A new millimeter-wave micro-fluidic temperature sensor for wireless passive radar interrogation

This paper presents a micro-fluidic temperature sensor that uses a variable radar echo to measure temperature at Ka-frequency band. The device is made up of a planar gap capacitor with a micro-fluidic channel situated between the plates. As the temperature changes, the water level in the channel moves across the capacitor plates. This rising level of high permittivity liquid within the capacitor modifies the capacitance and as a result modifies the scattering parameter S11. The sensor can then be integrated with an antenna and interrogated at distance by a reader. The detected radar echo level changes in proportion to temperature when illuminated by the wave reader. Simulations were performed in order to optimize the design for the capacitor and channel, and to verify the change of S11. The micro-fluidic device was fabricated using an Su-8 micro-machining process. Measurements were performed using a VNA and a probe measurement station in order to verify the change in S11 with respect to the level of water in the micro-fluidic channel. The S11 measurements yield an 8dB range which is in agreement with the theoretical calculations that are explained in the paper. For the radar echo measurements, a 4dBm range is obtained. This corresponds to a 10° K measurable temperature range. This design allows wireless temperature sensing at a distance, thus making it an effective solution for distant temperature monitoring applications.