Studies On Performance And Process Development Of Novel Metal Oxide Thin Film Transistors

High-performance thin-film transistor (TFT) backplane technology is the basictechnology and center element in flat panel display (FPD) industry, such as active matrixorganic light-emitting diode (AMOLED). It is also the common key technology in otherFPDs industry to improve the quality of the display and reduce the production costs. With thelarge-size, high-resolution,3D displsy technologies rapid development, those increasinglyhigh requirements for TFT backplane. However, the traditional a-Si TFT has low carriermobility, so it is hard to achieve high-resolution display; and polysilicon (p-Si) TFT hashigher mobility, be widely used in high-resolution AMOLED, but the manufacturing processis complexity, the production costs is high, the uniformity is poor, and the yield is low, whichrestrict its development in the large-size FPDs. In recent years, the research on MOTFT hasgained huge progress. MOTFT is famous for its high mobility, good uniformity and goodstability. It is one of the most competitive TFTs in AMOLED. Therefore, it is very impormantto research the new AMOLED technologies based on MOTFT.This work firstly focuses on gate insulator. As the gate insulator layer determines thebreakdown voltage, the leakage currnet, and other important device operating parameters,thus obtaining a high dielectric constant, high-quality gate insulator layer is extremelyimportant. In order to meet the low cost, suitable for the preparation of large arearequirements in industry, we use the high dielectric constant Al2O3as the insulator anddeveloping a new anodizing process that avoids the use of expensive vacuum equipment.Anodized Al2O3prepared by using the improved anodic oxidation process exhibites a highdielectric constant (~10), a high breakdown electric field (~6MV/cm), and a low leakagecurret (<10-8A/cm2), which is very suitable for the production of large-size AMOLEDdisplays.However, the device has poor reliability because the Al hillocks damage the Al2O3insulator layer, due to the poor thermal stability of pure Al film under heat treatment. Thus,we propose Al alloy, such as Nd or Ce, as a gate to improve the thermal stability of Al/Al2O3structure, and achieved good results under high temperature. Experiments shows that Nd andCe ions are existence in the oxide film obtained by anodizing Al alloy, and they would diffuse into IZO film. The Ce ion could act as an electron trap, so the IZO-TFTs with Al-Ce gatewould experience seriously degradation. On the other hand, Nd ion is stable and would notproduce electron traps. Moreover, the existence of Nd would suppress undesirable freeelectron formation in the channel. Therefore, rare earth element Nd has a good compatibilitywith IZO, and anodizing Nd:Al2O3dielectric layer can actually be used in MOTFT devices,which has a great application potential in AMOLED displays.With the development of large-size display, Al alloy gate began to show the drawbacksof high resistivity, hence increase the signal delay of the displays. Therefore, we proposeusing buried thick Al gate structure to meet the low resistivity and solve the problem of signaldelay, which use only the thicking of pure Al and an auxiliary material named JSR-NN901.Pure Al film has mature preparation process and low cost, and thick Al further enhance thethermal stability of the pure Al can effectively suppress the generation of hillocks. MOTFTsprepared by this method exhibit high mobility, high electrical stability, high reliability.Therefore, MOTFTs with buried thick Al gate structure is a simple and effective technique,which has a great opportunity in the large-size TFT backplane manufacturing process.In order to obtain low cost and high resolution displays, the back-channel-etch (BCE)structure need to be used in the preparation of source/drain (S/D) electrodes. However, it isdifficulty to pattern them because metal oxide semiconductor materials are very susceptibleto most commonly used etchants and plasma treatment employed in wet-etch and dry-etchprocesses, respectively. Thus, we propose two methods to improve the performance for TFTbased on BCE structure: one is using H2O2-based etchant in combination with SF6plasmatreatment; the other is using an amorphous carbon (C) nanofilm inserted into the interfacebetween IZO and S/D electrodes as a barrier layer. The second method is not restricted byetchants, which has the universality characteristics. Experimental results show that thesemethods are simple and effective to fabricate high performance, high reliability MOTFT andwill be more and more attention in the production of high resolution MOTFT backplane.Then, we are inspired by the above results that the MoO3-residue layer may be associatedwith the operation mechanisms of some nonvolatile memories, thus providing the opportunityto extend the applications of metal oxide semiconductors to transparent memory devices thatcan be integrated with other transparent circuit elements. This also makes it possible to design freedom for system-on-panel applications for real, fully transparent displays.Based on the research on the MOTFT, we improve the traditional manufacture processof the MOTFT backplane, and develop a new type of process technology. Thus the number ofphotolithographic mask decreases from7times to5times. Then, we successfully achieved theproduction of AMOLED display. Finally, we develop a variety of2to7inch AMOLED basedon MOTFT backplane by using our new semiconductor material system, which hasindependent intellectual property rights, and realize images and video including monochrome,full color, transparent, and flexible display. Thus, we can see the future of the MOTFT.