Monolithically Stacked Two Layers of a-IGZO-Based Transistors Upon a-IGZO-Based Analog/Logic Circuits

In this work, back end of line (BEOL)-compatible amorphous indium–gallium–zinc oxide (a-IGZO) transistors are monolithically stacked on top of first-layer a-IGZO-based analog/digital circuits, including a single-stage amplifier and a five-stage ring oscillator (RO). The second-layer a-IGZO transistors are fabricated with a low thermal budget ( $< 200~^{\circ }\text{C}$ ), demonstrating an ultralow subthreshold swing of 75.7 mV/dec, ultralow leakage current ( $< 10^{-{12}}$ A), and ultrahigh ON/OFF ratio ( $10^{{9}}$ ). After 3-D integration, the performance of a-IGZO-based amplifiers and RO circuits in the first layer shows negligible degradation and exhibits a maximum voltage gain larger than 100 at ${V}_{{\text {DD}}} =10$ V and a maximum oscillation frequency of 3.3 kHz at ${V}_{{\text {DD}}} =5.5$ V. This work proves that a-IGZO-based transistors and circuits can be monolithically stacked without performance degradation and shows great prospects for potential high-density and high-performance monolithic 3-D integration applications.