Previously, we have introduced Four main molding methods for ITO targets, mainly about their definitions and characteristics. And today I think it necessary to talk about the advantages and disadvantages of each method, which is helpful for you to make choices between them when you want to prepare ITO targets.
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Hot Pressing
The hot pressing method is a simultaneous process of press molding and heat sintering, and the advantages of this process are:
(1) During the process of hot pressing, since the powder is in a thermoplastic state, the deformation resistance is small, and plastic flow and densification are easy, so the required molding pressure is small. (2) Simultaneous heating and pressurization help the contact, diffusion and flow between the powder particles, reduce the sintering temperature and shorten the sintering time, and suppress the growth of crystal grains. (3) The hot pressing method can easily obtain a sintered body which is close to the theoretical density and whose porosity is close to zero, and it is easy to obtain a fine grain structure.
The main disadvantages of this method are: (1) The obtained target size is small due to the limitation of the hot pressing equipment and the size of the mold. The current maximum size of the target is 400 × 300mm. (2) This method has high requirements on mold materials (generally high-purity high-strength graphite). Moreover, the hot pressing equipment needs to be imported, and is not suitable for industrial continuous production. The production efficiency is low and the cost is high. (3) The uniformity of the target grain is poor.
Hot isostatic pressing
Hot isostatic pressing is the most common method for preparing ITO sputtering targets. It has the following advantages: (1) It can overcome the disadvantages of hot pressing in a graphite mold. (2) In the heated and pressurized state, the product is simultaneously pressed in all directions, so that the obtained product has a very high density (almost a theoretical density). (3) The hot isostatic pressing strengthens the pressing and sintering process and lowers the sintering temperature, thereby avoiding grain growth, so that the product can obtain excellent physical and mechanical properties.
The disadvantages are: (1) The size of the target is limited by the pressure of the equipment and the size of the cylinder, and it is difficult to prepare a large-sized target. (2) The equipment is expensive and the investment cost is high. (3) Low production efficiency and high production cost make the product uncompetitive.
Room temperature sintering
The main advantages of the room temperature sintering method are: (1) The size of the target is not limited by the equipment, so it is possible to produce a large-sized target. (2) Low equipment investment, low production cost, high production efficiency, excellent target performance, and easy industrial production. (3) This method is suitable for the performance requirements of coated targets for high-end displays.
Its weaknesses are: (1) This method is the most difficult sintering method compared with other methods. To obtain a dense sintered body, it is often necessary to add a sintering aid. However, the sintering aid is difficult to remove from the sintered body, thus lowering the purity of the product. (2) The shape, particle size and particle size distribution of the powder are strictly required. In order to meet the requirements, the powder is generally subjected to ball milling, jet milling and sedimentation classification. (3) The target produced is generally thin.
Cold isostatic pressing
The advantages of cold isostatic pressing for the preparation of ITO targets are: (1) Compared with mechanical pressing, since the pressure of cold isostatic pressing is large and the workpiece is uniformly stressed, it is particularly suitable for pressing large-sized powder products. (2) The pressed powder product has the advantages of high density and uniformity. (3) The pressed powder does not require the addition of a lubricant or the like. (4) Its production cost is low and it is suitable for mass production.
However, the use of hyperbaric oxygen makes the process dangerous.
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The full name of ITO is indium tin oxide, a composition of indium, tin, and oxygen with different proportions. The substance of ITO and ITO sputtering target is the same, the latter of which is actually a black-gray ceramic semiconductor formed by mixing indium oxide and tin oxide powder in a certain ratio.
At present, there are mainly four molding methods for preparing the ITO target, and each of them has its advantage in a specific application.
Vacuum hot pressing densifies the ITO powder by utilizing thermal energy and mechanical energy, and it can produce a high-density ITO ceramic target with a density of 91% to 96%. This method can easily obtain an ITO target that is close to the expected density and whose porosity is close to zero. However, limited by the equipment and the mold size, vacuum hot pressing is less advantageous in preparing a large-sized sputtering target.
Hot isostatic pressing (HIP) prepares ITO sputtering target by sintering under pressure or pressing at high temperatures. Similarly with vacuum hot pressing, HIP can obtain a product with high density (almost a theoretical density) and excellent physical and mechanical properties in the heated and pressurized state. But it is also limited by the pressure of the equipment and the size of the cylinder.
Room temperature sintering prepares a high-density target performed by slurry casting or pre-pressing first and then sinters it under a certain atmosphere and temperature to obtain the ITO target. Its biggest advantage over others is the ability to produce large-size sputtering targets. But the purity of the target made by this method is lower, compared to other sintering methods.
Cold isostatic pressing (CIP) uses rubber or plastic as a mold covering material at a normal temperature and uses a liquid as a pressure medium to transfer ultra-high pressure. CIP can also prepare ITO sputtering targets of a larger size. And it is cheap, which is suitable for mass production. But CIP requires the materials to be sintered at a high temperature of ~ °C in a 0.1~0.9 MPa pure oxygen environment, which has a higher risk.
These three terms are co-related. In short, ITO conductive glass is made by coating a layer of ITO film on a piece of ultra-thin glass by sputtering or evaporation. Here, the ITO target is the source that provides indium tin oxide atoms; ITO film is obtained when these indium tin oxide atoms are deposited on the substrate (the glass), and the glass coated with ITO film is called ITO conductive glass.
ITO sputtering target, along with its derivatives such as ITO film and ITO glass, has many applications in various industries. ITO target is often used to make transparent conductive coatings for displays such as liquid crystal displays (LCD), flat panel displays, plasma displays, and touch panels. ITO thin film is used in organic light-emitting diodes, solar cells, and antistatic coatings. Apart from the electronics industry, ITO target is also used for various optical coatings, most notably infrared-reflecting coatings and sodium vapor lamp glasses for automotive.
The future of the ITO target is prosperous, but there are still difficulties that we need to overcome. The developing trend of ITO target can be concluded into four points:
1) large size,
2) low resistivity,
3) high density and
4) high utilization.
Point 4 has been one of the hotspots and difficulties in this field. At present, the planar target still dominates the sputter coating industry due to its multiple advantages such as simple structure, strong versatility, and good uniformity. However, the utilization rate of the planar target is low. Recently, sputtering target manufacturers are actively researching and developing a new type of sputtering target (such as rotatory target) to achieve high utilization.
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