Tomography of Intrinsic Filamentation in Silica and Hsq Based Reram Devices with Conductive Atomic Force Microscopy

Resistive RAM (ReRAM) devices have shown promise in delivering the next generation of electronic memories as flash approaches its scaling limit. Their low operating voltages, small sizes and simple structure enable low power consumption and dense device packing. Although many dielectric materials such as transition metal oxides and perovskites have demonstrated desirable properties in ReRAM architectures, silicon based memories facilitate the route for integration into existing CMOS infrastructures. This makes them particularly suited for investigation and optimisation. In recent work we have specifically looked at devices sandwiching amorphous silicon suboxide or hydrogen silsesquioxane (HSQ) active layers between different electrode materials, including platinum and titanium nitride. We here present characterisation results probing the three-dimensional structure of conducting filaments in our silicon memories. These filaments are thought to be composed of oxygen vacancies and govern the switching mechanism that enables dielectric materials to operate in ReRAM devices. Imaging these features with conventional topographical and cross-sectioning techniques is difficult, owing to the low contrast with the background pristine material. Our results demonstrate the importance of conductive atomic force microscopy in three-dimensional device analysis and show that filament growth conforms closely to the pristine dielectric structure. Formation pathways follow the intrinsic film inhomogeneities, a feature that may produce defect clustering and promote oxygen vacancy generation under electrical stress. We are also able to demonstrate, for the first time in intrinsic switching devices, the competition between multiple growths in which only a single pathway find success.