S. Ilias, F. Picard, P. Topart, C. Larouche, H. Jerominek "Enhancement of structural stiffness in MEMS structures" Proc. of SPIE, 6109, art. no. 610907, 2006
Résumé: Many optical applications require smooth micromirror reflective surfaces with large radius of curvature. Usually when using surface micromachining technology and as a result of residual stress and stress gradient in thin films, the control of residual curvature is a difficult task. In this work, two engineering approaches were developed to enhance structural stiffness of micromirrors. 1) By integrating stiffening structures and thermal annealing. The stiffening structures consist of U-shaped profiles integrated with the mirror (dimension 200×300 µm2). 2) By combining selective electroplating and flip-chip based technologies. Nickel was used as electroplated material with optimal stress values around ±10 MPa for layer thicknesses of about 10 µm. With the former approach, typical curvature radii of about 1.5 cm and 0.6 cm along mirror width and length were obtained, respectively. With the latter approach, an important improvement in the micromirror planarity and flatness was achieved with curvature radius up to 23 cm and roughness lower than 5 nm rms for typical 1000×1000 µm2 micromirrors.
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