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Thin film deposition techniques utilizing the anodic vacuum arc
343
Surface and Coatings Technology, 54/55 (1992) 343-348
Thin film deposition techniques utilizing the anodic vacuum arc S. Meassick, C. Chan and R. Allen Department of Electrical and Computer Engineering, 235 Forsyth Building, Northeastern University, Boston, M A 02115 (USA)
Abstract The properties of thin films produced with a steady state low current (I < 100 A) anodic arc deposition apparatus have been investigated. The anodic arc allows for the rapid deposition (faster than 5 urn min-I) of macro particle free coatings. Coatings produced with the anodic vacuum arc are either amorphous or exhibit grains with a grain size of less than I nm. The resulting coatings do not have macroparticle inclusions and exhibit good adherence of the film to the substrate. Deposition results from, stainless steel indicate that alloy coatings can be readily deposited with the stoichiometry of the source material being well preserved, while experiments with nickel, copper and aluminum indicate that the thin films have a very low impurity content. Investigations of the corrosion resistance of stainless steel, nickel, and aluminum coatings on iron samples indicate that coatings of less than 1.0 um offer excellent corrosion protection.
1. Introduction
Plasma assisted deposition techniques, including vacuum arcs, are playing an increasingly important role in the deposition of thin films for optical thin films, semiconductor processing and corrosion protection due to the fact that these coatings exhibit superior properties to those produced by non-plasma assisted means. Deposition techniques utilizing plasmas include plasma assisted chemical vapor deposition (PACVD) [1], ion-assisted evaporation [2], and cathodic vacuum arc deposition [3]. All of these deposition technologies exhibit certain drawbacks. PACVD and ion-assisted evaporation usually exhibit slow deposition rates, are costly to implement, and have problems with the adherence of the thin films and the inclusion of impurities. The cathodic vacuum arc exhibits many desirable qualities for the deposition of coatings and thin films such as low cost, rapid deposition and simplicity of design. The major problems with using the cathodic vacuum arc for the deposition of thin films is the inclusion of numerous small (less than 10 urn) macroparticles in the thin films. The inclusion of macro particles limits the usefulness of coatings deposited by the cathodic arc to applications where surface topology, and the inclusion of macroparticles and voids created by these macroparticles is not an important consideration. For corrosion protection, voids created by the macro particles requires that the coating be substantially thicker than the size of the macroparticles [4] in order to assure that there is not a path through the coating. Many attempts have been made to overcome the problem of macroparticle inclusion, including magnetic control of cathode
0257-8972/92/$5.00
spot movement [5, 6], separation of the ions from the neutral particles by magnetic means [5, 6], and the bombardment of the cathode with vapors from external sources [7]. The anodic vacuum arc differs from the cathodic vacuum arc in that the arc is sustained by material evaporated from the anode, as opposed to the cathode. In the anodic vacuum arc, the cathode is either totally inactive without eroding cathode spots or has many rapidly moving spots on the cathode surface. All of the material that sustains the arc is emitted by the anode. Until recently, steady state arcs sustained by anodic material were only known for currents exceeding 400 A [8, 9]. In these high current arcs, anodic evaporation occurs in large luminous spots at the surface of the anode. High current anodic arcs were known predominantly in vacuum breakers where the aim of investigations was to minimize the erosion of the anode through elimination of the anodic arc. Several investigators have reported on the characteristics of low current microsecond-duration anode spots (Bacon [10] and Grissom and Newton [11]). In these experiments the arc was pulsed for approximately 10 us at currents as low as 20 A. Due to the short pulse length, very little material evaporation from the anode occurred and the usefulness of anodic evaporation for the deposition of coatings was not investigated. Recently, however, several investigators [12, 13] have found a new steady state mode of operation of the anodic vacuum arc at much lower currents, typically less than 100 A. In these low current anodic arcs the anode is tailored in order to enhance evaporation. The cathode is designed either for minimal erosion, by using
©
1992 - Elsevier Sequoia. All rights reserved
344
S. Meassick et at. / Thin film deposition techniques
a refractory material such as carbon or tungsten, or is manufactured of the same material as the anode. In either case the cathode is designed so as not to heat up appreciably. By tailoring the anode it has been possible to achieve rapid evaporation of the anode material without macroparticle inclusion that occurs in the cathodic arc. The anodic vacuum arc produces a metal vapor plasma that, unlike the fully ionized plasma of the cathodic arc, is only partially ionized (approximately 20%). In the anodic are, the ions are singly ionized and have a directed energy of approximately 5 eV while the electrons have a temperature ofless than 1 eY. Near the anode, the density of the expanding plasma is approxirna tely I x 1018 em- 3 while the neutral density is an order of magnitude higher. Coatings deposited with the low current anodic vacuum arc exhibit all of the desirable qualities of coatings deposited with plasma assisted deposition techniques, and cathodic arcs in specific, but do not suffer from many of the problems that these methods entail. In particular, the anodic arc rapidly produces coatings that are of a very high quality and do not suffer from macroparticle inclusion. The crystallographic structure strongly affects the mechanical, electrical and corrosion properties of the coating. In this paper we present results from the deposition of type 303 stainless steel, nickel, aluminum and copper utilizing the low current anodic vacuum arc. In particular, the arc voltage and deposition rate as a function of arc current are presented. In addition, energy dispersive X-ray analysis (EDS) was used to determine the stoichiometry of the deposited thin films as a function of arc current, film thickness and angle from normal to the anode surface. Scanning electron microcospy (SEM), transmission electron microcospy (TEM) and X-ray diffraction techniques are used to ascertain the crystallographic structure of the deposited thin films. The corrosion protective properties against salt sprays for type 303 stainless steel, aluminum, and nickel coatings on iron substrates are presented.
2. Experimental details
A schematic representation of the apparatus [14, 15J used to investigate anodic arc phenomena is shown in Fig. l. The vacuum chamber in which the experiments are performed is 50 em in diameter and 65 em long and is pumped via a diffusion pump to a base pressure of approximately 1 x 10- 6 Torr. During operation of the anodic arc the pressure in the chamber rises to approximately 1 x 10- 5 Torr. The cathode consists of either a carbon or tungsten rod 2.5 em in diameter surrounded with a ceramic shield so that there is no line of sight to the target. The anode is constructed of a crucible 40 mm long, 8 mm in width and 4 mm in depth into which the
Target Bias Supply
...-------1~ Material \ (303 SS)
Target Ceramic Shield
\
Anode (Tungstenl\
/' Cathode / (Carbon)
'-----,
1/4 Ohm Ballast Resistor Power Supply Fig. 1. Schematic diagram of anodic arc deposition apparatus.
material to be evaporated is placed. The use of a crucible allows for the use of a substantial amount of material (on the order of 109), and allows the direction of the metal vapor plasma to be tailored. The crucible can be constructed of either tungsten or ceramic materials. In the case of tungsten, part of the arc current is intercepted by the crucible, thereby indirectly heating the crucible. This indirect heating places limits of the current at which the arc can be run so that the material to be evaporated does not overheat and causing nucleate boiling which results in material being spattered out of the crucible. The crucible can also be constructed of a ceramic material, such as A1 2 0 3 , in which case a separate tungsten electrode must be used to carry current to the anode material. This arrangement has the advantage that all of the current flows into the anode material, thereby heating its surface and greatly diminishing the chances of nucleate boiling. The anodic arc is powered by a low voltage de power supply (100 V, 100 A), operated in a current regulating mode, through a current limiting resistor. During anodic arc operation, the arc voltage is approximately 17-20 V, and in conjunction with radiation at the most intense spectral line of the evaporated material, is characteristic of proper arc operation. Ignition of the anodic arc occurs by physically contacting the anode and cathode together, thereby initiating a cathodic arc which transitions to the steady state anodic arc after approximately 1 s. After contact of the electrodes, one or more slowly moving cathode spots appear on the cathode, severely eroding it. In this phase the arc is sustained by cathodic material. The anode is heated due to ohmic heating and ion bombardment. Eventually, the anode heats up sufficiently so that anode material is evaporated at which time the color of the
S. Meassick er al. I Th ill film deposition techniques
arc changes to that of the most intense spectral line of the anode material and the cathode spots transform into many, rapidly moving spots that no longer erode the cathode. The targets are arranged on a biasable substrate holder that is located so that it is not in line of sight of the cathode. The anode to substrate distance is adjustable to approximately 45 em. The data presented in this paper were taken with ·a 20 em substrate to anode separation. The substrate holder is biasable positive or negative to approximately 300 V, with a negative bias usually being applied in order to accelerate ions into the target. The d.c. bias is applied via a low voltage (approximately 300 V) d.c. power supply. Films were deposited on to silicon wafers for the stoichiometry tests and onto formvar TEM sample holders for analysis of grain sizes and orientations. X-ray diffraction results were obtained from coatings that were deposited onto glass and copper substrate holders, while corrosion tests were made with iron substrates.
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20
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60
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Arc Current (A) Fig. 2. Deposition rate of nickel, aluminium and copper as a function of anodic arc current. There is a threshold current for deposit ion that is materi al dependent, with higher threshold currents corresponding to higher boiling temperatures of the anode material.
3. Results and discussion The rate of evaporation, and therefore deposition, were dependent on the arc current. Evaporation rates of approximately 0.04 g min -) A-I (3.2 g min -1 at 80 A) were achieved for type 303 stainless steel while evaporation rates of 0.06 g min -) A-I were achieved for aluminum. The maximum evaporation rate from the anode was only limited by the current available from the d.c, power supply (100 A). The coatings deposited with the anodic vacuum arc exhibited good adherence to the substrate. Cellophane tape tests were used for the initial analysis. Indentation tests were performed for a more objective measure of the adherence of the film. In these tests, the surface of the substrate was dented with a diamond tipped probe and the cracking, delamination or spalling of the coatings near the dent was visually observed. The coatings deposited with the anodic vacuum arc showed no delamination or spalling and only minimal cracking around the edge of the dent. Deposition rates of up to 6 urn min - ) were achieved at a distance of approximately 20 em from the anode with an arc current of 80 A. The deposition rate decreases as the square of the anode to target separation. Figure 2 shows the deposition rate for nickel, aluminum and copper as a function of current. It is evident that there is a material dependent threshold current for deposition. The threshold current ranges from 15 A for aluminum to 30 A for nickel. The threshold current increases as the boiling point of the material increases (2467, 2567 and 2732 °C for aluminum, copper and nickel respectively) . Above this threshold current, the deposition rate increases linearly with current. Figure 3 shows the arc
345
30
28 • Nickel • Aluminum .. Copper
26
~ c
24
ClO
5
~ ~
22 20
18
16 0
10
20
30
40
50
60
70
Arc Current (A) Fig. 3. Voltage-current characteristic far nickel, aluminum and copper. A material dependent threshold current exists where the resistance changes from negative to positive.
voltage drop (anode to cathode) as a function of arc current for nickel, aluminum and copper. Below a threshold current the voltage-current characteristic is negative (negative resistance). This threshold current is again material dependent and occurs at approximately the same current as the threshold current for deposition. The threshold current for deposition is probably due to the fact that the anode must be heated sufficiently in order to allow evaporation to occur. Since radiation and conduction will be the primary cooling mechanisms of the anode before there is rapid evaporation, a larger
S. Meassick et at. / Thin }ilm deposition techniques
346
heat ing curre nt will be required for materials with a higher boiling tem perature. Below this thres hold voltage the a rc is operating at least partially as a cat hodic arc with cathode sp ots tha t cause erosion of the cathode. Above the thre shold voltage, in addition to their rapid moti on over the cathode, the cathode spots no longer cause erosion and the inclusion of cathode material in the coating. Th us, when operated in the anodic mode, the cathode spots are different in nature. The stoichiometry of Type 303 stainless steel, aluminum , copper an d nickel films dep osited on silicon were analyzed utilizing EDS . Films of at least 3 urn in thickness were used to ensu re that the X-ray signal measured originated in the film and not in the silicon substrate. For these experiments, the cathode consisting of tungsten instead of carbon was used so that any cathode material included in the film could be detected with the EDS system. The electron beam energy was fixed at 15 keV in order to ensure that the electron energy was at least three times the highest X-ray energy of interest. The source stoichiometry of the type 303 stainless steel was determined to be 68.3% Fe, 18.9% Cr, 9.5% Ni, 1.9% Mn and 0.7% Si while the samples of aluminum, copper and nickel were pure as far as could be determined with the EDS system. Figure 4 shows the concentration of cathode material (tungsten) as a function of arc current for aluminum, cop per and nickel films. For low arc currents, up to the same material dependent threshold current as previously seen, there is some inclusion of cathode material in the deposited film. Above this current there is no measurable inclusion of cathode material. These data a lso indicate
that below a threshold voltage the arc is operating at least partially as a cathodic arc. Figure 5 shows the concentrations of the type 303 stainless steel film constituents normalized to the stoichiometry of the source material as a function of the arc current. It is evident that as the arc current increases, the stoichiometry of the deposited film approached that of the source material. Even for very low currents (30 A) there is only a small difference between the source material and the resulting film. For arc currents above 60 A, there is less than a 2% difference in the stoichiometry of the source material and the deposited film while for arc currents above 80 A, there is no measurable difference in th e stoichiometry of the source material and the deposited film. The deposited thin films were examined with a scan ning electron microscope in order to ascertain the surface roughness and defect densities of the films. Figure 6 shows a scanning electron micrograph of a nickel film deposited on a silicon substrate for an arc curren t of 80 A. There are no discernible surface features down to the resolution limit of the instrument (approximately 4 nm). In addition, the surface was found to be free of macroparticle inclusions. The crystalline structure of the steel thin films was investigated with transmission electron micrographs and X-ray diffraction techniques . Figure 7 shows a transmission electron micrograph of the steel deposited on formvar substrates for an arc current of 80 A. The th ickness of the films used for the transmission electron micrographs is approximately 200 A. It is evident that the grains are randomly orientated with a size of less than I nm. In addition to the TEM and SEM images
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Ar c Current (A) Fig. 4. Inclusion of ca thode material in the deposi ted coating as a func tion of arc curre nt. Above a threshold current there is negligible inclus ion of cathode material in the coating.
::- - ~ ,.-
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.'
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Arc current (A) Fig . 5. Stoichiometry of a type 303 stainless steel thin film as a function of arc current. The stoichiometry is normalized to tha t of the source material.
s. Meassick et ill.
/ Thin jilm deposition techniques
0.25 /-lm Fig. 6. Scanning electron micrograph of a nickel film deposited on a silicon substrale shows no discernible surface features other than the latex sphere used as a focusing aid.
347
on iron substrates. The arc current for all samples used in the corrosion tests was 80 A. The corrosion inhibiting properties of these coatings against humid and marine environments was evaluated using saIt spray fog testing according to standards ASTM BII7 [16] and MILSTD 202F method 1010 [17]. Each sample was exposed to a salt spray environment at a temperature of 35°C, a humidity of 85% and a spray solution consisting of 5% NaCl dissolved in water. Tests were conducted for up to 48 h on the samples. Figure 8 shows aluminum coated (uncoated, 0.25, 0.5 and 0.75 urn respectively) iron samples exposed to the salt spray environment for 5 h. It is evident that the 0.75 urn thick aluminum coating provides almost complete protection from corrosion. In order to obtain a more objective evaluation of the protective qualities of the coatings, each sample was given a rating R based on the percentage A of the surface covered with corrosion and based on the scheme of ASTM B537 [18J:
R=
{
3(2- l0g (A ))
A~O.I
9
O~A~O.1
to
A=O
Figure 9 shows the protection rating as a function of coating thickness for aluminum, nickel, and type 303 stainless steel. All samples were exposed to the saIt spray environment for 48 h. It is evident that aluminum gives complete protection (R = 10) for coating thicknesses of approximately 1.5 urn while the thickness required for nickel and type 303 stainless steel is considerably thicker. This is in stark contrast to coatings deposited with a cathodic arc which require coating thicknesses on the order of 20 urn in order to provide similar corrosion
Fig. 7. Transmission electron micrograph of type 303 stainless steel deposited on a formvar substrate shows an almost amorphous coating with feature sizes less than I nm.
of thin films, the crystalline structure of the deposited steel was examined with the aid of X-ray diffraction techniques for film thicknesses of up to I urn, The diffraction patterns showed no peaks arising from a crystalline structure for thin films, indicating that the deposited film is amorphous. For very thick layers, with a corresponding long arc duration and therefore heating of the sample, the X-ray diffraction pattern showed peaks for a [110](;( and [III] and [200]')' grain structure with the y phase becoming more pronounced as the heating increased. In order to evaluate the protective qualities of coatings deposited with the anodic vacuum are, coatings of type 303 stainless steel, aluminum and nickel were deposited
Fig. 8. Picture of iron samples coated with aluminum that were subjected to salt spray corrosion testing. The uncoated sample is labeled (a), while samples coated with 0.25 urn, 0.5 urn and 0.75 urn are labeled (b), (c) and (d) respectively.
S. Meassick et al. I Thinfilm deposition techniques
348
12,-----r----,----.----,-----.
10
produced with the anodic arc had no discernible surface texture and were amorphous in nature. An investigation of the protective properties of anodic arc coatings against salt spray indicates that even thin coatings can reliably protect a sample against corrosion.
8
Acknowledgments
6 o
4
The authors are grateful to Joe Genevich for construction of the apparatus, to Bill Fowle for undertaking the TEM measurements and to Bill Giessen and the Army Materials Technology Laboratory for the X-ray diffraction analysis.
Type 303 S5
• Nickel + Aluminum
1
2
3
4
5
Coating thickness (pm) Fig. 9. Corrosion protection rating of aluminum, nickel and type 303 stainless steel after 48 h of salt spray.
protection [4]. While the conditions cited in ref. 4 are for deposition rates substantially higher than used in commercial cathodic arc coaters, resulting in a large number of macropartic1es that are large in size, the thickness of the protective coating must be greater than the macroparticle size for effective corrosion protection. The relatively poor protective properties of cathodic arc coating are due to the voids that are created by the inclusion of macroparticles in the coating for the cathodic arc, allowing the salt spray to penetrate to the substrate.
4. Conclusions
We have investigated the properties of thin films produced by the anodic vacuum arc. A material dependent threshold voltage was found below which the arc operates at least partially as a cathodic arc with the characteristic erosion of the cathode and the inclusion of cathode material in the coating. As the arc current is increased, the stoichiometry of stainless steel coatings approached that of the source material. The coatings
References 1 1. E. Greene and S. A. Banelt, J. Vac. Sci. Technol., 21 (1982) 285. 2 S. M. Rossnagel, J. J. Cuomo and W. D. Westwood, Plasma Based Processing, Noyes, 1989. 3 R. L. Boxman and S. Goldsmith, IEEE Trans. Plasma sa; 17 ([989) 705. 4 S. Bababeygy, Corrosion protection by vacuum arc coatings, Proc. Interfinish 88, Paris, October 1988. 5 D. M. Sanders, J. Vac. Sci. Technol. A, 7 (1989) 2339. 6 G. V. Kljuchko, V. G. Padalka and L. P. Sablev, Plasma arc apparatus for applying coatings by means of a consumable cathode, US Patent 4492 845. 7 R. Buhl, Verfahren und Vorrichtung zum Vakumbeschichten mittels einer elektrischen Bogenentladung zwischen einer Anode und einer Kathod, European patent application 0285 745. 8 H. C. Miller, IEEE Trans. Plasma Sci., 13, (1985) 242. 9 C. W. Kimblin, J. App/. Phys., 40 (1969) 1744. 10 F. M. Bacon, J. Appl. Phys., 46 (1975) 4750. II J. T. Grissom and J. C. Newton, J. Appl. Phys., 45 (1974) 2885. 12 H. Ehrich, J. Vac. Sci. Technol. A, 6 (1988) 134. 13 H. Ehrich, B. Hasse, K. G. Muller and R. Schmidt, J. Vac. Sci. Technol. A, 6 (1988) 2499. 14 S. Meassick, C. Chan, T. Sroda and R. AlIen, Anodic vacuum arc deposition processes, Int. Can! Metali. Coatings and Thin Films, San Diego, CA, April 22-26, 1991. 15 S. Meassick, 1. Kumpf, R. Allen, C. Chan and T. Sroda, Investigation of the properties of type 303 stainless steel thin films deposited with the anodic vacuum arc, Mater. Lett., (1992) in the press. 16 ASTM Standard B1I7, Salt Spray Fog Testing, 1971. 17 MIL-STD 202F, Sait Spray Testing, April 1981. 18 ASTM standard B537, Rating of Electroplated Panels Subjected to Atmospheric Exposure, 198!.
Surface and Coatings Technology, 54/55 (1992) 343-348
Thin film deposition techniques utilizing the anodic vacuum arc S. Meassick, C. Chan and R. Allen Department of Electrical and Computer Engineering, 235 Forsyth Building, Northeastern University, Boston, M A 02115 (USA)
Abstract The properties of thin films produced with a steady state low current (I < 100 A) anodic arc deposition apparatus have been investigated. The anodic arc allows for the rapid deposition (faster than 5 urn min-I) of macro particle free coatings. Coatings produced with the anodic vacuum arc are either amorphous or exhibit grains with a grain size of less than I nm. The resulting coatings do not have macroparticle inclusions and exhibit good adherence of the film to the substrate. Deposition results from, stainless steel indicate that alloy coatings can be readily deposited with the stoichiometry of the source material being well preserved, while experiments with nickel, copper and aluminum indicate that the thin films have a very low impurity content. Investigations of the corrosion resistance of stainless steel, nickel, and aluminum coatings on iron samples indicate that coatings of less than 1.0 um offer excellent corrosion protection.
1. Introduction
Plasma assisted deposition techniques, including vacuum arcs, are playing an increasingly important role in the deposition of thin films for optical thin films, semiconductor processing and corrosion protection due to the fact that these coatings exhibit superior properties to those produced by non-plasma assisted means. Deposition techniques utilizing plasmas include plasma assisted chemical vapor deposition (PACVD) [1], ion-assisted evaporation [2], and cathodic vacuum arc deposition [3]. All of these deposition technologies exhibit certain drawbacks. PACVD and ion-assisted evaporation usually exhibit slow deposition rates, are costly to implement, and have problems with the adherence of the thin films and the inclusion of impurities. The cathodic vacuum arc exhibits many desirable qualities for the deposition of coatings and thin films such as low cost, rapid deposition and simplicity of design. The major problems with using the cathodic vacuum arc for the deposition of thin films is the inclusion of numerous small (less than 10 urn) macroparticles in the thin films. The inclusion of macro particles limits the usefulness of coatings deposited by the cathodic arc to applications where surface topology, and the inclusion of macroparticles and voids created by these macroparticles is not an important consideration. For corrosion protection, voids created by the macro particles requires that the coating be substantially thicker than the size of the macroparticles [4] in order to assure that there is not a path through the coating. Many attempts have been made to overcome the problem of macroparticle inclusion, including magnetic control of cathode
0257-8972/92/$5.00
spot movement [5, 6], separation of the ions from the neutral particles by magnetic means [5, 6], and the bombardment of the cathode with vapors from external sources [7]. The anodic vacuum arc differs from the cathodic vacuum arc in that the arc is sustained by material evaporated from the anode, as opposed to the cathode. In the anodic vacuum arc, the cathode is either totally inactive without eroding cathode spots or has many rapidly moving spots on the cathode surface. All of the material that sustains the arc is emitted by the anode. Until recently, steady state arcs sustained by anodic material were only known for currents exceeding 400 A [8, 9]. In these high current arcs, anodic evaporation occurs in large luminous spots at the surface of the anode. High current anodic arcs were known predominantly in vacuum breakers where the aim of investigations was to minimize the erosion of the anode through elimination of the anodic arc. Several investigators have reported on the characteristics of low current microsecond-duration anode spots (Bacon [10] and Grissom and Newton [11]). In these experiments the arc was pulsed for approximately 10 us at currents as low as 20 A. Due to the short pulse length, very little material evaporation from the anode occurred and the usefulness of anodic evaporation for the deposition of coatings was not investigated. Recently, however, several investigators [12, 13] have found a new steady state mode of operation of the anodic vacuum arc at much lower currents, typically less than 100 A. In these low current anodic arcs the anode is tailored in order to enhance evaporation. The cathode is designed either for minimal erosion, by using
©
1992 - Elsevier Sequoia. All rights reserved
344
S. Meassick et at. / Thin film deposition techniques
a refractory material such as carbon or tungsten, or is manufactured of the same material as the anode. In either case the cathode is designed so as not to heat up appreciably. By tailoring the anode it has been possible to achieve rapid evaporation of the anode material without macroparticle inclusion that occurs in the cathodic arc. The anodic vacuum arc produces a metal vapor plasma that, unlike the fully ionized plasma of the cathodic arc, is only partially ionized (approximately 20%). In the anodic are, the ions are singly ionized and have a directed energy of approximately 5 eV while the electrons have a temperature ofless than 1 eY. Near the anode, the density of the expanding plasma is approxirna tely I x 1018 em- 3 while the neutral density is an order of magnitude higher. Coatings deposited with the low current anodic vacuum arc exhibit all of the desirable qualities of coatings deposited with plasma assisted deposition techniques, and cathodic arcs in specific, but do not suffer from many of the problems that these methods entail. In particular, the anodic arc rapidly produces coatings that are of a very high quality and do not suffer from macroparticle inclusion. The crystallographic structure strongly affects the mechanical, electrical and corrosion properties of the coating. In this paper we present results from the deposition of type 303 stainless steel, nickel, aluminum and copper utilizing the low current anodic vacuum arc. In particular, the arc voltage and deposition rate as a function of arc current are presented. In addition, energy dispersive X-ray analysis (EDS) was used to determine the stoichiometry of the deposited thin films as a function of arc current, film thickness and angle from normal to the anode surface. Scanning electron microcospy (SEM), transmission electron microcospy (TEM) and X-ray diffraction techniques are used to ascertain the crystallographic structure of the deposited thin films. The corrosion protective properties against salt sprays for type 303 stainless steel, aluminum, and nickel coatings on iron substrates are presented.
2. Experimental details
A schematic representation of the apparatus [14, 15J used to investigate anodic arc phenomena is shown in Fig. l. The vacuum chamber in which the experiments are performed is 50 em in diameter and 65 em long and is pumped via a diffusion pump to a base pressure of approximately 1 x 10- 6 Torr. During operation of the anodic arc the pressure in the chamber rises to approximately 1 x 10- 5 Torr. The cathode consists of either a carbon or tungsten rod 2.5 em in diameter surrounded with a ceramic shield so that there is no line of sight to the target. The anode is constructed of a crucible 40 mm long, 8 mm in width and 4 mm in depth into which the
Target Bias Supply
...-------1~ Material \ (303 SS)
Target Ceramic Shield
\
Anode (Tungstenl\
/' Cathode / (Carbon)
'-----,
1/4 Ohm Ballast Resistor Power Supply Fig. 1. Schematic diagram of anodic arc deposition apparatus.
material to be evaporated is placed. The use of a crucible allows for the use of a substantial amount of material (on the order of 109), and allows the direction of the metal vapor plasma to be tailored. The crucible can be constructed of either tungsten or ceramic materials. In the case of tungsten, part of the arc current is intercepted by the crucible, thereby indirectly heating the crucible. This indirect heating places limits of the current at which the arc can be run so that the material to be evaporated does not overheat and causing nucleate boiling which results in material being spattered out of the crucible. The crucible can also be constructed of a ceramic material, such as A1 2 0 3 , in which case a separate tungsten electrode must be used to carry current to the anode material. This arrangement has the advantage that all of the current flows into the anode material, thereby heating its surface and greatly diminishing the chances of nucleate boiling. The anodic arc is powered by a low voltage de power supply (100 V, 100 A), operated in a current regulating mode, through a current limiting resistor. During anodic arc operation, the arc voltage is approximately 17-20 V, and in conjunction with radiation at the most intense spectral line of the evaporated material, is characteristic of proper arc operation. Ignition of the anodic arc occurs by physically contacting the anode and cathode together, thereby initiating a cathodic arc which transitions to the steady state anodic arc after approximately 1 s. After contact of the electrodes, one or more slowly moving cathode spots appear on the cathode, severely eroding it. In this phase the arc is sustained by cathodic material. The anode is heated due to ohmic heating and ion bombardment. Eventually, the anode heats up sufficiently so that anode material is evaporated at which time the color of the
S. Meassick er al. I Th ill film deposition techniques
arc changes to that of the most intense spectral line of the anode material and the cathode spots transform into many, rapidly moving spots that no longer erode the cathode. The targets are arranged on a biasable substrate holder that is located so that it is not in line of sight of the cathode. The anode to substrate distance is adjustable to approximately 45 em. The data presented in this paper were taken with ·a 20 em substrate to anode separation. The substrate holder is biasable positive or negative to approximately 300 V, with a negative bias usually being applied in order to accelerate ions into the target. The d.c. bias is applied via a low voltage (approximately 300 V) d.c. power supply. Films were deposited on to silicon wafers for the stoichiometry tests and onto formvar TEM sample holders for analysis of grain sizes and orientations. X-ray diffraction results were obtained from coatings that were deposited onto glass and copper substrate holders, while corrosion tests were made with iron substrates.
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• Nickel
4
• Aluminum
3.5
+ Copper
3
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20
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60
70
Arc Current (A) Fig. 2. Deposition rate of nickel, aluminium and copper as a function of anodic arc current. There is a threshold current for deposit ion that is materi al dependent, with higher threshold currents corresponding to higher boiling temperatures of the anode material.
3. Results and discussion The rate of evaporation, and therefore deposition, were dependent on the arc current. Evaporation rates of approximately 0.04 g min -) A-I (3.2 g min -1 at 80 A) were achieved for type 303 stainless steel while evaporation rates of 0.06 g min -) A-I were achieved for aluminum. The maximum evaporation rate from the anode was only limited by the current available from the d.c, power supply (100 A). The coatings deposited with the anodic vacuum arc exhibited good adherence to the substrate. Cellophane tape tests were used for the initial analysis. Indentation tests were performed for a more objective measure of the adherence of the film. In these tests, the surface of the substrate was dented with a diamond tipped probe and the cracking, delamination or spalling of the coatings near the dent was visually observed. The coatings deposited with the anodic vacuum arc showed no delamination or spalling and only minimal cracking around the edge of the dent. Deposition rates of up to 6 urn min - ) were achieved at a distance of approximately 20 em from the anode with an arc current of 80 A. The deposition rate decreases as the square of the anode to target separation. Figure 2 shows the deposition rate for nickel, aluminum and copper as a function of current. It is evident that there is a material dependent threshold current for deposition. The threshold current ranges from 15 A for aluminum to 30 A for nickel. The threshold current increases as the boiling point of the material increases (2467, 2567 and 2732 °C for aluminum, copper and nickel respectively) . Above this threshold current, the deposition rate increases linearly with current. Figure 3 shows the arc
345
30
28 • Nickel • Aluminum .. Copper
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~ c
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22 20
18
16 0
10
20
30
40
50
60
70
Arc Current (A) Fig. 3. Voltage-current characteristic far nickel, aluminum and copper. A material dependent threshold current exists where the resistance changes from negative to positive.
voltage drop (anode to cathode) as a function of arc current for nickel, aluminum and copper. Below a threshold current the voltage-current characteristic is negative (negative resistance). This threshold current is again material dependent and occurs at approximately the same current as the threshold current for deposition. The threshold current for deposition is probably due to the fact that the anode must be heated sufficiently in order to allow evaporation to occur. Since radiation and conduction will be the primary cooling mechanisms of the anode before there is rapid evaporation, a larger
S. Meassick et at. / Thin }ilm deposition techniques
346
heat ing curre nt will be required for materials with a higher boiling tem perature. Below this thres hold voltage the a rc is operating at least partially as a cat hodic arc with cathode sp ots tha t cause erosion of the cathode. Above the thre shold voltage, in addition to their rapid moti on over the cathode, the cathode spots no longer cause erosion and the inclusion of cathode material in the coating. Th us, when operated in the anodic mode, the cathode spots are different in nature. The stoichiometry of Type 303 stainless steel, aluminum , copper an d nickel films dep osited on silicon were analyzed utilizing EDS . Films of at least 3 urn in thickness were used to ensu re that the X-ray signal measured originated in the film and not in the silicon substrate. For these experiments, the cathode consisting of tungsten instead of carbon was used so that any cathode material included in the film could be detected with the EDS system. The electron beam energy was fixed at 15 keV in order to ensure that the electron energy was at least three times the highest X-ray energy of interest. The source stoichiometry of the type 303 stainless steel was determined to be 68.3% Fe, 18.9% Cr, 9.5% Ni, 1.9% Mn and 0.7% Si while the samples of aluminum, copper and nickel were pure as far as could be determined with the EDS system. Figure 4 shows the concentration of cathode material (tungsten) as a function of arc current for aluminum, cop per and nickel films. For low arc currents, up to the same material dependent threshold current as previously seen, there is some inclusion of cathode material in the deposited film. Above this current there is no measurable inclusion of cathode material. These data a lso indicate
that below a threshold voltage the arc is operating at least partially as a cathodic arc. Figure 5 shows the concentrations of the type 303 stainless steel film constituents normalized to the stoichiometry of the source material as a function of the arc current. It is evident that as the arc current increases, the stoichiometry of the deposited film approached that of the source material. Even for very low currents (30 A) there is only a small difference between the source material and the resulting film. For arc currents above 60 A, there is less than a 2% difference in the stoichiometry of the source material and the deposited film while for arc currents above 80 A, there is no measurable difference in th e stoichiometry of the source material and the deposited film. The deposited thin films were examined with a scan ning electron microscope in order to ascertain the surface roughness and defect densities of the films. Figure 6 shows a scanning electron micrograph of a nickel film deposited on a silicon substrate for an arc curren t of 80 A. There are no discernible surface features down to the resolution limit of the instrument (approximately 4 nm). In addition, the surface was found to be free of macroparticle inclusions. The crystalline structure of the steel thin films was investigated with transmission electron micrographs and X-ray diffraction techniques . Figure 7 shows a transmission electron micrograph of the steel deposited on formvar substrates for an arc current of 80 A. The th ickness of the films used for the transmission electron micrographs is approximately 200 A. It is evident that the grains are randomly orientated with a size of less than I nm. In addition to the TEM and SEM images
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s. Meassick et ill.
/ Thin jilm deposition techniques
0.25 /-lm Fig. 6. Scanning electron micrograph of a nickel film deposited on a silicon substrale shows no discernible surface features other than the latex sphere used as a focusing aid.
347
on iron substrates. The arc current for all samples used in the corrosion tests was 80 A. The corrosion inhibiting properties of these coatings against humid and marine environments was evaluated using saIt spray fog testing according to standards ASTM BII7 [16] and MILSTD 202F method 1010 [17]. Each sample was exposed to a salt spray environment at a temperature of 35°C, a humidity of 85% and a spray solution consisting of 5% NaCl dissolved in water. Tests were conducted for up to 48 h on the samples. Figure 8 shows aluminum coated (uncoated, 0.25, 0.5 and 0.75 urn respectively) iron samples exposed to the salt spray environment for 5 h. It is evident that the 0.75 urn thick aluminum coating provides almost complete protection from corrosion. In order to obtain a more objective evaluation of the protective qualities of the coatings, each sample was given a rating R based on the percentage A of the surface covered with corrosion and based on the scheme of ASTM B537 [18J:
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Figure 9 shows the protection rating as a function of coating thickness for aluminum, nickel, and type 303 stainless steel. All samples were exposed to the saIt spray environment for 48 h. It is evident that aluminum gives complete protection (R = 10) for coating thicknesses of approximately 1.5 urn while the thickness required for nickel and type 303 stainless steel is considerably thicker. This is in stark contrast to coatings deposited with a cathodic arc which require coating thicknesses on the order of 20 urn in order to provide similar corrosion
Fig. 7. Transmission electron micrograph of type 303 stainless steel deposited on a formvar substrate shows an almost amorphous coating with feature sizes less than I nm.
of thin films, the crystalline structure of the deposited steel was examined with the aid of X-ray diffraction techniques for film thicknesses of up to I urn, The diffraction patterns showed no peaks arising from a crystalline structure for thin films, indicating that the deposited film is amorphous. For very thick layers, with a corresponding long arc duration and therefore heating of the sample, the X-ray diffraction pattern showed peaks for a [110](;( and [III] and [200]')' grain structure with the y phase becoming more pronounced as the heating increased. In order to evaluate the protective qualities of coatings deposited with the anodic vacuum are, coatings of type 303 stainless steel, aluminum and nickel were deposited
Fig. 8. Picture of iron samples coated with aluminum that were subjected to salt spray corrosion testing. The uncoated sample is labeled (a), while samples coated with 0.25 urn, 0.5 urn and 0.75 urn are labeled (b), (c) and (d) respectively.
S. Meassick et al. I Thinfilm deposition techniques
348
12,-----r----,----.----,-----.
10
produced with the anodic arc had no discernible surface texture and were amorphous in nature. An investigation of the protective properties of anodic arc coatings against salt spray indicates that even thin coatings can reliably protect a sample against corrosion.
8
Acknowledgments
6 o
4
The authors are grateful to Joe Genevich for construction of the apparatus, to Bill Fowle for undertaking the TEM measurements and to Bill Giessen and the Army Materials Technology Laboratory for the X-ray diffraction analysis.
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1
2
3
4
5
Coating thickness (pm) Fig. 9. Corrosion protection rating of aluminum, nickel and type 303 stainless steel after 48 h of salt spray.
protection [4]. While the conditions cited in ref. 4 are for deposition rates substantially higher than used in commercial cathodic arc coaters, resulting in a large number of macropartic1es that are large in size, the thickness of the protective coating must be greater than the macroparticle size for effective corrosion protection. The relatively poor protective properties of cathodic arc coating are due to the voids that are created by the inclusion of macroparticles in the coating for the cathodic arc, allowing the salt spray to penetrate to the substrate.
4. Conclusions
We have investigated the properties of thin films produced by the anodic vacuum arc. A material dependent threshold voltage was found below which the arc operates at least partially as a cathodic arc with the characteristic erosion of the cathode and the inclusion of cathode material in the coating. As the arc current is increased, the stoichiometry of stainless steel coatings approached that of the source material. The coatings
References 1 1. E. Greene and S. A. Banelt, J. Vac. Sci. Technol., 21 (1982) 285. 2 S. M. Rossnagel, J. J. Cuomo and W. D. Westwood, Plasma Based Processing, Noyes, 1989. 3 R. L. Boxman and S. Goldsmith, IEEE Trans. Plasma sa; 17 ([989) 705. 4 S. Bababeygy, Corrosion protection by vacuum arc coatings, Proc. Interfinish 88, Paris, October 1988. 5 D. M. Sanders, J. Vac. Sci. Technol. A, 7 (1989) 2339. 6 G. V. Kljuchko, V. G. Padalka and L. P. Sablev, Plasma arc apparatus for applying coatings by means of a consumable cathode, US Patent 4492 845. 7 R. Buhl, Verfahren und Vorrichtung zum Vakumbeschichten mittels einer elektrischen Bogenentladung zwischen einer Anode und einer Kathod, European patent application 0285 745. 8 H. C. Miller, IEEE Trans. Plasma Sci., 13, (1985) 242. 9 C. W. Kimblin, J. App/. Phys., 40 (1969) 1744. 10 F. M. Bacon, J. Appl. Phys., 46 (1975) 4750. II J. T. Grissom and J. C. Newton, J. Appl. Phys., 45 (1974) 2885. 12 H. Ehrich, J. Vac. Sci. Technol. A, 6 (1988) 134. 13 H. Ehrich, B. Hasse, K. G. Muller and R. Schmidt, J. Vac. Sci. Technol. A, 6 (1988) 2499. 14 S. Meassick, C. Chan, T. Sroda and R. AlIen, Anodic vacuum arc deposition processes, Int. Can! Metali. Coatings and Thin Films, San Diego, CA, April 22-26, 1991. 15 S. Meassick, 1. Kumpf, R. Allen, C. Chan and T. Sroda, Investigation of the properties of type 303 stainless steel thin films deposited with the anodic vacuum arc, Mater. Lett., (1992) in the press. 16 ASTM Standard B1I7, Salt Spray Fog Testing, 1971. 17 MIL-STD 202F, Sait Spray Testing, April 1981. 18 ASTM standard B537, Rating of Electroplated Panels Subjected to Atmospheric Exposure, 198!.