Flexible electronics and displays are emerging fields made possible by advancements in materials and manufacturing. They can be applied to skin electronics, wearables, and curved screens.
Micro-LEDs are considered the most promising candidate for the next-generation display technology. They have excited the research community and the industry because of their efficiency, response time, brightness, reliability, and lifespan.
Potential of Micro-LEDs
Micro-LEDs are a light source that utilizes inorganic LED chips measuring less than 100μm. They have garnered attention for their superior electrical and optical properties, unlike existing displays such as LCD, QD, and OLED.
In commercializing micro-LEDs, transfer printing is used to rearrange micro-LED dies from a growth substrate to the final substrate with precise alignment. Mass transfer with high controllability and selectivity is also essential to gain the desired layout and precise alignment required to reduce the cost of the final product.
However, previous transfer methods pose numerous challenges, including chip damage, poor transfer performance, and misalignment. There are different transfer techniques like laser assist, stamping, and fluidic assembly, but they still face manufacturing issues. Micro-vacuum force has also been utilized, but there is still a need for better selective control.
Universal Selective Transfer Process
To address this concern, experts from the Korea Advanced Institute of Science and Technology (KAIST) developed a micro-vacuum-assisted selective transfer printing technology (μVAST). The study "Universal, selective transfer printing via micro-vacuum force" describes a method that transfers micro-LED chips in large numbers using adjustable micro-vacuum suction force.
Led by KAIST professor Keon Jae Lee, the research team used a laser-induced etching method on glass substrates. Then, they connected the laser-drilled glass to vacuum channels and controlled the micro-vacuum force at the desired hole arrays to selectively pick up and release micro-LEDs.
After that, a vacuum-controllable module made of microchannels above micro-holes was constructed with microelectromechanical systems (MEMS) technology. This enables the researchers to control the micro-vacuum force for the pick-and-place of microchips selectively.
The transfer mechanism and reliability of the μVAST were investigated theoretically using a finite element method (FEM) simulation. In this method, different inorganic thin-film semiconductors were transfer-printed from the donor wafer to the final substrates using micro-vacuum suction.
The simulation realized selective transfer printing and heterogeneous integration with diverse device shapes. The scientists also demonstrated a flexible, high-performance micro-LED device on a polyimide substrate with an average transfer yield of 98.1%. It also shows mechanical stability and uniform optical power intensity. Additionally, multiple selective transfers were implemented by two separate vacuum channels with independent pressure control.
It was found that the high adhesion switchability of the μVAST system facilitates the pick-up and release of thin-film semiconductors without additional chip or adhesive damage. It achieves a higher adhesion switchability than previous transfer methods, allowing the assembly of micro-sized semiconductors with different heterogeneous materials, shapes, sizes, and thicknesses onto arbitrary substrates with high transfer yields. This means the micro-vacuum-assisted transfer process offered a tool for large-scale, selective integration of microscale, high-performance inorganic semiconductors.
Currently, the team is investigating how transfer printing of commercial micro-LED chips with an ejector system can commercialize next-generation displays like wearable phototherapy patches, flexible and stretchable devices, and large-screen televisions.
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