Scanning transmission electron microscopy of gate stacks with HfO2 dielectrics and TiN electrodes

High-angle annular dark-field (HAADF) imaging and electron energy-loss spectroscopy (EELS) in scanning transmission electron microscopy were used to investigate HfO2 gate dielectrics grown by atomic layer deposition on Si substrates, and their interfaces with TiN electrodes and silicon, as a function of annealing temperature. Annealing at high temperatures (900°C) caused significant roughening of both bottom (substrate) and top (electrode) interface. At the bottom interface, HAADF images showed clusters of Hf atoms that protruded into the interfacial SiO2 layer. Low-loss EELS established that even crystalline HfO2 films exposed to relative high temperatures (700°C) exhibited significant differences in their electronic structure relative to bulk HfO2. Further annealing caused the electronic structure to more closely resemble that of bulk HfO2, with the most significant change due to annealing with the TiN electrode.