Ke Long

Stability of amorphous-silicon TFTs deposited on clear plastic substrates at 250/spl deg/C to 280/spl deg/ C

Amorphous-silicon (a-Si) thin-film transistors (TFTs) were fabricated on a free-standing new clear plastic substrate with high glass transition temperature (T/sub g/) of >315/spl deg/ C and low coefficient of thermal expansion of <10 ppm/ /spl deg/ C. Maximum process temperatures on the substrates were 250/spl deg/C and 280/spl deg/C, close to the temperatures used in industrial a-Si TFT production on glass substrates. The first TFTs made at 280/spl deg/C have dc characteristics comparable to TFTs made on glass. The stability of the 250/spl deg/C TFTs on clear plastic is approaching that of TFTs made on glass at 300/spl deg/C-350/spl deg/C. TFT characteristics and stability depend only on process temperature and not on substrate type.

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.

Stress control for overlay registration in a‐Si:H TFTs on flexible organic‐polymer‐foil substrates

— Mechanical stress in hydrogenated amorphous-silicon (a-Si:H) thin-film transistors (TFTs) is becoming an important design parameter, especially when the TFTs are made on compliant substrates. Excessive stress always has been avoided to prevent film fracture and peeling. Now, attention is turning to the effects of stress on the TFT backplane dimensions and hence on the overlay alignment. The goal is to keep the size of the circuit-on-substrate composite structure the same at successive critical photolithographic steps. This is done most easily by keeping the structure flat. We show that a compensating stress can be dialed into the silicon nitride SiNx) gate dielectric to also keep the substrate size constant. Varying the stress in the SiNx gate dielectric did not significantly change the as-fabricated TFT characteristics.

Active matrix OLED using 150ºC a-Si TFT backplane built on flexible plastic substrate

Flexible displays fabricated using plastic substrates have a potential for being very thin, light weight, highly rugged with greatly minimized propensity for breakage, roll-to-roll manufacturing and lower cost. The emerging OLED display media offers the advantage of being a solid state and rugged structure for flexible displays in addition to the many potential advantages of an AM OLED over the currently dominant AM LCD. The current high level of interest in flexible displays is facilitating the development of the required enabling technologies which include development of plastic substrates, low temperature active matrix device and backplane fabrication, and display packaging. In the following we will first discuss our development efforts in the PEN based plastic substrates, active matrix backplane technology, low temperature (150°C) a-Si TFT devices and an AM OLED test chip used for evaluating various candidate designs. We will then describe the design, fabrication and successful evaluation and demonstration of a 64x64 pixel AM OLED test display using a-Si TFT backplane fabricated at 150°C on the flexible plastic substrate.

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

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.

High mobility nanocrystalline silicon transistors on clear plastic substrates

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.

Active-Matrix Amorphous-Silicon TFTs Arrays at 180

An amorphous-silicon thin-film transistor (TFT) process with a 180 degC maximum temperature using plasma-enhanced chemical vapor deposition has been developed on both novel clear polymer and glass substrates. The gate leakage current, threshold voltage, mobility, and on/off ratio of the TFTs are comparable with those of standard TFTs on glass with deposition temperature of 300 degC-350 degC. Active-matrix pixel circuits for organic light-emitting displays (LEDs) on both glass and clear plastic substrates were fabricated with these TFTs. Leakage current in the switching TFT is low enough to allow data storage for video graphics array timings. The pixels provide suitable drive current for bright displays at a modest drive voltage. Test active matrices with integrated polymer LEDs on glass showed good pixel uniformity, behaved electrically as expected for the TFT characteristics, and were as bright as 1500 cd/m2

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We fabricated the first bottom-gate amorphous silicon (a-Si:H) thin-film transistors (TFTs) on a clear plastic substrate with source and drain self-aligned to the gate. The top source and drain are self-aligned to the bottom gate by backside exposure photolithography through the plastic substrate and the TFT tri-layer. The a-Si:H channel in the tri-layer is made only 30 nm thick to ensure high optical transparency at the exposure wavelength of 405 nm. The TFTs have a threshold voltage of /spl sim/3 V, subthreshold slope of /spl sim/0.5 V/dec, linear mobility of /spl sim/1 cm/sup 2/V/sup -1/ s/sup -1/, saturation mobility of /spl sim/0.8 cm/sup 2/V/sup -1/s/sup -1/, and on/off current ratio of >10/sup 6/. These results show that self-alignment by backside exposure provides a solution to the fundamental challenge of making electronics on plastics: overlay misalignment.

on Clear Plastic and Glass Substrates for Organic Light-Emitting Displays

Dry dye printing and solvent-enhanced dye diffusion were used to locally dope a previously spin-coated poly(9-vinylcarbazole) (PVK) polymer film with different dyes to fabricate side-by-side red, green, and blue (RGB) organic light-emitting device pixels. The fabrication details and the resolution and stability of this patterning technique are discussed. The technique was then used to make combined polymer/small-molecule devices, in which the printability of polymer for color integration was combined with the superior transport properties and thin-layer capabilities of small molecules for high efficiency and low leakage current. To reduce reverse leakage current and raise efficiency, a blanket tris-8-hydroxyquinoline aluminum (Alq3) electron transport layer was deposited on top of the polymer layer after the dye diffusion step, along with a 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline hole/exciton blocking layer between the Alq3 and the PVK to ensure that all light emission occurred from the doped polymer and not from the Alq3. Devices with this hybrid doped polymer/small molecule structure have an extremely low reverse leakage current (with a rectification ratio of 106 at plusmn10 V). The electroluminescence efficiency of the devices was optimized by varying the dye concentration of the printing plate. A three-color passive-matrix test array with 300 mumtimes1 mm RGB subpixels was demonstrated with this structure

Self-aligned amorphous-silicon TFTs 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.