We are developing MEMS-based programmable reflective slit masks for future generation infrared multi-object spectroscopy (MOS) for space and ground-based telescopes. These devices are composed of monocrystalline silicon micromirrors of size 200 x 100 um(2) which can be tilted by electrostatic actuation yielding a tilt-angle of 20 degrees. An electromechanical clamping mechanism has been demonstrated providing uniform tilt-angle within one arc minute precision over the whole array (5 x 5 micromirrors). Slit masks of different sizes have been produced; the largest one measures 25 x 22 mm(2) and is composed of 20'000 micromirrors. Thanks to the architecture and the fabrication process of these slit masks; the micromirror peak-to-valley deformation (PTV) is uniform over the device and was measured being below 10 nm for uncoated micromirror. A slit mask of size 5 x 5 micromirrors was successfully tested in cryogenic conditions at 92 K; the micromirrors were actuated before, during and after the cryogenic experiment. To achieve for the large arrays a better fabrication yield and a higher reliability, the architecture, the process flow, the assembly and the electronics are being optimized. Optical characterizations as well as experiments of the large devices are underway.