Effect of Excimer Laser Fluence Gradient on Lateral Grain Growth in Crystallization of a-Si Thin Films
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Effect of Excimer Laser Fluence Gradient on Lateral Grain Growth in Crystallization of a-Si Thin Films Minghong Lee, Seungjae Moon, Mutsuko Hatano1, Kenkichi Suzuki2, and Costas P. Grigoropoulos Department of Mechanical Engineering, University of California, Berkeley, CA 94720-1740, U.S.A. 1 Hitachi Laboratory, Hitachi Ltd., Tokyo 185-8601, JAPAN 2 Electron Tube & Devices Division, Hitachi Ltd., Mobara 297, JAPAN ABSTRACT In order to clarify the relationship between excimer laser fluence gradient and the length of lateral grain growth, the laser fluence is modulated by a beam mask. The fluence distribution is measured by using a negative UV photoresist. The lateral growth length and the grain directionality are improved with increasing fluence gradient. Lateral growth length of about 1.5 µm is achieved by using a single laser pulse without substrate heating on a 50 nm-thick a-Si film by enforcing high fluence gradient. Electrical conductance measurement is used to probe the solidification dynamics. The lateral solidification velocity is found to be about 7 m/s. INTRODUCTION Conventional excimer laser crystallization (ELC) can produce grains of hundreds of nanometer in size depending on the a-Si film thickness [1]. However, the processing window for the conventional technique is narrow because large grains can only be obtained in the so-called superlateral growth (SLG) regime [2]. In addition, the grain size produced by the conventional ELC is highly non-uniform with randomly oriented grain boundaries which results in nonuniform device characteristics. Therefore recent research efforts on ELC have been focusing on developing spatially and directionally controlled crystallization methods. Several methods have been shown to give laterally oriented grain growth. These methods include the use of beam mask [3], diffraction mask [4], anti-reflective coating [5], phase shift mask [6], and the interference effect induced by a frequency-doubled Nd:YAG laser [7]. The working principle in all these techniques relies on shaping the laser energy profile that is irradiated onto an a-Si sample. In order to induce lateral grain growth, a fluence gradient must be enforced such that the a-Si film completely melts at the area exposed to higher laser fluence and partially melts at the adjacent area exposed to lower laser fluence. Under this condition, grains grow laterally towards the completely molten region. However, relationship between fluence gradient and lateral growth length has not been demonstrated. The purpose of this study is therefore to experimentally clarify this relationship by shaping the excimer laser beam via a beam mask to produce the desired fluence gradient. Electrical conductance measurement was also performed to probe the solidification dynamics and estimate the lateral solidification velocity. A solidification model is proposed to qualitatively explain the grain microstructure produced by the crystallization process.
Q7.6.1
EXPERIMENT The excimer laser used in the crystallization experiment is a KrF excimer las
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