Focused gold ion beam for the fabrication of sub-100 nm length InGaZnO thin film transistors on flexible substrates

The realization of thin-film electronics on flexible substrates utilizing metal oxide semiconductor, specifically indium-gallium-zinc oxide (IGZO), holds great potential for the advancement of lightweight and flexible systems [1-2]. IGZO devices exhibit a favorable balance between large-area fabrication, low-temperature manufacturing, and electrical performance. However, a limitation in the development of high-speed thin-film devices for data transmission and communication arises from the relatively low carrier mobility of IGZO thin film transistors (TFTs), typically around 10 cm2/Vs when manufactured at room temperature [3]. To address the need for high-speed IGZO-based thin-film devices on these substrates, various scaling strategies have been employed to produce MOS devices with ultra-thin channel length? [4-6]. To achieve higher resolution, Ga-ion based focused ion beam (FIB) techniques have been utilized to mill a top-metal contacts, resulting in scaling down to a channel length of 160 nm, which currently represents the shortest length achieved in IGZO TFTs [5]. Further reduction in channel length can be accomplished by employing alternative ions in FIB instrumentation. In this research, ultra-scaled IGZO TFTs were fabricated on a free-standing polyimide foil by utilizing Au+ ions for metal contacts milling, as depicted in Fig.1a. Optimization of the beam parameters was crucial to prevent excessive ion contamination and damage to the underlying IGZO layer [7-8]. The patterning process was performed using an Au+ beam accelerated at 35 kV, with a beam current of 8.2 pA and an ion fluence of 6000 µC/cm2, resulting in a channel length of 78 nm, as illustrated in Fig.1b-c. The reduction in channel length through FIB milling led to a drain current value of 685 µA at a gate-source voltage of 12 V. The high drain current value obtained can be attributed not only to the scaling process but probably also to the implantation of Au ions in the IGZO. Monte Carlo simulations have confirmed that the Au ion contamination contributes to an enhanced IGZO conductivity, resulting in significantly improved carrier mobility of 3.06 cm2 V−1 s−1. Scaling the channel length of IGZO TFTs using Au FIB has demonstrated to be a viable approach for enhancing carrier mobility to a level compatible with data transmission applications. Furthermore, the increase in carrier mobility is not solely due to the scaling of the channel size but also the augmented IGZO conductivity resulting from gold implantation, indicating the multifaceted nature of FIB as a scaling method.