Cathodic Arc Deposition of Films

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CATHODIC ARC DEPOSITION OF FILMS

Ian G. Brown Lawrence Berkeley National Laboratory, Berkeley, California 94720; e-mail: [email protected]

KEY WORDS: vacuum arcs, ﬁlms, plasma deposition, surface modiﬁcation

ABSTRACT Cathodic arc deposition is a plasma-based technology for the fabrication of films. The process can be carried out either at high vacuum or in a low pressure gaseous environment, and films can be formed for example of metals, ceramics, diamond- like carbon, some semiconductors and superconductors, and more. The plasma stream can be filtered to remove microdroplet contamination, and the ion energy can be controlled by substrate bias, thereby transforming the straightforward deposition method into hybrids with other surface modification processes such as ion beam–assisted deposition, ion beam mixing, and ion implantation. The method provides a versatile and powerful plasma tool for the synthesis of novel and technically important surfaces.

INTRODUCTION The cathodic arc is a low-voltage, high-current plasma discharge that takes place between two metallic electrodes in vacuum. The earliest work in this area dates back to the late 1800s. Reviews of vacuum arc plasma discharges have been presented by Cobine (1), Lafferty (2), and more recently by Boxman et al (3). A remarkably complete bibliography containing much of the his- tory of the vacuum arc research literature up to 1986 has been compiled by Miller (4). Biennial conferences in the ﬁeld have been held regularly for many years (5), and many of the papers presented have been published in archived journals (6). The published proceedings of other, related conference series also offer a rich source of material in the ﬁeld (7, 7a–c). The cathodic arc is also called a metal vapor vacuum arc or simply a vacuum arc, but cathodic arcs also occur at elevated gas pressure when the gas 243 0084-6600/98/0801-0243$08.00

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participates significantly in the discharge processes; another kind of vacuum arc is the anodic arc with ionization processes quite different from the cathodic arc. Nevertheless, the terms cathodic arc and vacuum arc are often correctly used interchangeably. In the present context, the importance of the cathodic arc plasma discharge lies in its copious and efficient production of metal plasma and its simplicity, features that make it well suited for film deposition applications. The suitability of the cathodic arc plasma for film production has been recognized and developed mostly in the last two decades. Earlier the phenomenon was viewed largely as a problem to be avoided, primarily in the fields of vacuum switching and high-voltage vacuum insulation. It was not until the 1970s that the research field of cathodic arc film deposition was taken up by a growing number of workers around the world (8–13). Industrial applications of the method have mushroomed, and today there are many equipment manufactur- ers and users of the technology throughout the United States, Russia and other FSU countries, Europe, and elsewhere. Excellent reviews of the field have been presented previously in the texts referred to above (1–3) and by Boxman et al (14, 15), Sanders (16), and others. By far the most developed technological application at the present time is the formation of films of TiN that are both hard and decorative—titanium nitriding. The superb tribology of TiN and its gold-simulating spectral response have led to its acceptance for coating applications in domains as diverse as machine tools and drills, low-cost jewelry, and building hardware; the excellent visual impact of titanium nitrided pieces has presumably played not an insignificant role in generating its wide acceptance. Although TiN films can be formed by methods other than cathodic arc-based approaches, for many purposes cathodic arc films are superior, and this technology is widely used. A great variety of other kinds of films are also amenable to the method, and the technology is now broadening rapidly. Industrially relevant processing parameters, such as size, throughput, and cost are steadily improving, and a growing understanding of the fundamental plasma physics and materials science involved is paving the way to new and better systems.

PLASMA PHYSICS OF THE CATHODIC ARC At the cathode of the vacuum arc the current is concentrated at a small number of discrete sites called cathode spots. The formation of cathode spots is a fundamental characteristic of the vacuum arc discharge, and the physics of the spots has been studied by a number of workers (2–6, 17–27). The spot size is estimated to be in the range 1–10 microns, and the current density at the spots is 6 8 2 of order 10 –10 Acm . Many of the parameters of the vacuum arc plasma are determined by the plasma physics within the cathode spot. The spot is formed

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by an explosive emission process (28, 29) and the lifetime (residence time) of an individual spot at a fixed location has been put at from about 10 ns (30) to about a microsecond (23). New daughter spots are formed at the edges of previous parent spot locations, giving rise to an apparent spot motion across the cathode surface that is typical and indicative of vacuum arcs (31). Small surface irregularities such as sharp edges or protuberances tend to anchor the spots to these regions. The plasma pressure within a cathode spot is high, and the strong pressure gradient causes the plasma generated there to plume away from the surface in a manner similar to the plasma plume generated by an intense focused laser beam at a solid surface. The current carried by a cathode spot is typically a few to a few tens of amperes, depending on the metal, and if the arc is caused to conduct a higher total current, then more cathode spots are formed; thus a typical cathodic arc discharge of several hundred amperes arc current might involve the participation of several tens of cathode spots. The assemblage of cathode spots gives rise to a dense plasma of cathode material. The plasma plumes away from the cathode, initially normal to the cathode surface and in the general direction of the anode, thereby allowing the arc current to flow and the arc to persist. There is a lower limit to arc current, called the chopping current, below which the spot will not persist; an upper limit is determined by source cooling requirements and the possible formation of anode spots (32). The arc voltage (when the arc is alight, i.e. the burning voltage) lies generally in the range 10–30 V and varies according to the cathode material used. Unlike the more common high-pressure gaseous arc discharge, the cathodic arc displays a positive resistance characteristic, i.e. the arc voltage increases with arc current, albeit only slowly. A basic feature of the vacuum arc discharge is the relationship between the metal ion flux that is generated at the cathode spots and the current that drives the arc (3, 20, 33). The arc current is composed of both an electron component and an ion component. For all cathode materials and for typical arc currents in the several hundred amperes range, the plasma ion current is a constant fraction of the arc current, Iion Iarc, where 0.10 0.02. Thus the electrical efficiency of plasma generation—the= ratio' of available metal ion plasma current to arc current—is about 10%. The plasma ions form the substance required for film deposition, and thus some of the ion flux generated at the cathode spots serves to carry the arc current (along with the much greater electron flux), and some is diverted to be used for deposition at a location distant from the arc itself. Along with the intense plasma flux that is generated at the cathode spots, there is also a component of cathode debris in the form of microdroplets, usually called macroparticles (34) (because they are macroscopic compared with the plasma particles). These metallic globules are ejected from the cathode in the molten state and rapidly solidified in flight; they are typically in the range

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of 0.1–10 m in diameter. Macroparticle generation is less for higher melting point materials, and there is also a natural separation of the macros from the plasma because the plasma flux is peaked in the forward direction whereas the macroparticle flux is peaked close to parallel to the cathode surface ( 20). Of- ten a magnetic duct, described below, is incorporated into the overall plasma gun configuration, thereby producing a metal plasma stream that is free of macroparticles, as well as of neutral atoms; the plasma exiting the magnetic duct filter is fully ionized. When the cathodic arc is operated in a gas background, as for example for the formation of metallic oxides or nitrides (e.g. alumina or titanium nitride), the macroparticle generation is observed to be less than in vacuum. The mass of plasma generated by the vacuum arc is of the order of several tens of micrograms per Coulomb of arc charge (arc current arc on-time), where the precise value depends primarily on the metal used (20, 33, 35). This mass generation efficiency is consistent with the ion current generation efficiency introduced above. Macroparticle generation is an additional source of cathode mass consumption; thus the cathode mass consumed is in general greater than the mass of plasma produced, the difference being largely the macroparticle mass. The metal (or carbon) plasma that is generated at the spots plumes away from 1 the cathode with an ion streaming velocity in the range 1 to 3 cm s [generally, for all cathode materials (36)] that corresponds to an ion drift energy of about 10 to 200 eV depending on the ion mass (37). This feature of the vacuum arc plasma—the high directed ion energy—is critically important to the virtue of cathodic arc film deposition. The high ion deposition energy provides a kind of pseudo-temperature to the growing film that in turn provides surface atom mobility and leads to high film quality. The physical phenomena involved here are the same as for ion-beam-assisted deposition (IBAD). This serendipity can be further extended by adding the feature of ion energy control; then the film morphology and structure can be optimized. This highly advantageous plasma tool is described and discussed in more detail below. The ion charge state distributions of cathodic arc plasmas have been investigated in some detail (37–40). The ions are in general multiply stripped with charge states Qi from 1 up to about 6 , depending on the metal species, and with a mean charge state+ of from 1 to+ about 3 . It is now possible to predict the charge state distribution expected+ from any elemental+ cathode material with some precision (40). Triggering of vacuum arcs is an important concern. Depending on the parameters and operational mode of the source involved, triggering can be done electronically, electromechanically, or purely mechanically, and it can be laser initiated, gas-breakdown initiated, or surface breakdown initiated, for example (41–45).

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Figure 1 Simpliﬁed schematic of basic cathodic arc conﬁguration.

Cathodic arc plasma sources have been made in many different embodiments for many research and industrial uses. They span the gamut from miniature finger-sized research tools (46) to massive human-sized installations that produce microsecond duration, low-repetition rate plasma pulses or large-area, steady state plasma streams. A simplified schematic of a basic cathodic arc configuration is shown in Figure 1, and a photograph of two kinds of sources in Figure 2. The upper photograph shows an electro-mechanically triggered dc source witha5cmdiameter titanium cathode that is cooled by direct water contact to the back side and a water-cooled cylindrical copper anode (shown removed). The lower photograph shows a small, uncooled source with a 3.175 mm diameter cathode (several shown) that operates typically with sub-millisecond pulses and duty cycle 0.1%. THE DEPOSITION PROCESS The film composition reflects the cathode composition, though not always accurately. Several different factors contribute to the non-equivalence of cathode

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Figure 2 Cathodic arc plasma sources. Upper: DC source with 5 cm diameter cathode. Lower: miniature, low duty cycle source with 3 mm diameter cathode.

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and film composition; a primary one is the different sputtering rates of the various elemental species. The growing film is bombarded by the incoming ion flux, some sputtering occurs, and the degree of sputtering is not the same for different constituent elements of the film. Thus the residual (net) film composition will differ from the plasma composition (47, 48). Also, the plasma itself may not accurately reflect the composition of the cathode, especially if some gaseous or nonmetallic species such as LaB6 or FeS2 are involved (49, 50). For the case of a single-element cathode, this is of course not a concern. Oxide and nitride films can be made by operating the source at, or at least locating the substrate in, an elevated background gas pressure. The reactive 4 gas pressure may be barely greater than the base pressure, e.g. in the 10 torr range, or as high as several hundred millitorr. Film thickness can be from monolayers up to microns or even many tens or hundreds of microns in some cases; thickness in the broad range 0.1–10 m is typical of the technique. Kim and coworkers (51) have formed super-thin films of Pt and Re for surface catalysis studies (thickness 10 A˚ and less) using a small filtered cathodic arc source operated in a short-pulse mode by which fractions of a monolayer are formed per pulse. It has been shown that the films can be of particularly high quality, having no islands or valleys (clusters of atoms, or local deficiencies of coverage) (52). At the other extreme, Boxman et al (53) have designed and operated a source at spectacularly high deposition rates. In their experiments, the substrate served also as the anode, thus achieving maximum efficiency in this respect. Coatings of Al on steel, Mo on Co, and TiN on steel were prepared, among others. Deposition rates of up to approximately 1 0.1 mm s were measured at the highest arc currents investigated of 2 kA. Maximum pulse length was about 100 ms. It is interesting to note that the power flux carried by the arc to the substrate under these conditions was as 2 high as several hundred MW m , an impressively high value and more than enough to cause local melting and intermixing of the deposited material with the substrate material, usually also with beneficial results. This work probably holds the record at present for highest cathodic arc deposition rate. More 1 typically, deposition rates of order 1–10 mh are achieved. Removal of macroparticles from the plasma (or at least reduction of macroparticle content) by passing the plasma stream through a magnetic duct macroparticle filter is becoming a common procedure, but for many applications (for example decorative coatings) it is unnecessary and is not done. The kind of setup employed for filtered cathodic arc deposition is indicated schematically in Figure 3. Good fundamental understanding of plasma transport through magnetic ducts is crucial to maximizing the filter efficiency, and much progress has been made recently in this subfield of vacuum arc plasma physics; some highlights of this work are discussed below.

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Figure 3 Simplified schematic of a basic filtered arc setup using a quarter-torus solenoid. KINDS OF FILMS Most of the solid metals of the Periodic Table can be accessed by cathodic arc methods (38, 39). Low-melting-point metals such as In, Sn, Pb, and others cannot be operated dc but must be used in a low duty cycle pulsed mode to keep the mean thermal load low. On the other hand, at typical operating pressures 6 in the 10 torr range, films of the highly reactive metals such as Li, Ca, Ba, etc, suffer gross oxidation, and many metals including Al and Ti are difficult to deposit in a completely oxygen-free state other than by a DC arc. Semiconduc- tors such as Si have been used, but with difficulty and only when highly doped so that the bulk resistivity is sufficiently low that the cathode is able to support conduction of the arc current. Carbon is the subject of much ongoing research for the formation of hydrogen-free diamond-like carbon films (see below). The rare earths, noble metals, and refractory metals in general work well. High-quality stoichiometric nitrides and oxides can be formed by operating at a judiciously selected background gas pressure, including, above all, TiN as well as others such as AlN, VN, CrN, ZrN, Al2O3, SiO2,TiO2,VO2,Y2O3,

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ZrO2,Nb2O5, to name just a few. Carbides, for example SiC, TiC, and WC, can be formed using a carbide cathode material or by mixing the plasmas formed from two separate plasma sources. Films of more complex composition, such as TiCN, TiCrN, TiAlN, TiZrN, TiAlZrN, and others, can be formed using an appropriate alloy cathode, with a background gas ambient at the appropriate pressure. In this case, the effect of preferential sputtering (47, 48) can cause the film composition to differ from the cathode alloy composition and to be dependent upon the applied substrate bias. Carbon films in which the bonding is a mixture of graphitic (sp2) and diamond (sp3) have been widely explored in recent years. This material has been given many different names, including diamond-like carbon or dlc; hard carbon; amorphous carbon or a-C; amorphous diamond; and tetrahedrally bonded amorphous carbon or ta-C. It can be made by many different methods including cathodic arc deposition. Some of the features of dlc made by cathodic arc deposition include its being inherently hydrogen free (compared with some other approaches that use a hydrocarbon gas as the basic feedstock), high sp3- content (up to 85%), and high surface hardness (over 85 GPa). Some recent developments in this area are discussed below. For a more complete perspective of the panorama of film materials and structures that have been made by cathodic arc deposition, see the last several years of the relevant conference proceedings (5–7, 7a–c).

RECENT ADVANCES IN PLASMA TECHNOLOGY Advances have been made in a number of areas of plasma technology that are important for the development of cathodic arc film deposition for research and industrial applications, including new approaches to macroparticle filtering, ion energy control, hybridization with other nearby plasma methods and technolo- gies, and understanding of the plasma properties and behavior at a fundamental level. Macroparticle Filtering Macroparticle contamination of the cathodic arc plasma is for most applications a disadvantage and for some a fatal defect. Most tribological and decorative applications would be better served by a macroparticle-reduced process, while others, including particularly most applications to the semiconductor and magnetic storage industries, call for essentially macroparticle-free film deposition. There is incentive for improved theoretical models and for the development of new macro-filtering schemes. Much progress has been made, and a number of new systems and approaches have been described.

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The approach to macroparticle filtering that has seen the most significant progress is the use of curved magnetic guide fields, first introduced by Aksenov and coworkers in the 1970s (10, 54, 55). Plasma from the cathodic arc source is injected into a bent solenoidal magnetic field region, a quarter-torus being typical, where the plasma stream is bent through an angle of 90 (Figure 3). The plasma is transported through the duct, with some loss, while the macroparticles are not magnetically guided and are lost from the plasma stream. The goal of duct research is thus to increase the plasma transport efficiency through the duct and to reduce the residual macroparticle flux at the substrate location. A detailed experimental investigation of duct plasma transport as a function of duct parameters was made by Anders and coworkers (56–58). The importance of the flow of magnetized electrons (electron gyro radius e < rd the duct minor radius) in determining the transport of unmagnetized ions (ion gyro radius i > rd) via ambipolar electric fields was pointed out. For good plasma transport the duct must be biased with respect to the plasma, typically by about 10 to 20 V, an effect noted in the pioneering work of Aksenov and coworkers.+ This feature+ was recognized also by Bilek and coworkers who showed that a similar effect can be produced by applying a positive bias to a strip electrode that is located near the outer wall of the interior of the duct (59, 60). These kinds of optimized 90 ducts can achieve a plasma transport efficiency up to about 25%. Anders has taken the concept further in the S-duct (61), shown schematically in Figure 4. Here the macroparticle transport is reduced to an unmeasurably low level by the use of two 90 ducts in series, but done in such a way as to offset the curvature drift (62, 63) of the metal plasma stream toward the duct wall. The plasma transport efficiency is also reduced to about (0.25)2 or about 6% for optimum operation. This draconian solution to macroparticle removal is suited to those applications where essentially no macroparticles can be tolerated and the required film thickness is small; the suitability of the approach for producing very thin, particulate-free, diamond-like carbon protective overcoats for magnetic storage hard disks has been demonstrated (64, 65). Somewhat offsetting the disadvantage of low plasma transport efficiency are the results of Schülke and coworkers, who investigated the transport of high current arc plasmas through a 90 duct (66). The peak arc current was up to several kiloamperes, and the plasma ion current at the filter exit was as much as 7% of the arc current, for a duct transport efficiency (ratio of ion current at duct exit to that at duct entrance) of up to 35%. Extrapolation of this result to an S-duct configuration would predict usable S-duct-filtered ion current in the tens of amperes range. These impressively large plasma currents could find industrial application. Aksenov and coworkers have described a novel kind of geometry involving an axially symmetric magnetic filter configuration (67), indicated schematically in

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Figure 4 S-duct described by Anders and coworkers.

Figure 5. Both mirror and ring-cusp magnetic geometries (adding and opposing magnetic field coils, respectively) were investigated. Good efficiency and high 1 2 film deposition rates were demonstrated (10 mh of TiN over 0.2 m substrate area). Recently an in-line magnetic filter system has been described by Ryabchikov and coworkers (68, 69). In this configuration a concentric, circular, “venetian blind” arrangement (a nested set of tapered sections) is used (see Figure 6). A reduction of two orders of magnitude in the macroparticle fraction and a plasma transport efficiency of up to 70% were reported. This kind of system could be nicely suited to large facilities. Ion Energy Control The ability to control the directed energy of the cathodic arc plasma ions can be advantageous in a number of ways, providing a means for doing ion surface modification similar to that of ion-beam-assisted deposition, or to ion mixing, or to ion implantation, depending on the energy regime employed. New hybrid processing schemes that combine the different ion energy regimes can then be

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Figure 5 Axially symmetric macroparticle ﬁlter conﬁguration of Aksenov and coworkers.

accessed and used in a single fabrication/modification process to make novel films of complex design. Substrate bias has been successfully used as a means for controlling the ion energy. Ions are accelerated at the plasma sheath that forms at the biased substrate, and the deposition energy is equal to the sum of the drift energy of ions in the plasma stream and the energy acquired in their acceleration through the sheath drop, Edep Edrift eQiVsheath. This straightforward approach was used as a means for the= control+ of microhardness of the deposited film by Aksenov and coworkers as long ago as 1977 (13). Internal compressive stress in the film that otherwise tends to lead to delamination and limitation of the achievable film thickness can also be controlled (47, 48, 70, 71). As the substrate voltage increases, so does the sheath thickness. When a high-voltage pulse is applied to a substrate immersed in a plasma, the sheath rapidly expands into the plasma and the system can be severely disrupted (e.g. plasma depletion and high-voltage breakdown) if the substrate is maintained at high voltage. One approach to solving this problem is to apply the bias voltage in a repetitively pulsed mode; in the pulse-on period ions are accelerated into the substrate as required, and in the pulse-off period the plasma recovers. This technique is called plasma source ion implantation or plasma immersion ion implantation (psii, piii or pi3) (72). Because of the surface retention of condensed metal plasma, the piii process in a metal plasma is quite different from that in a gaseous plasma, and qualitatively new and different consequences

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Figure 6 Ryabchikov ﬁlter.

follow (73–76). Ions that are accelerated from the plasma into the substrate during the high-voltage bias pulse suffer collisions with metal atoms previously deposited on the substrate during the voltage-off part of the cycle, and thus also produce recoil implantation. So the depth profile of the implantation is different from the usual shifted-Gaussian-like shape of direct implantation and extends roughly uniformly from the surface down to the maximum range. By varying the proportions of the deposition (voltage-off) and implantation (voltage-on) parts of the cycle (i.e. the duty cycle of the pulse biasing), one can tailor the shape of the profile, and the range is determined by the amplitude of the applied pulse voltage. The process involves both film deposition as well as ion implantation [or what has been termed subplantation (71) when the ion energy is much below the usual implantation regime] and can be called metal plasma immersion ion implantation and deposition, or mepiiid. This model needs to be modified for the case of a plasma with substantial streaming velocity, as for the cathodic arc plasma where drift velocity is about the same as or perhaps greater than the ion acoustic speed in the plasma (37), but the general idea remains intact. Substrate

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bias provides control over an important deposition parameter—the ion energy. Although the global need for the bias to be applied in a repetitively pulsed mode, as opposed to dc, is not yet completely clear for the case of a plasma with high drift velocity, it is certain that repetitively pulsed operation offers a closer control over the high-voltage sheath behavior, and the risk of breakdown or other unwanted plasma behavior can be avoided. The method provides a tool for nano-engineering the film in some important ways; the interface width can be tailored, and the film morphology and structure can to a large extent be controlled. In a clear demonstration of the interface mixing that can be brought about in this way, carbon plasma from a filtered cathodic arc source was deposited onto a silicon substrate with a repetitive pulse bias of 2 kV at 25% duty cycle so as to form a thin film of diamond-like carbon with an atomically mixed interface with the underlying silicon (OR Monteiro, unpublished data). A high-resolution TEM image of this region of the sample is shown in Figure 7, together with the results of a calculation of the mixing using the T-DYN code (77–79). The carbon is implanted into the silicon substrate to a depth of about 150 A.˚ The intermixed region can be seen in the TEM image by the damage layer that extends down to this same depth. Columnar growth of films is characteristic of low-energy methods such as evaporation or sputtering, and for many applications it is a negative feature that

Figure 7 TEM image of the graded interface between a diamond-like carbon ﬁlm and silicon substrate, together with the results of a calculation of the mixing using the T-DYN code. A repetitive pulse bias of 2 kV at 25% duty cycle was applied to the substrate during the early part of the deposition. An intermixed region of thickness about 150 A˚ can be seen.

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needs to be removed. The columnarity can be removed by adding a concur- rent energetic ion beam bombardment, i.e. IBAD. The inhibition of columnar growth by the enhanced adatom mobility brought about by ion bombardment of sputtered films has been nicely shown by Olbrich & Kampschulte in a series of edge-on scanning electron microscope (SEM) photographs (80). Cathodic arc deposition has an inherent IBAD-like feature due to the high ion drift energy, as described above, and this feature can be vastly expanded via substrate biasing. This important attribute of the approach was noted by Martin and coworkers (81–83). Good control over the columnarity can be achieved. The film-substrate adhesion can be enhanced by atomic mixing at the interface. In this case the ion energy is kept high, at some keV or greater. At this energy ions are implanted into the substrate to a depth of 100 A˚ or more, depending on the film and substrate atomic species, and a highly mixed interface is produced. When a film thickness of the same order as the interface width has accumulated, the pulse bias voltage is reduced since intermixing with the substrate is no longer a factor. Higher ion energy would simply sputter away the already deposited film, and the film is built up by deposition at a lower but optimized energy. The effect of substrate bias changes as the bias increases, and thus conven- tionally used terminology also changes. For low bias (low ion energy)—in the range tens of eV up to several hundred eV—the phenomena involved are similar to IBAD; in this range the film morphology and structure can be tailored. For intermediate bias (intermediate ion energy)—several hundred eV to several keV—sputtering is important and ion penetration becomes significant; this range is appropriate for control of the film-substrate interface and layer-to-layer interfaces of a multilayer structure. For yet higher bias (high ion energy)—from several keV to 100 keV—the appropriate way of viewing the interaction is as a more-or-less straightforward energetic ion implantation. It is a powerful method that should engender many advances with many variants. For a more complete survey of this subfield, see References 72–76. Some ex- amples of the use of these methods to specific film synthesis applications are given below. Thickness Uniformity Film thickness uniformity is a general concern. Mechanical scanning of the substrate or magnetic scanning of the plasma column are two approaches that have been used. Another method is the use of an array of permanent magnets in the form of a magnetic bucket (84, 85). This kind of magnetic geometry has been long used for the confinement of laboratory plasmas of various kinds (86–88), and it also works well for cathodic arc plasmas. The deposition profile (radial distribution of deposited film thickness) follows the plasma density

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profile (radial distribution of plasma density in the region immediately adjacent to the substrate). The radial density profile of the cathodic arc plasma is generally bell shaped, with the detailed distribution (Gaussian, cosine, cosine2, etc.) depending on the details of the plasma generation and transport; the profile of the plasma exiting a magnetic duct is different from the profile for the case of a magnetic-field-free plasma gun, for example. The magnetic bucket plasma confinement configuration used here is an array of high-strength (rare earth) permanent magnets that are arranged to form a short cylindrical multicusp magnetic field region. The magnets are fixed on the outer circumference to a short length of steel tubing. In one configuration, a steel tube of diameter 22 cm, length 8 cm, and thickness 6 mm was used to support (and provide a return flux path for) a total of 20 samarium-cobalt permanent bar magnets of length 8 cm (axial direction), thickness 2 cm (radial direction), and width 1 cm (circumfer- ential direction), magnetized in the 2 cm direction. The configuration and the magnetic field so established are shown schematically in Figure 8. The high- order multipole field has an almost field-free inner region with a sharply rising magnetic wall at large radius. The effect on the plasma is to limit radial expan- sion, and the radial density uniformity is greatly improved. An example (85) of the effect of a bucket on the plasma distribution is shown in Figure 9. Titanium plasma from a repetitively pulsed source was used, with and without a 16 cm inner diameter multicusp, and the radial profile of deposited film thickness at the multicusp exit plane was measured. The device is clearly highly effective in improving the radial uniformity of plasma density and thereby of deposited film thickness. Larger configurations of this kind might well find application in industrial equipment.

Figure 8 End view of a magnetic multicusp, with schematic plasma distribution.

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Figure 9 Radial proﬁle of ﬁlm deposition thickness with and without the use of a magnetic multicusp.

Plasma Mixing Plasmas from two separate cathodic arc sources have been combined to form a blended plasma of ion species from both sources. Films can be formed of composition that can be tailored over a wide range. The method has long been used to form alloy coatings, for example of TiC, by running an arc on cathodes of Ti and C simultaneously (15). A schematic of the setup used by Monteiro and coworkers (89–92) is shown in Figure 10. Two repetitively pulsed cathodic arc plasma sources, with their separate magnetic ducts for macroparticle filtering, are mixed together before reaching the substrate. Optionally, a short magnetic multipole of the kind described above has been used to better blend the two plasmas and to produce a more uniform plasma density profile; in this situation, the magnetic multipole configuration has been called a plasma ho- mogenizer. The deposition can be carried out synchronously or asynchronously, i.e. the plasma sources can be operated simultaneously or sequentially. When operated synchronously, the film material is a controlled mixture of species from both cathodic arc sources; the plasma guns may be operated with different pulse lengths and the stoichiometry of the film constituents controlled. When operated sequentially, multilayer structures can be formed. Some of the novel kinds of films that have been made in this way are described below.

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Figure 10 Schematic of the dual plasma mixing conﬁguration of Monteiro and coworkers.

Hot Cathodes

The use of hot cathodes (up to about 1000C) has been explored by several workers. There are two reasons for interest in this mode of operation—the possibility of macroparticle-free operation as the arc runs in a spotless, diffuse mode rather than the more usual spot-mode (93), and the possibility of accessing normally insulating cathode materials, particularly boron, by virtue of their increased electrical conductivity at high temperature. Veerasamy and coworkers (94) have operated a carbon vacuum arc to form hard carbon films on silicon substrates using three configurations and operational modes: unfiltered cathodic spot arc, filtered cathodic spot arc, and unfiltered cathodic “distributed” arc; filtering, when done, was by a 90 magnetic duct of conventional design. The films were examined by a number of methods,

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including atomic force microscopy, for their surface morphology. The films formed using the distributed arc mode and without any macroparticle filtering were found to be similar in structure to and as dense as films grown from the filtered cathodic spot arc. Chhowalla and coworkers (94a, b) have pursued this work further and conclude that the low macroparticle content of the carbon plasmas generated by these cathodes, and also of the diamond-like carbon films formed, is a consequence not of the high cathode temperature but of the deep erosion holes that are formed in the compressed graphite cathode. They refer to this arc mode as a stationary carbon arc because the discharge remains local- ized throughout its lifetime within a single high-aspect ratio hole in the cathode. The plasma formed has increased stability and a greatly reduced macroparticle content. This method for the deposition of diamond-like carbon films with low or reduced macroparticle content could have industrial importance. Richter and coworkers have made an important advance in demonstrating that boron plasma can be formed by the cathodic arc process using a hot boron cathode (95–97). The cathode was heated up to about 1000C by a surrounding oven, and as a result of additional power input during arc operation, parts of the surface rose further to over 2000C. A combined diffuse mode and spot mode was observed. Macroparticles continued to be generated and were removed from the boron plasma stream by subsequent magnetic filtering. The facility was used to form cubic boron nitride (cBN) thin films by reactive deposition of boron plasma in a nitrogen background (97). This demonstration that dense boron plasma can be formed in copious quantity in this way could have important implications for hard film synthesis and possibly also to the semiconductor community.

NOVEL FILMS AND APPLICATIONS The diversity of the kinds of film materials and structures, and the applications to which the films have been put, have both seen impressive advances in recent years. Films of metals and alloys, including non-equilibrium alloys, high- quality diamond-like carbon, ceramics of many different kinds, semiconductors, high-temperature superconductors, and others have been formed, both as single- layer films and as multilayer structures. In the following, a number of novel areas are briefly described. (See also 3, 6, 7, 7a–c, 98 for complete and detailed discussion of these and other kinds of cathodic arc films and application areas). Diamond-Like Carbon Diamond-like carbon (dlc) films have been the subject of a great deal of research effort over the past decade or so (7, 7a–c, 98). This material is a mixture of graphitically bonded (sp2) and diamond-bonded (sp3) carbon atoms, and

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assumes progressively more of a diamond character as the sp3 :sp2 ratio increases. The films are hard, wear resistant, smooth, and have low friction, features that make the material highly attractive for a range of industrial applications. Cathodic arc techniques have been used to form dlc thin films with excellent results (10, 64, 65, 70, 71, 99–107). For optimum formation conditions of ion deposition energy about 100 eV, the hydrogen-free dlc films can have sp3 content up to 85% and hardness up to 85 GPa, which is significantly greater than the 20 GPa found for hydrogenated, sputter-deposited carbon films used in hard disk application (104), for example, and approaches the hardness of natural diamond, about 100 GPa. An illustration of the wear resistance of this material compared with the more conventional hydrogenated, sputtered amorphous carbon films is shown in Figure 11 (108). Hard disks coated with 100 A˚ of cathodic arc deposited dlc and lubricated after deposition with 15 A˚ 1 of perfluoropolyether were tested using sliders in contact at a speed of 13 m s

Figure 11 Wear as a function of time for sliders in contact with cathodic arc dlc-coated and sputtered carbon-coated disks. Wear on a disc in contact with the cathodic arc-coated slider is almost 20 times less than for a similar slider in contact with a sputter-coated disk.

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Figure 12 TEM image of a hard-soft dlc multilayer structure made by cathodic arc deposition with modulated ion energy.

and a load of 40 mg; the worn volume is plotted as a function of time and is a factor of 20 lower for the cathodic arc dlc than for the sputter-coated disk. Multilayer structures of high sp3 content and low sp3 content dlc also have been made (108, 109), as shown in Figure 12. By periodically varying the carbon ion energy, the layers of the structure can be modulated in their properties such as density, hardness, and internal stress. This kind of hard-soft dlc multilayered structure provides an additional degree of freedom to the film characteristics, and the overall compressive growth stress of the film can be reduced while still maintaining a very hard and wear-resistant surface. Other kinds of multilayered dlc structures have also been made, for example with TiC/dlc layers (110) and with layers doped with Ti or W (111). Cubic Boron Nitride Cubic boron nitride (cBN) is of interest for a variety of applications because of its extreme hardness, high thermal conductivity, high electrical resistance, chemical inertness, and optical transparency. cBN films have been made by many different PVD and CVD methods, but a cathodic arc approach has been difficult because boron is not an electrical conductor and thus cannot be used as a cathode material, at least not at and near room temperature. However, an important advance made recently by Richter and coworkers is that they have

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made and operated a cathodic arc source with a boron cathode operating at about 1000C, i.e. when boron becomes a good conductor (as described above) (95–97). Films were made that were almost 100% cubic phase, of quality com- parable to the best films made by other methods such as magnetron sputtering. Ceramics Ceramic films of many different kinds have been made for many years by cathodic arc deposition, but this broad applications area has been enhanced greatly by the growing using of filtered plasmas to form macroparticle-free (or -reduced) ceramic films such as Al2O3 (74, 82, 112–114) and many other metal oxides such as VO2, ZrO2,TiO2, and Nb2O5. Highly adherent 3Al2O3 2SiO2 alumina-silica (mullite) films that maintain their integrity and adhesion throughout repeated high-temperature cycling have been made using the methods described above (89, 90), e.g. mixed plasmas of aluminum and silicon in the presence of a low-pressure oxygen background, with repetitively pulsed substrate bias for ion energy control. In the early stages of the process, the ion energy was held in the keV range to produce atomic mixing at the film-substrate interface, and in the latter stages of deposition, the energy was reduced to 100 eV to optimize the film structure and morphology. The film-substrate adhesion was typically greater than 70 MPa (the limit of measurement), and the films maintained their adhesion after repetitive cycling in temperature between ambient and 1000C. Semiconducting Materials Films of silicon and other semiconductors can be formed by cathodic arc deposition under somewhat restrictive conditions. The poor electrical conductivity of undoped silicon makes it unsuitable as a vacuum arc cathode material, but highly doped material has a sufficiently low resistivity that it can be used (115). Boxman and coworkers (116) have made and explored in some detail films of heavily boron-doped amorphous silicon and of undoped amorphous SnO2 x (an n-type semiconductor), and conclude that cathodic arc deposition methods could provide a valuable technology in some specialized semiconducting thin film applications such as for solar cells, flat panel displays, windshield defrosters, and other devices. In the related field of rare earth optoelectronic materials, rare earth elements such as Er are used to dope thin film materials, for example, alumina to produce the required optical characteristics. Raoux and coworkers (117) have made Al2O3 and Al2 xErxO3 thin films as a preliminary demonstration of the use of pulsed vacuum arc methods for the fabrication of multilayer waveguide structures such as Al2O3/Al2 xErxO3/Al2O3/Si. An Al1.92Er0.08O3/Si film, 2000 A˚ thick, was made using an Al/Er(10%) cathode.

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Superconducting Materials Both low-temperature and high-temperature superconducting thin films have been made by cathodic arc methods. Bendavid and coworkers (83) have formed niobium thin films on silicon. The films were of cubic phase and had relatively high superconducting critical temperatures (Tc) of about 9 K and residual resistivity ratios of up to 28.2. NbN films were also made, with a Tc of 11.0 K and residual resistivity ratio of 1.2. High-Tc superconducting films, including YBa2Cu3O7 films by Studer and coworkers (118) and Bi2Sr2CaCu2Oy/Ag (50 wt.%) films by Chae and coworkers (119), have been made by cathodic arc deposition followed by a post-deposition annealing phase. Tc onset was approximately 85 K in both cases. In spite of the encouraging results found in these preliminary investigations, the method has not been adopted as a general superconducting thin film synthesis technology. Biomaterials Applications of cathodic arc methods to a number of different biomedical areas have been explored to some extent. One important field is for the formation of biologically compatible hard coatings for orthopedic implants such as prosthetic hip and knee joints, although at present, the main concern is wear of the ultra- high molecular weight polyethylene (UHMWPE) mating component rather than the metallic part itself. Diamond-like carbon could have particular promise in this application (120), as well as for broader biological applications, because of its very high surface hardness and low wear as well as its biocompatibility. Zirconia is another material that has received some attention. In other work, several different kinds of biocompatible and bioactive surface coatings that can promote and stabilize cell attachment to medical instruments have been formed and explored by Ignatius and coworkers (121). The films were tested on primary central nervous system neurons and the growth response was very good. The method and the materials could have ramifications in a number of areas of research and biotechnology, for example for chronic implantation of microelectrode arrays in the cerebral cortex for neuroprosthetic and neural monitoring application, and for research on the human central nervous system.

SUMMARY Research and development in cathodic arc ﬁlm deposition has been active and growing in recent years. Understanding of the basic plasma physics involved continues to advance, and numerous important new ways of manipulating and controlling the plasma and the deposition process have been developed. Ac- ceptance of the technology throughout industry remains somewhat limited, although use in a number of particular areas is growing. The ﬁlm material

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properties can be excellent, and the range of kinds of materials that can be produced is impressive and still growing. The presence of macroparticles in the plasma stream is a disadvantage for some, but not all, applications, and there is a need for simple and high-efficiency macroparticle filter systems; some of the filter systems described here may fill this need for some applications. Scale-up to greater plasma througput and higher film deposition rates is needed for some applications, and future advances in this area could engender a greater appeal for the technology from certain industries. As an area of scientific research, the field is very healthy.

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