Measurement of liquid complex dielectric constants using non-contact sensors Jun Wan Kim, Praveen Pasupathy, Sheng Zhang, and Dean P. Neikirk Department of Electrical and Computer Engineering Microelectronics Research Center The University of Texas at Austin Austin, TX 78712 USA Abstract— Our primary objective in this work is the accurate measurement of complex dielectric constant and conductivity of unknown liquids. In this paper we present design, fabrication, and testing results for an IDC (interdigitated capacitor) electrode sensor for use as a fluid monitoring component that can be integrated into a microfluidic system. Unlike prior work we show how to accurately extract both the conductivity and permittivity of liquids, for loss tangents ranging from much less than one to greater than one (i.e., from lossy dielectrics to conductors); this has not been shown in previous work. In addition, we also demonstrate a method of remotely accessing the IDC sensor by wireless inductive coupling similar to EAS (Electronic Article Surveillance) tags. I. INTRODUCTION The miniaturization of chemical and biological sensors has received considerable attention in recent years for medical diagnostics, environmental monitoring, pharmaceutical screening, military applications, etc. One interesting area of development in microfluidic systems is detecting dielectric properties of a MUT (Material Under Test) using IDC electrodes (Fig. 1). IDC-based chemical sensors have been investigated by many researchers because they are cheap to manufacture and can be easily integrated with other sensing components and signal processing electronics. Unfortunately the methods discussed previously for extraction of dielectric constant are valid only when the MUT is lossless. In this paper we present a data extraction method and experimental results for materials ranging from essentially lossless (e.g., air, isopropyl alcohol, oil, etc.) to very lossy (e.g., salt water and nano-wire suspensions). II. MODELS FOR PARAMETER EXTRACTION A. Confomral mapping for IDCs A useful method for the calculation of the capacitance of IDC structures is the conformal mapping technique. Conformal mapping provides closed form expressions for the computation of the capacitance of IDC electrodes based on the geometry and property of the sensor. In 1977 Wei first evaluated the capacitance of an IDC with an infinite top layer based on conformal mapping [1]. This model was extended and improved by Veyres and Hanna [2] to evaluate the capacitance for a sensor having a finite layer structure in 1980. Since then, the model used by Veyres et al. has played an important role in analyzing the IDC structure in a variety of scientific applications. In our lab, Wentworth et al. used the model to characterize tape automated bonding (TAB) interconnect in 1989 [3]. In 1996 Gevorgian et al. [4] proposed a different model for an IDC with a multilayered top structure based on the same conformal mapping technique. The more general form for multilayered IDC electrodes was discussed by Igreja in 2004 [5]. A major limitation of these approaches, however, is their applicability only to lossless layers (i.e., layers with zero conductivity). Signal Electrode Ground Electrode ε sub h 1 w d h 2 ε ins S S G G ε sub ε ins l Figure 1: Illustration of insulated interdigitated capacitor structure. To measure the dielectric properties of liquid materials, it is advantageous to insulate the electrodes from the liquid in order to reduce electrical or electrochemical interference. Therefore, the capacitance effect of the insulation layer must be taken into account when performing impedance analysis of an IDC sensor. In the case of a multilayer structure where both the conductivities and dielectric constants of each layer vary simple conformal mapping approaches are problematic at best. For instance, in the conventional multilayer conformal The sponsors of this work include the Office of Naval Research under the University Affiliated Research Center (UARC) Basic Research Program and the Advanced Energy Consortium. 978-1-4244-5335-1/09/$26.00 ©2009 IEEE 2017 IEEE SENSORS 2009 Conference