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- 1. The recent development of the microLED manufacturing process Zhao, Mengxuan Advisor: Chen, Haijie; Li, Zhuoxuan Tsinghua University (e.g., Feb 2nd, 2021)
- 2. AGENDA 1. Research question 1.1. Introduction 1.2. Research motivation 1.3. Advantages of this research 2. Research Methodology 3. Achievement of this investigation 3.1. Traditional manufacturing process 3.1.1. Flow path 3.1.2. Emphasis---- mass transfer 3.1.2.1. Self-assembly 3.1.2.2. Pick and place 3.1.2.3. Selective release 3.2. Monolithic integration 3.2.1. How to grow LED materials 3.2.2. How to colourization 3.2.3. How to restrain optical crosstalk 4. Conclusion
- 4. 1. Research question 1.1. Introduction MicroLED is a display technology that gives out light by tiny-sized LED, which is smaller than 50 μm. It can be applied in the area of augmented reality, extra-large displays, smartwatches, and so on, appealing to many companies from all over the world.
- 5. 1. Research question 1.2. Research motivation A review of microLED technology is important for both academia and industry to gain a fuller picture of the materials and processes that are explored, the performance and potential of each technology route.
- 6. 1. Research question 1.2. Research motivation—— Why is microLED important? Pixels in applications like AR glasses need to be minified. (why “micro”) Physical properties of microLED is better. (why “LED”)
- 7. 1. Research question 1.3. Advantages of this research To date, several microLED reviews have been published, however as the field is progressing rapidly, many of these works are not up to date. Key literature reviews studied in this investigation are mainly published from 2018 to 2021. Besides, this review combines both traditional and novel manufacturing routines, drawing a whole picture of this technology.
- 8. 2. Research methodology • Methodology: • investigation method(Search for published literatures and so on) • literature research method • quantitative analysis method(draw a table to compare different techniques) • qualitative analysis method(conclude the advantages of different display technologies)
- 9. 3. Achievement of this investigation 3.1. Traditional manufacturing process 3.1.1. Flow path
- 10. 3. Achievement of this investigation 3.1.2. Emphasis---- mass transfer Since LED of three colours cannot be grown directly on the device substrate, mass transfer is indispensable in the whole process before more cutting-edge technologies arise, being the most difficult. Therefore it is of great importance to sort the development of this technology, by the composition of several camps. However, some of them which are not cut-edging and promising such as roll printing will not be included.
- 11. 3. Achievement of this investigation 3.1.2.1. Self-assembly Direct self-assembly: To precisely install micro LED devices at the correct position, the constraint has to be built on the target substrates, which is in most conditions achieved by a magnetic field. The devices are about to be magnetified and poured onto the platform, being guided by a magnetic field to get to where they suppose to be.
- 12. 3. Achievement of this investigation 3.1.2.1. Self-assembly Fluidic self-assembly: There are wells on the targets substrates that trap micro LED devices and settle them down by their gravity. To attach to the substrates, the chips are stuck by alloy, which is molten in advance and cooled down after catching chips. The shape of the chips is changed into a circular design instead of a traditional rectangular shape with the aim not to concern the orientation of each chip.
- 13. 3. Achievement of this investigation 3.1.2.2. Pick and place apply silicon electrodes to remove devices from a source substrate to receiving substrate may be, but is not limited to, a display substrate, a lighting substrate, a substrate with functional devices such as transistors or integrated circuits (ICs), or a substrate with metal redistribution lines. The silicon substrates are able to be charged positively or negatively to pick or place the devices.
- 14. 3. Achievement of this investigation 3.1.2.3. Selective release The advantage of this technology is that a process of detection and repairing is involved, enhancing the field of micro LED products. This technique has a similar process as the pick and place, except the device can be removed to receiving substrate from the source substrate directly by photoetching. After detecting, we can just pick up the addressed MicroLED chips mapping to defect positions from the LED wafer, then selective mass transfer to the backplane. The whole detecting and transferring process are achieved by laser and related technology.
- 15. 3. Achievement of this investigation 3.2. Monolithic integration Since micro LEDs have been put into application, more and more challenges are keeping coming out, such as the process of mass transfer being exorbitant in cost or inconvenient in producing photolithography equipment such as AR glasses. Monolithic integration is a routine that layers of LED materials can be grown on device substrates directly by some kinds of wafer-level monolithic hybrid integration technologies. Applying this, we can produce micro LED screens on devices such as AR glasses directly without mass transfer.
- 16. 3. Achievement of this investigation 3.2.1. How to grow LED materials To avoid any damage to the GaN layer, monolithic integration can be adopted to circumvent dry-etching. Rather than dry etching, SiO2 microhole masks are made on the template by etching SiO2 film grown on the template. Epitaxy is able to take place in these microholes afterwards.
- 17. 3. Achievement of this investigation 3.2.1. How to grow LED materials Besides, photoetching is another way to realize direct growth. A layer of indium tin oxide (ITO, 115 nm) was deposited on the p-GaN layer by e-beam evaporation and patterned by wet etching in diluted aqua regia using photoresist as etching mask. Then the photoresist that protects materials beneath it was remained as masks to define individual micro- LEDs by dry etching GaN.
- 18. 3. Achievement of this investigation 3.2.2. How to colourization The quantum dots which absorb light and give out light of different colours are applied, to make blue LED red or green. The technique mainly uses atomized QDs ink by ultra-sonic/pneumatic and sprays uniform and size-controlled QDs materials with a sprayer and airflow control.
- 19. 3. Achievement of this investigation 3.2.3. How to restrain optical crosstalk The optical crosstalk between neighbouring micro-LED pixels can be a factor to decrease contrast, which is able to be suppressed by some ingenious adjustments for monolithic integration. To reduce the optical cross-talk effect, a mold with open windows and blocking walls is available. The window size here is the same as the micro-LEDs, and the unopened region forms the blocking wall that is just the shape of the trench between micro-LEDs.
- 20. 3. Achievement of this investigation 3.2.3. How to restrain optical crosstalk By applying inductively coupled plasma etching technology, we can dig gullies on the sapphire substrate as microreflectors, which reflect emitting lights and focus them. Each micro-LED pixel owns an individual sapphire microreflector that can directly concentrate rays from an emitting source.
- 21. 4. Conclusion In conclusion, we have made an introduction to micro LED’s manufacturing process. Especially, we present the mass transfer and monolithic integration technologies which are the keys to achieving micro LED volume production. On the one hand, monolithic integration technology may attract more researchers for its advantage in feasibility, on the other hand, the traditional mass transfer routine will be still developed for its market maturity.
- 22. 4. Conclusion As for mass transfer, pick and place is the mainstream method, developing by companies like Mikro Mesa, Cooledge, and Apple’s subsidiary corporation LuxVue. However, selective release and self- assembly is early-stage but potential respectively because of its repairing capability and high assembly efficiency.
- 23. 4. Conclusion However, in the aim to be applied in AR, micro LED’s chip size should be no larger than 5 microns. While smaller size leads to lower quantum efficiency and harder transfer methods so material scientists will keep discovering means to enhance EQE.
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- 25. ACKNOWLEDGEMENT
- 26. thanks
- 27. APPENDIX • Introduction of colorization 1.RGB three colors There are three colors LED in one unite. three colors of different light intensity make different colors light, which colorize the screen. However, due to the wavelength of LED’s light Is not always accurate, there might be errors in The colors. 27
- 28. APPENDIX • Introduction of colorization UV/blue LED’s light changes into other colors(such as red), after passing Through the buffer layer which is also Called color conversion layer. LEDs can be integrated in the way of RGB three colors. 28
- 29. APPENDIX • Introduction of colorization Create three plates with LEDs of different color, and then overlap their images in the use of a lens. Each of the plates is concolorous such as red or green or blue. 29