silicon adjacent and underlying the arms 5 and the portions penetrated by holes 70 remain an isotropic etch is then performed the result of which after etching mask 90 on the front face and layer 7 on the back face is represented in cross section view in fig7 f and in plan view in fig1 a of which a portion 100 is enlarged in fig1 b the etch is for example a dry etch performed by means of a plasma comprising sulfur hexafluoride sf 6 the etch extends under the arms 5 simultaneously laterally from each side from below and from the edges 72 of holes 70 at holes 70 the etch thus follows the perimeter 72 and confers a rounded contour 74 to the silicon at the level of the fulcrums of arms 5 at the edge of wafer 3 and at the edge of the micromirror the extension of the etch to the thin portion of the wafer 3 via the holes 70 allows the known formation of points 65 of fig3 e 6 a and 6 b to be avoided the removal of points 65 allows problems observed in the known structures to be avoided according to a further embodiment of the invention illustrated in plan view in fig1 a network 107 of parallel lines is formed on the top surface of micromirror 1 the network 107 is formed outside the region in which the arms 5 are fixed the parallel lines of network 107 are of a number a width and a spacing chosen to form a diffraction grating adapted for diffracting a light beam received by the micromirror to a.