  We report on the fabrication and characterization of nanopatterned dc SQUIDs
with grain boundary Josephson junctions based on heteroepitaxially grown
YBa$_2$Cu$_3$O$_7$ (YBCO)/ SiTrO$_3$ (STO) superlattices on STO bicrystal
substrates. Nanopatterning is performed by Ga focused-ion-beam milling. The
electric transport properties and thermal white flux noise of superlattice
nanoSQUIDs are comparable to single layer YBCO devices on STO bicrystals.
However, we find that the superlattice nanoSQUIDs have more than an order of
magnitude smaller low-frequency excess flux noise, with root-mean-square
spectral density $S_\Phi^{1/2}\sim 5-6\,\mu\Phi_0/\sqrt{\rm Hz}$ at 1 Hz
($\Phi_0$ is the magnetic flux quantum). We attribute this improvement to an
improved microstructure at the grain boundaries forming the Josephson junctions
in our YBCO nanoSQUDs.
