Novel LTPS technology for large substrate

We developed partial laser anneal silicon (PLAS) thin‐film transistor (TFT) of novel low‐temperature polycrystalline‐silicon (LTPS) technology, which had the mobility of 28.1 cm²/Vs lager than that of mass produced oxide TFT and photo‐stability comparable with that of LTPS TFT in bottom gate structure. This innovative technology enables the conversion from an α‐Si TFT to a high‐mobility TFT most easily and inexpensively. Moreover, there is no limit of substrate size, such as Gen10 and more. Photo‐stability of PLAS will be suitable to organic light‐emitting diode backplane, high‐dynamic range TV, and outdoor IDP.     

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.     

Microcrystalline‐silicon thin films by physical vapor deposition for wide‐area low‐temperature polysilicon production

A sputtered microcrystalline‐silicon thin film deposited on unannealed Corning Code 1737 glass has been shown to transform to polycrystalline material after excimer‐laser annealing at low energy densities, and below the full‐melt threshold. The homogeneous large‐grain polysilicon films obtained show promise for high‐yield manufacturing of large‐area LTPS displays. The material properties of the films will be presented and the properties of devices fabricated in the films will be discussed.     

Active‐matrix OLED backplanes based on LTPS for small molecules and polymers

Abstract— A four‐mask low‐temperature poly‐Si (LTPS) TFT process for p‐ and n‐channel devices has been developed. PECVD‐deposited amorphous silicon was recrystallized to polycrystalline‐silicon with single‐area excimer‐laser crystallization, while the gate dielectric was fabricated by PECVD deposition of a SiH₄‐N₂O‐based silicon oxide. Formation of drain and source was carried out with self‐aligned ion‐beam implantation. To prove the potential capability of these devices, which are suitable for conventional and inverted OLEDs alike, several functional active‐matrix backplanes implementing different pixel circuits have been produced. This active‐matrix backplane process has been customized to drive small molecules as well as polymers regardless if its structure is top or bottom emitting.     

Hysteresis analysis in excimer‐laser‐annealed low‐temperature polycrystalline‐silicon thin‐film transistors

To analyze the hysteresis phenomenon in p‐channel low‐temperature polycrystalline‐silicon thin‐film transistors (LTPS TFTs), the direct correlation between the hysteresis and the interface (N_it) and the grain‐boundary trap density (N_trap) has been investigated. To fabricate LTPS TFTs with different electrical properties and trap types, the thickness of a‐Si was varied from 30 to 80 nm and crystallized by the excimer‐laser‐anneal (ELA) method. The interface trap density is extracted from the subthreshold slope (SS) and low‐high‐frequency C‐V analysis, while the grain‐boundary trap density is extracted by the Levinson and Proano method. The LTPS TFTs with smaller hysteresis exhibited a lower trap density. From the correlation between extracted parameters, the hysteresis seems to be more dependent on N_it and decreases when the film thickness increases to 80 nm while the N_trap is almost the same in all devices.     

Digital‐to‐analog converter with gamma correction on glass substrate for TFT‐panel applications

Abstract— Low‐temperature polysilicon (LTPS) technology has a tendency towards integrating all circuits on glass substrate. However, the poly‐Si TFTs suffered poor uniformity with large variations in the device characteristics due to a narrow laser process window for producing large‐grained poly‐Si TFTs. The device variation is a serious problem for circuit realization on the LCD panel, so how to design reliable on‐panel circuits is a challenge for system‐on‐panel (SOP) applications. In this work, a 6‐bit R‐string digital‐to‐analog converter (DAC) with gamma correction on glass substrate for TFT‐panel applications is proposed. The proposed circuit, which is composed of a folded R‐string circuit, a segmented digital decoder, and reordering of the decoding circuit, has been designed and fabricated in a 3‐μm LTPS technology. The area of the new proposed DAC circuit is effectively reduced to about one‐sixth compared to that of the conventional circuit for the same LTPS process.     

High‐performance thin‐film transistor using silicide‐mediated crystallization

Abstract— We studied the silicide‐mediated crystallization of a‐Si for low‐temperature polycrystalline‐silicon (LTPS) on glass. By controling the heating method and Ni density on the a‐Si, the grain size could be increased to 40 μm. Radial grain growth from a NiSi₂ crystalline nucleus gives rise to a large‐grain poly‐Si without amorphous phase inside. A field‐effect mobility of over 200 cm²/V‐sec was achieved by using LTPS.     

A flexible 2.1‐in. active‐matrix electrophoretic display with high resolution and a thickness of 100 μm

Abstract— A paper‐thin QVGA, flexible 2.1‐in. active‐matrix electrophoretic display (AMEPD) that features 100‐μm thick and a 192‐ppi resolution has been developed. An LTPS‐TFT backplane with integrated peripheral driver circuits was first fabricated on a glass substrate and then transferred to a very thin (30‐μm) plastic film by employing surface‐free technology by laser ablation/annealing (SUFTLA®). A micro‐encapsulated electrophoretic imaging sheet was laminated on the backplane. A supporting substrate was used to support the LTPS‐TFT backplane. Fine images were successfully displayed on the rollable AM‐EPD. The integrated driver circuits dramatically reduce the number of external connection terminals, thus easily boosting the reliability of electrical connections even on such a thin plastic film.     

High refresh rate and low power consumption AMOLED panel using top‐gate n‐oxide and p‐LTPS TFTs

A pixel circuit and a gate driver on array for light‐emitting display are presented. By simultaneously utilizing top‐gate n‐type oxide and p‐type low‐temperature polycrystalline silicon (LTPS) thin‐film transistors (TFTs), the circuits provide high refresh rate and low power consumption. An active‐matrix LED (AMOLED) panel with proposed circuits is fabricated, and driving at various refresh rate ranging from 1 to 120 Hz could be achieved.     

A self‐reset ambient‐light sensor system for low‐temperature polycrystalline‐silicon active‐matrix displays

Abstract— A new digital ambient‐light sensor system has been designed and fabricated on a glass substrate using a conventional low‐temperature polycrystalline‐silicon (LTPS) technology. In the proposed system, analog‐to‐digital conversion (ADC) is performed in the time domain instead of the voltage domain and is combined with a light‐detection process. The proposed system employs self‐reset architecture and requires only one comparator for n‐bit digital output. Because the complex analog circuitry is eliminated from the system, it can be readily integrated on the glass substrate.     

Threshold‐voltage‐compensation methods for AMOLED pixel and analog buffer circuits

Abstract— New pixel‐circuit designs for active‐matrix organic light‐emitting diodes (AMOLEDs) and a new analog buffer circuit for the integrated data‐driver circuit of active‐matrix liquid‐crystal displays (AMLCDs) and AMOLEDs, based on low‐temperature polycrystalline‐silicon thin‐film transistors (LTPS‐TFTs), were proposed and verified by SPICE simulation and measured results. Threshold‐voltage‐compensation pixel circuits consisting of LTPS‐TFTs, an additional control signal line, and a storage capacitor were used to enhance display‐image uniformity. A diode‐connected concept is used to calibrate the threshold‐voltage variation of the driving TFT in an AMOLED pixel circuit. An active load is added and a calibration operation is applied to study the influences on the analog buffer circuit. The proposed circuits are shown to be capable of minimizing the variation from the device characteristics through the simulation and measured results.     

Current‐mode ambient‐light sensing circuit using p‐type low‐temperature polycrystalline‐silicon TFTs and p‐i‐m diodes

Abstract— A current‐mode ambient‐light sensing circuit, which is composed of p‐intrinsic‐metal (p‐i‐m) diodes and p‐type low‐temperature polycrystalline‐silicon (LTPS) thin‐film transistors (TFTs) for autobrightness control of display panels. The proposed sensing circuit exhibits a wide dynamic range of 56 dB, while the output signal range is 1.8 times wider than that of a previously reported sensing circuit.     

Optimization of low‐temperature poly‐Si TFT‐LCDs and a large‐scale production line for large glass substrates

Abstract— An update of the progress of inherently low‐temperature poly‐Si (LTPS) technologies, such as ELA, ion doping, and activation in conjunction with chemical vapor deposition (CVD) and photolithography will be given. We will also discuss whether LTPS LCDs will be applied to a large‐scale production line using a large motherglass substrate. It was found that a more‐powerful excimer laser as well as photolithography with higher‐resolution and a more‐precise overlaid arrangement would enable a large‐scale production line handling motherglass of 4th generation size to be constructed in the very near future with reasonable investment and productivity costs.     

Value‐added integration of functions for silicon‐on‐glass (SOG) based on LTPS technologies

Abstract— Low‐temperature polycrystalline‐silicon (LTPS) TFT‐LCDs are on their way to becoming advanced displays, which will lead to the use of system‐on‐glass (SOG) technology. Improvement in poly‐Si TFT performance is essential in order for them to become value‐added displays, where circuits for various functions are integrated onto the substrate. In this paper, the key processes of the applications for future displays will be described, including the recent development of SOG.     

LTPS circuit integration for system‐on‐glass LCDs

Abstract— A 3.5‐in. QVGA‐formatted driving‐circuit fully integrated LCD has been developed using low‐temperature poly‐Si (LTPS) technology. This display module, in which no external ICs are required, integrates all the driving circuits for a six‐bit RGB digital interface with an LTPS device called a “FASt LDD TFT” and achieves a high‐quality image, narrow frame width, and low power consumption. The LTPS process, device, and circuit technologies developed for system‐on‐glass LCD discussed. The development phase of LTPS circuit integration for system‐on‐glass LCDs is also reviewed.     

Current and future technology of low‐temperature poly‐Si TFT‐LCDs

Abstract— The state of the art of large‐area low‐temperature TFT‐LCDs will be reported in this paper. High‐performance poly‐Si TFTs are expected to realize various applications such as system display where various signal‐processing functions are added to the display. In the past few years, low‐temperature poly‐Si thin‐film‐transistor (LTPS TFT) technology has made great progress, especially in the areas of excimer laser annealing (ELA) of high‐quality poly‐Si film, ion doping for large‐area doping, and high‐quality gate SiO₂ film formation by using the low‐temperature PE‐CVD method. Also, technology trends and possible applications, such as a system displays, will be discussed.     

Comparison of a‐Si and Poly‐Si for AMOLED displays

Abstract— The characteristics of OLED backplanes including the intrinsic properties of a‐Si TFTs and LTPS TFTs will be reviewed. While LTPS TFTs reveal satisfactory stability in AMOLED‐display applications, a‐Si AMOLEDs show better uniformity and are capable of driving OLEDs. However, the stability of a‐Si TFTs under long‐term operation is still unacceptable and remains to be the key issue constraining the commercialization of a‐Si TFT AMOLEDs.     

A highly sensitive and low‐noise IR photosensor based on a‐SiGe as a sensing and noise filter: Toward large‐sized touch‐screen LCD panels

Abstract— The a‐SiGe TFT photosensor for embedded touch‐screen panels (TSPs) was characterized by comparison with an a‐Si sensor. The photoresponse of an a‐SiGe sensor at a 850‐nm wavelength was much higher than that of a‐Si, indicating that a‐SiGe is a strong candidate material for an IR sensor. In order to increase the signal‐to‐noise ratio, the incident visible light was filtered by incorporating a bandpass‐filter layer. An a‐SiGe IR‐sensor‐embedded LCD panel was successfully demonstrated, showing an excellent multitouch property independent of ambient‐light conditions. This technology can be widely used in multifunctional TSPs.     

Active‐matrix sensor in AMLCD detecting liquid‐crystal capacitance with LTPS‐TFT technology

Abstract— An active‐matrix capacitive sensor for use in AMLCDs as an in‐cell touch screen has been developed. Pixel sensor circuits are embedded in each pixel by using low‐temperature polycrystalline‐silicon (LTPS) TFT technology. It detects a change in the liquid‐crystal capacitance when it is touched. It is thin, light weight, highly sensitive, and detects three or more touch events simultaneously.     

Alleviation of abnormal NBTI phenomenon in LTPS TFTs on polyimide substrate for flexible AMOLED

This letter investigates the negative‐bias temperature instability (NBTI) behavior of p‐channel low‐temperature polycrystalline silicon thin‐film transistors (LTPS TFTs) on plastic substrate. The measurements reveal that the threshold‐voltage positive shift is highly correlated to the passivation of grain boundary trap states. By applying the established phenomenon such as NBTI recovery and H diffusion from PI substrate, a new model is introduced to explain the mechanism and verified by the experiment. With the thick buffer and bottom metal layer or newly processed PI substrate, we succeeded in adjusting the NBTI behavior of LTPS TFTs on plastic substrate.