Learn the basics of MEMS capacitive humidity sensors, namely their structure, performance metrics, dew point, and applications.
For many applications, advances in MEMS IC technology have transformed the complex art of humidity measurement into a simple, low-cost sensor. These sensors may not serve every application; however, they do work well in relatively clean extended-ambient conditions. Weather monitoring and air conditioning might be the obvious choice, though others include humidity control for food storage, warehousing, and printing. Furthermore, industrial processes such as chemical and pharmaceutical manufacture are best done in controlled humidities, and electronics assembly needs to avoid low humidities to prevent static discharge damage. In this article, we'll go over some basics of humidity measurement using capacitance sensing, typical types of MEMS humidity sensors, their performance, applications, and other considerations. Capacitive Humidity Sensor Measurement Basics Humidity is sensed by capacitance using moisture-sensitive dielectrics. Figure 1 illustrates the high-level construction basics of a capacitive humidity sensor. Figure 1. A capacitive humidity sensor's basic structure. Image used courtesy of XY When creating a capacitive humidity sensor, an electrode is first deposited on a substrate, usually silicon. Next, a thin humidity-sensitive dielectric layer, usually polymer, is deposited, and the second electrode, moisture-permeable, is added on top. Finally, the sensor is covered with a permeable layer to protect it from contamination and condensation. MEMS IC sensors include the circuitry necessary to convert the capacitance measurement to a digital or analog output and are manufactured using IC methods. The water molecule is highly polarized (with a dielectric constant of around 80), which can be much higher than polymers. When the dielectric absorbs water vapor, its dielectric constant increases, thus increasing the capacitance. At lower humidity, the dielectric gives up some water, and the capacitance goes back down. The change is nearly linear with RH…