A new silicon dioxide-silicone hybrid insulator for amorphous silicon thin film transistors

Active-matrix organic light-emitting diode (AMOLED) displays will become the dominant technology for the next generation of flexible displays, due to their light weight and flexibility. Organic light-emitting diodes (OLEDs) require a permeation barrier to protect them from the rapid degradation upon exposure to moisture or oxygen. The large experience and manufacturing base of a-Si:H thin-film transistors (TFTs) make their upgrade to AMOLED displays very desirable. Each pixel of an AMOLED needs TFT switch, and a TFT that can source sufficient current to drive the OLED, and is sufficiently stable in dc operation to surpass the OLED's lifetime.;A SiO2-silicone hybrid material was developed as an effective permeation barrier for the protection of OLEDs. The physical and chemical properties of the hybrid material suggest that the hybrid is a homogeneous SiO2-like material with residual silicone character. It combines the electrical properties of SiO2 and the mechanical flexibility of silicone polymer. A study of hybrid growth shows that the hybrid covers trenches and similar non-uniform topographies conformably, which is very important for the uniform coverage of the relief of thin-film circuits.;The SiO2-silicone hybrid also functions well as the gate dielectric for a-Si:H TFTs. The hybrid gate dielectric enables bottom-gate inverted TFTs with record high field-effect mobilities of 2 cm2/V·s for electrons and 0.1 cm2/V·s for holes. The hybrid dielectric has a breakdown field Ebd ≅ 8 MV/cm and a dielectric constant of 4.0. When used in a thickness of ∼100 nm, which is 1/3 of the usable thickness of the conventional 300-nm SiN x dielectric, the hybrid has ∼ 1.8 the gate capacitance of SiN x. The high gate capacitance and mobility raise the transistor current to 4 times that of an otherwise identical a-Si:H/SiNx TFT.;High stability of a-Si:H TFTs is important for use in AMOLED displays. The stability of a-Si:H/SiNx TFTs can be raised by modifying the a-Si:H channel layer and the SiNx gate dielectric. However, the high stability of a-Si:H/SiN x TFTs relies on a high deposition temperature of SiN x of ≥300°C. The high deposition temperature limits the use of flexible plastic substrates, which are required for flexible OLEDs displays. Measurements of the threshold voltage shift under high-field gate bias, of a-Si:H TFTs with hybrid deposited at room temperature, is approximately one-half of that in conventional a-Si:H/SiNx TFTs fabricated at 300°C.;The SiO2-silicone hybrid also was used to raise the flexibility of a-Si:H TFTs on plastic foil. TFTs made with the hybrid on 50-m thick polyimide foil can be bent down to 0.5 mm radius (5% strain) in tension, which is ∼ 10 times higher that of conventional a-Si:H/SiNx TFTs, and down to 1mm radius (2.5% strain) in compression, which is somewhat higher than that of conventional a-Si:H/SiNx TFTs. The pronounced flexibility shifts the criterion for reversible bending away from a-Si:H TFT backplanes and toward the materials for substrate and encapsulation. It qualifies a-Si:H TFTs for pull-out display screens in handheld devices.;Thus the SiO2-silicone hybrid enables a-Si:H TFTs that can source high current, are electrical stabile, and mechanically flexible. The SiO2-silicone hybrid material positions a-Si:H TFTs well for applications of flexible electronics.