We study the filament structure in 50 nm x 50 nm Resistive Random Access Memory (ReRAM) cells in the forming/set state with a Hf/HfO2/TiN metal-insulator-metal stack by scanning transmission electron microscopy in cross section view. We reveal the filament morphology and, by the measurement of filament size and electrical resistance, evaluate the average resistivity of the filament material. The combination of the various data indicates the nanostructure of the conductive filament. (C) 2015 Elsevier Ltd. All rights reserved.