Mechanism of TCO thin film removal process using near-infrared ns pulse laser: Plasma shielding effect on irradiation direction

Substrate side irradiation is widely used for a thin film removal process because high absorption at the film/substrate or film/film interface leads to complete isolation of thin film by single shot irradiation of laser pulse with low energy. However, in the transparent thin film removal process, large thermal expansion or local phase change at the interface cannot be created by substrate side irradiation because of its large optical penetration depth compared to its small thickness. Nevertheless, substrate side irradiation works obviously for single shot film isolation process compared to film side irradiation, and the mechanism of the process was not clear in terms of difference in the irradiation direction. In order to investigate the effect of the irradiation direction, this study focused on the transient interaction between the material and nanosecond laser pulse. Experimental results showed that film was thermally ablated. Variation of temporal profile of nanosecond laser pulse during the process was experimentally investigated to detect plasma shielding. Pulse width and energy transmittance of transmitted pulse decreased by plasma shielding as pulse energy increases regardless of irradiation direction. In addition, temperature distribution in the film during the process was investigated using a 2-dimensional thermal model, which accounts for melting, vaporization, and laser induced plasma shielding. Calculated temperature distribution was used to support the scenario of the process mechanism which was investigated in the experiments. Our findings demonstrate that laser induced backward ablation is a single shot TCO film removal mechanism, and plasma shielding is dominant factor to interrupt absorption of beam thorough the film in the film side irradiation process. (C) 2016 Elsevier Ltd. All rights reserved.