TCO (Transparent Conductive Oxide) thin film first appeared in the early 20th century, Badeker made cdos Transparent conducting film for the first time in 1907, began the development and application of Transparent Conductive film, InSn oxides and InSn alloy was reported in 1968, in the theoretical research and applied research caused widespread interest.These oxides are heavily doped and highly degenerate semiconductors, whose mechanism is stoichiometric ratio migration and doping, and whose bandgap width is generally larger than 3eV and varies with different components. Their photoelectric performance depends on the oxidation state of the metal and the characteristics and quantity of dopants.

Conductive mechanism and properties of ITO films

In2O3 is a direct transition wide gap semiconductor material whose crystal structure is cubic ferromanganese ore.In the formation process of In2O3, there is no complete ideal chemical ratio structure, and the crystal structure lacks oxygen atoms (oxygen vacancy), so there are excess free electrons, showing a certain electronic conductivity.Meanwhile, if the high-priced cation such as Sn doping is used to replace the position of In^3+ In the lattice of In2O3, the concentration of free conducting electrons will be increased, thereby improving the conductivity of indium oxide.In ITO films, Sn is generally In the form of Sn^2+ or Sn^4+. Since In is positively trivalent In In2O3, the presence of Sn^4+ will provide an electron to the conduction band, while the presence of Sn^2+ will reduce the electron density In the conduction band.In addition, SnO itself is dark brown, and its transmittance to visible light is poor.In the low-temperature deposition process, Sn mainly exists in the form of SnO in ITO, leading to low carrier concentration and high film resistance.After annealing treatment, on the one hand, SnO can be converted to SnO2 to further oxidize the film, and on the other hand, excess oxygen in the film can be desorbed, so as to reduce the film resistance and improve the visible light transmittance of the film.

Characteristics of ITO transparent conductive film:

(1) conductive performance is good, the resistivity can be up to 10 ^ - 4 Ω · cm;

(2) high visible light transmittance, up to 85%;

(3) the absorption of ultraviolet radiation, the absorption rate ≥ 85%;

(4) Reflectivity to infrared rays, reflectivity ≥ 80%;

5] it has attenuation rate on microwave, the attenuation rate is ≥ 85%;

· high film hardness, wear resistance, chemical corrosion resistance;

Good processing performance for lilug coating, which is convenient for etching, etc.

Magnetron sputtering deposition

Magnetron sputtering deposition can be divided into dc magnetron sputtering deposition and rf magnetron sputtering deposition.

Dc magnetron sputtering is a widely used coating method at present. Generally, conductive indium tin alloy targets are used. After vacuum extraction of sputtering chamber, inert gas Ar and reactive gas O2 should be injected.Sputtering basic process: the target material needs sputtering material as the cathode, as the substrate of the anode plus several thousand volts of voltage.After the pre-vacuum of the system, inert gas with appropriate pressure is charged, such as Ar, as the carrier of gas discharge, and a small amount of O2 as the reaction gas. The total pressure is generally within the range of 10^ -1 ~ 10Pa.Under the action of high pressure of the positive and negative electrodes, electrode gas atoms will be a large number of ionization, make Ar atomic ionization ionization process for Ar + ion and independence movement of electrons, including electronic flew to anode, positively charged Ar + ion in the acceleration of high voltage electric field under the action of high speed to fly to target material as a cathode, and with the impact of target material release energy in the process, the impact of one result is that a large number of target material surface atoms gain very high energy, make it out of the original lattice and fly to the substrate, and highly active O plasma reaction and deposition on the ITO thin film is formed on the substrate.

Sputtering film after the general need for heat treatment.There are two types of film forming processes.If the deposited film is anoxic and opaque ITO film, generally heat treatment should be carried out in an oxidizing atmosphere such as oxygen atmosphere or air.On the contrary, if the deposited film contains more oxygen, high transparency and low conductivity, it should be in a vacuum or nitrogen hydrogen mixture reduction atmosphere.Considering that indium tin alloy target should be prevented from "poisoning" as far as possible in industrial production, the film forming rate and substrate temperature should not be too high, so that the deposition film in the state of hypoxia is a better choice.

The process for continuous ITO film plating layer, ITO film with film thickness uniformity, easy to control, repeatability, stability, is suitable for continuous production, and can be widespread, substrate and target position according to the ideal of design at any place, can be preparing dense layer of thin film at low temperature, the process is suitable for large-scale industrial production, is currently the most widely used method of coating.What needs to be improved is that this process requires higher vacuum of equipment.The photoelectric properties of the film are sensitive to the changes of various sputtering parameters, so it is difficult to adjust the process, and the utilization rate of the target material is also low (about 20%).

During the coating process, ITO film is mainly transparent and conductive. The most important process parameters affecting these two indexes include sputtering voltage, deposition rate, substrate temperature, total sputtering pressure, oxygen partial pressure and Sn/ In component ratio of target material (generally 1/9).