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Metallic film deposition using a vacuum arc plasma source with a refractory anode
Surface & Coatings Technology 204 (2009) 865–871
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Metallic film deposition using a vacuum arc plasma source with a refractory anode Isak I. Beilis ⁎, Raymond L. Boxman Electrical Discharge and Plasma Laboratory, School of Electrical Engineering, Fleischman Faculty of Engineering, Tel Aviv University, P.O.B. 39040, Tel Aviv 69978, Israel
a r t i c l e
i n f o
Available online 9 September 2009 Keywords: Metallic film Vacuum arc Macroparticles Filtering Refractory anode Deposition rate HRAVA Plasma source
a b s t r a c t Conventional metallic film deposition techniques are compared with the hot refractory anode vacuum arc (HRAVA) developed in the last decade. In the HRAVA process, the anode is heated by the arc, and a dense plasma plume of cathode material is formed by re-evaporation of cathode material from the anode. The steadystate HRAVA mode is reached when the anode is sufficiently hot and a plasma plume expands radially. HRAVA processes using high arc current (I = 145–340 A), metal cathodes (Cu, Ti, Cr), refractory anodes (graphite, Mo, W) and 5–18 mm inter-electrode gaps were investigated theoretically and experimentally. It was found that the anode surface temperature increased approximately linearly with the arc current: on a graphite anode from 1950 K to 2200 K when I increased from 175 to 340 A, and on a W anode from 2250 to 2500 K when I increased from 150 to 250 A. High quality, macroparticle free metal thin films formed on substrates arranged radially around the electrode axis and having a direct line of sight to the anode, but not to the cathode. The deposition rate was determined in 300 A arcs with Cu cathodes, at distances of L = 110 and 80 mm from the arc axis, to be 2.0 and 3.6 μm/min, respectively. The Cr deposition rate at L = 80 mm for I = 200 to 300 A was 0.72 to 1.4 μm/min, and for Ti at L = 100 mm, 0.88 to 1.8 μm/min. The main advantages of the HRAVA source is simplicity of the construction, generation of highly energetic and ionized metallic vapor, and high deposition rate. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In the last decades metal plasma deposition techniques have progressed and have been used in different applications, including diffusion barriers and conducting metal layer interconnections in high aspect ratio vias and trenches in integrated circuits [1]. Physical vapor deposition (PVD) techniques including evaporation, magnetron sputtering (dc, r.f., and pulsed), and different types of arc deposition, are used for coating applications in the metal, biomedical, optical and electrical industries. Various ionized PVD sputtering techniques have been developed that can achieve a high degree of ionization of the sputtered atoms and were recently reviewed by Helmersson et al. [2]. These techniques not only increased the ionization, but also the deposition rate which is larger than conventional dc magnetron sputtering by factors of 2–4 depending on the target material [2]. To increase the power level, the power may be applied in pulses. In high power impulse magnetron sputtering (HIPIMS), the power has been brought to extremely high instantaneous levels of >1000 W/cm2 using a pulse length of typically 50–500 μs. HIPIMS has been successfully developed to produce high plasma densities, of the order of 1013 cm− 3. A novel biasing technique for magnetron sputtering that controls the ion flux and energy to the insulated samples was considered by Barnet [3]. Self-sputtering magnetron deposition (SSMD) was discussed by Radsimski ⁎ Corresponding author. E-mail address: [email protected] (I.I. Beilis). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.09.003
et al. [4] for dc and Posadowski et al. [5] for pulsed magnetrons. This technique can work without gas, and can reach about 2 µm/min at a distance of 10 cm with 80 W/cm2 for Cu film. Self-sputtering with a reactive gas has been investigated for the formation of the Al2O3 layers — deposition rates were in the range of 0.02–0.4 µm/min depending on source power. Electron beam evaporation is extensively used to produce thick films of metallic and non-metallic materials with thickness from several micrometers to several millimeters [6]. Chemical vapor deposition (CVD) techniques are important for formation of thin films on complex structures and selective deposition and hot-filament chemical vapor deposition are used in mechanical, optical, electronic and other applications. Typically the deposition rate is low — hundreds of Å/min at optimal temperatures of 200–300 °C [7]. Good CVD Cu films have been demonstrated by Bollmann et al. [8]. Babayan et al. [9] reported using plasma enhanced CVD with deposition rates of ~ 0.1 µm/min. Plasma assisted CVD is used when low deposition temperatures (<200 C) are required [10]. Electroplating is suitable for filling complexes structures. Electroplating insulating wafers requires a conductive seed layer, which can be deposited by CVD or plasma deposition techniques. Electroless deposition is a method for depositing from a solution where the electrochemical reaction consists of two reactions in which one generates electrons, while the second neutralizes the metal ions with the electrons from the first reaction [11,12]. Electroless copper deposition is a low-cost, highly selective process and widely used for depositing Cu films with low resistivity (~2 µΩ cm) [11].
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Various deposition techniques have been described in the literature for vacuum arc deposition (VAD) [13,14], filtered VAD [15– 18], shielded VAD [19], high-current arc (HCA) deposition [20], hot anode vacuum arc (HAVA) [21,22], and hot cathode vacuum arc (HCVA) [23–25] using an electron beam to heat the cathode. During the past decade, a VAD variant, the hot refractory anode vacuum arc (HRAVA), has been investigated at Tel Aviv University. In the present work HRAVA plasma source measurements will be summarized in Section 2, HRAVA deposition characteristics (including trench filling) will be compared with other deposition techniques in Section 3, and Section 4 will provide a summary of the state-of-the-art and an outlook for future developments. 2. Hot refractory anode vacuum arc (HRAVA) deposition The conventional cathodic vacuum arc is a discharge where the arc plasma is attached to the cathode by luminous “cathode spots” [16,26,27]. These 10–100 µm spots have a current density of 105– 108 A/cm2 and move randomly across the cathode surface with velocities of 10–103 cm/s. Most of the atoms in the plasma are ionized, and these have multiple charge states [28]. The ion current fraction (ion current to the total arc current ratio) is 0.07–0.15 in the plasma jet generated by the cathode spots. Beilis proposed the main mechanism of cathode spot operation on different cathode metals, including plasma jet generation [29] and expansion [30]. The arc also produces macroparticles [31] which must be separated from the plasma jet to deposit high quality films. In the HRAVA deposition, the macroparticles are converted to plasma in an arc with a refractory anode and the plasma can be used to deposit films of the cathode material [32]. The anode heat regime [33], plasma radiation [34] and deposition characteristics [35] of the HRAVA were investigated experimentally. A theoretical model of the arc was proposed that described self-consistently the anode and plasma phenomena [36]. It was found that initially the cathode plasma jet heats the anode and deposits cathode erosion products including macroparticles on the anode surface. As the anode temperature increases with
time, all the condensed cathodic material on the anode re-evaporates and a dense plasma fills the interelectrode gap. In steady state, the macroparticles are heated in the dense plasma or on the hot anode surface, and evaporate. Investigations in the last decade of the HRAVA as a plasma source for depositing metallic coatings and films are described in the following paragraphs. Experiments were conducted in two cylindrical stainless steel chambers: (a) 400 mm length, 160 mm diameter and (b) 530 mm length and 400 mm diameter, pumped down to a pressure of 4 × 10− 3 Pa before arc initiation [35,37,38]. The arc was sustained between a water cooled 30 mm (Cu, Cr) or 60 mm (Ti) diameter cylindrical cathode, and a refractory anode (Fig. 1). The anode was made of graphite (DFP-1 Poco Graphite Inc.), Mo or W and had a cylindrical shape, 32 mm diameter and 30 mm height. A 60 mm diameter and 15 mm height W anode was used with the Ti cathode. Both symmetric and asymmetric anodes were investigated. The front surface of the asymmetric anodes was inclined so that the maximal and minimal anode lengths (L1, L2) were: (1) (30, 25), and (2) (30, 20) mm, whereas the length of the symmetrical anode was 30 mm. Two cylindrical Mo radiation shields, 60 and 70 mm diameter, surrounded the anode to reduce radiative heat losses. A shutter was used to determine the deposition time window. A substrate holder kept the substrate in the mid-plane of the arc, facing normally to the plasma flux (Fig. 1). The inter-electrode gap h was varied in the range of 5–18 mm. The arc current I was 145–340 A for periods up to 180 s. The anode temperature was measured using high-temperature thermocouples (W/Re26%, W/Re5%) [33] at three points (T1, T2, and T3 at 2, 12 and 23 mm from the front surface of symmetric anodes) inside the anode body, [38,39] during arcs (Fig. 1). In asymmetric anodes, two thermocouples were placed 2 mm from the front anode surface near the apex and anti-apex, and 7 mm from the back anode surface [39,40]. Fig. 2 shows a steady state temperature of T1 = 2530 K near the front surface and T3 = 2180 K near the rear surface (W anode) [38]. Initially T1 increased identically for graphite, Mo and W anodes (Fig. 3) [38,39], but then diverged to a lower steady state temperature for graphite (e.g. ~ 2000 K for I = 175 A) than for W and Mo
Fig. 1. Schematic diagram of the chamber and electrode assembly with a symmetric anode.
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Fig. 4. Arc current dependence of the steady state temperatures for a symmetric W anode (Cu cathode, h = 10 mm). Fig. 2. Temperature evolution at three locations in a symmetrical W anode with Cu cathode, I = 250 A and gap distance h = 10 mm.
coupling the equation of plasma flow [43] and equations from the thermal model of the anode [33].
(~2320 K). The symmetric W anode temperature increased approximately linearly with the arc current (Fig. 4). It was also found that the anode temperature weakly decreased with the gap distance. The temperature distribution on the asymmetric anodes was asymmetric, with the highest temperature at the apex, which was ~ 50–100 K higher than the temperature of the symmetric anode under the same conditions [39,40]. The gap distance mainly influenced the anode apex temperature and L2 influenced the temperature distribution in the anode body. Anode plasma generation, development and expansion were investigated experimentally [41,42] and theoretically [43]. The plasma density, electron temperature and ion current were measured using probes [41,44,45]. An effective anode voltage Uef was defined as the ratio of the arc power dissipated in the anode to the arc current. Uef was determined from a thermal model using the time dependant anode temperature distribution [33].
2.2. Measurements and calculations
2.1. Model The time dependent anode plasma density n(T(t)) was determined, taking into account that the anode vapor is highly ionized and the plasma leaves the inter-electrode region in the radial direction [43]. The anode plasma was described by a system of hydrodynamic equations assuming that the plasma density and temperature are uniform within most of the inter-electrode space, and that the plasma accelerates at the exit of the gap to the sound speed [43]. The plasma energy balance was determined by the heat flux to the anode from electron bombardment and the heat losses by thermal conduction and radiation. A self-consistent system of equations was solved by
Fig. 3. Time evolution of the surface temperature T1 of symmetric graphite, Mo and W anodes, with a Cu cathode, I = 175 A, and h = 9 mm.
The Cu cathode erosion rate G [defined as the ratio of material lost to the charge transferred by the arc, ∫Idt], was measured by weighing the cathode before and after arcing (110 µg/C) [35] and it weakly depended on the gap [44]. The calculated anode temperature evolution [43] agreed well with measurements [33,39,40]. The measured and calculated graphite anode effective voltage Uef began to decrease after 30 and 15 s for 175 and 340 A respectively, when the anode temperature was about 1500 K, indicating that the cathode plasma jet dissipated in the plasma generated in the material reevaporated after previously condensing on the anode [43]. Thus the gap plasma density increased in time, asymptotically approaching a steady state level. The steady state plasma density increased from 1.3 to 2.5 × 1014 cm− 3 when I increased from 175 to 340 A (Fig. 5) [41,45]. The calculated time dependent electron temperatures Te agreed with the measurements; Te decreased with time from 1.6 to ~1 eV during 80 s after arc ignition. The plasma density decreased from 2 × 1013 to 2 × 1011 cm− 3 as the distance from the electrode edge was increased from 3 to 18 cm [42]. The radial expansion significantly accelerated the plasma (up to 20 eV), by a gas dynamic mechanism, similar to that occurring in a plasma jet emitted from the cathode spot [27,42]. 2.3. Ion current The ion current extracted from the radially expanded plasma, generated from a Cu cathode and a graphite anode, was measured [44]. Its axial distribution had a maximum ~2 cm from the plane of the cathode front surface in the anode direction. The maximum is attributed to the direct contribution of the cathode plasma jet (Fig.6). The maximal ion current increased with arc current I and gap distance h. The ion
Fig. 5. Measured (solid) and calculated (dotted) dependencies of the steady state plasma density on arc current with a Cu cathode and Mo anode, h = 10 mm.
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Fig. 6. The steady state distribution of the ion current per unit length ji (A/cm) in the radial direction, as a function of the axial direction x (x = 0 is at the plane of the cathode surface) (L = 5 cm, h = 10 mm, Cu cathode, graphite anode). The arc current I is the parameter.
current fraction fi (the ratio of the ion current to the total arc current) in the initial stage, when only cathode jets supplied plasma, was about 8%, equal to that measured in the conventional cathodic vacuum arc [46]. However in the steady-state HRAVA stage, fi was about 10.5% at I = 145 A, increasing to 11.5% at I = 350 A. The increased fi during the HRAVA stage may be explained by an increased ion density in the gap from ionization of the atoms re-evaporated from the anode surface and by macroparticle evaporation in the dense plasma and at the anode surface [35,36,44]. 2.4. Deposition 20 × 20 × 0.5 mm stainless steel and glass coupons and 75 × 26mm glass microscope slides were used to collect depositions from the arc. They were mounted along the mid-plane of the electrodes at distances
of L = 80–165 mm from the electrode axis. Macroparticles deposited on the substrates were observed by optical microscopy, which could detect macroparticles larger than ~ 1 µm. The film thickness was measured by profilometry. After Cu deposition, two distinct regions were observed on the substrate surface, characterized by different macroparticle densities [35,37]: (1) a matt region facing the cathode with high macroparticle density (C-region) and (2) a mirror-like region facing the anode with almost no macroparticles (A-region). Typical micrographs of films from the C- and A-regions are shown in Fig. 7A. The observed location of the A–C boundary at different distances from the electrode axis is presented in Fig. 7B. The boundary loci lay along a straight line extrapolated from the line passing through the edge of the cathode and the edge of the cathode shield. 2.5. Macroparticle contamination The macroparticle size distribution was determined by analyzing micrographs of the coated substrate surfaces obtained with an optical microscope and digital camera. The macroparticle distribution function was defined according to Daalder [31] as the number of macroparticles (ΔNMP) per µm diameter (ΔDMP) and per Coulomb charge transfer (ΔC) in the arc per mm cylindrical height (Δz). The macroparticle size distributions in the C- and A-regions (near the A–C boundary) are presented in Fig. 8 for L = 110 mm, I = 200 A Cu-cathode and a Moanode. The macroparticle density was approximately 2.5 orders of magnitude less in the A-region than in the C-region. The largest macroparticle size found in the C-region was 45 µm, in comparison to 17 µm in the A-region. The macroparticle flux to the A-region was on the order of 10− 1 mm− 2 s− 1 in comparison to about 103 mm− 2 s− 1 observed in 0.1–1 s cathode arc [31] for a Cu cathode (I = 200 A). The macroparticle flux to the C-region decreased with time, by a factor of ~4 from arc ignition until steady state. The macroparticle density in the A-region decreased approximately linearly with arc current [38]. 2.6. Deposition rate Mass gain of stainless steel substrates in the first measurements indicated a Cu deposition rate (averaged over the deposition time) of 2 µm/min (graphite anode, I = 200 A, h = 10 mm, L = 100 mm) [35]. Later, the film thickness (H) was measured by profilometry on glass substrates a few mm into the A-region after a succession of arcs, with duration increased in Δt = 15 s increments [37,38,47] to obtain the deposition rate Vdep = ΔH/Δt, where ΔH is the difference in film thickness between successive arcs. Vdep(t) with a W anode for Cu and Cr cathodes increased with arc time and reached a steady state value at 60–80 s depending on the cathode material and arc current (Fig. 9). Vdep increased with I; with a Cu cathode it reached 2.3 µm/min at
Fig. 7. A. Micrograph of the A-region (left) and C-region (right) showing macroparticle contamination in a Cu film on a glass substrate placed x = 110 mm and exposed for 60 s from the beginning of a 200 A arc. B. Location of the A- and C-regions, with low and high macroparticle density, respectively. ○ — experimental points (approximated by straight line) indicate the location of the A–C boundary as a function of the distance r from the electrode axis.
Fig. 8. Macroparticle size distribution in the C- and A-regions (L = 110 mm, I = 200 A, h = 10 mm). The C-region (solid curve) of the substrate was deposited for 10 s, and the A-region (dotted curve) for 30 s, beginning 60 s after arc ignition.
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3. Comparison with other deposition techniques In this section the main features of various deposition techniques are compared with HRAVA deposition. Different aspects including the system complexity and deposition rate will be considered. 3.1. HRAVA compared to non-arc deposition systems
Fig. 9. Temporal evolution of the deposition rate at L = 80 mm for Cr (I = 200, 250 and 300 A) and Cu (I = 200 A) cathodes, and at L = 100 mm for Ti (I = 200 and 300 A) cathode with W anode, h = 10 mm.
200 A and 3.6 µm/min at 300 A [38]. The deposition rate for Ti and Cr cathodes decreased approximately as L− 2. Vdep in the optimal direction for an asymmetric anode was larger than that obtained with a symmetric anode [39]. Measuring the total vapor flux from the deposition rate and comparing it with the ion flux, the ion fraction in the radial vapor flux was determined to be ~ 0.6 [44]. 2.7. Trench filling Cu was deposited on 10 × 10 mm Si wafers with a 1.2 µm thick SiO2 top layer in which 300 nm deep by 100 nm wide trenches were etched. The substrate was pulsed-biased, with peak voltages of −100 V, a duty cycle of 80%, and a pulse frequency of 60 kHz [48]. The substrates were located at 74 mm < L < 122 mm in the A-region, on a holder oriented so that the substrate was perpendicular to the direction of radial plasma flux propagation from the interelectrode gap in the anode region. The shutter was opened 60 s after arc initiation. A SEM micrograph of a sectioned Cu coated substrate (Fig. 10) shows a row of fully filled trenches after 2 min exposure to the plasma. The maximum deposition rate was 425 nm/min, and the minimum average resistivity was 5.48 µΩ cm (111) and (200) texturing was observed in the deposited Cu films, and the ratio between the (111) and (200) orientation intensity was 3.4, more than twice that of the standard random sample (~1.5 on JCPDS card no.04-0836). This is important and beneficial for circuit life times because (111) oriented Cu films have better electromigration reliability than (200) oriented films [48].
Fig. 10. Cross-sectional SEM micrograph showing a row of trenches fully filled with Cu deposited by 2 min exposure to a HRAVA (L = 84 mm, I = 200 A, h = 10 mm, Vdep = 425 nm/min).
Conventional PVD includes evaporation and sputtering. In evaporation systems, the source material is held in a crucible or hearth, which is heated resistively, inductively or by an electron beam. The deposition rate is limited by the evaporation rate of the source material. Usually the source material is molten, requiring upward orientation of the source. Large power supplies are required to maintain refractory deposition materials at sufficiently high temperatures to obtain a useful deposition rate. The deposition is effective when the sample surface is normally oriented to the vapor stream, which propagates in a straight line from the evaporation source. Consequently, the coverage on vertical walls is poor. It is generally difficult to evaporate alloys from a single source, if its various components have different vapor pressures. The energy of the evaporated atoms is determined by the source surface temperature and is relatively low. Consequently the coating density and adhesion are also low. Electron beam evaporation can have deposition rates in range of 0.1–100 μm/min on relatively cool substrates with very high material utilization efficiency. However the hardware is complex as radiation damage, and X-ray and secondary electron emission must be addressed. Magnetron sputtering can be conducted at lower substrate temperatures comparing to other systems, but it leaves residual compressive stress in the film. In magnetron sputtering systems, ions from a low pressure glow discharge bombard the target and eject mostly neutral atoms. Conventional magnetron sputtering cannot fill vias and trenches — often voids are formed due to the broad angular distribution of the sputtered atoms especially for trenches with high aspect ratio [1,2]. The deposition rate and deposition quality for samples having surface structures were significantly improved by using relatively complex plasma assisted ionized PVD or self-sputtering magnetron techniques [1,4,5]. In the high power impulse magnetron sputtering discharge, the deposition rate was generally found to be lower than in a conventional dc magnetron sputtering at the same average power (typically of the order of 25–35% of the rates in conventional dc magnetron sputtering) [2]. However, the film quality in terms of smoothness and density were significantly improved by the use of high power impulse magnetron sputtering. CVD, electroless and electroplating techniques have some advantages over PVD technology. Highly conformal films can be deposited on very complexly shaped substrates. CVD film composition can be controlled by the gas composition or the substrate temperature. However in CVD, the film grows at high temperatures, and corrosive gaseous products may be formed and incorporated into the film. The energy of the depositing atoms is low in chemical techniques. Film morphology, texture and other properties, depend strongly on the deposition parameters. The Cu film structure in electroplating has more randomly oriented grains than in magnetron sputtering PVD [20]. Electroplating baths must be disposed, posing both practical and environmental concerns. In comparison, the HRAVA deposition system (Fig. 1) is relatively simple, and does not need magnetic fields, plating baths and complex power sources. A HRAVA radial plasma source generates highly energetic (~ 20 eV) and ionized (~ 0.6) metallic plasma. This radial expanding plasma can metallize different substrate shapes including bands, rings and internal surfaces. High deposition rates can be achieved, e.g. ~3.6 μm/min for Cu films. The deposition rate obtained with various technologies is summarized in Table 1. It can be seen that the HRAVA deposition rate is exceeded only by electron beam
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Table 1 Deposition process properties. Method
Coating material
Deposition rate, µm/min
Substrate temperature, °C
References
Physical vapor deposition (magnetron sputtering) Ionized physical vapor deposition High power impulse magnetron sputtering
Ti Al on SiO2 Cu and Ti films AlOx from an Al Cu Cu on Si wafer Reactive: Al2O3 Metals and alloys, Oxides, carbides and borides Cu on TiN Cu on Si wafer Cu on TiN Si2O2 on Si(100) substrate Cu on SiO2, Al, Ta and TiN samples Cu on Si wafer Cu on Si wafers C on Si wafer Cu and Al TiN Cu on Si wafer Al, Ti and Cr Al Ti- stainless plate Ti and TiN- on carbon steel FeCrNi on steel Cu on glass
0.017 0.01 By factor of 2 lower By factor of 3–4 lower than with dc sputtering 2 0.14 0.02–0.4 30–50 15–20 0.2–0.05 0.05 0.05–0.01 0.1 0.02–0.004 0.1–0.16 0.1–0.12 0.1 0.18 0.016 Few 0.06–0.6 0.48 0.132 3–60 0.1–2 2.3 μm/min (I=200A), 3.6 μm/min (I=300A)
25 25 – – 25 25 25 – – 250–200 190–140 140–110 115–350 240–160 70 55–90 –
[1] [3] [2] [2] [4] [5] [5] [6] [6] [7] [8] [8] [9] [10] [11] [12] [13] [14] [19] [20] [21] [22] [23] [24] [25] [38]
Self-sputtering magnetron deposition
Electron beam evaporation Chemical vapor deposition
Plasma assisted chemical vapor deposition Electroless deposition Vacuum arc deposition with straight duct Shield filtered arc Filtered high current arc Hot anode vacuum arc Arc-like (electron beam gun) or spotless arc deposition
HRAVA
evaporation. However the electrical power required to reach the cited deposition rate is much larger (~100 kW) [24] than needed for HRAVA deposition (~ 6 kW).
– – <70 – – 220, 350, 500 120–200 –
The mass utilization rates obtained with other FVAD systems reported in the literature was also examined. Their cathode mass utilization was calculated from the reported ion current assuming that the depositing flux is fully ionized from:
3.2. HRAVA and other vacuum arc based deposition systems Two vacuum arc deposition systems using different techniques, Filtered Vacuum Arc Deposition (FVAD) with a quarter-torus macroparticle filter, and HRAVA, were directly compared experimentally using I =200 A, deposition time 120 s and a Cu-cathode in both systems [49]. The deposited area, cathode utilization efficiency, thickness, deposition rate, mass deposition rate, ion flux fraction in the total depositing flux and macroparticle contamination were compared. The HRAVA films were deposited on a cylindrical region co-axial with the electrode axis with a characteristic area of S=2πRΔh=100 cm2, where Δh=2 cm is the characteristic half-width of the thickness distribution in the axial direction (A-region). The mass deposition rate Δmtot/Δt was calculated by integrating the measured film thickness on the whole deposited area. The mass deposition in the FVAD system was calculated taking into account its axially symmetric thickness distribution. The average cathode mass utilization efficiency fm was found as the ratio between the total mass of the whole deposited film to the total mass eroded from the cathode [49]. The measured HRAVA mass deposition rate (400 mg/min) was about 40 times higher than for the FVAD system (9.5 mg/min) and the cathode mass utilization efficiency fm for the HRAVA system was about 36% in comparison with 0.8% for FVAD. The HRAVA coated a much larger substrate area than the FVAD system (100 cm2 compared to 30 cm2), azimuthally uniform around the electrode axis [49]. Vdep was an order of magnitude greater by HRAVA deposition than by FVAD, 2.3 and 0.25 µm/min respectively. In the FVAD system, the substrate is placed at a relatively large distance from the cathode to reduce the macroparticle flux, resulting in considerable plasma losses to the filter walls, as well as a bulky system. In the HRAVA system almost macroparticle free plasma flux was obtained directly from the anode plasma plume; a large source– substrate separation was not required. The plasma jet in the FVAD system was sometimes not stable due to the presence of the magnetic field near the cathode surface [50]. In contrast, in the HRAVA system, a magnetic field was not required.
fm =
Iion Mi ⋅ ; Iarc Z jejEr
where Er and Z are the erosion rate and ion charge [16,28]. Table 2 shows that fm = 36% for HRAVA exceeds the highest value for a quarter-torus FVAD system [51] by 3 times. The ratio of ion current at the filter exit to Iarc is less than 2.5% for most FVAD systems [52], corresponding to fm = 8.8%. Data for steady state [53] and pulsed [54] FVAD sources are also presented for comparison in Table 2. Small macroparticles have been observed to be transmitted with plasma flow even in curved filter ducts [55]; this may be explained theoretically by macroparticle reflection from the duct walls [56]. In the HRAVA most of the macroparticles are evaporated in the dense plasma and on the hot anode surface. The highest deposition rate and a relatively low resistivity were obtained for Cu films filling trenches with an aspect ratio of 3 using the HRAVA source. It should be noted that thin Cu films with lower electrical resistivity (decreased from 3.6 to 1.8 µΩ cm when the film thickness increased from 25 to 135 nm) have deposited on glass
Table 2 Comparison of the cathode utilization efficiency for the HRAVA system and several FVAD systems [49]. References Apparatus
Operation type
Cathode Iarc, A
Iion, A
[49] [49] [53] [50] [15]
Steady-state Steady-state Steady-state Steady-state Steady-state wall bias+20 V pulsed pulsed
Cu Cu Cu Ti Ti
200 200 110 320 100
– 36 0.37 0.8 0.25 0.8 0.9 1.3 0.85 4
Ti Ti
150 3.8 300 4.5
[51] [54]
HRAVA Quarter torus
Cathode utilization efficiency, %
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substrates using a dc filtered cathodic vacuum arc technique [57,58]. Trench and vias filling using a pulsed vacuum arc technique was demonstrated by Witke and Siemroth [59] (deposition rate of 0.6 µm/ min with 4 kA arc current) and Monteiro [60] (the deposition rate was not reported). Let us compare the HRAVA deposition rate with deposition rates of other arc techniques. Table 1 also presents the deposition rate using straight ducts, systems using filtering shields, and evaporators using hot cathode and hot anode vacuum arcs. A relatively high deposition rate is observed with f-HCA and the spotless hot cathode arc, comparable with HRAVA deposition. However very high EB power (up to 300 kW) is required for intense cathode evaporation, and hence the apparatus is relatively complex. The plasma flux generated in the low current hot anode source is weakly ionized. 4. Summary and outlook A comparison of deposition techniques including PVD, CVD and their plasma enhanced variants shows that they mostly produce films with relatively low deposition rates. The exception is a technique using electron beam heating; this, however, requires high power and complex equipment. In comparison, the HRAVA apparatus is simple, and the main disadvantage of cathodic vacuum arc deposition, macroparticle contamination, is avoided. In the HRAVA, the macroparticles are converted to plasma by evaporation at the hot refractory anode surface as well as in the dense gap plasma, and subsequent ionization of the neutrals. This increases the extracted ion current and the deposition rate in comparison to those using cathodic arcs. The HRAVA apparatus requires no additional ducts or magnets, the plasma losses are minimal and the cathode mass utilization efficiency is relatively high. The high degree of ionization permits effective substrate biasing to increase film adhesion. The radial plasma flow [61] deposits a film over a relatively large cylindrical area placed around the source. Macroparticle contamination can be almost eliminated by mounting components only in the A-region. Rapid Cu trench filling without macroparticle contamination was demonstrated. In the future, HRAVA deposition of more refractory materials (e.g. Ti) and more volatile materials (e.g. Sn and Zn), as well as deposition in reactive gasses to obtain nitride and oxide coatings should be investigated. The materials thus formed have practical applications, e.g. TiN for barrier and wear protective coatings, and SnO2 and ZnO for conductive transparent films in electro-optical devices. Acknowledgements The work was supported by a grant from the Israel Science Foundation (#727/07). The authors gratefully acknowledge S. Goldsmith, H. Rosenthal, M. Keidar, J. Heberlein, E. Pfender, V. Paperny A. Shashurin, D. Arbilly, A. Nemirovsky, A. Snaiderman and D. Grach for their significant contributions at different stages of our HRAVA investigation. References [1] S.M. Rossnagel, J. Vac. Sci. Technol., B 16 (5) (1998) 2585. [2] U. Helmersson, M. Lattemann, J. Bohlmark, A.P. Ehiasarian, J.T. Gudmundsson, Thin Solid Films 513 (2006) 1. [3] E. Barnat, T. Lu, J. Vac. Sci. Technol., A 17 (6) (1999) 3322. [4] Z.J. Radsimski, W.M. Posadowski, S.M. Rossnagel, S. Shingubara, J. Vac. Sci. Technol., B 16 (3) (1998) 1102. [5] W.M. Posadowski, Thin Solid Films 343–344 (1999) 8549. [6] B.A. Movchan, Surf. Eng. 22 (1) (2006) 35.
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[7] N.-I. Cho, Thin Solid Films 308–309 (1997) 465. [8] D. Bollmann, R. Merkel, A. Klumpp, Microelectron. Eng. 37/38 (1997) 105. [9] S.E. Babayany, J.Y. Jeongy, V.J. Tuy, J. Parkz, G.S. Selwynz, R.F. Hicksyx, Plasma Sources Sci. Technol. 7 (1998) 286. [10] S.K. Lakshmanan, W.N. Gill, J. Vac. Sci. Technol., A 16 (4) (1998) 2187. [11] Y. Shacham-Diamand, S. Lopatin, Microelectron. Eng. 37/38 (1997) 47. [12] M. Hasegawa, Y. Okinaka, Y. Shacham-Diamand, T. Osaka, Electrochem. Solid-State Lett. 9 (8) (2006) C138. [13] B. Rother, J. Siegel, J. Vetter, Thin Solid Films 188 (1990) 293. [14] R.L. Boxman, S. Goldsmith, Surf. Coat. Technol. 43/44 (1990) 1034. [15] I.I. Aksenov, V.A. Belous, V.G. Padalka, V.M. Khoroshikh, Sov. J. Plasma Phys. 4 (4) (1978) 425. [16] R.L. Boxman, P. Martin, D. Sanders (Eds.), Handbook of Vacuum Arc Science and Technology, Noyes Publishing, Ridge Park NJ, 1995. [17] S. Falabella, D.A. Sunders, J. Vac. Sci. Techhol., A 10 (2) (1992) 394. [18] P.J. Martin, A. Bendavid, Thin Solid Films 394 (2001) 1. [19] K. Miernik, J. Walkowicz, J. Bujak, Plasmas Ions 3 (2000) 41. [20] P. Siemroth, T. Schulke, Surf. Coat. Technol. 133–134 (2000) 106. [21] H. Ehrich, B. Hasse, M. Mausbach, K.G. Muller, J. Vac. Sci. Technol., A 8 (3) (1990) 2160. [22] H. Ehrich, J. Vac. Sci. Technol., A 6 (1) (1988) 134. [23] H. Kajioka, J. Vac. Sci. Technol., A 15 (1997) 2728. [24] K. Goedicke, B. Sheffel, S. Shiller, Surf. Coat. Technol. 68/69 (1994) 799. [25] B. Sheffel, C. Metzner, K. Goedicke, J.-P. Heiness, O. Zywitzki, Surf. Coat. Technol. 120–121 (1999) 718. [26] B. Juttner, J. Phys. D, Appl. Phys. 34 (2001) R103. [27] I.I. Beilis, IEEE Trans. Plasma Sci. 29 (N5) (2001) 657. [28] W.D. Davis, H.C. Miller, J. Appl. Phys. 40 (1969) 2212. [29] I.I. Beilis, in: R.L. Boxman, P.J. Martin, D.M. Sanders (Eds.), Handbook of Vacuum Arc Science and Technology, Noyes Publications, Park Ridge, N.J, 1995, p. 208. [30] I.I. Beilis, M. Keidar, R.L. Boxman, S. Goldsmith, J. Appl. Phys. 83 (2) (1998) 709. [31] J.E. Daalder, J. Phys. D, Appl. Phys. 9 (1976) 2379. [32] I.I. Beilis, R.L. Boxman, S. Goldsmith, M. Keidar, H. Rosenthal, “Deposition of coatings and thin films using a vacuum arc with hot a non-consumable anode”, Israeli patent #130650, 21/02/2006, USA patent#6391164, 21 May 2002. [33] H. Rosenthal, I.I. Beilis, S. Goldsmith, R.L. Boxman, J. Phys. D, Appl. Phys. 28 (1) (1995) 353. [34] H. Rosenthal, I.I. Beilis, S. Goldsmith, R.L. Boxman, J. Phys. D, Appl. Phys. 29 (6) (1996) 1245. [35] I.I. Beilis, R.L. Boxman, S. Goldsmith, Surf. Coat. Technol. 133–134 (1–3) (2000) 91. [36] I.I. Beilis, R.L. Boxman, S. Goldsmith, The Plasma-Model of New Mode Hot Anode Vacuum Arc, XXIII ICPIG, Toulouse, France, vol. II, July- 1997, p. II-84. [37] I.I. Beilis, A. Shashurin, D. Arbilly, R.L. Boxman, S. Goldsmith, Surf. Coat. Technol. 177–178 (2004) 233. [38] I.I. Beilis, A. Snaiderman, A. Shashurin, R.L. Boxman, S. Goldsmith, Surf. Coat. Technol. 202 (1–3) (2007) 925. [39] I.I. Beilis, A. Shashurin, A. Nemirovsky, S. Goldsmith, R.L. Boxman, Surf. Coat. Technol. 188–189 (2004) 228. [40] I.I. Beilis, A. Shashurin, A. Nemirovsky, S. Goldsmith, R.L. Boxman, IEEE Trans. Plasma Sci. 33 (5) (2005) 1641. [41] I.I. Beilis, M. Keidar, R.L. Boxman, S. Goldsmith, Phys. Plasmas 7 (7) (2000) 3068. [42] I.I. Beilis, R.L. Boxman, S. Goldsmith, V.L. Paperny, J. Appl. Phys. 88 (11) (2000) 6224. [43] I.I. Beilis, R.L. Boxman, S. Goldsmith, Phys. Plasmas 9 (7) (2002) 3159. [44] A. Shashurin, I.I. Beilis, R.L. Boxman, Plasma Sources Sci. Technol. 17 (2008) 015016. [45] A. Shashurin, I.I. Beilis, R.L. Boxman, Plasma Sources Sci. Technol. 18 (2009) 045004. [46] C. Kimblin, J. Appl. Phys. 44 (1973) 3074. [47] I.I. Beilis, A. Snaiderman, R.L. Boxman, Surf. Coat. Technol. 203 (2008) 501. [48] I.I. Beilis, D. Grach, A. Shashurin, R.L. Boxman, Microelectronic Eng. 85 (2008) 1713. [49] A. Shashurin, I.I. Beilis, Y. Sivan, S. Goldsmith, R.L. Boxman, Surf. Coat. Technol. 201 (2006) 4145. [50] V.N. Zhitomirsky, R.L. Boxman, S. Goldsmith, J. Vac. Sci. Technol., A 13 (4) (1995) 2233. [51] S. Anders, A. Anders, I. Brown, J. Appl. Phys. 74 (6) (1993) 4239. [52] P.J. Martin, A. Bendavid, Thin Solid Films 394 (2001) 1. [53] C.A. Davis, Ph.D. Thesis, University of Sydney, Australia, 1993. [54] M.M.M. Bilek, A. Anders, Plasma Sources Sci. Technol. 8 (1999) 488. [55] M. Keidar, I.I. Beilis, B.R. Aharonov, R.L. Boxman, S. Goldsmith, J. Phys. D: J. Appl. Phys. 30 (1997) 2972. [56] M. Keidar, I.I. Beilis, R.L. Boxman, S. Goldsmith, IEEE Trans. Plasma Sci. 24 (1) (1996) 226. [57] S.P. Lau, Y.H. Cheng, J.R. Shi, P. Cao, B.K. Tay, X. Shi, Thin Solid Films 398 (2001) 539. [58] J.R. Shi, S.P. Lau, Z. Sun, X. Shi, B.K. Tay, H.S. Tan, Surf. Coat. Technol. 138 (2001) 250. [59] T. Witke, P. Siemroth, IEEE Trans. Plasma Sci. 27 (4) (1999) 1039. [60] R. Monteiro, J. Vac. Sci. Technol., B 17 (3) (1999) 1094. [61] I.I. Beilis, M. Keidar, R.L. Boxman, S. Goldsmith, J. Heberlein, E. Pfender, J. Appl. Phys. 86 (1) (1999) 114.
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Metallic film deposition using a vacuum arc plasma source with a refractory anode Isak I. Beilis ⁎, Raymond L. Boxman Electrical Discharge and Plasma Laboratory, School of Electrical Engineering, Fleischman Faculty of Engineering, Tel Aviv University, P.O.B. 39040, Tel Aviv 69978, Israel
a r t i c l e
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Available online 9 September 2009 Keywords: Metallic film Vacuum arc Macroparticles Filtering Refractory anode Deposition rate HRAVA Plasma source
a b s t r a c t Conventional metallic film deposition techniques are compared with the hot refractory anode vacuum arc (HRAVA) developed in the last decade. In the HRAVA process, the anode is heated by the arc, and a dense plasma plume of cathode material is formed by re-evaporation of cathode material from the anode. The steadystate HRAVA mode is reached when the anode is sufficiently hot and a plasma plume expands radially. HRAVA processes using high arc current (I = 145–340 A), metal cathodes (Cu, Ti, Cr), refractory anodes (graphite, Mo, W) and 5–18 mm inter-electrode gaps were investigated theoretically and experimentally. It was found that the anode surface temperature increased approximately linearly with the arc current: on a graphite anode from 1950 K to 2200 K when I increased from 175 to 340 A, and on a W anode from 2250 to 2500 K when I increased from 150 to 250 A. High quality, macroparticle free metal thin films formed on substrates arranged radially around the electrode axis and having a direct line of sight to the anode, but not to the cathode. The deposition rate was determined in 300 A arcs with Cu cathodes, at distances of L = 110 and 80 mm from the arc axis, to be 2.0 and 3.6 μm/min, respectively. The Cr deposition rate at L = 80 mm for I = 200 to 300 A was 0.72 to 1.4 μm/min, and for Ti at L = 100 mm, 0.88 to 1.8 μm/min. The main advantages of the HRAVA source is simplicity of the construction, generation of highly energetic and ionized metallic vapor, and high deposition rate. © 2009 Elsevier B.V. All rights reserved.
1. Introduction In the last decades metal plasma deposition techniques have progressed and have been used in different applications, including diffusion barriers and conducting metal layer interconnections in high aspect ratio vias and trenches in integrated circuits [1]. Physical vapor deposition (PVD) techniques including evaporation, magnetron sputtering (dc, r.f., and pulsed), and different types of arc deposition, are used for coating applications in the metal, biomedical, optical and electrical industries. Various ionized PVD sputtering techniques have been developed that can achieve a high degree of ionization of the sputtered atoms and were recently reviewed by Helmersson et al. [2]. These techniques not only increased the ionization, but also the deposition rate which is larger than conventional dc magnetron sputtering by factors of 2–4 depending on the target material [2]. To increase the power level, the power may be applied in pulses. In high power impulse magnetron sputtering (HIPIMS), the power has been brought to extremely high instantaneous levels of >1000 W/cm2 using a pulse length of typically 50–500 μs. HIPIMS has been successfully developed to produce high plasma densities, of the order of 1013 cm− 3. A novel biasing technique for magnetron sputtering that controls the ion flux and energy to the insulated samples was considered by Barnet [3]. Self-sputtering magnetron deposition (SSMD) was discussed by Radsimski ⁎ Corresponding author. E-mail address: [email protected] (I.I. Beilis). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.09.003
et al. [4] for dc and Posadowski et al. [5] for pulsed magnetrons. This technique can work without gas, and can reach about 2 µm/min at a distance of 10 cm with 80 W/cm2 for Cu film. Self-sputtering with a reactive gas has been investigated for the formation of the Al2O3 layers — deposition rates were in the range of 0.02–0.4 µm/min depending on source power. Electron beam evaporation is extensively used to produce thick films of metallic and non-metallic materials with thickness from several micrometers to several millimeters [6]. Chemical vapor deposition (CVD) techniques are important for formation of thin films on complex structures and selective deposition and hot-filament chemical vapor deposition are used in mechanical, optical, electronic and other applications. Typically the deposition rate is low — hundreds of Å/min at optimal temperatures of 200–300 °C [7]. Good CVD Cu films have been demonstrated by Bollmann et al. [8]. Babayan et al. [9] reported using plasma enhanced CVD with deposition rates of ~ 0.1 µm/min. Plasma assisted CVD is used when low deposition temperatures (<200 C) are required [10]. Electroplating is suitable for filling complexes structures. Electroplating insulating wafers requires a conductive seed layer, which can be deposited by CVD or plasma deposition techniques. Electroless deposition is a method for depositing from a solution where the electrochemical reaction consists of two reactions in which one generates electrons, while the second neutralizes the metal ions with the electrons from the first reaction [11,12]. Electroless copper deposition is a low-cost, highly selective process and widely used for depositing Cu films with low resistivity (~2 µΩ cm) [11].
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Various deposition techniques have been described in the literature for vacuum arc deposition (VAD) [13,14], filtered VAD [15– 18], shielded VAD [19], high-current arc (HCA) deposition [20], hot anode vacuum arc (HAVA) [21,22], and hot cathode vacuum arc (HCVA) [23–25] using an electron beam to heat the cathode. During the past decade, a VAD variant, the hot refractory anode vacuum arc (HRAVA), has been investigated at Tel Aviv University. In the present work HRAVA plasma source measurements will be summarized in Section 2, HRAVA deposition characteristics (including trench filling) will be compared with other deposition techniques in Section 3, and Section 4 will provide a summary of the state-of-the-art and an outlook for future developments. 2. Hot refractory anode vacuum arc (HRAVA) deposition The conventional cathodic vacuum arc is a discharge where the arc plasma is attached to the cathode by luminous “cathode spots” [16,26,27]. These 10–100 µm spots have a current density of 105– 108 A/cm2 and move randomly across the cathode surface with velocities of 10–103 cm/s. Most of the atoms in the plasma are ionized, and these have multiple charge states [28]. The ion current fraction (ion current to the total arc current ratio) is 0.07–0.15 in the plasma jet generated by the cathode spots. Beilis proposed the main mechanism of cathode spot operation on different cathode metals, including plasma jet generation [29] and expansion [30]. The arc also produces macroparticles [31] which must be separated from the plasma jet to deposit high quality films. In the HRAVA deposition, the macroparticles are converted to plasma in an arc with a refractory anode and the plasma can be used to deposit films of the cathode material [32]. The anode heat regime [33], plasma radiation [34] and deposition characteristics [35] of the HRAVA were investigated experimentally. A theoretical model of the arc was proposed that described self-consistently the anode and plasma phenomena [36]. It was found that initially the cathode plasma jet heats the anode and deposits cathode erosion products including macroparticles on the anode surface. As the anode temperature increases with
time, all the condensed cathodic material on the anode re-evaporates and a dense plasma fills the interelectrode gap. In steady state, the macroparticles are heated in the dense plasma or on the hot anode surface, and evaporate. Investigations in the last decade of the HRAVA as a plasma source for depositing metallic coatings and films are described in the following paragraphs. Experiments were conducted in two cylindrical stainless steel chambers: (a) 400 mm length, 160 mm diameter and (b) 530 mm length and 400 mm diameter, pumped down to a pressure of 4 × 10− 3 Pa before arc initiation [35,37,38]. The arc was sustained between a water cooled 30 mm (Cu, Cr) or 60 mm (Ti) diameter cylindrical cathode, and a refractory anode (Fig. 1). The anode was made of graphite (DFP-1 Poco Graphite Inc.), Mo or W and had a cylindrical shape, 32 mm diameter and 30 mm height. A 60 mm diameter and 15 mm height W anode was used with the Ti cathode. Both symmetric and asymmetric anodes were investigated. The front surface of the asymmetric anodes was inclined so that the maximal and minimal anode lengths (L1, L2) were: (1) (30, 25), and (2) (30, 20) mm, whereas the length of the symmetrical anode was 30 mm. Two cylindrical Mo radiation shields, 60 and 70 mm diameter, surrounded the anode to reduce radiative heat losses. A shutter was used to determine the deposition time window. A substrate holder kept the substrate in the mid-plane of the arc, facing normally to the plasma flux (Fig. 1). The inter-electrode gap h was varied in the range of 5–18 mm. The arc current I was 145–340 A for periods up to 180 s. The anode temperature was measured using high-temperature thermocouples (W/Re26%, W/Re5%) [33] at three points (T1, T2, and T3 at 2, 12 and 23 mm from the front surface of symmetric anodes) inside the anode body, [38,39] during arcs (Fig. 1). In asymmetric anodes, two thermocouples were placed 2 mm from the front anode surface near the apex and anti-apex, and 7 mm from the back anode surface [39,40]. Fig. 2 shows a steady state temperature of T1 = 2530 K near the front surface and T3 = 2180 K near the rear surface (W anode) [38]. Initially T1 increased identically for graphite, Mo and W anodes (Fig. 3) [38,39], but then diverged to a lower steady state temperature for graphite (e.g. ~ 2000 K for I = 175 A) than for W and Mo
Fig. 1. Schematic diagram of the chamber and electrode assembly with a symmetric anode.
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Fig. 4. Arc current dependence of the steady state temperatures for a symmetric W anode (Cu cathode, h = 10 mm). Fig. 2. Temperature evolution at three locations in a symmetrical W anode with Cu cathode, I = 250 A and gap distance h = 10 mm.
coupling the equation of plasma flow [43] and equations from the thermal model of the anode [33].
(~2320 K). The symmetric W anode temperature increased approximately linearly with the arc current (Fig. 4). It was also found that the anode temperature weakly decreased with the gap distance. The temperature distribution on the asymmetric anodes was asymmetric, with the highest temperature at the apex, which was ~ 50–100 K higher than the temperature of the symmetric anode under the same conditions [39,40]. The gap distance mainly influenced the anode apex temperature and L2 influenced the temperature distribution in the anode body. Anode plasma generation, development and expansion were investigated experimentally [41,42] and theoretically [43]. The plasma density, electron temperature and ion current were measured using probes [41,44,45]. An effective anode voltage Uef was defined as the ratio of the arc power dissipated in the anode to the arc current. Uef was determined from a thermal model using the time dependant anode temperature distribution [33].
2.2. Measurements and calculations
2.1. Model The time dependent anode plasma density n(T(t)) was determined, taking into account that the anode vapor is highly ionized and the plasma leaves the inter-electrode region in the radial direction [43]. The anode plasma was described by a system of hydrodynamic equations assuming that the plasma density and temperature are uniform within most of the inter-electrode space, and that the plasma accelerates at the exit of the gap to the sound speed [43]. The plasma energy balance was determined by the heat flux to the anode from electron bombardment and the heat losses by thermal conduction and radiation. A self-consistent system of equations was solved by
Fig. 3. Time evolution of the surface temperature T1 of symmetric graphite, Mo and W anodes, with a Cu cathode, I = 175 A, and h = 9 mm.
The Cu cathode erosion rate G [defined as the ratio of material lost to the charge transferred by the arc, ∫Idt], was measured by weighing the cathode before and after arcing (110 µg/C) [35] and it weakly depended on the gap [44]. The calculated anode temperature evolution [43] agreed well with measurements [33,39,40]. The measured and calculated graphite anode effective voltage Uef began to decrease after 30 and 15 s for 175 and 340 A respectively, when the anode temperature was about 1500 K, indicating that the cathode plasma jet dissipated in the plasma generated in the material reevaporated after previously condensing on the anode [43]. Thus the gap plasma density increased in time, asymptotically approaching a steady state level. The steady state plasma density increased from 1.3 to 2.5 × 1014 cm− 3 when I increased from 175 to 340 A (Fig. 5) [41,45]. The calculated time dependent electron temperatures Te agreed with the measurements; Te decreased with time from 1.6 to ~1 eV during 80 s after arc ignition. The plasma density decreased from 2 × 1013 to 2 × 1011 cm− 3 as the distance from the electrode edge was increased from 3 to 18 cm [42]. The radial expansion significantly accelerated the plasma (up to 20 eV), by a gas dynamic mechanism, similar to that occurring in a plasma jet emitted from the cathode spot [27,42]. 2.3. Ion current The ion current extracted from the radially expanded plasma, generated from a Cu cathode and a graphite anode, was measured [44]. Its axial distribution had a maximum ~2 cm from the plane of the cathode front surface in the anode direction. The maximum is attributed to the direct contribution of the cathode plasma jet (Fig.6). The maximal ion current increased with arc current I and gap distance h. The ion
Fig. 5. Measured (solid) and calculated (dotted) dependencies of the steady state plasma density on arc current with a Cu cathode and Mo anode, h = 10 mm.
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Fig. 6. The steady state distribution of the ion current per unit length ji (A/cm) in the radial direction, as a function of the axial direction x (x = 0 is at the plane of the cathode surface) (L = 5 cm, h = 10 mm, Cu cathode, graphite anode). The arc current I is the parameter.
current fraction fi (the ratio of the ion current to the total arc current) in the initial stage, when only cathode jets supplied plasma, was about 8%, equal to that measured in the conventional cathodic vacuum arc [46]. However in the steady-state HRAVA stage, fi was about 10.5% at I = 145 A, increasing to 11.5% at I = 350 A. The increased fi during the HRAVA stage may be explained by an increased ion density in the gap from ionization of the atoms re-evaporated from the anode surface and by macroparticle evaporation in the dense plasma and at the anode surface [35,36,44]. 2.4. Deposition 20 × 20 × 0.5 mm stainless steel and glass coupons and 75 × 26mm glass microscope slides were used to collect depositions from the arc. They were mounted along the mid-plane of the electrodes at distances
of L = 80–165 mm from the electrode axis. Macroparticles deposited on the substrates were observed by optical microscopy, which could detect macroparticles larger than ~ 1 µm. The film thickness was measured by profilometry. After Cu deposition, two distinct regions were observed on the substrate surface, characterized by different macroparticle densities [35,37]: (1) a matt region facing the cathode with high macroparticle density (C-region) and (2) a mirror-like region facing the anode with almost no macroparticles (A-region). Typical micrographs of films from the C- and A-regions are shown in Fig. 7A. The observed location of the A–C boundary at different distances from the electrode axis is presented in Fig. 7B. The boundary loci lay along a straight line extrapolated from the line passing through the edge of the cathode and the edge of the cathode shield. 2.5. Macroparticle contamination The macroparticle size distribution was determined by analyzing micrographs of the coated substrate surfaces obtained with an optical microscope and digital camera. The macroparticle distribution function was defined according to Daalder [31] as the number of macroparticles (ΔNMP) per µm diameter (ΔDMP) and per Coulomb charge transfer (ΔC) in the arc per mm cylindrical height (Δz). The macroparticle size distributions in the C- and A-regions (near the A–C boundary) are presented in Fig. 8 for L = 110 mm, I = 200 A Cu-cathode and a Moanode. The macroparticle density was approximately 2.5 orders of magnitude less in the A-region than in the C-region. The largest macroparticle size found in the C-region was 45 µm, in comparison to 17 µm in the A-region. The macroparticle flux to the A-region was on the order of 10− 1 mm− 2 s− 1 in comparison to about 103 mm− 2 s− 1 observed in 0.1–1 s cathode arc [31] for a Cu cathode (I = 200 A). The macroparticle flux to the C-region decreased with time, by a factor of ~4 from arc ignition until steady state. The macroparticle density in the A-region decreased approximately linearly with arc current [38]. 2.6. Deposition rate Mass gain of stainless steel substrates in the first measurements indicated a Cu deposition rate (averaged over the deposition time) of 2 µm/min (graphite anode, I = 200 A, h = 10 mm, L = 100 mm) [35]. Later, the film thickness (H) was measured by profilometry on glass substrates a few mm into the A-region after a succession of arcs, with duration increased in Δt = 15 s increments [37,38,47] to obtain the deposition rate Vdep = ΔH/Δt, where ΔH is the difference in film thickness between successive arcs. Vdep(t) with a W anode for Cu and Cr cathodes increased with arc time and reached a steady state value at 60–80 s depending on the cathode material and arc current (Fig. 9). Vdep increased with I; with a Cu cathode it reached 2.3 µm/min at
Fig. 7. A. Micrograph of the A-region (left) and C-region (right) showing macroparticle contamination in a Cu film on a glass substrate placed x = 110 mm and exposed for 60 s from the beginning of a 200 A arc. B. Location of the A- and C-regions, with low and high macroparticle density, respectively. ○ — experimental points (approximated by straight line) indicate the location of the A–C boundary as a function of the distance r from the electrode axis.
Fig. 8. Macroparticle size distribution in the C- and A-regions (L = 110 mm, I = 200 A, h = 10 mm). The C-region (solid curve) of the substrate was deposited for 10 s, and the A-region (dotted curve) for 30 s, beginning 60 s after arc ignition.
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3. Comparison with other deposition techniques In this section the main features of various deposition techniques are compared with HRAVA deposition. Different aspects including the system complexity and deposition rate will be considered. 3.1. HRAVA compared to non-arc deposition systems
Fig. 9. Temporal evolution of the deposition rate at L = 80 mm for Cr (I = 200, 250 and 300 A) and Cu (I = 200 A) cathodes, and at L = 100 mm for Ti (I = 200 and 300 A) cathode with W anode, h = 10 mm.
200 A and 3.6 µm/min at 300 A [38]. The deposition rate for Ti and Cr cathodes decreased approximately as L− 2. Vdep in the optimal direction for an asymmetric anode was larger than that obtained with a symmetric anode [39]. Measuring the total vapor flux from the deposition rate and comparing it with the ion flux, the ion fraction in the radial vapor flux was determined to be ~ 0.6 [44]. 2.7. Trench filling Cu was deposited on 10 × 10 mm Si wafers with a 1.2 µm thick SiO2 top layer in which 300 nm deep by 100 nm wide trenches were etched. The substrate was pulsed-biased, with peak voltages of −100 V, a duty cycle of 80%, and a pulse frequency of 60 kHz [48]. The substrates were located at 74 mm < L < 122 mm in the A-region, on a holder oriented so that the substrate was perpendicular to the direction of radial plasma flux propagation from the interelectrode gap in the anode region. The shutter was opened 60 s after arc initiation. A SEM micrograph of a sectioned Cu coated substrate (Fig. 10) shows a row of fully filled trenches after 2 min exposure to the plasma. The maximum deposition rate was 425 nm/min, and the minimum average resistivity was 5.48 µΩ cm (111) and (200) texturing was observed in the deposited Cu films, and the ratio between the (111) and (200) orientation intensity was 3.4, more than twice that of the standard random sample (~1.5 on JCPDS card no.04-0836). This is important and beneficial for circuit life times because (111) oriented Cu films have better electromigration reliability than (200) oriented films [48].
Fig. 10. Cross-sectional SEM micrograph showing a row of trenches fully filled with Cu deposited by 2 min exposure to a HRAVA (L = 84 mm, I = 200 A, h = 10 mm, Vdep = 425 nm/min).
Conventional PVD includes evaporation and sputtering. In evaporation systems, the source material is held in a crucible or hearth, which is heated resistively, inductively or by an electron beam. The deposition rate is limited by the evaporation rate of the source material. Usually the source material is molten, requiring upward orientation of the source. Large power supplies are required to maintain refractory deposition materials at sufficiently high temperatures to obtain a useful deposition rate. The deposition is effective when the sample surface is normally oriented to the vapor stream, which propagates in a straight line from the evaporation source. Consequently, the coverage on vertical walls is poor. It is generally difficult to evaporate alloys from a single source, if its various components have different vapor pressures. The energy of the evaporated atoms is determined by the source surface temperature and is relatively low. Consequently the coating density and adhesion are also low. Electron beam evaporation can have deposition rates in range of 0.1–100 μm/min on relatively cool substrates with very high material utilization efficiency. However the hardware is complex as radiation damage, and X-ray and secondary electron emission must be addressed. Magnetron sputtering can be conducted at lower substrate temperatures comparing to other systems, but it leaves residual compressive stress in the film. In magnetron sputtering systems, ions from a low pressure glow discharge bombard the target and eject mostly neutral atoms. Conventional magnetron sputtering cannot fill vias and trenches — often voids are formed due to the broad angular distribution of the sputtered atoms especially for trenches with high aspect ratio [1,2]. The deposition rate and deposition quality for samples having surface structures were significantly improved by using relatively complex plasma assisted ionized PVD or self-sputtering magnetron techniques [1,4,5]. In the high power impulse magnetron sputtering discharge, the deposition rate was generally found to be lower than in a conventional dc magnetron sputtering at the same average power (typically of the order of 25–35% of the rates in conventional dc magnetron sputtering) [2]. However, the film quality in terms of smoothness and density were significantly improved by the use of high power impulse magnetron sputtering. CVD, electroless and electroplating techniques have some advantages over PVD technology. Highly conformal films can be deposited on very complexly shaped substrates. CVD film composition can be controlled by the gas composition or the substrate temperature. However in CVD, the film grows at high temperatures, and corrosive gaseous products may be formed and incorporated into the film. The energy of the depositing atoms is low in chemical techniques. Film morphology, texture and other properties, depend strongly on the deposition parameters. The Cu film structure in electroplating has more randomly oriented grains than in magnetron sputtering PVD [20]. Electroplating baths must be disposed, posing both practical and environmental concerns. In comparison, the HRAVA deposition system (Fig. 1) is relatively simple, and does not need magnetic fields, plating baths and complex power sources. A HRAVA radial plasma source generates highly energetic (~ 20 eV) and ionized (~ 0.6) metallic plasma. This radial expanding plasma can metallize different substrate shapes including bands, rings and internal surfaces. High deposition rates can be achieved, e.g. ~3.6 μm/min for Cu films. The deposition rate obtained with various technologies is summarized in Table 1. It can be seen that the HRAVA deposition rate is exceeded only by electron beam
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Table 1 Deposition process properties. Method
Coating material
Deposition rate, µm/min
Substrate temperature, °C
References
Physical vapor deposition (magnetron sputtering) Ionized physical vapor deposition High power impulse magnetron sputtering
Ti Al on SiO2 Cu and Ti films AlOx from an Al Cu Cu on Si wafer Reactive: Al2O3 Metals and alloys, Oxides, carbides and borides Cu on TiN Cu on Si wafer Cu on TiN Si2O2 on Si(100) substrate Cu on SiO2, Al, Ta and TiN samples Cu on Si wafer Cu on Si wafers C on Si wafer Cu and Al TiN Cu on Si wafer Al, Ti and Cr Al Ti- stainless plate Ti and TiN- on carbon steel FeCrNi on steel Cu on glass
0.017 0.01 By factor of 2 lower By factor of 3–4 lower than with dc sputtering 2 0.14 0.02–0.4 30–50 15–20 0.2–0.05 0.05 0.05–0.01 0.1 0.02–0.004 0.1–0.16 0.1–0.12 0.1 0.18 0.016 Few 0.06–0.6 0.48 0.132 3–60 0.1–2 2.3 μm/min (I=200A), 3.6 μm/min (I=300A)
25 25 – – 25 25 25 – – 250–200 190–140 140–110 115–350 240–160 70 55–90 –
[1] [3] [2] [2] [4] [5] [5] [6] [6] [7] [8] [8] [9] [10] [11] [12] [13] [14] [19] [20] [21] [22] [23] [24] [25] [38]
Self-sputtering magnetron deposition
Electron beam evaporation Chemical vapor deposition
Plasma assisted chemical vapor deposition Electroless deposition Vacuum arc deposition with straight duct Shield filtered arc Filtered high current arc Hot anode vacuum arc Arc-like (electron beam gun) or spotless arc deposition
HRAVA
evaporation. However the electrical power required to reach the cited deposition rate is much larger (~100 kW) [24] than needed for HRAVA deposition (~ 6 kW).
– – <70 – – 220, 350, 500 120–200 –
The mass utilization rates obtained with other FVAD systems reported in the literature was also examined. Their cathode mass utilization was calculated from the reported ion current assuming that the depositing flux is fully ionized from:
3.2. HRAVA and other vacuum arc based deposition systems Two vacuum arc deposition systems using different techniques, Filtered Vacuum Arc Deposition (FVAD) with a quarter-torus macroparticle filter, and HRAVA, were directly compared experimentally using I =200 A, deposition time 120 s and a Cu-cathode in both systems [49]. The deposited area, cathode utilization efficiency, thickness, deposition rate, mass deposition rate, ion flux fraction in the total depositing flux and macroparticle contamination were compared. The HRAVA films were deposited on a cylindrical region co-axial with the electrode axis with a characteristic area of S=2πRΔh=100 cm2, where Δh=2 cm is the characteristic half-width of the thickness distribution in the axial direction (A-region). The mass deposition rate Δmtot/Δt was calculated by integrating the measured film thickness on the whole deposited area. The mass deposition in the FVAD system was calculated taking into account its axially symmetric thickness distribution. The average cathode mass utilization efficiency fm was found as the ratio between the total mass of the whole deposited film to the total mass eroded from the cathode [49]. The measured HRAVA mass deposition rate (400 mg/min) was about 40 times higher than for the FVAD system (9.5 mg/min) and the cathode mass utilization efficiency fm for the HRAVA system was about 36% in comparison with 0.8% for FVAD. The HRAVA coated a much larger substrate area than the FVAD system (100 cm2 compared to 30 cm2), azimuthally uniform around the electrode axis [49]. Vdep was an order of magnitude greater by HRAVA deposition than by FVAD, 2.3 and 0.25 µm/min respectively. In the FVAD system, the substrate is placed at a relatively large distance from the cathode to reduce the macroparticle flux, resulting in considerable plasma losses to the filter walls, as well as a bulky system. In the HRAVA system almost macroparticle free plasma flux was obtained directly from the anode plasma plume; a large source– substrate separation was not required. The plasma jet in the FVAD system was sometimes not stable due to the presence of the magnetic field near the cathode surface [50]. In contrast, in the HRAVA system, a magnetic field was not required.
fm =
Iion Mi ⋅ ; Iarc Z jejEr
where Er and Z are the erosion rate and ion charge [16,28]. Table 2 shows that fm = 36% for HRAVA exceeds the highest value for a quarter-torus FVAD system [51] by 3 times. The ratio of ion current at the filter exit to Iarc is less than 2.5% for most FVAD systems [52], corresponding to fm = 8.8%. Data for steady state [53] and pulsed [54] FVAD sources are also presented for comparison in Table 2. Small macroparticles have been observed to be transmitted with plasma flow even in curved filter ducts [55]; this may be explained theoretically by macroparticle reflection from the duct walls [56]. In the HRAVA most of the macroparticles are evaporated in the dense plasma and on the hot anode surface. The highest deposition rate and a relatively low resistivity were obtained for Cu films filling trenches with an aspect ratio of 3 using the HRAVA source. It should be noted that thin Cu films with lower electrical resistivity (decreased from 3.6 to 1.8 µΩ cm when the film thickness increased from 25 to 135 nm) have deposited on glass
Table 2 Comparison of the cathode utilization efficiency for the HRAVA system and several FVAD systems [49]. References Apparatus
Operation type
Cathode Iarc, A
Iion, A
[49] [49] [53] [50] [15]
Steady-state Steady-state Steady-state Steady-state Steady-state wall bias+20 V pulsed pulsed
Cu Cu Cu Ti Ti
200 200 110 320 100
– 36 0.37 0.8 0.25 0.8 0.9 1.3 0.85 4
Ti Ti
150 3.8 300 4.5
[51] [54]
HRAVA Quarter torus
Cathode utilization efficiency, %
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substrates using a dc filtered cathodic vacuum arc technique [57,58]. Trench and vias filling using a pulsed vacuum arc technique was demonstrated by Witke and Siemroth [59] (deposition rate of 0.6 µm/ min with 4 kA arc current) and Monteiro [60] (the deposition rate was not reported). Let us compare the HRAVA deposition rate with deposition rates of other arc techniques. Table 1 also presents the deposition rate using straight ducts, systems using filtering shields, and evaporators using hot cathode and hot anode vacuum arcs. A relatively high deposition rate is observed with f-HCA and the spotless hot cathode arc, comparable with HRAVA deposition. However very high EB power (up to 300 kW) is required for intense cathode evaporation, and hence the apparatus is relatively complex. The plasma flux generated in the low current hot anode source is weakly ionized. 4. Summary and outlook A comparison of deposition techniques including PVD, CVD and their plasma enhanced variants shows that they mostly produce films with relatively low deposition rates. The exception is a technique using electron beam heating; this, however, requires high power and complex equipment. In comparison, the HRAVA apparatus is simple, and the main disadvantage of cathodic vacuum arc deposition, macroparticle contamination, is avoided. In the HRAVA, the macroparticles are converted to plasma by evaporation at the hot refractory anode surface as well as in the dense gap plasma, and subsequent ionization of the neutrals. This increases the extracted ion current and the deposition rate in comparison to those using cathodic arcs. The HRAVA apparatus requires no additional ducts or magnets, the plasma losses are minimal and the cathode mass utilization efficiency is relatively high. The high degree of ionization permits effective substrate biasing to increase film adhesion. The radial plasma flow [61] deposits a film over a relatively large cylindrical area placed around the source. Macroparticle contamination can be almost eliminated by mounting components only in the A-region. Rapid Cu trench filling without macroparticle contamination was demonstrated. In the future, HRAVA deposition of more refractory materials (e.g. Ti) and more volatile materials (e.g. Sn and Zn), as well as deposition in reactive gasses to obtain nitride and oxide coatings should be investigated. The materials thus formed have practical applications, e.g. TiN for barrier and wear protective coatings, and SnO2 and ZnO for conductive transparent films in electro-optical devices. Acknowledgements The work was supported by a grant from the Israel Science Foundation (#727/07). The authors gratefully acknowledge S. Goldsmith, H. Rosenthal, M. Keidar, J. Heberlein, E. Pfender, V. Paperny A. Shashurin, D. Arbilly, A. Nemirovsky, A. Snaiderman and D. Grach for their significant contributions at different stages of our HRAVA investigation. References [1] S.M. Rossnagel, J. Vac. Sci. Technol., B 16 (5) (1998) 2585. [2] U. Helmersson, M. Lattemann, J. Bohlmark, A.P. Ehiasarian, J.T. Gudmundsson, Thin Solid Films 513 (2006) 1. [3] E. Barnat, T. Lu, J. Vac. Sci. Technol., A 17 (6) (1999) 3322. [4] Z.J. Radsimski, W.M. Posadowski, S.M. Rossnagel, S. Shingubara, J. Vac. Sci. Technol., B 16 (3) (1998) 1102. [5] W.M. Posadowski, Thin Solid Films 343–344 (1999) 8549. [6] B.A. Movchan, Surf. Eng. 22 (1) (2006) 35.
871
[7] N.-I. Cho, Thin Solid Films 308–309 (1997) 465. [8] D. Bollmann, R. Merkel, A. Klumpp, Microelectron. Eng. 37/38 (1997) 105. [9] S.E. Babayany, J.Y. Jeongy, V.J. Tuy, J. Parkz, G.S. Selwynz, R.F. Hicksyx, Plasma Sources Sci. Technol. 7 (1998) 286. [10] S.K. Lakshmanan, W.N. Gill, J. Vac. Sci. Technol., A 16 (4) (1998) 2187. [11] Y. Shacham-Diamand, S. Lopatin, Microelectron. Eng. 37/38 (1997) 47. [12] M. Hasegawa, Y. Okinaka, Y. Shacham-Diamand, T. Osaka, Electrochem. Solid-State Lett. 9 (8) (2006) C138. [13] B. Rother, J. Siegel, J. Vetter, Thin Solid Films 188 (1990) 293. [14] R.L. Boxman, S. Goldsmith, Surf. Coat. Technol. 43/44 (1990) 1034. [15] I.I. Aksenov, V.A. Belous, V.G. Padalka, V.M. Khoroshikh, Sov. J. Plasma Phys. 4 (4) (1978) 425. [16] R.L. Boxman, P. Martin, D. Sanders (Eds.), Handbook of Vacuum Arc Science and Technology, Noyes Publishing, Ridge Park NJ, 1995. [17] S. Falabella, D.A. Sunders, J. Vac. Sci. Techhol., A 10 (2) (1992) 394. [18] P.J. Martin, A. Bendavid, Thin Solid Films 394 (2001) 1. [19] K. Miernik, J. Walkowicz, J. Bujak, Plasmas Ions 3 (2000) 41. [20] P. Siemroth, T. Schulke, Surf. Coat. Technol. 133–134 (2000) 106. [21] H. Ehrich, B. Hasse, M. Mausbach, K.G. Muller, J. Vac. Sci. Technol., A 8 (3) (1990) 2160. [22] H. Ehrich, J. Vac. Sci. Technol., A 6 (1) (1988) 134. [23] H. Kajioka, J. Vac. Sci. Technol., A 15 (1997) 2728. [24] K. Goedicke, B. Sheffel, S. Shiller, Surf. Coat. Technol. 68/69 (1994) 799. [25] B. Sheffel, C. Metzner, K. Goedicke, J.-P. Heiness, O. Zywitzki, Surf. Coat. Technol. 120–121 (1999) 718. [26] B. Juttner, J. Phys. D, Appl. Phys. 34 (2001) R103. [27] I.I. Beilis, IEEE Trans. Plasma Sci. 29 (N5) (2001) 657. [28] W.D. Davis, H.C. Miller, J. Appl. Phys. 40 (1969) 2212. [29] I.I. Beilis, in: R.L. Boxman, P.J. Martin, D.M. Sanders (Eds.), Handbook of Vacuum Arc Science and Technology, Noyes Publications, Park Ridge, N.J, 1995, p. 208. [30] I.I. Beilis, M. Keidar, R.L. Boxman, S. Goldsmith, J. Appl. Phys. 83 (2) (1998) 709. [31] J.E. Daalder, J. Phys. D, Appl. Phys. 9 (1976) 2379. [32] I.I. Beilis, R.L. Boxman, S. Goldsmith, M. Keidar, H. Rosenthal, “Deposition of coatings and thin films using a vacuum arc with hot a non-consumable anode”, Israeli patent #130650, 21/02/2006, USA patent#6391164, 21 May 2002. [33] H. Rosenthal, I.I. Beilis, S. Goldsmith, R.L. Boxman, J. Phys. D, Appl. Phys. 28 (1) (1995) 353. [34] H. Rosenthal, I.I. Beilis, S. Goldsmith, R.L. Boxman, J. Phys. D, Appl. Phys. 29 (6) (1996) 1245. [35] I.I. Beilis, R.L. Boxman, S. Goldsmith, Surf. Coat. Technol. 133–134 (1–3) (2000) 91. [36] I.I. Beilis, R.L. Boxman, S. Goldsmith, The Plasma-Model of New Mode Hot Anode Vacuum Arc, XXIII ICPIG, Toulouse, France, vol. II, July- 1997, p. II-84. [37] I.I. Beilis, A. Shashurin, D. Arbilly, R.L. Boxman, S. Goldsmith, Surf. Coat. Technol. 177–178 (2004) 233. [38] I.I. Beilis, A. Snaiderman, A. Shashurin, R.L. Boxman, S. Goldsmith, Surf. Coat. Technol. 202 (1–3) (2007) 925. [39] I.I. Beilis, A. Shashurin, A. Nemirovsky, S. Goldsmith, R.L. Boxman, Surf. Coat. Technol. 188–189 (2004) 228. [40] I.I. Beilis, A. Shashurin, A. Nemirovsky, S. Goldsmith, R.L. Boxman, IEEE Trans. Plasma Sci. 33 (5) (2005) 1641. [41] I.I. Beilis, M. Keidar, R.L. Boxman, S. Goldsmith, Phys. Plasmas 7 (7) (2000) 3068. [42] I.I. Beilis, R.L. Boxman, S. Goldsmith, V.L. Paperny, J. Appl. Phys. 88 (11) (2000) 6224. [43] I.I. Beilis, R.L. Boxman, S. Goldsmith, Phys. Plasmas 9 (7) (2002) 3159. [44] A. Shashurin, I.I. Beilis, R.L. Boxman, Plasma Sources Sci. Technol. 17 (2008) 015016. [45] A. Shashurin, I.I. Beilis, R.L. Boxman, Plasma Sources Sci. Technol. 18 (2009) 045004. [46] C. Kimblin, J. Appl. Phys. 44 (1973) 3074. [47] I.I. Beilis, A. Snaiderman, R.L. Boxman, Surf. Coat. Technol. 203 (2008) 501. [48] I.I. Beilis, D. Grach, A. Shashurin, R.L. Boxman, Microelectronic Eng. 85 (2008) 1713. [49] A. Shashurin, I.I. Beilis, Y. Sivan, S. Goldsmith, R.L. Boxman, Surf. Coat. Technol. 201 (2006) 4145. [50] V.N. Zhitomirsky, R.L. Boxman, S. Goldsmith, J. Vac. Sci. Technol., A 13 (4) (1995) 2233. [51] S. Anders, A. Anders, I. Brown, J. Appl. Phys. 74 (6) (1993) 4239. [52] P.J. Martin, A. Bendavid, Thin Solid Films 394 (2001) 1. [53] C.A. Davis, Ph.D. Thesis, University of Sydney, Australia, 1993. [54] M.M.M. Bilek, A. Anders, Plasma Sources Sci. Technol. 8 (1999) 488. [55] M. Keidar, I.I. Beilis, B.R. Aharonov, R.L. Boxman, S. Goldsmith, J. Phys. D: J. Appl. Phys. 30 (1997) 2972. [56] M. Keidar, I.I. Beilis, R.L. Boxman, S. Goldsmith, IEEE Trans. Plasma Sci. 24 (1) (1996) 226. [57] S.P. Lau, Y.H. Cheng, J.R. Shi, P. Cao, B.K. Tay, X. Shi, Thin Solid Films 398 (2001) 539. [58] J.R. Shi, S.P. Lau, Z. Sun, X. Shi, B.K. Tay, H.S. Tan, Surf. Coat. Technol. 138 (2001) 250. [59] T. Witke, P. Siemroth, IEEE Trans. Plasma Sci. 27 (4) (1999) 1039. [60] R. Monteiro, J. Vac. Sci. Technol., B 17 (3) (1999) 1094. [61] I.I. Beilis, M. Keidar, R.L. Boxman, S. Goldsmith, J. Heberlein, E. Pfender, J. Appl. Phys. 86 (1) (1999) 114.