Conductive AFM Topography of Intrinsic Conductivity Variations in Silica Based Dielectrics for Memory Applications

As flash memory approaches its scaling limit, resistive RAM (ReRAM) devices are showing promise in providing the next generation of electronic memories. This comes about as a result of their simple structure, small size and low operating voltages, enabling denser device packing and lower power consumption. Many materials have shown desirable properties as thin film active layers in ReRAM architectures, yet silicon based memories allow for facile integration into existing CMOS infrastructures, making them a particularly suitable class of devices for development. We have looked specifically at devices containing amorphous silicon suboxide or hydrogen silsesquioxane (HSQ) active layers sandwiched between a number of different electrode materials, including titanium nitride and platinum. Here, we present recent characterisation results in which we have probed the three-dimensional structure of conducting filaments in our silicon memories. These filaments govern the switching mechanism that gives dielectric materials their memory function and are thought to be composed of oxygen vacancies, generated as the device is subjected to an electric field. Such features are difficult to image with conventional topographical and cross-sectioning techniques, owing to their low contrast against the background of pristine material. Our results demonstrate the importance of conductive atomic force microscopy for three-dimensional analyses and show that the filament formation is closely linked to the pristine structure of the dielectric. Growth pathways conform to the intrinsic inhomogeneities of the films, a feature that is thought to produce defect clustering and thus promote the generation of oxygen vacancies under electrical stress. We have also been able to demonstrate, for the first time in an intrinsic switching material, that multiple growths compete to bridge the active layer while only a single pathway is finally successful.