The  plasma, known as the fourth state of matter, consists of a large number of free electrons, ions and neutral active groups. With its unique electrical, thermal and optical properties, plasma has shown great application value in many fields such as semiconductor industry, material processing, environmental protection, biomedicine, diamond preparation and military industry.

Artificial plasma generation methods include DC, RF and microwave discharge. Among them, microwave plasma technology efficiently transfers the microwave energy generated by the microwave source to the resonant cavity through a waveguide or antenna, which accelerates the initial electrons in the discharge gas, and then inelasticly collides with the gas molecules and causes ionization. By adjusting the microwave power, a continuous and stable discharge can be achieved. Compared with other discharge methods, microwave discharge shows a series of significant advantages. Firstly, microwave has strong penetration, high energy conversion rate and good controllability. As an energy source, microwave generator is easy to operate, stable and pollution-free. Secondly, since the plasma generation area maintains a certain distance from the microwave energy source, the direct damage of the energy source is effectively avoided. In addition, microwave discharge does not require electrodes, thus eliminating the potential pollution of metal electrode materials to plasma. More importantly, the microwave-excited plasma region is not limited by the electrode gap and can produce large-scale, high-concentration plasma. In view of the above advantages, the research and application fields of microwave plasma source technology are receiving extensive attention.

Microwave plasma has been widely used in many fields such as physics, chemistry and engineering. Specifically, there are the following :

( 1 ) Spectral analysis : Using the energy of microwave as an excitation source, the sample is excited and ionized. The radiation generated by the non-radiative excitation of the sample atoms becomes the spectral information of the sample elements. The main advantage of the microwave discharge plasma jet elemental analysis method is that it can use helium as the working gas to determine all the detection elements, and its operation is relatively simple, the required instrument cost is low, and the operation cost is low.

( 2 ) Synthesis and preparation of materials : Nano-materials can be prepared by atmospheric pressure microwave discharge plasma through nitrogen and oxygen. In the preparation of nanoparticles, nano-metal nitrogen and oxide particles can be mainly prepared, and their particle size can reach nanometer level. In the preparation of nanotubes and nanorods, good results have also been achieved. These nanomaterials are widely used in related fields such as photocatalysts and field emission devices.

( 3 ) Environmental protection : With the rapid development of society, people pay more attention to environmental issues while developing production, and the concept of green environmental protection is more deeply rooted in the hearts of the people. However, in industrial production and daily life, there are still a large number of solid-liquid-gas wastes and toxic substances, which not only endanger the environment, but also pose a great threat to human health. In this context, the use of microwave discharge plasma technology for waste treatment is particularly important. Microwave plasma technology has the following advantages : First, microwave plasma has high energy density and high reaction temperature, which can produce more active substances, promote the excitation of gas molecules, and have strong production capacity. 2.There is no need for the use of catalyst, no combustion fuel, and the generated gas volume is small. The reactor has a large temperature gradient, flexible reaction, can effectively reduce pollutants.

( 4 ) Etching technology : Etching technology has been widely used in semiconductor devices, the basic manufacturing process of integrated circuits, thin film circuits, microelectronics and other important fields, and plays an important role. Compared with other plasma etching techniques, microwave discharge plasma etching has a higher power frequency. Its etching reaction chamber and discharge chamber are separated, which effectively avoids the influence of light and heat on the substrate.