What Is Thin Film Design?

ITO film is an n-type semiconductor material with high electrical conductivity, high visible light transmittance, high mechanical hardness, and good chemical stability. It is the most commonly used film material for transparent electrodes of liquid crystal displays (LCD), plasma displays (PDP), electroluminescence displays (EL / OLED), touch panels (TouchPanel), solar cells and other electronic instruments.

ITO film

Tin-doped indium oxide (IndiumTinOxide), generally referred to as ITO.

The actual research work on ITO films was at the end of the 19th century, when very thin metal films were obtained on photoconductive materials. Research on transparent conductive materials has entered a new era or should be mainly used in aircraft de-icing window glass during the Second World War. In 1950, the second transparent semiconductor oxide, In2O3, was made for the first time, especially after tin was doped in In2O3, which made this material widely used in transparent conductive films and has broad application prospects.

I. Basic properties of ITO thin film The microstructure of ITO (In2O3: SnO2 = 9: 1). After doping Sn in In2O3, the Sn element can replace the In element in the In2O3 lattice and exist in the form of SnO2. In element is trivalent. When SnO2 is formed, it will contribute an electron to the conduction band, and at the same time, it will generate oxygen holes in a certain anoxic state, forming a carrier concentration of 1020 to 1021 cm-3 and a migration of 10 to 30 cm2 / vs. rate. This mechanism provides a low film resistivity in the order of 10-4 .cm, so the ITO film has the conductivity of a semiconductor. ITO is a wide band film material with a band gap of 3.5-4.3ev. The excitation absorption threshold for the forbidden band in the ultraviolet region is 3.75ev, which is equivalent to a wavelength of 330 nm, so the light transmittance of the ITO film in the ultraviolet region is extremely low. At the same time, the near-infrared region is reflected due to the carrier's plasma vibration phenomenon, so the light transmittance of the ITO film in the near-infrared region is also very low, but the transmittance of the ITO film in the visible region is very good. Physical and chemical properties, ITO film has good conductivity and high light transmittance in the visible light region.

2. Several factors affecting the conductive properties of ITO thin films The area resistance (R), film thickness (d), and resistivity () of ITO thin films are interrelated. The calculation formula between these three is: R = / d. It can be seen from the formula that in order to obtain ITO films with different surface resistances (R), it is actually necessary to obtain different film thicknesses and resistivities.

Generally speaking, it is relatively easy to obtain different film thicknesses when preparing ITO thin films. The required film thickness can be obtained by adjusting the deposition rate and deposition time during film deposition, and the corresponding process methods and means can be used. For precise film thickness and uniformity control.

The resistivity () of the ITO film is the key to the manufacturing process of the ITO film, and the resistivity () is also an important index to measure the performance of the ITO film. The formula = m / ne2T gives several main factors affecting the resistivity () of the thin film, and n and T represent the carrier concentration and carrier mobility, respectively. When n and T are larger, the resistivity () of the film is smaller, and vice versa. The carrier concentration (n) is related to the composition of the ITO film material, that is, the tin content and oxygen content of the ITO film itself are related. In order to obtain a higher carrier concentration (n), the tin content of the ITO deposition material can be adjusted by And oxygen content to achieve; and the carrier mobility (T) is related to the crystalline state, crystal structure and defect density of the ITO film, in order to obtain a higher carrier mobility (T), you can reasonably adjust Factors such as deposition temperature, sputtering voltage, and film formation conditions during film deposition.

So from the perspective of the ITO film preparation process, the resistivity of the ITO film is not only related to the composition of the ITO film material (including tin content and oxygen content), but also to the process conditions (including substrate temperature, Sputtering voltage, etc.). A large number of scientific literatures and experiments have analyzed the relationship between the resistivity of ITO thin films and the contents of Sn and O2 elements in ITO materials, as well as the process conditions such as substrate temperature during the preparation of ITO thin films.

Process and method for preparing ITO thin film by low sputtering voltage

1. Preparation of ITO film by low-voltage sputtering. Because ITO film itself contains oxygen element, during the process of preparing ITO film by magnetron sputtering, a large amount of oxygen anions will be generated. The oxygen anions will be bombarded by a certain particle energy under the action of an electric field. The surface of the deposited ITO film causes structural defects in the crystal structure and crystal state of the ITO film. The larger the sputtering voltage, the greater the energy of oxygen anion bombarding the surface of the film layer, so the greater the probability of causing such structural defects, and the more serious the crystal structure defects, resulting in an increase in the resistivity of the ITO film. In the case, the sputtering voltage when the ITO film is deposited by magnetron sputtering is about -400V. If the sputtering voltage is lowered below -200V using a certain process method, the resistivity of the deposited ITO film will be reduced by more than 50%. This not only improves the product quality of the ITO film, but also reduces the production cost of the product.

2. Two effective ways to reduce the sputtering voltage of the thin film when preparing ITO thin films by DC magnetron sputtering. The effect of magnetic field strength on the sputtering voltage. When the magnetic field strength is 300G, the sputtering voltage is about -350v; When it rises to 1000G, the sputtering voltage drops to about -250v. In general, the higher the magnetic field strength and the lower the sputtering voltage, but when the magnetic field strength is above 1000G, the influence of the magnetic field strength on the sputtering voltage is not obvious. Therefore, in order to reduce the sputtering voltage of the ITO film, it can be achieved by reasonably increasing the magnetic field strength of the sputtering cathode. Effect of RF + DC power supply on sputtering voltage In order to effectively reduce the voltage of magnetron sputtering to achieve the purpose of reducing the resistivity of ITO film, a special set of sputtering cathode structure and sputtering DC power supply can be used. A set of 3KW RF power supply was reasonably matched and stacked on a set of 6KW DC power supply, and the process of reducing the sputtering voltage of ITO film was studied under different DC sputtering power and RF power. When the magnetic field strength is 1000G and the power of the DC power supply is 1200W, by changing the power of the RF power supply, a large number of process experiments have been obtained: "When the RF power is 600W, the sputtering voltage of the ITO target can be reduced to -110V" in conclusion. Therefore, the application of the new RF + DC power supply and the design of the special sputtering cathode structure can also effectively reduce the sputtering voltage of the ITO film, thereby achieving the purpose of reducing the film resistivity.

3. A new deposition method to reduce the resistivity of ITO film-HDAP method HDAP method uses high-density arc plasma (HDAP) discharge to bombard the ITO target material, evaporate the ITO material, and deposit it on the base material to form an ITO film. Due to the action of high-energy arc ions, In and Sn in the ITO particles are completely ionized, thereby enhancing the reactivity during deposition, reducing the crystal structure defects and reducing the resistivity.

Using the same composition of ITO materials, other process conditions remain the same, and at the same substrate temperature, respectively, "DC magnetron sputtering", "DC + RF magnetron sputtering", "HDAP method for preparing ITO thin films" experiment.

The experimental results show that the ITO film with lower resistivity can be obtained by the HDAP method, especially when the ITO film is prepared on a material whose substrate temperature cannot be too high, the ITO film prepared by the HDAP method can obtain a more ideal ITO film. When the substrate temperature reaches about 350 ° C, the three deposition methods have less effect on the resistivity of the ITO film.

The ITO thin films prepared by magnetron sputtering and HDAP were analyzed by scanning electron microscope. It is obvious that the surface of the ITO film prepared by the HDAP method is flat and uniform. The HDAP method for preparing ITO thin films is mainly applicable to those substrates that cannot be heated, and at the same time, the resistivity of ITO thin films is relatively suitable.

With the rapid development of the display device industry, new requirements are imposed on the performance characteristics of ITO films. At the same time, the in-depth development of ITO thin film manufacturing technology has made the needs of display devices possible. ITO films with different properties can be applied in different display devices.

In China, the manufacturing and development of ITO thin film equipment began in the 1980s, mainly some single-type vacuum coating equipment. Due to the limitations of the ITO process and manufacturing methods, the product quality was poor and the output was small. The products are mainly used as ordinary transparent electrodes and solar cells.

In the early 1990s, with the rapid development of LCD devices, the demand for ITO thin film products has also increased dramatically. Some domestic manufacturers have begun to introduce a series of complete ITO coating production lines from abroad. However, due to the high price of imported equipment, The inconvenience of technical services and other factors make many manufacturers discourage.

At the end of the 1980s, China produced the first ITO continuous coating production line for TN-LCD. The process line used in this production line is to deposit indium tin alloy material on the surface of the substrate using the principle of DC magnetron sputtering, and perform high temperature oxidation treatment to convert the indium tin alloy film into the required ITO film. This type of production line is characterized by low equipment capacity, poor quality, and complex process regulation.

In the mid-1990s, with the development of the domestic LCD industry, while the demand for ITO products increased, new requirements were placed on the quality of the products, so a second-generation ITO coating production line appeared. This production line not only has a significant increase in output over the first generation production line, but also because the ITO film is directly deposited using ITO ceramic targets, and is compatible with the process of RF magnetron sputtering deposition of SiO2 film, making the production line Compared with the first generation production line, the process controllability and other aspects have made a qualitative leap.

In 1999, a series of problems such as the slow deposition rate of SiO2 films deposited by RF magnetron sputtering, which affected the production line's production capacity and equipment utilization, were effectively solved, and a third-generation large-scale high-end ITO film production line appeared. This production line has successfully applied the intermediate frequency reactive sputtering SiO2 thin film process, an all-molecular pump oil-free vacuum system, and an independent fully automatic carriage return mechanism. This production line has the ability to produce ITO film materials for high-end STN-LCD.

With the development of reflective LCD, anti-reflection LCD, LCOS image projector rear projection television and other display devices, higher requirements have been placed on ITO thin film products. SiO2 / ITO two-layer film structure of ITO thin film materials cannot meet the requirements It is necessary, and the optical performance requirements of the product such as high reflectivity or high transmittance have been achieved by using a multilayer composite film system. With years of experience in design and development, domestic manufacturers have launched the fourth generation of large-scale multilayer film production lines. The production line consists of 15 vacuum chambers. It uses an all-molecular pump oil-free vacuum system, uses three RF / MF / DC magnetron sputtering processes, and uses PEM / PCV for process gas control. This production line has the ability to deposit five layers of films continuously.

With the quiet rise of touch input electronic products such as PDAs and e-books, equipment for making corresponding materials has also emerged. Due to the special working principle of touch products, the required ITO film must be made of flexible material (PET). The deposition temperature of the film must not be too high (less than 120 ° C). At the same time, the ITO film layer is thin, The sheet resistance is high and uniform, so strict requirements are imposed on the ITO film deposition process.

With the development of organic electroluminescence displays (OLED) and other display devices, there will be newer and higher requirements for the manufacturing process and equipment of ITO films, and it will also vigorously promote the development of ITO film manufacturing equipment.