Evaluation of polycrystalline silicon after Excimer laser annealing by Retardation measurement method Nakcho Choi, Eungtaek Kim, Kie Hyun Nam, Deokhoi Kim and Beohm Rock Choi Samsung Display Co., Ltd., Yongin-City, Korea Abstract The excimer laser annealing (ELA) process is one of the important processes in Low Temperature Polycrystalline Silicon (LTPS) and the optimization of laser intensity and scan pitch in ELA process are the key technology to make good performance in OLED panel. However, the visual inspection through human eye has been used to find the Optimum ELA energy density (OPED) because of the fast inspection speed and accuracy. As the size of glass in LTPS is increasing, the need of a new inspection method has been raised. So the measurement of retardation which has been used in liquid crystals’ cell gap measuring was applied to evaluate the degree of crystallization in polycrystalline silicon(p-Si) made from the ELA process for the first time. The origin of the retardation is estimated as a Laser-induced periodic surface structure (LIPSS). Author Keywords Excimer laser annealing (ELA), Laser-induced periodic surface structure (LIPSS) 1. Introduction After the study of p-Si using ELA [1] James S. Im and H. J. Kim discussed how grain growth and abrupt grain size decrease with ELA energy. [2] He explained that to maximize the average grain size, it is necessary to irradiate laser to amorphous silicon layer with a sufficient amount of energy so that a completely non-nucleated nucleus is present in the lower part of the a-Si. Huck Lim et al. [3] and James B. Boyce et al. have shown that the intensity of the (111) peak of Si and the grain size and mobility are proportional. [4] However, since such an assay method for destroying or damaging the substrate is not suitable as a real-time inspection method in the ELA manufacturing process, the inspection used in manufacturing is a visual test in which inspectors watch an image reflected by a light diagonal to a periodic protrusion at Optimized Process Energy Density (OPED) was determined by observing vertical line stain and reflectance uniformity. In this paper, we introduce optical retardation analysis method which has physical meaning of alignment of Si lattice in protrusions as well as has quantitative values 2. Experiment and Result Transistor (TR) design is decided according to the resolution and requirement characteristics of AMOLED products. Due to the periodicity between the pitch of TR and the pitch of ELA, moiré stains are generated, so the ELA scan pitch is limited. Therefore, the energy of the OPED region during the ELA process differs when the pixel pitch is changed for each product. In general, the shape of the Laser-induced periodic surface structure (LIPSS) is shown in Figure. 1. Regular periodic intervals exist in the long axis direction of the laser, but there are irregular intervals in the scanning direction. This is explained by H. M. van Driel [5] and S. E. Clark [6], which is generated by p- polarized laser and s-polarized laser, respectively. (a) (b) Figure 1. SEM image of a Laser-induced periodic surface structure (a) tilted (b) top In this paper, we propose a hypothesis that micro-stress induced retardation occurs when Si is melted and crystallized, and that this is concentrated in the vicinity of the hillock and has a large phase difference. 2.1. Polarizer Micro Scope (POM) Generally, Dark field reflection microscope is used to observe the uniformity of the protrusions after ELA. In particular, stripe strain in the scan direction occur at intervals of about 1.5 to 3 μm, which can be observed by the reinforcement and destructive interference of light reflected from the periodic protrusions. In order to observe the striations more highly, Paul C. van der Wilt reported that polarizing microscopy is preferred and that the brightness of blue light is similar to grain size according to ELA Energy. [7] However, he did not explain why the polarized light microscope image is good to observe the striations on p-Si, but In this paper, I proposed that this is because of the optical anisotropy of poly crystalline silicone that amorphous silicon does not have. In Figure. 2(a), the a-Si region without laser irradiation is black in the cross polarizers, whereas the p-Si region is brightly lighted, which means that retardation occurs in the crystallization process. Also, when the sample of the p-Si region was rotated, it was bright and dark, which was the same phenomenon as when the liquid crystal molecules were rotated on the aligned LCD in Figure. 2(b),(c). This is due to the equation (1) that the transmittance varies with the angle between the slow axis of the anisotropic molecule and the transmittance axis of polarizer with the transmission axis of the orthogonal polarizer. In p-si, the scan direction of the laser is slow axis.   2 2 Re sin 2 sin 2 1             T (1) axis of molecule and transmittance axis of polarizer, retardation, and wavelength, respectively. [8] As rotating sample at the fixed polarizer, POM image at 0 degree of sample (b) and POM image at 60 degree of sample (c) ISSN 0097-996X/19/4802-0885-$1.00 © 2019 SID Late-News Paper 62-4 / N. Choi SID 2019 DIGEST • 885