Plasma is the fourth state of matter except solid, liquid and gas. 

It is mainly dissociated by gas under the action of high electromagnetic field to form a highly active 

substance consisting of electrons (with negative charge), positive ions (with positive charge), 

free radicals and neutral gas molecules. In general, the positive charge number in plasma 

is equal to the negative charge number, so the whole is electrically neutral (Quasi-neutral). 

Lightning, fluorescent lights and neon lights in daily life are plasma luminescence.

There are many ways to produce plasma, but both generation and maintenance 

require external energy input. 

The electrons obtain energy from the applied electromagnetic field, and then 

inelastic collisions occur to produce excitation, dissociative attachment, 

dissociation , ionization, recombination,resulting in secondary electrons, 

atoms, molecules, ions, etc. 

However, in dissociation collisions, electrons may recombine with gas ions 

or disappear from the electromagnetic field because of drift and diffusion, 

so the plasma is stable when the rate of electron is equal to the rate of disappearance. 

Here list of four ETCH commonly used methods to produce plasma, will be explained later.

Three、Plasma parameters

（Mean Free Path，MFP）：

The average distance a particle can move before colliding with a particle. 

The MFP is inversely proportional to the environmental pressure P. 

the higher the pressure, the shorter the MFP, the lower the pressure, and the longer the MFP, 

which is why the plasma production is produced in a low-pressure approximate vacuum environment, 

because at atmospheric pressure (760 Torr), the electron MFP is very short.