Compared to conventional light sources, such as OLEDs and LCDs, micro-LEDs — inorganic LED chips Mass transfer with high selectivity and controllability is essential for rearranging micro-LED dies from a growth substrate onto a final substrate with the desired layout and precise alignment needed to reduce the final product costs. However, existing transfer methods have many challenges, including the need for additional adhesives, misalignment, low transfer yield, and chip damage. A conceptual drawing of micro-vacuum assisted selective transfer printing (μVAST). Courtesy of KAIST. To address this, researchers at Korea Advanced Institute of Science and Technology (KAIST), led by professor Keon Jae Lee, developed a micro-vacuum-assisted selective transfer printing technology (µVAST). The technique transfers micro-LED chips in large numbers with an adjustable micro-vacuum suction force. Using a laser-induced etching method on glass substrates, µVAST produces μ-hole arrays (20 to 50 μm in size) with a high aspect ratio greater than 5, at fabrication speeds of up to 7000 holes per second. The laser-drilled glass is connected to vacuum channels and controls the micro-vacuum force at the desired hole arrays to selectively pick up and release micro-LEDs. A vacuum-controllable module, composed of micro-channels on top of micro-holes, was constructed with microelectromechanical systems (MEMS) technology to selectively control the micro-vacuum force for the pick-and-place of microchips. The module enables selective modulation of the micro-vacuum suction force on microchip arrays. Using pressure control during the micro-vacuum transfer procedure, μVAST achieved selective and massive transfer of thin-film semiconductors with high adhesion switchability of 3.364 × 106 — three orders of magnitude higher than other transfer technologies. Universal transfer printing of thin-film semiconductors via micro-vacuum assisted selective transfer printing (μVAST). Courtesy of KAIST. The researchers investigated the transfer mechanism and reliability of the μVAST theoretically, using a finite element method (FEM) simulation to adjust the adhesion force balance during the pick-up and release steps. They transfer-printed various inorganic thin-film semiconductors, including silicon and III-V semiconductors, from donor wafers to final substrates via micro-vacuum suction and without any additional adhesives. Using μVAST, the simulations realized heterogeneous integration and selective transfer printing with diverse device shapes. The researchers also demonstrated a high-performance, flexible micro-LED device on a polyimide substrate with an average transfer yield of 98.1%, uniform optical power intensity, and mechanical stability. Multiple selective transfers were implemented by independent pressure control of two separate vacuum channels. The μVAST system’s high adhesion switchability facilitates the pick-up and release of thin-film semiconductors without additional adhesives or chip damage. The µVAST achieves a higher adhesion switchability than previous transfer methods, enabling the assembly of micro-sized semiconductors with various heterogeneous materials, sizes, shapes, and thicknesses onto arbitrary substrates with high transfer yields. “The micro-vacuum-assisted transfer provides an interesting tool for large-scale, selective integration of microscale, high-performance inorganic semiconductors,” Lee said. Lee’s team is currently investigating the transfer printing of commercial micro-LED chips with an ejector system for commercializing next-generation displays such as large-screen TVs, flexible and stretchable devices, and wearable phototherapy patches. The research was published in Nature Communications (www.doi.org/10.1038/s41467-023-43342-8).
