The Excessive Humidity Effect on Capacitive Distance Sensors for Precision Positioning

In the high tech precision industry capacitive sensors often are used to measure distances with sub-nanometre accuracy and stability. It is well known that the dielectric constant of the medium between the sensor electrodes changes with the environment and is of influence on the measurement results. However, measurements performed at the Delft University of Technology in an environment with changing humidity, with an electrode spacing ranging from 10 ?m to 100 ?m, showed disturbances of more than hundred times the expected value; in those measurements up to 250 nm was found instead of the expected disturbance of 1.7 nm. After inquiry also Physik instrumente could confirm such behaviour. Because such anomaly is of severe influence on the stability and accuracy of capacitive sensors in this report the influence of humidity is studied and described in detail. A possible explanation for the disturbances is the adsorption of water on the capacitive electrodes. Water is a dipole molecule that easily adsorbs on almost all surfaces. The amount of adsorbed water as well as the shape (smooth layer or droplet) depends on the humidity level as well as on the material type (hydrophobic, hydrophilic), roughness and contamination. When water molecules adsorb on the capacitive electrodes, the average dielectric constant between the electrodes increases. Each molecular layer of water disturbs the capacitive measurement with an amount equal to its layer thickness of 0.31 nm. In a number of publications the presence of adsorbed water is recognized, but the anomalous effect is attributed to short circuiting due to water layers adsorbing on the insulators. From most studies in other fields it appears that only a few nanometres of water is adsorbed. Those who reported on thicker layers described that when the adsorbed layer thickness passes 1 nm, droplets start to form, with heights up to 100 nm. Since no clear relation could be found in literature, a measurement setup was designed and built. This setup consisted of a capacitive sensor and an optical reference sensor; a laser interferometer. The capacitive sensor contained one aluminium electrode and one semi-transparent electrode of gold-palladium sputtered on a fused silica surface, which enabled the interferometer to measure at exactly the same location as the capacitive sensor. When measuring at the same location the same adsorbed water droplets could be measured. It follows from theory that both measuring concepts should respond in opposite direction to water layers. This difference could be used to determine the total water layer thickness. ii When using this experimental setup with an electrode spacing of 50 ?m while changing the humidity level from 30 to 90 %RH, differences in sensor output up to 400 nm were found. These differences were proven to be dependent on the relative humidity and can be described with a power function. From measurements conducted with different electrode spacings it appeared that the effect did not depend on the electrode spacing. Therefore the assumption of adsorbed water on the capacitive electrodes holds. The effect of changing water layers on the laser interferometer was much smaller than expected. This can be explained by water droplets that diffuse the laser beam; the beam only reflects on the electrode at locations where the (wetted) surface is smooth. All in all the effect of adsorption of water on capacitive measurement is proven to be more than known in literature. However, since in practice the humidity is changing far slower than during the measurements in this report, sub-nm stability still is possible when using capacitive sensors.Precision and Microsystems EngineeringMechanical, Maritime and Materials Engineerin