Electron Beam Induced Ductile Enhancement and Large Strain Plasticity of Cu Thin Films

Abstract

As flexible devices become a new paradigm of electronic devices in the future, researches for improving the reliability of flexible devices are attracting attention. Since the reliability of the flexible device is closely related to the reliability of the metal thin film underlying the electrical characteristics of the device, the reliability improvement of the metal thin film leads to the improvement of the reliability of the flexible device. Unlike metal thin films deposited on conventional rigid and unmodified substrates, metal thin films deposited on flexible devices are exposed to continuous mechanical deformation. The destruction of the metal thin film due to the mechanical deformation soon leads to the deterioration of the electrical properties and causes the destruction of the device. Therefore, it is essential to understand the development of metal thin films with excellent mechanical reliability and the deformation mechanism to make them possible. The rapid destruction of the metal film is caused both in material and structural aspects. In the material aspect, plastic deformation is difficult because the displacement of the dislocations within the crystal is limited due to the reduction of the grain size due to the thinning of the material. In addition, the number of dislocation sources is also small, and it is impossible to generate dislocations continuously for plastic deformation. Therefore, a material having a small grain size does not undergo plastic deformation and undergoes a straight failure after elastic deformation. On the structural level, on the other hand, the dislocations in the material easily escape to the surface therefore plastic deformation is inhibited, as well as stress localization at the curvatures and defects of the surface causes rapid destruction of material. The maximization of the surface effect can be solved to some extent by a method of increasing the thickness of the structure by depositing the thin film on the substrate. In this case, the fracture mechanism of the metal film is affected by the difference in the mechanical properties of the two materials, since it must move with a flexible substrate having a lower modulus and a higher elastic limit strain than the metal film. Particularly, the difference in mechanical properties causes delamination of the metal thin film during tensile deformation. Since the delaminated metal thin film follows the deformation in the state of free-standing film, delamination causes destruction of the metal thin film. Therefore, in order to design a metal thin film having excellent mechanical reliability, it is necessary to design a structure that suppresses delamination. When the metal thin film is uniformly deformed along the flexible substrate without separation, the plastic deformation is evenly caused, and deformation can be performed without destruction. In addition, when such a structure is made, the potential must be steadily supplied to the inside of the metal so that the plastic deformation can be made steadily. That is, it is necessary to design a microstructure capable of continuously applying dislocations to the inside. For this purpose, a comprehensive understanding of how the microstructure of the thin film changes during tensile deformation should be given priority. However, since the grains existing in the thin film are smaller than the resolution of the electron backscattering diffraction (EBSD), which is a commonly used microstructure analysis instrument, there is a problem that it is difficult to analyze the microstructure with the existing EBSD. In this study, the design of the structural layer to improve the reliability of the copper thin film, which is a typical conductor used in electronic devices, was conducted through a new method called electron beam irradiation. When the electron beam is irradiated onto the copper thin film, the adhesion between the copper thin film and the flexible substrate is improved, and the peeling of the copper thin film is suppressed. Therefore, the fracture of the thin film due to the peeling was suppressed, so that the surface showed almost no crack even under the tensile strain of 30 %. Improvement of interfacial adhesion of electron beam irradiation was confirmed by nanoscratch test and nanoindentation test, and it was confirmed that the flexible substrate deformed by electron beam causes improvement of adhesion. Based on that the excellent mechanical properties lead the excellent electrical properties, it is possible to fabricate a phosphorescent organic light-emitting diode (OLED) that does not break down in the irradiated area under deformation, and it can be used as a patterning technology for fabricating a flexible device and a responsive device in the future The microstructure deformation mechanism of copper thin films was also investigated by mechanical deformation of copper thin films. For microstructure analysis of grains of size near 100 nm, microstructure analysis was carried out using ASTARTM, a microstructure analysis technique based on a transmission electron microscope instead of the conventional EBSD. As a result, it was confirmed that the high density twin boundary existing inside the copper thin film helps to relieve the stress and strain energy inside the material through the microstructure change rather than the crack. The twin boundaries not only cause coarsening of grains in the process of propagation between grains, but also cause refinement of grains by making twin boundary - dislocation structure through interaction with dislocations generated at twin boundaries. Through the microstructural change, the copper thin film irradiated with the electron beam could continue the plastic deformation continuously without breaking even under the high tensile strain. In this study, it is shown that the microstructures can be improved by plastic deformation if the steady dislocation supply and strain localization are solved even in materials with fine microstructure, which are known to be brittle. Based on the work, the design criteria of metal thin film materials to improve the performance and reliability of flexible devices are presented.