Resistance Decrease of Phosphorus Ion Implanted Poly-Si Thin Films During Low Temperature Annealing
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ABSTRACT During low temperature (below 450°C) annealing, the sheet resistance of phosphorus implanted poly-Si thin films (film thickness : 50nm) decreased to lE+30/square. The sheet resistance after annealing decreased with annealing time and became lower when the dosing level was high enough. At the dose of 8E+14 ions/cm 2 we obtained a sheet resistance of 1E+3Q/square by annealing at 450'C for 180min. We analyzed ellipsometry data assuming a two-layer model where the surface layer consists of a-Si and the lower layer of poly-Si. This analysis indicated that the surface of implanted films was amorphized by ion implantation and the amorphized layer thickness increased with dosing level. Also, it turned out that the lower poly-Si layer thickness increased from 30nm to 50nm after annealing accompanied by the conversion of a-Si layer to poly-Si layer. During annealing at low temperature, activation of phosphorus ions implanted into poly-Si and recrystallization of a-Si took place simultaneously and the sheet resistance after annealing decreased with the increase in thickness of the recrystallized region. INTRODUCTION For active matrix liquid crystal display (LCD), polycrystalline silicon thin film transistors (poly-Si TFTs) have several advantages including high mobility to achieve the integrability of driving circuit devices and pixel driving devices on a substrate and the reduction of pixel size. Poly-Si TFTs fabricated by high temperature (above 600°C) processes have already been used in LCDs because of their high performance and reliability. Although high temperature process technology can be only applied to fabricate TFTs on quartz substrates, not on glass substrates. Recently, the fabrication of poly-Si TFTs at temperature below 450'C has been developed for the use of glass substrates[ 1]-[3] The low temperature formation of TFTs' source/drain region has been studied[4],[5]. Impurity doping and activation processes below 450TC have been required for source/drain formation . Ion implantation is used to dope an impurity into films. Impurity activation has been done by thermal annealing or laser annealing after impurity doping. In this paper, the resistance decrease of poly-Si thin films using mass-selected ion implantation and thermal annealing below 450'C has been investigated. Spectroscopic ellipsometry measurements and ultraviolet reflectance measurements have been performed to analyze structure changes of implanted films during low temperature annealing. A relationship between resistance and structure is also discussed. EXPERIMENTAL Poly-Si films were prepared by the method described below. Amorphous Si films were deposited by low pressure chemical vapor deposition method at 485°C using Si2H6 on glass substrate. Film thickness was approximately 50nm. The amorphous Si films were then crystallized by annealing at 650 0 C for 5h using a conventional furnace. Then, mass selected phosphorus ions were implanted into the poly-Si films with an accelerating voltage of 10keV and four dosing levels ranging from 8E+13/cm
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