Process gases

In general, a process gas is any gas or gas mixture used in an industrial, chemical, or manufacturing process to enable, support, or alter a production sequence. Unlike utility gases (which are used generally across a facility for power or heating), a process gas actively participates in the core production mechanism.

In the case of plasma processing, we can discriminate several types of process gases contributing to the technological outcome:

• Ionization gas

It is the gas that undergoes the gaseous discharge, producing large amounts of electrons, ions, and excited species. The main part of the electrical energy is consumed by ionization gas. Typically, Ar is used for this purpose, but in special cases, other noble gases are also of importance, e.g., He. In many processing schemes, the function of the ionization gas is combined with that of the reactant. It is the case, e.g., when air (compressed dried air – CDA or synthetic air) or forming gas (nitrogen with hydrogen) is used to produce the plasma.

• Reactant gas (input)

This is a chemically active gas that is mixed with, added to, or used as the ionization gas. Typical examples are oxygen added for oxidation-based processes, nitrogen for nitriding, and hydrogen for reduction processes. The reactant gases also comprise the precursors used for coating processes – frequently, vapors of organic materials.

• Carrier gas

It is used to transfer non-gaseous materials into plasma, such as precursor droplets or powder. The main characteristic of the carrier gas is a minimal contribution to chemical reactions and thermal balance in plasma. Most frequently, it is He, Ar, or N₂.

• Plasma gas (product)

The outcome of the gaseous discharge is plasma gas containing numerous chemical radicals that can interact with substrates. In atmospheric-pressure processes, the plasma gas is guided from the plasma reactor onto the substrate surface and must be neutralized before being released into the ambient.

• Ambient gas

Typically, the ambient gas is air, including the humidity. To reduce the secondary impact of reactive gases in air (e.g., oxygen, water), an artificial ambient consisting of passive or dry gases such as Ar or N₂ is frequently used.

Various gas sources for the process are known. To provide the CDA, compressors with a filter and drying stages are used. The membrane separators can be used to increase the concentration of either oxygen or nitrogen in the technical gas. Pure gases are delivered in pressurized containers (e.g., 10 L or 50 L bottles), typically with 200 or 300 bar.  At high consumption levels, liquefied gases (e.g., liquid nitrogen, LN) are used to improve economic efficiency. It is also common to use solidified carbon dioxide (CO₂ ice).

Depending on the process requirements, the gas purity level is defined.