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The ability to manufacture high quality, flat panel displays has been key to the recent rapid development of portable electronic devices, such as laptop computers, PDAs, mobile telephones and digital cameras. Most of these are liquid crystal displays (LCDs) with 'active matrix' control in which a thin film transistor (TFT) sits underneath each pixel to control the liquid crystal. These TFTs use a thin layer of silicon, known as hydrogenated amorphous silicon (a-Si:H), as the semiconductor in the TFTs with silicon nitride as a gate insulator, both of which are produced by exposing the surface to be coated to an activated gas at high temperature in a process known as rf plasma enhanced chemical vapour deposition (rf-PECVD). Currently, temperatures in excess of 200 C are required for this, and so these displays must be made on fragile, glass substrates. It would be much better if portable displays could be made on plastic substrates, as this would make the display more robust if dropped. However, most transparent, colourless plastics cannot withstand high temperatures, and therefore it will be necessary to produce a good insulator layer at a temperature below 150 C.The insulator in a TFT-LCD display would serve two purposes: to insulate the semiconductor in the TFT from the controlling (gate) electrode (known as the gate insulator) and to act as a diffusion barrier layer between the plastic substrate and the TFT. Most plastics contain a high number of impurities and this diffusion barrier is necessary to prevent impurities from transferring from the plastic into the sensitive TFT where they would change its electrical properties. A new, low temperature insulator must fulfil both of these requirements.Several new insulator layers have been developed recently for other electronic applications. High-k dielectrics, such as HfO2, are being investigated to replace silicon dioxide as the insulator layer in crystalline silicon metal-oxide-semiconductor field effect transistors. Low-k dielectrics, such as FOx flowable oxides, which can be spin coated from a liquid, are being investigated as the insulator between interconnects in integrated circuits.This work will investigate the deposition of these new materials together with silicon nitride produced by very high frequency (VHF-) PECVD at temperatures below 150 C. Their properties both as a gate dielectric in an a-Si:H TFT and as a diffusion barrier will be tested for the first time with the aim of identifying the most promising materials for this application. To enable this, a new vacuum measurement system will be constructed to perform diffusion barrier measurements on these thin film materials by measuring the change in mass as an impurity is absorbed using a sensitive quartz crystal microbalance. An existing rf-PECVD deposition system will also be adapted to allow VHF-PECVD of silicon nitride at low temperatures. Facilities already exist to allow fabrication and testing of TFTs. The stability, interfacial properties and threshold voltage of these devices will be measured and the result correlated with the type of dielectric employed.This work will not only permit the development of the next generation of TFT-LCD displays on plastic substrates, but is also key to the development of organic light emitting diode (OLED) displays in which UK industry has a significant investment. In this technology, the liquid crystal and backlight are replaced by a printed organic polymer material which emits light when a current is passed through it. However, these polymers are extremently sensitive to water vapour contaminstaion, and a good, transparent, low temperature diffusion barrier is required to coat the device to ensure a long lifetime. The quality of diffusion barriers is currently limiting the development of this new type of display - this research will have a direct impact in this area and should unlock the potential of OLED technology.
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