Additional Vaporization Methods

Vaporization can also be produced by vacuum arc, laser, electric field, and polymer evaporation methods.

Vacuum Arc Vaporization. Arc vaporization was first reported by Robert Hare in 1839 and has been used to deposit carbon (Ref 67) and metal (Ref 68) films. Arc vaporization in vacuum occurs when a high-current, low-voltage arc passes between slightly separated electrodes in a vacuum, vaporizing the electrode surfaces and forming a plasma of the vaporized material (Ref 69, 70, 71, 72). In order to initiate the arc, a high voltage "trigger" arc is used. A high percentage of the vaporized material is ionized in the arc, and the ions are often multiply charged. A negative space charge is generated in the plasma, and the ions are accelerated away from the plasma to energies that are much higher than thermal energies. This means that the deposition is accompanied by concurrent bombardment from the high-energy film ions. This concurrent bombardment can have beneficial effects on the film density, as is found in ion plating.

If the arc anode is cooled or has a large area, vaporization primarily occurs from the solid cathode surface by arc erosion. At the present time,the principal arc vapor source in thin-film technology is the solid cathode. Problems with this deposition technique include stabilization and movement of the arc on the solid surface and the formation of globules of the ejected material. Arc confinement and controlled arc movement using magnetic fields have given rise to the "steered arc" source. The globules can be filtered from the arc using various means (for example, the "plasma duct") (Ref 73), all of which reduce the deposition rate. Carbon ions (500 eV) from a vacuum arc source have been used to deposit hydrogen-free, diamond-like carbon films (Ref 74).

If the anode is allowed to melt, material evaporates from the molten (consumable) anode surface. This vaporization technique is essentially the same as the vacuum-arc-remelting (VAR) process (Ref 75, 76). Some studies have been done using the vapor from the molten anode of a vacuum arc. This has the advantage that globules are formed to a lesser degree than in cathodic erosion (Ref 77, 78). A commercial source of metal ions from a vacuum arc is the metal vapor vacuum arc (MVVA) source (Ref 79).

Additional information is available in the article "Arc Deposition" in this Volume.

Globules. The number of globules produced from the cathode surface depends on the melting point of the cathode material and the arc movement. The distribution of globule emission is nonisotropic, with the maximum number being found at angles greater than 60° from the normal to the surface. The neutral atoms found in the arc vapor are thought to be produced by thermal evaporation from the ejected globules. This effect causes the composition of the deposited film to vary with thickness and position when an alloy material is deposited (Ref 80).

Laser Vaporization. Flash evaporation can also be done with pulsed laser heating of surfaces (Ref 81, 82). This technique is sometimes called laser ablation deposition (LAD) (Ref 83). Typically an excimer laser (yttrium-aluminum-garnet, or YAG, or argon-fluorine) is used to deposit energy in pulses. The YAG lasers typically deliver pulses (5 ns pulse length, 5 Hz frequency) with an energy of about 1 J/pulse (9 x 10-4 Btu/pulse), and the argon-fluorine lasers typically deliver pulses (20 ns, 50 Hz) with about 300 nJ/pulse (2.7 x 10-10 Btu/pulse). The vaporized material forms a plume above the surface, where some of the laser energy is adsorbed and ionization and excitation occur. In laser vaporization the ejected material is highly directed; this creates a problem with forming a uniform thickness over large areas. During vaporization molten globules are ejected, and these can be eliminated by using a velocity filter. Laser vaporization, combined with the passage of a high electrical current along the laser-ionization path to give heating and ionization, has been used to deposit hydrogen-free, diamond-like carbon films at an ablation energy density greater than 5 x 1014 W/m2 (1.6 x 1014 Btu/ft2 • h) (Ref 83). Laser vaporization with concurrent ion bombardment has been used to deposit a number of materials (Ref 84, 85), including high-quality, high-temperature superconductor oxide films (Ref 86), at low substrate temperatures. Laser vaporization can be used to vaporize material from a film on a transparent material onto a substrate facing the film, by shining the laser through the "backside" of the transparent material, vaporizing a controlled film area, and thus depositing a pattern directly onto the substrate (Ref 87).

Field Evaporation. Surface atoms of metals can be vaporized by a high electric field. This technique is known as field evaporation and can be directly observed in the field ion microscope (Ref 88). This vaporization technique is used to clean emitter tips in field ion microscopy and to form metal ions from liquid-metal-coated tips. Field evaporation has been used to directly deposit nanometer-size gold structures (Ref 89). The very sharp tips necessary to obtain the high field can be formed in a variety of ways (Ref 90).

Polymer Evaporation. Many monomers and polymers can be evaporated to produce thin organic films on a substrate surface. Some organics can be cross-linked in the vapor phase inside a heated furnace before condensing on the substrate surface (paralyene process) (Ref 91). Condensed polymers can be cross-linked on the surface by exposing them to an electron beam (Ref 92) or ultraviolet radiation (Ref 93).

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