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Sintering Technology and Application of ITO Target

Sintering Technology and Application of ITO Target

ITO target is a mixture of indium trioxide and tin dioxide, which is an important raw material for ITO thin film preparation.   

ITO targets are mainly used in the production of ITO transparent conductive glass, which is the main material for the production of planar liquid crystal display, and has a wide range of important applications in the electronic industry and information industry. The theoretical density of ITO target is 7.15g/cm3, and the relative density of high quality finished ITO target should be more than 99.5%. The target has lower resistivity, higher thermal conductivity and higher mechanical strength. High-density targets can sputter on glass substrates at lower temperatures to obtain conductive films with lower resistivity and higher transmittance, and even ITO conductive films on organic materials.

Preparation of ITO Targets

High performance ITO targets must have the following properties:

1. High density, the relative density of commercial products should be at least 98%, and the density of high-end products should be about 99.5%.

2. High heat shock resistance;

3. Organizational homogeneity without segregation;

4. Fine and uniform grain size;

5. The purity reached 99.99%.

At present, the production technology and equipment of ITO target materials are mature and stable. The main preparation methods are atmospheric sintering, hot isostatic pressing, vacuum hot pressing and cold isostatic pressing.

1 Atmospheric pressure sintering

Atmospheric pressure sintering process is rising in the early 1990 s a target material preparation methods, it is adopts the preloading method or slip casting method of preparation of high density of billet, target material in a certain     atmosphere and temperature to sintering of grey target element, through the control of sintering temperature and sintering atmosphere, make the target material, to effectively control the growth of the grain, achieve high densification of target material and the uniformity of the grain size distribution. The main process of atmospheric sintering method is as follows: mix In2O3 powder with SnO2 powder with certain vibration-solid density, prepare slurry for slurry casting or mold pressing or cold isostatic pressing (molding pressure is generally 100~300MPa) with a small amount of molding agent or both. By slip casting or add a blank forming agent in 300 ~ 500 ℃ temperature range for a long time of dehydration and degreasing process, finally, in at least 1 ATM pure oxygen or air pressure sintering atmosphere, sintering temperature is 1450 ~ 1550 ℃, the target material obtained for a single ITO solid solution phase, its relative density is above 95%, the grain size is 1 ~ 20 microns.

The main advantages of atmospheric sintering method are: target size is not limited by equipment, can produce large size target, and equipment investment is small, production cost is low, production efficiency is high, target performance is excellent, easy to achieve industrial production. It is suitable for coating target material used in high-end display. Disadvantages: compared with other methods, this method is the most difficult method for sintering. To obtain compact sintered body, it is often necessary to improve the sintering activity of raw material powder or add sintering assistant. It is difficult to remove the active agent and sintering assistant from the sintered body. The target material prepared is generally thin, so the deformation control of the target material is very strict, and it is generally required to have a good net forming ability.

The difficulty of ITO target in atmospheric sintering process is reflected in the increase of density, because In2O3 and SnO2 in ITO solid solution will decompose and sublimate in gaseous state at high temperature (especially above 1200℃). Thus it can be seen in a low oxygen pressure especially owe oxygen atmosphere, due to the decomposition of In2O3 and SnO2 and In2O3 and SnO2 volatilization in the form of gas, gaseous material escaping from the ITO solid solution formed a lot of air flow channel and pore, hinder the improvement of the density of ITO target materials, thus in the sintering process of ITO target materials zhongtong into certain pressure pure oxygen or air atmosphere to improve the partial pressure of oxygen, inhibit the decomposition and evaporation process is very important to get a high density of target.

2 Vacuum hot pressing sintering

Vacuum thermocompression is a process of densification of ceramic materials by means of thermal energy and mechanical energy. ITO ceramic targets with high density up to 91%~96% can be prepared. The process flow of hot pressing method is that after the mold is heated and processed, the test material is injected, the model is fixed on the heating plate under pressure, and the melting temperature and time of the test material are controlled to harden and cool down after melting, and then the finished product of the model is taken out.

The advantages of this process are as follows :(1) when pressing, the powder is in thermoplastic state, the deformation resistance is small, easy to plastic flow and densification, and the molding pressure is small. (2) due to the simultaneous heating and pressurization, it is helpful for the mass transfer process such as the contact, diffusion and flow of powder particles, reducing sintering temperature and shortening sintering time, and inhibiting the growth of grain. (3) it is easy to obtain sintered bodies with close to theoretical density and close to zero porosity by hot pressing method, and it is easy to obtain fine grain structure.

The disadvantages of this method are as follows: the target size is small due to the limitation of the hot pressing equipment pressure and mold size; High requirements for mold materials (generally high purity and high strength graphite), short die life, large loss, and at high temperature and ITO target easy reduction reaction; Not suitable for industrial continuous production, low production efficiency and high product cost; The uniformity of target grain is poor.

3 Hot isostatic sintering

Hot isostatic pressure (HIP) can be considered as sintering under pressure or high temperature. Compared with traditional pressureless sintering, hot isostatic pressing method at relatively low temperatures (usually around 0.5 ~ 0.7 times of material melting point) to obtain fully densification, organizational structure and can be very good control, inhibit the growth of grain, obtain homogeneous and isotropic organization, can be processed into a certain "net forming" complex shape product. The process flow of ITO target prepared by hot isostatic pressure method is mainly as follows: Will single-phase ITO solid solution powder in certain reducing atmosphere (such as mixture of H2 and N2 and H2) and temperature (300 ~ 500 ℃) under partial reduction, reduction degree of control between 0.02 ~ 0.2, reoccupy mould or cold isostatic pressing and forming method of a combination (100 ~ 300 mpa) will restore after the powder pressed into initial billet, placed at the beginning of billet in the package of stainless steel and supplemented by isolation between the two materials in the set, then the coating and vacuum sealing, finally put in hot isostatic pressing furnace in 800 ~ 1050 ℃, ITO target materials were prepared by isostatic pressure of 50~200MPa for 2~6h.

Hot isostatic pressure method is the most commonly used method to prepare ITO sputtering targets abroad. The advantages of hot isostatic pressure method are :(1) it can overcome the disadvantages of hot pressure in graphite mold and is not easy to restore. (2) under the condition of heating and pressurizing, the products are pressurized in all directions at the same time, so the product density is very high (almost up to the theoretical density) and can be made into large-sized products. (3) hot isostatic pressure strengthens the pressing and sintering process, reduces the sintering temperature, avoids grain growth, and obtains excellent physical and mechanical properties. Its disadvantages are: limited by the equipment pressure and cylinder, can not produce large size target; Expensive equipment and high investment cost; Low production efficiency, high production cost, product competitiveness is not strong.

Application and development trend of ITO target

ITO transparent conductive film has been widely used in the field of optoelectronics by virtue of its excellent optical and electrical properties, especially in the high-speed development of FPD industry. Besides LCD, there are HDTV, PDP, Touch Panel, EL and so on. In addition, ITO film glass, as a surface heating element, is used in automobile, train, aircraft windshield, airship dazzle window, tank laser rangefinder, airborne optical reconnaissance instrument, periscope observation window, etc., not only plays a role in heat insulation and cooling, but also can remove frost after power, so it has been widely used. If used as building curtain wall, can heat insulation in summer, winter can be cold. The attenuation of ITO film to microwave can be used for transparent window of electromagnetic shielding. ITO films can also be used in solar cells, protection and other fields.

In recent years, with the development of large-sized flat panel display, the requirements for ITO target size and density are getting higher and higher. Hot pressing equipment and technology are far from meeting their requirements. Therefore, the generation of large-sized and high-density ITO target has become the focus of research and development of major target manufacturers in China. With the development of LCD, LED, PDP, OLED and other display technologies, it is required to improve the quality of ITO target, reduce the cost, and develop towards the direction of large size, low resistivity, high density, target body integration, and improve the utilization rate.

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