Paper EM-WeM5
Role of Active and Inert Electrodes in Filament Formation in Resistive Switching Devices (RRAM)

Wednesday, October 21, 2015, 9:20 am, Room 210E

Reliable filamentary resistive switching (RS) depends largely on the electrochemical properties of the active and inert electrodes. A resistive switching memory cell in a RRAM is generally built as a capacitor-like MIM Cu/TaOx/Pt structure, comprised of an insulating or resistive material sandwiched between two electron conductors. Cu electrode produces via a redox reaction (Cu -> Cu⁺+e^-) highly mobile Cu⁺ cations that drift in TaO_x and discharge at the inert Pt electrode forming a conductive filament (CF) of the ON state. To assess the role of active and inert electrodes, we report manufacturing and characterization of 4 derivative devices: Cu/TaO_x/Ta, Cu/TaO_x/Ti, Ta/TaO_x/Pt, Ti/TaO_x/Pt. For a possible integration of RRAM in CMOS back-end, two new metals Ta and Ti used in CMOS metalization are selected. In all four derivative cells a CF formation could be observed at voltages 2V-5V, comparable with Cu/TaO_x/Pt. However, Cu/TaO_x/Ti could not be reset, being permanently damaged. Cu/TaO_x/Ta could be reset but only a few times displaying noisy reset behavior. Ta/TaO_x/Pt device shows resistive switching with a low forming voltage of 2V. The best resistive switching behavior was shown by Ti/Ta₂O₅/Pt device. Main conclusions of the study: 1) In Cu/TaO_x/Ti Cu CFs are formed with a weak base at the Ti electrode due to Cu dissolution in Ti. Cu CF has low resistance and cylindrical shape being difficult to rupture. 2) Cu/TaO_x/Ta device shows better switching properties than Cu/TaOx/Ti, because Ta is better diffusion barrier for Cu than Ti. 3) Ta/TaO_x/Pt is a potential candidate for RS but suffers from: i) the redox reaction Ta -> Ta⁺+e^- is much weaker than for Cu, ii) the diffusion of Ta in TaO_x is faster than of Cu in TaO_x. The result of these two competing mechanisms are fragile CFs with no resistive constrictions. Hence, the RS is observed but its operation is unreliable. 4) Ti/TaO_x/Pt displays if noisy RS. This is due to Ti being known as a getter for oxygen. The Ti CF is therefore fragile as Ti is easily and permanently incorporated in the oxide matrix forming a Ta_xTi_yO_z compounds. The overall conclusion from this comparative study is a better understanding of conditions for reliable RS: a metallic filament has to be formed with a resistive constriction, e.g. in the form of truncated cone with a sharp apex. Cylindrical CFs are hard to rupture and hence undesirable for RS. Cone-like CFs for reliable RS require: a) copious supply of metal ions, b) high metal ion diffusivity in the dielectric, c) high ion stopping power of the counter electrode, and d) moderate compliance currents to be applied during the set operation to assure cone-like shape of the CFs.