Performance of poly-Si TFTs fabricated by SELAX

Selectively enlarging laser crystallization (SELAX) has been proposed as a new crystallization process for use in the fabrication of thin-film transistors (TFTs). This method is capable of producing a large-grained and flat film of poly-Si. The average grain size is 0.3/spl times/5 /spl mu/m, and the surface roughness of the poly-Si layer is less than 5 nm. The TFTs fabricated with this method have better performance and are more uniform than those produced with the conventional excimer laser crystallization (ELC) method. The average values of field-effect mobility are 440 cm/sup 2//Vs (n-type), and 130 cm/sup 2//Vs (p-type). The subthreshold slope for both types is 0.20 V/dec. Values for standard deviation of threshold voltage are 0.03 V (n-type) and 0.20 V (p-type). The delay time of the CMOS-inverter of SELAX TFTs is less than half that of ELC TFTs.

I. Introduction

Thin-Film transistors (TFTs) with better performance will be essential to the next generation of mobile displays, since not only the pixel array but functional circuits, such as controllers, drivers, and so on, will be integrated on the same glass substrate. The displays will require TFTs that have low power consumption and thus low threshold voltage, a steep subthreshold slope (S-value), high field-effect mobility, and little deviation in any of these values. The poly-Si TFT has the higher mobility than any kind of TFT (for instance, amorphous Si TFTs or organic TFTs) and is thus the most promising device in terms of achieving these goals. In general, a poly-Si TFT is fabricated by excimer laser crystallization (ELC) [1]. However, the performance of the conventional ELC TFT is not sufficient for the integration of functional circuits. ELC leads to a poly-Si layer with lots of grain boundaries, which constitute defects and lead to poor performance for TFTs thus fabricated.