Effective strategies for current boosting in a mesa-shaped In–Ga–Zn–O vertical-channel thin-film transistor with a short-channel length of 40 nm

Abstract

To improve the device characteristics of mesa-shaped vertical thin-film transistors (VCTs) with channel lengths (L_CH) below 100 nm, using In–Ga–Zn–O (IGZO) channels prepared with atomic layer deposition (ALD), we analyzed the physical origins of the degradation in device performance when the L_CH was scaled down to 40 nm. The impact of this strategic approach in simultaneously scaling down both channel (T_CH) and gate insulator (T_GI) thicknesses was also investigated. Al₂O₃ spacers prepared by ALD were patterned using a hybrid-etching technique to form a vertical sidewall corresponding to the L_CH with a sound profile. The increase in the T_CH was found to have a detrimental impact on the device characteristics. The number of trap states within the channel was found to significantly increase due to the back-channel effect (BCE) and the asymmetric geometry effect (AGE), attributed to the process conditions for forming a mesa-shaped 40-nm vertical-channel structure. The T_CH was strategically scaled down to reduce additional trap sites by mitigating both the BCE and the AGE, resulting in improved device characteristics, including a turn-on voltage (V_ON) of −1.1 V and a subthreshold swing of 231.6 mV dec⁻¹. The hydrogen incorporated into the IGZO channel from the Al₂O₃ spacer was also verified to markedly improve the current drivability from 24 to 34 μA μm⁻¹. The T_GI was also scaled down to enhance the immunity against the short-channel effect, resulting in an insignificant drain-induced-barrier-lowering effect. The V_ON shift was suppressed to a value of +0.3 V under positive-bias-temperature stress conditions. As a result, a T_CH of 5 nm and a T_GI of 15 nm have been identified as optimal conditions for the implementation of an IGZO VCT with an L_CH of 40 nm. These findings provide an understanding of new key parameters for the realization of high-performance IGZO VCTs when the L_CH is reduced to 40 nm and below.