Introduction

ION PLATING is a generic term applied to film deposition processes in which the substrate surface and the growing film are subjected to a continuous or periodic flux of energetic massive bombarding particles (ions, radicals, atoms, or molecules--reactive or inert) sufficient to cause changes in the film formation process and the properties of the deposited film (Ref 1, 2, 3, 4). The bombarding species and the depositing species can be from a number of sources. Bombardment can take place in a plasma or vacuum environment. When a beam of energetic particles is used in vacuum, the process is often called ion-beam-assisted deposition (IBAD).

A vacuum can be defined as an environment where the gas density is low and the mean free path for collision is very long. In vacuum-barrel deposition processing, this means that the pressure is lower than about 1.3 mPa (1 x 10-5 torr). A plasma is a low-pressure gas that contains enough ions and electrons to have an appreciable electrical conductivity. This requires a gas pressure of greater than 13 mPa (1 x 10-4 torr). Plasmas can be low-pressure (less than about 0.4 Pa, or 3 mtorr) or higher-pressure (greater than about 0.4 Pa, or 3 mtorr), depending on whether or not collisions in the gas phase are sufficient to "thermalize" high-energy particles leaving a source (Ref 5, 6). This differentiation is important when using sputtering as a source for deposition because reflected high-energy neutrals from the target can have an important effect on the growing film. Figure 1 shows the distance traveled by particles of differing masses (12 and 400 atomic mass unit, or amu) and energies (5 eV, or 0.8 aJ, and 1 keV, or 160 aJ) in argon before becoming thermalized. It should be noted that the gas density in a plasma system can vary with position due to preferential gas motion and particle temperature distribution (Ref 7, 8, 9).

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Fig. 1 Plot of distance traveled vs. argon gas pressure to show effect of variations in mass and energy of particles before thermalization. Results calculated from test data obtained at room temperature. Source: Ref 5

There are two basic versions of the ion plating process. In plasma-based ion plating the substrate is in contact with a plasma, and the ions are accelerated from the plasma and arrive at the surface with a spectrum of energies. In vacuum-based ion plating the film material is deposited in a vacuum and the bombardment is from an ion or plasma "gun." The plasma-based ion plating process was first described in the technical literature in 1963 (Ref 10, 11, 12), and the first reference to vacuum-based ion plating was in 1973 (Ref 13). In reactive ion plating, the plasma or ion/plasma gun can form ions of a reactive species to both bombard and react with the depositing material to form a compound film material. In some cases, such as when using low-voltage, high-current electron-beam evaporation or arc vaporization, an appreciable portion of the vaporized source material can be ionized to allow bombardment by "film ions." Often the term ion plating is accompanied by modifying terms such as sputter ion plating, reactive ion plating, chemical ion plating, alternating ion plating, arc ion plating, and so on, which indicate the source of depositing material, the method used to bombard the film, or other particular conditions of the deposition.

Figure 2(a) shows a simple plasma-based ion plating configuration using a resistively heated vaporization source, and Figure 2(b) shows a simple vacuum-based system using an electron-beam evaporation source. In plasma-based ion plating, the substrate can be positioned in the plasma generation region or in a remote or downstream location outside the active plasma generation region.

Figure 2(a) shows a simple plasma-based ion plating configuration using a resistively heated vaporization source, and Figure 2(b) shows a simple vacuum-based system using an electron-beam evaporation source. In plasma-based ion plating, the substrate can be positioned in the plasma generation region or in a remote or downstream location outside the active plasma generation region.

Fig. 2 Schematic showing typical ion plating installations. (a) Plasma-based configuration with resistively heated vaporization source. (b) Vacuum-based configuration with electron-beam evaporation source

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