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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

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 isolator and metal films is necessary due to the electrical working principles used. The different physical properties and deposition conditions of the films lead to thermal and intrinsic strains. This strains tend to deform micromechanical elements (bending).

The action of the mechanical strains of a vertical actuator for atomic force microscopy (AFM) has been analysed. The actuator consists of two torsional beams, a short comb element for force generation and a long cantilever beam for scanning (Fig. 1). The different electrostatic principles for actuation [1] as well as the fabrication technology [2] have been described in detail elsewhere. Fig. 2 shows the cross section of a cantilever beam with a high aspect ratio. Regardless of the higher stiffness in the z-direction vertical bending occurs also. An effect which is noticeable especially for long beams (more than 100 µm). Thermal and intrinsic strains are effective on the top, on the sidewalls and on the sidewall overhang. They depend on the Youngs modulus, the thermal expansion coefficient and the thickness of the layers. The final beam bending is a result of these strains in combination with the dimensions of the SCS core.

FEM simulations have been carried out in order to calculate the vertical beam bending dependent on the film thickness’ and the length of the sidewall overhang. A simulation example is shown in Fig. 3 illustrating the resulting beam deformation after a combined oxide and aluminium depo-sition. Thus, a defined pre-bending or negligible bending can be achieved by competing films: compressive (metal) against tensile (oxide) strain but also by adjustment of the ratio of thermal strain of the top film compared to the sidewall overhang. The latter one is of special interest because an actuator position adjustment independent of the material properties becomes possible. This z-direction tuning has been proven for a given silicondioxide deposited beam which nor-mally bends down due to the compressive strain. It will be shown that only by the application of a special dry etch process this bending can be converted into bending up representing a method for tuning the out-of-plain bending of free movable SCS structures independent of the deposited film thickness’.

REFERENCES

[1] Kerst Griesbach, Andreas Bertz, Klaus Wolf, Jürgen Wibbeler, Matthias Küchler, Thomas Geßner: Single Crystal Silicon Vertical Actuators based on Surface Micromachining Proceedings of the 8th International Fair and Congress for Sensors, Transducers & Systems, Nürnberg, 1997, vol. 3, pp. 7 - 11

[2] T. Gessner, A. Bertz, C. Steiniger, U. Wollmann: Material and Technology Approaches of Surface Micromachining, Micro Materials 1997, Berlin, Germany

MK, 19. November 1999