Sources of Depositing Species

Thermal Vaporization. The various thermal vaporization sources can be used in ion plating. For plasma-based ion plating, the resistively heated sources are most often used. Low-energy electron-beam heating from hollow cathode sources and thermionic sources can be used, often with a magnetic confining field. This allows the electrons to heat the material to be vaporized and also to create the plasma. High-energy electron-beam heating can be used, but this requires isolating the electron-emitting filament from the plasma by the use of a conductance baffle with a hole to allow the electron beam to enter the plasma/crucible region (differentially pumped e-beam) (Ref 1, 81).

At high vaporization rates, gas phase nucleation generates ultrafine particles in the plasma (Ref 1). These particles become negatively charged, are suspended in the plasma, and do not deposit on the substrate. However, when the plasma is extinguished, the ultrafine particles will deposit on surfaces in the system.

Physical sputtering is often used as a source of depositing material. However, when using dc magnetron sputtering configurations, the plasma is confined in a region near the target and is not available as a supply of ions for substrate bombardment. In this case, the plasma used to supply these ions can be from an unbalanced magnetron configuration, the use of rf in conjunction with the dc magnetron, or the use of an auxiliary plasma source. A hot filament auxiliary plasma source and an unbalanced magnetron plasma source configurations are shown in Fig. 7(a) and 7(c), respectively.

Arc Vaporization. An arc can also be established with a gas present, giving a plasma arc. In a plasma arc, both the vaporized material and gaseous species are ionized (Ref 82, 83, 84). Either a solid cathodic arc surface or a molten anodic arc surface (Ref 85, 86) can be used. Ion species are then accelerated to the substrate under an applied bias. A problem with the cathodic arc vaporization source is that the arc causes the emission of molten globules (that is, macroparticles or macros) that deposit on the film surface. Various techniques are used to eliminate the globules from the plasma. Arc vaporization sources are widely used in tool coating, even though they present a source of globules. The arc source and a sputtering source can be combined into one design (Ref 87). It has been found that by using the arc discharge for sputter cleaning, the cleaning and heating can be performed much faster than by using a dc diode discharge, due to the high ionization and the multiply charged heavy metal ions in the arc discharge (Ref 88). In addition, the surface smoothness is increased during arc-sputter cleaning (Ref 74). Figure 8 shows some arc-source ion plating configurations.

Fig. 8 Typical arc sources used in ion plating. (a) Vacuum arc/molten anode source. In the vacuum, the ions are accelerated away from the positive space charge in the plasma. (b) Cathodic arc vaporization source. In the plasma, the film ions are thermalized in the plasma, but both the film ions and the gas ions are accelerated to the substrate under an applied bias.

Fig. 8 Typical arc sources used in ion plating. (a) Vacuum arc/molten anode source. In the vacuum, the ions are accelerated away from the positive space charge in the plasma. (b) Cathodic arc vaporization source. In the plasma, the film ions are thermalized in the plasma, but both the film ions and the gas ions are accelerated to the substrate under an applied bias.

Chemical Vapor Precursor Gas. Gaseous chemical vapor precursor species containing the material to be deposited can be used as a deposition source. Using a chemical vapor precursor species in a plasma is very similar to plasma enhanced chemical vapor deposition (PECVD), in which the plasma is used to decompose the chemical species and to bias PECVD where ions from the plasma are accelerated to the substrate surface (Ref 43). Typical chemical vapor precursor gases are TiCl4 for titanium, SiH4 for silicon, and C2H for carbon, diamond-like carbon (DLC), and diamond.

The chemical vapor precursor may not be completely dissociated, so that it will deposit a compound material or material that has some of the original compound-forming material in it. For example, SiH4 can be used to deposit amorphous silicon containing hydrogen. The chemical vapor precursor can be injected into the plasma (Ref 1, 89) in plasma-based ion plating or into a confined plasma ion source in vacuum-based ion plating (Ref 90, 91, 92).

Laser vaporization with concurrent ion bombardment has been used to deposit high-quality high-temperature superconductor films at relatively low substrate temperatures (Ref 93). Laser vaporization creates a large number of ions in the vapor "plume," and these can be accelerated to the substrate surface. This technique has been used to deposit hydrogen-free DLC films (Ref 94).

For more information about sources of depositing species, see the other articles in this Section.

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