  Increasing circuit complexity within quantum systems based on superconducting
qubits necessitates high connectivity while retaining qubit coherence.
Classical micro-electronic systems have addressed interconnect density
challenges by using 3D integration with interposers containing through-silicon
vias (TSVs), but extending these integration techniques to superconducting
quantum systems is challenging. Here, we discuss our approach for realizing
high-aspect-ratio superconducting TSVs\textemdash 10 $\mu$m wide by 20 $\mu$m
long by 200 $\mu$m deep\textemdash with densities of 100 electrically isolated
TSVs per square millimeter. We characterize the DC and microwave performance of
superconducting TSVs at cryogenic temperatures and demonstrate superconducting
critical currents greater than 20 mA. These high-aspect-ratio, high critical
current superconducting TSVs will enable high-density vertical signal routing
within superconducting quantum processors.
