Next-generation infra-red astronomical instrumentation for space and ground-based telescopes requires MOEMS-based programmable slit masks for multi-object spectroscopy (MOS) which has to work in cryogenic environment. A first prototype of micromirror arrays (MMA) of 5 x 5 single-crystal silicon micromirrors was successfully designed, fabricated and tested. 100 x 200 mu m(2) micromirrors can be tilted by electrostatic actuation yielding 20 degrees mechanical tilt-angle. The MMA were successfully actuated before, during and after cryogenic cooling, below 100 K. A MMA is composed of two different chips fabricated on silicon on insulator (SOI) wafers: the mirror chip and the electrode chip. The array was obtained by assembling these two chips. For the assembly step of large array (100 x 200 micromirrors) we needed high precision alignment as well as the suppression of manual handling. Therefore we developed a technique of assembly for such devices and we designed and fabricated a dedicated XYZ tip/tilt stage. This stage allows aligning the electrodes towards the micromirrors with a micrometer precision. Large MMA of 100 x 200 micromirrors, measuring 22 mm x 25 mm, for large field of view were microfabricated and assembled using the above setup. No additional deformations were observed due to the assembly step. The peak to valley (PTV) deformation of the micromirrors was found to be 14 nm PTV. The first actuation tests were carried out.