| In recent years, Amorphous Indium-Gallium-Zinc-Oxide(a-IGZO) thin film transistors(TFTs) have been regarded as one of the most promising candidates for the addressing devices in next-generation flat-panel displays including active-matrix liquid crystal display(AMLCD) and active-matrix organic light-emitting diode(AMOLED) due to their outstanding features such as high field-effect mobility, good large-area uniformity, high light transmittance and low temperature fabrication ability. Despite of the progress in device manufacture development, further study on the fabrication process and device stability principle is required. In this thesis, we studied the thermal stability of N-type silicon wafer based a-IGZO TFTs experimentally and theoretically. We discussed the influence of the oxygen contained active layer and the passivation layer thickness on the transfer characteristics and thermal stability of devices. We also successfully developed a-IGZO TFTs with good transfer characteristics and thermal stability by photolithography and wet etching technologies, which can be references for the mass production of a-IGZO TFT. Main experimental results and conclusions are as following:Firstly, we prepared the TFTs without passivation layer by RF magnetron sputtering deposition technology. Then, we studied the influence of argon and oxygen flow rate ratio on properties of a-IGZO films and the corresponding devices, especially the thermal stability. As the increase in oxygen flow rate, deposition rate of a-IGZO film reduced and the surface roughness increased. For a-IGZO TFTs, deterioration of device characteristics was observed with the increasing oxygen flow rate. Four kinds of a-IGZO TFTs fabricated with the active layers prepared at a fixed Ar flow and various O2 flow rates were measured at the temperature varying from 298 K to 398 K. The result showed the transfer characteristic curves of the devices shifted negatively with the rise of oxygen flow rate.Secondly, four kinds of TFTs with differently thick passivation-layers were fabricated and measured at temperature varying from 298 K to 573 K. It was found that the variation of passivation-layer thickness significant influenced both the electrical properties and thermal stability of devices. The passivation-layer isolated the back channel from the water and oxygen in air, which resulted in the improvement of thermal stability of the TFTs under the low temperature. With the increase in protective layer thickness, the isolation effect was more obvious, and the TFTs became more stable. However, when the test temperature was higher than 473 K, the transfer characteristics of the four kinds of TFTs deteriorated dramatically, which was due to thermally excited carriers through intrinsic excitation and oxygen vacancy formation. A qualitative model was proposed to effectively ascertain the aforementioned two physical mechanisms. With passivation-layer thickness decreasing oxygen vacancy formation became more and more evident while intrinsic excitation could apparently worsen the characteristics of a-IGZO TFTs under the temperature higher than 473 K. In addition, the "passivation-layer thickness effect" for thermal stability of a-IGZO TFTs was theoretically explained by the variation of defect formation energy with the device passivation-layer thickness.Finally, we also employed the photolithography and wet etching process development to optimize the performance and thermal stability for a- IGZO TFTs. It was found that the thickness of gate insulate(GI) had great effect on the electrical characteristics of devices. The decrease in GI thickness improved the subthreshold swing(SS) and the low surface roughness, resulting in the good front channel. The continuous deposition of films for active layer and source/drain electrodes produced good ohmic contact between them. At last, we produced a-IGZO TFTs(BCE) with wet-etched tantalum gate and S/D electrodes whose electrical performance and thermal stability were relatively satisfactory. |
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