Process Utilization

Arc sources can be classified according to the duration of operation, the type of electrode that provides the metal vapor, and whether the arc comprises discrete spots or is distributed over a larger area of the electrode. There are pros and cons associated with using each type of arc. For example, in a pulsed cathodic arc source, the arc duration is typically short enough that direct cathode cooling is not needed, which simplifies the design and makes it easier to change target materials. In addition, the confinement of the arc spot to the cathode surface is not problematic, because the short arc duration typically means that there is not enough time for the arc spot to leave the cathode surface. Short arc durations also permit deposition of materials, such as silicon, that are vulnerable to cracking because of relatively poor thermal conductivity, coupled with a negative resistivity coefficient with respect to temperature, which slows arc spot motion. However, pulsed sources that have low duty cycles also have correspondingly low integrated coating rates.

Continuous cathodic arc sources are typically sustained by a low-voltage, high-current power supply, such as an arc welding supply. In this arc mode, the arc spots that appear to move rapidly on the negative electrode are actually separate arcing events that occur in rapid succession. The rate of apparent motion of the arc spot(s) is a strong function of:

• Cathode composition

• The presence and composition of any working gases

• The component of any magnetic fields parallel to the cathode surface

Continuous cathodic arc sources typically provide higher coating rates, but the cathode must be designed to dissipate the heat generated by the arc. Because the heat is concentrated in a small spot, direct water cooling is usually required. This leads to difficulty in changing cathodes and limits the use of some low thermal conductivity materials. In addition, arc confinement is essential, because damage to support components and contamination of the coating can occur if the arc spot leaves the cathode surface. More details on arc initiation, confinement, and other aspects of cathodic arc source design are provided in Ref 1, 2, and 3.

The cold cathodic arc source typically produces droplets of cathode material (macroparticles). These macroparticles result from the extreme localized heating of the cathode, which is due to the high current densities that are found in cold cathode arcs (104 to 108 A/cm2). Unless the macroparticles can be removed from the plasma stream, they become lodged in the coating and are usually considered to be defects.

Macroparticle Filtering. An extensive body of knowledge that describes the filtering of these macroparticles in cases where such defects are unacceptable is now available. Two useful sources are Ref 5 and 6. Although the design of macroparticle filters is also beyond the scope of this article, one example is shown in Fig. 1. Briefly, this approach uses magnetic fields in order to constrain arc-produced electrons to follow a curved path from cathode to workpiece. This sets up an electrostatic field that channels the ions through the filter. The macroparticles, however, follow straight-line trajectories into baffles and are stopped. Although there are numerous designs for such filters, they all lose at least half of the desired coating material during transit through the filter, which leads to a corresponding decrease in deposition rate.

Fig. 1 Generic filter for removing macroparticles from a cathodic arc

A second general approach, which is appropriate in cases where a small number of macroparticles can be tolerated, is given by Coll (Ref 7). The cathode is placed behind a magnet structure that focuses the ion stream strongly at the coil location and then diverges outwardly toward the workpiece location. Arc confinement is provided by the magnetic field from the focusing coil (Fig. 2). The focusing action is thought to vaporize any macroparticles that pass through the plasma, reducing their number substantially. The source, however, requires a background gas pressure on the order of 1 Pa to operate, which can affect coating quality.

Fig. 2 Approach for reducing number of macroparticles from a cathodic arc

A third approach for avoiding macroparticles is to use an arc source that does not produce them. A broad class of such sources is based on a different type of arc, which is characterized by a much lower current density (~10 A/cm2). This is about five orders of magnitude lower than that found in cold cathode arcs. The same arc currents are made possible because the arc is distributed over a much larger area of the electrode, which leads to the term distributed discharge arc (Ref 8). Although such distributed discharge arcs have been reported for both electrodes, most of the investigations to date have been on arcs that vaporize the positive electrode or anode.

Arc Source Types. Anodic arc sources can be classified according to the method by which ionization electrons are supplied (Ref 3). The sources can be hot filament (Ref 9), hollow cathode (Ref 10, 11, 12, 13), or cathodic arc (Ref 14, 15, 16). Typical configurations are shown in Fig. 3 and 4.

Fig. 3 Anodic arc device. Source: Based on Ref 11

Substrate shutter

Movable cathodic shield

m «1 ~

~L™J

o

Expanding

♦ —1

Movable cathode with cylindric stainless steel shield

Masked glass substrate and diagnostic u- ■

Tungsten crucible (represents anode and contains metal to be evaporated)

Fig. 4 Arc discharge apparatus. Source: Based on Ref 18

The hollow cathode arrangement described by Dorodnov (Ref 10, 11) operates as a self-sustaining arc (Fig. 3). The material to be evaporated, which forms the anode, is located within a hollow cathode. The anode material evaporates under a low-voltage (10-50 V), high-current (~100 A) electron beam, which, along with electrons trapped within the hollow cathode, ionizes the vapor (Ref 7). Because voltages are relatively low, sputtering of the cathode is minimized. Dorodnov reports using this source for chromium, carbon, magnesium, titanium, molybdenum, silicon, germanium, and copper (Ref 11). Saenko (Ref 13, 17) used this source to process substrates with diameters up to 150 mm (6 in.). Derkach and Saenko (Ref 12) incorporated a divergent plasma lens, which resulted in a 150 mm (6 in.) diameter copper vapor plasma with an ion-component beam uniformity of about 2%. The deposition rate was reported to be approximately 100 nm/s (4 pin./s).

The approach taken by Ehrich et al. (Ref 16) used a cathodic arc to supply ionization electrons to the vapor stream (Fig. 4). In this configuration, copper and zinc films were deposited at rates between 23 and 65 nm/s (0.9 and 2.6 pin./s) (below that reported by Dorodnov). The films were found to be homogeneous, with film purities up to 99.9% (Ref 14). The researchers determined an ion temperature of approximately 0.7 eV at plasma densities between 1015 and 1017/m3 (Ref 18). These films were found to have densities that were lower than bulk material densities by 0 to 10%. Overall, this technique produced compact films with physical properties that were close to those of the bulk materials.

The use of shielding eliminated contamination of the deposited material that was due to cathodic bombardment (Fig. 4). This type of contamination, along with source contamination from the containment vessel, are issues that need to be addressed when designing this arc source. Dorodnov (Ref 11) suggests using a hearth made of the same material as that being evaporated. For vapors originating from sources that sublimate, such as carbon, actively cooled supports outside the heat-affected zone have been used. However, the relatively low evaporation rate (compared with the cathodic arc technique), short run times, and issues associated with filament lifetime have hindered the commercial development of the anodic arc source.

The utility of the anodic arc lies in its ability to generate a flow of predominant monocharged ions without macroparticles. This flow of single-charge-state ions facilitates deposition by allowing greater control of deposition energies. This can be contrasted with the cathodic arc, which produces multiple-charge-state ions. Deposition onto a biased substrate in the presence of multiple-charge-state ions may result in sputter damage to the substrate. Because the charge-state distribution in the anodic arc is nearly single valued, substrate sputtering can readily be controlled. Similarly, the monocharged nature of the anodic-arc-generated ions simplifies stream focusing and control. This facilitates usage in materials processing and as a plasma source in space research.

The rate of deposition, using arc technology in a laboratory environment can range from several angstroms/minute to 0.1 mm/s (4 mils/s) depending on the specific process chosen and the coating quality required. High-quality optical coatings of aluminum oxide and zirconium oxide have been produced using a cathodic arc with a macroparticle filter at rates of 35 pm/h and 20 pm/h, respectively (Ref 19).

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