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

Abstract

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 SA2/1 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.