  We show elasto-capillary folding of silicon nitride objects with accurate
folding angles between flaps of 70.6$\pm$0.1{\deg} and demonstrate the
feasibility of such accurate micro-assembly with a final folding angle of
90{\deg}. The folding angle is defined by stop-programmable hinges that are
fabricated starting from silicon molds employing accurate three-dimensional
corner lithography. This nano-patterning method exploits the conformal
deposition and the subsequent timed isotropic etching of a thin film in a 3D
shaped silicon template. The technique leaves a residue of the thin film in
sharp concave corners which can be used as an inversion mask in subsequent
steps. Hinges designed to stop the folding at 70.6{\deg} were fabricated
batchwise by machining the V-grooves obtained by KOH etching in (110) silicon
wafers; 90{\deg} stop-programmable hinges were obtained starting from silicon
molds obtained by dry etching on (100) wafers. The presented technique is
applicable to any folding angle and opens a new route towards creating
structures with increased complexity, which will ultimately lead to a novel
method for device fabrication.
