1. The definition of plasma

　　Plasma is an ionized gaseous substance, composed of ions, electrons and neutral atoms/molecules, and maintains a neutral charge as a whole. Electrons and ions are close enough to each other that they can influence each other on many nearby charged particles within a radius called the Debye shield length. As a result, the charged particles in the plasma collectively react to the external electromagnetic field. Due to the high density of free ions and electrons, plasma is very conductive. Except for the surrounding area between the plasma and the electrode, the plasma has the same positive and negative charges. There is no space charge in most plasmas.

　　2. How to generate plasma? How is plasma generated?

　　The ions and electrons in the plasma disappear through diffusion and recombination. In order to maintain a stable plasma, external stimulation will generate more electrons and ions, so the rate of generation and loss must be balanced. Most plasma generation methods are heating free electrons to decompose  neutral atoms and molecules into ions and electrons with sufficiently high energy.

　　3. DC Glow Discharge Plasma

　　DC glow discharge has a fixed cathode and an anode. The electrons accelerate from the cathode to the anode and gain more energy. Along the anode, electrons collide with other ions and neutral substances multiple times. During the impact, the electrons transfer their energy to the ions and bound electrons in the neutralizer. Once the electric field strength reaches a certain level, the freely moving electrons will gain enough energy to separate the bound electrons from the neutral particles.

　　4, the principle of atmospheric pressure plasma surface activation

　　①. Free radicals in N2 plasma react with carbon on the surface

　　②. Oxygen free radicals, carbon, hydrogen, metals, etc. form an oxygen layer on the surface.

　　③. The surface is activated by plasma

　　5. Oxygen plasma application

　　Oxygen plasma has many applications, such as photoresist stripping, dross removal, organic pollutant removal and surface functional group (hydroxyl, carboxyl part) generation to improve adhesion and coating ability. It converts hydrophobic surfaces such as silicon wafers, PDMS, metals, plastics, polymers, and composite materials into hydrophilic surfaces. Oxygen plasma etching also improves adhesion by increasing the surface roughness of polymers, and is widely used in device packaging, such as wire bonding, chip connection, flip chip underfill and device packaging. By increasing the wettability and adhesion of silicon, glass, and PDMS, oxygen plasma treatment is an important step in the manufacture of microfluidic devices.