박리기술을 이용한 플렉서블 금속 필름의 형성 기구

Abstract

Flexible electronics have potential applications as large-area displays, solid-state lighting, energy harvesters, and sensors. Consequently, flexible substrates have been investigated for use in next-generation flexible electronics. Typically, many flexible devices are based on flexible plastic substrate such as polyethylene terephthalate (PET), polyether sulfone (PES), or polyimide (PI). However these substrates have critical issues of their weak stability, due to high water vapor permeation rate (10 ~ 10-1 g m-2 d-1 at 25 °C), and temperature limitation (< 350 °C) during fabrication of thin film transistors (TFT). Moreover, the difference of coefficient of thermal expansion (CTE) between plastic and Si base material causes cracks in devices during heating processes. To actualize flexible electronics, new flexible substrates and methods to fabricate them should be developed simultaneously. Therefore, as an alternative approach, metal substrates can be used to fabricate the flexible electronics with their durability, thermal stability and chemical stability for mass production. However, metal films have not been commercialized because their high surface roughness (generally Ra > 50 nm) causes contact resistance and leakage current in the electronic device, which is of prime challenge for real commercialization as flexible electronic device substrates. Such a challenge can be mitigated by peel-off a thin-metal foil from the mother substrates having a flat surface roughness. Chemical post-treatment could decrease the adhesion between the targeting metal layer and the mother substrate. Also, excimer laser has been used for metling the sacrificial layer underneath a transferring film or device. Even though a number of works on have been conducted, such adhesion weakening is achieved by etching a sacrificial layer, which has a risk creating undercuts to reduce the overal contact area. Moreover, such chemical peel-off and laser peel-off techniques have a limitation in mass production. Meanwhile, a number of direct peel-off (DPO) approaches have been limited within non-reactive metals based on van der waals bonding system. In this study, we demonstrated a novel physical mechanism for direct peel-off (DPO) at the Ti/In2O3 system, by explainnig the chemical evolution with peel-off behaviors. Thermodynamic reaction is induced at the metal/ceramic interface to bond the metal layer with the anion of the ceramic layer (especially the oxide) to induce a thermodynamically stable phase at the interface. In order to control peel-off behaviors, it is required to optimize the selection and deposition process of the metal material to control the thermodynamic interfacial reaction. We showed that the hydrophobicity of the flat n-type transtion metal oxide can lead to stable peel-off phenomenon. However, no studies have been conducted on the use of Ti as a functional layer for peel-off process. With the DPO process at the Ti/In2O3 system, the following were successfully achieved: (1) suitable adhesion for peel-off process; (2) defect-free morphology over entire surface; (3) very low surface-roughness. Consequently, we fabricated Extremely Flat (EF) Ti/Cu foil with Ra < 3 nm and evaluated its applicability to flexible electronics by demonstrating a large-area flexible OLED device on the EF Ti/Cu foil.