Invited Paper TF-ThM1
Thin-Film Encapsulation Based on ALD Technology for Organic Light-Emitting Diodes

Thursday, November 2, 2017, 8:00 am, Room 21

Thin-film encapsulation of organic electronic circuits like Organic Light-Emitting Diodes (OLED) consists in the addition of vacuum deposited thin mineral barrier directly onto the organic circuit. The main challenge today is to achieve high encapsulation levels with WVTR ~ 10^-6 g/m²/day. The ALD technology is well-suited to reach that goal because the technology allows the deposition of defect-free thin barrier films, mainly of Al_xO_y. However, the main limitation of the ALD technology is the relative stability of the Al_xO_y layer when it is exposed to hot humid atmospheres. Due to its high C and H atoms content, resulting from the low temperature deposition used to deposit onto the fragile circuits (maximum process temperature allowed for OLED ~ 100 °C), the Al_xO_y layer corrodes very quickly and loses its barrier properties prematurely. An additional passivation is therefore mandatory in order to protect the Al_xO_y from water condensation onto its surface. Different works have illustrated the benefits of depositing an additional ALD layer on top of the Al_xO_y, an additional PECVD or PVD layer on top of the Al_xO_y or using hybrid ALD/MLD (Al_xO_y/metalcone) nanocomposites. In each case, the stability of the aluminum oxide is improved because it is not exposed directly to moisture condensation. Alternatively for thin-film encapsulation purposes, other materials of interest can be deposited by low-temperature ALD: ZrO₂, TiO₂, SnO₂, SiN_x, ZnO, or ZnO:Al (AZO). After a review of solutions to stabilize Al_xO_y films, performances of new single barrier materials will be described in this work and compare to Al_xO_y. It is well-known also that the quality of the encapsulation relies also on the cleanliness of processes. Defects can be described as exogenous particles but they can be seen also as the circuit’s surface topography, as well as its surface nature. Getting rid of defects is an important feature for thin-film encapsulation. ALD can provide high surface conformity and can be deposited almost onto every surface. However, as thin-film barriers deposited by ALD remain very thin (dozens of nm), it is mandatory to grow them onto a particle-free surface to ensure a reliable encapsulation. Otherwise, particles can create pinholes that act as a pathway for moisture to reach the fragile device underneath. In the case of OLEDs, black spot features then appear. Other encapsulation processes, based on other deposition technologies like PECVD or PVD, described the use of thick planarization layers to fix the particle issue. Similar strategies can be applied with ALD. The technology developed at CEA-Leti will be presented in the second part of this work.