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Comparison of filtered high-current pulsed arc deposition (φ-HCA) with conventional vacuum arc methods
Surface and Coatings Technology 126 Ž2000. 81᎐88
Comparison of filtered high-current pulsed arc deposition ž -HCA/ with conventional vacuum arc methods T. Witke a,U , T. Schuelke b, B. Schultrich a , P. Siemrotha , J. Vetter c a
b
Fraunhofer-Institute Material and Beam Technology (IWS), Winterbergstr. 28, D-01277 Dresden, Germany Fraunhofer USA, Center for Surface and Laser Processing, One Technology Plaza, 211 Fulton Street, Peona, IL 61602 c METAPLAS IONON, Am Bottcherberg 30-38, D-51427 Bergisch Gladbach, Germany ¨ Received 9 December 1999; accepted in revised form 31 January 2000
Abstract Special features of the filtered high-current pulsed arc deposition Ž -HCA. are presented in comparison with the conventional dc-arc method. The high-current arc technique allows pulse currents of some kiloamperes and averaged arc currents of approximately 1 kA. Due to the high spot velocity the droplet density is already markedly reduced. The remaining particles may be completely eliminated by combination with a magnetic filter. The optimum design of this device together with the high efficiency of the pulsed arc source yields ion currents at the filter exit above 100 A during a pulsed arc and above 10-A averaged current. High quality films with deposition rates of industrial relevance may be produced by advanced arc techniques. Various coatings such as metal refractory nitrides, oxides and hard coatings are investigated and compared with the conventional dc-arc deposition. It is shown that the -HCA module is a promising supplementary device that can be attached to industrial standard equipment for various applications demanding smooth mirror-like metallic hard coatings. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Vacuum arc; Plasma filter; Hard coatings; Optical films; Metallic films
1. Introduction One of the established coating technologies in pulsed vapor deposition ŽPVD. is the dc vacuum arc source. Between a cathode, made of the material that is evaporated and the anode a low voltage, high-current vacuum arc discharge is operated. The arc discharge is concentrated to cathodic spots of a few micrometres in diameter and evaporates effectively the target material such as titanium, chromium, aluminum᎐titanium, zirconium. Because of the high energy density, the evaporated material is ionized, i.e. in the plasma state. This results in effective reactions with additional gas atmospheres such as nitrogen or acetylene. U
Corresponding author. Tel.: q49-351-2583-412. E-mail addresses: [email protected] ŽT. Witke., [email protected] ŽT. Schuelke., [email protected] ŽJ. Vetter.
Typical coatings are TiN, CrN, TiŽC,N., ŽAlTi.N and ZrN. Typically, the deposited film thickness is in the range of 1᎐5 m. Film properties are the substantial hardness, the low coefficient of friction, and the low tendency of the coatings to stick and weld. This results in superior performance of all types of metal cutting tools, metal forming tools, and plastic industry tools. A new field of application are decorative coatings. Standard PVD equipment and applications of hard coatings are shown in Fig. 1. An unwanted but intrinsic feature of vacuum arc based deposition technologies is the emission of macroparticles, originating from cathode spots and flying across the deposition chamber. These so-called droplets have dimensions up to tens of micrometers w1x and can easily hit the substrates. The resulting droplet accumulation on coated surfaces limits the fields of application. For example, contamination with macroparticles
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 5 4 4 - 2
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T. Witke et al. r Surface and Coatings Technology 126 (2000) 81᎐88
Fig. 1. PVD equipment for hard coatings Žlinks. deposition and typical applications of hard coatings.
is not acceptable for protective coatings of magnetic storage disks, optical applications, as well as film deposition in semiconductor industry. Fig. 2 shows TiN films deposited applying different vacuum arc technologies. The left image gives a typical example for dc-arc deposited coatings. It shows dense and void-free films. The particle distribution on top of this film generates a micro-roughness of approximately 140 nm. A micro-roughness of less than 15 nm can be achieved by extracting these particles out of the plasma before they get in contact with the substrates. The result is shown in the right image of Fig. 2.
2. Filtered high-current arc evaporator In the last years a new type of vacuum arc plasma source Žhigh-current pulsed arc, HCA. was developed to overcome the limits of surface smoothness and deposition rate achievable with conventional dc sources. The HCA offers the possibility to deposit homogeneous thin films with a rate up to 200 nmrs over a substrate area of approximately 100 cm2 w2,3x. Fig. 3 shows typical light patterns as they can be observed on the cathode surface while the arc discharge operates. Compared to the dc-arc discharge the
number of simultaneously existing spots and their moving velocity across the cathode are much higher in the HCA system. The emission probability of droplets decreases with increasing spot velocities w2x. Applying high-current discharges with high spot velocities is the first step towards the deposition of macroparticle free films. This type of arc source is highly recommended for plasma filter applications requiring high deposition rates and minimum micro-roughness. High-current vacuum arc carbon plasmas are able to form extremely thin, dense and void-free films. The film properties even satisfy requirements for hard disk and readrwrite head coatings, assumed there are no macroparticles on top of the coating. Different methods were proposed to minimize the macroparticle flux at substrates. The amount of particles could be markedly reduced by an increased cathode spot velocity using self magnetic fields w2x, external magnetic fields w4x or fast rotating cathodes w5x. But the best results were achieved by using curved magnetic ducts to separate plasma from particles w6᎐10x. Plasma losses occur due to ion recombination processes at the duct wall. In the last years the transmission Žplasma outputrplasma input. of 90⬚-filters could be improved up to 25% by optimizing the duct design. However, when using pulsed arc currents in the range from 50 to 300 A w9,11x, the
Fig. 2. Surface quality of TiN films Žthickness approx. 1.0 m. produced by different arc sources.
T. Witke et al. r Surface and Coatings Technology 126 (2000) 81᎐88
83
Fig. 3. Discharge patterns and parameters of dc-arc and HCA systems.
resulting time-averaged deposition rates at the substrates are far from the demands of industrial applications. To achieve deposition rates of industrial relevance, a new source of metal ion beams, consisting of a highcurrent pulsed arc evaporator combined with a sectioned filter unit was developed and successfully tested w12x. This arrangement, called -HCA consists of a compact HCA-source and a variable number of mag-
netic filter segments. It can be arranged ᎏ with or without filters ᎏ on any vacuum equipment having a 100-mm flange. To meet different demands, from one up to six filter units can be used, each of them with a curvature of 30⬚. The investigations, described in the present paper have been carried out with a two-segment filter Ž60⬚.. Fig. 4 shows the scheme and a view of the experimental arrangement used for the deposition tests de-
Fig. 4. Scheme and a view of the experimental arrangement used for the deposition tests.
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T. Witke et al. r Surface and Coatings Technology 126 (2000) 81᎐88
scribed in the following. For the present investigations flat, cylindrical cathodes of 50-mm diameter have been used. The anode was a copper plate at ground potential with a central hole of 5 cm for plasma escape. The pulse power supply delivers pulses of 500-s duration and peak currents of up to 5 kA. Ion fluxes were measured by ‘ion collectors’ Žextended metal plates, operating as Langmuir-probes in the ion saturation regime.. To measure the filter transmission and the plasma flux inside the filter the collectors were placed behind the anode hole Žwithout filter. and behind every filter segment. Because the transferred charge is decisive for coating rate, it was convenient and reasonable to use the time-integrated ion collector and arc currents Ži.e. charges. for the comparison of the results of different duct parameters. The duct wall bias and the axial magnetic field strength were varied in the range from 0 to 25 V and from 20 to 80 mT, respectively. Fig. 5 shows cross-sections of plasma density distributions at different filter positions. Interestingly, the density maximum shifts steadily along the flight path through the filter in both vertical and horizontal direction. The given numbers are averaged ion pulse currents. The cathode material was carbon. Fig. 6 shows the filter transmission vs. the duct wall bias for different magnetic fields. For carbon plasma, the best transmission was about 65% using a duct wall bias of approximately 12.5 V and a magnetic induction on the filter axis of approximately 80 mT. With increas-
ing ion mass and energy the transmission decreases remarkably Že.g. the best transmission for copper was approx. 25%.. The transport efficiency of higher than 25% is comparable with the best dc-filters, but due to the high arc current in the kiloampere-range deposition rates of approximately 10 nmrs were achieved. The deposited layers are without any particles, holes and pits Žsee Figs. 8 and 9.. Films of approximately 50-nm thickness have been deposited homogeneously Žthickness variation below 5%. on a 4-inch substrate after 300 pulses. Repetition rates are adjustable from more than 100 Hz down to single pulses depending on the allowed thermal input. The plasma characterizations of different vacuum arc vaporizer were determined by optical emission spectroscopy. By investigating the lines of spectrum with the Boltzmann-plot the electron temperature and ionization rate of the plasma could be determined. According to the profile of selected lines the electron density of the plasma is apparent. With these parameters, it is possible to estimate the ionization rate of the plasma with the Saha-equation. The detected emission spectra of the different carbon plasmas are shown in Fig. 7. They are dominated by the carbon ions. According to the plasma parameters of the carbon plasma the ionization rate equals one or is approximately one ŽTable 1.. The continuous radiation background in the dc- and laser-arc spectra is emitted by hot macroparticles. Due
Fig. 5. Plasma transportation in the -HCA arrangement, plasma flow distribution before, inside and after the filter ŽUBI AS s 15 V, magnetic fields 40 mT..
T. Witke et al. r Surface and Coatings Technology 126 (2000) 81᎐88
85
Fig. 6. Transmission vs. the duct bias for different axial magnetic fields Žcarbon plasma and two filter segments..
Fig. 7. Emission spectrum of carbon arc plasmas from the area of plasma formation Žcathode spot..
to the reduced macroparticle emission in the pulsed high-current arc discharge ŽHCA. its spectrum contains much less background. The intensity of the lines at the filter exit in the -HCA spectrum is less compared to the HCA spectrum Žfilter entrance .. Additionally, C 2molecule band radiation can be observed at the filter exit. The root cause for this transition are cooling mechanisms during the filtered plasma transport. The results of optical emission spectroscopy are summarized in Table 1. Besides dc-arc and HCA, the laser-arc plasma is included. This plasma is basically a high-current vacuum arc plasma ignited by a short laser pulse plasma w13x. The typical electron temperature is
1.5 eV. The laser-arc particle emission ranks in between dc-arc and HCA. Kinetic energies were measured by retarding field collectors. The HCA discharge produces the highest energetic ions. Plasma filtering decreases the average ion energy.
Table 1 Plasma parameters of carbon plasma, produced by arc evaporation of carbon
Arc current Pulse duration Electron temperature ŽeV. Composition Ž%. Neutrals C2 Ions Cq Ion energy ŽeV.
dc-arc
Laser-arc
HCA
-HCA
80 A DC Te ( 1.5
1 kA 80 s Te ( 1.5
5 kA 0.5᎐1 ms Te ( 2.0
5 kA 0.5 ms Te ( 1.5
5 95 20
0 100 20
0 100 45
5 95 25
3. Results Fig. 8 shows examples of SEM pictures of thin carbon films Žapprox. 100 nm. deposited with unfiltered and filtered HCA. Our dc-arc equipment is not capable of establishing a stable arc discharge on carbon cathodes. The small amount of particles emitted by a HCA carbon discharge can be completely filtered applying the -HCA arrangement. The TEM cross-section of a film made by filtered technique is presented in Fig. 9. It shows the amorphous film structure obviously. Mechanical and optical properties of the deposited carbon films are given in Table 2. Hard, amorphous carbon films with a hardness up to above 70 GPa have been produced. These films are suitable as protective layers for hard disks, compact discs and for optics and tools as well. Using the described filter arrangement, diamond-like carbon ŽDLC. films with a thickness of approximately 50 nm were
Table 2 Mechanical and optical properties of amorphous carbon films dc-arc Youngs-modulus ŽGPa. Extinction coefficient s 400 nm: a, k s 600 nm: a, k:
Insufficient film quality
Laser-arc 450᎐500 110 000 cmy1 , 0.35 42 000 cmy1 , 0.2
HCA 750 31 000 cmy1 , 0.1 - 2100 cmy1 , - 0.01
-HCA 550 31 000 cmy1 , 0.1 - 2100 cmy1 , - 0.01
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Fig. 8. Surface quality of amorphous carbon films Žthickness approx. 0.2 m..
deposited on silicon. Three-hundred pulses were necessary to achieve this thickness. Young’s modulus was measured using an ultrasonic technique described in Schneider et al. w14x, and values between 400 and 700 GPa have been found corresponding to a hardness between 40 and 70 GPa. These high values indicate the diamond-like characteristics of the films. Smooth, droplet- and void-free carbon films as thin as 5 nm were reproducibly deposited. An elastic modulus of 550 GPa Žhardness: 55 GPa. was consistently measured by applying laser acoustic wave technology w14x. The combination of the illustrated deposition and measurement capabilities opens the way to establish high quality manufacturing lines in DLC production. The number of DLC applications is steadily growing. Examples are coated tools for lubricant free highspeed processing of non-iron materials and surface protection of optical components in CO 2 laser equipment Fig. 10. Other application fields for filtered discharge are TiN-and Al 2 O 3 coatings. Filtered HCA deposition of droplet-free TiN films are useful for hard coatings on advanced tools and diffusion barrier layers in microelectronics. An example of a droplet free TiN film is
Fig. 9. TEM cross-section of an amorphous carbon film.
shown in Fig. 2. Transparent layers of Al 2 O 3 with medium hardness for protection of optical elements made of organic glasses are in discussion. With the dc-arc the inclusion of droplets reduces the transparency strongly. In current semiconductor manufacturing, the structure size is scaled down below 0.35 m. Trenches or vias with aspect ratios of up to 5 are filled by a chemical vapor deposition ŽCVD. process with tungsten for the vertical interconnection of different conductive layers. In the near future, the line width and via area have to be further minimized and consequently the aspect ratio will increase to at least 7᎐8. The deposition technology will be challenged by dual damascene approaches that require to fill via and trench structures on top of each other in one deposition step. Such deep cavities have to be completely filled with a highly conductive and electromigration resistant material. Several methods have been proposed to fill steep cavities with aluminum or copper. Up to now it is difficult to deposit these metals using CVD techniques. Specifically, the deposition of electromigration resistant AlrCu-alloy-films is unsolved. Thus, there have been several new PVD techniques proposed for the metallisation of sub-micrometre high aspect trenches and vias. Physical vapor deposition techniques, especially magnetron sputtering, have been widely used for the deposition of thin metallic films in microelectronics. Conventional PVD sources Ževaporators or magnetron sputter targets. cannot be used due to void formation and the deposition of low density films on the sidewalls. The best results of nearly void free PVD-based filling with aluminum or copper have been obtained with collimated sputtering w15x, or with sources employing highly ionized plasmas, such as self-sputtering w16x or ECR Želectron cyclotron resonance. plasma source w17x. Using the HCA evaporation a fully ionized metal flux is produced. The first results show an excellent filling behavior. Besides the 100% ionization, the high particle energy and the high deposition rate are the main advantages of this method w18,19x.
T. Witke et al. r Surface and Coatings Technology 126 (2000) 81᎐88
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Fig. 10. DLC-coatings Žpulsed arc. for lubricant-free machining of aluminum and copper Žleft. and on CO 2 -laser optics.
HCA deposition has been carried out for copper, pure aluminum and an aluminum alloy AlCu4Mg1. The filling procedure was performed in SiO 2 trenches. The adhesion was found to be excellent for copper as well as for aluminum even without any additional adhesion layer Že.g. Ta.. The substrate temperature was held near room temperature by back-side cooling. The average deposition rate was in the range of 100 nmrs Žunfiltered HCA. and 10 nmrs for the filtered HCA. The films have been characterized by scanning electron microscopy, sheet resistance measurements, X-ray microanalysis and Auger electron spectroscopy. Already without filtering copper, HCA deposition results in high quality films. The surface quality as well as the trench filling was further improved by using the filtered HCA. As an example in Fig. 11, a completely filled 0.4-m wide trench with an aspect ratio of approximately 3 is shown. This deposition was carried out onto a cooled substrate without substrate bias. Investigations of the thin film quality demonstrate high purity of the layer material. Defects were not observed in the trench.
Deposition experiments with Al and AlCuMg cathodes have been done, to test the filling behavior and to prove whether the stochiometry of the film reflects that of the cathode or not. Due to the huge difference in the atomic numbers of Cu and Al and Mg compositional changes in the thin film can be easily characterized. In both cases no marked grain growth, as typically for sputtering and evaporation could be observed. The thin film surface was found to be very smooth. Auger electron spectroscopy was used to characterize the purity of the thin films. No contamination was found after a short sputter cleaning the surface. Due to some problems for quantitative determination of low element concentration with AES, X-ray microanalysis was used to measure the copper and magnesium concentration in the alloy. The cathode composition was determined to 93.9 at.% Al, 3.7 at.% Cu and 2.4 at.% Mg. For the deposited thin films an average concentration of 94 at.% Al, 4 at.% Cu and 2 at.% Mg was found. This result supports the expectation that no essential compositional changes should occur in the arc deposition of alloys. This corresponds with the fact that
Fig. 11. Droplet-free deposition of copper by the -HCA arrangement, right picture.
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in the arcing process the target material is completely transformed to a flux of ionized particles.
4. Summary Using the described filter arrangement a large amount of different films have been deposited, metallic films, and films consisting of oxides and nitrides as well. Inspection by scanning electron microscopy ŽSEM. showed no macroparticles, holes and pits indicating that the separation of plasma and macroparticles works well with the scaled-up filter. The pulsed plasma beam of some hundred amperes was produced by transporting the fully ionized plasma vacuum arc plasma through a curved magnetic duct. Deposition rates of approximately 10 nmrs are achievable. The repetition rates are adjustable from 300 Hz down to single pulses in dependence on the acceptable thermal input. Promising applications are the deposition of hard, amorphous carbon films, droplet-free TiN films for advanced applications, transparent Al 2 O 3 protective coatings and metallisation for microelectronics. References w1x S. Anders, A. Anders, I.G. Brown, IEEE Trans. Plasma Sci. 21 Ž1993. 440᎐446. w2x P. Siemroth, T. Schulke, T. Witke, Surf. Coatings Technol. ¨ 68r69 Ž1994. 314᎐319. w3x B. Schultrich, P. Siemroth, H.-J. Scheibe, Surf. Coatings Technol. 93 Ž1997. 64᎐68.
w4x M. Ives, J. Brooks, J. Cawley, W. Burgmer, Surf. Coatings Technol. 49 Ž1991. 244. w5x G.H. Kang, H. Uchida, E.S. Koh, Surf. Coatings Technol. 68r69 Ž1994. 141᎐145. w6x I.I. Axenov, V.G. Bren, V.G. Padalka, L.P. Sablev, R.I. Stupak, V.M. Khoreshikh, US Pat. 4551221, 15 August 1984. w7x P.J. Martin, R.P. Netterfield, T.J. Kinder, US Pat. 5433836, 18 July 1995. w8x I.I. Axenov, V.G. Belous, V.G. Padalka, V.M. Khoreshikh, Sov. J. Plasma Phys 4 Ž1978. 425᎐428. w9x S. Falabella, D.M. Sanders, J. Vac. Sci. Technol. A10 Ž1992. 394᎐397. w10x S. Anders, A. Anders, I.G. Brown, J. Appl. Phys. 7 Ž1993. 4239᎐4241. w11x A. Anders, S. Anders, I.G. Brown, J. Appl. Phys. 75 Ž1994. 4895᎐4899. w12x T. Schulke, A. Anders, P. Siemroth, IEEE Trans. Plasma Sci. ¨ 25 Ž1997. 660᎐664. w13x H.-J. Scheibe, D. Drescher, B. Schultrich, M. Falz, G. Leonhardt, R. Wilberg, Surf. Coatings Technol. 85 Ž1996. 209᎐214. w14x D. Schneider, H.-J. Scheibe, P. Hess, Diamond Related Mater 2 Ž1993. 1396᎐1401. w15x A. Kobayashi, Advanced metallization and interconnect systems for ULSI application, in: R. Haveman, J. Schmitz, H. Komiyama, K. Tsubouchi ŽEds.., Mater. Res. Soc. Proceed., Pittsburgh, 1996, pp. 177᎐183. w16x N. Motegi, J. Vac. Sci. Technol. B13 Ž1995. 1906᎐1909. w17x C.A. Nichols, S.M. Rossnagel, S. Hamaguchi, J. Vac. Sci. Technol. B14 Ž1996. 3270᎐3275. w18x C. Wenzel, N. Urbansky, P. Siemroth, T. Schulke, Advanced ¨ metallization and interconnect systems for ULSI application, in: R. Havemann, J. Schmitz, H. Komiyama, K. Tsubouchi ŽEds.., Mater. Res. Soc. Proceed., Pittsburgh, 1996, pp. 185᎐190. w19x C. Wenzel, N. Urbansky, P. Siemroth, T. Witke, Proc. Advanced Metallization and Interconnect Systems for ULSI Application, San Diego, 1997.
Comparison of filtered high-current pulsed arc deposition ž -HCA/ with conventional vacuum arc methods T. Witke a,U , T. Schuelke b, B. Schultrich a , P. Siemrotha , J. Vetter c a
b
Fraunhofer-Institute Material and Beam Technology (IWS), Winterbergstr. 28, D-01277 Dresden, Germany Fraunhofer USA, Center for Surface and Laser Processing, One Technology Plaza, 211 Fulton Street, Peona, IL 61602 c METAPLAS IONON, Am Bottcherberg 30-38, D-51427 Bergisch Gladbach, Germany ¨ Received 9 December 1999; accepted in revised form 31 January 2000
Abstract Special features of the filtered high-current pulsed arc deposition Ž -HCA. are presented in comparison with the conventional dc-arc method. The high-current arc technique allows pulse currents of some kiloamperes and averaged arc currents of approximately 1 kA. Due to the high spot velocity the droplet density is already markedly reduced. The remaining particles may be completely eliminated by combination with a magnetic filter. The optimum design of this device together with the high efficiency of the pulsed arc source yields ion currents at the filter exit above 100 A during a pulsed arc and above 10-A averaged current. High quality films with deposition rates of industrial relevance may be produced by advanced arc techniques. Various coatings such as metal refractory nitrides, oxides and hard coatings are investigated and compared with the conventional dc-arc deposition. It is shown that the -HCA module is a promising supplementary device that can be attached to industrial standard equipment for various applications demanding smooth mirror-like metallic hard coatings. 䊚 2000 Elsevier Science S.A. All rights reserved. Keywords: Vacuum arc; Plasma filter; Hard coatings; Optical films; Metallic films
1. Introduction One of the established coating technologies in pulsed vapor deposition ŽPVD. is the dc vacuum arc source. Between a cathode, made of the material that is evaporated and the anode a low voltage, high-current vacuum arc discharge is operated. The arc discharge is concentrated to cathodic spots of a few micrometres in diameter and evaporates effectively the target material such as titanium, chromium, aluminum᎐titanium, zirconium. Because of the high energy density, the evaporated material is ionized, i.e. in the plasma state. This results in effective reactions with additional gas atmospheres such as nitrogen or acetylene. U
Corresponding author. Tel.: q49-351-2583-412. E-mail addresses: [email protected] ŽT. Witke., [email protected] ŽT. Schuelke., [email protected] ŽJ. Vetter.
Typical coatings are TiN, CrN, TiŽC,N., ŽAlTi.N and ZrN. Typically, the deposited film thickness is in the range of 1᎐5 m. Film properties are the substantial hardness, the low coefficient of friction, and the low tendency of the coatings to stick and weld. This results in superior performance of all types of metal cutting tools, metal forming tools, and plastic industry tools. A new field of application are decorative coatings. Standard PVD equipment and applications of hard coatings are shown in Fig. 1. An unwanted but intrinsic feature of vacuum arc based deposition technologies is the emission of macroparticles, originating from cathode spots and flying across the deposition chamber. These so-called droplets have dimensions up to tens of micrometers w1x and can easily hit the substrates. The resulting droplet accumulation on coated surfaces limits the fields of application. For example, contamination with macroparticles
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 5 4 4 - 2
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T. Witke et al. r Surface and Coatings Technology 126 (2000) 81᎐88
Fig. 1. PVD equipment for hard coatings Žlinks. deposition and typical applications of hard coatings.
is not acceptable for protective coatings of magnetic storage disks, optical applications, as well as film deposition in semiconductor industry. Fig. 2 shows TiN films deposited applying different vacuum arc technologies. The left image gives a typical example for dc-arc deposited coatings. It shows dense and void-free films. The particle distribution on top of this film generates a micro-roughness of approximately 140 nm. A micro-roughness of less than 15 nm can be achieved by extracting these particles out of the plasma before they get in contact with the substrates. The result is shown in the right image of Fig. 2.
2. Filtered high-current arc evaporator In the last years a new type of vacuum arc plasma source Žhigh-current pulsed arc, HCA. was developed to overcome the limits of surface smoothness and deposition rate achievable with conventional dc sources. The HCA offers the possibility to deposit homogeneous thin films with a rate up to 200 nmrs over a substrate area of approximately 100 cm2 w2,3x. Fig. 3 shows typical light patterns as they can be observed on the cathode surface while the arc discharge operates. Compared to the dc-arc discharge the
number of simultaneously existing spots and their moving velocity across the cathode are much higher in the HCA system. The emission probability of droplets decreases with increasing spot velocities w2x. Applying high-current discharges with high spot velocities is the first step towards the deposition of macroparticle free films. This type of arc source is highly recommended for plasma filter applications requiring high deposition rates and minimum micro-roughness. High-current vacuum arc carbon plasmas are able to form extremely thin, dense and void-free films. The film properties even satisfy requirements for hard disk and readrwrite head coatings, assumed there are no macroparticles on top of the coating. Different methods were proposed to minimize the macroparticle flux at substrates. The amount of particles could be markedly reduced by an increased cathode spot velocity using self magnetic fields w2x, external magnetic fields w4x or fast rotating cathodes w5x. But the best results were achieved by using curved magnetic ducts to separate plasma from particles w6᎐10x. Plasma losses occur due to ion recombination processes at the duct wall. In the last years the transmission Žplasma outputrplasma input. of 90⬚-filters could be improved up to 25% by optimizing the duct design. However, when using pulsed arc currents in the range from 50 to 300 A w9,11x, the
Fig. 2. Surface quality of TiN films Žthickness approx. 1.0 m. produced by different arc sources.
T. Witke et al. r Surface and Coatings Technology 126 (2000) 81᎐88
83
Fig. 3. Discharge patterns and parameters of dc-arc and HCA systems.
resulting time-averaged deposition rates at the substrates are far from the demands of industrial applications. To achieve deposition rates of industrial relevance, a new source of metal ion beams, consisting of a highcurrent pulsed arc evaporator combined with a sectioned filter unit was developed and successfully tested w12x. This arrangement, called -HCA consists of a compact HCA-source and a variable number of mag-
netic filter segments. It can be arranged ᎏ with or without filters ᎏ on any vacuum equipment having a 100-mm flange. To meet different demands, from one up to six filter units can be used, each of them with a curvature of 30⬚. The investigations, described in the present paper have been carried out with a two-segment filter Ž60⬚.. Fig. 4 shows the scheme and a view of the experimental arrangement used for the deposition tests de-
Fig. 4. Scheme and a view of the experimental arrangement used for the deposition tests.
84
T. Witke et al. r Surface and Coatings Technology 126 (2000) 81᎐88
scribed in the following. For the present investigations flat, cylindrical cathodes of 50-mm diameter have been used. The anode was a copper plate at ground potential with a central hole of 5 cm for plasma escape. The pulse power supply delivers pulses of 500-s duration and peak currents of up to 5 kA. Ion fluxes were measured by ‘ion collectors’ Žextended metal plates, operating as Langmuir-probes in the ion saturation regime.. To measure the filter transmission and the plasma flux inside the filter the collectors were placed behind the anode hole Žwithout filter. and behind every filter segment. Because the transferred charge is decisive for coating rate, it was convenient and reasonable to use the time-integrated ion collector and arc currents Ži.e. charges. for the comparison of the results of different duct parameters. The duct wall bias and the axial magnetic field strength were varied in the range from 0 to 25 V and from 20 to 80 mT, respectively. Fig. 5 shows cross-sections of plasma density distributions at different filter positions. Interestingly, the density maximum shifts steadily along the flight path through the filter in both vertical and horizontal direction. The given numbers are averaged ion pulse currents. The cathode material was carbon. Fig. 6 shows the filter transmission vs. the duct wall bias for different magnetic fields. For carbon plasma, the best transmission was about 65% using a duct wall bias of approximately 12.5 V and a magnetic induction on the filter axis of approximately 80 mT. With increas-
ing ion mass and energy the transmission decreases remarkably Že.g. the best transmission for copper was approx. 25%.. The transport efficiency of higher than 25% is comparable with the best dc-filters, but due to the high arc current in the kiloampere-range deposition rates of approximately 10 nmrs were achieved. The deposited layers are without any particles, holes and pits Žsee Figs. 8 and 9.. Films of approximately 50-nm thickness have been deposited homogeneously Žthickness variation below 5%. on a 4-inch substrate after 300 pulses. Repetition rates are adjustable from more than 100 Hz down to single pulses depending on the allowed thermal input. The plasma characterizations of different vacuum arc vaporizer were determined by optical emission spectroscopy. By investigating the lines of spectrum with the Boltzmann-plot the electron temperature and ionization rate of the plasma could be determined. According to the profile of selected lines the electron density of the plasma is apparent. With these parameters, it is possible to estimate the ionization rate of the plasma with the Saha-equation. The detected emission spectra of the different carbon plasmas are shown in Fig. 7. They are dominated by the carbon ions. According to the plasma parameters of the carbon plasma the ionization rate equals one or is approximately one ŽTable 1.. The continuous radiation background in the dc- and laser-arc spectra is emitted by hot macroparticles. Due
Fig. 5. Plasma transportation in the -HCA arrangement, plasma flow distribution before, inside and after the filter ŽUBI AS s 15 V, magnetic fields 40 mT..
T. Witke et al. r Surface and Coatings Technology 126 (2000) 81᎐88
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Fig. 6. Transmission vs. the duct bias for different axial magnetic fields Žcarbon plasma and two filter segments..
Fig. 7. Emission spectrum of carbon arc plasmas from the area of plasma formation Žcathode spot..
to the reduced macroparticle emission in the pulsed high-current arc discharge ŽHCA. its spectrum contains much less background. The intensity of the lines at the filter exit in the -HCA spectrum is less compared to the HCA spectrum Žfilter entrance .. Additionally, C 2molecule band radiation can be observed at the filter exit. The root cause for this transition are cooling mechanisms during the filtered plasma transport. The results of optical emission spectroscopy are summarized in Table 1. Besides dc-arc and HCA, the laser-arc plasma is included. This plasma is basically a high-current vacuum arc plasma ignited by a short laser pulse plasma w13x. The typical electron temperature is
1.5 eV. The laser-arc particle emission ranks in between dc-arc and HCA. Kinetic energies were measured by retarding field collectors. The HCA discharge produces the highest energetic ions. Plasma filtering decreases the average ion energy.
Table 1 Plasma parameters of carbon plasma, produced by arc evaporation of carbon
Arc current Pulse duration Electron temperature ŽeV. Composition Ž%. Neutrals C2 Ions Cq Ion energy ŽeV.
dc-arc
Laser-arc
HCA
-HCA
80 A DC Te ( 1.5
1 kA 80 s Te ( 1.5
5 kA 0.5᎐1 ms Te ( 2.0
5 kA 0.5 ms Te ( 1.5
5 95 20
0 100 20
0 100 45
5 95 25
3. Results Fig. 8 shows examples of SEM pictures of thin carbon films Žapprox. 100 nm. deposited with unfiltered and filtered HCA. Our dc-arc equipment is not capable of establishing a stable arc discharge on carbon cathodes. The small amount of particles emitted by a HCA carbon discharge can be completely filtered applying the -HCA arrangement. The TEM cross-section of a film made by filtered technique is presented in Fig. 9. It shows the amorphous film structure obviously. Mechanical and optical properties of the deposited carbon films are given in Table 2. Hard, amorphous carbon films with a hardness up to above 70 GPa have been produced. These films are suitable as protective layers for hard disks, compact discs and for optics and tools as well. Using the described filter arrangement, diamond-like carbon ŽDLC. films with a thickness of approximately 50 nm were
Table 2 Mechanical and optical properties of amorphous carbon films dc-arc Youngs-modulus ŽGPa. Extinction coefficient s 400 nm: a, k s 600 nm: a, k:
Insufficient film quality
Laser-arc 450᎐500 110 000 cmy1 , 0.35 42 000 cmy1 , 0.2
HCA 750 31 000 cmy1 , 0.1 - 2100 cmy1 , - 0.01
-HCA 550 31 000 cmy1 , 0.1 - 2100 cmy1 , - 0.01
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Fig. 8. Surface quality of amorphous carbon films Žthickness approx. 0.2 m..
deposited on silicon. Three-hundred pulses were necessary to achieve this thickness. Young’s modulus was measured using an ultrasonic technique described in Schneider et al. w14x, and values between 400 and 700 GPa have been found corresponding to a hardness between 40 and 70 GPa. These high values indicate the diamond-like characteristics of the films. Smooth, droplet- and void-free carbon films as thin as 5 nm were reproducibly deposited. An elastic modulus of 550 GPa Žhardness: 55 GPa. was consistently measured by applying laser acoustic wave technology w14x. The combination of the illustrated deposition and measurement capabilities opens the way to establish high quality manufacturing lines in DLC production. The number of DLC applications is steadily growing. Examples are coated tools for lubricant free highspeed processing of non-iron materials and surface protection of optical components in CO 2 laser equipment Fig. 10. Other application fields for filtered discharge are TiN-and Al 2 O 3 coatings. Filtered HCA deposition of droplet-free TiN films are useful for hard coatings on advanced tools and diffusion barrier layers in microelectronics. An example of a droplet free TiN film is
Fig. 9. TEM cross-section of an amorphous carbon film.
shown in Fig. 2. Transparent layers of Al 2 O 3 with medium hardness for protection of optical elements made of organic glasses are in discussion. With the dc-arc the inclusion of droplets reduces the transparency strongly. In current semiconductor manufacturing, the structure size is scaled down below 0.35 m. Trenches or vias with aspect ratios of up to 5 are filled by a chemical vapor deposition ŽCVD. process with tungsten for the vertical interconnection of different conductive layers. In the near future, the line width and via area have to be further minimized and consequently the aspect ratio will increase to at least 7᎐8. The deposition technology will be challenged by dual damascene approaches that require to fill via and trench structures on top of each other in one deposition step. Such deep cavities have to be completely filled with a highly conductive and electromigration resistant material. Several methods have been proposed to fill steep cavities with aluminum or copper. Up to now it is difficult to deposit these metals using CVD techniques. Specifically, the deposition of electromigration resistant AlrCu-alloy-films is unsolved. Thus, there have been several new PVD techniques proposed for the metallisation of sub-micrometre high aspect trenches and vias. Physical vapor deposition techniques, especially magnetron sputtering, have been widely used for the deposition of thin metallic films in microelectronics. Conventional PVD sources Ževaporators or magnetron sputter targets. cannot be used due to void formation and the deposition of low density films on the sidewalls. The best results of nearly void free PVD-based filling with aluminum or copper have been obtained with collimated sputtering w15x, or with sources employing highly ionized plasmas, such as self-sputtering w16x or ECR Želectron cyclotron resonance. plasma source w17x. Using the HCA evaporation a fully ionized metal flux is produced. The first results show an excellent filling behavior. Besides the 100% ionization, the high particle energy and the high deposition rate are the main advantages of this method w18,19x.
T. Witke et al. r Surface and Coatings Technology 126 (2000) 81᎐88
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Fig. 10. DLC-coatings Žpulsed arc. for lubricant-free machining of aluminum and copper Žleft. and on CO 2 -laser optics.
HCA deposition has been carried out for copper, pure aluminum and an aluminum alloy AlCu4Mg1. The filling procedure was performed in SiO 2 trenches. The adhesion was found to be excellent for copper as well as for aluminum even without any additional adhesion layer Že.g. Ta.. The substrate temperature was held near room temperature by back-side cooling. The average deposition rate was in the range of 100 nmrs Žunfiltered HCA. and 10 nmrs for the filtered HCA. The films have been characterized by scanning electron microscopy, sheet resistance measurements, X-ray microanalysis and Auger electron spectroscopy. Already without filtering copper, HCA deposition results in high quality films. The surface quality as well as the trench filling was further improved by using the filtered HCA. As an example in Fig. 11, a completely filled 0.4-m wide trench with an aspect ratio of approximately 3 is shown. This deposition was carried out onto a cooled substrate without substrate bias. Investigations of the thin film quality demonstrate high purity of the layer material. Defects were not observed in the trench.
Deposition experiments with Al and AlCuMg cathodes have been done, to test the filling behavior and to prove whether the stochiometry of the film reflects that of the cathode or not. Due to the huge difference in the atomic numbers of Cu and Al and Mg compositional changes in the thin film can be easily characterized. In both cases no marked grain growth, as typically for sputtering and evaporation could be observed. The thin film surface was found to be very smooth. Auger electron spectroscopy was used to characterize the purity of the thin films. No contamination was found after a short sputter cleaning the surface. Due to some problems for quantitative determination of low element concentration with AES, X-ray microanalysis was used to measure the copper and magnesium concentration in the alloy. The cathode composition was determined to 93.9 at.% Al, 3.7 at.% Cu and 2.4 at.% Mg. For the deposited thin films an average concentration of 94 at.% Al, 4 at.% Cu and 2 at.% Mg was found. This result supports the expectation that no essential compositional changes should occur in the arc deposition of alloys. This corresponds with the fact that
Fig. 11. Droplet-free deposition of copper by the -HCA arrangement, right picture.
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in the arcing process the target material is completely transformed to a flux of ionized particles.
4. Summary Using the described filter arrangement a large amount of different films have been deposited, metallic films, and films consisting of oxides and nitrides as well. Inspection by scanning electron microscopy ŽSEM. showed no macroparticles, holes and pits indicating that the separation of plasma and macroparticles works well with the scaled-up filter. The pulsed plasma beam of some hundred amperes was produced by transporting the fully ionized plasma vacuum arc plasma through a curved magnetic duct. Deposition rates of approximately 10 nmrs are achievable. The repetition rates are adjustable from 300 Hz down to single pulses in dependence on the acceptable thermal input. Promising applications are the deposition of hard, amorphous carbon films, droplet-free TiN films for advanced applications, transparent Al 2 O 3 protective coatings and metallisation for microelectronics. References w1x S. Anders, A. Anders, I.G. Brown, IEEE Trans. Plasma Sci. 21 Ž1993. 440᎐446. w2x P. Siemroth, T. Schulke, T. Witke, Surf. Coatings Technol. ¨ 68r69 Ž1994. 314᎐319. w3x B. Schultrich, P. Siemroth, H.-J. Scheibe, Surf. Coatings Technol. 93 Ž1997. 64᎐68.
w4x M. Ives, J. Brooks, J. Cawley, W. Burgmer, Surf. Coatings Technol. 49 Ž1991. 244. w5x G.H. Kang, H. Uchida, E.S. Koh, Surf. Coatings Technol. 68r69 Ž1994. 141᎐145. w6x I.I. Axenov, V.G. Bren, V.G. Padalka, L.P. Sablev, R.I. Stupak, V.M. Khoreshikh, US Pat. 4551221, 15 August 1984. w7x P.J. Martin, R.P. Netterfield, T.J. Kinder, US Pat. 5433836, 18 July 1995. w8x I.I. Axenov, V.G. Belous, V.G. Padalka, V.M. Khoreshikh, Sov. J. Plasma Phys 4 Ž1978. 425᎐428. w9x S. Falabella, D.M. Sanders, J. Vac. Sci. Technol. A10 Ž1992. 394᎐397. w10x S. Anders, A. Anders, I.G. Brown, J. Appl. Phys. 7 Ž1993. 4239᎐4241. w11x A. Anders, S. Anders, I.G. Brown, J. Appl. Phys. 75 Ž1994. 4895᎐4899. w12x T. Schulke, A. Anders, P. Siemroth, IEEE Trans. Plasma Sci. ¨ 25 Ž1997. 660᎐664. w13x H.-J. Scheibe, D. Drescher, B. Schultrich, M. Falz, G. Leonhardt, R. Wilberg, Surf. Coatings Technol. 85 Ž1996. 209᎐214. w14x D. Schneider, H.-J. Scheibe, P. Hess, Diamond Related Mater 2 Ž1993. 1396᎐1401. w15x A. Kobayashi, Advanced metallization and interconnect systems for ULSI application, in: R. Haveman, J. Schmitz, H. Komiyama, K. Tsubouchi ŽEds.., Mater. Res. Soc. Proceed., Pittsburgh, 1996, pp. 177᎐183. w16x N. Motegi, J. Vac. Sci. Technol. B13 Ž1995. 1906᎐1909. w17x C.A. Nichols, S.M. Rossnagel, S. Hamaguchi, J. Vac. Sci. Technol. B14 Ž1996. 3270᎐3275. w18x C. Wenzel, N. Urbansky, P. Siemroth, T. Schulke, Advanced ¨ metallization and interconnect systems for ULSI application, in: R. Havemann, J. Schmitz, H. Komiyama, K. Tsubouchi ŽEds.., Mater. Res. Soc. Proceed., Pittsburgh, 1996, pp. 185᎐190. w19x C. Wenzel, N. Urbansky, P. Siemroth, T. Witke, Proc. Advanced Metallization and Interconnect Systems for ULSI Application, San Diego, 1997.