In this paper we present the application of a computationally efficient Rigorous Coupled-Wave Analysis method to solve the electromagnetic radiation scattering problem in devices featuring nano-structured interfaces and multilayer structures. The method, already successfully used to simulate photon propagation and absorption in thin-film solar cells, is used in this work to compute the optical reflectivity of micro- mirrors in Micro-Opto-Electro-Mechanical Systems (MOEMS).