ADVANCED PROCESSING STRATEGIES FOR SOLUTION-BASED FERROELECTRIC HFO2 FILMS

Ferroelectric hafnium oxide (HfO2) has emerged as a promising candidate for scalable, leadfree ferroelectric technologies due to its compatibility with complementary metal-oxidesemiconductor (CMOS) fabrication processes. Its ferroelectric behavior originates from the stabilization of a metastable orthorhombic Pca21 phase. Although vacuum-based deposition techniques such as Atomic Layer Deposition (ALD) and Pulsed Laser Deposition (PLD) provide excellent control over stoichiometry and film thickness, they exhibit fundamental limitations in sustaining ferroelectricity in films thicker than ∼40 nm, where the non-polar monoclinic phase becomes energetically favored. Thick ferroelectric films are of particular interest in this PhD, as we aimed to assess the piezoelectric response of HfO2 for resonator applications.
Chemical Solution Deposition (CSD) offers a promising route to overcome these constraints by enabling ferroelectric films approaching 1 µm in thickness. This is achieved through fine-grained microstructures, where small grain sizes help to kinetically stabilize the orthorhombic phase. Nonetheless, solution-processed HfO2 films still face challenges related to reproducibility and film density, which can in turn limit their dielectric strength.
This thesis systematically investigates solution-derived ferroelectric HfO2 films from
processing to application. First, annealing atmosphere control and layer-by-layer deposition strategies were developed to enhance fabrication reproducibility and ferroelectric properties. Second, interface engineering using HfO2–ZrO2 multilayer heterostructures was explored to improve ferroelectric wake-up dynamics and break-down strength. Third, the integration of solution-processed thick HfO2 films into high-overtone bulk acoustic resonators was attempted to assess their piezoelectric potential for device applications. The findings establish a framework for solution-derived ferroelectric HfO2 and identify pathways for future device application.