Nigel D. Young

Novel fingerprint scanning arrays using polysilicon TFT's on glass and polymer substrates

Novel fingerprint scanning arrays based upon capacitance sensing have been made. Each sensor element consists of a capacitor electrode and two poly-Si thin film transistors for addressing and read out. The devices were fabricated on glass, polyimide and polyethersulphone substrates using a low temperature (<250/spl deg/C) process.

An investigation of laser annealed and metal-induced crystallized polycrystalline silicon thin-film transistors

We report results on thin-film transistors (TFTs) made from a new hybrid process in which amorphous silicon (a-Si) is first converted to polycrystalline silicon (poly-Si) using Ni-metal-induced lateral crystallization (MILC), and then improved using excimer laser annealing (laser MILC or L-MILC). With only a very low shot laser process, we demonstrate that laser annealing of MILC material can improve the electron mobility from 80 to 170 cm/sup 2//Vs, and decrease the minimum leakage current by one to two orders of magnitude at a drain bias of 5 V. Similar trends occur for both p- and n-type material. A shift in threshold voltage upon laser annealing indicates the existence of a net positive charge in Ni-MILC material, which is neutralised upon laser exposure. The MILC material in particular exhibits a very high generation state density of /spl sim/10/sup 19/ cm/sup -3/ which is reduced by an order of magnitude in L-MILC material. The gate and drain field dependences of leakage current indicate that the leakage current in MILC transistors is related to this high defect level and the abruptness of the channel/drain junction. This can be improved with a lightly doped drain (LDD) implant, as in other poly-Si transistors.

The fabrication and characterization of EEPROM arrays on glass using a low-temperature poly-Si TFT process

The fabrication and optimization of poly-Si thin-film transistors and memory devices on glass substrates at temperatures of 200/spl deg/C-400/spl deg/C is described, and the device characteristics and stability are discussed. The devices were formed using PECVD amorphous silicon, silicon dioxide, and silicon nitride films, and the crystallization of the amorphous silicon was achieved with an excimer laser. The performance of 16/spl times/16 EEPROM arrays with integrated drive circuits formed using this technology is presented.

Defects and leakage currents in BF2‐implanted preamorphized silicon

High‐dose silicon implants have been used to preamorphize the surface of single‐crystal silicon prior to the implantation of low‐energy BF2. The preamorphization results in shallow junction formation with minimal channeling of the boron, but high concentrations of electrically active defects are formed, leading to excessive reverse bias leakage currents. Measurements of leakage current and deep‐level defects indicated that two distinct types of electrically active defects were important: those associated with what are probably complexes or clusters of point defects located near the far end of the range of the implanted silicon, and those associated with extended defects (loops) at the edge of the regrown amorphous region. The former defects were deep‐level donors present in high concentrations (>1017 cm−3) after regrowth of the amorphous layer at 600 or 700 °C and resulted in leakage currents >10−4 A/cm2. These centers could be annealed out at 800 °C reducing the leakage current to values between 5×10−8 a...

Explanation for the leakage current in polycrystalline-silicon thin-film transistors made by Ni-silicide mediated crystallization

The source of the leakage current in polycrystalline-silicon (poly-Si) thin-film transistors (TFTs) made by Ni-mediated crystallization has been investigated. Studies of TFTs and of the crystallization process by in situ transmission electron microscopy show that the crystallization process is a two-stage process and that the cause of the leakage problem is associated with incomplete crystallization of amorphous-Si. By removing the last pockets of amorphous-Si, for instance, by long anneals, poly-Si TFTs can be made with adequately low leakage current <1 pA/μm (at a source–drain voltage of 5 V) for display applications, despite the presence of Ni up to 2.5×1019 atoms/cm3.

Characterisation of low temperature poly-Si thin film transistors

Abstract There is developing interest in using thin film transistors as active elements in a range of large area electronics applications. The characteristics of poly-Si thin film transistors (TFTs), processed at glass compatible temperatures, have been investigated. The particular features examined were the leakage current, hydrogenation mechanism and mobility. The hydrogenation was found to proceed by a lateral penetration through the gate oxide around the edges of the poly-Si gate finger. This led to a channel length dependence of sub-threshold slope in partially hydrogenated devices. In contrast, the leakage current, which was shown to be a generation current at the drain junction, did not require hydrogen penetration into the centre of the channel and hence passivation of the generation centres was channel length independent. The hydrogen diffusion coefficient in fine grain poly-Si was estimated at 350°C to be ∼1–10 × 10 −14 cm 2 / s depending upon the detailed material properties. Thermal crystallisation of LPCVD and PECVD amorphous silicon was found to be comparable with both leading to large dendritic grains and enhanced carrier mobility.

Passive and active matrix addressed polymer light-emitting diode displays

Polymer LEDs provide a new alternative to LCDs for many display applications, and are particularly attractive because of their high brightness, near-perfect viewing angle, and very fast response time. In this paper, the basic technology used to form the LED structures, and the performance of these devices is presented. Then, the fabrication and driving of passive addressed matrix displays formed using this technology is discussed. Finally, the necessity for active matrix addressing for larger size and higher resolution displays is demonstrated, and it is shown that this is best achieved using low temperature poly-Si technology. The state-of-the-art poly-Si technology used for active matrix addressed LED displays is described, with particular reference to transistor variation, and the resulting non-uniformities in images on displays. A variety of different addressing techniques, and pixel circuits can be used to drive the LEDs in the active matrix, and the performances of these schemes are compared. These include the basic current source circuit; the modified current source circuit; transistor current mirror circuits; and circuits with optical feedback and correction for uniformity variation. Consideration is given both to analogue and to digital drive methods.

Thin-film-transistor- and diode-addressed AMLCDs on polymer substrates

Abstract— Amorphous-silicon-rich nitride (αSiNx:H) thin-film diodes (TFDs) have been fabricated on glass, polyethersulphone (PES), polyacrylate (PAR), and polyethelyenenaphthalate (PEN) substrates at temperatures of 150–200°C, and a TFD-addressed TN-LCD fabricated on polyethersulphone is presented. Furthermore, poly-Si thin-film-transistors (TFTs) have been fabricated on glass, polyimide (PI), and polyethersulphone at temperatures of 200–275°C, and active-matrix arrays with drive circuitry have been realized. This work is an important step leading to a compact, lightweight, robust, system on a panel.

38.1: Optical Feedback for AMOLED Display Compensation using LTPS and a-Si:H Technologies

New optical feedback pixel circuits for a-Si:H and LTPS technologies will be presented. The circuits will enable correction of threshold voltage drift of the drive TFT and degradation of the OLED. In the a-Si:H case this will be achieved with a standard a-Si:H process and for LTPS an a-Si NIP photodiode is integrated. Operation, technology and measurements will be presented.

32.1: Invited Paper: A Comparison of Pixel Circuits for Active Matrix Polymer/Organic LED Displays

In this paper measurements on several types of active matrix polymer LED (AMPLED) displays will be presented. The issues ofimage uniformity and polymer aging will be addressed by pixel circuit designs.