Amorphous-Silicon TFTs with Self-Aligned Poly-Silicon Source and Drain
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AMORPHOUS-SILICON TFTs WITH SELF-ALIGNED POLY-SILICON SOURCE AND DRAIN Chang-Dong Kim, Osamu Sugiura and Masakiyo Matsumura Dept. of Physical Electronics , Tokyo Institute of Technology 2-12-1 Q-okayama Meguro-ku, Tokyo, 152 JAPAN ABSTRACT A new self-aligned a-Si TFT has been proposed. The TFT has the highly conductive n' poly-Si source and drain with self-aligned manner. The key process technique employed to this TFT is of excimer-laser selective recrystallization. Two ways have been succcessfully demonstrated to acheive the selectivity of recrystallization. The fabricated TFT had the fieldeffect mobility of 0.8cm2/Vs and high on/off current-ratio more than 107. INTRODUCTION Amorphous silicon thin-film transistors (a-Si TFTs) are widely used as the switching device of liquid-crystal displays (LCDs) [1], due to their low off-current and high on/off current ratio. It is well known that, for high quality LCDs, the TFT gate pattern edge should be aligned with the TFT drain pattern edge in order to reduce the overlap capacitance satisfactorily. This demand is satisfied by various self-alignment technologies proposed to date [2]. It is worthy to note that there is another important demand; the parasitic resistancc should be also small. Dominant origins of the parasitic resistance are in the n' a-Si layer of source and drain, and at contact between n' a-Si and metal. In this paper, we have proposed an ideal a-Si TFT with very small parasitic resistance by the effects of n' poly-Si source and drain. Another advantage of this TFT is of large allowance to an offset between channel edge and metal. Detailed process and TFT characteristics are also presented.
PROPOSED TIT STRUCTURE Cross-section of the proposed TFT is shown schematically in Fig.1. After depositing the a-Si layer on the insulator, the layer is selectively recrystallized by excimer-laser
n+poly-Si'
[a-Si I
n+poly-Si
INSULATOR
SUBSTRATE
Fig.1. The cross-section of new a-Si TFT with self-aligned n' poly-Si source and drain.
Mat. Res. Soc. Symp. Proc. Vol. 297. ©1993 Materials Research Society
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irradiation to form the self-aligned source and drain of highly conductive n' poly-Si. Since sheet resistance of the poly-Si layer is very low, large margin is allowed to the offset between the metal and channel edges. Carrier density is very high in the n' poly-Si, and thus the contact resistance is also small. Current needs not cross the a-Si layer near the source and drain. Therefore, there is no non-linear parasitic resistance caused by the current crossing. There is a key technology for fabricating this structure, i.e., excimer-laser selective recrystallization method [3] for n' poly-Si source and drain. We have invented two selective recrystallization methods as shown in Fig.2. The method 1 is of excimer-laser irradiation from the backside of the glass substrate as shown in (a). The gate metal is used as the mask to the laser-light. The a-Si layer except on the gate electrode is selectively recrystallized by the laser light passing-through the substrate.
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