Atomic-scale stress modulation of nanolaminate for micro-LED encapsulation

Micro/nano-LEDs for augmented reality (AR) and virtual reality (VR) applications face the challenge that the edge effect in micro-LEDs becomes significant as the size of devices shrinks. This issue can be effectively addressed through thin-film encapsulation, where zero stress of the thin film is a crucial factor, apart from the barrier property. Herein, a stress-modulation strategy was developed through a binary-cycle atomic-layer deposition (ALD) process combining PEALD SiO₂ (compressive stress) and thermal ALD Al₂O₃ (tensile stress) in the same process window. The hybrid ALD process allows avoiding extra thermal stress generation and enables precise modulation of the atomic-scale thickness, thereby allowing the fabrication of nanolaminates with modulated stress. The optical nanolaminate developed herein achieved a stress level of near-zero, representing one of the best among reported studies. The structural design, characterized by a high-low refractive index, tortuous permeation path, and ultra-thin thickness, remarkably improved the optical transmittance and barrier properties (8.68 × 10^-6 g m^-2 day^-1) of the nanolaminate. Moreover, the micro-LED encapsulated with SA_2/1 exhibited excellent stability under thermal cycling, damp heat, and applied stress conditions. The mechanical stability of the nanolaminate was due to the strong interaction between Si-O and Al-O and the abundance of Si-O-Al bonding in the interface. Overall, the ALD-coating process provides a new avenue for accurately controlling the stress on nanolaminates, and has potential application to bolster the reliability of optoelectronic devices.