Fully Vertical Chip Structure

P1.25-P0.6: Four advantages stand out

Liang Fubo, general manager of Crystal Optoelectronics Jiangxi, introduced at the scene of 2021 Jibang Consulting New Display Industry Seminar. Through experiments, Crystal Optoelectronics compared the performance of its vertical 5×5mil chip with JD's formal 5×6mil chip. The results show that, compared with the front mounted chip, the vertical chip produces less light interference due to the single side light and no side light. In other words, the smaller the spacing, the less the brightness loss. Therefore, the smaller the distance between the vertical chip and the luminous intensity and display clarity have obvious advantages.

Specifically, the vertical chip has the shape of radiant light, uniform light output, easy light distribution, good heat dissipation performance, so the display effect is clear;

In addition, the vertical electrode structure, the current distribution is more uniform, IV curve consistency is good, while the horizontal chip because of the electrode on the same side, the current is blocked, the uniformity of light spot is poor.

In terms of production yield, the vertical structure can beat two less wires than the ordinary formal structure, and the area of the wire inside the device is more sufficient, which can effectively increase the production capacity of the equipment and reduce the defect rate caused by the welding wire of the device by an order of magnitude.

In display applications, the "caterpillar" phenomenon has long been a major problem for manufacturers, and the root cause of this phenomenon is metal migration.

According to Liang, metal migration is closely related to the temperature, humidity, potential difference and electrode material of the chip, and is more likely to occur in the display screen with smaller spacing. The all-vertical chip structure also has a natural advantage in addressing metal migration.

First, the distance between the positive and negative electrodes of the vertical structure chip is greater than 135μm. Because the distance between the positive and negative electrodes is larger in the physical space, the life of the vertical chip lamp beADS can be more than 4 times longer than that of the horizontal chip even if metal ions migrate, which greatly improves the reliability and stability of the product.

Second, the vertical structure of blue-green chip surface is fully inert metal electrode Ti/Pt/Au, it is difficult to metal migration, the main performance is the same as red vertical chip.

Three is the vertical structure of the chip with silver glue, good thermal conductivity, the lamp temperature is much lower than the level of the formal installation, can greatly reduce the migration speed of metal ions.

It can be seen that compared with the ordinary formal solution, the vertical solution has better performance, higher yield and reliability.

At this stage, in the P1.25-P0.9 applications, while normal formal solutions dominate the market due to their low price, flip and vertical solutions play a major role in the high-end applications due to their higher performance.

In terms of cost, the RGB price of a set of chips of the vertical scheme is 1/2 of that of the flip scheme, so the cost performance of the vertical structure is higher than that of the flip scheme.

In the application of P0.6-P0.9mm, the normal forward assembly scheme is limited by the limit of physical space, which is difficult to guarantee the yield, and the possibility of mass production is low, while the flip and vertical chip scheme can meet the requirements.

It is important to note that for the assembly house, selection of inversion structure scheme to increase the quantities of equipment, and due to the two flip chip bonding pad tiny solder paste welding yield is not high, while vertical chip solution encapsulation process maturity is high, the existing assembly house equipment can be general, plus a set of vertical chip RGB costs only half of flip chip, a set of RGB, the overall cost performance of the vertical scheme is also higher than that of the inverted scheme.

Therefore, under the condition of the same level of performance, the cost performance of the vertical structure is higher than that of the flip structure. In general, vertical chip architectures will dominate the market for 1.25-p0.6 display applications with four advantages.

P0.6-P0.3: two major technical routes support

For P0.6-P0.3 applications, it focuses on the Thin Film chip technology without substrate, covering vertical structure and flip structure. Thin film LED generally refers to the Thin film LED chip after substrate stripping. After substrate stripping, a new substrate can be fixed or the substrate can be made into a Vertical structure, which is called Vertical Thin Film, or VTF for short. At the same time, you can also do not set the substrate, made of flip structure, called the Thin Film Flip Chip, referred to as TFFC.

Liang Fubo, for P0.6-P0.3 and other ultra-small spacing display demand, crystal energy based on Thin Film LED technology, developed two relatively mature technical route scheme.

Technical route 1: VTF/TFFC chip + quantum dot red light (QD+ blue InGaN LED)

Under the extremely small chip size, traditional AlGaNP red LED is very easy to be broken in the transfer process due to its poor mechanical properties after substrate removal, which makes it difficult to carry out subsequent batch production.

Therefore, one solution is to use printing, spraying, printing and other technologies to place quantum dots on the surface of GaN blue LEDs to obtain red LEDs.

Technical route 2: InGaN LED is used for all RGB colors

Due to the insufficient mechanical strength of the existing four-element red light without the substrate, it is difficult to carry out the subsequent production process. Another solution is that all three RGB colors are InGaN LEDs, and the unification of epitaxy and chip manufacturing process is realized at the same time.

According to the introduction, Crystal Energy has started to grow gallium nitride red light research and development on silicon substrate, and silicon-based InGaN red light LED has made some achievements, which provides the possibility for this technology.

It is worth noting that by comparing the advantages and disadvantages of TFFC, FC and MICRO in the aspects of substrate, chip separation, luminous efficiency and massive transfer, a conclusion is drawn that combining the technical route of MICRO with MICRO's MINI chip can greatly reduce the chip cost while reducing the technical difficulty. This also means that 4K, 8K MINI ultra-high definition display LED large screen products are expected to enter thousands of households.

Mini LED: technology development and patent layout

As early as the second half of 2018, the company began to develop a 5×5mil silicon substrate vertical chip for MINI RGB display, with a luminous region of 90×90um. During the research, it overcame the problems of process yield and ESD of the epizaxial chip.

According to Liang Fubo, the blue and green chip of vertical MINI LED and the red chip have the same chip height, and both of them emit light from one side, showing excellent performance in contrast and luminous Angle. At the same time, the blue and green chip with vertical structure can completely avoid the black spots on the screen caused by the migration of metal ions generated by the client for a long time. In addition, thanks to the mature packaging process, most of the existing equipment in the packaging plant can be seamlessly connected, which greatly reduces the fixed asset investment in the packaging plant.

At present, the 5×5mil silicon substrate vertical chip has officially started mass production in July 2020, and is currently in the stage of small batch shipment. At the same time, we will continue to cooperate with more customers for product testing and verification.

According to the plan, the company expects the 4×4mil silicon vertical blue-green chip with the P1.25-P0.7 spacing to be in mass production by the end of the fourth quarter of 2021. The 2×4mil silicon vertical red-blue-green chip is expected to achieve mass production by the end of the fourth quarter of 2023 and is suitable for P0.6-P0.3 spacing, which can be applied up to P0.7 based on the actual cost performance.

Obviously, crystal energy technology research and development work is progressing step by step, and the patent technology layout is also all in control.

At present, the company has more than 420 global patents, covering the entire industrial chain of epitaxial materials and chip applications. Among them, the vertical structure MINI LED chip based on silicon substrate has technical patent protection both at home and abroad.

Based on the feedback from strategic customers on the silicon substrate MINI LED products, we will consider further production expansion in the future.

Summary: At present, 4K and 8K MINI ultra-high-resolution display screens are unstoppable driven by 5G technology. The vertical MINI LED chip on silicon substrate has the opportunity to become a super cost-effective light source solution.