Plasma Sources

The use of plasma is at the core of numerous technologies in industry, enabling applications in microelectronics, materials, and medicine. This is possible because plasma, being a reactive mixture of ions, electrons and neutrals, opens new pathways for reactions which are otherwise difficult or impossible by conventional chemistry. A key benefit of plasma stems from its non-equilibrium: electrons, ions and neutrals maintain separate temperatures. High temperature chemistry being accessible in room temperature environments has advanced materials science and enables applications such as medical device coatings, functionalization of the surfaces of delicate materials, and for the precise construction of semiconductor devices.

The plasma generation environment depends on application requirements and pressure ranges. This has led to the development of various kinds of plasma sources. The three most common plasma generation methods are via constant electric fields (DC), alternating electric fields (RF) and electromagnetic fields (GHz). The fundamental properties of a plasma span a wide range of particle temperatures and densities depending on the source used. Below is a summary of the main differences between DC, RF  and microwave plasmas.

DC Plasma Sources

This is the simplest configuration of a plasma reactor, consisting of two electrodes situated inside a vacuum vessel. Plasma is created by applying a DC voltage to the gas filled gap between the electrodes. A large DC current flows between the electrodes hence these plasmas are also referred as DC discharges. These are also knows as Townsend discharges, as the ion and electron generation methods are described by the Townsend model.

Find out more about DC Plasma Sources here.

RF Plasma Sources

When the frequency of applied electric field lies in the range of radio waves, only the electrons can follow the electric field, whereas the ions remain at rest. These electrons collide with the gas atoms/molecules and ionize them to generate plasma. RF plasma sources are the most commonly used in the semiconductor industry. By changing the frequency used, or by modulating the RF power (similar to AM radio), control can be gained over the ion energies and ion fluxes processing the substrate surfaces.

Find out more about RF Plasma Sources here.

Microwave Plasma Sources

In the GHz frequency range, only electrons can follow the oscillations of electric field. However, the electromagnetic wave cannot fully penetrate the plasma because of the skin depth phenomenon. Application of an external magnetic field allow better coupling of the electron with wave field, creating a resonance between two which efficiently delivers energy to the plasma. Such plasmas are called electron cyclotron resonance (ECR) plasmas, and these generate particularly high plasma densities.

Find out more about Microwave Plasma  Sources here.

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