Sheng Tao

Thin film imaging technology on glass and plastic

Hydrogenated amorphous silicon (a-Si:H) technology offers a viable technological alternative for improved imaging of optical signals and high energy radiation. This paper reviews X-ray imaging technology in terms of detector operating principles, including optoelectronic characteristics, and fabrication process issues related to pixel (Schottky diode detector plus thin film transistor) integration. Recent results which describe the extension of the current fabrication processes to low (/spl sim/120/spl deg/C) temperature are also presented. The low temperature processing enables fabrication of thin electronics on flexible (polymer) substrates.

Top-gate TFTs using 13.56 MHz PECVD microcrystalline silicon

Top-gate thin-film transistors (TFTs) with microcrystalline silicon (/spl mu/c-Si) channel layers deposited using standard 13.56 MHz plasma-enhanced chemical vapor deposition were fabricated at a maximum processing temperature of 250/spl deg/C. The TFTs employ amorphous silicon nitride (a-SiN) as the gate dielectric layer. The 80-nm-thick /spl mu/c-Si channel layer showed a dark conductivity of the order of 10/sup -7/ S/cm and a crystalline volume fraction of over 80%. The /spl mu/c-Si TFTs showed a field effect mobility of 0.85 cm/sup 2//V/spl middot/s, a threshold voltage of 4.8 V, a subthreshold slope of 1 V/dec, and an ON/OFF current ratio of /spl sim/10/sup 7/. More importantly, the TFTs were very stable under gate bias stress, offering promise for organic light-emitting display (OLED) applications.

A-Si Amoled Display Backplanes on Flexible Substrates

In view of its maturity and low-cost, the amorphous silicon (a-Si) technology is an attractive candidate for active matrix organic light emitting diode (AMOLED) display backplanes on flexible substrates. However, the a-Si material comes with significant intrinsic shortcomings related to speed (mobility) and stability of operation, requiring novel threshold-voltage-shift (δVT) compensated thin-film transistor (TFT) pixel circuits and architectures to enable stable OLED operation. But given the dramatic progress in efficiency of OLED materials over recent years, the drive current requirement has been significantly lowered, thus relaxing the constraints on a-Si TFTs. For compatibility to plastic substrates, the a-Si TFT process temperature must be reduced from the conventional 300°C to ∼150°C or below, which tends to compromise the integrity of thin-film materials and device performance. Hence, optimizing the TFT process for high device performance with limited thermal budget is a necessary step towards flexible AMOLEDs with a-Si backplanes. This paper reviews the design and process challenges, and specifically examines the performance of TFTs and δVT- compensated integrated pixel driver circuits on plastic substrates with respect to current driving ability and long term stability. More importantly, lifetime tests of circuit degradation behaviour over extended time periods demonstrate highly stable drive currents and its ability to meet commercial standards.

57.2: Extreme AMOLED Backplanes in a‐Si with Proven Stability

Instability has long been a barrier to the use of a-Si AMOLED backplanes. We present here the first demonstration of proven stability of a-Si AMOLED pixels. Over 7000h of stability data is shown for pixel circuits that compensate for threshold-voltage shift, temperature, and OLED degradation (extreme compensation). This demonstrates that stable AMOLED backplanes are achievable using well-established and proven a-Si TFT technology in mainstream use by the flat panel display industry.

Resolution enhancement and performance characteristics of large-area a-Si:H x-ray imager with a high-aspect-ratio SU-8 micromold

Hydrogenated amorphous silicon is known for its large area imaging applications because of its high photoconductivity and high absorption coefficient in the visible light range. This material can be also applied to X-ray imaging when coupled with a uniform scintillation (e.g. Gd2O2S phosphor) film integrated on top of a 2-D detection array. A thick phosphor layer is the prerequisite for high X-ray conversion efficiency. In reality, however, there may be significant crosstalk between adjacent pixels thus undermining spatial resolution. This paper introduces a high aspect ratio microstructure with the new photoresist SU-8 epoxy, which limits the phosphor to regions above the photodiodes. The differences between the above scheme and that of a continuous phosphor layer are compared in terms of the absorption efficiency, the conversion efficiency, and the modulation transfer function (MTF). The measurements are carried out in a medical testing environment with X-ray source voltages of 40-120kVp. The results show a great improvement in the spatial resolution.

Mechanically strained a-Si:H AMOLED driver circuits

This paper examines the variations in performance of amorphous silicon (a-Si:H) thin-film transistor (TFT) pixel driver circuits for active-matrix organic light-emitting diode (AMOLED) displays, which are subject to compressive or tensile mechanical strain. The external strain is induced by bending of the TFT substrate, and is measured by the observed changes in resistance of in-situ strain gauges. Mechanical strain impacts the performance of the circuit in terms of its drive current, which may be attributed to mobility and Fermi energy shifts in the individual TFTs. The effect of strain-induced shifts in the TFTs as a function of strain orientation (longitudinal or transverse) with respect to direction of current flow is also examined. Our measurements show that the variation in the drain current of a longitudinally oriented TFT can be as much as ∼ 1.5% for strains as high as 10 −3 . Proper layout and circuit design can suppress the effect of strain-induced shifts, and should be taken into consideration when designing stable TFT driver circuits for mechanically flexible AMOLED displays.

ITO/a-SiN x :H/a-Si:H Photodiode with Enhanced Photosensitivity and Reduced Leakage Current Using Polycrystalline ITO Deposited at Room Temperature

We report an ITO/a-SiN x :H/a-Si:H MIS photodiode structure based on room temperature deposition of optically transparent polycrystalline ITO for applications in large area optical and x-ray imaging. The photodiode structure exhibits device characteristics with reduced leakage current and enhanced photosensitivity giving rise to a hundred-fold improvement in dynamic range. This notable improvement in performance is believed to be due to the reduced diffusion of oxygen from the ITO to the a-Si:H layer, and thus reducing the density of defect states inside the a-Si:H layer. The behavior of photo and dark current is consistent with an elaborate transport model for the Schottky barrier. The model agrees reasonably well with measurement data for the dark current and provides a consistent picture in terms of the photo current behavior in the MIS structure, where the insulating layer serves to reduce the oxygen diffusion.

Accelerated Stress Testing of a-Si:H TFTs for Amoled Displays

In this work, we have developed a method for accelerated stress testing of TFT driver circuits for AMOLED display backplane applications. Based on high current and temperature stress measurements, acceleration factors have been retrieved, which can be used to significantly reduce the testing time required to guarantee a 20000-hour display backplane lifespan.