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ITO Thin film
Nov 08, 2016

TCO (Transparent Conductive Oxide) film first appeared at the beginning of twentieth Century 1907, Badeker CdO made the first transparent conductive film, and led to the development and application of transparent conductive film, InSn oxide and InSn alloy in 1968 was reported, caused widespread interest in its theoretical and applied research. These are heavy, high oxide doped degenerate semiconductor, semiconductor mechanism as a stoichiometric offset and doping, the band gap is generally greater than 3eV, and with different components and change their optical properties depend on the metal oxidation state and dopant characteristics and quantity.

ITO films have cubic bixbyite structure complex, the lowest resistivity close to 10^- 5. The order of CM, the visible range average light transmittance above 90%, its excellent photoelectric properties make TCO films with practical value.

ITO transparent conductive film has high visible light transmittance and high conductivity, but also have other excellent properties, such as high infrared reflectivity, and glass has strong adhesion, good mechanical strength and chemical stability, acid solution by wet etching process to form electrode, is widely used in flat-panel displays, microwave with the RF shielding device, sensitive devices and solar cells and other fields. Especially in recent years, the rise of flat panel display devices such as liquid crystal, but also promoted the research and demand of ITO thin films.

2. The conduction mechanism and properties of ITO thin films

In2O3 is a direct transition wide band gap semiconductor material, and its crystal structure is cubic iron manganese ore structure. Due to the formation of In2O3 in the process does not constitute a complete ideal chemical composition, crystalline structure of the lack of oxygen atoms (oxygen vacancies), there is a surplus of free electrons, showing a certain electronic conductivity. At the same time, if the high price of cations such as Sn doping in the In2O3 crystal lattice in place of In^3+, it will increase the concentration of free conductive electrons, and thus improve the conductivity of indium oxide. In ITO thin films, Sn is generally in the form of Sn^2+ or Sn^4+, because In is positive in In2O3, the presence of Sn^4+ will provide an electron to the conduction band, in contrast to the presence of Sn^2+ will reduce the density of electrons in the conduction band. In addition, SnO itself is dark brown, the transmittance of visible light is poor. In the process of low temperature deposition, Sn mainly exists in the form of SnO in ITO, which leads to lower carrier concentration and higher film resistance. After annealing treatment, on the one hand can make the transition to SnO2 SnO, the film further oxidation, on the other hand to film the excess oxygen desorption, so as to reduce the membrane resistance, improve film visible light transmittance of the objective.

Properties of ITO transparent conductive films:

A good electrical conductivity, resistivity of 10^- 4. Cm;

The high visible light transmittance, can reach more than 85%;

With the UV absorption, absorption rate greater than 85%;

The reflective of infrared reflectivity greater than 80%;

With the decay rate of the microwave attenuation rate greater than 85%;

The film made of high hardness, wear resistance, chemical corrosion;

The film, good processing performance, easy etching.

3, ITO thin film preparation method and technology

ITO film can be used to prepare a lot of film forming technology, such as magnetron sputtering deposition, vacuum evaporation deposition and sol gel (-Gel Sol), etc..

3.1 magnetron sputtering deposition

Magnetron sputtering deposition can be divided into direct current magnetron sputtering deposition and radio frequency magnetron sputtering deposition.

DC magnetron sputtering is a widely used coating method, the general use of conductive indium tin alloy target, after the sputtering chamber vacuum, in addition to the inert gas to be pumped into Ar, but also through the reaction gas O2. The basic process of sputtering target is needed: sputtering materials as cathode, anode as substrate with thousands of volts voltage. After the system pre pumping, the proper pressure of inert gas, such as Ar, as the carrier of the gas discharge, and a small amount of O2 as the reaction gas, the total pressure is generally in the range of 10^- 1 ~ 10Pa. The positive and negative electrodes with high pressure gas atoms between the electrodes will be a large number of ionization, ionization process of Ar atomic ionization electronic Ar+ ion and the independence movement, in which electrons towards the anode, Ar+ positively charged ions in the accelerating effect of high voltage electric field under high speed to the target as the cathode, and releases energy in the process of impact with the target, one of the impact is the result of a large number of target surface atoms to obtain very high energy, out of the shackles of the original lattice to the substrate, and O plasma reaction with high activity and deposited ITO thin film on a substrate.

After sputtering, the film is generally treated by heat treatment. For different film forming process, there are two ways. If the deposited film is ITO film hypoxia, opaque, general heat treatment shall be conducted in an oxygen atmosphere or air oxidation atmosphere; if the deposited film containing more oxygen, high transparency and low conductivity, it should be a reducing atmosphere in a vacuum or nitrogen hydrogen mixture. Taking into account the industrial production should be as far as possible to prevent the indium tin alloy target "poisoning", improve the film rate and substrate temperature should not be made high requirements, the deposition film in the anoxic state is a better choice.

The process is suitable for continuous plating ITO film, ITO film with uniform thickness, easy control, good repeatability, stable film, continuous production, and is suitable for large area and the location of each plating substrate and the target can be in any place, can design ideal to obtain compact film layer at low temperature, the process is suitable for large-scale industrialization production, is currently the most widely used coating method. Need to improve the process for the equipment of higher vacuum requirements; film photoelectric

3.2 vacuum evaporation deposition

The traditional vacuum evaporation method is widely used in the preparation of aluminum film and optical thin films for packaging production, because it has the advantages of simple equipment, high deposition rate, this method can be used for the preparation of ITO film.

One method is to directly heat the mixture of In2O3 and SnO2, the evaporation temperature of the film is too high, so it must be heated by electron beam bombardment, and it is not suitable to be used in industrial production. Another approach is to use resistance heating evaporation boat evaporation of low melting point of the In and Sn mixture, while the reaction chamber through the oxygen, through the reaction to produce ITO film. This method is simple and the cost is low. But to the excellent performance of the film deposition, the substrate must be heated to a higher temperature, and must be subjected to heat treatment.

In recent years, in order to improve the quality of the film and reduce the substrate temperature, the development of methods for plasma assisted preparation of ITO film evaporation system, namely the creation of the electrode in the vacuum chamber, applying DC voltage into DC glow discharge plasma. Due to the bombardment of the plasma to the substrate and the activation of the film material, the film quality is improved, and the substrate temperature is reduced. But the substrate temperature is still maintained at more than 200 degrees Celsius, and due to the limitation of the DC glow discharge conditions, oxygen partial pressure must be maintained at 100Pa above (in the lower oxygen partial pressure, discharge will be extinguished). We know that one of the most important parameters determining the electrical properties of ITO films is the concentration of oxygen vacancies, the low oxygen partial pressure is likely to form a high concentration of oxygen vacancies in order to obtain a high conductivity.

3.3 sol gel (Sol-Ge) method

Sol gel method is a new method of high performance fiber particles, and thin films prepared by the sol-gel method at the beginning of 80s will be applied to the deposition of ITO film, the isopropanol indium [In (OC3H7) 3] and [Sn (OC3H7 4] tin isopropyl alcohol soluble in alcohol, ultrasonic mixing sol, and then rotating or plating coating method in the glass surface, after aging for 400 ~ 500 degrees of heat treatment to remove the organic ingredients, and then in a reducing atmosphere is cooled to 200 DEG C. The film of 10 ~ 12m^2 large area can be deposited by sol gel method in order to prepare low radiation (LE) glass and hollow glass.

This method is easy to control the film composition, doping can be controlled at the molecular level, suitable doping levels require accurate film, also can make the raw material closely at the molecular level, highly uniform thin film, through the selection of solvent, concentration, catalyst dosage adjustment, can be easily controlled sol, film thickness control, Czochralski method can also double coating.

In short, the sol gel method does not need vacuum equipment, the process is simple, suitable for large area and complex shape of the matrix, no damage to the substrate, the ITO thin film of the large-scale industrialization has a very important role.

Prepared by the sol-gel method with good photoelectric property of ITO films is influenced by many factors, including: Sn doped proportion, metal ion concentration, pulling speed, firing temperature etc.. Only select the appropriate Sn doping ratio (about 12%), the concentration of metal ions is as large as possible (about 0.64M), the appropriate growth rate, the highest possible temperature to prepare ITO films with good.

4, application

ITO thin film is widely used because of its excellent properties of transparency and conductivity. At present, the main application fields of a flat liquid crystal display (LCD) and electroluminescence (ELD), transparent electrode of solar battery; it due to light (selective high visible light transmittance, high reflectivity of infrared light) can be used as Low-E glass, architectural glass window for the thermal barrier effect in cold area the low radiation glass, heat transmission loss can be reduced by about 40% in the high latitudes; because the ITO can be used in conductive glass, need to shield the electromagnetic wave shielding places, such as reserve computer room, even the radar stealth aircraft, can do a transparent screen window or shielding layer for preventing electromagnetic interference; because the refractive index of the ITO film the rate (in the 1.8 to 1.9 range) and conductivity, it is suitable for antireflection coating for silicon solar cells and collect photocurrent, the photothermal conversion and utilization, utilization as too The heat of the sun is selectively through the membrane, and the heat energy is effectively "captured" into the solar collector.

Based on the DC magnetron sputtering, metal organic chemical vapor deposition and newly developed sol-gel technology development mature, ITO film has been applied in many fields, industrialization and sustainable development, maturity. But because of indium is scattered elements, store less, higher prices in nature, researchers are also looking for more cost-effective transparent conductive films, ZnO films doped with aluminum is recognized as one of the most promising materials.

At present, with a large screen, high definition LCD popularization, need the whole world energy shortage and environmental protection, but also reflects the good prospects of the solar cell, the near future theoretical research and practical application of transparent conductive film represented by ITO will enter a new stage.