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.
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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:
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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.
Magnetron sputtering depends on the presence of a magnetic field above the target’s surface. Ferromagnetic targets (Fe, Ni, Co, Permalloy, etc) of normal thickness have high magnetic permeabilities which trap the flux from the source’s normal magnet set. However, reducing the target’s thickness causes it to saturate allowing the magnetic field to penetrate. A nickel target’s permeability is such that a target thickness of 0.100 (or less) saturates. An iron target, with its much higher permeability, must be approximately 0.002 to saturate in the normal magnet set’s field. Sputtering very thin targets brings its own problems: first the power must be very low to prevent target melting and burn-through; second, thin targets tend to bend away from the cooling well’s surface exacerbating the chances of melting and burn-through; and third there may be no obvious signs when burn-through occurs and the cooling well’s surface starts to sputter. For these reasons, SAM strongly recommends the customer use a high-strength magnet set when sputtering ferromagnetic materials.
There are many factors that influence deposition rate. The two common ways to increase rate within an existing sputtering system are to either increase the applied power to the target or reduce the throw distance. There is a linear relationship between sputtering rates and the applied target power. Doubling the applied power will double the sputtering rate. However, increasing the power to the target must be carefully planned since it is easy to damage a fragile target or melt a metallic target if the power is too high. Decreasing the source-to-substrate distance (r) is the most powerful way to influence the deposition rates because distance and rate have a power-law relationship. For a point source, the sputtering rate is a function of 1/r^2. However, decreasing the distance (r) may decrease in film thickness uniformity on the substrate. If neither ‘power’ nor ‘distance’ modification is possible in your vacuum system, contact SAM for further options. Slow deposition rates are expected when RF sputtering oxide or nitride targets. In some instances, rates can be dramatically increased by sputtering the metal target using pulsed DC power and adding O2 or N2 to the argon to produce the intended film composition. Contact SAM with any questions regarding this ‘reactive’ sputtering method.
Roughly three-quarters of the power applied to a sputter gun ends up heating the coolant water. Understanding the heat transfer processes and heat dissipation is critical to successful sputtering. For a given sputter gun, four factors determine the maximum power density the target will accept: (1) the type of target material; (2) its thermal conductivity; (2) heat transfer from target to gun’s cooling water; and (4) the water’s flow rate. The type of material determines its brittleness, thermal expansion coefficient, fragility, etc which determines the target’s response to thermal stress. Applying a high power density to a material with a poor thermal conductivity (compared to, say Cu or Au) creates a large temperature difference between the target’s top and bottom surfaces causing thermal stress. For fragile materials (such as some ceramics and semiconductors) stress may crack the target. For stronger, ductile materials (many metals) the target may melt. The efficiency of target cooling depends on the ‘thermal resistance’ between the target’s back surface (the surface not being sputtered) to the gun’s cooling water. Directly-cooled targets, where the water flow is in contact with the target’s back surface, have low thermal resistance. By comparison, indirectly-cooled targets (which are clamped to a copper plate in the sun and cooled by water flowing across the plate’s back surface, not the target’s) have high thermal resistance. At the atomic level, the interface between target’s back surface and copper plate’s front surface has a small contact area for thermal conduction. Over the remaining area, heat transfer is by radiation. (Interestingly, the thermal resistance of such an interface under vacuum may be 4-10 higher than its thermal resistance in air.) Water flow through the sputtering gun should be sufficient to remove the total power. As a guideline 1 gal/min (~4 L/min) dissipating 4 kW power causes a rise in the water temperature of ~15C. For coolant liquids other than water, remember to factor in the liquid’s specific heat. Suggested guidelines for calculating maximum power are: for a high thermal conductivity, directly-cooled, metal target such as Al, the maximum power density is ~250 W/in^2 for the target’s (front-side) surface area. For other types of (directly-cooled) targets, de-rate the maximum power density based on the target materials thermal conductivity compared to aluminum. As an example, for a target with a thermal conductivity 1/10th that of Al, the maximum power density should be 25 W/in^2. For a high thermal conductivity, indirectly-cooled, metal target such as Al, the power density limit is ~100 W/in^2 for the target’s (front-side) surface area. Again, de-rate this power density for materials with lower thermal conductivities. Two important final points: (1) To prevent target cracking, fragile targets may need to have maximum power density de-rated even further than indicated by the relative thermal conductivities; and (2) some target materials crack no matter what power density is used.
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