Polysilicon TFT technology for active matrix OLED displays

The integration of active matrix polysilicon TFT technology with organic light emitting diode (OLED) displays has been investigated with the goal of producing displays of uniform brightness. This work identifies and addresses several process integration issues unique to this type of display which are important in achieving bright and uniform displays. Rapid thermal processing has been incorporated to achieve uniform polysilicon microstructure, along with silicides to reduce parasitic source and drain series resistance. Using these processes, TFT drain current nonuniformity has been reduced below 5% for 90% of the devices. This work also introduces transition metals to produce low resistance contacts to ITO and to eliminate hillock formation in the aluminum metallization. These processes, along with spin on glasses for planarization, have been used to produce functional active matrix arrays for OLED displays. The final array pixel performance is also presented.

I. Introduction

A variety of technologies are currently in competition for a share of the flat panel display market. Currently, the dominant technology is the liquid crystal display (LCD), which operates by modulating light generated by a back-light. A new emissive technology, known as organic light emitting diode displays (OLED displays), recently has shown promise for high luminous efficiency (12 Lum/W), low voltage (10 V) and large viewing angle displays [1]. In these displays, current is passed through a thin multilayer organic material where it is converted into light. One of the layers is an electron transporting material while the other is a hole transporting material. At the junction between the two is a layer optimized for luminous recombination where the carriers recombine and the excess energy is released as light. The wavelength of the light can vary depending on the dopant species used in this light-emitting layer [1], [2]. The organic layers are sandwiched between a hole-injecting electrode consisting of a large work function material and an electron-injecting electrode consisting of a small work function material.