Indium Alloying as a Mobility Booster in IGZO Thin-Film-Transistor via Plasma-Enhanced Atomic Layer Deposition Approach for BEOL Compatible Circuits

Abstract

Indium Gallium Zinc Oxide (IGZO) thin-film transistors (TFTs) offer promising potential for next-gen monolithic three - dimensional (M3D) integrated chips due to their decent mobility, excellent electrostatic characteristics, and low-temp processing. However, mobility in IGZO TFT is still underwhelming. Here, accurate modulation of the Indium (In) ratio via a plasma-enhanced atomic-layer deposition (PEALD) approach is investigated, together with a post-deposition annealing (PDA) process over multiple temperatures. Through detailed material characterization, it is elucidated that a high In ratio with PDA favorｓ more In─O bonding and reduction of defects state. While a higher PDA temperature results in a higher mobility, other electrical performance is degraded due to unwanted defects generation. Optimized IGZO (In (5): Ga (1): Zn (1)) TFT is obtained after a PDA of 350 °C with a significant enhancement in field-effect mobility (µ_FE) from 25.10 to 66.04 cm² V·s⁻¹. In addition, the TFT achieves other excellent electrical performance, including an I_on-off of 10⁷, low subthreshold swing (SS) of 79.25 mV dec^−1, and threshold voltage (V_TH) of −0.39 V. Building upon the optimized IGZO TFT, a n- IGZO TFT-based pseudo enhancement load (PEL) inverter is presented, achieving robust rail-to-rail operation, thus demonstrating its suitability for integration in CMOS backend-of-line (BEOL) for high-performance circuitries.