Spectroscopic Sensing Method of Liquid Permittivity with On-Chip Capacitor

Abstract

A liquid permittivity sensing method is proposed, which is based on measuring the S-21 of an on-chip capacitor submerged in a material under test (MUT). The real part of permittivity can be estimated from epsilon' - vertical bar S-21 vertical bar 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 mu m X 195 mu m. A simple circuit model of the capacitor is used to develop epsilon' - vertical bar S-21&VERBAR relation with a fitting parameter. It shows less than 6.4% root-mean-square (rms) error of epsilon' for propanol (C3H8O) at 100 MHz-5 GHz.