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 μm to 100 μm, 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 th