CMOSens® Opens Up New Applications

Sensor technology is of great importance in this regard as different devices and systems can trade information only when data is available. Sensors are the sensory organs within the IoT. The development of sensor technology has increased rapidly in recent years. Fifteen years ago, the standard humidity sensor was larger than a matchbox, which would have made many modern developments in IoT impossible. As a pioneer in the sensor industry, Sensirion has made a significant contribution to miniaturization. The key to this development is the integration of the sensor elements into standard CMOS technology. Sensors manufactured with this technology, known as CMOSens® Technology, are very small. Even the very first temperature sensor that Sensirion launched in 2001 was only 5 x 7.5 x 2.5 mm³ in size. It can then be integrated in electronics manufacturing in the usual way. Through integration of sensor elements into conventional CMOS technology, sensors have benefited from the continuous miniaturization in electronics. Sensors based on CMOSens® Technology can also be produced in very large volumes, making them particularly cost-effective.

Humidity sensors use a capacitive principle, whereby the sensor element is built out of a capacitor. The dielectric is a polymer that absorbs or releases water proportionally to the relative environmental humidity and thus changes the capacitance of the capacitor. An electronic circuit measures this change in capacitance, allowing the relative air humidity to be determined. A patented, “micro-machined” finger electrode system forms the capacitance. Applied in several processing steps, the different protective and polymer cover layers simultaneously protect the sensor from interference. Together with the humidity sensor, a temperature sensor is also included on the CMOS. This also enables an accurate determination of the dew point, without incurring errors due to temperature gradients between the humidity and temperature sensors.

Microsensor systems manufactured with CMOSens® Technology have a high degree of integration and functionality. The analog and digital signal processing as well as the calibration data are integrated on the semiconductor chip along with the sensor element, covering an area of a few square millimeters. In addition to functions for linearization, digitization and temperature compensation of the measurement values, other functionalities like a self-test are also included directly on the chip. Thanks to this integration, signal processing electronics can strengthen and digitize the sensitive sensor signals directly on site. Solder joints that are prone to failure are also omitted, which further contributes to long-term stability. At the same time, interferences of the analog sensor signals are virtually eliminated, thus achieving outstanding sensor precision. A digital interface is also integrated on the CMOS element, making it easy to integrate into the application.

Challenge of Series Production and Packaging

The technical challenge was to integrate the sensor element into standard CMOS production. The sensitive polymer must not be damaged during the CMOS sensor production steps and subsequent processing. At the same time, it was necessary to make all production steps scalable so that series production is possible as this is the only way to carry over the economies of scale resulting from the production of semiconductors to the production of the sensors. An important step during manufacturing is sensor calibration, which must be maintained even after production. Long-term stability of the sensor also has to be ensured.

Another major challenge that Sensirion had to solve in the development of CMOSens® Technology was packaging. The standard procedure of encapsulating the semiconductor structure in a plastic is not suitable for humidity and temperature sensors as the sensor element has to come into contact with the ambient air. Sensirion’s solution was to develop what is known as open cavity molding, in which – simply put – an opening is left in the plastic. Once again, the challenge was that the sensor element must not be damaged during the production process and that all processes are scalable for series production. The latest range of humidity and temperature sensors makes use of a chip-scale package that can be processed with the flip chip method, allowing an even greater level of miniaturization. The smallest humidity and temperature sensor currently offered by Sensirion is a mere 1.3 x 0.7 x 0.5 mm3.