Precursor ligand chemistry-driven engineering of SiNx gate insulators for enhanced IGZO TFTs

Abstract

Silicon nitride (SiNx) thin films were deposited by plasma-enhanced atomic layer deposition (PEALD) using two aminosilane-based precursors with distinct molecular structures: diisopropylaminosilane (DIPAS) and trisilylamine (TSA). The bulky single silyl group in DIPAS limits each pulse to a single ligand exchange, often leaving residual byproducts. In contrast, the three silyl groups of TSA enable multiple reactions per pulse, leading to superior film continuity and uniformity. Consequently, TSA-based SiNx exhibited a growth per cycle (GPC) of 0.76 Å/cycle—about four times higher than DIPAS (0.19 Å/cycle). X-ray photoelectron spectroscopy (XPS) revealed that the TSA film contained 9.6 at% O, compared to 15.8 at% for DIPAS, with no detectable C. Secondary ion mass spectrometry (SIMS) indicated that the H content of the DIPAS film was more than twice that of TSA, attributable to differences in byproduct removal efficiency linked to ligand structure. SiNx was applied as the gate insulator of In–Ga–Zn–O (IGZO) thin-film transistors (TFTs), with the cation ratio of In/Ga/Zn set to 1:1:1. TSA-based devices demonstrated a field-effect mobility (μFE) of 61.8 cm2/V·s and a stable threshold voltage (VTH) of –0.98 V. Conversely, DIPAS-based devices exhibited higher mobility but a substantial negative VTH shift (–8.3 V) due to excessive H incorporation. Under positive bias temperature stress (PBTS), TSA devices showed no VTH shift, while DIPAS devices shifted –0.35 V. Under negative bias temperature stress (NBTS), TSA devices shifted –1.2 V, while DIPAS devices shifted –0.11 V. These results underscore the pivotal role of precursor ligand structure in determining SiNx film quality, which in turn critically impacts oxide TFT performance and reliability.