Defect Reduction by Tilted Zone Crystallization of Patterned Silicon Films on Fused Silica
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DEFECT REDUCTION BY TILTED ZONE CRYSTALLIZATION OF PATTERNED SILICON FILMS ON FUSED SILICA. L. E. FENNELL, M. D. MOYER, D. K. BIEGELSEN, A. CHIANG AND N. M. JOHNSON Xerox Palo Alto Research Center, Palo Alto, CA 94304. ABSTRACT It is well known that crystal growth and defect propagation occur along the local thermal gradient at a liquid - solid interface, It is shown here that this property can be used to greatly reduce the density of structural defects in zone crystallized silicon thin films on amorphous substrates, A solidification front perpendicular to the direction of zone motion has its in - plane component of the thermal gradient parallel to the direction of zone motion. In this case, the defects, once nucleated, propagate along this zone path and the defect density reaches a steady state value. By using striped polysilicon patterning and a solidification front tilted relative to the molten zone path, a defect-free region is shown to occur. It is also shown that defects, once nucleated, are swept laterally from the region of interest and are terminated at the stripe boundary. Methods of forming a tilted interface are proposed and demonstrated for CO 2 laser crystallization. INTRODUCTION The ability to produce single crystal silicon thin films controllably on bulk glass is being pursued by many research groups. One technique involves patterning the polysilicon layer into stripes, encapsulating the patterned polysilicon with deposited Si0 2 , and subsequently scanning the composite film with a cw laser beam [1,2]. One of the problems common to this and similiar scanning techniques has been the nucleation and propagation of low angle or sub-grain boundaries during the crystallization process. When this material is processed to make large area thin -film transistors, sub-grain boundaries are often found transiting the active area of the devices, Enhanced diffusion of dopants along these sub-grain boundaries during device processing has been demonstrated [3] and correlations of device characteristics with sub-boundaries have been made [1]. In each of the various crystallization techniques efforts are being made to reduce or eliminate sub-grain boundary defects [4]. This paper describes significant a modification of the laser crystallization technique which results in low defect and defect-free single-crystal silicon for use in large - area thin - film transistor and integrated circuit applications, CRYSTALLIZATION TECHNIQUE One method of crystallizing patterned polysilicon stripes with a laser is to focus the laser beam as a Gaussian spot on the center of a stripe and scan the beam along the stripe. An alternative method is to image the beam to form a line image perpendicular to the stripes and scan this beam along the stripes which can result in Both of these methods produce a crystalizing several stripes at one time. crystallization interface that is symmetric about the stripe axis and the resulting crystalline film frequently contains defects such as microtwins and low-angle grain boundaries. Furthermore these
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