A Low-Power Nonvolatile Switching Element Based on Copper-Tungsten Oxide Solid Electrolyte

We describe the materials aspects and electrical characteristics of W-(Cu/WO₃)-Cu switching elements. These materials are compatible with back-end-of-line processing in CMOS integrated circuits where both tungsten and copper already play a significant role. Devices based on Cu/WO₃ solid electrolytes formed by photodiffusion of copper into tungsten oxide switch via the electrochemical formation of a conducting filament within the high resistance electrolyte film. They are able to switch reversibly between widely spaced nonvolatile resistance states at low voltage (<1 V) and current (<10 muA). Electrical characterization revealed that devices consisting of plasma-grown oxides have a variable initial threshold voltage and poor retention, whereas devices based on deposited oxide exhibit a stable switching threshold and good retention, even at elevated operating temperature (>125 degC). This difference in behavior was attributed to the observation that the copper tends to oxidize in the plasma-grown oxide whereas the copper in the deposited oxide exists in an unbound state and is, therefore, more able to participate in the switching process