Anisotropic I–V characteristics of spontaneously emerged periodic stripes of superconducting NbN thin films on Si trench sidewall by RF magnetron sputtering

To realize a highly integrated thin film superconductive coil, sputter-deposition of NbN on a trench sidewall of Si substrate was examined. A trench of 0.2 mm in depth was fabricated by Si Deep-RIE. A Nb target was sputtered in Ar/N2 mixed gas flow. Thanks to the periodic corrugated structure formed by repeated Deep-RIE process, it was found that NbN superconducting lateral stripes connected with each other with thinner NbN thin film were spontaneously emerged. The superconducting properties along and across the stripes were completely different. Along the stripes, the I–V curves showed supercurrent. Additionally, the undulations were observed in I–V curves below Tc indicating the existence of the inhomogeneity of such as thickness, having different Jc depending on the position in the stripes. The normal conductivity along the stripes is almost five times higher than that across the stripes. Across the stripes, clear supercurrent was observed in the films formed in N2 flow rate lower than 12%, indicating the stripes were connected with a thin superconductive layer in between. In the films formed in N2 flow rate over 14%, no supercurrent was observed but the conductance increased with temperature indicating tunneling conduction. The obtained novel structure can be looked upon as a spontaneously emerged multifilamentary superconducting wire possibly applicable to produce high magnetic field.

► Sputter-deposition of NbN on a trench wall of Si substrate was examined.
► NbN superconducting lateral stripes were spontaneously emerged.
►  The anisotropic superconducting properties have been observed.
► Along the stripes, the undulations were observed in I–V curves below Tc.
►  Across the stripes, the conductance increased with temperature.