||The present paper discusses the application of the silicon-aluminum and its chemical compound thin film applied on non conductive vacuum metallization (NCVM) and passivation layer of the a-IGZO thin-film transistors (a-IGZO TFTs). The first part of the paper compares the advantages of the liquid phase process and solid phase process in preparing silicon-aluminum target, and tries to establish a mass production technique with uniform texture. The second part uses the silicon-aluminum target to investigate the optical and electrical properties of silicon-aluminum thin film. The third part focuses on reactive sputtering technique, deposits silicon-aluminum nitric oxide (SiAlNO) thin film, and studies the effect and mechanism of improving the ambient atmosphere stability of the a-IGZO TFTs.|
The research firstly produces target of which shrinkage and macro segregation defects are significantly reduced by the mould designed and of casting parameter controlled; however, the micro segregation and coarse casting texture still affect the thin film quality during sputtering test. Due to the poor wetting property of liquid aluminum and silicon, either infiltration method or liquid-phase sintering method can reach the 99% theoretical density standard for sputtering target. Hot press process can obtain silicon-aluminum alloy with better density, micro structural and uniform texture via proper adjustment of parameters, but the process window is limited and not available to mass production. Silicon-aluminum target made by hot isostatic pressing process has surpassed the casting and hot press process not only on density, but also the consistency of microstructure as well. It successively established silicon-aluminum target production technique capable of mass production, and effectively improved consistency of sputtering thin film in the composition and structure.
In development of the thin film with high reflectivity and high resistance, the research attempts to develop a new NCVM film based on the opinion of metal nano particles distributed on non-conductive substrate. A dual-phase microstructure composed of nano-sized Al particles dispersed in an amorphous Si matrix was obtained which is different from the solid solution amorphous structure of traditional co-sputtering. When the aluminum content is at 15-30%, and the thickness of silicon-aluminum thin film with a range of 20-50nm, its sheet resistance can reach 106 Ω／□, also reflectivity of the visible light is over 60%, which comply with the NCVM requirements. In theoretical calculation, EMA is only applicable for metallic character, i.e. silicon-alumium thin film of which alumium content is larger than percolation threshold, within this range, the simulation result is quite similar to the experimental result.
The research extends the silicon-aluminum target application from NCVM to the passivation layer of a-IGZO TFTs, and finds out the deposited SiAlNO thin film produced by the sputtering process not only can protect the a-IGZO from being affected by the oxygen and moisture to improve the ambient atmosphere stability, but also reduce shift of threshold voltage after illumination process, which have advantage of reducing damage of activation layer. The research suggests that the oxygen vacancies shift to gate with negative bias stress, which reduces the recombination rate of electrons and hole makes electrons jump to conductive band easier, thus shift of threshold voltage is larger than illumination process without negative bias stress. The research also finds out that the SiAlNO thin film has the ability to repair the defect of active layer, so as to prevent the oxygen and moisture from impact on component electrical stability. Therefore, it can explain the reason that a-IGZO shift of threshold voltage decreases when SiAlNO thin film is as passivation layer, after illumination process.
SiAlNx thin film after annealing will form SiAlNO thin film, not only repair a-IGZO defect, also greatly reduce the dangling bonds of SiAlNO thin film, so as to restore the device from conducting to semiconducting property. The research finds out it can achieve the same protective effect as the Al2O3 through increasing the aluminum content or simply increasing the thickness of the SiAlNO thin film. From the overall evaluation of the experiments, passivation layer can protect the active layer by isolating oxygen, moisture and light, also takes integrate production efficiency and so on into account, Si-30AlNO thin film is the best choice for a-IGZO passivation layer.