Electric-Field-Induced Metal Filament Formation in Cobalt-Based CBRAM Observed by TEM

Conductive-bridging random access memory (CBRAM) using a cobalt (Co) electrode has recently featured a CMOS-compatible process, excellent data retention, and a sub-μA operating current level, which are difficult to achieve by conventional CBRAM. However, the resistive switching (RS) mechanism of Co CBRAM has not been extensively explored compared to that of the conventional CBRAM cells using Ag, Cu, or Ni as active metal electrodes. Because only implicit inferences based on electrical measurements have been made, the formation of Co filaments is not yet clearly understood. This study presents evidence of Co filament formation in Co/10 nm SiO_x/TiN resistive random access memory (RRAM) using direct transmission electron microscopy (TEM) observations. Co protrusions larger than 5 nm are observed, and their exact atomic composition is investigated by spectroscopy. We explain the RS operation of Co/SiO_x/TiN cells based on the Co electromigration-mediated filament development process through Co electrode deformation, protrusion, and subsequent Co conductive bridge formation. An asymmetric RS operation and negative temperature correlation of the resistance are found in low resistance state (LRS) cells, which further supports the Co-involved RS operation in Co/SiO_x/TiN devices.