as well as in the thermionic arc process, an arc discharge is anchored anodically on the crucible. However, the HCD is a self-sustaining discharge, because the hollow cathode, after an r.f. start up, is heated (and sputtered!) by ion impact. The HCD evaporator for ion plating was invented by Morley in 1968 [56, 57]. He applied a guiding magnetic field to realize a 900 deflection of the confined arc. Evaporators with 1800 deflection were afterwards developed both at Dow Chemical [58, 59] and VEB Hochvakuum Dresden [60, 61]. The application of an HCD device to a reactive process (chromium carbides and nitrides) was first reported by Komiya and coworkers from Ulvac [62, 63]. After an analysis of the incident ions at the substrate [64], they published a paper in 1978 on the production of TiC and TiN coatings [651. The principles of the commercial Ulvac system are shown in Fig. 9. A small replaceable hollow cathode is situated in front of the anodic crucible Vacuum

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so that it shadows the substrates above as little as possible. There is no magnetic confinement or guidance. The argon gas is introduced through the hollow cathode tube, and a manifold is used for the reactive gas. A d.c. bias can be applied to the substrate holder. 2.5.3. Cathode spot arc evaporation The mechanisms of cathode spot arc evaporation have been studied extensively over the past few decades [66 69]. Several detailed descriptions have been published covering the details of spot initiation, spot evolution, spot extinction, and the ensuing movement and macroparticle emission [70, 711. While earlier work concentrated on the feasibility of vacuum switches [72], research on the cathode arc as an evaporation device was for a long time confined to the U.S.S.R. [73 76] and only recently has work started elsewhere [77 86]. In a relatively recent study it was shown that there are two mechanisms of spontaneous spot initiation [87]. (i) Impurities and asperities lead to higher local electric fields that develop into a spot arc if they can draw sufficient energy from the discharge; this is probably the most common case arising in industrial practice. (ii) Sometimes, the area around a burning spot arc becomes energetically more favorable than the area occupied by the spot, causing the ignition of a new spot arc and the extinction of the preceding one. Figure 10 is a schematic diagram of a cathode spot. A very dense plasma cloud hangs above a small area of several microns in diameter. Below the cloud, ion bombardment and local Joule heating has caused the cathode surface to melt, forming a micro-auto-crucible. The micro-pool emits large quantities of metal vapor which is extensively ionized during its passage through the plasma cloud. It should be kept in mind that up to 100 A of current are drawn through such a spot. The molten metal is controlled by the high electron pressure of the cloud, in a similar way to EDM. But this same pressure deforms the expanding pool until it disintegrates splashing large quantities of microdroplets onto the substrates, mostly during the heating and etching phase at low pressure, if ion-bombardment heating and etching is used. During -

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TiN deposition, the number of large microdroplets can be reduced by an order of magnitude if the cathodes themselves are coated with TiN [881. The operating parameters of a random cathode spot arc depend largely on the thermal properties of the cathode material: vapor pressure, boiling point, melting point, heat of melting, and thermal and electrical conductivity. These quantities determine a maximum current that can be drawn from a single spot and the minimum current required for stable arc operation. A further increase in the current leads to the coaxistence of several spots on the target surface. Systems with large and small targets have been designed and are in practical operation (see, for example, Figs. 11 and 12). One of the problems to be dealt with is the confinement of the arc to the target surface. Several methods can be used either separately or in combination: (i) trenches in the target surface that the arc cannot jump over too often, (ii) a ring of magnetically permeable material such as soft iron which repels the arc by Biot—Savart forces [80, 89, 90]; (iii) a ring of an insulating material having a secondary emission ratio less than unity at the mean energies of the charged arc particles and a surface energy less than that of the evaporated target material (the best choice is boron nitride [911); (iv) a magnetic field whose axis is normal to the target surface [92, 93]; and (v) steering magnetic fields [94]. Several authors have shown [95 98] that magnetic fields can be used to anchor the cathode spot or limit its movement to predefined tracks. In this way, the “steered arc” may avoid some of the splashing problems of the “random arc”. Recently, large developmental efforts have gone into the reduction of the share of microdroplets (Fig. 13) in a typical arc coater [71, 99]. One of -

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the most promising routes appears to be a kind of magnetic mass separator [100 103]. The emitted ion current is deflected around an obstruction, Fig. 14, or behind a curved plasma guide, Fig. 15. Magnetic and electric fields are arranged in such a way that only ions reach the substrate while -

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all clusters splash onto the walls. We are not aware of any industrial users of such devices. If the surface of the anode is limited such as when it is a small tungsten boat the anode becomes very hot and starts vaporizing its contents, e.g. an aluminum wire. The vapor density produced by such an anode is sufficient to provide stable arc evaporation. This allows coating at very low residual pressures. The experimental system developed by Ehrich [104] hides the cathode spot optically from the substrates in order to avoid contamination, and uses mechanical ignition. —

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3. Design criteria for industrial PVD systems Reactivity, which is particularly influenced by the degree of ionization of the condensing vapor, is the most important criterion for the selection of the type of process. We have therefore focused this paper on arc processes. Other important criteria include multilayer capability, length uniformity, lifetime, source orientation, control, energy efficiency, cost, suitability for different substrates, and chamber geometry. 3.1. Multilayer capability Some applications require sandwich coatings. While in principle such multilayer coatings can be achieved with any technology, they are more easily produced in machines incorporating sputter sources. 3.2. Length uniformity Length uniformity, for example within ~3% for the length of the erosion zone, can be achieved with sputter sources using standard components. Similar results should be possible with cathodic arc sources. Evaporation sources using crucibles can only achieve this type of length uniformity with a substrate holder design that does not allow the vacuum chamber to be filled optimally, naturally a disadvantage from a production point of view. Good uniformity on three-dimensional bodies requires multiple substrate motion with all types of source. 3.3. Lifetime In principle, the cathode lifetime of anodic evaporators is limited by the occurrence of cathode sputtering or straight alloying. In the case of filaments these problems have been reduced to a negligible level by the space-charge distribution in the cathode chamber. In the case of the hollow cathode discharge gun, sputtering of the cathode is an intrinsic feature. The problem of the fluctuating lifetime of the tantalum cathode tubes (which alloy with evaporated titanium) has been solved by designing equipment with multiple evaporators and quick exchange modules.

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3.4. Source orientation Electron-beam and thermionic arc evaporators are based on large molten pools which limit the equipment design. Cathode spot evaporators use micropools and so these vapor sources can be mounted at any angle. 3.5. Control In all types of ARIP processes, gas supply and control of gas flow are not very critical. They are quite tolerant of operator errors. None of the industrial systems needs a partial-pressure monitor incorporated, although these can be useful. These are important advantages compared with reactive magnetron sputtering which needs such high partial pressures of reactive gas that target poisoning, i.e. nitration or oxidation of the erosion track, cannot be avoided. This problem arises for all transition metal nitrides and oxides. Special partial-pressure control systems and reactive-gas inlet systems have been designed to remedy this problem [105]. 3.6. Energy efficiency and cost Low power consumption is a strong argument in favor of evaporation systems [106]. The main problem with cold sources such as the planar magnetron is that most of the energy has to be cooled away. This energy is expensive in three ways: (i) as consumed power it increases the variable cost; (ii) as an additional investment in power supplies it increases the capital outlay; (iii) cooling water is not free at most sites, which means that expensive recycling systems are required. A simple cost consideration can be made: in a sputter system for hard films, the power supply costs almost match the cost of the rest of the equipment; electricity costs for sputter coating match the total cost for galvanic coatings. These limitations can only be overcome by using hot vapor sources. As a rule of thumb, hot vapor sources need about 10% of the electrical energy needed to evaporate comparable amounts of metal from cold sources.

3.7. Suitability for different substrates It should be kept in mind that coating systems are just that they are systems. Their performance does not depend simply on the choice of the vapor source or the coating process. The success of a coating system also depends on how well its design accommodates the peculiarities of a part to be coated or how easily it accepts mixed loads. This fact is illustrated in Fig. 16. Some of these aspects will be treated below. Essential elements in this context are the substrate holders and their suitability for a certain combination of vapor source and mechanical part For a long time, it was believed that the PVD coating of wear parts would always require customized equipment, making it mainly a business for equipment manufacturers. Indeed, the 1970s saw a large variety of —

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equipment designs. However, the 1980s have revealed that what is required is not equipment variety but load mixing capability. Most equipment is used in job coating centers whose priorities are (i) fast service: regular turnaround times below 3 days with a 24 h express service; and (ii) unpredictable daily loads from tool makers whose coatings orders are linked to tool orders. Even the largest twist drill, tap, or end mill producers do not produce continuously enough of any one tool to make dedicated equipment economically feasible. Two facts illustrate this situation very well. (i) Approximately 80% of all wear coatings are produced in only three types of machines. (ii) Substrate holder costs amount to roughly one quarter of the total equipment costs for most users. This situation might change again for the coating of automotive parts. 3.8. Chamber geometry We now discuss some criteria for the chamber design in general purpose systems. Upright vessels do have genuine advantages over horizontal vessels. They permit easier construction of substrate carriers that can rely on gravity for holding parts. They allow rotating substrate holders which hold the parts at one end without exerting any torque on their bearings; this gives them a clear advantage for the coating of shank-type tools. There are only four reasons why horizontal equipment is built. (i) Evaporation sources do not allow the coating of long parts in vertical vessels. (ii) Story height is limited in many factories which makes it difficult to install equipment exceeding 3 m in height. (iii) Specific floor loads are usually limited. (iv) Maintenance becomes increasingly difficult the taller the equipment becomes. Front loading appears attractive on first consideration. However, it is not so attractive in practice since it requires either that the operator mount heavy parts in the chamber at arm’s length or that loading aids be added, with all their disadvantages: coupling to the mechanical drive is not easily accessible after loading; torque occurs during loading, etc. Top loading usually accompanies suspended parts, bottom loading is associated with candle-type holders. In practice, bottom loading is much

501

more convenient; it allows loading by simply laying down the parts and, when combined with a lateral movement of the stage, complete and easy access to the substrate holder.

Fig. 17. Balzers model B,~l830.

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Fig. iS. Balzers model B.U 1-100.

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Acknowledgments We would like to thank C. Bergmann, formerly with Multi-Arc Corp., and H. Randhawa, Vac-Tec Systems Inc., for critical reading of the manuscript, for many valuable comments and references, and for the photographs of and data on their equipment. We also thank both J. J. Collin, Hauzer Techno Coating Europe By, and W.-D. Münz, Leybold AG, for correcting the table on commercial machines, as well as E. Ertürk, Interatom GmbH, S. Komiya, Ulvac Japan Ltd., D. A. Levien, Tecvac Ltd., and R. Wilberg, VEB Hochvakuum Dresden, for photographs and corrections to the table. References 1 D. M. Mattox, Sandia Corp. Rep. SC-DR-281-63 (1963). 2 D. M. Mattox, Film deposition using accelerated ions, Electrochem. Technol., 2 (1964) 295 - 298. 3 D. M. Mattox and J. E.. McDonald, Interface formation during thin film deposition, J. Appi. Phys. 34 (1963) 2493 - 2494. 4 B. Berghaus, Improvements in and relating to the coating of articles by means of thermally vaporised material, Br. Patent 510,933 (1937). 5 D. M. Mattox, Recent advances in ion plating. In Proc. 6th mt. Vacuum Congr., Jpn.J. Appi. Phys., Suppl. 2(1) (1974) 443 -450. 6 D. W. B. Martinson, P. J. Nordlander and S.-E. Karlsson, AES investigations of the interface between substrate and chromium films prepared by evaporation and ion plating, Vacuum 27 (3) (1977) 119 - 123. 7 J. A. Thornton and D. W. Hoffmann, The influence of discharge current on the intrinsic stress in Mo films deposited using cylindrical and planar magnetron sputtering sources, I Vac. Sci. Technol. A, 3 (3) (1985) 576 - 579. 8 C. T. Wan, D. L. Chambers and D. C. Carmichael, Investigation of hot-filament and hollow-cathode electron-beam techniques, J. Vac. Sci. Technol., 8 (6) (1971) VM 99 104. 9 K. D. Kennedy, G. R. Scheuermann and H. R. Smith, Jr., Gas-scattering and ionplating deposition methods, Res./Dev., (1971) 40 - 44. 10 H. R. Harker and R.-J. Hill, The deposition of multicomponent phases by ion plating, J. Vac. Sci. Technol., 9 (6) (1972) 1395 - 1399. 11 E. D. Erikson, Thickness distribution of a metal-alloy from a high-rate electronbeam source, J. Vac. Sd. Technol., 11(1) (1974) 366 - 370. 12 D. G. Teer, The energies of ions and neutrals in ion plating, J. Phys. D, 9 (1976) L187 - L189. 13 W. R. Stowell and D. Chambers, Investigation of hollow cathode effects on the structure of bulk films, J. Vac. Sci. Technol. 11(4) (1974) 653 - 656. 14 G. A. Baum, R. L. Beno and Th. van Vorous, Apparatus for vacuum deposition on a negatively biased substrate, U.S. Patent 3,428,546 (1966). 15 M. Matsubara and H. Murayama, Method for ionizing electrostatic plating, U.S. Patent 3,953,619 (1972). 16 C. T. Wan, D. L. Chambers and D. C. Carmichael, Effect of processing conditions on characteristics of coatings vacuum deposited by ion plating. in Proc. 4th Intern. Conf. on Vacuum Metallurgy, Tolyo, Japan (1973) pp. 231 - 237. 17 Y. Enomoto and K. Matsubara, Structure and mechanical properties of ion-plated thick films, J. Vac. Sci. Technol., 12 (4) (1975) 827 -829.

506 18 19 20 21 22

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507 46 H. K. Pulker and H. Daxinger, Method of producing gold-color coatings, U.S. Patent 4,254,159 (1977) assigned to Balzers AG. 47 R. Buhl, H. K. Pulker and E. Moll, TiN coatings on steel, Thin Solid Films, 80 (1981) 265 - 270. 48 E. Moll, W. Haag and K. Ruhm, unpublished work, Balzers AG. 49 M. Tancula, W. Oleszkiewicz and J. Zdanowski, Ion plating in a system with a hot cathode, J. Vac. Sci. Technol. A 1(2) (1983) 241 - 243. 50 M. Y. Kanev, S. V. Todorova, D. Dochev and D. Kazandjiev, Chrome-plating by ion assisted technology, Proc. mt. Ion Engineering Congr. ISIAT 83 and IPAT 83 Kyoto, September 12 - 16, 1983, pp. 1269 - 1274. 51 R. Buhl, E. Moll and H. Daxinger, Method and apparatus for evaporating material under vacuum using both an arc discharge and electron beam, U.S. Patent 4,448,802 (1981) assigned to Balzers AG. 52 E. Moll, H. K. Pulker and W. Haag, Method and apparatus for the reactive vapor deposition of layers of oxides, nit rides oxynitrides and carbides on a substrate, U.S. Patent 4,619,748 (1985), assigned to Balzers AG. 53 H. K. Pulker, W. Haag, M. Bühler and E. Moll, Properties of ion plated oxide films, J. Vac. Sci. Technol., A, 3 (6) (1985) 2700 - 2701. 54 H. K. Pulker, W. Haag, M. Bühler and E. Moll, Optical and mechanical properties of ion plated oxide films. In Proc. 5th mt. Conf. Ion and Plasma Assisted Techniques, Munich, May 1985, 299 - 306. 55 Y. S. Kuo, R. F. Bunshah and D. Okrent, Hot hollow cathode and its applications in vacuum coating: a concise review, J. Vac. Sci. Technol., A, 4 (3) (1986) 397 - 402. 56 J. R. Morley, Vacuum vapor deposition utilizing low voltage electron beam, U.S. Patent 3,562,141 (1968). 57 J. R. Morley and H. R. Smith, High rate ion production for vacuum deposition, J. Vac. Sci. Technol., 9 (6) (1972) 1377 - 1378. 58 D. G. Williams, Vacuum coating with a hollow cathode source, J. Vac. Sci. Technol., 11(1) (1974)374-376. 59 G. Mah, P. S. McLeod and D. G. Williams, Characterization of silver coatings deposited from a hollow cathode source, J. Vac. Sci. Technol., 11 (4) 633-665 (1974). 60 W. Fleischer, D. Schulze, R. Wilberg, A. Lunk and F. Schrade, Reactive ion plating (RIP) with auxiliary discharge and the influence of the deposition conditions on the formation and properties of TiN films, Thin Solid Films, 63 (1979) 347 - 356. 61 R. Basner, G. Ebersbach, A. Lunk and G. Richter, Hohlkathoden-Bedampfungseinrichtung zur funktionellen TiN-Beschichtung, Beitràge zur 8. Tagung Hochvakuum, Grenzfldchen/DUnne Schichten, Dresden March 5 - 7 1984, pp. 434 - 437. 62 S. Komiya and K. Tsuruoka, Production and measurement of dense metal ions for PVD by hollow cathode discharge, Jpn. J. Appl. Phys., Suppl. 2, (1) (1974) 415. 63 5. Komiya and K. Tsuruoka, Physical vapor deposition of thick Cr and its carbide and nitride films by hollow-cathode discharge, J. Vac. Sci. Technol., 13 (1) (1976) 520 524. 64 S. Komiya, K. Yoshikawa and S. Ono, Mass and energy analysis of incident ions to substrate during deposition by hollow cathode discharge, J. Vac. Sci. Technol., 14 (5) (1977) 1161 - 1164. 65 S. Komiya, N. Umezu and T. Narusawa, Formation of thick titanium carbide films by the hollow cathode discharge reactive deposition process, Thin Solid Films, 54 (1978) 51 - 60. 66 R. Tanberg, Phys. Rev., 35 (1930) 1080. 67 A. A. Snaper, Arc deposition process and apparatus, U.S. Patent 3,625,848 (1968). 68 A. S. Gilmour, Jr. and D. L. Lookwood, Pulsed metallic-plasma generators, Proc. IEEE, 60 (1972) 977. 69 A. S. Gilmour, Jr., Vacuum arc gettering pump, U.S. Patent 3,437,280. 70 T. Utsumi and J. H. English, Study of electrode products mitted by vacuum arcs in form of molten metal particles, J. Appi. Phys., 46 (1975) 126 - 131.

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