Due to the smoothness of the surfaces in surface micromachining, large adhesion forces between fabricated structures and the substrate are encountered. Four major adhesion mechanisms have been analysed: capillary forces, hydrogen bridging, electrostatic forces and van der Waals forces. Once contact is made adhesion forces can be stronger than the restoring elastic forces and even short, thick beams will continue to stick to the substrate. Contact, resulting from drying liquid after release etching, has been successfully reduced. In order to make a fail-safe device stiction during its operational life-time should be anticipated. Electrostatic forces and acceleration forces caused by shocks encountered by the device can be large enough to bring structures into contact with the substrate. In order to avoid in-use stiction adhesion forces should therefore be minimized. This is possible by coating the device with weakly adhesive materials, by using bumps and side-wall spacers and by increasing the surface roughness at the interface. Capillary condensation should also be taken into account as this can lead to large increases in the contact area of roughened surfaces. N Tas et al

This paper describes the design, fabrication and testing of laterally driven linear electrostatic micromotors. The motors employ mechanical leverage with the aim to increase the force from the order of 1 μN up to the order of 0.1 mN, in combination with walking motion to increase the stroke to virtually unlimited. Three designs have been made and tested. Although walking motion has been shown to be feasible in MEMS technology, improvement of the clamping is needed to benefit from the use of levers to increase the generated force.

Laterally driven linear electrostatic micromotors have been fabricated by standard surface micromachining. We attempt to employ mechanical leverage with the aim to increase the force from the order of 1 µN up to the order of 0.1 mN, in combination with walking motion to increase the stroke to virtually unlimited. Three designs have been made and tested. We conclude that mechanical levers with proper stiffness characteristics to be driven by electrostatic actuators are feasible. Friction as a function of the applied electrostatic clamp force has been measured, showing that there is a significant adhesion in the clamps. Walking motion has been successfully generated in one of the designs, generating a stroke of 15 µm and a force of 3 µN. Improvement of the clamping is needed to benefit from the implemented levers to increase the generated force.