Crystallization of Amorphous Si Thin Films Using a Viscous Ni Solution
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CRYSTALLIZATION OF AMORPHOUS Si THIN FILMS USING A VISCOUS Ni SOLUTION Jin Hyung Ahn, Sung Chul Kim, and Byung Tae Ahn Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Taejon, 305-701, Korea ABSTRACT We prepared a viscous Ni solution by dissolving NiCl2 in 1N HCl and mixing it with propylene glycol to control the amount of Ni on Si surface. A uniform film was formed after spin coating and oven dry. The a-Si films deposited by LPCVD with Si2H6 gas were crystallized more uniformly and more reproducibly. And the crystallization was enhanced from 600°C, 30h to 500°C, 10h. The surface roughness of poly-Si film crystallized with the viscous solution was much smaller than that of poly-Si film crystallized from Ni/Si direct contact. The TFT mobility was improved even though the crystallization temperature was much lower. INTRODUCTION It generally takes tens of hours to crystallize a-Si films at 600°C. So, there have been many studies to enhance the solid-phase crystallization. One of the methods is to deposit metals on Si surface. Metals can be deposited using sputtering or evaporation. Lee fabricated highperformance thin film transistors by laterally crystallizing the channel area from the source/drain area on which 5Å thick Ni was deposited by sputtering [1]. But metals also can be deposited using metal solutions. The amount of metal deposited on Si surface can be controlled by the concentration of the solution. Sohn reported that SPC was enhanced using various metal solutions [2]. Yoon reported that an a-Si film could be crystallized at 500°C in 20h using a Ni solution [3]. Kalkan reported that crystallization time could be reduced from 18h to 10min at 600°C using a Ni solution [4]. Sohn and Yoon spin-coated metal solutions and Kalkan dropped the solution onto a 200°C a-Si film and dried it. They all used a diluted acid like HCl and HNO3 as the solvent. But the solutions using diluted acid as the solvent have some limitations. Metals such as Ni and Al of which electronegativity is not larger than Si cannot be deposited on the Si surface [2]. And the solution exists on a-Si films in uncontrollable forms of droplet because Si surface is hydrophobic, resulting in non-uniform and irreproducible deposition. In this study we made a viscous Ni solution dissolving NiCl2 in a 1N HCl+propylene glycol solvent. The viscous solution formed a uniform film when spin coated, and enough amount of Ni could be uniformly deposited after oven dry. As a result, crystallization temperature could be lowered and the crystallization process could be more uniform and reproducible. EXPERIMENT 100nm thick a-Si films were deposited by low-pressure chemical vapor deposition at 530°C using Si2H6 on oxidized Si wafers. The films were cleaned in boiling H2SO4+H2O2 solution and dipped in diluted HF solution to remove oxide on the surface before solution coating. Ni solution
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was prepared by dissolving NiCl2 in a 1N HCl+propylen glycol solvent. Propylene glycol is an organic solvent that makes the
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