For Mini/Micro LED display technology, an important step towards the next generation of high energy efficiency lies in innovating technology and materials to achieve higher efficiency, smaller size, lighter weight, and lower overall costs.

As a high-tech company that provides high-end LED lighting, micro displays, and GaN HEMT device material products and solutions, LatticePower has conducted in-depth research on gallium nitride materials and integrated them well with Mini/Micro LED display technology. At the 2022 Jibang Consulting Compound Semiconductor New Application Prospective Analysis Conference, Guo Xiao, R&D Manager of Crystal Energy Optoelectronics, introduced the research and development progress of Crystal Energy Optoelectronics in silicon substrate gallium nitride Mini/Micro LED display technology.

Guo Xiao, R&D Manager of Crystal Energy Optoelectronic Epitaxy

Low cost and high reliability Mini LED achieved by silicon substrate vertical chip technology

Guo Xiao pointed out that from a market perspective, Mini LED has shown signs of dawn, and Samsung, LG, Xiaomi, TCL, LeEco and others have all launched mass-produced Mini LED terminal products. However, at the current level of technological development, the price of Mini LED products is still relatively high, which is not conducive to product promotion, and the industrial chain urgently needs to further reduce costs.

At present, there are three main implementation schemes for RGB small pitch display screens:

Horizontal plan: R uses vertical chips, G and B use regular formal horizontal chips;

Vertical scheme: R, G, and B can all use vertical chips, with G and B using silicon substrate vertical chips;

Inversion scheme: R, G, and B all use inverted chips.

Guo Xiao introduced that between P1.25-P0.9, the 1010 solution is mainly used. Among them, the ordinary TS solution occupies the main market due to its low price, while the vertical and inverted solutions compete in the high-end application market. In terms of current development level, the vertical chip packaging process has high maturity, and the equipment of existing packaging factories can be fully interchangeable. However, the flip chip solution not only requires a large number of additional equipment, but also has high chip prices - it is understood that the cost of vertical chips is half of flip chips.

Between P1.25-P0.9, ordinary TS schemes are difficult to ensure yield and overcome physical space limitations, so only vertical and inverted schemes can be used. The cost of vertical solutions is relatively low.

Between P0.6-P0.3, ordinary flip chip schemes are difficult to achieve, while thin film flip chips can be made by removing the substrate and evaporating a tin column with a height of about 20um between two PADs, resulting in a thickness of 3-5 μ The thin film chip of m, combined with QD quantum dot film, KSF phosphor or silicon-based GaN red light, can achieve a spacing of P0.6-P0.3.

At present, the potential of vertical structure RGB technology has been fully demonstrated in the display field. Guo Xiao introduced that compared to the horizontal plan, the vertical plan has the following advantages:

1. In terms of display effect, vertical chips emit light on one side without side lighting. Compared to horizontal chips, as the spacing decreases, vertical chips have less light interference, and the smaller the spacing, the less brightness loss. Meanwhile, vertical chips have significant advantages in display clarity and are more realistic in dynamic displays.

2. From the perspective of yield, horizontal chips are more prone to ion migration due to factors such as temperature, humidity, potential difference, and electrode material, which can lead to black spots on the screen. However, crystalline photovoltaic vertical chips have the ability to resist ion migration, so such problems will not occur. Guo Xiao also stated that through experiments, the vertical chip of LatticePower will not fail even after 500 hours of aging in room temperature saline immersion.

3. In addition, the vertical chip scheme also has advantages such as more uniform current distribution, good I-V curve consistency, high luminescence consistency, less need for two wires, and better heat dissipation.

LatticePower has conducted extensive and in-depth research on Mini LED display technology. It is reported that LatticePower began developing Mini RGB display screens in the second half of 2018, using 5 × Vertical chip with a 5mil silicon substrate and a 90% light emitting region × ninety μ M. In July 2020, LatticePower officially achieved mass production of Mini RGB chips, and as of now, the mass production of chips can reach 4.2 × 4.2mil.

According to different point spacing, Crystal Energy Optoelectronics adopts different vertical chip application schemes on silicon substrates. Among them, P1.25 adopts the chip1010/0808 scheme; Use the Top1010 scheme between P1.25-P0.9; P0.9 adopts a 4-in-1 scheme; P0.6 adopts the COB scheme.

At present, LatticePower is developing a solution for the P0.6-P0.3 spacing. In stages P0.6-P0.3, LatticePower adopted TFFC chips (thin film flip chip, abbreviated as TFFC).

It is reported that sapphire substrate technology is one of the more mature industrial technologies in the current LED preparation process. However, sapphire substrates have high hardness and poor heat dissipation, and can only be removed by laser peeling. When using sapphire substrates to produce high-power LEDs and TFFCs, there are significant limitations. LatticePower has been deeply rooted in GaN technology on silicon substrates for many years, and silicon substrates can be wet etched to remove the substrate, which has lower cost, higher yield, and higher stability. It has great advantages in the production of TFFC chips and other aspects.

In the process of chip fabrication, as the chip size decreases, traditional AlGaInP red LED has poor mechanical properties after removing the substrate, and is prone to breakage during transfer, making it difficult to carry out subsequent process production. There are two main technical solutions.

One is that InGaN TFFC LED chips are used for all RGB colors to ensure the uniformity of RGB color LED epitaxy and chip manufacturing process. Guo Xiao pointed out that the significant breakthrough in research and development of silicon based InGaN red LED has provided the possibility for the implementation of this technology.

The second solution is the TFFC chip+quantum dots/KSF red light scheme, which uses printing, spraying, printing and other technologies to place quantum dots or KSF fluorescent powder on the surface of the blue light LED to obtain a red LED.

In terms of mass production progress and planning, LatticePower achieved mass production of P1.25-P0.7 in July 2020; Mass production of P1.25-P0.6 was achieved in July 2021, and it is expected to achieve mass production of P0.6-P0.3 by December 2023. At that time, 2 will be adopted × A 4mil TFFC chip.

Guo Xiao pointed out that for epitaxy, Mini/Micro LED displays have strict requirements for wavelength yield and I-V curve consistency. At present, many major industrial chain enterprises adopt QD display technology, one of the reasons is that the epitaxial of blue light LED has excellent uniformity.

Therefore, developing efficient gallium nitride based red micro LEDs has become an urgent task. And LatticePower has made significant breakthroughs in the development of gallium nitride based red micro LEDs.

A mainstream research direction for Micro LEDs is the use of silicon substrate GaN LED as a substrate technology. Due to the physical compatibility between silicon substrate gallium nitride and silicon semiconductor wafers, the existing resources of crystal energy optoelectronics can be maximized while avoiding the problem of massive transfer. Crystal energy optoelectronics prioritizes research on Micro LED micro displays, bonding silicon based GaN with silicon based CMOS driver circuits at the wafer level, and continuing the GaN chip process on CMOS wafers after removing the silicon substrate. In terms of research direction, the first step is to conduct monochromatic Micro LED arrays, followed by full-color transformation, with application directions focused on AR/VR/HUD/HMD, etc.

In September 2021, LatticePower successfully prepared a red, green, and blue silicon substrate Micro LED array, taking a crucial step in the development of Micro LED full-color chips. At present, the pixel spacing of InGaN red green blue Micro LED arrays on crystalline photovoltaic silicon substrates is 14 microns, and the pixel density reaches 1800 PPI.

The third generation of semiconductors is an important development direction determined by the National 2030 Plan and the 14th Five Year National R&D Plan, and is seen as an opportunity for China's semiconductor industry to overtake in a bend.

LatticePower has been focusing on silicon substrate gallium nitride technology for nearly 20 years, and has taken the lead in realizing the industrialization of silicon substrate GaN based LEDs worldwide. With the rapid rise of GaN in more fields such as 5G, fast charging, and Mini/Micro LED, LatticePower will create greater value for customers. (Text: Lin Xifeng from the Compound Semiconductor Market)

(Reprinted from: Compound Semiconductor Market)