TFT Processes Research Articles

Improved Electrical Performance of MILC Poly-Si TFTs Using $\hbox{CF}_{4}$ Plasma by Etching Surface of Channel

In this letter, a new manufacturing method for metal-induced lateral crystallization (MILC) polycrystalline silicon thin-film transistors (poly-Si TFTs) using CF4 plasma was proposed. It was found that CF4 plasma effectively minimizes the trap-state density by etching away the top surface of MILC and passivating the trap states, leading to superior electrical characteristics such as high field-effect mobility, low threshold voltage, low subthreshold slope, low leakage current, and high ON-/ OFF-current ratio. CF4-plasma MILC TFTs also possess high immunity against the hot-carrier stress and thereby exhibit better reliability than that of conventional MILC TFTs. Moreover, the manufacturing processes are simple (without any additional thermal annealing step) and compatible with MILC TFT processes.

Improvement of Silicon-Based Thin Film Transistor Performances by Modifying Technological Fabrication Process Steps: a Similar Approach with ULSI Technology

The silicon based thin film transistors technological process evolution is governed by electrical performance improvements that are needed for the numerous fields of large area electronic applications. In fact, the electrical properties of the TFTs are directly linked to many aspects combining efforts to improve the process steps by several ways. All these approaches are similar to the ultra large scale integration technology (ULSI) ones. The goal of this paper is to highlight these similarities. After the presentation of the evolution during the last twenty years of the TFT processes, we will discuss on the common approaches between nano- and giga- electronics in order to show that their basic scientific principles are roughly the same.

P-24: High-Temperature (250°C) Amorphous-Silicon TFT's On Clear Plastic Substrates

silicon (a-Si) thin film transistors (TFT's) were fabricated on free-standing, clear plastic substrates with a maximum process temperature of up to 250° C. An a-Si TFT backplane for active matrix OLED (AMOLED) application was also made on such substrates. The performance of both the TFT's and the AMOLED backplane are excellent. These results will enable the fabrication of flexible AMLCD or AMOLED displays on clear plastic substrates with the TFT processes currently used for glass substrates.

49.3: A 200-dpi Transparent a-Si TFT Active-Matrix Phosphorescent OLED Display

We have fabricated a 120×160 high-resolution (200dpi) a-Si TFT active-matrix transparent phosphorescent OLED (PHOLED™) display with novel pixel architecture to maximize transparency and aperture ratio and also ensure comparable light emission from both sides of the display. The a-Si backplane was selected as the technology that would most easily enable the pathway toward achieving high-resolution flexible transparent AMOLEDs (T-AMOLEDs) on polymeric substrates. A-Si TFTs are preferred for fabrication on polymeric substrates since lower process temperatures can be used in comparison to poly-Si TFT processes. As a TOLED generally emits less light from a transparent cathode than anode, a standard 2T pixel was designed with both an opaque, reflective anode region on top of the TFTs as well as a conventional transparent ITO anode to equal the emission from both contacts. This design achieves a total pixel aperture ratio of 64% with a display transparency of 23% in the off-state.

A-Si TFT OLED Fabricated on Low-Temperature Flexible Plastic Substrate

AbstractActive matrix organic light emitting diode (AM OLED) display fabricated on a flexible plastic substrates has a potential for being lower cost, lighter weight, lower power and highly rugged with superior image quality, compared to the current glass substrate based AM LCDs. Amorphous silicon (a-Si) technology offers the potential for fabricating the required active matrix backplanes on low temperature (< 150°C) plastic substrates for producing flexible AM OLED displays. We have developed low-temperature a-Si TFT processes, and backplane electronics designs to fabricate and successfully demonstrate AM OLEDs using 4”-size flexible plastic substrates. Flexible AM OLED test displays with sizes up to 2×2-inch and resolutions up to 80 cgpi equivalent have been designed and fabricated. The required enabling technologies and fabrication and evaluation of these displays are discussed.

Thermal dimensional stability of glass substrates for poly‐Si TFT‐LCD application

Abstract— Thermal dimensional stability of Fusion‐drawn Corning Code 1737 glass was investigated at simulated thermal cycles for low‐temperature poly‐Si TFT fabrication. For low‐temperature poly‐Si TFT processes between 550 and 600°C, annealed Code 1737 is required to meet a typical thermal shrinkage requirement of less than 20 ppm. For super‐low‐temperature poly‐Si TFT processes between 400 and 450°C, Code 1737 meets the requirement in the unannealed state. Code 1737 glass having a high strain point of 666°C provides thermal capabilities as a substrate for low‐temperature poly‐Si TFT‐LCD applications.

50.1: Invited Paper: Future Application Potential of Low Temperature p‐Si TFT LCD Displays

AbstractAs information technology rapidly grows, the development of LCDs with higher specifications is demanded for use as information display devices. Since the LTPS TFT enables high performance circuit integration in the LCD, it is a suitable technology for mobile information devices. Here we will discuss past and present LTPS TFT LCD circuit design technologies, devices and processes technologies, and will propose a next‐generation LTPS TFT LCD technology road map.

46.1:Invited Paper: Developments in p‐Si TFT LCD Projectors

AbstractDevelopments in high‐brightness and high‐resolution technologies for p‐Si TFT LCD projectors are reviewed. Recent topics relating to the improvement of image quality in areas such as contrast ratio and color uniformity are introduced. P‐Si TFT LCD Projectors should be dominant in consumer applications with continuous efforts to improve TFT processes, LCD technology and optical components for smaller size LV.

Recent Progress of Low Temperature Poly Si Tft Technology

ABSTRACTSince being introduced to the production line in 1996, replacing the first generation a-Si TFT line, low-temperature poly-Si production technology aimed at manufacturing small and medium size LCD products has improved steadily corresponding to customers' requirements for rapid growth of the DVC and DSC markets. In the future, this production technology must progress to actual industry technology levels in order to cope with production applied not only to large size displays, which have a major market share in the present display market, but also to a large glass substrate, which effectively cuts the cost of products, although improvement of production yield and productivity in terms of pursuing cost reduction must be proceeded.This paper has described existing problems of inherent low-temperature poly-Si TFT processes and their relating additional processes in present production methods. We have also discussed updating production technologies. To cope with production for a large size display, it is necessary to establish fabrication technology of higher performance TFTs with electron mobility larger than 200cm2/V·s. We believe that one key technology is to fabricate a large-scale and highly-uniform recrystallized poly-Si film with smooth surface morphology as well as precisely-controlled grain size in production. To cope with production using a large glass substrate, it is essential to develop ELA equipment with laser power greater than 200W.

Recent Progress of Low Temperature Poly Si TFT Technology

AbstractSince being introduced to the production line in 1996, replacing the first generation a-Si TFT line, low-temperature poly-Si production technology aimed at manufacturing small and medium size LCD products has improved steadily corresponding to customers' requirements for rapid growth of the DVC and DSC markets. In the future, this production technology must progress to actual industry technology levels in order to cope with production applied not only to large size displays, which have a major market share in the present display market, but also to a large glass substrate, which effectively cuts the cost of products, although improvement of production yield and productivity in terms of pursuing cost reduction must be proceeded.This paper has described existing problems of inherent low-temperature poly-Si TFT processes and their relating additional processes in present production methods. We have also discussed updating production technologies. To cope with production for a large size display, it is necessary to establish fabrication technology of higher performance TFTs with electron mobility larger than 200cm2/V s. We believe that one key technology is to fabricate a large-scale and highly-uniform recrystallized poly-Si film with smooth surface morphology as well as precisely-controlled grain size in production. To cope with production using a large glass substrate, it is essential to develop ELA equipment with laser power greater than 200W.

Minimal Glass Deformations with Rapid Thermal Annealing Control

A critical step in the fabrication of polycrystalline silicon thin-film transistors (TFT's) is to crystallize the amorphous silicon layer deposited on glass. This paper describes a complete rapid thermal annealing (RTA) process applied to the crystallization of the amorphous layer. The RTA heating and cooling rates have been optimized to minimize thermal gradients and to improve glass substrate stability. Numerical simulations taking into account the lamp characteristics, the annealing chamber configuration and the sample properties have been developed and allow elucidation of thermal nonuniformities in the sample during the RTA process. Although the transition point of a glass (T g) is often considered as an upper limit in usual polysilicon TFT's processes, this paper shows that if the cooling rate is accurately controlled, annealing temperatures can be above T g without thermal damage to the glass.

(Invited) Silicon TFTs for Flat Panel Displays

A general investigation of flat panel displays, leads to the conclusion that an electrooptic transducer (liquid crystal for instance) operates in optimized conditions when directly driven by an active matrix system. Such a matrix can be fabricated by different procedures, but one of the most promising is the a-Si TFT technology. a-Si TFT seems quite suitable for LC driving, nevertheless polysilicon TFT should be preferred for fast peripheral addressing circuits. Two basic TFT processes are proposed, using a-Si and SiO2 films deposited on glass by the glow discharge technique. In the first process, silicon was deposited at 300°C instead of 500°C in the second. Only in this last case silicon could be easily recrystallized to get polysilicon TFT on glass. In this paper, the major attention was focused on amorphous Si TFT. Static and dynamic characteristics are presented putting forward the predominant role of traps, and a LC switching simulation is shown. Finally a large area circuit is proposed and the relevant problems discussed.

Silicon TFTs for flat panel displays

Liquid crystal displays work best when directly driven by an active matrix system. Such matrices can be made in different ways, but one of the most promising is the amorphous silicon thin film transistor (a-Si TFT) technology. Although an a-Si TFT technique seems quite suitable for LCD drives, polysilicon TFTs should be preferred for fast peripheral addressing circuits. Two basic TFT processes are proposed, using a-Si and SiO 2 films deposited on glass by the glow discharge technique. Silicon was deposited at 300°C in the first process and at 500°C in the second. Only in this latter case could silicon be easily recrystallized to give a polysilicon TFT on glass. This paper devotes most of its attention to the amorphous Si TFT. Static and dynamic characteristics are presented emphasizing the predominant role of traps, and a LC switching simulation is demonstrated. Finally a large area circuit is proposed and its problems discussed.