Deflection of surface-micromachined devices due to internal, homogeneous or gradient stresses

Devices surface-micromachined out of a thin film containing an internal stress will be deformed upon release. The stress of a film can be approximated in second order by a biaxial homogeneous stress superimposed on a linear biaxial stress gradient. In order to model and reduce the stress-induced deformation, the stress state of a thin film is evaluated with surface-micromachined test devices. The homogeneous stress component is measured with deflecting indicators, and the gradient component is measured from the curvature of cantilever beams. A finite element model of a cantilever beam is developed and the parameters influencing the stress-induced deflection are investigated. The model is then extended to calculate the deflection of a doubly supported bridge and the result is compared with experiment. Finally, by using the finite element model of the bridge, the importance of the supports is demonstrated, indicating that reducing the undercut of the supports is an effective method to reduce the stress-induced deflection. (C) 1999 Elsevier Science S.A. All rights reserved.