Flexible IGZO TFT Technology for Shift Register Drivers

Abstract

Active sensing matrices play a pivotal role in various electronic devices, including optical and X-ray imaging arrays, electronic skins, and artificial tactile arrays, among others. These matrices function through a thin-film active switching mechanism, allowing for the scanning of rows and columns by external circuitry to read the sensory signals of individual pixels. Recently, indium–gallium-zinc oxide thin-film transistors (IGZO TFTs) have emerged as highly promising technology in the realm of flexible electronics. They enable the large-scale integration of functional circuits on flexible substrates. Shift registers are commonly employed as peripheral scanning circuits to sequentially address active-matrix arrays. To enhance system compactness and minimize external electrical connections, it is imperative to seamlessly integrate shift registers within the active matrices. However, contemporary flexible IGZO-based shift registers suffer from high operating voltages and low frequencies, which constrain their applicability in high-performance flexible sensors and displays. In response to this challenge, a breakthrough is presented in the form of low-voltage, high-frequency bootstrap shift registers implemented with flexible IGZO technology. The approach involves utilizing SU-8 buffered polyimide (PI) polymer foils as substrates. These foils boast an exceptional level of surface smoothness, significantly increasing the yield and performance of electronic components, including vias, IGZO TFTs, and capacitors used in the shift register circuitry. Additionally, HfO2/Al2O3/HfO2 sandwich structures are employed as high-k dielectric layers to reduce the operational voltage. Thanks to the innovative circuit design and optimized fabrication methods, the 16-stage shift register can operate at just 1.8 V with a frequency of 15 kHz. This breakthrough promises to have a profound impact on a wide range of applications for driving flexible active-matrix electronic systems.