Sharp in 2012 for the first time in the world mastered the production of TFT LCD displays with an active matrix based on IGZO material. IGZO HD LCD panels use thin-film transistors based on an amorphous semiconductor, which is an oxide of indium gallium and zinc (Indium Gallium Zinc Oxide).
The IGZO transistor technology was developed by Sharp in collaboration with their colleagues at the Semiconductor Energy Laboratory. IGZO transistors provide features that can significantly reduce power consumption, increase display resolution, and improve touch screen performance. The article provides an overview of IGZO-TFT technology and new LCD displays based on it.
Introduction
The first actively addressable LCDs (1972) used cadmium selenide to form a thin-film transistor array. Then, for many years, amorphous silicon (a-Si) became the main active matrix material in LCD displays. It was clear that this was far from the best material for forming a transistor matrix: it had a lot of drawbacks. As a result, to provide acceptable drive currents, the active matrix transistor in LCDs had to be large enough.
However, a-Si TFT technology had other advantages that ensured its dominance for many years. This is simplicity, low cost and high repeatability of the process. Active matrix technology based on a-Si has now been mastered by hundreds of manufacturers. Polycrystalline silicon matrices are also used, but this technology is much more complex and expensive than a-Si technology.
The search for alternative materials never stopped. Over the years, technologists have tried many semiconductor materials as alternatives to a-Si.
The main goal is to ensure the best performance of active matrix transistors while retaining the a-Si technology where possible. First of all, it was necessary to increase the current of the transistor in the open state (Ion) and reduce the current of the transistor in the closed state (Doff).
The IGZO material was first synthesized and proposed by Japanese researchers N. Iizuka and T. Mohr in 1985. In the future, many groups of technologists around the world were actively engaged in studying the properties of this material. The IGZO material for a TFT matrix was first tested by the development team of Hideo Hooson in 2004.
It was with metal oxides that have semiconductor properties that Hideo Hooson experimented. While having an amorphous structure, oxide semiconductors have properties more suitable for the production of TFT transistors than hydrogenated a‑Si. Hydrogenated a-Si is deposited by sputtering a silicon target in a hydrogen-containing atmosphere.
H. Hooson experimentally found the most successful mixture of oxides for creating TAOS-TFT. It consists of oxides of indium, gallium and zinc (Ignaz), abbreviated as IGZO. Ignaz films are formed on quartz glass using a pulsed laser deposition (PLD) process, where the target is a mixture of Ga2O3, In2O3 and Zone powders in a ratio of 1:1:8. The sintering of the mixture of powders to form the target is carried out in a tube furnace at atmospheric pressure.
The technological process of applying an IGZO layer to a glass substrate is rather complicated. A combination of technological processes is used to create the required film structure (Fig. 1). On fig. 2 shows the structure of a TFT IGZO transistor.
The process of forming the IGZO TFT structure is actually exactly the same as for the traditional a‑Si based.
The current Id is normalized with respect to the parameter W/L (gate zone width/length). Although LTPS transistors have a very high on-state current due to high carrier mobility, the off-state current for them is unacceptably high, and even higher than that of a-Si transistors. On the other hand, due to the higher mobility of charge carriers, the IGZO material is more than 20 times higher than that of a-Si.
This quality alone makes it possible to significantly reduce the size of the matrix transistors, while maintaining the same parameters as compared to a‑Si. In addition, since hole mobility at negative bias is very low, it is clear that IGZO transistors provide an ideal off-state characteristic. Based on this, we can conclude that the ON / OFF characteristics provided by IGZO transistors are significantly higher compared to traditional a-Si and LTPS.
On fig. 4a shows the performance of TFT transistors using channel etching (CE) technology. In this case, the IGZO transistors have poor uniformity due to the instability of the etching process.
If you use technology with an additional stop process (etch stopper, ES), instability can be significantly reduced. When using the stop process, the threshold voltage (Vth) in the range of ±1 V is stable and unchanged for G8 size substrates. The current values for the off state at negative gate biases (Vg < 0) were so small that they could not even be measured.
On fig. 5 shows the results of testing the reliability of the tests for constant displacement combined with temperature. At low and high temperatures, a low dependence of the Vth bias was recorded compared to a-Si.
On fig. 6 shows the results of checking the stability of the transfer characteristic of the transistors of the LCD module at an operating temperature of +60 °C. A slight change was found after 1000 hours of operation. Thus, IGZO-TFT transistors have proven to be highly reliable when operating in a real LCD.
Accordingly, it is possible to make the size of the row and column lines of the matrix smaller and increase the resolution of the panel compared to a‑Si.
Sharp has developed a blueprint for high-resolution LCD displays for various application sectors, taking advantage of IGZO technology. A 32-inch IGZO TV panel has been developed with a resolution of 3840×2160 pixels, which corresponds to a density of 140 pixels per inch. For mobile computers, a 10-inch IGZO panel with a resolution of 2560 × 1600 pixels is intended. The seven-inch IGZO panel for tablets has a resolution of 800×1280 pixels, with a density of 217 pixels per inch.