Hyang-Shik Kong

42.1: A Novel Four-Mask-Count Process Architecture for TFT-LCDs

High production cost of the TFT-LCDs is closely related to the complication of the fabrication process. To simplify the fabrication process, it is critical to reduce the number of mask-count. In this paper, 4-mask-count TFT-array process using slit (or gray-tone) photolithography technology is introduced. Various kinds of the geometrical slit designs and the exposure conditions have been simulated and experimentally optimized. By adopting this novel process, TFT channel is defined without using additional photolithography.

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

— 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.

Characteristics of a-SiGe:H Thin Film Transistor Infrared Photosensor for Touch Sensing Displays

Photo-generation properties in an amorphous SiGe:H thin film transistor (TFT) were characterized by analyzing the electrical performances under illumination. The a-SiGe:H TFT showed a higher photo sensitivity at the wavelength of IR regime, and as a-SiGe:H layer was thicker, the generated photo currents proportionally increased due to the higher photon absorption. The increase of Ge composition led to the reduction of optical energy band gap. However, the photo sensitivity was not increased with lowering of energy band gap due to the creation of defects in the layer. The photo generation was dependent on the geometry of TFT, such as channel width and length. In particular, it was observed that regions under the source/drain electrode played a critical role in generating the photo currents which was demonstrated by technology computer-aided design simulation. Based on these, the generation of photo current and the photo field effect in a-SiGe:H TFT were interpreted using band diagrams. A-SiGe:H IR photosensor-embedded liquid crystal display panel was successfully demonstrated, indicating that these can be useful to broad use in the photo-type touch sensor, photo detector, and solar cell devices.

Overlay accuracy on a flexible web with a roll printing process based on a roll-to-roll system

For high-quality flexible devices from printing processes based on Roll-to-Roll (R2R) systems, overlay alignment during the patterning of each functional layer poses a major challenge. The reason is because flexible substrates have a relatively low stiffness compared with rigid substrates, and they are easily deformed during web handling in the R2R system. To achieve a high overlay accuracy for a flexible substrate, it is important not only to develop web handling modules (such as web guiding, tension control, winding, and unwinding) and a precise printing tool but also to control the synchronization of each unit in the total system. A R2R web handling system and reverse offset printing process were developed in this work, and an overlay between the 1st and 2nd layers of ±5μm on a 500 mm-wide film was achieved at a σ level of 2.4 and 2.8 (x and y directions, respectively) in a continuous R2R printing process. This paper presents the components and mechanisms used in reverse offset printing based on a R2R system and the printing results including positioning accuracy and overlay alignment accuracy.

Effect of Self-Assembled Monolayer (SAM) on the Oxide Semiconductor Thin Film Transistor

In this paper, we proposed the self-assembled monolayer (SAM) as a protection layer against plasma and chemically induced damages to the back interface of an oxide semiconductor during the deposition of the passivation layer. When a thin-film transistor (TFT) is passivated with plasma-enhanced chemical-vapor deposition (PECVD) SiOx and solution-based materials, the back interface of the oxide semiconductor could be exposed to plasma and chemically induced damages, respectively. We employed SAMs on the back surface of the oxide semiconductor prior to the passivation process to suppress such damage. The hydrophobic Cl-SAM (3-chloropropyltriethoxysilane) suppressed the degradation in mobility and subthreshold slope (SS) due to ion bombardment during plasma treatment. The hydrophobic CH3-SAM (octyltriethoxysilane) successfully blocked chemically induced damage due to solution-based passivation.

High-mobility material research for thin-film transistor with amorphous thallium–zinc–tin oxide semiconductor

The applicability of thallium–zinc–tin oxide (TlZnSnO) as a channel material for a thin-film transistor (TFT) was investigated by first-principles simulation and cosputtering experiment with XZnSnO (X = Al, Ga or In). The electron effective mass (m*) of Tl0.4ZnSnO was simulated to be ~0.153, which is much smaller than that of In0.4ZnSnO (0.246). An In0.4ZnSnO TFT exhibited a mobility (μ) of 32.0 cm2 V−1 s−1 in the experiment; therefore, the Tl0.4ZnSnO TFT was expected to have a higher mobility of approximately 50 cm2 V−1 s−1 following the relation (μ ∝ 1/m*). Moreover, the Tl-related oxide semiconductor would provide better TFT stability because its oxide vacancy is more stable than that of an In-related oxide semiconductor.

Abnormal Behavior of Threshold Voltage Shift in Bias-Stressed a-Si:H Thin Film Transistor under Extremely High Intensity Illumination

We report on the unusual behavior of threshold voltage turnaround in a hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT) when biased under extremely high intensity illumination. The threshold voltage shift changes from negative to positive gate bias direction after ∼30 min of bias stress even when the negative gate bias stress is applied under high intensity illumination (>400 000 Cd/cm(2)), which has not been observed in low intensity (∼6000 Cd/cm(2)). This behavior is more pronounced in a low work function gate metal structure (Al: 4.1-4.3 eV), compared to the high work function of Cu (4.5-5.1 eV). Also this is mainly observed in shorter wavelength of high photon energy illumination. However, this behavior is effectively prohibited by embedding the high energy band gap (∼8.6 eV) of SiOx in the gate insulator layer. These imply that this behavior could be originated from the injection of electrons from gate electrode, transported and trapped in the electron trap sites of the SiNx/a-Si:H interface, which causes the shift of threshold voltage toward positive gate bias direction. The results reported here can be applicable to the large-sized outdoor displays which are usually exposed to the extremely high intensity illumination.

Ring-shaped stain patterns driven by solute reactive mesogens in liquid crystal solution

We report on the formation of ring-shaped stain patterns in a polymer-stabilized patterned vertical alignment mode liquid crystal display (LCD) during the cell filling process. Through the interpretation of the formation mechanism, an effective way to control its development is provided. Systematic trace of the reactive mesogens reveals that the formation of patterns is strongly related to the segregation of solute mesogens in the stain area. These undesirable patterns can be avoided or controlled by reducing the drop volume at each droplet using an inkjet printing technique, meaning that the printing technique could be a useful solution in display technology. For the formation of ring-shaped patterns, the dragging of reactive mesogens during the spreading of the liquid crystal solution plays a key role in the closed LCD cell.