Implementation of oxide vertical channel TFTs with sub-150 nm channel length using atomic-layer deposited IGZO active and HfO2gate insulator

Abstract

We fabricated vertical channel thin film transistors (VTFTs) with a channel length of 130 nm using an ALD In-Ga-Zn-O (IGZO) active channel and high-k HfO₂gate insulator layers. Solution-processed SiO₂thin film, which exhibited an etch selectivity as high as 4.2 to drain electrode of indium-tin oxide, was introduced as a spacer material. For the formation of near-vertical sidewalls of the spacer patterns, the drain and spacer were successively patterned by means of two-step plasma etching technique using Ar/Cl₂and Ar/CF₄etch gas species, respectively. The SiO₂spacer showed smooth surface morphology (R_q = 0.45 nm) and low leakage current component of 10^-6A cm^-2at 1 MV cm^-1, which were suggested to be appropriate for working as spacer and back-channel. The fabricated VTFT showed sound transfer characteristics and negligible shifts in threshold voltage against the bias stresses of +5 and -5 V for 10⁴s, even though there was abnormal increase in off-currents under the positive-bias stress due to the interactions between hydrogen-related defects and carriers. Despite the technical limitations of patterning process, our fabricated prototype IGZO VTFTs showed good operation stability even with an ultra-short channel length of 130 nm, demonstrating the potential of ALD IGZO thin film as an alternative channel for highly-scaled electronic devices.

Keywords: ALD-grown In–Ga–Zn–O; nanoscale device; oxide TFT; vertical channel.