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Matthias Küchler, Kerst Griesbach, Andreas Bertz, Thomas Gessner, Wolfgang Faust, Rainer Dudek:
Stress-caused Deflections of Asymmetrical Coated Single-Crystal Silicon Elements
Proceedings of the MRS Fall Meeting 98, Symposium AA, Boston, MA, USA, 1998, vol. 546
ABSTRACT
Microelectromechanical systems (MEMS) based on Single Crystal Silicon (SCS) have a wide variety of applications because of the well-known excellent mechanical properties. However, in most cases the deposition / growing of insulator and metal films (necessary due to the electrical working principles used) lead to thermal and intrinsic stresses which tend to deform micromechanical elements (bending).
Up to now the vertical deflection of long beams with equal wide trenches surrounding the beams leading to a symmetrical sidewall deposition were analysed [1] and chances for tuning were discussed [2]. However, for several applications, e.g. differential capacitor accelerometers trenches with differing widths are required. Depending on the step coverage of the used deposition methods (PECVD, sputtering) the varying trench width surrounding the beam results in asymmetrical sidewall depositions of the beam causing its lateral deflection.
This deflection in vertical and lateral direction has been analysed on single beams and complex cantilever structures. Firstly, FEM simulations have been carried out in order to calculate the vertical and lateral beam bending dependent on the different trench widths. Secondly, the vertical deflection of fabricated beams has been measured using a phase-shifting interferometer. The cross section of the beams and the lateral deflection are visible by a SEM inspection after a special cleave preparation (Fig. 1).
The knowledge of the dimensional stability of the beam depending on the degree of beam bending due to differing trench widths and cross sections is essential for the dimensioning of free standing elements just as a defined prebending or defined pre-contact force.
REFERENCES
[1] M.T.A. Saif et. al.: Planarity of large MEMS.
Journal of Microelectromechanical Systems , Vol. 5, No. 2, June 1996
[2] Kerst Griesbach, Andreas Bertz, Wolfgang Faust, Rainer Dudek, Matthias Küchler, Thomas Geßner: Investigation and Modification of the Mechanical Stress of Single Crystal Silicon Actuators Based on Surface Micromachining.
Proceedings of the 6th International Conference on New Actuators, Bremen, Germany, 1998, pp. 82 - 85
Fig. 1: SEM of bent beams due to asymmetrical sidewall deposition
MK, 26. Februar 2001