A study on reducing image‐sticking artifacts in wide‐screen TFT‐LCD monitors

Abstract— In order to improve the image quality of flat‐panel displays, the image retention of a display has to be reduced. But at the same time, display devices also have to produce durable images while they are used for long periods of time in a single session. To achieve high‐quality images with reduced image sticking in TFT‐LCD devices, we have increased their voltage‐holding ratio (VHR). This technology ensures the achievement of durable images over long periods of time and also reduces the image‐sticking problem. There are many ways to achieve increased VHR. In terms of the characteristics of the TFT devices, reducing the leakage‐current effects is the most frequently applied method in the display industry.     

In‐cell capacitive‐type touch sensor using LTPS TFT‐LCD technology

Abstract— An LTPS TFT‐LCD with an in‐cell capacitive‐type touch sensor has been proposed and prototyped. The embedded sensor in the pixel was designed to amplify the voltage change caused by capacitive coupling between the detection electrode and conductive object (user's finger). No touch force is needed for sensor actuation and no extra electrical connection for the counter‐substrate is needed. The validity of the observed voltage difference of the sensor output on the TFT substrate was examined. The proposed architecture is considered to be applicable to larger LCDs for various applications such as smartphones, automotive navigation systems, and mobile internet devices.     

Development of rollable silicon thin‐film‐transistor backplanes utilizing a roll‐to‐roll continuous lamination process

Abstract— Rollable silicon thin‐film‐transistor (TFT) backplanes utilizing a roll‐to‐roll process have been developed. The roll‐to‐roll TFT‐backplane technology is characterized by a glass‐etching TFT transfer process and a roll‐to‐roll continuous lamination process. The transfer process includes high‐rate, uniform glass‐etching to transfer TFT arrays fabricated on a glass substrate to a flexible plastic film. In the roll‐to‐roll process, thinned TFT‐glass sheets (0.1 mm) and a base‐film roll are continuously laminated using a permanent adhesive. Choosing both an appropriate elastic modulus for the adhesive and an appropriate tension strength to be used in the process is the key to suppressing deformation of the TFT‐backplane rolls caused by thermal stress. TFT backplanes that can be wound, without any major physical damage such as cracking, on a roll whose core diameter is approximately 300 mm have been sucessfully obtained. Incorporating the TFT‐backplane rolls into other roll components, such as color‐filter rolls, will make it possible to produce TFT‐LCDs in a fully roll‐to‐roll manufacturing process.     

Solution‐processed oxide semiconductors for low‐cost and high‐performance thin‐film transistors and fabrication of organic light‐emitting‐diode displays

Abstract— High‐performance solution‐processed oxide‐semiconductor (OS) thin‐film transistors (TFTs) and their application to a TFT backplane for active‐matrix organic light‐emitting‐diode (AMOLED) displays are reported. For this work, bottom‐gated TFTs having spin‐coated amorphous In‐Zn‐O (IZO) active layers formed at 450°C have been fabricated. A mobility (μ) as high as 5.0 cm²/V‐sec, ?0.5 V of threshold voltage (VT), 0.7 V/dec of subthreshold swing (SS), and 6.9 × 10⁸ of on‐off current ratio were obtained by using an etch‐stopper (ES) structure TFT. TFTs exhibited uniform characteristics within 150 × 150‐mm² substrates. Based on these results, a 2.2‐in. AMOLED display driven by spin‐coated IZO TFTs have also been fabricated. In order to investigate operation instability, a negative‐bias‐temperature‐stress (NBTS) test was carried out at 60°C in ambient air. The IZO‐TFT showed ?2.5 V of threshold‐voltage shift (ΔVT) after 10,800 sec of stress time, comparable with the level (ΔVT = ?1.96 V) of conventional vacuum‐deposited a‐Si TFTs. Also, other issues regarding solution‐processed OS technology, including the instability, lowering process temperature, and printable devices are discussed.     

Ultra‐FFS TFT‐LCD with super image quality,fast response time,and strong pressure‐resistant characteristics

Fringe‐field‐switching (FFS) devices using liquid‐crystal (LC) with a negative dielectric anisotropy exhibit high transmittance and wide viewing angle simultaneously. Recently, we have developed an “Ultra‐FFS” thin‐film‐transistor (TFT) LCD using LC with a positive dielectric anisotropy that exhibits high transmittance, is color‐shift free, has a high‐contrast ratio in a wide range, experiences no crosstalk and has a fast response time of 25 msec. In this paper, the device concept is discussed, and, in addition, the pressure‐resistant characteristics of the devices compared with that of the twisted‐nematic (TN) LCD is discussed.     

All‐additive ink‐jet‐printed display backplanes: Materials development and integration

Abstract— Methods used to deposit and integrate solution‐processed materials to fabricate TFT backplanes by ink‐jet printing are discussed. Thematerials studied allow the development of an all‐additive process in which materials are deposited only where their functionality is required. The metal layer and semiconductor are printed, and the solution‐processed dielectric is spin‐coated. Silver nanoparticles are used as gate and datametals, the semiconductor used is a polythiophene derivative (PQT‐12), and the gate dielectric is an epoxy‐based photopolymer. The maximum processing temperature used is 150°C, making the process compatible with flexible substrates. The I_ON/I_OFF ratio was found to be about 10⁵?10⁶, and TFT mobilities of 0.04 cm²/V‐sec were obtained. The influence of surface treatments on the size and shape of printed features is presented. It is shown that coffee‐stain effects can be controlled with ink formulation and that devices show the expected pixel response.     

Evaluation of single‐, pre‐emphasis,and dual‐driving methods in large‐sized TFT‐LCDs

Abstract— As the panel size and the frame frequency of TFT‐LCDs increases, driving issues become much more important for larger‐sized and higher‐resolution TFT‐LCDs. In our previous paper, the pre‐emphasis driving method was proposed to shorten the driving time of the data line with heavy loads of the large‐sized TFT‐LCDs. This paper proposes a simulation model based on the evaluation results of the developed pre‐emphasis source driver, and the issues of driving the data line with heavy loads are reviewed. The single‐, pre‐emphasis, and dual‐driving methods are compared in terms of their driving time and power consumption for large‐sized TFT‐LCDs with various resistances and capacitances of the data lines. At a panel load of 250‐pF capacitance and 15‐kΩ resistance in full‐HD resolution, the pre‐emphasis driving can reduce the pixel driving time to 66% with a 54% increase in the analog power consumption.     

A low‐cost low‐temperature thin‐film‐transistor backplane based on oxide semiconductor

Abstract— An indium‐gallium‐zinc‐oxide (IGZO) thin‐film transistor (TFT) based on an anodized aluminum‐oxide gate dielectric and photoresist passivation has been fabricated. The TFT showed a field‐effect mobility of as high as 18 cm²/V‐sec and a threshold voltage of only 0.5 V. A 50 × 50 AMOLED display based on this type of TFT was designed and fabricated. The average luminance of the panel was 150 cd/m², and the maximum pixel luminance was 900 cd/m².     

Vt Compensated voltage‐data a‐Si TFT AMOLED pixel circuits

Abstract— Active‐matrix organic light‐emitting‐diode (AMOLED) displays are now entering the marketplace. The use of a thin‐film‐transistor (TFT) active matrix allows OLED displays to be larger in size, higher in resolutions and lower in power dissipation than is possible using a conventional passive matrix. A number of TFT active‐matrix pixel circuits have been developed for luminance control, while correcting for initial and electrically stressed TFT parameter variations. Previous circuits and driving methods are reviewed. A new driving method is presented in which the threshold‐voltage (V_t) compensation performance, along with various circuit improvements for amorphous‐silicon (a‐Si) TFT pixel circuits using voltage data, are discussed. This new driving method along with various circuit improvements is demonstrated in a state‐of‐the‐art 20‐in. a‐Si TFT AMOLED HDTV.     

ITO lift‐off technique for TFT mask‐reduction process

Abstract— In recent years, reducing the number of TFT‐manufacturing steps has become an unavoidable technology development stream for all TFT‐LCD makers for the purpose of cost reduction. In this paper, an advanced photomask‐process‐reduction technique, a three‐mask TFT process, by chemical lift‐off which is inherent of the conventional four‐mask TFT fabrication process, is proposed. The major feature of this three‐mask technique is the combining of the passivation‐layer and pixel‐electrode formation within one photolithography process. A new halftone mask (HTM) design has been applied to the photolithography process. With this new HTM design, a small SiN_x island bridge was formed, located at the TFT source contact‐edge border. And it provided an ITO pixel electrical conducting path and avoided the undercut issue where ITO breaks from the gate insulator (GI). In order to enhance the chemical lift‐off efficiency, different process and structure designs were also implemented and introduced. Furthermore, a new laser lift‐off technology was adopted to improve the ability of ITO lift‐off. By using this new laser lift‐off technology, unnecessary ITO film could be easily lift‐off before photoresist stripping. Finally, the first HTM lift‐off panel was successfully demonstrated by using our new three‐mask TFT design scheme.     

IGZO‐TFT technology for large‐screen 8K display

We succeeded in G8 factory for mass production of Indium–Gallium–Zink–Oxide thin‐film transistor (IGZO‐TFT) for the first time in the world. The initial TFT process was an etching stop‐type TFT, but now, we are mass producing channel etching‐type TFTs. And, its application range is smartphones, tablets, PCs, monitors, TV, and so on. In particular, because of recent demands for high‐resolution and narrow frame, our IGZO display has been advanced in technology development with gate driver in panel. In this paper, we report development combining low resistance technology and the latest IGZO‐TFT (IGZO5) for large‐screen 8K display.     

Flexible AMOLEDs with low‐temperature‐processed TFT backplane technologies

Developments of backplane technologies, which are one of the challenging topics, toward the realization of flexible active matrix organic light‐emitting diodes (AMOLEDs) are discussed in this paper. Plastic substrates including polyimide are considered as a good candidate for substrates of flexible AMOLEDs. The fabrication process flows based on plastic substrates are explained. Limited by the temperature that plastic substrates can sustain, TFT technologies with maximum processing temperature below 400 °C must be developed. Considering the stringent requirements of AMOLEDs, both oxide thin‐film transistors (TFTs) and ultra‐low‐temperature poly‐silicon TFTs (U‐LTPS TFTs) are investigated. First, oxide TFTs with representative indium gallium zinc oxide channel layer are fabricated on polyimide substrates. The threshold voltage shifts under bias stress and under bending test are small. Thus, a 4.0‐in. flexible AMOLED is demonstrated with indium gallium zinc oxide TFTs, showing good panel performance and flexibility. Further, the oxide TFTs based on indium tin zinc oxide channel layer with high mobility and good stability are discussed. The mobility can be higher than 20 cm²/Vs, and threshold voltage shifts under both voltage stress and current stress are almost negligible, proving the potential of oxide TFT technology. On the other hand, the U‐LTPS TFTs are also developed. It is confirmed that dehydrogenation and dopant activation can be effectively performed at a temperature within 400 °C. The performance of U‐LTPS TFTs on polyimide is compatible to those of TFTs on glass. Also, the performance of devices on polyimide can be kept intact after devices de‐bonded from glass carrier. Finally, a 4.3‐in. flexible AMOLED is also demonstrated with U‐LTPS TFTs.     

A 2‐in. a‐Si:H TFT‐LCD with embedded backlight control TFT sensors with various channel widths

Abstract— A 2.0‐in. a‐Si:H TFT‐LCD with embedded TFT sensors for the control of the backlight intensity according to the ambient light intensity has been developed. Two types of a‐Si:H TFT sensors with various channel widths were embedded into a TFT backplane with bottom‐ and top‐gate structures for measuring the ambient light and backlight illumination, respectively. The output signal, measured by a readout IC, increased with backlight intensity until 20,000 lux.     

Dual‐plate OLED display (DOD) based on amorphous‐silicon (a‐Si) TFT backplane

Abstract— An improved AMOLED with an a‐Si TFT backplane based on a unique structure is reported. The new structure is refered to as a dual‐plate OLED display (DOD). While a top‐emission OLED array is directly fabricated on a TFT backplane, the DOD consists of an upper OLED substrate and a lower TFT substrate, which are independently fabricated. Because the OLED substrate, which is fabricated through the process flow of bottom emission, is attached to the TFT substrate, the light is emitted in the opposite direction to the TFT backplane. The DOD enables the design of large‐sized TFTs and a complicated pixel circuit. It can also not only achieve higher uniformity in luminance in large‐sized displays due to the low electrical resistance of the common electrode, but also wider viewing angles.     

Thermal dimensional stability of glass substrates for poly‐Si TFT‐LCD application

Abstract— Thermal dimensional stability of Fusion‐drawn Corning Code 1737 glass was investigated at simulated thermal cycles for low‐temperature poly‐Si TFT fabrication. For low‐temperature poly‐Si TFT processes between 550 and 600°C, annealed Code 1737 is required to meet a typical thermal shrinkage requirement of less than 20 ppm. For super‐low‐temperature poly‐Si TFT processes between 400 and 450°C, Code 1737 meets the requirement in the unannealed state. Code 1737 glass having a high strain point of 666°C provides thermal capabilities as a substrate for low‐temperature poly‐Si TFT‐LCD applications.     

Design of dual‐outputs‐single‐stage a‐Si:H TFT gate driver for high resolution TFT‐LCD application

A new gate driver has been designed and fabricated by amorphous silicon technology. With utilizing the concept of sharing the noise free block in a single stage for gate driver, dual‐outputs signals could be generated in sequence. By increasing the number of output circuit block in proposed gate driver, number of outputs per stage could also be adding that improves the efficiency for area reduction. Besides, using single driving thin‐film‐transistor (TFT) for charging and discharging, the area of circuit is also decreased by diminishing the size of pulling down TFT. Moreover, the proposed gate driver has been successfully demonstrated in a 5.5‐inch Full HD (1080xRGBx1920) TFT‐liquid‐crystal display panel and passed reliability tests of the supporting foundry.     

Amorphous‐oxide TFT backplane for large‐sized AMOLED TVs

Abstract— Amorphous‐oxide thin‐film‐transistor (TFT) arrays have been developed as TFT backplanes for large‐sized active‐matrix organic light‐emitting‐diode (AMOLED) displays. An amorphous‐IGZO (indium gallium zinc oxide) bottom‐gate TFT with an etch‐stop layer (ESL) delivered excel lent electrical performance with a field‐effect mobility of 21 cm²/V‐sec, an on/off ratio of >10⁸, and a subthreshold slope (SS) of 0.29 V/dec. Also, a new pixel circuit for AMOLED displays based on amorphous‐oxide semiconductor TFTs is proposed. The circuit consists of four switching TFTs and one driving TFT. The circuit simulation results showed that the new pixel circuit has better performance than conventional threshold‐voltage (VTH) compensation pixel circuits, especially in the negative state. A full‐color 19‐in. AMOLED display with the new pixel circuit was fabricated, and the pixel circuit operation was verified in a 19‐in. AMOLED display. The AMOLED display with a‐IGZO TFT array is promising for large‐sized TV because a‐IGZO TFTs can provide a large‐sized backplane with excellent uniformity and device reliability.     

Full‐color active‐matrix OLED based on a‐Si TFT technology

Abstract— We have successfully demonstrated a 4‐in. full‐color active‐matrix OLED display based on amorphous‐Si (a‐Si) TFT technology. With improvements in the TFT manufacturing process and structure, a‐Si TFTs provide abundant capability to drive OLEDs. This demonstration clearly shows the possibility of using a‐Si TFTs as driving backplanes in the manufacture of full‐color AMOLEDs.     

Microcavity white‐emitting OLED devices

Abstract— Microcavity designs for OLED devices with an unpatterned white emitter have the potential to provide greater brightness and larger color gamut than non‐microcavity designs while still enabling lower‐cost large‐format manufacturing. In this paper, such microcavity and non‐microcavity designs are compared. Color filters must still be employed to provide an adequate color gamut. Top‐emitter structures have somewhat greater on‐axis luminance and color gamut, but increased angular change, than bottom‐emitter designs. In a single‐stack bottom‐emitter active‐matrix TFT device using an RGBW format, the use of microcavities is estimated to reduce the average power usage by 35% and the peak power by 58%, while increasing the NTSC ratio for color gamut area by about 10%. Angular luminance and color change is likely to be acceptable, especially for hand‐held applications. Tandem devices employing multiple emitter stacks increase the lifetime of OLED devices but require larger driving voltages; for such devices, microcavity structures are useful although the percentage reduction obtained in power usage is not quite as large. Generally, tandem devices with microcavities have a slightly stronger cavity effect yielding slightly larger color gamut, but also greater angular color and luminance shift. Therefore, microcavity architectures are less appealing for tandem devices.     

Instability of light illumination stress on amorphous In–Ga–Zn–O thin‐film transistors

Amorphous In–Ga–Zn–O thin‐film transistors (TFTs) have attracted increasing attention due to their electrical performance and their potential for use in transparent and flexible devices. Because TFTs are exposed to illumination through red, green, and blue color filters, wavelength‐varied light illumination tests are required to ensure stable TFT characteristics. In this paper, the effects of different light wavelengths under both positive and negative V_GS stresses on amorphous In–Ga–Zn–O TFTs are investigated. The TFT instability that is dependent on optical and electrical stresses can be explained by the charge trapping mechanism and interface modification.