Poly-Si TFT fabricated by laser-induced in-situ fluorine passivation and laser doping

A new poly-Si TFT has been fabricated by employing laser-induced in-situ fluorine passivation and a laser-doping method. With only one excimer laser annealing, we have successfully fabricated the device using one step to crystallize, passivate and dope simultaneously. Although no additional plasma post-passivation was performed, the on-state and the off-state leakage properties of TFTs with fluorine passivation were improved compared with those without fluorine passivation. The device with in-situ fluorine passivation has the maximum transconductance of 13.3 μA/V for a C₂F/sub 6/ flow rate of 100 sccm, whilst that for a device without fluorine passivation is 8.4 μA/V. The device reliability under electrical stress was remarkably improved in the in-situ fluorine-passivated devices due to the fluorine passivation of trap states in the poly-Si channel and at the SiO₂/poly-Si interface.

I. Introduction

Polycrystalline silicon thin film transistors (poly-Si TFTs) employing excimer laser annealing have attracted considerable attention because of their possible applications in active matrix liquid crystal displays (AMLCDs) due to their high mobility [1]. However, it is well known that grain boundaries and intra-grain defects in the poly-Si layer degrade the electrical properties of poly-Si TFTs [2]. Hydrogenation is widely used to passivate the defects, but it takes a long processing time to obtain the satisfactory performance improvement [2], [3]. Moreover, the reliability of hydrogen-passivated TFTs is rather poor because weak Si–H bonds may not be stable under electrical stress [3].