Active Matrix Liquid Crystal Research Articles

Employ Present Five Masks Amorphous Silicon Thin-Film Transistor Design and Process Flow to Realize 5-in. InGaZnO Active-Matrix Liquid Crystal Display with Improved Stress Stability

The present five masks bottom gate and staggered amorphous silicon (a-Si) thin-film transistor (TFT) process flow is not suitable for indium–gallium–zinc-oxide (IGZO) TFT because of its vulnerability to post etching process during the source/drain pattern. Bottom gate and coplanar IGZO TFTs were made feasible through the reverse of second (channel layer) and third (source/drain electrodes) masks process flow to avoid etching damage problem. Besides, the post IGZO nitrous oxygen (N2O) plasma treatment was employed to improve the stress instability. On the basis of secondary ion mass spectrometry (SIMS) and X-ray spectroscopy (XPS) results, it is believed that the post N2O plasma treatment passivates the interface states and converts the inhomogeneous and low quality IGZO to the homogeneous and high quality IGZO. In the end, a 5-in. IGZO active-matrix liquid crystal display was demonstrated via five masks bottom gate and coplanar TFT configuration.

An analytical capacitance model for a hydrogenated amorphous silicon based thin-film transistor

This paper deals with the analysis of the dynamic behavior of a hydrogenated amorphous silicon based thin-film transistor and with amorphous silicon technology as an alternative technique for the implementation of amorphous silicon based digital systems for active matrix liquid crystal displays and solar cells. It presents a model that considers the channel charge and its transient characteristics using the charge oriented model. The calculated dependences of capacitances on bias voltages are given taking into account the density of states of the amorphous silicon. An amorphous silicon based inverter is taken as a case study. The proposed analysis will contribute to develop accurate and efficient models that help simulators to target amorphous silicon devices in a practical design.

Investigation of Polysilicon Thin-Film Transistor Technology for RF Applications

The low-temperature polycrystalline silicon (LTPS) thin-film transistor (TFT) is an emerging technology to manufacture active matrix liquid crystal displays. With the TFT's maximum frequency of oscillation f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> exceeding 3.5 GHz, it becomes feasible to develop integrated circuits (ICs) in LTPS TFT technology to facilitate system on panel or system on display. This paper investigates the LTPS TFT characteristics for developing RF ICs. The dc and ac equivalent-circuit models were developed for LTPS TFT RF integrated-circuit design. A phase-locked loop (PLL) was demonstrated using the 3-μm LTPS TFT technology. The supply voltage and power consumption of the PLL are 8.4 V and 25 mW, respectively. The operation frequency range of the TFT PLL is from 2 to 10 MHz, and the measured root-mean-square jitter is 235 ps.

Glass Substrates for Liquid Crystal Displays

We present a brief history of substrate glasses developed by Corning Incorporated (Corning) for use in Active matrix liquid crystal display (AMLCD) displays. The most basic attributes required of AMLCD substrates include thermal and mechanical stability, precise geometry control, a surface that is basically perfectly smooth, and no inclusions large enough to block a pixel in the final display. In addition, the glasses used as substrate materials must be essentially alkali‐free so that they do not interact chemically or electronically with thin‐film transistors (TFT). Thin, precision sheet was first made at Corning via the slot‐draw process, but was eventually moved to the fusion‐draw process; neither process was originally intended for this application. Alkali‐free glasses were originally developed for electronic applications and lamp envelopes, and considerable research was required to invent compositions that were compatible with the high‐viscosity fusion‐draw process. Examples of the technical challenges presented by the evolving industry requirements are provided, including eliminating arsenic from the substrate glass and reducing the dimensional change during high‐temperature processing of polysilicon‐based TFT.

Touch-Sensitive Active-Matrix Display with Liquid-Crystal Capacitance Detector Arrays

We present two types of touch in-cell technology for active-matrix liquid crystal displays (AMLCDs) with liquid-crystal capacitance detector arrays embedded in a hydrogenated amorphous silicon (a-Si:H)-based backplane. The scanning method can determine and provide multiple simultaneous touch locations on the basis of the information extracted from each sensing element. On the other hand, the crossing method provides the position information from the coordinate information such that the system complexity and power consumption are expected to be considerably reduced. In both cases, by limiting the operation of sensors to the subthreshold region, we expect that the degradation of a-Si:H thin-film transistors (TFTs) will be suppressed and reliability will be ensured. Their relatively simple architecture enables the proposed touch-panel function to be readily integrated within large-area displays.

Liquid crystal applications in photonics

We developed new optical switches based on nematic and ferroelectric liquid crystal (LC) cells for photonics applications. Certain new LC switches based on the effect of total internal reflection in nematic LC and deformed helix ferroelectric effect ferroelectric LC with very fast response time were developed. Fast bistable optical switches of the light polarization based on ferroelectric liquid crystal cells were proposed. The switches are characterized by 100 is switching time and 26 dB crosstalk at the wavelength of 632.8 nm and bistable, i.e., required zero power consumption in the switch state. High frequency hysteretic free electrically controlled 0–2π phase modulation of light has been proposed using a very short helix pitch (less than 400 nm) deformed helix ferroelectric liquid crystal. The electrically controlled 0–2π hysteretic free phase modulation was achieved at the driving voltage frequency up to 4 kHz and the voltage amplitude of 32 V. The application of fast V-shaped deformed helix ferroelectric ferroelectric LC (DHF-FLC) for new active-matrix liquid crystal display (LCD) and optical data processing devices is envisaged. Photoalignment technology can be very useful for the new generation of liquid crystal devices as well as in new photovoltaic, optoelectronic and photonic devices based on highly ordered thin organic layers. We have investigated the LC photoalignment in superthin tubes, which are basic elements of switchable photonic crystal/liquid crystal structures and obtained the order parameter comparable with usual homogeneous nematic LC cells. We studied LC alignment on silicon surfaces with submicrometer-sized straight and curved waveguide profiles. The liquid crystal cladding refractive index was then varied according to the applied voltage, and subsequently the microresonator resonance wavelengths were tuned. Based on our initial measurements, the free spectral range (FSR) wavelength shift within the range of 20 nm was obtained, which is comparable with a thermooptic effect. The new voltage controllable Si-based add drop filters are envisaged based on this principle.

Integrated Ambient Light Sensor with an LTPS Noise-Robust Circuit and a-Si Photodiodes for AMLCDs

Ambient light sensors have been used to reduce power consumption of Active Matrix Liquid Crystal Displays (AMLCD) adjusting display brightness depending on ambient illumination. Discrete sensors have been commonly used for this purpose. They make module design complex. Therefore it has been required to integrate the sensors on the display panels for solving the issue. So far, many kinds of integrated sensors have been developed using Amorphous Silicon (a-Si) technology or Low Temperature Polycrystalline Silicon (LTPS) technology. These conventional integrated sensors have two problems. One is that LTPS sensors have less dynamic range due to the less photosensitivity of LTPS photodiodes. The other is that both the LTPS and a-Si sensors are susceptible to display driving noises. In this paper, we introduce a novel integrated sensor using both LTPS and a-Si technologies, which can solve these problems. It consists of vertical a-Si Schottky photodiodes and an LTPS differential converter circuit. The a-Si photodiodes have much higher photosensitivity than LTPS ones, and this contributes to wide dynamic range and high accuracy. The LTPS differential converter circuit converts photocurrent of the photodiodes to a robust digital signal. In addition it has a function of canceling the influences of the display driving noises. With the circuit, the sensor can stably and accurately work even under the noises. The performance of the sensor introduced in this paper was measured to verify the advantages of the novel design. The measurement result showed that it worked in a wide ambient illuminance range of 5-55, 000lux with small errors of below 5%. It was also verified that it stably and accurately worked even under the display driving noise. Thus the sensor introduced in this paper achieved the wide dynamic range and noise robustness.

Threshold Voltage Shift Variation of a-Si:H TFTs With Anneal Time

AbstractHydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) are widely used in many areas and the most important application is in active matrix liquid crystal display. However, the instability of the a-Si:H TFTs constrains their usability. These TFTs have been annealed at higher temperatures in hope of improving their electrical performance. But, higher anneal temperatures become a constraint when the TFTs are grown on polymer-based flexible substrates. This study investigates the effect of anneal time on the performance of the a-Si:H TFTs on PEN. Thin-film transistors are annealed at different anneal times (4 h, 24 h, and 48 h) and were stressed under different bias conditions. Sub-threshold slope and the off-current improved with anneal time. Off-current was reduced by two orders of magnitude for 48 hours annealed TFT and sub-threshold slope became steeper with longer annealing. At positive gate-bias-stress (20 V), threshold voltage shift (∆Vt) values are positive and exhibit a power-law time dependence. High temperature measurements indicate that longer annealed TFTs show improved performance and stability compared to unannealed TFTs. This improvement is due to reduction of interface trap density and good a-Si:H/insulator interface quality with anneal time.

The Characteristics of n- and p-Channel Poly-Si Thin-Film Transistors with Fully Ni-Salicided S/D and Gate Structure

n- and p-channel poly-Si thin-film transistors with fully Ni-self-aligned silicided (fully Ni-salicided) source/drain (S/D) and gate structure (n- and p-channel FUSA-TFTs) have been successfully fabricated on a thick channel layer. The conventional poly-Si gate is replaced by the fully Ni-silicided gate, and the parasitic S/D resistance of the FUSA-TFTs is significantly reduced by the fully Ni-silicided S/D structure. The fully Ni-salicidation process is executed at a low temperature of for a short rapid thermal annealing time. Experimental results show that the FUSA-TFTs give increased on/off current ratio, improved subthreshold characteristics, less threshold voltage roll-off, lower parasitic S/D resistance, higher gate capacitance, and larger field-effect mobility than conventional TFTs. The FUSA-TFTs effectively suppress the floating-body effect and parasitic bipolar junction transistor action. The characteristics of the FUSA-TFTs are suitable for three-dimensional integration applications and high performance driver circuits in the active-matrix liquid crystal displays.

Large Area PECVD Technology

The rapid growth of the Active Matrix Liquid Crystal Display market since the late 1990's has been driven by demand for ever larger displays that are widely affordable. Therefore, LCD manufacturers have steadily driven their industry toward lower and lower manufacturing cost per unit area of display. The current generation, Generation 10, which is just starting, is based on a glass substrate that's over 9m2 in size. This paper discusses the challenges that were faced and how they were overcome to produce high-quality PECVD layers at high rates on such large substrates. Also discussed is how the PECVD technology that was developed for AMLCD manufacturing, was adapted to manufacture Thin Film Silicon Solar panels 5.7m2 in size. By scaling up the panel size, the cost per watt of energy from a square meter of panel has been made commercially viable in today's photovoltaic energy market.

New method for extracting the capacitance-voltage characteristics of an active-matrix liquid crystal display and its application to overdrive technology

We propose a new technique to extract the steadystate capacitance-voltage (C-V) characteristics of an active-matrix liquid crystal display (AMLCD) using its transient luminance responses. We show how to determine the capacitance of pixels in an AMLCD using the charge conservation law in a pixel and the one-to-one relation between the luminance and capacitance of LC molecules. The C-V data extracted at the ends of the first and second frame times match very well with each other, which confirms that our technique is valid. By using the extracted C-V data, we calculate overdrive voltage values according to transitions. The results show that the overdrive voltages have a linear relation with the transition voltage difference. The linear characteristics can help the overdrive hardware to be extremely simple. The ease of implementation of the overdrive technology with the linear voltage relation should facilitate further adoption and expansion of LCD products with faster response times.

Thermally stable very thin Ag films for electrodes

Low-resistivity metals such as Cu and Ag have been investigated for substitution of Al alloy films in the electrodes of thin film transistor used in active matrix liquid crystal displays. Though Ag has the drawback of agglomerating easily during heat treatment, improved thermal stability by modification of the Ag film into an Al∕Ag∕Al structure has been confirmed. In this paper, the surface morphology and electrical resistivity of this structure with various Ag layer thicknesses (from 95to50nm) are investigated. The Al∕Ag∕Al structure showed excellent stability after annealing at 600°C in vacuum, even with reduced thickness. The resistivity of the Al∕Ag(95nm)∕Al film and of the Al∕Ag(50nm)∕Al film after annealing were 1.8 and 2.4μΩcm, respectively. For comparison, properties of Ag films with the same thickness were investigated, but these films became discontinuous after annealing due to agglomeration. Modified Ag films maintained excellent properties after annealing, even when the Ag layer thickness was reduced.

Asymmetric electrical properties of fork a-Si:H thin-film transistor and its application to flat panel displays

Inverted stagger hydrogenated amorphous silicon (a-Si:H) fork-shaped thin-film transistors (TFTs) were fabricated with a five-photomask process used in the processing of the active-matrix liquid crystal displays (AM-LCDs). We investigated the asymmetric electrical characteristics of a fork-shaped a-Si:H TFT under different bias conditions. To extract the electrical device parameters, we developed asymmetric geometrical factors. Current-voltage measurements indicate that the ON-OFF current ratio of fork TFT can be enhanced significantly by choosing the outer electrode as the drain, while the field-effect mobility and threshold voltage have the identical values when different drain bias conditions are used. Finally, we developed concepts of its possible application to AM-LCDs.

Optical Sensing Circuit Using Low-Temperature Polycrystalline Silicon p-Type Thin-Film Transistors and p–Intrinsic–Metal Diode for Active Matrix Displays with Optical Input Functions

An optical sensing circuit composed of low-temperature polycrystalline silicon (LTPS) p-type thin-film transistors (TFTs) and a p–intrinsic–metal (p–i–m) diode is proposed for image scanning and touch sensing functions. Because it is a very difficult challenge to integrate both display pixels and optical sensing circuits into the restricted pixel area, the number of additional devices and control signal lines must be minimized. Therefore, two p-type TFTs, one p–i–m diode, one capacitor, and one signal line are added to display pixel for the proposed optical sensing circuit. Active matrix liquid crystal display (AMLCD) and active matrix organic light-emitting diode (AMOLED) pixels with the proposed optical sensing circuit have image scanning and touch sensing functions, respectively. Through the measurement of the proposed circuit under the condition of incident light varying from 0 to 10,000 lx, we verified that the dynamic and output ranges of the proposed circuit are 30 dB and 1.5 V, respectively.

Novel Twin Poly-Si Thin-Film Transistors EEPROM With Trigate Nanowire Structure

This letter demonstrates a novel twin poly-Si thin-film transistor (TFT) electrical erasable PROM (EEPROM) that utilizes trigate nanowires (NWs). The NW TFT EEPROM has superior gate control because its trigate structure provides a higher memory window and program/erase (P/E) efficiency over those of a single-channel one. For endurance and retention, the memory window can be maintained at 1.5 V after 103 P/E cycles and 25% charge loss for ten years of NW twin poly-Si EEPROM. This investigation explores its feasibility in future active matrix liquid crystal display system-on-panel and 3-D stacked Flash memory applications.

Temperature Coefficient of Poly-Silicon TFT and its Application on Voltage Reference Circuit With Temperature Compensation in LTPS Process

The temperature coefficient (TC) of n-type polycrystalline silicon thin-film transistors (poly-Si TFTs) is investigated in this paper. The relationship between the TC and the activation energy is observed and explained. From the experimental results, it is also found that TC is not sensitive to the deviation of the laser crystallization energy. On the contrary, channel width can effectively modulate the TC of TFTs. By using the diode-connected poly-Si TFTs with different channel widths, the first voltage reference circuit with temperature compensation for precise analog circuit design on glass substrate is proposed and realized. From the experimental results in a low-temperature poly-Si process, the output voltage of voltage reference circuit with temperature compensation exhibits a very low TC of 195 ppm/degC , between 25degC and 125degC. The proposed voltage reference circuit with temperature compensation can be applied to design precise analog circuits for system-on-panel or system-on-glass applications, which enables the analog circuits to be integrated in the active-matrix liquid crystal display panels.

Dynamic Response of Normal and Corbino a-Si:H TFTs for AM-OLEDs

The dynamic characteristics of normal and Corbino hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) have been investigated. Top- and bottom-gate normal a-Si:H TFTs and bottom-gate Corbino a-Si:H TFTs were fabricated with a five-photomask process used in the processing of the active-matrix liquid crystal displays. The charging time and feedthrough voltage DeltaVP measurement indicates that the normal a-Si:H TFT shows a similar behavior regardless of its TFT geometrical structure. Using a simple gate-to-source capacitance CGS model, the dependence of DeltaVP on gate-to-source overlap and storage capacitor has closely been estimated using analytical calculation. Due to a unique electrode geometry, the Corbino a-Si:H TFT shows a small deviation from an analytical model used for the normal a-Si:H TFT, and consequently, a modified analytical model was developed. We also developed concepts of its possible application as a switching device to active-matrix organic light-emitting displays.

A Fast and Compact Analog Buffer Design for Active Matrix Liquid Crystal Displays Using Polysilicon Thin-Film Transistors

This paper presents a new source-follower type analog buffer for active-matrix liquid crystal displays applications, which exhibits high immunity to the threshold voltage variations of the polysilicon (poly-Si) thin-film transistors (TFTs). The functionality of the buffer was verified through simulations. In order for the simulations to be realistic, parameters extraction from fabricated poly-Si TFTs were used.

42.2: World's Largest (15‐inch) XGA AMLCD Panel Using IGZO Oxide TFT

AbstractThe world's largest (15‐inch) XGA active matrix liquid crystal display (AMLCD) panel made with IGZO TFTs (W/L=29.5/4 μm) was fabricated and evaluated with the field effective mobility of 4.2±0.4 cm2/V‐s, Vth of −1.3±1.4V and sub‐threshold swing (SS) of 0.96±0.10 V/dec. for a manufacturing‐oriented process, the main factors affecting threshold voltage (Vth) of the IGZO thin film transistors (TFT) are investigated. On the glass surface, thicker regions of IGZO film have a negative threshold voltage shift. A dry etching process of molybdenum source and drain (S/D) causes negative shift of the average threshold voltage compared to wet etching in the bottom gate back channel etched TFTs. However, optimization of SiOx passivation and subsequent annealing shift average Vth positively and reduce Vth variation.

Common voltage control technology for highly reliable active matrix liquid crystal displays

The relationship between flicker and a common voltage (Vcom), which directly affects the image sticking problem of liquid crystal displays, is explained. A simple symmetry of flicker with respect to Vcom is introduced. A new circuit utilizing a digital interface to generate Vcom is proposed. The circuit easily controls Vcom through the digital interface, enabling automation of the Vcom optimization. The development of an automatic Vcom-tuning system plus the Vcom optimization algorithms using the symmetry of flicker is presented. The total tuning time was 0.35 seconds regardless of screen size.