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.