Research on mechanical properties and fracture behavior of ALD thin films for flexible electronics

Abstract

With the recent commercialization of foldable smartphones and rollable TVs, the market's interest in various flexible devices is exploding. Many inorganic thin films have been widely used as encapsulation layers, transparent electrodes, dielectric layers in various flexible devices. Among various deposition methods of inorganic thin films, atomic layer deposition (ALD) attracts attention as an advanced deposition method due to multiple advantages such as precise thickness control, conformal deposition, and low-temperature deposition. However, the brittleness of the inorganic thin film may lead to mechanical failures such as cracking and delamination. Therefore, in order to prevent such mechanical problems, systematic and elaborate studies on the mechanical properties and fracture behavior of ALD thin films is essential. However, since indirect methods such as nanoindentation and bulge tests have been mainly used in previous studies, it was difficult to measure the inherent mechanical properties accurately. In particular, no systematic study was conducted on the elongation and strength, directly related to mechanical reliability. In this dissertation, the mechanical properties of ALD thin films were accurately measured by using a tensile test on the water surface. Studies were conducted on the effects of three factors on the mechanical properties and fracture behavior of the ALD thin films. First, the effect of the thickness on the mechanical properties of the ALD Al$_2$O$_3$ thin film was investigated, and a mechanism for the change in the mechanical properties was proposed. Also, the mechanical properties of the ALD Al$_2$O$_3$/TiO$_2$ nanolaminate according to the sublayer thickness were measured, and the fracture behavior of the nanolaminate was also analyzed. Through this, an optimal condition for the sublayer thickness at which the elongation is maximized was proposed. Finally, the degradation of the ALD Al$_2$O$_3$ thin film by the hygro-thermal environment on the mechanical properties was investigated. In addition, a single layer coating of organic materials was proposed as a method of preventing mechanical deterioration due to the hygro-thermal environment, and this method proved that it could effectively prevent the elongation degradation by the hygro-thermal environment. The results of this dissertation are expected to contribute significantly to designing mechanically-robust flexible devices containing ALD thin films, which cause mechanical failures.