OLEDs (Organic Light-Emitting Diodes) are already being used in many mobile devices and TVs, and the next generation of OLED displays will be flexible and foldable. These foldable and flexible OLED displays tend to reduce the radius of curvature under 2.5R and require excellent encapsulation characteristics at thin thicknesses. To satisfy these requirements, multilayer barrier films are considered to be indispensable for the OLEDs. Unlike a single thin film such as SiO₂ and SiN_x, which is currently applied, the multi-layer itself can produce an antireflection film effect due to a difference in refractive index of each thin film in laminated thin films, thereby reducing diffuse reflection. Also It is effective for blocking ultraviolet rays and loss stress.^{[1], [2]}. Recently, The ALD (Atomic Layer Deposition) method is used for developing thin film encapsulation technology, due to advantages of self limiting surface reaction and controlling the fraction of reaction.

In this paper, SiO₂, SiN_x and multi-layer thin films were deposited on PEN (polyethylene naphthalate) films by using the NSi-01 single precursor by PEALD (Plasma Enhanced Atomic layer Deposition) technique at low temperature (90℃), and thin film thickness and refractive index were measured using a M2000D Spectroscopic Ellipsometer from Woollam. Also, the WVTR (Water Vapor Transmission Rate) characteristics of the deposited films were confirmed using a MOCON Aquatran 2. The refractive index of the SiO₂ and SiN_x film was 1.47 and 1.85 respectively, which confirmed the possibility of antireflection effect through the multi-layer thin film. The WVTR characteristics were measured according to the thickness of SiO₂ and SiN_x film over 100hrs. The both deposited SiO₂ and SiN_x thin film below 150Å showed poor or breakdown encapsulation property (Fig.1). In order to overcome this, the SiO₂ film and SiN_x film were stacked in order. The deposited multi-layer film showed excellent WVTR characteristics with a breakdown prevention of encapsulation property at thin thickness. Also, 2 to 3% of the carbon in the deposited SiNx film is expected to lower film stress and maintain the flexibility of the entire film (Fig. 2). ^[3] From this research, we confirmed the possibility of deposit multi-layer thin film consist of silicon oxide film and nitride film in one chamber using a one precursor by PEALD. Especially, multilayer type thin film deposition suggested the possibility of preventing reflection and water vapor simultaneously.

References

1. Hemant Kumar Raut,*a V. Anand Ganesh,a A. Sreekumaran Nairb and Seeram Ramakrishna*acd “Anti-reflective coatings: A critical, in-depth review” Energy Environ. Sci.,3784, 2011
2. S. Chhajed, M. F. Schubert, J. K. Kim, and E. F.Schubert, “Nanostructured Multilayer Graded-indexAntireflection Coating for Si Solar Cells with Broadbandand Omnidirectional Characteristics”, Applied Physics Letters, vol. 93, p. 251108, 2008
3. KwanDo Kim, SeokHee Jang, JongMin Kim* and SangMok Chang, “Characterization of Thin Film Passivation for OLED by PECVD” , Korean Chem. 575, 2012