[en] Er₂O₃ (erbia) is considered a promising hydrogen isotope barrier and electrical insulation coating for the applications in some sub-systems of future nuclear fusion research reactor designs. Considering harsh environment of the reactor, structural and microstructural stability of such coatings in large range of temperature is utmost important. The polymorphs of erbia are reported in cubic, monoclinic and hexagonal phases depending on the ambience of the formation. Cubic is the most stable phase among these as it does not transform up to 2327 °C. Hence,it is important to study the evolution of structural phases under the adopted deposition process. Systematic erbia deposition experiments are performed through magnetron sputtering of erbium by argon plasma in presence of controlled oxygen, which is commonly known as reactive sputter coating. The structural phases and microstructure of the deposited films are studied using X-Ray Diffraction (XRD), Grazing Incidence Diffraction (GID), Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM). The variation in these properties is correlated with the variation in process parameters such as, temperature, oxygen proportion, post annealing, buffer metallic layer deposition, etc. This helps in deriving important understanding on evolution of cubic and monoclinic phases and their effect on microstructure and intactness of coating and the details of the same will be covered in this paper. (author)