Development of micro-vacuum transfer printing for flexible optoelectronics

Abstract

Inorganic micro light-emitting diodes (μLEDs) is one of the most promising candidate for a light source of next-generation optoelectronics such as ultrahigh-resolution displays for AR/VR applications, flexible displays, and optogenetics, owing to their excellent electrical/optical properties (contrast ratio, brightness, response time, power efficiency) and superior thermal/mechanical stability compared to conventional liquid crystal displays (LCDs) and organic light-emitting diodes (OLEDs), and quantum dot displays (QDs). To fabricate μLEDs-based flexible optoelectronics, micro-sized thin-film semiconductor chips have to be transfer-printed from the brittle growth wafer to deformable target substrates. Conventional transfer technologies such as elastomeric transfer, laser-based transfer, electrostatic/electromagnetic transfer, and fluidic self-assembly methods still have several issues including a need for additional adhesives, low transfer yields, misalignment, and chip damage or contamination. Here, the micro-vacuum assisted transfer printing process is developed with a vacuum transfer module, fabricated via laser-induced microelectromechanical systems (MEMS) technologies. The vacuum transfer module, composed of 20 μm-sized micro-hole (μ-hole) arrays with high aspect ratio and micro-channel (μ-channel) structure, is fabricated using a glass and quartz substrate by laser-induced etching (LIE) method. Ultrahigh adhesion switchability, which is 1,000 times higher than that of previous transfer technologies, is acheived by a pressure control of the vacuum transfer module during the transfer printing process. The μLED and Si arrays with a chip size of 80 μm are selectively and repeatedly transfer-printed from the mother wafers to flexible substrates with high transfer yield. The transferred microchip arrays are electrically interconnected with the bottom electrodes via anisotropic conductive films (ACF), realizing flexible optoelectronics with excellent electrical/optical characteristics and mechanical stability under repeated bending tests.