Reliability considerations for electrostatic polysilicon actuators using as an example the REMO component

The REMO (REflective MOdulator) component is a micromechanical light modulator fabricated by polysilicon surface micromachining. During operation the maximum tensile stress is 130 MPa and the maximum homogeneous electric field is 60 V mu m(-1). The effects of these high operation stresses in combination with humid air have been studied. In dry air the mechanical properties of polysilicon meet the requirements for a highly reliable modulator, i.e., the fracture strength is well above the operation stress and slow crack growth is not expected to occur. In dry air no change of the mechanical or the electrical properties can be observed, The combination of cyclic mechanical stress, high electric field and humid air can lead to membrane rupture. On specimens where the stress cycling is stopped before rupture, an increase of the resonance frequency and a permanent electrical polarization of the air-gap capacitor can be observed. This is explained by the formation of a surface oxide. The oxide stiffens the membrane, which results in an increase of the resonance frequency, Trapped charges in the oxide lead to a permanent polarization of the air-gap capacitor.