A New Poly-Si TFT Employing XeCl Excimer Laser Annealing with Pre-patterned Al Laser Beam Mask
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A New Poly-Si TFT Employing XeCl Excimer Laser Annealing with Pre-patterned Al Laser Beam Mask Min-Koo Han, Sang-Hoon Jung, and Jae-Hong Jeon School of Electrical Engineering, Seoul National University, Seoul 151-742, KOREA Phone : +82-2-880-7248, Fax : +82-2-883-0827, E-mail : [email protected]
ABSTRACT We report new lateral grain growth mechanism by XeCl excimer laser annealing on a-Si film with pre-patterned Al layer. 2000Å-thick Al pattern on 800Å-thick PECVD a-Si film successfully reflects the incident laser beam and results in temperature gradient during the recrystallization process. The TEM images show that grain growth near the boundary between the liquid and the solid region exhibits a different mechanism compared with the conventional ELA. About 1.5 m-long lateral grain has been successfully obtained by single laser irradiation A poly-Si TFT with single grain boundary in the channel has been fabricated by the proposed excimer laser annealing. An excellent device characteristics such as mobility more than 250cm2/Vsec, high On/Off current ratio of 6.3x106 and low threshold voltage less than 1 V has been obtained. The experimental results show that the mobility, threshold voltage and subthreshold slope of proposed TFTs are superior to those of the conventional TFTs.
INTRODUCTION Polycrystalline silicon thin film transistor (poly-Si TFT) recrystallized by excimer laser annealing (ELA) is a promising device for active matrix liquid crystal displays (AMLCDs) due to high field-effect mobility and current driving capability [1]. Although the ELA poly-Si TFTs exhibit high field effect mobility and a large driving current, a variation of TFT characteristics in the large panel may be the serious problem besides a large leakage current [2]. These problems are originated from the inherent disordered grain structure of poly-Si film, and various efforts has been reported to control the grain structure of ELA poly-Si film [3]. Recently repoted sequential lateral solidification (SLS), which successfully grow continuous lateral grains, may be the most powerful tool to improve poly-Si film. However, SLS requires troublesome accurate substrate motion control so that the reproducibility and the uniformity may be guaranteed. The purpose of our work is to propose a simple ELA method employing a laser beam mask in order to increase the grain size and to control the grain boundary simultaneously with only one excimer laser irradiation. XeCl laser irradiation on a-Si film with the pre-patterned laser beam D4.2.1
mask results in selective melting of a-Si film. A large temperature gradient between the liquid and the solid regions successfully initiates the nucleation preferentially at the edge of masking pattern and the lateral grain size increases up to micron order. Our method does not require any accurate motion control and is carried out by only single laser irradiation while the SLS requires elaborated continuous beam scanning. We have investigated various excimer laser annealing method to improve the grain structure of p
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