Sources of Bombarding Species

The energetic species used to bombard the growing film can be either ions or neutrals, although acceleration of charged ions is the most common way to obtain a controlled bombardment.

Plasmas. A common source of energetic bombarding species is ions accelerated from an inert or reactive gas plasma. The plasma can be formed using a number of configurations. The most common configuration is the dc diode with an electrically conductive substrate serving as the cathode. When the substrate or depositing film is an electrical insulator, the plasma can be formed by making the substrate an rf electrode in an rf plasma system.

Bombardment can be enhanced by having a superimposed rf and dc potential on the substrate. In some cases, the plasma can be formed by the electrons used to vaporize the source material (Ref 64, 65, 66, 67, 68).

In some cases, auxiliary plasmas are used to provide the ions. Often these auxiliary plasmas are formed using a hot electron-emitting filament (Ref 69), a hollow cathode (Ref 70, 71), or a plasma arc source. The electrons can be confined with a magnetic field, as shown in Fig. 7.

Fig. 7 Schematic showing selected methods used to provide a plasma near the substrate. (a) Hot cathode configuration. (b) Magnetically confined plasma configuration. (c) Unbalanced magnetron configuration

Gas Ions and Film Ions Generated by Arc Sources. Low-voltage, high-current arcs are a source of ions. If the arc is in a vacuum, then the vaporized electrode material is highly ionized and is often multiply charged (Ref 72, 73). In a vacuum, a positive space charge in the plasma between the electrodes accelerates the ions away from the electrodes. If the gas pressure is low, the ion can bombard a surface with appreciable energy. An arc can also be established with a gas present to generate a plasma arc. In a plasma arc, both the vaporized material (film ions) and gaseous species are ionized and can be accelerated to bombard the growing film. The ions from the arc can be used to sputter clean the surface at a high particle energy. If the accelerating voltage is high enough, the ion bombardment can prevent any net deposition on the substrate (Ref 65, 74).

High Energy Neutrals. In sputter deposition, ions bombarding a sputtering cathode can be neutralized and be reflected with an appreciable portion of their incident energy. If the gas pressure is low (~< 0.4 Pa, or 3 mtorr, as shown in Fig. 1), the high-energy reflected neutrals can bombard the growing film and affect the film properties (Ref 75, 76).

High-energy neutrals are also formed by charge exchange processes in the higher-pressure dc diode plasma configurations where the substrate is the cathode (Ref 77, 78, 79, 80).

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