- Email: [email protected]
Comparison of DC and AC arc thin film deposition techniques
Surface and Coatings Technology 120–121 (1999) 226–232 www.elsevier.nl/locate/surfcoat
Comparison of DC and AC arc thin film deposition techniques Thomas Schuelke a, *, Thomas Witke b, Hans-Joachim Scheibe b, Peter Siemroth b, Bernd Schultrich b, Otmar Zimmer b, Jo¨rg Vetter c a Fraunhofer USA, Center for Surface and Laser Processing, Bradley University, College of Engineering and Technology, 1501 W. Bradley Avenue, Peoria, IL 61625, USA b Fraunhofer Institute Material and Beam Technology, Winterbergstr. 28, 01277 Dresden, Germany c Metaplas Ionon GmbH, Metaplas Ionon Oberfla¨chenveredelungs GmbH, Am Bo¨ttcherberg 30–38, 51427 Bergisch-Gladbach, Germany
Abstract DC and AC vacuum arc evaporation comprise two techniques currently implemented for thin film deposition, each with specific technical aspects with regard to the discharge systems and their application fields. The DC arc discharge at several 10 A up to 300 A is a well-established PVD technique to deposit hard coatings for a variety of applications. Examples of applications benefiting from this technique are cutting and forming tools. The primary function of hard coatings in this instance is to reduce friction and wear, and the standard coating materials employed are TiN, CrN, TiCN and AlTiN. In today’s manufacturing industries, machines are designed to be fully automated to simplify the coating process which contains several steps like heating, cleaning and deposition. In contrast, AC arc discharges are characterized by the repetition of short current pulses up to several 1000 A. They offer new possibilities including an increase of average current. This is especially important for applications that require plasma filtering to reduce or even avoid droplet deposition on substrates. Yet another useful property is the increase of plasma ionization compared to DC discharges. Applications like hard-amorphous carbon deposition and metal-interconnect deposition schemes in the semiconductor industry benefit from these superior plasma conditions. AC systems could be effectively introduced to industry by using the already well-established DC equipment base. Attaching AC evaporators to conventional systems serves customers in the coating market with new flexibility while maintaining the production proven reliability of their investments. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Diamond-like carbon; Hard coatings; Thin film deposition; Vacuum arc technology
1. Introduction The use of physical vapor deposition (PVD) in thin film applications is state-of-the-art in a variety of established markets. Wear reducing hard coatings on tools and components, metallic shiny and scratch resistant covers for decorative accessories as well as the deposition of semiconductor metal interconnection systems are well-defined and developed markets. Vacuum arc deposition ( VAD) is one of the competing technologies that rapidly gained market share since it was introduced on an industrial scale in the late 1970s. Features like high deposition rates and the possibility of low temperature deposition processes made it popular. During the past decade, the equipment manufactur* Corresponding author. Tel.: +1-303-677-2720; fax: +1-309-677-3670. E-mail address: [email protected] ( T. Schuelke)
ers invested in improving the handling as well as the reliability of VAD systems. Leading VAD equipment manufacturers operate their own coating service centers to provide customers with high quality coatings. This ensures a short customer feedback loop to improve process and equipment development. Larger cutting tool manufacturers as well as companies that run a large number of cutting processes in their production lines tend to operate their own coating facilities to keep delivery times as short as possible. Generally, flexibility in batch size and delivery service are becoming critically important to customers. In order to respond as fast as possible and to keep low costs, service centers need to run the coating processes at high efficiency in terms of throughput and quality control. This requires: $ running different customer’s parts in the same batch to optimize the volume utilization of the coating chamber;
0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 48 0 - 6
T. Schuelke et al. / Surface and Coatings Technology 120–121 (1999) 226–232
stable process windows to ensure low batch to batch and within batch variations of the coated film properties to ensure consistent film quality for a variety of differently shaped and sized parts. By introducing quality management systems (ISO 9000 series) coating centers try to improve their service and to satisfy their customers’ requirements for reliability and consistency. Besides quality the pricing is a key factor. Part price to coating price ratios of more than 40:1 for high-end tools or components are common for high volume coating services. For example, the lifetime of a tool worth several hundred dollars can be severely extended while the coating price stays below ten dollars. The reflected PVD/VAD market observations refer to systems based on DC arc discharges. Although this technology has proven to be reliable for a wide range of applications, there are intrinsic characteristics of the arc discharge that limit its further market expansion by influencing the performance of the deposition process and the deposited films. $ The maximum deposition rate with DC arc discharges per evaporation unit is intrinsically limited. At the same substrate–cathode distance, substrate temperature and arc current determine the deposition rate per evaporator. Usually, the temperature has to be in a substrate dependent range to ensure film adhesion on steel parts or to prevent decomposition of substrate materials that cannot stand higher temperatures (e.g. polymers). Another option to increase the deposition rate would be to increase the arc current. Arc discharges tend to split their cathodic foot points (cathode spots) exceeding a material specific current level. Several parallel existing spots influence each other at higher currents. Their intrinsic interaction drives them towards the cathode edge and finally shuts off the discharge. A common method to reduce this effect is the application of external magnetic fields. Still, the deposition rate is limited because too strong magnetic fields cause difficulties in igniting the arc discharge itself. This is caused by an increase of the necessary voltage to support the discharge in a magnetic field. The used maximum operating currents per evaporator rarely exceed 300 A. There are DC systems available that can run an evaporator source up to 1000 A. This is possible due to a special cathode design that requires a rather large amount of space inside the deposition chamber and limits the possible number of evaporators per chamber [1]. Generally, the limited deposition rate determines the throughput of PVD systems. $ While evaporating the cathode material, the vacuum arc discharge ejects microscopic droplets from the cathode towards the substrates which cause an increase in micro-roughness on the coated parts. This effect can be reduced by increasing the speed of $
227
cathode spots, e.g. by applying external magnetic fields. Presently, there is no method in existence that completely prevents the droplet deposition on the substrates without special plasma optical handling systems [2]. $ PVD implies generally a line-of-sight deposition process. This can be partly overcome by applying a voltage bias to conductive substrates. Nevertheless, there are difficulties in coating parts with a homogeneous thin film that have complicated geometry such as tubes or sharp edges/corners. Applications like high aspect ratio tube coatings are out of the range of conventional DC arc PVD. $ Another critical factor that limits PVD application is related to corrosion resistance [3]. A variety of attempts were made to replace chromium/nickel electroplated films with pure PVD coatings. Salt-spray tests reveal insufficient protection against corrosion and prevent the direct replacement of electroplating technology by PVD. PVD chromium would have to be deposited with unusually high thickness for hard coatings (several 10 mm) to provide similar protection. Because of limited deposition rates this approach is too expensive. $ Precision coatings are a concern in leading edge applications like wafer metallization in the semiconductor industry. A typical requirement is the deposition of 500±50 nm aluminum films before etching metal interconnection layouts. This deposition is usually done by sputtering but chemical vapor deposition (CVD) becomes important as well. VAD is not used at all in this market, mainly because of the droplet concern. This listing of limitations reflects the reason for the steadily ongoing research and development in the field of vacuum arc plasma sources. A large number of developments were introduced in the last few years to tackle the limitations. A comparison of DC and AC VAD can demonstrate the potential of AC techniques for the massive extension of conventional markets served by VAD.
2. Conventional DC VAD 2.1. State-of-the-art DC VAD systems In today’s PVD coating service centers a fully equipped DC VAD batch production system features the following options. $ The system is integrated into the production line including transport and storage of the parts to coat, ex situ cleaning technology, PVD process, post-treatment and quality control. $ The entire PVD process covering vacuum system check, in situ cleaning steps (heat treatment, pre-
228
$
$
$
T. Schuelke et al. / Surface and Coatings Technology 120–121 (1999) 226–232
sputtering) and coating steps is completely computer controlled. Critical parameters like substrate temperature, background pressure, gas flows, arc currents and substrate bias voltages are continuously monitored to allow high quality coatings with minimum batch to batch variations. For instance, in the coating service centers of Metaplas Ionon the monitored data are stored in a batch linked file system to ensure ISO 9000 quality system satisfying documentation. The rotating substrate fixtures feature up to three axle planetary motions to ensure homogeneous film properties and to minimize within batch variations. Electronically controlled power supplies are designed to prevent destructive arcing effects on the substrate surface. High speed turbo pump systems are used to shorten the pumping time of larger chamber volumes.
2.2. Established markets for conventional DC VAD The major markets for conventional DC VAD are the cutting and cold forming tools industries. Cutting tools like twist drills and end mills made of high speed steel or solid tungsten carbide are coated to increase the tool life. Films like TiN, TiCN, CrN, ZrN or AlTiN dominate [4]. To reduce the abrasive wear of cold forming tools, manufacturers employ mechanical, thermal or thermochemical treatments. Mechanical surface rolling and heat treatment are standard methods. Very common are processes like salt bath or plasma ion nitriding (PIN ). In terms of thermo-chemical treatments, CVD is the classic process. PVD and especially DC VAD became interesting in the 1980s [5]. TiC, TiN and Cr are adequate films for most of the common tool steels [6 ]. Hybrid processes which involve application of PIN and DC VAD showed significant improvements in terms of lifetime and corrosion protection compared to isolated application of the technologies [7]. Machine parts and components such as pistons, gears, bearings, valves and others define another growing market for DC VAD coatings. Friction reduction and especially corrosion resistance become important. A general trend is to attempt replacing classical Cr/Ni electroplating processes by CVD/PVD techniques because of environmental reasons. Especially here the line-of-site nature of PVD processes becomes limiting. The third rapidly growing market for DC VAD concerns decorative applications. The combination of scratch resistance and decorative metallic shine (golden for TiN, ZrN, chrome–silver for CrN and other colors like rainbow effects) makes these film systems very attractive for bathroom fixtures, taps, door handles, cases and other applications. Unmachined brass or zinc parts are polished, Cr/Ni electroplated and finally PVD coated. As in the case of forming tools and component
coatings, the trend in decorative coatings is to get rid of environmentally harmful electroplating processes. DC VAD is also considered as a deposition process for coating plastics. Major concerns in the field of decorative coatings are throughput and the consistency of the achieved metallic shine, the latter influenced by droplets as well as process stability. Other markets like biomedical or medicine–technical applications are beginning to develop as a potential target for DC VAD techniques. 2.3. Development trends in DC VAD techniques Analyzing the major markets for DC VAD reveals trends that challenge equipment manufacturers: $ throughput for large volume production; $ new film composition for high speed and lubricant free machining; $ improvement of PVD film corrosion protection to enable replacement of Cr/Ni electroplating for forming tools, components and decorative applications; $ design of a special vacuum arc plasma source to address issues related to complex geometry to be coated; $ precision coatings with high thickness uniformity; $ droplet reduction; $ hybrid technologies (PIN+PVD). To address the throughput issue equipment manufactures are moving toward large production systems with several evaporator units per chamber. New film stacks combining hard coatings with rather soft films (MoS ) 2 address high speed and lubricant free cutting applications [8]. Carbon containing metal films are introduced to reduce the friction coefficient [9]. To eliminate droplet deposition on the substrates plasma filters are getting ready for industrial environments [10].
3. VAD employing AC arc discharges 3.1. AC discharge principle Through the attempt to optimize the controllability of VAD for multi-layer structures, to increase the deposition rates and to build high current ion sources, AC arc discharges came into the picture [11]. The AC vacuum arc is a sequence of individually ignited pulsed discharges. AC cycle parameters like pulse width and height are determined by the pulse forming network (PFN ). During the first part of the cycle the PFN capacitors discharge their energy into the vacuum arc. During the second part there is no arc burning and the remaining PFN energy recharges the capacitors. The energy consumed by the arc discharge gets compensated by the external power supply and the next cycle can be initiated. To strike the arc discharge, a short pulse
229
T. Schuelke et al. / Surface and Coatings Technology 120–121 (1999) 226–232
(a)
(b)
Fig. 1. Cathodic arc for (a) DC discharge and (b) AC discharge with high current arc (HCA) evaporator. The conditions for the DC discharge were: Ti cathode, arc current 100 A, exposure time 60 ms and evaporation mass 3 mg. The discharge conditions for the HCA evaporator were: Cu cathode, peak arc current 5000 A (average arc current at 300 s−1 repetition rate 1000 A), pulse duration 1 ms, evaporated material 2 mg.
ignition plasma is necessary. This plasma can be generated for instance mechanically (trigger pin), utilizing a pulsed laser beam or high voltage ignition sparks, however, for higher repetition rates, mechanical triggers are rather unqualified. The AC VAD technique applies high current vacuum arc discharges of several thousand amperes with repetition rates of several hundreds of pulses per second. Both parameters, current and repetition frequency are tailored to the specific application. They allow us to significantly increase the average arc current and subsequently the deposition rate compared to conventional DC VAD [12]. No external magnetic field is necessary to control the cathode spot dynamics. Fig. 1 shows pictures of cathodes in DC and AC vacuum arc discharge modes. 3.2. AC VAD — characteristics and applications In addition to the higher deposition rate, AC VAD offers other possibilities. Extensive investigation of vacuum arcs applying optical emission spectroscopy (OES) and high speed photography (HSP) showed significant differences in the plasma composition and cathode spot dynamics between DC and AC discharges
[13,14]. Plasmas carrying high current arc discharges are generally higher ionized and have higher electron temperatures than DC arc plasmas. Cathode spots in high current discharge move considerably faster over the cathode surface. Table 1 compiles plasma composition and cathode spot dynamics data as well as resulting film properties for different plasma sources (arc currents) (see Fig. 2). The following list summarizes characteristics of AC VAD compared to DC VAD and possibilities to utilize them addressing application relevant issues. $ High current arc discharges are utilized to enable AC VAD resulting in significantly higher plasma emission and deposition rates. Throughput can be improved. It also qualifies AC VAD for filtered arc applications to get rid of droplets/microparticles while maintaining a considerable deposition rate. Metal and glass sheet band coatings are other applications that demand high deposition rates. $ The energy content deposited into AC VAD plasmas is higher, resulting in increased electron temperatures and ionization degrees. In practice, this feature enables the deposition of special films like amorphous carbon films with a large content of sp3 bonds.
Table 1 Plasma composition, spot dynamics and film properties depending on the plasma source Plasma source
DC, 63 mm cylindrical cathode
AC laser arc
AC high current arc
Peak current (A) Ionization (%) Electron temperature (eV ) Ion energy (eV ) Cathode spot velocity (m s−1) Young’s modulus of amorphous carbon (GPa) Macroparticles
80 95 1.5 20 10 not measured Fig. 3(a)
1000 100 1.5 20 – 450–500 –
5000 100 2.0 45 50 750 Fig. 3(b)
230
T. Schuelke et al. / Surface and Coatings Technology 120–121 (1999) 226–232
Fig. 2. (a) Schematic of a filtered AC VAD system (HCA), (b) tilted and (c) straight cross-section of copper filled silicon dioxide trenches for metal interconnect in microelectronics.
$
$
Furthermore, it also allows the moderation of the ion energy distribution that substrates finally experience. Mechanical film properties such as Young’s modulus can be tailored with respect to application requirements. AC VAD cathode spots are moving much faster over the cathode surface. This reduces the droplet emission to a level that cannot be achieved by DC VAD without plasma filtering. Applications that need high deposition rates as well as improved surface finish (e.g. decorative coatings) are potentially benefiting. The cathode erosion area is defined by the ignition point and PFN parameters. This keeps the handling of the discharge simple and allows flexible evaporator designs.
is rotating while the laser beam scans the ignition plasma pulses along a line. This configuration allows the deposition of multi-layer film stacks by using different materials on the same cathode cylinder. The typical discharge current pulse carries 1000 A for 100 ms at a repetition rate of 1000 s−1. A second AC VAD system design was introduced with the HCA evaporator. Millisecond current pulses of 5000 A with repetition rates of 300 s−1 enabled an averaged arc current of 1000 A for the very first time in VAD. The arc discharges are triggered in the center of a circular shaped cathode by flashing high voltage sparks [16 ]. A ring of fast moving and simultaneously existing cathode spots erodes the cathode surface in two dimensions. This design allows the integration of the evaporator into a very compact flange unit (see Fig. 3).
3.3. AC VAD technology at Fraunhofer IWS 3.4. Development trends in AC VAD techniques During the last decade two basic AC VAD evaporator designs were developed at Fraunhofer IWS in Germany. The first development was the LaserArc@ technique [11,15]. The industrial prototype system proved its reliability for the deposition of superhard amorphous carbon films for a variety of applications. The basic evaporator design consists of a rotating cylindrical cathode, an anode plate as well as an optical lens system that guides a pulsed laser beam to the desired arc ignition point on the cathode. The cathode material gets eroded in two dimensions because the cathode cylinder
Today, AC VAD is mainly used on a laboratory scale. Prototype equipment is available that shows the potential of the technology in preparation for industrial scale production. Research activities are concentrated on two sides, process and equipment development. 3.4.1. Application and process development The process development makes use of the superior plasma conditions of high current discharges. These stimulate the development of deposition processes with
231
T. Schuelke et al. / Surface and Coatings Technology 120–121 (1999) 226–232
(a)
(b)
Fig. 3. (a) TiN film deposited with DC arc; (b) TiN film deposited with HCA.
respect to outstanding film properties. An example is the deposition of diamond-like carbon films (DLC ). The ability to control DLC film deposition opens applications ranging from cutting tools over laser mirror coatings to surface protection layers in the hard disk industry. DLC films are considered for cutting applications of non-ferrous materials in lubricant free environments. Compared to diamond CVD, the DLC AC VAD process offers lower cost due to much higher deposition rates and much better adhesion on tungsten carbide tools with more than 6% cobalt. Another example are disk and read/write head overcoats in the hard disk industry. Requirements are hard, smooth and corrosion protective films of less than 5 nm thickness with excellent
adhesion and low friction coefficient. An adequate technology was developed using filtered AC VAD [17]. The third example would be the deposition of copper interconnect patterns for semiconductor production backend processes [18].
3.4.2. Equipment development Besides ‘shop hardening’ of the prototype equipment the second area of development concerns the design of compact evaporator units that can be retrofitted to existing PVD equipment. This concept allows for the upgrade of production proven deposition equipment with the new possibilities of AC VAD inclusive of the
Table 2 Available VAD technologies in the PVD market Discharge type
DC
Plasma treatment
unfiltered
filtered
unfiltered
filtered
30–300 30–300
100–200 100–200
1000–5000 200–1000
1000–5000 200–1000
DC <20
DC <10
0.08–1 800
0.08–1 200
Plasma parameters Electron temperature (eV ) Ionization (%) Ion energy (eV )
1.5 95 20
– – –
1.5–2.0 100 45
1.5 95 25
Equipment example Type Manufacturer Typical film systems
MZR 303U Metaplas Ionon AlTiN, TiN, CrN, ZrN, TiCN
LaserArc@ Fraunhofer IWS DLC
Q-HCA Fraunhofer IWS DLC, copper, oxides
Typical film thickness (mm) Application
1–5 Tools, components, decorative
– – DLC, metals, hard coatings 0.05–1 Tools, components, electronics
0.002–0.050 Hard disk, R/W heads, semiconductor interconnection
Comments
Large-scale production
0.05–1 Components, tools for lubricant free machining, laser mirrors Prototype production ready
Discharge parameters Typical arc current (A) Averaged arc current per source (A) Pulse duration (ms) Maximum deposition rate per source (mm h−1)
AC
Special application, small lot
Development
232
T. Schuelke et al. / Surface and Coatings Technology 120–121 (1999) 226–232
combination with plasma filters to deposit droplet free PVD films with high deposition rates. The evidence that AC VAD systems and the developed processes can be utilized in a production environment will shift the technique from research and development laboratories to industry.
4. Available technologies for VAD Since both techniques, DC and AC VAD systems, can be equipped with plasma filters, there are four possible VAD configurations available. Table 2 compiles an overview.
5. Summary Summarizing the present situation in the field of DC and AC VAD PVD we consider the following. $ The DC VAD market is well established in the tools and components industry, it is growing in the field of decorative coatings. New markets, like biomedical applications, are in development. $ The AC VAD can overcome intrinsic limitations of DC VAD related to the deposition rate per evaporator unit. This makes VAD considerable for high rate coating processes that were classically served by electron beam evaporation, like steel band coating or architectural glass coating. The plasma source development is concentrating on retrofit systems that open a cost-effective way to add AC features to standard DC systems. $ The AC VAD has also proven to be able to extend the conventional application fields because of the superior plasma composition resulting in film systems for new applications. In combination with plasma filtering systems, AC VAD has the potential to gain market share in semiconductor and hard disk industry. AC VAD techniques extend the possibilities of VAD by enabling new applications that cannot be achieved by DC VAD. The introduction of AC VAD on an industrial scale is the short-term goal. This promotes VAD itself to be an even more competitive technology as it stands, compared to the variety of PVD/CVD techniques available on the market.
References [1] R.P. Welty, Linear magnetron arc cathode and macroparticle filter, Int. Conf. on Metallurgical Coatings and Thin Films, San Diego, CA, April 12–15, (1999). [2] A. Anders, Approaches to rid cathodic arc plasmas from macroand nanoparticles, Int. Conf. on Metallurgical Coatings and Thin Films, San Diego, CA, April 12–15, (1999). [3] P.Y. Park, E. Akiyama, A. Kawashima, K. Asami, K. Hashimoto, The corrosion behavior of sputter-deposited Cr–Mo alloys in 12 M HCl solution, Corros. Sci. 37 (11) (1995) 1843–1860. [4] J. Vetter, Vacuum arc coatings for tools: potential and application, Surf. Coat. Technol. 76/77 (1995) 719–724. [5] K. Lange, Lehrbuch der Umformtechnik, 2nd edn., Springer, Berlin, 1984. [6 ] R.L. Boxman, P.J. Martin, D.M. Sanders, Handbook of Vacuum Science and Technology: Fundamentals and Application, Noyes Publications, Park Ridge, NJ, 1995. [7] T. Schuelke, T. Krug, Plasma assisted surface technologies to improve tool properties, Int. Body Engineering Conf. (IBEC ), Detroit, MI, September 29–October 1, (1998). [8] I.L. Singer, S. Fayeulle, P.D. Ehni, Wear behavior of triode-sputtered MoS coatings in dry sliding contact with steel and ceramics, 2 Wear 195 (1/2) (1996) 7–20. [9] J. Vetter, T. Michler, H. Steuernagel, Hard coatings on thermochemically pretreated soft steels: application potential for ball valves, Surf. Coat. Technol. 111 (2/3) (1999) 210–219. [10] D. Bhat, Large-area filtered cathodic arc deposition (LAFAD@) technology for intelligent processing of materials, National Center for Manufacturing Sciences (NCMS) Fall Workshop Series, Dearborn, MI, September 21–22, (1998). [11] H.-J. Scheibe, P. Siemroth, Film deposition by laser-induced vacuum arc evaporation, IEEE Trans. Plasma Sci. 18 (6) (1990) 917–922. [12] B. Schultrich, P. Siemroth, H.-J. Scheibe, High rate deposition by vacuum arc method, Surf. Coat. Technol. 93 (1997) 64–68. [13] P. Siemroth, T. Schu¨lke, T. Witke, Investigation of cathode spots and plasma formation of vacuum arcs by high speed microscopy and spectroscopy, IEEE Trans. Plasma Sci. 25 (4) (1997) 571–579. [14] T. Witke, H. Ziegele, Plasma state in pulsed laser, arc and electron deposition, in: PSE’96, Fifth Int. Conf. on Plasma Surface Engineering, Garmisch-Partenkirchen (1996), Surf. Coat. Technol. 97 (1997) 414–419. [15] H.-J. Scheibe, D. Drescher, B. Schultrich, M. Falz, G. Leonhardt, R. Wilberg, The laser-arc: a new industrial technology for effective deposition of hard amorphous carbon films, Surf. Coat. Technol. 85 (1996) 209–214. [16 ] P. Siemroth, T. Schu¨lke, T. Witke, High-current arc — a new source for high-rate deposition, Surf. Coat. Technol. 68/69 (1994) 314–319. [17] T. Schu¨lke, A. Anders, P. Siemroth, Macro-particle filtering of high-current vacuum arc plasma, IEEE Trans. Plasma Sci. 25 (1997) 660–664. [18] C. Wenzel, N. Urbansky, P. Siemroth, T. Witke, New results of HCA based metal fill for ULSI application, Proc. Advanced Metallization and Interconnect Systems for ULSI Application, San Diego, (1997).
Comparison of DC and AC arc thin film deposition techniques Thomas Schuelke a, *, Thomas Witke b, Hans-Joachim Scheibe b, Peter Siemroth b, Bernd Schultrich b, Otmar Zimmer b, Jo¨rg Vetter c a Fraunhofer USA, Center for Surface and Laser Processing, Bradley University, College of Engineering and Technology, 1501 W. Bradley Avenue, Peoria, IL 61625, USA b Fraunhofer Institute Material and Beam Technology, Winterbergstr. 28, 01277 Dresden, Germany c Metaplas Ionon GmbH, Metaplas Ionon Oberfla¨chenveredelungs GmbH, Am Bo¨ttcherberg 30–38, 51427 Bergisch-Gladbach, Germany
Abstract DC and AC vacuum arc evaporation comprise two techniques currently implemented for thin film deposition, each with specific technical aspects with regard to the discharge systems and their application fields. The DC arc discharge at several 10 A up to 300 A is a well-established PVD technique to deposit hard coatings for a variety of applications. Examples of applications benefiting from this technique are cutting and forming tools. The primary function of hard coatings in this instance is to reduce friction and wear, and the standard coating materials employed are TiN, CrN, TiCN and AlTiN. In today’s manufacturing industries, machines are designed to be fully automated to simplify the coating process which contains several steps like heating, cleaning and deposition. In contrast, AC arc discharges are characterized by the repetition of short current pulses up to several 1000 A. They offer new possibilities including an increase of average current. This is especially important for applications that require plasma filtering to reduce or even avoid droplet deposition on substrates. Yet another useful property is the increase of plasma ionization compared to DC discharges. Applications like hard-amorphous carbon deposition and metal-interconnect deposition schemes in the semiconductor industry benefit from these superior plasma conditions. AC systems could be effectively introduced to industry by using the already well-established DC equipment base. Attaching AC evaporators to conventional systems serves customers in the coating market with new flexibility while maintaining the production proven reliability of their investments. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Diamond-like carbon; Hard coatings; Thin film deposition; Vacuum arc technology
1. Introduction The use of physical vapor deposition (PVD) in thin film applications is state-of-the-art in a variety of established markets. Wear reducing hard coatings on tools and components, metallic shiny and scratch resistant covers for decorative accessories as well as the deposition of semiconductor metal interconnection systems are well-defined and developed markets. Vacuum arc deposition ( VAD) is one of the competing technologies that rapidly gained market share since it was introduced on an industrial scale in the late 1970s. Features like high deposition rates and the possibility of low temperature deposition processes made it popular. During the past decade, the equipment manufactur* Corresponding author. Tel.: +1-303-677-2720; fax: +1-309-677-3670. E-mail address: [email protected] ( T. Schuelke)
ers invested in improving the handling as well as the reliability of VAD systems. Leading VAD equipment manufacturers operate their own coating service centers to provide customers with high quality coatings. This ensures a short customer feedback loop to improve process and equipment development. Larger cutting tool manufacturers as well as companies that run a large number of cutting processes in their production lines tend to operate their own coating facilities to keep delivery times as short as possible. Generally, flexibility in batch size and delivery service are becoming critically important to customers. In order to respond as fast as possible and to keep low costs, service centers need to run the coating processes at high efficiency in terms of throughput and quality control. This requires: $ running different customer’s parts in the same batch to optimize the volume utilization of the coating chamber;
0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 48 0 - 6
T. Schuelke et al. / Surface and Coatings Technology 120–121 (1999) 226–232
stable process windows to ensure low batch to batch and within batch variations of the coated film properties to ensure consistent film quality for a variety of differently shaped and sized parts. By introducing quality management systems (ISO 9000 series) coating centers try to improve their service and to satisfy their customers’ requirements for reliability and consistency. Besides quality the pricing is a key factor. Part price to coating price ratios of more than 40:1 for high-end tools or components are common for high volume coating services. For example, the lifetime of a tool worth several hundred dollars can be severely extended while the coating price stays below ten dollars. The reflected PVD/VAD market observations refer to systems based on DC arc discharges. Although this technology has proven to be reliable for a wide range of applications, there are intrinsic characteristics of the arc discharge that limit its further market expansion by influencing the performance of the deposition process and the deposited films. $ The maximum deposition rate with DC arc discharges per evaporation unit is intrinsically limited. At the same substrate–cathode distance, substrate temperature and arc current determine the deposition rate per evaporator. Usually, the temperature has to be in a substrate dependent range to ensure film adhesion on steel parts or to prevent decomposition of substrate materials that cannot stand higher temperatures (e.g. polymers). Another option to increase the deposition rate would be to increase the arc current. Arc discharges tend to split their cathodic foot points (cathode spots) exceeding a material specific current level. Several parallel existing spots influence each other at higher currents. Their intrinsic interaction drives them towards the cathode edge and finally shuts off the discharge. A common method to reduce this effect is the application of external magnetic fields. Still, the deposition rate is limited because too strong magnetic fields cause difficulties in igniting the arc discharge itself. This is caused by an increase of the necessary voltage to support the discharge in a magnetic field. The used maximum operating currents per evaporator rarely exceed 300 A. There are DC systems available that can run an evaporator source up to 1000 A. This is possible due to a special cathode design that requires a rather large amount of space inside the deposition chamber and limits the possible number of evaporators per chamber [1]. Generally, the limited deposition rate determines the throughput of PVD systems. $ While evaporating the cathode material, the vacuum arc discharge ejects microscopic droplets from the cathode towards the substrates which cause an increase in micro-roughness on the coated parts. This effect can be reduced by increasing the speed of $
227
cathode spots, e.g. by applying external magnetic fields. Presently, there is no method in existence that completely prevents the droplet deposition on the substrates without special plasma optical handling systems [2]. $ PVD implies generally a line-of-sight deposition process. This can be partly overcome by applying a voltage bias to conductive substrates. Nevertheless, there are difficulties in coating parts with a homogeneous thin film that have complicated geometry such as tubes or sharp edges/corners. Applications like high aspect ratio tube coatings are out of the range of conventional DC arc PVD. $ Another critical factor that limits PVD application is related to corrosion resistance [3]. A variety of attempts were made to replace chromium/nickel electroplated films with pure PVD coatings. Salt-spray tests reveal insufficient protection against corrosion and prevent the direct replacement of electroplating technology by PVD. PVD chromium would have to be deposited with unusually high thickness for hard coatings (several 10 mm) to provide similar protection. Because of limited deposition rates this approach is too expensive. $ Precision coatings are a concern in leading edge applications like wafer metallization in the semiconductor industry. A typical requirement is the deposition of 500±50 nm aluminum films before etching metal interconnection layouts. This deposition is usually done by sputtering but chemical vapor deposition (CVD) becomes important as well. VAD is not used at all in this market, mainly because of the droplet concern. This listing of limitations reflects the reason for the steadily ongoing research and development in the field of vacuum arc plasma sources. A large number of developments were introduced in the last few years to tackle the limitations. A comparison of DC and AC VAD can demonstrate the potential of AC techniques for the massive extension of conventional markets served by VAD.
2. Conventional DC VAD 2.1. State-of-the-art DC VAD systems In today’s PVD coating service centers a fully equipped DC VAD batch production system features the following options. $ The system is integrated into the production line including transport and storage of the parts to coat, ex situ cleaning technology, PVD process, post-treatment and quality control. $ The entire PVD process covering vacuum system check, in situ cleaning steps (heat treatment, pre-
228
$
$
$
T. Schuelke et al. / Surface and Coatings Technology 120–121 (1999) 226–232
sputtering) and coating steps is completely computer controlled. Critical parameters like substrate temperature, background pressure, gas flows, arc currents and substrate bias voltages are continuously monitored to allow high quality coatings with minimum batch to batch variations. For instance, in the coating service centers of Metaplas Ionon the monitored data are stored in a batch linked file system to ensure ISO 9000 quality system satisfying documentation. The rotating substrate fixtures feature up to three axle planetary motions to ensure homogeneous film properties and to minimize within batch variations. Electronically controlled power supplies are designed to prevent destructive arcing effects on the substrate surface. High speed turbo pump systems are used to shorten the pumping time of larger chamber volumes.
2.2. Established markets for conventional DC VAD The major markets for conventional DC VAD are the cutting and cold forming tools industries. Cutting tools like twist drills and end mills made of high speed steel or solid tungsten carbide are coated to increase the tool life. Films like TiN, TiCN, CrN, ZrN or AlTiN dominate [4]. To reduce the abrasive wear of cold forming tools, manufacturers employ mechanical, thermal or thermochemical treatments. Mechanical surface rolling and heat treatment are standard methods. Very common are processes like salt bath or plasma ion nitriding (PIN ). In terms of thermo-chemical treatments, CVD is the classic process. PVD and especially DC VAD became interesting in the 1980s [5]. TiC, TiN and Cr are adequate films for most of the common tool steels [6 ]. Hybrid processes which involve application of PIN and DC VAD showed significant improvements in terms of lifetime and corrosion protection compared to isolated application of the technologies [7]. Machine parts and components such as pistons, gears, bearings, valves and others define another growing market for DC VAD coatings. Friction reduction and especially corrosion resistance become important. A general trend is to attempt replacing classical Cr/Ni electroplating processes by CVD/PVD techniques because of environmental reasons. Especially here the line-of-site nature of PVD processes becomes limiting. The third rapidly growing market for DC VAD concerns decorative applications. The combination of scratch resistance and decorative metallic shine (golden for TiN, ZrN, chrome–silver for CrN and other colors like rainbow effects) makes these film systems very attractive for bathroom fixtures, taps, door handles, cases and other applications. Unmachined brass or zinc parts are polished, Cr/Ni electroplated and finally PVD coated. As in the case of forming tools and component
coatings, the trend in decorative coatings is to get rid of environmentally harmful electroplating processes. DC VAD is also considered as a deposition process for coating plastics. Major concerns in the field of decorative coatings are throughput and the consistency of the achieved metallic shine, the latter influenced by droplets as well as process stability. Other markets like biomedical or medicine–technical applications are beginning to develop as a potential target for DC VAD techniques. 2.3. Development trends in DC VAD techniques Analyzing the major markets for DC VAD reveals trends that challenge equipment manufacturers: $ throughput for large volume production; $ new film composition for high speed and lubricant free machining; $ improvement of PVD film corrosion protection to enable replacement of Cr/Ni electroplating for forming tools, components and decorative applications; $ design of a special vacuum arc plasma source to address issues related to complex geometry to be coated; $ precision coatings with high thickness uniformity; $ droplet reduction; $ hybrid technologies (PIN+PVD). To address the throughput issue equipment manufactures are moving toward large production systems with several evaporator units per chamber. New film stacks combining hard coatings with rather soft films (MoS ) 2 address high speed and lubricant free cutting applications [8]. Carbon containing metal films are introduced to reduce the friction coefficient [9]. To eliminate droplet deposition on the substrates plasma filters are getting ready for industrial environments [10].
3. VAD employing AC arc discharges 3.1. AC discharge principle Through the attempt to optimize the controllability of VAD for multi-layer structures, to increase the deposition rates and to build high current ion sources, AC arc discharges came into the picture [11]. The AC vacuum arc is a sequence of individually ignited pulsed discharges. AC cycle parameters like pulse width and height are determined by the pulse forming network (PFN ). During the first part of the cycle the PFN capacitors discharge their energy into the vacuum arc. During the second part there is no arc burning and the remaining PFN energy recharges the capacitors. The energy consumed by the arc discharge gets compensated by the external power supply and the next cycle can be initiated. To strike the arc discharge, a short pulse
229
T. Schuelke et al. / Surface and Coatings Technology 120–121 (1999) 226–232
(a)
(b)
Fig. 1. Cathodic arc for (a) DC discharge and (b) AC discharge with high current arc (HCA) evaporator. The conditions for the DC discharge were: Ti cathode, arc current 100 A, exposure time 60 ms and evaporation mass 3 mg. The discharge conditions for the HCA evaporator were: Cu cathode, peak arc current 5000 A (average arc current at 300 s−1 repetition rate 1000 A), pulse duration 1 ms, evaporated material 2 mg.
ignition plasma is necessary. This plasma can be generated for instance mechanically (trigger pin), utilizing a pulsed laser beam or high voltage ignition sparks, however, for higher repetition rates, mechanical triggers are rather unqualified. The AC VAD technique applies high current vacuum arc discharges of several thousand amperes with repetition rates of several hundreds of pulses per second. Both parameters, current and repetition frequency are tailored to the specific application. They allow us to significantly increase the average arc current and subsequently the deposition rate compared to conventional DC VAD [12]. No external magnetic field is necessary to control the cathode spot dynamics. Fig. 1 shows pictures of cathodes in DC and AC vacuum arc discharge modes. 3.2. AC VAD — characteristics and applications In addition to the higher deposition rate, AC VAD offers other possibilities. Extensive investigation of vacuum arcs applying optical emission spectroscopy (OES) and high speed photography (HSP) showed significant differences in the plasma composition and cathode spot dynamics between DC and AC discharges
[13,14]. Plasmas carrying high current arc discharges are generally higher ionized and have higher electron temperatures than DC arc plasmas. Cathode spots in high current discharge move considerably faster over the cathode surface. Table 1 compiles plasma composition and cathode spot dynamics data as well as resulting film properties for different plasma sources (arc currents) (see Fig. 2). The following list summarizes characteristics of AC VAD compared to DC VAD and possibilities to utilize them addressing application relevant issues. $ High current arc discharges are utilized to enable AC VAD resulting in significantly higher plasma emission and deposition rates. Throughput can be improved. It also qualifies AC VAD for filtered arc applications to get rid of droplets/microparticles while maintaining a considerable deposition rate. Metal and glass sheet band coatings are other applications that demand high deposition rates. $ The energy content deposited into AC VAD plasmas is higher, resulting in increased electron temperatures and ionization degrees. In practice, this feature enables the deposition of special films like amorphous carbon films with a large content of sp3 bonds.
Table 1 Plasma composition, spot dynamics and film properties depending on the plasma source Plasma source
DC, 63 mm cylindrical cathode
AC laser arc
AC high current arc
Peak current (A) Ionization (%) Electron temperature (eV ) Ion energy (eV ) Cathode spot velocity (m s−1) Young’s modulus of amorphous carbon (GPa) Macroparticles
80 95 1.5 20 10 not measured Fig. 3(a)
1000 100 1.5 20 – 450–500 –
5000 100 2.0 45 50 750 Fig. 3(b)
230
T. Schuelke et al. / Surface and Coatings Technology 120–121 (1999) 226–232
Fig. 2. (a) Schematic of a filtered AC VAD system (HCA), (b) tilted and (c) straight cross-section of copper filled silicon dioxide trenches for metal interconnect in microelectronics.
$
$
Furthermore, it also allows the moderation of the ion energy distribution that substrates finally experience. Mechanical film properties such as Young’s modulus can be tailored with respect to application requirements. AC VAD cathode spots are moving much faster over the cathode surface. This reduces the droplet emission to a level that cannot be achieved by DC VAD without plasma filtering. Applications that need high deposition rates as well as improved surface finish (e.g. decorative coatings) are potentially benefiting. The cathode erosion area is defined by the ignition point and PFN parameters. This keeps the handling of the discharge simple and allows flexible evaporator designs.
is rotating while the laser beam scans the ignition plasma pulses along a line. This configuration allows the deposition of multi-layer film stacks by using different materials on the same cathode cylinder. The typical discharge current pulse carries 1000 A for 100 ms at a repetition rate of 1000 s−1. A second AC VAD system design was introduced with the HCA evaporator. Millisecond current pulses of 5000 A with repetition rates of 300 s−1 enabled an averaged arc current of 1000 A for the very first time in VAD. The arc discharges are triggered in the center of a circular shaped cathode by flashing high voltage sparks [16 ]. A ring of fast moving and simultaneously existing cathode spots erodes the cathode surface in two dimensions. This design allows the integration of the evaporator into a very compact flange unit (see Fig. 3).
3.3. AC VAD technology at Fraunhofer IWS 3.4. Development trends in AC VAD techniques During the last decade two basic AC VAD evaporator designs were developed at Fraunhofer IWS in Germany. The first development was the LaserArc@ technique [11,15]. The industrial prototype system proved its reliability for the deposition of superhard amorphous carbon films for a variety of applications. The basic evaporator design consists of a rotating cylindrical cathode, an anode plate as well as an optical lens system that guides a pulsed laser beam to the desired arc ignition point on the cathode. The cathode material gets eroded in two dimensions because the cathode cylinder
Today, AC VAD is mainly used on a laboratory scale. Prototype equipment is available that shows the potential of the technology in preparation for industrial scale production. Research activities are concentrated on two sides, process and equipment development. 3.4.1. Application and process development The process development makes use of the superior plasma conditions of high current discharges. These stimulate the development of deposition processes with
231
T. Schuelke et al. / Surface and Coatings Technology 120–121 (1999) 226–232
(a)
(b)
Fig. 3. (a) TiN film deposited with DC arc; (b) TiN film deposited with HCA.
respect to outstanding film properties. An example is the deposition of diamond-like carbon films (DLC ). The ability to control DLC film deposition opens applications ranging from cutting tools over laser mirror coatings to surface protection layers in the hard disk industry. DLC films are considered for cutting applications of non-ferrous materials in lubricant free environments. Compared to diamond CVD, the DLC AC VAD process offers lower cost due to much higher deposition rates and much better adhesion on tungsten carbide tools with more than 6% cobalt. Another example are disk and read/write head overcoats in the hard disk industry. Requirements are hard, smooth and corrosion protective films of less than 5 nm thickness with excellent
adhesion and low friction coefficient. An adequate technology was developed using filtered AC VAD [17]. The third example would be the deposition of copper interconnect patterns for semiconductor production backend processes [18].
3.4.2. Equipment development Besides ‘shop hardening’ of the prototype equipment the second area of development concerns the design of compact evaporator units that can be retrofitted to existing PVD equipment. This concept allows for the upgrade of production proven deposition equipment with the new possibilities of AC VAD inclusive of the
Table 2 Available VAD technologies in the PVD market Discharge type
DC
Plasma treatment
unfiltered
filtered
unfiltered
filtered
30–300 30–300
100–200 100–200
1000–5000 200–1000
1000–5000 200–1000
DC <20
DC <10
0.08–1 800
0.08–1 200
Plasma parameters Electron temperature (eV ) Ionization (%) Ion energy (eV )
1.5 95 20
– – –
1.5–2.0 100 45
1.5 95 25
Equipment example Type Manufacturer Typical film systems
MZR 303U Metaplas Ionon AlTiN, TiN, CrN, ZrN, TiCN
LaserArc@ Fraunhofer IWS DLC
Q-HCA Fraunhofer IWS DLC, copper, oxides
Typical film thickness (mm) Application
1–5 Tools, components, decorative
– – DLC, metals, hard coatings 0.05–1 Tools, components, electronics
0.002–0.050 Hard disk, R/W heads, semiconductor interconnection
Comments
Large-scale production
0.05–1 Components, tools for lubricant free machining, laser mirrors Prototype production ready
Discharge parameters Typical arc current (A) Averaged arc current per source (A) Pulse duration (ms) Maximum deposition rate per source (mm h−1)
AC
Special application, small lot
Development
232
T. Schuelke et al. / Surface and Coatings Technology 120–121 (1999) 226–232
combination with plasma filters to deposit droplet free PVD films with high deposition rates. The evidence that AC VAD systems and the developed processes can be utilized in a production environment will shift the technique from research and development laboratories to industry.
4. Available technologies for VAD Since both techniques, DC and AC VAD systems, can be equipped with plasma filters, there are four possible VAD configurations available. Table 2 compiles an overview.
5. Summary Summarizing the present situation in the field of DC and AC VAD PVD we consider the following. $ The DC VAD market is well established in the tools and components industry, it is growing in the field of decorative coatings. New markets, like biomedical applications, are in development. $ The AC VAD can overcome intrinsic limitations of DC VAD related to the deposition rate per evaporator unit. This makes VAD considerable for high rate coating processes that were classically served by electron beam evaporation, like steel band coating or architectural glass coating. The plasma source development is concentrating on retrofit systems that open a cost-effective way to add AC features to standard DC systems. $ The AC VAD has also proven to be able to extend the conventional application fields because of the superior plasma composition resulting in film systems for new applications. In combination with plasma filtering systems, AC VAD has the potential to gain market share in semiconductor and hard disk industry. AC VAD techniques extend the possibilities of VAD by enabling new applications that cannot be achieved by DC VAD. The introduction of AC VAD on an industrial scale is the short-term goal. This promotes VAD itself to be an even more competitive technology as it stands, compared to the variety of PVD/CVD techniques available on the market.
References [1] R.P. Welty, Linear magnetron arc cathode and macroparticle filter, Int. Conf. on Metallurgical Coatings and Thin Films, San Diego, CA, April 12–15, (1999). [2] A. Anders, Approaches to rid cathodic arc plasmas from macroand nanoparticles, Int. Conf. on Metallurgical Coatings and Thin Films, San Diego, CA, April 12–15, (1999). [3] P.Y. Park, E. Akiyama, A. Kawashima, K. Asami, K. Hashimoto, The corrosion behavior of sputter-deposited Cr–Mo alloys in 12 M HCl solution, Corros. Sci. 37 (11) (1995) 1843–1860. [4] J. Vetter, Vacuum arc coatings for tools: potential and application, Surf. Coat. Technol. 76/77 (1995) 719–724. [5] K. Lange, Lehrbuch der Umformtechnik, 2nd edn., Springer, Berlin, 1984. [6 ] R.L. Boxman, P.J. Martin, D.M. Sanders, Handbook of Vacuum Science and Technology: Fundamentals and Application, Noyes Publications, Park Ridge, NJ, 1995. [7] T. Schuelke, T. Krug, Plasma assisted surface technologies to improve tool properties, Int. Body Engineering Conf. (IBEC ), Detroit, MI, September 29–October 1, (1998). [8] I.L. Singer, S. Fayeulle, P.D. Ehni, Wear behavior of triode-sputtered MoS coatings in dry sliding contact with steel and ceramics, 2 Wear 195 (1/2) (1996) 7–20. [9] J. Vetter, T. Michler, H. Steuernagel, Hard coatings on thermochemically pretreated soft steels: application potential for ball valves, Surf. Coat. Technol. 111 (2/3) (1999) 210–219. [10] D. Bhat, Large-area filtered cathodic arc deposition (LAFAD@) technology for intelligent processing of materials, National Center for Manufacturing Sciences (NCMS) Fall Workshop Series, Dearborn, MI, September 21–22, (1998). [11] H.-J. Scheibe, P. Siemroth, Film deposition by laser-induced vacuum arc evaporation, IEEE Trans. Plasma Sci. 18 (6) (1990) 917–922. [12] B. Schultrich, P. Siemroth, H.-J. Scheibe, High rate deposition by vacuum arc method, Surf. Coat. Technol. 93 (1997) 64–68. [13] P. Siemroth, T. Schu¨lke, T. Witke, Investigation of cathode spots and plasma formation of vacuum arcs by high speed microscopy and spectroscopy, IEEE Trans. Plasma Sci. 25 (4) (1997) 571–579. [14] T. Witke, H. Ziegele, Plasma state in pulsed laser, arc and electron deposition, in: PSE’96, Fifth Int. Conf. on Plasma Surface Engineering, Garmisch-Partenkirchen (1996), Surf. Coat. Technol. 97 (1997) 414–419. [15] H.-J. Scheibe, D. Drescher, B. Schultrich, M. Falz, G. Leonhardt, R. Wilberg, The laser-arc: a new industrial technology for effective deposition of hard amorphous carbon films, Surf. Coat. Technol. 85 (1996) 209–214. [16 ] P. Siemroth, T. Schu¨lke, T. Witke, High-current arc — a new source for high-rate deposition, Surf. Coat. Technol. 68/69 (1994) 314–319. [17] T. Schu¨lke, A. Anders, P. Siemroth, Macro-particle filtering of high-current vacuum arc plasma, IEEE Trans. Plasma Sci. 25 (1997) 660–664. [18] C. Wenzel, N. Urbansky, P. Siemroth, T. Witke, New results of HCA based metal fill for ULSI application, Proc. Advanced Metallization and Interconnect Systems for ULSI Application, San Diego, (1997).