Highly flexible cover window using ultra-thin glass for foldable displays

• Published in: Journal of mechanical science and technology Vol. 35; no. 2; pp. 661 - 668
• Main Authors: Ha, Myeong-Hun, Choi, Jong-Keun, Park, Byung-Min, Han, Kwan-Young
• Format: Journal Article
• Language: English
• Published: Seoul Korean Society of Mechanical Engineers 01.02.2021; Springer Nature B.V
• Subjects: Bend radius; Buffer layers; Control; Design optimization; Displays; Dynamical Systems; Electronic materials; Engineering; Finite element method; Flexibility; Industrial and Production Engineering; Light transmittance; Mechanical Engineering; Mechanical properties; Multilayers; Optical properties; Original Article; Substrates; Surface hardness; Vibration; Neutral plane; Foldable display; Cover window; Multilayer structure; Ultra-thin glass; Optical clear adhesive
• ISSN: 1738-494X; 1976-3824
• DOI: 10.1007/s12206-021-0126-y

The market for next-generation flexible displays is rapidly expanding with the launch of foldable electronic products. Consequently, the flexible electronic material and device industries are continuously growing. Specifically, the cover window used in foldable displays is a very important component that must have excellent optical, physical, and mechanical characteristics while withstanding high external stresses caused by the bending radius, unlike the existing rigid-type cover windows. Considerable research efforts have been dedicated toward improving their performance. In this study, a cover window substrate for a foldable display having high flexibility was developed. To this end, ultra-thin glass (UTG) with excellent flexibility and transparency was used to overcome the low surface hardness of the typical polymer substrate used in existing foldable substrates. In addition, for efficient stress control, the design of the multilayer structure was optimized by generating multiple neutral planes through the use of an optical clear adhesive (OCA) buffer layer. The structure of the cover window was designed using the finite element simulation technique, and actual samples of the cover window with the optimized structure were produced to evaluate their physical, mechanical, and optical characteristics. As a result, the optimized foldable cover window showed a surface hardness of 9 H and a light transmittance of 90 %; especially, it exhibited an excellent bending reliability with 200000 bending repetitions without failure at the small bending radius of 1.5 R.