Development of Highly Stretchable and Impermeable Thin Film Encapsulation by Applying Wavy Structure to Thermally Grown Silicon Dioxide

Department of Materials Science and EngineeringIn recent, Stretchable displays are being actively researched as a next-generation display following flexible displays. Organic light-emitting diodes (OLEDs) has excellent display performance and a simple and thin device structure, making it suitable for use in flexible displays. However, encapsulation materials with water vapor transmission rate (WVTR) below 10-5 g/m2/day must be used for long-term reliability of OLEDs, but these encapsulation materials are consisting of an amorphous inorganic thin film with low elastic deformation limit and brittle fracture, such as SiO2, Al2O3, SiNx. Thus, development of stretchable encapsulation should be preceded to develop a stretchable display. Fortunately, stretchability can be increased by structure design of material. These stretchable structures allow large deformation of system under small local strain below the elastic deformation limit of constitutive material. Among various stretchable structures, a three-dimensional wavy structure is best structure for stretchable encapsulation materials because it is impossible to have a structure with opened area that can be a fast water vapor permeation path in a large area. This research aims to develop the high stretchable and ultra-low permeable encapsulation material was developed by applying a wave structure to thermally grown silicon dioxide (SiO2). Thermally grown silicon dioxide deposited by oxidation of single crystalline silicon wafer at 1100??? has amorphous structure, high density, and no defects. Thus, it has the high fracture strength, high elastic deformation limit and the superior barrier properties among inorganic thin films. Furthermore, various wavy structures can be applied to the thermally grown silicon dioxide, through surface texturing of the silicon wafer. To apply a wavy structure to silicon dioxide, a process for etching a silicon single crystalline wafer in a wavy structure was developed. By controlling the size of the wavy structure and the thickness of SiO2, a stretchable encapsulation material with a stretchability of up to 20% and WVTR of 10-6 g/m2/day was carried out. By theoretically analyzing the mechanical behavior of wavy structured film, the universal deformation model was carried out. This model expects the stretchability depending on the characteristics of wavy structured inorganic thin film. The wavy structured SiO2 with the optimized structure was fabricated by designing the most optimal wavy structure and film thickness for high stretchability and ultra-low permeability. Through comparing the WVTR at acceleration condition of the wavy structured SiO2 before and after cyclic deformation with 90% of stretchability during 1000 cycles, it was confirmed that the barrier properties and reliability of the wavy structured SiO2 at the cyclic deformation. Furthermore, the barrier properties and lifetime of barrier at atmospheric condition were discussed by Arrhenius approach.clos