Low-Thermal-Budget Ferroelectric Hf1– x Zr x O2: Crystallization Pathways, Phase Selection, and Process Design

Hf1–xZrxO2 (HZO) is a complementary metal–oxide–semiconductor (CMOS)-compatible ferroelectric for scaled nonvolatile memories and three-dimensional integrated electronics. However, its integration is constrained by the thermal exposure required to crystallize and retain polar orthorhombic Pca21. Building on established transformation kinetics, this Review organizes low-thermal-budget HZO processing as a path-dependent phase-selection problem. The pathway is organized into crystallization readiness, phase-selective nucleation of a metastable pseudo-cubic parent, competitive grain growth, nonpolar-to-polar conversion, suppression of monoclinic relaxation, and polar-state retention and activation. Precursor chemistry, internal interfaces, mechanics, oxygen-deficiency timing, composition, dopants, and photon- or field-assisted treatments are assessed by the bottleneck they primarily remove. Representative processes near 300 °C and within the 350–400 °C back-end-of-line window are benchmarked using deposition and annealing histories, stack geometry, pristine vs cycled response, measurement conditions, and reliability. The literature favors coupled or sequential interventions rather than a universally optimal interface, stress state, vacancy concentration, or dopant. Priorities include standardized thermal-budget reporting, phase-specific in situ characterization, quantitative descriptors of the pre-crystalline state, and predictive links from the as-deposited film to reliable device operation.