Ion Plating Bombardment Parameters

A variety of ion plating configurations can be used (Fig. 9). Each configuration will have somewhat different parameters that need to be controlled. Ideally, the ion plating parameters that should be controlled are:

• Ion species and ionization state

• Particle energy

• Gas composition and mass flow

• Bombardment uniformity

• Substrate temperature

• Gas incorporation

Fig. 9 Typical configurations used in ion plating

Ion Species. The mass of the bombarding species is important to the energy and momentum transferred during the collision. From the laws of conservation of energy and the conservation of momentum, the energy, E, transferred by the physical collision between hard spheres is given by:

where E = energy, M = mass, i = incident particle, t = target particle, and j = the angle of incidence as measured from a line joining their centers of masses. The maximum energy transfer occurs when Mi = Mt and the motion is along a path joining the centers (j = 0°). In some cases, bombardment by self-ions (that is, ions of the target material) can result in self-sputtering. This occurs when atoms of the sputtered material become ionized and bombard the target.

The most common inert gas species used for plasma formation and ion bombardment is argon, because it is the least expensive of the inert gases. Krypton is sometimes used, and historically mercury was used. Common reactive gases used in plasmas are nitrogen and oxygen. A mixture of inert gas and reactive gas is often used to increase the momentum transfer efficiency in reactive deposition. Helium is sometimes mixed with other gases to increase the thermal conductivity of the gas mixture to aid in substrate cooling.

Particle Energy. The energetic particle energy and energy distribution are important parameters. The energy should be high enough to give appreciable energy transfer on collision, but it should not be high enough to be physically implanted and trapped in the depositing film where it can precipitate and form voids. Neither should it be high enough to cause excessive sputtering. For low-temperature deposition, the ion energy should not exceed about 300 eV (48 aJ). If the substrate is heated to 400 °C (750 °F) or greater, the energy can be increased, because the implanted ions are continually being rapidly desorbed. For low-pressure sputter deposition, the presence of high-energy reflected neutrals from the sputtering target can be an important parameter.

Flux Ratio. The ratio of depositing atoms to bombarding species is important to the film properties. Typically, to complete the disruption of the columnar morphology of the growing film and to obtain the maximum density and least microporosity, the energy deposited by the bombarding species should be about 20 eV (3 aJ) per depositing atom or about 20 to 40% resputtering (Ref 95, 96, 97).

Gas Composition and Mass Flow. Gas composition is an important processing variable in ion plating. In reactive deposition, the gas mass flow can be an important variable that is sensitive to the fixture/system geometry. The gas used for an inert plasma should be free of contaminants (for example, water vapor and oxygen) that will become activated in the plasma. Inert gases can be purified using heated reactive surfaces (for example, titanium or uranium chip beds). Reactive plasmas should be free of contaminants. For example, in reactive gases or gas mixtures, water vapor can be removed by cold traps using zeolite adsorbers.

Mixtures of gases can be used to deposit films having differing compositions and properties such as color. For example, titanium deposited in a nitrogen plasma to form titanium nitride is a gold color, but with a mixture of nitrogen and methane the color can be made bronze, rose, violet, or black as the TiCxNy varies in composition.

The gas distribution into the deposition system is an important factor in obtaining uniform bombardments over a surface and uniform activation of a reactive gas.

Bombardment Reproducibility. In vacuum-based ion plating, the ion and atom fluxes can be measured directly by using a Faraday cup ion collector and a mass deposition meter (for example, a quartz crystal deposition monitor). The presence of high-energy reflected neutrals from the sputtering target in the vacuum environment is difficult to measure and can be an unknown processing parameter when a sputtering source is used. In plasma-based ion plating, the ion flux and flux energy distribution are difficult to measure directly. In both vacuum-based and plasma-based ion plating bombardment and deposition, consistency, uniformity, and reproducibility are controlled by having a consistent vaporization source, system geometry, fixture motion, gas composition, gas flow, and substrate power (that is, voltage and current).

Fixturing is often the key to bombardment and deposition uniformity. Fixtures can be in the form of holders that move the substrates or move the deposition sources (Ref 98). Often a three-dimensional object is rotated in front of the deposition source to randomize the deposition angle of incidence, increase uniformity of bombardment, and give a more uniform morphology to the deposit (see the article "Growth and Growth-Related Properties of Films Formed by Physical Vapor Deposition" in this Volume). In some cases, especially when using an rf bias, irregularly shaped objects are surrounded by a grid that is electrically tied to the object, thus giving a smooth equipotential surface around the object. Figure 10 shows the use of a rotating "cage" to hold loose parts to be coated (Ref 99).

Fig. 10 Schematic showing key components of a barrel-plating configuration used in ion plating. The grid allows the acceleration of ions through the grid-holes to bombard the small parts enclosed within the rotating barrel (cage). Source: Ref 99

Substrate Temperature. In some cases, ion plated films are deposited without deliberate heating of the substrate. This is particularly advantageous when the substrate is thermally sensitive (for example, a plastic). In the extreme, the deposition can be periodic to allow cooling of the substrate between depositions. For example, the substrates can be mounted on a drum and alternately rotated in front of a deposition source and allowed to cool between depositions (Ref 100, 101, 102).

For the highest-density deposit and the most complete reaction, an elevated temperature is generally desirable (Ref 103). Substrate heating can be done by ion bombardment prior to and during deposition (Ref 74), but often a more controllable technique is to have an auxiliary heating source, such as a radiant heater or electron-bombardment heating. In tool coating, for example, the tool is often heated to just below the tempering temperature.

Gas Incorporation. At low substrate temperatures, bombarding gas can be incorporated into the growing film, particularly if the bombarding energy is high. To minimize gas incorporation, the bombarding energy should be kept low (that is, less than 300 eV, or 50 aJ), or a heavy bombarding particle (for example, krypton or mercury) can be used, or the substrate temperature can be kept high (that is, greater than 300 °C, or 570 °F). Low-temperature bombardment can be used to deliberately incorporate gas into the surface of a depositing film. In sputter-ion vacuum pumps, for example, trapping is used to pump inert gases and to incorporate insoluble light and heavy gases in depositing films (Ref 104, 105).

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