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OLED key process: Introduction to evaporation technology

Return list Reffer:UIV Click number:- Release time:2018-11-30

AMOLED market outlook

Although the growth momentum is not up to 2017's expectations, but with the efforts of major panel manufacturers and terminal manufacturers, AMOLED screens are gradually infiltrating on mobile phones and TVs. At the same time, they are slowly developing such as car displays etc. It can be seen that with the further maturity of AMOLED technology, the market share of AMOLED will be further expanded.

According to IHS Markit's forecast, devices carrying AMOLED displays will reach 750 million units in 2022. Among them, the AMOLED screen shipment of mobile phones accounts for more than 92% of the total AMOLED screen device shipments.
The rise of the AMOLED market is also reflected in the investment market for display devices. Thanks to the significant investment in AMOLED and LCD by Chinese and Korean panel makers in 2016-2017, the revenue of equipment manufacturers reached a record high in 2017. However, after 2018, LCD equipment will gradually slow down due to the saturation of the overall market capacity and the slowdown in capacity expansion. At the same time, the investment in AMOLED has gradually slowed down, which has led to a gradual decline in the overall equipment and equipment supply revenue market from 2017. During the downturn, the revenue composition of equipment manufacturers is different from that of 2017. In 2017, most of the equipment manufacturer's revenue came from new production line projects.

In 2018, the source of revenue for equipment manufacturers will be divided into three main categories:

• A portion of the revenue comes from existing construction in progress orders: Many of the plants planned for 2016 and 2017 are still under construction and capacity expansion. Equipment manufacturers can rely on PO orders previously received in these projects to maintain revenue.

• A portion of the revenue comes from new projects. Although in 2018, many panel manufacturers have experienced different degrees of slowdown in their capacity expansion plans. However, some Chinese manufacturers continue to expand their production capacity. This new capacity will give equipment manufacturers an additional source of revenue.

• The last part comes from the retrofit of the AMOLED line. Although the expansion of small and medium-sized AMOLED capacity has gradually slowed down, with the transformation and reconstruction of some LCD Gen 8 generation lines, there are more equipment requirements in the market. However, since the WOLED + Oxide TFT technology is used in the fabrication of a large-sized AMOLED panel, the device composition of the Oxide TFT and the a-Si TFT is similar. Therefore, in the production line transformation, more equipment needs come from the evaporation section, not the front section.
Although at this stage, the Korean panel factory takes advantage of its better cost control and production technology to occupy a near monopoly position in the market of small and medium size and large size OLED screens. However, as Chinese panel makers gradually increase their investment in AMOLED technology and the government subsidies increase, the technology and cost gap between Chinese panel makers and Korean manufacturers will gradually decrease, thus making the future in AMOLED panels. The industry presents a phenomenon of contending.

The existing mainstream AMOLED production technology is mainly based on evaporation technology, and the fabrication of the device is completed by means of an evaporation machine, an evaporation source and a mask (FMM or OPM). A brief discussion of existing evaporation techniques will be given below.

Introduction to evaporation technology

Whether it is for RGB color separation AMOLED display for small and medium size applications or WOLED display for large size applications, the manufacturing process is vacuum evaporation technology. Therefore, in these devices, functional layers such as HIL, HTL, EML (R & G & B colors), ETL, EIL, Cathode, and Charge Generation Layer are also continuously deposited on the TFT substrate by vacuum evaporation.

Due to the need for process doping and in order to avoid cross-contamination, different functional layers need to be vapor-deposited in different vapor deposition chambers, and the substrate is transferred between different cavities by the robot after the evaporation is completed.

In the production of display panels, the material film formation can be roughly divided into PVD physical Vapor Deposition and Chemical Vapor Deposition. The evaporation technique is one of the physical meteorological deposition methods.

Unlike CVD technology, vacuum evaporation as one of the PVD technologies has the following characteristics in the process of film formation:
• Solid or molten materials are required as an evaporation source for the deposition process.
• When the film is deposited, the evaporation source material enters the gas phase through a physical evaporation process.
• When evaporating into a film, the moving path of the gas phase molecules is approximately a straight line.
• When filming on the bottom and on the substrate, the vapor phase of the evaporation source does not chemically react when it is deposited.

Compared to the sputter film formation method which belongs to the PVD method, the evaporation technique requires a higher degree of vacuum to increase the mean free path of its gaseous molecules. However, the vapor deposition technique has a higher film deposition rate and a better film purity.

The principle of evaporation can be substantially simplified as a process in which the material is sublimated by heat and then re-deposited on a cooler substrate. When the material is heated and separated, the number J of molecules evaporated per unit time (m2s) per unit of distillation area and the vapor pressure, the molecular mass M of the material and the temperature T exhibit a certain proportional relationship. It can be seen that as the vapor pressure increases, the deposition rate also increases. At the same time, the temperature T is also an important factor in determining the evaporation rate.
There are evaporation source, screen machine, glass substrate and cooling plate in the vapor deposition chamber. In the organic thin film vapor deposition, it is necessary to invert the substrate with the TFT first, and then align the Mask and the substrate with a netting machine, and then open the vapor deposition source to vapor-deposit the substrate.

In the entire evaporation system, the vaporizer, mask, and evaporation source are supplied by different equipment suppliers. The largest steaming machine used in small and medium-sized AMOLED panel production lines is Japan's Cannon Tokki, while the main suppliers of FMM are DNP and Darwin. In the market of vapor deposition machine, although Cannon Tokki is also the preferred supplier of evaporation machine for panel manufacturers, other vapor deposition equipment factories such as Ulvac are also actively trying to enter this market. At the same time, FMM technology is one of the core technologies of AMOLED evaporation technology. In recent years, some Chinese manufacturers have also increased their investment and layout in FMM technology. For example, Shanghai Hehui Optoelectronics has established its own FMM research production line in the Shanghai Gen 4.5 plant.

Introduction to evaporation source

The evaporation source is the core component of another evaporation technology. According to the difference in the phase state of the material when the evaporated material evaporates, the evaporation material can be roughly divided into:
• Melting-type material: Generally, the molten material is made of metal. This type of material melts first when evaporated, that is, when the temperature reaches the melting point, the equilibrium vapor pressure is also lower than 10-1 Pa.
• Sublimation-type material: For sublimation materials, the evaporation of the material does not have to undergo a melting process and directly sublimate. When the material is heated below the melting point, the equilibrium vapor pressure is already relatively high. Common concentrated materials are Cr, Ti, Mo, Fe, Si. For OLEDs, most of the materials used are small molecular organics, which are sublimation materials.
From a purely academic point of view, there are three theoretical sources of evaporation:
• Free evaporation source: The evaporation rate depends not only on the equilibrium vapor pressure Pe of the material, but also on the actual partial pressure Ph of the evaporated material.
• Knudsen Cell: This type of evaporation source is characterized by small evaporation area, low evaporation rate, good directional flow of the evaporating beam, and accurate control of temperature and evaporation rate.
• Crucible evaporation source: This type of evaporation rate controllability is between the above two evaporation sources. This type of evaporation source is typically used in small laboratories.
It is assumed that the distance between the evaporation source and the substrate is r. Simply speaking, the deposition rate of the film in the process of evaporation of the ordinary free evaporation source is inversely proportional to the distance r2 from the evaporation source to the substrate, and is affected by the angle θ between the substrate and the evaporation source. When θ =0 and r is small, the sinking bottom is close to the evaporation source, and the deposition rate is large. In the Knudsen evaporation source, the evaporation of the material follows the Knudsen cosine law, that is, the material mass at the Φ angle is proportional to cosΦ, so that the material beam evaporated by this type of evaporation source has strong directivity and Controllability.

Thanks to the good material beam directionality and the controllability of evaporation, the Knudsen evaporation source is used in a large number of high-precision evaporation processes. In order to obtain better material thickness and uniformity, the vapor deposition unit equipped with the Knudsen evaporation source can be further increased by increasing the target-source distance (Target Source Distance. T/S), rotating the substrate and evaporating. The deposition uniformity of the film is improved by placing the source and the substrate surface on the same circumference.

The above is the classification of evaporation sources from a purely theoretical perspective. However, from the perspective of display industry production, evaporation sources can be classified according to their macroscopic shapes:

• Point Source: Primarily a single Knudsen Cell. When designing the equipment, it is necessary to find the optimal process window between evaporation uniformity, source and substrate distance, off-axis location, material utilization rate, deposition rate and other parameters.
• Line Source: Mainly a parallel Knudsen Cell. Its technical feature is that the evaporation source or substrate is moved during evaporation. Compared to Point Source, theoretically, the line source has a better deposition effect because of the smaller Source-to-substrate spacing, better deposition uniformity, and better material utilization. Line Source can be further divided into Top Heater and Side Heater depending on the location of the Heater Heater.
• Planar/Area Source and its extension technology: Unlike Line Source and Point Source, this technology features evaporation of the OLED material to a planar Substrate, which is then evaporated onto the target substrate. . Compared with other evaporation structures, the device design of the structure is simpler, and in theory, the area of the Dead Zone/Shadow Area will be smaller when the AMOLED device is fabricated. But because it requires evaporation of the material to an intermediate plane, the process steps are more complicated. The technology has not yet entered the production line for the time being.

Future OLED production technology development

Although the AMOLED technology is matured by evaporation, it has been applied on a large scale. However, the AMOLED technology still has its limitations, and the OLED display production technology also has a broad space for innovation. One of the development directions of OLED production technology innovation is the printing OLED process.
Compared with traditional AMOLED evaporation technology, printed OLED technology has several advantages:

• Not inferior to the display effect of AMOLED evaporation technology: At this stage, by printing the OLED, the printed OLED has achieved a demo of 400 PPI AMOLED. With the further advancement of technology, printed OLEDs can theoretically produce about 500 PPI AMOLED devices. This resolution has reached the needs of existing small and medium size display screen applications. Unlike existing vapor-deposited AMOLED Pentile pixel arrangements, printed OLED devices with barrier layers have separate RGB tri-color sub-pixels in each pixel. This represents a device made with printed OLEDs that consumes less energy and has a slightly longer lifetime than vapor-deposited devices in the same white light display scenario. At the same time, by cutting the same color pixel in the same pixel and further illuminating independently, so that after the PWM dimming is adopted under low brightness conditions, each of the monochrome sub-pixels can be independently cross-illuminated and blackened to realize gray scale display. . By this method, the problem that the existing small-sized AMOLED panel adopts PWM black insertion under low-light conditions and causes eye discomfort can be theoretically reduced.

• Lower production costs: Compared to traditional AMOLED evaporation technology, printed OLED technology is better at cost control over equipment and consumables. In theory, the price of a printing printer is lower than the price of a Cannon Tokki vaporizer. At the same time, only a photo mask is needed to make an ink bank layer, and a large amount of FMM is not required, and the material utilization rate can be increased by 90% - 95%. If the printed OLED can achieve the yield of the conventional AMOLED evaporation technology, the printed AMOLED product has a cost advantage.

• Lower technical barriers and opportunities for overtaking in corners: Although supported by the policy, each panel manufacturer has developed a positive layout in the AMOLED industry and achieved good results. However, compared with the industry leaders, domestic panel makers still have a lot of gaps in terms of technical strength and upstream and downstream raw material control. For industry leaders to lay out and set up barriers in equipment and materials in advance, it will undoubtedly increase the difficulty for Chinese manufacturers to compete in the AMOLED market. Compared with the long-term dip dyeing of traditional AMOLED evaporation technology by Korean manufacturers, printing OLED is still a new field. There is a large opportunity space and low barriers to entry for upstream and downstream raw material supply, device structure and pixel layout. With the further development of printed OLED technology, it will undoubtedly become an opportunity for Chinese panel makers to catch up and surpass display technology.

Although the printed display has many potential advantages, the technology is still in the development stage. Globally, the units that invest in R&D in printing and display technology mainly include South Korea's Samsung and LG, Japan's JOLED, China's BOE Hefei Zhuon, Huaxing Optoelectronics and Juhua Printing. Although each has invested heavily in print display technology, the slightly longer process flow, lower device life and complex drying process have temporarily restricted its use in mass production. However, with the development of printing display technology and the difficulty of overcoming these technical difficulties, printing OLED is expected to partially replace the evaporation technology, thereby further increasing the penetration of AMOLED into the traditional LCD display market.

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