Measurement of Inductance in Niobium Nitride Films for Single Flux Quantum Circuits

In recent years, rapid single flux quantum (RSFQ) circuits, which are promising for the development of high-speed digital electronics, have attracted widespread interest. Niobium nitride (NbN) films have also attracted significant attention for high-frequency applications in superconducting electronics because of their superior material parameters, including their high critical temperature Tc and large gap voltage Vg. To enable the development of all-NbN SFQ circuits, we have developed a fabrication process based on NbN junctions and inductors made of NbN strip lines. It is important to evaluate the inductance because it can affect the performance of an RSFQ circuit and is essential for the RSFQ design. We measured the inductance properties of both epitaxial and polycrystalline NbN strip lines using a superconducting quantum interference device modulation technique at 4.2 K and obtained the width dependence on the inductance of the NbN films. The penetration depth was then calculated from these measurements. The NbN inductance values were calculated numerically with respect to the penetration depth and were then compared with the measured values from the experiments. It was demonstrated that the differences between these values were within 5%. The results reported here will be useful for the design and fabrication of all-NbN RSFQ circuits.