Optimization of high-quality YBa₂Cu₃O_7-δ superconducting nanowire fabrication by post-thermal annealing

The fabrication of high-quality YBa₂Cu₃O_7−δ (YBCO) nanowires has garnered significant attention in the field of high-temperature superconductivity due to their potential applications in quantum communication, deep space exploration, and various other fields. Cl₂-assisted reactive ion etching (RIE) stands out as a more effective and efficient method for patterning scalable thin films. However, neither RIE nor high-density RIE has achieved superconducting YBCO nanowires with a width smaller than 3 μm. Here, we delve into the factors that limit the line width of Cl₂-assisted inductively coupled plasma reactive ion etching (ICP-RIE) processing and the method to elimiate them. Our approach involves utilizing Cl₂/Ar as etching gas and incorporating a specialized vacuum heating process after etching. Our experimental results demonstrate the successful realization of 10 nm-thick YBCO nanowires with widths as small as 0.15 μm, exhibiting excellent performance in terms of their intrinsic superconducting properties. The mechanism is evidenced by X-ray photoelectron spectroscopy (XPS) analysis in comparison of nanowires with and without heating treatment, in which the residual Cl₂ on the sidewall of nanowires evaporates and oxidizes Cu⁺ back into Cu²⁺ in an unetched state.