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

Abstract

We describe the materials aspects and electrical characteristics of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ W-( Cu/WO_3)- Cu$</tex> switching elements. These materials are compatible with back-end-of-line processing in CMOS integrated circuits where both tungsten and copper already play a significant role. Devices based on <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$ Cu/WO_3$</tex> solid electrolytes formed by photodiffusion of copper into tungsten oxide switch via the electrochemical formation of a conducting filament within the high resistance electrolyte film. They are able to switch reversibly between widely spaced nonvolatile resistance states at low voltage ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$≪$</tex> 1 V) and current <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(≪10 muhbox A)$</tex> . Electrical characterization revealed that devices consisting of plasma-grown oxides have a variable initial threshold voltage and poor retention, whereas devices based on deposited oxide exhibit a stable switching threshold and good retention, even at elevated operating temperature <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(≫125$</tex> <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$^circ C)$</tex> . 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.