(Invited) Technology Trend of Poly-Si TFT from Viewpoints of Crystallization and Device Performance

Abstract

Thin-film transistors (TFTs) utilizing the Low-Temperature Polycrystalline-Si (LTPS) as their active areas are very crucial for the system-on-glass (SOG). The liquid-crystal display (LCD) or organic light emitting diode display are fabricated in the display area and their controls are performed by the micro-computer that is formed in the peripheral area. The application of high-performance poly-Si TFTs to the high-definition LCD light valves for projectors was reported for the first time[1]. Therefore, the requirements for the recrystallized Si film are a high quality, a large grain formation and a position control of the nucleation. To satisfy those requirements, a super lateral growth (SLG)[2] using the excimer laser annealing (ELA), a phase-modulated ELA[3], a solid-phase crystallization followed by ELA[1], a low-temperature solid-phase excimer laser crystallization[4], a Pd:YAG laser crystallization[5], a soft X-ray crystallization (SXC)[6] have been studied. For the device structure, the double gate TFT[7] and the tunnel TFT[8] have been developed. In our group, the low-temperature crystallization methods and the novel TFT utilizing tunnel effect have been researched. Low-temperature crystallization: For the ELA method, the mechanism of multi-pulse ELA crystallization has been clarified and a large grained film with a high-quality[4] was provided. In addition, hydrogens in a-Si film greatly influenced the crystallinity and grain size of the poly-Si[9,10] and we discovered the advantage in ELA of hydrogen–modulation-doped a-Silicon Layer[11]. For the SXC, a low-temperature crystallization of a-Si, Ge and SixGe1-x film was researched and developed using the short undulator as a light source. The fundamental process of the SXC-method is a local excitation of core electrons to vacuum level by photons followed by the atom movement due to the Coulomb repulsion, which results in the lower temperature process. The SXC-method reduced the threshold temperature of crystallization for a-Si, Ge and SixGe1-x film by 100 - 150 degree C as compared with the conv. thermal crystallization. Novel TFT: Although the high-performance TFT with the high carrier mobility and low leakage current utilizing the large single crystalline grains has been intensively researched, it does not successfully operates from the productive viewpoint[12]. Tunneling Dielectric Thin-Film Transistor (TDTFT) was proposed and fabricated in a bottom-gate structure. Using a 1.7-nm-thick SiNx film, it reduced the gate-off current less than 1/10 in comparison with a conv. TFT. In addition, the TDTFT improved the hump effect as shown in Fig.1[13]. It is considered that the fixed charge in the SiNx film suppresses the formation of the parasitic channel in the poly-Si edge by the Coulomb repulsion. We conclude that TDTFT will suitable as a candidate of the next generation TFT structure.