A first electrode is integral to a beam having two ends securedly affixed to a support fixed onto a substrate. Residual stresses cause buckling of the beam so that the arch of the beam is close to at least one second electrode. The support can be formed by a ring fixed to the substrate by means of two fixing bases arranged on each side of the ring on a fixing axis passing through the centre of the ring, the beam being arranged on a diameter of the ring perpendicular to the fixing axis. The ring comprises at least one layer tensile stressed along the fixing axis. The beam can comprise at least one layer stressed in compression along a longitudinal axis of the beam. The second electrode can be integral to an additional support, the beam being arranged between the substrate and the second electrode.