ZnO is considered one of the most promising materials for producing next-generation transparent and flexible electronics for displays, such as thin film transistors (TFTs) on plastic substrates. Over the past decade, TFTs produced from amorphous or polycrystalline Si have been important components in flat-panel displays. However, these TFTs (especially amorphous Si-TFTs) have several critical problems, including being light sensitive and having low mobilities (≤1 cm2/Vs) under light reduction. One approach to resolving these problems is to use an oxide semiconductor as the channel layer in TFTs. Intrinsic ZnO has been proposed recently for the active channel layer in transistors. An important advantage of these transistors is their high electron channel mobility (1 to 70 cm2/Vs with an on/off current ratio between 105 and 106), which results in higher drive currents and faster device operating speeds. Furthermore, the characteristics of ZnO-TFTs do not degrade on exposure to visible light due to ZnO’s wide band gap of ~3.37 eV, thereby eliminating the need to shield the active channel layer from visible light. Furthermore, since ZnO is one of the few oxides that can be grown as a crystalline material at relatively low deposition temperatures, fabrication of high-quality ZnO films at low temperatures will enable the production of flexible TFTs on plastic substrates for next-generation displays.
Traditional post annealed method in the furnace at a high temperature of more than 700 ℃, which represents undesirable side effects for device fabrication, and an obstacle to the fabrication of flexible electronic devices with high thermal budget. Therefore, to overcome these problems, we took two methods on post annealing process. One is a low temperature process of TFT controlling annealing ambient, and the other is a laser annealing process using excimer laser. Many papers have discussed quality improvement of ZnO films by annealing in oxygen, nitrogen ambient, air or vacuum to reduce defects and enlarge the grain size. Not many papers have focused on the effects of completed device by controlling annealing ambient in the low-temperature.
Also, excimer laser annealing (ELA) overcomes these problems. ELA is a well-known technique that is commonly used in the flat panel display industry. It is possible to laser-crystallize ZnO films deposited at low temperatures, thereby satisfying the requirement of low temperature when fabricating flexible substrates. Several papers have reported that laser annealing improves the characteristics of Si-TFTs, but few studies have examined the effects of laser annealing on ZnO-TFTs.
In our experiment, we fabricated bottom-gate-type TFTs that use ZnO as the active channel layer. After fabrication of ZnO-TFTs, we adopted post annealing process in low temperature and using eximer laser to reduce defects and enlarge the grain size of the ZnO channel layer for improvement of TFT characteristics. The effects of post annealing process on ZnO films and the performances of the ZnO-TFTs were discussed.