Thin Film Encapsulation of Aluminum Oxide by Ozone-based Atomic Layer Deposition

Abstract

The environmental instabilities of organic light emitting diodes (OLEDs) remain a primary obstacle to commercialization. The development of an effective moisture permeation barrier has been a key issue for OLEDs. Atomic layer deposition (ALD) has emerged as a promising method that meets the water vapor transmission rate (WVTR) requirement of 1.0 × 10-6 g/m2/day because it enables the deposition of ultra-thin, uniform, and conformal films. Nevertheless, the effectiveness of conventional ALD technique in thin film encapsulation (TFE) of OLEDs is limited by absence of oxygen precursor with high oxidation potential, low deposition rate, presence of defects in single layer. Therefore, it is indispensable to develop an alternative ALD process suitable for TFE of OLEDs. In this study, the moisture barrier properties of Al2O3 multi-density layer structure deposited by ozone-based atomic layer deposition were suggested. This structure uses one material and consist of at least two layers with different densities. In order to optimize the process of Al2O3 multi-density layer structure, three studies were preceded. First, the influence of ozone (O3) concentration on moisture barrier properties of Al2O3 single density layer was studied. Al2O3 prepared at higher O3 concentration (≥300 g/m3) showed better moisture barrier properties than those at lower O3 concentration (≤200 g/m3) due to increase of amounts of reactive oxygen species. Next, thickness dependence on moisture permeation performance of Al2O3 single density layer was examined. Al2O3 thin films had two distinctly different permeation regimes, which confirmed the existence of a critical thickness (25 nm) at which the WVTR decreased drastically. Finally, the effect of scan speed on the moisture barrier properties of Al2O3 single density layer was investigated. Despite the increase of scan speed (from 100 to 800 mm/s), there was little change in the WVTR value of Al2O3 (from 1.4 × 10-3 to 3.0 × 10-3 g/m2/day). Based on above-mentioned results, Al2O3 multi-density layer structures with a thickness of 100 nm was comprised of two alternating 25-nm-thick Al2O3 thin films with different densities. The O3 concentration was maintained at 300 g/m3. The substrate was placed on the substrate carrier, which was moved at a scan speed of 800 mm/s using vacuum single linear motion. The lowest WVTR of the multi-density layer structure was 5.3 × 10-5 g/m2/day, which is two order of magnitude less than WVTR for the reference single density layer. This improvement is attributed to the location mismatch of paths for atmospheric gases. This mechanism is analyzed by high resolution transmission electron microscopy and X-ray photoelectron spectroscopy. These results confirmed that the multi-density layer structure exhibits very good characteristics as a moisture permeation barrier via location mismatch of paths for oxidative species.