No abstract

display backplanes and creating multifunctional flexible circuitry with thin-film technologies are considered. Within this scenario, amorphous oxide semiconductor (AOS) became one of the most competitive TFT technologies, enabling excellent uniformity in large areas, high transparency in visible spectrum, and field-effect mobility (µ FE ) exceeding 10 cm 2 V −1 s −1 even when fabricated below 200 °C. [1][2][3] The potential of these materials as semiconductors in TFTs started to be recognized in 2004 with the work by Nomura et al. on flexible indium-gallium-zinc oxide (IGZO) transistors. [4] Despite the establishment of IGZO TFTs as a key technology for large-area electronics, indium and gallium are critical raw materials, imposing important constrains regarding the sustainability of this approach. [5] Therefore, the ideal route for the next-generation AOS-based transistor technology should comprise an indium-and gallium-free semiconductor material, providing at least comparable performance and processing temperature to IGZO. Zinc-tin oxide (ZTO) has been recognized as a likely choice. Chiang et al. [6] reported in 2005 the first successful integration of sputtered ZTO as semiconductor layer in TFTs. Their staggered bottom-gate, top contact devices showed µ FE ≈ 20-50 cm 2 V −1 s −1 , turn-on voltage (V on ) between −5 and 5 V, and on/off ratio > 10 7 when annealed at 600 °C. However, µ FE drastically decreased to 5-15 cm 2 V −1 s −1 when lower annealing temperature (300 °C) was used, which is still too high for temperature-sensitive polymeric substrates as polyethylene naphthalene (PEN). Since then more than 150 articles on ZTO TFTs have been published, following both physical and solution-processing routes. Focusing on sputtering, which is the processing technique with easier penetration on an industrial TFT baseline process, aspects as different Zn:Sn ratios, [7][8][9] chamber pressure, oxygen flow ratio, and RF power during sputtering have been studied. [10] Nonetheless, for all these experiments a processing or post-processing temperature exceeding 300 °C was always used to achieve proper device operation (e.g., µ FE > 5 cm 2 V −1 s −1 in devices properly patterned, where overestimation of mobility due to fringing effects can be neglected. [11] A very recent work by Han et al. [12] is the exception to this, where a remarkable saturation mobility (µ sat ) of 67 cm 2 V −1 s −1 is achieved for polycrystalline tin-doped Zinc-tin oxide (ZTO) is widely invoked as a promising indium and galliumfree alternative for amorphous oxide semiconductor based thin-film transistors (TFTs). The main bottleneck of this semiconductor material compared to mainstream indium-gallium-zinc oxide (IGZO) is centered in the larger processing temperatures required to achieve acceptable performance (>300 °C), not compatible with low-cost flexible substrates. This work reports for the first time flexible amorphous-ZTO TFTs processed at a maximum temperature of 180 °C. Different aspects are explored to obtain performance levels comparable ...

This paper presents a study concerning the role of channel length scaling on IGZO TFT technology benchmark parameters, which are fabricated at temperatures not exceeding 180 • C. The parameters under investigation are unity currentgain cutoff frequency, intrinsic voltage-gain, and on-resistance of the bottom-gate IGZO TFTs. As the channel length varies from 160 to 3 μm, the measured cutoff frequency increases from 163 kHz to 111.5 MHz, which is a superior value compared to the other competing low-temperature thin-film technologies, such as organic TFTs. On the other hand, for the same transistor dimensions, the measured intrinsic voltage-gain is changing from 165 to 5.3 and the on-resistance is decreasing from 1135.6 to 26.1 kΩ. TFTs with smaller channel length (3 μm) have shown a highly negative turnon voltage and hump in the subthreshold region, which can be attributed to short channel effects. The results obtained here, together with their interpretation based on device physics, provide crucial information for accurate IC design, enabling an adequate selection of device dimensions to maximize the performance of different circuit building blocks assuring the multifunctionality demanded by system-on-panel concepts. Index Terms-Intrinsic voltage-gain, unity current-gain cutoff frequency, IGZO TFTs and channel length scaling. I. INTRODUCTION I NDIUM-GALLIUM-ZINC oxide thin-film transistors (IGZO TFTs) are becoming a reference technology for flexible, ultra-definition or even transparent display backplanes.