Mitsushi Ikeda

Development and evaluation of a large-area selenium-based flat-panel detector for real-time radiography and fluoroscopy

The x-ray flat panel detector (FPD) is a key component of the next generation x-ray imaging systems which promote digitization of x-ray images. By developing FPD applicable to both fluoroscopy and radiography, it is expected that x-ray diagnostic systems will change dramatically not only in terms of performance, but also in shape and form. The purpose of this research is to develop a selenium-based FPD applicable to both radiography and fluoroscopy. This report presents the results of the work on new technology which can be applied to create clinically-useful view size detector. The prototype detector adopts 1000 micron thick selenium as the photoconductor, 23 cm square of field of view, 150 X 150 micron pixel pitch, 1536 X 1536 pixel number, and is capable of capturing images at up to 30 frames/second. The features of this prototype detector are: simple-structure selenium suitable for real-time readout, and a high voltage protection structure for the TFT array which acts at x-ray overexposure. To realize large field of view, the field uniformity performance of the selenium and TFT array has been improved, and noise from the TFT array has been minimized. In this paper, new physical performance related to MTF, input- and-output characteristics, image lag, blooming, etc., are discussed.

Characterization of Photo Leakage Current of Amorphous Silicon Thin-Film Transistors

The photo leakage current of amorphous silicon thin-film transistors (a-Si TFTs) for switching elements in active-matrix liquid crystal displays (AMLCDs) is studied to achieve high-image-quality LCDs. The position dependence of photo leakage current generation in the a-Si:H TFT is evaluated using a slit light from the channel side. The generated photo leakage current is composed of a peak at the junction region and a gradual part at channel region, both of which are larger at the source electrode side than at the drain electrode side. This large photo leakage current at the source electrode side can be explained by the diffusion and tunnel current increase caused by the variation of the quasi Fermi level by photogenerated carriers in the reverse bias source junction and the larger electron mobility than the hole, respectively. The results of this study indicate the importance of the source junction for the TFT off-current, in contrast to studies in the past which put forth that the off-current is limited by the generation-recombination current at the drain junction. Our results indicate the importance of front-side illumination by the reflected-light illumination from the high brightness backlight of AMLCD displays.

SiOx/TaOx Gate Insulator a-Si TFT for Liquid Crystal Displays

A double layer of anodized TaOx and plasma CVD SiOx was applied to the gate insulator for an a-Si TFT active matrix LCD. This double layer was effective in eliminating interlayer short circuits between address bus lines and data bus lines in an LCD. No deterioration was observed in the TFT characteristics or the reliability upon adding a TaOx layer to a SiOx gate insulator. An LCD with no interlayer short circuits has been fabricated with this double layer.

Low resistivity quasi-epitaxial Mo−Ta alloy and high-quality anodic oxide for a-Si thin-film transistor liquid-crystal display

Sputtered Mo‐Ta alloy and its anodic oxide have been studied in a Mo composition range from 0 to 30 at. %. Resistivity abruptly changed as Mo composition increased above 10 at. % from 185 μΩ cm to about 35 μΩ cm. The crystal structure transformed from tetragonal to cubic at this Mo composition. For higher Ta compositions, the crystal structure varied according to the under‐layer polycrystalline film crystal structure, which may be denoted as quasi‐epitaxial deposition, and the resistivity decreased to as low as 22 μΩ cm. Anodic oxide films of Mo‐Ta alloy were superior to conventional Ta anodic oxide films in regard to resistivity and breakdown field, and the best insulator was obtained at Ta 95 at. %. This quasi‐epitaxial Mo‐Ta alloy and anodic oxide were applied for thin‐film transistor matrix substrates.

Semi-epitaxial bcc Ta Growth on Metal Nitride

The crystal structure change of sputtered Ta metal according to the nonmetal underlayer film was studied. The bcc-structure Ta was grown on TaNx, NbNx and bcc Mo underlayer films, whereas tetragonal-structure Ta was grown on glass, Ta2O5 and some metals. The crystal structure of Ta varied from tetragonal to bcc according to the N composition of the TaNx and NbNx underlayer films. The bcc Ta growth on TaNx was explained in terms of crystal structure compatibility of the Ta lattice arrangement between TaNx and bcc Ta, and tetragonal Ta growth on Ta2O5 film could be explained by partial Ta lattice matching between Ta2O5 and Ta crystals. Bcc Ta on the TaNx film gave a low resistivity of 25 µΩcm, in contrast to that of conventional tetragonal Ta of 180 µΩcm. The resistivity of Ta anodic oxide was increased by five orders of magnitude by adding N compared to that of conventional Ta anodic oxide. Low-resistivity bcc Ta on TaNx and high-resistivity TaN anodic oxide are applied as gate bus lines of TFT-LCD.

Low Resistivity Mo-W Alloy for A-Si Tft Gate Electrode

Among various requirements for the a-Si TFT-LCD gate electrode, low resistivity is becoming more emphasized with the increase of display size and information content as well as process feasibility. We have developed a low resistivity Mo-W alloy as a material for gate buslines. The Mo-W film was formed by DC magnetron sputtering using Ar or Kr as the working gas. The resistivity of the fabricated film was 16 μΩ-cm when deposited with Ar, and decreased to a value as low as 13 μΩ-cm with Kr, which was less than a half that of the conventional Mo-Ta film. Inverted staggered a-Si TFTs having Mo-W gate electrodes formed with Kr were fabricated, and good transfer performance with thermal- and electrical stability was obtained. The applicability of the new alloy to LCDs with large area and high resolution was shown.

A high aperture and high density a-Si TFT-LCD with a new pixel structure

The authors have developed a TFT-LCD with new pixel structure which enhances aperture ratio. The new structure has a transparent electrode between the signal line and the pixel electrode, which shields the capacitive coupling between the signal line and the pixel electrode and reduces the liquid crystal disclination lines caused by the transverse electric field between them. A high aperture ratio of 40% has been achieved on a 40 /spl mu/m square pixel, which is the largest value for this size of a-Si TFT-LCD.<<ETX>>

Pixel-defect-tolerant design based on visibility for TFT-LCDs

— The repair of pixel defects on TFT-LCDs, which is generally carried out with laser processing or by adopting a redundant structure, is very effective for high-yield production. However, repair may result in abnormal pixel performance, usually due to the asymmetry of the applied pixel voltage. In this study, based on consideration of the dependence of the threshold contrast on the background luminance, the authors are able to determine the visibility of a repaired pixel. As a result, good agreement between measured and calculated visibility was obtained. Based on these results, we have proposed a new pixel design rule which enables complete repair of pixel defects that cannot be detected by the human eye. This design can also suppress the reduction of the aperture for TFT-LCD panels. Furthermore, we demonstrate a new redundant structure which reduces the change in pixel performance after repair to a lower level than is attainable with any other reported technique.

Photolithographic pattern transformation by backside exposure in a-Si:H thin-film transistor liquid crystal displays

Resist pattern transformation by backside exposure, which is a key process for a self-alignment technique is investigated. The light intensity and a-Si:H thickness markedly affect the pattern transformation, while the effect of gate insulator thickness is small. Numerical simulations based on Fresnel diffraction showed fairly good agreement with the experimental results.

Drain current reduction by source electrode overlap in hydrogenated amorphous silicon thin‐film transistors

The characteristics of hydrogenated amorphous silicon (a‐Si:H) thin‐film transistors are investigated as a function of the overlap of the source electrode Lovs on top of the passivation layer. The drain current is reduced by Lovs and the effect is dependent on drain voltage and a‐Si:H layer thickness. The effect is enhanced at high drain voltage and at thin a‐Si:H layer thickness. At high drain voltage, field‐effect mobility and threshold voltage decrease as Lovs increases. Numerical calculation of carrier concentration in a‐Si:H layer showed that the carrier is depleted under Lovs and that the effective channel length is shortened by the size of Lovs. The nonlinear resistance formed by Lovs is discussed in terms of drain voltage and a‐Si:H layer thickness.