  We present a "nanoladder" geometry that minimizes the mechanical dissipation
of ultrasensitive cantilevers. A nanoladder cantilever consists of a
lithographically patterned scaffold of rails and rungs with feature size $\sim$
100 nm. Compared to a rectangular beam of the same dimensions, the mass and
spring constant of a nanoladder are each reduced by roughly two orders of
magnitude. We demonstrate a low force noise of $158 (+62)(-42)\,$zN and $190
(+42)(-33)\,$zN in a one-Hz bandwidth for devices made from silicon and
diamond, respectively, measured at temperatures between 100--150 mK. As opposed
to bottom-up mechanical resonators like nanowires or nanotubes, nanoladder
cantilevers can be batch-fabricated using standard lithography, which is a
critical factor for applications in scanning force microscopy.
