Spectroscopic Sensing Method of Liquid Permittivity with On-Chip Capacitor

A liquid permittivity sensing method is proposed, which is based on measuring the S21 of an on-chip capacitor submerged in a material under test (MUT). The real part of permittivity can be estimated from ɛ′ – |S21| fitted relation in the frequency range of 100 MHz–5 GHz, which are pre-calibrated with four kinds of known materials. The on-chip capacitor is realized with a top metal in a complementary metal oxide semiconductor (CMOS) process, which has an inter-digitized structure featuring a small size of 150 μm × 195 μm. A simple circuit model of the capacitor is used to develop ɛ′ – |S21| relation with a fitting parameter. It shows less than 6.4% root-mean-square (rms) error of ɛ′ for propanol (C3H8O) at 100 MHz–5 GHz.