Flow-through Microfluidic Relative Permittivity Sensor Using Highly-Doped Silicon Sidewall Electrodes

This paper reports a flow-through sensor for the real-time measurement of the relative permittivity of a flowing fluid. The proposed sensor contains multiple parallel microchannels. Each channel acts as a parallel plate capacitor in which the electrodes are isolated from the fluid. This approach results in a low pressure drop and high sensitivity while the internal volume is only 50 nL. The fabrication technology allows for controllable scaling of the channel geometry. Equally deep microchannels with widths varying between 5 µm and 50 μm are fabricated. The fabrication technology is compatible with other flow-through sensors, enabling further on-chip integration. The sensor has been characterised using 10 different fluids with relative permittivity values ranging from 1 to 80 – including water and water-containing mixtures – at a measurement frequency of 300 kHz. The results show an accuracy within 3% of full scale and the repeated measurements with nitrogen and water show standard deviations of 1.20 and 1.03, respectively.