Measurement of the Surface Force in Micro Structures and Its Reduction

Abstract

In microstructures such as microsensors and microactuators, a small surface force can occur between two contacting surfaces which affects operations. Therefore, it is very important to clarify the generation mechanism of this surface force and to find a way to decrease it in the devices.In this study, a microstructure used in evaluating surface force was fabricated by a silicon micromachining technique. This device has a movable plate supported by cantilever beams and two opposing electrodes positioned above and below the movable plate. The surface force acting at the contact area of the movable plate and the opposing electrodes was evaluated as follows: The movable plate made contact with one of the electrodes when an electrostatic force was applied. Then another electrostatic force was applied to remove the movable plate by applying a voltage between it and the other electrode. The surface force was calculated from the voltage needed to separate them. This method was applied to measure the surface force under various conditions in which the amount of water adsorbed on the surface was different.It was confirmed that the surface force could be lowered by such measures as thermal treatment at about 200°C and hydrophobic treatment of the surface, which decreased the amount of water adsorbed on the surface from the atmosphere. This meant that interaction between the water molecules adsorbed on the surface was one of the main causes of the surface force.Another way to lower surface force was to make the contact area of the movable plate and the electrodes samller. For example, pyramid type stoppers, formed on the movable plate, were very effective to decrease the surface force.

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