Polysilicon devices for large-area electronics applications: OLED displays and x-ray sensors

This dissertation investigates polysilicon devices for large area electronics. In this work, a new large area method to crystallize the polysilicon has been investigated. The polysilicon material properties are investigated and the impact of the polysilicon microstructure on thin film transistor performance is discussed. The performance of polysilicon thin film transistors as well as lateral polysilicon diodes has been investigated. To investigate the utility of this new material for emerging large area electronics applications, large area polysilicon circuits and displays have been fabricated. The impact of the new crystallization method has been traced from the polysilicon material properties to the device characteristics and finally to the circuit and system level.; The new polysilicon crystallization method investigated in this work is entitled rapid thermal processing (RTP). This method has advantages in large area processing compared with currently used excimer laser crystallization techniques, but the crystallization temperature of the process needs to be reduced. A set of statistically designed experiments were conducted in order to investigate the effect of various amorphous silicon deposition and annealing parameters on the resulting polysilicon microstructure and the crystallization temperature of the material. The goal of the materials work was to achieve large polysilicon grain sizes and crystallization temperatures compatible with large area substrates such as glass. Thin film transistors were fabricated to investigate the effect of the polysilicon microstructure on the device performance. It was found that the devices with the highest performance were fabricated at the lowest crystallization temperatures, which is a promising result for the RTP crystallization process.; In addition to investigating a new polysilicon crystallization process for large area electronics, this work introduces two new large area electronics applications requiring polysilicon devices: active matrix organic light emitting diode displays and diode based x-ray sensors. The AM-OLED displays require polysilicon TFTs as a pixel control element and the x-ray sensors require lateral polysilicon diodes for each pixel-sensing. element. Also, both of these large area applications can utilize the polysilicon devices for integrating driving or sensing circuitry onto the panel to reduce system cost and improve the robustness of the panel. In this work, the first polysilicon AM-OLED display has. been demonstrated [1] and lateral polysilicon diodes with performance suitable for x-ray sensors have been fabricated and analyzed.; Finally, in order to demonstrate the application of the RTP crystallization process for display and sensor applications, driver circuit components were fabricated at glass compatible temperatures. The circuit performance was correlated to the material properties as well as to the thin film transistor performance. A variety of analog and digital circuits such as op-amps, shift registers, and DACs designs were designed, fabricated and characterized.