  In force sensing, optomechanics, and quantum motion experiments, it is
typically advantageous to create lightweight, compliant mechanical elements
with the lowest possible force noise. Here we report wafer-scale batch
fabrication and characterization of high-aspect-ratio, nanogram-scale
Si$_3$N$_4$ "trampolines" having quality factors above $4 \times 10^7$ and
ringdown times exceeding five minutes (1 mHz linewidth). We measure a thermally
limited force noise sensitivity of 16.2$\pm$0.8 aN/Hz$^{1/2}$ at room
temperature, with a spring constant ($\sim$1 N/m) 2-5 orders of magnitude
larger than those of competing technologies. We also characterize the
suitability of these devices for high-finesse cavity readout and optomechanics
applications, finding no evidence of surface or bulk optical losses from the
processed nitride in a cavity achieving finesse 40,000. These parameters
provide access to a single-photon cooperativity $C_0 \sim 8$ in the
resolved-sideband limit, wherein a variety of outstanding optomechanics goals
become feasible.
