A New Double Laser Recrystallization Technique to Induce Ultra-Large Poly-Si Grains
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A New Double Laser Recrystallization Technique to Induce Ultra-Large Poly-Si Grains Minghong Lee, Seungjae Moon, Mutsuko Hatano1, 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 ABSTRACT A new double laser recrystallization technique that can produce lateral grains of tens of micrometers is presented. A nanosecond laser (excimer or Nd:YLF laser) and a pulse modulated Ar+ laser are used in the experiment. The effect of different parameters on lateral grain growth is investigated. These parameters include the time delay between the two lasers, the excimer laser fluence, the Ar+ laser power and the pulse duration. This process has wide process window and is insensitive to both the excimer laser fluence and the Ar+ laser power fluctuations. Preheating and melting of the a-Si film with the Ar+ laser before firing the excimer laser is found to be necessary for inducting lateral grain growth. The transient excimer laser irradiation is believed to generate nucleation sites for initiating the subsequent lateral grain growth. The solidification dynamics of the process is probed by high spatial and temporal resolution laser flash photography. A lateral solidification velocity of about 10 m/s is observed. INTRODUCTION In order to improve poly-Si grain size, uniformity and hence TFT device performance for advance flat panel display applications, several methods have been developed utilizing spatially selective melting and lateral temperature modulation [1-3]. In this investigation, a new recrystallization technique is employed to obtain lateral grains of tens of micrometers with two lasers. Specifically, this technique superposes a temporally modulated CW Ar+ laser and a spatially homogenized KrF excimer laser pulse to achieve lateral grain growth. This technique is different from the previous reported, in that instead of scanning an Ar+ laser beam [4], it utilizes a second laser of shorter pulse duration to trigger nucleation and promote the lateral growth at a stationary position. It will also be shown that the double laser technique has a wide process window and is insensitive to both Ar+ laser power and excimer laser fluence fluctuations. Although the interface kinetics in melting and solidification of silicon has been studied in theoretical modeling [5-6], it has not been visualize experimentally. In order to visualize the liquid/solid interface dynamics during the lateral grain growth process, high spatial and temporal resolution laser flash photography is used. EXPERIMENT The schematics of the double laser recrystallization and the laser flash photography experimental setups are shown in figure 1 (a) and (b), respectively. A KrF excimer laser (λ = 248nm, FWHM = 25ns) and an Ar+ laser (λ=514.5nm) were used in the double laser recrystallization experiment. The continuous Ar+ laser beam was temporally modulated by an acousto-optical modulator. A pair of knife-edges were placed downstream of th
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