Toshimasa Eguchi

Temperature-Dependent Transfer Characteristics of Amorphous InGaZnO4 Thin-Film Transistors

The transfer characteristics of amorphous InGaZnO4 thin-film transistors (a-IGZO TFTs) were measured at temperatures ranging from 298 to 523 K in order to analyze the behavior of the above-threshold (ON state) and subthreshold regions. For comparison, the transfer characteristics of a hydrogenated amorphous silicon TFT (a-Si:H TFT) were measured in the same temperature range. We developed a simple analytical model that relates the threshold voltage (Vt) decrease due to increasing temperature to the formation of point defects in a-IGZO. It is well known that the formation of point defects results in the generation of free carriers in oxide semiconductors. Incorporating the analytical model with the experimental transfer characteristics data taken at high temperatures over 423 K, we estimated the formation energy to be approximately 1.05 eV. The Vt decrease because of the generation of point defects is peculiar to a-IGZO TFTs, which is not observed in a-Si:H TFTs. The results for the ON-current activation energy suggested that the density of tail states for a-IGZO is much lower than that for a-Si:H.

Comparison of Ultraviolet Photo-Field Effects between Hydrogenated Amorphous Silicon and Amorphous InGaZnO4 Thin-Film Transistors

We discuss the ultraviolet (UV) photo-field effects in amorphous InGaZnO4 thin-film transistors (a-IGZO TFTs) compared with those in hydrogenated amorphous silicon (a-Si:H) TFTs. It is shown that the UV illumination induces a much more significant threshold voltage (Vt) decrease and OFF-current increase for the a-IGZO TFTs than for the a-Si:H TFTs. The significant Vt decrease is found to take several tens of min to return to the initial state after switching off the UV light. A qualitative model is introduced to explain the photoresponse unique to the a-IGZO TFTs.

Flexible High-Performance Amorphous InGaZnO4 Thin-Film Transistors Utilizing Excimer Laser Annealing

We have investigated an excimer laser annealing (ELA) process for use in fabricating high-performance amorphous InGaZnO4 (IGZO) thin-film transistors (TFTs) on flexible plastic substrates. We numerically estimate the temperature increase of the IGZO film and substrate as a function of laser energy density. This is one of the most important measures for optimizing ELA conditions in order to apply plastic-based TFT fabrication. Because the optical absorption coefficient of IGZO film is three orders of magnitude higher than that of plastic substrates with respect to 308-nm laser light, it is possible to selectively increase the temperature of the IGZO film. The temperature of the IGZO film is estimated to increase to approximately 1500 °C at typical laser energy levels. Furthermore, incorporating a SiO2 buffer layer (some hundreds of nm) between the IGZO film and the plastic substrate is found to effectively suppress thermal damage to the substrate. We have experimentally investigated the properties of IGZO films irradiated with various excimer-laser energy densities. X-ray diffraction patterns and carrier densities of the IGZO films are found to significantly vary with laser energy density. We have used calculations and experimental results to optimize the ELA process; this has enabled us to produce high-performance IGZO-TFTs having a field-effect mobility of 15.6 cm2 V-1 s-1.

High-rate glass etching process for transferring polycrystalline silicon thin-film transistors to flexible substrates

We report on a process technology that makes possible the transfer of polycrystalline silicon thin-film transistor (poly-Si TFT) arrays from an original rigid glass substrate to another flexible plastic substrate. The transfer technology is characterized by its high-rate glass etching process, using a newly developed apparatus. After a description of the transfer sequence free from adhesive contamination, we present experimental observations for high-rate glass etching in hydrofluoric (HF) and hydrochloric (HCl) acid solution mixtures. The etching apparatus provides jets of the solution mixtures to the glass surface in order to achieve good circulation of the solutions in the bath, as well as to remove etch products effectively from the surface. We successfully achieved etch rates as high as 6 /spl mu/m/min with the etched surfaces almost as smooth as the original glass. In order to gain insight into the chemical mechanism of the etching, we developed a simplified kinetic etching model based on a Langmuir isotherm. The model and experimental etch-rate data are generally in good agreement, indicating that the basic modeling approach captures much of the essential chemistry for the high-rate glass etching. The transfer technology allows us to obtain TFT flexible substrates with good electrical characteristics and flexibility even after an annealing process at as high as 150/spl deg/C. These results demonstrate that the transfer technology is a promising candidate for achieving entirely new lightweight electronic devices such as flexible displays and radio frequency identification tags based on TFT flexible substrates.

Effects of Thermal Annealing on ZnO Thin-Film Transistor Characteristics and the Application of Excimer Laser Annealing in Plastic-Based ZnO Thin-Film Transistors

In this paper, we present the effects of thermal annealing and excimer laser annealing (ELA) on the characteristics of zinc-oxide (ZnO) thin-film transistors (TFTs) fabricated by magnetron sputtering at room temperature. The transfer characteristics of the ZnO-TFTs are found to improve with increasing thermal annealing temperature. Results of in situ high-temperature X-ray diffraction and electron back scattered diffraction pattern analyses indicate that this phenomenon arises from the crystallization of a low-crystallinity region present in as-deposited ZnO films. Almost all low-crystallinity regions crystallize at temperatures above 500 °C. Furthermore, to achieve such high-performance TFTs on flexible plastic substrates, ZnO-TFTs on a plastic substrate were irradiated with an excimer laser to raise the temperature of the ZnO films. Because of the extremely short pulse width of the laser (25 ns), it is possible to selectively increase the temperature of the ZnO with negligible thermal damage to the plastic substrate. The ELA process has enabled us to produce flexible high-performance ZnO-TFTs with a field-effect mobility of 12 cm2 V-1 s-1.

Application of Exponential Tail-State Distribution Model to the Above-Threshold Characteristics of Zn-Based Oxide Thin-Film Transistors

Our experimental results for amorphous InGaZnO4 and polycrystalline ZnO TFTs show a gate-voltage-dependent transconductance that is somewhat different from that for crystalline MOSFETs. We show that a model of the exponential distribution of band tail states can be used to describe the above-threshold behavior of Zn-based oxide TFTs.

Study on Current Crowding in the Output Characteristics of Amorphous InGaZnO4 Thin-Film Transistors Using Dual-Gate Structures with Various Active-Layer Thicknesses

We investigate current crowding in the output characteristics of amorphous InGaZnO4 thin-film transistors (a-IGZO TFTs) using dual-gate structures with various active-layer thicknesses in comparison with that of hydrogenated amorphous silicon (a-Si:H) TFTs. As the active-layer thickness becomes more than 150 nm, current crowding is found to begin to occur for both types of TFTs. We show that the current crowding for a-IGZO TFTs is less significant than that for a-Si:H TFTs at the same active-layer thickness. The current crowding is also found to gradually decrease with increasing temperature. To gain an insight into the mechanism of current crowding, we investigate the temperature dependence of the current–voltage characteristics for an ITO/a-IGZO/ITO sandwich structure, which we assume to be equivalent to an electrode–intrinsic–channel (e–i–c) structure in the source/drain regions of a-IGZO TFTs. Results suggest that current crowding arises from space-charge-limited current in the e–i–c structure. The smaller degree of current crowding for a-IGZO TFTs makes it possible to utilize a relatively thick a-IGZO active layer, which might be desirable both for commercial applications such as photodetecting devices and for fabrication processes in mass production.

Effect of Zinc Oxide Film Deposition Position on the Characteristics of Zinc Oxide Thin Film Transistors Fabricated by Low-Temperature Magnetron Sputtering

We report on the effect of zinc oxide (ZnO) film deposition position on the characteristics of ZnO thin-film transistors (TFTs) fabricated by magnetron sputtering with no intentional heating of the substrate. We evaluate the properties of ZnO (channel semiconductor) films deposited at various positions with respect to the target position. We show that the film deposition at a position off-centered from the target results in good TFT characteristics. This might be due to the fact that the off-centered deposition position is effective for suppressing the effect of energetic negative ions in the plasma.

Transfer Processes for Thermally Stable Large-Size TFT Flexible Substrates

We report on glass etching transfer processes to obtain thermally stable large-size TFT flexible substrates on plastic film bases. The transfer processes include high-pressure jet etching that allows us to achieve good etch-rate uniformity over the large area and the utilization of an adhesive having a low elastic modulus. From the experiments and simulations, we find that using an adhesive having a low elastic modulus is effective in reducing bend of the flexible substrate under thermal stresses. The simulations also predict that the reduction in the bend corresponds to the reduction in the principal stress of the adhesive, leading to the suppression of the film peeling off from the thinned glass. Using the transfer processes, we have successfully fabricated thermally stable TFT flexible substrates (300 mmtimes350 mm times200 mum) that have satisfactory electrical characteristics

48.3: Continuous Coating of Photo‐Alignment Layer on a Flexible Color Filter for LCDs Using a Roll‐to‐Roll Manufacturing Process

We applied a photo-alignment method to the color filter instead of a rubbing method to form an orientation layer continuously. We confirmed that the column spacers with the size of 5.5 μm in height and 20-μm-square did not affect both the coating and alignment process.