Stable TFT Research Articles

P‐108: High conductivity transparent electrode with In2O3 – ZnO periodic structure and gradient oxygen concentration

Multilayer structures based on the In‐Zn‐O (IZO) system with modulated oxygen doping have been developed as a TCF (transparent conductive film) alternative to ITO or IGZO layers in the LC and OLED displays. The minimum resistivity (3.37 Ohm*cm) and the maximum concentration of free charge carriers correspond to the layers synthesized in an atmosphere of pure argon, while the maximum mobility (39.18 cm2/V*s) corresponds to the layers synthesized in the atmosphere of both argon and oxygen 99.6%/0.4% mixture. Thin‐film periodic structures with gradient modulation of oxygen content in thickness have been fabricated by periodically changing the oxygen concentration in the composition of the working gas during spraying at a substrate temperature of 100 °C. The structures consist of alternating layers with a high concentration of free charge carriers and layers with high mobility. As a result of the single layers' thicknesses optimization the thin‐film periodic structures with a minimum resistivity of 2.89 Ohm*cm have been made. The resistivity is lower than in homogeneous layers of the same composition. The alternation of layers of optimal thickness provides high carrier mobility, it is inherent to layers synthesized in the atmosphere of 99.6/0.4 argon and oxygen mixture at high concentration. The mobility value is close to the value achieved in the layers synthesized in the pure argon atmosphere.This material provides fabrication of high quality, stable and low cost transparent electrodes or TFT layers with performances corresponding to parameters of traditional materials.

88‐3: Late‐News Paper: IGZO Backplane for Full‐color Electrophoretic Display

E Ink's Gallery™ 3 with SDTC's IGZO backplane are being developed to have short page update time and good color image quality for full‐color EPD. Since the backplane for Gallery™ 3 must be driven by high voltage approximately close to +/‐ 30V, stable TFT characteristics and low TFT off‐leakage under high voltage application are important. SDTC's IGZO TFT was confirmed to be superior in terms of stability against high voltage application. An 8‐inch 300dpi Gallery™ 3, which is now in mass production, achieves page update time of less than 1 second in full color mode and 0.35 seconds in monochrome mode. Beyond the IGZO TFT employed in the 8‐ inch Gallery™ 3 panel, we have also proved the IGZO TFT with further high mobility, whose mobility is 3‐times higher than that of the current 8” panel, developed at SDTC has excellent characteristics stability against high voltage application as well, and it is currently being developed for mass production of EPD. We are also developing flexible IGZO EPD and large screen size IGZO signage EPD as the future technologies.

Stretched-Exponential Trends in a-Igzo Tfts

A bias temperature stress (BTS) investigation of amorphous InGaZnO thin-film transistors (a-IGZO TFTs) is undertaken using the stretched-exponential (SE) relation to model the threshold voltage shift (ΔVTH) as a function of time, temperature, and applied gate voltage. Explicitly, the SE equation employed herein is described as ΔVTH (t, T, VG) = (VG-VTH0) (1-exp(-(t/τ)β)), where VTH0 is the initial unstressed TFT threshold voltage, τ is a trapping time, τ = τ0exp(Eτ/kBT), where τ0 is a trapping time prefactor, Eτ is an energy barrier, and β is an energy barrier parameter [1, 2]. Simulation indicates that a larger τ and/or β results in less ΔVTH, i.e., a more stable a-IGZO TFT. An alternate form of the SE voltage prefactor (VG-VTH0) involves the use of V0, which is identified as a saturation voltage [3, 4]. Either prefactor formulation can be used to accurately fit ΔVTH trends for stress times less than ~3 hours. However, as shown in Fig. 1, fitting of a sample data set restricted to stress times less than 3 hours leads to dramatically different saturated threshold voltage shifts, even though both prefactors precisely fit the initial 2 hours of data (see insert of Fig. 1). Physically, this is associated with the fact that the (VG-VTH0) prefactor corresponds to the applied overvoltage which explicitly depends on VG, while V0 is independent of VG. Simulation is useful for revealing various kinds of ΔVTH (t, T, VG) trends. For example, Fig. 2(a) shows that the relevant energy barrier, Eτ, associated with ΔVTH (10-5 s, 300 K, VG = 30 V) is very narrow for β = 1 and broadens dramatically as β → 0. The Eτ-β relationship is better represented by taking the derivative of ΔVTH with respect to Eτ, as shown in Fig. 2(b), where the magnitude of the peak increases and the distribution of Eτ narrows as β increases. As a larger β is indicative of a more stable TFT, it is reasonable to theorize that a narrow Eτ distribution is meaningful in predicting stable long-term TFT reliability. [1] M. Mativenga, et al., “High stable amorphous indium-gallium-zinc-oxide thin-film transistor using an etch-stopper and a via-hole structure,” Journal of Information Display, Vol. 12, No. 1, 47-50 (2011)[2] T. Y. Hsieh, et al., “Investigating degradation behavior of InGaZnO thin-film transistors induced by charge-trapping effect under DC and AC gate bias stress,” Thin Solid Films, 528, 53-56 (2013)[3] J. M. Lee, et al., “Bias-stress-induced stretched-exponential time dependence of threshold voltage shift in InGaZnO thin film transistors,” Applied Physics Letters, 93, 093304 (2008)[4] S. Park, et al., “Threshold voltage shift prediction for gate bias stress on amorphous InGaZnO thin film transistors,” Microelectronics Reliability, 52, 2215-2219 (2012) Figure 1

Paper No S12.1: Architectures of Oxide TFT for High Resolution Display (Invited Paper)

AbstractIn this paper, we present the architecture of oxide TFT selected from diverse TFT structures for the application to the high resolution mobile display. Due to the selection of proper materials and process, we can achieve highly stable BCE TFT and show the promise of vertical oxide TFT with the smallest TFT footprint.

Novel Crosslinked PVA Without Photoinitiator for Organic Passivation Layers of Pentacene Thin-Film Transistors

We report a new polyvinyl alcohol (PVA)-tetraethoxysilane (TEOS) hybrid layer to protect an organic thin-film transistor (OTFT). PVA/TEOS hybrid thin film was successfully fabricated through hydrolysis and condensation mechanism. The surface roughness of PVA/TEOS film was measured by atomic force miscroscophy (AFM) and it showed a good surface roughness with a root-mean-squae value of about 0.23 nm. Thermally crosslinked PVA with TEOS was successfully adaped as a solution processable passivation layer for pentacene TFT. In case of well known photo-crosslinked PVA/ammonium dichromater (ADC) passivation, extremely large initial performance drop (almost 52% mobility drop) was observed after passivation. PVA/TEOS hybrid passiavtion, however, no significant initial performance drop was found after passivation process. In addition, pentacene TFT with PVA/TEOS passivation layer exhibited very stable TFT operation with almost no field mobility drop or threshold voltage shift up to 980 h.

(Invited) Novel Integration Process for IGZO MO-TFT Fabrication on Gen 8.5 PECVD and PVD Systems - A Quest to Improve TFT Stability and Mobility

IGZO is one of the promising materials being used to make high-mobility TFTs for high-quality displays. However, a-IGZO TFTs have stability issues in addition to limited mobility (<10 cm2/V·s). AKT has made a stable a-IGZO TFT through ESL optimization and plasma treatment of the IGZO interface. AKT a-IGZO ES-TFT shows SS 0.4 ±0.1 V/dec, mobility 10 ±1.0 cm2/V·s, Von 0.25 ± 1.5V over 2200 x 2500 mm2 substrate. BTS stability range under 80oC, ±50V conditions is ~1.5V. Furthermore, another AKT MO ES-TFT shows >30 cm2/V·s mobility using multi-layer structure.

(Invited) Instability Assessment and Modeling Of Amorphous InGaZnO Thin Film Transistors Under Alternating Pulse Bias Stresses

Amorphous InGaZnO (a-IGZO) TFTs have attracted more demands due to the transparency and high mobility. Since TFTs are always exposed to both positive and negative gate bias, the alternating DC bias stress tests are required to ensure stable TFT characteristics. Moreover, the analysis about scaling-down behavior under bias stress condition is required. In this paper, the effects of alternating pulse bias stress on the threshold voltage shift (ΔVth) of a-IGZO TFTs with respect to the stress time duration (t d) are analyzed by the variation of interface trap charges (Qit) and the channel electric field (E-field). In addition, the channel length-dependent tendency is verified using the concepts of the subgap density of states (DOS) and density of total trap states (NT).

Thermally Crosslinked Polyvinyl Alcohol (PVA) Layers for the Passivation of Pentacene Thin-Film Transistors

We report a new crosslinked polyvinyl alcohol (PVA) layer to protect organic thin-film transistors (OTFTs) using the glutaraldehyde (GA) as a crosslinking agent. After thermal crosslinking reaction of PVA with GA, the water contact angle was increased from 9.8° to 55.1° as more hydrophobic state. This clearly means that hydroxyl group of PVA surface was disappeared by the crosslinking reaction. Thermally crosslinked PVA/GA was successfully adapted as solution processable passivation layer for pentacene TFT. In case of well known photo-crosslinked PVA/ammonium dichromate (ADC) passivation, extremely large initial performance drop (almost 52% mobility drop) was observed after passivation. PVA/GA passivation, however, only 3.8% initial mobility drop was found after passivation process. In addition, pentacene TFT with PVA/GA passivation layer exhibited very stable TFT operation with almost no field effect mobility drop or threshold voltage shift up to 1100 h.

Low Voltage Driven, Stable Solution-Processed Zinc-Tin-Oxide TFT with HfOy and AlOx Stack Gate Dielectric

We report on low voltage driven, solution processed zinc-tin oxide (ZTO) transistors with a spin-coated hafnium oxide (HfOy) and aluminum oxide (AlOx) stack gate insulator (HfOy/AlOx). The ZTO TFT with a HfOy/AlOx dielectric processed at the maximum temperature of 400°C exhibits the field-effect mobility of 3.84 cm2/V s, subthreshold swing of 117 mV/dec., and threshold voltage (Vth) of 0.84 V. The positive gate bias stress degradation of Vth with time follows a stretched exponential behavior with a time constant of 1.13 × 107 s, indicating stable TFT.

Highly Stable IGZO TFT with Electro-Less Ni Plated Cu Electrodes

not Available.

Effects of Alternating Pulse Bias Stress on Amorphous InGaZnO Thin Film Transistors

Amorphous InGaZnO (a-IGZO) TFTs attracted more demands due to the transparency and high mobility. Since TFTs are always exposed to both positive and negative gate bias, the alternating pulse bias stress tests are required to ensure stable TFT characteristics. In this paper, the effects of alternating pulse bias stress on the threshold voltage shift (ΔVth) of a-IGZO TFTs with respect to the channel length (L) and the stress time interval (Tinterval) are investigated using the concepts of the stretched-exponential function and the density of total trap states (NT).

Threshold voltage shift under electrical stress in amorphous, polymorphous, and microcrystalline silicon bottom gate thin‐film transistors

AbstractThe crucial influence of the interface between the gate dielectric and intrinsic layer has been studied in detail for amorphous silicon, polymorphous silicon and microcrystalline silicon bottom gate thin‐film transistors. We show that the electrical parameters of the TFTs depend directly on the quality of this interface, which can be strongly degraded by a vacuum break. A hydrogen plasma pretreatment of this interface greatly improves the electrical characterstics of polymorphous silicon TFTs, the mobility increases from 0.4 to 0.75 cm2/V s while their subthreshold slope decreases from 1 to 0.7 V/dec. Moreover, we show that these improvements also translate into more stable TFT characteristics. Finally, microcrystalline silicon TFTs show the best stability and a nitrogen‐plasma treatment of the dielectric improves their stability.

Highly Stable ZnO TFT: Effect of Substrate Buffer Layer

We presented effect of substrate buffer layer on the staggered ZnO TFT performance. Thermal oxide with smooth surface and negligible hydrogen amount forms good back channel with the ZnO active layer deposited by PEALD to result in high performance of ZnO TFT. On the other hand, PECVD SiO2 buffer layer generates some defects in the back channel and make PEALD ZnO have higher carrier density to result in much negatively shifted Von and reduced mobility of TFT. ZnO TFT having sputtered SiO2 as a buffer layer showed similar performance to that with thermal oxide.

Highly Stable ZnO TFT

not Available.

P‐259L: Late‐News Poster: A 14inch Uniform AMOLED Display with Low Cost PECVD Based Microcrystalline Silicon TFT Backplanes

AbstractThis paper discusses development of uniform 14.1 inch AMOLED display using PECVD based microcrystalline silicon (mc‐Si) TFTs. Microcrystalline silicon was deposited using conventional 13.56 MHz Plasma Enhanced Chemical Vapor Deposition (PECVD) with novel gas precursors. The mc‐Si TFT's show a field effect mobility of around 1cm2/V.s and off‐current less than 1pA. Electrical stress on mc‐Si TFTs for a long time shows no significant threshold voltage shift indicating a stable TFT backplane. Significant uniformity improvements were made with novel TFT structure to give uniform AMOLED display. The deposition time for mc‐Si TFTs was significantly reduced by using a thin mc‐Si layer. Mc‐Si TFT backplanes using conventional PECVD equipment offers significant cost advantages over other competing laser and non‐laser polysilicon TFT technologies for AMOLEDs.

Transfer Processes for Thermally Stable Large-Size TFT Flexible Substrates

We report on glass etching transfer processes to obtain thermally stable large-size TFT flexible substrates on plastic film bases. The transfer processes include high-pressure jet etching that allows us to achieve good etch-rate uniformity over the large area and the utilization of an adhesive having a low elastic modulus. From the experiments and simulations, we find that using an adhesive having a low elastic modulus is effective in reducing bend of the flexible substrate under thermal stresses. The simulations also predict that the reduction in the bend corresponds to the reduction in the principal stress of the adhesive, leading to the suppression of the film peeling off from the thinned glass. Using the transfer processes, we have successfully fabricated thermally stable TFT flexible substrates (300 mmtimes350 mm times200 mum) that have satisfactory electrical characteristics

PECVD‐based nanocrystalline‐silicon TFT backplanes for large‐sized AMOLED displays

Abstract— A 14.1‐in. AMOLED display using nanocrystalline silicon (nc‐Si) TFTs has been developed. Nanocrystalline silicon was deposited using conventional 13.56‐MHz plasma‐enhanced chemical vapor deposition (PECVD). Detailed thin‐film characterization of nc‐Si films was followed by development of nc‐Si TFTs, which demonstrate a field‐effect mobility of about 0.6–1.0 cm2/V‐sec. The nc‐Si TFTs show no significant shift in threshold voltage when over 700 hours of constant current stress is applied, indicating a stable TFT backplane. The nc‐Si TFTs were successfully integrated into a 14.1‐in. AMOLED display. The display shows no significant current decrease in the driving TFT of the 2T‐1cap circuit because the TFTs are highly stable. In addition to the improved lifetime of AMOLED displays, the development of nc‐Si TFTs using a conventional 13.56‐MHz PECVD system offers considerable cost advantages over other laser and non‐laser polysilicon‐TFT technologies for large‐sized AMOLEDs.

70.4: A 14.1inch AMOLED Display using Highly Stable PECVD based Microcrystalline Silicon TFT Backplane

We have developed a 14.1 inch AMOLED display using microcrystalline silicon (mc-Si) TFTs. Microcrystalline silicon was deposited using conventional 13.56MHz Plasma Enhanced Chemical Vapor Deposition (PECVD). Detailed thin film characterization of mc-Si films was followed by development of mc-Si TFTs which show a field effect mobility of around 0.7∼1.0cm2/V.s. The mc-Si TFTs show no significant shift in threshold voltage when applied with a long time constant current stress, indicating a stable TFT backplane. The mc-Si TFTs were successfully integrated in a 14.1 inch AMOLED display. The display shows no significant current decrease in the driving TFT of the 2T circuit, even after long time of lifetime tests. Along with improved lifetime for AMOLED display, the development of mc-Si TFTs using conventional 13.56 MHz PECVD system offers significant cost advantages over other laser or non-laser polysilicon TFT technologies for AMOLED.

P-type thin-film transistors with vacuum-deposited crystallized copper-doped germanium films

Copper-doped 10-nm-thick vacuum-deposited Ge films between vitreous aluminosilicate insulator films can be crystallized at 400°C, with hole mobilities of 80 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V.s. They yield stable p-type TFT's with 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> on/off ratio which are process-compatible with n-type CdSe TFT's and thus usable for complementary on-board shift registers in active matrix displays.