Oxidation of Hafnium Thin Films on Amorphous and Crystalline Substrates Studied In Situ Using Transmission Electron Microscopy

<i>In situ </i>environmental transmission electron microscopy (ETEM) enables the observation of controlled metal oxidation, a process of broad relevance to applications ranging from aeronautics to microelectronics. We focus on the oxidation of hafnium due to its role in high-performance alloys that operate in extreme conditions and the use of hafnia as a high-k dielectric for microelectronics. We present <i>in situ</i> results from ETEM, results from supporting experiments including atom probe tomography (APT), and data from large-scale and long-time molecular dynamics (MD) simulations. Our overall goal is to achieve process control for materials in microelectronics and predictive simulations of material failure in extreme conditions.<br/><br/>We deposit Hf films via magnetron sputtering on amorphous substrates built into ETEM heating chips. These films show columnar grains with Hf(0001) film normal texture. TEM imaging during sample heating and oxygen exposure reveals sequences of phase transformations during oxidation, and concurrent secondary electron collection (<i>i.e</i>., SEM) aids in separating processes that take place within the film and at the surface. Similarly deposited Hf films are also investigated by complementary <i>ex situ</i> studies, carrying out oxidation in a tube furnace with controlled atmosphere. Combining atomic force microscopy (AFM) and high-resolution cross-section TEM reveals morphology, oxide thickness, and oxide-metal orientation relationship as a function of oxidation time and temperature. X-ray photoelectron spectroscopy (XPS) depth profiling demonstrates the presence of a suboxide phase at the oxide-metal interface. APT is used to measure the dissolved oxygen concentration profile in the unoxidized metal found below the oxide growth front. Diffusivity of oxygen in Hf metal and morphology changes in ETEM are compared with MD simulation for evaluation refinement of machine-learned interatomic potentials.<br/><br/>The Hf-O system contains many oxide phases that may be metastable. <i>In situ</i> ETEM experiments allow transient phases to be detected, but analysis can be challenging given the polycrystalline nature of the underlying Hf film. To improve the visibility of oxide phases, we deposit single crystal Hf thin films epitaxially on crystalline 2D material substrates, such as graphene, using ultra-high vacuum (UHV) electron-beam evaporation. These crystals are characterized in an TEM connected by UHV to the evaporator, and are then transferred through air and oxided <i>in situ</i> in the ETEM. We find an oxidation sequence for these crystals that includes an amorphous phase and a crystalline, hexagonal phase that we label h-HfO_x. Automated phase quantification allows us to interpret the time evolution of our films, enabling selection of phase and orientation during oxidation.<br/><br/>By studying oxidation of both epitaxial and non-epitaxial Hf films, we identify oxidation mechanisms that are relevant to the performance of alloys in extreme conditions. Using simulation results to refine state-of-the-art atomistic simulations improves the ability of computation to model and predict failure mechanisms. Insights from modelling and ETEM together can also reveal processing routes to select the properties of hafnium thin films. In particular, we include MoS₂among our 2D substrates studied to evaluate the use of ultra-thin hafnium metal films as seed layers to improve fabrication of semiconductor-dielectric layers for 2D microelectronics.