Plasma, the fourth state of matter

Definition of plasma

Plasma is an ionized gas phase substance that consists of ions, electrons and neutral atoms and/or molecules that grossly maintain charge neutrality. Electrons and ions should be close enough so that each of them can influence many nearby charged particles within a radius called Debye screening length. As a result, charged particles in plasma response collectively to external electromagnetic field. With high density of free moving ions and electrons, plasma is highly electrically conductive. Except boundary regions between plasma and electrodes, plasma contains same amount of positive and negative charges. There is no space charge within the bulk of plasma.

Plasma and plasma sheath
Plasma and plasma sheath

Plasma sheath

Electron temperature is usually equal or higher than that of ions. Since electrons are much lighter than ions, they can escape from plasma at much faster speed than ions if there is no confining potential barrier. Once electrons are mostly depleted from the boundary interface between plasma and electrodes or samples, a region with only positive ions and neutrals will be formed. This usually dark boundary region is called plasma sheath. Positive charges in plasma sheath can push more ions to diffuse out of plasma. It also creates a potential barrier to prevent electrons from diffusing out of plasma. Eventually the loss rate of electrons and ions will reach an equilibrium state. Plasma sheath also creates a positive plasma potential with respect to the grounded chamber walls. The voltage drop across plasma sheath can accelerate ions and create ion sputtering effect in many applications.

How to generate plasma

Electrons and ions in plasmas can disappear through diffusion or recombination. To sustain a stable plasma, external excitation is required to create more electrons and ions so that their creation rate can reach a balance with the loss rate. Most of the plasma generation methods rely on heating free electrons to high enough energy to break down neutral atoms or molecules into ions and electrons. The following sections will give a brief introduction to some of the most common plasma generation methods