Basic introduction to plasma

1、 What is plasma?

 

Plasma is the fourth material state except for solid, liquid, and gas. It is mainly formed by the dissociation of gas under the action of a high electromagnetic field to form a highly active substance composed of electrons (with a negative charge), positive ions (with a positive charge), free radicals, and neutral gas molecules. It will emit light under the condition of common production. Generally, the number of positive charges in plasma is equal to negative The number of charges, so the whole is quasi-neutral. The lightning, fluorescent lamp, and neon lamp in our daily life are the phenomena of plasma luminescence.

 

2、 Generation of plasma

 There are many ways to generate plasma, but both generation and maintenance need external energy input. The electrons get energy from the external electromagnetic field, and then inelastic collision produces excitation and dissociative adsorption Secondary electrons, atoms, molecules, and ions are produced by the reactions of attachment, disassociation, ionization, and recombination. However, in the dissociation collision, the electrons may reabsorb with gas ions, or disappear due to drift and diffusion away from the electromagnetic field. Therefore, when the electrons are generated and disappeared, they will not be able to form a new type of material When the rate is equal, the plasma is stable. Here are four commonly used plasma generation methods for etching

 

CCP：Capacitively Coupled Plasma

 

ICP：Inductively Coupled Plasma

 

TCP：Transformer Coupled Plasma

 

ECR：Electron Cyclotron Resonance

 

 

 3、 Parameters of plasma

 a. Mean free path (MFP)

 The average distance a particle can move before it collides with it. The MFP is inversely proportional to the ambient pressure P. the higher the pressure, the shorter the MFP, the lower the pressure, and the longer the MFP. This is why plasma is produced in a low-pressure approximate vacuum environment, because at atmospheric pressure (760 Torr), the MFP of electrons is very short, and it is difficult to obtain enough ability to use gas ionization.

b. Ionization degree:

 The ratio of electron concentration to the sum of ion and neutral particle concentration in plasma. The ionization degree mainly depends on the energy of electrons in the plasma. The ionization degree in most plasma reaction chambers is less than 0.01%, the ionization degree of high-density plasma is about 1% ~ 5%, and the ionization degree at the center of the sun is about 100%.

 

 

4、 Application of plasma

 At present, plasma technology is widely used in various industries, such as semiconductor industry, biomedical industry, food and chemical industry, automobile parts industry, etc,