2017-2018 is a pivotal period for the display industry. Companies are scrambling for new display technologies to bring better user experience with devices that sport them. Micro LED display technology is believed to combine the LED and display industries together to create a supply chain rolling out screen products more appealing, power-saving, and refined. This chain is even anticipated to replace a part of the current LCD chain worth USD 30-40 billion in the future (see here). The Korean Institute of Machinery and Materials (KIMM) revealed two months ago a refined roll-transfer technology which enables a successful mass Micro LED transfer onto flexible target substrates.
Korea is as of now synonymous with high end display technology and a country that probably holds most of the display-related patents. It has undefeatable OLED giants Samsung and LG Display, on which even the iPhone maker has become reliant. In KIMM, a government-funded research institute under the Ministry of Science and ICT, the Department of Applied Nano Mechanics studies the mechanical behaviors of nanoscale structures. LEDinside got in touch with the department’s head Dr. Jae-Hyun Kim to get more details about the roll-to-plate transfer technology.
|The developing team at the Department of Applied Nano Mechanics (Image: KIMM)|
A Potential Game Changer: the Roll-to-plate Transfer Technology
Approaches companies currently use to transfer a large amount of Micro LED chips onto the receiver substrates are pretty much stamping via electrostatic forces, electromagnetic forces, the van der Waals forces, or mechanical deformation. Fluid assembly and positioning is also heard to be an option. Based on the understanding of the nanoscale mechanical behaviors, Dr. Kim and his group developed a roll-based transfer technology for transferring nanoscale objects from a donor substrate to a target substrate with high yields and productivity. It has been applied to the transfer of backplane thin film transistors, silicon memory, III-V solar cells, and, of course, Micro LEDs.
|The roll-based transfer machine (top); a part of the roll transfer process (bottom) (Images: KIMM)|
Inspired by the PDMS stamp method, the team create disposable transfer films and coat the roll stamp with them to transfer components. The roll stamp first picks up an array of thin film transistors and places them on a temporary substrate. With the TFT elements on it, the substrate is then delivered to go through the second transfer, in which Micro LED chips are lifted off from a semiconductor wafer and precisely located on the substrate to connect with previously deposited transistors after a simultaneous solder bonding at room temperatures. Carrying arrays of two interconnected components, the entire structure is then moved onto a target substrate to become an active matrix Micro LED display.
During the process, the pressure between the roll stamps and the horizontal delivering plate is under thorough control to prevent unwanted stretch and damage of the electronic components. The rotational movement of the roll stamp should be in sync with the translational movement of the delivering plate to balance the downward vertical force given by the roll stamp and the horizontal force. The machine is also calibrated to achieve a high precision for aligning and overlaying transistors and Micro LEDs. “The precision should be able to realize a considerable high resolution of a pixel size 10 times less than that of Micro LED,” said Dr. Kim. In addition, considering properties of adhesion of Micro LED and other materials, the team optimize the contact pressure for the pick-and-place process.
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|Micro LED display production using the roll transfer technology (Video: KIMM)|
Leveraging properties of the transfer films and the equipment, the whole transfer process, according to Dr. Kim, can transfer more than 10,000 Micro LEDs per second, while incumbent die bonders can only mount a very limited amount of chips, 1-10 Micro LEDs, at a time. “It takes more than a month to produce a 100-inch full HD 2-megapixel digital signage with a die bonder, but that can be done in less than an hour with the roll transfer process,” explained Dr. Kim. “The speed of the production has been dramatically improved and the production cost can be incomparably low.”
Will Flexible Micro LED Displays See Growth first?
Dr. Kim continued, “The roll transfer technology can transfer Micro LEDs of less than 100um in size and thin film LED devices of less than 10um in thickness. Such a reliability and precision could enable flexible, stretchable and light displays.” In the name of the project ‘The Development of Infrastructure for Flexible Devices with High Performance Using Nanomaterials’, KIMM intends to employ this technology in the manufacturing of digital signage, automotive transparent displays, and stretchable Micro LED lighting for medical applications, wearable devices, and smartphones.
|The department head Dr. Jae-Hyun Kim is powering a Micro LED display (Image: KIMM)|
In comparison to entrants in the Micro LED market that are currently working towards the mass transfer of LEDs sized around 150 microns, KIMM’s roll transfer technology indeed has an edge in terms of producing sophisticated Micro LED displays with extremely high pixel density. “Producing finer flexible Micro LED displays within a relatively short time will be the biggest advantage for the roll-to-plate transfer technology to differentiate itself from solid-based transfer methods,” commented Simon Yang, Assistant Research Manager at LEDinside.
With over 20 patents regarding the process, equipment, and the disposable transfer films, KIMM lately licensed the roll-based transfer technology for digital signage applications to Lumens, a Korean company engaged in the manufacture of LED products. However, the technology licensed includes a process to transfer thick Micro LEDs (100um in thickness) onto a rigid PCB substrate with solder paste. The institute will soon be licensing the technology able to transfer thinner and/or smaller Micro LEDs (the thickness is less than 10 um and lateral size less than 100 um) onto a flexible and/or transparent substrate.
Before KIMM’s announcement of the roll transfer technology, the industry anticipated displays and projection modules featuring 150-micron LEDs will be first available on the market as early as 2018. When the mass transfer for LEDs of this size matures, market entrants will then invest in processes for making smaller products. “Once the roll transfer technology is optimized and licensed for the production of Micro LED displays, the market might start to see a different dynamic. It could be expected to bring down the overall cost if both the unit per hour (UPH) and the yield of the transfer process improve. When the cost reduces, it is more likely to expedite the mass production of flexible Micro LED displays that can be used in a myriad of applications. Flexible displays have more potential than rigid ones do, which means a bunch of business opportunities will be waiting ahead,” analyzed Yang.
(Dr. Jae-Hyun Kim, Head of the Department of Applied Nano Mechanics, was interviewed by Roger Chu, Joanne Wu, and Evangeline Huang. The article was written by Evangeline Huang, Editor, LEDinside)
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