  A superconducting quantum interference device (SQUID) miniaturized into
nanoscale is promising in the inductive detection of a single electron spin. A
nano-SQUID with a strong spin coupling coefficient, a low flux noise, and a
wide working magnetic field range is highly desired in a single spin resonance
measurement. Nano-SQUIDs with Dayem-bridge junctions excel in a high working
field range and in the direct coupling from spins to the bridge. However, the
common planar structure of nano-SQUIDs is known for problems such as a shallow
flux modulation depth and a troublesome hysteresis in current-voltage curves.
Here, we developed a fabrication process for creating three-dimensional (3-D)
niobium (Nb) nano-SQUIDs with nano-bridge junctions that can be tuned
independently. Characterization of the device shows up to 45.9 % modulation
depth with a reversible current-voltage curve. Owning to the large modulation
depth, the measured flux noise is as low as 0.34 \mu\Phi$_0$/Hz$^{1/2}$. The
working field range of the SQUID is greater than 0.5 T parallel to the SQUID
plane. We believe that 3-D Nb nano-SQUIDs provide a promising step toward
effective single-spin inductive detection.
