Fabrication and characterization of an attonewton-sensitivity Si cantilever with an Nb micro-ring

We have fabricated ultrasensitive silicon cantilevers integrated with a superconducting Nb microring as a key part of a flux quantum-based force measurement system and have characterized both their mechanical and superconducting properties by using mechanical methods. A 50-nm-thick Nb ring with inner and outer radii of 5 μm and 10 μm was mounted by DC magnetron sputtering and lift-off on the end of a 410-μm-long, 4-μm-wide and 340-nm-thick single-crystalline silicon cantilever. The cantilever showed a force sensitivity as good as 4.2 aN/ Hz1/2 at 3 K, which was estimated from its spring constant of 2.6 x 10-5 N/m and a quality factor of 31000. The superconducting property of the Nb micro-ring was observed from the variation of the cantilever's vibration amplitude while the vibration was driven by an excitation coil, showing a transition temperature of 7 K. The superconducting transition was also confirmed through an electrical transport measurement for a separately-prepared Nb strip film on a silicon wafer.