Vacuum Evaporation for OLED:
The source material is evaporated into atoms or molecules in a vacuum by heating with electric current, electron beam bombardment or laser etc. Then they move linearly with long free path, collide with the surface of substrate and condense on it to form a thin film. So the free path of residual molecules in evaporation chamber should be longer than the distance from evaporation source to substrate in order to ensure purity and fastness of the coating.
The devices for measuring vacuum are vacuum gauges, and commonly used ones are thermocouple vacuum gauge and ionization vacuum gauge. Thermocouple vacuum gauge measures pressure between 0.1~10 Pa based on the principle that the thermal conductivity of gas is proportional to the pressure at low pressure; ionization vacuum gauge is made based on the principle that the ionization current generated from collision between electrons and gas molecules varies with the pressure, and its measuring range is 10-1~10-6 Pa. Please note that ionization vacuum gauge must be used at pressure lower than 0.1 Pa, otherwise the device may be damaged.
The film is formed by deposition with vacuum evaporation. It includes three basic processes as below:
1. Heating/evaporation process
2. Transport of gasified atoms or molecules between evaporation source and substrate
3. Deposition of evaporated atoms or molecules on surface of substrate
Procedure for Preparing Thin Film:
● Loading of ITO substrate
● Pre-processing of substrate
● Coating of hole injection layer
● Alignment of metal mask
● Coating of light-emitting material
● Coating of electron transport layer
● Coating of electron injection layer
● Coating of electrode layer
In comparison with LCD, OLED has many excellent features such as self emission of light, no need of backlight, high contrast, thin thickness, light weight, wide viewing angle, quick response, usability on flexible panel, wide range of working temperature, simple construction & process, and it is considered as one of major trends in next-generation flat-panel display devices, thus attracting increasing attention. Currently, many manufacturers have engaged in R&D and manufacturing of OLED devices and equipment. Although OLED technology is regarded as the ideal display technology, to achieve true commercialization, it still faces a range of technical problems to be solved, such as even coating on large substrate. Unevenness of organic film will lead to uneven brightness and color of light emission, affecting the display performance; high cost of devices due to inefficient utilization of organic materials is also a major obstacle to mass production of OLED.
Characteristics of Vacuum Evaporation for OLED
Currently, the commonly used OLED coating process is evaporation, and in the vast majority of cases, it is point-source evaporation. The vacuum in conventional thermal evaporation is generally higher than 10-4 Pa; higher vacuum leads to less defects in thin film formation and higher purity of materials in film. When organic materials are heated in a vacuum, depending on the properties of materials, some materials are liquefied and then gasified, while some are directly sublimed; then they fly away from the surface of materials at certain initial velocity, move to the ITO surface, cool down and are deposited on it to form a thin film.
If the vacuum is lower than 10-4 Pa, the vacuum chamber will be filled with water molecules, oxygen molecules and other foreign gases which may collide with organic small-molecule materials in evaporation process; it will greatly reduce the quality of film formation, and even result in lower performance or failure of the devices.
In early stage of OLED research, two-stage vacuuming system which combines mechanical pump and molecular pump was commonly used to ensure high vacuum. In recent years, by using molecular pump followed by sputter-ion pump, it is able to produce ultrahigh vacuum so as to manufacture high-performance OLED.
There are two types of device for measuring vacuum in a chamber: the thermal conductivity gauge that measures a vacuum lower than 0.1 Pa, i.e. thermocouple gauge and thermistor gauge; the ionization gauge that measures a vacuum higher than 0.1 Pa. The thickness of functional layers is measured by using a crystal resonator; generally, the evaporation rate of organic materials is 0.5~2 Å/s, the evaporation rate of metals is 2~5 Å/s, and the thickness is 80~100 nm.
Selection of Sensor Quartz Crystals Monitor in Vacuum Evaporation for OLED
OLED vacuum film measuring system: quartz crystal microbalance (QCM); it works on the principle that the mass of a quartz crystal increases as the materials are evaporated in evaporation process thus changing the natural resonance frequency of quartz crystal; by installing a quartz crystal resonator in a resonant circuit, the change in mass of thin film can be read out as the change in frequency. OLED materials have granular amorphous structure so that they cannot be firmly bound with the electrode of crystal. This structure attenuates the energy of motion, resulting in short life of the crystal. It is recommended to use an alloy crystal with low thermal impact. Some OLED materials are deposited at very low rate. It is important to maximize the stability of rate and minimize the noise. An alloy crystal will offer best stability of rate and minimum noise.