poly-Si Thin Film Transistor Technology Research Articles

Introduction to the Focus Issue on Oxide Thin Film Transistors

The thin film transistor (TFT) operates on the same basic principle as the metal–oxide–semiconductor field-effect transistor (MOSFET). TFT-based liquid crystal displays (LCDs) and MOSFET-based integrated circuits (ICs) are two of the largest semiconductor products currently in production. They have a similar history of market growth, as shown in Fig. 1. The main advantages of the TFT over the MOSFET are a low fabrication temperature and the sizeand wafer-independent substrate. In addition to LCDs, TFTs have been used in a large number of products, including organic light-emitting diodes (OLEDs), other types of flat panel displays, X-ray imagers, radio-frequency identification, finger printers, sensors, etc. In spite of the successful demonstration of functional TFTs since the 1960s, the mass production of TFT products, e.g., large-area LCDs, has never been realized. The breakthrough in the field came from the report of the first functional amorphous silicon (a-Si:H) TFT with a silicon nitride gate dielectric layer in 1979. Today’s TFT LCDs are dominated by this technology, specifically large area displays. A small portion of the business, such as small mobile displays, is based on the polycrystalline silicon (poly-Si) TFT technology. The a-Si:H TFT suffers from low mobility. The poly-Si TFT is hindered by the large area, high throughput production problem. The dream of having a TFT technology that can be easily fabricated on a large-area, low-temperature substrate with a high throughput has never died. In recent years, metal oxide TFTs that can be prepared by sputter deposition at room temperature have been reported. This kind of TFT can have a mobility one order of magnitude larger than that of the a-Si:H TFT or similar to that of the poly-Si TFT. The oxide TFTs may also be useful for transparent electronics. This result attracted immediate attention from many academic and industry researchers. Many papers on oxide TFTs have been published in various types of journals and presented at conferences, such as the ECS Journal of Solid State Science and Technology, ECS Solid State Letters, and ECS Transactions volumes. The ECS TFT symposium series, which have been held regularly for more than 24 years, are popular platforms for presenting new results. Because we are still in the early production stage for oxide TFTs, there are many unsolved issue that remain under intensive study. For that reason, it was felt that a focus issue covering up-to-date research efforts would be valuable for those working in these areas. This issue of the ECS Journal of Solid State Science and Technology is devoted to the publication of scientific and technological papers

Channel Thickness Effect on High-Frequency Performance of Poly-Si Thin-Film Transistors

In this letter, we present the high-frequency performances of polycrystalline silicon (poly-Si) thin-film transistors (TFTs) with a nominal gate length of 0.22 μm. Owing to the short gate length and adoption of salicide process, cutoff frequency (fT) of 17 GHz and maximum oscillation frequency of ~ 21 GHz are obtained. The result suggests that the poly-Si TFT technology is applicable to RF integrated circuits up to 2 GHz. In addition, we also investigate the effects of channel thickness on the high-frequency characteristics of poly-Si TFTs. We find that the variation of fT with channel thickness is mainly due to the change in transconductance.

Metal-Induced Continuous Zonal Domain (CZD) Polycrystalline Silicon Thin-Film Transistors and Its Application on Field Sequential Color Liquid Crystal Display

Metal induced polycrystalline silicon (poly-Si) films composing of continuous zonal domain (CZD) have been obtained through pre-defined crystalline nucleation lines (CNL). The crystallization process is precisely controllable and repeatable. P-channel thin-film transistors (TFTs) built on CZD poly-Si present high performance and high uniformity. Based on this CZD poly-Si TFT technology, which provides fast addressing characteristics and a large aperture ratio, together with a fast liquid crystal mode, a field sequential color liquid crystal display (FSC-LCD) prototype is designed and fabricated successfully. Excellent color purity and fast moving image can be obtained.

Embedded NVM Devices with Solid-Phase Crystallized Poly-Si Film on a Glass Substrate for System-on-Panel Applications

Embedded nonvolatile memory (NVM) devices with solid-phase crystallized polycrystalline silicon (poly-Si) films and an oxide–nitride–oxynitride (ONOn) stack structure on a glass panel were fabricated and investigated for system-on-panel applications. Memory-in-pixel and memory blocks are expected to be integrated in display panels as the integration of display systems progresses. Poly-Si thin-film transistor technology and a low temperature method to deposit an ultrathin tunneling layer using plasma-assisted oxynitridation were used to fabricate embedded poly-Si NVM devices on glass panels. A memory window from was obtained at a low operating voltage with an erasing voltage of and a programming voltage of . Moreover, an extrapolation of the performance of the fabricated poly-Si NVM device suggests that it retains a threshold voltage window of more than 80% between the programming and erasing states up to . The results demonstrate that the proposed devices with poly-Si layers using solid-phase crystallization and ONOn stack structures have suitable switching and retention characteristics for data storage applicable to real flat-panel display applications.

Design and fabrication of an ultrahigh-luminance field-emission display

An ultrahigh-luminance field-emission display (FED) is designed in which a field emitter array (FEA) and memory function are integrated in every pixel of the display panel. The memory function consists of an emission-control thin-film transistor (TFT), a memory capacitor, a write-enable TFT, and a memory-discharging TFT. The operation mechanism is that the time of luminance of the pixel is made longer by the memory function; thus, luminance becomes higher. In principle, the maximum luminance of such a display is expected to be more than 100 000 cd/m2. The ultrahigh-luminance FED is applicable for daylight-readable displays used in mobile electronic equipment. The authors fabricated a prototype of the ultrahigh-luminance FED using Spindt-type FEA and poly-Si TFT technology. It was confirmed that the luminance of the display panel is controlled by the signal for the memory-discharging TFT, which means that the luminance can be dynamically controlled by the input signal. The maximum luminance of 1700 cd/m2 was achieved at a relatively low anode voltage of 1.5 kV. The higher luminance can be expected by applying higher anode voltage.

Divide-by-four frequency divider in poly-Si thin film transistor technology

The development of a low-temperature poly-silicon (LTPS) process for thin film transistors (TFT) has led to the potential of embedding integrated circuits on active-matrix liquid-crystal displays. A divide-by-four frequency divider with a wide operation range is realised in 3 µm LTPS TFT technology. The operating frequency range of the divider covers from 1 kHz to 2.8 MHz. The core circuit area of the divider is 0.21 × 2.3 mm.

Three-dimensionally stacked poly-Si TFT CMOS inverter with high quality laser crystallized channel on Si substrate

3D stacked poly-Si CMOS inverters with a high quality laser crystallized channel were fabricated on bulk Si wafers. In order to fabricate 3D stacked poly-Si CMOS inverters, the PMOS thin-film-transistor (TFT) at upper poly-Si layer were stacked on the NMOS TFT at lower poly-Si layer and interlayer dielectric film. After laser crystallization, grains in poly-Si films were very uniform and the dominant crystalline orientation was (1 1 1) direction. The sub-threshold swing of NMOS and PMOS TFTs was very good, showing 78 mV/dec. and 86 mV/dec., respectively. And the maximum/minimum current ratio of both TFTs was larger than 10 7 which is equivalent to those at the bulk or SOI MOSFET. The DC voltage transfer characteristics and transient characteristics of stacked poly-Si CMOS inverter were good enough for the vertical integrated CMOS applications. We verified the feasibility of 3D stacked CMOS inverter circuit by poly-Si TFT technology.

Fabrication and characterization of metal-oxide-nitride-oxynitride-polysilicon nonvolatile semiconductor memory device with silicon oxynitride (SiOxNy) as tunneling layer on glass

A nonvolatile semiconductor memory (NVSM) device with a metal-oxide-nitride-oxynitride-polysilicon (MONOS) structure on a rough polysilicon (poly-Si) surface was fabricated using a low-temperature process and poly-Si thin film transistor (TFT) technology on glass. For the fabrication of the NVSM device on glass, plasma-assisted oxynitridation was carried out using nitrous oxide (N2O) as a reactive gas, due to the very rough surface of the poly-Si on glass annealed using an excimer laser. The ultrathin SiOxNy films obtained using the N2O plasma have a very uniform distribution on poly-Si and similar contents of oxygen and nitrogen in the peaks and valleys of the grains. The NVSM devices having a MONOS structure with a tunneling layer of ultrathin SiOxNy on glass have suitable switching and charge retention characteristics for data storage. The results demonstrate that the NVSM device made using low-temperature poly-Si TFT technology on glass reported in this paper can be used in various types of display devices.

A low-ripple poly-Si TFT charge pump for driver-integrated LCD panel

A low-ripple charge pump for voltage up-conversion is described which was implemented in a poly-Si thin-film-transistor (TFT) technology for driver-integrated liquid-crystal-display (LCD) panels. The ripple voltage of the charge pump is reduced by employing a multi-phase clocking method. The charge pump was implemented in a poly-Si TFT technology. For output current of 100/spl mu/A, filtering capacitor of 100 pF and pumping clock frequency of 1 MHz, the measured ripple voltage of the proposed charge pump is 135 mV while that of conventional Dickson charge pump is 590 mV. The measured pumping efficiencies of the Dickson and proposed 4-phase clocking charge pump are 59% and 65%, respectively.

Process-independent analogue data driver for polysilicon TFT AMLCD

Polycrystalline silicon (poly-Si) thin film transistor (TFT) technology is very suitable for driving an active matrix LCD (AMLCD) panel as the driver circuit, and the panel can be integrated on the same substrate. This allows the entire display system to be thin and makes the concepts of ‘TV on wall’ and ‘sheet computer’ possible. However, the large variation of threshold voltage of poly-Si TFT across the wafer makes it difficult to obtain analogue amplifiers with constant gain and phase margin. In this paper, an analogue data driver for the poly-Si TFT AMLCD is proposed. An operational amplifier with a gate bias-voltage generation circuit for this analogue data driver, with characteristics independent of variations in threshold voltage, will be presented. In Hspice simulation, with threshold voltage varying from 2.5 V to 4.5 V, gain variations of the proposed amplifier were reduced from ±10 dB to ±0.2 dB and phase margin variations were reduced from 10° to 0.37° compared with typical operational amplifier design. This enables the analogue data driver for AMLCD to be implemented in poly-Si TFT technology.

Large Area Electronics for Flat Panel Imagers Progress and Challenges

AbstractThe technology of large area electronics has made significant progress in recent years because of the fast maturing excimer laser annealing process. The new thin film transistors based on laser processed poly silicon provide unprecedented performance over the traditional thin film transistors using amorphous silicon. They open up the possibility of building flat panel displays and imagers with higher integration and performance. In this paper, we will review the progress of poly-Si thin film transistor technology with emphasis on imager applications. We also discuss the challenges of future improvement of flat panel imagers based on this technology.

Excimer laser processing for a-Si and poly-Si thin film transistors for imager applications

Pulsed excimer laser annealing (ELA) has become an important technology by which to produce high performance thin film transistors (TFTs) for large area electronics. The applications of these advanced TFTs in flat-panel displays and flat-panel imagers for two-dimensional x-ray imaging have attracted much interest. TFTs made from excimer laser crystallized poly-Si thin films with mobilities higher than 100 cm2/V s and off-state leakage currents lower than 2 fA/μm at drain voltage VD=5 V have recently been achieved. The improved off-state leakage current enables one to consider making flat-panel imagers based on poly-Si TFT technology. Laser doping or dopant activation is another important application of the ELA process. Using a laser doping process, we have fabricated a-Si TFTs with self-aligned poly-Si source/drain contacts. These new devices have reduced source/drain parasitic capacitance and their channel length can easily be scaled down. In this article, we will review the excimer laser processing technology for both the poly-Si TFT technology and self-aligned a-Si:H TFT technology and report the current status of developing flat-panel imagers using these new TFT technologies.

Current status and future prospects of poly-Si devices

Thin-film polysilicon insulated-gate field effect transistors deposited on glass substrates are the subject of worldwide research and development activity. The greatest motivation for this is their application to flat-panel displays, including, in particular, active-matrix liquid crystal displays (AMLCDs), where they offer several important advantages over the more mature amorphous silicon thin-film transistor (TFT) technology. One of these is the ready availability of both n-type and p-type poly-Si TFTs. Polysilicon-on-glass CMOS TFT technology may be used to fabricate AMLCD drivers on the display substrate; it can also be used in a variety of other applications such as printers, scanners, smart sensors and neural networks. This review identifies the major achievements and key issues in the development of poly-Si TFT technology for both display and nondisplay applications.