A new design concept for a dynamically focusing silicon membrane mirror with 6-mm diameter and electrostatic actuation was realized. With this concept, membrane buckling by residual compressive stress inside the membrane can be avoided, which is observed even in crystalline membranes fabricated in silicon-on-insulator (SOI) technology and leads to severe distortion of stress-sensitive devices, such as membrane-based micromirrors. To eliminate the influence of residual stress (compressive or tensile), a membrane suspension with a new stress-relief design was developed by the use of finite element (FEM) simulations. The improvement was achieved by a special tangential-beam suspension, which allows an in-plane expansion or contraction of the membrane, which reduces the stress-induced deformation and leads to substantially flat and distortion-free micromirrors (distortion<λ/10 ). Measurements of realized devices are in very good agreement with the prediction of the FEM simulation. A comparison of membranes with the new stress-relief suspension shows, for example, for a membrane with 6-mm diameter and 10-μm thickness, a distortion of 54 nm compared to 340 nm for a conventional rigidly clamped membrane. A focus range between 97 mm and infinity (flat position) can be used in accordance with the simulation.