Alex Z. Kattamis

Highly stable amorphous-silicon thin-film transistors on clear plastic

Hydrogenated amorphous-silicon (a-Si:H) thin-film transistors (TFTs) have been fabricated on clear plastic with highly stable threshold voltages. When operated at a gate field of 2.5×105V∕cm, the threshold voltage shift extrapolated to only ∼1.2V after ten years. This stability is achieved by a high deposition temperature for the gate silicon nitride insulator which reduces charge trapping and high hydrogen dilution during a-Si:H growth to reduce defect creation in a-Si:H. This gate field of 2.5×105V∕cm is sufficient to drive phosphorescent organic light emitting diodes (OLEDs) at a brightness of 1000Cd∕m2. The half-life of the TFT current is over ten years, slightly longer than the luminescence half-life of high quality green OLEDs.

Amorphous-Silicon Thin-Film Transistors Fabricated at 300

We have made hydrogenated amorphous-silicon thin-film transistors (TFTs) at a process temperature of 300degC on free-standing clear-plastic foil substrates. The key to the achievement of flat and smooth samples was to design the mechanical stresses in the substrate passivation and transistor layers, allowing us to obtain functional transistors over the entire active surface. Back-channel-passivated TFTs made at 300 degC on glass substrates and plastic substrates have identical electrical characteristics and gate-bias-stress stability. These results suggest that free-standing clear-plastic foil can replace display glass as a substrate from the points of process temperature, substrate and device integrity, and TFT performance and stability.

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We have fabricated active-matrix organic light emitting diode (AMOLED) test arrays on an optically clear high-temperature flexible plastic substrate at process temperatures as high as 285 degC using amorphous silicon thin-film transistors (a-Si TFTs). The substrate transparency allows for the operation of AMOLED pixels as bottom-emission devices, and the improved stability of the a-Si TFTs processed at higher temperatures significantly improves the reliability of the light emission over time.

on a Free-Standing Foil Substrate of Clear Plastic

We demonstrate nanocrystalline silicon (nc-Si) top-gate thin-film transistors (TFTs) on optically clear, flexible plastic foil substrates. The silicon layers were deposited by plasma-enhanced chemical vapor deposition at a substrate temperature of 150/spl deg/C. The n-channel nc-Si TFTs have saturation electron mobilities of 18 cm/sup 2/V/sup -1/s/sup -1/ and transconductances of 0.22 /spl mu/S/spl mu/m/sup -1/. With a channel width to length ratio of 2, these TFTs deliver up to 0.1 mA to bottom emitting electrophosphorescent organic light-emitting devices (OLEDs) which were fabricated on a separate glass substrate. These results suggest that high-current, small-area OLED driver TFTs can be made by a low-temperature process, compatible with flexible clear plastic substrates.

Reliability of Active-Matrix Organic Light-Emitting-Diode Arrays With Amorphous Silicon Thin-Film Transistor Backplanes on Clear Plastic

The stability of thin-film transistors (TFTs) of hydrogenated amorphous-silicon (a-Si:H) against gate-bias stress is improved by raising the deposition power and temperature of the silicon nitride gate dielectric. We studied the effects of power density between 22 and 110 mW/cm2 and temperature between 150degC and 300degC . The time needed to shift the threshold voltage by 2 V varies by a factor of 12 between low power and low temperature, and high power and high temperature. These results highlight the importance of fabricating a-Si:H TFTs on flexible plastic with the SiNx gate dielectric deposited at the highest possible power and temperature.

High mobility nanocrystalline silicon transistors on clear plastic substrates

The transition of thin-film transistor (TFT) backplanes from rigid plate glass to flexible substrates requires the development of a generic TFT backplane technology on a clear plastic substrate. To be sufficiently stable under bias stress, amorphous-silicon (a-Si:H) TFTs must be deposited at elevated temperatures, therefore the substrate must withstand high temperatures. We fabricated a-Si:H TFT backplanes on a clear plastic substrate at 200degC. The measured stability of the TFTs under gate bias stress was superior to TFTs fabricated at 150degC. The substrate was dimensionally stable within the measurement resolution of 1, allowing for well-aligned 8 times 8 and 32 times 32 arrays of pixels. The operation of the backplane is demonstrated with an electrophoretic display. This result is a step toward the drop-in replacement of glass substrates by plastic foil.

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— The direct voltage programming of active-matrix organic light-emitting-diode (AMOLED) pixels with n-channel amorphous-Si (a-Si) TFTs requires a contact between the driving TFT and the OLED cathode. Current processing constraints only permit connecting the driving TFT to the OLED anode. Here, a new “inverted” integration technique which makes the direct programming possible by connecting the driver n-channel a-Si TFT to the OLED cathode is demonstrated. As a result, the pixel drive current increases by an order of magnitude for the same data voltages and the pixel data voltage for turn-on drops by several volts. In addition, the pixel drive current becomes independent of the OLED characteristics so that OLED aging does not affect the pixel current. Furthermore, the new integration technique is modified to allow substrate rotation during OLED evaporation to improve the pixel yield and uniformity. The new integration technique is important for realizing active-matrix OLED displays with a-Si technology and conventional bottom-anode OLEDs.

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— An active-matrix organic light-emitting diode (AMOLED) display driven by hydrogenated amorphous-silicon thin-film transistors (a-Si:H TFTs) on flexible, stainless-steel foil was demonstrated. The 2-TFT voltage-programmed pixel circuits were fabricated using a standard a-Si:H process at maximum temperature of 280°C in a bottom-gate staggered source-drain geometry. The 70-ppi monochrome display consists of (48 × 4) × 48 subpixels of 92 ×369 μm each, with an aperture ratio of 48%. The a-Si:H TFT pixel circuits drive top-emitting green electrophosphorescent OLEDs to a peak luminance of 2000 cd/m2.

Gate Dielectric Deposition Power and Temperature on a-Si:H TFT Stability

We developed a hydrogenated amorphous silicon (a-Si:H) thin-film transistor (TFT) process on stainless steel (SS) foil substrates for high uniformity of TFT characteristics. The a-Si:H TFTs with channel length of 5μm showed a mobility of ∼0.3cm2/Vs and a threshold voltage of ∼4.5 V, which are similar to those of TFTs on glass substrates. We designed a pixel circuit with two a-Si:H TFTs and fabricated a 70ppi active-matrix organic light-emitting display (AMOLED) backplane on 75μm thick SS foil substrates. The pixel circuit can provide OLED current of up to 9.2μA at VDD=20V, Vscan=20V, and Vdata=15V. This current level can produce a luminance of greater than 500cd/m2 by an AMOLED using a white OLED with a luminous efficiency greater than“12cd/A and RGBW color filters.

Amorphous Silicon Thin-Film Transistor Backplanes Deposited at 200

Interest is widespread in flexible thin-film transistor backplanes made on clear polymer foil, which could be universally employed for a variety of applications. All ultralow process temperatures, plastic compatible thin film transistor (TFT) technologies battle short or long term device instabilities. The quality and stability of amorphous silicon thin-film transistors (a-Si:H TFTs) improves with increasing process temperature. TFT stacks deposited at less than 250oC by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) exhibit higher threshold voltage shifts after gate bias stressing than stacks deposited at ~300C in the active matrix liquid-crystal display (AMLCD) industry [1]. Therefore, optically clear plastic (CP) substrates are desired that tolerate high process temperatures. The first step in a-Si:H TFT fabrication on a polymer is the deposition of a planarizing barrier and adhesion layer. For this purpose we have been using silicon nitride (SiNx) grown by PECVD. This paper discusses the substrate preparation and SiNx deposition for the process temperature of 250oC. We study the mechanical strain in the SiNx film on the CP substrate, as a function of RF power. Earlier work has shown that SiNx films deposited at low RF power are under tensile strain, and become increasingly compressed as the deposition power is raised [2]. Additionally, at very high deposition power the substrate is bombarded at the beginning of film growth to achieve good film adhesion. The goal is to identify the correct processing conditions at which the total mismatch strain between the film and the substrate is minimized, to keep the film/substrate composite flat and avoid mechanical fracture as well as peeling due to poor adhesion. Optimal deposition conditions were identified to obtain crack-free SiNx barrier layers. The barrier layers were tested by fabricating a-Si:H TFTs on them at 250C.