Ferroelectric HfO2 films have gained great attention due to their large polarization & coercive field, scalability down to sub-10 nm scale, and their compatibility with conventional silicon-based devices. HfO2 films are known to exhibit ferroelectricity at nanometer-scale thicknesses, but questions about size, structure, distribution of ferroelectric domains within the film, and characteristics of grain boundaries that govern the overall ferroelectric properties of the HfO2 film are yet to be answered. In particular, the stabilization mechanisms of the ferroelectric phase in HfO2 are of our interest. We analyze the HfO2 films with various electron microscopy methods such as cross-sectional (S)TEM, in-plane (S)TEM, iDPC-STEM to determine the relationship between physical properties and crystal structure. In addition, we are developing next-generation ferroelectric devices with fabrication techniques.