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Plasma Basics


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A gas plasma may form whenever gas is exposed to an electric field. If the field is sufficiently strong, a high percentage of gas atoms will surrender an electron or two and become ionized. The resultant ionized gas and liberated energetic electrons comprise the gas plasma or plasma. The ionized gas atoms have relatively little kinetic energy unless they are accelerated through an electric field. When accelerated, they will bombard a surface with sufficient force to dislodge an atom from a target material. Plasma effects sputtering and etching processes by momentum transfer of material, and the use of reactive gases produces chemical modification as well.

The term sputtering refers to the process of dislodging atoms from a target material to coat a thin film onto a material of interest. The term etching refers to the use of plasma to remove atoms from a material of interest in order to clean the material or to modify the surface or thickness of the material. In both sputtering and etching, the plasma can be diffused or it can be directed for a more concentrated effect.

Plasma processing has several advantages over wet chemical processing. Surface tension phenomena of the liquid are eliminated, as is air-bubble formation, which may cause incomplete wetting. Plasma etching concludes rapidly upon termination of process, whereas liquid etching processes are difficult to end with precision. Disposal of toxic and corrosive liquids is obviated. The primary disadvantage of plasma processing is lower throughput.

Anatech works in the area of plasma processing which uses gases present at ambient temperature without thermal phase change. These gases can be either inert or reactive. In the case of reactive gases, processing uses momentum transfer as well as the chemical properties of the reaction of the gas to the material of interest. Anatech’s sputter process is primarily physical, mostly diffused, and slightly directed. The ashing/etching process is diffused and relies more on chemical effects than on physical sputtering effects. Anisotropic etching is directional and is induced by electrically controlling ions at some specific energy, or range or energies.

Thin film deposition services that Anatech offers, are based upon these same plasma processes. Gas is introduced to a vacuum chamber, cracked into a plasma, and deposited on to the substrate a molecule at a time. Deposition is induced by molecules condensing from a hotter plasma to a cooler substrate, and by electrical bias.