Planarity of large MEMS

A systematic approach is developed to study the planarity of large (few mm long) micromechanical cantilever beams made of μm-size features. The beams are made by the SCREAM (single crystal reactive etching and metallization) process. SCREAM beams consist of a single crystal silicon (SCS) core coated on top and sides by oxide or nitride and a metal. The sidewalls overhang the SCS core. The beams deform out of plane due to thermal and intrinsic strains of the coating films. These strains are defined and measured for plasma deposited SiO₂ and sputtered aluminum films. A linear elastic model of SCREAM cantilever beams is then developed to evaluate the deformation of the beams caused by film strains. The model predicts that the beams may bend up or down or remain planar depending on their cross-sectional design. Also, the greater the depth of the beams, the more planar they are, and a change in temperature (room temp-100°C) has little influence on planarity for beams with thin (~.2 μm) metallization. The model is validated by fabricating large (up to 2 mm long) cantilever beams, 1 and 2 μm wide, with PECVD SiO₂ and sputtered Al coatings. The deformations of the beams prior to metallization as well as before and after annealing of the metallized beams are measured. Good agreement is obtained between the experimental deformations and those predicted by the model. The paper is concluded with an example of a working, large (4×5 mm²), planar MEM device fabricated by the SCREAM process