Active Matrix Liquid Crystal Display Research Articles

P-168 : LTPS PMOS Four-Mask Process for AMLCDs

Simple p-channel poly-Si TFT (Thin Film Transistor) structure (PMOS 4mask structure) for AMLCD (active matrix liquid crystal display) was developed. Compared with a conventional PMOS 6mask process, two photo mask steps, passivation-hole mask and pixel mask, has been eliminated to create 4mask process. Both ELA (Eximer Laser Annealing) and SLS (Sequential Lateral Solidification) method were used for the formation of a poly-Si. By using PMOS 4mask process, 10.4-inch XGA (1024×768) AMLCD panel with integration of the gate-driver and demultiplexer was successfully realized.

A study of different types of current mirrors using polysilicon TFTs

Polysilicon thin-film technology has become of great interest due to the demand for large area electronic devices. Active matrix liquid crystal displays (AMLCDs) and active matrix organic light emitting displays (AMOLEDs) are among the fields where polysilicon thin-film transistors (poly-Si TFTs) are most commonly used. Such devices, generally, require analog signal processing. This fact makes the performance of basic analog blocks, such as current mirrors implemented with poly-Si TFTs, crucial. This paper examines the performance of various current mirror designs through simulation. Finally, a novel design of a current mirror is proposed aimed to be used in low voltage applications.

Dual select diode AMLCDs: A path towards scalable two-mask array designs

— In this paper an alternative active-matrix LCD technology is described with scalable, low-cost processing. The Dual Select Diode AMLCD needs less than 40% of the capital investment in the array process compared to a-Si TFT arrays and results in 20% lower-cost LCD modules. Development at several AMLCD manufacturers is in progress.

Goniometric and conoscopic measurements of angular display contrast for one‐, three‐, five‐, and nine‐million‐pixel medical liquid crystal displays

Active-matrix liquid crystal displays (AMLCDs) have become the preferred choice for displaying digital diagnostic images because of their low cost of ownership, high contrast, and small footprint. However, the contrast and luminance of AMLCDs changes significantly with viewing direction. In this paper, we use a goniometric and a conoscopic method to measure angular contrast and luminance in one-, three-, five-, and nine-million-pixel medical AMLCDs. The viewing angle characteristics of the displays are compared by evaluating the compliance with the desired grayscale and recommended tolerance limits. Using the measured contrast responses, we determined the angle along each of the orthogonal and diagonal axes for which the luminance ratio drops to 0.5, 0.2, and 0.1 of the maximum ratio, seen at perpendicular viewing. The results show a reduction as large as 54% in available JND levels between the perpendicular and off-normal viewing directions. All of the display systems proved to have better angular response in the horizontal and vertical direction as compared to the diagonal directions, and none were able to satisfy a 20% contrast tolerance limit for any viewing direction lying in a cone at an angle of 45 degrees from the normal. Additionally, we show that the measured contrast can be significantly affected by the measurement method used to record the angular change in luminance.

LCD vs. CRT: Effects on Performance of Office Work

The present display technology transition in computing from CRT to LCD monitors poses many human factors research questions. In this study, computer monitor technologies were compared, using two CRT and two active-matrix LCD monitors under four lighting levels. Ten participants took part in a 4times4 within-subjects experimental design where they performed word processing, data entry, and web-browsing tasks. Results show that one of the LCD monitors was associated with poorer performance on the word processing task than an equivalent grouping of the other three monitors. Thus, it is concluded that active-matrix LCD monitors are capable of equitable performance with the traditional CRT technology, but it is uncertain whether the technologies themselves are equivalent. Lighting was another significant effect, with the dark (0 lux) and very bright (1200 lux) conditions causing poorer performance. There was no interactive effect between monitors and lighting. The battery of tasks used in this study, representative of office work, is also discussed.

Effect of Viewing Angle on Luminance and Contrast for a Five-Million-Pixel Monochrome Display and a Nine-Million-Pixel Color Liquid Crystal Display

Digital imaging systems used in radiology rely on electronic display devices to present images to human observers. Active-matrix liquid crystal displays (AMLCDs) continue to improve and are beginning to be considered for diagnostic image display. In spite of recent progress, AMLCDs are characterized by a change in luminance and contrast response with changes in viewing direction. In this article, we characterize high pixel density AMLCDs (a five-million-pixel monochrome display and a nine-million-pixel color display) in terms of the effect of viewing angle on their luminance and contrast response. We measured angular luminance profiles using a custom-made computer-controlled goniometric instrument and a conoscopic Fourier-optics instrument. We show the angular luminance response as a function of viewing angle, as well as the departure of the measured contrast from the desired response. Our findings indicate small differences between the five-million-pixel (5 MP) and the nine-million-pixel (9 MP) AMLCDs. The 9 MP shows lower variance in contrast with changes in viewing angle, whereas the 5 MP provides a slightly better GSDF compliance for off-normal viewing.

Organic Thin Film Transistor-Driven Liquid Crystal Displays on Flexible Polymer Substrate

In this report, organic thin film transistor (OTFT)-driven active-matrix liquid crystal display (AM-LCD) on flexible polymer substrate is demonstrated. A polymer material, poly (3-hexylthiophene) (P3HT) was used as the active layer and printed by a rubber stamp printing method using a prepatterned silicone elastomer stamp. With this method, the active layer was easily printed on the device without further patterning process. The saturation field-effect mobility of the rubber-stamp-printed TFT array was 0.025 cm2/V·s and the on/off ratio was 104. From the results, a 2'' OTFT-LCD panel with 35×24 pixels was successively fabricated and operated using a polycarbonate substrate. In addition, we investigated the reliability of the OTFT device under various conditions with or without a passivation layer to realize more stable devices.

AAPM/RSNA tutorial on equipment selection: PACS equipment overview: display systems.

Display systems are key components of the digital radiology department. Current display systems for medical imaging are based on cathode-ray tubes (CRTs) or active-matrix liquid crystal displays (AMLCDs). The CRT is a cathodoluminescent display: Light is generated by exciting a luminescent material with energetic electrons. AMLCDs are light-modulating devices that form the image in the screen by controlling the transparency of individual display pixels. Many image quality aspects of CRTs are determined by the way the pixel luminance is generated in the cathodoluminescent screen. The resolution properties of AMLCDs are much better than those of CRTs. In CRT devices, phosphor granularity and raster scanning patterns are the main components of spatial noise. In AMLCDs, the most notable feature of the noise characteristic is the subpixel structure of complex pixel designs used in medical displays. The small-spot contrast of CRTs is dominated mainly by veiling glare and reflections of ambient illumination. In addition to display reflectance, the contrast of medical AMLCDs is affected by crosstalk and by variations of the luminance at off-normal viewing angles.

3.5: Novel Touch Sensitive In‐Cell AMLCD

AbstractA touch sensitive Active Matrix Liquid Crystal Display (AMLCD) that obviates the need to have a transparent touch panel mounted on top of the LCD, in the viewer side, is realized. This is achieved through the introduction of a resistive touch‐sensitive mechanical switch into the pixel. The combined display driving and touch sensing electronics allow for a limited change in the display driving architecture.

High-performance polycrystalline silicon thin-film transistor with multiple nanowire channels and lightly doped drain structure

This investigation examines polycrystalline silicon thin-film transistors (TFTs) with multiple nanowire channels and a lightly doped drain (LDD). A device with an LDD structure exhibits low leakage current because the lateral electrical field is reduced in the drain offset region. Additionally, multiple nanowire channels can generate fewer defects in the polysilicon grain boundary and have more efficient NH3 plasma passivation than single-channel TFTs, further reducing leakage current. They exhibit superior electrical characteristics to those of single-channel TFTs, such as a higher ON/OFF current ratio (>108), a better subthreshold slope of 110 mV/decade, an absence of drain-induced barrier lowering, and suppressed kink-effect. Devices with the proposed TFTs are highly promising for use in active-matrix liquid-crystal display technologies.

Process-independent analogue data driver for polysilicon TFT AMLCD

Polycrystalline silicon (poly-Si) thin film transistor (TFT) technology is very suitable for driving an active matrix LCD (AMLCD) panel as the driver circuit, and the panel can be integrated on the same substrate. This allows the entire display system to be thin and makes the concepts of ‘TV on wall’ and ‘sheet computer’ possible. However, the large variation of threshold voltage of poly-Si TFT across the wafer makes it difficult to obtain analogue amplifiers with constant gain and phase margin. In this paper, an analogue data driver for the poly-Si TFT AMLCD is proposed. An operational amplifier with a gate bias-voltage generation circuit for this analogue data driver, with characteristics independent of variations in threshold voltage, will be presented. In Hspice simulation, with threshold voltage varying from 2.5 V to 4.5 V, gain variations of the proposed amplifier were reduced from ±10 dB to ±0.2 dB and phase margin variations were reduced from 10° to 0.37° compared with typical operational amplifier design. This enables the analogue data driver for AMLCD to be implemented in poly-Si TFT technology.

Fabrication and electro-optic characteristics of polymer-stabilized V-mode ferroelectric liquid crystal display and intrinsic H-V-mode ferroelectric liquid crystal displays: their application to field sequential fullcolor active matrix liquid crystal displays

Shunsuke KobayashiJun XuHirokazu FurutaYuji MurakamiSatoru KawamotoMasafumi Oh-kouchiTokyo University of ScienceLiquid Crystal Institute and the GraduateSchool of Engineering ScienceYamaguchi, OnodaYamaguchi 756-0884, JapanHiroshi HasebeHaruyoshi TakatsuDainippon Ink. and Chemical Ind.Ina-machi, Kitaadachi-gunSaitama 362-8577, JapanAbstract. We fabricate ferroelectric liquid crystal display (FLCD) de-vices exhibiting a thresholdless V-shaped switching by adopting a me-sogenic liquid crystal polymer stabilization and also FLCDs exhibitinghalf-V-shaped switching using an undoped FLC material. These FLCDsare respectively called polymer-stabilization (PS)-V-mode FLCDs and in-trinsic H-V-mode FLCDs. By using especially developed polyimide ma-terials such as RN-1199 and RN-1411 series (from Nissan Chem., Ind.)for the surface alignment and together with a well-controlled rubbingtechnique, photoalignment, and ion-beam alignment, we succeed in fab-ricating zigzag defect-free FLCDs. In particular, the PS-V-mode FLCDfabricated in this way exhibits continuous operation with a high contrastratio of 700:1, and the intrinsic H-V-mode FLCD has good temperaturestability by using the combined alignment technique. Particularly, a field-sequential full-color (FS-FC) LCD using PS-V-mode FLCD shows goodlong-term stability for a continual operation. Using these FLCDs, we alsodevelop several prototype models of FS-FC active matrix LCDs with thespecification of video graphics arrays (VGAs) and extended graphicsarrays (XGAs), where we take advantage of their high-speed response(time constants 100 to 500 ms).

MODELING AND SIMULATION OF PARASITIC CAPACITANCES FOR ACTIVE MATRIX LIQUID CRYSTAL DISPLAYS (AMLCDs)

In this paper, we present a compact approach for modeling a parasitic capacitance in active matrix liquid crystal displays (AMLCDs) as a function of voltage applied at pixel electrodes. The director distribution as well as the potential distribution is estimated from a three-dimensional finite element method (FEM), while the parasitic capacitance within a liquid crystal cell is calculated with an energy moment method. Furthermore, we applied our proposed approach on the VA (vertical alignment) mode LC cell with a chevron-type electrode structure in order to understand the dynamic behavior of the cell.

Liquid Crystal Mixtures with Low Birefringence for Active-Matrix LCD

The liquid crystal (LC) mixtures with fused ring systems such as Decahydronaphthalenes and Tetrahydronaphthalenes for reflective or transflective active-matrix liquid crystal display (AM-LCD) have higher durability than LC mixtures with ester-components. These LC mixtures also have some problems such as low nematic-isotropic transition temperature TNI and high driving voltage. The improvement for driving voltage was carried out by using Tetrahydronaphthalene-component with moderate birefringence Δn and large dielectric anisotropy Δ˜. However, It was difficult to obtain LC mixtures with sufficiently low driving voltage, because of low TNI and the comparatively high Δn of tetrahydronaphthalene. We have improved these problems by introducing some newly developed LC components and optimization. We obtained the LC mixtures with low Δn and low driving voltage; less than 0.075 of Δn and about 4 V or less of driving voltage. These LC mixtures have high durability with stable nematic phase at low temperature.

Design, Synthesis and Physical Properties of New Liquid Crystal Materials Having a Fluoro-Substituted Tetrahydronaphthalene Structure for Active Matrix LCD

Liquid crystal materials having a fluoro-substituted tetrahydro-naphthalene structure were designed for active matrix LCD. 6-Aryl-5-fluoro-1,2,3,4- tetrahydronaphthalenes and 5,6-difluoro-1,2,3,4-tetrahydronaphthalenes were prepared by regioselective fluorination and hydrogenation of 2-naphthols. 6-Aryl-5,7-difluoro-1,2,3,4-tetrahydronaphthalenes were synthesized from 5,7-difluoro-1,2,3,4-tetrahydronaphthalene-2-one. The liquid crystal mixture containing these compounds with very good co-solubility exhibits a wide nematic temperature range with low melting point and very large value of dielectric anisotropy. From these results these compounds are useful for design of new liquid crystal mixtures for TFT-displays.

Directional nickel-induced fielded aided lateral crystallization of amorphous silicon

For application to active matrix liquid crystal displays (AMLCDs), a low temperature (<900 K) process for the production of polycrystalline silicon is required to permit the use of inexpensive glass substrates. This would allow the integration of drive electronics onto the display panel. Current low temperature processes include excimer laser annealing and solid phase crystallization, both of which are currently unsuitable for the fabrication of low cost, large area devices. The addition of small amounts of metal (e.g., Ni) to the amorphous silicon has been shown to significantly reduce the solid phase crystallization temperature. The rate of this solid phase metal induced crystallization can also be increased as a result of the presence of an electric field. The work presented here reports directional crystallization of amorphous silicon thin films during heat treatment in the presence of an electric field. Models are proposed for metal induced crystallization with and without an applied electric field in which a reaction between Ni and Si to produce NiSi is the rate-limiting step. The crystallization rate is increased through the application of an electric field that leads to the drift of Ni ions to the growth front. The model and experimental data reveal that the crystallization rate increases exponentially with temperature and linearly with electric field strength.

AMLCD flicker model considering the V/sub T/ shift in a-Si:H TFT

The observed development of flicker with time in active matrix liquid crystal display (AMLCD) panels has been correlated to the threshold voltage shift of the thin film transistors (TFTs) used in panels. The effect of a threshold voltage shift on the panel flicker was calculated using a flicker model developed here. One set of AMLCD panels used for the flicker characterization employed top-gate TFTs, while a second set employed bottom-gate TFTs. Optical measurements to determine the flicker voltage as a function of aging were performed on both types of panels. Electrical characterization of the TFTs extracted the threshold voltage from the C-V measurement. Samples of the TFTs were available on the kerf strips for both panel types, and ensured that the optically characterized panels and the TFTs electrically characterized were manufactured in the same batch for each of the two panel types.

Angular dependence of the luminance and contrast in medical monochrome liquid crystal displays.

Active-matrix liquid crystal displays (AMLCDs) are light-modulating devices that generate images by differentially transmitting a nearly uniform luminous field provided by a backlight. While emissive displays exhibit a quasi-Lambertian emission with almost constant contrast at off-normal viewing, the anisotropy of the electro-optic effect that controls light transmission in AMLCDs causes a pixel luminance that varies, sometimes strongly, with viewing angle. These variations are not identical for all gray levels and can eventually cause grayscale inversions. In this paper, we measured the luminance emission of a monochrome medical AMLCD, a medical cathode-ray tube monitor, and a color desktop AMLCD, using a collimated photopic probe positioned on a manual rotation arm, and a research radiometer with automatic readout. The probe measures luminance with a small acceptance angle and provides optical shielding from emissions at other viewing directions that contaminate the readings. We obtained luminance response curves versus angle in the vertical, horizontal and at 45 degrees diagonal directions. The display systems were calibrated to reflect the DICOM Part 3.14 standard grayscale display function (GDF) when measured using the manufacturer's probe and software tools. We analyzed the measurements at different viewing directions with respect to their departure from the GDF by computing the normalized contrast (deltaL/L) as a function of the DICOM just-noticeable difference index. Although cathode-ray tubes are known to be quasi-Lambertian emitters, the luminance at normal viewing is higher than the luminance observed at large angles. This decrease in luminance is however proportionally similar for all gray levels, resulting in a relatively flat contrast response for all angles. In addition to being more pronounced, the angular variation in AMLCDs does not follow the same profile at different intensities with the subsequent variation in the achieved display contrast. The changes due to off-normal viewing are substantial at large angles in the horizontal and vertical directions, and much worse in the diagonal viewing directions.

Novel liquid‐crystal materials for AMLCDs

Abstract— We have developed novel liquid‐crystal (LC) materials with a difluoromethyleneoxy CF2O) moiety as the linkage group in order to satisfy the various requirements of active‐matrix liquid‐crystal displays (AMLCDs). We measured the physical properties of the CF2O LC materials. The novel CF2O LC materials have excellent physical properties, that is, high dielectric anisotropy, low viscosity, a broad nematic range, high reliability, etc. It was revealed that several advantages can be obtained by the introduction of CF2O moiety into the LC molecules. The mixture containing CF2O LC materials for various applications indicated a decrease in rotational viscosity of about 15–25% compared with that for conventional LC mixtures. Also, these novel mixtures with CF2O LCM demonstrated a higher voltage‐holding ratio (VHR). It was confirmed that mixtures containing CF2O LC materials have suitable characteristics for various applications, for example, low‐driving‐voltage note‐book PCs, quick response monitors, and transflective applications.

Threshold‐voltage drift of amorphous‐silicon TFTs in integrated drivers for active‐matrix LCDs

Abstract— The threshold‐voltage drift of a‐Si TFTs for AMLCD integrated drivers was studied. Analysis of the drift shows two different kinetics. Both charges trapped in the insulator and defect generation in the a‐Si layer occur simultaneously. However, after a period of time charge trapping in the insulator becomes predominant.