Ultrasmooth Sub-Nanometer Chemical SiO 2 Surfaces and Enhanced Interface Characteristics via High-pH Fluoride Treatment for Si/HfO 2 MOS Devices

Interface engineering at the silicon/high-κ dielectric interface is a critical issue for achieving a superior electrical performance in Si-based MOS devices. The conventional HF-last process produces a hydrogen-terminated Si surface, which leads to complex interfacial reactions during ALD deposition and postannealing, resulting in the formation of silicide, silicate, and oxygen vacancies that degrade electrical performance. Here, we demonstrate an ultrasmooth, subnanometer (∼2 monolayers) chemical SiO2 layer formed by a simple high-pH fluoride solution treatment following standard RCA cleaning. The tailored fluoride chemistry (NH4F/(NH4)2SO3/NH4OH) provides an ultralow, nonlinear, and self-limiting etching characteristics that effectively eliminate hillock-like surface features, yielding an atomically smooth SiO2 surface (Rq = 0.06 nm). HfO2 gate stacks fabricated on this surface exhibit markedly improved interfacial properties compared to the HF-last substrate without substantial compromise in equivalent oxide thickness (EOT), indicating a significant (∼80%) reduction in interface trap density, suppressed fixed oxide charges (∼60% reduction) and hysteresis, enhanced breakdown voltages, and a transition from trap-controlled space-charge-limited conduction to tunneling-dominant carrier transport. Cross-sectional high-resolution scanning transmission electron microscopy (HRSTEM) and energy-dispersive X-ray spectroscopy (EDS) confirmed an atomically smooth and abrupt SiO2/HfO2 interface without element intermixing, even after postmetallization annealing. This work establishes a simple, industry-compatible strategy for achieving an atomically smooth SiO2 interlayer, overcoming the challenges of heterogeneous growth and roughness in chemical SiO2, thereby enabling robust interface control in Si/high-κ gate stacks and providing a scalable pathway for advanced Si CMOS technology.