How to unlock HD codeless Micro LED?
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Five years ago, the Apple iPhone X began to be equipped with OLED (small organic molecule electroluminescent display) screens; in 2021, Apple equipped the new MacBook Pro with a new display technology: Mini LED (submillimeter light-emitting diode). But Mini LED is not the end point. Another reserve technology that Apple is involved in, the rookie Micro LED (micron light-emitting diode) display technology may be expected to dominate the world.
The reason for upgrading the display technology is to see more clearly and restore the real world. But at present, products equipped with Micro LED technology are too expensive, such as Samsung's 110-inch Micro LED TV, the price has exceeded one million yuan, and ordinary people have no luck at all. When will the price of Micro LED products be affordable, and when will Micro LED screens be available on mobile phones and tablets?
In this article, you will learn: why Micro LED is called the ultimate display technology, the difficulties of Micro LED industrialization, and the market situation of Micro LED.
Display technology that changes human life
Simply put, you can understand Micro LED as a new display technology that scales LEDs to extremely small sizes. Its future applications are expected to involve displays, TVs, mobile phone tablet displays, augmented reality/virtual reality/mixed reality (AR/VR/MR), space display, flexible transparent display, wearable/implantable optoelectronic devices, optical communication/optical interconnection , medical detection, smart car lights and other fields.
The industry uses two methods to define the technical specifications of Micro LED.
One is to define the size of the chip and scale the LED chip to less than 100 μm, which can be called a Micro LED chip (some scholars believe that 50 μm should be used as the boundary). Depending on the viewing distance and the limit resolution of the human eye, the requirements for chip size are also different:
The viewing distance of VR/AR applications is about 5cm, and the pixel density should reach about 1800PPI. At this time, the size of the Micro-LED chip is 3~5 μm;
A 10-inch to 12-inch flat panel must have a pixel density of at least 300PPI, corresponding to a chip size of 20 to 30 μm;
A 75-inch TV only needs 43PPI, and the chip size is often about 200μm.
The other is defined by the process. Since the size of the Micro LED chip is very small, its length and width are even smaller than the height of the chip, resulting in the height of the chip being larger than the size of the die-bonding surface, which is not conducive to fixing the chip on the substrate. Therefore, it will increase compared with traditional LEDs. One-step laser lift off chip substrate operation.
Throughout the display industry, technologies continue to emerge. Currently, mainstream technologies in the market include TFT-LCD (Thin Film Transistor Liquid Crystal Display), AMOLED (Active Matrix Organic Light Emitting Diode Display), QD-OLED (mixed OLED and QLED technology), QNED (based on QD-OLED display with blue LED as light source), Mini LED, Micro LED, Micro-OLED.
The current display technology can be simply divided into four categories: LCD, OLED, QLED, and Micro LED. Other technologies are improved or transitional versions based on this. Micro LED is considered to be an ideal display technology and a new type of display technology. The ultimate goal of the era is expected to replace LCD and OLED, which are widely used at this stage.
From black and white to high-definition color, CRT technology has fulfilled a generation's dream of influencing the world. From thick and heavy to flat panel displays, LCD and OLED meet people's requirements for thin, light, portable, and eye-friendly. At this stage, the only next-generation display technology that can be closer to reality and even deceive the human eye is Micro LED.
Industry insiders believe that when multiple display technologies explode, only one or two technologies may eventually dominate the market, and Micro LED is undoubtedly the most promising.
Why can Micro LED get such a high evaluation? Because it not only has most of the advantages of LED, but also has the characteristics of high brightness, high resolution, high response speed, low power consumption, small size, easy disassembly, high flexibility, and no seams, etc., and can cover most display screens. Application scenarios [5]. At the same time, its response speed can reach tens of nanoseconds, there is no afterimage and there is no life problem.
In addition, compared with LCD, OLED, QLED and other predecessors, the main performance parameters of Micro LED are relatively excellent.
Transition technology has been commercialized on a large scale
Micro LED also has a close relative: Mini LED, which can be understood as a transitional technology between LED and Micro LED. Because the technical principles are very similar, the industry generally attributes the two to one route, and in some cases they are collectively referred to as MLED.
As a pre-order technology, Mini LED mainly solves the problem of chip size faults. The development of related technologies has a certain reference effect on Micro LED, and its chip size is generally 75 μm to 300 μm.
Mini LED is much easier to realize than Micro LED. Not only is it easier to realize in terms of chip size, but also there are no scientific problems in materials. It has been mass-produced and put into the market. Brands such as Apple, Samsung, and TCL have already Realize large-scale mass production.
However, Mini LEDs and Micro LEDs have different application scenarios. Mini LEDs on the market generally appear as LCD backlights, combined with quantum dot technology to achieve high dynamic range display, while Micro LEDs are directly used to make display pixels [7]. To put it simply, commercially available Mini LED displays are still LCDs in essence, and Mini LEDs are just a foil for LCDs, while Micro LED displays directly emit light and form images.
Since 2018, panel manufacturers have successively released prototypes using Mini LEDs as backlights. The product sizes have reached 152.4mm~685.8mm (6~27 inches). So far, Mini-LED backlight displays have 1398mm (55 inches), 1651mm (65 inches), 1905 mm (75 inches) and 2159 mm (85 inches) and other models, can achieve high indicators of 8K resolution and 120Hz refresh rate, and can increase the contrast ratio of existing displays from 10000:1 to 1000000:1 .
The industrialization of Mini LED is clear and has already achieved a certain amount of shipments. In 2021, the global shipment of medium-sized Mini LED backlight panels will reach 7.4 million pieces. By 2023, smart cars will become the new frontier of Mini LED.
As an intermediate transition, the Mini LED industry is thriving, but Micro LED is still a minority, so what is the difficulty in getting Micro LED out of the circle?
It's not that easy to make
Micro LED is good in everything, but expensive. The reason is that there are many problems in the manufacturing process, which makes it difficult to produce in large quantities, and the yield rate is low, which causes the price to rise.
On the surface, Micro LED chips are just miniaturization of LED chips, but the actual situation is far more difficult than imagined. The process of converting from the millimeter level to the micron level is essentially the miniaturization, thinning and matrixing of LEDs. During this process, steps such as material growth, device preparation, driving technology, and production technology have all changed, resulting in qualitative changes.
Micro LED adopts the RGB (red, green and blue) color standard. Because it is a self-luminous component, if you want Micro LED to display images, you must make light-emitting chips of three primary colors to form pixels.
Micro LED chips of different colors can be prepared with different materials, such as InGaN/GaN-based materials for the preparation of green/blue Micro-LED arrays, AlInGaP/GaAs-based materials for the preparation of red Micro LED arrays, sapphire, gallium arsenide Epitaxial layers are grown on substrates such as silicon and silicon to prepare Micro LED arrays.
From the substrate to the product, the Micro LED display will go through four major processes: epitaxial growth, chip manufacturing, mass transfer, and performance testing.
From a technical point of view, the materials and equipment related to chip preparation are relatively mature, and the symptoms of the problem lie in the manufacturing process. Specifically, the barriers of Micro LED include the following points:
it's all petty fault
For chips, the smaller the size, the more difficult it is to manufacture, and the same is true for Micro LEDs. The size of the Micro LED chip is too small. When making it into various display devices, the impact of the small size must be considered.
To obtain a small-sized Micro LED, it is necessary to use a micro-scale process technology to shrink the LED chip to a size that meets the application requirements. Microscale process technology includes three paths: chip bonding, wafer bonding, and film transfer. The better the technology is implemented, the higher the pixel density will be.
There is also a fatal problem that Micro LED has to face: during the miniaturization process, side wall defects often occur. For example, for the same error defect of 2μm, the remaining usable rate is 97% on a 250μm×250μm LED, but only 4% on a 5μm×5μm Micro-LED.
Not only that, the smaller the size of the Micro LED chip, the larger the ratio of the inductively coupled plasma (ICP) etching area (sidewall) to the volume of the active area, and the higher the proportion of defects formed by etching damage[15] . As a result, the proportion of non-radiative recombination gradually increases, and the luminous efficiency and service life decrease. If the size is reduced from 400 μm to 20 μm, the reduction in current density and light efficiency can reach 50% [7]. This will also lead to increased Shockley Reid Hall (SRH) non-radiative recombination probability in the active region, reduced radiative recombination probability and luminous efficiency, and the introduction of new leakage channels to aggravate the reverse leakage of the device. These problems are especially noticeable on Micro-LEDs with a size smaller than 10 μm.
The problem of chip size will also make it difficult to improve the efficiency and stability of driver integration with Micro LED arrays. In short, the mass production process of LED or Mini LED may no longer be applicable to Micro LED.
The Mass Transfer Dilemma
Each pixel of the LED display is composed of a dot matrix, and the spacing of the array of small LED chips is basically the same. Micro LED is also the same. After the chip is manufactured, a large number of Micro LED chips must be transferred to the circuit substrate in a fixed-point and massive amount. This process is called "mass transfer".
Different from traditional LEDs, the mass transfer of Micro LED not only has higher requirements on the control panel display of transfer accuracy, transfer rate, and color uniformity, but also has a larger transfer quantity. A 4K Micro LED display needs to complete more than 20 million Micro LED displays. LED chips are flipped, and 8K has reached hundreds of millions.
At present, there are many technical solutions for mass transfer technology, including precise pickup technology, laser release technology, fluid self-assembly technology, and roller transfer printing technology. Different technologies have different characteristics, but they all have certain disadvantages.
Specifically, the precision pick-up technology requires extremely high accuracy and stability of the transfer equipment; the laser release technology may cause damage to the chip surface during the implementation process, reducing the yield rate, and the laser equipment is expensive; the fluid assembly technology requires 3 years of experience. The transfer can be completed only once, and the efficiency is low; the roller transfer technology can realize the transfer on the flexible substrate, but it also needs 3 transfers to complete the huge transfer.
Some methods can achieve a yield rate of 99.99% in the laboratory, but compared with the industrialization requirement of at least 99.9999% yield rate, there is a large gap. The industry is looking for an easy-to-implement, high-yield and relatively cheap technical solution.
The Challenge Behind the Colors
Why are displays so colorful? Because the RGB (red, green, and blue) color system is widely used in the display industry, most of the colors visible to the naked eye can be realized through the superposition and synthesis of the three primary colors of RGB. If you want Micro LED to be able to correctly depict the picture, you need to make a color scheme, that is, full color.
There are two main solutions for Micro LED colorization: one is the blue source color conversion scheme, but the color conversion material has problems such as coating uniformity and reliability, and the application is seldom; the other is the RGB triad that directly uses LED However, for Micro LED, this solution is not ready to use, and the size of the LED needs to be adjusted accordingly. In addition, due to the difference in the uniformity of the wavelength of each color, it will also face the problem of insufficient light efficiency and yield.
Difficulties in the industrial chain
The creation of Micro LED chips does not mean the end. To make it truly visible to people, breakthroughs must be made in multiple technical fields.
Like other chip technologies, Micro LED will also involve a huge industrial chain and raw material issues. From raw materials to lighting screens, it is divided into upstream, midstream and downstream. Each link is closely related to the final product. Among them, the key technologies include driver IC, driver backplane and packaging.
The industry is gaining momentum
As the ultimate goal, Micro LED is undoubtedly the star in the display field.
Since 2000, a professor at Texas Tech University proposed the concept of Micro LED, and Kansas State University prepared Micro LED based on group III nitrides. Since then, the academic community has set off a wave of research.
The reason for upgrading the display technology is to see more clearly and restore the real world. But at present, products equipped with Micro LED technology are too expensive, such as Samsung's 110-inch Micro LED TV, the price has exceeded one million yuan, and ordinary people have no luck at all. When will the price of Micro LED products be affordable, and when will Micro LED screens be available on mobile phones and tablets?
In this article, you will learn: why Micro LED is called the ultimate display technology, the difficulties of Micro LED industrialization, and the market situation of Micro LED.
Display technology that changes human life
Simply put, you can understand Micro LED as a new display technology that scales LEDs to extremely small sizes. Its future applications are expected to involve displays, TVs, mobile phone tablet displays, augmented reality/virtual reality/mixed reality (AR/VR/MR), space display, flexible transparent display, wearable/implantable optoelectronic devices, optical communication/optical interconnection , medical detection, smart car lights and other fields.
The industry uses two methods to define the technical specifications of Micro LED.
One is to define the size of the chip and scale the LED chip to less than 100 μm, which can be called a Micro LED chip (some scholars believe that 50 μm should be used as the boundary). Depending on the viewing distance and the limit resolution of the human eye, the requirements for chip size are also different:
The viewing distance of VR/AR applications is about 5cm, and the pixel density should reach about 1800PPI. At this time, the size of the Micro-LED chip is 3~5 μm;
A 10-inch to 12-inch flat panel must have a pixel density of at least 300PPI, corresponding to a chip size of 20 to 30 μm;
A 75-inch TV only needs 43PPI, and the chip size is often about 200μm.
The other is defined by the process. Since the size of the Micro LED chip is very small, its length and width are even smaller than the height of the chip, resulting in the height of the chip being larger than the size of the die-bonding surface, which is not conducive to fixing the chip on the substrate. Therefore, it will increase compared with traditional LEDs. One-step laser lift off chip substrate operation.
Throughout the display industry, technologies continue to emerge. Currently, mainstream technologies in the market include TFT-LCD (Thin Film Transistor Liquid Crystal Display), AMOLED (Active Matrix Organic Light Emitting Diode Display), QD-OLED (mixed OLED and QLED technology), QNED (based on QD-OLED display with blue LED as light source), Mini LED, Micro LED, Micro-OLED.
The current display technology can be simply divided into four categories: LCD, OLED, QLED, and Micro LED. Other technologies are improved or transitional versions based on this. Micro LED is considered to be an ideal display technology and a new type of display technology. The ultimate goal of the era is expected to replace LCD and OLED, which are widely used at this stage.
From black and white to high-definition color, CRT technology has fulfilled a generation's dream of influencing the world. From thick and heavy to flat panel displays, LCD and OLED meet people's requirements for thin, light, portable, and eye-friendly. At this stage, the only next-generation display technology that can be closer to reality and even deceive the human eye is Micro LED.
Industry insiders believe that when multiple display technologies explode, only one or two technologies may eventually dominate the market, and Micro LED is undoubtedly the most promising.
Why can Micro LED get such a high evaluation? Because it not only has most of the advantages of LED, but also has the characteristics of high brightness, high resolution, high response speed, low power consumption, small size, easy disassembly, high flexibility, and no seams, etc., and can cover most display screens. Application scenarios [5]. At the same time, its response speed can reach tens of nanoseconds, there is no afterimage and there is no life problem.
In addition, compared with LCD, OLED, QLED and other predecessors, the main performance parameters of Micro LED are relatively excellent.
Transition technology has been commercialized on a large scale
Micro LED also has a close relative: Mini LED, which can be understood as a transitional technology between LED and Micro LED. Because the technical principles are very similar, the industry generally attributes the two to one route, and in some cases they are collectively referred to as MLED.
As a pre-order technology, Mini LED mainly solves the problem of chip size faults. The development of related technologies has a certain reference effect on Micro LED, and its chip size is generally 75 μm to 300 μm.
Mini LED is much easier to realize than Micro LED. Not only is it easier to realize in terms of chip size, but also there are no scientific problems in materials. It has been mass-produced and put into the market. Brands such as Apple, Samsung, and TCL have already Realize large-scale mass production.
However, Mini LEDs and Micro LEDs have different application scenarios. Mini LEDs on the market generally appear as LCD backlights, combined with quantum dot technology to achieve high dynamic range display, while Micro LEDs are directly used to make display pixels [7]. To put it simply, commercially available Mini LED displays are still LCDs in essence, and Mini LEDs are just a foil for LCDs, while Micro LED displays directly emit light and form images.
Since 2018, panel manufacturers have successively released prototypes using Mini LEDs as backlights. The product sizes have reached 152.4mm~685.8mm (6~27 inches). So far, Mini-LED backlight displays have 1398mm (55 inches), 1651mm (65 inches), 1905 mm (75 inches) and 2159 mm (85 inches) and other models, can achieve high indicators of 8K resolution and 120Hz refresh rate, and can increase the contrast ratio of existing displays from 10000:1 to 1000000:1 .
The industrialization of Mini LED is clear and has already achieved a certain amount of shipments. In 2021, the global shipment of medium-sized Mini LED backlight panels will reach 7.4 million pieces. By 2023, smart cars will become the new frontier of Mini LED.
As an intermediate transition, the Mini LED industry is thriving, but Micro LED is still a minority, so what is the difficulty in getting Micro LED out of the circle?
It's not that easy to make
Micro LED is good in everything, but expensive. The reason is that there are many problems in the manufacturing process, which makes it difficult to produce in large quantities, and the yield rate is low, which causes the price to rise.
On the surface, Micro LED chips are just miniaturization of LED chips, but the actual situation is far more difficult than imagined. The process of converting from the millimeter level to the micron level is essentially the miniaturization, thinning and matrixing of LEDs. During this process, steps such as material growth, device preparation, driving technology, and production technology have all changed, resulting in qualitative changes.
Micro LED adopts the RGB (red, green and blue) color standard. Because it is a self-luminous component, if you want Micro LED to display images, you must make light-emitting chips of three primary colors to form pixels.
Micro LED chips of different colors can be prepared with different materials, such as InGaN/GaN-based materials for the preparation of green/blue Micro-LED arrays, AlInGaP/GaAs-based materials for the preparation of red Micro LED arrays, sapphire, gallium arsenide Epitaxial layers are grown on substrates such as silicon and silicon to prepare Micro LED arrays.
From the substrate to the product, the Micro LED display will go through four major processes: epitaxial growth, chip manufacturing, mass transfer, and performance testing.
From a technical point of view, the materials and equipment related to chip preparation are relatively mature, and the symptoms of the problem lie in the manufacturing process. Specifically, the barriers of Micro LED include the following points:
it's all petty fault
For chips, the smaller the size, the more difficult it is to manufacture, and the same is true for Micro LEDs. The size of the Micro LED chip is too small. When making it into various display devices, the impact of the small size must be considered.
To obtain a small-sized Micro LED, it is necessary to use a micro-scale process technology to shrink the LED chip to a size that meets the application requirements. Microscale process technology includes three paths: chip bonding, wafer bonding, and film transfer. The better the technology is implemented, the higher the pixel density will be.
There is also a fatal problem that Micro LED has to face: during the miniaturization process, side wall defects often occur. For example, for the same error defect of 2μm, the remaining usable rate is 97% on a 250μm×250μm LED, but only 4% on a 5μm×5μm Micro-LED.
Not only that, the smaller the size of the Micro LED chip, the larger the ratio of the inductively coupled plasma (ICP) etching area (sidewall) to the volume of the active area, and the higher the proportion of defects formed by etching damage[15] . As a result, the proportion of non-radiative recombination gradually increases, and the luminous efficiency and service life decrease. If the size is reduced from 400 μm to 20 μm, the reduction in current density and light efficiency can reach 50% [7]. This will also lead to increased Shockley Reid Hall (SRH) non-radiative recombination probability in the active region, reduced radiative recombination probability and luminous efficiency, and the introduction of new leakage channels to aggravate the reverse leakage of the device. These problems are especially noticeable on Micro-LEDs with a size smaller than 10 μm.
The problem of chip size will also make it difficult to improve the efficiency and stability of driver integration with Micro LED arrays. In short, the mass production process of LED or Mini LED may no longer be applicable to Micro LED.
The Mass Transfer Dilemma
Each pixel of the LED display is composed of a dot matrix, and the spacing of the array of small LED chips is basically the same. Micro LED is also the same. After the chip is manufactured, a large number of Micro LED chips must be transferred to the circuit substrate in a fixed-point and massive amount. This process is called "mass transfer".
Different from traditional LEDs, the mass transfer of Micro LED not only has higher requirements on the control panel display of transfer accuracy, transfer rate, and color uniformity, but also has a larger transfer quantity. A 4K Micro LED display needs to complete more than 20 million Micro LED displays. LED chips are flipped, and 8K has reached hundreds of millions.
At present, there are many technical solutions for mass transfer technology, including precise pickup technology, laser release technology, fluid self-assembly technology, and roller transfer printing technology. Different technologies have different characteristics, but they all have certain disadvantages.
Specifically, the precision pick-up technology requires extremely high accuracy and stability of the transfer equipment; the laser release technology may cause damage to the chip surface during the implementation process, reducing the yield rate, and the laser equipment is expensive; the fluid assembly technology requires 3 years of experience. The transfer can be completed only once, and the efficiency is low; the roller transfer technology can realize the transfer on the flexible substrate, but it also needs 3 transfers to complete the huge transfer.
Some methods can achieve a yield rate of 99.99% in the laboratory, but compared with the industrialization requirement of at least 99.9999% yield rate, there is a large gap. The industry is looking for an easy-to-implement, high-yield and relatively cheap technical solution.
The Challenge Behind the Colors
Why are displays so colorful? Because the RGB (red, green, and blue) color system is widely used in the display industry, most of the colors visible to the naked eye can be realized through the superposition and synthesis of the three primary colors of RGB. If you want Micro LED to be able to correctly depict the picture, you need to make a color scheme, that is, full color.
There are two main solutions for Micro LED colorization: one is the blue source color conversion scheme, but the color conversion material has problems such as coating uniformity and reliability, and the application is seldom; the other is the RGB triad that directly uses LED However, for Micro LED, this solution is not ready to use, and the size of the LED needs to be adjusted accordingly. In addition, due to the difference in the uniformity of the wavelength of each color, it will also face the problem of insufficient light efficiency and yield.
Difficulties in the industrial chain
The creation of Micro LED chips does not mean the end. To make it truly visible to people, breakthroughs must be made in multiple technical fields.
Like other chip technologies, Micro LED will also involve a huge industrial chain and raw material issues. From raw materials to lighting screens, it is divided into upstream, midstream and downstream. Each link is closely related to the final product. Among them, the key technologies include driver IC, driver backplane and packaging.
The industry is gaining momentum
As the ultimate goal, Micro LED is undoubtedly the star in the display field.
Since 2000, a professor at Texas Tech University proposed the concept of Micro LED, and Kansas State University prepared Micro LED based on group III nitrides. Since then, the academic community has set off a wave of research.
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