Analysis of Micro LED large screen display technology - Chip and encapsulation structure
In the information society, people receive a large amount of information each day. As the main interface facilitating exchange between people and information, display products have a significant impact on people’s lives.
Given the rapid increase of information—in particular the replacement of traditional single text video information by the multimedia digital audio —higher demand of display quality and resolutuion is proposed for the display device. Therefore, various new display technologies are booming, especially the new generation of display technology, Micro LED, which has become a technical highland for display manufacturers. In response to this shift, Absen released the Micro LED display technology worldwide on 12 March 2021.
Comparison of mainstream display technologies
Diﬀerent scenarios propose diﬀerent performance requirements for display devices, and the corresponding mainstream display technologies selected are also diﬀerent. According to usage scenarios, display devices can be divided into 2C display and 2B display.
2C display mainly refers to TVs, monitors, laptops, in‐vehicle products, mobile phones, wearable products etc. LCD is the traditional technical solution mainly applied to 2C display, while it suﬀers from issues such as low contrast, small color gamut and limited viewing angle. With the development of material technologies, OLED technology has emerged. Compared with LCD, OLED boasts significant advantages including simpler frames, thinner overall thickness, higher brightness and contrast, larger viewing angle, wider color gamut range and lower power consumption. These strengths have quickly enabled OLED display products to dominate the consumer market.
However, OLED display products also have disadvantages such as short life. Micro LED is characterized by display technology that minimizes the LED display to a micron scale, which matches the long life and reliability of inorganic LED. This technology results in displays that feature high brightness, high contrast, wide color gamut, long service life and high reliability. Due to these qualities, the micro LED display is considered to be a near perfect display technology.
The high brightness and energy‐saving capabilities favor broad application prospects of micro LED in 0.x‐inch and x‐inch wearable products, while Apple, as a representative, expects to apply Micro LED technology to 2C products such as the Apple Watch. On the other hand, the advantage of seamless splicing renders the Micro LED the best solution for large screen TV products above 85‐inch. Currently, Sony, Samsung and some domestic display manufacturers have launched large screen consumer products based on Micro LED technology.
2B display mainly refers to outdoor and indoor large screens in the theaters, shopping malls, meeting rooms, control rooms, etc. The mainstream display technologies selected for these use scenarios include LCD splicing display, projection display and LED large screen display. The imposssibility of seamless splicing for LCD and the poor brightness and contrast for projection display limit their application, while LED display characterized by high brightness, high contrast, wide color gamut, available seamless splicing and other advantages becomes the most promising display technology. With the decrease of dot pitch, especially the development of Micro LED technology, the ultra‐high definition of LED display has become a trend, and the display quality is getti ng more and more delicate.
Consequently, the application scenarios for LED display are also expanding from the traditional engineering display to the commercial display and in‐home display. It can be seen that Micro LED is highly competitive in both 2C and 2B markets because of its unique advantages. Accordingly, both traditional panel manufacturers and LED display manufacturers are actively developing this technology.
As a global leading supplier for true LED display applications and services, Absen is no exception. At present,
Absen has launched the following Micro LED commercial display products worldwide: 2K 55‐inch (P0.6), 2K 73‐inch (P0.8), 2K 82‐inch (P0.9), 4K 110‐inch (P0.6), 4K 138‐inch (P0.7), 4K 165‐inch (P0.9) and 8K 220‐inch (P0.6).
Micro LED display technology can be deemed as the micro pitch and HD of traditional LED displays, and tiny LED particles are used as the pixel light‐ emitti ng points. Accordingly, the structure and encapsulation of LED chip directly aﬀect the performance of Micro LED display devices.
Structural comparison of Micro LED light emitting chips
Generally, the LED chip is composed of the substrate, P‐type semiconductor layer, N‐type semiconductor layer,
P‐N junction, positive electrode and negative electrode. When the positive voltage is applied between the positive electrode and negative electrode, the holes injected from the P area to the N area and the electrons injected from the N area to the P area recombine at the PN junction, converting the electrical energy into light with diﬀerent wavelength.
The structure of LED chips is mainly divided into the following types: wire‐bonding structure, flip structure and verticalstructure.
Figure 1 (below) shows the schematic design of three chip structures.
1) Structure of wire‐bonding chip As the earliest chip structure, the wire‐bonding chip is composed of the following from top to bottom: electrodes, P‐type semiconductor layer, light emitting layer, N‐type
semiconductor layer and substrates. In this structure, the heat generated at the PN junction in this structure needs to pass through the sapphire substrate to be conducted to the heat sink.
The poor thermal conductivity of the sapphire leads to poor thermal conductivity of the structure, thereby reducing the luminous eﬃciency and reliability of the chip. In the structure of the wire‐bonding chip, the p‐electrode and n‐electrode are both located on the light‐emitting surface of the chip, and the shielding of the electrodes will result in low luminous eﬃciency of the chip. Furthermore, locating the p‐ electrode and n‐electrode on the same side of the chip is also prone to current crowding, which will reduce the luminous eﬃciency. Additionally, electrode metal drift may be caused by factors such as temperature, humidity and etc, which will result in short‐circuit problems as the reduction of the chip size and the distance between positive and negative electrodes.
2) Structure of flip chip
The flip chip is composed of the following from top to bottom: sapphire substrate, N‐type semiconductor layer, light emitti ng layer, P‐type semiconductor layer and electrodes. Compared with the wire‐
bonding chip, the heat generated at the P‐N junction of this structure can be directly conducted to the heat sink without passing through the substrate, resulting in excellent heat dissipation performance and thus excellent luminous eﬃciency and reliability.
In the flip structure, both p electrode and n electrode are located on the bottom, which avoids the obstruction of light emitted from the top and contributes to a high light output eﬃciency. Additionally, the long distance between positive and negative electrodes in the flip structure can reduce the risk of short circuit caused by metal migration of the electrodes.
3) Structure of vertical chip Diﬀerent from the wire‐bonding chip, the substrate with high thermal conductivity (Si, Ge or Cu substrate) instead of sapphire substrate is used in the vertical chip, which greatly improves the heat dissipation performance of the chip. Moreover, the positive electrode and negative electrode of the vertical chip are located on the top and bottom of the chip separately, which contributes to more uniform distribution of the current and heat and further improved reliability of the chip. At the present however, the current vertical chip suﬀers from high cost and low mass production capacity.
Table 1 (above) outlines the performance comparison of these three chips. It can be seen that the flip chip is more suitable for micro LED display products due to the high luminous eﬃciency, good heat dissipation performance, high reliability and mass production capacity.