High-$T_{\mathrm{c}}$ Josephson Junction Arrays Fabricated by He-FIB

The development of a fabrication process for Josephson junctions (JJs) using a direct-write helium-focused ion beam (He-FIB) technique provides the opportunity to control parameters such as the critical current <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$I_{\mathrm{c}}$</tex-math></inline-formula> and normal-state resistance <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$R_{\mathrm{n}}$</tex-math></inline-formula> of individual JJs in YBa<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{2}$</tex-math></inline-formula>Cu<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{3}$</tex-math></inline-formula>O<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{7-\mathrm{x}}$</tex-math></inline-formula> JJ arrays. Such arrays could be used as a voltage calibration standard that is operated at higher temperatures and higher driving frequencies than the currently used niobium JJ arrays. This study presents the fabrication process of 79 YBa<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{2}$</tex-math></inline-formula>Cu<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{3}$</tex-math></inline-formula>O<inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_{7-\mathrm{x}}$</tex-math></inline-formula> JJs in a meander array design using a direct writing He-FIB process with the same irradiation dose for all JJs. In this design, each of the JJs can be measured individually, and adjacent JJs that exhibit similar and suitable parameters for a voltage standard array can be measured in series. The scattering of the JJ parameters was investigated at 5 K and resulted in a spread in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$I_{\mathrm{c}}$</tex-math></inline-formula> of about 60% and in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$R_{\mathrm{n}}$</tex-math></inline-formula> of 25%. However, a couple of subarrays with a spread lower than 10% could be found. In addition, these JJ subarrays were irradiated with high-frequency radiation in the GHz and THz range at a temperature of 20 K and synchronization of up to three JJs was observed in both frequency ranges.