Yuta Sugawara

Reliability Analysis of Ultra Low-Temperature Polycrystalline Silicon Thin-Film Transistors

We investigated the reliability of polycrystalline silicon (poly-Si) thin-film transistors (TFTs) fabricated by an ultra low-temperature process below 200 °C. Their comparatively high reliability was confirmed. It was found by emission observation that hot carriers were hardly generated under the drain avalanche condition. The Joule heating effect was also hardly observed under DC stress at high drain and gate voltage stress. These results show that the hot carrier and Joule heating effects are not the predominant causes of the degradation of ultra low-temperature TFTs. The new degradation mode of the threshold voltage shift by a vertical electric field was found to be dominant in ultra low-temperature TFT.

Polycrystalline silicon thin-film transistors on quartz fiber

We demonstrate the fabrication of polycrystalline silicon (poly-Si) thin-film transistors (TFTs) on a thin quartz fiber for the first time. The poly-Si used in the active layer of the TFTs was prepared by excimer laser annealing of an amorphous Si thin film deposited on the fiber. Top-gated TFTs were fabricated on the fiber, and a field effect mobility of 10cm2∕Vs was obtained. The proposed TFTs on a thin quartz fiber, named fiber TFTs, have potential application in microelectronic devices using TFTs fabricated on one-dimensional substrates.

Reliability of Low Temperature Polycrystalline Silicon Thin-Film Transistors with Ultrathin Gate Oxide

We analyzed the reliability against the hot-carriers effect and Joule heating effects of low-temperature polycrystalline silicon (poly-Si) thin-film transistors (TFTs) with an ultrathin gate oxide film deposited by sputtering. For the hot-carrier effect, although the generation of hot-carriers was enhanced in TFT with a thin gate oxide film, no marked degradation due to direct constant voltage (DC) stress was observed in the electric characteristics even when the thickness of the gate oxide film was 9.5 nm. Regarding the effect of Joule heating on the reliability, the thickness of the oxide film markedly affected the TFT characteristics. A new degradation mode different from the conventional mode with a shift in threshold voltage owing to heat generation was observed when the oxide film was extremely thin in spite of small amount of heat generated during the application of DC stress. Regarding the reliability of the low-temperature poly-Si TFT with an ultrathin gate oxide film, the results revealed that a new degradation mode develops due to heat generation.

Improvement of Reliability in Low-Temperature Polycrystalline Silicon Thin-Film Transistors by Water Vapor Annealing

Low-temperature polycrystalline silicon (poly-Si) thin film transistors (TFTs) are promising for realizing future displays such as a system-on-panel. Reliability is one of the most important issues for realizing system-on-panel. We performed water vapor annealing to improve reliability. To investigate the effect on the hot carrier effect, we imposed DC stress on a TFT with and without water vapor annealing. Degradation by the hot carrier effect was improved by the treatment. Analysis by emission microscope revealed that hot electron generation was enhanced by the treatment. To study the effect on thermal degradation, we also imposed DC stress on the TFT with and without the treatment. Degradation induced by Joule heating was also greatly improved. Analysis by thermal infrared imaging system revealed that operating temperature increased due to improved performance. We discussed the model for the treatment. Water vapor annealing improved the hot carrier effect and thermal degradation by reinforcing the Si–Si or Si–O bonding with activated oxygen.

Crystallinity Evaluation by Microwave Photoconductivity Decay in Double-Layered Polycrystalline Silicon Thin Films Crystallized by Solid Green Laser Annealing

The crystallinity of double-layered polycrystalline silicon thin films crystallized by solid green laser annealing (GLADLAX poly-Si) was evaluated in comparison with that of the conventional green-laser-annealed single-layer poly-Si (GLA SL poly-Si) by reflectance peak mapping measurement of microwave photoconductivity decay (µ-PCD), which measures the recombination lifetime of photoexcited carriers. As a result, the anisotropy of the poly-Si was observed as a difference in the reflectance-peak. Furthermore, the upper layer of the GLADLAX poly-Si had a larger microwave reflectance-peak compared with the GLA single-layer poly-Si, and correlation was confirmed between the reflectance-peak of the poly-Si and the electrical performance of thin film transistor (TFT). Thus, it can be said that crystallinty evaluation by µ-PCD measurement are promising for application to next-generation low-temperature poly-Si TFTs.

Crystallization of Double-Layered Silicon Thin Films by Solid Green Laser Annealing

A new laser annealing crystallization method for enhancing the crystallinity of polycrystalline Si (poly-Si) is demonstrated. This method utilizes double-layered amorphous Si films and the high penetration property of the green laser in poly-Si films. The simultaneous crystallization of the upper and lower a-Si layers of the double-layered substrate is achieved, with the upper a-Si layer becoming poly-Si with very large crystal grains. Furthermore, the crystallization laser energy of this double-layered substrate is reduced by approximately 30% compared with that of a conventional single-layer a-Si substrate, suggesting a thermal reserving role of the lower a-Si layer when it crystallizes upon the irradiation of green laser light.